Symposium NM04—Advanced Manufacturing of 2D Materials at the Atomic Scale

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.

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

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

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.

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.

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.

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.