Inelastic electron tunneling spectroscopy of an alkanethiol self-assembled monolayer using scanning tunneling microscopy

We report inelastic electron tunneling spectroscopy (IETS) of a C8 alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.     

Localized molecular constraint on electron delocalization in a metallic chain

An artificial quantum structure consisting of a single CO molecule adsorbed on a Au chain was assembled by manipulating single Au atoms on NiAl(110) at 12 K with a scanning tunneling microscope (STM). The CO disrupts the delocalization of electron density waves in the chain, as it suppresses the coupling between neighboring chain atoms. The possibility to specify the CO position on the chain allows controlled modification of the electronic properties in a quantum system. Inelastic electron tunneling spectroscopy with the STM provides vibrational characterization of the adsorbed CO.     

Direct measurement of electron transport features in cytochrome c via V–I characteristics of STM currents

The morphology of and electron tunneling through single and cluster cytochrome c molecules deposited on self-assembled dodecanthiol monolayer film on a gold substrate have been studied experimentally using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy. STM images of a single cytochrome c molecule revealed a globular structure with a diameter of 4 nm and height of 1.5 nm. A spectroscopic study obtained by recording tunneling current–bias voltage (V–I) curves revealed that the STM current increases stepwise at asymmetric threshold sample bias voltages of +100 mV and –200 mV.     

Au(111)表面单个钴酞菁分子的去氢过程

利用低温超高真空扫描隧道显微镜对单个钴酞菁分子实现了选键化学反应.通过对吸附于Au(111)表面的单个钴酞菁分子外围H原子的"剪裁",并用实验图像和谱学方法,结合第一性原理理论计算研究了逐步去除钴酞菁分子8个外围H原子的过程.理论计算结果再现了实验中所观测到的分子空间构型的变化,并阐明了吸附体系中局域自旋的恢复和变化过程.     

Single-molecule reaction and characterization by vibrational excitation

Controlled chemical reaction of single trans-2-butene molecules on the Pd(110) surface was realized by dosing tunneling electrons from the tip of a scanning tunneling microscope at 4.7 K. The reaction product was identified as a 1,3-butadiene molecule by inelastic electron tunneling spectroscopy. Threshold voltage for the reaction is approximately 365 mV, which coincides with the vibrational excitation of the C-H stretching mode. The reaction was ascertained to be caused by C-H bond dissociation by multiple vibrational excitations of the C-H stretching mode via inelastic electron tunneling process.     

Vibrational characterization of the oxidation products on Si(111)-(7 x 7)

The oxidation products on Si(111)-(7x7) are investigated at 82 K by means of high-resolution electron energy loss spectroscopy. The isotope-labeled vibrational spectra with 16O2, 18O2, and 16O 18O show that, in the initial stage of the oxidation, an O2 molecule dissociates to form a metastable product with an O atom bonding on top of the Si adatom and the other inserted into the backbond. The metastable product is observed as a dark site in the topographic scanning tunneling microscopy (STM) image and can be transformed to a stable product by the STM manipulation. Our results are in good agreement with recent theoretical calculations.     

扫描隧道显微镜诱导单分子发光热荧光现象的全量子理论

使用量子主方程方法从理论上研究了STM诱导分子电致发光现象,并特别考虑了分子和量子化等离激元相互作用对于单分子光学性质的影响. 分子-等离激元相互作用大大增加了分子的自发辐射速率和分子发光的的强度,而且当分子自发辐射速率和振动弛豫速率可比拟的时候,将会出现热荧光现象.     

Spatially dependent inelastic tunneling in a single metallofullerene

We have measured the elastic and inelastic tunneling properties of isolated Gd@C(82) molecules on Ag(001) using cryogenic scanning tunneling spectroscopy. We find that the dominant inelastic channel is spatially well localized to a particular region of the molecule. Ab initio pseudopotential density-functional theory calculations indicate that this channel arises from a vibrational cage mode. We further show that the observed inelastic tunneling localization is explained by strong localization in the molecular electron-phonon coupling to this mode.     

Control of molecular rotors by selection of anchoring sites

We demonstrate a new method to switch on and off the rotational motion of a long-chain molecule by controlling the bonding geometry between the molecule and a substrate. An azobenzene derivative molecule adsorbed on a Au(111) surface is immobile only when its three rotation centers, comprised of two phenyl rings and a nitrogen-nitrogen bond, are located at hollow sites of the Au(111) surface, as observed by scanning tunneling microscopy. Rotational motion can be activated by exciting the vibrational modes and inducing hopping motion away from the immobile site with a voltage pulse.     

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.     

Control and characterization of a multistep unimolecular reaction

Electrons from a scanning tunneling microscope were used to break a C-H bond in a single acetylene (HCCH) molecule adsorbed on Cu(001) at 9 K. The product ethynyl (CCH) was characterized by imaging, inelastic electron tunneling spectroscopy, and variable temperature measurements of the rotation rate. Significant changes in the vibrational spectra and bonding geometry accompanied HCCH dissociation. The ethynyl was further dehydrogenated to form dicarbon (CC). Dissociation studies of the isotopes HCCD and DCCD permitted unambiguous identification of the reaction products.     

Single quantum dots as scanning tunneling microscope tips

We report the use of single quantum dot structures as tips on a scanning tunneling microscope (STM). A single quantum dot structure with a diameter of less than 200 nm and a height of 2 μm was fabricated by reactive ion etching. This dot was placed on a 40 μm-high mesa and mounted on the tip of a STM. The topography of large structures such as quantum wires or gold test substrates is clearly resolved with such a tip. To check the transport properties of the tip, quantum dot arrays were fabricated on resonant tunneling double barrier structures using the same process parameters. Conventional tunneling spectroscopy clearly resolved the 0D states in our samples. Using a metal substrate as second electrode such STM tips can be used to perform high resolution energy spectroscopy on single dots and free standing wire structures.     

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.     

Nonclassical behavior in the capacitance of a nanojunction

The capacitance of a nanojunction formed by a scanning tunneling microscope (STM) tip and a two-dimensional gold cluster was measured through the single electron tunneling spectroscopy of a double-barrier tunnel junction. By decreasing the STM tip-cluster separation, it was observed that the capacitance first increases and then decreases at short separation. This characteristic clearly deviates from the classical behavior and provides evidence for potential quantum effects on the capacitance.     

纳米隧道结的量子电容现象

利用STM针尖和二维Au纳米团簇构造的双隧道结，通过对单电子隧穿谱的测量，研究了纳米隧道结的电容随隧道结宽度的变化，发现电容随结宽度的变化偏离了经典电容的行为，为纳米隧道结的量子电容效应提供了实验证据。     

Scanning probe microscopy characterization of gold-chemisorbed poplar plastocyanin mutants

Two poplar plastocyanin mutants adsorbed onto gold electrodes have been characterized at single molecule level by scanning probe microscopy. Immobilization of the two redox metalloprotein mutants on Au(1 1 1) surface was achieved by either a disulphide bridge (PCSS) or a single thiol (PCSH), both the anchoring groups having been introduced by site-directed mutagenesis. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) analysis gives evidence of a stable and robust binding of both mutants to gold. The lateral dimensions, as estimated by STM, and the height above the gold substrate, as evaluated by AFM, of the two mutants well agree with crystallographic sizes. A narrower height distribution is observed for PCSS compared to PCSH, corresponding to a more homogeneous orientation of the former mutant adsorbed onto gold. Major differences between the mutants are observed by electrochemical STM. In particular, the image contrast of adsorbed PCSS is affected by tuning the external electrochemical potential to the redox levels of the mutant, consistent with some involvement of copper active site in the tunneling process. On the contrary, no contrast variation is observed in electrochemical STM of adsorbed PCSH. Moreover, scanning tunneling spectroscopy experiments reveal asymmetric I–V characteristics for single PCSS proteins, reminiscent of a rectifying-like behaviour, whereas an almost symmetric I–V relation is observed for PCSH.     

Spatial variations in submolecular vibronic spectroscopy on a thin insulating film

Scanning tunneling spectroscopy on single naphthalocyanine molecules adsorbed on an ultrathin aluminum oxide film exhibits electron-vibronic coupling that varies with the position of tunneling over the molecule. The spectra at different positions are composed of several series of equally spaced peaks, which are interpreted as progression of progressions of molecular vibrational modes. The spatial variations correlate with the molecular orbital structure, revealing spatially dependent electron-vibronic coupling and selective vibrational excitation.     

Symmetric versus nonsymmetric structure of the phosphorus vacancy on InP(110)

The atomic and electronic structure of positively charged P vacancies on InP(110) surfaces is determined by combining scanning tunneling microscopy, photoelectron spectroscopy, and density-functional theory calculations. The vacancy exhibits a nonsymmetric rebonded atomic configuration with a charge transfer level 0.75+/-0.1 eV above the valence band maximum. The scanning tunneling microscopy (STM) images show only a time average of two degenerate geometries, due to a thermal flip motion between the mirror configurations. This leads to an apparently symmetric STM image, although the ground state atomic structure is nonsymmetric.     

The structure of monolayer SiO2 on Mo(1 1 2): A 2-D [Si-O-Si] network or isolated [SiO4] units?

In this letter, atomically resolved scanning tunneling microscopic (STM) images obtained from monolayer SiO₂/Mo(1 1 2) are presented. The results are consistent with a previously proposed structural model of isolated [SiO₄] units based on vibrational features observed by high-resolution electron energy loss spectroscopy (HREELS) and infrared reflection-absorption spectroscopy (IRAS), and oxygen species identified by ultra-violet photoemission spectroscopy (UPS). These results are inconsistent with a structural model that assumes a two-dimensional (2-D) [Si-O-Si] network. These data illustrate that a metal substrate, although coated with an oxide thin layer, can be directly imaged at the atomic-scale with STM.     

Emergence and visualization of an interface state during contact formation with a single molecule

Contact formation dynamics and electronic perturbations arising from the interaction of a metallic probe and a single molecule (1,3 cyclohexadiene) bound on the Si (100) surface are examined using a series of plane wave, density functional theory calculations. The approach of the probe induces a relaxation of the molecule that ultimately leads to the formation of an interface state due to a specific interaction between the probe apex atom and the C=C bond of the molecule. The calculated interface state is located 0.2 eV above the Fermi energy, in agreement with low temperature scanning tunneling spectroscopy local density of states data (0.35 eV), and is responsible for the contrast observed in low bias empty-state STM images.