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.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

Selective analysis of molecular states by functionalized scanning tunneling microscopy tips

Selective analysis of molecular states in scanning tunneling microscopy (STM) has so far been achieved in a few cases by tuning the bias range of the STM in high-resolution measurements. Correspondingly, perylene adsorbed in a close-packed monolayer on Ag(110) is imaged mainly through the pi states of the molecule. By contrast, functionalizing the STM tip with a perylene molecule leads to a mismatch between the energy levels of the STM tip and the molecule adsorbates and, instead, images only the metal states of the underlying silver surface. The observation opens a route for better energy selectivity in electron transport measurements through organic interfaces.     

One-dimensional structural and electronic properties of magnetite Fe3O4(1 1 0)

We study the structure and the electronic properties of the (1 1 0) surfaces of magnetite Fe₃O₄ thin films by scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images show a surface reconstruction consisting of ridges along the direction. Based on atomically resolved STM images we present a model for the observed ridge reconstruction of the surface, in agreement with a bulk-truncated layer containing both octahedral and tetrahedral iron ions. The metallic and semiconductor-like shapes of the measured current-voltage (I-V) curves indicate a non-uniform segregation of magnesium through the film. The weak contrast between the tops and valleys of ridges measured in the STS current maps is attributed to tetrahedral and octahedral coordination at the tops and the valleys, respectively. This attribution is in agreement with the proposed structure model. We observe a contrast enhancement at a tip change accompanied by a corrugation enhancement. This tip change is induced by picking up material from the sample, resulting in a magnetic tip. Thus, the contrast enhancement is attributed to detection of spin polarized current.     

Reversible local-modification of surface structure on clean Ge(0 0 1) by scanning tunneling microscopy below 80 K

The structure of the clean Ge(0 0 1) surface is locally and reversibly changed between c(4×2) and p(2×2) by controlling the bias voltage of a scanning tunneling microscopy (STM) below 80 K. It shows hysteresis for the direction of the sample bias voltage change. The c(4×2) structure is observed with the sample bias voltage V_b?−0.7 V. This structure is maintained at V_b?0.7 V with increasing the bias voltage from −0.7 V. When V_b is higher than 0.8 V, the structure changes to p(2×2). This structure is then maintained at V_b?−0.6 V with decreasing the bias voltage from +0.8 V. The area of the structure change can be confined in the single dimer row just under the STM tip using a bias voltage pulse. In particular, the minimum transformed length is four dimers along the dimer row in the transformation from p(2×2) to c(4×2). The observed local change of the reconstruction with hysteresis is attributed to the energy transfer process from the tunneling electron to the Ge lattice in the local electric field due to the STM bias voltage. A phenomenological model is proposed for the structure changes. It is based on a cascade inversion of the dimer buckling orientation along the dimer row.     

Room temperature observation of nitric oxide on Ir(1 1 1) by scanning tunneling microscopy

Room temperature (RT) adsorption of nitric oxide (NO) on Ir(1 1 1) was studied by scanning tunneling microscopy (STM). At low exposures, NO molecules can not be imaged by STM, because at RT the diffusion of NO is much faster than the STM scanning speed. At high exposures near the saturation coverage, however, a well-ordered 2 × 2 structure is observed. The coverage of the major 2 × 2 species is 0.25 and they can be assigned to the NO molecules adsorbed on the Ir ontop sites. A small number of less bright spots are assigned to nitrogen atoms produced by dissociation. Their number increases by annealing the NO-saturated surface at 380 K. A small number of another dissociation product, oxygen, are observed as black lines, indicating that the diffusion of oxygen atoms is fast. Scratch-like noise features were also detected by the STM, which suggests that a mobile precursor state exists, which was clearly shown by the effects of electron irradiation from the STM tip. These results are consistent with the previous molecular beam studies. Hopping of the 2 × 2 ordered NO species was frequently observed at the anti-phase domain boundaries and edges of the 2 × 2 islands.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(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 TiO₂(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 Ti₂O₃.     

Non-contact atomic force microscopy studies of (2 × 4) InP(0 0 1) surface

A sputter-cleaned indium-rich (2 × 4) InP(0 0 1) surface was investigated by non-contact scanning atomic force microscopy (NCAFM). Atomically-resolved images of the surface exhibit two different patterns. The patterns can be interpreted within the mixed dimer model of (2 × 4) reconstructed InP(0 0 1) surface. It is shown that due to contrast formation mechanism in NCAFM the features resolved are in close correspondence to scanning tunnelling microscopy (STM) data. Due to chemical interaction a P-terminated tip gives the image similar to an empty-state STM image, whereas an In-terminated tip gives the image resembling a filled-state STM one. Moreover, it is shown that due to dipole-dipole interaction, NCAFM can be sensitive to orientation of In-P dimers.     

How does ultrasonic cavitation remove dental bacterial biofilm?

Bacterial biofilm accumulation is problematic in many areas, leading to biofouling in the marine environment and the food industry, and infections in healthcare. Physical disruption of biofilms has become an important area of research. In dentistry, biofilm removal is essential to maintain health. The aim of this study is to observe biofilm disruption due to cavitation generated by a dental ultrasonic scaler (P5XS, Acteon) using a high speed camera and determine how this is achieved. Streptococcus sanguinis biofilm was grown on Thermanox™ coverslips (Nunc, USA) for 4 days. After fixing and staining with crystal violet, biofilm removal was imaged using a high speed camera (AX200, Photron). An ultrasonic scaler tip (tip 10P) was held 2 mm away from the biofilm and operated for 2 s. Bubble oscillations were observed from high speed image sequences and image analysis was used to track bubble motion and calculate changes in bubble radius and velocity on the surface. The results demonstrate that most of the biofilm disruption occurs through cavitation bubbles contacting the surface within 2 s, whether individually or in cavitation clouds. Cleaning occurs through shape oscillating microbubbles on the surface as well as through fluid flow.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

Gd adsorption on the Mo(2 1 1) surface

The adsorption of Gd thin layers on the Mo(2 1 1) face was investigated by using Auger electron spectroscopy (AES), low electron energy diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoemission spectroscopy (XPS) and measurements of the work function changes (Δφ). It was found that at 300 K Gd does not form any dilute chain structures and from the very beginning of the adsorption process Gd forms a densely packed layer. The dilute p(4 × 1) chain structure was observed by LEED after annealing thin layers (θ < 1 ML) to temperatures above 770 K. STM images confirm the existence of the p(4 × 1) structure islands. The intermixing of the substrate and adsorbate atoms takes place.     

STM tip-enhanced photoluminescence from porphyrin film

Tunneling electrons-induced molecular fluorescence in organic film is enhanced by the surface plasmons. The plasmon enhancement can be expected not only by the plasmons of the substrate but also by the noble metal tip of scanning tunneling microscope (STM). In this report we investigate the tip effect in photoluminescence of meso-tetrakis(3,5-di-tertiarybutyl-phenyl)porphyrin (H₂TBPP) film on indium tin oxide (ITO) combined with a STM. The experimental result shows the PL of molecules is enhanced by an Ag tip. This enhancement factor is evaluated larger than 2000.     

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.     

Fano fingerprints of Majoranas in Kitaev dimers of superconducting adatoms

We investigate theoretically a Fano interferometer composed by STM and AFM tips close to a Kitaev dimer of superconducting adatoms, in which the adatom placed under the AFM tip, encloses a pair of Majorana fermions (MFs). For the binding energy Δ of the Cooper pair delocalized into the adatoms under the tips coincident with the tunneling amplitude t between them, namely Δ=t, we find that only one MF beneath the AFM tip hybridizes with the adatom coupled to the STM tips. As a result, a gate invariance feature emerges: the Fano profile of the transmittance rises as an invariant quantity depending upon the STM tips Fermi energy, due to the symmetric swap in the gate potential of the AFM tip.     

Atomic resolution noncontact atomic force and scanning tunneling microscopy of TiO2(110)-(1 x 1) and - (1 x 2): simultaneous imaging of surface structures and electronic states

We present simultaneous imaging of TiO2(110)-(1 x 1) and - (1 x 2) using noncontact atomic force microscopy (NC-AFM) and scanning tunneling microscopy (STM). The surface topography was imaged under NC-AFM feedback, while the surface electronic states were imaged by STM. The image contrasts of NC-AFM and STM were antiphase in (1 x 1) and in phase in (1 x 2). The uppermost oxygen and Ti atoms underneath were, respectively, imaged by NC-AFM and STM. The NC-AFM image contrast was close to the true surface topography in (1 x 2), but reduced in (1 x 1).     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Signatures of epitaxial graphene grown on Si-terminated 6H-SiC (0 0 0 1)

Epitaxial graphene layers thermally grown on Si-terminated 6H-SiC (0 0 0 1) have been probed using Auger electron spectroscopy, Raman microspectroscopy, and scanning tunneling microscopy (STM). The average multilayer graphene thickness is determined by attenuation of the Si (L₂₃VV) and C (KVV) Auger electron signals. Systematic changes in the Raman spectra are observed as the film thickness increases from one to three layers. The most striking observation is a large increase in the intensity of the Raman 2D-band (overtone of the D-band and also known as the G′-band) for samples with a mean thickness of more than ∼1.5 graphene layers. Correlating this information with STM images, we show that the first graphene layer imaged by STM produces very little 2D intensity, but the second imaged layer shows a single-Lorentzian 2D peak near 2750 cm⁻¹, similar to spectra acquired from single-layer micromechanically cleaved graphene (CG). The 4-10 cm⁻¹ higher frequency shift of the G peak relative to CG can be associated with charge exchange with the underlying SiC substrate and the formation of finite size domains of graphene. The much greater (41-50 cm⁻¹) blue shift observed for the 2D-band may be correlated with these domains and compressive strain.     

Preparing and regulating a bi-stable molecular switch by atomic manipulation

We present a scanning tunneling microscopy (STM) investigation into the influence of the STM tip on the adsorption site switching of polychlorinatedbiphenyl (PCB) molecules on the Si(111)-7?×?7 surface at room temperature. From an initially stable adsorption configuration, atomic manipulation by charge injection from the STM tip prepared a new bi-stable configuration that switched between two bonding arrangements. No switching rate bias dependence was found for +?1.0 to +?2.2?V. Assuming a thermally driven switching process we find that the measured energy barriers to switching are influenced by the exact location of the STM tip by more than 10%. We propose that this energy difference is due the dispersion interaction between the tip and the molecule.