Several large-scale superperiodicities on highly oriented pyrolytic graphite observed by scanning tunneling microscopy

On freshly cleaved highly oriented pyrolytic graphite we observed large-scale superperiodicities by a scanning tunneling microscope at room temperature in air. Several hexagonal superstructures with periods of 30 nm, 4.2 nm, 2.4 nm, and 2.0 nm, respectively, and a strip-like superstructure with a period of 1 nm were obtained. With exception of the largest hexagonal superperiodicity (30 nm spacing), all other superstructures are superimposed on the atomic corrugation of graphite. The origin of these superstructures is not clear yet. We assume that they arise from crystal defects in graphite. The hexagonal superstructure may be caused by the Moiré effect due to the rotational misorientation of the two top layers or of two successive layers near the surface.     

Simultaneous observation of the triangular and honeycomb structures on highly oriented pyrolytic graphite at room temperature: An STM study

Scanning tunneling microscopy (STM) was used to study the surface structures of dry-prepared and di-chloroethane-treated HOPG samples. Both triangular and honeycomb structures were simultaneously observed with the same tip at room temperature around a strand (grain boundary) on the HOPG surface. This observation did not support the tip effect in STM imaging explanation for HOPG in literature. A general layer-sliding model was utilized to explain the experimental results: sliding of the HOPG topmost layer was used to explain the origins of the triangular and honeycomb structures, and molecule intercalation into inter-layer spacing between the first and second layers of HOPG induced inhomogeneous deformation of the HOPG topmost layer that accordingly generated the Moiré patterns of the HOPG sample in di-chloroethane.     

Two-dimensional spectral-spatial image of highly oriented pyrolytic graphite

Experimentally observed electron paramagnetic resonance (EPR) spectra of highly oriented pyrolytic graphite samples can be considered as a sum of three signals coming from two adjacent faces and crystal edges. Contributions from faces can be analyzed analytically by Dyson’s theory based on an infinite flat conductive plate, while the contribution from edges is not described by this theory. Overlapping of these signals makes it difficult to get useful information about the sample from spectra observed. Implementation of two-dimensional spectral-spatial imaging technique proved to be helpful to solve this problem. It permits the characterization of the EPR spectrum from a selected flat spatial region located far from crystal edges where the model of the infinite flat conductive plate can be applied. By analyzing the EPR signal from spatial slices by Dyson’s equation we have obtained the values of the diffusion coefficient and the surface relaxation rate.     

VUV photochemistry of oriented molecules: methylchloride on highly oriented pyrolytic graphite

The photoionization of CH₃Cl adsorbed on highly oriented pyrolytic graphite has been investigated as a function of the photon energy in the range 13–35 eV. Two ions were detected — CH⁺₃ and H⁺ — which show differences in their coverage and energy dependences. At very low coverages only H⁺ ions are produced, while at higher coverages (still in the monolayer regime) CH⁺₃ ions are also observed, reflecting a change in the orientation of the molecule on the surface as a function of coverage. The cross-section curves as a function of the photon energy for both ions show a maximum at 22 eV, while the onsets are at 16 eV for CH⁺₃ and 19 eV for H⁺.     

Intercalation of lithium ions into bulk and powder highly oriented pyrolytic graphite

The electrochemical reactions of highly oriented pyrolytic graphite (HOPG) bulk and powder electrodes in 1 M LiPF₆ 1:1 EC/DMC solution were investigated and the results show that the intercalation reaction of lithium ion into HOPG electrode occurs only at the edge plane and SEI formation reaction on the basal plane is negligible in comparison with that on the edge plane. The active surface area of HOPG powder electrode could be deduced by comparing the peak area (consumed charge for SEI formation) at potential of 0.5 V on voltammograms with that of bulk HOPG edge electrode. The diffusion coefficients of lithium ion in HOPG bulk layers and in HOPG powder was for the first time measured by use of electrochemical impedance spectra and potential step chronamperameter methods. It was found that the diffusion coefficients of lithium in HOPG were in the range of 10⁻¹¹-10⁻¹² cm² s⁻¹ for the lithium-HOPG intercalation compounds at potentials from 0.2 (vs. Li/Li⁺) to 0.02 V, decreasing with the increase of lithium intercalation degree. A good agreement was obtained between the results from bulk and powder HOPG electrodes by electrochemical impedance spectra method.     

高度取向石墨的巨磁电阻效应

石墨是准二维半金属材料，然而在通常细晶粒、无取向的石墨中并没有发现很大的磁电阻效应.在高度取向的石墨中发现了巨大的正磁电阻效应，在8.15 T的外磁场中，4.2，300 K温度下的磁电阻分别高达85300%和4950%.生产这一巨磁电阻效应的机制除正常磁电阻效应外，可能源于磁场诱导的类金属-绝缘体的转变 关键词： 磁电阻效应 石墨     

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.     

高定向热解石墨表面局域导电增强现象的扫描探针显微学研究

基于导电型原子力显微镜和扫描隧道显微镜的对比观察,研究高定向热解石墨表面上残留石墨片的导电增强现象.根据样品电阻的测量数据,将这种现象归结为导电针尖与石墨表面的点接触问题,并且发现接触电导和接触点处局域的电子密度成正比,从而确定石墨表面的局域导电增强现象的原因在于残留在石墨表面的石墨片具有较高的电子密度. 关键词： 扫描隧道显微镜 导电原子力显微镜 高定向热解石墨 导电性     

The bonding geometry of alkylsilanes on gold: Relation to surface pattern development and STM image contrast

The adsorption of methylsilane (MeSiH₃) on Au(1 1 1) was investigated using density functional theory (DFT) within a generalized gradient approximation (GGA). Two preferred chemisorption sites are identified: the hollow site and an atop site with an ejected gold substrate atom. Both of these configurations result in a tetracoordinate Si with a distorted tetrahedral geometry. The energy of adsorption is calculated allowing an analysis of the preferred binding geometry as a function of coverage. The relation of the supermolecular length scale pattern that emerges from a spinodal decomposition of the two phases arising from the two binding geometries is discussed. The pattern observed in the STM images is shown to be consistent with the local density of states calculated for the two different binding geometries.     

A direct observation on the surface assembling and ordering of coumarin derivatives on the graphite surface

The 2D monolayer structures of two coumarin derivatives, 4-heptadecyl-7-hydroxycoumarin (HHC) 1 and 3-(2-benzo thiazolyl)-7-octadecyloxy-coumarin (BOC) 2, have been studied by STM and different sensitivity to thermal annealing for these two systems has been observed. The results show that the arrangements of these two molecules are obviously different in the self-assembled monolayer at 20 °C on the graphite surface. Furthermore, only BOC is sensitive to thermal annealing. The assembly of BOC becomes more stable and ordered after annealing at 65 °C for 1 h.     

A combined scanning tunneling microscopy and reflectance anisotropy spectroscopy investigation of tetraphenylporphyrin deposited on graphite

The ordering properties of tetraphenylporphyrin molecules sublimated on a highly oriented pyrolitic graphite surface have been investigated. The morphological information obtained by scanning tunneling microscopy has been linked with the optical spectra by reflectance anisotropy spectroscopy. Measurements were performed in situ as a function of the nominal coverage.     

STM/STS observation of ferrocene derivative adduct to C60 on HOPG

The C₆₀ONCFn cycloadduct (Fn=ferrocene) was prepared in the reaction between C₆₀ and ferrocene oxime, the ferrocene derivative was bound to C₆₀ at the 6–6 bond by a heterocyclic oxygen–nitrogen–carbon ring; the compound was stable in air. The compound dissolved in dichloroethane was deposited on HOPG and observed by UHV STM/STS methods. The molecules of C₆₀ONCFn formed several-microns-long straight chains with clearly visible adducted groups pointing to one side of the chain. The STM/STS observations are discussed within the terms of semiempirical quantum chemical molecular modeling.     

Effect of temperature on the physical sputtering of highly oriented pyrolytic graphite

The results of experimentally investigating the sputtering and erosion of the basal plane of highly oriented pyrographite UPV-1T under irradiation with 30-keV Ar⁺ in the range from room temperature to 400°C are presented. It has been found that ion-induced surface-relief evolution at higher temperatures results in a two-fold increase in the sputtering yield (Y = 2) in comparison with sputtering of a surface with a nanosized relief at temperatures less than that of the texture transition T _t ≈ 150°C. Sputtering simulation using the OKSANA code for a surface with a sinusoidal nanorelief, which reflects the instability of the basal plane of UPV-1T under ion irradiation, permits to estimate the ratio of the amplitude to the relief period at T < T _t.     

Residence sites of fluorine in crystalline silicon and highly oriented pyrolytic graphite

Measurements of the electric field gradient (efg) at F impurity sites in crystalline silicon and highly oriented pyrolytic graphite (HOPG) have been compared with cluster model calculations using both Hartree-Fock (HF) and density functional theory (DFT) formalisms. The technique of time-differential perturbed angular distributions of -ravs was employed to derive the efg parameters following implantation of¹⁹F via the¹⁹F(p, p)¹⁹F^* reaction. For the case of HOPG the DFT method gave closer agreement with the experimental values of ¦V _zz¦=3.24(14)×10²²V/m² and=0.16(3), yieldingV _zz=–3.09×10²²V/m²and=0.13 for¹⁹F at a site between the layers with point group symmetry C_2h and inter-layer spacingd=3.70 Å. For¹⁹F implantation in silicon three sites were found corresponding to quadrupole frequencies 23.2(3) MHz, 35.2(3) MHz and 37.1(5) MHz. Both HF and DFT calculations are consistent with the assignment of interstitial antibonding and bond centre sites for the 23.2 and 35.2 MHz, respectively. In the former case, the F atom is located 1.81 Å along a <111> direction from a silicon atom; in the latter situation the Si-Si bond length is found to expand by 1.02 Å from its normal lattice value. It is speculated that the third interaction, which occurs at only the 10% level, possibly arises from sites associated with a defect or other impurity. To achieve a reduction in cluster size, the completion of dangling bonds with atoms other than hydrogen was investigated. The results were found to be comparable.     

Studies of the adsorption of tetraphenylporphyrin molecules on graphite

Combined scanning tunneling microscopy, reflection electron energy loss spectroscopy and X-ray photoelectron studies have been performed in situ under ultra high vacuum condition, on tetraphenylporphyrin molecules (H₂TPP) vacuum sublimated on highly oriented pyrolitic graphite. The experimental studies were performed at room temperature, as a function of the amount of deposited porphyrins.The propensity of H₂TPP to self-assembly on the graphite surface could be detected after a threshold of deposited material. In this case tetraphenylporphyrin molecules arranged according to a quasi-hexagonal lattice separated from their nearest neighbours by a minimum distance of about 1 nm. The formation of an additional incomplete layer, at a slightly higher coverage, was also detected where the quasi-hexagonal symmetry is retained. Finally, subsequent tetraphenylporphyrins depositions gave molecular aggregates randomly distributed on the graphite surface with subsequent loss of order.     

STM and AFM of bio/organic molecules and structures

Applications of scanning tunneling and atomic force microscopes in bio/organic researches are reviewed with a special emphasis on the types of researches that are expected to contribute to the creation of a new field of “single molecule biochemistry” in the near future. The reviewed articles within the scope as stated above actually include a fairly broad spectrum of researches. It is, therefore, a hope of the author that this review will be useful to those who are considering biological applications of the probe microscopy techniques but are not quite familiar with the types of experiments that have been done in the past. In the section on scanning tunneling microscopy, identification of chemically distinct functional groups by the difference in their tunneling properties will be discussed as a main focus because it is fundamental for biochemists to identify molecules by their shapes and properties. In the section on atomic force microscopy, recent progresses in the imaging techniques of proteins and DNAs are closely reviewed, and rapidly advancing technologies of single molecule measurements and manipulation of nanometer sized structures are given extensive coverage because the author considers that such new applications are extremely promising to open an entirely new field in biological sciences.     

An artefact in scanning tunneling microscopy images of highly oriented pyrolytic graphite

An artefact with a structure that appears as an ordered array of chains, several 1000 Å long and with distances of between 10 Å and 30 Å in STM images of HOPG is reported. The artefact, which emerges from a too low point density in the scanning grid during data aquisition in the scanning tunneling microscope, is explained using the atomically resolved image, the orientation of the substrate in the STM, the sequential character of data acquisition in scanning tunneling microscopy together with the limited number of data points for the reconstruction of STM images and related image-processing problems, and the specific noise spectrum of the tunneling current. The identification of the artefact is verified by a computer simulation of the STM images, which reproduces the experimental data in detail.     

Low-energy μSR and SQUID evidence of magnetism in highly oriented pyrolytic graphite

Low-energy muon spin rotation and SQUID magnetization measurements were performed on proton-irradiated and non-irradiated highly oriented pyrolytic graphite samples. The samples were found to be ferromagnetic above and below room temperature and to include a substantial temperature-dependent surface contribution. Assuming uniformity, the thickness of the magnetic surface layer was estimated to be 13(2) nm. The discovered surface magnetism is intrinsic and not due to irradiation.     

Influence of Interorbital Interference on Tunneling Process in STM

We present a theoretical study of the influence of intra-atomic interorbital interference on the formation of STM images of metal surfaces. The obtained results show that this kind of interference may modify significantly the tunneling current by the increase or decrease of the current contributions flowing through different orbitals of the surface atoms. STM simulations performed for aluminium and nickel surfaces indicate that the height of corrugation and the topographies of STM images of different surfaces depend considerably on this interference.     

STM investigations of local potential barrier dist ribution of the atomic surface of graphite

To modulate the tunneling gap with the lock-in amplifier in the scanning tunneling microscopy(STM), information of the tunneling current variation can be obtained. The local potential barrier distribution of graphite surface atoms is got by means of such technology. Compared with STM image under topography observation mode, the local potential barrier image has higher resolu tion and less influence on the tip and better anti-interference capability. Obs erved results of the graphite are given and discussed in this paper.