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。     

Distance-dependent STM-study of the W(110)/C-R(15×3) surface

The W(110)/C-R(15×3) reconstructed surface has been studied by STM at variable tip-surface separation controlled by the tunneling gap resistance. A pronounced dependence of the STM image contrast as a function of tip height has been observed which is explained by the suppression of higher Fourier components, i.e. small wavelength features, with increasing tip height and an additional spatial dependence of the decay length of the surface wavefunction. As an important implication of our study we have found that STM images of non-trivial surface structures can depend critically on the tunneling gap resistance.     

Scanning tunneling microscopy of a luttinger liquid

Explicit predictions for scanning tunneling microscopy (STM) on interacting one-dimensional electron systems are made using the Luttinger liquid formalism. The STM current changes with the distance from an impurity or boundary in a characteristic way, which reveals the spin-charge separation and the interaction strength in the system. The current exhibits Friedel-like oscillations, but also carries additional modulated behavior as a function of voltage and distance, which shows the spin-charge separation in real space. Moreover, very close to the boundary the current is strongly reduced, which is an indication of the interaction strength in the system.     

Inducing all steps of a chemical reaction with the scanning tunneling microscope tip: towards single molecule engineering

All elementary steps of a chemical reaction have been successfully induced on individual molecules with a scanning tunneling microscope (STM) in a controlled step-by-step manner utilizing a variety of manipulation techniques. The reaction steps involve the separation of iodine from iodobenzene by using tunneling electrons, bringing together two resultant phenyls mechanically by lateral manipulation and, finally, their chemical association to form a biphenyl molecule mediated by excitation with tunneling electrons. The procedures presented here constitute an important step towards the assembly of individual molecules out of simple building blocks in situ on the atomic scale.     

Time and energy dependent dynamics of the STM tip — graphene system

Probability current and probability density of wave packets was calculated by solving the three dimensional time-dependent Schrödinger equation for a local potential model of the scanning tunneling microscope (STM) tip — graphene system. Geometrical and electronic structure effects of the three dimensional tunneling process are identified by studying three models of increasing complexity: a jellium half space, a narrow jellium sheet, and a local one electron pseudopotential. It was found that some of the key characteristics of the STM tip — graphene tunneling process are already present at the simple jellium models. In the STM tip — jellium half space system the direction of the momentum does not change during the tunneling event, hence this setup is characterised by introducing an effective distance. For the STM tip — narrow jellium sheet system the direction of the momentum is changed from vertical to horizontal during the tunneling event. The wave packet preferentially tunnels into the bound state of the jellium sheet. For the atomistic model of the graphene sheet an anisotropic spreading of the wave packet was found for hot electrons. This may open new opportunities to build carbon based nanoelectronic devices.     

Role of stress in thin film alloy thermodynamics: competition between alloying and dislocation formation

Using scanning tunneling microscopy (STM) and first-principles local-spin-density-approximation calculations to study submonolayer films of Co (1-c)Ag (c)/Ru(0001) alloys, we have discovered a novel phase-separation mechanism. When the Ag concentration c exceeds 0.4, the surface phase separates between a dislocated, pure Ag phase and a pseudomorphically strained Co(0.6)Ag (0.4) surface alloy. We attribute the phase separation to the competition between two stress relief mechanisms: surface alloying and dislocation formation. The agreement between STM measurements and our calculated phase diagram supports this interpretation.     

Current-distance-voltage characteristics of electron tunneling through an electrochemical STM junction

We have studied the electron tunneling process through an electrochemical scanning tunneling microscopic (STM) junction formed by a gold tip and a gold electrode immersed in an inert NaClO₄ solution. Current-distance-voltage characteristics of the tunneling process are examined by simultaneous measurement of tunneling current, voltage, and distance. The results indicate that the tunneling voltage across the junction changes with tunneling distance; however, tunneling conductance is an inverse exponential function of distance over the entire investigated range of tunneling current, voltage, and distance. The results provide clear evidence for the validity of a one-dimensional tunneling model for the aqueous tunneling process. Implications of the observation are mentioned with regard to the distance-dependent STM imaging and the origin of a low tunneling barrier height.     

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.     

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.     

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.     

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.     

Identifying the electronic properties of the Ge(111)-(2×1) surface by low-temperature scanning tunneling microscopy

We present the results of our low-temperature scanning tunneling microscopy (STM) investigation of the clean Ge(111) surface. Our experiments enable, for the first time, STM observation of one-dimensional surface screening around surface defects. We identify the dominating role of surface states in the low-temperature STM imaging as well as the important influence of nonequilibrium kinetics on the measured tunneling spectra.     

STM tunneling through a quantum wire with a side-attached impurity

The STM tunneling through a quantum wire (QW) with a side-attached impurity (atom, island) is investigated using a tight-binding model and the non-equilibrium Keldysh Green function method. The impurity can be coupled to one or more QW atoms. The presence of the impurity strongly modifies the local density of states of the wire atoms, thus influences the STM tunneling through all the wire atoms. The transport properties of the impurity itself are also investigated mainly as a function of the wire length and the way it is coupled to the wire. It is shown that the properties of the impurity itself and the way it is coupled to the wire strongly influence the STM tunneling, the density of states and differential conductance.     

Photo-induced tunneling spectroscopy of ReS2: Dramatic increase of the quantum efficiency by chemical treatment

In this work we present a systematic study of the local photovoltaic properties of ReS₂, using a scanning tunneling microscope (STM). The tunneling junction of the STM was optically illuminated during the tunneling process. The phase sensitive detected photo-induced tunneling current (PITC) was studied as a function of wavelength and surface topography. In order to improve the performance of ReS₂ solar cells, the samples were treated with NaI/I₂ and EDTA solutions. Relative to the untreated sample, the EDTA-treated samples show an increase in the photo-induced tunneling current by a factor of 8–10 in the whole spectral range, the NaI/I-treated samples by 2–3. Two dimensional mapping of the PITC was performed on an atomic scale and compared to the surface topography.     

Adsorption behavior of binary mixed alkanethiol molecules on Au: Scanning tunneling microscope and linear-scan voltammetry investigation

Adsorption behavior of binary mixed alkanethiol molecules of octanethiol (OT) and dodecanethiol (DDT) on Au substrate has been investigated with a variation of molar fraction in solution at room temperature. Linear-scan voltammograms (LSV) taken from the binary mixed self-assembled monolayer (SAM) on Au/Si showed a single reductive desorption peak, continuously shifting with a variation of mixing ratio in solution. Molecule-resolved scanning tunneling microscope (STM) images obtained from the binary SAM on Au/mica showed the phase separation with a few nanometer sized domains. The difference in the chain length between OT and DDT was not large enough to induce the phase separation with much larger domains, which could be well detected by LSV. There was no preferential adsorption of DDT to OT.     

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.     

Channel selective scanning tunneling spectroscopy

In this paper we simulate STM and STS experiments for CO monomers and dimers on Cu(1 1 1) surface. We show that the contrast of STM images can be attributed to interference effects between tunneling channels, and suggest that functionalizing the microscope tip improves the channel selectivity of STM. Furthermore, we show that voltage and position dependent tunneling spectra also reflect the same interference effects, but adds the energy resolution to the channel analysis. Especially in the case of nonresonant tunneling, STS measures local density of states only indirectly. The present study suggests that STS in constant height mode can be used in investigating the phase and energy sensitivity of tunneling channels in adsorbate molecules and nanostructures.     

FOURIER ANALYSIS OF TEMPORAL AND SPATIAL OSCILLATIONS OF TUNNELING CURRENT IN SCANNING TUNNELING MICROSCOPY

Partially oxidized Si(111) surfaces and surfaces of highly oriented pyrolytic graphite (HOPG) were studied by two different ultrahigh vacuum scanning tunneling microscope (UHV-STM) systems and by an STM system working under ambient conditions, respectively. The STM current images of partially oxidized Si(111) surfaces and HOPG surfaces were analyzed by one/two-dimensional fast Fourier transformation (1D-FFT/2D-FFT). The phenomenon of temporal oscillations of tunneling current on the partially oxidized Si(111) surfaces was detected with both UHV-STM systems. Temporal as well as spatial oscillations of tunneling current appeared in highly resolved STM current images of the Si(111) surfaces simultaneously, but both kinds of oscillations could be discriminated according to their different influence on the 2D-FFT spectra of the current images, while varying the scanning range and rate. On clean HOPG surfaces only spatial oscillations of tunneling current induced by the surface structure were observed.     

Vacuum phonon tunneling

Field-induced phonon tunneling, a previously unknown mechanism of interfacial thermal transport, has been revealed by ultrahigh vacuum inelastic scanning tunneling microscopy (STM). Using thermally broadened Fermi-Dirac distribution in the STM tip as in situ atomic-scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric field and thermally vibrating image charges, and its rate is enhanced by surface electron-phonon interaction.