This research explores room-temperature measurements of SCI at the interface between an archetypal Dirac semimetal (Cd3As2) and a conventional metallic ferromagnet (Ni0.8Fe0.2).
While previously, experimental studies of one-dimensional transform have been largely limited to earlier studies of thermal transport through single-walled carbon or boron nitride nanotubes, recent attempts have been made to probe thermal transport in quasi-1D van der Waals crystal nanowires, revealing interesting observations in the process.
Magnetic proximity effects have been studied in a variety of magnetic heterostructures for many applications including spintronics, valleytronics, and topological phenomena.
In order to expand the ReaxFF capability for support material simulations, the 2DCC group has extended a previously developed ReaxFF parameter set for BaTiO3 to SrTiO3 (STO) – a highly relevant support material for 2D-chalcogenide growth.
The interplay between topology in momentum space and topology in real space creates a vibrant playground for studying emergent phenomena in condensed matter physics.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
We present modulator designs based on graphene-metal hybrid plasmonic metasurfaces with highly enhanced light- graphene interaction in the nanoscale hot spots at pump and probe (signal) wavelengths. Based on this design concept, we have demonstrated high-speed all optical modulators at near and mid-infrared wavelengths (1.56 um and above Gum) with significantly reduced pump fluence (1-2 orders of magnitude) and enhanced optical modulation.
This study not only offers a way to build nanoscale junctions but also provides fundamental understandings of the electronic and optoelectronic properties of vdW nanowires and their heterojunctions.
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.
Through an in-depth investigation by ARPES experiment and the numerical calculations, an upper bound of 3 meV for the gap size of the topological surface state (TSS) is estimated. Furthermore, we also reveal band chiralities for both the TSS and quasi-2D bands, which can be well reproduced in a band hybridization model based on the circular dichroism measurements.
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.
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.
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 paper reports the first clear evidence for the THE in heterostructures that combine a model topological insulator (Bi2Se3) with a ferromagnetic insulator (BaFe12O19).
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.
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.
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.
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 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.
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.”
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.
To assess the potential of transition metal dichalcogenides (MDs) 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 MoS, and WS.
In this highlight, we identified newly discovered low-frequency (LF) (<100 cm−1 ) Raman features due to the formation of unique 2D polar metals (Ag, Cu, Pb, Bi, Ga, In) or metal alloys (InxGa1−x) intercalated at an epitaxial graphene (EG)/silicon carbide (SiC) interface and demonstrate that 2D-Ag and 2DGa can have spatially distinct phases with their own unique Raman responses. Additionally, we establish that the 2D-Ga exhibits a structural evolution as a function of temperature, independent of the SiC and EG, that can lead to nucleation of secondary phases.
Professor Shuolong Yang group at the University of Chicago has set up a new integrated platform for multiresolution photoemission spectroscopy (MRPES) that integrates a helium discharge lamp, a narrow bandwidth 6 eV laser, and a tunable ultrafast laser, which effectively combines static ARPES, time-resolved ARPES (trARPES), and micro-ARPES (μARPES).
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.
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.
While previous studies show a large discrepancy of the charge-spin conversion efficiency in topological insulators (TI), the 2DCC research group reports spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature.
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.
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.
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.
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. 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.
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 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.
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.
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.
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.
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).
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.
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.
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).
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.
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.
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.
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.
Prof. Lanzara' group at Lawrence Berkeley National Laboratory recently studied the anomalous CDW phase transition of TaTe using APES and low-energy electron diffraction. Their data reveal the presence of a surprising quasi- one-dimensional Fermi surface with nesting condition. They find that the wave vectors of the Fermi surface nesting and periodic lattice distortion are different, suggesting the decoupled formation between charge and lattice orders. This work provides routes for forging unconventional CW phases and charge- lattice entanglement that would otherwise not be available in materials with fixed dimensionality.
A multi-dimensional optical imaging technique that combines scattering was developed to map subdiffractional distributions of doping and strain in MoS2 and MoS2 /graphene vertical heterostructures. The variations in doping and strain were correlated to electronic properties and were used to understand the behavior of biosensors fabricated using the 2D material.
This study demonstrates a facile route to obtain wafer-scale monolayer amorphous MoO, using monolayer 2D MoS, grown by metalorganic chemical vapor deposition (MOCVD) as a starting material, following by UV-ozone oxidate at substrate temperatures as low as 120-C. The process yields smooth, continuous, uniform and stable monolayer oxide with wafer-scale homogeneity.
In this highlight, 2DCC and PARADIM team up to work with the inventor of remote epitaxy, Prof. Kim (MIT), to unveil the respective roles and impacts of the substrate material, graphene, substrate–graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single crystal epitaxy in the overlying film.
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 paper reports the first clear evidence for the THE in heterostructures that combine a model topological insulator (Bi2Se3) with a ferromagnetic insulator (BaFe12O19).
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.
In this study, we measured the nonequilibrium carrier dynamics in an air-stable 2-D polar metal heterostructures. Using transient absorption spectroscopy, the mechanism for energy dissipation was determined to involve contributions from the various components of the heterostructure, including the 2-D metal layer, SiC substrate, and graphene capping layer.
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.
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