STM Tip Research Articles

STM探針からの電子・正孔注入による電子輸送制御

STM探針からの電子・正孔注入による電子輸送制御

Imaging of ferroelectric vinylidene fluoride and trifluoroethylene copolymer films by scanning tunneling microscopy

In this paper, we reported the possibility to image non-conducting P(VDF-TrFE) copolymer films by STM. The films had the thickness of ∼25.0 nm and were spin-coated onto Au or graphite substrates. For films deposited on Au substrates, STM images showed grain structures of ∼100 nm, much larger than the grains of bare Au substrate. With increased scan rate, the film surface was damaged by STM tip and extreme protrusions and holes were observed. For films deposited on graphite substrates, we only obtained an image of very flat plane and could not observe the topography of the film surface. It seemed that the tip had pierced through the uppermost P(VDF-TrFE) layers and only imaged the layers nearest to the substrate. Asymmetrical current–voltage curves were observed from copolymer films deposited on HOPG. Experimental results were discussed.

Density functional non-equilibrium Green's function (DFT-NEGF) study of the smallest nano-molecular switch

We report a density functional non-equilibrium Green's function (DFT-NEGF) study of electrical transport and switching behavior in a single molecular conductor consisting of a 1,4-benzene-dithiolate (BDT) molecular wire with one sulfur end group bonded to an Au(1 1 1) substrate and the other to a monatomic Au-scanning tunneling microscope (STM) tip. The I– V characteristics of the various configurations of the BDT between gold electrodes are calculated. We find that the conductance of the molecule varies dramatically upon the lateral motion of the STM tip, which suggests that this system has potential application as a molecular device. The projection of the density of states (PDOS) and the transmission coefficients ( T( E)) of the two-probe system at zero bias are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to switching-behavior. Furthermore, the transmission coefficients of the system at various external voltage biases are also investigated. The results show that, the broadening of the transmission coefficient spectrum with increasing of the external voltage bias indicates a strong coupling between the molecular orbitals in the BDT and the incident states from the electrodes, thus the current increases with increase of the bias voltage.

Hopping hydrogens

A flat organic molecule resting on a surface can be reversibly switched between two different conductance states with an STM tip

Investigation of structure and ORR reactivity of fuel cell catalysts by in-situ STM

In order to provide insight into the interfacial relationships at fuel cell electrodes, a measuring technique based on a scanning tunneling microscope working in an electrochemical cell has been developed. The structure of model electrodes consisting of carbon supported Pt and ionomer mixtures as well as MEA electrode surfaces is imaged by the electrochemical scanning tunneling microscope (EC-STM) from microscale to nanoscale. Images with subnanometer resolution are obtained indicating that some ordering of the particles on the carbon support is present in the model system and in the MEA. It is demonstrated that in both cases nanometer structures can be imaged reliably and that the interface is rather clean and active under reaction conditions. In addition, a technique has been developed to measure the local reactivity by using the STM tip as a sensor electrode for the ORR.

First-principles analysis of the STM image heights of styrene on Si(100)

We report on theoretical investigations of scanning tunneling spectroscopy (STM) image heights on Si(100). Calculations are performed using density functional theory (DFT) within the Keldysh nonequilibrium Green's function (NEGF) formalism. The nonequilibrium potential drop between Si(100) and a STM tip is determined self-consistently. This potential drop is found to play an important role in the calculated image height characteristics of adsorbed hydrocarbons by lowering the vacuum barrier and shifting molecular levels. Numerical data collected for image heights of styrene against a hydrogen passivated Si(100) background are found to agree quantitatively with the corresponding experimental results. We also present a comparison between results obtained by the NEGF-DFT formalism and the Tersoff-Hamann approximation, showing that nonequilibrium analysis can be important in the study of STM image heights of molecules.

Construction of a versatile ultralow temperature scanning tunneling microscope

We constructed a dilution-refrigerator (DR)-based ultralow temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR-based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in an UHV environment to a STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 h. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance, and applications for low temperature physics.

Thermoelectric effects in STM tunneling through a monoatomic chain

AbstractWe study thermoelectric properties of the system composed of a monoatomic chain on a surface and additional electrode coupled to the chain, which can be an STM tip. In particular, we are interested in thermopower, electric and thermal conductance, Wiedemann–Franz relation and thermoelectric figure of merit, which is a direct measure of the usefulness of the system for applications. We discuss the modifications of the STM wire topography due to temperature difference between the electrodes. Finally, we also make connection to STM experiment, in which the thermopower has been directly measured, showing different structure, not visible in topography spectra. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silver oligomer and single fullerene electronic properties revealed by a scanning tunnelling microscope

Clusters on surfaces have been investigated with low-temperature scanning tunnelling microscopy and spectroscopy. Constant current spectra acquired on Ag oligomers and one-dimensional chains on a Ag(111) reveal a single resonance peak whose energy shifts towards the Fermi level with increasing cluster size. Next, controlled and reproducible contact between a STM tip and a C 60 molecule adsorbed on Cu(100) is reported. The transition from tunnelling to contact is discussed in terms of local heating of the tip-molecule junction.

STM investigation of the charge transport mechanisms to nanoscale metallic islands on a semiconductor substrate

Scanning tunneling spectroscopy (STS) was performed at room temperature on disconnected two-dimensional nanoscale metallic islands made of erbium disilicide $(\mathrm{Er}{\mathrm{Si}}_{2})$ grown on Si(111). Our aim was to investigate how the charges injected by the STM tip were transferred from the metallic island to the semiconductor substrate, and how the charge transport mechanisms were reflected in the STS data. For that purpose, the relative importance of the two possible conduction paths, parallel or perpendicular to the surface, was modified by oxygen adsorption. The spectra of oxygen-dosed samples were found to change with setpoint current on $p$-type substrates, whereas no shift was observed for clean samples. Since oxygen exposure only affects the Si surface, this effect was interpreted as a change in the easy conduction path between the islands and the bulk substrate from parallel to perpendicular to the surface (i.e., through the metal-semiconductor interface). We propose and detail a method based on the analysis of STS spectra to determine the characteristics of the nanoscale metal-semiconductor contact. The results are in agreement with previous point contact experiments, which validates our analysis.

Molecular machines: synthesis and characterization of two prototypes of molecular wheelbarrows

Technomimetic molecules are molecules designed to imitate macroscopic objects at the molecular level, also transposing the motions that these objects are able to undergo. This article focuses on the synthesis of two polyaromatic hydrocarbons designed by analogy with macroscopic wheelbarrows. The molecular wheelbarrows are synthesized following a modular strategy based on sequential double Knoevenagel and Diels–Alder reactions. Our strategy allowed to easily vary the chemical nature of the handles, which is crucial for subsequent manipulation with an STM tip.

Dipole-Driven Ferroelectric Assembly of Styrene on Au{111}

By studying styrene, a simple hydrocarbon with a weak dipole moment, we have investigated ferroelectric ordering and ferroelectric transitions at the single-molecule level. Low-temperature scanning tunneling microscopy imaging of individual styrene molecules reveals their internal structure and hence the orientation of their dipole moment parallel to the Au{111} surface. At near monolayer coverages, both local ferroelectric ordering of the molecules and long-range antiferroelectric ordering of the domains are observed, and the dipole−dipole interaction energies are quantified. A piezoelectric transition from ferroelectric to paraelectric ordering is observed upon further increase of the coverage. The effect of the STM tip in randomizing ordered domains is also discussed. This work demonstrates that important ferroelectric properties such as spontaneous polarization, long-range ordering, and piezoelectricity can be achieved in nanoscale domains of a weakly polar molecule on a metal surface.

Measurement of Tunneling Current of Self-Assembled Viologen Derivative Molecules Using Scanning Tunneling Microscopy

The electrical properties of viologen derivatives with various lengths were studied using self-assembly and ultrahigh-vacuum scanning tunneling microscopy (UHV-STM). In addition, in this study, we produced the vertical junction structure of an STM tip/molecule/Au(111) substrate using an STM tip. We used Au(111) as the base substrate. Also, the current–voltage (I–V) and differential conductance (dI/dV–V) characteristics were measured while the electrical properties of the self-assembled viologen derivatives were scanned by scanning tunneling spectroscopy (STS). We investigated electron movement, which is caused by height variation, because we can observe height variation between the barrier height of organic materials and the STM tip from the energy gap (Eg) of organic materials. The effective barrier heights of viologen derivative N-methyl-N'-(8-mercaptooctyl)-4,4'-bipyridinium (VC8SH), N-methyl-N'-(10-mercaptodecyl)-4,4'-bipyridinium (VC10SH), N-methyl-N'-di(8-mercaptooctyl)-4,4'-bipyridinium (HSC8VC8SH), and N-methyl-N'-di(10-mercaptodecyl)-4,4'-bipyridinium (HSC10VC10SH) self-assembled monolayers (SAMs) were calculated from the decay constant (β) to be 0.81, 0.46, 0.28, and 0.26 Å-1, respectively.

The aqueous electrochemistry of carbon-based surfaces-investigation by scanning tunneling microscopy

Electro-oxidation of carbon-based materials will lead to conversion of the solid to CO2/CO at the anode, with H2 being produced at the cathode. Recent voltammetric investigations of carbon nano-tubes and single crystal graphite have shown that only edge sites and other defect sites are electrochemically active. Local oxidation of diamond-like carbon films (DLC) by an STM tip in moist air followed by imaging allows correlation of topographical change with electro-chemical conditions and surface reactivity. The results may have implications for lithographic processing of carbon surfaces, and may have relevance for electrochemical H2 production.

Special features in self-assembled monolayer revealed by functionalised STM tips

Some functional groups in self-assembled monolayers have recently been identified in STM images, due to the chemical modification of gold tips [1]. In those studies, different molecules presenting different functional groups were investigated, resolving several molecular organizations [1-2, 4]. In this paper, we aimed at demonstrating that such chemically modified STM tips can be used to investigate phenomenon routinely observed with standard Pt/Ir tips, such as domain boundary dynamics in hexadecanol SAMs. It also allows transitory features to be identified. Moreover, rarely observed phenomena are also presented below, i.e. bilayers composed of wax esters molecules physisorbed at the phenyloctane/graphite interface.

Methods of Analysis for Imaging and Detecting Ions and Molecules

Abstract For imaging molecular processes in living cells, we developed novel intracellular fluorescent and bioluminescent indicators for second messengers, protein phosphorylation, protein/protein interactions, and protein localizations that work in single living cells. Key molecules and steps of cellular signaling pathways were visualized in target live cells under fluorescent microscopy using developed fluorescent indicators. A second new approach to molecular imaging is also described. Chemically modified STM tips (molecular tips) were developed for molecular imaging at the interfaces. The molecule-specific imaging is based on an increase in a tunneling current due to the overlap of electronic wave functions induced by the chemical interactions between tip and sample molecules. Chemically selective imaging using STM molecular tips can be described as intermolecular tunneling microscopy.

On the possibility of STM-control of dissociative electron transfer

A novel physical mechanism of control of the rate of the electrochemical dissociative electron transfer with the use of the in situ STM arrangement is proposed. The mechanism consists of the electron tunneling between the substrate electrode and STM tip via the anti-bonding orbital of the molecule. A model for the theoretical description of this system is presented allowing calculation of the adiabatic Gibbs energy surface and the average occupation of the anti-bonding orbital of the molecule.

Reaction threshold and decoherence: current induced desorption of CO on Cu(111) in STM

The transfer of a CO molecule from the Cu(111) surface towards the STM tip, induced by the tunnelling current, occurs only if a threshold voltage of 2.4 eV is exceeded (Bartels L, Meyer G, Rieder K-H, Velic D, Knoesel E, Hotzel A, Wolf M and Ertl G 1998 Phys. Rev. Lett. 80 2004). We present a theory based on the Quantum Nano Dynamics (QND) approach, treating the competition between a coherent and a decoherent mechanism of current induced CO desorption via inelastic tunnelling and vibrational excitation. The keypoint of the theory is to regard the tunnelling process as driving the system in a transient excited state and to include explicitely many-particle relaxation effects in the excited state. Only a decoherent desorption mechanism via a transient negative ion resonance of the sample surface, which is created through electron injection and interacts with the CO molecule, provides an explanation for the existance of the voltage threshold for CO desorption and reproduces the quantum yield and the isotope effect as observed in the STM experiment.

Local stabilization of single-walled carbon nanotubes onSi(100)-2 × 1:H via nanoscale hydrogen desorption with an ultrahigh vacuum scanning tunnellingmicroscope

An ultrahigh vacuum scanning tunnelling microscope (UHV-STM) was used to modify the interface betweenisolated ≈10 Å-diameter single-walled carbon nanotubes (SWNTs) and the hydrogen-passivated Si(100)surface. Room-temperature UHV-STM desorption of hydrogen at the SWNT/H-Si(100)interface resulted in the local mechanical stabilization of tubes originally perturbed by therastered STM tip under nominal imaging conditions. For the section of the SWNTcontacted by depassivated Si, a topographic depression of 1.5 Å (1 Å) was measured inthe case of parallel (nearly perpendicular) alignment between the tube axis andthe Si dimer rows, in agreement with existing first-principles calculations. Thecompatibility of hydrogen-resist UHV-STM nanolithography with SWNTs adsorbed onH-Si(100) would enable the atomically precise placement of single molecules inproximity to the tube for the bottom-up fabrication of molecular electronic devices.

Regularly-Shaped Diacetylene Nano-Structures on Surfaces

When 10,12-pentacosadiynoic acid (PENTA), dissolved in phenyloctane, is brought into contact with graphite, it immediately builds a film on the surface. The molecular arrangement of this ordered film has been characterized with an STM. The same packing was observed in a PENTA 2D-domain adsorbed from the solution into a small artificial hole scratched with an STM tip in a perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) layer formed in advance by vacuum-deposition. Thereby, a nano-scale domain of diacetylene molecules is embedded in the PTCDA film. In the posterior imaging scans, the original irregular scratched area becomes regular due to the readsorption of PTCDA into the gap.