Adsorbate dynamics induced by STM

Microscopic models to describe adsorbate dynamics induced by STM, e.g. the dynamics of STM-induced desorption of CO from Cu(111) and the dynamics of STM-induced rotation of acetylene on Cu(100) are presented. In these models, the relation between the change of the electronic state caused by an electron tunneling from an STM tip and the transition of the vibrational state for the molecular/intramolecular motion is taken into account from a microscopic point of view. Calculated probabilities for inducing desorption and rotation in these models agree with recent experimental results.     

用扫描隧道显微镜测量局域功函数

用扫描隧道显微镜在Cu（111）-Au和Pt（111）-Ag表面上对局域功函数进行了测量．在扫描的同时通过测量隧道电流对针尖样品间距离变化的响应，可以在得到扫描隧道显微镜（STM）图的同时得到功函数图．用这种方法，成功地观察到Au，Ag覆盖层与Cu，Pt衬底间的功函数的差别．结果表明：Au覆盖层的功函数介于Cu（111）和Au（111）的功函数之间，这与其它方法的结果一致．在Pt（111）-Ag表面观察到了局域功函数随覆盖层厚度的变化．本工作表明：扫描隧道显微镜在研究功函数与表面结构的关系方面是十分有用的；用测量局域功函数的方法还可以区分表面不同种的物质 关键词：     

Mapping the first electronic resonances of a Cu phthalocyanine STM tunnel junction

Using a low temperature, ultrahigh vacuum scanning tunneling microscope (STM), dI/dV differential conductance maps were recorded at the tunneling resonance energies for a single Cu phthalocyanine molecule adsorbed on an Au(111) surface. We demonstrated that, contrary to the common assumption, such maps are not representative of the molecular orbital spatial expansion, but rather result from their complex superposition captured by the STM tip apex with a superposition weight which generally does not correspond to the native weight used in the standard Slater determinant basis set. Changes in the molecule conformation on the Au(111) surface further obscure the identification between dI/dV conductance maps and the native molecular orbital electronic probability distribution in space.     

Scattering of surface state electrons at large organic molecules

The scattering of surface state electrons at Lander-type molecules on Cu(111) is investigated by means of scanning tunneling microscope (STM) experiments at low temperature and model calculations. Specific information concerning the electronic interaction of the different internal groups of the molecule with the surface is obtained. Remarkably, the central molecular wire of the molecule, although decoupled from the surface by spacer groups and therefore not visible in STM images, is the main one responsible for scattering of surface state electrons.     

甘氨酸在Cu(111)表面吸附的扫描隧道显微镜研究

用超高真空扫描隧道显微镜(UHV-STM)研究了室温下甘氨酸在Cu(111)表面的吸附行为.实验发现,在低覆盖度下甘氨酸分子在表面表现为二维气体.当覆盖度足够高时,甘氨酸分子最终会形成二维固相结构,为(4×8)超结构.针对这种结构提出了两种可能的结构模型,模型能够很好地解释STM图.当覆盖度介于气相和固相之间时,根据蒸镀条件和退火条件的不同,表面可能出现两种不同的中间相,一种为条纹结构,另一种为六角结构,对于中间相有待于进一步的研究 关键词： 表面吸附 甘氨酸 铜 扫描隧道显微术     

局域功函数图像及其在Cu(111)-Au/Pd表面的应用

用扫描隧道显微镜（STM）对Cu(111)-Au和Cu(111)-Pd表面的局域功函数进行了研究.通过 测量隧道电流对针尖样品间距的响应，得到了与STM形貌图一一对应的表面局域功函数图像. 实验发现，Au/Pd覆盖层和Cu衬底间的功函数有明显的不同.Pd薄膜的功函数甚至超过了其体 本征值，且功函数在台阶处变小.用偶极子的形成解释了台阶处功函数的降低.这一工作表明 ，用测量局域功函数的方法容易区分表面上不同的元素，并具有纳米尺度的空间分辨率. 关键词： 扫描隧道显微镜 局域功函数 台阶     

STM observation of the origin of electrochemical promotion on metal catalyst-electrodes interfaced with YSZ and β″-Al2O3

Scanning tunneling microscopy (STM) was used to investigate the surfaces of Pt(111) single crystals interfaced with YSZ and β″-Al₂O₃ at atmospheric pressure. In both cases the STM imaged the reversible electrochemically controlled dosing (backspillover) of O²⁻ species and of Na⁺ species on Pt(111) surface respectively, which both form a (12 × 12) hexagonal structure on the Pt(111) surface. On the mechanistic side, the STM has confirmed the backspillover mechanism of electrochemical promotion and metal support interactions.     

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.     

STM images of a large organic molecule adsorbed on a bare metal substrate or on a thin insulating layer: Visualization of HOMO and LUMO

An isomer of the methylterrylene molecule was adsorbed both on Cu(111) and on a NaCl bilayer deposited on Cu(111) and imaged by ultra high vacuum scanning tunneling microscopy at low temperature (5 K). On the bare metal surface, the STM images do not reveal any intramolecular resolution and do not depend on the applied tunnel bias. On the contrary, the images acquired at specific bias voltages for the molecule on the salt layer show a striking similarity with the spatial distribution of the electronic probability density in the highest occupied molecular orbital (HOMO) and in the lowest unoccupied molecular orbital (LUMO) of free methylterrylene. They are well reproduced by elastic scattering quantum chemistry calculations. These data provide a direct view of the hyperconjugative interaction between the methyl group and the frontier orbitals of terrylene.     

Low temperature electron energy loss spectra of acetylene chemisorbed on metal single-crystal surfaces; Cu(111), Ni(110) and Pd(110)

Vibrational spectra of acetylene chemisorbed on Cu(111), Ni(110) and Pd(110) at 110–120 K were measured using electron energy loss spectroscopy. Loss peaks were assigned to vibrational modes of the non-dissociatively adsorbed molecules with the aid of the corresponding C₂D₂ spectra. The spectra show that the molecules undergo significant rehybridisation on adsorption. Comparisons are made with the spectra of acetylene adsorbed on a range of other transition metal surfaces at low temperature. Taking into account these and earlier literature results, two distinct patterns of spectra are observed (Type A and Type B) for specular spectra. The Cu(111) spectrum is classified as Type A while the Ni(110) and Pd(110) spectra are classified as Type B. Suggestions are made for the structures of the surface species corresponding to the two spectral types.     

The interaction of acetylene with Ni(111), chemisorbed oxygen on Ni(111), and NiO(111); the formation of CH species on chemically modified Ni(111) surfaces

Ultraviolet photoelectron spectroscopy, temperature programmed thermal desorption and low-energy electron diffraction have been used to study the interaction of acetylene with a clean Ni(111) surface, with a Ni(111) surface having co-adsorbed oxygen and with an epitaxially grown NiO(111) surface produced by room temperature oxidation ofNi(111). The adsorption of a (2 × 2) overiayer of π-bonded acetylene or oxygen on the Ni(111) surface markedly alters the subsequent interaction and reaction of the surface with incident acetylene. We find that in the presence of either a (2 × 2) overiayer of oxygen or π-bonded acetylene, a new more strongly bound hydrocarbon phase forms at room temperature. We identify this new phase from its ionization levels as a CH species, and for saturation coverages we find approximately twice as many of these species as the number of π-bonded acetylene molecules in the (2 × 2) structure. Preadsorption of oxygen limits the adsorption of π-bonded acetylene but does not affect the subsequent formation of this CH species. Exposure of acetylene to NiO at room temperature produces only CH species. Based upon these results we propose idealized models for the bonding geometry of π-bonded acetylene and CH species on the Ni(111) surface. The conditions for the formation of CH species and the significance of CH species to surface reactions on Ni are also discussed.     

Monolayered adatom aggregation induced by metallofullerene molecules on Cu(100)

The adsorption and self-assembly of Gd@C₈₂ molecules on Cu(100) surface have been investigated using scanning tunneling microscopy (STM). The metallofullerene molecules in the assemblies showed two characteristic apparent heights in the STM images. STM manipulation and spectroscopy was performed and revealed the formation of Cu adatom islands underneath the Gd@C₈₂ molecules. The monolayered Cu aggregates were resulted from the adatom–molecule complexation, which is supported by density functional theory (DFT) calculations that show charge transfer and electrostatic interactions between Gd@C₈₂ and adatoms. In addition, sub-molecularly resolved STM images demonstrated the structural and orientational ordering of Gd@C₈₂ assemblies upon thermal annealing. DFT calculations demonstrated that Gd atom located at the lower part of the carbon cage is a favored adsorption configuration for Gd@C₈₂ molecules adsorbed on Cu(100).     

Bias voltage dependent shift of the atomic-scale structure of a Ge(111)-(2 × 1) reconstructed surface measured by low temperature scanning tunneling microscopy

The results of low temperature scanning tunneling microscopy (STM) investigations of a clean Ge(111) surface are presented. Bias dependent shifts of the atomic-scale structure caused by the (2 × 1) reconstruction of the Ge(111) surface are observed. A detailed comparison of experimental data with theoretical predictions based on the π-bonded chain model indicates that inelastic tip-sample interaction plays a significant role in STM imaging of the Ge(111)-(2 × 1) reconstructed surface. The text was submitted by the authors in English.     

Local spectroscopy of image-potential-derived states: from single molecules to monolayers of benzene on Cu(111)

Stark-shifted image-potential states were measured with an STM tip for benzene adsorbed on a Cu(111) surface. A single benzene molecule locally shifts the position of the first image state toward the Fermi level by 0.2 eV relative to its position on the clean surface. The energetic position of this molecule-modified state shifts to lower energy with increasing coverage of benzene on the surface. This is attributed to local surface potential changes that are correlated with the lowering of the crystal work function due to adsorption of benzene.     

Growth of Cu Films on Si(111)-7×7 Surfaces at Low Temperature: A Scanning Tunnelling Microscopy Study

Morphologies of Cu(111) films on Si(111)-7×7 surfaces prepared at lowtemperature are investigated by scanning tunnelling microscopy (STM) andreflection high-energy electron diffraction (RHEED). At the initial growth stage, Cu films are flat due to the formation of silicide at the interface that decreases the mismatch between Cu films and the Si substrate. Different from the usual multilayer growth of Cu/Cu(111), on the silicide layer a layer-by-layer growth is observed. The two dimensional (2D) growth is explained by the enhanced high island density at low deposition temperature. Increasing deposition rateproduces films with different morphologies, which is the result of Ostwald ripening.     

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.     

Long-range electronic interactions at a high temperature: bromine adatom islands on Cu(111)

Long-range electronic interactions between Br adatom islands, which are formed at approximately 600 K, on Cu(111) are mediated by substrate surface-state electrons at that elevated temperature. Using scanning tunneling microscopy at 4 K, we have quantified nearest neighbor island separations and found favored spacings to be half-multiples of the Fermi wavelength of Cu(111). The strong interaction potential and decay length of the interisland interactions are discussed in terms of the interaction of Br with the substrate surface state.     

Triphenylene adsorption on Cu(111) and relevant graphene self-assembly

Investigations on adsorption behavior of triphenylene(TP) and subsequent graphene self-assembly on Cu(111) were carried out mainly by using scanning tunneling microscopy(STM).At monolayer coverage,TP molecules formed a longrange ordered adsorption structure on Cu(111) with an uniform orientation.Graphene self-assembly on the Cu(111) substrate with TP molecules as precursor was achieved by annealing the sample,and a large-scale graphene overlayer was successfully captured after the sample annealing up to 1000 K.Three different Moire patterns generated from relative rotational disorders between the graphene overlayer and the Cu(111) substrate were observed,one with 40 rotation between the graphene overlayer and the Cu(111) substrate with a periodicity of 2.93 nm,another with 70 rotation and 2.15 nm of the size of the Moire supercell,and the third with 100 rotation with a periodicity of 1.35 nm.     

Ethylene and acetylene adsorption on Cu(111) and Pt(111) studied by Auger spectroscopy

High resolution, electron impact excited, carbon Auger spectra of ethylene and acetylene adsorbed on Cu(111) and Pt(111) are compared. The spectra of ethylene on the two metals provide the first example of the sensitivity of AES to the nature of metal-adsorbate bonding for molecular adsorbates. The acetylene spectra are identical on the two metals. The changes in the carbon Auger spectra resulting from thermal decomposition of the two adsorbates on Pt(111) are discussed in the context of results from electron energy loss spectroscopy.     

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.