Single Crystal Graphite Research Articles

GEO-MIX-SELF calculations of the elastic properties of a textured graphite sample at different hydrostatic pressures

The recently developed GEO-MIX-SELF approximation (GMS) is applied to interpret the pressure dependence of the longitudinal ultrasonic wave velocities in a polycrystalline graphite sample that has already been investigated in a wide range of experimental contexts. Graphite single crystals have extremely anisotropic elastic properties, making this sample a challenging test to demonstrate the potential of the GMS method. GMS combines elements of well known self-consistent algorithms and of the geometric mean approximation. It is able to consider mixtures of different polycrystalline phases, each with its own nonspherical grain shape and preferred orientation (texture). Pores and `cracks', typical for bulk graphite, are modeled as phases with `empty' grains. The pressure dependence (up to 150 MPa) of the experimental wave velocities can be well explained using the known texture of the sample by fitting the shape parameters and volume fractions of the graphite grains, cracks and spherical pores. The pressure dependence of these parameters describes a reasonable scenario for the closing of the cracks and pores with increasing pressure.

Surface morphology of flux-grown graphite single crystals

Surface morphology of flux-grown graphite single crystals

Synthesis of carbon films by magnetron sputtering of a graphite target using hydrogen as plasma-forming gas

During magnetron sputtering of graphite using hydrogen as plasma-forming gas, a material sputtered from the target deposited only on heated substrates. When using currentheated Ni-Fe foils as substrates, carbon deposits of different crystal structures were obtained. The conditions for growing single-crystal graphite layers and single- and multilayer graphenes are found by analyzing deposits by Raman scattering.

Defect modulated Raman response of KC8 single crystals

AbstractIn situ experiments of graphite intercalation with potassium have been performed under high vacuum (HV) conditions. A detailed Raman lineshape analysis of the G‐line was applied to determine the intrinsic response of the stage one KC8 compound. Four defect‐modulated graphite single crystals were analyzed. We observe five vibrational modes (∼560, 1260, 1510, 1547, and 1563 cm−1). From a Breit–Wigner–Fano (BWF) lineshape analysis we elucidate the contribution of each vibrational mode observed. The intensity of the z‐axis mode (CZ) around ∼560 cm−1 varies depending on the defect modulated nominal KC8 crystal chosen. We prove that the intrinsic G‐line of KC8 is at 1510 cm−1, and it is strongly dependent on the actual defect content in the sample, which has important implications for the electron phonon coupling (EPC) responsible of superconductivity in this system.

Carbon isotope anatomy of a single graphite crystal in a metapelitic migmatite revealed by high-spatial resolution SIMS analysis

High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated from a partially melted metamorphic rock. SIMS 13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed to have resulted from the difference in diffusivity between 12C and 13C in the growth medium to the interface of graphite precipitation in different growth sectors.

How to organize a neutron imaging user lab? 13 years of experience at PSI, CH

PSI has a relatively long tradition in neutron imaging since the first trials were done at its formerly existing research reactor SAPHIR with film methods. This reactor source was replaced after its shutdown in 1994 by the spallation neutron source SINQ in 1996, driven by the 590 MeV cyclotron for protons with presently up to 2.3 mA beam current. One of the first experimental devices at SINQ was the thermal neutron imaging facility NEUTRA, which was designed from scratch and has been the first device of its kind at a spallation source. Until now, NEUTRA has been successfully in use for many investigations in a wide range of studies covering fuel cell research, environmental behavior of plants, nuclear fuel inspection and the research on cultural heritage objects. It has been the host of PhD projects for students from all over Europe for years. In a previous meeting it has been offered as a European reference facility. Some of its features were really adapted to the layout of new installations. In 2004, it was possible to initiate the project of a second beam line at SINQ for imaging with cold neutrons. Previous studies have shown the potential of this option in order to broaden the user profile and to extend the scientific basis for neutron imaging. It was inaugurated with a workshop at PSI in 2005. The user service was started at the facility ICON in 2006. Beside the setup, installation and optimization of the facilities, the organization of the user program plays an important role. The two neutron imaging beam lines are equal installations at SINQ among the 14 scientific devices. Therefore, the user approach is organized via “calls for proposals”, which are sent out each half year via the “Digital User Office (DUO)” (see http://duo.web.psi.ch). The evaluation of the proposals is done by the “Advisory Committee for Neutron Imaging (ACNI)” consisting of 6 external and PSI internal members. Further requests are given by industrial collaborations. This beam time allocation is handled more directly and in time in order to fulfill the companies’ demands. Here, the confidentiality plays a more important role than in scientific studies that are done with the aim of a free publication. It has been possible to earn money regularly from the industrial projects in order to cover the salary cost of some positions within the NIAG group. The permanent improvement of the methodology and performance in neutron imaging is a third major activity of the NIAG team. Running projects in this direction are the permanent insert of a grating interferometry device, improved energy selection with the help of single graphite crystals and utilization of the beam line BOA at SINQ for the energy range between 4 and 15 Å.

Aharonov-Bohm effect on columnar defects in thin graphite and graphene

We studied Aharonov-Bohm effect on thin graphite single crystals containing columnar defects (nanoholes) produced by irradiation with heavy ions. The thickness of irradiated single crystals has been varied from 50 nm down to less then 1 nm by thinning the thicker samples with beam-plasma etching. For irradiated samples we found the existence of oscillating, field-periodic contribution of magnetoresistance with a periodicity corresponding to the flux quantum hc/e per area of nanohole independently of sample thickness. The results point to the significant contribution of Dirac fermions to the oscillating part of magnetoresistance and also to the existence of the edge states located near the nanohole perimeter.

Electric-field-induced formation of a conducting channel in a dielectric polymer film

The composition of the products of destruction of a poly(styrene) film at the edge and inside the electric breakdown channel has been investigated using Raman spectroscopy. It has been demonstrated that, apart from the compounds with double carbon-carbon bonds, the breakdown initiates the formation of aromatic cycles having two to six aromatic rings, graphite nanocrystals with a linear size not above ten aromatic rings, and, finally, graphite single crystals. This suggests that the temperature in the breakdown channel exceeds 1800 K and that the current density in it is ∼106 A/cm2.

Oxidation Stability of Multiwalled Carbon Nanotubes for Catalytic Applications

The oxidation of multiwalled carbon nanotubes (CNTs) was investigated with regard to a detailed prediction of the lifetime of this material as a catalyst for oxidative dehydrogenations. A power-law kinetics is found to be adequate for the description of CO2 formation in the temperature range of 623−823 K and under O2 partial pressures of 0.025−0.6 bar. The stability against oxidation can be enhanced by passivation with B2O3 or P2O5 and by high temperature treatment. The progress of oxidative degradation was monitored by TEM and Raman spectroscopy. A mechanistic study supported by high pressure XPS and SSITKA reveals full agreement with the established model of the oxidation of conventional carbon materials; however, the theory of sequential layer degradation as observed for single crystal graphite is not transferable to a technical grade CNT material, and instead, various modes of propagation of combustion sites are identified.

Polysynthetic nature of stage-one graphite-metal intercalation compounds prepared from graphite single crystals

Polysynthetic nature of stage-one graphite-metal intercalation compounds prepared from graphite single crystals

Electrocapillary maximum and potential of zero charge of carbon aerogel

We present an electrochemical study of carbon aerogel (CA) in aqueous sodium fluoride solutions, focusing on the comparison of two quantities that are related to the potential of zero charge (pzc): the capacitance minimum and the 'electrocapillary maximum' of the surface forces. Capacitance minima are well resolved in our samples. Their potential emerges reproducibly as around 90 mV (vs. Ag/AgCl in KCl), similar to the value, 70 mV, of bulk glassy carbon which we use for comparison, and similar to previous reported pzc values for carbon materials. Significantly, no electrocapillary maximum is found in this potential range. This demonstrates that the pzc does not necessarily coincide with the potential of the maximum of surface stress. We also determined the area-specific capacitances, c(a) = 2.8 microF cm(-2), which agrees well with reports for the basal-plane of graphite single crystals. Our experiments yield large reversible strain amplitudes, up to 0.45%.

To the description of multi-layered nanotubes in models of cylindrically anisotropic elasticity

The atomic structure and mechanical behavior of multi-layered carbon nanotubes are in many ways similar to the structure and behavior of graphite single crystals. Therefore, just as various methods of discrete atomic simulation can successfully be applied to nanotubes, so also can methods of continuum mechanics be applied to carbon nanotubes with rather thick walls and diameters greater than several nanometers. In continual description of nanotubes, the model of a thin cylindrical shell has gained wide acceptance. An alternative analytical model in recent use is the model of cylindrically anisotropic hollow rods elaborated by S. Lekhnitskii in the framework of classical anisotropic elasticity. In the works based on the above model, torsion and tension problems for cylindrical tubes made of graphite-like materials with rhombohedral (in particular, hexagonal) symmetry were solved. In the paper, we perform a more thorough analysis of two different solutions of torsion and tension problems for nanotubes with two types of cylindrical anisotropy. Consideration is given to a strong dependence of the elastic characteristics of carbon nanotubes on the type of curvilinear anisotropy. In particular, it is shown that the discrepancy in the results found in relevant papers is due to the difference in the types of curvilinear anisotropy used.

New Cavity Design for Broad-Band Quasi-Optical HF-EPR Spectroscopy

The design and performance of two new devices adapted for high-frequency electron paramagnetic resonance (HF-EPR) in a broad frequency range are described. Both systems, a Fabry–Perot resonator and a rotating sample holder, rely on similar construction schemes and on the use of submicrometer piezoelectric positioners. A study of a single crystal of graphite allows illustrating the operation of these two new systems associated with our quasi-optical HF-EPR spectrometer.

Field-periodic magnetoresistance oscillations in thin graphite single crystals with columnar defects

The magnetoresistance of thin graphite single crystals with columnar defects has been studied. The field-periodic contribution to the magnetoresistance with a period of 7.5 T has been revealed. The atomic force microscope estimation of the columnar defect diameter shows that the period of the contribution oscillatory as a function of the flux is close to hc/e per defect. The result agrees with the measurements of Aharonov-Bohm magnetoresistance oscillations in graphene mesoscopic rings [S. Russo et al., Phys. Rev. B 77, 085413 (2008)].

Formation of quasi-free graphene on the Ni(111) surface with intercalated Cu, Ag, and Au layers

This paper reports on a study by angle-resolved photoelectron and low-energy electron energy loss spectroscopy of graphene monolayers, which are produced by propylene cracking on the Ni(111) surface, followed by intercalation of Cu, Ag, and Au atoms between the graphene monolayer and the substrate, for various thicknesses of deposited metal layers and annealing temperatures. It has been shown that the spectra of valence-band π states and of phonon vibrational modes measured after intercalation become similar to those characteristic of single-crystal graphite with weak interlayer coupling. Despite the strong coupling of the graphene monolayer to the substrate becoming suppressed by intercalation of Cu and Ag atoms, the π state branch does not reach at the K point of the Brillouin zone the Fermi level, with the graphene coating itself breaking up partially to form graphene domains. At the same time after intercalation of Au atoms, the electronic band structure approaches the closest to that of isolated graphene, with linear π-state dispersion near the K point of the Brillouin zone, and the point of crossing of the filled, (π), with empty, (π*), states lying in the region of the Fermi level, which makes this system a promising experimental model of the quasi-free graphene monolayer.

Topology of nanometric graphite films

This work submits the results of a study in which the combinational scattering (CS) spectroscopy and electronic and optical microscopy of nanometric single-crystal graphite films obtained by precipitating in gas phase were used. It is shown the graphite material samples synthesized in this manner are fine, highly streamlined films that grow alongside of the substrate surface. The films feature a fairly smooth surface and thicknesses of 100-1 atomic layers. The study of the topologic properties of the material, such as the formation of folds along the entire surface and the occurrence of waveform ripples on some surface areas, showed that they may appear due to large differences in the carbon film and nickel substrate thermal expansion factors. The film thickness was assessed for the observed parameters of the periodic structures and the results comply with data obtained using other techniques. In some areas of the material in study, the graphene layers spontaneously split and form bubbles. The results of the CS analysis of various bubbles on studied film surfaces are evidence that, similar to the film itself, the walls of bubbles are composed of different numbers of graphite layers. It is shown that transparent bubbles in which meshlike topology is observed are formed with a single atomic layer.

Implicit spatial averaging in inversion of inelastic x-ray scattering data

Inelastic x-ray scattering (IXS) is now a widely used technique for studying the dynamics of electrons in condensed matter. We previously posed a solution to the phase problem for IXS [P. Abbamonte et al., Phys. Rev. Lett. 92, 237401 (2004)] that allows explicit reconstruction of the density propagator of a system. The propagator represents, physically, the response of the system to an idealized, point perturbation, so provides direct, real-time images of electron motion with attosecond time resolution and $\text{\AA{}}$-scale spatial resolution. Here we show that the images generated by our procedure, as it was originally posed, are spatial averages over all source locations. Within an idealized, atomiclike model, we show that in most cases a simple relationship to the complete, unaveraged response can still be determined. We illustrate this concept for recent IXS measurements of single-crystal graphite.

Effect of hydrogenation on structure and superconducting properties of CaC6

We report the effects of hydrogenation on the structure and superconducting properties of good quality bulk superconducting CaC6 samples prepared from Madagascar graphite single crystals using a Ca-metal vapour transport method. The hydrogenation of a CaC6 sample takes several hours during which, at different hydrogen exposure times, the structural and superconducting studies were carried out using X-ray diffraction, Raman scattering, and temperature and field dependence of magnetization measurements. These studies reveal that only a mixture of CaH2 and graphite is obtained upon hydrogenation of CaC6. The successive suppression of superconducting volume fraction and critical temperature is observed when increasing the hydrogenation time. The results reveal that there is a definite relation between structure and superconductivity in the hydrogenated CaC6 samples. The changes in stoichiometry and inevitable Ca-sub-lattice disorder during the deintercalation process are considered to be the main sources for the change in the superconducting transition temperature.

Nanoscale Mechanical Drumming Visualized by 4D Electron Microscopy

With four-dimensional (4D) electron microscopy, we report in situ imaging of the mechanical drumming of a nanoscale material. The single crystal graphite film is found to exhibit global resonance motion that is fully reversible and follows the same evolution after each initiating stress pulse. At early times, the motion appears "chaotic" showing the different mechanical modes present over the micron scale. At longer time, the motion of the thin film collapses into a well-defined fundamental frequency of 1.08 MHz, a behavior reminiscent of mode locking; the mechanical motion damps out after approximately 200 micros and the oscillation has a "cavity" quality factor of 150. The resonance time is determined by the stiffness of the material, and for the 75 nm thick and 40 microm square specimen used here we determined Young's modulus to be 1.0 TPa for the in-plane stress-strain profile. Because of its real-time dimension, this 4D microscopy should have applications in the study of these and other types of materials structures.

Peeling Single-Stranded DNA from Graphite Surface to Determine Oligonucleotide Binding Energy by Force Spectroscopy

We measured the force required to peel single-stranded DNA molecules from single-crystal graphite using chemical force microscopy. Force traces during retraction of a tip chemically modified with oligonucleotides displayed characteristic plateaus with abrupt force jumps, which we interpreted as a steady state peeling process punctuated by complete detachment of one or more molecules. We were able to differentiate between bases in pyrimidine homopolymers; peeling forces were 85.3 - 4.7 pN for polythymine and 60.8 +/- 5.5 pN for polycytosine, substantially independent of salt concentration and the rate of detachment. We developed a model for peeling a freely jointed chain from the graphite surface and estimated the average binding energy per monomer to be 11.5 +/- 0.6 k(B)T and 8.3 +/- 0.7 k(B)T in the cases of thymine and cytosine nucleotides, respectively. The equilibrium free-energy profile simulated using molecular dynamics had a potential well of 18.9 k(B)T for thymidine, showing that nonelectrostatic interactions dominate the binding. The discrepancy between the experiment and theory indicates that not all bases are adsorbed on the surface or that there is a population of conformations in which they adsorb. Force spectroscopy using oligonucleotides covalently linked to AFM tips provides a flexible and unambiguous means to quantify the strength of interactions between DNA and a number of substrates, potentially including nanomaterials such as carbon nanotubes.