Broader Impacts Highlights 2018

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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.

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.

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)

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.”