Local-density-functional calculations are used to study the change of work functions induced by a layer of adsorbates. We investigated and compared the work function of a monolayer of Mo, Ag, Au, Fe, Co, Ni, Nb, Li, N, and O on W͑100͒, W͑110͒, W͑211͒, and W͑111͒ surfaces. While many systems obey the commonly accepted rule that electronegative adsorbates increase the work function of the surface, we find some exceptions. For example, overlayers of Fe, Co, and Ni increase the work function of W͑100͒, W͑211͒, and W͑111͒, but decrease the work function of the W͑110͒ surface, although the charge transfer is the same in all orientations. We found that even a layer of oxygen can decrease the work function of W͑100͒, although there are always electrons transferred from the W substrate to the oxygen adsorbates. In order to understand these results, we established the relationship between surface dipole density and work function within the framework of local-density formalism. It turns out that subtle details of the charge transfer can determine the sign and magnitude of surface dipole change, leading to a strong dependence on the orientation of the substrate, with the consequence that the work-function changes are not always governed by the sign and quantity of adsorbate induced charge transfer.

Carbon nanotubes can be viewed as rolled-up graphene sheets. As such, their work functions should be closely related to those of graphene due to geometric and structural similarities. In this paper, we have systematically investigated the work functions of single-walled and multiwalled carbon nanotubes by density functional calculations. The work functions of single-walled carbon nanotubes ͑SWCNTs͒ are very close to those of graphene in the armchair conformation, while for the zigzag and chiral conformations, the work functions are close to those of graphene as the diameter is larger than a certain threshold. When the diameter of the tube is smaller than 10 Å, the work functions of zigzag and chiral tubes increase dramatically as the diameter decreases. The deviation in the work function from that of graphene for small tubes can be explained and qualitatively estimated by the downshift of the Fermi level due to the curvature effect. For multiwalled carbon nanotubes ͑MWCNTs͒, we only consider zigzag and armchair MWCNTs. We find that the work functions of all armchair and zigzag MWCNTs with inner tube diameters larger than 10 Å are very close to those of graphene. The work functions of zigzag MWCNTs with inner tube diameters smaller than 10 Å exhibit significant variations depending on the diameters of the inner and outer tubes. Using a very simple model, we find that the work functions of MWCNTs can be successfully estimated from the work functions and electronic structures of the constituent SWCNTs.

Using first-principles calculations, we calculated the atomic structures and surface energies of molybdenum surfaces in the ͑100͒, ͑110͒, ͑111͒, and ͑211͒ orientations. The equilibrium crystal shape of molybdenum is then found using the Wulff construction. We find that all four orientations appear on the Wulff plot and hence they are stable. ͓S0163-1829͑98͒06803-9͔

Using first principles calculations, we studied the overlayer growth mode and the substrate stability when ultrathin layers of various metals are grown on a Mo(111) substrate. We found that the growth mode is Stranski-Krastanov, and the overlayer can induce the substrate to facet, in accordance with recent experimental observations. The growth-induced instability of the substrate towards faceting is driven by the enhancement of the surface energy anisotropy. However, faceting can be forbidden in some cases if the overlayer adsorption does not lower the surface formation energy significantly.[ S0031-9007(97)04532-8] PACS numbers: 68.35. Bs, 68.35.Md, 71.15.Nc Ultrathin metal films supported on metal substrates can have novel physical and chemical properties that render them useful in applications such as magnetic technology, catalysis, and material science. These novel properties depend on factors such as the overlayer-substrate interaction, the overlayer growth mode, and the substrate morphological change. Recently, some metal overlayers are observed to induce the substrate to facet [1]. Such a flat to hilland-valley surface reconstruction involves morphological changes in a macroscopic scale, and is quite different from the more familiar atomic scale adatom-induced surface reconstruction [2]. The main objective of this paper is to use first principles calculations to study the overlayer growth and the induced substrate faceting. We focus on various metallic overlayers on bcc (111) substrates, where there exist comprehensive experimental data [1,3,4]. These systems are particularly worth studying since they involve simultaneously the physics of the surfaces, thin films, bimetallic interfaces, and reconstructions in macroscopic length scales. Each of these aspects is interesting in its own right and when combined together, they present a complex and challenging problem that mandates the use of ab initio calculations for an accurate description at the atomic level. This is to our knowledge the first attempt using first principles calculations to address directly the problem of overlayer-induced faceting.When thin metal layers are grown on the (111) The general phenomenon of faceting has attracted much attention for almost a century [5,6], and the thermodynamic driving force is attributed to the surface energy anisotropy [5][6][7][8]. Low index clean metal surfaces seldom facet since the anisotropy is usually small, but stable metal surfaces can facet upon adsorption of O and Cl [9]. Embeddedatom [10] and earlier local-density approximation (LDA) results [11] indicate that metal overlayers can also enhance surface energy anisotropy significantly, although these phenomena are actually rather subtle since isoelectronic metals like Cu, Ag, and Au can behave differently [1].We seek to understand these observations by first principles calculations. We choose Mo as the substrate, and we consider the energetics of the adsorption and growth of pseudomorphic layers of different fcc metals (Cu, Ag, Au, Pd, Pt) on diff...

We report the joint studies of experimental and theoretical surface band structures of Nb(001).Angle-resolved photoelectron spectroscopy was used to determine surface-state dispersions along three