Symposium NM04—2D MXenes—Synthesis, Properties and Applications

MXenes are potentially the largest class of 2D materials discovered so far. With a general formula of M_n+1X_nT_x, M is an early transition metal (Ti, V, Nb, Ta, etc.), X is C and/or N, T_x represents the surface groups (-O, -OH, -F, -Cl), and n = 1–4, over 30 stoichiometric phases have already been discovered, with many more predicted computationally. MXenes have been widely studied owing to their exceptional properties, including hydrophilicity, scalability, mechanical strength, thermal stability, redox capability, and ease of processing. While the majority of research has focused on Ti₃C₂T_x, each MXene composition has its own unique optical, electrical, and mechanical properties. The incorporation of two metals into MXenes further imparts novel properties, and stabilized structures that cannot be formed otherwise. For ordered double transition metal MXenes, this allows for the incorporation of elements such as Cr, giving these MXenes unique magnetic properties. One other important class of MXenes are solid-solution MXenes, where multiple elements are randomly distributed within the M layers. These MXenes exhibit tunable properties that are directly related to their chemistry, whether optical, electronic or electrochemical. Moreover, solid-solution MXenes allow for the synthesis of MXenes with unique structures, such as M₅X₄T_x. By studying these, and other MXenes, it allows us to understand why MXenes have the properties they do, allowing us to rationally produce MXenes with desired properties.

A new two-dimensional (2D) materials family of transition metal carbo-chalcogenides (TMCCs) can be considered as a merge between two well-known families of 2D materials: MXenes and transition metal dichalcogenides (TMDCs), at the atomic scale. In this presentation, we will report on the synthesis, characterization, and properties of 2D Nb₂S₂C and Ta₂S₂C which are the first two members of TMCCs. Both are delaminated from their parent multilayered counterpart via electrochemical lithiation followed by liquid exfoliation in water. Electronic and magnetic properties of Nb₂S₂C multilayers were studied, a superconductivity transition is observed at 7.55 K. The effect of delamination on the electrochemical performance of Nb₂S₂C as electrode materials for Li-ion batteries was examined, and the delaminated Nb₂S₂C was found to surpass both multilayered Nb₂S₂C and delaminated NbS₂ in terms of capacity and stability. In addition, Ab initio calculations predicted elastic constant of TMCCs to be 50% higher than that of TMDCs.

Anhydrous synthesis of MXene facilitates the control over the surface functionalization and the final physicochemical properties. Here, we propose a novel anhydrous etching solution, consisting of high-boiling-point dimethylsulfoxide as organic medium, NH₄HF₂ as etchant, CH₃SO₃H as acid, and NH₄PF₆ as intercalant. This anhydrous solution does not only enable one to increase the reaction temperature up to 100 °C for accelerating the etching and delamination of Ti₃AlC₂ MAX crystals, but also suppresses the destructive side reaction of the produced Ti₃C₂T_x MXene flakes. Consequently, the etching reaction is completed in 4 h at 100 °C and produces high-quality monolayer Ti₃C₂T_x MXene with an electrical conductivity of 8,200 S cm^-1 and a yield of over 70%. The synthesized Ti₃C₂T_x exhibits more F-terminations than conventional aqueous Ti₃C₂T_x. The atypical surface structure of Ti₃C₂T_x MXene leads to an exceptionally high ultimate tensile strength (167 ± 8 MPa), which is about 5 times larger than the ones synthesized in aqueous HF solution (31.7 ± 7.8 MPa).

One- (1D) and two-dimensional (2D) materials offer advantages that their 3D counterparts do not. To produce such materials in bulk there are two conventional approaches: bottom-up and top-down. MXenes are an excellent example of the latter. The main drawback of this approach is high cost in first making the layered solid and then the many steps needed to etch and delaminate them. The bottom-up approach is in principle much more scalable. There are several bottom-up approaches, the one of interest here is the sol-gel approach common in the processing of ceramic powders, especially oxides of Ti and Si. The man disadvantages of this approach are the need to rely on the solubility of the precursor salt or compound in water and the need for a calcination and/or hydrothermal step that is usually needed to crystallize the products made at lower temperatures. In trying to etch the MAX phases without HF, we stumbled on a very powerful method to make 1D and 2D materials. The general idea is to dissolve inexpensive, non-water soluble, precursors in quaternary ammonium cation salts, or quats, at temperatures < 100 °C under ambient pressures for a few days. The aim of this talk is to describe the resulting materials and some of their properties in two systems: Ti and Mn. In the former, we converted 10 binary and ternary titanium carbides, nitrides, borides, phosphides, and silicides into anatase-based, 1D sub-nano filaments ≈ 6x10 Å in cross-section that self-assemble into 2D flakes by immersing them in a tetramethylammonium hydroxide, TMAH, solution at temperatures in the 50 to 85 °C range. The 2D flakes are quite well-ordered in the stacking direction. In some cases, the conversion is 100 % precluding the need for centrifuges, filters, etc. We currently routinely make 100 g batches in a lab setting. In some cases, we also make mesoscopic powders. Electrodes made from some of filtered films performed well in lithium-ion and lithium-sulphur systems. These materials also biocompatible and reduce the viability of cancer cells thus showing potential in biomedical applications. In the Mn-case, five water-insoluble Mn-bearing precursors, viz. Mn₃O₄, Mn₂O₃, MnB, Mn₅SiB₂, and Mn₂AlB₂, were converted to birnessite 2D MnO₂ flakes, that are remarkably crystalline. Here again, the precursor powders are immersed in 25 wt. % TMAH aqueous solutions at 50 °C to 80 °C, for 4 to 2 days, respectively. The 2D sheets demonstrate reversible O₂ electrocatalysis with activities comparable to those of a commercial Pt/C catalyst. Synthesizing 1D and 2D materials in bulk, at near ambient conditions, starting with non-layered precursors (e.g., TiC, Mn₃O₄) could usher a new age where 1D and 2D materials can be mass produced inexpensively using earth abundant elements and a green process.

Molybdenum Nitride (MoN), hardest superconducting material is estimated to have a critical superconducting temperature of 16 K. Although that these properties attract the attention of many researchers, there are limited studies investigating the effects of process parameters of chemical vapor deposition synthesis of MoN crystals. The melt catalyst method in which a catalyst (Cu or In) is placed on top of a metal foil (Mo) and after reaching the melting temperature of the catalyst, synthesis gasses (CH4, NH3) are supplied to produce 2D Mxene materials such as Mo2C and MoN. This method is advantageous to produce large area 2D crystals with reduced defects and once the process is optimized, the reproducibility and reliability of the method is makes it perfect for advanced electronic applications. We studied the effects of process parameters (effect of catalyst, catalyst thickness, catalyst type, reaction temperature, growth duration, gas amounts) on the growth of 2D MoN crystals and discussed the results. SEM, EDS and OM studies are used to investigate the morphology of these crystals and to further understand the structure of the produced crystals Raman spectroscopy, XPS, TEM and XRD studies are used. We also showed that it is possible to synthesize Molybdenum CarboNitride structures by treating previously synthesized Mo2C crystals with ammonia gas. This is the first study that investigates the transition of priorly synthesized Mo2C crystals to MoCN and MoN. Depending on the ammonia exposure duration, Mo2C crystals can be replaced by MoN crystals completely. For future work, it is aimed to find the corelation between superconducting transition temperature with the different nitriding durations. This study is based on work supported by the Air Force Office of Scientific Research under Award Number FA9550-19-1-7048.

The size of individual flakes is critical for 2D materials and their devices. For materials produced via a top-down approach, such as graphene oxide produced from graphite, every step of the synthesis process, including the particle size of the precursor and the exfoliation protocol, affects the size of the resultant material. The size, in turn, affects the properties of assembled films and fibers, and the synthesis of ultra-large flakes has been a target for multiple dedicated studies.
Titanium carbide MXene (Ti₃C₂T_x) is one example of 2D material produced via a top-down approach by selectively etching Al from the parent Ti₃AlC₂ phase, followed by delamination of the multilayer material into individual flakes. Ti₃C₂T_x has found promise in multiple applications, including energy storage, flexible electronics, and membranes, where the size of the flakes is an important parameter, and the increased average size can be beneficial. In this work, we will discuss the factors affecting the lateral size of MXene and ways to synthesize ultra-large individual flakes. Starting with the synthesis of the MAX precursor, etching, and delamination, each step of the process affects the resulting flake size. In particular, the delamination process is critical and must be controlled to achieve large individual flakes of MXene. We will demonstrate a new soft delamination protocol, which preserves the size of the flakes and provides an approach to fabricating individual flakes of up to 40 µm in lateral size.

Formidable efforts are invested world-wide on two-dimensional (2D) materials research. Most 2D materials are based on van der Waals crystals, that can be mechanically exfoliated (cf. graphene). The primary exception, two-dimensional metal carbides and nitrides (MXene), is produced by chemical exfoliation, i.e. selective etching of recurring layers in 3D layered precursor solids. The first MXene, Ti₃C₂, was first reported in 2011, and since then the family has increased drastically, to also include members with in-plane and out-of-plane ordering of metal atoms, and high entropy metal configurations. Synthesis of new MAX phases designed as precursors for specific MXenes have been crucial for this development, being inspired from exploratory theoretical predictions in uncharted structural and compositional space. The present talk will briefly summarize our work in this area, and show some light on how MXene development can serve as inspiration for selective etching of other layered solids. One such example is a new type of chemically ordered, quaternary metal borides, that can be chemically exfoliated into 2D sheets, and selectively prepared in multilayer form, as delaminated single-layer sheets in colloidal suspension, or as additive-free filtered films, for property evaluation. Furthermore, future two-dimensional materials from selective etching will be discussed, looking beyond MXenes and 2D materials derived from metal borides. Based on the vast library of layered solids, new families of 2D materials are expected through an interplay of computationally driven atomistic design and development of new chemical exfoliation pathways.

MXenes are attracting ever growing attention for applications in energy storage, electromagnetic interference (EMI) shielding, superconductivity, and catalysis. Traditional preparation of MXenes involves high-temperature synthesis the MAX phase followed by its chemical etching and exfoliation. This synthetic route has high energy consumption, and uses large amounts of hazardous HF or Lewis acidic molten salts. The development of fast, facile, and atom-economic synthetic methods would greatly facilitate widespread practical applications of the rapidly developing family of functional MXenes. An ideal approach would involve direct interconversion of inexpensive precursors into MXenes bypassing intermediate MAX phases.
Here we report direct synthesis of Ti₂CCl₂ MXene via novel chemical reactions. The reactions are easily scalable, inexpensive, energy efficient, and more environmentally friendly compared to the traditional route. The directly synthesized MXene also showed excellent energy storage capacity for Li-ion intercalation. Besides convenience and scalability, the direct synthesis route offers new synthetic modalities not compatible with traditional MAX phase etching methods. Thus, we demonstrated chemical vapor deposition (CVD) synthesis of Ti₂CCl₂, Ti₂NCl₂, Zr₂CCl₂, and Zr₂CBr₂ MXenes, producing carpets of MXene sheets oriented perpendicular to substrate. Such orientation makes MXene surface easily accessible for ion intercalation and exposes catalytically active edge sites.
Overall, direct synthesis and CVD of MXenes opens up new options in making previously unrealized MXenes and fabricating MXene devices. The development of vapor phase synthesis further expands the capabilities for integration of 2D MXenes into electronic and optoelectronic devices, including cross-platform integration with silicon microelectronics.

With growing interest in two-dimensional (2-D) materials group and their outstanding properties, considerable attention has been paid to a family of 2-D transition metal carbides (MXene). Herein we summarize our recent works on electrically conductive MXene, especially for Ti₃C₂T_x in thin-film state at humid conditions. Our scale-up production system provides high quality crystalline MXene flakes to exhibit remarkable electrical conductivity as high as 18000 S/cm at 5 μm thickness. Several techniques were successfully used to monitor the stability of MXene dispersion and the fabricated films. X-ray crystallographic studies indicated that the interlayer spacing change by water uptake has a significant effect on the conductivity, and therefore further investigation to prevent degradation are highly required for applications under humid condition at high temperature.

2D ultrathin transition-metal carbides (TMCs), nitrides (TMNs), and carbonitrides are a family of important functional nanomaterials for various applications due to their properties that are directly dependent on the structure of the crystals, which can be controlled by processing. MXenes formed by wet etching which may consist of defects and surface functional groups (due to the use of acids) are especially useful for applications like composites, energy storage; however, for advanced electronics it is necessary to develop a versatile technique to produce high quality MXenes with controlled thickness, area, and high reproducibility.

Biased CVD is a promising technique to produce large area MXenes. In Biased CVD, a stack of metal substrate (e.g. Mo and Cu) is exposed to a volatile precursor (e.g., CH₄) which decomposes on the substrate surface (e.g., Cu) and reacts with the Metal (e.g., Mo) that is biased towards the surface to produce the desired deposit (e.g., Mo₂C). It is called biased because the catalyst layer (Cu) is used like a diode: Cu lets Mo atoms to diffuse through Cu to the surface but does not let carbon atoms to diffuse through Cu (due to the difference in solubilities). Hence the diffusion is simply called biased.

Studies in literature showed that the morphology and the thickness of Mo₂C crystals are strongly affected by catalyst type, thickness, composition, and the simultaneous graphene growth during CVD. To produce controlled morphology (thin, large area, high quality MXene crystals), the understanding of the growth mechanism via biasing reaction is crucial. However, it is still not clear and reproducible control of thickness and morphology is still a problem. Although it is revealed that simultaneous graphene growth affects the morphology and the thickness of Mo₂C crystals; there are conflicts in literature and the effect of its presence on the Mo₂C formation is not well understood. Moreover, fundamental physico-chemical and kinetic parameters related to the bulk and surface diffusion (associated with graphene, M and X) are missing in the literature and there is lack of data on the nucleation and growth kinetics of MXenes for physico/chemical modelling.

In this presentation, we present an investigation on the controlled growth of thin Mo2C crystals on both Cu and In substrates via CVD using CH4. SEM, EDS, Raman spectroscopy, XPS, and XRD studies show that hexagonal Mo2C crystals, which are orthorhombic, grow along the [100] direction together with graphene and an amorphous carbon thin film on Cu and In, respectively. The growth mechanism is examined and discussed in detail, and a model is proposed. AFM studies agree well with the proposed model.

This material is based on the work supported by the Air Force Office of Scientific Research (Award number: FA9550-19-1-7048).

2D transition metal carbides, nitrides, and carbonitrides (MXenes) have unique and tunable physical, chemical, and mechanical properties that drive their application in chemical sensing, energy storage, and water purification. Among these properties, the excellent dispersion of Ti₃C₂ MXene in water remains one the most important for applications that require solution processibility. However, MXene susceptibility to oxidation in aqueous environments remains a challenge for suspension storage and environmental applications. While there are many computational works that predict property changes for MXenes with altered surface chemistry, successful and controlled experimental manipulation of MXene surfaces remains challenging. This work investigates new methods of controlled MXene surface functionalization that take advantage of the material’s intrinsic -OH and -O surface terminations. These methods are based on past group work involving the formation of organic molecular monolayers on metal oxides with hydroxylated surfaces. The organic molecules were bound to the metal oxide surface through alkoxy bonds, and the monolayers were found to be stable in aqueous environments. Current work utilizes X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy to elucidate the intrinsic and altered surface chemistry of lab synthesized 2D Ti₃C₂ MXene. This work is important for the development of a simple and controlled functionalization method for MXenes with the potential to bind a wide variety of molecules to the surface.

The transition metal carbide (TMC) family, historically studied as refractory metals for chemical and mechanical applications, is receiving increasing attention in the ultrathin limit. The study of TMCs has been extended to this limit first with the isolation of MXenes, atomically-thin layers of TMCs bound by interlayer van der Waals (vdW) forces;¹ and second with the development of a liquid-metal-assisted chemical vapor deposition (LMCVD) approach to produce nonlayered, ultrathin TMC (UThTMC) crystals with tunable lateral size (1-10 μm) and controllable thickness (less than 100 nm).² MXenes are typically produced as high yield, textured powders, while UThTMCs are high quality, single crystal platelets, ideal for electronic transport studies. Tungsten carbides have received little attention in the ultrathin limit, never isolated in the MXene family due to the lack of proper tungsten precursors. LMCVD, then, is an alternative method to produce ultrathin tungsten carbides as demonstrated in two studies.^3,4 These LMCVD studies typically produce one carbide phase, and to promote the study of novel UThTMCs phases, we probe the growth parameters to determine whether different tungsten carbide phases can be isolated. In this work, we use LMCVD to synthesize WC and W₂C phases via copper/tungsten (Cu-W) and gallium/tungsten (Ga-W) substrates, respectively. Moreover, we perform detailed structural and transport property characterization to understand the carbide phase-dependent transport properties in these ultrathin crystals.

In our LMCVD approach, we use two substrates, Cu-W and Ga-W, and tune remaining growth parameters to form tungsten carbide nanoplatelets. Scanning electron microscopy reveals differences in the carbide morphology, with triangular and hexagonal crystalline platelets formed on Cu-W and Ga-W, respectively. Moreover, we find that these crystals are ultrathin via atomic force microscopy measurements. To determine the crystal structure of these platelets, we carry out selected area electron diffraction, X-ray diffraction, and planar and cross-sectional transmission electron microscopy, and we find that the tungsten carbide is isolated in two phases with high crystallinity: WC (P-6m2) on Cu-W, and W₂C (Pbcn, P6₃/mmc, or P-3m1) on Ga-W, corroborated with first principles density functional theory calculations of the tungsten carbide equation of state. We also measure the superconducting transition temperatures and critical fields of both tungsten carbide phases.

Our work highlights some of the challenges inherent to the phase and polymorph identification of UThTMCs. While we can distinguish between WC and W₂C phases, further studies are required to determine the W₂C polymorph in the Ga-W system, and we conclude that we have one or more of the Pbcn, P6₃/mmc, and P-3m1 polymorphs. Our results suggest that proper choice of substrate in LMCVD can lead to novel, single crystal UThTMCs, and these carbide phases require careful structural characterization to identify accurately.

References
1. Anasori, B., Lukatskaya, M. R., & Gogotsi, Y. Nat. Rev. Mater. 2, (2017).
2. Xu, C. et al. Nat. Mater. 14, 1135–1141 (2015).
3. Zeng, M. et al. Nano Energy 33, 356–362 (2017).
4. Wang, C. et al. Adv. Electron. Mater. 5, 1–7 (2019).

MXenes are the novel two-dimensional metal carbides or nitrides derived from their MAX phases. Recently discovered Mxenes, attracted the attention of many researchers and widely studied for the applications of energy storage, catalysis, gas sensing, wastewater treatment etc. More than twenty Mxenes have been successfully synthesized and Mo₂CT_x holds a great potential due to its theoretically predicted electrochemical, thermoelectric properties and chemical stability. Nonetheless, the long synthesis duration which can amount up to several days due to the strong bonding between the Mo-Ga atoms is challenging. In order to overcome this problem, scientist have used hydrothermal etching and lowered the duration drastically. Additionally, due to the hazardous nature of hydrofluoric acid, there have been studies on using HCl as etchant in hydrothermal conditions to increase the chemical potential between gallium and chloride ions which have proven successful. In this study, the hydrothermal etching of Mo₂Ga₂C to produce Mo₂CT_x using different fluoride salts with hydrochloric acid and only using hydrochloric acid is investigated and effects of different salts (LiF, BaF₂, MgF₂ and MnF₄) and no salt is discussed. The fluoride salts are mixed with hydrochloric acid to obtain in-situ hydrofluoric acid, and two different reaction temperatures for the hydrothermal etching process is studied (140 °C and 160 °C) for 24 hours (5-7 days for HCl). The synthesized Mxenes are characterized by; SEM and EDS for investigating morphology and chemical composition, and XRD for structural analysis. After studying the process parameters, such as temperature and etchant type on the synthesis of Mo₂CT_x powders, Mo₂CT_x powders are hand-mixed with epoxy resin with differing weight percentages to produce epoxy/Mo₂CT_x composites. Then these samples’ electromagnetic interference shielding effectiveness performance is measured. The EMI Shielding properties of Mo₂C Mxene composite is reported for the first time in this study. EMI Shielding properties of the epoxy/Mxene composites are reported in accordance with "ASTM D4935 Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials" using a vector network analyzer in the frequency range of 1.5–10 GHz. This study is supported by Air Force Office of Scientific Research grant number FA9550-19-1-7048.

The family of 2D carbides and nitrides called MXenes has grown to encompass numerous structures and compositions. These materials have been explored in a variety of applications such as energy storage, wireless communication, optoelectronics, and medicine because of their high electrical conductivity, redox-active surfaces, plasmonic behavior and other attractive properties. MXenes are typically derived via topochemical etching of atomically thick layers from precursor layered MAX phases using aqueous corrosive etchants. Knowledge of the reaction mechanism and process kinetics are of fundamental importance for synthesis and property control of MXenes. Prediction of the optimal synthesis approaches will facilitate new MXene composition discovery and prediction of optimal processing time as a function of various parameters will also facilitate scaling up wet chemical synthesis of MXenes for industrial use. Despite their importance, such studies have been challenging because of the atomic thickness of the A-element layers being etched and the aggressive etchants that hinder in-situ studies. Herein, we explore various ex-situ techniques to probe the effect of critical etching parameters on the structure, chemistry, and properties of the resulting MXenes; as well as develop an in-situ analytical technique to quantitatively measure the etching kinetics through byproduct hydrogen gas collection and tracking; and finally develop an in-situ optical microscopy and profilometry technique to directly visualize the structural evolution during the etching reaction. Through these methods, we are able to derive empirical models for prediction of etching kinetics, and gain a fundamental understanding of how the etching reaction starts, proceeds, and finishes.

MXenes, two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides, are a rapidly growing family of nanomaterials that have potential uses ranging from energy storage, catalysis, and electromagnetic interference shielding to medical applications. To recognize these applications, it is known that etching and subsequent delamination play a vital role in the quality and scalability of MXenes. Here, we present the experimental strategies to synthesize the most studied MXene, Ti₃C₂T_x, using different precursor powder, etching conditions, and delamination methods. For etching, we compare different etchants, including hydrofluoric acid (HF)/hydrochloric acid (HCl) mix and HCl/lithium fluoride (LiF) mix, and conditions such as time and temperature. For delamination, we investigate the effect of time, temperature, and environment (air or argon) on the quality of the Ti₃C₂T_x MXene. We analyze the resultant Ti₃C₂T_x MXene using x-ray diffraction, scanning electron microscopy, and electrical conductivity to illustrate the relation between different synthesis methods on the quality of single-to-few layer MXene flakes of Ti₃C₂T_x. With this detailed synthesis study on etching and delamination methods, we suggest updated optimal parameters for the synthesis of high quality Ti₃C₂T_x MXenes.

Among the burgeoning of novel MXene materials, Molybdenum carbide stands out as they are successfully synthesized by conventional selective chemical etching and chemical vapour deposition growth methods. Molybdenum carbide (Mo2C) is garnering growing research attention due to their interesting chemical and electronic properties. Their properties make them a promising material for future electrochemical energy storage applications. In this work, we show the 3D self-assembly architectures of 2D Molybdenum carbide grown successfully using a chemical Vapor deposition for the first time, to the best of our knowledge. Our controllable CVD approach allows to engineer several aspects of the as grown Mo2C flakes, in particular their orientation of growth with respect to the surface of growth substrate. The as synthesized Mo2C was further investigated by X-ray diffractometry, Raman spectroscopy, scanning electron microscopy and aberration-corrected scanning transmission electron microscopy to gain understanding of the influence of growth conditions on their chemical and structural characteristics.

MXenes is a family of transition metal carbides, nitrides or carbonitride materials that includes more than 20 different chemical compounds, that can be each synthesized with different methods and post-processed in multiple ways, enabling them to be used in a variety of applications. Consequentially, several thousand articles have been published that either mention MXenes or target them. Herein, we have gathered 15,116 articles since 2011, including the first publication about MXene, up to 31 December 2021 from 12 main scientific journal publishers (1. Cambridge University Press, 2. Elsevier (ScienceDirect), 3. Institute Of Physics Publishing (IOP), 4. John Wiley & Sons: Wiley Online (Wiley), 5. Nature Publishing Group (Nature), 6. Oxford Academic Press, 7. SAGE, 8. Springer, 9. Taylor & Francis, 10. American Association for the Advancement of Science (AAAS), 11. Royal Society of Chemistry (RSC) and 12. American Chemistry Society (ACS)). We visualize the results in a range of graphs and figures and try to find trends, correlations between them, but also to identify the gaps that this domain may have for future investigations. We probe the growth of MXenes through the year, statistical occurrence of keywords, MXenes’ properties, cluster of applications, specific applications, synthesis methods, which MXenes have been investigated the most over the last decades, 3D architectures, theoretical against experimental articles and combinations of these areas. On one hand, it is obvious that the MXenes research community grows every year dramatically, but on the other hand the investigations are very limited towards specific aspects of this 2D family. Initially, the scientific body has related MXenes with the Ti₃C₂ and there is very little attention for the rest of the materials. Hence, the properties and the applications that are targeted are also limited and on the way that Ti₃C₂ can achieve high performance. There is the need to investigate MXenes further than the titanium carbides, to gain a more comprehensive knowledge for these 2DMs from their synthesis to in what applications can be used. In addition, the problems that occur from the HF-based synthesis also must addressed either with post-process of the MXenes or by using a different path to synthesize these extortionary 2DMs, such as alkali treatment, electrochemical-etching and molten salts; especially the last one, due to the homogeneous surface chemistry of MXenes which is the most crucial advantage and disadvantage of these materials at the moment.

Two-dimensional (2D) MXene has shown enormous potential in scientific fields, including energy storage and electromagnetic interference (EMI) shielding. Unfortunately, MXene-based material structures generally suffer from mechanical fragility and vulnerability to oxidation. Herein, mussel-inspired dopamine successfully addresses those weaknesses by improving interface interaction and ordering in MXene assembled films. Dopamine undergoes in situ polymerization and binding at MXene flake surfaces by spontaneous interfacial charge transfer, yielding an ultrathin adhesive layer. Resultant nanocomposites with highly aligned tight layer structures achieve approximately seven times enhanced tensile strength with a simultaneous increase of elongation. The ambient stability of MXene films is also greatly improved by the effective screening of oxygen and moisture. Interestingly, angstrom thick polydopamine further promotes the innate high electrical conductivity and excellent EMI shielding properties of MXene films. This synergistic concurrent enhancement of physical properties proposes MXene/polydopamine hybrids as a general platform for MXene-based reliable applications. Furthermore, tight interfacial integration between MXene and polydopamine can be extended to MXene-based three-dimensional nanocomposite capacitive pressure sensor applications.

In recent years, multiple principal elements of two-dimensional transition metal carbides (MXenes) and its precursor high entropy (HE) MAX phase have been synthesized successfully. Mixing multiple transition metals opened a new ideology to explore the attainable chemistries and led to the discovery of new properties of the MAX & MXene family. It has been reported that HE MXenes show improved conductivity, charge storage capacity, and catalytic ability, as well as superior mechanical properties, as compared with single transition metal MAX. The unprecedented physicochemical properties were found to be closely related to short-range orderings, local lattice distortion, sluggish diffusion, and other features resulted from high configuration entropy.

This work aims to provide fundamental information on structural & chemical configurations at the atomic scale via the S/TEM characterization of HE MXene & MAX materials. The primary equipment used for characterization is JEOL ARM 200F and FEI Titan Themis 300, equipped with a spherical aberration corrector to achieve sub-angstrom imaging resolution and sub-nano spatial resolution. The STEM shows a hexagonal crystal structure of the MAX phase (space group P63/mmc, 194), and stress analysis indicates a strong correlation between lattice distortion and configurational entropy. The detailed chemical arrangement of multiple transition metals was revealed by atomic resolution EDS mapping while the HE MXene lattice and defect configuration was analyzed by Z-contrast (HAADF) images. The apparent atoms exhibit a stochastic arranged variation in brightness, revealing the randomly multiple principal atomic occupation. Our preliminary results offer a chance to garner in-depth understanding of the complex mechanism among multicomponent HE MAX & MXene materials.

MXenes are potentially the largest class of 2D materials discovered so far. With a general formula of M_n+1X_nT_x, M is an early transition metal (Ti, V, Nb, Ta, etc.), X is C and/or N, T_x represents the surface groups (-O, -OH, -F, -Cl), and n = 1–4, over 30 stoichiometric phases have already been discovered, with many more predicted computationally. This class of materials has been widely studied owing to their exceptional properties, including hydrophilicity, scalability, mechanical strength, thermal stability, redox capability, and ease of processing. Because MXenes inherit their structure from MAX phase precursors, understanding MAX phase synthesis leads to control over flake size, defect density, and chemical composition of the resultant MXene. One understudied, yet important class of MXenes are solid-solution MXenes, where multiple elements are randomly distributed within the M layers. Herein, a set of multi-M chemistries (Mo, V, Ti, Nb) are used to study the effect of structure and chemistry on MXenes. While solid-solution MXenes have unique and tunable chemical, optical, and electronic properties, they also enable the formation of novel MXenes that cannot exist otherwise. By choosing specific chemistries, we can then begin to understand fundamental aspects of MXene chemistry and structure.

Technical developments that have improved the optical performance in transmission electron microscopes have also enabled significant developments of in situ and in operando experiments under the electron beam. In the field of MXenes, in situ investigations have hitherto remained scarce, although there is much to learn about the interactions between MXene and ambient gas or liquid.
In the present talk, I will review state of the art technical possibilities and their boundary conditions. I will also review our efforts to understand the interactions between MXene, gas and vapor, and how results from these interactions help build an understanding for the stability of MXene and MXene terminations.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) show impressive performance in applications, such as supercapacitors, batteries, electromagnetic interference shielding, or electrocatalysis. These materials combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, and surface-engineered MXenes represent an ideal platform for fundamental and applied studies of interfaces in 2D functional materials.
The comprehensive understanding of MXene surfaces will be required for prescriptive engineering of their physical and chemical properties. We discuss general strategies to install and remove surface groups by performing substitution and elimination reactions. Successful synthesis of MXenes with oxo-, imido-, thio-, seleno-, or telluro- terminations, as well as bare MXenes (no surface termination), and hybrid organic-inorganic MXenes are demonstrated. The description of MXene surface structure requires an intricate mix of concepts from the fields of coordination chemistry, self-assembled monolayers and surface science. MXene surface groups control biaxial lattice strain, phonon frequencies, electrochemical performance, the strength of electron-phonon coupling, making MXene surfaces not spectators but active contributors to conductivity, superconductivity and catalytic activity.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) show impressive performance in applications, such as supercapacitors, batteries, electromagnetic interference shielding, or electrocatalysis. These materials combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, and surface-engineered MXenes represent an ideal platform for fundamental and applied studies of interfaces in 2D functional materials. A natural step in structural engineering of MXene compounds is the development and understanding of MXenes with various organic functional groups covalently bound to inorganic 2D sheets. Such hybrid structures have the potential to unite the tailorability of organic molecules with the unique electronic properties of inorganic 2D solids.
Here, we introduce a new family of hybrid MXenes (h-MXenes) with amido- and imido-bonding between organic and inorganic parts. The description of h-MXene structure requires an intricate mix of concepts from the fields of coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic components. h-MXenes also show superior stability against hydrolysis in aqueous solutions.

Transition metal-based carbides that belong to the family of MAX phases are an intriguing class of materials and have attracted a lot of attention as precursors for the relatively young class of 2D materials, the family of MXenes. Our group utilizes a diverse set of synthesis techniques to prepare new MAX phases as well as known ones with unique morphologies. Cr₂GaC in the shape of carbonaceous microwires, hollow and full microspheres as well as hitherto unknown carbonitride phases, such as Cr₂GaC_1-xN_x and V₂GaC_1-xN_x, are a few examples of our recent works. We strive to exfoliate these compounds into the respective 2D structures to study their electrochemical properties. All products are structurally characterized by diffraction and electron microscopy techniques.

Being a single member of carbonitride family, Ti₃CNT_x exhibited unique properties and behavior compared to its carbide counterpart in applications including energy storage, electromagnetic shielding, optics, sensing, etc. It had been the only carbonitride MXene for more than 10 years until we reported on Ti₂C_0.5N_0.5T_x in 2021 which exhibited the highest specific capacity among all reported multilayer MXene electrodes. Previously, we have also shown that Ti₃CNT_x is an excellent electrocatalyst for hydrogen evolution reaction with an onset potential of 56 mV which is significantly lower than that of Ti₃C₂T_x. All of the reports of carbonitride MXenes support the hypothesis that replacing carbon partially with nitrogen in X cites can increase the number of electrons as well as active sites in MXenes. Hence, we focused on enriching the carbonitride MXene family with new members to gain a further understanding of the synthesis and property of carbonitrides. It is noteworthy to mention that synthesis of carbonitride MXene is challenging due to the lower stability of nitride MXenes compared to carbides and also the lower number of Carbonitride MAX Phases to start with. Herein, for the first time, we report on the synthesis of new four titanium carbonitride MXenes Ti₄C_(3-y)N_yT_x (y = 2.2, 2.4, 2.6, and 2.8) derived from four new 413 MAX phases. By tuning the synthesis conditions using potassium fluoride and hydrochloric acid and changing various etching parameters we realized Ti₄C_(3-y)N_yT_x MXenes from Ti₄AlC_(3-y)N_yT_x (y = 2.2, 2.4, 2.6, and 2.8). MXene thin films were also achieved by delaminating the multilayers. The synthesis of Ti₄C_(3-y)N_yT_x (y = 2.2, 2.4, 2.6, and 2.8) MXenes is unique because this is the first study of carbonitride MXenes' where carbon to nitrogen ratio has been modified systematically in X cites. Finally, we will present our new findings on their performances in electrochemical energy storage.

Despite numerous prior reports of molten salt etching of MAX phases, there have been few successful reports of water-dispersible MXene nanosheets, and none for Nb-based MXenes. We recently demonstrated that molten salt etching can be used to create water-dispersible Ti3C2Tx nanosheets. Here we demonstrate the synthesis and aqueous dispersion of Nb2CTx nanosheets via molten salt etching by utilizing a KOH wash to add hydroxyl group to obtain water-dispersible Nb2CTx nanosheets. However, little is known about the oxidation of molten salt etched MXenes compared to acid-etched MXenes. Our results indicate slower oxidation kinetics for MXenes etched by molten salts, which may be due to the decreased amount of oxygen-containing terminal groups.

A large family of two-dimensional transition metal carbides and nitrides (MXenes) raises interest for many applications due to their high electrical conductivity, mechanical properties [1], potentially tunable electronic structure [2], nonlinear optical properties [3], and the ability to be manufactured in the thin film state [4]. However, their chemistry that is key to development of these applications, still remains poorly understood [5,6]. In this presentation we will discuss recent progress in understanding fundamental MXene chemistry and harnessing it for suppressing unwanted reactions and prolonging stability of these materials.
For example, suppressing oxidation and hydrolysis at high pH was demonstrated as an effective way to prolong shelf-life and stability of MXene aqueous colloids [7].
Other selected examples illustrating connections between understanding MXene chemistry and development of their applications will also be considered.

References
1. Y. Li, S. Huang, C. Wei, C. Wu, V. N. Mochalin, Nature Communications, 10, 3014 (2019)
2. M. Naguib, V. N. Mochalin, M. W. Barsoum, Y. Gogotsi, Advanced Materials, 26(7), 992-1005 (2014)
3. J. Yi, L. Du, J. Li, L. Yang, L. Hu, S. Huang, Y. Dong, L. Miao, S. Wen, V. N. Mochalin, et al., 2D Materials, 6, 045038 (2019)
4. Y. Dong, S. Chertopalov, K. Maleski, B. Anasori, L. Hu, S. Bhattacharya, A. M. Rao, Y. Gogotsi, V. N. Mochalin, R. Podila, Advanced Materials, 30(10), 1705714 (2018)
5. S. Huang, V. N. Mochalin, Inorganic Chemistry, 58(3), 1958 (2019)
6. S. Huang, V. N. Mochalin, ACS Nano 14(8), 10251-10257 (2020)
7. S. Huang, V. N. Mochalin, Combination of High pH and Antioxidant Improves Chemical Stability of Two-Dimensional Transition Metal Carbides and Carbonitrides (MXenes) in Aqueous Colloidal Solutions, Inorganic Chemistry, (2022 in press)

Understanding the interlayer interaction in two-dimensional (2D) transition metal carbides and nitrides (MXenes) is important to improve their exfoliation/delamination process and application in (nano)-tribology. The layer-substrate interaction is also essential in (nano)-tribology as effective solid lubricants should be resistant against peeling-off during rubbing. In our study, Ti-based MXenes with both homogeneous and mixed terminations are modeled using density functional theory (DFT). An ad-hoc modified dispersion correction scheme is used, capable of reproducing the results obtained from a higher level of theory. The nature of the interlayer interactions, comprising van der Waals, dipole-dipole, and hydrogen bonding, is discussed along with the effects of MXene sheet’s thickness and C/N ratio. Our results demonstrate that terminations play a major role in regulating MXenes’ interlayer and substrate adhesion to iron and iron oxide and, therefore, lubrication, which is also affected by an external load. Using graphene and MoS₂ as established references, we verify that MXenes’ tribological performance as solid lubricants can be significantly improved by avoiding –OH and –F terminations, which can be done by controlling terminations via post-synthesis processing. Our study on the interaction of Ti-based MXenes with water molecules confirms their extremely high hydrophilicity. The presence of -OH groups and vacancies on the surface inevitably strengthens their interaction with water. The dissociative chemisorption of water is found to be a favorable process when it occurs on both MXene’ edges and surface’ vacancies cluster, leading to material degradation. Our results try to explain the mechanisms of the spontaneous oxidative degradation of MXenes under aqueous environment.
These results are part of the ”Advancing Solid Interface and Lubricants by First Principles Material Design (SLIDE)” project that has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Grant agreement No. 865633).

The mechanical properties of MXenes are of crucial importance in a number of applications currently being studied, including electrochemical energy storage, flexible electronics, and structural composites. However, the difficulty of performing mechanical characterization experiments on 2D materials has limited the study of these important properties. Ab initio computational techniques have helped to fill in some knowledge gaps, but the length scale limitations of these methods preclude the study of more realistic MXenes containing heterogeneities such as point defects and surface terminations. Atomistic simulations, operating at larger length-scales, provide a solution, enabled by the recent development of a highly scalable bond-order interatomic potential for the titanium carbide MXenes. Large-scale simulations utilizing this potential can be used to study both in-plane and out-of-plane elastic properties as well as fracture behavior, crucially eliminating any system size effects. Results indicate a typical tradeoff between in-plane strength and out-of-plane flexibility depending on MXene structure and composition. The inclusion of surface vacancies has a deleterious effect on MXene mechanical properties, which may be unavoidable due to the harsh nature of MXene synthesis. However, via appropriate surface termination engineering, these effects can be mitigated and, in some cases, even overcome entirely. These newly gained insights at the atomic-scale should help inform synthesis and post-processing techniques seeking to produce mechanically superior MXenes.

2D transition metal carbides (MXenes) are promising as additives in ceramic systems due to their high temperature phase stability of their resulting carbides and in-plane stiffness. High temperature annealing studies on Ti₃C₂T_x MXene have shown phase transformation to lamellar, non-stoichiometric cubic TiC_y. Silicon carbide (SiC) is known for the high hardness, high wear resistance and stability for a wide range of temperatures. However, the low fracture toughness limits monolithic SiC applications. A ceramic structure with layers of TiC can potentially improve the fracture toughness of SiC. In the present work, we investigated the tailorable preparation of green bodies of nanometer thin Ti₃C₂T_x MXenes with SiC powders via electrostatic self-assembly, in a surfactant-free, scalable aqueous media. Because of negative surface charges (zeta potential of -35 to -50 mV) of MXene flakes, they can be mixed with SiC in pH ~ 5 and lead to a uniform self-assembly of Ti₃C₂T_x and SiC, thereby eliminating the need for additional surfactants. Prepared green bodies are sintered by pressure-assisted pulsed current sintering. Addition of 2D MXene fillers into SiC system improves the densification and lowers the sintering time and temperature. The effect of temperature, dwell time and pressure on the densification of the green bodies are studied, which can provide insights on the role of MXenes in improving SiC mechanical properties and their stability.

The US Army DEVCOM Soldier Center is the Army’s Warfighter-centric research facility. To continuously improve protection and effectiveness of our Soldiers, new technologies, particularly in advanced materials are needed. The MXene research community has continued to demonstrate the vast potential and feasibility of MXenes in accelerating the performance of many relevant technologies. Of particular interest to the Department of Defense community, and specifically at Soldier Center, are water filtration and remediation, portable power and energy generation and storage, sensors, environmental protection, and signature management. This presentation will focus on providing an overview of these critical needs areas, highlighting prior and on-going MXene research with direct relevance to the Soldier Center mission.

Ordered Double Transition Metal MXenes (DTMs) and their MAX phases are a more recent addition to the rapidly growing MXene family of 2D materials. The few reports published on the electrochemical charge storage properties of DTMs suggests that they perform much better than their mono metal counterparts. This is true for both ordered and solid solution DTMs. Despite the theoretical predictions on the stability of a number of double transition metal MAX phases, only a few have been synthesized and etched into their corresponding MXenes. In attempt to scale up the synthesis and processing of DTMs, we present the synthesis and structural studies of ordered Mo₂Ti AlC₂, Cr₂TiAlC₂, Cr₂Ti₂AlC₃ and solid solution Ti₂NbAlC₂ MAX phases and their etching into corresponding MXenes. In-situ high temperature XRD data collected on the admix of activated metal powders was used to optimize and model the MAX phase synthesis in high temperature tube furnace. Electrochemical studies such as CV, EIS and CCD of the DTMs compared with Ti₃C₂Tx will be presented and discussed.

Note: This abstract was submitted initially to the MRS-spring meeting but couldn’t be presented due to late issuance of US visa to the author.

David Pinto et al., “Synthesis and Electrochemical Properties of 2D Molybdenum Vanadium Carbides – Solid Solution MXenes,” J. Mater. Chem. A 8, no. 18 (2020): 8957–68, https://doi.org/10.1039/D0TA01798A.

Likui Wang et al., “Adjustable Electrochemical Properties of Solid-Solution MXenes,” Nano Energy 88 (2021): 106308, https://doi.org/https://doi.org/10.1016/j.nanoen.2021.106308.

Babak Anasori et al., “Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes),” ACS Nano 9, no. 10 (October 27, 2015): 9507–16, https://doi.org/10.1021/acsnano.5b03591.

V₂C MXene theoretically has one of the highest energy capacitance properties compared to other MXenes and as a result there is big interest toward this area. Herein, we report one of the purest V₂C results without any impurities (V₂AlC, AlF₃, etc.) through “in-situ” HF synthesis at <60^oC (No need for autoclave) with high delamination rate and low oxidation level due to the self-assembling nanoscrolls of single and/or few flakes of V₂C during the post-synthesis procedure of the intercalation/delamination step with an organic solvent (e.g. quaternary ammonium salt: TMAOH, TPAOH, TBAOH). The morphology and structure of the V₂CT_Z MXene was characterized by X-ray diffraction, scanning/transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and BET Surface Area Analysis. Furthermore, the papyrus-like V₂C structure capacitance properties are investigated by cyclic voltammetry, galvanostatic charge–discharge and electrical impedance spectroscopy.

MXenes, a large family of two-dimensional materials, have attracted a lot of interest due to their large chemistry space and the diverse chemical, electrical, mechanical, and optical properties. In particular, MXenes’ metallic conductivities and redox-active surfaces make them attractive for electrochemical energy storage. Recently, we demonstrated that partial oxidization of Ti₃C₂T_x MXene led to enhanced pseudocapacitance in water-in-salt electrolyte by cycling the material at 1.2 V vs. Ag wire. However, there is a fine line between partial oxidation of Ti₃C₂T_x MXene and complete oxidation that leads to the formation of titania and carbon. The extent of oxidation is difficult to measure with conventional X-ray and electron-based techniques. Raman spectroscopy has proven to be a powerful technique for detecting the degradation of Ti₃C₂T_x MXenes. This work focuses on using in situ electrochemical Raman spectroscopy to analyze the oxidation processes in Ti₃C₂T_x MXenes at a high anodic potential. The findings allow us to expand the knowledge of MXene electrochemistry and its degradation processes to achieve higher energy density in MXene-based energy storage devices.

Electrochemical nitrogen reduction reaction (NRR) is used to convert atmospheric nitrogen (N₂) to ammonia (NH₃) at ambient conditions. In general, acidic electrolytes are used for NRR to provide protons (H⁺) for the electrocatalytic reduction process. However, this leads to a low NH₃ selectivity due to the formation of the H₂ by-product via the hydrogen evolution reaction (HER). Recently, we showed that a Ti₂N MXene is active and selective for NRR in acidic electrolyte. We demonstrated that this new catalyst works through a Mars-van Krevelen (MvK) mechanism instead of the conventional associative/dissociative mechanisms. However, the effect of pH and electrolyte on the catalytic activity and selectivity was not well understood. Here, we investigate these effects by performing experiments in varying electrolytic conditions and pH values and develop relations between pH, electrolyte choice, and performance. The Ti₂N nitride MXene was synthesized via an oxygen-assisted molten salt fluoride etching technique and characterized through XRD, SEM, and FTIR spectroscopy. Our findings showed that changing pH does not affect the onset potentials for NRR, but does affect yield and selectivity. Alongside this, it was also found that pH changes the charge storage and HER performance of the material. It was found that as pH increases, selectivity towards NH₃ production increases due to the lower concentration of H⁺ able to participate in the HER. Favorable NRR selectivity values were obtained in non-aqueous protic electrolytes, such as 2-picoline/trifluoro-acetic acid because of the suppression of HER. These findings are being expanded to other MXene materials to advance our knowledge of the reactivity of MXenes in NRR, and to design optimal electrolytic conditions for the production of NH₃ through NRR.

Transition metal carbides and nitrides (MXenes) materials have distinctive properties that make them suitable for storing various mobile metal ions. For instance, MXenes have layered structures with large interlayer spacing, high electrical conductivity, and chemical diversity. However, MXenes lose electrical conductivity and redox activity due to restacking of the layers. Additionally, the uncontrolled attachment of surface functional groups like O, OH, and F influence their pristine electrochemical characteristics.
It is possible to take advantage of the properties of MXene by deriving hybrid materials from MXene. We developed a facile but efficient method to convert V₂CT_x MXene into V₂S₃@C@V₂S₃ heterostructures within 1 minute. Our results indicated that hybrid materials synergistically improve the performance of NIBs by combining the advantages of individual 2D materials.
The synergistic effect is shown in this unique design by alleviating conductivity, structural degradation, and sluggish kinetics of V₂S₃. Whereas the carbon interlayer in the anode behaves like flexible conductive support and an anchoring network. Moreover, the ultrathin V₂S₃ nanosheets and the V₂S₃-C heterointerfaces enhance the Na⁺ adsorption and migration abilities. As a result, this V₂S₃@C@V₂S₃ anode achieves a highly reversible capacity (628 mAh/g at 0.1 A/g) and excellent rate performance (477 mAh/g at 10A/g), and impressive cycling stability (2000 cycles at 20 A/g, record-high value).
Considering the rich compositional diversity of MXene, the in-situ conversion strategy developed here can be extended to construct a wide range of high-performance electrode materials for advanced batteries.

MXene-class materials have emerged as promising candidates in areas currently dominated by graphene, transition metal dichalcogenide (TMD) and related 2D materials, and hold several assets for applications to energy related fields including storage and conversion. Since the first report of Ti₃C₂T_x synthesis in 2011 [1], this ever growing family of materials has garnered significant attention from the research community owing to their remarkably tunable physicochemical properties and ease for large area synthesis using low-cost methods. Ti₃C₂T_x MXene has been utilized as a functional layer/additive in various opto-electronic devices such as solar cells, photodetectors etc. [2]. Ti₃C₂T_x has particularly shown remarkably promising results when used as charge selective contacts in conventional c-Si and Perovskite solar cells [3]. The high conductivity, fully tunable work function, and thickness dependent transparency coupled with a high chemical stability make Ti₃C₂T_x an ideal material to partner with Si and possibly other emerging chalcogenide thin film absorbers as a replacement of the standard p-n junction.
In the present work, we aim at investigating the interplay of a model MXene/c-Si interface with different organic molecular dipole interlayers in terms of carrier transport, carrier selectivity, and chemical compatibility. Initially, a screening of different organic dipoles is performed to identify the most suitable candidates out of PEDOT, PEI, PAMAM, APTES. Multilayer MXene is synthesized from the bulk Ti₃AlC₂ MAX phase through Minimum Intensive Layer Delamination (MILD) technique and 2D sheets are subsequently deposited by drop casting and spray coating on the n-Si/dipole stack. Optimization routes are investigated to tune the band alignment at the MXene/n-Si interface by using the self-assembled mono layers of different organic dipoles. An in-depth characterization of the as prepared MXene sheets by means of XPS, Raman Spectroscopy, AFM and other methods is presented to assess the chemical and structural properties of the heterojunctions, along with the complete electrical characterization of the MXene/n-Si Schottky diodes where the fundamental properties of the interface formed between the 2D charge selective contact and the 3D semiconductor are investigated. Additionally, optoelectronic properties of first proof of concept devices, both solar cells as well as photodetectors, are reported and the complete set of results is presented in regard to the state of the art. Finally, the use of MXene in combination with emerging Sb₂Se₃ photovoltaic absorbers is investigated using the c-Si as reference framework.
References:
[1] Naguib, Michael, et al. "Two Dimensional Nanocrystals: Two Dimensional Nanocrystals Produced by Exfoliation of Ti₃AlC₂ (Adv. Mater. 37/2011)." Advanced Materials 23.37 (2011): 4207-4207.
[2] Wang, Yizhou, et al. "MXenes for energy harvesting." Advanced Materials (2022): 2108560.
[3] Aydin, Erkan, et al. "Scaled Deposition of Ti₃C₂T_x MXene on Complex Surfaces: Application Assessment as Rear Electrodes for Silicon Heterojunction Solar Cells." ACS nano 16.2 (2022): 2419-2428.

Two-dimensional (2D) transition metal carbides (MXenes) are of great interest as electrode materials for a variety of applications, including photovoltaics (PV), thanks to their tunable optoelectronic properties, high metallic conductivity, and attractive solution-processability. However, thus far, MXene electrodes have only been exploited for small-scale solar cells with modest power conversion efficiencies (PCEs). Hence, herein, to unveil the potential of MXene electrodes at the industrial level, we implemented a scalable spray coating technique to deposit highly conductive (ca. 8000 S/cm) Ti₃C₂T_x MXene films via an automated spray system. We successfully employed these Ti₃C₂T_x films as rear electrodes for silicon heterojunction (SHJ) solar cells. The conformal alignment of the MXene flakes on the random pyramidal textured silicon wafers has provided an intimate contact with the underneath ITO, leading to >20% PCE over both small (4.2 cm²) and large (243 cm², i.e., using industry-sized 6-inch pseudo-square wafers) cell areas. Notably, the Ti₃C₂T_x-rear contacted devices have almost retained 99% of their initial PCE after 600+ days of ambient air storage. Their performance is comparable with state-of-the-art solar cells contacted with sputtered silver electrodes, offering a potential cost reduction for SHJ solar cells. Besides, such high-throughput fabrication of large-scale MXene-based devices brings this technology closer to industrial maturity.

In current research on free-form displays to provide hyper-interconnection between users via e-textiles or e-skins, 2D Titanium Carbide MXene is emerging as an attractive material, specifically as an alternative electrode in flexible devices to brittle ITO and other transparent conducting polymers. MXenes provide high transparency, metallic conductivity, tunable work function, and solution-processability.^[1–4] The use of MXene electrodes in OLED devices is expected to drive an exceptional technical advance in the display research field. OLED devices have demonstrated excellent compatibility with free-form displays, by virtue of their high flexibility, low power consumption and heat, light weight and nanometer-scale thickness. The synergetic effect of the MXene material and OLED devices offers a new paradigm in free-form displays. However, the low moisture-stability of the MXene OLED remains a major challenge. The Mxene electrode and organic constituent layers are easily oxidized under water or high humidity conditions. Therefore, a reasonable passivation system which can protect them from the moisture molecules is required. If it can be realized, the advanced development of matrix-form transparent MXene OLEDs which are capable of displaying complex information will follow.
In this work, we developed a highly moisture-stable MXene OLED by exploiting an encapsulation system that combines a water-resistant TiO₂ based nanolaminate layer/SiO₂-siloxane polymer/attachable PET. Despite the near-room temperature process (40°C), the TiO₂ based nanolaminate layer showed outstanding impermeability (WVTR on the order of 10^-5 g m^-2day^-1) even with nanoscale thickness (30 nm) due to its low residual stress.^[5] The SiO₂-siloxane polymer contained compressive stress that could be used to offset the tensile residual stress of the TiO₂-based nanolaminate layer, reducing the total residual stress of the barrier to zero.^[5,6] And then, to achieve not only physical scratch resistance but also water endurance, ultra-thin PET (30m) was attached to the top of the device. Using the encapsulation system, a highly stable MXene OLED was fabricated which exhibited a long life-time in ambient conditions (> 2,000 hours). In addition, the MXene OLED displayed a stable J-V-L (current density-voltage-luminance) curve after repetitive high curvature (1.000 times / 1.5 mm) folding deformation, and maintained 70% of its initial luminance after water immersion for 6 hours. In conclusion, the proposed development is expected to be applicable to deformable see-through displays in smart windows, automotive displays and augmented reality systems and provide a technical model that can advance MXene applications.

[1] M. Alhabeb, K. Maleski, B. Anasori, P. Lelyukh, L. Clark, S. Sin, Y. Gogotsi, Chem. Mater. 2017, 29, 7633.
[2] S. Ahn, T. H. Han, K. Maleski, J. Song, Y. H. Kim, M. H. Park, H. Zhou, S. Yoo, Y. Gogotsi, T. W. Lee, Adv. Mater. 2020, 32, 1.
[3] S. Lee, E. H. Kim, S. Yu, H. Kim, C. Park, T. H. Park, H. Han, S. W. Lee, S. Baek, W. Jin, C. M. Koo, C. Park, Adv. Funct. Mater. 2020, 30, 1.
[4] S. Lee, E. H. Kim, S. Yu, H. Kim, C. Park, S. W. Lee, H. Han, W. Jin, K. Lee, C. E. Lee, J. Jang, C. M. Koo, C. Park, ACS Nano 2021, 15, 8940.
[5] S. Y. Jeong, H. R. Shim, Y. Na, K. S. Kang, Y. Jeon, S. Choi, E. G. Jeong, Y. C. Park, H. E. Cho, J. Lee, J. H. Kwon, S. G. Im, K. C. Choi, npj Flex. Electron. 2021, 5, 1.
[6] A. Behrendt, J. Meyer, P. Van De Weijer, T. Gahlmann, R. Heiderhoff, T. Riedl, ACS Appl. Mater. Interfaces 2016, 8, 4056.

Layered metal carbides and nitrides (MXenes) are a rapidly expanding family of 2D material that have attracted substantial research interest due to their unique properties of hydrophilicity, good metallic conductivity, and structural diversity. Surface terminations are a natural consequence of the MXene synthesis, traditionally consisting of O, OH, and F. The location of these surface-terminating functional groups and/or their chemical nature play a crucial role on the overall MXene properties and affects their performance in different applications. Therefore, to better understand how MXenes properties are governed by their surface terminations, and to develop design strategies that are independent of surface composition, a detailed atomic resolution understanding of electronic structure of MXene is needed. In this contribution, we present fundamental studies aimed at understanding the role of surface functional groups on modifying the electronic structure and related optical properties of single and multilayer Ti₃C₂T₂ (T refers to surface termination group) MXene sheets using aberration-corrected STEM-EEL. This work furthers our understanding of MXene chemistry-property relationships and opens the door to termination-engineered MXenes.

Discovered in 2013, 2D niobium carbide (Nb₂C) MXene has been shown to have many extraordinary properties, such as high photothermal conversion efficiency, strong electron-phonon interactions, strong optical absorption in the near-infrared, and even saturable optical absorption. These unique properties of Nb₂C render this MXene potentially useful for a variety of applications, including photonic and optoelectronic devices and even photothermal cancer therapy. Here, we employ both terahertz time-domain spectroscopy (TDS) and time-resolved terahertz spectroscopy (TRTS) to investigate intrinsic and photoinduced conductivity and dynamics of optically injected carriers with 1.55 eV and 3.1 eV excitations in order to understand the photoinduced processes taking place in Nb₂C. We find that the photoinduced conductivity in this MXene shows an initial rapid decay over a picosecond time scale, followed by a much longer-lived component that lasts for nanoseconds. We also observe that the long-range conductivity is strongly limited by the nanoflake boundaries. We explore the impact of post-synthesis treatments on the carrier lifetime and mobility with the goal of optimizing its optical response for applications in photocatalytic and optoelectronic devices. A. M. Fitzgerald acknowledges support by NSF NRT CEDAR Fellowship, Award Number 2021871.

MXenes are two-dimensional (2D) transition metal carbides, nitrides and carbonitrides with a general formula M_n+1X_nT_z, where M is a metal, X = C or N, and n = 1–3; T_z denotes surface terminations such as –OH, –O, and/or –F. Unlike most other 2D materials, MXenes are intrinsically metallic. They have a high density of states at the Fermi due to a large contribution from transition metal d-orbitals. Many of them also exhibit strong surface plasmon modes in the visible to infrared range. High conductivity, pronounced plasmonic properties and optical nonlinearities, high damage thresholds for exposure to high-intensity laser light suggest MXenes as a promising platform for optoelectronic devices and underscore the need to understand dynamics of photoexcitations in these materials.
We investigate the photoexcited carrier dynamics in three different MXenes, in Ti₃C₂T_z, Mo₂Ti₂C₃T_z, and Nb₂CT_z, using ultrafast transient optical absorption, which provides access to excited state dynamics and changes in the optical absorption the visible range. We find that absorption at the proposed plasmon peak is suppressed by the photoexcitation, with an absorption bleach recovery time increasing with increasing intrinsic free carrier density: recovery time is the fastest in Nb₂CT_z, and it is slower in Mo₂Ti₂C₃T_z and in Ti₃C₂T_z.
Uncovering the exact mechanisms behind the observed rich palette of MXene optoelectronic properties and their relationship to MXene chemistry, structure, and morphology, will lay foundations for MXene-based and visible photonic devices.

The rapid development of Internet of Things (IOT) and new generations of telecommunication networks bring forward a demand for thin, lightweight shielding materials and microwave circuit components. MXenes, two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, are a large family of 2D materials that are promising for use in EMI shielding, antennas, transmission lines, etc., owing to their high electrical conductivity. However, the underlying mechanisms are currently unknown. This work focuses on experimental and theoretical methods for extracting electromagnetic properties of MXene films in the microwave region. This, coupled with the variety of chemistries and structures available to MXenes gives rise to unprecedented tunability of their properties, creates the possibility to adjust MXenes for specific applications and provides a roadmap to their stable performance.

Due to the growing need for personal safety applications for emergency communication purposes, two-dimensional (2D) and one-dimensional (1D) nanomaterials-based physical sensors have been actively examined for communication applications owing to their captivating properties such as tunable semiconducting properties, high electrical conductivity/mobility, etc.[1] The combination of Tellurium (Te) nanowires (NWs) and MXene (2D/1D nanohybrid) has not been explored as the active material in fabricating flexible, piezoresistive sensors which provide exciting electromechanical properties.[2], [3] In this perspective, 2D Ti₃C₂T_X film was deposited on cellulose paper substrate followed by 1D Te NWs using the vacuum filtration method to fabricate the piezoresistive pressure sensor. The fabricated sensor was later encapsulated in polydimethylsiloxane (PDMS) and utilized as a wireless communication system button. The fabricated sensor displayed an admirable sensitivity of 6.032 kPa^-1 (applied pressure ~1.477 – 3.185 kPa) and outstanding stability of ~2500 cycles. The underlying transduction mechanism of the TeNWs/ Ti₃C₂T_X nanohybrid pressure sensor for enhancement in sensor current at the metal-semiconductor junction was explained by the increment of tunneling current upon applying external pressure owing to the decrement in an interlayer resistance of the 2D Ti₃C₂T_X and tunneling distance between 1D Te NWs. Furthermore, an android application was developed to wirelessly receive data via Bluetooth from the fabricated piezoresistive sensors connected to a microcontroller. In addition, the application displayed the pattern based on the pressure load acting on the sensors as a Morse dash or dot. Also, it can be used similarly to a telegraph to send complex messages such as “HELP.” The effective demonstration of a TeNWs/MXene nanohybrid-based flexible, low-cost and biodegradable physical sensor opens new avenues in wireless monitoring and communication applications.
References:
[1] S.-T. Han et al., “An Overview of the Development of Flexible Sensors,” Adv. Mater., vol. 29, no. 33, p. 1700375, 2017.
[2] X. Li, C. Wang, Y. Cao, and G. Wang, “Functional MXene Materials: Progress of Their Applications,” Chem. – An Asian J., vol. 13, no. 19, pp. 2742–2757, 2018.
[3] F. Liang and H. Qian, “Synthesis of tellurium nanowires and their transport property,” Mater. Chem. Phys., vol. 113, no. 2, pp. 523–526, 2009.

Photothermal modulation is a remote and non-genetic technique to control neural activities with high spatiotemporal resolution and cell-type specificity. During the modulation, the interface between photothermally active nanomaterial and neuron is illuminated with light pulses, leading to transient local temperature rise which will elicit an action potential. Recently, we demonstrated that two-dimensional nanoflake, Ti₃C₂T_x (MXene), is an outstanding candidate for photothermal modulation of neural activity with low needed incident energy per pulse. However, the safety of Ti₃C₂T_x flakes for neural modulation is unknown. Namely, cytotoxicity of Ti₃C₂T_x flakes, phototoxicity, and hyperthermia damage. Here, we investigated the biosafety of Ti₃C₂T_x flakes across multiple assays, including viability, cellular stress (mitochondria membrane potential), and oxidative stress to determine the safety threshold of photothermal modulation. We demonstrated that culturing neurons on a ca. 100 nm Ti₃C₂T_x film and illuminating it with laser pulses (635 nm) with incident energy from 2 - 14 µJ per pulse and different pulse frequencies (1 pulse, 1 Hz, and 10 Hz pulses) neither reduced the cell viability nor generated cellular stress. The upper limit of this safety range is higher than the needed incident energy for neural excitation (< 10 µJ). However, applying laser pulses with high incident energy (e.g., 22 µJ per pulse) will result in irreversible damage. We further investigated the generation of reactive oxygen species (ROS) and revealed that culturing neurons on Ti₃C₂T_x film will not induce ROS generation. The safety evaluation with multiple assays will guide the design of the operation parameters and protocols of photothermal modulation both in vitro and in vivo.

2D titanium carbide (Ti₃C₂) MXene is a suitable candidate as a transparent conducting electrode (TCE) for optoelectronic devices due to its high conductivity, high work function (~5.1eV), and decent optical transparency. Although MXene TCE with high conductivity is a promising electrode for optoelectronic devices, their eligibility as a TCE for displays is still challenging due to their high sheet resistance compared to conventional TCE such as indium tin oxide (ITO). To fabricate the highly conductive MXene TCE, well exfoliated two-dimensional MXene with a large flake size of MXene is essential. Herein, we synthesized 2D MXene through the minimally intensive layer delamination (MILD) method with an additional delamination process with Couette-Taylor reactor which can realize the well delaminated 2D MXene. Ascribed to the ultra exfoliated 2D MXene formation, the MXene TCE film which is achieved by simple solution processing showed high optical transparency and low sheet resistance of 75.8% at 550nm and 132.5ohm/sq. Furthermore, we introduced the 2D Mxene/Silver Nanowire (AgNW) hybrid TCE to enhance the conductivity further. The optimized hybrid TCE showed 72.6% optical transparency at 550nm and 18.03ohm/sq sheet resistance respectively which are 6.04 times improved values in terms of the figure of merit (FoM). In order to adapt MXene/AgNW hybrid TCE as a transparent bottom electrode for organic light-emitting diodes (OLEDs), we patterned hybrid TCE by using the photolithography process and lift-off technique. The hybrid TCE-based OLEDs exhibited clear red electroluminescence (EL) peak at 620nm. They showed a maximum external quantum efficiency (EQE), current efficiency (CE), and luminance of 5.25%, 5.25cd/A, and 7563cd/m² respectively. Furthermore, the hybrid TCE was readily formed on a flexible poly(ethylene terephthalate) (PET) substrate by using a solution process. The hybrid TCE gave rise to the enhancement of flexibility of OLEDs. These works demonstrate the potential of 2D MXene/AgNW TCE in flexible optoelectronic devices.

Non-invasive electrochemical immunosensor is developed by fabricating L-cysteine/gold nanoparticle/titanium carbide (L-cys/AuNPs/Ti₃C₂-MXene) onto the thread electrode surface for the detection of sweat cortisol to indicate adrenal gland disorders. Herein, Ti₃C₂-MXene and AuNPs offer high surface area, high electrochemical conductivity and facilitate antibody immobilization leading to enhanced immunosensor sensitivity. The morphologies of MXene are characterized using transmission electron microscope (TEM) and the surface morphologies of the L-cys/AuNPs/Ti₃C₂-MXene modified thread are investigated by scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX). For the functionalization of antibodies, a direct conjugation between antibody and antigen is performed. Anti-cortisol antibodies are covalently immobilized onto L-cys/AuNPs/Ti₃C₂-MXene modified on thread-based electrode using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-hydroxysulfosuccinimide coupling agents to enhance the sensor specificity. The electrochemical measurements are performed on a potentiostat using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for the electrode characterization and amperometry for determination of cortisol from the current change by using a solution of 5 mM [Fe (CN)₆]^3−/4− in 0.5 M KCl. The detection principle is based on the decrease of oxidation current towards the antigen-antibody binding interaction owing to blocking of electron transfer process by cortisol. This system offers two linear ranges from 5 to 40 ng/mL (R² = 0.9880) and from 40 to 180 ng/mL (R² = 0.9560) with a limit of detection (LOD) of 0.54 ng/mL with negligible effect from interferences. Furthermore, this immunosensor provides high specificity towards cortisol detection with high reproducibility, and long-term storage stability. Ultimately, this immunosensor is successfully applied for the detection of cortisol in artificial sweat with satisfactory results.

The discovery of two-dimensional transition metal carbides and carbonitrides (MXene) has laid the foundation of layered materials for number of potential future applications. The material embodies countless desired properties such as hydrophilic surfaces, high electrical conductivity, large surface to volume ratio and efficient absorption of electromagnetic waves. The unique properties of these 2D layered MXene materials have been explored largely in energy storage devices and electromagnetic shielding applications for decades. More recently, the material has attracted enormous interest in the field of chemical and biomolecule sensing owing to its varied chemical structure and ease of functionalization for sensitive and selective detection of different analytes. Phosphorus in the form of phosphates is an essential nutrient for many biological, environmental and agricultural processes that often needs strict monitoring to avoid detrimental effects. However, the complex oxidation states of phosphate in various pH conditions and its interferences with many ions in natural waters and the human body have made it difficult to develop a reliable and accurate method of sensing. Hence, the development of low cost, highly sensitive sensor materials are imperative for the rapid detection of phosphates. In this study, we explored the large-scale exfoliation of the widely studied Ti₃C₂T_x MXene materials by adopting minimally intensive layer delamination (MILD) etching route. Controlled chemical exfoliation conditions lead to selective removal of Al layer from Ti₃AlC₂ to form few layer MXene that are confirmed from structural and morphological conditions such as XRD, SEM, and AFM. The exfoliated Ti₃C₂T_x was used as an active electrochemical sensing platform for detection of phosphate by exploiting the redox behavior of phosphate complexed with molybdenum. The sensing of phosphates in the presence of interfering ions indicated low detection limit and high sensitivity along with a large linear detection range with reliable performance. Such an application of 2D MXene could make it a potential candidate for future environmental monitoring and sensing.

Recent research on the synthesis of MXene shows a strong dependence of MXene properties on the synthesis conditions. MXenes are synthesized by selective etching of A-layer atoms from the structures of MAX phases. So far, several etching processes have been introduced, and recent reports show that the morphology of the synthesized MXenes, their size, their surface chemistry, and the amount and the type of various defects in their structures depend on their synthesis conditions. Therefore, synthesis conditions inevitably control the physical and chemical properties of MXenes, and understanding these processing-structure-property relationships may lead to innovative new ways to tune the properties of MXenes for a variety of applications. This talk will first provide a short review of the effects of synthesis methods and conditions on the structure and morphology of MXenes. Then, some of our recent work on tuning various properties of MXenes by changing synthesis conditions will be discussed. In particular, the effects of etching parameters on the electrical, electrochemical, and mechanical properties of the various MXenes will be presented.

Two-dimensional transition metal carbides and nitrides (MXenes) have shown great potential in electrochemical energy storage applications. Their high electrical conductivity coupled with surface redox reactions make them suitable candidates for high-rate pseudocapacitive energy storage. However, MXene supercapacitors often exhibit a limited voltage at higher anodic potentials due to possible oxidation. One of the possible strategies to expand their voltage is by pairing them with suitable candidates such as oxides to design asymmetric supercapacitors. Among them, lithium intercalated bilayered V₂O₅ (Li-V₂O₅) is one of the promising candidates thanks to its high capacity and improved structural stability. In this study, we have explored free-standing films of lithium intercalated V₂C MXene (Li-V₂C) as negative electrode with Li-V₂O₅-CNT as positive electrode in aqueous LiCl electrolyte. This results in an electrochemically stable 2 V supercapacitor with high energy density and good cycling stability. The work highlights that MXenes, beyond Ti₃C₂, have great possibilities in electrochemical energy storage applications.

MXenes are two-dimensional (2D) transition metal carbides, nitrides and/ or carbonitrides.^[1] It is well-established that MXenes can be intercalated by a variety of cations, both chemically and electrochemically.^[2] Predominant attention so far has been devoted to the studies of the intercalation of alkaline and alkaline earth cations such as Li⁺, K⁺, Na⁺, Mg²⁺ or alkylammonium cations into Ti₃C₂T_x MXenes. ^[3][4] Transition metal ions and clusters are known for their attractive electrochemical response.^[5][6] However, the effects of intercalation of transition metal ions on electronic and electrochemical properties of MXenes have not been explored in detail.
Herein, we studied the effects of intercalated Cu ions on Ti₃C₂T_x and vice versa, with the purpose of providing a comprehensive understanding on how the chemical, physical and electrochemical properties of both pre-intercalated transition metal ion and MXene itself were altered. Specifically, in-situ Raman and electrochemical quartz crystal microbalance (EQCM) were used to track the changes of chemical speciation in between the confined environment of Cu-Ti₃C₂T_x layers during charging/ discharging in aqueous electrolytes. In addition, in-situ X-ray absorption spectroscopy (XAS) was applied to track the oxidation changes of both intercalated Cu ions and Ti atoms of the MXene under different applied potentials, providing insights into the charge transfer between Cu ions and Ti₃C₂T_x. We discovered that the interactions between Ti₃C₂T_x and Cu ions resulted in altered electronic and electrochemical properties compared to the Cu²⁺ ions in solution and pristine Ti₃C₂T_x. Using Cu²⁺ and Ti₃C₂T_x MXene as an example, this work offers mechanistic understanding of the interactions between multi-valent transition metal and MXene layers, providing foundation for the rational design and utilization of transition metal ion intercalants to tune properties of MXenes for their applications in different electrochemical systems and beyond.

[1] B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nature Reviews Materials 2017, 2, 16098.
[2] M. R. Lukatskaya, O. Mashtalir, E. Ren Chang, Y. Dall’Agnese, P. Rozier, L. Taberna Pierre, M. Naguib, P. Simon, M.W. Barsoum, Y. Gogotsi, Science 2013, 341, 1502-1505.
[3] J. Li, H. Wang, X. Xiao, Energy & Environmental Materials 2020, 3, 306-322
[4] M. Ghidiu, S. Kota, J. Halim, A. W. Sherwood, N. Nedfors, J. Rosen, V. N. Mochalin, M. W. Barsoum, Chemistry of Materials 2017, 29, 1099-1106.
[5] D. Göhl, H. Rueß, M. Pander, A. R. Zeradjanin, K. J. J. Mayrhofer, J. M. Schneider, A. Erbe, M. Ledendecker, Journal of The Electrochemical Society 2020, 167, 021501.
[6] M. Esmaeilirad, A. Baskin, A. Kondori, A. Sanz-Matias, J. Qian, B. Song, M. Tamadoni Saray, K. Kucuk, A. R. Belmonte, P. N. M. Delgado, J. Park, R. Azari, C. U. Segre, R. Shahbazian-Yassar, D. Prendergast, M. Asadi, Nature Communications 2021, 12, 5067.

MXenes are a family of two-dimensional (2D) transition metal carbides, nitrides and carbonitrides, with a general structure of M_n+1X_nT_x, where M is the transition metal, X is carbon or nitrogen, T represents the surface terminations (F, O, OH), and n can vary from 1-4. MXenes have shown promise in multiple electrochemical systems including aqueous supercapacitors and non-aqueous batteries containing monovalent cations. However, there have been limited works studying the electrochemical properties of these materials in aqueous electrolytes with multivalent cations such as Zn²⁺ and Mg²⁺. MXenes have high (electro)chemical stability in halide zinc electrolyte systems unlike many oxide materials. Herein, the charge storage mechanism of Ti₃C₂T_x was studied in multiple Zn containing aqueous electrolytes. In-situ XRD and in-situ UV-Vis were employed to monitor the intercalating species and redox response in the electrodes. Ti₃C₂T_x electrodes could deliver battery-like performance at low rates with supercapacitor performance at high rates. This work demonstrates the potential of using MXenes in zinc ion capacitors and zinc hybrid energy storage devices.

Successful implementation of wearable electronics requires practical wearable energy storage systems that can meet certain power and energy metrics. However, flexible, stretchable, and truly textile-grade energy storing platforms have so far remained missing from most e-textile systems due to the insufficient performance metrics of current available materials and technologies. Two-dimensional (2D) transition metal carbides and nitrides (MXenes) offer unique combinations of properties including metallic conductivity, high specific capacitance, hydrophilicity, and solution processability, as well as mechanical flexibility and robustness that render these materials promising for flexible wearable energy storage technologies. Here we demonstrate textile-based electrochemical capacitor devices with high areal loading of Ti₃C₂T_x that can be integrated in series via a stacked design approach and meet the real-world power requirements for wearable electronics. A demo textile supercapacitor with 5 cells in series and a footprint area of 25 cm² and a MXene loading of 24.2 mg cm^-2 could operates in a 6 V voltage window delivering an energy density of 0.401 mWh cm^-2 at a power density of 0.248 mW cm^-2, and an areal capacitance of 146 mF cm^-2 at a 0.16 mA cm^-2 discharge current. The MXene textile supercapacitor powers a temperature monitoring system requiring high current densities with wireless data transmission to a receiver for 96 minutes. This initial report of a MXene textile supercapacitor powering a practical peripheral electronics system demonstrates the potential of this family of 2D materials to support a wide range of devices such as motion trackers and biomedical monitors in a flexible textile form factor.

As demand for Li-ion batteries (LIBs) continues to rise, concerns about the scarcity of raw materials like lithium present a challenge for large-scale distribution of energy storage devices. Potassium ion (K-ion) batteries (PIBs) present an attractive alternative to devices based on lithium ions due to the abundance of potassium in the earth’s crust and potassium’s lower cost to lithium. However, many traditional electrode materials cannot reliably accommodate potassium ions due to the ion’s large ionic radius. As a result, materials with interlayer regions tailored for K⁺ ion accommodation and diffusion are necessary to fully realize potassium-ion batteries.
Potassium-preintercalated bilayered vanadium oxide (KVO) has been previously identified as a suitable cathode material for K-ion batteries due to its wide d-spacing of 9.65 Å and high theoretical capacity through multiple reversible vanadium reduction steps. Aqueous synthesis of KVO from an α-V₂O₅ precursor produces particles with one-dimensional (1D) nanorod/nanobelt morphologies with promising performance as electrodes in PIBs, but substantial capacity fading during cycling. Alternatively, use of vanadium-based 2D MXene as a precursor has been shown to produce chemically preintercalated bilayered vanadium oxides (BVOs) with centrally nucleated 2D nanosheet morphologies, akin to a flower. Cycling of these nanoflower BVOs resulted in improved cycle-life stability and rate performance in Li-ion batteries as compared to the performance of α-V₂O₅-derived BVOs. In this work we explored if similar improvements may be possible for K-ion batteries. We report, for the first time, the electrochemical behavior of MXene-derived δ-K_xV₂O₅●nH₂O (KVO) electrodes in non-aqueous K-ion cells. Through this study, the KVO synthesis, processing, and cycling conditions that produced the most promising performance have been identified. Initial cycling of the KVO between 2.0 – 3.7 V showed stable cycling with comparable capacities (~70 mAh/g) of other KVOs, but it was found that water content in the hydrated KVO limited cycle stability when the potential limits were extended. Interlayer water dynamics and reactivity can cause parasitic reactions, such as electrolyte decomposition, poorly reversible formation of foreign compounds and vanadium oxide structure transformation, leading to the capacity decay. Vacuum drying the KVO powder at 200°C and vacuum drying the prepared electrode film at 120°C significantly stabilized cycling performance, even when cycling down to 1.5 V, due to the partial removal of the interlayer water molecules. Cycling of the vacuum-dried materials/electrodes up to 4.3 V appeared to introduce new electrochemical behavior with minimal improvements to stability. In the interest of highlighting cathodic performance,2.0 – 3.7 V was kept as the operating potential window. We show that by adequately drying before and after electrode fabrication, the MXene-derived δ-K_xV₂O₅●nH₂O electrodes demonstrated enhanced electrochemical stability while maintaining improved specific capacities in K-ion batteries.

Silicon-based nanomaterials are promising candidates for rechargeable anode material in Lithium-ion batteries (LIBs), due to their higher theoretical specific capacity, and low lithium insertion potential. The challenge to utilize these tremendous properties of Si for LIBs is to decrease volume changes and capacity loss during the lithiation/delithiation process along with dealing with their poor electrical conductivity and low lithium-ion diffusivity. Dispersing Si in a carbon like matrix is one of the approaches to tackle these challenges. Titanium carbide (Ti₃C₂ -T_x), where T_x are surface terminations, is a class of two-dimensional materials called MXenes. Ti₃C₂ have high conductivity and excellent Li-ion diffusion properties. Herein, PDMS (Polydimethylsiloxane) is intercalated in Ti₃C₂ (with d-spacing ~1 nm) for introducing the silicon within layers of MXene. The intercalated MXene (with d-spacing ~12 nm) is further processed by a unique process of photonic curing to convert intercalated PDMS to silica (SiO₂), leaving behind Si nanomaterial within the MXene layers. The as-converted silica ash on outer surface of MXene will protect it from oxidation due to its oxygen diffusion resistant ability. Photonic curing of 5K and 25K PDMS intercalated MXene is carried out at 350 V, 450 V and 550 V bank voltage for 7 pulses. Our results show that using PulseForge photonic curing of PDMS intercalated MXene changes the d-spacing from 12 nm to ~ 5 nm using a 450 V bank voltage, 7 pulses, and 28 Jcm^-2 overall fluence. The change in d-spacing and loud photoacoustic signal during the process implies that photonic curing removed organic part of PDMS and left mostly SiO₂. Cyclic voltammetry of cured PDMS intercalated MXene shows that reversible electrochemical reactions are stable with continued cycling. We aim to cure the intercalated MXene under vacuum and reactive gas environment, with varying pressure for controlling nucleation and growth of intercalated material. The present work shows that the photonic curing process is a promising instantaneous and roll-to-roll amenable synthetic route for low-cost large scale-controlled synthesis of Si-MXenes composite with the ability to tune the Si nanostructure within the MXenes layers.

Two-dimensional transition metal carbides, known as MXenes, have shown great promise as active materials in catalytic applications such as the hydrogen evolution reaction. Double transition metal (DTM) MXenes, in which two different transition metals occupy the metal sites, can enhance the tunability of MXenes electrocatalytic properties. The transition metal occupying the outer atomic layers controls the HER performance based on their basal plane activity. We investigated out-of-plane ordered DTM MXenes, with inner layers of transition metal (M″: Ti) sandwiched by outer layers of different transition metals (M′: Mo, W, Cr) in a layered M'₂M"C₂T_x structures. We demonstrate the role of exposed basal plane transition metals on the HER catalytic activity of these out-of-plane ordered DTM MXenes and compare their behaviour to a mono-transition metal MXene, Ti₃C₂T_x. W₂TiC₂T_x MXene shows the lowest HER overpotential under acidic conditions over Mo₂TiC₂T_x, Cr₂TiC₂T_x and Ti₃C₂T_x MXenes. The improved electrocatalytic performance of W₂TiC₂T_x can be due to the presence of tungsten atoms in the outer M' layers. Additionally, we assessed W₂TiC₂T_x MXene for its long-term stability of more than 24 hours. Our findings, further demonstrate MXenes are precious-metal-free 2D flakes with highly HER active basal planes as well as promising and tunable electrocatalysts for clean energy applications.

Abstract for NM04 - 2D MXenes—Synthesis, Properties and Applications
Copper-Modified 2D Transition Metal Carbide (MXene) Electrocatalysts for CO₂ Reduction
Shiba P. Adhikari¹, Alice Gao^{1, 2}, Sixbert Mohuza¹, and Zachary D. Hood^1
1Applied Materials Division, Argonne National Laboratory, Lemont, IL 60439, USA
²University of Illinois Urbana-Champaign, Champaign, IL 61801, USA

Carbon capture and sequestration, as well as industrial decarbonization, are among the primary methods to tackle rising carbon dioxide (CO₂) concentrations in the atmosphere. The CO₂ reduction reaction (CO₂RR) has long been recognized as a promising strategy to capture and convert industrial CO₂ which may help to reduce atmospheric pollution strongly associated with climate change.^[1] Although multiple strategies have shown CO₂ reduction to C1 and C2 liquid fuels, two major hurdles still exist for the most recent examples of catalysts for this reaction, namely 1) the preparation of the catalysts is generally complicated, and 2) the selectivity of the catalysts towards specific C1 and C2 products requires better control.^[2] Here, we present recent results highlighting new Cu-based MXene electrocatalysts that overcome the preparation and CO₂RR selectivity issues. The preparation of these electrocatalysts leverages reductive adsorption of Cu ions directly onto hydroxy-terminated Ti₃C₂T_x (T_x = -O, -OH, etc.) ^[3] that yields unique metal-ceramic heterostructures with tunable nanoscale hierarchy.^[4] This hierarchy is important towards the structure-property relationships for C1 and C2 production during the CO₂RR. The Cu-based MXene electrocatalysts involved in this study reveal the importance of electrocatalyst selection for enhanced selectivity on electrocatalytic C2 production. These insights suggest new approaches for controlling the MXenes’ behavior for optimal characteristics for the CO₂RR.
Acknowledgements:
This material is based upon work supported by Laboratory Directed Research and Development (LDRD) funding from Argonne National Laboratory, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC02-06CH11357.
References:
Bushuyev, O.S., De Luna, P., Dinh, C.T., Tao, L., Saur, G., van de Lagemaat, J., Kelley, S.O. and Sargent, E.H., What should we make with CO2 and how can we make it? Joule, 2018, 2(5), 825-832.
Handoko, A. D.; Chen, H.; Lum, Y.; Zhang, Q.; Anasori, B.; Seh, Z. W., Two-dimensional titanium and molybdenum carbide MXenes as electrocatalysts for CO₂ reduction. Iscience 2020, 23 (6), 101181.
Naguib, M.; Mochalin, V. N.; Barsoum, M. W.; Gogotsi, Y., 25th anniversary article: MXenes: a new family of two-dimensional materials. Advanced Materials 2014, 26 (7), 992-1005.
Adhikari, S., Hood, Z.D., In preparation, 2022.

The surface chemistry of MXene can be modified and tuned in order to tune its properties by covalent functionalizations and chemical modifications. The surface chemistry of MXene including introduction of various heteroatoms like chalcogens (S, Se, Te) as well as covalent functionalization of MXene surface will be discussed. Changes of MXene termination significantly improve performance of MXene for supercapacitor applications as well as water splitting. Introduction of zwitterion molecules was used to significantly enhanced capacitance of MXene in both aqueous and non-aqueous electrolytes including ionic liquids. For these modification were used trietoxysilane linkers which is universal tools for introduction of different functionalities on various MXene surface (like Ti₃C₂T_x, V₂CT_x, Nb₂CT_x) including various zwitterion molecules and complex supramolecular structures like alfa- and beta- cyclodextrin. Functionalized MXene were used to improve compatibility with polymer matrix for 3D printing of MXene using FMD method providing highly conductive materials for energy storage applications and electromagnetic shielding.

Two-dimensional (2D) MXenes are well known for their excellent photocatalytic properties. However, their oxidation stability is far from satisfactory, which makes controlling the photocatalytic processes challenging. Herein, we investigate the influence of oxidative stabilization of delaminated MXene on its optical and photocatalytic properties. The model 2D Ti₃C₂T_x MXene was synthesized via two well-established approaches such as HF/TMAOH (TMAOH-MXene) and minimum intensive layer delamination with HCl/LiF (MILD-MXene) coupled with L-ascorbic acid (Vit.C) stabilization. Our approach revealed that the TMAOH-MXene showed a wider optical band gap than MILD-MXene. The MILD-MXene was therefore most efficient, showing almost 100 % effectiveness in the photocatalytic decomposition of model dyes. The industrial viability was achieved on a commercial textile dye, having a 100-times higher concentration than model dyes. The MILD-MXene needed only several seconds of UV-light and simulated white light irradiation to fully decompose the commercial dye. Further in-detail analysis at various visible light ranges showed that irradiation with 625 nm (red light) gave the highest efficiency of dye decomposition with MILD-MXene catalyst. The photocatalytic mechanism was associated with the interplay between surface dye adsorption and reactive oxygen species (ROS) generated by the MXene under light irradiation. However, excellent dye adsorption was a major driving force in the process. Importantly, both MXenes could be further successfully reused while retaining about 70 % of their activity. Collectively, the obtained results revealed that oxidative stabilization with Vit.C preserved MXene’s original surface functionalization. It was further possible to understand MXene’s genuine properties such as optical activity and performance in the photocatalytic process, thus supporting its practical use in industrial water remediation technologies.

Traditional synthesis of Ti₃C₂T_z MXene obtained by the selective etching of aluminum from Ti₃AlC₂ MAX phase with hydrofluoric acid, requires a large amount of water to obtain a colloidal suspension of few to single flake Ti₃C₂T_z. Typically, the amount of water needed is about 640-800 mL per gram of MXene, as this volume is thought to both wash away residual salts formed in the etching process, and to bring the solution pH to neutral at which point a stable colloidal MXene suspension forms. While MXenes have been shown to be quite promising in terms of their performance across a wide variety of applications, their widescale production has lagged. One of the limiting steps at this time is the large cost of their production and the need for fluorine in the etching medium. Additionally, the treatment of HF containing wastewater is both a major industrial and environmental concern. For MXene production to occur at industrial scales, the large amount of acidic wastewater that is currently a byproduct of its methods of production must be addressed. Furthermore, the presence of F- in this wastewater is problematic as this treatment is quite costly and produces a large amount of industrial waste sludge that becomes difficult to dispose of due to environmental regulations. Herein we show that by modifications made to the synthesis process, we can obtain Ti₃C₂T_z colloidal suspensions with 75% less water utilized in the washing procedure. Additionally, the fluorine in the remaining wastewater can also be recovered through the precipitation of cryolite, Na₃AlF₆, a value-added compound used extensively in the aluminum production cycle. This cryolite can then be used as a quantitative tool to determine the efficiency of aluminum extraction from Ti₃AlC₂ MAX phase, allowing for optimization of parameters such as etching time, temperature, reagent concentrations, and/or pH of etchant.

Thermoelectric (TE) materials are important for sustainable development because they can directly convert heat into electricity. Compared with inorganic TE materials, conductive polymers have demonstrated unique advantages. But their TE properties, particularly the Seebeck coefficient, must be greatly enhanced for practical application. Here, I will present the significant enhancement in the Seebeck coefficient and thus the overall thermoelectric properties of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) that is hte most popular thermoelectric polyme through the incorporation of MXene. MXene can enhance the Seebeck coefficient from 23 up to 57.3 μV K⁻¹ and the power factor from 44.1 to 155 μW m⁻¹ K⁻². Tjhis is the first time to achieve the enhancement in the Seebeck coefficient of a p-type TE polymer by an n-type filler.

MXenes are highly regarded for their localized surface plasmons (SPs), broadband optical absorption, and tunable work functions. These unique features strongly depend on the dipole moments between the transition metal (e.g., Ti) and the functional surface species (T_x) of MXenes. Hence, tailoring the surface terminations of MXenes is expected to alter their work functions, plasmon energies, and optical properties simultaneously, making it hard to modify one property (e.g., work function) without affecting the other properties. In this work, we introduce an alternate method for surface-termination-independent controlling of the work function of Ti₃C₂T_x MXene via a well-controlled molecular doping process. Utilizing Tris(4-bromophenyl)ammoniumyl hexachloroantimonate, viz. magic blue (MB), as a dopant, we finely tune the work function of Ti₃C₂T_x, achieving an increase of ~ 500 meV. The incremental changes in the work function of our spray-coated Ti₃C₂T_x films (20-nm-thick) are proven to be entirely independent of T_x, as denoted by the invariant absorption and relaxation dynamics of the transversal (out-of-plane) plasmonic band throughout the doping process. Using ultraviolet and X-ray photoemission spectroscopy (UPS and XPS) along with electron paramagnetic resonance (EPR) spectroscopy, we attribute the work function modulation to an electron transfer from the energy states below the Fermi level in Ti₃C₂T_x to the dopant cations, accompanied by the physisorption of the dopant anions (antimony hexachloride) on the MXene surface, leading to the formation of polarons as additional charge carriers. The demonstrated ability to engineer the work function of MXenes without altering their surface-termination-dependent properties (e.g., SPs) allows for higher operational degrees of freedom, which is alluring for MXene-based device applications.

This paper presents a transparent conducting electrode (TCE), which is an environmentally-stable two-dimensional titanium carbide (Ti₃C₂T_x, MXene) that has a high work function WF. High-temperature annealing reduced the d-spacing of Ti₃C₂T_x from 1.33 to 1.06 nm; the result was a highly-compacted thin film that can resist inward diffusion of moisture intercalants. The MXene had high WF = 5.84 eV, which is the highest yet; it was achieved by p-type molecular doping using perfluorosulfonic acid. With benefits from the highly compacted structure of Ti₃C₂T_x and superior chemical stability of overcoated PFSA, the p-doped Ti₃C₂T_x exhibited only a 4.8% increase in sheet resistance with WF maintained > 5.60 eV even after 22-day exposure to ambient air. Lastly, the Ti₃C₂T_x TCE was successfully applied as the anode for both large-area and ten-by-ten passive matrix flexible organic light-emitting diodes on 6 cm × 6 cm substrates. The significant increases in environmental stability and WF provides a new method to develop practical applications of MXene as TCEs for optoelectronics and photovoltaics.

MXenes, a fascinating large family of two-dimensional (2D) layered transition metal carbides and nitrides, have aroused substantial interest in various applications owing to their unprecedented ultrahigh metallic conductivity, mechanical properties, and hydrophilicity. This work demonstrates scalable, straightforward, continuously controlled, additive-free wet-spinning assembly of 2D titanium carbide (Ti₃C₂T_x) MXene fibers. MXene flakes are perfectly aligned under a mechanical drawing force using concentrated MXene colloids in a coagulation bath to produce Ti₃C₂T_x MXene fibers via a wet-spinning process. The wet-spun Ti₃C₂T_x MXene fibers demonstrated excellent electrical conductivity and mechanical properties. As a result, Ti₃C₂T_x MXene fiber-based miniaturized devices are promising for portable applications. Furthermore, the Ti₃C₂T_x MXene hybrid fibers with the synergistic effect of gas-adsorption and electronic properties significantly improved NH₃ sensing response (ΔR/R0 = 6.77%) at room temperature for wearable gas sensors. Additionally, fabricated wet-spun Ti₃C₂T_x MXene fibers are integrated into electrical wires to switch on a light-emitting diode light and transmit electrical signals to earphones to demonstrate their application in electrical devices. Furthermore, the wet-spun electroconductive Ti₃C₂T_x MXene fibers exhibited an enhanced electrochemical performance when assembled as supercapacitor applications. Thus, we envisage that these exciting features of 2D Ti₃C₂T_x MXene hybrid fiber materials will provide a novel pathway for designing next-generation portable miniaturized devices.

Surface chemistry controls not only the physicochemical, electrochemical, and optoelectronic properties, but also environment stability and processability of nanomaterials. MXenes have been attracted in many electronic, electrochemical, and optoelectronic applications, due to their high electronic conductivity (~15000 S/cm), hydrophilicity, and solution processability. MXenes, transition metal carbides/carbonitrides/nitrides, are a very large family of 2D materials with the general formula Mn+1XnTx, where M, X, Tx, and n represent transition metal(s), carbon or nitrogen, surface terminal groups such as –OH, -O and -F, and integers ranging from 1 to 4, respectively. The surface terminations determines the surface properties of MXenes. In this presentation, the presenter will briefly demonstrate two ways to control the surface chemistry of MXenes, including anhydrous synthesis and post-surface functionalization methods. The developed surface chemistry not only control the surface functionality and physicochemical properties of MXenes, but also enables preparation of stable MXene dispersions with good oxidation stability. The surface chemistry also provides an opportunity to prepare printable flexible MXene electrodes for various applications including EMI shielding, flexible joule heater, LED display and energy storage.

2D Transition Metal Carbides and Carbonitrides (MXenes) for Electromagnetic Interference (EMI) Shielding
Aamir Iqbal¹, and Chong Min Koo¹*

¹School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do 16419, Republic of Korea


*Corresponding author: [email protected] (C. M. Koo)


Two-dimensional (2D) MXenes are a newly discovered family of transition metal carbides, nitrides, and carbonitrides. Their outstanding electrical conductivities of >10⁴ S cm⁻¹ in 2D morphology, low density, mechanical flexibility, easy processability, and structural controllability extends their application scope to compete with existing conventional 2D materials. Ti₃C₂T_x, the representative candidate of the MXene family, exhibits an EMI shielding effectiveness of 92 dB at 45 micrometer thickness, whereas a 55-nanometer thick film can provide 99% shielding against EMI, which is outstanding for highly compacted electronics. A transition metal carbonitride, Ti₃CNT_x MXene, with a moderate electrical conductivity provides a shielding effectiveness of 116 dB at 40 micrometer thickness, higher than the more-conductive Ti₃C₂T_x or metal foils of the same thickness. This exceptional shielding performance of Ti₃CNT_x was achieved by thermal annealing and is attributed to an anomalously high absorption of electromagnetic waves in the layered metamaterial-like porous structure. With high absorption of EM waves at minimal thickness, MXenes are ideal for real applications in smart 5G electronics.


Keywords: Transition metal carbides, carbonitrides, MXenes, electromagnetic shielding, porous structure.

Scaling up and environmental impacts associated with nanomaterials synthesis is a major challenge in the industries nowadays. As one of the cutting-edge nanostructures, MXenes, have significantly impacted materials science and nanotechnology since their discovery in 2011. Theoretical calculations have predicted more than 100 possible compositions of MXenes, and lab-scale fabrication of more than 40 MXene structures has been reported to date. The unique characteristics of MXenes have made them an ideal fit for a wide variety of applications, including energy storage, environmental, electronics, communications, gas, and liquid separations and adsorption, biomedical, and optoelectronics. By 2021, MXenes have already shown promise in several research areas, including energy storage devices, electromagnetic interference shielding, nanocomposites, and hybrid materials. In parallel, new applications are emerging where MXenes outperform other nanomaterials, such as in tribology. Despite all the new development in the MXenes field, there is no study of the environmental impacts of MXene synthesis. Here, we investigate the environmental impacts associated with the fabrication of Ti₃C₂T_x MXene as a coating layer for electromagnetic interface (EMI) shielding of communications satellites. We use aluminum and copper as an external normalization alternate to show the gaps and recommend solutions for improving the scalability of the Ti₃C₂T_x MXene. Two Ti₃C₂T_x synthesis systems, a small laboratory scale (19.2 g per batch) and a large laboratory scale (800 kg per batch) were compared. We also investigated the use of TiO₂ instead of Ti for the synthesis of the MXene precursor as well as the effects of laboratory location on the environmental impacts of MXene. In summary, this work would help through the more efficient and environmentally friendly synthesis of MXenes, which is vital for considering the scale-up of MXenes.

With high metallic conductivity, a new family of two-dimensional transition metal carbides (MXene) is a promising candidate for electronics. However, one of challenges in MXene application is poor stability to moisture, degradation of electrical properties by swelling (lattice expansion by water intake) and oxidation [1, 2]. For applications in the field of electronics, MXene with improved environmental stability (e.g. no significant degradation under 60°C/RH85%) is highly required. A number of works has been reported to improve MXene stability in humid conditions, including surface modification [3], but most of them have resulted in ruining their superior properties derived from diversity in surface terminating groups. In this work, we propose a facile strategy to fabricate MXene films exhibiting excellent conductivity and stability under 60°C/RH85% without tailoring of MXene surface functional groups.

References
[1] Voigt, C. A.; Ghidiu, M.; Natu, V.; Barsoum, M. W. Anion Adsorption, Ti₃C₂T_z MXene Multilayers, and Their Effect on Claylike Swelling. J. Phys. Chem. C 2018, 122 (40), 23172–23179.
[2] Iqbal, A.; Hong, J.; Ko, T. Y.; Koo, C. M. Improving oxidation stability of 2D MXenes: synthesis, storage media, and conditions. Nano Converg. 2021, 8 (1), 1–22.
[3] Chen, W. Y.; Lai, S. N.; Yen C. C.; Jiang X.; Peroulis D.; Stanciu A. Lia. Surface Functionalization of Ti₃C₂T_x MXene with Highly Reliable Superhydrophobic Protection for Volatile Organic Compounds Sensing. ACS Nano 2020, 14 (9), 11490–11501.

Emerging soft machines require high-performance strain sensors to achieve closed-loop feedback control. Predicting the performance of a soft robotic sensor from its composition and morphology is nearly impossible with traditional computational approaches. Machine learning (ML) is a versatile tool to uncover complex correlations between fabrication recipes and sensor performance, yet the limited acquisition rate of high-quality data hinders the development of high-accuracy prediction models at the device level. In this talk, I will demonstrate our recent work of using an ML model to predict device-level performance and recommend new material compositions for soft machine applications. I will present a three-stage ML framework to construct a prediction model capable of automating the design of strain sensors across a wide strain range from <0.5% to 350%. First, a support-vector machine classifier is trained by using 351 compositions of various nanomaterials, including Ti3C2TX MXene nanosheets, single-walled carbon nanotubes, and polyvinyl alcohol. Second, through 12 active learning loops, 125 strain sensors are stagewise fabricated to enrich the multi-dimensional dataset. Third, data augmentation is implemented to synthesize >10,000 virtual data points followed by genetic algorithm-based selection to optimize the prediction accuracy of ML model. An ultimate prediction model is finally constructed and able to (1) predict sensor characteristics based on fabrication recipes and (2) recommend feasible fabrication recipes for adequate strain sensors. As final demonstrations, model-suggested strain sensors are integrated into soft gripper and batoid-like swimmer to endow them with real-time sensing capabilities.

Fibers, yarns, and textiles have long established themselves in many sectors of society from our daily wear to medicine and structural composites. The rise of the Internet of Things (IoT) has prompted incorporation of conductive materials into textiles that enable real-time monitoring of external stimuli (e.g., stretch, pressure, temperature, and humidity) and body responses. Sensors, antennas, energy storage sources need to be incorporated into textiles. New functions, such as IR stealth, EMI shielding or passive and active temperature control, can be enabled by conductive or optically active coatings. Among the many conductive materials and fabrication methods demonstrated, few met the performance and cost requirements of real-world applications. MXenes are an emerging class of 2D nanomaterials that possess excellent electrical conductivity, attractive optical/plasmonic properties, solution processability and hydrophilic surfaces, making them ideal for imparting conductivity to textiles in a simple dip coating fashion. In this work, a high throughput continuous coating method was demonstrated with a customized automatic yarn dip coater. Thanks to the shear force present in the dip coater and the preferred interactions between Kevlar and MXene, viscous MXene solutions and fast coating speeds could be used. Kilometers of highly conductive MXene coated Kevlar cords were produced with consistent quality. Tensile test with in-situ electrical resistance tracking showed the multifunctionality of Ti₃C₂T_x coated Kevlar for composites, both as structural reinforcement and damage sensing.

Detecting neurotransmitters in the system is critical to diagnosing nervous system diseases. Many studies have been reported to produce high-performance sensors for sensing dopamine. Among the various materials, 2-dimensional Ti₃C₂ MXene, which is highly conductive and electrochemically active, has been investigated to detect neurotransmitters including dopamine, uric acid, and serotonin. However, MXene oxidizes quickly in aqueous conditions, hindering MXene from being widely applicable for biosensing applications.
Herein, we report a composite of MXene and poly (ethylene glycol) diacrylate (PEGDA), enabling selective detection of neurotransmitters. PEGDA is a porous, UV-cross-linkable material with a low degradation rate, protecting MXenes from oxidation in aqueous conditions. The composite thickness was easily controllable by changing UV irradiation time and the guide mold. Also, the composite structure was porous enough to assist small molecules diffused inside, facilitating electrochemical detection of neurotransmitters.
Electrochemical detection of dopamine was confirmed via cyclic voltammetry (CV) and differential pulse voltammetry (DPV), confirming that the composite could detect dopamine in aqueous conditions up to 50 nM. The limit of detection (LOD) is estimated to be lower than 50 nM, suggesting out MXene composite hydrogel would be further applicable for detecting neurotransmitters sensitively but selectively.

There is a critical unmet need to detect and image two-dimensional (2D) materials within single cells and tissues while surveying a high degree of information from single cells. Here, we propose a versatile multiplexed label-free single-cell detection strategy based on single-cell mass cytometry by time-of-flight (CyTOF) and ion-beam imaging by time-of-flight (MIBI-TOF). [1] This strategy, “Label-free sINgle-cell tracKing of 2D matErials by mass cytometry and MIBI-TOF Design” (LINKED), enables nanomaterial detection and simultaneous measurement of multiple cell and tissue features. As a proof of concept, we selected a set of 2D materials, transition metal carbides, nitrides, and carbonitrides (MXenes), to ensure mass detection within the cytometry range while avoiding overlap with more than 70 currently available tags, each able to survey multiple biological parameters. In addition, MXenes have received substantial attention for their promise as biomedical nanotools. [2-5]
First, we demonstrated their detection and quantification in 15 primary human immune cell subpopulations. Together with the detection, mass cytometry is used to capture several biological aspects of MXenes, such as their biocompatibility and cytokine production after their uptake. In vivo biodistribution experiments using a mixture of MXenes in mice confirmed the versatility of the detection strategy and revealed MXene accumulation in the liver, blood, spleen, lungs, and relative immune cell subtypes. Finally, we applied MIBI-TOF to detect MXenes in different organs revealing their spatial distribution. The label-free detection of 2D materials by mass cytometry at the single-cell level, on multiple cell subpopulations, and in multiple organs simultaneously, is expected to enable exciting new opportunities in biomedicine.

References:

1. Fusco L. et al. Immune Profiling and Multiplexed Label-Free Detection of 2D MXenes by Mass Cytometry and High-Dimensional Imaging. Advanced Materials, 2022, 2205154.
2. Gogotsi, Y. & Anasori, B. The Rise of MXenes. ACS Nano, 2019, 13, 8491.
3. Fusco L. et al. Graphene and other 2D materials: a multidisciplinary analysis to uncover the hidden potential as cancer theranostics. Theranostics, 2020, 10, 5435.
4. Unal M.A. et al. 2D MXenes with antiviral and immunomodulatory properties: a pilot study against SARS-CoV-2. Nano Today 2021, 38, 101136.
5. Gazzi A. et al. Photodynamic Therapy Based on Graphene and MXene in Cancer Theranostics. Front. Bioeng. Biotechnol., 2019, 7, 295.

The rapidly growing family of ultrathin layered two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, referred to as MXenes are steadily advancing as novel inorganic nanosystems for various electronic and optoelectronic applications. The metallic conductivity, hydrophilic nature, and other unique physiochemical properties qualifies 2D MXenes to meet the requirements of biomedicine such as in stem cell research. Stem cell therapies show great promise for the cure of neurological disorders, as stem cells can serve as cell replacement, while also secreting factors to enhance endogenous tissue regeneration. We investigate the impact of outstanding electrical and surface functional properties of MXenes in the application areas of stem cells. Our synthesis strategies allow us to synthesize highly conductive and stable colloidal aqueous dispersions of MXenes that are coated or drawn as films. These freestanding films of MXenes exhibit electrical conductivities ranging from 10 S.cm^-1 (V₄C₃T_x MXene) to 20,000 S.cm^-1 (Ti₃C₂T_x MXene). By the virtue of such high electrical conductivities, MXene nanosheets are explored as interfaces for regulating stem cells. The stem cells grown using a technology that allows for the culture of stem cells in combination with 3D printing technology can be translated to novel therapies for bone regeneration and reconstruction. Stem cells cultured on MXene films enable higher synchronous electrical activities and higher proliferative ability. Our preliminary results suggest that owing to the 2D network, MXenes can be engineered as guided bone regeneration membranes that serve as a scaffold to induce stem cell attachment. This study is an important step toward understanding the potential of MXenes in stem cell research and biomedicine, in general.

Recently, Ti₃C₂T_x MXene has attracted increasing interest for use in sensing and stimulation applications for monitoring electrophysiological activity in the body. The electrochemical properties of MXene-based electrode arrays have consistently demonstrated superior recording quality with increased signal to noise ratios (SNR) compared to clinically standard arrays. Further, the high capacitance of the MXene nanostructure paired with the large charge injection capacity (CIC) make these devices very desirable for high-quality, micron-scale recording and electrical stimulation. For MXene wearable and implantable devices to become clinically and commercially viable, however, they must be able to withstand routine sterilization procedures. Standard sterilization modalities used widely across the world include steam autoclave sterilization at 120 °C, Ethylene Oxide (EtO) gas, and hydrogen peroxide (H₂O₂) plasma. Here, we present a comprehensive analysis of the structural, functional, and electrical effects of these common sterilization protocols on thin-film spray-coated Ti₃C₂T_x neural microelectrode arrays as well as Ti₃C₂T_x infused ‘MXtrodes’. To confirm that bacteria could be effectively eradicated from the Ti₃C₂T_x MXene devices, E. Coli inoculated samples were first tested in each sterilization machine, and the resulting colony-forming units (CFU) were counted. Across all sterilization modalities, the bacterial colony units were below the threshold of detection which confirmed the devices could be effectively sterilized. Post-sterilization analysis reveals that for both autoclave and EtO sterilized devices, the crystalline structure, contact adhesion, and electrical properties remain unchanged. Furthermore, there were no changes in the XRD and Raman spectra for autoclave or EtO sterilized thin-films or MXtrodes. Finally, electrochemical impedance and conductivity were statistically unchanged, and SEM did not reveal any delamination or morphological changes to the devices. Hydrogen Peroxide plasma sterilization, however, resulted in severe damage of Ti₃C₂T_x MXene films as confirmed by SEM, XRD, and Raman. Further, plasma sterilization significantly increased the recoding impedance measurements and revealed a significant loss in conductivity. Ultimately, our findings indicate that EtO and Autoclave sterilization do not alter Ti₃C₂T_x MXene devices and should be considered for all future MXene wearables and invasive implants.

Breast cancer is the most commonly diagnosed invasive cancer among women globally. Current therapies (e.g. chemotherapy) show numerous limitations due to the lack of selectivity and the consistent side effects. This led to investigating novel approaches aimed at improving tumor-killing efficacy. We previously demonstrated the effect of new bioactive nanomaterials with promising photophysical properties, the MXenes, used to increase the efficiency of targeted breast cancer Photothermal Therapy (PTT). In this work, we investigated the effect of Few and Multi layers MXenes mediated-PTT on two different three-dimensional reliable models of breast cancer; conventional spheroids and bioprinted spheroids. We performed PTT on the two models of breast cancer by using a non-toxic concentration of MXenes (50 µg/mL). After PTT, a significant reduction in cell viability was observed along with a strong increase in reactive oxygen species (ROS). We furthermore investigated the effect of PTT on the migration of macrophages and endothelial cells towards cancer region in both 3D models. Our results indicate that PTT mediated by both Few and Multi layers MXenes significantly modulate tumor progression by inducing the cells death by increasing the temperature, particularly in the bioprinted model.

MXenes include transition metal carbides/nitrides with intercalated cations, such as Li-Ti₃C₂T_x and K-Ti₃C₂T_x, where T_x represents surface terminations (typically O, OH and F). They have shown promising performance for energy storage, which is highly dependent on the surface chemistry and the Ti oxidation state. Nevertheless, contributions of different oxygen bonds in the local chemistry of MXene flakes and interlayers are difficult to disentangle without high chemical and spatial sensitivity.

In this presentation, we will show that Scanning Transmission X-ray Microscopy (STXM) is well suited for chemical imaging on individual MXenes particles. This synchrotron-based method allows the recording of soft X-ray Absorption Spectra (sXAS) with <60 nm spatial resolution, hence enabling the characterization of single MXene particles with high chemical sensitivity to Ti oxidation state and oxygen bonding, as well as bulk sensitivity. We will present here the first STXM characterization of Ti₃C₂T_x , Li -Ti₃C₂T_x, and K-Ti₃C₂T_x MXenes at the Ti L-, C K- and O K-edges. The data analysis with unsupervised machine learning algorithms of STXM measurements for intercalated MXenes achieved the identification of different oxygen-containing compounds. Moreover, STXM measurements in transmission and total electron yield detection mode at Ti L- and O K-edge allowed the comparison between surface and bulk properties for Ti₃C₂T_x MXene.

A decisive step towards understanding the migration of oxygen as a termination and water species was also achieved with in situ temperature-dependent STXM measurements for these MXenes. The local behaviour of oxygen during annealing up to 650°C was mapped at the Ti L- and O K-edge. These are the first bulk-sensitive measurements able to map oxygen redistribution inside MXene interlayer during annealing. The aforementioned experimental results advocate for the importance of spaces between MXene layers and overlapping flakes in the formation and control of oxygen-containing species in MXenes.

Currently, the conversion of captured CO₂ into value-added chemicals or fuels is considered a key strategy to mitigate the yet increasing anthropogenic CO₂ emissions, in particular when combined with H₂ obtained using renewable energy. Depending on the catalyst and the reaction conditions used, thermocatalytic CO₂ hydrogenation can give CO, methanol, dimethyl ether (DME), methane, or heavier hydrocarbons. MXene materials, that is, a family of two-dimensional (2D) carbides, nitrides and carbonitrides with the general formula of M_{n +1}X_nT_x (where M is an early transition metal, n = 1, 2, 3, X is C and/or N and T are surface –O–, –OH and/or –F groups), are currently emerging in thermocatalytic applications as catalysts or supports with reactive metal–support interactions. Enabled by the scalable synthesis of MXenes, we report a gram-scale synthesis of a phase-pure multilayered hexagonal 2D-Mo₂C material with only Mo-terminated basal planes. 2D-Mo₂C is by a factor of six per mass of catalyst more active for CO formation than the reference β-Mo2C catalyst and shows no deactivation on stream for more than 100 h. We also report that silica-supported, dispersed, reducible nanosheets of a delaminated molybdenum MXene, Mo₂CT_x, can be used to engineer a Cu/Mo₂CT_x interface that shows an at least six times increased intrinsic formation rate of methanol by the direct hydrogenation of CO₂ compared to Cu/SiO₂.

MXenes are a large family of 2D materials [1] with excellent potential for energy storage applications [2]. Ti₃C₂T_x (where T_x refers to surface terminations -O, -OH, -F, etc, present in varying amounts depending on synthesis conditions) is one of the most studied MXenes to date. Due to hydrophilic surfaces and weak attractive forces between the negatively charged layers, MXenes can retain significant amounts of water between the layers, and they can be intercalated with a variety of cations and molecules [3]. Understanding the structure and dynamics of intercalated species within MXene electrodes is important for the development of practical energy storage devices. Infrared spectroscopy is sensitive to the hydrogen bonding environment of water molecules and thus well suited for the study of water structures [4]. We have recently demonstrated that this method can be applied to probe proton intercalation into the MXene interlayer space [5].

Here, we use infrared spectroscopy in the attenuated reflectance mode to probe confined water between Ti₃C₂T_x MXene layers as a function of relative humidity. The MXene was intercalated with various cations (Li, Na, K, Cs, …) or treated with an acidic solution to remove all intercalants [6]. Clear changes in the amount and hydrogen-bonding structure of the confined water are observed depending on the identity of the intercalated cation as well as the relative humidity. Because anions do not intercalate into MXenes [7], we are able to study the hydration shell of the cation without interference of anions or contact ion pairs. This work provides new understanding on the solvation of cations confined between MXene layers.

References
[1] Naguib et al. Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv. Mater. 2011, 23, 4248−4253
[2] Jun et al. Review of MXenes as new nanomaterials for energy storage/delivery and selected environmental applications. Nano Res. 2019, 12, 471–487
[3] Osti et al. Effect of metal ion intercalation on the structure of MXene and water dynamics on its internal surfaces. ACS Appl. Mater. Interfaces 2016, 8, 8859−8863
[4] Ohno et al. The effect of cooperative hydrogen bonding on the OH stretching-band shift for water clusters studied by matrix-isolation infrared spectroscopy and density functional theory. Phys . Chem. Chem. Phys. 2005, 7, 3005–3014
[5] Lounasvuori et al. Vibrational signature of hydrated protons confined in MXene interlayers. Under review 2022
[6] Chen et al. Pristine Titanium Carbide MXene Films with Environmentally Stable Conductivity and Superior Mechanical Strength. Adv. Funct. Mater. 2020, 30, 1906996
[7] Shpigel et al. Can Anions Be Inserted into MXene? J. Am. Chem. Soc. 2021, 143, 32, 12552–12559

As electronic technology advances, the need in safe and long-lasting energy storage devices that occupy minimum volume arises. Short charging times of several seconds to minutes, with energy densities comparable to those of batteries, can be achieved in pseudocapacitors. These are sub-class of supercapacitors, where capacitance is mediated by fast redox reactions and can enable at least an order of magnitude more energy to be stored than in typical electrical double layer capacitors. Transition metal oxides (e.g. RuO₂, MnO₂) and conducting polymers (e.g. polyaniline) serve as typical examples. However, these materials are often high in cost and/or suffer from low cycling stability. As a result, the search for new pseudocapacitive materials constitutes an important direction today.
In this talk, I will discuss how the key performance metrics of pseudocapacitors – capacitance and charging rates – can be pushed to the limits in the materials that combine good electrical and ionic conductivities (ensuring fast charge transfer and hence charging rates) with high density of redox-active and ionically accessible sites (enabling high capacitance and charging rates). In particular, the electrochemistry of 2D transition metal carbides (MXenes), with an emphasis on the mechanism of charge storage and factors affecting the overall capacitive response in these materials will be discussed. Moreover, the role of the cation intercalation in layered MXenes as means to manipulate its electrochemical response are examined.

Titanium carbide MXene being the most studied member of the 2D transition metal carbides/nitrides (MXene) family, has found applications in the field of energy storage and energy generation, due to its dynamic but tunable surface chemistry, good hydrophilicity and electrical conductivity. Conductive and clay like, in situ HF etched titanium carbide MXene can be used as supercapacitor electrodes and are prospective candidates in flexible electronics for commercial supercapacitor application. Commercial supercapacitors also require high energy storage capability, for which a high mass loading is necessary. Herein a high mass loading electrode with high areal capacitance has been fabricated using a simple brush coating of clay like titanium carbide MXene on carbon cloth. A controlled alkalization of titanium carbide using a weak base, ammonium hydroxide has been carried out in the synthesis of titanium carbide MXene. The in situ alkalization process reduces the time of synthesis and causes an inherent expansion of the interlayer spacing, making the diffusion of ions easier. The structural, morphological, optical, and electronic properties have been studied using X-ray diffraction, scanning electron microscope, UV visible absorption spectroscopy, X-ray photoelectron spectroscopy analysis and Fourier transform infrared spectroscopy analysis. The prepared MXene has the characteristic (002) diffraction peak and a layered morphology, similar to the open accordion morphology of HF etched MXene. The optical studies indicated ease of delamination from the evolution of the plasmonic peak in the NIR region. XPS and FTIR studies indicated presence of –O, -F and –OH functional groups. Titanium carbide MXene electrode on glassy carbon and on carbon cloth were fabricated. Nafion-assisted coating in glassy carbon electrode was replaced by binder free coating of MXene clay, while using carbon cloth. A simple brush coating onto pre-heated carbon cloth produced uniform films, with good electrochemical performance. The prepared electrode exhibited good gravimetric capacitance of 109 F/g at a current density of 0.5 A/g and areal capacitance of 2500 mF/cm² at a current density of 11 mA/cm². High mass loading combined with high areal capacitance make the prepared electrode a potential candidate for commercial applications.

It is essential to investigate novel electrode materials to advance the development of supercapacitors and their implementations in electronic applications. A group of two-dimensional (2D) transition metal carbides and nitrides known as MXenes (Ti₃C₂T_z) have earned more attention as electrode materials for industrial supercapacitors. It has the distinctive features that bulk materials lack, such as a large specific surface area, thermal resistive properties, a low energy barrier for electron transport, and a short ion diffusion path, which make desirable electrodes for energy storage devices.
Titanium Carbide (Ti₃C₂T_z) is synthesized in this study using a relatively simple chemical method to develop low defects and high-quality MXene while keeping the stacking and interlayer spacers intact. The crystalline structure and surface morphology are investigated by XRD, SEM, TEM and BET analyses. An electrode is fabricated using a synthesized Ti₃C₂T_z as an active electrode material and Ni-foil as a current collector. The layered morphology is confirmed using a field emission scanning electron microscope, and the presence of the elements of the synthesized MXene is verified using energy-dispersive spectroscopy analysis. The specific surface area of MXene is estimated to be 38 m²g^-1 using the BET measurement by nitrogen adsorption and desorption isotherm, with a pore volume of 0.4029 cm³g^-1 and average pore size of 4 nm. The electrochemical performance of a Ti₃C₂T_x electrode is tested using cyclic voltammetry with a 6M KOH electrolyte.
When used as an electrode for supercapacitors, Ti₃C₂T_x MXene demonstrated an exceptional specific capacitance of 831 Fg-1 at a 2 mVs^-1 scan rate in the voltage window of 0.0-0.6 V. The layered structures of MXene improve electrolyte ion transport while also providing transition metal active redox sites on the surface. The electrode has remarkable cyclic stability of 97% over 10000 continuous cycles at 50 mV s^-1. The findings suggest an appealing strategy for producing two-dimensional nanomaterials-based electrodes and composites for improved implementation and innovative supercapacitor designs.

Keywords: MXene, Energy Storage, 2D materials

References:
R. Li, L.Zhang, L. Shi, and P. Wang, MXene Ti₃C₂: an effective 2D light-to-heat conversion material, ACS nano., 2017, 11, 3752-3759.
S. Nam, J.N. Kim, S. Oh, J. Kim, C.W. Ahn, and I.K. Oh, Ti₃C₂T_x MXene for wearable energy devices: Supercapacitors and triboelectric nanogenerators, APL Material., 2020, 8, 110701.
B. Anasori, M.R. Lukatskaya, and Y. Gogotsi, 2D metal carbides and nitrides (MXenes) for energy storage, Nature Reviews Materials., 2017, 2,1-17.
X.Wang, Y. Li, S. Wang, F. Zhou, P. Das, C. Sun, S. Zheng and Z.S. Wu, MXene for energy storage: present status and future perspectives, Journal of Physics: Energy., 2020, 2, 032004.

Photocatalytic dye degradation is a most promising and cost-effective way to remove organic pollutants from water in various industries. MXene, a new group of two-dimensional materials, has recently gained tremendous attention as a photocatalyst due to its unique surface functionalities, chemical composition, and physicochemical and visible light absorption properties. These properties make MXene a promising catalyst for the photodegradation of organic molecules. In this work, we have synthesized V₂CT_x (T_x = -OH, -F, Cl) MXene by acid etching method and have been characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy to confirm its structural and surface morphological properties. The chemical composition of catalysts was studied by X-ray photoelectron spectroscopy(XPS). The potentiality of photocatalytic degradation of V₂CT_x MXene was studied by UV-Visible spectroscopy using methylene blue (MB) and crystal violet(CV) under visible light irradiation. The degradation performance for 0.01 M MB dye is 97.9% in 45 minutes and for 0.25 M CV is 93% in 60 min under visible light. This work opens a path for studying the catalytic property of V₂CT_x MXene.

2D Materials-based 3D architectures are promising materials for advanced systems of future technologies. Among 2D materials, MXenes (especially their most studied member, Ti₃C₂T_x) present a unique opportunity for application via colloidal processing, as they are electrically conductive and chemically active, whilst still being easily dispersed in water. This talk will present an overview of the progress on the development of colloidal approaches towards 3D architectures of tuned microstructures built from 2D MXenes, with emphasis on ice-templating and extrusion-based additive manufacturing (AM) methods. The potential of these methods and materials will be exemplified in specific applications such as in the development of vertically aligned MXene and multi-material architectures for high-rate performance supercapacitors and the development of advanced composites for electrothermal heating.

Two-dimensional transition metal carbides, known as MXenes, have found wide use in energy storage and catalysis applications. Although MXenes have been used in these applications, few studies have investigated the use of MXenes’ interior transition metal carbide/nitride core and abundant surface groups as nanosized building blocks toward extreme environment nanoceramics. In this presentation, we illustrate the high-temperature behavior of Ti₃C₂T_x and Mo₂TiC₂T_x MXenes up to 2,000 °C. In addition, we exhibit the role of surface-adsorbed alkali metal cations (Na⁺, K⁺, Ca²⁺) on improving the phase stability and controlling the phase transformation of these MXenes using in situ two-dimensional x-ray diffraction (XRD²) up to 1,100 °C using and ex situ XRD² up to 2,000 °C. We also present the changes in phase transformation of non-decorated and alkali metal cation decorated Mo₂TiC₂T_x MXenes using in situ scanning transmission electron microscopy (STEM) up to 800 °C. The combination of in situ XRD² and STEM methods with ex situ XRD², x-ray photoelectron spectroscopy, and thermogravimetric analysis demonstrate the effect of alkali metal cations on improving the phase stability and controlling the phase transformation of MXenes. This presentation assists in furthering development of MXenes as a diverse and tunable family of nanoceramics for ultra-high temperature applications.

Zirconium diboride (ZrB2) is one of the ultra-high temperature ceramics (UHTC), which are used for applications at temperatures above 2000 C. However, a significant drawback is its poor mechanical properties such as fracture toughness, poor densification behavior and oxidation resistance since monolithic boride is prone to the formation of volatile oxides (such as B₂O₃) at temperatures above 1100 C. In this project, we have investigated a one-pot, surfactant-free, aqueous mixing method to develop homogeneous ZrB₂-Ti₃C₂T_x MXene green bodies and evaluated their sintering behavior. We present the phase transformation of Ti₃C₂T_x MXene to TiC_y at the grain boundaries, the interface interactions between the ZrB₂-TiC_y grains and their densification mechanism. A nominal relative density of 96% is achieved with the addition of 0.5 wt% of MXene to ZrB₂ and sintering of the resulting powder mixture at 1900 C with 50 MPa pressure under vacuum. We discuss a fundamental understanding of the intermediate phase formation and evolution, and their effect on the mechanical properties of the resulting UHTC. This study lays the groundwork for 2D MXenes as compatible materials for UHTC applications that can be used as precursors for carbides in ultra-high temperature applications such as hypersonics, nuclear, and extraterrestrial travel.

MXenes are a family of two-dimensional (2D) transition metal carbonitrides, with a general structure of M_n+1X_nT_x, where M is the transition metal, X is carbon or nitrogen, and T represents the surface terminations (F, O, OH) and n = 1-4 indicates to the number of atomic layers in their structure. Since their discovery, MXenes have shown promising properties for electrochemical charge storage applications and various MXene compositions have been tested in different electrochemical systems, both in their multilayered and delaminated forms. However, usually these materials show capacitive-like response in aqueous electrolytes with the exception of protic electrolytes such as H₂SO₄ where redox peaks and a pseudocapacitive behavior is observed. Particularly, MXenes tested in aqueous neutral electrolytes exhibit quasi-rectangular cyclic voltammograms. In this presentation, we demonstrate that through changing MXenes’ electrode structure by hybridizing them with conducting large modifiers, these materials can show anomalous electrochemical response with distinct redox couples in neutral aqueous electrolytes. We primarily show this behavior on hybrid V₂CT_x electrodes, where the free-standing films exhibit a distinct cyclic voltammograms in various alkali cation containing electrolytes with broad cathodic and anodic peaks, unlike pristine V₂CT_x which has a capacitive response. The observed phenomena in hybrid structures indicates a different charge storage mechanism in these electrodes while maintaining the overall amount of the charge that can be stored. Our results open new avenues in understanding the charge storage mechanism in MXene electrodes which can be used to tailor the desired electrochemical response of these materials in different electrolyte systems.

Friction and wear occur in all moving mechanical systems thus greatly contributing towards energy losses and downgraded efficiencies. These aspects together with global warming, diminishing material and raw oil resources as well as more stringent restrictions for the use of certain additives in lubricating oils urgently ask for advanced lubrication concepts. In this regard, solid lubrication has emerged as a promising strategy to tackle these challenges. Very recently, MXene solid lubricant coatings have shown an outstanding wear performance and durability with the potential to outperform all state-of the-art solid lubricants including graphene and MoS₂. Although showing an excellent wear performance, MXene solid lubricant coatings still demonstrate a rather poor frictional performance, which needs further optimization and improvement.
Therefore, we aim at fabricating innovative sandwich-like composite coatings consisting of two different 2D nano-materials (Ti₃C₂T_x and MoS₂) on stainless steel substrates for solid lubrication purposes. As an additional variable, we vary the respective order of the different coating layers (Ti₃C₂T_x/MoS₂ and MoS_2/ Ti₃C₂T_x). We envision that these sandwich-like composite coatings exhibit enhanced mechanical properties as well as good friction and wear properties at the same time due to the designed sandwich-like structure and synergetic effects originating from individual nano-sheets.
As a first step, single-material solid lubricant coatings (either only Ti₃C₂T_x or MoS₂) with a thicknesses of 600 nm are spray-coated on stainless steel substrates with a particular emphasis on the coatings’ homogeneity. Subsequently, the as-deposited coatings are holistically studied regarding their overall quality (roughness and thickness), physicochemical properties (surface chemistry) by complementary materials characterization. Afterwards, the effect of the material (Ti₃C₂T_x or MoS₂) on friction and wear under dry sliding conditions is evaluated in short- (running-in) and long-term experiments (durability). Particular emphasis is laid on the underlying energy-dissipating friction and wear mechanisms. In this regard, the worn surfaces (coatings and counter-bodies) will be fully characterized by complementary characterization to elucidate the formation of potential tribolayers.
After having studied the effect of the underlying material, sandwich-like composite coatings combining Ti₃C₂T_x and MoS₂ are spray-coated on stainless steel substrates. Regarding deposition, concentrations of 4 mgmL^-1 for MoS₂ and 5 mgmL^-1 for Ti₃C₂T_x were used, modifying the applied volume to ensure the fabrication of high-quality, thickness and homogeneous sandwich-like composite coatings. To allow for a fair and direct comparison, the sandwich-like composite coatings will be evaluated using the same experimental conditions (materials characterization and tribological testing conditions). The worn composite coatings will be studied by advanced, complementary materials characterization to elucidate the underlying friction and wear mechanisms as well as potential synergetic effects stemming from the different 2D nano-sheets.
Summarizing, sandwich-like composite coatings (MoS_2/Ti₃C₂T_x and Ti₃C₂T_x/MoS₂) are expected to improve the interfacial properties and enhance the system's mechanical properties. This is hypothesized to induce synergetic tribological properties thus creating solid lubrication coatings with an enhanced friction and wear performance at the same time.

The ever-increasing demand for renewable energy has urged the development of new materials for the new-generation energy storage and conversion. Among all the candidature materials, 2D transition metal carbides, carbonitrides and nitrides (often referred to as MXene) have drawn intensive attention owing to their excellent electrical conductivity and unique chemical and physical properties. Our research shows that MXene can be applied as host materials for anchoring single atoms to be used in electrochemical hydrogen generation [1] and carbon dioxide conversion [2], as well as composite materials for the application in new-generation batteries with excellent electrochemical performance such as in lithium-sulfur batteries [3, 4], lithium-iodine batteries [5], lithium/sodium-selenium batteries [6], sodium-ion batteries [7], sodium-sulfur batteries [8], and potassium-sulfur batteries [9]. Moreover, the careful design of the material architecture of MXenes has endowed the application for the desalination of saline water [10]. The excellent performances indicate that the MXene-based materials are the promising materials for the applications in the field of energy conversion and storage.

References:
[1] J. Zhang, Y. Zhao, X. Guo, C. Chen, C.-L. Dong, R.-S. Liu, C.-P. Han, Y. Li, Y. Gogotsi, G.X. Wang, Nature Catalysis, 1 (2018) 985.
[2] D. Zhao, Z. Chen, W. Yang, S. Liu, X. Zhang, Y. Yu, W.-C. Cheong, L. Zheng, F. Ren, G. Ying, X. Cao, D. Wang, Q. Peng, G.X. Wang, C. Chen, J. Am. Chem. Soc., 141 (2019) 4086.
[3] W. Bao, D. Su, W. Zhang, X. Guo, G.X. Wang, Adv. Funct. Mater., 26 (2016) 8746-8756.
[4] W. Bao, L. Liu, C. Wang, S. Choi, D. Wang, G.X. Wang, Adv. Energy Mater., 8 (2018) 1702485.
[5] X. Tang, D. Zhou, P. Li, X. Guo, C. Wang, F. Kang, B. Li, G. Wang, ACS Cent. Sci., 5 (2019) 365.
[6] F. Zhang, X. Guo, P. Xiong, J. Zhang, J. Song, K. Yan, X. Gao, H. Liu, G. Wang, Adv. Energy Mater., 10 (2020) 2000446.
[7] X. Guo, W. Zhang, J. Zhang, D. Zhou, X. Tang, X. Xu, B. Li, H. Liu, G. Wang, ACS Nano, 14 (2020) 3651-3659.
[8] W. Bao, C. E. Shuck, W. Zhang, X. Guo, Y. Gogotsi, G. Wang, ACS Nano, 13 (2019) 11500-11509.
[9] X. Tang, D. Zhou, P. Li, X. Guo, B. Sun, H. Liu, K. Yan, Y. Gogotsi, G. Wang, Adv. Mater., 32 (2020) e1906739.
[10] W. Bao, X. Tang, X. Guo, S. Choi, C. Wang, Y. Gogotsi, G. Wang, Joule, 2 (2018) 778-787.

Electrochemical actuators (ECAs) can generate deformation and force under very low input voltages, and are promising for various applications such as artificial muscles and soft robotics. Ti₃C₂T_x MXene is a potential active material for ECAs due to its large volumetric capacitance and high electrical conductivity. However, MXene film suffers from unfavorable in-plane actuation performance in aqueous electrolytes because of the high in-plane alignment of MXene sheets and poor wet strength. Here, we fabricated molecular-level methylcellulose/MXene hybrid films with enlarged layer distances by a simple mixing and vacuum filtration process. The hybrid films show improved ambient stability and wet strength in 1 M aqueous Li₂SO₄. Though the charge storage capacity was improved slightly, the hybrid films generated significantly higher in-plane actuation displacements in the aqueous electrolyte. Based on the direct strain monitoring as well as in-situ XRD and ex-situ XPS investigations, the enhanced actuation is ascribed to the enlarged interlayer distance allowing more ions/water to be intercalated/deintercalated and more importantly the large sliding of MXene sheets induced by methylcellulose during the charge/discharge. The assembled soft ECA has a high Young’s modulus of 1.93 GPa and generated a peak-to-peak strain difference up to 0.541% and a blocking force of 4.7 times its gravity under 1-V triangular and DC voltages, respectively. The significant actuation enhancement induced by the insulating methylcellulose can inspire the rational design of high performance ECAs based on other nanomaterials.
Reference
Chen S., Ciou J.-H., Yu F., Chen J., Lv J., Lee P. S. Molecular-Level Methylcellulose/MXene Hybrids with Greatly Enhanced Electrochemical Actuation. Advanced Materials. 2022, 2200660.

All-solid-state energy storage devices with non-flammable inorganic solid electrolytes are a key technology for addressing the safety issues of lithium-ion batteries with flammable organic liquid electrolytes. However, conventional electrode materials suffer from substantial volume change during lithium-ion (de)intercalation, leading to the failure of the interface between the electrode materials and solid electrolytes and then severe performance degradation. In this work, we report pseudocapacitance through a strain-free interface between a solid electrolyte and a transition-metal carbide nanosheet (MXene), where MXene electrodes show negligible structural changes during lithium-ion (de)intercalation. As a proof of concept, we demonstrate the stable operation of an all-solid-state hybrid capacitor (Ti3C2Tx|Li3PS4 glass|LiCoO2).

The emergence of two-dimensional (2D) materials has captured the imagination of researchers since graphene was first exfoliated from graphite in 2004. Their exotic properties give rise to many exciting potential applications in advanced electronic, optoelectronic, energy and biomedical technologies. Scalable growth of high quality 2D materials is crucial for their adoption in technological applications the same way the arrival of high-quality silicon single crystals was to the semiconductor industry. A huge amount of effort has been devoted to grow large-area, highly crystalline 2D crystals such as graphene and transition metal dichalcogenides (TMDs) through various methods.
While CVD growth of wafer-scale monolayer graphene and TMDs has been demonstrated, considerable challenges still remain. In this talk, we first advocate for the focus on the crystal growth morphology as an underpinning for understanding, diagnosing and controlling the CVD process and environment for 2D material growth. Like snowflakes in nature, 2D crystals exhibit a rich variety of morphologies under different growth conditions. The mapping of crystal shapes in the growth parameter space “encodes” a wealth of information, the deciphering of which will lead to better understanding of the fundamental growth mechanism and materials properties. However, the morphology pattern evolution of 2D crystals such as MoS₂ monolayers under a practical CVD growth condition is highly complicated due to the entanglement of multiple growth factors. The ability to directly monitor it in real-time would be substantial to provide first-hand data to lay the groundwork for most advanced tools such as machine-learning to unravel those threads. A customized system with the function of observing and recording the CVD growth of MoS₂ in a miniature furnace is developed in this work. Image processing techniques are utilized to convert the real-time growth footage into frame-wise digital numbers and machine-learning is deployed to uncover the importance of multiple controlling factors in the growth. The model successfully guides the discovery of experimental control parameters to grow ultra-large size MoS₂monolayers. The model also demonstrates the possibility to trace back experimental condition by analyzing the crystal morphology parameters. In a parallel effort, we also demonstrate that the widely used powder-processing technique of dry ball-mill, can be improved to produce high-quality 2D flakes in large scale and with low cost. Seventeen types of commonly seen polymers, including both artificial and natural ones, have been examined as additive to dry ball-mill of hexagonal boron nitride. The potential of polymer-assisted ball-mill exfoliation as a universal way to produce ultra-thin 2D nanosheets is also demonstrated.

The urgent demand for Ohmic contact on a large scale has been raised since a milestone has been achieved in the large-scale synthesis of two-dimensional (2D) Molybdenum disulfide (MoS₂), which is the most studied 2D semiconductor owing to its dangling-bond-free surface and atomic thickness. Molybdenum nitride (Mo₅N₆), one of 2D metallic MXenes families which possess an excellent electronic conductivity comparable with graphene, can be a competitive candidate for the electrode material. However, the high nitridation temperature of MoS₂ hinders the utilization of Mo₅N₆ in the preparation of in-plane heterojunction with MoS₂. In this work, by weakening the Mo-S bonds using an oxygen plasma pretreatment, the conversion temperature of MoS₂ to Mo₅N₆ was effectively lowered to 600 Celsius degree, which is a sustainable temperature for MoS₂. Through a shadow mask method, the preparation of 2D Mo₅N₆-MoS₂ in-plane metal-semiconductor heterostructure was successfully achieved, which is verified by the Raman mapping. The transfer measurements revealed a Schottky barrier of about 30 meV, which provides solid evidence of Ohmic contact. Furthermore, the scale-up pattern synthesis of the in-plane Mo₅N₆-MoS₂ heterostructure was also demonstrated for the first time. Our method provides a scalable production of in-plane 2D metal-semiconductor heterojunction with Ohmic contact and sheds light for the large-scale electronic application of 2D semiconductors.

MXenes are an emerging class of 2D materials with unique properties including excellent metallic conductivity, mechanical flexibility, hydrophilicity, and tunable surface functionality, which ensure their utility for diverse applications. However, the synthesis of stoichiometric MXenes with high quality in a low-cost process is still challenging because of the difficulty in controlling the oxygen substitution in the precursors and final products of MXenes. Here, we report a novel cost-effective method to synthesize Ti₃C₂T_x MXene with controlled contents of substitutional oxygen by optimizing the amount of excess carbon in carbothermal reduction of TiO₂ precursor. The highest used content (2 wt.%) of excess-carbon yields TiC with minimal oxygen substitutes, which leads to Ti₃AlC₂ MAX phase with improved crystallinity and stoichiometry, and, finally, Ti₃C₂T_x MXene with a high electrical conductivity (>11700 S cm^-1) and superior electromagnetic shielding effectiveness. Additionally, the effects of substitutional oxygen on the physical properties of TiC, and Ti₃AlC₂ are elucidated by density-functional theory calculations. This TiO₂-based method of synthesizing high-quality Ti₃C₂T_x MXene substantially lowers the material cost and can facilitate large-scale manufacturing, thus accelerating global research on MXenes and their introduction into industrial products.

Borophene refers to synthetic two-dimensional (2D) boron, which has attracted significant attention due to its anisotropic metallicity, correlated electron phenomena, and diverse superlattice structures.^[1] Reducing the dimensionality of nanomaterials imposes additional quantum confinement effects that unlock new physical phenomena, such as one-dimensional (1D) confined plasmons, spin-polarized edge states, and Luttinger liquid behavior. However, the realization of quantum-confined borophene nanoribbons (BNRs) is hampered by limited boron precursor options for bottom-up synthesis. In this work, we present a substrate mediation strategy to synthesize quantum-confined BNRs on vicinal Ag(977) substrates. The resulting BNRs inherit the high degree of polymorphism present in borophene, resulting in v_1/5 and v_1/6-BNR lattice configurations in addition to phase intermixing.^[2] Through atomic-scale imaging, spectroscopy, and first principles calculations, the edge structures of BNR polymorphs are shown to possess reconstructed armchair edges for v_1/6-BNRs and sawtooth zigzag edges for v_1/5-BNRs. The confined electron wave functions in 1D BNRs lead to the observation of energy level quantization and spatial nodes characteristic of quantum-well states.

Acknowledgement:
This work was supported by the Office of Naval Research (ONR N00014-17-1-2993) and the National Science Foundation Materials Research Science and Engineering Center (NSF DMR-1720139).

References:
[1] Mannix, A. J., et al., Synthesis of borophenes: Anisotropic, two-dimensional boron polymorphs. Science, 2015, 350 (6267), 1513-1516.
[2] Liu, X., et al., Intermixing and periodic self-assembly of borophene line defects. Nature Materials, 2018, 17 (9), 783-788.

Twisted bilayer 2D materials exhibit a wide range of intriguing physical properties, such as superconductivity, ferromagnetism, and superlubricity. Depending on the twist angle, periodic moire superlattices form in twisted bilayer graphene, with inhomogeneous interlayer coupling and lattice deformation. For a small twist angle, each moire supercell contains a large number of atoms (>10,000), making it computationally expensive for first-principles and atomistic modeling. In this work, a finite element method based on a continuum model is used to simulate the inhomogeneous interlayer and intralayer deformations of twisted bilayer graphene and MoS2. The van der Waals interactions between the 2D layers are described by a periodic potential energy function, whereas each 2D layer is treated as a continuum membrane with effective elastic properties. Our simulations show that structural relaxation and the induced strain localization are most significant at small twist angles, where the strain distribution is highly localized as shear strain solitons along the boundaries between neighboring domains of commensurate AB stacking. Moreover, it is found that there exist many metastable equilibrium configurations at particular twist angles, depending on the specimen size. The nonlinear mechanics of twisted bilayer 2D material is thus expected to be essential for understanding the strain distributions in the moire superlattices and the strain effects on other physical properties.

Direct fabrication of matter on atomic level is the North Star goal for areas spannign quantum computing, quantum communications, biological sensing, and fundamental physical research. Over the last decade, it has been shown that electron beams in Scanning Transmission Electron Microscopy can be used not only to probe structure and electronic properties of materials on atomic level, but also to modify materials on the atomic level. Harnessing electorn beam changes for direct atomic farication however requires synergy between machine learning methods and microscope control. In this presentation, I will illustarte the progression of automated electron microscopy from real-time data analysis to physics discovery to atomic manipulations. Here, the applications of classical deep learning methods in streaming image analysis are strongly affected by the out of distribution drift effects, and the approaches to minimize though are discussed. The robust approach for real-time analysis of the scanning transmission electron microscopy (STEM) data streams, based on the ensemble learning and iterative training (ELIT) of deep convolutional neural networks, is implemented on an operational microscope, enabling the exploration of the dynamics of specific atomic configurations under electron beam irradiation via an automated experiment in STEM. Combined with beam control, this approach allows studying beam effects on selected atomic groups and chemical bonds in a fully automated mode. We demonstrate atomically precise engineering of single vacancy lines in transition metal dichalcogenides and the creation and identification of topological defects graphene. The ELIT-based approach opens the pathway toward the direct on-the-fly analysis of the STEM data and engendering real-time feedback schemes for probing electron beam chemistry, atomic manipulation, and atom by atom assembly. We further illustarte how deep kernel learning (DKL) methods allow to realize both the exploration of complex systems towards the discovery of structure-property relationship, and enable automated experiment targeting physics (rather than simple spatial feature) discovery. The latter is illustrated via experimental discovery of the edge plasmons in STEM/EELS. Jointly, these developments open the pathway for creation and characterization of designed defect configurations and artificial molecules in 2D materials.

Recent advances in metal additive manufacturing (AM) have provided new opportunities for prompt designs of prototypes and facile personalization of products befitting the fourth industrial revolution. In this regard, its feasibility of becoming a green technology, which is not an inherent aspect of AM, is gaining more interests. A particular interest in adapting and understanding of eco-friendly ingredients can set its important groundworks. Here, we demonstrate a water-based solid-phase binding agent suitable for binder jetting 3D printing of metals. Sodium salts of common fruit acid chelators form stable metal-chelate bridges between metal particles, enabling elaborate 3D printing of metals with improved strengths. Even further reductions in the porosity between the metal particles are possible through post-treatments. A compatibility of this chelation chemistry with variety of metals is also demonstrated. The proposed mechanism for metal 3D printing can open up new avenues for consumer-level personalized 3D printing of metals.

Bilayers of 2D materials offer unique opportunities for creating devices with tunable electronic, optical, and mechanical properties. In van der Waals heterostructures (vdWHs), where the constituent monolayers have different lattice constants, aligned bilayers foster a superlattice moiré length scale that is larger than the lattice constant of either constituent material. Here, we report new Raman modes in WSe₂-WS₂ vdWHs in the range of 240-260 cm^-1, which are absent in both monolayers and homobilayers of WS₂ and WSe₂. Using first-principles calculations and geometric arguments we show that these new modes are a consequence of the large moiré length scale, which results in zone-folded phonon modes that are Raman active. These modes are sensitive to changes in twist angle, but notably, they occur at identical frequencies for a given small twist away from either the 0-degree or 60-degree aligned heterostructure. Our measurements also show that the Raman intensity in the frequency range of interest, which includes both the new Raman modes and the WSe₂ A_1g peak, exhibits a markedly different dependence on excitation energy in near-0 vs. near-60 degree vdWHs. Temperature-dependent reflectivity measurements demonstrate the significant Raman intensity modulation arises from resonant Raman effects

The surging of air pollutants is a major concern for mankind in the modern era affecting climate change, public and individual health [1]. A soaring demand have been pressed for sophisticated gas sensing devices at indoor areas, aviation sectors, road transportation and industries for the detection of toxic domestic gases and environmental monitoring [2]. One of the solutions may lie in the nanotechnology of 2D materials to develop hybrid structures as the ultra-sophisticated sensing medium. The artificial stacking of 2D heterostructures brings unprecedented properties useful to develop selective and sensitive chemical and gas sensors that are also applicable to several disciplines of mechanical, biological and corrosion science [3-5]. The high surface-to-volume ratio of these layered materials facilitates efficient response to the surrounding environment and promotes ultrafast carrier mobility of surface atoms [6].
Here, I will present a systematic study of the growth of vertically oriented MoS₂ nanoplatelets over graphene sheets. The advantage of having an aligned structure is to generate the availability of numerous active sensing sites due to cross-section thickness (» 20 nm) to attain large-area adsorption of analytes. The synergy between the transport property of graphene sheets and selectivity from MoS₂ edge atoms can develop hybrid sensors as chemiresistor which is superior than their individual counterparts. Additionally, I will demonstrate the mechanism of doping from the aqueous analytes through Raman spectroscopy and high-resolution scanning probe techniques, especially to distinguish the role of edge atoms than basal plane of MoS₂. The present design of hybrid structures will be useful for detecting a broad range of analytes in extremely low concentrations of parts per billions (ppb). The present study strengthens the possibility of sensing the interaction between several organic and biomolecules toward different 2D materials.

References:
1. I. Manisalidis, E. Stavropoulou, et al., Frontiers in public health, 2020, 14.
2. S. M. Majhi, A. Mirzaei, et al., Nano Energy, 2021, 79, 105369.
3. M. Tripathi, R. Garriga, et al., 2D Materials, 2022, 9, 045033.
4. G. Chilkoor, K. Jawaharraj,…M.Tripathi et al., ACS nano, 2020, 14, 14809-19.
5. A. Mescola, G. Paolicelli,…M.Tripathi., Small, 2021, 17, 2104487.
6. M. Tripathi, F. Lee, et al., ACS nano, 2021, 15, 2520-31.

Pristine van der Waals (vdW) interfaces between two-dimensional (2D) and other materials are essential for emerging optical and electronic devices. Their fabrication is, however, challenged by weak vdW forces that are constrained by the intrinsic material properties and hence not readily tunable to accommodate formation of arbitrary heterostructures. Conventional techniques overcome this limitation by facilitating transfer using sacrificial layers, solvents and high temperatures, steps which could introduce damage and contaminants. In addition, often, post-transfer fabrication steps are required for device integration, which could further introduce damage. An ideal fabrication approach would accommodate a single-step 2D material-to-device integration.

To realize such a platform, here, we present the adhesive matrix transfer approach. This approach enables sacrificial layer-free, dry, and pristine fabrication of arbitrary vdW interfaces, and their integration into functional devices. Core to this approach is decoupling the vdW forces defining the interface of interest from the forces responsible for 2D material transfer. This is achieved by embedding the substrate of interest in a matrix whose surface adhesive forces can be independently tuned to allow for the transfer of arbitrary layers. With this approach we have demonstrated conventionally forbidden, direct vdW integration of diverse 2D materials, including MoS₂, WSe₂ and GaS, with bulk dielectrics (SiO₂, Al₂O₃), core building blocks of functional devices. We use this technique to then demonstrate single-step integration of 2D materials into devices, using an example of arrays of monolayer-MoS₂ transistors. With no exposure to polymers and solvents, our approach enables clean interfaces and pristine surfaces with reduced fermi-level pinning, which can be further engineered to demonstrate high-quality n- and p-type behavior. Beyond serving a platform to probe intrinsic properties of 2D and other sensitive nanomaterials without the influence of processing steps, our fabrication technique further allows efficient formation of devices with unconventional form-factors, here we discuss its application in 2D flexible electronics.

Two-dimensional (2D) materials have been studied for use in a variety of emerging electronic, optoelectronic, and photonic applications. Device fabrication often involves depositing various three-dimensional (3D) materials, such as metals or dielectrics, onto the 2D material, which creates a buried interface. Interfacing materials significantly affect the nature and homogeneity of the electronic and phononic structure in 2D materials. To date, spectroscopic studies of the doping and strain in supported 2D materials have mostly been limited to unrealistic cases where the 2D material is transferred or exfoliated onto a substrate of interest [1,2], with a few exceptions [3]. A polymer-free transfer technique capable of exposing any buried 2D-3D material interface would enable direct physical characterization of realistic heterojunction properties.
In this presentation, we present a new polymer-free transfer technique that allows direct characterization of the buried interface in almost any 2D-3D heterostructure. The method is enabled by employing a water soluble GeO₂ substrate and wafer bonding process. We demonstrate the diversity of the technique by fabricating various heterostructures on monolayer MoS₂, including inert and reactive metals and oxide dielectrics, and expose the initially buried interfaces for direct characterization. We also fabricate the same heterostructures by exfoliating MoS₂ onto metals and oxides to directly compare strain, doping, and chemistry in MoS₂ across fabrication techniques. We employ Raman spectroscopy to measure the strain and doping profile in MoS₂ heterostructures. The surface chemistry and band alignment (where applicable) are measured using X-ray photoelectron spectroscopy (XPS).
Reactively sputtered GeO₂ films exhibit a 2:1 O:Ge ratio according to XPS. After heterostructure fabrication and wafer bonding, the GeO₂ layer is completely dissolved in deionized H₂O within a few hours, exposing the buried interface. In general, peak shifts in MoS₂ Raman spectra indicate the degree of strain and doping imparted on MoS₂ significantly depends on the interfacing material and the fabrication method. When the interfacing material is directly deposited on MoS₂, the strain imparted on the MoS₂ is more significant than the degree of doping. Reactions between MoS₂ and reactive metals (Ti, Ni) are significant when the heterostructure is fabricated by direct deposition according to XPS and Raman spectroscopy. In contrast, when heterostructures are fabricated by exfoliating MoS₂ onto a substrate of interest, doping dominates over strain. Our novel method enables direct characterization of buried 2D-3D interfaces and expanded control over the strain profile in 2D materials, which could lead to new discoveries and functionalities.
This work was supported by a Laboratory Directed Research & Development program at Sandia National Laboratories. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science.
[1] B. G. Shin et al. Adv. Mater. 2016, 28 (42), 9378-9384
[2] Y. Chen et al. ACS Nano 2018, 12, 2569-2579
[3] K. Jo et al. ACS Nano 2021, 15, 5618-5630

The inorganic nanosheets have 2 dimensional layered structure which have attracted great attention due to their novel electronic, thermal, and chemical properties. Also, 2 dimensional materials have really tiny thickness and have weak coupling between layers, so it can be easily exfoliated to single layer. It is already crystalline phase so thermal annealing is not neccesary. Also, it is easy to scale up and very thin, so it can be applied transparent and flexible devices too. We particularly focused on the Dion-Jacobson structure, which is relatively easy to insert and exfoliate and to obtain nanosheets with parent composition properties. In layered perovskite oxides of the Dion-Jacobson (DJ) type, perovskite blocks (corner-shared BO₆ octahedra) are well known to be layered in an n:1 manner with an alkali metal cation layer, which is greatly influenced by their composition. In this study KSr_1.8A_0.2Na_n-3Nb_nO_{3n + 1} (n = 3 to 5) (KSANNO) having a DJ structure was synthesized using a solid solution reaction. Nanosheet solution was obtained from KSANNO through soft chemical method. The obtained nanosheets were filmed by Langmuir Blodgett method. We confirmed that the electrical properties of the nanofilm were changed depending on the number of layers of the perovskite block belonging to the monolayer.

Hexagonal boron nitride (h-BN) is a promising ultrawide-bandgap semiconductor for emerging optoelectronic and high-power electronic devices. Nevertheless, ohmic current injection into h-BN remains a significant challenge in developing these current-driven devices. Interestingly, the hybridized two-dimensional structure containing boron, carbon, and nitrogen, which was reported to have modifiable electrical properties by controlling C concentration [1], is a potential material to overcome this challenge. It is thus possible that a small C concentration introduced to hybridize with h-BN could improve the electrical conductivity of h-BN.

In this study, the effect of C on the electrical conductivity of h-BN was investigated. To minimize the influences from other C-unrelated defects, the single-crystal h-BN synthesized under high pressure and high temperature (HPHT) was selected. C was introduced by post-growth diffusion with two different conditions: at 2.5 GPa and 3100°C for 30 minutes and at 1 atm and 2000°C for 4 hours. The former condition results in gray-colored h-BN and the latter one gives yellow-colored h-BN. According to SIMS profiles, the gray C-doped h-BN (h-BN:C) shows ~10 at.% C, which is ~100 times higher in C concentration than the yellow h-BN:C. The transverse dielectric breakdown of both h-BN:C is comparable to that of undoped h-BN. Raman spectra show the apparent differences between these two types of h-BN:C. At 3 K, the E_2g vibrational mode of h-BN is observed at ~1368 cm^-1 for both h-BN:C, while the peaks corresponding to G and 2D modes in graphene are detected only in the gray h-BN:C, indicating the existence of C–C bonds. It can be expected that C and h-BN domains coexist in the gray h-BN:C. The electrical conductivity was investigated by forming metal contact following the method described in Ref. 2. The I-V characteristic of the gray h-BN:C initially exhibited nonohmic conduction within an applied voltage of ±100 V at 298 K and transformed to ohmic conduction after the breakdown-like behavior at 598 K. The surface topology change of the h-BN channel can be seen in most of the devices with ohmic conduction after the high-T measurements. On the other hand, the yellow h-BN:C requires at least ±50 V to inject current with nonohmic conduction for all T, and no breakdown-like behavior nor topology change in the channel is observed. After the T-dependent I-V measurements, the devices were characterized by electron-beam-induced current (EBIC) mapping, cross-sectional TEM, and EELS. The EBIC results indicate the formation of a conductive path in the gray h-BN:C channel. The EELS mapping and its energy loss spectra reveal the existence of discrete graphite/graphene layers and randomly distributed C domains in the gray h-BN:C, which is consistent with the Raman spectra. These results suggest the possibility that C structures existing in h-BN induced the local breakdown of h-BN, which functions as a conductive path. Therefore, to enable the development of h-BN:C-based high-power electronics, the formation of randomly distributed C domains and graphite/graphene layers must be controlled.

[1] L. Ci, et al., Nat. Mater 9, 430 (2010).
[2] S. Ngamprapawat, et al., ACS Appl. Mater. Interfaces 14, 25731 (2022).

Tellurene is an emerging 2D p-type semiconductor. By contact engineering and post-metallization annealing process, the ambipolar Te-FETs were fabricated in this study. Interestingly, thinner Te flakes tend to have a p-type characteristic while thick Te possesses n-FET property. Here, based on experiments and simulations, we propose a new transport model for 2D Te FETs. This model helps us understand the thickness-polarity dependence. Such phenomena in other 2D semiconductors were previously interpreted using the theory of band structure in different thicknesses. However, the subtle change in the band structure of Te is not capable of tuning the polarity from p-type to n-type. In other similar p-type 2D materials like BP, the role of surface native oxide was often ignored in FET transport. We further demonstrated for the first time a monolithic CMOS inverter based on tellurene, leveraging the symmetric operation of p- and n-type tellurene FETs. Our results not only demonstrate robust and high-performance tellurene CMOS inverters, but also provide a new transport model to similar 2D semiconductors, shedding light on high-performance 2D electronics and CMOS design.

One of the central goals of nanotechnology is to directly fabricate nanoscale architectures with atomic-scale precision. With recent advancements in aberration corrected scanning transmission electron microscopy (STEM), the sub-Å sized electron beam can be used to manipulate the atomic structure of materials. Previous work has shown the electron beam is capable of locally and controllably creating defect structures within 2D materials such as nanopores and atomic edge configurations that have enhanced electronic, magnetic, optical and catalytic properties. In the model MoS₂ materials system, several such defect structures can be fabricated through the localized irradiation of sulfur sites within the lattice through the knock-on process. In this work, an emphasis was placed on the controlled fabrication and characterization of two distinct defect structures. First, the sulfur vacancy line (SVL) defect structure, composed of a line of single sulfur vacancy sites, that has previously been shown to conglomerate together to form a local metallic MoS area within the MoS₂ monolayer. Second, the metallic Mo₆S₆ (MoS-NW) nanowire-type defect structure that forms along the edge of nanopores within the MoS₂ monolayer. Initial work has demonstrated that these MoS-NW form preferentially along the zig-zag sulfur terminated direction within the lattice. Through precise control over the electron beam irradiation of S sites along this direction, edge decorated nanopores can be fabricated. To increase the reliability of this fabrication method, Python-based codes have been developed that allow for in situ data acquisition, decoding and automated beam control in-line with the electron microscope workflow following the ensemble learning iterative training (ELIT) approach for deep convolutional neural networks (DCNNs). Using ELIT DCNNs, during fabrication, the image is decoded in real time into the atomic coordinates and identities allowing for precise targeting of atomic species and locations. In this regard, automated electron beam manipulation experiments have been developed that can form these beneficial defect structures that can lead to direct bandgap engineering.

Microscopy research was performed at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, which is a US Department of Energy (DOE), Office of Science User Facility.

Quantum Information Science (QIS) is an emerging research area that promises to revolutionize computing, communication, and sensing. Two-dimensional (2D) materials are one of the most versatile motifs for generating new QIS materials since one layer in arbitrary stacks forms heterostructures unconstrained by epitaxy. Since manual mechanical exfoliation, or “tape exfoliation,” produced mono and few-layered graphene in 2004, researchers have made tremendous progress in 2D material research. However, it has been challenging to study and utilize 2D materials because of a time- and labor-consuming fabrication process that highly depends on the hands-on experience of researchers. Here, I present the ongoing development of a high throughput 2D material fabrication platform, Quantum Material Press (QPress), an integrated cluster tool including three custom machines: an exfoliator, cataloger, and stacker. The exfoliator is a fully automated machine for tape exfoliation using commercial pressure-sensitive adhesive (PSA) tape and a roller assembly. The mechanism of the exfoliator is the same as that of the manual tape exfoliation. However, the PSA tape is compressed onto a substrate by a press roller and peeled off by a tape rewinder instead of human hands. After the exfoliation, the cataloger detects and classifies the exfoliated flakes, and makes a data library for users. Finally, we use the fully motorized stacker to stack selected flakes into heterostructure, with 3-axis and 6-axis manipulation stages for a substrate and a stamp, respectively. These machines allow us to investigate underlying mechanisms of 2D material fabrication. We believe that QPress will help us to provide a more reliable method of 2D material fabrication by developing optimum conditions for higher yield and reproducibility.

Two-dimensional (2D) van der Waals (vdW) materials and their heterostructures provide excellent platforms for exploring fascinating physical phenomena and implementing intriguing applications. One of the most attractive features of this family of materials is the tunability of their quantum behavior depending on the layer number. Different exotic physical properties, such as magnetic, electronic, and optoelectronic properties, were observed in samples with different thicknesses, especially in a thickness range from monolayer through several nanometers. It is recognized that lattice structural transformations often accompany electronic and spin structural changes that provoke exotic quantum phenomena. It is however unclear whether the same structural transformation as bulk material occurs in thin flake samples. Indeed, a consistent result is often missing regarding the structure-property relationship in several-layer thick 2D vdW materials. In this talk, we reveal the thickness dependency of low-temperature phase transformation and exciton state change in several model 2D vdW materials by utilizing cryogenic atomic resolution scanning transmission electron microscopy (STEM) and monochromated electron energy loss spectroscopy (EELS). Our results allow a precise understanding of relevant layer-number-dependent properties, information critical to harnessing unique quantum characteristics in thin layer 2D vdW materials for device applications.

Scanning transmission electron microscopy (STEM) provides uniquely powerful tools to study the interfacial structure and transformations of 2D materials up to atomic resolution. Here, we utilize 2D multilayer stacks to create nanoscale laboratories for studying the atomic structure, structural transformations, and properties of 2D interfaces inside the STEM. We utilize graphene encapsulation in combination with a MEMS-based heating holder to conduct in-situ studies of solid-solid phase transformations and interfacial restructuring in 2D transition metal dichalcogenides. The graphene encapsulation protects the 2D materials from the vacuum environment of the STEM, enabling high temperature studies up to 1000°C. We use these structures to directly visualize the layer-by-layer phase transformation of MoTe₂ and the lattice reconstruction of 2D moirés.

First, we study the interfacial reconstruction mechanisms of twisted bilayer transition metal dichalcogenides (MoSe₂ and WSe₂) using in situ aberration-corrected STEM and dark-field transmission electron microscopy (DFTEM). We discover a new mechanism of local reconstruction from moiré patterns to parallel (3R) and/or antiparallel (2H) domains in twisted bilayer 2D transition metal dichalcogenides (TMDCs). Unlike previous reports, where the twist angle is altered by the rotation of a whole flake, we observe the nucleation and growth of commensurate domains through the formation and migration of grain boundaries. We capture the dynamic nucleation and growth of the aligned structure through a series of atomic resolution STEM images combined with atom-tracking analysis. These results provide new insights into the mechanisms of interlayer rotation at the atomic scale and potential method into engineering the structure of moiré superlattices.

Second, we examine the atomic mechanisms of phase transitions of MoTe₂. MoTe₂ exhibits multiple structural and electronic phases at room temperature (2H, 1T’, and T_d) that can be easily manipulated with different stimuli, making it a promising candidate for phase-change memories, high-performance transistors, and broadband photodetectors. In order to capture the dynamics of the T_d-to-2H phase transition, we apply heat pulses as short as 0.5s to "freeze" the growth front of the 2H phase at different temperatures. Next, we combine dark-field TEM and aberration-corrected STEM to image the phase transitions of few-layer MoTe₂ in situ from the micro- to atomic scales. We find that T_d-to-2H phase transitions initiate at the phase boundary at temperatures as low as 200-225 C, much lower than previously thought. We individually measure the phase front of each layer and find that the linear growth rate of the 2H phase at 275^oC varies from 5 to 14 nm/sec between different layers, which suggests that factors such as strain and local defect densities affect the energy barrier of the phase transition. In addition, the 2H-T_d interfaces propagate primarily along the b-axis of the T_d grain, producing an anisotropic, layer-by-layer phase transformation. These results should lead to new methods to manipulate the quantum and electronic properties within individual atomic layers.

Since the discovery of monolayer graphene in 2004, two-dimensional (2D) materials have attracted considerable attention owing to their potential applications in various fields such as optics, electronics, and catalysis. Numerous 2D materials have been developed in the past decade. Among them, 2D transition metal chalcogenides (TMCs) attract most of the attention, mainly attributed to their large spectrum of fascinating properties along with various crystal structures. In parallel with the discovery of new candidate materials and exploration of their unique characteristics, engineering 2D TMCs into designated structures and phases are also of great importance for meeting the requirement for different applications. In this talk, I will discuss how we use the chemical vapor deposition (CVD) method to engineer the structure and phase of 2D TMCs and as a result, to tune their electronic and magnetic properties. Several strategies will be particularly introduced including construction of heterostructure, alloying, phase-selective growth, and dimension tuning. Additionally, the feasibility and capability of using state-of-the-art machine learning (ML) techniques to guide the synthesis and engineering of 2D TMCs will also be discussed.

For decades Silicon based field-effect transistors (FETs) have been the foundation of modern semiconductor technology. Advancements in device fabrication have reduced the size of FETs down the nanoscale while improving performance and efficiency. Silicon however has reached its scaling limit and is no longer suitable for next generation devices due to inadequate carrier mobility, heat generation, and atomic scale thickness limitations. Transition metal dichalcogenides (TMDs) are intrinsic semiconductors that transition from indirect to direct band gaps in the near infrared to visible spectrum, making them excellent candidates for optoelectronic applications. However, the synthesis of these materials is not a trivial exercise, and the method of mechanical exfoliation, that jumpstarted the field, is not well suited for deeper research into the properties of these materials. Additionally, mechanical exfoliation has the potential to degrade the crystalline structure of the layers and is the source of significant defects. Interface quality and layer integrity are of vital importance when producing heterostructures, which is a key step in the functionalization of next-generation devices. To this end, direct epitaxial growth techniques such as metal oxide chemical vapor deposition (MOCVD) facilitates improved control of growth conditions by precise delivery of precursor gases at consistent reactor temperature and pressure, which enables the modulation of material properties. In this study, we demonstrate wafer scale growth of MoS₂ and WS₂ in a horizontal MOCVD reactor as part of the National Science Foundation’s 2-dimensional Crystal Consortium. We use molybdenum hexacarbonyl, tungsten hexacarbonyl, and hydrogen sulfide precursors and grew MoS₂ at 9, 10, and 11 minutes at 50 torr reactor pressure. We also reduced the metal flow rate by 20% while increasing growth time. For WS₂ we grew for 12 and 15 minutes at 200 torr without varying the metal flow rate. Samples were characterized via Raman Spectroscopy and Atomic Force Microscopy to analyze the quality of the growth and bilayer coverages. Using imaging processing techniques in MatLab, we found that the percent of bilayer coverage of MoS₂ varied between 57% and 24%. For WS₂, the 15 minute growth yielded a fully coalesced monolayer with approximately 16% bilayer coverage while the 12 minute growth did not fully coalesce. Raman spectroscopy of the MoS₂ samples also revealed an average 21.4 cm^-1 difference between the A1g and E2g phonon modes, confirming monolayer growth with varying percentages of bilayer coverage. These same layers were further characterized through tip-enhanced Raman spectroscopy (TERS) and kelvin probe force microscopy (KPFM). These techniques provided insight on chemical composition at the nanometer scale and the variation of the TMD material’s work function across the layer surface respectively. Our results provide fundamental insights into the conditions for nucleation and growth kinetics of wafer scale 2-dimensional TMDs and explore the chemical and electrical consistency across samples grown via the MOCVD technique.

Control over matter has allowed physics, condensed matter, and materials science communities to realize a wide range of properties useful in many fields and applications. Accessing control of nanometer and smaller scales allows to shape materials properties in fundamentally new and exciting ways. The scanning transmission electron microscope (STEM), like scanning probe microscopy (SPM), routinely enables direct visualization of matter at the atomic scale. The atom-sized electron probe in the STEM, however, also contains enough energy and momentum to structurally and chemically alter matter at the same atomic scale, where this electron-matter interaction is typically considered “beam damage.” It is argued that understanding and harnessing this interaction to manipulate matter at atomic scales will allow a more complete design and control of materials.

While manipulation of matter at the level of single atoms has been demonstrated in both STEM and SPM, this has primarily been performed manually without reproducibility or useful precision. Beam-induced effects in the STEM have been notoriously difficult to control, and the ordinarily stochastic changes have also been challenging to characterize analytically via electron energy loss spectroscopy (EELS) or 4D-STEM methods.

Artificial intelligence is leveraged to guide the microscope towards both atomic fabrication and autonomous characterization. The first step towards accurate atomic manipulation is to obtain knowledge of the atomic landscape in as close to real time as possible, while the second step is instructing the electron beam where to go relative to these positions. The atomic coordinate extraction – from image data - must be performed both quickly and reliably. Several methods exist that can detect atoms, but here we use deep convolutional neural networks. However even though a trained neural network is fast, it, like other methods, usually requires fine tuning of several parameters to ensure a reliable extraction. The extra time this incurs cannot be exhausted during a live experiment; further, each new image might require different parameter tuning. To overcome this and ensure robust predictability, deep ensembles are used, which handle the issues - also encountered in self-driving vehicles - of so-called “out of distribution” shifts.

Given atomic coordinates, one can classify different atomic or defect classes based on mixture models, intensities, or graph analysis, which allows to experiment with beam-induced effects precisely at the single-defect level. Different strategies of beam control are demonstrated in both graphene and MoS₂ and are shown to produce structures that cannot be fabricated by any other means.
Meanwhile, characterization of atomic and mesoscale features in the STEM is largely carried out by operator intuition and expertise such that a small region of interest is selected for detailed analyses, e.g., EELS or 4D-STEM, by inspection of the structural image, i.e., the annular dark field (ADF) signal. For several reasons, including handling beam sensitive materials, autonomous exploration of a larger sample space via EELS or 4D-STEM is useful. Deep kernel learning is utilized here to develop and learn a relationship between image data (structure) and a physical response by either EELS or 4D-STEM (property). These structure-property relationships are constructed from scratch and on-the-fly, with no prior information being supplied to the model. In this way, a material system can be explored in a completely autonomous fashion, without operator bias and without imparting too much unnecessary dose to the specimen.

Therefore, in leveraging AI and deep learning routines, atomic fabrication and autonomous characterization are made possible in the STEM. In the former, beam-matter interaction is exploited, while in the latter, it is minimized to only what is necessary. Both schemes are explored, and the challenges and practical limitations are discussed.

Recently, two dimensional transition metal dichalcogenides (TMDs) have been widely applied in electronic devices because of their excellent electronic, optical and mechanical properties. Nowadays, due to the scaling demand of electronic devices, it is necessary to better investigate the performance of materials in electronic and optoelectronic devices. In this regard, electrical breakdown associated with material damage is a critical issue in device failure. In this research, the device behaviors of MoSSe samples prepared under different selenization temperatures are revealed via in-situ biasing experiments and recorded by transmission electron microscope (TEM) at the atomic scale. The crystalline atomic-layer MoSSe was synthesized through chemical vapor deposition (CVD) system and plasma-assisted chemical vapor reaction (PACVR) process, followed by transferring the as-grown samples onto specialized TEM electrical chips. The TEM characterization of initial sample was conducted for the basic understanding of as-grown MoSSe. The advanced in situ technique was employed to record the structural evolution during biasing. Furthermore, we utilized atomic resolution scanning transmission electron microscope (STEM) images to identify the propagation of cracks and voids. It is noted that the selenization temperature is the key factor of defect concentration on the surface, which further affect the mechanical property of MoSSe.

Two-dimensional materials possess distinctive electrical and physical properties, providing great opportunities towards the next-generation semiconductor devices. Compared with unipolar transistors, ambipolar devices that allows both the electron and hole transport provide more flexibility in circuit applications. The potential of the ambipolar device can be maximized in a dual-gate structure, where one of the gates can tune the ambipolar transistor from p-type to n-type, or vise versa. The major benefit of these transistors is that due to the two independent gate inputs, the number of transistors needed in a circuit can be reduced. Previously, ambipolar dual-gate transistors based on low-dimensional materials such as graphene, carbon nanotube and transition metal dichalcogenides (TMD), and their application in logic gates, are demonstrated [1][2]. But the problem exists, for example, in the unsymmetric n- and p-branch transport, lack of ability to form logic gates that can be cascaded, inequivalent gate control and so on. Till now, a well-designed ambipolar transistor that can be used in cascadable, standard, non-pass-transistor logic circuits is never developed.
In this work we designed and fabricated high-performance ambipolar dual-gate transistors based on TMDs and demonstrated their applications in logic circuits. Through the hBN sandwiched structure, the well-engineered contacts for both electron and hole injection, and the control of threshold voltages, for the first time, the transistors have achieved simultaneously the correct logic input and output, low contact resistance, high on-off ratio, well-balanced p- and n-branch transport, and low hysteresis. We then connected the devices and formed logic gates such as XOR, NAND, etc. All gate stages are connected between a Vdd and Vss, to prevent the problems brought by other pass transistor logics. We found that the logic input and output of these gates are stable, and they can work in the same operation voltages ranges, which means these logic gates can be cascaded. The total number of transistors are reduced, comparing with the traditional CMOS technology. Then the ability of forming more complicated logic circuits is demonstrated through a full adder. This work shows the potential of 2D material based transistors, and paves the way of using 2D based ambipolar transistors in practical application of logic circuits.
References:
[1] Pan, Chen, et al. Nature Electronics 3.7 (2020): 383-390.
[2] Resta, Giovanni V, et al. ACS nano 12.7 (2018): 7039-7047.

Thin films and nanoparticles are ubiquitous components of many devices such as membranes for filtration or electrodes in electrochemical energy storage and reaction centers for catalysis. As the use of techniques based on high energy electron or x-ray probes for microscopy and spectroscopy is particularly relevant to study such materials operando, issues arise from material degradation by the probe radiation. Here, by using an x-ray and electron transparent graphene film as the sample support, we observe a remarkable reduction of the radiation damage in thin films. In such geometry, the amount of secondary electrons originating from the substrate vanishes, and thus the main source of damage disappears. Demonstration using highly focused x-ray beams evidences the capabilities of the technique, paving the way for the development of experimental setups with crucial damage control/reduction.

Graphene oxide (GO) sheets have attracted increasingly high interest due to their relative ease of synthesis and water processability for a wide range of applications. Understanding how the sheets interact with each other is a crucial and foundational piece of knowledge guiding the production, processing, and use of GO in many applications. Here, we report that dehydration, such as by vacuum drying, triggers irreversible self-crosslinking of GO sheets and drastically alters their solution processability. Dehydrated GO films maintain structural integrity in water, and they can no longer redisperse as single layers even after agitation. At the bilayer level, dehydration fixes the stacked GO sheets together and prevents them from dissociation after sonication. Spectroscopical studies support the formation of new ester bonds, suggesting a condensation-esterification reaction between GO sheets. This new insight about a fundamental property of GO sheets should have broad implications in how the material should be processed and used.

Straining nanomaterials to break their lattice symmetry is perhaps the most efficient approach toward realizing bandgap tunability. Graphene has a great potential to replace silicon in prospective semiconductor industries due to its outstanding electronic and transport properties; nonetheless, its lack of energy bandgap is a substantial limitation for practical applications. In this talk, a large-scale strain engineering technique to control the local strains in 2D materials and their heterostructures will be discussed. First, laser shock nanostraining will be introduced to generate three-dimensional (3D) nanostructures and thus induce local strains in the graphene sheet. The size dependent straining limit of the graphene and the critical breaking pressure will be discussed. Moreover, laser shock nanostraining induces modulated inhomogeneous local asymmetric elastic–plastic strains as results of 1-100 GPa-level shock loading at a high strain rate of 10⁶–10⁸ s⁻¹. Currently, due to the weak lattice deformation induced by uniaxial or in-plane shear strain, most strained graphene studies have yielded bandgaps <0.5 eV, laser shock nanostraining can induce tunable bandgaps in graphene of up to 2.1 eV. Finally, laser shock nanostraining will be discussed to controllably tune the interlayer distance between VdW heterostructures and generate strain coupling between layers. The strains in the 2D heterolayers, providing a simple and effective way to modify their optic and electronic properties.

Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS₂, WS₂, MoSe₂, and WSe₂ via proton beam irradiation. Defect-induced emissions are found to occur 100~200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high excitation rate and long lifetime. Using scanning transmission electron microscopy, we show that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and GW-BSE calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.
Besides the optically active property, the defects generated by proton irradiation also induce a robust n-type doping effect in 2D TMDs. For example, we demonstrated a reliable and long-time air stable n-type doping scheme with WSe₂ by proton irradiation. The irradiated WSe₂ remains an n-type semiconductor even after it is exposed to ambient conditions for a year. Localized ion irradiation with a focused beam can directly pattern on the sample to make high performance homogenous p-n junction diodes.
Controllable proton irradiation provides both optically active and N-type doping effects, which is also compatible with current CMOS processes, thus would be an ideal platform for future complementary high-performance electronics and optoelectronics applications.

As transistor features move towards the atomic scale, the ability to study and design for atomic scale effects become critical for next-generation microelectronics. Atomic precision advanced manufacturing (APAM) of electrical devices, fabricated using hydrogen depassivation lithography in a scanning tunneling microscope (STM), offers a way to explore atomic scale physics with the ultimate degree of control. While almost all previous APAM work has focused on exploring applications in quantum physics, particularly with a focus on qubits, we have recently demonstrated successful integration of microelectronic-focused APAM devices within Sandia’s 0.35-micron CMOS node [1]. However, demonstration of the compatibility of APAM material with CMOS at relevant operational temperatures and device drive current conditions is lacking.

To establish APAM + CMOS operational compatibility, we performed accelerated lifetime testing of both standard photolithographically patterned APAM delta layer devices (>5 μm width) as well as STM patterned APAM devices (<1 μm width). Variable width devices allow for testing at higher current densities along with connecting robustness results to a wide range of different application spaces (transistor channel, interconnect material, etc.). Analysis of the APAM devices shows that all but one device survived for several weeks at 300 °C at high current densities (minimum of 2.5 MA/cm²) while all three implanted phosphorus samples failed under the same conditions [2]. Further characterization demonstrated that the failure mechanism was a result of the metal bond-wire interconnects used to package the devices, and not the APAM devices themselves. Modeling using Sandia’s open-source TCAD code Charon validates that the current flow is tightly confined around the delta layer, resulting in an estimate of current density of >8 MA/cm², almost 10x higher than copper (~1 MA/cm² at 300 °C). Demonstration of the robustness of APAM devices relative to wire-bonds used to make device contacts coupled with the previous demonstration of APAM’s integrability [1] with a CMOS process opens the door for utilizing APAM to enhance CMOS transistors along with providing wider manufacturing interest.

This work was supported by the Laboratory Directed Research and Development Program at Sandia National Laboratories and was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government.

[1] Leenheer, A.; Halsey, C.; Ward, D.; Campbell, D.; Mincey, J. S.; Anderson, E. M.; Schmucker, S. W.; Ivie, J. A.; Scrymgeour, D.; Gao, X.; Lepkowski, W.; Misra, S. ECS Meeting Abstracts 2021, MA2021-02, (30), 918-918.

[2] Halsey, C.; Depoy, J.; Campbell, D. M.; Ward, D. R.; Anderson, E. M.; Schmucker, S. W.; Ivie, J. A.; Gao, X.; Scrymgeour, D. A.; Misra, S. IEEE Transactions on Device and Materials Reliability 2022, 22, (2), 169-174

Two-dimensional (2D) materials are poised for the atom-precise engineering of materials morphology, composition, and phase that has established them as platforms to investigate new physical, chemical, and quantum phenomena. Given the range of exciting physics and chemistries that these materials offer, realizing diverse 2D platforms is critical to push the frontiers of next generation materials and devices. Here, we introduce 2D molecular crystal (2DMC) composed of single organic molecules in an atomically flat geometry. We develop a gas phase synthetic technique to generate high quality, high fidelity molecular crystals with wafer-scale homogeneity. Room temperature scanning tunneling microscopy (STM) together with cross-polarized microscopy reveal “superatomic” unit cells within single crystalline domains as large as a few hundred microns. Optical imaging and spectroscopic characterization reveal significant optical anisotropy in terms of polarization and birefringence in the 2DMCs, which is attributed to the diverse superatomic unit cell geometries and compositions. Further optoelectronic and time-resolved photoemission studies will be discussed to substantiate the unique excitonic properties of the 2DMCs and their heterostructures. This work expands the state-of-the-art in 2D materials through the realization of atomically thin, wafer-scale molecular films for new molecule-based nanostructures and device fabrication.

Wafer-scale atomically thin films are inherently polycrystalline, and therefore criss-crossed by defect laden grain boundaries. It is well understood that a single, isolated crystalline defect creates long range deformations in a thin film, much like an electron that sources an electric field. Yet, unlike Maxwell's equations, the elasticity governing a buckled sheet is nonlinear. Thus, when multiple defects are present, superimposing their undulations will not give the sheet's final shape. This poses a challenge for understanding the shape of atomically thin films when they are not confined to a substrate. Here we perform spatially resolved measurements of the wrinkling of monolayer MoS₂ in a relatively force free environement. In doing so, we directly investigate the relationship between grain size and emergent corrugations of single layer polycrystalline MoS₂. We show that polycrystallinity, even with free boundaries, gives rise to micron scale wrinkles. Using universal scaling properties of thin sheets, we rationalize the large-scale features of the topography, including wrinkle wavelength and amplitude. Though the wrinkles we observe originate from a network of topological defects, our continuum arguments depend only on average strain and grain size, and not on atomistic details. The scaling relations we obtain suggest guidelines for tailoring grain size for specific application requirements.

Site selective deposition (SSD), in which substrate and deposition parameters are adjusted to allow growth in one region of a surface while avoiding growth in adjacent regions, is a promising strategy for circumventing the limitations of top-down fabrication processes and enabling etch free pattering of nanoscale features. Layered two-dimensional (2D) materials provide an excellent platform for SSD integration: their van der Waals surfaces are generally unreactive but exhibit strongly tuneable properties if heterogeneities are present. In particular, deliberate introduction of defects or other heterogeneous nucleation sites can control the diffusion and nucleation of materials grown on the surface. SSD of metals on 2D materials has been demonstrated at micrometer scales, but not at the length scales required in future devices. Nano- and atomic- scale guided deposition on 2D materials would meet three needs: etch-free selective deposition to form 2D/metal, 2D/dielectric, and 2D/semiconductor interfaces at specific locations; pushing the resolution limits of conventional lithography; and fabrication of structures that that require ultra-precise (<1 nm) alignment between structures, such as nanoscale interconnects, on-chip optical systems, and emerging quantum information sources.

Here, we explore the local structural properties and routes towards SSD for device-relevant 2D/3D Au/Ti interfaces using a combination of atomic resolution scanning transmission electron microscopy (STEM), in situ ultra-high vacuum (UHV) TEM, and monochromated high-energy resolution electron energy-loss spectroscopy (EELS). First, we demonstrate ultra-low nucleation density, single crystal, epitaxial Ti nanoislands using slow metal evaporation rates onto clean suspended 2D materials in UHV. We characterize the nucleation, epitaxy, island morphology and formation mechanism. We find that the island morphologies depend on the substrate compliance, such that thinner 2D substrates result in Ti islands that curve to accommodate strain, while thicker 2D substrates are less compliant and result in Ti islands with ordered arrays of dislocation networks. We then utilize these Ti islands as selective nucleation sites for Au metal deposition. We find 100% selectivity for Au nucleation under conditions where the substrate remains free of heterogeneous nucleation sites. We discuss the alignment and defect structures in Au grown on Ti and the stability of Au on the Ti surface on in situ heating. The importance of UHV conditions in the fabrication of these heterostructures is discussed. Lastly, we explore further routes towards SSD of tailored metallic nanostructures on 2D materials, such as ion beam patterning and utilizing printed nanoparticle seeds for UHV growth.

We report isotropic plasma atomic layer etching (ALE) of titanium nitride (TiN) using sequential and self-limiting oxidation and etching steps. TiN is oxidized to TiO₂ via exposure to O₂ gas which is subsequently spontaneously etched by exposure to a H₂ and SF₆ plasma. The process exploits the selectivity of spontaneous etching of TiO₂ over TiN with the plasma. A 4:1 ratio of H₂:SF₆ is shown to be highly selective, etching TiO₂ but exhibiting negligible etching of TiN over 50 cycles. TiN ALE was observed at temperatures between 200°C and 300°C, with a maximum etch rate of 0.8 Å/cycle observed at 300°C, measured using ex-situ ellipsometry. After ALE, the etched surface was characterized using X-ray photoelectron spectroscopy and atomic force microscopy. These findings have relevance for applications of TiN in microwave kinetic inductance detectors and superconducting qubits.

Moiré superlattices formed from independently twisting trilayers of graphene have been proposed as an ideal model for studying electronic correlation. Multilayered moiré systems offer several advantages over their twisted bilayer analogs, including more robust and tunable superconductivity, a wide range of twist angles associated with flat band formation, and larger scale moiré patterns. Atomic reconstruction, which strongly impacts the electronic structure of twisted graphene structures, has been suggested to play a major role in the relative versatility of super-conductivity in trilayers. Despite this, reconstruction in graphene trilayers has been only probed using indirect measurements or those only applicable to exposed samples. Herein, we exploit an inteferometric 4D-STEM approach to obtain displacement and strain maps in a representative range of twisted trilayer graphene structures. This methodology correlates local intensity modulations to stacking order and allows us to selectively probe bilayer interfaces within the material. The resulting mechanism we present informs a more complete understanding of how atomic reconstruction scales with layer number in graphene moirés and modulates symmetries crucial for establishing superconductivity.

Visible-range single photon emitters (SPEs) from monolayer hexagonal boron nitride has demonstrated exceptional optical performance as a candidate for quantum optical technology application. However, the controllability of the carbon defect engineering in hBN has remained elusive, including the unadjustable SPE density, unconfined wavelength, and low uniform yield. Thus, the integration into on-chip quantum devices has suffered. Here, by precisely controlling the carbon incorporation via diffusion from molten copper during chemical vapor deposition (CVD) growth, we demonstrate a strategy to engineer the defect density in hBN, and achieve high uniform SPEs with confined emission wavelengths in hBN. By controlling the carbon diffusion rate, we observed an increase in carbon doping level and provided evidence for achieving high-yield uniform single photon emitters in hBN down to monolayer limit. Our method enables the density of SPE creation to become adjustable, from hBN monolayer samples, with high uniformity and confined wavelength. This method stimulates a significant step forward to integrating advanced two-dimensional (2D) material engineering into on-chip quantum devices.


The authors acknowledge the support from RGC (16304421).

Carbon nanotubes (CNTs) with different chirality are great models to study electronic properties that influence ion transport through single-channel nanotubes. Based on band gap theory, CNTs in different chirality with different curvatures of tube surface may have different conductivity. Herein we report ion transport through chirality-separated pure (6,5), (7,4), and (7,5)/(8,4) CNT porins (CNTPs). Single channel conductance from chirality-pure, yet almost identical in diameter CNTPs indicates that electronic properties have a weak effect on ion transport. Furthermore, we measured ion transport through fluorescence ultrashort nanotubes (FUNs) – functionalized channels with defective walls. These channels have significantly lower conductance compared to the pure (6, 5) CNTPs. Besides, ions can successfully transport through CNTP dispersed by surfactants such as CHAPS and SDOC, the ion conductance of CNTPs is independent of the surfactant used to cut the CNTs.

The synthesis of homogeneous monolayer molybdenum disulfide (MoS₂) thin films has been an important benchmark for enabling its usage in industrial scale electronic and optoelectronic devices. Recently, molecular O₂ has emerged as a reagent for scalable production of MoS₂ films through either gradual etching of an Mo source or evolution from MoO₃ preceding sulfurization. However, oxidation of MoS₂ and its precursors is not well understood and is a frequent subject of study. Here, we invert the order of traditional MoS₂ sulfurization to demonstrate that balancing the etch rate of partially formed MoS₂ films as opposed to an unreacted Mo source is an alternative pathway towards producing homogeneous monolayer MoS₂ films. Further, this growth enables adjacent regions of either multilayer or monolayer MoS₂ to be patterned dependent on whether the respective region was deposited with a Mo metal film or was left as exposed substrate. We study growth effects at 550 °C and 650 °C as well as different O₂ flow rates of 1 sccm and 5 sccm. This study suggests that MoS₂ formation through O₂ etching traverses two separate mechanisms based on the pristine quality of the partially formed MoS₂ source film. As suggested in theoretical studies, defect-filled MoS₂ lowers the activation energy of O₂ adsorption to 0.8 eV and leads to growth of monolayer-bilayer MoS₂ flakes. Alternatively, O₂ adsorption onto pristine MoS₂ raises the activation energy to 1.59 eV and generates monolayer MoS₂ films that spread homogeneously across a 1x5 cm area. The films were characterized as MoS₂ using Raman spectroscopy. XPS analysis also confirms peaks for MoS₂ that are shifted to higher energies due to the presence of trace MoO₃. This demonstrates that the joint adsorption of oxygen and sulfur species onto Mo is advantageous to the creation of the observed homogeneous monolayer MoS₂. Beyond this, Mo-O bonds present in the film could be a significant factor in increasing its photoluminescence compared to traditional MoS₂ syntheses.

Single-metal-atom chains (SMACs), as the smallest one-dimensional structure, have intriguing physical and chemical properties. Although several SMACs have been realized so far, their controllable fabrication remains challenging due to the need to arrange single atoms in an atomically precise manner. Here we develop a chemical vapour co-deposition method to construct a wafer-scale network of platinum SMACs in atom-thin films. The obtained atomic chains possess an average length of up to about 17nm and a high density of over 10 weight percentage. Interestingly, as a consequence of the electronic delocalization of platinum atoms along the chain, this atomically coherent one-dimensional channel delivers a metallic behaviour, as revealed by electronic measurements, first-principles calculations and complex network modelling. Our strategy is potentially extendable to other transition metals such as cobalt, enriching the toolbox for manufacturing SMACs and paving the way for the fundamental study of one-dimensional systems and the development of devices comprising monoatomic chains.

Heterointerfaces between two-dimensional (2D) materials and bulk metals determine the electrical and optical properties of their heterostructures. Although deposition of various metals on 2D materials were studied, there is still lack of studies on the interaction at the van der Waals (vdW) heterointerface between 2D materials and metals. Here, we quasi-van der Waals (qvdW) epitaxial recrystallization of a gold thin film into crystallographically aligned single crystals by encapsulation annealing of gold thin film with hexagonal boron nitride (hBN). When a polycrystalline gold thin film passivated with hBN was annealed, it was recrystallized into single gold crystals with planar shape and crystallographic alignment with hBN due to a strong interaction between the gold film and hBN at the heterointerface. This reflects that a weak vdW force at the heterointerface is sufficiently strong to induce epitaxial recrystallization. Using this method, we fabricated a gold nanocrystal array with the same crystalline orientation and smooth top surface. Our work demonstrates a new method for epitaxial recrystallization of bulk crystals and provides a deep understanding of the interaction at the vdW heterointerface of 2D materials and metals.

Recently, transition-metal dichalcogenides (TMDs) have attracted much attention as new materials for electronics devices, electrocatalysts, photocatalysts, sensors, batteries, and bio-applications. Most TMDs are two-dimensional (2D) materials with a single layer. Bonds between each layer are made up of Van der Waals bonds, while intra-layer atoms bind together as covalent bonds.
Chemical vapor deposition (CVD) with sulfur gas is the most popular method for synthesizing large- scale 2D materials with high quality. Various MoS₂ can be obtained from this method using various precursors with different properties, process temperatures, and substrate materials. Solution process methods show advantages for preparing films with large size, high throughput, low cost, thickness control, and an environmentally friendly process. Even though there is sulfur in the precursors of the solution-process synthesis methods, supplementing the sulfur that is lost in the high-temperature CVD process is unavoidable.
Electrohydrodynamic (EHD) jet printing is a technique that uses electric fields to yield fluid flows for delivering solution/paste materials to a target substrate. EHD printing can create smooth areas or patterns with a large range of material viscosity, even with low-viscosity solutions or high-viscosity pastes, which are better by far than inkjet printing for this concern. Moreover, the merit of EHD jet printing is that MoS₂ TFTs can be patterned simply without using any shadow masks compared to other methods such as E-beam or thermal evaporation.
Directly printed transistors have been in the limelight due to low cost and an environmentally friendly technique. An electrohydrodynamic (EHD) jet printing technique was employed to pattern both MoS₂ active layer and Ag source/drain electrodes. Printed MoS₂ lines were patterned on a silicon wafer using a precursor solution and simple annealing, and the patterns were transferred on other SiO₂ substrates for TFT fabrication. On top of the patterned MoS₂, Ag paste was also patterned for source and drain electrodes using EHD jet printing. The printed TFTs had a high on-off current ratio exceeding 10⁵, low subthreshold slope, and better hysteresis behavior after transferring MoS₂ patterns. This result could be important for practical TFT applications and could be extended to other 2D materials.

Exhibiting excellent electrical, mechanical, and thermal properties, complimented by its unique atomic-scale thickness, graphene is often composited with a plethora of materials for almost every application to achieve improved performances. However, the reliability and cost-effectiveness of existing methods to fabricate and prepare large-area graphene-based devices are still challenges that often hinder their use for industrial applications. As a derivative of graphene, graphene oxide (GO) inherits the structure but with added oxygen functional groups. In our recent and on-going work, we used a novel and straightforward approach to deposit GO flakes suspended in air. To achieve this, we performed electrochemical reduction of GO to produce reduced GO (RGO) membranes deposited and suspended on gold mesh. Being suspended in air enables the possibility to probe the RGO sheets in the absence of an underlying substrate. When observed under scanning electron microscopy (SEM), several RGO membranes exhibited patterns formed by distinct differences in contrast, which we ascribe to variations in oxidation across the surface of the RGO membrane, as a result of an intermediate process in the electrochemical reduction. This phenomenon of sub-microscale lateral variation led us to employ Kelvin probe force microscopy (KPFM) in order to investigate the surface behavior. The patterns obtained in SEM were also observed in the surface potential (analogous to work function) maps taken from KPFM, with values ranging from 5.55 to 5.70 eV. This spatial distribution of work function supports the interpretation of the SEM patterns being related to the variations in oxidation level across the surface of the RGO membrane. When measured under varying humidity conditions, we found that the work function decreased from a mean average of 5.65 eV at 5% RH down to 5.38 eV at 85% RH. Interestingly, when we probed the chemical composition of an RGO membrane using X-ray absorption spectroscopy (XAS) under controlled humidity conditions, we found that the increased RH is accompanied by an increase in C-O-C signal. The prevailing hypothesis for adsorption of water in GO is that it is a physical process, where water molecules traverse through the GO laminates, with the process being dependent largely on the GO interlayer distance. Our XAS results show that the accepted mechanism of water adsorption in GO-based devices warrants a much deeper investigation. These findings will be useful in understanding and tuning the performance of GO-based humidity sensors and materials for dehumidification.

Ideal van der Waals (vdW) metal contact with weak interaction can achieve high performance of the two-dimensional (2D) electrical and optical devices and stable interface states since it is possible to prevent Fermi level pinning (FLP) and interface defects occurring between 2D semiconductors and metals. Recently, there have been many reports to demonstrate ideal vdW metal contact by avoiding damage from metal deposition such as representative method of transferred metal contact. However, the transferred contact method through handwork technology is vulnerable from forming uniform and stable vdW interface. To overcome this limitation, we choose the vdW contact method using a selenium (Se) buffer layer that consisting of conventional fabrication processes. In this study, in order to explore the ideal vdW metal contact, we compare vdW metal contact, transferred metal contact and conventional metal contact (direct metal contact) in Schottky diode structure with graphite as 2D vdW bottom contact that can analyze difference of Schottky barrier height through rectification and photovoltaic effect. As a result, we derived that Se buffer layer method is the most suitable way to form stable and ideal vdW contact between 2D semiconductors and metal, which can prevent FLP. Using vdW Au and graphite contact, we fabricate tungsten diselenide (WSe₂) Schottky diode that exhibit excellent operation with ideality factor about 1, on/off ratio over 10⁷ and coherent properties.

Seebeck coefficient serves as an important indicator for understanding the thermoelectric (TE) properties. In order to derive thermoelectric properties, it is inevitable to measure the exact Seebeck coefficient. However, in certain cases, it is very difficult to evaluate Seebeck coefficient of a 2D film with a large area when the electrical resistance is extremely large. Here, we report a simple way to measure the in-plane Seebeck coefficient of the 2D transition metal dichalcogenide (TMDC) film, whose electrical resistance is over 2 MW, through two-step thermal annealing process. By the two-step annealing at 573 K in PtSe₂ film on sapphire substrate, it is possible to obtain the effect in two ways, first, between the 2D TMDC layers, and second, interface of metal and film. As a result, we observe the electrical resistance is lowered to ~400 kW and the Seebeck coefficient is also measurable, and its value is exceeding ~160 mV/K, which is a 400% improvement compared to the single-crystalline bulk of PtSe₂. In addition, it is confirmed that the Seebeck coefficient of the annealed samples are independent of the top electrode materials. We discuss the necessity of annealing process in measuring Seebeck coefficient of having high resistance 2D PtSe₂ films by atomic crystallographic characteristics and contact resistance between metal and PtSe₂ film. Our finding is an important accomplishment in measuring and understanding the Seebeck effect and provide a promising guideline with a high TE property in 2D TMDC material.

Tungsten diselenide (WSe₂) is one of the transition metal dichalcogenide materials which shows ambi-polar semiconducting property. WSe₂ is a material of which semiconducting polarity can be easily convertible n- to p-type by surface oxidation using oxygen-based treatment such as O₂ plasma and UV-ozone (UVO) due to the formation of tungsten oxide (WO_x). Various studies have been conducted on p-doping by using both the plasma and UVO treatments onto top layer of WSe₂. Among them, UVO is found to be the more effective and facile technique to form WO_x which induces p-doping in WSe₂. Furthermore, WSe₂ shows a self-limited oxidation property in that only one layer of WSe₂ can be oxidized without affecting on the underlying layer. Here, we propose a new double-side doping technique to form top and bottom WO_x, using a polymeric material subjected to the UVO treatment. The proposed method has a great potential to contribute to the development of doping technology of two-dimensional materials.

Acknowledgement
This work was supported by the Global Research Laboratory (GRL) Program (2016K1A1A2912707) and the Basic Science Research Program (2021R1A2C2010869), funded by the National Research Foundation of Korea (NRF).

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) -based homo- and hetero-structures result in unusual route to drastically increase the Seebeck coefficient through the interface-induced Seebeck effect. Such intriguing properties have led to us to study the in-plane Seebeck coefficient and electrical conductivity of thickness-controlled a relatively new class of metallic 2D TMDCs, 2D platinum ditelluride (PtTe₂) thin films. We followed a series of processes, including a simple tellurization of the Pt thin films with ~5, ~10, ~20, and ~30 nm-thick and a wet chemical transfer on sapphire substrates, to measure in-plane thermoelectric properties of the 2D PtTe₂ thin films. We observe the higher in-plane Seebeck coefficient and power factor of ≥ 3.0 μV/K and ≥ 16.0 μW/mK² in the thicker 2D PtTe₂ (≥ 20 nm) films, which is related to the more semimetallic transition as increasing PtTe₂ film thickness. These in-plane TE properties of the thickness-modulated 2D PtTe₂ thin films that are anticipated to offer an alternative approach for fabricating high-efficiency TE devices comprising vertically stacked 2D/2D hetero-structures (i.e., PtTe₂/MoS₂ and PtTe₂/PtSe₂)

Red phosphorus (RP) is one of the allotropes of phosphorus and has many structural phases. Type-I RP is amorphous, and other types (type II, III, IV, and V) are known to be crystalline phases. The structures of Type-IV (fibrous RP) and Type-V (Hittorf’s phosphorus) have been identified through previous experiments, but the structures of Type-II and Type-III have yet to be reported. Here we unravel the crystal structure of type-II RP by complementary structure characterization techniques via powder X-ray diffraction, 3D electron diffraction, and atomic-resolution scanning transmission electron microscopy (STEM) imaging. The type-II RP crystals were synthesized by the chemical vapor transport method and identified through powder XRD and Raman spectroscopy in comparison with previous literature. From our study, we successfully identify that type-II RP has approximately 250 phosphorus atoms in a triclinic unit cell. Moreover, STEM imaging clearly confirms the helical tubular structure is the building block of type-II RP. The electrical properties of type II RP toward optoelectric device applications are also experimentally verified.

The new class of two-dimensional (2D) material, “MXene,” has displayed state of the art of performance in energy storage, membranes, electromagnetic shielding, and chemical sensors. To further improve the performance of MXenes for diverse applications, there is a need to develop a method for the alteration of surface properties of MXene. Theoretical studies suggested that manipulation of the MXene surface termination (such as =O, -Cl, -F, -OH) strongly alter their functional properties; however, experimental control of the MXene surface termination is still in the developmental stage. Here, we demonstrated that the terminal groups of salt-etched Ti₃C₂T_z MXene surface can be removed by annealing under an inert atmosphere. The removal of terminal groups of MXene was confirmed by XRD, EDS, and XPS measurements. The free active sites available after the annealing can be re-functionalized by a varied species. In this work, the annealed Ti₃C₂T_z were re-functionalized by -OH groups and (3-Aminopropyl) triethoxysilane (APTES), which FTIR confirmed. The surface-modified Ti₃C₂T_z were utilized as a solid lubricant showing about a 50 % reduction in friction. This paper discusses an effective approach to alter the surface functionalization of MXene, thus providing a way to utilize surface-dependent properties.

Due to the high density of interfacial defect-induced trap states within the bandgap of two-dimensional (2D) materials, 2D material-based semiconductor devices exhibit inferior electrical performance compared to theoretical predictions. The interface traps existing in the 2D material-based devices affect their electrical properties in various forms: e.g. hysteresis in I-V characteristics, subthreshold swing, Schottky barrier heights, Fermi-level pinning, and device polarity. It is found that the interface traps, leading to the generation of the defects states inside band-gap of the semiconductor, are more susceptible to the ultrathin 2D semiconductors. Here, we perform multi-frequency capacitance and conductance measurements to study the modulation of density of interface trap () as a function of gate voltage by varying the thickness of Molybdenum disulfide (MoS₂) placed on ultra-clean hexagonal boron nitride (hBN) gate insulator in a metal-insulator-semiconductor (MIS) structure. In this study, the typical MIS capacitors are prepared to generate different capacitance values depending on the gate voltage, where D_it are extracted from the difference in capacitance values at high and low frequencies. C-V measurements revealed that decreases with increasing the thickness of WSe₂. It is found that the density of in our study is lower than of SiO₂/Si.

This work was supported by the Global Research Laboratory (GRL) Program (2016K1A1A2912707) and the Basic Science Research Program (2021R1A2C2010869), It was also supported by Ministry of Trade, Industry and Energy (20011582).

2D-Transition Metal oxides (TMOs), although less studied than the well-known 2D transition metal dichalcogenides(2D-TMCs) are appealing due to some specific advantages. Like the TMCs they are semiconductors, however they are less contaminant and offer a large potential for tuning their electro-optical properties by varying their stoichiometry. In particular, MoO₃ is transparent and shows a wide bandgap (>3 eV) and a high dielectric constant k ~ 500. Additionally, orthorhombic α-MoO₃ possesses the well-known layered crystal structure of MoO₃ which offers the possibility to create two dimensional (2D) morphologies.[1] In this context, 2D MoO₃ has shown extraordinary properties such as anisotropic polariton propagation,[2] and is an ideal material for electronic applications for high power electronics and short wavelength optoelectronics.

Currently most of the work in 2D-semiconductors is still made using small flakes obtained from mechanically exfoliated single crystals, however there is no doubt that it will be advantageous to deposit large 2D-strucutres as this will have tremendous impact both for the manipulation an integration of these materials.[3] In context we show the successful preparation of nanometer thick thin films formed by 2D MoO₃ crystals by a pulsed laser deposition-based process that starts with the deposition in vacuum from a MoO₃ target of dense, amorphous and continuous substoichimetric MoO_3-X layers. Subsequently, the films are annealed in air up to 300 C while following the evolution of their optical properties in-situ in the UV-VIS wavelength region by spectroscopic ellipsometry. When the temperature reaches 250 C a clear change in the optical properties starts that is related to the films crystallization. Analysis of the optical properties shows how the initially absorbing films with a metallic-like behavior after the annealing become transparent in the NIR and VIS regions, and shows a band gap >3 eV. This optical change is related to the formation of large rectangular micron size α-MoO₃ single crystals with a thickness of the order 10 nm easily observable by optical microscopy. A full characterization of the morphology, structure and stoichiometry has been confirmed by X-ray diffraction analysis, Raman spectroscopy, AFM and transmission electron microscopy. We will show the distinct fetures of the measured dielectric constant of the 2D MoO₃ microcrystals and compare them to that of the bulk in terms of defects and lattice stress. Finally, we analyze the implications for different optoelectronic applications.

[1] I. A. de Castro, et al., . Molybdenum Oxides – From Fundamentals to Functionality. Adv. Mater. 29, 1-31 (2017).
[2] W. Ma, et al In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal. Nature. 562, 557–562 (2018).
[3] J-H Kim, et al. Van der Waals epitaxial growth of single crystal α-MoO3 layers on layered materials growth templates. 2D Mater. 6 (2019).

Two-dimensional (2D) transition metal dichalcogenides (TMDc) is rapidly emerging as a material for thermoelectric (TE) devices. The energy conversion efficiency of the thermoelectric materials is figure of merit, ZT=PF×T/κ (where PF=S²×σ, PF is power factor, T is absolute temperature, κ is thermal conductivity, S is Seebeck coefficient, σ is electrical conductivity). In order to achieve a high ZT, it is important to obtain a high PF, and a low κ. Except for T and κ, it is important to improve PF=S²×σ. Unfortunately, Seebeck coefficient and electrical conductivity are through trade-off relations. In this report, we develop a PtTe₂/MoS₂ hetero-junction structure to break this trade-off relationship. The 2D PtTe₂ and MoS₂ films are synthesized by using low-pressure chemical vapor deposition (LP-CVD) method. The 2D PtTe₂ film exhibits a semi-metallic property with low resistance (≤ 2 kΩ), whereas the MoS₂ is considered as an insulator with high resistance (≥ 10 MΩ). We prepared five sets of samples, including PtTe₂ (5-nm), PtTe₂ (5-nm)/MoS₂ (7-nm), PtTe₂ (5-nm)/MoS₂ (7-nm)/MoS₂ (7-nm), PtTe₂ (5-nm)/MoS₂ (21-nm), and PtTe₂ (5-nm)/MoS₂ (21-nm)/MoS₂ (21-nm). As a result, we observe that the in-plane Seebeck coefficient for PtTe₂ (5-nm)/MoS₂ (7-nm)/MoS₂ (7-nm) hetero-junction structure is -19 μV/K at 300K. It indicates an increase of ~200% compared to that of the PtTe₂ (5-nm) thin film. Also, the electrical conductivity (5.3 kS×cm) is improved by ~170%. Consequently, the power factor (190 μW/K²m) is improved by ~720%. These findings represent that the 2D TMDc hetero-junction structures have good advantages in improving the power factor in future TE applications.

Two-dimensional transition metal carbides/carbonitrides/nitrides also known as MXenes are promising candidates for many applications due to their high electrical conductivity, mechanical flexibility, surface tunability, and easy processability. Carbide and carbonitride MXenes have been explored for multiple applications, however, a few reports are available on nitride MXenes due to higher formation energy and poor stability against etchant, typically HF. In this work, we optimize synthesis conditions of Ti₄N₃T_x MXene by using molten salt etching method in air environment. Successful synthesis of Ti₄N₃T_x MXene was verified by XRD, XPS, SEM, TEM, and AFM techniques. Moreover, freestanding Ti₄N₃T_x films were obtained by vacuum-assisted filtration which exhibit 99.99% shielding efficiency at 150 µm thickness. Electrical and electronic properties of Ti₄N₃T_x can be tailored to expand areas of applications of nitride MXenes.

Cerium oxide nanoparticles (CNPs) are a well-known functional material with broad applications in catalysis, gas sensor, and nanomedicine due to their unique shape-dependent properties. In recent years, solutions-based, sol-gel, and template-assisted synthesis routes have been explored for CNPs synthesis. Among these methods, hydrothermal synthesis has been considered one of the most effective and scalable routes thanks to its merit of single-step, one-pot synthesis. Regarding hydrothermal synthesis, cerium (III) nitrate/chloride/sulfate, sodium hydroxide, and ammonium hydroxide are the most commonly used precursor and oxidizers in CeO₂ hydrothermal synthesis. However, the application of the nanoparticles synthesized from a harsh high-pH alkaline synthetic environment usually suffers from vigorous nanoparticle aggregation, which could severely affect their performance.
In this study, we report a facile hydrothermal synthesis method to fabricate cerium oxide nanoparticles with different shapes. The ceric ammonium nitrate (CAN) was used as a cerium precursor to synthesize a highly dispersed CNPs in a low-pH acidic environment to obtain a cubic fluorite structure with narrow size distribution. The octahedron shape of CNP was synthesized by introducing only CAN into the aqueous precursor. The strong shape transformation of CNPs from octahedron to truncated octahedron to cubical is achieved by controlling the introduction of acetic ion which is done by varying the concentration between sodium acetate and acetate acid in precursor. CNP synthesized in pH >10 environments was considered a control in this study to compare the physiochemical properties difference between CNPs synthesized in acidic or alkaline precursor environments. One of the key properties of CNPs is its cyclic change of redox potential Ce³⁺/ Ce⁴⁺ on its surface. Hence the variation in the morphology of CNPs changed its redox potential, which was measured and analyzed using X-Ray photoelectron spectroscopy. CNP's change in size and shape affects its superoxide dismutase activity (SOD). The result, which showed better SOD activity, will be studied for their application in biomedical therapy.

Van der Waals (vdW) heterostructures of transition metal dichalcogenides (TMDs) have attracted great interests because a variety of combinations in vdW heterostructures provide a platform to explore novel physics and fabricate electronic devices with superior performance. In this regard, effective methods for large-scale growth and patterning of vdW heterostructure are required. Here, we report a patterned growth method of vertical MoS₂/WS₂ heterostructure by controlling nucleation sites by strain. We generated a large strain in the local region of growth substrate, WS₂, by transferring it on a patterned SiO₂ substrate with etched steps. When MoS₂ was grown by chemical vapor deposition (CVD) on the WS₂, the strain-concentrated regions of WS₂ serve as favorable nucleation sites for MoS₂. Therefore, the vertical vdW heterostructure of MoS₂/WS₂ was successfully grown at desired positions. Our work shows a novel way to control nucleation of 2D materials by strain and to selectively produce the vertical vdW heterostructure at the patterned areas.

Graphene is a nanostructure of great interest due to its extraordinary properties such as high surface area, high thermal conductivity, high mobility of charge carriers and excellent mechanical properties. However, the chemical stability of Graphene causes it to be a very inert material in chemical interactions with its environment and therefore of little catalytic activity. However, Nitrogen-doped Graphene has emerged as a great alternative to metal catalysts due to its catalytic properties. The charge distribution of the Carbon atoms is influenced by the neighboring-Nitrogen atoms doping the material, which induces active regions on the Nitrogen-doped Graphene (N-rGO) surface. These regions or active sites, together with its property as a good electrical conductor, allow it to participate in electrocatalytic reactions of great interest such as: the oxygen reduction reaction (ORR) [1], the iodide reduction reaction (IRR) [2] or charge storage; which are essential for fuel cells, dye-sensitized solar cells (DSSC), supercapacitors or Metal-Air batteries.
These types of applications require considerable amounts of material, so it is vital to implement and understand the mechanisms involved in scalable treatments to achieve N-doping. In this sense, thermal treatments as a post-synthesis method correspond to a simple and easily scalable technique. Furthermore, it is well known that Nitrogen atoms can be incorporated into the sp² Carbon lattice in different configurations: pyridine nitrogen, graphitic nitrogen, pyrrolic nitrogen or even as oxidized nitrogen species [1-2]. However, each of these configurations or Nitrogen species may have different roles in the reactions of interest [1]; therefore, being able to modify or modulate the proportion between the Nitrogen species present in the N-rGO would be very useful.
The present work analyzes in detail the doping of Graphene with Nitrogen through post-synthesis thermal treatments in an inert atmosphere, using Graphene Oxide (GO) as a precursor, seeking to understand at atomic level and propose a reaction mechanism. Special emphasis has been placed on the role of oxygen species present in GO as precursor and its importance in the incorporation of Nitrogen atoms into the N-rGO network. This analysis has been carried out using different sets of samples with a systematic monitoring of their physico-chemical and structural properties using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), optical microscopy and atomic force microscopy (AFM). Even more, it is shown how the temperature in a second thermal treatment on the N-rGO obtained, plays a key role to modify or modulate the proportion of the different Nitrogen species present in the final N-rGO.

[1] Fernandez-Escamilla H.N. et al. Advanced Energy Materials 11 (3): 2002459 (2021).
[2] Ruiz-Marizcal J.M. et al. Carbon 167: 209-218 (2020).

Acknowledgements: The authors thank CONACyT for financial support through project A1-S-17539 and UNAM for DGAPA-PAPIIT project IN111223 and DGTIC-UNAM projects LANCAD-UNAM-DGTIC-382. J.M.R.-M. thanks CONACyT for his Ph.D. scholarship. The authors thank Francisco Ruiz, Jaime Mendoza, Eduardo Murillo, Israel Gradilla, Eloisa Aparicio and Jesus A. Diaz for technical assistance.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), such as molybdenum tellurides (MoTe_x), exhibit optoelectronic and physicochemical properties strongly dependent on the material crystalline structure and polymorphic phase. Although the phase-dependent properties of 2D MoTe_x are conducive for additional degrees of freedom of control at the device level, synthesis of large-area, phase-pure crystals is challenging due to the small ground-state energy differences between the different polymorphs. While established methods in synthesizing and/or isolating 2D MoTe_x have enabled fundamental studies of their unique properties, they are limited in their control over crystallinity, stoichiometry, and manufacturing scalability. This work focuses on the deterministic synthesis of 2D MoTe_x on epitaxial, van der Waals epitaxial, and non-epitaxial substrates via ultra-high vacuum (UHV) molecular beam “epitaxy” (MBE) and combines in situ/in operando and ex situ characterization techniques.

Research into two-dimensional van der Waals (vdW) materials continues to flourish due to the large selection of crystals available. In stacks of vdW materials, the interlayer interactions prove to be a crucial variable in determining the electronic properties of a given structure. It has been shown that using hydrostatic pressure, the interlayer coupling can be finely tuned in magic angle twisted bilayer graphene (MATBG) to induce superconductivity, which is strong evidence of an induced electronic flat band, effectively simulating changing the twist angle in the heterostructure after it has been fabricated. Magic angle twisted trilayer graphene (MATTG) could prove to be an interesting platform to further study the effects of tuning interlayer coupling via hydrostatic pressure, owing to the fact that it has been shown to have a similar flat band to MATBG. Using MATTG devices inside a pressure cell loaded with hydrostatic pressure on the order of a few GPa, superconductivity could potentially be induced in an MATTG device which otherwise did not exhibit correlated phases owing to a relative twist angle that deviates too far from MATTG’s magic angle of ~1.6°. Such a result could further confirm both the utility of hydrostatic pressure as a method of changing interlayer coupling, and TTG as a tunable heterostructure.

The MXene family of two-dimensional transition metal carbides and nitrides already includes ~50 members with distinct numbers of atomic layers, stoichiometric compositions and solid solutions, in-plane or out-of-plane ordering of atoms, and a variety of surface terminations. MXenes have shown properties that make them attractive for applications ranging from energy storage to electronics and medicine. Although this compositional variability allows fine-tuning of the MXene properties, it also creates challenges during the analysis of MXenes because of the presence of multiple light elements (for example, H, C, N, O, and F) in close proximity. Here, we show depth profiling of single particles of MXenes and their parent MAX phases with atomic resolution using ultralow-energy secondary-ion mass spectrometry. We directly detect oxygen in the carbon sublattice, thereby demonstrating the existence of oxycarbide MXenes. We also determine the composition of adjacent surface termination layers and show their interaction with each other. Analysis of the metal sublattice shows that Mo₂TiAlC₂ MAX exhibits perfect out-of-plane ordering, whereas Cr₂TiAlC₂ MAX exhibits some intermixing between Cr and Ti in the inner transition metal layer. Our results showcase the capabilities of the developed secondary-ion mass spectrometry technique to probe the composition of layered and two-dimensional materials with monoatomic-layer precision.

Two-dimensional (2D) materials have attracted significant interests due to the large surface-to-volume ratio, electron confinement, and various surface-enhanced applications. Although significant advances have been made in the synthesis of 2D nanomaterials, an understanding of the formation mechanisms is crucial for the future success in design, synthesis, and applications of novel 2D materials. We use liquid cell Transmission Electron Microscopy (TEM) to study model 2D systems at the atomic resolution. Our direct observations allow to uncover the nucleation, formation of intermediates, and dynamic structural evolution leading to 2D nanostructures. Assisted with theoretical calculation, the underlying mechanisms have been revealed. In this talk, progress made in our recent in situ studies of 2D nanomaterials formation at the atomic level will be presented, including the growth of 2D nanosheet with nanoclusters as the building blocks, and the dynamic formation of 2D molecular crystals.

New quantum materials and architectures are synthesized via precise multi-stage chemical synthesis strategies. The discovery of optimal synthesis routes for predictive synthesis is the primary bottleneck in realizing such structures. Several iterations of self-driving microfluidic laboratories have been demonstrated for high-throughput screening of schedules for liquid-phase molecular and nanoparticle synthesis. However, there is no comparable scheme for exploring synthesis of condensed phases. As a result, synthesis strategies for promising materials are currently identified by time-consuming trial and error and there are no known predictive schemes for condensed phases and architectures.

In this talk, I will use offline reinforcement learning (RL) to predict optimal synthesis strategies for manufacturing 2D quantum materials using chemical vapor deposition. The RL agent, trained on molecular-dynamics-based computational synthesis, learns important fabrication parameters including threshold temperatures and chemical potentials for onset of chemical reactions and predicts previously unknown synthesis schedules for 2D heterostructures. The RL scheme can be extended to predict long-time behavior of reacting systems, far beyond the domain of molecular dynamics simulations, making these predictions directly relevant to experimental synthesis.

Two-dimensional van der Waals materials are promising material platform for quantum information technology because of the ability to control their electronic structures through isolation of atomic layer & interface engineering forming heterostructures. Such materials with antiferromagnetic characteristics are interesting for spintronic applications. Nanostructural counterparts of these van der Waals antiferromagnets could have different magneto-optical properties compared to the bulk since one can expect the influence of finite size effects and surface effects. Here we show versatile fabrication and synthesis of magnetic nanostructures of metal thiophosphates using both top-down and bottom-up approaches. These include electron beam lithography, focused ion beam, solution phase chemical synthesis and liquid phase exfoliation. We will discuss structural, magnetic and optical characterization of the magnetic nanostructures.

Atomically-thin hexagonal boron nitride (hBN) has gained attentions as a material platform for broad applications in nano-photonics and electronics, including near-infrared waveguide, deep-ultraviolet emitter and dielectric film. Recent studies show that the dielectric and electronic properties of multilayers (ML) can be tuned with control of the interlayer rotational angle, θ. While layer by-layer assembly of singly crystalline hBN can enable fabrications of uniform, polytypical structures, large-scale films for atomically clean assembly is still missing. Here, we report the epitaxial growth of single-layer (SL) hBN fims on wafer-scale, which can be exfoliated and assembled by van der Waals interactions. Chemical vapor deposition on a vicinal (110) surface of germanium with miscut toward the <001> direction results in SL films with a single crystallographic orientation. Density functional theory calculations show that it is energetically favorable for the nitrogen-terminated zig-zag edges of hBN to be aligned to the step edges along the <110>, guiding the unidirectional growth. The SL hBN can be repeatedly grown on a same substrate after mechanical exfoliations, and assembled into twisted MLs with controlled θ. Optical absorption spectra show a clear θ-dependency, suggesting effective modulations of the band structures of hBN. Furthermore, the ML films can be integrated with a semiconducting MoS2 channel with tunable dielectric properties by θ.

Two-dimensional (2D) materials, which possess atomic or nanometer thickness and infinite planner dimensions, thrive on the rich variety of features and properties that are vastly distinctive from their bulk counterparts. Conventionally, bulk synthesis of 2D materials has predominantly been through selective etching of layered solids. Herein and for the first time, we convert – near ambient conditions – a dozen of cheap, earth-abundant, water-insoulble precursors into one-dimensional (1D) nanofilaments than self-assemble into 2D flakes. In that, the precursor is simply heated in tetramethylammonium hydroxide aqueous solution at 50 -80 ° C for tens of hours. The structure, composition, oxidation state, and morphology of the resulting sheets are resolved by density functional theory, X-ray diffraction, X-ray photoelectron, electron energy loss, Raman, X-ray absorption near edge structure spectroscopies, atomic force microscope, scanning, transmission and high-resolution transmission electron microscopies and selected area diffraction. The resulting flakes showed enhanced electrochemical performance as cathodes in both Li-ion battery and Li-S battery. They also found to be biocompatible to human cells but fatal to come types of cancer cells, thus showing potential in biomedical applications. The developed synthesis protocol of 1D and 2D materials in bulk-scale at near ambient conditions is paradigm shifting and will undoubtedly open new and exciting avenues of research and applications.

The family of two-dimensional (2D) transition metal carbides and nitrides (MXenes) has grown to encompass numerous structures and compositions. MXenes have shown properties that make them attractive for applications ranging from energy storage to electronics and medicine, including high electrical conductivity, redox-active surfaces, and plasmonic behavior. MXenes are typically produced through topochemical etching of atomically thick layers from precursor layered MAX phases using corrosive aqueous etchants. However, the harsh synthesis conditions lead to various defects in the MXene lattice that make them susceptible to degradation and limit their practical storage, distribution, and application. Herein, we investigate the chemistry of the precursor MAX phases and demonstrate the existence of oxycarbides; and explore the influence of MAX stoichiometry on the chemical and thermal stability of the resulting MXenes. We also probe the mechanism and kinetics of the MAX etching reaction for MXene synthesis using in situ analytical and microscopic techniques and demonstrate the influence of critical etching parameters on the structure, chemistry, and properties of the resulting MXenes. We further develop kinetic models that allow prediction of the optimal synthesis conditions, and which make the synthesis more scalable, environmentally friendly, and efficient.

Aluminum is the third-most abundant element in Earth’s crust. As a result, aluminum hydroxide polymorphs such as gibbsite and bayerite are abundant minerals in soils and dominate aluminum ores. They have also been widely used as adsorbents, fire retardants, coatings, catalysts, and luminescence powders, as well as comprising important precursors to various alumina products. Understanding the crystallization pathways of the aluminum hydroxide polymorphs is thus important to geochemistry, environmental science, energy storage, catalysis, biomedicine, industrial processing, and even nuclear waste treatment. Here we use in situ magic angle spinning nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM), scanning/transmission electron microscopy (S/TEM), atomic force microscopy (AFM), small angle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS), electrospray ionization (ESI)-mass spectrometry, X-ray diffraction (XRD), and X-ray pair distribution function (PDF) techniques to probe the nucleation and crystal growth mechanisms of gibbsite nanoplates in detail. By focusing on understanding the role of aluminum coordination change dynamics from tetrahedral in solution to octahedral in solids and vice versa, and by quantifying intermediate polyoxoaluminate cluster formation, some unifying principles governing these transformations emerge. Furthermore, various advanced techniques reveal the transformation and assembly of aluminum-oxo 2D clusters during the hydrothermal process, which indicate cluster assembly pathways to gibbsite nucleation and crystal growth. These findings are important for developing new methods for morphology and size-controlled synthesis of 2D aluminum hydroxide materials and may aid in the design of novel metal oxide 2D materials.

Recently the negatively charged boron vacancies ( V_B^-) in hexagonal boron nitride (hBN) have been shown as spin defects that have great potential in quantum sensing. However, so far the sensitivity is limited by either photoluminescence (PL) brightness or the optically detected magnetic resonance (ODMR) contrast, and linewidth. In this work, we demonstrate the generation of these spin defects using high energy helium ion beams and perform ODMR measurements with different laser and microwave powers. The spin defects generated by high energy helium ions exhibit a high PL brightness and ODMR contrast while keeping a small linewidth, hence a good sensitivity. By comparing different fluences of helium irradiations, we determine an optimal fluence which is sufficient in creating spin defects without damaging the overall crystal lattice structure. With this optimal fluence, we can obtain a high signal-to-noise ratio ODMR spectrum with an accurate measurement of zero field splitting frequency, and a best sensitivity as ~2.55μT/sqrt (Hz). Moreover, with a focused beam, we can deterministically create such spin defects with nanometer precision.
Hexagonal boron nitride (hBN) has been a centre of interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability as well as the lack of atomic defect structure information. In this work, we report two new blue quantum emitters with zero phonon line (ZPL) centred around 460 nm and 490 nm in hBN powders. We further demonstrate that the new emissions can be created by high temperature annealing or high energy ion irradiation in exfoliated hBN flakes. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion irradiated sample are vacancy type and adatom (intercalant) type. Through first principle GW-BSE (Bethe-Salpeter equation) calculation, we attribute the observed emissions at 460nm and 490nm to originate from boron intercalant and nitrogen intercalant respectively. Our results not only discover a new group of blue quantum emissions in hBN, but also provide an insight on the physical origin of the emissions by correlating the emission wavelength with local atomic structures in hBN.

A novel strategy to synthesize molybdenum blue (MB) nanorings by irradiating MoO₃ suspended in water/ethanol mixtures with intense femtosecond laser pulses is demonstrated. It is found that the MoO₃ can be dissolved in the water during laser irradiation to form molybdic acid, which provides an acidic environment for the formation of MB. Concentrations of ethanol as low as 1% result in the formation of MB and by adjusting the concentration of ethanol in the solvent, the absorption band can be tuned due to the modification of Mo^V-O-Mo^VI entities. The MB synthesized in 30% ethanol appears the darkest blue and assembly of vesicles 115 nm in size is observed. At high concentrations of ethanol (>70%), H_XMoO₃ and MoO_3-X are preferentially formed instead of MB. The photothermal conversion efficiency of the MBs synthesized in 1% and 30% ethanol is above 40%.

Cross-correlation studies using nano angle-resolved photoelectron spectroscopy (nARPES) coupled with scanning tunneling microscopy and scanning tunneling spectroscopy (STM/STS) measure a band gap renormalization on purposefully induced defects in monolayer tungsten disulfide (WS₂). Mirror twin boundary (MTB) defects are identified with non-contact atomic force microscopy, which enables structural comparison to local studies performed with nARPES and STM. Both nARPES and STS studies a formation of Tomonaga–Luttinger Liquids within mirror twin boundary (MTB) defects, which are created through the growth of one-dimensional (1D) MTBs from point defects tht are induced into WS₂ with Ar⁺ bombardment. Chalcogen defect creation also provides a path to study novel substitutional defects. 1D MTBs are substantially charged at the nanoscale.

Modulation of thicknesses and atomic structures can broadly program the physical properties of crystalline films. The layer-by-layer assembly of atomically thin crystals provides a powerful means to arbitrarily design films at the atomic level, which are unattainable with existing growth technologies. However, atomically clean assembly of the materials with high scalability and reproducibility remains challenging. Here, we report programmed crystal assembly of graphene and monolayer hexagonal boron nitride, assisted by van der Waals interactions, to form wafer-scale films of pristine interfaces with near-unity yield. The atomic configurations of the films are tailored with layer-resolved compositions and in-plane crystalline orientations. We demonstrate batch-fabricated tunnel device arrays with modulation of the resistance over orders of magnitude by thickness control of the hexagonal boron nitride barrier with single-atom thick precision and large-scale, twisted multilayer graphene with programmable electronic band structures and crystal symmetries. Our results constitute an important development in the artificial design of large-scale films.

Transition metal dichalcogenides (TMD) is a promising candidates as a channel material for next-generation electrical and optoelectrical applications. The atomic thin body and outstanding properties of it, however, are highly affected by integration processes such as transfer, photolithography and metallization. Here, we proposed novel synthesis method for utilizing TMD having ultraclean heterojunctions by employing confined interfacial chalcogenization (CIC). We synthesized TMD at the junction of two metal, Au and transition metal (TM) and the embedded TMD exhibit high crystallinity and atomically clean interfaces with adjacent metals. Chalcogenization at confined space suppressed defect generation and guided lateral growth of TMD. In addition, top Au acts as an epitaxial template for growing underling TMD and enables the chalcogenization at low temperature. We proved that CIC is a general synthesis method for TMD including vertical heterostructure. We elucidated that TM dissolved in Au accelerated the diffusion of chalcogen inside of Au and enables the fast and controlled CIC. Moreover, we achieved the site-selective CIC and demonstrated vertical memristor with excellent device uniformity. This method, therefore, offers preparation platform for the electrical devices utilizing TMD as a vertical channel with ensuring ideal contact of metal and TMD.

Among semiconductor colloidal nanocrystals, 2D nanoplatelets (NPLs) are geometrically seen as well-defined flexible substrates for the self-assembly of molecules. In the presence of mechanical stress brought by surface stabilizers, helical structures are formed according to the parameters of the initial material after internal energy relaxation.
Here, we demonstrate the tuning of the NPLs helices radii through the organic ligands, described as an anchoring group and an aliphatic chain of a given length. A perfect control in surface chemistry allows the switch between different morphological features of these nanohelices. Nonetheless, their optical properties in the visible region are well-preserved upon such modifications. Numerical simulations and as well as structural X-ray diffraction studies done on these anisotropic nanoparticles, unveil a strong preferential orientation effect on their resulting scattered patterns.
A mechanical model accounting for the misfit strain between the inorganic core and the organic ligands, enables to predict the nanohelices radii. The model treats these different layers of substrate, anchoring group and aliphatic chain contributions individually and demonstrates good agreement for all studied homo- and hetero-structure cadmium-based NPLs. It reveals ultimately that the self-assembly of surface molecules is equivalent to a layer, whose Young modulus in lateral compression can be estimated close to 0.9 GPa.
Furthermore, the chirality of the semiconductor nanohelices shown in this work is dictated by the ligands anchoring group and can be inverted from one population to another. Chiroptical properties exhibited in circular dichroism measurements show a special interest in investigating this type of mirror-symmetry-breaking particles and could undoubtedly, lead to the emission of circularly polarized light in the visible spectrum.

Grain boundaries (GBs) of two-dimensional (2D) materials host unique atomic defects that reveal novel properties, but deterministic control of the defect structures has not been achieved. Here, we report selective formations of dislocation cores at GBs of monolayer (ML) MoS₂ by vapor-liquid-solid (VLS) growth with Na-Mo-O eutectic alloys. The alloys function as a growth catalyst and provide Mo-rich chemical environments during the growth, which guide the formation of pentagon-heptagon defects with Mo-Mo homo-elemental bond (Mo 5|7) by yield > 95 %. In particular, we found that GBs with high tilt angle (θ_t > 34°) relieve strains by deformation-twining, instead of forming a dense arrays of defects including derivatives of S 5|7 defects. Polycrystalline MoS₂ films grown by VLS mode present about 8 times higher photoluminescence (PL) intensity than the films grown by vapor-solid-solid (VSS) mode. Density-functional theory (DFT) calculations suggest that the enhancement of PL intensity is associated with suppression of S 5|7 donor-type derivatives and electron doping. Using eutectic alloys will be useful to precisely determine atomic defects of various polycrystalline 2D films for engineering the material properties.

Recently, demanding for OLEDs (organic light-emitting diode) has increased rapidly, many researchers are researching to develop better OLED products. Currently, it is possible to manufacture a device that can fold and bend the display, but it is still very difficult to realize flexible and stretchable displays. One of these difficulties is absence of gas barrier films. Graphene has been considered as one of the best candidates for gas barrier films, because it is theoretically ideal for flexible and transparent gas barriers. However, large-scale graphene synthesized using chemical vapor deposition (CVD) contains numerous defects, and therefore has poor gas barrier properties (water vapor transmission rate WVTR = 1.57 g/(m²-d) for 3-layer graphene). In order to solve this problem, we introduced two-dimensional (2D) self-assembled alkane layers on graphene. The scanning tunneling microscopy (STM) study revealed that a self-assembled hexatriacontane (HTC) monolayer on graphene covers nanometer-size defects due to the flexible nature and strong intermolecular van der Waals interactions of alkane. In addition, HTC multilayers between graphene layers greatly improve their adhesion, indicating that HTC multilayers between graphene layers can effectively block the lateral pathway between graphene layers by filling open space with close-packed self-assembled alkanes. By these mechanisms, alternately-stacked composites of graphene and self-assembled alkane layers greatly increase gas-barrier property to WVTR as low as 1.2 x 10^-3 g/(m²-d), whereas stacked graphene layers generally show WVTR > 0.5 g/(m²-d). Furthermore, the self-assembled alkane layers have superior crystallinity and wide band gap, so they have little effect on the transmittance. However, the limitation of this method is that defects in graphene which are larger than the length of the alkane molecule might not be covered, because absent molecule-graphene interaction impedes alkane self-assembly.

The composition dynamics regulate the accessible capacity and rate performance of rechargeable batteries. Heterogeneous Li reactions can lead to non-uniform electrochemical activity and amplify mechanical damage in the cell. We develop operando optical microscopy as a laboratory tool to map the spatial composition heterogeneity in a solid-solution cathode for Li-ion batteries. We observe that the active particles charge asynchronously with reaction fronts propagating on the particle surfaces during the 1st charging, while the subsequent (dis)charge cycles transition to a synchronous behavior for the same group of particles. Such transition is understood by computational modeling which incorporates the dependence of Li diffusivity and interfacial reaction on the state of charge. Furthermore, we use synchrotron experiments to visualize the morphological defects at multi-scales ranging from the macroscopic composite, particle ensembles, to individual single particles. We find that the interfacial failure reconstructs the conductive network and redistribute the electrochemical activities that render a dynamic nature of electrochemistry and mechanics evolving over time in the composite electrodes.

Ruddlesden-Popper perovskite (RPP), a quasi two-dimensional phase of organic-inorganic hybrid halide perovskites have been considered as promising candidates for next generation opto-electronic applications. In addition to their broad controllability of bandgap between 2 and 3 eV, their inert surface nature can realize high-performance optoelectronic devices even at nanometer scale. Moreover, phase-pure monocrystalline RPPs can be synthesized by using solution processes, which exhibit significantly enhanced optoelectronic characteristics. However, the multi-layer structure of fabricated RPP crystals and the high solubility in typical solvents have been major challenges to obtain monolayer RPP for device fabrication. Conventional layer splitting and photolithography-based patterning processes which involves solvents are incompatible with RPPs. Therefore, a dry process for monolayer manipulation of RPP must to be established to realize nanoscale device array.
In this contribution, we develop a novel all-dry layer manipulation process through adhesion control of RPP. We first synthesize inch-scale phase-pure monocrystalline RPP crystals. Using the RPP crystals, we characterize the adhesion between RPP and various inorganic materials for adhesion layer by using atomic force microscopy. We find out that adhesion between RPP and inorganic materials surface can be tailored depending on the surface roughness as well as the material of adhesion layer. We investigate the fracture mechanics of RPP monolayers by measuring the out-of-plane adhesion to in-plane toughness ratio. Simultaneous processes of deterministic monolayer spitting and lateral patterning with micrometer-scale resolution are demonstrated. Furthermore, we demonstrate RPP monolayer photodetector array fabricated by the developed monolayer manipulation process.

Electronics miniaturization has been a driving force for the development of new electronic devices. Nanotechnology can be able to contribute not only to the synthesis of new nanostructures but to the analysis of their physical properties, including the appearance and understanding of new phenomena due to the dimensions in the nano regime.
Since several decades ago, carbon nanotubes (CNTs) have shown excellent electrical conduction properties, which has allowed them to be used as efficient interconnects decreasing the required space. Following the pioneering proposes of assembling 1D nanostructures into ordered arrays as a potential alternative for developing electronic nanocircuits, Ordered Networks based on Carbon Nanotubes (ON-CNTs) were designed [1] and their electronic transport properties studied [1-2] theoretically by computational calculations. This type of CNTs arrays can be seen as electronic nanocircuits, where the CNTs junctions (network nodes) could behave as the electronic nanodevices with different functions, which are interconnected by the CNTs with good electrical conduction properties behaving as the conducting wires.
In such type of nanocircuits, the electronic nanodevices functions would be directly dictated by the physical properties of the junctions between the pair of individual CNTs. In this sense, as the first stages towards developing this type of nanocircuits based on CNTs experimentally, it is of great importance to start studying the electrical conduction properties of a single cross-junction between a pair of individual CNTs and even the electrical conduction properties of an individual CNT under the presence of spots of different materials (e.g. Platinum, amorphous Carbon or even holes), whose disturbance should be different on the electronic transport properties along the CNT, therefore dictating different functions for the potential nanodevices.
Here, we will show some of our recent results on the analysis of the electrical conduction properties of individual CNTs influenced by the presence of spots of different sizes of holes, amorphous Carbon or Platinum. These spots are generated on individual CNTs either by ion milling (holes) or by controlled deposition (amorphous Carbon or Platinum) with a gas injection system (of an organometallic molecule as precursor) and a focused ion beam (FIB) in a dual-beam scanning electron microscope (FIB-SEM).

[1] Romo-Herrera J.M. et al. NanoLetters 7 (3): 570 (2007).
[2] Romo-Herrera J.M. et al. ACS Nano 2 (12): 2585 (2008).

Acknowledgements: The authors thank CONACyT for financial support through project A1-S-17539 and UNAM for DGAPA-PAPIIT project IN111223 and DGTIC-UNAM projects LANCAD-UNAM-DGTIC-382. The authors thank Israel Gradilla for technical assistance.

Aligning atomically thin two-dimensional materials vertically to the bottom has received much attention to utilize its structural anisotropy. In particular, various strategies have been reported to achieve vertical MXene, early transition metal carbide and nitride, for high capacitive performance. However, controlling the azimuthal orientation of the vertical MXene nanosheets cannot be achived with the previous methods, so its application was limited predominantly to capacitors. Here, vertical alignment induced by applying an alternating electric field to MXene colloid is presented. The reversible alignment switching of the colloid depending on the ON and OFF states of the electric field is observed through a polarized optical microscope. The immobilized nanosheets maintaining the colloidal orientation under the electric field can be obtained through in-situ freeze-drying. The oriented vertical MXene nanosheets exhibit an anisotropic electrical conductivity with a difference of nearly 9 times depending on the direction. In addition, various patterns composed of vertical MXenes are devoloped using diversified electrode designs. Based on these results, a new paradigm is suggested for various applications of vertical MXene such as sensors, optical polarizers, and heat-dissipating films, not limited to high-performance storage batteries.

Lithium-ion battery technology has seen immense success in consumer electronics and is becoming a leading technology for the mobility electrification with increasingly more attention being paid to battery safety problems¹. Detecting gas molecules (including hydrogen, carbon monoxide, carbon dioxide, ethylene, and methane) released from battery thermal runaway using gas sensors is one of the effective strategies to achieve early safety warnings for Lithium-ion batteries. Benefiting from the superiorities of small size/compactness, high sensitivity, and stable performance, resistive gas sensors are considered promising candidates in this field.
Two-dimensional (2D) materials have demonstrated great potential in the field of gas sensing due to their layered structures. Particularly, 2D Transition Metal Dichalcogenides (TMDs) possess high surface areas and exhibit semiconducting properties with tunable band gaps making them excellent candidates for sensing applications². Among the different TMDs materials, MoS₂ has gas sensing ability comparable to graphene and WS₂ can sense both toxic and non-toxic gases. In combination with the general benefits of 2D nanomaterials, the incorporation of one-dimensional (1D) nanomaterials with 2D TMDs is a recent approach for improving the gas sensing performance of these materials by the synergistic effects of hybridization. Indeed, hybrid (2D/1D) nanostructured materials provide the opportunity to fabricate and test the synergy between two or more different materials to achieve new functions. The use of single-wall carbon nanotubes (SWCNTs) as a skeleton for the TMDs enables the tuning of the electrical properties as the d orbitals from the transition metal M may partially overlap with the Π orbitals in the SWCNTs.
The purpose of this work is to fabricate a gas nanosensor based on TMDs@SWCNTs resistive channel and to further couple it to a Lithium-ion battery with the aim of detecting toxic byproducts released due to battery thermal runaway.
In this work, high-quality WS₂@SWCNTs and MoS₂@SWCNTs are fabricated using a bottom-up approach. First, the SWCNTs are synthesized via a chemical vapor deposition (CVD) method on a quartz substrate where the characteristics of the SWCNTs, i.e., diameter, density, and length are controlled by controlling the catalyst and CNT growth conditions. Next, mono and bilayer WS₂ and MoS₂ nanoflakes with diameters ranging between 10-20 nm are directly synthesized on the walls of the CNTs using homemade molecular beam evaporation/epitaxy (MBE) system.
To test the sensing abilities of WS₂@SWCNTs and MoS₂@SWCNTs, the electrical measurements are first performed under tunable relative humidity environments. An opposite electrical response towards humidity is obtained for the two types of heterostructures. The WS2@SWCNTs have shown a positive response followed by a negative response which highlights the change in the intrinsic properties of the material depending on the injected percentage of relative humidity. Whereas, the MoS₂@SWCNTs have shown only a positive response with the change in the percentage of relative humidity. This opposite response can be explained by the fact that intrinsically WS₂ behaves as an n-type semiconductor while MoS₂ behaves as a p-type semiconductor.
Further measurements will be performed to evaluate the selectivity, sensibility, and the ability to perform monitoring functioning of TMDs@SWCNTs nanosensors in the presence of different byproduct gas molecules. Finally, the TMDs@SWCNTs nanosensors will be coupled to a homemade Li-ion pouch cell battery and will be tested under harsh environments.

References:
1 Essl, C., Seifert, L., Rabe, M. & Fuchs, A., Batteries 7, 25 (2021)
2 Late, D. J. et al. , ACS Nano 7, 4879–4891 (2013)

With the significant improvements and developments in transmission electron microscopes (TEM) techniques, accurate observation and identification is now more commonly adopted in angstrom-level precision, which reveals as sequential images about the real-space information. However, it still has difficulties to analyze the spatial defect distribution, phase transformation and dynamic phenomena since the TEM images can only record the appearances of the surface atoms while the out-of-plane information is severely missing. In this work, the healing process, characterized by the regeneration of crystalline structures within the e-beam burned holes, has been experimentally observed using TEM images. Modeling based on density functional theory and molecular dynamics was conducted to study the energy path as well as the stabilized phases of all stages before and after the healing of MXene. Furthermore, a first principle based deep learning algorithm is proposed and adopted to reconstruct the 3D transition states of a MXene healing process experimentally observed. Starting from the TEM image. Around hole atoms are assumed to have crystalline structure and set as the input along with the in-plane information of atoms that can be observed directly within the hole. Out-of-plane information of the latter group of atoms are then treated as the output of the algorithm along with the spatial distribution of all underlying atoms. The input and output information are reformed into graphical representations and treated as a graph reference problem. The trunk of this algorithm is set as a deep convolutional neural network incorporated with attention-based and non-attention-based components. To conform to the distance prediction, smooth potentials are constructed for Ti-Ti, Ti-C and C-C interactions based on first principle calculation. The minimum potential energy accumulation of all atoms within the hole is set as one of the convergence conditions of the algorithm along with the fitting of the training data sets. Our models are trained on defected MXene structures optimized with DFT, where different sizes and number of atoms are considered. The predictions of this algorithm fit well with the TEM images and help with the understanding of the healing processes observed in MXene materials.

Aberration-corrected scanning transmission electron microscopy (STEM) has long been thought as a cutting-edge analytical tool for materials characterization, which offers valuable structural and spectral information with single atom sensitivity. The capability of STEM for engineering materials, however, is underestimated and much less explored. The art of STEM for engineering materials is like sculpting, in which the electron probe can be used as an atomic knife to generate and excite defects in parent materials. Further, novel structures and 2D, 1D materials can be constructed via controlled defect engineering and electron beam induced phase transitions. Such studies, especially when combined with first-principles calculations and electron beam lithography technique, shed light on atom manipulation and novel materials synthesis via electron beam engineering.

In this presentation, I will discuss our recent efforts in applying electron beam engineering to construct a range of novel 1D and 2D nanostructures that could not be feasibly obtained via conventional synthesis. Examples include the in-situ sculpting of 1D metallic nanowires within 2D semiconducting dichalcogenide films [1-2], electron-beam-induced formation of single-atomic-layer CuO on graphene surface [3] and monolayer SiC in graphene nanopores, and atom-by-atom fabrication of freestanding monolayer molybdenum membranes [4].

I will also present a recent study on the successful fabrication of single-atomic-layer chromium oxide (CrO) through electron beam engineering of monolayer chromium oxychloride (CrOCl) in STEM. First-principles calculations show that the generation of CrO from CrOCl may stem from the ionization damage of CrOCl under electron beam irradiation. The whole fabrication process can be precisely controlled by imaging and adjusting the dose rate in STEM and should in principle be applicable to other transition metal oxyhalogen compounds. Intriguingly, the as-fabricated CrO can self-heal and follows a well-defined orientation relationship with the parent CrOCl phase, which is a feature of lattice distortion type Martensite transformation. The size of the as-fabricated CrO single crystal can reach several hundred nm². Valence electron energy loss spectroscopy (VEELS) results verify the theoretical prediction that CrO is an indirect semiconductor with a bandgap about 2.1 eV.
[1] J. Lin et al., Nature Nanotechnology, 9, 436 (2014).
[2] J. Lin et al., ACS Nano, 10, 2782 (2016).
[3] K. Yin et al., 2D Materials, 4, 011001 (2017).
[4] X. Zhao et al., Advanced Materials, 30, 1707281 (2018).

Choice of substrate is one of important factors for synthesis of thin films. For oxide materials, Schlom et.al. comprehensively investigate the thermodynamic stability of more than 80 binary oxides for epitaxial growth on silicon substrates , however thermodynamics stability literary evaluates the thermal stability between oxide and silicon, and does not include any crystallographic factors such as lattice mismatch and crystal orientation. The thermal stability is literally potential use for epitaxial growth as Schlom mentioned in his report[1].

In previous report, we employed an absorption energy between oxide and silicon (Si) substrate to predict the direction of crystal growth of oxide films, and experimentally deposited oxide films on Si substrates[2]. In this study, molecular dynamics was applied on carbon clusters placed on variety of substrates with vacuum slab (supercell), and the absorption energy was estimated on the supercell. Carbon films were experimentally deposited on the target substrates by pulsed laser deposition (PLD) in carbon dioxide atmosphere[3].

For evaluation of absorption energy, supercell was consisted of carbon clusters placed on variety of substrates with vacuum slab. As carbon clusters, (1) carbon atom, (2) six-membered ring (6-ring) and (3) seven six-membered rings (nanographene) were placed on SrTiO, silicon and sapphire substrates. On the surface of SrTiO(001), for an example, carbon clusters were placed on either Sr, Ti or O atom. And the absorption energy was estimated by using the density functional theory (DFT) with a semi-core pseudopotential. The generalized gradient approximation (GGA) method was used to obtain the electron density. The energy difference for self-consistent field was set at 1.0x10-6 Ha. Crystal structure of each substrates was optimized with periodic condition, and supercell consisting of slab and vacuum layer was used to optimize the top layer of substrate. After optimal surface was prepared on each substrates, supercell with graphitic cluster and slab was optimized, and total energy was estimated on supercell combined cluster and substrate (Etot_combined). Absorption energy was evaluated to be

Etot_sub + Etot_cluster - Etot_supercell,

where Etot_sub and Etot_cluster) were total energy of subs and graphitic cluster, respectively.

Among target substrates, the absorption energy were in range from 500 to 1000 J/mol. On the surface of Si(001), nanographene was stable with the absorption energy of ~940 J/mol, however 6-ring did not flatly cover the surface. The 6-ring vertically stood up on the Si(001), which impeded flat growth of graphene. Carbon clusters flatly covered only on SrTiO(001)[4].

For demonstration deposition, PLD method was employed with carbon dioxide atmosphere and the surface morphology was observed by atomic force microscopy (AFM). As expected, the surface morphology of SrTiO(001) shows flat surface with Ra ~ 63 pm, which is comparable with superflat CVD graphene annealed in carbon dioxide atmosphere (Ra∼76 pm)[5]. In general, the deposited clusters migrate on the target surface, and the clusters are captured by a kink. Interestingly, graphene showed layer by layer growth and grew not from kink but the terrace edge. We will show optimized structure of supercell with candidate substrates, and compared with experimental results.

Acknowledgments
This study was supported in part by the Amada foundation under contract no. AF-2020227-B3, Tokyo Ohka Foundation for The Promotion of Science and technology no. 22117 and the Collaborative Research Project of the Institute of Fluid Science, Tohoku University. Special acknowledgment to the National Cheng Kung University 90 and beyond (NCKU’90).

References
[1] D.G. Schlom et. al., J. Mater. Res. 11, 2757 (1996).
[2] S. Kaneko et. al., Appl. Surf. Sci. 586, 152775 (2022).
[3] S. Kaneko et. al., ACS Omega 2, 1523 (2017).
[4] S. Kaneko et. al., Sci. Rep. 12, 15809 (2022).
[5] J. Zhang et. al, Angew. Chem. Int. Ed. 58, 14446 (2019).

Biosurfactant-aided liquid-phase exfoliation (LPE) is emerging as a biocompatible, green, economical, safe, and efficient approach to prepare two-dimensional (2D) materials for biomedical applications. However, relatively little is known about the molecular mechanisms of this process. Herein, we present the first study of how flavin mononucleotide (FMN) interacts with hexagonal boron nitride (hBN) nanosheets in the context of LPE. We demonstrate that FMN molecules can self-assemble on hBN via π-π interactions, as well as intermolecular hydrogen bonds (H-bonds) between the isoalloxazine moieties. Binding free energy analysis has shown FMN to be an efficient surfactant for LPE of hBN in water. According to the theoretical simulations, stable water suspension of hBN were experimentally obtained by LPE using FMN. With this work, we aim to exemplify how molecular dynamics (MD) simulation can predict and guide empirical LPE experiments, direct the surfactant screening and improve scalable production of 2D materials for biomedical applications.

Recent studies of mechanically-synthesised two-dimensional (2D) sheets from otherwise three-dimensional non-van der Waals (vdW) crystals are harbingers of a new approach for materials engineering. Yet, the nature in which 2D sheets can be derived from a non-layered, isotropic crystal remains unknown. In the case of boron, the complex crystal structure of the bulk material and its abundance of phases, as well as the influence of crystal orientation and thickness on structure interpretation, make it challenging to determine the nature of the exfoliated 2D sheets. Here we demonstrate, using aberration-corrected scanning transmission electron microscopy (STEM) and extensive structural modelling, that liquid phase exfoliated boron sheets consist of a planar arrangement of icosahedral subunits, formed from the cleaving of the {001} planes of β-rhombohedral boron. Our experimental results supported by density functional theory (DFT) calculations show that planar defects in the form of stacking faults — which form parallel layers of faulted planes in the same orientation — play a significant role in the exfoliation of 2D sheets from bulk boron, reducing the energy required to cleave the bulk {001} surface by ~ 36%. We demonstrate that planar defects are key to an engineerable pathway for the exfoliation of 2D sheets from boron, and more broadly of other covalently-bonded materials. This provides opportunities for the design of new 2D materials from non-vdW crystals.

Twisted van der Waals heterostructures are a highly tunable platform due to the many degrees of freedom available for controlling their electronic and chemical properties. Here, we focus on the local stacking order of low-degree twisted graphene heterostructures as a platform for manipulating the surface chemistry of this class of materials. We report the emergence and engineering of stacking domain-dependent surface adhesion in twisted few-layer graphene. Minimally twisted double bi- and tri-layer graphene heterostructures were fabricated and imaged using mid-infrared near-field optical microscopy and atomic force microscopy to identify rhombohedral and Bernal stacking domains. We then observed that metallic nanoparticles and liquid water exhibit a domain-selective adhesion on these heterostructures, with preference for the rhombohedral stacking configurations. Finally, we used an atomic force microscope to manipulate nanoparticles located at certain stacking domains, resulting in a local reconfiguration of the moiré superlattice near the nanoparticles at the μm-scale. Our findings establish a new approach to controlling moiré chemistry and nanoengineering.

Unlike metals and alloys with high ductility, inorganic semiconductors are mostly ceramics with brittle nature due to covalent/ionic bonding. Recent studies showed that some layered/van der Waals semiconductors could exhibit substantial room-temperature ductility, despite that the underlying mechanisms remain to be understood. Here we report that van der Waals semiconductor GaSe can have crystal orientation-dependent large plasticity at room temperature. Through in situ tensile and compressive experiments inside electron microscopes, we demonstrate that microfabricated GaSe can have substantial ductility loaded along and slanted with the intralayer direction, while showing predominantly elastic deformation perpendicular to the intralayer direction until brittle fracture. We further reveal that, despite the interlayer gliding as the main mechanism, cross-layer slips induced by buckling associated with stacking faults also contribute to the plasticity. This study offers insights to understand the ductility and plasticity of van der Waals semiconductors and shows promising flexible/deformable electronics and energy device applications.

The MXenes are a family of 2D materials which have since their discovery in 2011 received ever increasing attention. MXenes consists of alternating monoatomic layers of carbon or nitrogen and transition metals, with the metal always as the outermost layers. These structural and compositional degrees of freedom of the MXenes let them display a wealth of properties and characteristics. Many different MXenes are investigated for a variety of applications, ranging from energy storage to catalysis and water desalination. The outermost transition metal layers of MXenes are in general decorated with additional atoms and molecules, so called terminations. These also influence the MXene properties, and thus increase even further the potential for materials design and property tailoring of MXenes.

At the beginning of 2022, a new way of synthesizing S-terminated MXene through delamination of van der Waals multilayer parent phases was reported, along with two novel 2D structures synthesized in this way. These 2D structures can be viewed either as MXenes with chalcogen terminations, or – as suggested by the authors – as transition metal carbo-chalcogenides (TMCCs), to emphasize their similarity to another large family of 2D materials: the transition metal dichalcogenides (TMDCs). This structural duality of the new sub-family of chalcogen-terminated MXenes further emphasizes their relevance and calls for more in-depth investigations. Thus, we consider the possibility to expand this sub-family, by computationally evaluating the stability of the corresponding van der Waals parent phases, as well as other related potential parent phases. We confirm the results reported in the original publication, discuss computational results for prediction of further members of the chalcogen-terminated MXenes, and present materials properties of identified stable phases.

The electronic and optoelectronic properties^1-3 of 2D materials and their stacking heterostructures^4-6 have inspired enormous interests. WSe₂, as a p-type semiconductor,^7,8 represents a foremost building block in the p-n junctions. WSe₂ shall possesses the features of large area, homogeneity, and precise layer control. To date, there is yet an ideal synthesis method ever reported. First, we present a facile approach to prepare high-quality wafer-scale homogenous WSe₂ full films. In brief, we first deposit a thin layer of metal and then do the selenization. We can control the film thickness of WSe₂ in a chemical equilibrium environment. Second, we present a facile approach to prepare high-quality large-area homogenous WSe₂ full films. In brief, we developed a pre-seeding strategy for depositing W-containing precursors over dielectric substrates, which form well-distributed particles as seeding center. Upon the salt-assisted sublimation of tungsten oxides, the WSe₂ forms over the seeded substrates (at high temperature) in the Se-rich atmosphere. Eventually the high quality of the synthetic film has been reflected on the high performances of photodetectors and field-effect transistors. Our finding may pave the way of wafer scale integration of transition metal dichalcogenides in the integrated flexible electronics.
Keywords: WSe₂; optoelectronics; photodetectors; transistors; transition metal dichalcogenides; PdSe₂; layered noble metal dichalcogenides
References
1. Pang J, Mendes RG, Wrobel PS, et al. Self-Terminating Confinement Approach for Large-Area Uniform Monolayer Graphene Directly over Si/SiOx by Chemical Vapor Deposition. ACS Nano. 2017;11(2):1946-1956.
2. Pang J, Bachmatiuk A, Yin Y, et al. Applications of Phosphorene and Black Phosphorus in Energy Conversion and Storage Devices. Adv Energy Mater. 2018;8(8):1702093.
3. Pang J, Mendes RG, Bachmatiuk A, et al. Applications of 2D MXenes in energy conversion and storage systems. Chem Soc Rev. 2019;48(1):72-133.
4. PangJ^#, ChangB^#, Liu H^*, ZhouW^*. Potential of MXene-Based Heterostructures for Energy Conversion and Storage. ACS Energy Lett. 2022;7:78-96.
5. Pang J*, Bachmatiuk A, Yang F, et al. Applications of Carbon Nanotubes in the Internet of Things Era. Nano-Micro Lett. 2021;13(1):191.
6. Wang Y, Pang J*, Cheng Q, et al. Applications of 2D-Layered Palladium Diselenide and Its van der Waals Heterostructures in Electronics and Optoelectronics. Nano-Micro Lett. 2021;13(1):143.
7. Cheng Q^#, Pang J^#*, Sun D^#, et al. WSe₂ 2D p-type semiconductor-based electronic devices for information technology: Design, preparation, and applications. InfoMat. 2020;2(4):656-697.
8. Zhang S^#, Pang J^#*, Cheng Q^#, et al. High-performance electronics and optoelectronics of monolayer tungsten diselenide full film from pre-seeding strategy. InfoMat. 2021;3:1455-1469.

Two-dimensional MoS₂ is one of the most promising materials for next-generation electric and optoelectronic devices due to its semi-conducting nature and plenty of other suitable properties. The primary approach to synthesizing large-scale and high-quality MoS₂ monolayers is chemical vapor deposition (CVD), where the gas phase process is a crucial step in CVD. However, few experiments have given a detailed explanation of the gas-phase reaction mechanism, which is essential for researching the reactive pathways to precursors and controllable growth of MoS₂. This motivates the development of atomic and molecular dynamic simulation to give micro-scale insights. Here we used reactive molecular dynamics (RMD) simulations to investigate the reaction mechanism of MoO₃ and S₂ precursors in the gas phase. Firstly, the bond-order-based algorithm was developed to automatically analyze products' real-time distribution and we used cluster analysis to screen the main intermediate product (Mo_xO_yS_z) statistically. Then we developed path-tracking tools based on target atoms to obtain the trajectory distribution of all Mo clusters in the entire reaction stage, and determine the most critical reaction pathway in statistics. Finally, we mapped the energy barrier pathways and did ReaxFF-NEB calculations to verify the feasibility of distributed reaction pathways, also explored their sensitivity to temperature and Mo/S ratio in the gas phase. These results provide valuable insight into guiding rational synthesis of MoS₂ and other transition metal dichalcogenides by the CVD process.

Xenes, the monoelemental family of two-dimensional (2D) materials, exhibit a variety of interesting physical properties with potential exploitation in nanotechnology [1]. In the framework of flexible electronics, photonics and related applications including wearable and strain-responsive devices, 2D bendable membranes are highly desired. We report here on the realization of Xenes-based membranes by taking benefit from the recently developed scheme of the Xene heterostructure, namely silicene-stanene grown by molecular beam epitaxy [2], where silicene is sandwiched in between an alumina encapsulation layer on the top surface and stanene on thin Ag(111) crystal at the bottom face. When the so-grown Xenes-base membrane is transferred onto a flexible substrate [3], the application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. Under tensile strain, the bendable membrane shows Raman frequency shift comparable to other promising flexible systems, like those based on transition metal dichalcogenides, and high stability up to one thousand bending cycles. We also show that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Moreover, similarly to the unstrained case [4], optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene. Finally, as compelling evidence of the technological potential of the membranes, we demonstrate that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures. Bendable Xenes-based memebranes may thus hold high potential for flexible and silicon-compatible applications [5]. The work is within the ERC-COG 2017 Grant N0. 772261 "XFab" and ERC-PoC 2022 Grant N. 101069262 “XMem”.
References
[1] A. Molle and C. Grazianetti, “Xenes: 2D Synthetic Materials Beyond Graphene”, Elsevier.
[2] D. S. Dhungana, C. Grazianetti, C. Martella, S. Achilli, G. Fratesi, A. Molle, Adv. Funct. Mater. 31, 2102797 (2021).
[3] C. Martella, G. Faraone, M. H. Alam, D. Taneja, L. Tao, G. Scavia, E. Bonera, C. Grazianetti, D. Akinwande, A. Molle, Adv. Funct. Mater. 30, 2004546 (2020).
[4] E. Bonaventura, D. S. Dhungana, C. Martella, C. Grazianetti, S. Macis, S. Lupi, E. Bonera, and A. Molle, Nanoscale Horiz. 7, 924 (2022).
[5] C. Martella, C. Massetti, D. S. Dhungana, C. Grazianetti, and A. Molle, submitted.

Monolayer phosphorous dopant devices with high dopant confinement are an emerging methodology that allows the measurement and manipulation of single or few-dopant atom quantum dots for quantum computation and arrayed devices for analogue quantum simulation. STM fabrication of these devices provides a unique ability to create near perfect atomic structures on clean atomically ordered silicon in ultrahigh vacuum conditions. The devices are encapsulated in situ with a UHV grown ~30 nm epitaxial silicon layer followed by metallization for contact to the buried device components and top gates. In this talk we will present a single electron transistor (charge sensor) coupled to a few donor cluster where the chemical potential of the central island and the charge occupation on the few atom cluster are controlled via two capacitively coupled in-plane gates while current through the source/drain-leads and the SET island are used to determine the charge state. An external aligned metal gate on the surface is integrated ex situ using HfO2 as a dielectric. We confirm the functionality of the surface gates and measure a near zero-leakage current over a large gate range. Based on charge stability measurements, the surface gates have a similar lever arm as compared to the in-plane gates and therefore greatly expand the effective device gating range. We demonstrate the tuning of a few-donor cluster coupled to an SET charge sensor using surface gates while we use in-plane gates to load/unload individual electrons from the donor-cluster as well as demonstrate spin sensitive readout in a magnetic field. We will discuss the design, fabrication, and use of surface top gates to individually or globally address individual components in few-donor/quantum dot devices as well as a 2x2 arrayed device.