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.     

Controlled lateral manipulation of single molecules with the scanning tunneling microscope

We report on the first successful lateral manipulation of molecules and controlled formation of nanostructures with the Scanning Tunneling Microscope (STM) at temperatures above 4 K as used by Eigler and collaborators. Among the first structures, we built the letters F and U forming the logo of our university at 30 K with CO molecules on a Cu(211) substrate. Our method to manipulate the molecules is analogous to that employed successfully up to now only by Eigler and co-workers. First experiences concerning the manipulation of the CO molecules on the highly anisotropic substrate are presented and the crucial role of the tip composition in imaging the CO molecules is demonstrated.     

Manipulation of atoms and molecules with a low temperature scanning tunneling microscope

Apart from its ability to image surfaces with atomic resolution the scanning tunneling microscope has evolved as a tool to manipulate single atoms and molecules. In this paper we present several examples of atomic manipulation and the formation of nanostructures with this technique. As examples for lateral manipulation, i.e. the sliding of atoms and molecules along the surface, we show CO, C₂H₄ and Pb on a stepped copper surface, the Cu(211). As examples for vertical manipulation, i.e. the transfer of atoms and molecules between the tip and the surface, we discuss Xe and C₃H₆. The design of our low temperature scanning tunneling microscope is shortly described.     

New mechanism for single atom manipulation

Scanning tunneling microscope (STM) investigations of the step roughening of Ag(110) have shown that the STM tip extracts atoms from otherwise stable steps even at typical imaging conditions. Detailed analyses of single STM scans reveal that none of the so far known lateral manipulation mechanisms (pushing, pulling, sliding) account for the observed atom extraction. The Ag atoms rather follow the energetically favorable path of a tip induced exchange process, similar to the concerted motion proposed previously for the diffusion on fcc(110) surfaces including a metastable and thus experimentally detectable dumbbell transition state.     

用扫描隧道显微术进行单原子操纵的机理研究

本文简要综述利用扫描隧道显微镜（ＳＴＭ）进行单原子操纵的物理机制。主要介绍了场增强的扩散、在表面上拖动（ｐｕｌｉｎｇ）推动（ｐｕｓｈｉｎｇ）原子、原子在针尖表面间接触和近接触转移、场致蒸发／脱附、隧道电子非弹性射激发和电子迁移的“电子风力”等过程。同时介绍了一些理论处理方法和对一些实验结果的解释。     

Vertical manipulation of native adatoms on the InAs(111)A surface

We achieved the repositioning of native In adatoms on the polar III-V semiconductor surface InAs(111)A-(2?×?2) with atomic precision in a scanning tunnelling microscope (STM) operated at 5?K. The repositioning is performed by vertical manipulation, i.e., a reversible transfer of an individual adatom between the surface and the STM tip. Surface-to-tip transfer is achieved by a stepwise vibrational excitation of the adsorbate-surface bond via inelastic electron tunnelling assisted by the tip-induced electric field. In contrast, tip-to-surface back-transfer occurs upon tip-surface point contact formation governed by short-range adhesive forces between the surface and the In atom located at the tip apex. In addition, we found that carrier transport through the point contact is not of ballistic nature but is due to electron tunnelling. The vertical manipulation scheme used here enables us to assemble nanostructures of diverse sizes and shapes with the In adatoms residing on vacancy sites of the (2?×?2)-reconstructed surface (nearest-neighbour vacancy spacing: 8.57??).     

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：     

Lateral and vertical manipulations of single atoms on the Ag(1 1 1) surface with the copper single-atom and trimer-apex tips

We study the lateral and vertical manipulations of single Ag and Cu atoms on the Ag(1 1 1) surface with the Cu single-atom and trimer-apex tips using molecular statics simulations. The reliability of the lateral manipulation with the Cu single-atom tip is investigated, and compared with that for the Ag tips. We find that overall the manipulation reliability (MR) increases with the decreasing tip height, and in a wide tip-height range the MR is better than those for both the Ag single-atom and trimer-apex tips. This is due to the stronger attractive force of the Cu tip and its better stability against the interactions with the Ag surface. With the Cu trimer-apex tip, the single Ag and Cu adatoms can be picked up from the flat Ag(1 1 1) surface, and moreover a reversible vertical manipulation of single Ag atoms on the stepped Ag(1 1 1) surface is possible, suggesting a method to modify two-dimensional Ag nanostructures on the Ag(1 1 1) surface with the Cu trimer-apex tip.     

Reversible Vertical Manipulations of Single Pt Adatom on Pt（111） Surface with a Triple-Apex Tip

With a triple-apex tip, we investigate theoretically the vertical manipulation of single Pt adatom on the Pt（111） surface. The adatom adsorbed on the f cc site of the flat Pt（111） surface can be transferred vertically to the tip by adjusting the tip height properly. Moreover, based on the strong vertical trapping ability and the relatively weak lateral trapping ability of the tip, we propose a simple method to realize a reversible vertical manipulation of the Pt adatom from the highly coordinated sites, the kink and the step sites, of the stepped Pt（111） surface. All the vertical manipulations are completed using only the atomic force between the tip and the adatom, without the electric field.     

CO dynamics induced by tunneling electrons: differences on Cu(110) and Ag(110)

The electronic current originating in a scanning tunneling microscope (STM) can be used to induce motion and desorption of adsorbates on surfaces. The manipulation of CO molecules on noble metal surfaces is an academic case that has received little theoretical attention. Here, we do thorough density functional theory calculations that explore the chemisorption of CO on Cu(110) and Ag(110) surface and its vibrational properties. The STM induced dynamics are explored after excitation of the highest lying mode, the C–O stretch. In order to give a complete account of this dynamics, the lifetime of the different CO modes is evaluated (by only including the mode decay into electronic excitations of the host surface) as well as the intermode coupling. Hence, after excitation of the stretch mode, the lower-energy modes are populated via intermode coupling and depopulated by electron-hole excitations. This study reveals the intrinsic features of the STM induced motion of CO on Cu(110) and Ag(110).     

CO在表面有序合金Cu{001}c(2×2)-Pd上的吸附

用高分辨电子能量损失谱研究CO在有序表面合金Cu{001}c(2×2)-Pd上的吸附位置,在240K,CO仅直立地吸附在Pd原子的顶位;在135K,低覆盖度时,CO仅直立地吸附于Pd原子顶住,覆盖度增加后,CO同时直立地也吸附于Pd和Cu原子的顶位。     

铜箔上生长的六角氮化硼薄膜的扫描隧道显微镜研究

利用扫描隧道显微镜研究了采用化学气相沉积法在铜箔表面生长出的高质量的六角氮化硼薄膜. 大范围的扫描隧道显微镜图像显示出该薄膜具有原子级平整的表面, 而扫描隧道谱则显示, 扫描隧道显微镜图像反映出的是该薄膜样品的隧穿势垒空间分布. 极低偏压的扫描隧道显微镜图像呈现了氮化硼薄膜表面的六角蜂窝周期性原子排列, 而高偏压的扫描隧道显微镜图像则呈现出无序和有序排列区域共存的电子调制图案. 该调制图案并非源于氮化硼薄膜和铜箔衬底的面内晶格失配, 而极有可能来源于两者界面处的氢、硼和/或氮原子在铜箔表面的吸附所导致的隧穿势垒的局域空间分布.     

Chemisorption of atomic oxygen on Pd(1 1 1) studied by STM

Atomic oxygen resulting from the dissociation of O₂ on Pd(1 1 1) at low coverage was studied in a variable temperature scanning tunneling microscope (STM) in the range from 30 to 210 K. Oxygen atoms, which typically appear as 30-40 pm deep depressions on Pd(1 1 1), occupy fcc hollow sites and form ordered p(2 × 2) islands upon annealing above 180 K. The mobility of the atoms diminishes rapidly below 180 K, with an approximate diffusion barrier of 0.4-0.5 eV. Oxygen atom pairs produced by thermal dissociation of O₂ at 160 K occupy both fcc and hcp hollow sites. The atoms travel approximately 0.25 nm after dissociation, and the distribution of pairs is strongly influenced by the presence of subsurface impurities within the Pd sample. At much lower temperatures, the STM tip can dissociate oxygen molecules. Dissociation occurs at sample bias voltages exceeding approximately 0.1 V. Following tip-induced dissociation, the product atoms occupy only fcc hollow sites. Oxygen atoms can be manipulated via short range repulsive interactions with the STM tip.     

Conformational changes of single molecules induced by scanning tunneling microscopy manipulation: a route to molecular switching

A detailed experimental and theoretical investigation of the processes involved in the manipulation of individual specially designed porphyrin-based molecules by scanning tunneling microscopy at low temperature is presented. On a stepped Cu(211) surface, the interaction between tip and molecule was used to locally modify in a reversible way the internal configuration of a single molecule, thus drastically changing the tunneling current passing through it. Model calculations confirm that this manipulation realizes the principle of a conformational molecular switch.     

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.     

Controlling adsorption status of individual fullerene at room-temperature

The influence of Ag atoms on the adsorption statuses of individual fullerene molecules on Si surface at room temperature has been investigated by scanning tunneling microscopy. For a fullerene molecule, its adsorption status can be switched from one initial state into multiple final states by attaching Ag atoms onto various parts of the molecule. Once silver atoms are removed from the adsorption sites by a STM tip, the adsorption statuses of the fullerene molecules are recovered as that for fullerene molecules on a bare Si surface.     

STM manipulation of a subphthalocyanine double-wheel molecule on Au(111)

A new class of double-wheel molecules is manipulated on a Au(111) surface by the tip of a scanning tunneling microscope (STM) at low temperature. The double-wheel molecule consists of two subphthalocyanine wheels connected by a central rotation carbon axis. Each of the subphthalocyanine wheels has a nitrogen tag to monitor its intramolecular rolling during an STM manipulation sequence. The position of the tag can be followed by STM, allowing us to distinguish between the different lateral movements of the molecule on the surface when manipulated by the STM tip.     

Recording intramolecular mechanics during the manipulation of a large molecule

The technique of single atom manipulation by means of the scanning tunneling microscope (STM) applies to the controlled displacement of large molecules. By a combined experimental and theoretical work, we show that in a constant height mode of manipulation the STM current intensity carries detailed information on the internal mechanics of the molecule when guided by the STM tip. Controlling and time following the intramolecular behavior of a large molecule on a surface is the first step towards the design of molecular tunnel-wired nanorobots.     

Preparing and regulating a bi-stable molecular switch by atomic manipulation

We present a scanning tunneling microscopy (STM) investigation into the influence of the STM tip on the adsorption site switching of polychlorinatedbiphenyl (PCB) molecules on the Si(111)-7?×?7 surface at room temperature. From an initially stable adsorption configuration, atomic manipulation by charge injection from the STM tip prepared a new bi-stable configuration that switched between two bonding arrangements. No switching rate bias dependence was found for +?1.0 to +?2.2?V. Assuming a thermally driven switching process we find that the measured energy barriers to switching are influenced by the exact location of the STM tip by more than 10%. We propose that this energy difference is due the dispersion interaction between the tip and the molecule.