Wenhao Lu

Toward the growth of an aligned single-layer MoS2 film.

Molybdenum disulfide (molybdenite) monolayer islands and flakes have been grown on a copper surface at comparatively low temperature and mild conditions through sulfur loading of the substrate using thiophenol (benzenethiol) followed by the evaporation of Mo atoms and annealing. The MoS(2) islands show a regular Moiré pattern in scanning tunneling microscopy, attesting to their atomic ordering and high quality. They are all aligned with the substrate high-symmetry directions providing for rotational-domain-free monolayer growth.

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

Coalescence of 3-phenyl-propynenitrile on Cu(111) into interlocking pinwheel chains.

3-phenyl-propynenitrile (PPN) adsorbs on Cu(111) in a hexagonal network of molecular trimers formed through intermolecular interaction of the cyano group of one molecule with the aromatic ring of its neighbor. Heptamers of trimers coalesce into interlocking pinwheel-shaped structures that, by percolating across islands of the original trimer coverage, create the appearance of gear chains. Density functional theory aids in identifying substrate stress associated with the chemisorption of PPNs acetylene group as the cause of this transition.