Current Imaging Tunneling Spectroscopy Research Articles

Inhomogeneous surface electronic properties and charge ordering in epitaxial Fe3O4 films on MgO(001)

AbstractWe report scanning tunneling microscopy (STM) study of surface electronic properties and charge ordering of epitaxially grown magnetite, Fe3O4, (001) films exhibiting high density of antiphase domain boundaries (APBs). STM measurements using a W tip reveal surface termination at B‐sites. Fe ions with a 0.3 nm periodicity, i.e., a single atomic distance are observed. Current imaging tunneling spectroscopy reveals the alternation of two kinds of current peaks with a 0.6 nm periodicty indicating the presence of charge ordering consisted of Fe dimers with different charge states. STM measurements using a magnetic Ni tip provide higher contrast of the charge ordering. The APBs can modify the charge ordering as ordered and disordered areas are observed on adjacent domains separated by an APB. These strongly indicate that APBs can induce inhomogeneous properties on the surface of Fe3O4(001) films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon carbide nanowires studied by scanning tunneling spectroscopy

The electronic structure of silicon carbide (SiC) nanowires was studied for the first time using scanning tunneling spectroscopy (STS) and current imaging tunneling spectroscopy (CITS). The STS spectra indicated that the surface of nanowires has an n-type semiconducting behavior which is attributed to nitrogen doping of the sample material. The local density of states (LDOS) showed characteristic peaks in occupied and unoccupied part of the spectra. The origin of the LDOS maxima were discussed in contexts of the dopant-induced states, the subsurface defects, and the surface states related to the local reconstruction process. The work shows the applicability to investigate the SiC nanowires by scanning tunneling microscopy and spectroscopy.

Charge storage in SnO2 nanoparticles: A method and mechanism for charge writing/erasing

In this work, we present and explain evidence of a charge confinement phenomena exhibited by SnO2 nanoparticles. By applying a voltage pulse from the scanning tunneling microscope (STM) tip or momentarily increasing the gap voltage, charged features can be written on the surface. The voltage dependence of the apparent height of the features, and current imaging tunneling spectroscopy experiments, supports the charge confinement theory put forward. This is substantiated by a numerical model illustrating how charge is confined in individual nanoparticles, and is used to explain the observations of STM experiments.

STS INVESTIGATIONS OF METALLIC NANOSTRUCTURES DEPOSITED ON Bi2Te3

Bi 2 Te 3 has attracted attention due to its potential applications in the microfabrication of integrated thermoelectric devices. It is also interesting to study the metallization process of this compound. Metallic nanostructures were deposited by means of an electron gun evaporator in ultra high vacuum (UHV) conditions (10-8 Pa) on the freshly cleaved 0001 surface of the crystal Bi 2 Te 3. Measurements were conducted using the commercially available Omicron UHV scanning tunneling microscope (STM). Scanning tunneling spectroscopy (STS) measurements were performed using current imaging tunneling spectroscopy (CITS), and subsequent calculation of the dI/dV maps. Metallic characteristics were observed on nickel islands since early stages of the growth. CITS and dI/dV maps showed distinct contrast between the substrate and metallic islands. Similar contrast was not observed in the case of titanium, most probably due to an intercalation process. Occurring of such a process was confirmed by the appearance of the superlattice structure.

Correlation between Surface Structure and Charge Ordering in Magnetite(001) Studied by Scanning Tunneling Microscopy and Spectroscopy

We report the observation on the charge ordering of Fe ions in a magnetite(Fe3O4)(001) film surface by scanning tunneling microscopy and spectroscopy (STM/STS) with a nonmagnetic W tip. The Fe3O4(001) film surface exhibits a B-plane termination with a (√2 × √2)R45° reconstruction. Fe cation rows running along the [110] direction show a wavelike structure. The (√2 × √2)R45° surface reconstruction is originated from the wavelike Fe rows. The current imaging tunneling spectroscopy image shows alternation of two different kinds of current peak with a separation of about 0.6 nm. Each current peak arises from the same local density of states (LDOS) of two Fe ions. The presence of Fe dimers with two different kinds of LDOS provides evidence for the charge ordering of Fe ions with different charge states. STM and STS using a nonmagnetic W tip allow us to correlate the charge ordering pattern with the surface structure.

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO 2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO 2(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO 2− x towards Ti 2O 3.

Cross-sectional scanning tunneling microscopy and spectroscopy of strain in buried heterostructure lasers

Cross-sectional scanning tunneling microscopy and spectroscopy have been used to probe the unreconstructed (1 1 0) surface of a commercially available buried heterostructure laser in ultra high vacuum. Complex re-growth above the non-linear blocking layers is shown to induce tensile strain in the device. Spectroscopic measurements show an increase in both the density of filled valence band states and empty conduction band states as a result of the strain, with a particularly large increase at −3.1 V. Current imaging tunneling spectroscopy measurements show an increase in the tunneling current in to and out of the strained regions at both gap voltage polarities, consistent with the spectroscopy. Moving towards tensile strain, InP is known to maintain much the same bandgap, with the split-off level and lower lying states being drawn up towards the valence band edge, consistent with the data.

Characterization of periodically nanostructured copper filaments self-organized byelectrodeposition

We report in this paper the electric properties of nanostructured copper filament arrays self-organizedby a novel electrochemical method. Due to the spontaneous oscillation of the concentration field of[Cu2+ ] in front of the growing interface, crystallites of copper and cuprous oxide appearalternately on the filaments of the electrodeposits. A conducting atomic forcemicroscope (CAFM) and current imaging tunnelling spectroscopy (CITS) were used tocharacterize the electric properties of the nanostructured copper filaments. Byapplying a constant voltage across the conducting probe of the CAFM and thesample, an electric current mapping is achieved, in which alternating low andhigh current regions correspond exactly to the periodic nanostructures on thefilaments. The profile of the electric current along the structured filament has beenanalysed, and no noticeable potential drop has been observed. A typical linearI–V curve for a metaland nonlinear I–V curve for a semiconductor were collected in the high and low current regionsrespectively. These results suggest that despite the periodic distribution ofCu2O crystallites on the deposited filament, there should exist a metallic core of coppercrystallites inside the filament. This type of structured metal–semiconductor filament mighthave potential application.

Local Electronic Structure of Al Nanocluster Array Fabricated on Si(111)7 ×7 Surface

We have applied scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) as well as current imaging tunneling spectroscopy (CITS) [1] to an Al nanocluster periodic array of a submonolayer-Al/Si(111) system. These clusters become visible with an Al deposition thickness of 0.24–0.5 ML at a substrate temperature of 550 °C. The site-resolved STS spectra of the Al nanocluster at 78 K show an insulating energy gap of 3.4 eV. In the CITS images, we have found different features of space-resolved tunneling current for the filled and empty states, which markedly depend on atomic sites and also on inequivalent half unit cells.

Scanning tunnelling microscopy and spectroscopy of the reduced TiO 2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO 2(1 0 0) surface. The STM results showed that the TiO 2(1 0 0) surface is capable to form (1 × 7) reconstruction which can transform to (1 × 3) reconstruction due to reoxidation of the surface. The CITS results showed that the (1 × 7) reconstruction is much more metallic in compared to the (1 × 3) reconstruction showing pronounced surface states at energy 1.3 eV and 0.8 eV below the Fermi level and at energy 1.0–1.2 eV above the Fermi level.

Supramolecular ‘flat’ Mn9grid complexes—towards functional molecular platforms

Flat, quantum dot like arrays of closely spaced, electron rich metal centres are seen as attractive subunits for device capability at the molecular level. Mn(II)9 grids, formed by self-assembly processes using 'tritopic' pyridine-2,6-dihydrazone ligands, provide easy and pre-programmable routes to such systems, and have been shown to exhibit a number of potentially useful physical properties, which could be utilized to generate bi-stable molecular based states. Their ability to form surface monolayers, which can be mapped by STM techniques, bodes well for their possible integration into nanometer scale electronic components of the future. This report highlights some new Mn(II)9 grids, with functionalized ligand sites, that may provide suitable anchor points to surfaces and also be potential donor sites capable of further grid elaboration. Structures, magnetic properties, electrochemical properties, surface studies on HOPG (highly ordered pyrolytic graphite), including the imaging of individual metal ion sites in the grid using CITS (current imaging tunneling spectroscopy) are discussed, in addition to an analysis of the photophysics of a stable mixed oxidation state [Mn(III)4Mn(II)5] grid. The grid physical properties as a whole are assessed in the light of reasonable approaches to the use of such molecules as nanometer scale devices.

Cover Picture: Scanning Tunneling Microscopy and Spectroscopy of Donor–Acceptor–Donor Triads at the Liquid/Solid Interface (ChemPhysChem 11/2005)

The cover picture shows a submolecularly resolved scanning tunneling microscopy current image and scanning tunneling spectroscopy curves of a mixture of two physisorbed donor (oligo‐p‐phenylenevinylene)–acceptor (perylene diimide)–donor (oligo‐p‐phenylenevinylene) triads at the liquid–solid interface. By means of bias‐dependent imaging and scanning tunneling spectroscopy, it is not only possible to distinguish both triads, which differ in the nature of the substituents in the bay area of the central perylene diimide acceptor, but also to identify the donor and acceptor moieties within a given triad. Find out more in the Article by Uji‐i et al. on page 2389.

Scanning tunneling microscopy investigation of nanostructures produced by Ar+ and He+ bombardment of MoS2 surfaces

Nanostructures were fabricated on natural MoS2 crystals by bombardment with low doses of Ar+ and He+ with energies ranging from 100to5keV. The bombarded surfaces were investigated with x-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM) in an ultrahigh vacuum environment. The ion exposures were low enough to ensure that the observed nanostructures can be associated with individual ion impacts. Argon ions (Ar+) with energies of 100eV or less remove very few, if any, sulfur atoms from the surface but STM and XPS studies reveal that the electronic structure of the MoS2 surface is altered. Ar+ with energies greater than 100eV has a higher probability of sputtering sulfur atoms from the surface. The apparent size of the nanostructures in the STM images increased with Ar+ energies up to about 1keV and was dependent on the angle of incidence of the Ar+. Helium ion (He+) sputtering of MoS2 produced similar but smaller nanostructures when compared to Ar+ at the same impinged ion energy. STM images showed bright ring-shaped features were created with He+ energies greater than 500eV. On the basis of XPS and current imaging tunneling spectroscopy investigations, the features are assigned to sulfur atom vacancies. A change in the surface doping type from n to p was observed upon light sputtering of the surface.

Local electronic edge states of graphene layer deposited on Ir(1 1 1) surface studied by STM/CITS

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to observe electronic structure of the edges of monolayer graphite film deposited on the Ir(1 1 1) surface. The electronic structure derived from the tunnelling spectra revealed peak in electron local density of states very close to the Fermi level. This electronic state was interpreted in terms of localised edge state caused by the topology of the π electrons networks typical for the zig-zag edges. The observed maximum of local density of states at about 0.2 eV above the Fermi level was ascribed to the presence of resonant state caused by the appearance of disclinations centres in the vicinity of the graphite edges.

Current image tunneling spectroscopy of boron-doped nanodiamonds

The electron field emission properties of the nanodiamond films were examined using scanning tunneling microscopic (STM) technique. Current image tunneling spectroscopic measurements reveal the direct dependence of electron tunneling/field emission behavior of the films on the proportion of grain boundaries present. Local tunneling current-voltage (It–V) measurements show that incorporation of boron species insignificantly alters the occupied state, but markedly modifies the empty state of the diamond films, viz. it induces the presence of impurity states for the films heavily doped with borons, resulting in smaller emission energy gap for the samples. Such a characteristic improves both the local electron field emission behavior of the diamond films measured by STM and the average electron field emission properties measured by conventional parallel plate setup. These results infer clearly that the presence of impurity states due to boron doping is a prime factor improving the field emission properties for these boron-doped nanodiamond films.

Nanoscale Measurements of Conducting Domains and Current−Voltage Characteristics of Chemically Deposited Polyaniline Films

Spatial variations in electric conductivity and evolutions of band structures of polyaniline (PANI) films have been studied by use of a so-called current-sensing atomic force microscope (CS-AFM) or atomic force microscope current image tunneling spectroscopy (AFM-CITS). PANI films were deposited chemically onto indium-tin oxide- (ITO-) glass substrates, and their thickness and doping levels were controlled by polymerization and acid-doping conditions. The conducting uniformity of the PANI films depends on their doping level and thickness. Conducting domains were observed in fully doped PANI film, even when the bias voltage was reduced to as small as 30 mV. High current flowing regions gradually disappeared when conducting PANI films were partially dedoped. The point-contact current-voltage (I-V) characteristics of conducting tip-polymer/ITO systems were investigated on PANI films with different thickness and degree of doping. Various types of I-V curves representing metallic, semiconducting, and insulating states were obtained depending on the aggregation of polymer chains and doping level of the polymer film. The band gap energies (estimated from the I-V or dI/dV-V curves) of emeraldine base (EB) (undoped polyaniline) films are all higher than 3.8 eV, and a wide distribution of the band gap energies (0-1.1 eV and 0.75-1.8 eV for fully and partially doped PANI thin films, respectively) was found in a single polymer film.

THE CONDUCTING HOMOGENEITY OF ITO ELECTRODE AND WATER-SOLUBLE CONDUCTING POLYANILINE FILMS

The morphology and conductivity of indium/tin oxide (ITO) film have a large impact on the performance of light emitting diode (LED) devices using ITO as an anode. Atomic Force Microscope and current image tunneling spectroscopy (CITS) were used to probe the surface morphology and scanning tunneling spectroscopy (STS) of ITO surface. The morphology of all ITO films studied revealed a spherical aggregation with various grain sizes of 20~50 nm, depends on clean methods. It was also found that the conductivity of ITO films is related to film thickness. In general, thicker film shows higher conductivity. Nevertheless, the conductivity within a single grain is very homogeneous. On the other hand, the topographies of poly(N-(4-sulfophenyl)aniline (PSA) films revealed a rod-shaped aggregation with a diameter of 50 nm. The corresponding CITS images revealed conducting islands with irregular shape and size. The conductivity within a single polymer rod is not homogeneous.

Scanning tunneling microscopy/spectroscopy observation of intrinsic hydrogenated amorphous silicon surface under light irradiation

Continuous visible light irradiation is used to observe intrinsic hydrogenated amorphous silicon (a-Si:H) surfaces by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). A deposited a-Si:H surface is wet-cleaned in order to terminate the a-Si:H surface with H atoms. A current-sample bias ( I– V) curve shows a rectifying behavior in the dark, and the tunneling current increases dramatically only under the reverse-bias condition with light irradiation. This is attributed to the photoexcited minority carriers that lead to the appearance of a higher voltage across the vacuum than that in the dark. Experiments with both p-type and n-type substrates justify the suggested model. High tunneling current under irradiation enables us to obtain highly resolved STM images. Current imaging tunneling spectroscopy observations show that the detected tunneling current in the reverse direction is increased over the entire surface with irradiation, and the increment of the tunneling current is different at each surface site.

Conductivity Measurements of Stilbene-Based Molecules Incorporated into Self-Assembled Monolayers by Conducting Probe Atomic Force Microscopy

We describe a technique for acquiring current-voltage (I–V) characteristics of a metal-molecule-metal junction using a conducting probe atomic force microscopy (CP-AFM) technique. To conduct a repetitive experiment efficiently, we have utilized the current imaging tunneling spectroscopy (CITS) mode of the CP-AFM system. We have prepared a self-assembled monolayer of 1-decanethiol on Au(111) substrate where 4,4'-bis(mercaptomethyl)-trans-stilbene (BMMS) molecules have been incorporated. We demonstrate that we have constructed a conductivity map derived from a set of CITS data. Conductance peaks presumably involved by BMMS molecules have been found in the conductivity map and the corresponding I–V curves have exhibited fluctuations in current. The causes of the current fluctuations are also discussed.

A study of the electronic states of Ni xMn 3− xO 4+δ thin films using scanning tunneling microscopy and current imaging tunneling spectroscopy

Ni x Mn 3− x O 4+δ (0.4⩽ x⩽1) are a series of spinel structured materials exhibiting negative temperature coefficient of resistance (NTCR) characteristics. Thin films were produced on <100> silicon substrates using rf magnetron sputtering in an oxygen/argon atmosphere (2.5% oxygen). Following deposition, the films were annealed at 800 °C in air and the crystalline structure studied using X-ray diffraction. Scanning tunneling microscopy (STM) was used to study the topology of the layers in both the as-deposited and annealed states. Annealing resulted in a more uniform and ordered structure. Tunneling conductance (d I/d V vs V) and normalised tunneling conductance {(d I/d V)/(1/ V) vs V} characteristics were measured and used to generate conductance maps {d I/d V ( x, y,eV)} and to study the local density of states (LDOS) of the film surface. The tunneling conductance maps showed islands of bright contrast in an otherwise relatively uniform background of dark contrast. There were substantially fewer bright contrast islands in annealed films and they appear to bear no relation to any topographical features. The distribution of the LDOS in the bright regions was similar to that normally associated with metallic behaviour, while in the regions of dark contrast, the LDOS was more characteristic of semiconductors (LDOS ∼0 at Fermi level). The resistance–temperature characteristics were measured and found to be consistent with conduction models based on electron hopping.