STM Tip Research Articles

磁性原子架橋の磁性と伝導性に関する理論的研究

We investigate the nano-physics of atom bridges made from magnetic atoms formed between a Scanning Tunneling Microscope-tip (STM-tip) and a solid surface within the density functional theory. We consider a twisted ladder structure to represent the atom bridge. In the case of an Fe atom bridge, we find that the magnetic and transport properties depend on the distance between the nearest neighbor atoms along the bridge axis. We consider the alloy atom bridges whose components are found on both the right and left hand side of the Pauling-Slater curves, and show their corresponding magnetic and transport properties. On the basis of our numerical results, we discuss the origin of these interesting properties and the possibility for designing the properties of the atom bridge by STM-tip manipulation and/or alloying.

Recording the intramolecular deformation of a 4-legs molecule during its STM manipulation on a Cu(2 1 1) surface

A Lander molecule is a polyaromatic molecular board supported by four legs. By means of a low-temperature scanning tunneling microscope (LT-STM), the controlled lateral manipulation in constant height mode of this molecule adsorbed on the Cu(2 1 1) surface is studied. Simulations based on elastic scattering quantum chemistry description combined with molecular mechanics calculations reproduce the STM current recorded during the manipulation. We demonstrate that the main tunneling current is flowing through the central molecular wire even if the STM tip is pushing only on a leg of the molecule. The measured signal of manipulation results principally from the mechanical deformation of the molecular board.

Controlled Polymerization of Substituted Diacetylene Self-Organized Monolayers Confined in Molecule Corrals

We have shown that STM-tip-induced chain polymerization of 10,12-tricosadiynoic acid (TCDA) in a self-organized monolayer at the liquid-solid interface of TCDA on highly oriented pyrolytic graphite is possible. The oligomers thus produced started at the point where a voltage pulse was applied between the STM tip and the sample during a short period when the feedback condition was momentarily suspended (as it is for scanning tunneling spectroscopy). Polymerization probabilities depended upon the length of the applied voltage pulse and were generally higher for longer pulse widths in the 10-ms to 100-micros time scales, approaching unit probability for the former and decreasing quickly to a few tens of percent for the latter. The polymerization could be confined to certain nanometer-sized areas by using "molecule corrals,"and polymerization appeared to be governed by topochemical constraints. Polymerization across domain boundaries, or over molecule corral edges, was never observed in over approximately 150 observations. Due to the constant supply of nonpolymerized molecules from the covering solution, a dynamic exchange between molecules on the surface and in the solution was possible. This exchange occurred on a time scale that was comparable to the image acquisition time (approximately 10(1) s), and appeared to depend weakly upon the length of the desorbing oligomer. The desorption process was probably also influenced by interactions with the STM tip.

Crystalline Organic Molecular Thin Film with Electrical Switching Property: Scanning Probe Microscopy and Optical Spectroscopy Study

We report on the growth and characterization of crystalline 1,1-dicyano-2,2-(4-dimethylaminophenyl)ethylene (DDME) thin film with electrical switching property. X-ray diffraction analysis indicates that bulk single-crystal DDME belongs to monoclinic, space group P21, and Z = 2 structure. Optical spectroscopy shows that the film chemical structure is consistent with that of the powder material, suggesting that DDME material is very stable during the vacuum thermal deposition process. Scanning probe microscopy and transmission electron microscopy observations reveal that the as-deposited film has extended crystal structure and smooth surface morphology in micrometer scale. Parallel molecular stacking structure is repeatedly observed in the film surfaces with the axis of the molecular stack parallel to the substrate. The ac crystal plane is found to be one of the most stable surfaces of the DDME film in ambient conditions. Nanometer scale data storage is realized on the film by applying pulse voltage between the STM tip and the substrate. The switching mechanism is briefly discussed.

STM investigations on a tetralactam macrocycle adsorbed on Au(1 1 1) and Cu(1 1 1) surfaces

A large tetralactam macrocycle (961.3amu), which is a frequently used building block for catenanes and rotaxanes, was deposited onto Au(111) and Cu(111) surfaces by vacuum sublimation under UHV. STM images were recorded for different coverages at room temperature. The macrocycles are found to be very mobile on both surfaces, which leads to pronounced STM tip effects. At low coverage, the macrocycles preferentially form small clusters at step edges and surface defects, leaving large terraces uncovered. At a higher coverage of about one monolayer, disordered closed films are observed on Au(111), which remain disordered after annealing to 423K. Presumably, the reason for these observations is a week interaction between the Au and Cu surfaces and the molecules, in combination with formation of hydrogen bonds between the molecules.

Fabrication of Nano-Structure by STM Tip Induced Atomic Diffusion

Fabrication of Nano-Structure by STM Tip Induced Atomic Diffusion

Molecular wire formation from viologen assemblies.

The adsorption behavior of viologen alpha,omega-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times. Single molecular wires could be formed between a gold STM tip and a surface with a low coverage of the adsorbed dithiols, and their electrical behavior was investigated. Molecular conductivity was determined either by the repeated measurement of I(s) curves or by recording I-V curves for different tip-sample separations. These methods concurred in producing a value of (0.5 +/- 0.1) nS for the single-molecule conductivity of the alpha,omega-viologen dithiol molecule HS-6V6-SH. The high conductivity of HS-6V6-SH, as compared to that of HS-C12-SH, may be related to the low-lying LUMO, which provides a barrier indentation for electron transport in a two-step electron-transfer mechanism.

Tip to substrate distances in STM imaging of biomolecules

STM images of single biomolecules adsorbed on conductive substrates do not reproduce the expected physical height, which generally appears underestimated. This may cause the tip to interfere with the soft biological sample during the imaging scans. Therefore, a key requirement to avoid invasive STM imaging is the knowledge, and the control, of the initial tip to substrate distance. This is connected to the setting of the tunnelling current and applied voltage, which define a tunnelling resistance.The height of the STM tip was measured by calibrating the tunnelling resistance, as a function of its vertical displacement until establishing a mechanical contact. At a tunnelling resistance of 4×109Ω, distances of about 3 and 6nm are estimated when flat Au substrates are imaged in water and in air, respectively. On such a ground, the relevance of the starting tip–substrate distance in determining a non-invasive imaging has been investigated for a plastocyanin mutant chemisorbed on Au(111) electrodes. At tunnelling distances sufficient to overcome the physical height of the imaged biomolecules, their lateral dimensions are found to be consistent with the crystallography, whereas they are significantly broadened for smaller distances.

Molecular-scale structure fabrication at liquid/solid interfaces

Nano-fabrication of organic molecules at liquid/solid interfaces is a promising method to form nano-structures of thermally unstable molecules on surfaces. Perylene-3,4,9,10-tetracarboxylic-dianhydride mono-molecular layer was formed on HOPG (0001) surface by vacuum-deposition as a canvas layer for nano-fabrication. After a droplet of 1-octylbenzene solution of 3-imino-4,5,6,7-tetrachloroisoindolin-1-one (ITCII) was placed on the surface, an STM tip was inserted into the droplet and a part of the canvas layer was scratched by the STM tip by increasing tunneling current. The scratched area was found to be immediately covered by ITCII molecules. In such way, ITCII mono-molecular layer with a designed shape could be fabricated. Structures of the formed layer are discussed in comparison to that of freely adsorbed layer.

STM patterning of SnO 2 nanocrystalline surfaces

In this work, we report on the ability to write features less than 15 nm in size on nanocrystalline SnO 2 by applying negative voltage pulses onto an STM tip. The fact that these features can be erased by scanning with a positive tip bias, and the strong dependence of the apparent height of the features with scanning bias after writing seems to indicate that a degree of charge confinement within the 8 nm nano-crystals is involved.

Molecular rectification: self-assembled monolayers in which donor-(pi-bridge)-acceptor moieties are centrally located and symmetrically coupled to both gold electrodes.

Self-assembled monolayers (SAMs) obtained from 1-(10-acetylsulfanyldecyl)-4-[2-(4-dimethylaminophenyl)vinyl]quinolinium iodide exhibit asymmetric current-voltage (I-V) characteristics. The rectification may be reversibly switched: it is suppressed when the film is exposed to HCl vapor, the intramolecular charge-transfer axis being inhibited by protonation, but restored when exposed to NH(3). The behavior is intrinsic to the donor-(pi-bridge)-acceptor moiety, and ambiguity in the assignment has been excluded by matching the alkyl tails on the substrate and contacting STM tip to locate the chromophore midway between the electrodes: Au-S-C(10)H(21)//D-pi-A-C(10)H(20)-S-Au. Films contacted by gold tips exhibit rectification ratios of ca. 18 at +/-1 V, whereas those contacted by pentanethiolate (Au-S-C(5)H(11))- and decanethiolate (Au-S-C(10)H(21))-coated tips have corresponding ratios of ca. 11 and 5, respectively. The I-V curves are different, but when adjusted for thickness the current versus electric field dependence is indistinguishable. Seven dyes are reported: SAMs with sterically hindered D-pi-A moieties, in which the donor and acceptor are twisted out of plane, exhibit rectification, whereas those that are planar or have a weak donor-acceptor combination do not.

Two-gap interplay in MgB 2: a tunneling spectroscopy study

Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap superconductivity was given with an inverted junction setup, in which a small crystal of MgB 2 was used as the STM tip and 2H-NbSe 2 as the sample. This technique allowed to show that both gaps close at the critical temperature of the bulk material and thus are intimately related to the superconductivity. The experiments performed in the standard N-I-S geometry evidenced for two strongly coupled gaps Δ L=7.0±1.0 meV and Δ S=3.0±1.0 meV at 4.2 K. STS on as-grown c-axis oriented thin films yielded only small gap which confirmed the identification of this gap as originating from 3D-like π-band and, by exclusion, that of the large gap from 2D-like σ-band. The low gap values Δ S=2.2±0.3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to Δ S=2.8±0.3 meV. The c-axis tunneling spectra are better fitted considering anisotropic superconductivity inside the π-band. The issues of our findings are discussed in terms of two-band superconductivity.

Dipolar and nonpolar altitudinal molecular rotors mounted on an Au(111) surface.

We describe the preparation of a compound whose molecules consist of two metal sandwich stands carrying tentacles with affinity to metal surfaces and holding an axle that carries a dipolar or a nonpolar rotator. The dipolar rotor exists as three pairs of enantiomers, rapidly interconverting at room temperature. When mounted on a gold surface, each molecule represents a chiral altitudinal rotor, with the rotator axle parallel to the surface. The surface-mounted rotor molecules are characterized by several spectroscopic and imaging techniques. At any one time, in about one-third of the dipolar rotors the rotator is free to turn and the direction of its dipole can be flipped by the electric field applied by an STM tip, as revealed by differential barrier height imaging. Molecular dynamics calculations suggest that electric field normal to the surface causes members of one pair of enantiomers to rotate unidirectionally.

Triggered fast relaxation of metastable Pb crystallites

A small fraction of Ru-supported Pb crystallites, prepared by quenching from just below the melting point (∼600 K), is found in a nearly spherical metastable configuration that relaxes after an STM tip interaction. The metastable structure could not be reproduced by exposure to hydrogen, water, carbon monoxide or oxygen. However, AES measurements demonstrate the segregation of oxygen from the Pb/Ru interface to the surface of the continuous Pb film upon post-deposition annealing, suggesting surface oxygen as a possible origin of the metastability. The rapid relaxation of the metastable crystallites has been measured using STM, providing access to the early stages of relaxation where crystalline layers are removed on the time scale of seconds, in comparison with the time scales of minutes to hours for quenched structures. The shape evolution (facet radius vs. time) for different crystallites is consistent with predictions of shape-preserving relaxation from an initial Pokrovsky–Talapov shape for T<400 K. Evaluation of the overall kinetics of single layer relaxation shows that the concentration gradients of the diffusing species across the top facet of the crystallite are on the order of 1% of the equilibrium concentration.

Magnetic and transport properties of Fe V1− atom bridge constructed between an STM tip and a solid surface

We have investigated the magnetic and transport properties of an atom bridge made from magnetic materials, which is an atom-scale wire constructed between a scanning tunneling microscopy tip and a solid surface , with the aid of ab initio calculations. In the case of Fe β V 1− β alloy atom bridges, we have found that the value of the mean magnetic moment is similar to that of the corresponding alloy bulk, and the quantized conductance contribution from both the majority and minority spin electrons changes as β changes. These properties are different from the case of Fe 1− α Ni α alloy atom bridge.

Ballistic Electron Emission Luminescence of InAs Quantum Dots Embedded in a GaAs/AlxGa1−xAs Heterostructure

ABSTRACTBallistic electron emission luminescence (BEEL) is a further development of ballistic electron emission microscopy (BEEM) combining three-terminal hot electron injection and interband radiative recombination in direct-gap semiconductor materials. By using a planar tunnel-junction emitter rather than a STM tip, a spectrographic analysis of the induced electroluminescence can be performed with the help of much higher current injection level. We demonstrate the operational principle of BEEL in a GaAs/AlxGa1−xAs heterostructure with a layer of InAs quantum dots (QDs) as the optical active layer. The wavelength-resolved BEEL spectra from planar tunnel-junction devices disclose the QD luminescence as a peak near 1.34 eV accompanied with a linear quantum-confined Stark shift. At higher collector voltage, luminescence from bulk states of GaAs peaked at 1.48 eV is observed. The spectrally integrated BEEL intensity as a function of collector voltage fits well with the results from STM tip injection, which is measured in a single-photon-counting mode. Measurement of ballistic electron current spectroscopy is made possible by freezing out the thermionic leakage current at low temperatures. Our results indicate that it is feasible to simultaneously acquire topographic, electronic and photonic information of buried light-emitting semiconductor heterostructures.

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

Inter-orbital interference in STM tip during electron tunneling in tip?sample system: influence on STM images

We present a theoretical study of the influence of interference effects connected with the tunneling through s and pz orbitals of the apex atom in STM tip on the formation of STM image. The results show that such an interference may modify significantly the tunneling current by changing the current contributions from the different orbitals in the tip–sample system. STM simulations have been performed for the Al(0 0 1) surface and different fcc-metal tips: they clearly indicate that the height and type of the STM corrugation depend considerably on this interference.

Electroluminescence spectra of an STM-tip-induced quantum dot

We analyze the electroluminescence spectrum of an STM-tip-induced quantum dot in a GaAs surface layer. A flexible model has been developed that combines analytical and numerical methods and describes the key features of many-particle states in the STM-tip-induced quantum dot. The dot is characterized by its depth and lateral width, which are experimentally controlled by the bias and current. We find, in agreement with experiment, that increasing the voltage on the STM tip results in a redshift of the electroluminescence peaks, while the peak positions as a function of electron tunneling current through the STM tip reveal a blueshift.

Scanning tunneling spectroscopy investigation of self-assembled plastocyanin mutants onto gold substrates under controlled environment

The study of the electronic conduction through plastocyanin (PC) mutants assembled on a gold surface has been addressed by scanning tunneling spectroscopy. The two mutants exploit a single thiol group (PCSH) or a disulfide bridge (PCSS) to covalently bind at gold surface. The I– V measurements were performed by positioning the STM tip on top of a single molecule and sweeping the bias potential between ±1 V, under both ambient and controlled atmosphere. For PCSS, under ambient conditions, asymmetric I– V characteristics were obtained, which disappear under nitrogen atmosphere. PCSH, instead shows a symmetric I– V relation in air and under nitrogen environment. Here, as factors underlying this distinct electron conductive behaviour, a potential role for hydration water molecules and for copper redox levels are discussed.