STM analysis of triosmium carbonyl cluster adsorption at HOPG

The study of metallic carbonyl clusters as precursors in tailoring the heterogeneous metal catalysts has been of great importance. The catalytic nature of the adsorbed clusters in thin film form depends on the chemical properties of the substrate used. The metal-support interaction will determine various properties such as the surface morphology, adsorption features and the structural orientations. We report a scanning tunneling microscopy (STM) study of an osmium carbonyl cluster (Os₃(CO)₁₁(NCCH₃)) adsorbed on highly oriented pyrolytic graphite (HOPG). STM measurements showed that the osmium carbonyl cluster interacts with HOPG in such a way that it adsorbs on the basal plane showing regular lattice structure, whereas the axial planes of the HOPG surface shows no ordered structure. The regular cluster lattice structure of the carbonyl cluster on the basal plane of the graphite has lattice parameters of a=1.4 nm and b=1.5 nm. We believe that the regular orientation of the cluster indicates a monolayer adsorption of the cluster on the graphite basal planes. Scanning tunneling spectroscopy (STS) measurements also indicated an insulating behavior for the cluster molecules on HOPG, with a significant energy gap value of ca. 300 mV. The cluster interaction at the active sites, i.e. axial planes of the graphite, was also observed by in situ STM measurements.     

Two-dimensional electron momentum distribution in graphite revealed by means of angular correlation of positron annihilation

Two-dimensional angular correlation measurements with positron annihilation radiation (2D-ACAR) on a single crystal of Kish graphite reveal that the motion of the annihilating electrons is nearly free along the graphite layers and restricted within some distance shorter than the lattice constant in the direction perpendicular to the layers. The hollows appearing on the 2D-ACAR surfaces are explained in terms of preferential annihilation of positrons with π-electrons in graphite.     

Negative c-axis magnetoresistance in graphite

We have studied the c-axis interlayer magnetoresistance (ILMR), Rc(B) in graphite. The measurements have been performed on strongly anisotropic highly oriented pyrolytic graphite (HOPG) and single crystalline Kish graphite samples in magnetic field up to B=9 T, and the temperature interval 2 K?T?300 K. We have observed negative magnetoresistance, dRc/dB<0, for B‖c-axis for both samples above a certain field Bm(T)>5.4 T and 0.2 T for HOPG and Kish graphite, respectively. The results can be understood consistently by assuming that ILMR is related to a tunneling between zero-energy Landau levels of quasi-two-dimensional Dirac fermions, in a close analogy with the behavior reported for α-(BEDT-TTF)₂I₃ [N. Tajima, et al., Phys. Rev. Lett. 102 (2009) 176403], another multilayer Dirac electron system.     

Self-organization of surfactant molecules on solid surface: an STM study of sodium alkyl sulfonates

Adsorption and self-organization of sodium alkyl sulfonates (STS and SHS) on HOPG have been studied by using in situ scanning tunneling microscopy (STM). Both SHS and STS molecules adsorb on HOPG surface and form long-range well-ordered monolayers. The molecular rows and the axes of alkyl chain of the molecules cross each other at angles of 60° and 90° in the STS and SHS layers, respectively. Molecular details such as sulfonate functional group (head) and alkyl chain are clearly imaged. The neighboring molecules in different rows form a “head to head” configuration. Structural models for the molecular arrangement of the two adlayers are proposed.     

Observation of pseudogap in SnSe2 atomic layers grown on graphite

Superconducting metal dichalcogenides (MDCs) present several similarities to the other layered superconductors like cuprates. The superconductivity in atomically thin MDCs has been demonstrated by recent experiments, however, the investigation of the superconductivity intertwined with other orders are scarce. Investigating the pseudogap in atomic layers of MDCs may help to understand the superconducting mechanism for these true two-dimensional (2D) superconducting systems. Herein we report a pseudogap opening in the tunneling spectra of thin layers of SnSe₂ epitaxially grown on highly oriented pyrolytic graphite (HOPG) with scanning tunneling microscopy/spectroscopy (STM/STS). A significant V-shaped pseudogap was observed to open near the Fermi level (E_F) in the STS. And at elevated temperatures, the gap gradually evolves to a shallow dip. Our experimental observations provide direct evidence of a pseudogap state in the electron-doped SnSe₂ atomic layers on the HOPG surface, which may stimulate further exploration of the mechanism of superconductivity at 2D limit in MDCs.     

Reentrant metallic behavior of graphite in the quantum limit

Magnetotransport measurements performed on several well-characterized highly oriented pyrolitic graphite and single crystalline Kish graphite samples reveal a reentrant metallic behavior in the basal-plane resistance at high magnetic fields, when only the lowest Landau levels are occupied. The results suggest that the quantum Hall effect and Landau-level-quantization-induced superconducting correlations are relevant to understand the metalliclike state(s) in graphite in the quantum limit.     

单晶石墨、多晶石墨电导行为的差异

报道了单晶石墨、多晶石墨电导行为的差异.在8.15T磁场下，4.5K处单、多晶石墨的正磁电阻效应分别为69400%，170%，同时在0T，8T磁场作用下，多晶石墨的电导行为呈现类绝缘体型性质，但在单晶石墨中我们发现了与磁场相关的类半金属-绝缘体型转变，通过分析，我们认为：单、多晶石墨电导行为存在较大差异的原因可能来源于库仑相互作用在高定向热解石墨中变得不可忽视，而多晶石墨样品却存在晶界散射. 关键词： 半金属石墨 类半金属-绝缘体型转变     

Density of states at each carbon layer of K-intercalated HOPG compounds --- high resolution 13C NMR experiment ---

We have observed high resolution ¹³C NMR spectra in k-intercalated stages 2, 3, 4 and 6 HOPG compounds. The sample was a block itself or sliced one of HOPG. From T₁T of each resolved line we obtained a distribution of density of states N(E_F) at the Fermi level among the graphite layers.     

Graphite,Graphene, and the Flat Band Superconductivity

Superconductivity has been observed in bilayer graphene [1, 2]. The main factor that determines the mechanism of the formation of this superconductivity is the “magic angle” of twist of two graphene layers, at which the electronic band structure becomes nearly flat. The specific role played by twist and by the band flattening has been earlier suggested for explanations of the signatures of room-temperature superconductivity observed in the highly oriented pyrolytic graphite (HOPG), when the quasi two-dimensional interfaces between the twisted domains are present. The interface contains the periodic array of misfit dislocations (analogs of the boundaries of the unit cell of the Moiré superlattice in bilayer graphene), which provide the possible source of the flat band. This demonstrates that it is high time for combination of the theoretical and experimental efforts in order to reach the reproducible room-temperature superconductivity in graphite or in similar real or artificial materials.     

Magnetic-Field-Induced Semimetal-Insulator-like Transition in Highly Oriented Pyrolitic Graphite

We report the extraordinarily large positive magnetoresistances （MR, 69400% at 4.5K under a magnetic field of 8.15 T）, de Hass-van Alphen oscillations effect at 10 K and the semimetal-Jnsulator-like transition in a wide range of temperature in highly oriented pyrolitic graphite （HOPG）. Besides a dominating ordinary MR （OMR） mechanism in the free-electron mode, it is realized from qualitative analysis that the Coulomb interacting quasiparticles within graphite layers play some roles. However it is difficult to associate the transition with the simple OMR theory. In order to investigate the possible origins of the transition, further analysis is carried out. It is revealed that the magnetic-field-induced behaviour is responsible for the semimetal-insulator-like transitions in HOPG.     

The Magnetic Order in Ion Irradiated Graphite

The magnetism of highly oriented pyrolytic graphite(HOPG) induced by ion implantation is investigated with electron spin resonance(ESR) spectroscopy and magnetization measurements.The results indicate that the ESR spectra of the HOPG sample correlate with ion species,incident energy and dose of implantation.The correlation of the ESR spectra and magnetism of the HOPG sample with ~(12)C~+ ion implantation and H~+ ion implantation are studied in detail.The ferromagnetism of the HOPG sample is likely related to the asymmetric L1 line,which may be attributed to the interaction between localized defects and itinerant electrons occupied in the 'impurity'band induced by ion implantation.     

Magnetoresistance studies of MeV ranged H and C ion irradiated HOPG flakes

2 MeV protons and 1 MeV carbon ions are bombarded on highly oriented pyrolytic graphite (HOPG) samples and their electronic transport measurements are carried out in the presence of magnetic field. The Magneto-Resistance (MR) measurements show measurable hysteresis in the resistance value after ion beam irradiation for the in-plane magnetic field direction as well as for the out-of-plane field direction. The MR depends on the thickness of the flake and the method of its separation from the bulk HOPG. The results substantiate that the ferromagnetic coupling between the magnetic moments at the vacancy defect sites in HOPG sensitively depends on the average defect separation. The average defect separation range of 1.7–0.5 nm allows only a part of the 40 μm thick proton beam irradiated sample to go for ferromagnetic ordering. Similar conclusions are drawn from carbon ion irradiated HOPG flake. The irradiation increases the resistance of the flake as well.     

Electron spectroscopy measurements on hydrogen implanted graphite and comparison to amorphous hydrogenated carbon films (a-C:H)

Highly oriented pyrolytic graphite (HOPG) as well as polycrystalline graphite (pc-graphite) were bombarded with 3.5 keV H⁺ ions by means of a Penning ion source. The implanted graphite was characterized by in situ electron spectroscopy techniques such as UPS, XPS and EELS. Our UPS valence band measurements of the hydrogen saturated graphite reveal it to be an insulating phase, and XPS measurements show a shift of the C1s core level to higher binding energy with respect to pristine graphite. This behavior is explained by a Fermi energy shift upon hydrogen bombardment of graphite.In addition, a close resemblance in the electronic structure of hydrogen bombarded graphite and amorphous hydrogenated carbon films (a-C:H) is shown which suggests the modification of pristine graphite to an amorphous network [1] of mostly tetrahedrally bonded carbon atoms by hydrogen implantation.     

Absence of field anisotropy in the intrinsic ferromagnetic signals of highly oriented pyrolytic graphite

We have measured the magnetization of bulk samples of highly oriented pyrolytic graphite (HOPG) at magnetic fields applied parallel and perpendicular to the graphene layers. Within experimental error the intrinsic ferromagnetic signals of the samples show similar magnetic moments at saturation for the two magnetic field directions, in contrast to recently published data (J. ?ervenka et al., Nat. Phys. 5 (2009) 840). To check that the SQUID device provides correctly the small ferromagnetic signals obtained after subtracting the 100 times larger diamagnetic background, we have prepared a sample with a superconducting Pb-film deposited on one of the HOPG surfaces. We show that the field dependence of the measured magnetic moment and after the background subtraction is highly reliable even in the sub-μ emu range providing the real magnetic properties of the embedded small ferromagnetic and superconducting signals.     

Temperature dependence of the muon Knight shift in graphite measured using a μSR\times 2 method

We describe a simple method by which time differential μSR spectra are collected on two different samples simultaneously. One application is to make accurate measurements of the muon precession frequency in a sample relative to a reference. As an example we report precise measurements of the muon Knight shift on HOPG graphite with H parallel to \hat c as a function of temperature. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

A comparative STM study of Ru nanoparticles deposited on HOPG by mass-selected gas aggregation versus thermal evaporation

Scanning tunneling microscopy was used to compare the morphologies of Ru nanoparticles deposited onto highly-oriented graphite surfaces using two different physical vapour deposition methods; (1) pre-formed mass-selected Ru nanoparticles with diameters between 2 nm and 15 nm were soft-landed onto HOPG surfaces using a gas-aggregation source and (2) nanoparticles were formed by e-beam evaporation of Ru films onto HOPG. The particles generated by the gas-aggregation source are round in shape with evidence of facets resolved on the larger particles. Annealing these nanoparticles when they are supported on unsputtered HOPG resulted in the sintering of smaller nanoparticles, while larger particles remained immobile. Nanoparticles deposited onto sputtered HOPG surfaces were found to be stable against sintering when annealed. The size and shape of nanoparticles deposited by e-beam evaporation depend to a large extent on the state of the graphite support and the temperature. Ru deposition onto unsputtered HOPG is characterised by bimodal growth with large flat particles formed on the substrate terraces and smaller diameter particles aligned along the substrate steps. Evaporation onto sputtered HOPG results in the formation of 2 nm round particles with a narrow size distribution. Finally, thermal deposition onto both sputtered and unsputtered HOPG at 660 °C results in larger particles showing a flat Ru(0 0 0 1) top facet.     

Laser scribing on HOPG for graphene stamp printing on silicon wafer

Highly oriented pyrolytic graphite (HOPG) was scribed by pulsed laser beam to produce square patterns. Patterning of HOPG surface facilitates the detachment of graphene layers during contact printing. Direct HOPG-to-substrate and glue-assisted stamp printing of a few-layers graphene was compared. Printed graphene sheets were visualized by optical and scanning electron microscopy. The number of graphene layers was measured by atomic force microscopy. Glue-assisted stamp printing allows printing relatively large graphene sheets (40×40 μm) onto a silicon wafer. The presented method is easier to implement and is more flexible than the majority of existing ways of placing graphene sheets onto a substrate.     

Adsorption and manipulation of carbon onions on highly oriented pyrolytic graphite studied with atomic force microscopy

Carbon onions produced by DC arc discharge method were deposited on highly oriented pyrolytic graphite (HOPG) surface and their adsorption and manipulation was studied using an atomic force microscopy (AFM). Well-dispersed adsorption of carbon onions on HOPG surface was obtained and aggregations of onions were not observed. The van der Waals interaction between the onion and HOPG surface and that between two onions, were calculated and discussed using Hamaker's theory. The manipulation of adsorbed onions on HOPG surface was realized using the AFM in both the raster mode and the vector mode. The controllability and precision of two manipulation modes were compared and the vector mode manipulation was found superior, and is a useful technique for the construction of nano-scale devices based on carbon onions.     

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.