Single-layer MoS 2 is proving to be a versatile material for a wide variety of electronic, optical, and chemical applications. Sulfur depletion, without destabilization of the single layer, is considered a prudent way for making the basal plane of the layer catalytically active. Based on the results of our density-functional-theory examination of vacancy structures on one side of an MoS 2 layer, we show that the formation energy per sulfur vacancy is the lowest (energetically favorable) when the vacancies form a row and that the longer the row, the lower the formation energy. In addition, we find that the lowest energy barrier for the diffusion of sulfur vacancy at the row structures through the exchange of a vacancy with a nearby sulfur atom is 0.79 eV and that this barrier increases as the row elongates. We also evaluate the propensity for catalytic activity of an MoS 2 layer with two types of sulfur-vacancy structures (row and patch) and find the energetics for alcohol synthesis from syngas to be more favorable for the layer with a sulfur-vacancy patch.

Our calculated vibrational properties of Ag nanocrystals show three novel, size dependent features which have broad implications for their thermodynamic properties and stability. There is an enhancement in the vibrational density of states at low frequencies and an overall shift of the high frequency band beyond the top of the bulk phonons. Additionally, the vibrational projected density of states of the surface atoms scales linearly with frequency, at low frequencies. The generality of these results for systems with bond-order -bond-length correlation, low average coordination, and large ratio of surface to bulk atoms helps explain qualitatively several recent experimental observations. [S0031-9007(98)06877-X] PACS numbers: 63.20.Dj, 61.46. + w, 68.35.Ja, 79.60.Jv For the last few decades, studies of surface structure and dynamics have yielded a wealth of information from which a detailed understanding of phenomena at and near the surface have been derived. An important aspect of this body of knowledge is information about structural and dynamical changes at surfaces. As a result of reduced coordination, most surfaces relax inward. The inward relaxation can be explained by the smoothing of the charge density at the surface known as the Smoluchowski [1] effect, as first presented by Finnis and Heine [2] for Al surfaces and put on a more comprehensive basis by Feibelman [3] based on the ideas of bond-orderbond-length correlation advanced by Pauling [4]. For an extended system with a surface, these structural changes will be localized at and near the surface, as the surfaceto-volume ratio is vanishingly small. The presence of the surface thus does not affect the thermodynamic properties of the extended system in a noticeable way. But what will happen to the structure and dynamics of a system with a finite surface-to-volume ratio? How would the properties of these mesoscopic systems scale with the surface-tovolume ratio? These and related questions about novel characteristics resulting from the large surface-to-volume ratio surround the intense investigations pursued these days on the subject of nanostructures.In this Letter we present results for the structure, dynamics, and thermodynamics of metallic nanocrystals with different sizes and show how the characteristics of the vibrational dynamics scale with size. For this purpose, we have investigated the structural properties of Ag, Cu, and Ni nanocrystals with diameters ranging between 2.0 and 5.0 nm. We find that in all cases there is a global shrinkage of the nearest neighbor distance which eventually leads to an overall shift of the high frequency band in the vibrational density of states (DOS) to beyond the top of the bulk phonons. We show that this shrinkage is a characteristic of metals which have a bond-orderbond-length correlation. Our calculated vibrational DOS for the nanocrystals also exhibits a higher population at low frequencies, as compared to that in the bulk. We show that this enhancement is due to the contribution of the surface atoms for which...

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphene's electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

We find that nearly monodisperse copper oxide nanoparticles prepared via the thermal decomposition of a Cu(I) precursor exhibit exceptional activity toward CO oxidation in CO/O 2 /N 2 mixtures. Greater than 99.5% conversion of CO to CO 2 could be achieved at temperatures less than 250°C for over 12 h. In addition, the phase diagram and pathway for CO oxidation on Cu 2 O (100) is computed by ab initio methods and found to be in qualitative agreement with the experimental findings.Nanoparticles offer a larger surface-to-volume ratio and a higher concentration of partially coordinated surface sites than the corresponding bulk materials. The unique properties of nanoparticles are due to a strong interplay between elastic, geometric, and electronic parameters, as well as the effects of interactions with the support. The result of these features is often improved physical and chemical properties compared to the bulk material. It is for these reasons that heterogeneous catalysis at nanoparticle surfaces is currently under intense investigation in the catalysis community at large. 1,2 Conventional supported catalysts are generally produced by impregnation of a support medium with the desired metal ions followed by thermal treatments that result in small and dispersed active catalytic sites. 3,4 Many traditional catalysts based on the impregnation method 5,6 rely on the noble metals, in particular platinum, as the source for high activity. Such metals are recognized as a scarce resource as well as a limiting step in the development of viable energy alternatives to petroleum. Automotive exhaust catalysts (the three way catalyst) and fuel cells are examples of this tenet. 5,6 Any new system that overcomes these limitations will be invaluable.There are several important processes in heterogeneous catalysis where removal of carbon monoxide is either desired or absolutely necessary, such as in the postprocessing of Syngas 7 to produce hydrogen as an energy source for use in fuel cells. 8,9 A byproduct of this reaction is CO; however, trace amounts of CO (>10 ppm) can poison a fuel cell electrode, drastically reducing its efficiency. [10][11][12] The CuCu 2 O-CuO system has been known to facilitate oxidation reactions in the bulk, suggesting it has potential as a costeffective substitute for noble metals in various catalytic systems. [13][14][15] Here we describe a cheap, effective method of using copper oxide nanoparticles loaded onto silica gel as an exceptional catalyst toward CO oxidation at relatively low temperatures. Over sustained periods of time, conversions of 99.5% of CO to CO 2 are routinely observed and the catalyst structure is retained during the reaction. For example, during a 50 h period with the same ∼10 mg sample of copper oxide nanoparticles, over 30 L of CO is converted to CO 2 with an average conversion of 98 ( 1%.With recent developments in nanoparticle synthesis leading to the ability to control size, reproducibility, and structural complexity, it becomes urgent to define specific target structu...

In the quest for increased control and tuneability of organic patterns at metal surfaces, more and more systems emerge that rely upon coordination of metal adatoms by organic ligands using endgroups such as carbonitriles, amines, and carboxylic acids.[1] Such systems promise great flexibility in the size and geometry of the surface pattern through choice of the ligand shape, the number and arrangement of ligating endgroups, and the nature of the metal centers. Planar (trigonal or square) arrangements of ligands around metal centers occur most commonly as a result of attractive interactions of the ligands with the substrate. In contrast, in the solution phase planar, and in particular trigonal planar, arrangements are quite rare and generally require ligands whose nature (for example bidentate, pincer shape) forces planarity.Given the relatively short history of the field of surface coordination chemistry, compared to its solution-phase counterpart, it is of great interest to know which information can be gleaned from the latter to predict that for the former.

Abstract. We propose an accurate tight-binding parametrization for the band structure of MoS 2 monolayers near the main energy gap. We introduce a generic and straightforward derivation for the band energies equations that could be employed for other monolayer dichalcogenides. A parametrization that includes spin-orbit coupling is also provided. The proposed set of model parameters reproduce both the correct orbital compositions and location of valence and conductance band in comparison with ab initio calculations. The model gives a suitable starting point for realistic large-scale atomistic electronic transport calculations.

Step edges and low-symmetry faces of metal crystals can restrict the diffusive motion of adsorbates, yet they offer little flexibility with regards to the location and/or direction of the guided motion. We show inherently unidirectional motion of an organic molecule on a high-symmetry thermodynamic-equilibrium metal surface [Cu(111)]. Sequential placement of the substrate linkers of 9,10-dithioanthracene prevents it from rotating or veering off course. A combination of low temperature scanning tunneling microscopy and density functional theory simulations provide atomistic insight.

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.