Step-lattice-induced band-gap opening at the fermi level

The interaction of the Shockley surface state with the step lattice of vicinal Cu(111) leads to the formation of an electronic superlattice state. On Cu(443), where the average terrace length forms a "shape resonance" with the Fermi wavelength, we find a step-lattice-induced band-gap opening at the Fermi level. A gap magnitude >200 meV is inferred from high resolution photoemission experiments and line shape analysis. The corresponding energy gain with respect to a gapless case is approximately 11 meV/unit cell, and is a substantial contribution to the stabilization of the step lattice.     

Cu台阶面多层弛豫的第一性原理研究

采用基于密度泛函理论的第一性原理赝势平面波方法对Cu(311),(511),(331)和(221)四个高指数台阶表面的弛豫结构和弛豫后表面各层的电子特性进行了系统研究.发现四个台阶面的层间弛豫规律依次为-+-…,--+-…,--+-…和---+-…,与其平台-阶梯n(hkl)×(uvw)的表示法2(100)×(111),3(100)×(111),3(111)×(111)和4(111)×(111)中的原子排数n相关,即     

Binding of In and Pb surfactants on Cu{1 1 1} surfaces

Surfactants generally affect both diffusion and nucleation on surfaces, and their function depends on their spatial distribution. Using density-functional-theory based ab initio calculations, the authors have determined the binding energies of In and Pb surfactants at various surface sites of Cu{1 1 1}. The calculation results show that In surfactant prefers incorporation inside/near surface steps or inside top terrace layer, in contrast to at adatom positions. The relative preference among step and terrace sites depends on the availability of surface vacancies, which form either during deposition without sufficient diffusion or by thermal activation. The preference of In surfactant inside top terrace layer makes it effective in the generation of strain on terrace and the slow-down of Cu adatom diffusion. In contrast, incorporation of Pb surfactant inside top terrace is energetically less preferable. As a result, Pb surfactant is less effective in the generation of strain on terrace.     

Reaction mechanism for CO oxidation on Cu(3 1 1): A density functional theory study

The microscopic reaction mechanism for CO oxidation on Cu(3 1 1) surface has been investigated by means of comprehensive density functional theory (DFT) calculations. The elementary steps studied include O₂ adsorption and dissociation, dissociated O atom adsorption and diffusion, as well as CO adsorption and oxidation on the metal. Our results reveal that O₂ is considerably reactive on the Cu(3 1 1) surface and will spontaneously dissociate at several adsorption states, which process are highly dependent on the orientation and site of the adsorbed oxygen molecule. The dissociated O atom may likely diffuse via inner terrace sites or from a terrace site to a step site due to the low barriers. Furthermore, we find that the energetically most favorable site for CO molecule on Cu(3 1 1) is the step edge site. According to our calculations, the reaction barrier of CO + O → CO₂ is about 0.3 eV lower in energy than that of CO + O₂ → CO₂ + O, suggesting the former mechanism play a main role in CO oxidation on the Cu(3 1 1) surface.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.     

Analysis of interactions between Co adatoms on the vicinal Cu(111) surface

The energies of magnetic interactions between Co adatoms at the vicinal Cu(111) surface are calculated in the framework of the density functional theory using the Korringa-Kohn-Rostoker type Green’s functions. It is demonstrated that the interactions between Co adatoms appreciably depend on the distance from a surface step. Our calculations show that the magnitude of the repulsive barrier related to the surface step is larger for Co adatoms located at the upper surface terrace than for those located at the lower surface terrace.     

Confining surface state electrons in less than two dimensions: A spectroscopic study

We observe the partial confinement of the twodimensional electron gas of ans-p-like surface state by the regular step distribution of several vicinal Cu(111) surfaces. The spatial distribution of the surface state is visualized by scanning tunneling microscopy. Local tunneling spectroscopy reveals a shift of the maximum of the density of states of the surface state towards the Fermi level, which correlates with the size of the terraces. The average shift is also measured by angle-resolved photoemission and found to be consistent with a one-dimensional Kronig-Penney model.     

Link between adatom resonances and the Cu(111) Shockley surface state

Low-temperature scanning tunneling microscopy and spectroscopy at 7 K was used to assemble and characterize native adatom islands of successive size on the Cu(111) surface. Starting from the single adatom we observe the formation of a series of quantum states which merge into the well known two-dimensional Shockley surface state in the limit of large islands. Our experiments reveal a natural physical link between this fundamental surface property and the sp(z) hybrid resonance associated with the single Cu/Cu(111) adatom.     

Size-dependent surface states of strained cobalt nanoislands on Cu(111)

Low-temperature scanning tunneling spectroscopy over Co nanoislands on Cu(111) showed that the surface states of the islands vary with their size. Occupied states exhibit a sizable downward energy shift as the island size decreases. The position of the occupied states also significantly changes across the islands. Atomic-scale simulations and ab initio calculations demonstrate that the driving force for the observed shift is related to size-dependent mesoscopic relaxations in the nanoislands.     

The magnetic phase transition in Cu-doped ZnO: From bulk to nanocluster

Using the first-principles calculations based on the density functional theory, we have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster. The calculated results show that the substitutional Cu ions are spin polarized and have a tendency to assemble. It is found that the ground state has shown a change from ferromagnetic phase to antiferromagnetic phase as the size for the doping system decreases from bulk to nanocluster. In bulk ZnO, the ferromagnetism is attributed to the strong hybridization between Cu 3d and O 2p states. In ZnO nanocluster, however, the antiferromagnetic exchange interaction is dominant because of the very close Cu–Cu distance.     

Lateral displacement by transient mobility in chemisorption of CO on Pt(997)

We investigated adsorbed states of CO on Pt(997) at 11 K using infrared reflection absorption spectroscopy. At 11 K, thermal migration is suppressed and thus the initial chemisorption at terrace sites and step sites is controlled by the transient mobility of the adsorbing molecule. The initial occupation ratio between atop CO on the terrace and atop CO at the step is directly determined to be 3.6:1. With a simple isotropic migration model, we estimated the mean lateral displacement from the first impact point to the initial chemisorption site to be 6.8 A. We also discuss the origin of transient mobility of CO on metal surfaces.     

Tunneling spectra at terrace boundaries on the bismuth surface

The scanning tunneling spectra of the trigonal bismuth surface are measured in the vicinity of the boundaries of terraces with a diatomic height. It is found that the tunneling spectrum of the planar surface begins to transform at a distance of 2–3 nm from the terrace boundaries, specific features that are characteristic of the planar surface far from the terrace boundaries disappear, and new peaks in the density of states are observed. An analysis of the behavior of the current-voltage characteristics has revealed that one-dimensional electronic systems with a width of the order of an interatomic distance with their own individual spectra are formed at the ends of the rows of the atomic planes.     

Probing the effect of nitrogen gas on electrical conduction phenomena of ZnO and Cu-doped ZnO thin films prepared by spray pyrolysis

Zinc oxide (ZnO) and Cu-doped ZnO (CZO) thin films were prepared on borosilicate glass substrates by spray pyrolysis technique. The X-ray diffraction study revealed that Cu doping caused a reduction in crystallite size. AFM study showed an increase in roughness with doping. This is attributed to the aggregation of particles to form clusters. From transmission electron microscopy analysis, the particle size is measured to be in the range 30–65 nm (average particle size 48 nm) for undoped ZnO, whereas it is in the range 24–56 nm (average particle size 40 nm) for CZO film. The electrical resistivity of the thin films was investigated in the presence of air as well as N₂ mixed air at different temperatures in the range 30–270 °C. The change in resistivity properties was explained on the basis of conduction phenomena within the grain along with the grain boundaries as well as Cu- and N₂-induced defect states. The thermal activation energy of ZnO was found to be in the range 0.04–0.7 eV and dependent on Cu doping and N₂ level in air.     

High-resolution angle-resolved photoemission from Cu(610)

Cu(610) has a mean terrace width of six atoms. Although the regular arrangement of steps was verified by SPALEED, no step-induced surface umklapp process could be identified in UPS spectra taken with HeI radiation. This fact allows an easy comparison with results from Cu(100). In particular, we demonstrate that the final state wave vector can be determined by triangulation using exclusively the Cu(610) surface, since equivalent k-space points contribute at different electron emission angles. However, spectra from equivalent points exhibit differences in the UPS linewidth and this gives additional information on the final state lifetime. Finally we discuss the modification of a Tamm state [its analogue is observed at on the (100) surface] due to confinement by the steps.     

Asymmetric peak broadening in photoemission of surface states as a consequence of lateral electron confinement

In addition to the inverse lifetime, the photoemission peak width of a surface state depends on the quality of the surface. For a stepped surface, in particular, there is an energy shift of the surface state owing to the confinement of electrons. A random distribution of steps causes an asymmetric peak broadening, which is examined as a function of the shift parameter, the inverse lifetime and the mean terrace width. The model is discussed for the Cu(111) sp-surface state for which an influence of steps on the peak shape is obtained particularly for mean terrace widths less than 100 Å.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

Ag-Cu离子注入玻璃后不同气氛退火的光吸收研究

采用MEVVA源（metal vapor vacuum arc ion source）引出的强束流脉冲Ag,Cu离子先后注入到SiO₂玻璃，x射线光电子能谱仪（XPS）分析显示Ag，Cu大多仍为金属态，有部分氧化态Cu存在.透射电镜观察分析和光学吸收谱都表明在衬底中形成了纳米合金颗粒.结合有效媒质理论，得到模拟的光学吸收谱，与实验结果基本符合，较好地验证了以上结论.样品退火后颗粒发生分解，分解的颗粒在氧化气氛下被氧化，且有部分向样品表面蒸发；在还原气氛下氧化态元素被还原并成核生长.故 关键词： 离子注入 纳米颗粒 退火 光学吸收率     

Cluster method density of states for noble metals and comparison with photoemission spectra

The multiple scattering cluster method is used to calculate densities of states for Cu, Ag, and Au. The results compare favorably with experimental photoemission spectra. The method converges rapidly with the cluster size used for the calculation.     

Precursor to the onset of the bulk oxidation of Cu(100)

Using density-functional theory within the generalized gradient approximation, we investigate the energetics of oxygen subsurface adsorption governing the onset of bulk oxidation of Cu(100) surface. It shows that the presence of boundaries formed from merged missing-row nanodomains mismatched by a half unit-cell leads to preferred oxygen adsorption at the subsurface tetrahedral sites. The resulting Cu-O tetrahedrons along the domain boundary strikingly resemble that of the bulk oxide phase of Cu(2)O. These results provide direct atomic-scale insight into the microscopic origin of the crystallographic orientation relationships for oxide overlayer growth. Our results also suggest that the oxidation of an atomically flat terrace can still be a heterogeneous nucleation process controlled by defects in the oxygen-chemisorbed adlayer.