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Highlights

User 2DCC Research Highlights 2021

Project Summary: The electronic properties of transition metal dichalcogenides (TMDs) vary dramatically depending on their phase (2H vs. 1T) and ans the energy to convert between the two phases is predicted to be relatively low. This has sparked interest in the use of the phase change properties of TMDs for active components in electronic devices. A combination of mechanical strain and charge transfer from a metal substrate has previously been suggested to induce the phase transition from 2H to 1T.  In this study, x-ray and ultraviolet photoelectron spectroscopy (XPS, UPS) were used to identify charge transfer processes and changes in the structural phase for MoS2, MoSe2 and MoTe2 monolayers on Au surfaces annealed up to 500oC. Although charge transfer does appear to occur, we do not find spectroscopic evidence for the 2H to 1T phase change in exfoliated TMDs. However, both helium-ion irradiated exfoliated TMDs and layers grown by metalorganic chemical vapor deposition (MOCVD) show shifts in their XPS spectra that could be interpreted as a phase change but instead are the result of TMD/Au hybridization, grain boundaries and defects interacting with the Au surface.

Published in: J. Phys. Chem. 2020, 124, 25361-25368.

2DCC Role: The wafer-scale MoS2 monolayer samples used for this study were grown by MOCVD in the 2DCC facility. The user PI (J.T. Robinson) and his postdoctoral scholars have been active participants in 2DCC activities including participating in annual User Committee meetings and presenting posters at Graphene and Beyond.

Project Summary: The topological Hall effect (THE) is a phenomenon that is a consequence of a Berry phase created by spin textures in real space. Interfacing a topological insulator with a magnetic insulator provides a model platform for studying this phenomenon in a well-controlled manner. This papers reports the first clear evidence for the THE in heterostructures that combine a model topological insulator (Bi2Se3) with a ferromagnetic insulator (BaFe12O19). The key signature of the THE is an “excess” Hall resistance when the perpendicular to the plane magnetization of the ferromagnet is reversed by an external magnetic field. These samples show a conventional anomalous Hall effect (AHE) at high temperature (T > 80 K) but a pure THE develops in the temperature range of T = 2–3 K. Over T = 3–80 K, the two effects coexist but show opposite temperature dependence. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions (spin textured “bubbles”) that arise due to a Dzyaloshinskii–Moriya interaction at the interface. The estimated strength of this interaction is substantially higher than that in more conventional heavy metal-based systems widely studied in the spintronics community. Publication: Nano Lett. 2021, 21, 1, 84–90

2DCC Role: The Bi2Se3 thin films were grown by MBE in the multi-module MBE system in the 2DCC facility on BaFe12O19 substrates provided by the user (Wu). The user and his graduate students have been active participants in 2DCC User Committee meetings and also participated in the reverse site visit for the 2DCC renewal.

Project Summary: To assess the potential of transition metal dichalcogenides (TMDs) for future circuits, it is important to study the variation in key device parameters across a large number of devices. Here we benchmark device-to-device variation in field-effect transistors (FETs) based on wafer-scale monolayer MoS2 and WS2. Our study involves 230 MoS2 FETs and 160 WS2 FETs with channel lengths ranging from 5 µm down to 100 nm. We use statistical measures to evaluate key FET performance indicators for benchmarking these TMD monolayers against existing literature as well as ultra-thin body Si FETs. Our results show consistent performance of the 2D FETs across 1×1 cm2 chips owing to high quality uniform layers and clean transfer onto device substrates. We demonstrate record high carrier mobility of 33 cm2V-1s-1 in WS2 FETs, which is a 1.5X improvement compared to the best literature report. Our results confirm the technological viability of 2D FETs in future integrated circuits. Published in Nature Communications 2021, 12, 1-12.

2DCC Role: The wafer-scale MoS2 and WS2 monolayer samples used for this study were grown by metalorganic chemical vapor deposition (MOCVD) in the 2DCC facility. The device results provide a benchmark for academic and industry users who are working with similar 2DCC material.

In-House 2DCC Research Highlights 2021

Project Summary: Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which form when oppositely oriented triangular domains coalesce. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition (MOCVD) using a multistep growth process to achieve epitaxial WS2 monolayers. Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that preserve the aligned domain structure. The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with optical and transport properties approaching those of exfoliated flakes.

Published in: ACS Nano 15,2, 2532-2541 (2021).

2DCC Role: The MOCVD growth studies were carried out in the 2DCC Thin Films facility (Redwing).  Structural and optical characterization of the layers were carried out by 2DCC-affiliated faculty (Alem, Terrones) and transport measurements were performed by an internal user (Das).

Understanding the superconductivity at the interface of FeSe/SrTiO3 is a problem of contemporary interest in condensed matter physics because of the significant increase in the critical temperature (Tc ~ 50 K) for the onset of superconductivity compared to that of bulk FeSe crystals (Tc ~ 9 K). Additional interest in this problem arises from the possibility of an unconventional pairing mechanism. We used the 2DCC multimodule molecular beam epitaxy (MBE) and surface characterization facility to study the influence of various capping layers on the Tc of ultrathin films of FeSe grown on SrTiO3. The multimodule facility’s in vacuo four-probe electrical resistance measurement capability provided critical information about the Tc of MBE-grown FeSe films in their pristine state, while ex situ magneto-transport measurements elucidated the key role of distinct charge transfer from different capping layers (compound FeTe, non-metallic Te, and metallic Zr). Our results show that FeTe provides an optimal cap that barely influences the inherent Tc found in pristine FeSe/SrTiO3, while the transfer of holes from a non-metallic Te cap completely suppresses superconductivity and leads to insulating behavior. We also used ex situ magneto-resistance measurements in FeTe-capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field. Although this does not show any obvious signature of exotic physics, our study provides insight into the symmetry and pairing mechanism of high temperature superconductivity in FeSe.

 

Published in: Phys. Rev. Materials 5, 034802 (2021).

Project Summary: The sticking coefficients of thermally evaporated chalcogen elements selenium and tellurium were experimentally determined as a function of temperature. Their direct and quantitative determination provides important insights to comprehend and realistically model the growth kinetics of chalcogenide-based film growth.

A direct way to determine the ratio of chalcogenide elements sticking to a film surface is to measure the rate of mass accumulated, which was achieved by depositing selenium and tellurium on a quartz crystal microbalance held at different temperatures. Pronounced reduction of the sticking coefficients by a factor of 4 in a very narrow temperature range of 20°C and 30°C around temperatures of 35°C and 115°C were found for selenium and tellurium, pointing towards the critical need of precise temperature control during chalcogenide film growth using molecular beam epitaxy. The results reveals that unlike tellurium, selenium is supplied in different chemical forms with different desorption characteristics.

J. Vac. Sci. Technol. A 2021, 39, 023413. DOI:10.1116/6.0000736

Project Summary: Dilute magnetic semiconductors, achieved through substitutional doping of magnetic atoms into semiconducting systems, enable experimental modulation of spin dynamics for novel magneto-electric or magneto-optical devices, especially in 2D transition metal dichalcogenides that accentuate interactions and activate valley degrees of freedom.

Project Summary: The first ReaxFF force field developed for 2D-WSe2 provides the community with a highly efficient means that describe material growth, phase transitions, defect formation and migration and thus can provide valuable atomistic insights into experimental efforts on growth, phase, and defect engineering as a function of the local chemical environment. This potential can elucidate further the morphological evolution of monolayers in different environments in terms of loading conditions and defect concentrations/distributions. Interactions between vacancies and ripples in a 2D layers (“ripplocations”) suggest that vacancies could stabilize buckled structures by modulating the strain energy and possibly open a venue for sweeping out undesirable defects such as vacancies from 2D WSe2.

2DCC User Research Highlights 2020

In this project, we use a novel approach for next generation ultra-low-power sensor design by embracing the evolutionary success of animals with extraordinary sensory information processing capabilities that allow them to survive in extreme and resource constrained environments. Stochastic resonance (SR) is one of those astounding phenomena, where noise, which is considered detrimental for electronic circuits and communication systems, plays a constructive role in the detection of weak signals. Here, we show SR in a photodetector based on wafer-scale monolayer MoS2 for detecting ultra-low-intensity subthreshold optical signals from a distant light emitting diode (LED). We demonstrate that weak periodic LED signals, which are otherwise undetectable, can be detected by a MoS2 photodetector in the presence of a finite and optimum amount of white Gaussian noise at a frugal energy expenditure of few tens of nano-Joules. The concept of SR is generic in nature and can be extended beyond photodetector to any other sensors.

Topological insulators are materials that are insulating in the bulk but have conducting surface states with a helical Dirac dispersion. These properties are created by the combined effects of strong spin-orbit coupling and time-reversal symmetry. When a topological insulator is interfaced with a magnetic insulator, the magnetic interactions across the interface can break time-reversal symmetry, opening a gap at the Dirac point of the topological surface states. Experiments demonstrating the impact of magnetism on topology in this manner have been well documented. An unresolved question is whether the inverse situation is possible: can topological surface states influence magnetism? In a user project led by Mingzhong Wu (Colorado State), 2DCC scientists used molecular beam epitaxy (MBE) to synthesize topological insulator (Bi2Se3) thin films on ferrimagnetic insulator Y3Fe5O12 (YIG) thin film substrates grown by sputtering at Colorado State. Wu’s group then used ferromagnetic resonance measurements to show that the magnetic anisotropy, gyromagnetic ratio, and damping of the YIG were modified by interfacing with Bi2Se3. To prove that the topological surface states in Bi2Se3 are crucial for the observed changes in the magnetic properties, the 2DCC team interfaced YIG with (Bi,In)2Se3, a “trivial” insulator, and found no significant modification of the ferromagnetic resonance signal.

Atomic defects are controllably introduced in suspended single layer molybdenum disulfide (1L MoS2) using helium ion beam. Vacancies exhibit one missing atom of molybdenum and a few atoms of sulfur. Quantification was done using a Scanning Transmission Electron Microscope (STEM) with an annular detector. Experimentally accessible inter-defect distance was employed to measure the degree of crystallinity in 1L MoS2. Correlation between the appearance of an acoustic phonon mode in the Raman spectra and the inter-defect distance was established, which introduces a new methodology for quantifying defects in 2D materials.

ZrS2 has received increased attention due to its indirect band gap that matches with the visible spectrum and which is predicted to undergo an indirect-to-direct transition with strain. Alloying with ZrSe2 to produce ZrSxSe2-x (x=0...2) would provide continuous control over key optical and electronic parameters for photonics. Here, we combine Raman spectroscopy, spectroscopic ellipsometry (SE), and density functional theory to conduct the first comprehensive exploration of phonons and excitons in this system. We find that long-range Coulomb interactions activate optically-silent infrared phonons that completely dominate Raman spectra in some alloys. SE measurements of excitons reveal strong light-matter interactions with low optical losses for near-infrared wavelengths. This study suggests ZrSxSe2-x alloys will find applications in atomically-thin, tunable photodetectors and photovoltaics.

This review provided an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and how they can be utilized to assist/guide the design and synthesis of 2D materials. It focuses on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. It also discusses how machine learning can be combined with computation and experiments to guide the discovery of new 2D materials.

Plasmon-exciton coupling in hetero-bilayer of WSe2 and WS2 transferred onto Au nanorod arrays is studied. Dark-field scattering measurements reveal that the in-plane dipole moment of excitons in monolayer WS2 allows only the narrow spectral range of 30 nm for the resonant coupling between the localized particle plasmons from Au nanorods and the bright excitons from WS2. We demonstrate that the q-parameter that represents the asymmetry of Fano resonances from plasmon-exciton coupling can be controlled by the polarization states of incident light. Surface lattice resonances in between individual Au nanorods play a role to diminish the damping factor of plasmon-exciton coupling in the arrays.  

Dirac fermions with highly-dispersive linear bands are usually considered weakly correlated, due to relatively large bandwidths (W) compared to Coulomb interactions (U). With the discovery of nodal-line semimetals, the notion of Dirac point has been extended to lines and loops in the momentum space. The anisotropy associated with nodal-line structure gives rise to greatly reduced kinetic energy along the line. However, experimental evidence for anticipated enhanced correlations in nodal-line semimetals is sparse. Through optical studies on the ZrSiSe single crystals provided by 2DCC, Basov’s group at Univ. of Columbia find prominent correlation effects in nodal-line semimetals.

They observed spectroscopic hallmarks of electronic correlations: strong reduction  of the Drude weight and the Fermi velocity.

2DCC Research Highlights 2020

Doping is the cornerstone of semiconductor technology, enabling the success of modern digital electronics. Successful realization of wafer-scale, electronic grade, intrinsic 2D TMDCs via common deposition methods is rapidly progressing, however, advances in scalable doping still remain in the “proof-of-concept” stage, delaying the large-scale fabrication of logic circuits based on extrinsic 2D semiconductors. This work is presenting a wafer-scale synthesis of rhenium doping of WSe2 films via MOCVD at front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures. By controlling the partial pressures of the precursors, doping concentrations as low as 0.0001% can be achieved. Rhenium atoms substitutionally replace W atoms in the WSe2 lattice and introduce discrete defect levels that lie close to the conduction band minima confirming the n-type nature of the dopants. However, the transport properties of field-effect transistors degrade as a function of doping concentration which is attributed to the large binding energy of electrons due to less effective dielectric screening in monolayer 2D films.

Among the high-mobility two-dimensional transition metal dichalcogenides PtSe2 is of particular interest due to its record high carrier mobility of 1,000 cm2/Vs, sizeable band gap and air stability to address the current need for low-power, high-performance and ultra-thin body electronics. A scalable fabrication process with highly precise thickness control on ultra-smooth insulating gate oxides however has not been achieved to date. This work is addressing this gap by presenting ultra-thin growth of Pt layers on insulating Al2O3 and the subsequent conversion into PtSe2 by direct selenization in molecular beam epitaxy. Due to the chosen growth technique, the approach is scalable and offers precise control over Pt and thus PtSe2 layer thickness. We engineered deposition parameters to fabricate smooth, single crystal Pt films in the mono-/few-layer limit and optimized selenization conditions to convert a maximum amount of Pt into PtSe2. A post selenization anneal in Se proved essential to enhance crystal quality of the PtSe2 layers.

MnBi2Te4 has recently been established as the first intrinsic antiferromagnetic (AFM) topological insulator. Although quantum anomalous Hall effect (QAHE) has been observed in MnBi2Te4, it is only realized with odd numbers of septuple layers due to the AFM interlayer coupling. Therefore, it is crucial to stabilize ferromagnetic (FM) phase in MnBi2Te4. We have discovered a new FM phase with the Curie temperature of 26 K in the MnSb1.8Bi0.2Te4 sample through tuning growth conditions, in contrast to the AFM order seen in the Mn(Bi1-xSbx)2Te4 family. We have investigated magnetotransport properties of the FM thin flakes and observed features similar to topological Hall effect. Our work pushes forward the realization of intrinsic FM topological insulator and establishes a new platform to explore novel topological quantum states arising from the interplay between magnetism and band topology.

  • Atomically-thin metals Ga, In, and Sn are stabilized at the interface of epitaxial graphene and SiC through confinement heteroepitaxy
  • Metal atoms generated via thermal evaporation of metal powders migrate through defects in graphene layers and passivate the surface of SiC
  • 2D Ga layers exhibit BCS-type superconductivity and enhanced Tc of ~4 K (compared to 2 K in bulk, α-Ga)
  • The process of stabilizing 2D superconductors via CHet can be applied to elements beyond p-block metals, opening opportunities to study unconventional properties that enable exploration of new physics and devices.

2DCC User Research Highlights 2019

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

LANL CINT scientists and their collaborators at UConn, AFRL, Penn State, and U. Oregon have discovered a method to create spatially localized quantum emission sites in a wafer-scale transition metal dichalcogenide film, WSe2, synthesized at the 2D Crystal Consortium (2DCC) facility. The team’s objective was to determine the role of strain in creating localized quantum emission sites in order to learn how to control their properties through strain.

Because the WSe2 was very thin, it conformed to the radius of ultra-sharp tips with sub-10 nanometer diameters, and bent towards the substrate. The resulting strain was enough to change the electronic structure, but only at the tips. The affected area emitted light that was fundamentally different in nature than that from the rest of the WSe2 film, where photons were ejected one-by-one, achieving what is referred to as deterministic quantum emission.

An undergraduate REU student at UConn participated in the research as is a co-author on the paper.

The team published the results as a Featured Article in the May 27th issue of Applied Physics Letters.

Link to LANL press release:

https://www.lanl.gov/discover/news-release-archive/2019/May/0529-quantum-information.php

Monolayer 2D transition metal dichalcogenides (TMDs) have been a focus of increasing interest due to their unique properties but the development of device technologies has been hampered by difficulties in synthesizing large area monolayer and few layer films. We developed a multi-step process involving nucleation, ripening and preferential lateral growth to achieve epitaxial WSe2 monolayers on sapphire by gas source chemical vapor deposition. This process enables control of the nucleation density of WSe2 on the sapphire and the lateral growth rate of domains resulting in coalescence of monolayer regions over wafer-scale areas with minimal multilayer growth. The process also provides insights into the fundamental kinetic and thermodynamic factors that impact monolayer growth.

Broader Impacts Highlights 2019

The Thin Films and In Situ Characterization Facility in the 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) user facility includes a multimodule ultra-high vacuum system for the synthesis and study of two dimensional (2D) chalcogenide films by molecular beam epitaxy (MBE) [upper panel].  The lower left panel shows a high resolution cross-sectional transmission electron microscope image of a complex 2D chalcogenide film synthesized in this facility. Such 2D films are characterized in their pristine state by transferring them in vacuo to surface sensitive characterization tools. The in vacuo environment avoids the modification of the inherent properties of these 2D films by exposure to ambient atmosphere. The electronic energy surface states in these 2D films are probed using angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy. The right lower panel shows ARPES measurements of a topological insulator thin film.

STEPFORWARD is a geospatial tool built on Google Maps API to identify minority serving institutions (MSIs), primarily undergraduate institutions (PUIs) and non-R1 institutions within a geographical radius of travel. Higher Education R&D data highlights institutions active in research fields of interest.

 

The tool enables 2DCC researchers and the broader community to identify a diverse range of institutions for outreach visits and research interactions. The 2DCC supports visits to these institutions as part of a travel extension program.

The 2DCC completed its 4th year of sponsorship of the Graphene and Beyond Workshop.  To date, >650 individuals from academia, government and industry across all career levels have attended.  The 2DCC has broadened participation by sponsoring travel and registration for dozens of users and potential users particular from MSIs and PUIs. The workshop enhances community synergy toward a strong future in 2D crystal science and technology. The workshop consists of tutorials and research talks focused on 2D material synthesis, properties, theory and optic/electronics as well as recent results from the 2DCC and facilitated discussion on commercialization opportunities for 2D materials.

In 2019, the 2DCC partnered with the 5th annual US/EU 2D Materials Workshop for a joint workshop that included speakers from across the globe including the Graphene Flagship.

LiST is a web-based data management software tool developed by the 2DCC to capture, organize and curate experimental and theory/simulation data produced by the facility and external users.

Under LiST, data follow the sample from substrate preparation to synthesis protocol, integrated UHV characterization, ex situ characterization, modeling, delivery, and publication, with fine-grained access control and community-accessible tools and data.

In-House 2DCC Research Highlights 2019

2-dimensional materials hold promise for next-generation electronics. However, in order to realize 2D-based technologies, key milestones must be identified and met. This article identifies areas of research which are fundamentally required for achieving electronic grade 2D materials and brings together experts in these respective areas to discuss key challenges. As a result, this article serves as a roadmap for the realization of 2D materials which can be used in the development of technology toward industries including healthcare, internet of things, high-performance computing, and economical solar energy.

 

This article can serve as a guide, and a source of inspiration and motivation for researchers and members of the 2D community. Additionally, it communicates the importance of 2D research to the greater scientific community, as well as industry players, demonstrating the  current and potential impact of these materials on technology and society.

A graphene lattice remains the same under a rotation of 180º. This special symmetry is lost in monolayer transition metal dichalcogenides (TMDs), e.g. every horizontal Mo-S bond would end up as S-Mo bonds. While this lack of inversion symmetry brings out fascinating properties such as spin-coupled valley degrees of freedom, it also means that when two grains related by 180º rotation merge, the two lattices cannot smoothly stitch together, leaving an unavoidable scar – an inversion domain boundary (IDB) – that degrades transport properties.

 

In two recent 2DCC publications, joint experimental and theory efforts identified a general method to achieve orientational selectivity, originating from a localized defect pair that amplifies the energetic distinction between the two orientations. For monolayer MoS2 or WSe2 grown on hexagonal boron nitride (hBN), the defect pair is a generalization of a so-called Frenkel pair – a interstitial-vacancy pair shared between adjacent layers of a 2D heterostack. MoS2 and WSe2 domains initiated from such defects exhibit an orientation uniformity of over 80%, leading to suppression IDBs upon coalescence, as confirmed by aberration-corrected scanning/transmission electron microscopy. Fully coalesced 2D chalcogenide monolayer films synthesized using this approach demonstrated superior optical and electrical properties compared to films grown on sapphire under similar conditions. The results motivate further efforts focused on the development of single crystal hBN substrates and epilayers for synthesis of wafer-scale single crystal TMD films.

The strongly spin-momentum coupled electronic states on the surfaces of topological insulators (TIs) exist because of time-reversal symmetry. The theoretical description of these states is fundamentally analogous to a picture used to describe particles known as ‘axions’ theoretically postulated to exist in Nature but never observed. Demonstrating and understanding this conceptual analogy is important for gaining new insights into how our universe works. By interfacing the two opposite surfaces of a TI thin film with different magnetic materials and measuring the magnetic field dependence of the electrical conductance,  we show that the ‘axion’ analogy is robust and leads to a new phase of matter known as an ‘axion insulator.’

Broader Impacts Highlights 2018

Under LiST, data follow the sample from substrate preparation to synthesis protocol, integrated UHV characterization, ex situ characterization, modeling, delivery, and publication, with fine-grained access control and community-accessible tools and data.

Geospatial tool built on Google Maps API to identify MSI, PUI and non-R1 institutions along existing travel itineraries. Facilitates user outreach to a diverse range of institutions through travel extension. Higher Education R&D data highlights institutions active in research fields of interest

In-House 2DCC Research Highlights 2018

One of the most useful ways that materials interact with light is Raman scattering, in which a photon creates or absorbs a lattice vibration. When the photon energy equals an electronic transition in the material (a “resonance”), the scattering can become much stronger. An outstanding mystery in 2D transition metal dichalcogenides has been the pronounced weakness of some excitonic resonances compared to others. The 2DCC-MIP team has developed an efficient first-principles way to calculate excitonic resonant Raman intensities, thereby explaining the puzzling near-absence of resonant Raman response around the A and B excitons (band-edge excitations with very strong optical absorption) and the pronounced strength of the resonant Raman response from the C exciton (which arises from parallel valence and conduction bands). These insights can now be carried to other semiconducting systems.

Transition metal dichalcogenides (TMDs) such as WS2 exhibit intriguing properties in monolayer form including direct bandgaps and large exciton binding energies. A major challenge in harnessing their potential is the uniform growth of high quality monolayers over large substrate areas. Metalorganic chemical vapor deposition (MOCVD) is a promising technique for TMD synthesis but the impact of precursor chemistry on TMD growth and properties is not well understood at present. In this work, the 2DCC-MIP team investigated the effect of the choice of chalcogen precursors in a cold-wall reactor geometry for the specific case of WS2. The investigation reveals that  diethyl sulfide (DES) results in incorporation of carbon in the films, which can be correlated to a reduced lateral growth rate and quenching of photoluminescence from the WS2 films compared to growth with hydrogen sulfide (H2S). As a part of this investigation, thermodynamic modelling was also employed to understand the possible reaction outcomes. The calculations predict the formation of carbon when DES is used for growth consistent with the experimental observations. These results help in identifying promising precursors for the synthesis of high quality TMD films.

The strongly spin-momentum coupled electronic states on the surfaces of topological insulators (TIs) exist because of time-reversal symmetry. The theoretical description of these states is fundamentally analogous to a picture used to describe particles known as ‘axions’ theoretically postulated to exist in Nature but never observed. Demonstrating and understanding this conceptual analogy is important for gaining new insights into how our universe works. By interfacing the two opposite surfaces of a TI thin film with different magnetic materials and measuring the magnetic field dependence of the electrical conductance,  we show that the ‘axion’ analogy is robust and leads to a new phase of matter known as an ‘axion insulator.’

The Two-Dimensional Crystal Consortium closely couples experimental synthesis and measurement with theoretical and computational work to close the discovery loop on the growth and characterization of 2D materials.

Origami with regular paper can produce a limitless number of fascinating, beautiful and useful shapes, but we lack “fingers” at the nanoscale to guide the folding. Researchers in the Two-Dimensional Crystal Consortium at Penn State have conceived and simulated a novel way to “program” a folding pattern by patterning the nanoscale sheet with complementary regions of n-type and p-type doping. Such regions prefer to adhere to each other more than they like to adhere to themselves, and the boundaries between these regions define the fold lines of the final origami structure. Forcing the sheet through a small hole is enough to start the folding process, and controlling the shape and depth of such holes provides another means to influence the order in which the folds occur.

Monolayer 2D materials, particularly the family of 2D transition metal dichalcogenides, have been a focus of increasing interest due to their unique properties. The development of device technologies based on 2D chalcogenide has, however, been hampered by difficulties in synthesizing large area monolayer and few layer films. We developed a multi-step process involving nucleation, ripening and preferential lateral growth to achieve epitaxial WSe2 monolayers on sapphire by gas source chemical vapor deposition. This process enables control of the nucleation density of WSe2 on the sapphire and the lateral growth rate of domains resulting in coalescence of monolayer regions over wafer-scale areas with minimal multilayer growth. The process also provides insights into the fundamental kinetic and thermodynamic factors that impact monolayer growth.

When an atomically thin 2D material is suspended as a membrane so that adsorbed atoms can stick to both sides, these atoms can interact through the membrane and thus act as two coupled adsorbate systems, with new properties that are absent in either system alone. Computational modeling in the 2D Crystal Consortium predicts new patterns that emerge, such as an infinite staircase of fractional coverages of opposing sides with simple rational fractions being favored, a so-called “Devil’s Staircase.” Since adsorbate patterns are possible at different spatial scales, it is possible to obtain parallel staircases with different step heights, and even come arbitrarily close to violating a revered principle of thermodynamics, the Gibbs Phase Rule. These systems provide a new window into the behavior of atoms on surfaces.

User Research Highlights 2017

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

Published in Phys. Rev. Lett. 119, 077702 (2017). Copyright from APS.

Also supported by: NSF DMR-1419807,  NSF Grant 1639921, SRC-NRI 2700.001.

Broader Impacts Highlights 2017

The Graphene and Beyond Workshop, in its 5th year (2nd year of joint sponsorship with 2DCC), is a collaboration between the Center for 2D and Layered Materials (2DLM) and the 2DCC-MIP at Penn State. The workshop aims to enhance synergy in the community and build toward a strong future in 2D crystal science and technology. The 2017 workshop consisted of morning tutorials and afternoon talks focused on 2D material synthesis, properties, theory and optic/electronics including talks on recent results from the 2DCC and other institutions and discussion on commercialization opportunities for 2D materials.

The poster session consisted of 35 student and 18 non-student entries. The 2DCC supported travel for 3 non-R1 institution attendees (1 PUI, 2 MSIs). A 2DCC user meeting was held prior to the workshop to evaluate facility progress, access and future directions from the user point of view.

Attendees Total: 178 (79 Students)
Unique Institutions: 56
Academic (30; including 8 non-R1)
Government (8)
Industry (17)
International (1)

In-House Research Highlights 2017

Light and lattice vibrations in a crystal do not interact strongly in general, but both interact strongly with electrons. This three-way interaction allows a characterization method known as double resonance Raman spectroscopy to map out the low-energy electronic structure of materials and assign vibrational signatures to different microscopic processes. Double resonance in two-dimensional MoS2 reveals the dynamics of excitons – robust elementary excitations of a 2D crystal – between two sets of low-energy states known as valleys. The accurate assignment of vibrational signatures elucidates the essential physics limiting the performance of a novel class of “valleytronic” devices exploiting the selectivity of valleys to incident light carrying different polarizations.

Identifying crystal defects in a 2D crystal usually requires an electron microscope to directly resolve atomic details, a complex and time-consuming process on expensive equipment that can damage the sample under electron irradiation. By establishing a correlation between the modified optical response and certain defects, the MIP team and collaborators have demonstrated a quick and non-destructive method of identifying defects in 2D crystals. The reason for this correlation is identified through first-principles calculations: electrons trapped by sulfur vacancies – the absence of a sulfur atom – have energies that are forbidden for electrons in defect-free regions, and therefore emit light at wavelengths different from that of the latter.

Doping modulates the electronic, chemical, and mechanical properties of materials. For a two-dimensional tungsten disulfide, although an isolated molybdenum substitution only perturbs the host lattice negligibly, it couples strongly to common lattice defects such as sulfur vacancies, as verified by state-of-the-art electron microscopy and atomistic modeling techniques. This coupling can be potentially exploited to controllably segregate undesirable defects away from the active areas of 2D crystalline devices.

Grain boundaries are borders that separate crystals of distinct orientations and are generally considered as inevitable by-products of crystalline regions nucleating at different locations during growth. The MIP team has predicted how grain boundaries in two-dimensional crystals can form within a single grain by introducing bumps onto the substrate – the “floor” on which a 2D crystal grows. A 2D crystal warps itself as growth advances past a bump, so much that it eventually runs into itself at an angle determined by steepness of the bump. Although aesthetically jarring, grain boundaries can endow 2D crystals with desirable mechanical, electronic, and magnetic properties. The predicted topographic control of grain boundaries offers the opportunity to engineer grain boundaries into 2D crystals with atomic-scale control.

Ferromagnetic topological insulators (TIs) have promise for applications in spintronics, metrology, and quantum computing.

However, TI materials are fragile and often incompatible with nanofabrication techniques. We have developed a technique that enables persistent, micron-scale optical control of both magnetization and chemical potential in Cr-(Bi,Sb)2Te3 grown by MBE on SrTiO3. This system is uniquely positioned to enable arbitrary routing of the quantized edge states recently discovered in magnetic TIs. We also use Kerr and photocurrent microscopies to image magnetic inversion dynamics, p-n junctions, and magnetic recordings that we make in these materials. This work may enable dynamic, reconfigurable control of 1D quantum channels.

Atomically thin two-dimensional layers such as molybdenum disulfide, MoS2, are promising materials for nanoelectronics due to their exceptional electronic and optical properties. An inter-atomic potential has been developed that can accurately describe the thermodynamic and structural properties of MoS2 sheets, including defects and transitions between different structural phases. A new type of “ripple” defect has been identified as a favorable host for sulfur vacancy defects. A train of moving ripplocation defects may be able to “sweep out” sulfur vacancy defects from key regions within 2D devices.

2DCC Research Highlights 2016

The chemical similarity of molybdenum & tungsten suggests they should randomly distribute in WxMo1-xS2, a material of great interest for next-generation elec-tronics. The 2DCC discovered that these atoms actually form thin chains, whose very different masses make properties like heat conduction anisotropic. Stripes form due to fluctuations in the availability of sulfur, the third element in WxMo1-xS2. Paradoxically, the disorder of random sulfur variations produces ordered stripes of metal. Such ‘order from disorder’ could be a general way to create atomic order in 2D materials, extending to the 2D domain the types of structures known from 3D semiconductor as “quantum wells.”