Novel nanostructuring of the O/Cu(110) surface by reaction to oxygen

We present a scanning tunneling microscopy (STM) study of the reactivity to oxygen of the O/Cu(110) surface nanostructured with alternating oxidized and clean Cu stripes leading to novel nanostructuring by troughs made of missing top-most Cu atoms. The copper atoms extracted from these troughs are participating to the added-row reconstruction of the surface. The oxidation of the nanostructured surface proceeds by enlargement of the oxidized stripes and by oxidation of the troughs until the surface is fully covered by oxygen. At saturation, the trough arrangement, templated by the oxygen-free stripes, led to a novel nanostructure of the O/Cu(110) surface made of the (2 × 1) phase only.A limited influence of the step density was found as the nanostructuration blocks almost all the primary sources of copper atoms at the step edges. In this case, the troughs became the spare source feeding the reconstruction. Careful analysis of the trough distribution in the vicinity of step edges and on terraces shows clear indication of an anisotropic diffusion of the copper adatoms at the surface.     

A SPALEED structural study of cesium adsorption on stepped copper surfaces Cu(211) and Cu(511)

The adsorption of cesium on stepped copper surfaces Cu(211) and Cu(511) at temperatures ranging from 130 to 300 K was studied using SPALEED. The two Cu substrates differ mainly in the low index orientation of the relatively short terraces and permit a comparison of the effect of orientation on the adsorption behaviour. For coverages between 0.6 and 1.8 ML on Cu(211) within the whole temperature range Cs forms quasihexagonal layers corresponding to rows of Cs atoms running along the step edges of the substrate. A strong 15% compression of the distance between Cs atoms along the step edge direction was found. Similar structures were found on Cu(511) where in addition at crystal temperatures in excess of 250 K and coverage of 1 monolayer, the Cs atoms induce a double step reconstruction of the substrate.     

H,Cl和F原子钝化Cu_2ZnSnS_4(112)表面态的第一性原理计算

采用基于密度泛函理论的第一性原理计算方法系统研究了Cu_2ZnSnS_4体相的晶格结构、能带、态密度及表面重构与H,Cl和F原子在Cu_2ZnSnS_4(112)表面上的吸附和钝化机理.计算结果表明:表面重构出现在以金属原子Cu-Zn-Sn终止的Cu_2ZnSnS_4(112)表面上,并且表面重构使表面发生自钝化;当单个H,Cl或F原子吸附在S原子终止的Cu_2ZnSnS_4(112)表面上时,相比于桥位(bridge)、六方密排(hcp)位和面心立方(fcc)位点,三种原子均在特定的顶位(top)吸附位点表现出最佳稳定性.当覆盖度为0.5 ML时,无论H,Cl还是F原子占据Cu_2ZnSnS_4(112)表面的2个顶位均具有最低的吸附能.以S原子终止的Cu_2ZnSnS_4(112)表面在费米能级附近的电子态主要由价带顶部Cu-3d轨道和S-3p轨道电子贡献,此即表面态.当H,Cl或F原子在表面的覆盖度达0.5 ML时,费米能级附近的表面态降低,其中H原子钝化表面态的效果最佳,Cl原子的效果次之,F原子的效果最差.表面态降低的主要原因在于吸附原子从S原子获得电子致使表面Cu原子和S原子在费米能级处的态密度峰几乎完全消失.     

Quasiclassical studies of Eley-Rideal and hot-atom reactions on a surface at 94 K: H(D) → D(H) + Cu(1 1 1)

Reactions and reaction dynamics of gas-phase H(or D) atoms with D(or H) atoms adsorbed onto a Cu(1 1 1) surface have been investigated by the quasi-classical molecular dynamics method. To simulate the H(D) → D(H) + Cu(1 1 1) system at a 94 K surface temperature, D(or H) adsorbates were disseminated arbitrarily on the surface of Cu(1 1 1) to form 0.50, 0.28 and 0.18 ML of coverages. The interaction of hydrogen atoms and the surface system is worked out by an LEPS function. LEPS parameters have been determined by using the total energy values which were calculated by a density functional theory (DFT) method and the generalized gradient approximation (GGA) for the exchange-correlation energy for various configurations of one and two hydrogen atoms on the Cu(1 1 1) surface. The Cu(1 1 1) surface, imitated by an embedded-atom method which is a many-body potential parameterized by Voter-Chen, is formed as a multilayer slab. The slab atoms are permitted to move. Various processes, trapping onto the surface, inelastic reflection of the incident projectile and penetration of the adsorbate or projectile atom into the slab, are examined. The dependence of these mechanisms on isotopic replacement has also been analyzed. Considerable contributions of the hot-atom pathways for the product formations are consequently observed. The rate of subsurface penetrations is obtained to be larger than the sticking rate onto the surface.     

NO dissociation on Cu(111) and Cu2O(111) surfaces: a density functional theory based study

NO dissociation on Cu(111) and Cu(2)O(111) surfaces is investigated using spin-polarized density functional theory. This is to verify the possibility of using Cu-based catalyst for NO dissociation which is the rate limiting step for the NO(x) reduction process. The dissociation of molecularly adsorbed NO on the surface is activated for both cases. However, from the reaction path of the NO-Cu(2)O(111) system, the calculated transition state lies below the reference energy which indicates the possibility of dissociation. For the NO-Cu(111) system, the reaction path shows that NO desorption is more likely to occur. The geometric and electronic structure of the Cu(2)O(111) surface indicates that the surface Cu atoms stabilize themselves with reference to the O atom in the subsurface. The interaction results in modification of the electronic structure of the surface Cu atoms of Cu(2)O(111) which greatly affects the adsorption and dissociation of NO. This phenomenon further explains the obtained differences in the dissociation pathways of NO on the surfaces.     

Role of adsorption on surface composition of Pd–Cu nanoparticles

Monte-Carlo (MC) simulation is used to study the role of adsorption of hydrogen, oxygen and carbon monoxide (CO) on the surface composition and surface bond geometry of Pd–Cu nanoparticles. For clean particles the surface is found to be enriched in Cu. But in the presence of adsorbed hydrogen and CO there is a segregation reversal from Cu segregation at low coverage to Pd segregation at high coverage. In the presence of adsorbed oxygen, on the contrary, the extent of Cu segregation increases with coverage. For a 586-atom nanoparticle with 50% Pd in the bulk the corner sites are found to be occupied by Cu atoms up to one monolayer adsorption. But, while the occupancy of 7, 8 and 9-coordinated sites by Cu atoms decreases with increase of H and CO coverage, for oxygen adsorption this occupancy increases with coverage. The relevance of such results in catalysis studies is discussed.     

Site-specific reactivity of oxygen at Cu(110) step defects: an STM study of ammonia dehydrogenation

Defects on surfaces, such as steps, are known to influence catalytic activity considerably, but few microscopic observations have been made of such site-specific reactivity. Using atomic-resolution scanning tunneling microscopy (STM), we have observed the reactivity of oxygen adsorbed in the (2 × 1) structure at step sites on the Cu(110) surface with ammonia. It is found that the reactivity is high at both the top and bottom of a [11¯0] step and at the bottom of a [001] step, whereas almost no reaction occurs at the sites bordering the top of a [001] step. Reaction at the bottom of [001] steps appears to lead to deposition of copper atoms at the step edge.     

Coverage-dependent absorption of atomic hydrogen into the sub-surface of Cu(1 1 1) studied by density-functional-theory calculations

With density-functional-theory calculations, we have studied coverage-dependent absorption of H atoms into the sub-surface below a face-centered-cubic (fcc) hollow site of Cu(1 1 1). Both frozen and relaxed surface lattices were considered when the atomic H migrated from the surface to the sub-surface. The potential energy curve for the absorbing H shows that the surface site is in general favored over the sub-surface site, and this trend varies little with the H coverage (0.11-0.67 ML). If the hexagonal-close-packed (hcp) hollow sites immediately vicinal to the absorbing H are pre-adsorbed with other H atoms, the surface adsorption potential is greatly increased, because of the repulsive H-H interaction, to a value near, or even greater than, the sub-surface absorption potential; when two or three H atoms (on the hcp sites) are beside the absorbing H, the energy barrier for the sub-surface absorption is decreased, whereas that for diffusion from the sub-surface to the surface is enhanced. These results indicate that, on an H-saturated Cu(1 1 1) surface (0.67 ML), the sub-surface sites below the fcc sites with two or three neighboring H atoms can trap the sub-surface H.     

A possible mechanism of copper corrosion in anoxic water

Recent experiments show that solid Cu reacts with anoxic water. The reaction is observed by measuring the hydrogen release. This release is continuous and stable over a period of months. We have since theoretically found that water adsorbs dissociatively at a copper surface. But this adsorption is not enough to explain the amount of hydrogen released in the experiment. This observation calls for the explanation of the removal of the reaction product from the surface to provide a clean Cu surface where the water dissociation takes place. In this paper we investigate, by first-principles calculations, two possible mechanisms for this removal: first the possibility of Cu–O–H nanoparticulate formation, and second the diffusion of the dissociation products into Cu. We show that while the formation of nanoparticulates is energetically unfavorable, the diffusion of OH along grain boundaries can be substantial. The OH being placed in a grain boundary of the Cu sample quickly dissociates and O and H atoms diffuse independently of each other. Such a diffusion is markedly larger than the diffusion in bulk Cu. Thus, grain boundary diffusion is a viable mechanism for providing a clean Cu surface for the dissociation of water at the Cu surface. An order-of-magnitude estimate of the amount of hydrogen released in this case agrees with experiment. But this mechanism is not enough to explain the result of the experiment. We propose the formation of nanocrystals of copper oxide as a second step. A decisive experiment is proposed.     

STM studies on the growth of monolayers: Co on Cu(110) with one half monolayer of preadsorbed oxygen

The growth of the first cobalt monolayer (ML) on the Cu(110)-(2×1)O surface was studied by scanning tunneling microscopy. Extensive exchange of Cu and Co atoms takes place in the first stages of the deposition. The displaced Cu atoms form new Cu---O---Co mixed islands, with the same structure as those of the terrace surface. At 0.25 ML Co, a new structure nucleates, which contains three Cu atoms, four Co atoms and two O atoms per 2×2 cell. The structure consists of rows in the [ 10] direction with an internal periodicity of two lattice units. The rows are separated from one another by two lattice units along the [001] direction, and are found both in-phase and out-of-phase relative to one another. The result is a mixed p(2×2) and c(2×4) surface. The fraction of the surface covered by the new structure increases with Co coverage, and completely covers the surface at 1 ML Co.     

Dynamic structure mediated by graphitelike Al nets on the Al2Cu (001) surface

A detailed study of the (001) surface of the Al(2)Cu crystal using both experimental and ab initio computational methods is presented in this work. The combination of both approaches gives many arguments to match the surface plane with a bulk truncated surface model terminated by incomplete Al planes. The missing rows of Al atoms lead to a 2√2×√2R 45° surface reconstruction with two domains rotated by 90° from each other. Ab initio calculations demonstrate that the energetic cost associated with the removal of pairs of Al atoms is the lowest for the two nearest surface Al atoms (covalentlike interaction). They reveal that the remaining atomic rows of various widths are oriented according to the graphitelike Al 6(3) nets used to describe the Al(2)Cu bulk structure. The surface dynamics observed at 300 K at the Al(2)Cu surface is also presented. Finally, configurational and vibrational entropies are introduced to discuss the reduced surface plane density.     

低能Cu原子簇沉积薄膜的分子动力学模拟

用分子动力学计算机模拟研究了能量为5—20eV/atorn,结构为正二十面体的(Cu)₁₃原子簇在Cu（001）表面的沉积过程.采用紧束缚势同Moliers势的结合描述Cu原子间相互作用通过原子簇-衬底相互作用的“快照”研究沉积的动态过程．结果表明,当入射能量较低时,轰击弛豫后,入射原子簇在衬底表面发生重构,生成很好的外延层,靶没有任何损伤．随着轰击能量的增加,原子簇原子穿入靶的深度增加.当入射能量达到20eV/atom时,原子簇完全穿入靶并开始造成辐照损伤,表面出现空位,靶内产生间     

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).     

Fabrication and modification of the supported Al nanoparticles on the Al(001) and (110) surfaces using the reversible vertical single-atom manipulation with the Cu and Pt trimer-apex tips: a first-principles study

First-principles simulations show that at the step edge of the stepped Al(001) and (110) surfaces and at the edge of the small supported nanoparticles like the dimer, trimer, and tetramer, single Al atoms can be extracted and repositioned using the Cu trimer-apex tip and the Pt tip of a Al apex, while a more weakly adsorbed single Al adatom on the plane terrace of the flat surface or of the nanocluster cannot be vertically picked up by these two tips. This result suggests in principle a non-electrically assisted method of fabricating and modifying metal nanoparticles at the atomic scale using the vertical single-atom manipulation. As an illustration, a pyramidal nanocluster of five Al atoms is assembled on the Al(001) surface in an atom-by-atom way with the Cu trimer-apex tip, moreover, it can be modified to be two-dimensional in shape.     

BTA在铜表面吸附的密度泛函理论研究

本文基于密度泛函理论(DFT)的第一性原理研究了苯并三氮唑(BTA)分子吸附于铜表面的反应活性特征及其吸附在三种不同取向晶面时的电荷转移以及成键情况,结果表明：BTA分子的亲电和亲核活性中心为N(1)、N(2)和C(5),在铜表面垂直吸附时为化学吸附,Cu原子的最外层价电子转移到N(2)原子上,两者形成配位键;BTA分子在三种不同取向的铜表面吸附时的吸附能大小为：Cu(110)_x>Cu(100)_x>Cu(111)_x(x=T、B、H),T表示顶位,B表示桥位,H表示空位;BTA吸附在Cu(111)面的转移电荷量：T(顶位)>B(桥位)>H(空位).     

Kinetic pathway for the formation of fe nanowires on stepped Cu111 surfaces

We report the discovery of a novel kinetic pathway for the formation of one-dimensional Fe nanowires of single atom width on stepped Cu(111) surfaces. This pathway, identified through extensive total-energy calculations within density functional theory, establishes that the stable structure involves a row of Fe atoms on the upper edge of a step. The formation of the surface wire is preceded by facile incorporation of an initial row of Fe atoms into the surface layer at one lateral lattice constant away from the step edge, which then acts as an attractor for the second exposed row of atoms. The resulting wire structure provides a natural interpretation of existing experimental results. We also explore the applicability of this mechanism in the formation of other related systems.     

Comparative study of H2 adsorption on W(1 0 0)-c(2 × 2)Cu and W(1 0 0): Surface alloying effects

The interactions of H and H₂ with W(1 0 0)-c(2 × 2)Cu and W(1 0 0) have been investigated through density functional theory (DFT) calculations to elucidate the effect of Cu atoms on the reactivity of the alloy. Cu atoms do not alter the attraction towards top-W sites felt by H₂ molecules approaching the W(1 0 0) surface but make dissociation more difficult due to the rise of late activation barriers. This is mainly due to the strong decrease in the stability of the atomic adsorbed state on bridge sites, the most favourable ones for H adsorption on W(1 0 0). Still, our results show unambiguously that H₂ dissociative adsorption on perfect terraces of the W(1 0 0)-c(2 × 2)Cu surface is a non-activated process which is consistent with the high sticking probability found in molecular beam experiments at low energies.     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Ba和Sr原子在ZnO(0001)表面吸附的密度泛函理论研究

我们采用密度泛函理论下的平面波赝势方法研究Ba和Sr原子在ZnO(0001)表面的吸附结构和性质,仔细研究了三个吸附位（T4, H3 and Top）。发现Ba和Sr吸附在表面的H3和T4位时,他们之间的结合能相差很小,且这两种金属更易于吸附在表面的T4位,我们把计算的结果与贵金属在ZnO(0001)表面的吸附行为及前人的实验结果进行了比较,理论上发现Ag和Au易于吸附在ZnO(0001)表面的H3位,而实验上观察到即使在很小吸附比的条件下ZnO(0001)表面上也能形成Cu的团簇结构,这主要是由于Cu和ZnO(0001)表面衬底强的相互作用所致。     

Bi对自旋阀钉扎场的影响及机理

实验发现将Bi插入自旋阀多层膜TaNiFeCuBi(x)NiFeFeMn中可以显著地提高自旋阀的钉扎场Hex.采用XPS对Cu,Bi元素的分布情况进行了研究,发现Bi的插入明显抑制了Cu原子在自旋阀的制备过程中在NiFeFeMn界面的偏聚.进一步研究表明:自旋阀钉扎层NiFeFeMn界面中,Cu原子的存在是导致自旋阀Hex小于TaNiFeFeMn多层膜Hex的主要原因. 关键词： 自旋阀 钉扎场 交换各向异性 表面活化剂