Chemical vapor deposition allows the preparation of few-layer films of MoTe2 in three distinct structural phases depending on the growth quench temperature: 2H, 1T′, and 1T. We present experimental and computed Raman spectra for each of the phases and utilize transport measurements to explore the properties of the 1T MoTe2 phase. Density functional theory modeling predicts a (semi-)metallic character. Our experimental 1T films affirm the former, show facile μA-scale source-drain currents, and increase in conductivity with temperature, different from the 1T′ phase. Variation of the growth method allows the formation of hybrid films of mixed phases that exhibit susceptibility to gating and significantly increased conductivity.Keywords: chemical vapor deposition; metal semiconductor transition; phase change material; thin-layer materials; transition-metal dichalcogenides;

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.Keywords: atomically thin films; bandgap engineering; CVD; molybdenum diselenide; molybdenum disulfide; sputtering; transition metal dichalcogenides

A graphene-supported Ag3PO4/Ag/AgBr water oxidation photocatalyst was prepared by a photoassisted deposition–precipitation reaction, followed by a hydrothermal treatment. The composite photocatalyst exhibits double the O2-production activity than that of bare Ag3PO4 under visible light irradiation. Moreover, it exhibits enhanced activity in comparison to unsupported Ag3PO4/Ag/AgBr, to graphene-supported bare Ag3PO4 powder as well as to Ag/AgBr powder. This increase in activity is attributed to a combination of depletion of the conduction band of the as-synthesized n-doped Ag3PO4 material and a downshift of the Ag3PO4 valence band due to the pinning of its conduction band at the silver Fermi level, a process that is assisted by charge transfer and distribution onto the graphene support.

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.

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 MoS2 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.

The application of steric blocking in surface science is exemplified by the control of surface patterns through the selective methylation of pentacenetetrone. Pentacenetetrones interact (with one another) on Cu(111) via intermolecular hydrogen bonding involving the carbonyl oxygen and the adjacent hydrogen atoms. Steric blocking of the intermolecular interaction by the successive insertion of inert methyl groups at terminal locations transforms a dense molecular pattern first into isolated double rows and eventually into single rows in a highly predictable fashion. Density functional theory modeling reveals the underlying energetics.

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.

The diffusion temperature of molecular ‘walkers’, molecules that are capable of moving unidirectionally across a substrate violating its symmetry, can be tuned over a wide range utilizing extension of their aromatic backbone, insertion of a second set of substrate linkers (converting bipedal into quadrupedal species), and substitution on the ring. Density functional theory simulation of the molecular dynamics identifies the motion of the quadrupedal species as pacing (as opposed to trotting or gliding). Knowledge about the diffusion mode allows us to draw conclusions on the relevance of tunneling to the surface diffusion of polyatomic organic molecules.

Abstract

We found that anthraquinone diffuses along a straight line across a flat, highly symmetric Cu(111) surface. It can also reversibly attach one or two CO₂ molecules as “cargo” and act as a “molecule carrier,” thereby transforming the diffusive behavior of the CO₂ molecules from isotropic to linear. Density functional theory calculations indicated a substrate-mediated attraction of ∼0.12 electron volt (eV). Scanning tunneling microscopy revealed individual steps of the molecular complex on its diffusion pathway, with increases of ∼0.03 and ∼0.02 eV in the diffusion barrier upon attachment of the first and second CO₂ molecule, respectively.

Abstract

Anthraquinone molecules self-assemble on a Cu(111) surface into a large two-dimensional honeycomb network ()R23° with pore diameters of ≈50 Å. The spontaneous formation of a pattern containing pores roughly five times larger than the size of the constituent molecules is unprecedented. The network originates from a delicate balance between substrate-mediated repulsion and intermolecular attraction involving an unusual chemical motif: hydrogen bonding between a carbonyl oxygen and an aromatic hydrogen atom. Substrate-mediated long-range adsorbate-adsorbate repulsion has been observed on anisotropic surfaces and in the context of the absence of pattern formation. Its applicability for the design of tailored molecular films is explored here.

A systematic study of the dehydrogenation of substituted thiophenols by controlled charge injection from the tip of a scanning tunneling microscope (STM) reveals a pronounced dependence of the reaction yield on the position and the chemical nature of the substituent. We evaluate the dehydrogenation rate of para-halo-substituted species within a linear free energy relationship, namely the Hammett equation. The resultant ρ value of 1.4 can faithfully predict the reaction rates of molecules that are meta-halo-substituted or para-methyl-substituted. The positive sign of ρ suggests a negatively charged transition state at the core of the STM-induced process, and the magnitude of the ρ value indicates that the presence of the substrate does not preclude substantial substituent effects. The applicability of the Hammett equation to single-molecule chemistry offers facile prediction of the rate of STM-based single-molecule chemistry in a field, which so far has been addressed by focusing on involved quantum-mechanical modeling of its underlying processes.