Jonathan Wyrick

Controlled argon beam-induced desulfurization of monolayer molybdenum disulfide

Sputtering of MoS2 films of single-layer thickness by low-energy argon ions selectively reduces the sulfur content of the material without significant depletion of molybdenum. X-ray photoelectron spectroscopy shows little modification of the Mo 3d states during this process, suggesting the absence of significant reorganization or damage to the overall structure of the MoS2 film. Accompanying ab initio molecular dynamics simulations find clusters of sulfur vacancies in the top plane of single-layer MoS2 to be structurally stable. Measurements of the photoluminescence at temperatures between 175 and 300 K show quenching of almost 80% for an ~10% decrease in sulfur content.

Do Two-Dimensional “Noble Gas Atoms” Produce Molecular Honeycombs at a Metal Surface?

Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.

Power of Confinement: Adsorbate Dynamics on Nanometer-Scale Exposed Facets

The diffusion and arrangements of CO adsorbates within nanometer-scale pores on a copper surface are investigated by low-temperature scanning tunneling microscopy. In contrast to extended terraces, confinement stabilizes dislocation lines that expose more than one-fourth of the adsorbate population to potentially more reactive adsorption configurations. Confinement allows correlation between adsorbate diffusivity and the number of adsorbates in the pore. A marked increase is found that coincides with the absence of dense films on the exposed facets. In combination, we find that in confinement CO molecules are much more likely to be at adsorption sites that allow lateral access, in contrast to the dense and uniform films on extended terraces.

An MoSx Structure with High Affinity for Adsorbate Interaction

MoS2 is an intriguing material: although its basal plane is quite inert, it is the key catalyst for petrochemical hydrodesulfurization (and hydrodenitrogenation) processes. Dow/ Union Carbide developed an MoS2-based catalyst [1] for the formation of higher alcohols from syngas, an application which is gaining increased importance with the emergence of biofuels. In these applications, MoS2 is used as a fine powder; cobalt or nickel (or mixtures thereof) activate the powder through incorporation into edges of the MoS2 [2] structures. Further promotion is achieved by alkali doping with carbon typically serving as the support. Quite recently, MoS2 has attracted increasing interest as an exfoliatable monolayer material for (opto-)electronic applications, and as a surface material for electrochemical reactions, among other applications. Several studies have succeeded in growing MoS2 on various substrates and have shown that its catalytic activity may be ascribed to a metallic electronic state at the brim of MoS2 triangular clusters, which can be readily identified in scanning tunneling microscopy (STM). We have recently developed a technique for growing MoS2—by evaporating molybdenum on a sulfur-preloaded Cu(111) surface—that leads to epitaxial MoS2 islands of sizes ranging from approximately 1 to 100 nm and for which we have confirmed the presence of the brim state. Herein, we demonstrate that another novel MoSx structure, reproducibly formed in the same fashion as in the growth of MoS2 we recently performed, is stable in the entire temperature range of our experiments (25 K to 650 K) and reverts to its pristine form after exposure to oxygen-containing adsorbates upon annealing. More importantly, this structure interacts far more strongly with these adsorbates than MoS2. Analysis of STM images and related electronic structure calculations confirm the metallic nature of this monolayer material, which we rationalize below to have the composition Mo2S3. We chose anthraquinone (AQ) as test adsorbate, because it is large and rigid enough that we can directly image its adsorption geometry, from which we can derive insight into the interaction of the surface with the adsorbate. The sample preparation described in the Supporting Information gives two thermally stable MoSx patterns (Figure 1a). The patches formed by both patterns are capable of extending across substrate steps. Regions not covered with an MoSx patch exhibit the well-known ffiffiffi

Quasiparticle scattering from topological crystalline insulator SnTe (001) surface states | NIST

Recently, the topological classification of electronic states has been extended to a new class of matter known as topological crystalline insulators. Similar to topological insulators, topological crystalline insulators also have spin-momentum locked surface states; but they only exist on specific crystal planes that are protected by crystal reflection symmetry. Here, we report an ultra-low temperature scanning tunneling microscopy and spectroscopy study on topological crystalline insulator SnTe nanoplates grown by molecular beam epitaxy. We observed quasiparticle interference patterns on the SnTe (001) surface that can be interpreted in terms of electron scattering from the four Fermi pockets of the topological crystalline insulator surface states in the first surface Brillouin zone. A quantitative analysis of the energy dispersion of the quasiparticle interference intensity shows two high energy features related to the crossing point beyond the Lifshitz transition when the two neighboring low energy surface bands near the point merge. A comparison between the experimental and computed quasiparticle interference patterns reveals possible spin texture of the surface states.

Acetylene on Cu(111): imaging a molecular surface arrangement with a constantly rearranging tip.

Acetylene on Cu(111) is investigated by scanning tunnelling microscopy (STM); a surface pattern previously derived from diffraction measurements can be validated, if the variation of the STM image transfer function through absorption of an acetylene molecule onto the tip apex is taken into account. Density functional theory simulations point to a balance between short-range repulsive interactions of acetylene/Cu(111) associated with surface stress and longer range attractive interactions as the origin of the ordering.