Quan Ma

2-Dimensional Transition Metal Dichalcogenides with Tunable Direct Band Gaps: MoS2(1–x)Se2x Monolayers

MoS2(1-x) Se2x single-layer films are prepared using a mixture of organic selenium and sulfur precursors as well as a solid molybdenum source. The direct bandgaps are found to scale nearly linearly with composition in the range of 1.87 eV (pure single-layer MoS2 ) to 1.55 eV (pure single-layer MoSe2 ) permitting straightforward bandgap engineering.

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.

Postgrowth Tuning of the Bandgap of Single-Layer Molybdenum Disulfide Films by Sulfur/Selenium Exchange

We demonstrate bandgap tuning of a single-layer MoS2 film on SiO2/Si via substitution of its sulfur atoms by selenium through a process of gentle sputtering, exposure to a selenium precursor, and annealing. We characterize the substitution process both for S/S and S/Se replacement. Photoluminescence and, in the latter case, X-ray photoelectron spectroscopy provide direct evidence of optical band gap shift and selenium incorporation, respectively. We discuss our experimental observations, including the limit of the achievable bandgap shift, in terms of the role of stress in the film as elucidated by computational studies, based on density functional theory. The resultant films are stable in vacuum, but deteriorate under optical excitation in air.

Occupied and unoccupied electronic structure of Na doped MoS2(0001)

The influence of sodium on the band structure of MoS2(0001) and the comparison of the experimental band dispersion with density functional theory show excellent agreement for the occupied states (angle-resolved photoemission) and qualitative agreement for the unoccupied states (inverse photoemission spectroscopy). Na-adsorption leads to charge transfer to the MoS2 surface causing an effect similar to n-type doping of a semiconductor. The MoS2 occupied valence band structure shifts rigidly to greater binding with little change in the occupied state dispersion. Likewise, the unoccupied states shift downward, approaching the Fermi level, yet the amount of the shift for the unoccupied states is greater than that of the occupied states, effectively causing a narrowing of the MoS2 bandgap.

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

Symmetry-resolved surface-derived electronic structure of MoS2(0 0 0 1)

We find a wave vector dependence of the band symmetries for MoS(2)(0 0 0 1) in angle-resolved photoemission. The band structures are found to be significantly different for states of even and odd reflection parities, despite the absence of true mirror plane symmetry away from Γ, the Brillouin zone center, along the line to the K point, at the Brillouin zone edge. Our measurements agree with density functional theory (DFT) calculations for each band symmetry, with the notable exception of the Mo 4d(x(2)-y(2)) contributions to the valence band structure of MoS(2)(0 0 0 1). The band structure is indicative of strong S 3p and Mo 4d hybridization. In particular, the top of the valence band is predominantly composed of Mo 4d(3z(2)-r(2)) derived states near Γ, whereas near K Mo 4d(x(2)-y(2)) as well as Mo 4d(xy) dominate. In contrast, the bottom of the valence band is dominated by Mo 5s and S 3p(z) contributions.

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.