Thomas Kempa, Johns Hopkins University
Zakaria Al Balushi, University of California, Berkeley
Ying Fang, National Center for Nanoscience and Technology
Deep Jariwala, University of Pennsylvania
NM07.01: Controlled Synthesis and Chemical Functionalization of 2D Materials I
Sunday AM, April 18, 2021
8:10 AM - *NM07.01.01
Wafer-Scale Epitaxial Growth of Unidirectional TMD Monolayers
Joan Redwing1,Haoyue Zhu1,Tanushree Choudhury1,Benjamin Huet1,Anushka Bansal1,Thomas McKnight1,Nicholas Trainor1
The Pennsylvania State University1Show Abstract
Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS2 and WSe2, results in two energetically equivalent domain alignments, often referred to as 0o and 60o domains, when grown on substrates such as c-plane sapphire and graphene. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to play a key role in domain alignment by altering the energy landscape for nucleation and adatom diffusion.
Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe2 and WS2 monolayers on c-plane sapphire in a cold-wall horizontal quartz-tube reactor. The as-received sapphire substrates, which are miscut ~0.3o toward <110>, consist of steps with sub-1 nm step height separated by 50-70 nm wide terraces. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850oC to 1000oC, was used to achieve epitaxial films using W(CO)6, H2Se and H2S as precursors in a H2 carrier gas. Nucleation was observed to occur at the terrace edge and the growing domains align epitaxially with the underlying (0001) sapphire lattice. As a result of the nucleation process, the domains grow with a zig-zag edge facing the terrace edge which imparts a preferential direction to the domains. The percentage of domains with a preferred direction ranges from 75%-86% depending on MOCVD growth conditions. Continued lateral growth for times ranging from 10-30 minutes results in fully coalesced TMD monolayers that are epitaxially oriented on the sapphire, as assessed by in-plane x-ray diffraction, with a reduced density of inversion domain boundaries. The results demonstrate the important role of surface structure in nucleation and epitaxial growth of TMD monolayers.
8:35 AM - *NM07.01.02
In-Plane Heterostructures of Graphene and Hexagonal Boron Nitride
Hyeon Suk Shin1
Ulsan National Institute of Science and Technology1Show Abstract
Two-dimensional (2D) heterostructures combining several individual 2D materials provide unique platforms to create unprecedented physical properties, thereby exploring new applications. In particular, heterostructures of hexagonal boron nitride (h-BN) and graphene have attracted a great deal of attention for potential applications. Although several methods have been developed to produce in-plane heterostructures of graphene and h-BN through the partial substitution reaction of graphene, the reverse reaction has not been reported. Though the endothermic nature of this reaction might account for the difficulty and previous absence of such a process, we demonstrated a new chemical route in which the Pt substrate plays a catalytic role. We also proposed that this reaction proceeds through h-BN hydrogenation; subsequent graphene growth quickly replaces the initially etched region. Importantly, this conversion reaction enabled the controlled formation of patterned in-plane graphene/h-BN heterostructures, without needing the commonly employed protecting mask, simply by using a patterned Pt substrate . It means that we could fabricate spatially controlled in-plane heterostructures of h-BN and graphene.
We expanded the spatially controlled conversion of h-BN to graphene on an array of Pt nanoparticles (NPs) to realize an array of uniform GQDs embedded in an h-BN sheet. A uniform Pt NP array was formed on a SiO2/Si substrate with the aid of self-patterning diblock copolymer micelles, and the h-BN sheet was transferred on the Pt NPs array, followed by the conversion of h-BN on Pt to GQDs. The size of the obtained GQDs corresponded with the sizes of the Pt NPs, because of the selective conversion of h-BN on top of Pt NPs. Uniform and precisely controlled size of the GQDs ranging from 7 to 13 nm was achieved. Finally, we demonstrated electron transport by the size-controlled GQDs isolated by insulating h-BN like a Coulomb blockade, indicating that the splitting energy of the GQD is 70–140 meV, compatible with its dimension .
In addition, a new photoluminescence peak in the GQD/h-BN heterostructures was observed at 410 nm. This blue-emitting photoluminescence occurs at 1D heterojunctions of h-BN and graphene, which is originated from the localized energy states at the disordered boundaries of h-BN and graphene .
 G. Kim, et al., Nano Letters 15, 4769 (2015)
 G. Kim, et al., Nature Communications 10, 230 (2019)
 G. Kim, et al., Nature Communications 11, 5359 (2020)
9:00 AM - NM07.01.03
Vertically-Oriented MoS2 and WS2 for Nonlinear Nanophotonics—From Nanosheets to Heterostructures
Maarten Bolhuis1,Javier Hernández1,Sabrya van Heijst1,Miguel Tinoco Rivas1,2,Kobus Kuipers1,Sonia Conesa-Boj1
Delft University of Technology1,Universidad Complutense2Show Abstract
Transition Metal Dichalcogenide (TMDC) materials such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) exhibit unique optoelectronic properties. This makes TMDCs particularly promising candidates for building blocks of ultra-thin nanophotonic devices. For such applications, the vertically-oriented configuration of TMDCs could be advantageous as compared to the conventional horizontal configuration, because the inherent broken symmetry of the vertically orientated MoS2 and WS2 would favor an enhanced nonlinear response. Further, these attractive optoelectronic properties of vertically-oriented MoS2 and WS2 can be augmented by fabricating heterostructures based on their combination. Up to now, most of these heterostructures have been fabricated by utilizing transfer techniques or by performing separate chemical vapor deposition (CVD) steps for each material.
As a starting point, we show that the direct sulfurization of predeposited Mo metal results in vertically orientated MoS2 (v-MoS2) nanosheets. A systematic study on these v-MoS2 nanosheets shows that the sulfurization rate is strongly dependent on the reaction temperature and reaction time. By systematically varying the reaction time and temperature we were able to determine both the activation energy and the diffusion constant for the sulfurization process. We verify an enhanced nonlinear response in the resulting v-MoS2 nanostructures as compared to their horizontal counterparts.
Building upon these results, we demonstrate the feasibility of creating vertically-oriented TMDC heterostructures via a one-step CVD process without relying on transfer techniques, utilizing the direct sulfurization of molybdenum (Mo) and tungsten (W) heterostructures and multilayers. By depositing different configurations of Mo and W metal and carefully controlling the sulfurization depth we can design different TMDC heterostructures and multilayers. Utilizing FIB tomography and 3D reconstructions, we determine that the grain boundaries at the interface between the Mo and W metals are continuous and that, after sulfurization, these continuous metal grains facilitate the synthesis of in-plane vertical heterostructures. HR-TEM analysis confirms that these in-plane heterostructures are continuous and uniformly distributed along the interface.
This versatile method of fabricating in-plane and vertically aligned MoS2 and WS2 heterostructures can be further extended to different arrangements of TMDC heterostructures and multilayers, thus represents a steppingstone towards the fabrication of low-dimensional TMD-based nanostructures for versatile nonlinear nanophotonic devices.
9:15 AM - NM07.01.04
Impact of Oxygen on CVD Grown Boron Nitride Layers
Mohammad Malik1,Bin Wang1,2,Sahar Jaddi1,Yiyi Yan1,Victor Reis1,Yun Zeng2,Thomas Pardoen1,Benoît Hackens1,Jean-Pierre Raskin1
UCLouvain1,Hunan University2Show Abstract
h-BN is an eminent member of the 2D materials family with exceptional and peculiar properties such as atomically smooth surface, ultra-high flexibility, and transparency. h-BN is an attractive candidate for application in flexible electronics, optoelectronics, and electro-mechanics devices. Moreover, h-BN encapsulated 2D materials  demonstrate elevated throughput and enhanced physical properties.
h-BN is a binary element material produced in various shapes via the CVD process . The coalescence of the different shape and orientation domains generate a range of grain boundaries (GBs) that can be composed of N-N, B-B, and N-B (5, 7) defects . Indeed, defective GBs lead to high leakage current and low mechanical strength, making it unsuitable for electro/mechanical devices and other applications. Therefore, the synthesis of large-area and defect-free h-BN is pivotal. Attempts have been made to grow large-area h-BN relying on pre-treatment of the substrate , the alloyed substrate , water vapors , and monocrystalline substrate . These processes have shown promising results, but the impact of oxygen to reduce the nucleation density has not been widely scrutinized in the literature.
In this work, we investigated the impact of oxygen on the growth of large-area h-BN. CVD growth of h-BN was carried out on polycrystalline copper (Cu) foils using ammonia borane as a precursor. Our investigation suggests that pre-oxidation of Cu foils by a few hundred ppm of oxygen leads to an increase of the h-BN domains size by at least a factor of 5 when compared to the h-BN domains synthesized by sole hydrogen annealing of Cu foils [4, 6]. Typical samples contain the various shapes of domains with a maximum lateral size of hundreds of microns (> 100 μm), which have not been reported so far.
The quality of h-BN is confirmed by Raman, XPS, Tof-SIMS, and UV-Vis spectroscopies. An intense peak is observed in the frequency range of 1360-1370 cm-1 that is the typical E2g mode of h-BN reported in the literature. The binding energy of boron and nitrogen is about 190 eV and 398 eV, respectively. We also confirm the stoichiometry of h-BN, i.e., the N: B ratio is close to 1 in good agreement with high-quality h-BN growth. UV-Vis spectroscopy shows strong absorption around 202 nm that corresponds to the 6.13 eV optical bandgap. The combined results of Raman spectra with UV-Vis spectroscopy confirm the formation of h-BN and reject the possibilities of other crystallographic BN structures such as r-BN, c-BN, and w-BN. The tunneling current was measured for hundreds of metal/h-BN/metal structures with an overlap area of 25 μm2. For a typical device, the thickness of h-BN is 2.6 nm, as confirmed by AFM. The normalized current density was found to be 1.79 nA/μm2 at a DC bias of 1 V, proving the good insulator potential of the synthesized h-BN.
The achieved large areas of grown h-BN are attributed to an optimum concentration of intrinsic carbon and organic species present in the Cu foil. These species presumably play a critical role in determining the size of the h-BN domains. An excessive degree of oxidation deteriorates surface quality during the process, leading to voids on the substrate and gives rise to defective growth, whilst an inadequate degree of oxidation results in smaller domains (< 5-10 μm). There is a tradeoff between the residual impurities and the h-BN grain size that can be tuned by the degree of oxidation of Cu foils, as demonstrated in this work.
1- J. Holler et al., 2D Mater. 7, 015012, (2019).
2- L. Wang et al., Mater. Chem. Front. 1, 1836-1840, (2017).
3- J. Strand et al., J. Phys.: Condens Matt., 32, 055706, (2019).
4- R. Y. Tay et al., Nano Lett., 14, 839-846, (2014).
5- G. Lu et al., Nat. Comm., 6, 6160, (2015).
6- L. Wang et al., Nat., 570, 91-95, (2019).
9:30 AM - NM07.01.05
Versatile Roles of Monolayered Inorganic Nanosheets in Multifunctional Nanohybrids
Yonsei University1Show Abstract
The monolayered 2D nanosheets of layered inorganic solids (layered metal oxides, layered double hydroxides, layered metal chalcogenides, and graphene) attract intense research interest because of their versatile roles in multifunctional nanohybrids applicable for energy and environmental technologies. The monolayered 2D nanosheets of inorganic solids can be synthesized by soft-chemical exfoliation reaction of the pristine layered materials. A great diversity in the chemical compositions and crystal structures of inorganic nanosheets provides this class of materials with a wide spectrum of physical properties and functionalities. The inorganic nanosheets can be used as powerful building blocks for exploring high performance hybrid photocatalysts and electrocatalysts. These materials can play a role as catalytically active components as well as conductive additives for improving the catalyst performance of hybridized species. In this talk, several practical examples of 2D monolayered nanosheet-based photocatalysts and electrocatalysts active for solar fuel production will be presented together with the discussion about the relationship between catalyst performance and chemical bonding nature.
9:45 AM - NM07.01.06
Synthesis and Properties of Supertwisted Spirals of 2D Materials Enabled by Non-Euclidean Surfaces
Yuzhou Zhao1,Chenyu Zhang1,Daniel Kohler1,Jason Scheeler1,John Wright1,Paul Voyles1,Song Jin1
University of Wisconsin–Madison1Show Abstract
Euclidean geometry is the fundamental mathematical framework of classical crystallography. Traditionally, layered materials are grown on flat substrates, but growing Euclidean crystals on non-Euclidean surfaces has rarely been studied. Here, we present a general model describing the growth of layered materials with screw-dislocation spirals on non-Euclidean surfaces and show that it leads to continuously twisted multilayer superstructures. This model is experimentally demonstrated by growing supertwisted spirals of tungsten disulfide (WS2) and tungsten diselenide (WSe2) draped over nanoparticles near the centers of spirals. Microscopic structural analysis shows that the crystal lattice twist is consistent with the geometric twist of the layers, leading to moiré superlattices between the atomic layers, which further influence the electronic structures of the material. The preliminary studies on the physical properties of such multilayer twisting 2D materials will be discussed. Our results introduce new concepts in the growth of exotic nanomaterials and a rational strategy for potentially controlling the twist angle in layered materials from direct synthesis, which opens up new opportunities for the study of twisted moiré superlattices and chirality-related properties.
NM07.02: Controlled Synthesis and Chemical Functionalization of 2D Materials II
Sunday PM, April 18, 2021
10:30 AM - *NM07.02.01
Ledge-Directed Epitaxy of Single-Crystalline Nanoribbons of Transition Metal Dichalcogenides
Vincent Tung1,Areej Aljarb1,Jui-Han Fu1,Thomas Anthopoulos1,Jeehwan Kim2,Lain-Jong Li1
King Abdullah University of Science and Technology1,Massachusetts Institute of Technology2Show Abstract
Two-dimensional (2D) transition metal dichalcogenides (TMDs) monolayers are considered promising for future extreme device downscaling. For wafer scale manufacturing of advanced logic and memory devices, dense arrays of single-crystal, globally aligned TMD monolayer nanoribbons that can be controllably grown, are highly desired. Demonstrated top-down approaches to form such nanoribbons require large-area, single-crystal TMDs and damage-free etching processes, which are not available currently. Bottom-up growth methodologies for TMD nanoribbons have been reported to individually achieve control of layer number, single-crystallinity, local alignment, and dimensionalities. However, controlled nanoribbon growth with all aforementioned properties synergistically remains a major challenge. In this talk, we will demonstrate a ledge-directed epitaxy (LDE) of dense arrays of continuous, self-aligned, monolayer and predominantly single-crystalline MoS2 nanoribbons on β-gallium (III) oxide (β-Ga2O3) (100) substrates. Experimental observation and density function theory (DFT) simulation indicate that the presence of intrinsic ledges on β-Ga2O3 (100) leads to the reduction in binding energy. The two previously indistinguishable and nearly degenerate configurations are now remarkably uncoupled from each other by ~2 eV, overcoming the atomic registry—the van der Waals (vdW) epitaxy constraint and thus ensuring the mono-orientated nucleation. Meanwhile, potential energy surface (PES) mapping sheds light on a surface diffusion limited pathway along the ledge, hence driving the energetically preferred and directionally modulated growth of aligned MoS2 domains into single-crystalline nanoribbons. The stitching of unidirectional seeds into continuous, single-crystal and -orientated MoS2 nanoribbons was confirmed by second harmonic generation (SHG) and dark field-scanning transmission electron microscopy (DF-STEM). The MoS2 nanoribbons can be readily transferred to arbitrary substrates while the underlying β-Ga2O3 can be re-used after mechanical exfoliation. Prototype MoS2 nanoribbon-based field-effect transistors exhibit high on/off ratios of 108 and an averaged room temperature electron mobility of 65 cm2V-1s-1, both on par with values reported for mechanically exfoliated MoS2 devices. We further demonstrate, for the first time, the LDE growth as a general strategy for single-crystal p-type WSe2 nanoribbons and lateral n-p-n heterostructures made of p-WSe2 and n-MoS2 nanoribbons. Our findings pave the way to direct the growth of single-crystalline TMDs and their heterostructures for futuristic electronics applications.
10:55 AM - NM07.02.02
Illuminating the Electronic Properties of WS2 Polytypism with Electron Microscopy
Sabrya van Heijst1,Masaki Mukai2,Eiji Okunishi2,Hiroki Hashiguchi2,Laurien Roest1,Louis Maduro1,Juan Rojo3,4,Sonia Conesa-Boj1
Delft University of Technology1,EMBU JEOL Ltd2,Nikhef Theory Group3,Vrije Universiteit Amsterdam4Show Abstract
Tailoring the specific stacking sequences (polytypes) of layered materials represents a powerful strategy to identify and design novel physical properties. While nanostructures built upon transition-metal dichalcogenides (TMDs) with either the 2H or 3R crystalline phases have been routinely studied, our knowledge of those based on mixed 2H/3R polytypes is far more limited. Here we report on the characterization of mixed 2H/3R free-standing WS2 nanostructures displaying a flower-like configuration by means of advanced transmission electron microscopy. We correlate their rich variety of shape-morphology combinations with relevant local electronic properties such as their edge, surface, and bulk plasmons. Electron energy-loss spectroscopy combined with machine learning reveals that the 2H/3R polytype displays an indirect band gap with EBG = 1.6 eV. Further, we identify the presence of energy-gain peaks in the EEL spectra characterized by a gain-to-loss ratio IG / IL > 1. Such property could be exploited to develop novel cooling strategies for atomically thin TMD nanostructures and devices built upon them. Our results represent a stepping stone towards an improved understanding of TMDs nanostructures based on mixed crystalline phases.
11:10 AM - NM07.02.03
Frank–van der Merwe Growth of Two-Dimensional Transition Metal Dichalcogenide by Strain-Engineering
Yi Wan1,Vincent Tung1
King Abdullah University of Science and Technology1Show Abstract
Two-dimensional transition metal dichalcogenides (TMDCs) have attracted widespread attention due to their prospective for the next-generation non-silicon electronics and optoelectronics. Among these two-dimensional TMDCs research, the most highlight has been given to single-layer TMDCs because of its intriguing electrical and optical properties. However, the fabrication demands and the physics of TMDCs suggest that few-layer TMDCs may be more attractive than single layer TMDCs for the industrial manufacturers because few-layer TMDCs equit more density of states for application in electronics, better light absorption and wider spectral response in optical application. To realize full potential of few-layer TMDCs, there is still much effort in the scalable and layer-controllable growth of highly crystalline TMDCs need to be achieved. In this work, WSe2 is treated as a representative of common TMDCs growth, and we report an experimental observation that strain presenting in the first WSe2 layers plays a critical role in the determination of growth manners for the following layers. Frank–van der Merwe Growth mode can be realized through inducing built-in tensile strain in first WSe2 layers during the CVD process. Moreover, we demonstrate the stacking angle between the successive layers and the first layers by second harmonic generation (SHG) measurement. The foundational understanding of growth mechanism is assessed by a comprehensive density functional theory (DFT) simulation. This work provides a crucial hint to realize uniform layer control over the large scale, which is an important step for TMDCs towards real applications.
11:25 AM - NM07.02.05
Stabilization of NbTe3, VTe3, and TiTe3 via Nanotube Encapsulation
Scott Stonemeyer1,2,3,Jeffrey Cain1,2,3,Sehoon Oh1,3,Amin Azizi1,2,Peter Ercius3,Marvin Cohen1,3,Alex Zettl1,2,3
University of California, Berkeley1,Kavli Energy NanoScience Institute, Berkeley2,Lawrence Berkeley National Laboratory3Show Abstract
One of the primary objectives of nanoscience is the precise control over the processing-structure-property relationship intrinsic to traditional materials science. Recently, the concept of dimensionality, and the engineering of materials’ structure and properties via changes in dimensionality, has emerged as an additional degree of freedom within this paradigm. To this end, the isolation of single atomic-planes (e.g. graphene and the transition metal dichalcogenides) and chains (e.g. transition metal trichalcogenides, TMTs) from quasi-low-dimensional materials has been extremely fruitful. The archetypal quasi-1D materials family is the TMTs, commonly referred to as MX3 compounds, with M a transition metal and X a chalcogen. Typically for these materials, 1D MX3 chains are weakly coupled by interchain vdW interactions to form coherent, but highly anisotropic, three-dimensional crystals. The bulk synthesis and properties of the TMTs has been well explored and they are canonical examples of superconductors and charge density wave materials. This structural motif is common across a range of M and X atoms (e.g. NbSe3, HfTe3,TaS3), but not all M and X combinations are stable. Here, we demonstrate the successful synthesis of three previously unreported MX3 TMT compounds: NbTe3, VTe3, and TiTe3. This is accomplished through nano-confined growth within the cavity of multi-walled carbon nanotubes (MWCNTs). Depending upon the inner diameter of the encapsulating MWCNTs, specimens ranging from many chains, to few chains (2-3), and even single chain, can be isolated and studied. The MWCNT sheath stabilizes the chainlike morphology, enabling synthesis and characterization with transmission electron microscopy (TEM) and aberration-corrected scanning transmission electron microscopy (STEM). It is found that few-chain specimens of the new TMTs can exhibit a coordinated interchain spiraling, while the single-chain limit exhibits a trigonal anti-prismatic (TAP) rocking distortion, which we experimentally resolve for the first time here. First principles calculations give insight into the integral role that the encapsulating CNT plays in stabilization and provide information regarding the electronic structure of the new materials.
11:40 AM - NM07.02.06
Deterministic Fabrication of Arbitrary Vertical Heterostructures of 2D Ruddlesden-Popper Halide Perovskites
Dongxu Pan1,Yongping Fu1,Natalia Spitha1,Yuzhou Zhao1,Christopher Roy1,Darien Morrow1,Daniel Kohler1,John Wright1,Song Jin1
University of Wisconsin–Madison1Show Abstract
Ruddlesden-Popper (RP) phase lead halide perovskites have emerged as a new class of 2D semiconductors that exhibit tunable electronic and optical properties, potentially offering unlimited heterostructure configurations for exploration. However, the promise of such heterostructures has not been fulfilled, because halide perovskites’ highly mobile and fragile crystal lattices make controllable direct synthesis or van der Waals integration extremely difficult. Here, we report the direct growth of large-area free-standing nanosheets of diverse phase-pure RP perovskites with thickness down to a monolayer at the solution-air interface and a gentle and reliable approach for transferring and stacking these nanosheets. These advances enabled the deterministic fabrication of arbitrary vertical heterostructures and multi-heterostructures of different RP perovskites with unprecedented structural degrees of freedom that define the electronic structures of the heterojunctions. Such rationally designed heterostructures exhibit interesting interlayer properties such as interlayer carrier transfer and reduction of photoluminescence linewidth, and would enable the exploration of exciton physics and optoelectronic applications.
11:55 AM - NM07.02.08
Late News: Ionic 2D Covalent Organic Frameworks for the Innovative Remediation of Toxic Chromium and Arsenic Oxyanions in Water
Ping Li1,Santa Jansone-Popova1,Josh Damron1,Vyacheslav Bryantsev1,Bruce Moyer1
Oak Ridge National Laboratory1Show Abstract
Accumulation of highly toxic oxyanions such as chromate and arsenate in ground and surface waters from the geological processes and/or man-made pollution has led to a wide range of health and environmental problems. Thus, the development of innovative materials that can selectively remove target toxic oxyanions has attracted enormous research interest in academia and industry. Ionic 2D covalent organic frameworks (iCOFs) offer a unique means to effectively incorporate well-defined anion binding motifs into chemically robust, highly periodic, low-density polymeric materials. Structural modifications can be easily implemented to fine-tune the local environments of anion binding sites inside iCOFs providing insights into the selectivity and efficiency of the ion exchange process. A series of guanidinium-based 2D iCOFs have been developed to systematically examine the impacts of steric, electrostatic, and H-bond groups on the anion exchange process of chromate and arsenate anions. Interesting conclusions include: 1) well-position H-bonds seem to play a crucial role in determining the weak binding of arsenate anions, 2) electrostatic tuning significantly impacts the nature and incorporation of guanidinium sites in iCOFs, 3) moderate steric alteration imposes little influences on chromate and arsenate selectivity. Results and knowledge from this study are expected to aid the development of new guanidinium-based polymeric and composite anion exchange materials for toxic oxyanion remediation and other extraction applications.
NM07.03: Controlled Synthesis and Chemical Functionalization of 2D Materials III
Sunday PM, April 18, 2021
1:00 PM - *NM07.03.01
Chemically Tailoring Interfaces in Two-Dimensional Heterostructures
Northwestern University1Show Abstract
As a result of their unique electronic, optical, and physical properties, two-dimensional (2D) materials are actively being explored for applications in optoelectronics , neuromorphic computing , quantum information science , and energy technologies . With exceptionally high surface-to-volume ratios, 2D materials are highly sensitive to their environment, resulting in a strong dependence of their properties on substrate effects, extrinsic adsorbates, and interfacial defects. Furthermore, the integration of 2D materials into heterostructure devices introduces further demands for controlling interfaces with atomic precision. With this motivation, this talk will explore emerging efforts to understand and utilize interfacial chemical functionalization to influence the properties of 2D heterostructures. For example, organic adlayers can tailor chemical reactivity to enable conformal atomic layer deposition of pinhole-free encapsulation layers that mitigate the deleterious effects of ambient exposure, particularly for ambient-unstable 2D materials such as black phosphorus and monochalcogenides . The integration of organic self-assembled monolayers with 2D semiconductors also allows for tailoring of electronic and optical properties such as photoinduced charge separation in fullerene/InSe heterojunctions  and mixed-dimensional excitonic states in phthalocyanine/MoS2 heterojunctions . By exploiting spatially inhomogeneous surface chemistry, seamless lateral 2D heterointerfaces can also be realized including perylene/borophene , graphene/borophene , and concentric borophene superlattices , each of which show atomically sharp electronic interfaces as confirmed by ultrahigh vacuum scanning tunneling microscopy and spectroscopy. Overall, by providing precise tailoring of interfaces, chemical functionalization presents opportunities for improved functionality in 2D heterostructure devices.
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 S. Li, et al., ACS Nano, 14, 3509 (2020).
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 X. Liu, et al., Science Advances, 3, e1602356 (2017).
 X. Liu, et al., Science Advances, 5, eaax6444 (2019).
 L. Liu, et al., Nano Letters, 20, 1315 (2020).
1:25 PM - NM07.03.02
Ledge-Directed Epitaxy of Continuously Self-Aligned Single-Crystalline Nanoribbons of Transition Metals Dichalcogenides
Areej Aljarb1,2,Jui-Han Fu1,Lain-Jong Li1,3,Vincent Tung1
King Abdullah University of Science and Technology1,King Abdulaziz University2,Taiwan semiconductor Manufacturing Company3Show Abstract
Two-dimensional (2D) transition metal dichalcogenides (TMDs) monolayers have been considered promising for future device scaling. For wafer-scale manufacturing, dense arrays of single-crystal, globally aligned TMD monolayer nanoribbons are desired for advanced logic and memory devices. The top-down approach to form such nanoribbons requires large-area, single-crystal TMD, and deliberate lithography/etching processes, which are not available currently. The bottom-up growth approaches toward TMD nanoribbons have been reported to individually achieve the control of layer number, single-crystallinity, local alignment, and dimensionalities. However, the nanoribbon growth with all the above-mentioned properties synergistically remains a major challenge. Here, we demonstrate a ledge-directed epitaxy (LDE) of dense arrays of centimeter-long, self-aligned, monolayer, and predominantly single-crystalline MoS2 nanoribbons on β-gallium(III) oxide (β-Ga2O3) (100) substrates. Experimental observations and density functional theory (DFT) simulation suggest that nucleation and growth of the nanoribbons follow the β-Ga2O3 ledges. The stitching of unidirectional seeds into continuous, single-crystal, and -orientation MoS2 nanoribbons was confirmed by second harmonic generation (SHG) and dark field-scanning transmission electron microscopy (DF-STEM). The MoS2 nanoribbons can be readily transferred to arbitrary substrates while the underlying β-Ga2O3 can be re-used after mechanical exfoliation. Our prototype MoS2 nanoribbon-based field-effect transistor exhibits an on-off ratio of 108 and room temperature carrier mobility of 65 cm2V-1s-1 comparable to those mechanically exfoliated benchmarks. We further demonstrate the LDE growth of p-type WSe2 nanoribbons and lateral heterostructures made of p-n WSe2-MoS2 nanoribbons. Our findings pave the way to miniaturization of single-crystalline TMDs and their heterostructures with potential applications not available by other means.
2:10 PM - NM07.03.05
Electron Wind Force Annealing of MXene Films
Aman Haque1,John Sherbondy1,Abu Rasel1,Brian Wyatt Jr.2,Babak Anasori2
The Pennsylvania State University1,Indiana University–Purdue University Indianapolis2Show Abstract
MXenes are a family of two-dimensional (2D) transitional metal carbides and nitrides, obtained by selective etching of the A element from the parent MAX phases, where M is an early transition metal, A is an element of the A group, mostly group 13 or 14 of the periodic table (in particular, Al, Si, and Ga), and X stands for carbon and/or nitrogen. Their atomic-scale nature, scalable processing, and attractive electrical properties (up to 20,000 S/cm conductivity) make them strong candidates for a diverse range of applications such next generation energy storage systems, electromagnetic interference (EMI) shielding, antibacterial agents, and water purification. However, MXenes are solution-based 2D materials with abundant surface functional groups and intra-flake separations, which decrease the overall electrical conductivity in film form. This renders film annealing a critical post processing step.
While annealing of MXene films is typically performed in the 700-1500 °C range, we present a novel, athermal process to achieve similar results. We explore annealing with the electron wind force (EWF) at near-room temperature conditions in contrast to traditional heating methods. EWF is an atomic scale mechanical force that acts only in the defective regions, which is proposed to provide very high defect mobility. The process is demonstrated on nominally 5 microns thick, freestanding Ti3C2Tx films. We report 30-50% decrease in resistance after applying only 20 amp/mm2 current density. The specimen temperature remains below 100 °C during the process. The resistance change was permanent and did not increase back when the current was removed. We acquired Raman spectroscopy on as-received and EWF-annealed specimens to observe remarkable decrease of intensity in the Tx, M-Tx and the Carbon regions. In the Tx (230−470 cm−1) largest intensity drop occurred at wave shift of 286 cm-1. This range represents the in-plane vibrations of surface groups attached to titanium atoms. Therefore, our remarkable intensity drop suggests elimination of the surface terminated bonds, which are helpful for the electrical conductivity. Further, the C region (580 – 730 cm−1) represents carbon vibrations, where we observed remarkable intensity drops in the 600 and 720 cm-1 peaks. We also observed remarkable decrease in the 123 cm-1 peak, which represents the surface plasmon resonance. The most remarkable feature of this annealing process is that no increase in disordered carbon peaks (typically seen in high temperature annealing) were observed. Rather, the decreased peak intensity in the C region indicates loss of carbon atoms. The Raman results were verified by Energy-dispersive Spectroscopy performed in a SEM. We also prepared cross-sectional specimens to highlight the decrease of the intra-layer defects (mostly micron scale voids).
To summarize, we have demonstrated a low temperature (<100 °C) process to anneal MXene thick films to decrease their resistivity up to 50%. Raman and EDS analyses suggest that such improvement in electrical conductivity comes from the removal of the surface terminated bonds. The advantages of this new process are (a) low temperature will allow temperature sensitive substrates, such as polymers and (b) absence of high temperature induced formation of TiO2 or disordered carbon.
2:25 PM - NM07.03.06
New Methods and Observations in Contact Scaling for 2D FETs
Zhihui Cheng1,2,3,Huairuo Zhang1,Hattan Abuzaid3,Jonathan Backman4,Yifei Yu5,Shreya Singh3,Albert Davydov1,Mathieu Luisier4,Linyou Cao5,Curt Richter1,Aaron Franklin3
National Institute of Standards and Technology1,Purdue University2,Duke University3,ETH Zürich4,North Carolina State University5Show Abstract
Atomically thin two-dimensional (2D) crystals are promising channel materials for extremely scaled field-effect transistors (FETs) for the 2030 era . In the quest of ultra-scaled transistors, both channel length (distance from source to drain contacts) and contact length (distance that the contacts overlap the 2D channel) must be scaled. However, contacting 2D materials at scaled contact lengths (Lc < 30 nm) has rarely been pursued or studied in-depth. In this work, we experimentally scaled contact length for Ni-contacted MoS2 FETs and use asymmetrical contact measurements (ACM) as a new approach for characterizing the devices. We found that, contrary to most previous reports, top contacts can be scaled down to ~30 nm without noticeable degradation in contact resistance. Surprisingly, we also observed significant self-heating in scaled contacts in the saturation regime. While the first observation is promising for extremely scaled FET technologies, the second illustrates that current crowding in metal-2D contacts is a challenge toward the development for future scaled devices.
 IEEE International Roadmap for Devices and Systems. https://irds.ieee.org/ (2020).
2:40 PM - NM07.03.07
Fabrication and Structural Characterisation of MoS2 Nanowires
Louis Maduro1,Maarten Bolhuis1,Sonia Conesa-Boj1
Delft University of Technology1Show Abstract
Molybdenum disulfide (MoS2) and MoS2-based nanowire arrays have garnered significant attention recently due to several potential applications, including being used as interconnects for nanoelectronic devices, for efficient electron emission, their structural stability, and in connection promoting hydrogen evolution reaction. In particular, MoS2 nanowires display decreasing resistivity with decreasing nanowire diameter. For this reason, relatively large electric currents can be sustained for nanowires with diameters in the tens of nanometer range, even though the resistivity of such nanosized MoS2 nanowires increases to values larger than bulk MoS2. This property makes MoS2 nanowires promising candidates to be deployed as interconnects for nanodevices.
Furthermore, arranging MoS2 nanowires into bundles whose axis is perpendicular to the substrate surface is a promising strategy for creating efficient electron emitters, where a lower voltage is required for electron extraction when decreasing the diameter of these nanowires. In order to create an ideal electron emitter device, the density of MoS2 nanowires has to be such that the field enhancement effect of single nanowires is preserved while having a large density of MoS2 nanowires to achieve a high electron current density.
Previous work in the fabrication of MoS2 nanowires mostly focuses on bottom-up approaches based on hydrothermal synthesis, self-assembly, and chemical vapour deposition. However, these bottom-up methods lack precise control of the nanowire shape, nanowire-to-nanowire distance, and nanowire orientation. We propose a top-down approach for the fabrication of MoS2 nanostructures which has not been widely explored. Here demonstrate the feasibility of a top-down approach to fabricate ordered arrays of MoS2 nanowires where the shape, diameter, pitch, and length of these nanowires can be controlled in detail by means of the combination of sputter-deposition, electron beam lithography, and reactive ion etching.
NM07.04: Controlled Synthesis and Chemical Functionalization of 2D Materials IV and Imaging, Spectroscopy, and Quantitative Analysis of 2 Materials I
Sunday PM, April 18, 2021
4:00 PM - *NM07.04.01
Controlled and Scalable Synthesis of Two-Dimensional Carbides (MXenes)
Drexel University1Show Abstract
Two-dimensional metal carbides and nitrides, known as MXenes, are the largest and yet quickly expanding family of 2D materials. Their diverse electronic, optical, and electrochemical properties have already distinguished them from other 2D materials. MXenes are produced by selective etching processes in which A layer atoms (i.e., Al, Si, Ga, etc.) are chemically removed from layered ceramics known as MAX phases or related layered carbides. MXenes have a general formula of Mn+1XnTx where M is a transition metal (i.e., Ti, V, Nb, Mo, etc.), X is carbon and nitrogen, n can be from 1 to 4, and Tx indicates to presence of mixed surface terminations (O, OH, Cl, F, etc.) on the surface of outer transition metal layers of MXenes. MXenes can also contain two or more transition metals in the M sites in a random solid solution (such as (Ti,V)2CTx) or ordered (in-plane or out-of-plane, such as Mo2TiC2Tx) structure, which further expands their range of compositions and therefore, properties. The surface functional groups of MXenes provide them with chemical stability and control their electronic properties. Titanium carbide MXene, Ti3C2Tx, has metallic conductivity reaching over 15,000 S cm-1, while some other MXenes showed 2-3 orders of magnitude lower values. The tunable electronic properties of MXenes along with their high mechanical stiffness (Young’s modulus reaching ~0.4 TPa for Nb4C3Tx MXene) and high bending rigidity as well as rapid ion-transport properties, redox-active surfaces, tunable electrochemical and optical properties have rendered these materials as promising candidates for various high-tech applications such as transparent conductive electrodes (TCE) for organic light-emitting diodes, smart and conductive textiles for sensing and energy-storing wearable devices, thin-film transistors, biomedical and biosensing, photovoltaics, and high energy and power density supercapacitors. Herein, we aim to discuss synthesis of MXenes, as well as effect of synthesis process parameters on surface chemistry and properties. Scalability of MXene synthesis and its processing from aqueous solutions by a variety of techniques will be addressed in detail.
4:25 PM - NM07.04.02
AFM Additive Nanopatterning on Ionic Liquid Monolayer-Decorated Surfaces by Combined Mechanical and Electrical Stimuli
Zixuan Li1,Filippo Mangolini1,Jerzy Sadowski2,Raluca Gearba1,Karalee Jarvis1,Oscar Morales-Collazo1,Andrei Dolocan1,Joan Brennecke1
The University of Texas at Austin1,Brookhaven National Laboratory2Show Abstract
The development of direct-write nanopatterning approaches enabling the accurate and reliable production of nanoscale architecture is critical for exploiting the unique functionalities of materials at reduced length scales. In the last few decades, several atomic force microscopy (AFM) lithography techniques, which use a sharp probe to write on a solid surface, have been developed as cost-effective methods for patterning with nanoscale resolution. Common AFM lithography techniques usually exploit the meniscus formed around the tip-substrate contact to dissolve and transfer the reactants for surface patterning. This mechanism can be problematic in large scale production due to the restricted ink capacity, as well as the inconsistency in the transfer process by varying experimental and environmental factors. Here we highlight a new AFM patterning method with which an adsorbed ionic liquid (IL) monolayer on the substrate will be used as the ink material. The unique interfacial properties of ILs combined with electrical and mechanical stimuli can realize nanoscale patterning without the reliance of the meniscus for ink transfer. The chemical nature and physical properties of the deposited material will be evaluated.
4:40 PM - NM07.04.03
Rational Synthesis and Assembly of Transition Metal Dichalcogenide Nanocrystals with Tunable Optical Properties
Tomojit Chowdhury1,Erick Sadler1,Kiyoung Jo2,Todd Brintlinger3,Deep Jariwala2,Thomas Kempa1
Johns Hopkins University1,University of Pennsylvania2,U.S. Naval Research Laboratory3Show Abstract
Reducing the dimensionality of semiconducting transition metal dichalcogenides (TMDs) substantially alters their physical properties. Two-dimensional (2D) TMD crystals also exhibit electronic, magnetic, and optical properties which are sensitive to any manipulation of edge and strain states. We have introduced a rational chemical synthesis strategy that drastically improves control over the dimensions, morphology, and crystalline edges of 2D TMDs without the need for lithography and etching. TMD nanoribbons prepared through gas-phase growth on our designer surfaces exhibit atomically sharp edges and have enabled the exploration of anomalous exciton emission features. Moreover, we show that judicious assembly of mixed-dimensional architectures containing 2D TMD crystals and Si nanowires enables the introduction of nanoscale strain fields. Detailed nanoscale structural and optical characterization of these systems reveals unique edge and strain states that manifest localized photoluminescence with anomalous shifts in energy. Our results open a path toward the rational design of inorganic low-dimensional architectures with clear relevance for enabling future photonic and optoelectronic device studies.
5:00 PM - *NM07.04.04
Toward 2D Heat and Light in 2D Crystals
The University of Chicago1Show Abstract
2D crystals and superlattices possess extremely anisotropic structures originating from different in-plane intralayer and out-of-plane interlayer bondings. This extreme structural anisotropy leads to anisotropic transport of electrons, phonons and photons. Indeed, 2D materials have recently shown several exciting 2D electronic transport phenomena. However, its impacts on the transport of heat and light in different directions have not been fully explored. In this talk, I will discuss two recent results related to this topic. First, we will discuss extremely anisotropic thermal conduction in stacked multilayers of transition metal dichalcogenides, where interlayer rotations lead to air-like thermal insulation along the through-plane direction. This leads to a giant thermal conductivity anisotropy ratio ~ 900, which is much larger than that of graphite. Second, we will discuss the long-range (~cm) guiding and non-linear switching of 2D photonic waves using a "delta-waveguide" built based on wafer-scale MoS2. We show that this becomes an optical analog of quantum mechanical delta potential trap with a single trapped wavefunction. If time allows, we will also discuss how to realize these novel anisotropic properties by synthesizing and integrating other, molecule-based 2D materials.
5:25 PM - NM07.04.05
Atomic-Step-Induced Screw-Dislocation-Driven Spiral Growth of PVD SnS
Yih-Ren Chang1,Chien-Ju Lee2,Tomonori Nishimura1,Wen-Hao Chang2,Kosuke Nagashio1
The University of Tokyo1,National Chiao Tung University2Show Abstract
The in-plane piezoelectricity or ferroelectricity of two-dimensional (2D) materials can vanish due to the appearance of inversion symmetry with increasing flake thickness, which drastically limits the development of their energy-harvesting application. On the other hand, although screw dislocation has long been regarded as an undesired line defect in three dimensional materials, screw dislocation driven growth in 2D materials could result in non-centrosymmetric structure even in bulk materials. The non-centrosymmetric structure in 2D materials can bring about plenty of intriguing properties such as vertical conductivity through the screw dislocation core and nonlinear optical generation, piezoelectricity and ferroelectricity and these properties might be further used in various applications like memory devices or piezoelectric generators. [1-3] However, despite the fact that the inversion symmetry breaking characteristic in spiral structure may the solve above-mentioned problem in 2D materials, the control of spiral growth remains immature owing to random occurrence of screw dislocation which limits the development of applications based on spiral 2D materials by poor nucleation site control and low percentage of spiral flakes.
In this research, a novel mechanism to achieve high percentage of spiral SnS flakes with superior control of nucleation position is demonstrated. By introducing atomic steps on substrates, the screw dislocation can be easily formed when SnS partially grows across these steps and leads to over 90% of spiral SnS flakes grown by physical vapor deposition (PVD). Furthermore, the preference for SnS to nucleate at steps can introduce remarkable nucleation site control of spiral growth even on substrates with artificially transferred graphene atomic steps. Through second harmonic generation (SHG) spectroscopy and cross sectional STEM analysis, it turns out that the spiral SnS structure exhibits inversion symmetry characteristic. Contrary to common understanding that spiral 2D material with single screw dislocation and Burgers vector equaling single layer thickness would show non-centrosymmetric structure, single-spiral SnS flakes with Burgers vector equaling the thickness of single layer SnS exhibits centrosymmetric structure. One possible reason that lead to centrosymmetric structure in spiral SnS could be ascribed to its ferroelectricity,  indicating that the orientation switching of SnS layer and the formation of AB stacking structure could be induced by strain.
This is the first work to control the formation position of spiral 2D materials and point out 2D materials with single spiral morphology and Burgers vector equaling single layer thickness do not guarantee non-centrosymmetric structure. High spiral flake percentage and precise control of nucleation sites in this study will facilitate future development of spiral 2D materials.
 T. H. Ly, et al., Advanced Materials 2016, 28, 7723.
 L. M. Zhang, et al., Nano Letters 2014, 14, 6418.
 W. Z. Wu, et al., Nature 2014, 514, 470.
 M. H. Wu, et al., Nano Letters 2016, 16, 3236.
5:40 PM - NM07.04.06
Late News: WS2 Films Obtained with a Low Thermal Budget Sulfurization Process
Claudio Radtke1,Dheryck Cabeda1,Bruno Ferreira1,Guilherme Rolim1,Gabriel Soares1
Transition metal dichalcogenides (TMDs) with lamellar structures similar to that of graphite have received significant attention because some of them are semiconductors with sizable bandgaps and are naturally abundant. This offers opportunities for fundamental and technological research in a variety of fields including catalysis, energy storage, sensing, and electronic devices. Among them, bulk WS2 exhibits an indirect band gap of 1.2 eV, while single-layer WS2 exhibits a direct band gap of 1.8 eV. In order to fully exploit this versatility, obtaining layers of this material over large areas with the desired properties (in particular the number of monolayers composing the stack) is mandatory. Despite recent advances in the growth techniques of such a material, further investigation is needed especially in the basic mechanisms involved in the growth of WS2 layers aiming at expanding their applicability. In view of this scenario, we sulfurized sputtered WO3 films on SiO2/Si substrates, varying the processing parameters. The objective was i) to identify the role of hydrogen added to the carrier gas in the sulfurization process and ii) to determine sulfurization parameters that result in fully sulfurized WO3 films. For that, WO3/SiO2/Si stacks were introduced in a quartz tube where they were heated for variable times and temperatures. Pure Ar and a Ar:H2 gas mixture were used as carrier gases. Sulfur powder was independently heated. X-ray Photoemission Spectroscopy (XPS), Rutherford Backscattering Spectrometry (RBS), and Raman Spectroscopy were used to characterize the products of the sulfurization process and also to infer about W loss from the sample during sulfurization. Results evidence that H2 promotes an efficient sulfurization of the WO3 film. Using pure Ar as the carrier gas, higher temperatures must be employed aiming at a complete sulfurization. Moreover, W loss takes place during sulfurization, if S is not supplied in a specific moment of the sample’s heating ramp. This latter effect can be strongly suppressed by the use of H2, which also significantly lowers the temperature needed for sulfurization. These results evidence a synthesis of WS2 layers with a low thermal budget as well as details of the sulfurization process of WO3 films. Such knowledge is fundamental for further improvements of WS2 films’ properties and to new applications of this material.
5:55 PM - NM07.04.07
The Phonon Dispersion Relation as a Unique Identifier of Ordered BC3 “Flower” Units in Graphene-Like Materials
Devin McGlamery1,Alexander Baker2,Yi-Sheng Liu3,Martin Mosquera1,Nicholas Stadie1
Montana State University1,Lawrence Livermore National Laboratory2,Lawrence Berkeley National Laboratory3Show Abstract
Boron doped graphene and related materials, especially crystalline BC3, have long been sought as promising materials for applications such as hydrogen and lithium-ion storage. Among the materials presented by experimentalists so far, boron presents as a notoriously difficult structural/chemical environment to unambiguously probe in the presence of carbon, frustrating the report of a successful synthesis of single-layer BC3. Herein we report that the hexagonally symmetric C6B6 flower-like units that make up the structure of BC3 exhibit a characteristic “breathing mode” detectable by Raman spectroscopy in the place of the usual D peak associated with graphene. This mode has a distinctly different dispersion relation, which is easily measured using a typical benchtop Raman spectrometer. Boron-substituted graphitic materials were synthesized by the direct route in the temperature range of 750 °C to 1100 °C across a range of compositions and levels of structural order, as characterized by a complement of techniques: Raman spectroscopy, X-ray diffraction, X-ray absorption spectroscopy, energy dispersive X-ray spectroscopy, and Auger electron spectroscopy. The dispersion relation was shown to be directly correlated with the structure and composition of the resulting materials, culminating in the detection of extended BC3 units in samples of composition ~BC5 prepared at 800 °C.
NM07.05: Imaging, Spectroscopy, and Quantitative Analysis of 2D Materials II
Sunday PM, April 18, 2021
6:30 PM - *NM07.05.01
Manipulation of Magnetic Properties of van der Waals Crystals by Protons and Photons
Srinivasa Rao Singamaneni1
The University of Texas at El Paso1Show Abstract
van der Waals (vdW) engineering of magnetism is a topic of increasing research interest in the community at present. In the first part of my talk, I will present and discuss our recent efforts1 in manipulating the magnetic properties of quasi-two-dimensional layered vdW Mn3Si2Te6 (MST) crystals upon proton irradiation as a function of fluence 1×1015, 5×1015, 1×1016, and 1×1018 H+/cm2. We find that the magnetization is significantly enhanced by 53% and 37% in the ferrimagnetic phase (at 50 K) when the MST was irradiated with the proton fluence of 5×1015, both in ab and c plane, respectively. The ferrimagnetic ordering temperature and magnetic anisotropy retained even after proton irradiation.
In the second part of my talk, I will discuss on the electron spin resonance (ESR) properties2 of CrCl3 and CrI3 single crystals upon photo-excitation in the visible range. We noticed remarkable changes in the ESR spectra upon illumination. In the case of CrCl3, at 10 K, the ESR signal is shifted from g = 1.492 (dark) to 1.661 (light), line width increased from 376 to 506 Oe, and the signal intensity is reduced by 1.5 times. Most interestingly, the observed change in the signal intensity is reversible when the light is cycled on/off. We observed almost no change in the ESR spectral parameters in the paramagnetic phase (>20 K) upon illumination. Upon photo-excitation of CrI3, the ESR signal intensity is reduced by 1.9 times; the g-value increased from 1.956 to 1.990; the linewidth increased from 1170 to 1260 Oe at 60 K. These findings are discussed by taking into account the skin depth, the slow relaxation mechanism and the appearance of low-symmetry fields at the photo-generated Cr2+ Jahn-Teller centers. Such an increase in the g-value as a result of photo-generated Cr2+ ions is further supported by our many-body wavefunction calculations. This work has the potential to extend to monolayer vdWs magnets by combining ESR spectroscopy with optical excitation and detection. Our work shows that it is possible to employ proton and photon excitation in tuning the magnetic properties of vdW crystals, and provide many opportunities to design desired magnetic phases.
1L. M. Martinez, H. Iturriaga, R. Olmos, L. Shao, Y. Liu, Thuc T. Mai, C. Petrovic, Angela R. Hight Walker, and S. R. Singamaneni, Enhanced magnetization in proton irradiated, Mn3Si2Te6 van der Waals crystals, Appl. Phys. Lett. 116, 172404 (2020); doi: 10.1063/5.0002168
2S. R. Singamaneni, L. M. Martinez, J. Niklas, O. G. Poluektov, R. Yadav, M. Pizzochero, O. V. Yazyev, and M. A. McGuire, Light Induced Electron Spin Resonance Properties of van der Waals CrX3 (X = Cl, I) Crystals, Applied Physics Letters 117, 082406 (2020); doi: 10.1063/5.0010888.
6:55 PM - NM07.05.02
2D Confinement Heteroepitaxy Metals—Atomistic Insights of Interface Structure and Layer Intermixing
Hesham El-Sherif1,Natalie Briggs2,Joshua Robinson2,Nabil Bassim1
McMaster University1,The Pennsylvania State University2Show Abstract
Confinement heteroepitaxy (CHet) is a fabrication technique for 2D heterostructure in which atoms are intercalated between defective epitaxial graphene and substrates of silicon carbide in a CVD process typically at 800 °C. The confined layers can be designed with the merit of intercalation in several combinations, including metals, refractory, nitrides, and oxides. With all of these combinations, the CHet layers attract interest for many next-generation 2D devices in the field of photonics, plasmonic, hot-electron transistors, and optical polarization substrates.
Using a double-corrected aberration electron microscope and many FIB cross-section samples, we noted that the CHet layers’ structure is not a superficial metallic layer on a perfect 6H-SiC stacking. We found that the SiC terminations vary from terrace-to-terrace, including a significant 3C-SiC termination on the topmost layer. Accordingly, this termination variation is found to affect the Chet layers structure and physical properties. For example, the 2D metals – called half-Van der Waals metals –exhibit a non-centrosymmetric structure with a lateral direction variance of the staking order, structure, and the number of layers, which adds to the vertical direction bonding variance; from covalent (SiC) to metallic (Ga or In) to Van der Waal (Graphene). This makes the 2D CHet surfaces a candidate for generating non-linear optical response and implemented as Surface-enhanced Raman Scattering substrates.
Besides, we observed a picometer-to-angstrom lattice spacing variation at the Si-metal interface. We also investigated epitaxial graphene samples, just before the intercalation process, to understand the Si-graphene and Si-carbon buffer layer interface. Here, we could provide the first direct microscopic evidence that the SiC termination induces Si sublimation from the topmost layer through controlled electron beam damage experiments. Our TEM observations were also consentient with many previous theoretical expectations that explained epitaxial graphene growth due to a pico-scale corrugation in the Si-face atoms due to a (√3 × √3)R30° reconstruction on hexagonal (0001) 6H-SiC.
With these observations, we could explain anomalous but repeatable STEM-HAADF images of 2D Gallium and indium interfaces. These images show a significant EELS signal of Ga and In co-located with the topmost SiC layer in addition to Si and C signal within the 2D Ga and In layer. This interface mixing is observed to be limited to the topmost SiC layer but could be propagated into few layers in severe cases. The Si-metal interface instabilities are essential to be fully understood to optimize the growth conditions to avoid them. However, the presence of few doped Si and C atoms within 2D Ga and In could also be exciting if it induces extra orbital hybridization that may generate local and confined magnetism from the metallic layer, which we are currently investigating.
7:10 PM - NM07.05.03
Ambipolar Thermoelectric Measurements of Multilayer WSe2
Victoria Chen1,Hye Ryoung Lee1,Cagil Koroglu1,Connor McClellan1,Alwin Daus1,Eric Pop1
Stanford University1Show Abstract
Layered, two-dimensional (2D) semiconducting materials are promising candidates for thermoelectrics [1,2]. Among them, WSe2 has one of the lowest thermal conductivities , in part due to its heavier atoms, which is an important consideration. In addition, WSe2 is one of the few 2D materials that has been demonstrated as both n- and p-type (i.e. ambipolar) , thus satisfying the need of a thermoelectric generator with one material. Despite these promising material characteristics, only a few experimental studies on the thermoelectric properties of 2D WSe2 have been performed [5,6], and their dependence on thickness and temperature is still unknown to date.
In this work, we conduct thickness- and temperature- dependent in-plane thermoelectric measurements of WSe2 films. In order to tune the Fermi level and modulate the carrier density in the channel, we use an ionic-liquid, 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI), for electrolyte gating. WSe2 flakes are exfoliated onto a glass substrate and contacted from the top with 50 nm of evaporated Pd. By using ionic-liquid gating, we are able to sweep across a large range of carrier densities with relatively low applied gate bias. Furthermore, the low thermal conductivity of both the ionic-liquid and substrate contribute to a larger lateral temperature gradient along the WSe2, which is important for an accurate measurement of the Seebeck voltage. The thermoelectric measurements are conducted in vacuum, with on-chip Pd metal lines serving as resistive heaters and thermometers.
We sweep the gate voltage and measure the in-plane Seebeck coefficient (Sn,p) and electrical conductance of WSe2 flakes with thicknesses from ~10 to 90 nm between 296 to 400 K. At room temperature, we measure Sp up to 700 μV/K for holes and Sn down to -400 µV/K for electrons in the same device, which are some of the largest reported for sub-100 nm WSe2 [5,6]. In addition, we measure power factor values up to 50 µW/K2/cm, which are comparable to bulk Bi2Te3, a common commercially used thermoelectric material, at room temperature . We examine the trends of thermoelectric performance vs. electron and hole density as well as temperature and observe an increasing power factor with decreasing flake thickness. Overall, our results demonstrate the potential for WSe2 to eventually compete with bulk materials for use in thermoelectric energy harvesters and we contribute to further understanding the fundamental properties of WSe2.
 M. Dresselhaus et al., Adv. Mat., 19, 1043 (2007)
 J. Wu et al., Adv. Elec. Mat., 4, 1800248 (2018)
 A. Mavrokefalos et al., Appl. Phys. Lett., 91, 171912 (2007)
 S. Das et al., Appl. Phys. Lett., 103, 103501 (2013)
 J. Pu et al., Phys. Rev. B, 94, 014312 (2016)
 M. Yoshida et al., Nano Lett., 16, 2061 (2016)
 I Witting et al., Adv. Elec. Mat., 5, 1800904 (2019)
7:25 PM - NM07.05.05
Pyrene-tethered Poly(4-vinylpyridine) for Liquid Phase Exfoliation of Hexagonal Boron Nitride Allotropes and Their Thermal Conductive Nanocomposite
Hyeokjung Lee1,Seung Won Lee1,Chanho Park1,Kyuho Lee1,Cheolmin Park1
Yonsei University1Show Abstract
Owing to its favorable solution processability, the development of a stable dispersion of two-dimensional (2D) boron nitride (BN) has received significant attention for cutting-edge optic/electronic applications. Herein, we report an efficient method to disperse BN nanosheets (BNNSs) in polar solvents via dual noncovalent interactions using pyrene-tethered poly(4-vinylpyridine) (P4VP-Py). As a dispersion agent, P4VP-Py enables dual-functionalization with BNNS through π−π and Lewis acid−base interactions arising from the pyrene and pyridine moiety, respectively, resulting in highly stable BNNS dispersions in different solvents. The blend of P4VP-Py-functionalized BNNS with the pristine P4VP matrix resulted in increased thermal conductivity and dielectric constant combined with superior thermal stability by forming a compatible interface between the P4VP-Py matrix and the BNNS adjacent to the P4VP-Py. We demonstrated that dual noncovalent functionalization of BNNSs based on molecular design presents a strategy to achieve high dispersion of 2D materials into various media, ranging from polar solvents to solid matrices, for the expansion of advanced optic/electronic applications using BNNSs.
7:28 PM - NM07.05.06
ALD Growth of Wafer-Scale WS2 Film and Device Applications with Non-Toxic and Less Corrosive Precursors
Hanjie Yang1,Yang Wang1,Tao Chen1,Rongxu Bai1,Zecheng Wu1,Zhongya Pang2,Xingli Zou2,Hao Zhu1,Lin Chen1,Qingqing Sun1,Li Ji1
Fudan University1,Shanghai University2Show Abstract
Tungsten disulfide (WS2), as a member of two-dimensional transition metal dichalcogenides family, has attracted great attention due to its tunable band structure, excellent electrical transportation, unique optical properties and good air-stability. Despite the fact that WS2 is a potential candidate for next-generation channel materials, preparing wafer-scale WS2 film (8 inch or larger) with good uniformity and film quality remains challenging. Moreover, the bandgap of WS2 with different layer numbers was tunable, which implied the necessity to precisely control the thickness. Chemical vapor deposition (CVD) is a common method to achieve high quality WS2 films .However, it is difficult to synthesize 2-inch or larger size WS2 films with precisely controllable thickness. Atomic layer deposition (ALD) is an effective method to achieve wafer-scale WS2 film. The self-limiting growth mode enabled the thickness of WS2 film to be accurately controlled through changing the cycle numbers. The S sources of most ALD WS2 research works were H2S, which was highly toxic and dangerous. In addition, F-based precursor were used in many studies[2-5], resulting in an accelerated degradation of deposition system due to the highly corrosive nature of F-based precursors. In this work, low-toxic and less-corrosive tungsten hexachloride and hexamethyldisilathiane ((CH3)3SiSSi(CH3)3, HMDST) were used as W and S precursors, respectively, to achieve wafer-scale and thickness-controllable WS2 films. The annealing process was carried out to improve the crystallinity of WS2 film at 800oC for 2h. The XPS spectra, Raman spectra and plane-view and cross-sectional TEM indicated the good quality and controllable thickness of WS2 film. Moreover, arrays of field-effect transistors based on WS2 channel were further fabricated showing homogeneous electrical properties. The on/off ratio of WS2 FET was nearly 105 and the average electron mobility of WS2 FET was 3.5 cm2V-1s-1. Our results paved a new path to synthesize wafer-scale and thickness-controllable WS2 film in use of low-toxic and less-harmful precursors through ALD process, while the electrical properties demonstrated the attractive and promising potentials for WS2 films in future nano-electronic device integrations and applications.
 C.Y. Lan, Z.Y. Zhou, Z.F. Zhou, et al. "Wafer-scale synthesis of monolayer WS2 for high-performance flexible photodetectors by enhanced chemical vapor deposition", Nano Res., 2018, 11, (6), pp. 3371-3384.
 A. Delabie, M. Caymax, B. Groven, et al. "Low temperature deposition of 2D WS2 layers from WF6 and H2S precursors: impact of reducing agents", Chem. Commun., 2015, 51, (86), pp. 15692-15695.
 B. Groven, M. Heyne, A. Nalin Mehta, et al. "Plasma-Enhanced Atomic Layer Deposition of Two-Dimensional WS2 from WF6, H2 Plasma, and H2S", Chem. Mat., 2017, 29, (7), pp. 2927-2938.
 B. Groven, A.N. Mehta, H. Bender, et al. "Two-Dimensional Crystal Grain Size Tuning in WS2 Atomic Layer Deposition: An Insight in the Nucleation Mechanism", Chem. Mat., 2018, 30, (21), pp. 7648-7663.
 B. Groven, A.N. Mehta, H. Bender, et al. "Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates", J. Vac. Sci. Technol. A, 2018, 36, (1), pp. 11.
7:31 PM - NM07.05.07
An Effective Lattice Engineering Way of the Defect and Stacking Structure of Inorganic Nanosheets for Optimizing Their Electrode and Electrocatalyst Performances
Tae-Ha Gu1,Seong-Ju Hwang2
Ewha Womans University1,Yonsei University2Show Abstract
An effective chemical way to optimize the electrochemical functionalities of layered inorganic solids is developed by controlled restacking of exfoliated 2D nanosheet (NS). The fine-control of the stacking number and oxygen defect of restacked inorganic NSs can be achieved by employing diverse intercalants having various ionic sizes and charge densities. In contrast to conventional expectation, the supercapacitor electrode and electrocatalyst performances of inorganic NSs are well-correlated with their stacking numbers and oxygen defects, rather than with their basal spacings. The application of appropriate intercalant ion is quite effective in enhancing the electrode/electrocatalyst performances of restacked NSs via the creation of oxygen defect and the decrease of stacking number, resulting in improvement of catalysis kinetics and charge transfer property, and electrochemical surface area (ECSA). The present study highlights that the controlled restacking of exfoliated inorganic NSs can provide an effective way to optimize its electrochemical functionalities.
7:34 PM - NM07.05.08
Exfoliated g-C3N4 Nanosheet as an Emerging Cationic Building Block for 2D Superlattice Bifunctional Catalyst
Nam Hee Kwon1,Seong-Ju Hwang1
Yonsei University1Show Abstract
The interstratified superlattices of graphitic carbon nitride (g-C3N4)-molybdenum disulfide (MoS2) monolayers with improved bifunctional catalytic performance are synthesized by an electrostatically-driven self-assembly between oppositely-charged exfoliated nanosheets (NSs). In contrast to other exfoliated inorganic NSs, the cationic form of g-C3N4 NS can be obtained via the tuning of suspension pH, which is applicable as a new type of cationic building block for superlattice nanohybrids. The resulting superlattice nanohybrids display strong interfacial interaction between restacked g-C3N4 and MoS2 NSs, leading to creation of nitrogen vacancy, stabilization of 1T'-MoS2 phase, and efficient interfacial electronic transition between the interstratified NSs. The superlattice g-C3N4-MoS2 nanohybrids display remarkably enhanced bifunctionality as electrocatalysts for hydrogen evolution reaction and photocatalysts for visible light-induced N2 fixation with high selectivity. The beneficial effect of hybridization on catalyst performances is attributable to the promoted adsorption of H+/N2, the provision of many active sites, and the enhancement of charge transfer kinetics, the charge separation, and the visible light absorptivity. The present study highlights that the application of g-C3N4 NS as a cationic building block provides valuable opportunity to widen the library of multifunctional NS-based superlattice nanohybrids.
7:37 PM - NM07.05.10
Synthesis and Structural Characterizatioon of Graphene and Boron Nitride
Angela Luis Matos1,Vladimir Makarov2,1,Brad Weiner1,Gerardo Morell1
University of Puerto Rico at Río Piedras1,University of Puerto Rico2Show Abstract
Graphene and Boron nitride are 2D materials that has been widely investigated due to their excellent features and potential applications in optoelectronic devices because of their high absorbance and transmittance properties. Graphene has been grown by several methods such as chemical vapor deposition (CVD), chemical or plasma exfoliation from natural graphite, mechanical cleavage (exfoliation) from natural graphite, microwave synthesis, etc.
CVD has demonstrated to be the most remarkable process for large-scale graphene fabrication. When the thermal CVD process is carried out in a resistive heating furnace, it is known as thermal CVD, and when the process consists of plasma assisted growth, it is called plasma enhanced CVD or PECVD. In this work it is presented graphene growth by hot filament CVD. Here methane gas it is used as a carbon source which is decomposed with a filament at high temperature. In this technique it is possible to obtain a single to few layers graphene by adjusting the growth parameters. To characterize graphene one of the main techniques used is Raman spectroscopy. A nondestructive tool for investigating atomic vibrational properties. Using the important features such as the D, G and 2D band of graphene, can be distinguish the number of layers, the presence of defects, edge orientation and to monitor the temperature. In this work will be presented how the Raman spectra change depending on the substrate where graphene has been grown or transferred, as well as the dependence on the temperature of the G band in bilayer graphene. Also, hexagonal-boron nitride (h-BN) has been known as the best substrate dielectric for studying 2D physics of graphene and for the high-performance of graphene electronics, due to its atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. H-BN have been used to encapsulate graphene to avoid the impurities and as well to tailor the electronic properties of graphene. In this work, it will be presented the synthesis of h-BN and its structural characterization.
7:40 PM - NM07.05.12
Alkalide-Assisted Direct Electron Injection for the Non-Invasive N-Type Doping of Graphene
Sanghwan Park1,Chang Young Lee1
Ulsan National Institute of Science and Technology1Show Abstract
Although the doping of graphene grown by chemical vapor deposition is crucial in graphene-based electronics, non-invasive methods of n-type doping have not been widely investigated in comparison with p-type doping methods. We developed a convenient and robust method for the non-invasive n-type doping of graphene, wherein electrons are directly injected from sodium anions into the graphene. This method involves immersing the graphene in solutions of [K(15-crown-5)2]Na prepared by dissolving NaK alloy in 15-crown-5 solution. The n-type doping of the graphene was confirmed by down-shifted G and 2D bands in Raman spectra and by the Dirac point shifting to a negative voltage. The electron-injected graphene showed no sign of structural damage, exhibited higher carrier mobilities than that of pristine graphene, and remained n-doped for over a month of storage in air. In addition, we demonstrated that electron injection enhances noncovalent interactions between graphene and metallomacrocycle molecules without requiring a linker, as used in previous studies, suggesting several potential applications of the method in modifying graphene with various functionalities.
7:43 PM - NM07.05.13
Salt-Assisted Growth of 2D Transition Metal Dichalcogenides
National Institute for Materials Science1Show Abstract
Chemical vapor deposition (CVD) of 2D transition metal dichalcogenides (TMDCs) always involves the conversion of vapor precursors to solid products in a vapor-solid-solid (VSS) growth mode (e.g., WO3 + S → WS2 + SO2). This often requires very high temperatures to sublimate metal oxide precursors (e.g., WO3).
Our pioneering work on salt-assisted CVD (Salt 1.0 technique) enables the growth of 2D WS2 and WSe2 monolayers in a mild condition (lower temperature and atmospheric pressure) [1,2]. In the last five years, the use of alkali (alkaline earth) metal halides (AH, A = Li, Na, K, Ba, Ca; H = F, Cl, Br, I) in CVD has demonstrated great success in growing tens of atomically thin metal chalcogenides, graphene & h-BN monolayers [3,4]. This is due to the formation of volatile MOuClv and non-volatile NaMyOz when alkali (alkaline earth) metal halides react with metal oxides. They are highly efficient precursors for growing 2D TMDC monolayers.
The recent discovery and use of non-volatile molten salts in CVD (Salt 2.0 technique) trigger the vapor-liquid-solid (VLS) growth of 1D/2D TMDC monolayers . The Salt 2.0 technique shows great improvements in the high-efficient and reproducible growth of large-area, uniform, and high-quality 2D TMDC monolayers. The Salt 2.0 technique also demonstrates great potentials in growing 2D TMDC materials in the following aspects: wafer-scale single crystals, patterns, heterostructure, and alloys . It represents a new trend in the CVD growth of 2D TMDC materials.
 S. Li, S. Wang, D.-M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg and G. Eda, Appl. Mater. Today 1 60-66 (2015).
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 J. Zhou, J. Lin, X. Huang, Y. Zhou, Y. Chen, J. Xia, H. Wang, Y. Xie, H. Yu, J. Lei, D. Wu, F. Liu, Q. Fu, Q. Zeng, C.-H. Hsu, C. Yang, L. Lu, T. Yu, Z. Shen, H. Lin, B. I. Yakobson, Q. Liu, K. Suenaga, G. Liu and Z. Liu, Nature 556 355 (2018).
 C. Liu, X. Xu, L. Qiu, M. Wu, R. Qiao, L. Wang, J. Wang, J. Niu, J. Liang, X. Zhou, Z. Zhang, M. Peng, P. Gao, W. Wang, X. Bai, D. Ma, Y. Jiang, X. Wu, D. Yu, E. Wang, J. Xiong, F. Ding and K. Liu Nat. Chem. 11, 730 (2019).
 S. Li, Y.-C. Lin, W. Zhao, J. Wu, Z. Wang, Z. Hu, Y. Shen, D.-M. Tang, J. Wang, Q. Zhang, H. Zhu, L. Chu, W. Zhao, C. Liu, Z. Sun, T. Taniguchi, M. Osada, W. Chen, Q. Xu, A. T. S. Wee, K. Suenaga, F. Ding and G. Eda, Nat. Mater. 17 535 (2018).
 S. Li, Y.-C. Lin, X.-Y. Liu, Z. Hu, J. Wu, H. Nakajima, S. Liu, T. Okazaki, W. Chen, T. Minari, Y. Sakuma, K. Tsukagoshi, K. Suenaga, T. Taniguchi and M. Osada, Nanoscale 11 16122 (2019).
7:46 PM - NM07.05.14
Layer-Controlled Nb Doped MoS2 Thin Film with Wafer-Scaled Uniformity
Jae-Hwan Jung1,Ahrum Sohn1,Changhyun Kim2,Kyung-Eun Byun2,Yeonchoo Cho2,Hyeon Jin Shin2,Sang-Woo Kim1
Sungkyunkwan University1,Samsung Advanced Institute of Technology2Show Abstract
Recently, due to the physical limitations of silicon-based transistors, studies using 2-dimensional (2D) materials as next-generation transistors have become more active. Graphene, widely known as a representative next-generation 2D material, has attracted a great deal of attention due to its high electrical conductivity, thermal conductivity and stiffness coefficient. However, since it does have zero-bandgap, it is limited to use as a semiconductor. Molybdenum disulfide (MoS2), one of the TMD materials consisting of chalcogenide elements on both sides of the transition metal, has been reported to have n-type property, a direct band gap of 1.9 eV in a single layer and high absorptivity. Synthesis of MoS2 films with a different carrier type is a very desirable technology because it can overcome the scaling issues in the further CMOS technology. However, most doping techniques of MoS2 are difficult to satisfy well-controlled the number of layers, and large-scaled uniformity at the same time. Herein, we propose that an outstanding method with two steps enables us to obtain 2-inch wafer sized MoS2 thin film with controlled the number of layers and even metal-doped MoS2 with different carrier type in significantly short time. Various experiments have verified the quality of grown MoS2 including thickness-dependent electrical behavior, and especially an innovative TEM analysis has confirmed an existence of dopants regardless of an atomic size.
7:49 PM - NM07.05.15
Controlling Two-Dimensional Growth Through Interfacial Interactions and Dynamics
Maria Sushko1,Jinhui Tao1,Biao Jin1,Praveen Thallapally1,James De Yoreo1,Jun Liu1
Pacific Northwest National Laboratory1Show Abstract
Solution synthesis may prove to be one of the most scalable synthesis methods of 2D materials providing that the thermodynamic and kinetic drivers that determine the phase, nucleation, and growth kinetics, and the degree of perfection, can be understood and controlled. A combination of in situ characterization and simulations of crystallization pathways demonstrated the dependance of specific pathways on local chemical environments. Using well-defined model systems these studies identified the factors for particle morphology control during homogeneous nucleation and growth and highlighted the importance of interfacial precursor kinetics and surface stress in inducing asymmetric growth. In particular, fast ion deposition kinetics at twin planes was shown to promote the growth of nanoplates in pure precursor solution. In ligand assisted homogeneous and heterogeneous growth ion- and ligand-induced interfacial strain relief was found to control the growth of asymmetric nanoparticles and single-layer metal organic framework films, respectively. The detailed analysis of the dependence of crystallization pathways on synthesis parameters (pH, precursor and ligand concentrations, temperature) and the systematic approaches to control synthesis of 2D materials using homogeneous and heterogeneous solution-based synthesis will be discussed.
NM07.06: Imaging, Spectroscopy, and Quantitative Analysis of 2D Materials III
Monday AM, April 19, 2021
9:00 PM - NM07.06.02
Late News: Imaging Strain Fields in Moiré Heterostructures
University of California, Berkeley1Show Abstract
Strain plays a key role in defining both the electronic and chemical behavior or materials. The properties of two-dimensional (2D) materials are particularly susceptible to strain and the controllable creation and analysis of strain in such systems is critically important to understand their behavior. In recent years, moiré superlattices that are formed by a small lattice mismatch or the azimuthal misorientation (interlayer twist) of two 2D layers have emerged as some of the most fascinating platforms to probe a host of emergent phenomena. Twisted bilayer graphene, with one sheet twisted relative to another, results in a moiré superlattice with a wavelength that is inversely proportional to the twist angle. Because of the variation in stacking of AA vs AB/BA sites in the two graphene layers, the individual sheets strain from the graphene hexagonal lattice in order to minimize free energy. This talk will discuss our work to precisely map this atomic reconstruction and measure the resultant strain in twisted bilayer graphene using a new methodology called Bragg interferometry based on 4-dimensional scanning transmission electron microscopy (4D-STEM). Intralayer strain and reconstrcution mechanics are determined from mapping the displacement vectors between the graphene lattices by acquiring diffraction patterns at each position of the scanning electron probe. By quantitatively mapping strain tensor fields we uncover two distinct regimes of structural relaxation—in contrast to previous models depicting a single continuous process—and we disentangle the electronic contributions of the rotation modes that comprise this relaxation. Further, we find that applied heterostrain accumulates anisotropically in saddle point regions to generate distinctive striped shear strain phases. Our results thus establish the reconstruction mechanics underpinning the twist angle dependent electronic behaviour of twisted bilayer graphene, and provide a new framework for directly visualizing structural relaxation, disorder, and strain in any moiré material.
9:15 PM - NM07.06.03
Enhanced Electronic Quality of Monolayer WS2 via Re-Doping
Leyi Loh1,Michel Bosman1,Goki Eda1
National University of Singapore1Show Abstract
It is known that the electronic quality of monolayer transition metal dichalcogenides (TMDs) is strongly compromised by the presence of chalcogen vancancy defects. These defects are commonly present in densities of > 1013 cm-2 and result in unintentional n-type doping and sub-gap optical emission features. In this study, we report suppression of sulfur vacancy formation during chemical vapor deposition of monolayer WS2 by means of intentional in-situ Re-doping. Using aberration-corrected scanning transmission electron microscopy (STEM), we reveal that the sulfur vacancy density consistently decreases by up to 52% with increasing Re content. Interestingly, despite the possible electron-doping behavior of substitutional Re, we find the Re-doped WS2 to be less n-type, indicating the suppresion of unintentional doping effects of sulfur vacancies and high activation energy of Re impurities. We further show that substrate-induced exciton line broadening and sub-gap emission features are strongly suppressed in Re-doped samples, demonstrating their high electronic quality.
9:30 PM - NM07.06.04
Strategies to Induce Selectivity in 2D Material Chemical Sensors Through Integration with Covalent Organic Frameworks
Lucas Beagle1,2,Ly Tran1,2,Rahul Rao1,Luke Baldwin1,Nicholas Glavin1
Air Force Research Laboratory1,UES, Inc.2Show Abstract
Chemical and biological sensing using 2-D nanomaterials has been an area of intense investigation including inorganic materials that exhibit high sensitivity, however it has been limited as a field due to the lack of selectivity among analytes. While 2D inorganic materials can readily decipher between donor/acceptor groups, strategies to incorporate selectivity in these devices is crucial. Organic 2D materials known as covalent organic frameworks are large porous and repeated organic frameworks which have been shown to act as selectivity agents due to functionalization and porosity. This research focuses on the association of COFs with 2-D nanomaterials using top-down microwave-assisted activation of TMDs and deposition of COF particles. Several known COF species have been examined in order to understand the fundamental surface interactions between 2-D nanomaterials and COFs. Kinetics and mechanistic studies were used to determine the manner of association of the inorganic and organic substrates.
9:45 PM - NM07.06.06
Metal Atoms on WTe2—Surface Diffusion, Interactions with Defects and Clustering
Peter Sushko1,Zexi Lu1,Micah Prange1,Yang Wang1,Zdenek Dohnalek1
Pacific Northwest National Laboratory1Show Abstract
Electronic properties of 2-dimensional (2D) van der Waals materials can be modified by adsorbing and intercalating metal atoms. Controlled aggregation of isolated atoms into clusters and other low-dimensional features can be used to generate new functional properties with potential applications in electronic devices and energy storage and conversion. Our ability to utilize these possibilities are predicated on the development of new synthesis strategies that, in turn, require detailed understanding of diffusion and aggregation pathways of adsorbed species on surfaces of these materials.
Here we report on binding, diffusion, and aggregation of transition metal atoms (Pd, Cu, Fe) adsorbed on a prototype 2D transition metal dichalcogenide (TMD) WTe2 investigated using ab initio (density functional theory) simulations. While we find four distinct surface binding sites, the diffusion activation barriers between them differ dramatically, creating an asymmetry in surface migration. Metal dimers are only marginally more stable (~0.1 eV) than isolated adsorbed atoms; however, clusters of three atoms are stabilized by as much as ~0.4 eV. Further increase of the cluster size up to 11 atoms is not significantly favored on the non-defective surface, which suggests that such clusters can decompose under mild thermal treatment. In contrast, adsorption of metal atoms at the Te vacancy sites is strongly favored. This is particularly evident in the case of Pd, which is slightly more electronegative than Te. As the size of the vacancy bound Pd cluster increases, the amount of the electron charge transferred to the cluster from the surface Te increases as well. This charge redistribution is accompanied by Pd atoms displacing Te atoms from their lattice sites leading to significant lattice damage. Finally, we investigated pathways for Te substitution with the adsorbed metal species on pristine surfaces. Our results suggest that formation of transient Pd dimers and trimers can trigger Te-to-Pd charge transfer sufficient to destabilize surface Te atoms and induce Pd for Te substitution. These results are consistent with the formation of two types of small Pd clusters on WTe2 observed using scanning tunneling microscopy and their responses to thermal treatment. We discuss the similarities and the differences in behavior of the adsorbed metal species and their effect on the electronic properties of WTe2 as well as implications for functionalization of other TMDs.
10:00 PM - NM07.06.08
Evidence of Large Absorption in the UV-Vis Region from Chemically Exfoliated MoS2 Nanoparticles
Wafa Alnaqbi1,Juveiriah M. Ashraf1,Ayman Rezk1,Aisha Alhammadi1,Sabina Abdul Hadi2,Ammar Nayfeh1
Khalifa University of Science and Technology1,University of Dubai2Show Abstract
Molybdenum disulphide (MoS2) is a transition metal dichalcogenide (TMD) semiconductor that has unique optical and electronic properties making it attractive for future electronic and photonic devices . Due to the quantum confinement effect in 2D MoS2, it has a tuneable band gap which increases as the size decreases. Bulk MoS2 has an indirect band gap of ~1.2eV, while 2D MoS2 has a direct band gap of ~1.8eV. In this work, we present MoS2 nanoparticles (NPs) synthesized using a chemical exfoliation method and investigate the optical properties . First, we disperse 0.5 grams of MoS2 powder in 50 mL of N-Methyl-2-pyrrolidone (NMP). Next, sonication using a probe set at 50% amplitude, and 10/2 seconds duty cycle for 6 hours in an ice bath maintained at 0 °C. The dispersion is then centrifuged at 1500 rpm for 60 min to remove unexfoliated particles, then at higher speed (7500 rpm) for 30 min to remove soluble impurities from the dispersion. The NMP is then removed, and the MoS2 nanoparticles are filtered and re-dispersed in 25 mL of IPA. In order to first characterize the chemically exfoliated MoS2 (ce-MoS2) nanoparticles, we first deposit them on 3 cm x 3 cm pieces of fused silica by spin coating. Three coats of 100 µL were deposited with a total of 300 µL. The samples were left to dry for 1 hour between each coat at room temperature. The spin coating was conducted at 150 rpm for 40s. Optical microscopy of the samples under ultra-violet (UV) illumination showed a homogenous distribution of the MoS2 NPs, exhibiting a red photoluminescence (PL) which is correlated to the quantum-confinement effect in MoS2 at the nanoscale where direct band gap transitions can occur with a stable tunable PL depending on the NPs size. The films showed a smooth layer at low particle densities with cluster sizes below the resolution of the microscope (< 300 nm). At higher particle densities, larger clusters begin to form, with an approximate size of 1 μm and above, with a majority of clusters in the range of 2-5 μm in diameter.
Both reflection and transmission spectra are measured after each coat using a UV–VIS spectrophotometer (Lambda 850, PerkinElmer) to investigate the behaviour of MoS2 NPs, and the effect of the number of coats applied on the reflectance of the substrate. The corresponding reflection spectra were captured as a function of the incident wavelength, ranging from 250 nm to 1500 nm. Coated samples show a severe reduction in transmission to almost ~0% at UV-Vis (250 to 850 nm) and ~ 10% at NIR (850 to 1500 nm) after three coats. However, reflectance was around ~20% for UV-Vis and increased to ~50% at NIR. Which got absorption to increase from ~0% to ~80% at UV-Vis and ~40% at NIR after the third coat. We also deposited the MoS2 on low temperature PECVD Ge films grown on (100) P doped (n-type) Si described in detail in our previous work . A large drop in reflectance below 800 nm is seen which can be attributed to absorption in the MoS2 nanoparticles. Above 800 nm, reflectance increases. This asymmetrical behaviour, which will be investigated further, can be explained by increased scattering in the non-absorption region. To summarize, in this work, we investigate the optical properties of chemically exfoliated MoS2 NPs on fused silica and PECVD Ge films. The results show a large reduction in reflectance due to absorption in the MoS2 in the UV-Vis region. Moreover, these results show that MoS2 can have potential applications in photonics and PV-based devices.
 Singh, E., Singh, P., Kim, K. S., Yeom, G. Y., & Nalwa, H. S. (2019). Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics. ACS Applied Materials & Interfaces, 11(12), 11061-11105.
 Ghada H. Dushaq, Mahmoud S. Rasras, Ammar M. Nayfeh, "Low temperature deposition of germanium on silicon using Radio Frequency Plasma Enhanced Chemical Vapor Deposition," Thin Solid Films, page 585-592, 2017
Thomas Kempa, Johns Hopkins University
Zakaria Al Balushi, University of California, Berkeley
Ying Fang, National Center for Nanoscience and Technology
Deep Jariwala, University of Pennsylvania
NM07.07: Fundamental Properties of 2D Materials and Heterostructures I
Monday AM, April 19, 2021
8:05 AM - *NM07.07.01
Electrical and Chemical Control of Magnetism in van der Waals Ferromagnetic Materials
National University of Singapore1,Centre for Advanced 2D Materials2Show Abstract
Recent discoveries of gate-tunable magnetism in ferromagnetic two-dimensional (2D) materials such as CrI3and Fe3GeTe2 highlight the unique potential of this class of materials for novel spintronic devices. Understanding the interplay between magnetic order, free electron density, and electric field in the 2D limit is essential for uncovering the full potential of these materials. In this talk, we will first discuss electrical control of magnetism in Cr2Ge2Te6 (CGT), a van der Waals ferromagnetic semiconductor, in an electric double-layer transistor (EDLT) geometry. We show that degenerately electron-doped CGT exhibits enhanced Curie temperature of up to 200 K, and rotation of its easy axis from out-of-plane to in-plane orientation. We demonstrate that similar changes in Curie temperature and easy axis can be induced by Na intercalation, which is accompanied by heavy electron doping of the host. We will further discuss our exploration of magnetism in other van der Waals materials such as NbFeTe2 and magnetically doped transition metal dichalcogenides.
8:30 AM - NM07.07.02
Thickness-Dependent Ambient Effects on the Curie Temperatures and Magnetic Domains of Metallic Two-Dimensional Magnets
Cheng Gong1,Ti Xie1,Yeonghun Lee2,Jinling Zhou1,3,Alemayehu S. Admasu4,Nagarajan Valanoor3,John Cumings1,Sang-Wook Cheong4,Ichiro Takeuchi1,Kyeongjae Cho2
University of Maryland1,The University of Texas at Dallas2,University of New South Wales3,Rutgers, The State University of New Jersey4Show Abstract
The emergent magnetic two-dimensional (2D) materials provide ideal solid-state platforms for a broad range of applications including miniaturized spintronics and magnetoelectric sensors. Owing to the general environmental sensitivity of 2D magnets, the understanding of ambient effects on 2D magnetism is critical. Apparently, the nature of itinerant ferromagnetism potentially makes metallic 2D magnets insensitive to environmental disturbance. Nevertheless, our systematic study showed that the Curie temperature of metallic 2D Fe3GeTe2 decreases dramatically in the air but thick Fe3GeTe2 exhibits self-protection. Remarkably, we found the air exposure effectively promotes the formation of multiple magnetic domains in 2D Fe3GeTe2, but not in bulk Fe3GeTe2. Our first-principles calculations support the scenario that substrate-induced roughness and tellurium vacancies boost the interaction of 2D Fe3GeTe2 with the air. Our elucidation of the thickness-dependent air-catalyzed evolution of Curie temperatures and magnetic domains in 2D magnets provides critical insights for chemically decorating and manipulating 2D magnets.
8:45 AM - NM07.07.03
Atomic-Scale Understanding of Defect Dynamics in Phosphorene Under Ion Bombardment
Saransh Gupta1,Prakash Periasamy2,Badri Narayanan1
University of Louisville1,LAM Research Corporation2Show Abstract
The unique opto-electronic properties of two-dimensional monolayer of phosphorus (namely phosphorene) makes them lucrative for a variety of applications in nano-scale electronic devices, including field-effect transistors, photovoltaic junctions and thin-film solar cells. In particular, the lure of phosphorene arises from its semiconducting nature, thickness-dependent electronic band gap (1.5 eV in monolayer to 0.3 eV in bulk), high hole mobility (~105 cm2/V/s), and large drain current modulation. The properties of phosphorene can be further engineered by precise introduction of defects via ion bombardment or electron-irradiation. For instance, isolated mono-vacancies can induce hole doping as well as local magnetic moments; which can be leveraged to achieve specific functionality in opto-electronic devices. Despite this promise, precise defect engineering of phosphorene via ion/electron beams has remained challenging due to lack of fundamental understanding of the atomic-scale mechanisms underlying production, accumulation, and subsequent annealing of defects during irradiation. Indeed, previous first-principles modeling and electron microscopy studies provide insights into the atomic structure of defects, their thermodynamic stability, and threshold ion energy required to produce them. However, the effect of dose on defect accumulation, atomistic details of temporal evolution of defects, and defect dynamics under irradiation as well as subsequent annealing are largely unclear. Here, we employed reactive molecular dynamics (MD) simulations to unravel the dynamical atomic-scale processes underlying formation, accumulation, and re-organization/annealing of defects during Ar-ion bombardment of phosphorene. We found that radiation dosage strongly influences the type of defects that form, as well as their subsequent annealing. At low doses (1013 Ar+ions/cm2 at 25 keV), the predominant defects that form are isolated mono-vacancies or Stone-Wales type defects that largely retain the planarity of the sheet. Interestingly, even small voids (1-2 nm) that form at these low doses can be healed by subsequent annealing; such healing proceeds via local re-arrangement of rings. Beyond a critical dosage (1014 Ar+ ions/cm2 at 25 keV), large nanopores form whose edges are stabilized by formation of 3D structures consisting of P4 tetrahedra. During subsequent annealing, these 3D structures facilitate the coalescence of nanopores, which further degrades the structure. These results will be discussed in the context of designing novel routes to precisely tune opto-electronic properties of phosphorene-based devices using ion beams.
8:50 AM - NM07.07.08
Spatial Defects Nanoengineering for Bipolar Conductivity in MoS2
University of Barcelona1,Institute for Bioengineering of Catalonia2Show Abstract
Two-dimensional transition metal dichalcogenides show great potential as a new class of atomically thin semiconductors for electronics and optoelectronics. Understanding the atomistic origin of defects in these materials and their impact on the electronic properties, as well as finding viable ways to dope them is matter of intense scientific and technological interest. In particular, controlling defects could be envisioned as a strategy for the design of ad-hoc electronic and optoelectronic properties. Here, we demonstrate a new integration of thermochemical scanning probe lithography (tc-SPL) with a flow-through reactive gas cell to achieve a nanoscale control of the local thermal activation of defects in monolayer MoS2. The tc-SPL activated nanopatterns can present either p- or n-type doping on demand, depending on the used gasses, allowing the realization of field effect transistors, and p-n junctions with precise sub-mm spatial control and a rectification ratio over 104. Doping and defects formation mechanisms are elucidated at the molecular level by means of X-Ray photoelectron spectroscopy, scanning transmission electron microscopy, and density functional theory. The p-type doping of locally heated MoS2 in HCl/H2O atmosphere is found to be related to the rearrangement of sulfur atoms and the formation of new protruding covalent S-S bonds on the surface, which produce a band structure with p-character. Alternatively, local heating MoS2 in N2 produces n-character.
NM07.08: Fundamental Properties of 2D Materials and Heterostructures II
Zakaria Al Balushi
Monday PM, April 19, 2021
10:30 AM - *NM07.08.01
TMD Alloys—Phase Diagrams, Synthesis, Optical and Electrical Properties
National Institute of Standards and Technology1Show Abstract
10:55 AM - NM07.08.02
Strong and Flaw-Insensitive Two-Dimensional Covalent Organic Frameworks
Qiyi Fang1,Chao Sui1,Chao Wang1,Tianshu Zhai1,Jing Zhang1,Jia Liang1,Hua Guo1,Emil Sandoz-Rosado2,Jun Lou1
Rice University1,U.S. Army Research Laboratory2Show Abstract
Recently, two-dimensional covalent-organic-frameworks (COFs), an structural analogue of graphene, have attracted great interests because of their tailorable structures lacking in other 2D materials. The topological design principles of monomers enable researchers to control the pore size and the unit cell geometry of 2D COFs allowing for highly tunable functionalities and properties. The unique structural features make 2D COFs possess exciting multifunctional properties with broad applications in liquid/gas separation, water filtration, energy storage/conversion, catalysis and ionic conduction, etc. However, the understanding of its mechanical properties and fracture mechanisms remains elusive. Here we report a quantitative in-situ tensile study of ultrathin COFs films. The fracture strength was measured to be 0.75±0.34 GPa, and the tensile modulus was measured to be 10.38±3.42 GPa, with a nominal density of 0.393 g/cc, thus having specific strength equivalent to Kevlar(2 GPa cc/g), and specific modulus comparable to titanium alloys (23 GPa cc/g). Additionally, the fracture toughness was measured to be 0.55±0.09 MPa√m, and it was found that the crack propagation could be insensitive to the pre-crack when the size of pre-crack is below a critical value, leading to intriguing flaw insensitivity in such ultrathin nanomaterials. This work provides in-depth insights into the fracture properties of 2D COF films and lays a foundation for their future applications.
11:10 AM - NM07.08.03
Recycling of Two-Dimensional Materials for van der Waals and Remote Epitaxy
Jinkyoung Yoo1,Dongheun Kim1,Yeonhoo Kim1,Eric Auchter1,Enkeleda Dervishi-Whetham1
Los Alamos National Laboratory1Show Abstract
The absence of surface dangling bonds on two-dimensional (2D) materials mitigates materials compatibility issues of heterostructuring. Conventional materials synthesis on 2D materials have been prepared by van der Waals (vdW) and remote epitaxy techniques. In principle conventional or other 2D materials epitaxially grown on a 2D materials can be feasibly delaminated from the 2D layer as a substrate via applying mechanical force. Previous reports have shown that semiconductor nanomaterials and thin films for device applications can be prepared on 2D materials, especially graphene. Then the grown materials were detached from the host substrate, and applied for flexible devices. However the 2D layer in between the grown materials and the host substrate has not been thoroughly investigated. The 2D layer as a substrate or interlayer for vdW and remote epitaxy techniques has been considered as damaged. Thus there was no systematic characterizations of 2D layer after the epitaxy procedures and no attempt to recycling the 2D layer multiple times.
In the presentation recycling graphene and transition metal dichalcogenides layers for vdW and remote epitaxy will be demonstrated and discussed. Surface morphology and optical characteristics of the recycled 2D layers along repetition of the vdW and remote epitaxy were studied. Moreover the electrical/optical characterizations of semiconductors/2D heterostructures were performed to obtain insights of 2D/conventional materials interfacial properties.
11:25 AM - NM07.08.04
Optimizing the Schottky Barrier at Metal-MoS2 Junctions Through Metal Selection and Annealing
Meghan Bush1,Timothy Ismael1,Kazi Islam1,Claire Luthy1,Matthew Escarra1
Tulane University1Show Abstract
Monolayer molybdenum disulfide (MoS2) is an attractive semiconductor for nano-optoelectronic devices due to its direct bandgap, open-air stability, and potential for large-scale synthesis. To optimize the electrical performance of these devices for applications such as photovoltaics, the relationship between the semiconductor and metal contacts must be fully understood and optimized.
The Schottky barrier height, denoted as ΦB, is a potential energy barrier that exists at the junction of a metal and a semiconductor due to the difference in work functions. This directly impacts device performance, as a higher barrier decreases minority carrier transport since electrons cannot easily flow back into the semiconductor. We report here the experimental measurements of ΦB between monolayer MoS2 and a number of metals with varying work functions and process conditions. Large-area monolayer MoS2 is grown using chemical vapor deposition (CVD) and metal contacts are deposited using electron beam evaporation on patterns written by electron beam lithography.
Different metals have different electrical performances and Schottky barriers resulting from their interaction with a semiconductor. The metals used for contacts are Ti and Sc (low work functions) and Pt (high work function). Here, the barrier height is extracted using the thermionic emission model, which implies that the Schottky barrier is temperature dependent. With an applied source-drain bias of 1 V, a source-drain sweep is taken for the nanoscale transistors over the gate voltage range -30 V to 30 V. These sweeps are repeated at 25 K temperature increments over 100-400 K. This data is plotted on an Arrhenius plot, with logarithmic gate voltages on the y-axis and the inverse of temperature on the x-axis. The initial linear slopes are related to the barrier height using the thermionic emission model, which results in an approximate barrier vs gate voltage plot from which the actual Schottky barrier height is extracted.1,2 So far, we have experimentally determined the Schottky barrier height of titanium, ΦTi = 0.21. We will be repeating this process with the other metal contacts listed above to fully document how our CVD grown 2D MoS2 interacts with these metals.
Thermal treatments have been used for decades to improve electrical performance in traditional III-V solar cells and semiconductor devices. Building on this, we posit that there are optimal annealing parameters that can improve the desired transport properties in electron and hole-selective Schottky barriers between 2D materials and metals. This annealing is done using a Rapid Thermal Annealer (RTA) to expose the device to high temperatures for short periods of time. Since RTA is scalable, it directly translates to manufacturing for large area 2D PV. By comparing the Schottky barrier heights without annealing and after various annealing conditions, we will establish a better understanding of how this process impacts the metal-semiconductor junction. The ability to tune the barrier height allows devices to be further customized with improved performance.
Understanding the barrier of the MoS2-metal junction will allow for more complex devices to be designed and fabricated, including but not limited to photovoltaics, photo-emitters, photodetectors, etc. Future work entails optimizing device preparation and treatment to result in Schottky barriers for different 2D electronic applications. These measured barrier heights are also being included in a computational device model to study various MoS2-based optoelectronic devices.
1 Das, S., Chen, H., Penumatcha, A. V., & Appenzeller, J. (2012). High Performance Multilayer MoS2 Transistors with Scandium Contacts. Nano Letters, 13(13), 100-105
2 Wang, W., Liu, Y., Tang, L., Jin, Y., Zhao, T., & Xiu, F. (2014). Controllable Schottky Barriers between MoS2 and Permalloy. Scientific Reports, Sci Rep 4, 6928 (2014)
11:30 AM - NM07.08.06
Collective Modes in Substitutionally Doped 2H-NbS2
Amin Azizi1,Mehmet Dogan1,2,Jeffrey Cain1,2,Kyunghoon Lee1,2,Xuanze Yu1,Marvin Cohen1,2,Alex Zettl1,2
University of California, Berkeley1,Lawrence Berkeley National Laboratory2Show Abstract
Metallic transition-metal dichalcogenides (TMD) are rich material systems in which the interplay between strong electron-electron and electron-phonon interactions results in a variety of instabilities such as charge density waves (CDWs). While most metallic TMDs exhibit coexistence of superconductivity and CDWs, 2H-NbS2, with a superconducting transition temperature (TC) of ~6K, shows no CDW order. Most recently, the existence of an incommensurate CDW pinned by atomic impurities (such as vacancies) has been reported in 2H-NbS2. Substitutional dopants, depending on their atomic number and electronic state, can considerably affect electronic behavior in these systems.
Here, we substitutionally dope 2H-NbS2 with heavy atoms via chemical vapor transport. Using aberration-corrected scanning transmission electron microscopy (STEM) imaging, we confirm the 2H phase of NbS2 and determine the distribution and concentration of dopant atoms. Using a combination of low temperature transport measurements and density functional theory calculations, we demonstrate how heavy dopant atoms can modify the collective electronic states (such as superconductivity and CDW) in 2H-NbS2.
11:45 AM - NM07.08.07
Late News: Exfoliation and Optical Properties of Near-Infrared Fluorescent Silicate Nanosheets
Gabriele Selvaggio1,2,Milan Weitzel2,Nazar Oleksiievets2,Tabea Oswald2,Robert Nißler1,2,Ingo Mey2,Volker Karius2,Jörg Enderlein2,Roman Tsukanov2,Sebastian Kruss1,2,3
Ruhr University Bochum1,University of Göttingen2,Fraunhofer Institute for Microelectronic Circuits and Systems3Show Abstract
The near-infrared (NIR) region of the electromagnetic spectrum presents optimal characteristics for imaging of complex (biological) samples due to reduced light scattering, absorption, phototoxicity and autofluorescence. However, despite their clear potential over commonly employed visible fluorophores, only few NIR fluorescent materials are known so far, and even less are suitable for biomedical applications. For this reason, it is of great interest to identify novel NIR fluorophores and use them for biomedical applications.
Here, we exfoliated a class of layered silicates: Egyptian Blue (CaCuSi4O10, EB), Han Blue (BaCuSi4O10, HB) and Han Purple (BaCuSi2O6, HP). These pigments fluoresce in the NIR (λemi ≈ 920-950 nm) with a long excited state lifetime (τ ≈ 10-100 µs). By milling, tip sonication and several centrifugation steps, small and monodisperse 2D nanosheets (NS) could be exfoliated. The morphology of such NS was then fully characterized by atomic force (AFM) and scanning electron microscopy (SEM). Most interestingly, the intense NIR fluorescence emission of the bulk counterparts is retained in these nanostructures. In comparison to state-of-the-art fluorophores, EB-NS show no bleaching while displaying outstanding fluorescence intensity. These qualities of EB-NS enabled us to inject them into systems of biological relevance such as developing Drosophila embryos, and perform in vivo single-particle tracking and microrheology measurements. Furthermore, because of their high biocompatibility, it is possible to use them for NIR imaging in plants.  The above mentioned three silicate NS are very bright and can be imaged through several cm of tissue phantoms, which demonstrates the potential for (bio)photonics. Finally, we used the microsecond fluorescence lifetimes of EB-NS, HB-NS and HP-NS for micro- and macroscopic fluorescence lifetime imaging (FLIM). The results show that lifetime engineering of these silicate nanostructures is possible and can be used for lifetime-encoded imaging. 
In summary, we present a new exfoliation route that yields NIR fluorescent nanosheets with high potential for bioimaging and photonics.
 G. Selvaggio et al., Nat. Commun. 11, 1495 (2020)
 G. Selvaggio et al., https://doi.org/10.26434/chemrxiv.13350728.v1 (2021)
12:00 PM - NM07.08.08
Late News: Strain Tuning of the Optoelectronic Properties of Two-Dimensional Crystals
Elena Blundo1,Marzia Cuccu1,Pettinari Giorgio2,Cinzia Di Giorgio3,Tanju Yildirim4,Paulo E. Faria Junior5,Marco Felici1,Fabrizio Bobba3,Antonio Polimeni1
Sapienza, University of Rome1,National Research Council2,University of Salerno3,National Institute for Materials Science4,Universität Regensburg5Show Abstract
The variegated family of two-dimensional materials comprises crystals that have attracted great interest for diverse characteristics and peculiarities. Among them, semiconducting transition-metal dichalcogenides (TMDs) possess alluring optoelectronic and spin properties when reduced to the single layer. In particular, TMD monolayers are characterised by a direct bandgap, resulting in an efficient light emission in the visible/infrared range, which renders them appealing for optoelectronic devices. Furthermore, a strong spin-orbit coupling makes them interesting candidates for valley- and spin-tronics. Indeed, the inherent plane-confined nature of these materials -coupled to their exceptional mechanical flexibility and robustness- makes them highly sensitive to external stimuli. Methods to tailor their unique properties on demand have been thus sought after, and protocols based on controllable external perturbations such as mechanical deformations have shown promise in this respect.
Here, we present a novel technique to induce controllable strain-fields in TMD monolayers and study their effect on the optical and spin properties. Localised strains are created via low-energy hydrogen-ion irradiation of bulk TMDs, leading to the production and accumulation of molecular hydrogen in the first interlayer region. The trapped gas coalesces, leading to a local blistering of the material, and thus to the formation of one-monolayer-thick micro/nano-bubbles, which stud the crystal surface and locally turn the dark bulk material into an efficient light emitter.  Electron-beam-lithography-based approaches allowed us to achieve control over the formation process of the bubbles, and to create them in ordered arrays and with the desired dimensions (from few tens of nm to few microns). [1-2] These bubbles are durable and incredibly robust  and host complex strain fields, evaluated numerically, that cause dramatic changes in the TMD optoelectronic properties. Photoluminescence steady-state and time-resolved studies enabled the characterisation of the strain-induced band-structure modifications and revealed intriguing phenomena, such as bandgap crossovers enabling the creation of exciton states with long lifetimes.  Magneto-optical experiments allowed us to achieve unprecedented information on the spin and valley properties of strained TMDs, and led to the observation of hybridisation mechanisms between different band states.
Recently, the interest for TMD monolayers has surged due to the possibility to stack them via weak van der Waals forces to create heterostructures. TMDs have also been coupled with other semiconducting two-dimensional materials, such as post-transition-metal chalcogenides. Novel excitonic states -referred to as interlayer excitons- have been observed in TMD-based heterostructures, in which the two oppositely charged carriers forming the exciton are localised in different monolayers. Here, we exploit the same approach used for the formation of single monolayer bubbles to strain-tune the electronic properties of novel van der Waals heterostructures, and show how strain can play a crucial role by changing the relative band alignment of the stacked materials.
Our results unveil unprecedented information on the strain effects on TMDs and their heterostructures, the understanding of which represents an essential step towards their integration into flexible electronic devices.
 D. Tedeschi, E. Blundo, et al., Adv. Mater. 31, 1903795 (2019).
 E. Blundo et al., Adv. Mater. Interfaces 7, 2000621 (2020).
 C. Di Giorgio et al., Adv. Mater. Interfaces 7, 2001024 (2020).
 E. Blundo et al., Phys. Rev. Res. 2, 012024 (2020).
NM07.09: Fundamental Properties of 2D Materials and Heterostructures III
Zakaria Al Balushi
Monday PM, April 19, 2021
1:00 PM - NM07.09.01
Late News: Zero-Dimensional Graphene and Its Behavior During Mechanochemical Activation of Nanoferrites
Monica Sorescu1,Alice Perrin2,Michael McHenry3
Duquesne University1,MIT2,Carnegie Mellon University3Show Abstract
Nickel ferrite nanoparticles were subjected to mechanochemical activation for ball milling times ranging from 0 to 12 hours. The milling was performed with and without the addition of equimolar concentrations of graphene nanoparticles. Characterization of resulting nano-powders was undertaken by Mossbauer spectroscopy and magnetic measurements. The hyperfine magnetic field was studied as function of milling time for octahedral and tetrahedral sites. An additional quadrupole split doublet represented the occurrence of superparamagnetic particles in the as-obtained and milled specimens. A new phase was obtained in the graphene-milled set of samples, which could be assigned to carbon-rich particles. The degree of inversion and canting angle were derived from the Mossbauer measurements and studied as function of ball milling time. The degree of inversion was found to decrease with milling time, especially for the set without graphene and evidenced a transition from inverse to normal spinels. The canting angle decreased with time for the graphene milled nanoparticles. The recoilless fraction was determined as function of milling time and was consistent with the observation – for the first time in literature – of a distribution of recoilless fractions in the studied specimens. The saturation magnetization, remanence magnetization and coercive field were derived from the hysteresis loops, recorded at 5K and 5T. The zero-field-cooling-field-cooling measurements were obtained in a magnetic field of 200 Oe and the blocking temperature was determined. Our results show new features of the behavior of nickel ferrite nanoparticles under mechanochemical activation with and without graphene.
Cobalt ferrite nanoparticles were exposed to mechanochemical activation, with and without equimolar amounts of graphene nanoparticles, for time periods ranging from 0 to 12 hours. Their structural and magnetic properties were detailed from Mossbauer spectroscopy and magnetic measurements. The Mossbauer spectrum corresponding to the unmilled cobalt ferrite powder was analyzed using 2 sextets, corresponding to the tetrahedral and octahedral sites of ferrites. The rest of the spectra was deconvoluted using an additional quadrupole-split doublet, with an abundance close to 30% and was assigned to superparamagnetic particles. Moreover, the spectra corresponding to milling with graphene at the longest times needed a third sextet, which could be assigned to iron carbide. The degree of inversion was determined from the Mossbauer spectra and found to decrease with milling time, both for the set with and that without graphene. The canting angle was derived and studied as function of the ball milling time for both sets of samples. Hysteresis loops were recorded at 5 K in an applied magnetic field of 5 T and was found to exhibit a constricted shape. Magnetization was plotted as function of temperature in the range 5-300 K with an applied magnetic field of 200 Oe using zero-field-cooling-field-cooling (ZFC-FC) measurements. These made it possible to determine the blocking temperature of the samples. Our data exhibit new characteristics of the cobalt ferrite nanopowders milled with and without graphene nanoparticles.
1:15 PM - NM07.09.02
Acoustic Transport of Room Temperature Excitons in Monolayer WSe2
Kanak Datta1,Zidong Li1,Zhengyang Lyu1,Takashi Taniguchi2,Kenji Watanabe2,Parag Deotare1
University of Michigan–Ann Arbor1,National Institute for Materials Science2Show Abstract
In recent years, monolayer transition metal dichalcogenides (TMD) have garnered widespread interest from the research community across the world due to their exciting optical properties and strong light-matter interaction. As the optical properties of these materials are extremely sensitive to external mechanical stimuli, controlled static strain has been successfully applied to tune the light-matter interaction in these materials . However, the interaction of excitons with the travelling strain and piezoelectric field generated by surface acoustic wave remains widely unexplored. Here, we experimentally investigate exciton transport in monolayer WSe2 under surface acoustic wave.
We generate high-frequency Rayleigh type surface acoustic wave (SAW) by patterning interdigitated electrodes (IDTs) on 1280 Y-cut piezoelectric lithium niobate (LiNbO3) substrate using standard photolithography methods. The SAW resonators are designed for a resonance frequency of 600 MHz (SAW wavelength ~ 6 μm). We characterize the transport of excitons in hexagonal boron nitride (hBN) encapsulated WSe2 monolayer using phase locked time correlated single photon counting (TCSPC). Upon RF excitation the travelling hydrostatic strain wave of the SAW modulates the conduction and valance band of the monolayer WSe2 in opposite phase resulting in localized potential pockets, commonly referred as type – II bandgap modulation . The resulting potential pockets can capture and result in long-range transport of excitonic species.
We observed strong coupling of monolayer exciton species to the travelling strain field generated by the surface acoustic wave that resulted in remote recombination of the transported excitons. Using measured exciton lifetime of 1.27 ns, we estimate the exciton velocity under acoustic modulation to be 0.78x103 ms-1. Using experimentally reported value of strain mobility in monolayer WSe2 , we estimate the hydrostatic strain on the monolayer to be 0.4%. Our observations show that the transport distance is mostly limited by the intrinsic exciton mobility and lifetime in monolayer WSe2.
 C. Martella, C. Mennucci, A. Lamperti, E. Cappelluti, F. B. de Mongeot, and A. Molle, Adv. Mater. (2018).
 J. Rudolph, R. Hey, and P. V. Santos, Phys. Rev. Lett. 99, 1 (2007).
 D. F. Cordovilla Leon, Z. Li, S. W. Jang, C. H. Cheng, and P. B. Deotare, Appl. Phys. Lett. (2018).
1:20 PM - NM07.09.04
Late News: Structural and Spectroscopic Investigation of Layered Semiconductor-Iron Phosphorus Trisulfide-FePS3
Adam Budniak1,Szymon Zelewski2,Magdalena Birowska3,Tomasz Wozniak2,Tatyana Bendikov4,Yaron Kauffmann1,Yaron Amouyal1,Robert Kudrawiec2,Efrat Lifshitz1
Technion - Israel Institute of Technology1,Wroclaw University of Science and Technology2,University of Warsaw3,Weizmann Institute of Science4Show Abstract
Binary layered semiconductors, especially transition metal dichalcogenides (TMDs), are among the most studied van der Waals (vdW) semiconductors. Recently ternary layered materials are drawing more attention due to their attractive properties.
Amid ternary vdW semiconductors, iron phosphorus trisulfide - FePS3 - is an interesting material. It belongs to a family of transition metal phosphorus trisulfides (TMTs) with general formula MPS3. FePS3 is magnetic semiconductor, absorbing in near IR, and it is also relatively stable in exfoliated form. Large FePS3 crystals (almost centimeter size) have been obtained in big amounts (more than 70% chemical yield) via chemical vapor transport (CVT). Bulk material is investigated by X-Ray and ultraviolet photoelectron spectroscopy (XPS and UPS). Optical properties are carefully studied with temperature change. The results are further corroborated with DFT calculations. FePS3 is both mechanically and liquid exfoliated and obtained products are compared by X-Ray diffraction (XRD) and then by conventional transmission electron microscopy (cTEM), carefully proceeded for chosen crystallographic directions with high-resolution analysis for both kinds of samples. Here, a simple, one-step protocol for mechanical exfoliation directly onto transmission electron microscope grid is used as an efficient sample preparation method.[1,2,3] Finally, atomic resolution elemental maps are registered with high-resolution scanning transmission electron microscope (HR-STEM) and reveal directly the atom columns arrangement.
At last, the large-scale project will be briefly described, where vdW materials are used for the first time for tunable X-Ray emission. The X-Ray radiation is produced from two combined mechanisms [called parametric coherent bremsstrahlung (PCB) collectively]: parametric X-ray radiation (PXR) and coherent bremsstrahlung (CBS). FePS3 with other ternary vdW materials CrPS4, MnPS3, CoPS3, NiPS3, and also binary WSe2, are mechanically exfoliated and transferred to electron microscopy grids. Next, they are irradiated by an electron beam inside a transmission electron microscope. The tunability of energy of emitted X-Ray is achieved both by adjusting electrons acceleration voltage or using different material as the emitter.
 A.K. Budniak, N.A. Killilea, S.J. Zelewski, M. Sytnyk, Y. Kauffmann, Y. Amouyal, R. Kudrawiec, W. Heiss, E. Lifshitz, “Exfoliated CrPS4 with promising photoconductivity”, Small, 2020, 16 (1), 1905924
 A.K. Budniak, S.J. Zelewski, M. Birowska, T. Wozniak, T. Bendikov, Y. Kauffmann, Y. Amouyal, R. Kudrawiec, E. Lifshitz, “Spectroscopic and structural investigation of bulk and exfoliated iron phosphorus trisulfide - FePS3”, manuscript in preparation
 M. Shentcis, A.K. Budniak, X. Shi, R. Dahan, Y. Kurman, M. Kalina, H. Herzig Sheinfux, M. Blei, M. Kamper Svendsen, Y. Amouyal, S. Tongay, K. Sommer Thygesen, F.H.L. Koppens, E. Lifshitz, F. J. García de Abajo, L.J. Wong, I. Kaminer, “Tunable free-electron X-ray radiation from van der Waals materials”, Nature Photonics, 2020, 14 (11), 686-692
1:35 PM - NM07.09.05
Late News: Mobile Charges and Intertube Excitons in 1D van der Waals Coaxial Nano-Cables
James Lloyd-Hughes1,Maria Burdanova1,Reza Kashtiban1,Yongjia Zheng2,Rong Xiang2,Shohei Chiashi2,Shigeo Maruyama2
University of Warwick1,The University of Tokyo2Show Abstract
Heterostructures built from atomically thin 2D crystals, and bound by the van der Waals force, are well known to exhibit attractive optoelectronic properties and exotic physics. For instance, the strong Coulomb interactions can create bind charges across an interface, creating interlayer excitons with longer lifetimes. However the quantum confinement of electrons in quasi-1D structures creates distinctly different possibilities than available in 2D, as can be seen by contrasting graphene (a semi-metal in 2D with no excitonic absorption) and carbon nanotubes (semiconducting for certain chiralities, and featuring strong excitonic absorption).
Here we report the structure, composition and optoelectronic properties of a 1D van der Waals heterostructure consisting of nano-scale coaxial cables [1,2]. Carbon nanotubes (CNTs) formed the “core” of the cable, and were wrapped by atomically-thin nanotubes of the insulator boron nitride, and then by molybdenum disulfide (MoS2). The high quality of the composite was directly evident on the atomic scale from high-resolution transmission electron microscopy, and on the macroscopic scale by a study of its equilibrium and ultrafast optoelectronics .
We used THz spectroscopy to establish the good conductance of the CNT cores within the coaxial nano-cable, which can be therefore be regarded as a radial metal/insulator/semiconductor heterojunction. Via optical pump, THz probe spectroscopy we investigated the electron mobility of the MoS2 nanotubes, finding that it was comparable to that of high-quality atomically-thin 2D crystals of MoS2. The high mobility of the MoS2 nanotubes highlights the huge potential of the coaxial “hetero-nanotube” growth platform for nanoscale optoelectronic devices .
Finally, we addressed the intriguing question: can the optical properties of such coaxial nano-cables be described by the average properties of each material in isolation, or do interactions between the materials lead to unique new properties? To investigate, we used multi-colour infrared pump, visible probe spectroscopy to selectively create excitons in the carbon nanotubes, while probing the response of the A and B excitons in the MoS2 nanotube sheath. We observed a rapid and strong excitonic response of the MoS2 to the presence of excitons in the carbon nanotube core. This observation is surprising as in the single-photon, single particle picture a photon from the infrared pump pulse (0.6eV) does not give an electron sufficient energy to cross the MoS2 excitonic gap (1.9eV). Further, charge transfer processes are prohibited by the intermediate BN insulating layer. However, our experimental findings can be understood by considering the strong Coulomb correlations between the carbon nanotube core and the MoS2 sheath, which created the intertube excitonic and biexcitonic absorption features observed in the transient spectra. This first observation of intertube excitons, analogous to the interlayer excitons of 2D heterostructures, further underscores the potential of 1D van der Waals materials for basic science and future applications.
 Xiang et al., Science (2020) 367, 537-542, DOI: 10.1126/science.aaz2570
 Burdanova et al., Nano Letters (2020) 20, 3560-3567, DOI: 10.1021/acs.nanolett.0c00504
 Xiang and Maruyama (2021), Heteronanotubes: Challenges and Opportunities. Small Sci. 2000039, DOI: 10.1002/smsc.202000039
1:50 PM - *NM07.09.06
Van der Waals Heterostructure Magnetic Josephson Junction
Harvard University1Show Abstract
When two superconductors are connected through a ferromagnet, spin configuration of the transferred Cooper pairs can be modulated due to the exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction, depending on the thickness of the ferromagnetic weak link. In this talk, we present Josephson phase engineering in van der Waals heterostructures of atomically thin magnetic insulator Cr2Ge2Te6 sandwiched between NbSe2 van der Waals superconductors. Employing a superconducting quantum interference device based on our magnetic insulator Josephson Junctions, we reveal a doubly degenerate nontrivial Josephson phase originating from the magnetic barrier. We find that these unusual magnetic Josephson junctions are formed by momentum conserving tunneling of Ising Cooper pairs of NbSe2 across the magnetic domains of atomically thin Cr2Ge2Te6. The doubly degenerate ground states in magnetic insulator Josephson junction provide a quantum two-level system which can be utilized as a new component for superconducting quantum devices
NM07.10: Fundamental Properties of 2D Materials and Heterostructures IV
Monday PM, April 19, 2021
4:00 PM - *NM07.10.01
Orbital Magnetism in Graphene Heterostructures
University of California, Santa Barbara1Show Abstract
Ferromagnetism arises from the interplay of the Coulomb repulsion between electrons and their fermionic statistics. The vast majority of magnets consist of ordered arrangements of the electron spins stabilized by the spin orbit interaction. In my talk, I will describe a new class of magnets based on the spontaneous alignment of electron orbitals. Such orbital ferromagnetism may be a generic phenomena, but has, to date, found its fullest expression in graphene heterostructures in which the two dimensional orbits of electrons in distinct momentum space valleys provide the underlying degree of freedom for magnetic order. These magnetic degrees of freedom arise directly from the band wavefunctions, making orbital magnets exquisitely sensitive to both the design of the electronic wavefunctions as well as in situ control parameters. For example, in systems in which interlayer lattice mismatches lead to a moire superlattice potential, the resulting superlattice band structure may feature nontrivial topology, which in conjunction with orbital magnetism can give rise to precise quantization off the Hall effect at zero magnet field. Remarkably, the conduction electrons themselves can also directly influence the magnetic order, leading to field-effect switchable magnetic moments. Finally, I will conclude with our progress towards using orbital magnetism to engineer "topologically ordered" states at zero magnetic field, in which the electron splits into fractionally charged anyons.
4:25 PM - NM07.10.02
Late News: Molybdenum Disulfide NanoribbonsScalable Fabrication, Structure-Properties Correlation, Precision Manipulation and Applications
Yun Huang1,Kang Yu1,2,Huaizhi Li1,Kai Xu3,Zexi Liang1,Debora Walker4,Paulo Ferreira1,2,5,Peer Fischer6,7,Donglei (Emma) Fan1
The University of Texas at Austin1,International Iberian Nanotechnology Laboratory2,University of Illinois at Urbana-Champaign3,Harvard University4,University of Lisbon5,Max-Planck-Institute for Intelligent Systems6,University of Stuttgart7Show Abstract
In the family of two-dimensional (2D) transition metal dichalcogenide (TMD) materials, molybdenum disulfide (MoS2) has received immerse attention owing to its desirable electrical, chemical, and mechanical properties. Similar to single-crystalline MoS2 nanostructures, polycrystalline MoS2 structures also possess many desirable properties for applications, such as outstanding catalytic properties. Recently we reported an innovative and scalable approach to synthesize MoS2 nanoribbons with tunable dimensions and excellent dispersibility in suspension . The obtained polycrystalline MoS2 nanoribbons exhibit high chemical purity which endows them with much-enhanced surface reactivity compared to those of their single-crystal counterparts. They can be readily grafted with UV-triggered click-chemistry ; they effectively react and remove mercury contaminates from water. They also possess an ultrafast optoelectronic response from 450 nm to 750 nm that is the same as the single-crystal forms. In this presentation, we highlight these properties and describe in detail our newly obtained unpublished result on the electronic properties of the MoS2 nanostructures and how these determine their manipulation and electro-rotation behavior. We show that the nanoribbons can be manipulated efficiently in a high-frequency electric field. They propel along arbitrary paths on a 2D surface, assemble rapidly on prepatterned microelectrodes, and rotate clockwise and counter-clockwise. Their mechanical behaviors reflect their semiconductor electronic type, distinct from those of metals or insulators, which provides additional support to their optoelectronic characterizations. The fabrication, manipulation, assembly, and unique properties of the obtained MoS2 is useful for device fabrication, and applications of the TMD family.
 Huang, Y., Yu, K., Li, H., Liang, Z., Walker, D., Ferreira, P., Fischer, P., Fan, D., "Scalable Fabrication of Molybdenum Disulfide Nanostructures and their Assembly", Adv. Mater., 2003439, 2020.
4:40 PM - NM07.10.03
Late News: Narrow Excitonic Lines and Large-Scale Homogeneity of Transition Metal Dichalcogenides Grown by MBE on hBN
Wojciech Pacuski1,Magdalena Grzeszczyk1,Karol Nogajewski1,Aleksander Bogucki1,Kacper Oreszczuk1,Aleksander Rodek1,Julia Kucharek1,Karolina Polczynska1,Bartlomiej Seredynski1,Rafal Bozek1,Slawomir Kret2,Takashi Taniguchi3,Kenji Watanabe3,Janusz Sadowski1,2,4,Tomasz Kazimierczuk1,Marek Potemski1,5,Piotr Kossacki1
University of Warsaw1,Institute of Physics of the Polish Academy of Sciences2,National Institute for Materials Science3,Linnaeus University4,Laboratoire National des Champs Magnétiques Intenses5Show Abstract
Monolayer transition metal dichalcogenides (TMDs) are two-dimensional materials with exceptional optical properties such as high oscillator strength, valley related excitonic physics, efficient photoluminescence, and several narrow excitonic resonances. However, above effects have been so far explored only for structures produced by techniques involving mechanical exfoliation and for the best results, encapsulation in hBN, both proceedures inevitably inducing considerable large-scale inhomogeneity. On the other hand, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of interesting optical effects.
We report for the first time on the MBE-grown TMD exhibiting narrow and fully resolved spectral lines of neutral and charged exciton. Moreover, our MBE-grown TMD exhibits unprecedented high spatial homogeneity of optical properties, with variation of the exciton energy as small as 0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth [1,2] is presented for MoSe2 and includes extremely slow growth rate, the use of atomically flat hexagonal boron nitride (hBN) substrate and the annealing at very high temperature. Importantly, good optical properties are achieved for as-grown sample, without any post growth exfoliation and encapsulation in hBN. This novel recipe opens a possibility of MBE growth of TMD and their heterostructures with optical quality, dimensions and homogeneity required for optoelectronic applications.
 W. Pacuski, M. Grzeszczyk, K. Nogajewski, A. Bogucki, K. Oreszczuk, J. Kucharek, K.E. Polczynska, B. Seredynski, A. Rodek, R. Bozek, T. Taniguchi, K. Watanabe, S. Kret, J. Sadowski, T. Kazimierczuk, M. Potemski, P. Kossacki; Nano Letters 20, (2020) pp. 3058-3066.
 Z. Ogorzalek, B. Seredynski, S. Kret, A. Kwiatkowski, K. P. Korona, M. Grzeszczyk, J. Mierzejewski, D. Wasik, W. Pacuski, J. Sadowski and M. Borysiewicz; Nanoscale 12 (2020) pp. 16535-16542.
4:55 PM - NM07.10.04
Electrical Control of Strong-Light Matter Interactions in MoTe2
Souvik Biswas1,Zakaria Al Balushi1,2,Eoin Caffrey1,Sergiy Krylyuk3,Joeson Wong1,Kenji Watanabe4,Takashi Taniguchi4,Albert Davydov3,Harry Atwater1
California Institute of Technology1,University of California, Berkeley2,National Institute of Standards and Technology3,National Institute for Materials Science4Show Abstract
Light matter interaction between excitonic resonances in atomically thin TMDCs, which are highly susceptible to electrostatic stimulus, and cavity polaritonic modes can enable novel electro-optic functionalities. In this work, we investigated electrical control of strong light-matter interactions in molybdenum ditelluride via opto-electronic modulation of an h-BN encapsulated monolayer MoTe2 gated heterostructure designed also as a ‘Salisbury-screen’ photonic resonant absorber.
Using low temperature reflection contrast and photoluminescence measurements at different Fermi-level values, we observed strong modulation of the A 1s, 2s and B 1s exciton and trion peaks. Our results can be primarily understood from increased electronic doping which gives rise to enhanced elastic Coulomb scattering and screening – resulting in oscillator strength shifts from excitons to trions and also spectral broadening of the individual resonances. Moreover, many body effects leading to band-structure renormalization were seen from the changes in the binding energy of the different excitonic species with increased charge density. We performed transfer matrix calculations to obtain the complex dielectric function of MoTe2 at each Fermi level position. Notably, we observed unity-order refractive index modulation at the A1s exciton. Additionally, we also studied the influence of optical pumping by performing laser fluence dependent emission measurements. Spectral broadening of the excitonic features was observed, indicating exciton-exciton interaction at higher pump powers. Our results shed light on the strongly correlated many-body interactions in MoTe2 and suggest paths for integration into active devices such as near infrared electro-optical modulators.
5:10 PM - NM07.10.05
Fracture Toughness and Fracture Strain of Single-Layer Freestanding Graphene Extracted by On-Chip Testing
Sahar Jaddi1,Bin Wang1,2,Mohammad Malik1,Yun Zeng2,Jean-Pierre Raskin1,Thomas Pardoen1
UCLouvain1,Hunan University2Show Abstract
Graphene has attracted the attention of the scientific community for the last two decades, thanks to an astounding set of properties such as high thermal conductivity (5000 W/mK), superior electron mobility (250,000 cm2/V-s), high modulus of elasticity (~1 T Pa), and ultra-high flexibility. Graphene’s C-C bonds are sp2 hybridized providing superior mechanical stability in the basal plane directions. Graphene appears as the perfect material constituent for a wide range of application fields, like in flexible electronics, in electromechanical devices, and in reinforced lightweight composites that rely on both electrical and mechanical performances. It remains crucial to determine the latter properties under realistic and well-controlled conditions to guide the fail-safe design of graphene-based applications.
While the mechanical behavior of graphene has been very much investigated numerically for both perfect and defected lattice structures [1,2], most experimental works remain partly inconclusive regarding the fracture resistance. Nanoindentation of a suspended single layer (SLG) and of multilayer graphene (MLG) on top of holes [3,4], and uniaxial tensile test using the so-called push-to-pull device, are mostly used to deform graphene [8,9]. Testing graphene using a valid fracture mechanics approach is very complex due to the nature of the fabrication, the difficulty of pre-cracking, and the delicate data analysis. Two reports on the fracture toughness of SLG have been published [5,6], while other data are for MLG [7, 8]. The aforementioned works have provided insight into the fracture mechanisms of graphene, using a notch as a starter crack. This can potentially lead to an overestimation of the fracture toughness with a dependence on the notch root radius, which is difficult to deconvolute. Furthermore, the notch is produced by a focus ion beam, which may introduce damage. These tensile and fracture tests relied on external equipment either for loading or sensing, leading to cumbersome manipulation and allowing only a few specimens to be tested, preventing statistical analysis.
In the present work, we report two novel extensions of methods developed in our laboratory to study the mechanical and fracture properties of freestanding SLG. The method relies on the use of residual tensile-stress involved in a beam layer called the actuator beam. This beam is attached to the graphene specimen. Removing the sacrificial layer induces the release of internal stress in the actuator, pulling then on the graphene specimen. Hence, the use of external (electro-)mechanical actuation is avoided. In a first configuration, the actuator layer is pulling on a dog-bone shape or rectangular graphene specimen, from which the uniaxial stress-strain curve is extracted up to fracture . In a second configuration, two actuator beams pull on a notched specimen but using a crack arrest method to determine the fracture toughness suppressing the problem of notch blunting effect at crack initiation . Benefiting from the lithography process, numerous devices have been fabricated hence providing statistically representative data. Our investigation shows fracture strains larger than 10% and tensile strength larger than 100 GPa for specimens with an area larger than 160 mm2. Brittle fracture behavior is observed with fracture toughness value equal to about 4 MPa m0.5 in agreement with literature.
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5:15 PM - NM07.10.07
Nanoscale Anisotropic Strain Dynamics in MoS2 Imaged with Ultrafast Electron Microscopy
Yichao Zhang1,David Flannigan1
University of Minnesota Twin Cities1Show Abstract
The large anisotropy of the bonding states in layered transition metal dichalcogenides (TMDs) provides an additional dimension to modulate materials properties [1,2]. For example, relatively weak interlayer van der Waals forces lead to distinctly different electronic, mechanical, vibrational, and electrochemical properties along the c-axis of MoS2 compared to the basal plane. Efforts have been focused on probing photoinduced structural dynamics in the basal plane, where strong covalent bonding dominates the response of the material [3,4]. Comparatively few ultrafast studies have shown the different nature of photoexcited structural dynamics along the c-axis as compared to the in-plane behaviors [5,6]. In addition, defects play an important role in the nucleation of coherent strain waves studied with the bright-field imaging modality of ultrafast electron microscopy (UEM) [7-9].
Here, we report the combined study of direct imaging of photoinduced localized c-axis acoustic-phonon dynamics of a free-standing, multilayer MoS2 flake with UEM and finite element transient deformation analysis with COMSOL. With nanometer-picosecond spatiotemporal resolutions in bright-field imaging, we observe and quantify the emergence and oscillation of two spatially localized interlayer breathing modes separated by a 10-nm thick step edge, with the difference in frequency due to the different thicknesses in the two respective regions. The onset of coherent contrast dynamics in the thicker region of the terrace is delayed by 2 to 3 ps, indicating a more rapid structural response to photoexcitation in the thin region of the terrace. Time-dependent finite element analysis on a terrace of the same dimensions as the specimen shows excellent agreement with experimental results. Photoexcitation was modeled by applying a spatially varying strain profile along the c-axis calculated based on the absorption of multilayer MoS2 and the thickness of the terrace, leading to a discontinuity of the c-axis strain profile at the step edge. Both experiment and simulation captured a gradual decrease of 1 to 2 GHz of the oscillation frequencies starting 50 to 100 nm away from the step edge on both sides of the terrace, likely due to the additional force component along the c-axis opposed to wave propagation because of the distortion of the basal-plane covalent bonds. These results provide insights into the local, nanoscale nature of transient lattice deformations and the associated impacts on phonon dynamics in layered materials .
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 This material is based upon work supported by the National Science Foundation under Grant No. DMR-1654318. This work was partially supported by the National Science Foundation through the University of Minnesota MERSEC under Award Number DMR-2011401.
5:20 PM - NM07.10.08
Unconventional Superconductivity Mediated by Spin Fluctuations in Single-Layer NbSe2
Paul Dreher1,Wen Wan1,Rishav Harsh1,Francisco Guinea1,Miguel Ugeda1
Donostia International Physics Center1Show Abstract
Van der Waals materials provide an ideal platform to explore superconductivity in the presence of strong electronic correlations, which are detrimental of the conventional phonon-mediated Cooper pairing in the BCS-Eliashberg theory and, simultaneously, promote magnetic fluctuations. Despite recent progress in understanding superconductivity in layered materials, the glue pairing mechanism remains largely unexplored in the single-layer limit, where electron-electron interactions are dramatically enhanced. Here we report experimental evidence of unconventional Cooper pairing mediated by magnetic excitations in single-layer NbSe2, a model strongly correlated 2D material. Our high-resolution spectroscopic measurements reveal a characteristic spin resonance excitation in the density of states that emerges from the quasiparticle coupling to a collective bosonic mode. This resonance, observed along with higher harmonics, gradually vanishes by increasing the temperature and upon applying a magnetic field up to the critical values (TC and HC2), which sets an unambiguous link to the superconducting state. Furthermore, we find clear anticorrelation between the energy of the spin resonance and its harmonics (Ωn) and the local superconducting gap (Δ), which invokes a pairing of electronic origin associated with spin fluctuations. Our findings demonstrate the fundamental role that electronic correlations play in the development of superconductivity in 2D transition metal dichalcogenides, and open the tantalizing possibility to explore unconventional superconductivity in simple, scalable and transferable 2D superconductors.
5:23 PM - NM07.10.14
Multiscale Phonon Green’s Function for Silicene with and without Lattice Defects and Its Applications
Vinod Tewary1,Edward Garboczi1
National Institute of Standards and Technology1Show Abstract
Since the advent of graphene, many new two-dimensional (2D) materials have been either fabricated or theoretically pro