天冬氨酸在Cu(001)表面吸附的扫描隧道显微镜研究

用扫描隧道显微镜(STM)研究了室温下天冬氨酸在Cu(001)表面的吸附行为.实验发现,在较 低的覆盖度下,天冬氨酸分子在Cu(001)表面存在两种吸附状态.从STM数据估算出两种吸附 状态下天冬氨酸分子在Cu(001)表面的扩散激活能分别为079±001eV,088±005eV. 随着覆盖度的提高,天冬氨酸分子最终在Cu(001)表面形成一均匀衬度的吸附层,但并不形 成有序吸附结构,也不能使台阶发生小面化.天冬氨酸分子的这些吸附特点是迄今研究过的 所有氨基酸在Cu(001)表面吸附时不具有的. 关键词： 表面吸附 扫描隧道显微镜 氨基酸     

室温下单个甘氨酸分子在Cu(111)表面的操纵研究

先用低能电子衍射(LEED)证明了甘氨酸(NH₂-CH₂-COOH)能在室温下在Cu单晶表面产生比较稳定的吸附，然后用扫描隧道显微镜(STM)进一步研究了其吸附情况，看到单个甘氨酸分子在Cu(111)面上吸附稳定并至少有三种吸附状态.分子操纵研究结果表明，甘氨酸分子是被针尖“推着”移动的，它在Cu(111)面有固定的吸附位，并且移动时其吸附状态可以不变.研究结果表明，甘氨酸适合做室温下小分子的可控操纵研究，并且也说明室温下小分子的可控操纵是可能的. 关键词：     

乙烯和乙炔基在Ni(110)表面上吸附结构的研究

采用平面波赝势方法，利用基于从头计算的软件包，对乙烯和乙炔基在Ni(110)表面上吸附的问题进行了计算. 在低覆盖度时，孤立的乙烯分子的吸附能比密集时高，乙烯分子的C-C 轴大致沿衬底的Ni原子链方向(即沿［110］晶向)，C-C轴与衬底Ni(110)表面有12°的倾斜角，乙烯分子的C—C键的键长为 0.147nm. 乙烯分子中接近顶位的C原子与衬底中距离最近的Ni原子为0.199nm. 在高覆盖度时，乙烯分子在Ni(110)表面上形成c(2×4)再构，每个表面二维元胞中有两个乙烯分子，每个乙烯分子的吸附位置与低覆盖度时相似，而C—C键长比低覆盖度时要短. 乙炔基是乙烯在Ni(110)表面上分解的产物. 关于乙炔基的计算结果表明：乙炔基的两个C原子的间距为0.131nm，比乙烯分子中C原子的间距更短. 与乙烯分子相比，乙炔基的吸附位置更靠近顶位. H原子与吸附在顶位上的C原子相连接，C—H键也大致沿衬底的Ni原子链方向，与Ni表面呈45°的倾斜角. 关键词： 乙烯和乙炔基 平面波赝势方法 吸附几何结构     

Dy@C82分子在Au(111)表面依赖于覆盖度的取向研究

利用超高真空扫描隧道显微镜, 在低温(80 K)下研究了同分异构体分子Dy@C₈₂在Au(111)表面的吸附与分子取向．在低覆盖度下,Dy@C₈₂分子优先吸附于台阶边缘形成分子团簇与分子链结构．这种吸附取决于分子-衬底的相互作用,并存在多种不同的分子取向．增大分子覆盖度后,Dy@C₈₂在金表面形成二维有序密排的单层膜结构．Dy@C₈₂分子在金表面的取向倾向于其C₂长轴与金表面近乎平行．具有三种取向的分子最具优势,而同种取向的分子组成许多局限于一个个小区域内的取向有序结构畴．随着覆盖度的增加,Dy@C₈₂分子在Au(111)表面趋向于短程有序取向排列,这是由分子-衬底作用与分子间的偶极-偶极作用共同决定的．     

乙烯在Ni（110）表面吸附的几何结构

用理论计算的方法研究了不同覆盖度的乙烯在Ni（110）表面吸附的位置.乙烯的吸附几何结构在团簇计算中进行了局部优化.在低覆盖度下，单个乙烯分子占据了短桥位和顶位之间的中间位置.乙烯分子的C—C轴大致沿衬底的Ni原子链排列（即沿<110>晶向），C—C轴与衬底Ni（110）表面有10°的倾斜角.乙烯分子的C—C键的键长为0151nm.在高覆盖度下(05ML)，乙烯在Ni (110)上形成了有序的c（2×4）相，在一个表面元胞内的两个乙烯分子的吸附位置类似于低覆盖度时的结果，但乙烯分子的C—C 键键长分别为0142和0143nm.     

长链烷烃和醇在石墨表面吸附的扫描隧道显微镜研究

用扫描隧道显微镜（STM）研究了室温下十八醇（1-C18H37OH）和十四烷（C14H30）在石墨表面的吸附行为.十八醇在石墨表面自组装形成条状结构.实验发现了十八醇分子在石墨表面的两种不同的密排方式，它们形成并列的不同宽度的条状结构.还发现，由于溶剂（正十四烷C14H30）的完全蒸发，留下的十八醇分子在石墨表面吸附的自组装结构与固液界面中十八醇分子在石墨表面的吸附形成的结构在分子方向、表面分子晶体的晶格常数及条状结构宽度等方面有很大的不同.此外，实验中也发现溶剂十四烷在某种情况下也能在石墨表面吸附形成可供STM观察的有较长时间稳定性的条状周期结构.实验观察到的十四烷在石墨表面形成的晶体结构与固液界面中观察到的有序结构的晶体结构常数和分子排列方向也是不同的.根据STM图像，提出了十八醇和十四烷在石墨表面吸附的结构模型. 关键词： 十四烷 十八醇 石墨 扫描隧道显微镜     

甘氨酸吸附对Ni-Cu合金(110)表面偏析的影响

通过构造无序二元合金NixCu1-x(x=0.4)(110)表面的原子集团模型用来研究表面偏析. 根据这个模型，按覆盖度μ=0.33，应用Recursion方法计算了甘氨酸在NixCu1-x无序二元合金(110)表面吸附的电子结构. 结果表明，Ni0:4Cu0:6合金表面存在着Cu的偏析，甘氨酸的吸附抑制了Cu在表面的偏析，并使得合金表面的态密度在费米能级附近发生了很大变化，吸附时合金表面与甘氨酸之间发生了电荷转移.     

甘氨酸在Cu(001)表面的吸附结构

结合低能电子衍射(LEED)及其消失斑点规律和扫描隧道显微镜(STM)手段确定了室温下甘氨酸在Cu(001)表面能形成c(2×4)和两种(2×4)吸附结构((2×4)₁和(2×4)₂),并推断出在两种(2×4)结构单胞中两甘氨酸分子的羧基相对于衬底的吸附取向一致,而它们的氨基则不同.实验中还观察到c(2×4)与(2×4)₂结构能相互转变成窄条相互穿插共存,这说明几种吸附结构能量相近. 关键词：     

Cu(100)表面CO分子单层膜的原子结构

利用第一性原理研究了覆盖度分别为1.00, 0.50和0.25 ML时CO分子单层膜在Cu(100)表面的吸附系统. 计算表明CO分子对不稳定. 获得了CO分子单层膜在虚拟Cu(100)表面的原子结构, 以及CO分子单层膜在Cu(100)表面吸附系统的原子结构. 当CO分子单层膜在Cu(100)表面的三个吸附位吸附, 覆盖度为1.00 ML时, 顶位和桥位都稳定, 而空心位不稳定; 覆盖度为0.50和0.25 ML时, 三个吸附位都稳定.比较吸附前后CO分子单层膜的原子结构, 可知CO分子和Cu(100)表面的相互作用强于CO分子单层膜之间的相互作用. 关键词： CO分子单层膜 自组装 CASTEP Cu(100)     

单层硅烯表面的CoPc分子吸附研究

由于低维材料表面上的单原子和分子具有丰富的物理化学性质,现已经成为量子器件及催化科学等领域的研究热点.单层硅烯在不同的衬底制备温度下,表现出丰富的超结构,这些超结构为实现有序的单原子或分子吸附提供了可靠的模板.利用原位硅烯薄膜制备,分子沉积,超高真空扫描隧道显微镜以及扫描隧道谱,本文研究了Ag(111)衬底上3种硅烯超结构((4×4),(■×■),(2■×2■))的电子态结构,表面功函数随超结构的变化,以及CoPc分子在这3种超结构硅烯上的吸附行为.研究结果表明,这3种超结构的硅烯具有类似的电子能带结构,且存在电子从Ag(111)衬底转移到硅烯上的可能性,从而导致硅烯的表面功函数增大,表面功函数在原子级尺度上的变化对分子的选择性吸附起着重要作用.此外,还观察到分子与硅烯的相互作用导致CoPc分子的电子结构发生对称性破缺.     

Adsorption of L-Alanine on Cu（111） Studied by Scanning Tunnelling Microscopy

The adsorption of L-alanine on Cu（111） surface is studied by means of scanning tunnelling microscopy under ultra-high vacuum conditions. The results show that the adsorbates are chemisorbed on the surface, and can form a two-dimensional gas phase, chain phase and solid phase, depending on deposition rate and amount. The adsorbed molecules can be imaged as individual protrusions and parallel chains in gas and chain phases respectively. It is also found that alanine can form （2 × 2） superstructure on Cu（111） and copper step facet to （110） directions in solid phase. On the basis of our scanning tunnelling microscopic images, a model is proposed for the Cu（111）（2 ×2）-alanine superstructure. In the model, we point out the close link between （110）-direction hydrogen bond chains with the same direction copper step faceting.     

ADSORPTION BEHAVIOR OF AMINO ACIDS ON COPPER SURFACES

The major results of a series of our recent investigations on the adsorption of eight amino acids on Cu(001) and (111) surfaces are reviewed in the present paper. In all studied cases the molecules adsorb onto the surface in their anionic form. With the increase of the coverage three different 2D phases of the adsorbates, that is, the 2D lattice gas, intermediate, and solid phases, appear sequentially, although for few systems one or two of them do not appear. In both the 2D lattice gas and intermediate phases the molecules "stand" with their two oxygen "feet" on the surface and the intermolecular interactions are repulsive, although in the former they can diffuse frequently whereas in the latter they are discommensurate in one direction with the substrate. In the solid phase the molecules "lie" down on the surface to form commensurate superstructures. Adsorption of amino acids may often induce step faceting as well as bunching to form facets. Adsorption of L-lysine on Cu(001) may cause steps bunching to form facets with all the same chirality. Our preliminary results show that it is possible to manipulate individual molecules with the STM tip even at room temperature. These results may have applications in nano-materials, nano-technology, and very likely also in chiral separations or enantioselective heterogeneous catalysis.     

Isotopic mixing for the determination of relative coverages in overlayer structures: CO on Cu(111)

An accurate determination of the relative uptakes of CO in the formation of three successive overlayer structures on Cu(111) has been achieved by sequential adsorption of ¹²CO and ¹³CO followed by isotopic analysis of the desorbing gas. The results identify the intermediate structure as (1.5 × 1.5)?18° at a coverage of 0.44, rather than a slightly distorted c(4 × 2) structure with a coverage of 0.50.     

Structural transformations on Cu(110) under molecular iodine action

Interaction of molecular iodine with the Cu(110) surface is investigated by the methods of ultrahigh vacuum scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). It is found that at the coverage θ =0.5 monolayer (ML) iodine forms a simple commensurate c(2×2) lattice. Further exposure of iodine leads to uniaxial compression of the c(2×2) lattice along the 〈110〉 direction of the substrate. The STM data indicate that compression of the iodine layer proceeds through formation of striped domain walls. As the coverage is saturated at θ = 0.63 ML, iodine forms a uniformly compressed quasi-hexagonal structure. Further exposure of iodine on the Cu(110) surface results in growth of a copper iodide film. STM images of thin (7 to 20 Å) CuI films reveal, in addition to atomic modulation, a superstructure with a period of 90 to 100 Å consisting of double stripes. A structure model of the copper iodide surface allowing for CuI lattice contraction and formation of double stripe domain walls is proposed.     

Absolute coverage of C2H2 and C2H4 on Pt(111)

Quantitative XPS measurements have been performed in order to determine the absolute coverage of acetylene and ethylene adsorbed on Pt(111) showing a 2 × 2 LEED pattern. This LEED pattern has so far been attributed to a 2 × 2 superstructure with a coverage of 0.25. A quantitative evaluation of the C(1s) peak intensities for these adsorbed layers in comparison with adsorbed CO shows that the coverage is 0.5 instead of 0.25. Therefore the 2 × 2 LEED pattern should be assigned to a 2 × 1 superstructure in three domains rather than a 2 × 2 superstructure.     

Chlorine monolayers on the low-index faces of silver

Chlorine chemisorption layers were obtained on the (100), (110) and (111) faces of silver single crystals by means of the reaction with dichloroethane. The structure and stability of the surface phases was studied mainly by LEED and Auger spectroscopy. On the (100) face, a c(2 × 2) superstructure was found at all coverages, which was interpreted as a reconstructed layer with a structure similar to that of the AgCl (100) planes. On the other two faces, different superstructures were observed, depending on the coverage. At low coverages, (2 × 1) and (√3 × √3)-R 30° superstructures were found on the (110) and (111) faces, respectively, which seem to be better interprétable as reconstructed layers. At maximum coverages, c(4 × 2) and (3 × 3) superstructures were observed respectively on the two faces. These phases were interpreted as mixed layers with a packing of the silver and chlorine atoms similar to that of the AgCl (111) planes. The proposed models are in agreement with the different thermal stabilities of the layers. The models of the maximum-coverage structures correspond to about the same chlorine amount on the three faces, in agreement with the Auger results. The structures are also discussed in relation to the known epitactic relations for the growth of AgCl on silver.     

Influence of copper crystal surface on the CVD growth of large area monolayer graphene

We study the influence of the surface structure of copper single crystals on the growth of large area monolayer graphene by chemical vapor deposition (CVD) in ultra-high vacuum (UHV). Using atomic-resolution scanning tunneling microscopy (STM), we find that graphene grows primarily in registry with the underlying copper lattice for both Cu(111) and Cu(100). The graphene has a hexagonal superstructure on Cu(111) with a significant electronic component,whereas it has a linear superstructure on Cu(100). Graphene on Cu(111) forms a microscopically uniform sheet, the quality of which is determined by the presence of grain boundaries where graphene grains with different orientations meet. Graphene grown on Cu(100) under similar conditions does not form a uniform sheet and instead displays exposed nanoscale edges. Our results indicate the importance of the copper crystal structure on the microstructure of graphene films produced by CVD.     

Na adsorption on the Si111-(7 x 7) surface: from two-dimensional gas to nanocluster array

We have systematically investigated Na adsorption on the Si(111)-(7 x 7) surface at room temperature using scanning tunneling microscopy (STM). Below the critical coverage of 0.08 monolayer, we find intriguing contrast modulation instead of localized Na adsorbates, coupled with streaky noise in the STM images, which is accompanied by monotonic work function drop. Above the critical coverage, Na clusters emerge and form a self-assembled array. Combined with first-principles theoretical simulations, we conclude that the Na atoms on the (7 x 7) surface are, while strongly bound ( approximately 2.2 eV) to the surface, highly mobile in "basins" around the Si rest atoms, forming a two-dimensional gas phase at the initial coverage, and that the cluster at the higher coverage consists of six Na atoms together with three Si adatoms.     

Formation of nanoislands in the course of copper deposition on a Cu(111)-(9 × 9)-Ag surface

The process of copper deposition on a structured Cu(111)-(9 × 9)-Ag surface, which represents a (9 × 9) loop dislocation network, is studied by scanning tunneling microscopy. It is found that, when the substrate temperature is 100 K and the copper coverage is 0.1–0.4 of a monolayer, islands of a size no greater than 50 Å are formed at the Ag/Cu(111) interface. The islands remain stable as the sample is heated to room temperature. The shape and boundaries of the nanoislands follow the initial surface superstructure and are determined by the nonuniformity of the interaction of the upper silver layer with the copper substrate. The mechanism of island formation and the origin of their stability are explained in terms of the atom exchange between the adsorbate and substrate.     

Adsorption of bismuth on Si(110) surfaces

The adsorption of bismuth on Si(110) surfaces has been studied by means of quadrupole mass spectrometry (QMS), LEED and AES. The existence of three main adsorption states (the first, second and third phases) were observed. Two of them (the first and second phases) are two-dimensional phases and the other (the third phase) is a three-dimensional phase. After the completion of the first phase at a high substrate temperature, the LEED pattern showed a clear Si(110) 2 × 3 structure. A shift of the saturation coverage of the two-dimensional phase with the change of the deposition condition was also observed.