RESUMEN
The effects of strain on the work functions of tungsten surfaces covered with a monolayer of adsorbates have been studied systematically using ab initio density functional theory. It has been found that the strain on tungsten surfaces due to different atomic coverages exhibits very interesting influences on the surface work function. For a clean tungsten surface, a compressive strain more profoundly increases the work function than a tensile strain, and the strain dependence of the work function shows a concave trend. With an atomic coverage of adsorbates, the strain dependence of the work function on the tungsten surface can be dramatically changed or modulated to a linearly increasing, linearly decreasing, convex, or sinusoidal behaviour, depending on the types of atoms. Using the framework of the well-developed surface dipole model [Phys. Rev. B, 2003, 68, 195408], the result of modulation of strain effects on the work function induced by different adsorbates can be well understood and attributed to two contributions, one from the relaxation of the substrate induced by the overlayer and the other from the surface dipole moment. These contributions are strongly correlated with the interlayer distances modulated by the adsorbates and strains. It is found that the O adsorbate-induced modulation of the strain effect on the work function exhibits a strong linear dependence on a uniaxial strain, and this may have applications in reducing the work function of cathodes by applying an external strain.
RESUMEN
The field screening effect on the electronic and field-emission properties of zigzag graphene nanoribbons (ZGNRs) has been studied using first-principles calculations. We have systematically investigated the effects of inter-ribbon distance and ribbon width on the work function, field enhancement factor, band gap and edge magnetism of zigzag graphene nanoribbons (ZGNRs). It is found that the work function of ZGNRs increases rapidly as the inter-ribbon distance Dx increases, which is caused by the positive dipole at the edge of the ribbon. For a given Dx, the work function of ZGNRs decreases as the ribbon width W increases. The wider the ribbon, the stronger the effect of inter-ribbon distance on the work function. Using a simple linear interpolation model, we can obtain the work function of ZGNRs of any ribbon-width. In the case of Dx < W, the field enhancement factor increases rapidly as the inter-ribbon distance increases. As we further increase Dx, the enhancement factor increases slowly and then tends toward saturation. The inter-ribbon distance of ZGNRs can modulate the magnitude of the band gap and edge magnetism. These observations can all be explained by the screening effect.
RESUMEN
The field screening effect on the field-emission properties of armchair graphene nanoribbons (AGNRs) under strain has been studied using first-principles calculations with local density approximation (LDA). Using the zone folding method with the effect of a dipole barrier along with the work function of strained graphene, we can obtain the work function of AGNR of any width under strain, confirmed with the LDA calculations. We have systematically investigated the effects of inter-ribbon distance and ribbon width on the work function of AGNR arrays. It is found that the work function of AGNR arrays increases rapidly as the inter-ribbon distance D x increases, which is caused by the positive dipole at the edge of the ribbon. Using a simple linear interpolation model, we can obtain the work function of AGNRs of any ribbon-width and inter-ribbon distance. The dependences of the inter-ribbon distance and strain on the field enhancement factor have been determined. The enhancement factor reaches about 90% of its saturated value as the inter-ribbon distance approaches two times the ribbon-width. For a tensile strain, the field enhancement factor increases with applied strain while for a compressive one, the field enhancement factor is nearly independent. The effects of inter-ribbon distance and strain on the enhancement factor can be explained by the interlayer and intralayer screening effects, respectively.
