Scanning probe microscopy study of exfoliated oxidized graphene sheets

Exfoliated oxidized graphene (OG) sheets, suspended in an aqueous solution, were deposited on freshly cleaved HOPG and studied by ambient AFM and UHV STM. The AFM images revealed oxidized graphene sheets with a lateral dimension of 5–10 μm. The oxidized graphene sheets exhibited different thicknesses and were found to conformally coat the HOPG substrate. Wrinkles and folds induced by the deposition process were clearly observed. Phase imaging and lateral force microscopy showed distinct contrast between the oxidized graphene and the underlying HOPG substrate. The UHV STM studies of oxidized graphene revealed atomic scale periodicity showing a (0.273 ± 0.008) nm × (0.406 ± 0.013) nm unit cell over distances spanning few nanometers. This periodicity is identified with oxygen atoms bound to the oxidized graphene sheet. I(V) data were taken from oxidized graphene sheets and compared to similar data obtained from bulk HOPG. The dI/dV data from oxidized graphene reveals a reduction in the local density of states for bias voltages in the range of ±0.1 V.     

Scanning tunneling microscopy of RF oscillating surfaces

Amplitude and phase of high frequency surface acoustic wave (SAW) fields are investigated by a novel scanning tunneling microscopy technique. The gap voltage is modulated at a slightly detuned high frequency. Due to the nonlinearity of the tunneling process a frequency mixing appears. For scanned areas with dimensions much smaller than the wavelength of the SAW a remarkable local variation of amplitude and phase of the tunneling current at the difference frequency is observed. Depending on the local morphology different components of the particle displacement vector are detected. Model calculations of amplitude and phase images are presented for a real topography.     

Scanning Hall probe microscopy of vortex matter

Scanning Hall probe microscopy (SHPM) is a novel scanned probe magnetic imaging technique whereby the stray fields at the surface of a sample are mapped with a sub-micron semiconductor heterostructure Hall probe. In addition an integrated scanning tunnelling microscope (STM) or atomic force microscope (AFM) tip allows the simultaneous measurement of the sample topography, which can then be correlated with magnetic images. SHPM has several advantages over alternative methods; it is almost completely non-invasive, can be used over a very wide range of temperatures (0.3–300 K) and magnetic fields (0–7 T) and yields quantitative maps of the z-component of magnetic induction. The approach is particularly well suited to low temperature imaging of vortices in type II superconductors with very high signal:noise ratios and relatively high spatial resolution (>100 nm). This paper will introduce the design principles of SHPM including the choice of semiconductor heterostructure for different measurement conditions as well as surface tracking and scanning mechanisms. The full potential of the technique will be illustrated with results of vortex imaging studies of three distinct superconducting systems: (i) vortex chains in the “crossing lattices” regime of highly anisotropic cuprate superconductors, (ii) vortex–antivortex pairs spontaneously nucleated in ferromagnetic-superconductor hybrid structures, and (iii) vortices in the exotic p-wave superconductor Sr₂RuO₄ at milliKelvin temperatures.     

Mapping charge-mosaic surfaces in electrolyte solutions using surface charge microscopy

A significant limitation of electrokinetic measurements is that only an average value of the zeta potential/streaming potential is measured—regardless of whether the surface charge distribution is homogeneous or otherwise. However, in real-world situations, nearly all solids (and liquids) of technological significance exhibit surface heterogeneities. To detect heterogeneities in surface charge, analytical tools which provide accurate and spatially resolved information about material surface potential—particularly at microscopic and sub-microscopic resolutions—are needed.A novel AFM-based technique for mapping surface charge domains on heterogeneous surfaces, which we call Surface Charge Microscopy (SCM), was recently introduced by our research team. It relies on recording colloidal force curves over multiple locations on the substrate surface using small probes. The surface charge characteristics of the heterogeneous substrate are determined from the recorded colloidal force curves, allowing for the surface charge variation to be mapped. In this communication, we briefly review the SCM technique. Examples of results of measurements of the surface interaction forces that were recorded between a silicon nitride AFM cantilever and a multi-phase volcanic rock and heterogeneous surface of bitumen are also given.     

Scanning tunneling microscopy/spectroscopy,etching and modification of surfaces of YbBa2Cu3O7-δ single crystals using an STM

The surface of YbBa₂Cu₃O_7-δ single crystals was investigated using a scanning tunneling microscope at room temperature in air. Growth steps with step heights equal to the lattice parameter c as well as steps with step heights of multiples of 1.35 c were obtained. On smooth surfaces atoms arranged in a 3.8 Å square lattice could be resolved. We observed a tunneling-induced etching effect leading to an etch pit on the surface. Furthermore, by applying voltage pulses with amplitude > 5.0 V and duration > 100 μs holes with diameters of ～? 150 nm could be produced. Scanning tunneling spectroscopic measurements were performed yielding I-V and ln I-s characteristics. The extremely small effective barrier height of ? = 0.3 eV inferred from d ln I/ds may be caused by dirt between the surface and the tip.     

Scanning tunnelling microscopy and spectroscopy of the reduced TiO2(1 0 0) surface

Scanning tunnelling microscopy and current imaging tunnelling spectroscopy were used to study the topographic and electronic structure of a reduced TiO₂(1 0 0) surface. The STM results showed that the TiO₂(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.     

Scanning tunneling microscopy of surface-adsorbed fullerenes: C60, C70, and C84

Scanning tunneling microscopy (STM) is used to characterize partial monolayers of C₆₀, C₇₀, and C₈₄ adsorbed on the Au(1 1 1) surface at room temperature and under ambient conditions. A high degree of structural polymorphism is observed for monolayers of each of these fullerenes. For C₆₀, three lattice packings are observed, including a previously unreported 7 × 7 R21.8° structure that is stabilized by adjacent surface step defects. For C₇₀, two lattice packings are observed, and analysis of molecular features in STM images allows molecular binding geometry to be determined. In one of the two observed lattice structures, C₇₀ molecules align their long axis along the surface normal, while in the other, molecules align parallel to the surface and along a gold lattice direction. The parallel geometry is also preferred for isolated and loosely packed molecules on the surface. C₈₄ exhibits a large number of lattice orientations and no long-range order, and likely binds incommensurately on Au(1 1 1). Time series of images of partial C₇₀ monolayers show progressive surface modification as a result of perturbation by the STM tip; this is in contrast to the behavior of C₆₀, where alterations in surface structure at room temperature are thermally driven.     

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.     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

Scanning tunneling microscopy of N2H4 on silicon surfaces

The chemistry of N₂H₄ on Si(100)2 × 1 and Si(111)7 × 7 has been studied using scanning tunneling microscopy. At low coverages on Si(100)2 × 1 at room temperature the adsorption sites are distributed randomly on the surface and are imaged as dark spots in the dimer row by the STM. Upon annealing the substrate at 600 K, both isolated reaction products, as well as clusters of reaction products are formed on the surface. The STM images show that the majority of the isolated reaction products are adsorbed symmetrically across the dimers. Based on previous HREELS data, these are most likely NH_x groups. However, the clusters are not well resolved. Because of this we speculate that they are not simply symmetrically adsorbed NH_x groups, but likely have a more complicated internal structure. At higher coverages, the STM images show that the predominant pathway for adsorption is with the N---N bond parallel to the surface, in agreement with HREELS studies of this system. On Si(111)7 × 7, the molecule behaves in a manner which is similar to NH₃. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Near molecular-scale growth of natural minerals: Experimental methods and errors in length-dependent step speeds with scanning probe microscopy

Experimental observation of step (i.e., a linear discontinuity in a surface) growth on calcium carbonate and barium sulfate surfaces was performed using in situ scanning probe microscopy (SPM) in aqueous solutions. Step speeds at very short step lengths (<300 nm) were found to increase with step length, but scatter in the data precluded comparison of different theoretical models with regard to the experimental data. Analysis of experimental errors revealed that the data scatter was not unusual and further suggested that significant experimental bias in the determination of step properties such as the step free energy and kinetics of kink propagation and nucleation are introduced if the instrument bandwidth is not appropriate for the specific dynamics of the step. In light of step growth models based on either equilibrium assumptions or non-equilibrium assumptions, these biases tend to favor one model over the other.     

Scanning tunneling microscopy studies in the heavy fermion system with even and odd hybridization

The effect of even and odd hybridization for scanning-tunneling microscopy process in the heavy fermion system is studied by the nonequilibrium Green function method. It is found that an even hybridization leads to symmetric density of states (DOS), while an odd hybridization leads to the vanishing of the DOS of the nondiagonal term. Besides, the differential conductance under the even hybridization is the natural consequence of the existence of van Hove singularities. In the odd hybridization situation, the differential conductance reflects the interference between normal tunneling and cotunneling is absent in the simplest tunneling models. The results obtained provide valuable theoretical guidance to the heavy fermion (HF) experiments.     

Vicinal and on-axis surfaces of 6H-SiC(0001) thin films observed by scanning tunneling microscopy

Surfaces of 6H-SiC(0001) homoepitaxial layers deposited on vicinal (3.5° off (0001) towards [11 0]) and on-axis 6H---SiC wafers by chemical vapour deposition have been investigated using ultra-high vacuum scanning tunneling microscopy. Undulating step configurations were observed on both the on-axis and the vicinal surfaces. The former surface possessed wider terraces than the latter. Step heights on both surfaces were 0.25 nm corresponding to single bilayers containing one Si and one C layer. After annealing at T>1100°C for 3–5 min in UHV, selected terraces contained honeycomb-like regions caused by the transformation to a graphitic surface as a result of Si sublimation. A model of the observed step configuration has been proposed based on the observation of the [ 110] or [1 10] orientations of the steps and energetic considerations. Additional deposition of very thin (2 nm) SiC films on the above samples by gas source molecular beam epitaxy was performed to observe the evolution of the surface structure. Step bunching and growth of 6H---SiC layers and formation of 3C---SiC islands were observed on the vicinal and the on-axis surfaces, respectively, and controlled by the diffusion lengths of the adatoms.     

Scanning tunneling microscopy and spectroscopy study of charge inhomogeneities in bilayer graphene

We report a room-temperature scanning tunneling microscopy and spectroscopy study of bilayer graphene prepared by mechanical exfoliation on a SiO₂/Si surface and electrically contacted with gold pads using a mechanical mask. The bulk conductivity shows contributions from regions of varying electron density, indicating significant charge inhomogeneity. Large-scale topographic images show ripple-like structures with a roughness of ∼1 nm, while the small-scale atomic resolution images show graphite-like triangular lattices. The local () tunnel spectra have an asymmetric V-shape with the minima location showing significant spatial variation, indicating inhomogeneity in electron density of order 10¹¹ cm⁻². The minimum in spectrum at a fixed location also shifts linearly with the gate voltage with a slope consistent with the field-induced carrier density.     

Scanning tunneling microscopy and spectroscopy of Mo clusters grown on TiO2(1 1 0)

Molybdenum was deposited in two steps (3 eq. ML and 1 eq. ML) on the light blue rutile TiO₂(1 1 0) (1 × 1) surface at room temperature, each Mo deposition cycle being followed by an annealing up to 950-1000 K. This procedure was found to lead to formation of separated clusters having a size in very wide range (1-20 nm). Scanning tunneling microscopy showed a dependence of the cluster morphology as a function of the size. The scanning tunneling spectra of Mo clusters was studied as a function of cluster dimensions and discussed in comparison with photoelectron spectroscopy results previously obtained for homogeneous Mo films. The dI/dV curves do not display the valence band structure of deposited material, which could be explained by the Schottky barrier formation.     

The importance of imaging conditions in scanning tunneling microscopy for the determination of surface texture and roughness

We show that the control conditions in the feedback loop of a scanning tunneling microscope (STM) affect the values of the surface texture parameters, including fractal characterization, when they are calculated from STM images of the surface. The main surface texture parameters (such as the r.m.s. roughness, kurtosis, skewness, and average wavelength) show a strong dependence on the conditions used in the feedback loop for imaging. Fractal character changes also with feedback parameters. The quality of the STM images can be measured quantitatively by using some of the surface texture parameters.     

Interactions of oxygen and CO with AgAu bimetallic nanoparticles on sputtered highly ordered pyrolytic graphite (HOPG) surfaces

Oxidation of AgAu bimetallic nanoparticles on sputtered HOPG by atomic oxygen and reduction of the oxidized surface by CO at room temperature were studied using X-ray photoelectron spectroscopy (XPS). For 2 nm-sized nanoparticles, prepared by postdeposition of Ag on Au-core, atomic oxygen exposure mostly leads to the formation of chemically inert oxygen species. This result is analogous to that of pure Ag and Au nanoparticles of similar sizes on the same substrate. In contrast, “Au on Ag-core” nanoparticles form chemically active oxygen species, suggesting that depending on detailed structures of bimetallic nanoparticles, diverse chemical properties can be obtained.     

3D atom probe study of gas adsorption and reaction on alloy catalyst surfaces I: Instrumentation

A versatile gas reaction cell has been developed for a 3D atom probe system to provide a new method for studying the reaction mechanisms of heterogeneous catalysis. We describe the features and advantages of this new system. The reaction cell enables exposure of metal specimens in high temperature (?873 K) and high pressure (?1 bar) environments to a wide range of single gases or gas mixtures. Following treatment, specimens are transferred into the UHV analysis chamber of the 3D atom probe, which provides atomic-scale positional and chemical information for adsorbed gas and metal atoms/oxides from selected regions of the specimen apex. Results demonstrating the effects of heating alloy catalysts in vacuo and in air are presented, which validate the instrument design, alongside a sample of data for exposing Pt and Pt-Rh alloys to NO, revealing the power of the 3D atom probe technique in this field.     

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.