电场下用扫描隧道显微镜对针尖原子扩散的观察

用扫描隧道显微镜（STM）研究了在金针尖和金样品间施加大偏压时所发生的各种不同的现象．在缓变大偏压的作用下，观察到针尖原子会发生场致扩散，导致针尖形状发生变化，并且还观察到了场发射和共振隧穿现象．提出了针尖原子的场致扩散是偏压电场使针尖表面极化引起的这一机理，并且指出了这种场致扩散在用大脉冲偏压作表面加工中起着重要的作用 关键词：     

Scanning tunneling microscopy of adsorbates on insulating films. From the imaging of individual molecular orbitals to the manipulation of the charge state

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope to study the electronic properties of single atoms and molecules that are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. In the case of molecules on ultrathin NaCl films, the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the case of individual pentacene molecules. PACS 68.37.Ef; 73.61.Ng; 73.20.Hb     

通过单分子化学调控单个离子磁性--单分子"手术"开启分子磁性"开关"

报道了如何通过改变单个磁性离子的化学环境来调控其白旋性质．利用扫描隧道显微镜，对吸附于Au（111）表面的单个钴酞菁分子进行化学修饰，通过在针尖上施加一定的电压脉冲，将分子配合体外层的8个氢原子“剪裁”掉，使其与金衬底形成稳定的化学键合．在这个新的人造分子结构中，其中心钴离子的电子态在费米面上出现强烈的共振峰；通过理论的模拟和分析，发现出现这一共振峰的原因是，脱氢和与衬底的化学结合使中心钴离子自旋性质发生改变，从而导致了输运特性中近藤效应（Kondo effect）的出现．     

Control of the charge state of metal atoms on thin MgO films

The arrangement of single gold and palladium atoms deposited on the surface of a 3 monolayer thin film of MgO was investigated using low-temperature scanning tunneling microscopy. While Pd atoms are arranged in a random fashion, Au forms an ordered array on the surface. The long-range ordering as well as the scanning tunneling microscopy appearance of single Au atoms on a 3 monolayer thin MgO film can be explained by partial charge transfer from the substrate to Au atoms as predicted recently by density functional theory calculations [Phys. Rev. Lett. 94, 226104 (2005)10.1103/PhysRevLett.94.226104]. In contrast with that, Au atoms on a thick film were found to be essentially neutral.     

用扫描隧道显微镜针尖操纵金岛

用扫描隧道显微镜，在小隧道阻抗的条件下（小偏压和大隧道电流），通过移动针尖，实现了在室温下对真空蒸镀在高定向石墨上的、由几万个原子组成的纳米尺度金岛的操纵．在大隧道阻抗的情形下，用同一个针尖可对操纵的结果进行观察，而不会对金岛产生扰动．这种可控的操纵是通过当钨针尖与金岛间距离很近时形成的金属间黏附力大于金岛与石墨间的摩擦力而实现的 关键词：     

石墨上纳米尺度金岛的形成

用扫描隧道显微镜研究了真空蒸镀在高定向热解石墨上金岛的形成和形状．随着蒸镀量的增加及时间的推移，金原子在表面通过扩散而逐渐合并成越来越大的原子团，以至岛，甚至岛群．虽然几个纳米大小的原子团仍十分可动，但在蒸镀量大于20单层时的岛或岛群已十分稳定．研究发现，同一蒸镀量下，各个金岛具有非常接近的宽度和高度，用薄膜成核的圆柱状模型计算岛的宽度与高度之比表明，金岛非常接近热力学平衡状态．这些形状各异的通过生长而自组装形成的纳米尺度金岛可用来进行介观物理的研究 关键词：     

Modeling of surfaces of constant force above a lattice of close-packed atoms in the repulsive mode

Surfaces of constant force (force contours) are calculated for the scanning of an AFM tip over a lattice of close-packed atoms in the repulsive mode. It is shown that discontinuities are observed on the force contours in the regions between the atoms of the surface lattice for sufficiently small initial scanning heights of a tip with a single atom at its end. A cluster model of the tip end, which ensures continuity of the scanning at arbitrary initial heights, is constructed. The dependence of the AFM images on the orientation of the cluster on the tip end relative to the crystallographic axes of the surface is investigated for both an unperturbed lattice of close-packed atoms and a lattice containing point defects. The diagnostic possibilities of the findings are discussed. Zh. Tekh. Fiz. 67, 77–85 (June 1997)     

Competition between surface reaction and diffusion of gold deposited onto ZrTe3

Surface reaction and diffusion of gold, deposited onto the (0 0 1) ZrTe₃ van der Waals (vdW) surface, is studied by transmission electron and scanning tunneling microscopy. It is shown that both processes compete at temperatures as low as room temperature. In case of diffusion the deposited gold mostly disappears from the surface and intercalates into the vdW gaps of the substrate. Residual unreacted gold agglomerates are rather mobile and are often displaced by the scanning tip along the [1 0 0] direction of the substrate. In case of reaction, which usually takes place at somewhat higher substrate temperatures, grains of Zr₃Te₂, AuTe₂ and/or Au₂Te₃ are formed. Contrary to unreacted gold, the reaction products are not mobile.     

Simultaneous imaging of the In and As sublattice on InAs(110)-(1×1) with dynamic scanning force microscopy

Distance-dependent dynamic scanning force microscopy (SFM) measurements of InAs(110)-(1×1) acquired in ultrahigh vacuum at low temperatures are presented. On this surface, the atoms of the As sublattice are lifted by 80 pm with respect to the In sublattice and terminate the surface. Thus, since in most dynamic SFM images only protrusions with the periodicity of one sublattice are observed, these protrusions are correlated with the positions of the As atoms. However, under certain conditions, an additional contrast is visible which can be attributed to an interaction between the foremost tip atoms and the In atoms. Possible contrast mechanisms are discussed in terms of tip–sample distance and tip structure.     

用扫描隧道显微镜操纵Cu亚表面自间隙原子

在超高真空环境下使用扫描隧道显微镜研究了吸附有双甘氨肽分子的Cu(001)表面.在一定的 偏压条件下，针尖在该表面扫描后会形成纳米尺度的Cu团簇，这些团簇可以根据意愿排列成 字母或图形.团簇的高度同偏压、隧道电流以及时间等条件有密切关系.在室温下可以稳定存 在的团簇为制造纳米器件提供了技术上的可能性.实验结果表明，形成团簇的Cu原子不是来 自Cu衬底表面或是针尖.化学吸附在Cu表面的双甘氨肽分子，受到隧道电场的作用会在Cu表 面形成张应变场，Cu亚表面自间隙原子在张应变场作用下迁移到表面是形成团簇的原因. 关键词： 扫描隧道显微镜 纳米尺度Cu团簇 自间隙原子     

Reconstruction at a metallic interface studied by field ion and field emission microscopy

Reconstruction of a tungsten surface by adsorbed layers of gold, silver and copper has been studied by field emission and field ion microscopy. Gold reconstructs the surface in three ways, termed the α, β and γ rearrangements. The α rearrangement, which results in a smoothing of the tungsten surface, takes place at around 400° K with gold coverages of 5 monolayers (5θ), and is thought to be an increase in structural perfection of the tungsten surface by gold-assisted surface diffusion of tungsten atoms, β-reconstruction takes place in the temperature range 480–950°K at coverages ? 1.7θ, producing a faceted surface which comprises {211} and {110} facets, and is thought to result from the need to minimise the free energy at the gold/tungsten interface. The γ structure, which appears above 1400°K, is believed to represent a change in the shape of the tip by transport of tungsten to the (110) locality. Adsorbed silver produces neither β nor γ structures, and the degree of α rearrangement is very small, being confined to the {230} regions of the substrate. Copper lies between silver and gold in its ability to rearrange the tungsten surface, some degree of α rearrangement is detectable, and the β structure is very poorly developed unlike the γ structure which is clearly formed. The binding strength of copper to tungsten is greater than that of silver, but less than that of gold; the capacity of an adsorbate, to reconstruct the tungsten substrate is therefore thought to be related to the strength of the adsorbate-substrate bond.     

A scanning tunnelling microscopy investigation of gold island formation from an octanethiol self-assembled monolayer on Au(1 1 1)

The release of gold atoms from an octanethiol monolayer on Au(1 1 1) and the subsequent formation of single-layer-high gold islands have been investigated using a scanning tunnelling microscope (STM) in air. When the bias voltage between the STM tip and the sample is above the threshold for water electrolysis, reactive desorption of the thiol molecules takes place leading to the release of gold adatoms. The number of released atoms has been evaluated as a function of exposure to the tip current under both positive and negative bias voltages. Tip-induced ripening of the gold islands, and more interestingly, tip-induced disintegration of small islands are observed.     

Fluorescence spectra of colloidal self-assembled CdSe nano-wire on substrate of porous Al_2O_3/Au nanoparticles

We present a self-assembly method to prepare array nano-wires of colloidal CdSe quantum dots on a substrate of porous Al_2 O_3 film modified by gold nanoparticles. The photoluminescence(PL) spectra of nanowires are in situ measured by using a scanning near-field optical microscopy(SNOM) probe tip with 100-nm aperture on the scanning near-field optical microscope. The results show that the binding sites from the edge of porous Al_2 O_3 nanopores are combined with the carboxyl of CdSe quantum dots' surface to form an array of CdSe nanowires in the process of losing background solvent because of the gold nanoparticles filling the nano-holes of porous Al_2 O_3 film. Compared with the area of nonself-assembled nano-wire, the fluorescence on the Al_2 O_3/Au/CdSe interface is significantly enhanced in the self-assembly nano-wire regions due to the electron transfer conductor effect of the gold nanoparticles' surface. In addition, its full width at half maximum(FWHM) is also obviously widened. The method of enhancing fluorescence and energy transfer can widely be applied to photodetector, photocatalysis, optical display, optical sensing, and biomedical imaging, and so on.     

LOCAL ELECTROCHEMICAL DEPOSITION OF METAL ISLANDS MECHANICALLY INDUCED WITH THE TIP OF AN ATOMIC FORCE MICROSCOPE

The investigation of electrochemical processes on the nanometer scale is of great scientific as well as technological interest. Here we study the electrodeposition of copper on a polycrystalline gold surface, and demonstrate that copper deposition can be locally induced by mechanical activation with the tip of an atomic force microscope (AFM). Whereas at higher values of the deposition voltage (>100mV), a solid copper film can grow on the gold surface without tip activation, at lower voltages (approx. 30-60mV), copper deposition only occurs at the position where the surface is activated by the AFM tip due to scanning in mechanical contact with the sample. With this mechano-electrochemical "writing" process, which can be performed at ambient conditions, the controlled local deposition of metallic islands is possible, at applied force loads of the order of 10nN. Both the size-dependence of the locally induced structures on the deposition time and the reversibility of the local deposition process are studied. Depending on the deposition parameters, individual copper islands between 50nm and 200nm in size were deposited at predefined locations on the gold surface. The investigations open perspectives for the controlled mechano-electrochemical writing of more complex nanostructures with the AFM tip.     

Atom manipulation with the scanning tunneling microscope: nanostructuring and femtochemistry

We briefly survey our recent studies on the ‘soft’lateral manipulation of atoms and small molecules with the scanning tunneling microscope (STM), whereby mainly the tip–surface forces are employed. Repulsive (pushing) as well as discontinuous (pulling) and continuous (sliding)attractive manipulation modes could be distinguished on Cu(211) for CO molecules and metal atoms, respectively. In the case of pulling of Cu atoms on Cu(111) even finer details could be discerned: the adparticle may show various movement patterns visiting different surface sites upon applying different tip forces. Lateral manipulation also allows modifications of the Cu(211) substrate itself in an atom-by-atom manner by releasing atoms from sixfold coordinated kink sites and even sevenfold coordinated regular step sites. Furthermore, investigations concerning controled vertical manipulation with emphasis on ‘picking-up’ single CO molecules are reported. The mechanism behind vertical transfer of CO molecules relates to ultrafast chemical processes. Vertical manipulation implies, besides extending the possibilities for the build-up of nanostructures, the important possibility of creating structurally and compositionally well-defined tips, which may eventually lead to chemical sensitivity with the STM.     

Electron transport across a quantum wire in the presence of electron leakage to a substrate

We investigate electron transport through a mono-atomic wire which is tunnel coupled to two electrodes and also to the underlying substrate. The setup is modeled by a tight-binding Hamiltonian and can be realized with a scanning tunnel microscope (STM). The transmission of the wire is obtained from the corresponding Green’s function. If the wire is scanned by the contacting STM tip, the conductance as a function of the tip position exhibits oscillations which may change significantly upon increasing the number of wire atoms. Our numerical studies reveal that the conductance depends strongly on whether or not the substrate electrons are localized. As a further ubiquitous feature, we observe the formation of charge oscillations.     

AES and ELS study of gold deposition on top of hydrogen saturated Si(1 1 1) surfaces

Gold was deposited on top of hydrogen saturated Si(1 1 1) clean surfaces. The electronic nature and atomic intermixing between gold overlayer and silicon substrate were studied by AES and ELS. It is claimed that there is a critical thickness (～ 2 ML), where 1 ML = 7.8 × 10¹⁴ atoms cm^?2 for Si(1 1 1), for gold to induce alloyed metallic overlayer formation on the surface of a specimen due to intermixing reaction independently whether a surface of Si(1 1 1) substrate is saturated with hydrogen atoms or not.     

Kondo effect of an adsorbed cobalt phthalocyanine (CoPc) molecule: the role of quantum interference

It has been shown that by distorting a CoPc molecule adsorbed on a Au(111) surface a Kondo effect is induced with a temperature higher than 200 K. We examine a model in which an atom with strong Coulomb repulsion (Co) is surrounded by four atoms on a square (molecule lobes), with two atoms above and below it representing the apex of the STM tip and an atom on the gold surface (all with a single atomic orbital). The Hamiltonian is solved exactly for the isolated cluster, and, after connecting the leads, the conductance is calculated by standard techniques. Quantum interference prevents the existence of the Kondo effect when the orbitals on the square do not interact (undistorted molecule); the Kondo resonance shows up after switching on that interaction. The weight of the Kondo resonance is controlled by the interplay of couplings to the STM tip and the gold surface and between the molecule lobes.     

MANIPULATION OF ATOMS, MOLECULES AND CLUSTERS FOR CONSTRUCTION OF NANOSYSTEMS

Controlled manipulations with the scanning tunneling microscope (STM) down to the scale of small molecules and single atoms allow to built molecular and atomic nanosystems, leading to the fascinating possibility of creating manmade structures on atomic scale. Here we present a short review on atomic scale manipulation investigations. Upon soft lateral manipulation of adsorbed species, in which only tip/particle forces are used, three different manipulation modes (pushing, pulling, sliding) can be discerned. We show that even the manipulation of highly coordinated native substrate atoms is possible and we demonstrate how this can be applied as local analytic and synthetic chemistry tools, with important consequences on surface structure research. Vertical manipulation of Xe and CO is demonstrated, leading to improved imaging with functionalized tips. With CO deliberately transferred to the tip, we have also succeeded to perform vibrational spectroscopy on single molecules. Furthermore, we describe how we have reproduced a full chemical reaction with single molecules, whereby all basic steps, namely preparation of the reactants, diffusion and association, are induced with the STM tip. Finally, we have extended the manipulation techniques to large specially designed molecules by performing lateral manipulation in constant height and realizing the principle of a conformational molecular switch.     

Electrochemical STM investigation of 1,8-octanedithiol monolayers on Au(1 1 1).: Experimental and theoretical study

The surface structure of Au(1 1 1) electrodes covered by 1,8-octanedithiol self-assembled monolayers (SAMs) was studied with in situ scanning tunnelling microscopy (STM) as a function of the electrode potential in acidic and alkaline electrolytes. We investigated the correlation between the dynamics of the SAM and the underlying gold atoms during the reductive desorption and oxidative readsorption processes. The reductive desorption of 1,8-octanedithiol is characterized by a transition from a compact monolayer with an upright molecular configuration to a striped phase with flat lying dithiol molecules, whereas during the oxidative readsorption process the surface first becomes increasingly covered by the striped phase until the transition to the compact phase occurs. We also considered under equivalent conditions 1-octanethiol/Au(1 1 1) SAMs which were used as a reference to evaluate the influence of the -SH terminal group on the structure and stability of dithiol SAMs. The desorption and readsorption of both the dithiol and the monothiol have a considerable influence on the substrate morphology which is manifested in the dynamics of vacancy islands, gold islands and indentation of step edges. Quantum mechanical calculations in the framework of density functional theory (DFT) show that adsorbed thiols greatly facilitate the detachment of gold atoms from step edges. The high mobility of gold atoms observed experimentally is compatible with the presence of a defected layer of gold atoms. The DFT results suggest that the formation of a monolayer may involve the diffusion and self assembly of thiolate-Au moieties rather than the diffusion of the bare thiolates across the surface. This mechanism explains the formation of a defected layer of gold atoms.