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.

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.

Coherently strained InAs quantum dots (QDs) with narrow inhomogeneous broadening were grown by molecular beam epitaxy (MBE) using the Stranski-Krastanov (SK) growth mode. The increase in the InAs dot size and decrease in the dot density were induced by surface migration enhancement due to the low arsenic pressure below 6×10-7 Torr. In addition, the low arsenic pressure and the low growth rate produced the self size-limiting effect, which was attributed to the inhibition of indium incorporation. As a result, the narrowest photoluminescence (PL) linewidth of 18.6 meV (14 K) was successfully obtained from the single InAs/GaAs QD layer.

An analytical in-situ technique was utilized to evaluate reactant gas transport resistance (R other ) in catalyst layers (CLs). It was found that R other was increased with the decrease of Pt-loadings of the CLs although their thickness was decreased. Effective Knudsen diffusion coefficient was calculated from the pore size distribution and the porosity. It was estimated almost steady regardless of the Pt-loadings. To understand the increase of R other , a simple transport model was established including Knudsen diffusion resistance in secondary pores and the local transport resistance around the Pt surface. Knudsen diffusion resistance should be decreased in lower Pt-loadings because of the shorter transport length. On the other hand, it was considered that the local transport resistance was increased with decreasing of the effective Pt surface area. Hence, that the effective Pt surface area might be one of the important factors for R other in the CLs, especially in the lower Pt-loadings.

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.