Carbon alloy catalysts (CACs) are promising oxygen reduction reaction (ORR) catalysts to substitute platinum. However, despite extensive studies on CACs, the reaction sites and mechanisms for ORR are still in controversy. Herein, we present rather general consideration on possible ORR mechanisms for various structures in nitrogen doped CACs based on the first-principles calculations. Our study indicates that only a particular structure of a nitrogen pair doped Stone-Wales defect provides a good active site. The ORR activity of this structure can be tuned by the curvature around the active site, which makes its limiting potential approaching the maximum limiting potential (0.80 V) in the volcano plot for the ORR activity of CACs. The calculated results can be compared with the recent experimental ones of the half-wave potential for CAC systems that range from 0.60 to 0.80 V in the reversible-hydrogen-electrode (RHE) scale.

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We demonstrate a hydrostatic pressure-induced hard-to-soft transition of an isolated single wall carbon nanotube, using classical and ab initio constant-pressure molecular-dynamics simulations and continuum elastic theory analysis. At low pressure, the carbon tube is hard. Above a critical pressure, the tube becomes much softer with a decrease of bulk modulus by two orders of magnitude. The hard-to-soft transition is caused by a pressure-induced shape transition of the tube cross section from circular to elliptical.

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We report in this Letter that when radiation is incident on a metal surface perforated with an array of ring-shaped subwavelength apertures, the phase difference between the propagating surface Bloch wave and the localized surface wave can be tailored by the geometrical parameters of the array so as to affect the shape of the transmission spectrum. Above the resonant frequency of the aperture, interference between the two kinds of surface waves leads to a minimum in the transmission spectrum, whereas below it, the interference leads to a maximum. We suggest that this feature provides flexibility in engineering surface-wave-based all-optical devices.

The self-assembly of iron dots on the insulating surface of NaCl (001) is investigated experimentally and theoretically. Under proper growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting layer. Furthermore, both the vertical and lateral sizes of the dots can be tuned with the iron dosage without introducing apparent size broadening, even though the clustering is clearly in the strong coarsening regime. These observations are interpreted using a phenomenological mean-field theory, in which a coverage-dependent optimal dot size is selected by strain-mediated dot-dot interactions.PACS numbers: 81.07. Ta,61.46.+w,68.37.Ps,68.35.Md Clustering on surfaces by nucleation and growth during atom deposition has been an important subject in basic and applied science for decades [1,2]. Recent efforts have been focused on searching for methods to obtain nanometer-scale clusters with narrow size distributions. Such clusters or quantum dots are potentially valuable for optical, electronic, and magnetic device applications, but mass production of such structures by lithography or etching-based fabrication has proved to be exceptionally challenging [3,4]. Alternatively, it has been realized that the strain energy associated with the lattice mismatch between the dot and the substrate materials can be exploited to induce self-assembled formation of quantum dots with narrow size distributions. This has generated much excitement, particularly in the area of semiconductor quantum dots [3,4,5,6,7,8]. In such cases, the growth of the dots often proceeds in the StranskiKrastanow (SK) mode, which is characterized by the presence of a wetting layer prior to three-dimensional (3D) clustering. To date, the precise mechanism for size selection in semiconductor quantum dot systems remains a subject of active debate [4,5,6,9]: some attribute them to strain-induced thermodynamic equilibrium states, while others associate them with metastable configurations due to kinetic limitations.Although improved size uniformity can be achieved in quantum dot formation via the SK growth mode, the presence of a wetting layer is often undesirable, particularly for electronic and magnetic device applications of metallic/magnetic quantum dots. For this reason, it is preferred to fabricate quantum dots in the VolmerWeber (VW) growth mode, which is characterized by immediate 3D clustering on the substrate surface. Indeed, considerable recent efforts have been devoted to metallic/magnetic quantum dot formation on various substrates [10,11,12], but no significant size uniformity has been achieved in such studies.In this Letter we investigate the self-assembly of iron dots on NaCl(001), an insulating substrate, by thermal deposition and variable-temperature atomic force microscopy in ultrahigh vacuum. We show that, by properly choosing the growth conditions, nanometer-scale magnetic iron dots with remarkably narrow size distributions can be achieved in the absence of a wetting l...

Using an empirical Tersoff-Brenner many-body potential for the carbon atoms and a model Lennard-Jones interaction between the adatom and carbon atoms, the diffusion of an adatom over curved surfaces of C nanotubes is studied by calculating the potential energy surface and performing molecular dynamics simulation. The average curvature of the surface is found to have an important influence on the diffusion of the adatom. Positive curvature increases the diffusion barrier and corrugates the potential energy surface, while the negative curvature smoothes the potential energy surface, therefore it lowers the diffusion barrier. We also find that nanotube helicity can play an important role on the diffusion path, thus the adatom has different diffusion path for the armchair and zigzag nanotube. The nature of the curvature effect on the surface diffusion is connected with the strain effect.

Comprehensive first-principle calculations on strained rutile TiO2(110) indicate that the formation energy of different types of oxygen vacancies depends on the external strain. For the unstrained state, the energetically favorable oxygen vacancy (EFOV) appears on the bridging site of the first layer; when 3% tensile strain along [11[over ]0] is applied, EFOV moves to the in-plane site, while 2% compressive strain along either [001] or [11[over ]0] shifts EFOV to the subbridging site. We therefore suggest that the distribution of oxygen vacancies can be engineered by external strain, which may help to improve the applications of a TiO2 surface where oxygen vacancy plays an important role.