Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.     

Model of electronically enhanced Raman scattering from adsorbates on cold-deposited silver

We propose an extension of the charge transfer model of surface enhanced Raman scattering (SERS) at silver surfaces. Within an incoherent approximation we include propagating hot electrons, created or annihilated by increased photon-electron coupling at disordered (internal) surfaces. The hot electrons are inelastically scattered by the adsorbates. In “shape resonances” of the free molecule the corresponding cross section is 14 orders of magnitude larger than the ordinary Raman cross section. We list results related to photon-electron coupling and inelastic scattering of electrons by molecules and discuss open problems.     

Atomic engineering of photon emission with a scanning tunneling microscope

We present measurements of photon emission from individual several-atom silver chains on the NiAl(110) surface, excited by tunneling electrons in a scanning tunneling microscope (STM). The chains were assembled by manipulating single silver atoms on the NiAl(110) surface with the STM. The photon energy of this emission can be tuned by appending a single atom to the chain. These changes in photon emission result from changes in the electronic structure of the silver chain, each electronic state inside the chain being associated with a distinct channel of emission.     

Relaxation between electrons and surface phonons of a homogeneously photoexcited metal film

The energy relaxation between the hot degenerate electrons of a homogeneously photoexcited metal film and the surface phonons (phonon wave vectors in two dimensions) is considered under Debye approximation. The state of electrons and phonons is described by equilibrium Fermi and Bose functions with different temperatures. Two cases for electron scattering by the metal surface, namely specular and diffuse scattering, are considered.     

Excitation of molecular vibrational modes with inelastic scanning tunneling microscopy processes: examination through action spectra of cis-2-butene on Pd(110)

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection rules in terms of resonance tunneling.     

Surface structure of ultrathin Fe films on Cu(001) revisited

The structure and magnetism of ultrathin Fe films epitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are observed only for cooling and H2 dosing.     

Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals

We show that quantization of image-potential state (IS) electrons above the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking-fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy, the n=1 IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons in the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect-induced scattering above nanostructured surfaces.     

Raman spectroscopy of surfaces

Raman scattering has usually a very low efficiency. Therefore, during the first five decades after its discovery, Raman spectroscopic investigations of adsorbate-covered surfaces (except surfaces of highly porous samples) were out of reach. This changed in 1970s, when for molecules adsorbed on some surfaces, very large increase of the intensity of Raman spectrum (denoted as surface-enhanced Raman spectroscopy – SERS) was reported. In the past decade, two other very important achievements in surface Raman spectroscopy have been made: observation of SER spectrum of a single molecule and coupling of Raman spectroscope with the scanning probe microscope (STM or AFM) allowing a significant increase in the spatial resolution of Raman measurements in so-called tip-enhanced Raman spectroscopy (TERS). In the latter approach, fine tip made of a metal that supports surface plasmon resonances (such tip may be treated as a very local electromagnetic resonator) is brought at the nanometer distance above the surface, which induces large increase of the Raman scattering from molecules adsorbed at a surface located underneath the tip. This short review presents an overview of the state of the art and further possible applications of Raman spectroscopy in surface analysis. We mainly focus on SERS and TERS. Future prospects in these fields are also discussed.     

Observing the lateral confinement of surface state electrons in room temperature stable metallic nanostructures

The lateral confinement of the surface state electrons of Cu(111) has been studied by Scanning Tunnelling Microscopy and Spectroscopy at low temperature. The confining nanostructures are Cu(111) islands embedded in a semiconducting Cu₃N(111) film which completely isolate them from each other. The standing wave pattern observed reflect the shape of the edge of the islands, i.e. the positions of the confining potential as long as the islands are larger than twice the Fermi wavelength of the surface electrons. The interference pattern in smaller islands is more complex, reflecting the collective behavior of the electrons. When the width of the islands is, at least in one dimension, smaller than the Fermi wavelength, there is a clear shift in the energy of the bottom of the surface band towards the Fermi level. The depopulation of the surface state produced by lateral confinement might have important consequences with respect to the reactivity of these nanostructures.Received: 15 December 2003, Published online: 10 August 2004PACS: 68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM) - 73.20.At Surface states, band structure, electron density of states - 73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals     

Electron-induced H atom desorption patterns created with a scanning tunneling microscope: Implications for controlled atomic-scale patterning on H-Si(1 0 0)

We have used scanning tunneling microscopy (STM) to explore the details of single and multiple H atom desorption from the H-Si(1 0 0)-2 × 1 surface induced by the inelastic scattering of electrons from an STM tip. The desorption of pairs of H atoms from individual Si dimers is rarely observed. Two-H atom desorption most often involves pairs of dimers, in the same or adjacent rows. This suggests that recombinative H₂ desorption via an interdimer reaction pathway, like that observed recently under nanosecond laser heating, may also be operative for electron-induced excitation using STM. Repeatable fabrication of desired size-selected dangling bond (DB) clusters is also achieved. The single atomic precision of the fabrication is a result of the intrinsically unfavorable paired H atom desorption from a single dimer, but does not result from the spatial localization of excitation energy of the Si-H bond under the STM tip as suggested in previous studies.     

Mechanisms of molecular manipulation with the scanning tunneling microscope at room temperature: chlorobenzene/si(111)-(7 x 7)

We report a systematic experimental investigation of the mechanism of desorption of chlorobenzene molecules from the Si(111)-(7 x 7) surface induced by the STM at room temperature. We measure the desorption probability as a function of both tunneling current and a wide range of sample bias voltages between -3 V and +4 V. The results exclude field desorption, thermally induced desorption, and mechanical tip-surface effects. They indicate that desorption is driven by the population of negative (or positive) ion resonances of the chemisorbed molecule by the tunneling electrons (or holes). Density functional calculations suggest that these resonant states are associated with the pi orbitals of the benzene ring.     

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.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Inelastic scattering of slow electrons in solids

A theory of the inelastic scattering of slow electrons in solids due to excitation of interband transitions is developed. It is shown that both nondirect and direct transitions occur which can be described by a generalization of the formalism used in solid state optics. Experiments with 30–200 eV electrons scattered from Si (111) surfaces with well defined surface structures as determined by low energy electron diffraction confirm the theoretical predictions. They indicate that the inelastic scattering of slow electrons can be understood in terms of the three-dimensional band structure of solids and suggest the use of inelastic low energy electron scattering as a tool for band structure analysis.     

Au-CdSe纳米异质结在Au(111)表面上的分散与STM表征

研究单个Au-CdSe纳米异质结在经过表面修饰的Au(111)上的分散并进行STM表征. 发现衬底表面的自组装分子层的顶端功能基团对于形成良好的异质结分散状态非常重要. STM表明,巯基和羧酸基都能与异质结的Au纳米颗粒部分形成强烈的相互作用,从而将异质结有效地抓住,避免了团聚现象,便于异质结形成孤立分散的样品结构. 还可以采用条件适中的氩离子溅射技术来清除样品表面残留的有机物,从而得到较清晰的单个Au-CdSe纳米异质结的STM图像.     

The angular behaviour of electrons inelastically scattered from the shape resonance for CO chemisorbed on Ni(110)

Recent vibrational high resolution electron energy loss experiments (HREELS) have shown evidence for molecular shape resonances in the inelastic scattering of electrons from chemisorbed molecules. Such resonances arise from the capture of the incident electron in a quasibound state of the molecule, leading to the formation of a temporary negative ion. They are manifest as an enhancement in the intensity of a specific vibrational mode at a characteristic incident electron energy. In contrast to gaseous species, the alignment which the surface provides for the chemisorbed species, can be exploited to determine the angular characteristics of the resonant state. In this work, we show evidence for a shape resonance, centred at an incident energy of 18 eV, for CO/Ni(110). The angular dependence of the scattered electron intensity has been measured for the CO stretching vibration. The results are discussed in terms of the spherical harmonic components of the resonant state, modified by vibrational broadening caused by low frequency bending modes associated with the bonding of the CO molecule to the surface.     

Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from a crystal surface

Abstract

Convoy electrons produced at glancing angle scattering of MeV HeH⁺ ions from an atomically clean (001) surface of SnTe crystal are observed. Energy spectrum of the convoy electrons shows a peak broader than that at scattering of atomic projectiles and the most probable energy of convoy electrons at HeH⁺ scattering is larger than those at scattering of isotachic He ions. This acceleration of convoy electrons is qualitatively explained by the force due to surface wake induced by Coulomb exploding fragment He²⁺ and H⁺.     

Interaction of CO molecules with surface state electrons on Ag(1 1 1)

Like other close-packed noble metal surfaces, Ag(1 1 1) exhibits an occupied Shockley-type surface state that is believed to influence the adsorption of atoms and molecules. Using low-temperature scanning tunneling microscopy, we have directly probed this interaction by investigating the local CO distribution dependent on the Ag(1 1 1) surface state standing wave pattern forming in the neighborhood of strong scattering centers such as step edges or hexagonal holes. A quantitative analysis of the STM data reveals that the CO molecules are not arbitrarily distributed upon adsorption at 5 K; they adsorb preferentially near the minima of the standing wave pattern.