In-House 2DCC Research Highlights 2018

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.