Pristine Highly Oriented Pyrolytic Graphite Research Articles

Evaluation of damage induced in graphite due to sample preparation by standard non-destructive techniques

Damage introduced to graphite by mechanical polishing, argon ion beam polishing, fracture, and neutron bombardment has been studied in polycrystalline graphites and Highly Oriented Pyrolytic Graphite (HOPG). Scanning Electron Microscopy and Atomic Force Microscopy, as well as X-Ray Diffraction and Raman spectroscopy, were employed. The least disturbed surfaces are observed in pristine HOPG or by fracturing techniques that exposed pre-existing defects. A decrease of the mean particle size of the abrading medium and the use of ions and neutrons increased the D band and decreased the pseudo–crystallinity.

Disorder in H+-irradiated HOPG: effect of impinging energy and dose on Raman D-band splitting and surface topography.

Disorder was induced in pristine highly oriented pyrolytic graphite (HOPG) by irradiation with H+ ions with energies of 0.4 MeV and 1 MeV, and doses of 1014 ions/cm2 and 1016 ions/cm2. Raman spectroscopy was used as the main technique to characterize different samples and gain new insights on the splitting of the D band into two components (D1 and D2), trying to correlate this feature of the vibrational spectrum with the impinging energy and dose. An increased ID2/IG ratio in comparison with ID1/IG was observed in the irradiated samples. This behavior indicates that the impinging energy mainly affects the D1 component, while the D2 component is strongly dominated by the dose. We expect a larger contribution of defects (originating from the rupture of C–C sp2 symmetry through the formation of C–H sp3 bonds) to the D2 component than to the D1 component. SQUID measurements of the irradiated samples showed an enhancement in the normalized remanence, as well as an increment in coercivity compared to pristine HOPG, consistent with H+-induced point-like defects as well as C–H bonds. AFM scanning after Raman and SQUID characterization showed a distribution of surface defects, which were ascribed to the burst of hydrogen blisters formed as a consequence of the irradiation process. The results presented in this work contribute to the current trend in nanotechnology in areas devoted to the control of properties by defect engineering in carbon-based materials.

Adsorption of Ultrathin Ethylene Carbonate Films on Pristine and Lithiated Graphite and Their Interaction with Li.

Aiming at a better understanding of the solid-electrolyte interphase formation in Li-ion batteries, we have investigated the interaction of ultrathin films of ethylene carbonate (EC), which is a key solvent of battery electrolytes, with pristine and lithiated highly oriented pyrolytic graphite (HOPG) and with postdeposited Li. Employing X-ray and ultraviolet photoelectron spectroscopy as well as Fourier transform infrared spectroscopy under ultrahigh-vacuum conditions, in combination with density functional theory (DFT)-based calculations, we find that EC adsorbs molecularly intact on pristine HOPG in the entire temperature range between 80 K and desorption at 200 K. Features in the ultraviolet photoelectron spectra could be related to the molecular orbitals of EC obtained from DFT calculations, and a similar adsorption/desorption behavior is obtained also on lithiated HOPG. In contrast, stepwise postdeposition of ∼0.5 and one monolayer of Li0 on a preadsorbed EC adlayer leads not only to stabilization of Li+/Liδ+ at the surface, possibly as adsorbed Li+(EC) n species, but also to EC decomposition, forming products such as Li2CO3, ROCO2Li (CH2OCO2Li)2, and Li2O. Consequences on the electronic surface properties and on the stabilization of the resulting adlayer are discussed. Upon annealing up to room temperature, some residual Li-containing decomposition products remain on the surface, which is considered as the initial stage of the solid|electrolyte interphase formation at the electrode|electrolyte interface.

Trace element content and magnetic properties of commercial HOPG samples studied by ion beam microscopy and SQUID magnetometry

In this study, the impurity concentration and magnetic response of nine highly oriented pyrolytic graphite (HOPG) samples with different grades and from different providers were determined using ion beam microscopy and SQUID magnetometry. Apart from sideface contaminations in the as-received state, bulk contamination of the samples in most cases consists of disk-shaped micron-sized particles made of Ti and V with an additional Fe contamination around the grain perimeter. The saturation magnetization typically increases with Fe concentration, however, there is no simple correlation between Fe content and magnetic moment. The saturation magnetization of one, respectively six, out of nine samples clearly exceeds the maximum contribution from pure Fe or Fe3C. For most samples the temperature dependence of the remanence decreases linearly with T – a dependence found previously for defect-induced magnetism (DIM) in HOPG. We conclude that apart from magnetic impurities, additional contribution to the ferromagnetic magnetization exists in pristine HOPG in agreement with previous studies. A comparative study between the results of ion beam microscopy and the commonly used EDX analysis shows clearly that EDX is not a reliable method for quantitative trace elemental analysis in graphite, clarifying weaknesses and discrepancies in the element concentrations given in the recent literature.

Bromine doping of multilayer graphene for low-resistance interconnects

Multilayer graphene (MLG) is expected to be a low-resistance and high-reliability interconnect material replacing copper (Cu) in nanoscale interconnects. To achieve low-resistance interconnect with MLG, carrier doping is necessary since the carrier concentrations in pristine MLGs are low. In this work, the effects of bromine (Br) doping by intercalation on the carrier concentration and sheet resistance of exfoliated highly oriented pyrolytic graphite (HOPG) were investigated by measuring the Fermi level shift using ultraviolet photoelectron spectroscopy (UPS) and the four-terminal method, respectively. The Fermi level was shifted downward up to 0.63 eV, and the sheet resistance was reduced to less than 10% of that of the pristine HOPG by increasing the Br concentration. A similar Fermi level shift was observed for the commercially available CVD-MLG. Br doping is thus promising for low-resistance MLG interconnects from the perspective of carrier doping.

Intermolecular interactions dictating adhesion between ZnO and graphite

Zinc oxide (ZnO) was recently demonstrated to strongly adhere to both pristine graphitic surfaces and oxidized surfaces; however, the bonding interaction between the two is unknown and has yet to be investigated. In this study, atomic force microscopy (AFM) liftoff in the dry state was performed with a ZnO coated AFM tip on highly oriented pyrolytic graphite (HOPG) such that the bonding interaction can be identified and used to design high strength interfaces. It is found that the ZnO film on the AFM tip becomes negatively charged after contacting an oxygen plasma treated HOPG surface, and once charged can strongly interact with other surfaces such as pristine HOPG through induced polarization to create high adhesive energy.

Production of sp3 hybridization by swift heavy ion irradiation of HOPG

Highly oriented pyrolytic graphite (HOPG) samples were irradiated by swift heavy ions (86Kr, 209Bi and 238U) with the fluence of 1011–1013ions/cm2 at room temperature. The production of sp3 hybridization by the irradiation process has been confirmed directly by X-ray photoelectron spectroscopy (XPS). In this work, both irradiated and pristine HOPG samples were investigated by XPS and Raman spectroscopy. The existence of sp3 component is confirmed on the surface of the irradiated HOPG samples. XPS result shows that the acreage ratio Isp3/Isp2 increases with the ion fluence and saturates at a higher value of irradiation. It is found that the amount of hybridization (Isp3/Isp2) strongly depends on the electronic energy loss in the sample. Raman spectra of the irradiated samples show the increasing of acreage ratio ID/IG with the ion fluence, which indicates the change of the atomic structure and the phase transition from sp2 to sp3.

Water desorption from nanostructured graphite surfaces

Water interaction with nanostructured graphite surfaces is strongly dependent on the surface morphology. In this work, temperature programmed desorption (TPD) in combination with quadrupole mass spectrometry (QMS) has been used to study water ice desorption from a nanostructured graphite surface. This model surface was fabricated by hole-mask colloidal lithography (HCL) along with oxygen plasma etching and consists of a rough carbon surface covered by well defined structures of highly oriented pyrolytic graphite (HOPG). The results are compared with those from pristine HOPG and a rough (oxygen plasma etched) carbon surface without graphite nanostructures. The samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The TPD experiments were conducted for H2O coverages obtained after exposures between 0.2 and 55 langmuir (L) and reveal a complex desorption behaviour. The spectra from the nanostructured surface show additional, coverage dependent desorption peaks. They are assigned to water bound in two-dimensional (2D) and three-dimensional (3D) hydrogen-bonded networks, defect-bound water, and to water intercalated into the graphite structures. The intercalation is more pronounced for the nanostructured graphite surface in comparison to HOPG surfaces because of a higher concentration of intersheet openings. From the TPD spectra, the desorption energies for water bound in 2D and 3D (multilayer) networks were determined to be 0.32 ± 0.06 and 0.41 ± 0.03 eV per molecule, respectively. An upper limit for the desorption energy for defect-bound water was estimated to be 1 eV per molecule.

Electrochemical behavior of N and Ar implanted highly oriented pyrolytic graphite substrates and activity toward oxygen reduction reaction

In this paper nitrogen and argon implanted highly oriented pyrolytic graphite (HOPG) electrodes have been studied, with the aim of distinguishing whether the electrocatalytic activity of the modified electrodes is preferentially driven by chemical defects or by morphological defects. Modified HOPG electrodes have been prepared by ion implantation and fully characterized by photoemission spectroscopy and scanning tunneling microscopy. Several different N-based defects have been identified and characterized in terms of their thermal stability. The modified electrodes were electrochemically characterized considering the electron-transfer kinetics of two redox probes Ru(III)(NH3)6Cl3 and K4Fe(II)(CN)6: both types of electrodes show an increased performance in terms of standard rate constant k0 with respect to pristine HOPG for the oxidation of Fe(CN)64− and this effect has been exclusively related to a morphological effect. N-implanted HOPG electrodes show an increased reactivity toward oxygen reduction reaction (ORR), the onset potential being more positive with respect to both Ar-implanted HOPG and pristine HOPG (0.09 and 0.28V, respectively) and follow a four electrons reduction pathway to H2O. The results indicate that morphological as well as chemical defects are important factors for influencing the ORR kinetics. However, the enhanced ORR activity at N-HOPG suggests a pivotal role played by the N-based chemical defects.

Comment on “Revealing common artifacts due to ferromagnetic inclusions in highly oriented pyrolytic graphite” by Sepioni M. et al.

This comment addresses several issues in the paper by Sepioni et al., where it is stated that the ferromagnetism in pristine highly oriented pyrolytic graphite (HOPG) reported by several groups in the previous years is most likely due to impurity contamination. In this comment, clear arguments are given why this statement is not justified. Furthermore, it is pointed out, that there are already measurements using element-sensitive microscopic techniques, e.g. X-ray Magnetic Circular Dichroism (XMCD) that directly proved the intrinsic origin of the ferromagnetism in graphite, also in pristine HOPG.

Surface modification of highly oriented pyrolytic graphite by reaction with atomic nitrogen at high temperatures

Dry etching of {0001} basal planes of highly oriented pyrolytic graphite (HOPG) using active nitridation by nitrogen atoms was investigated at low pressures and high temperatures. The etching process produces channels at grain boundaries and pits whose shapes depend on the reaction temperature. For temperatures below 600°C, the majority of pits are nearly circular, with a small fraction of hexagonal pits with rounded edges. For temperatures above 600°C, the pits are almost exclusively hexagonal with straight edges. The Raman spectra of samples etched at 1000°C show the D mode near 1360cm−1, which is absent in pristine HOPG. For deep hexagonal pits that penetrate many graphene layers, neither the surface number density of pits nor the width of pit size distribution changes substantially with the nitridation time, suggesting that these pits are initiated at a fixed number of extended defects intersecting {0001} planes. Shallow pits that penetrate 1–2 graphene layers have a wide size distribution, which suggests that these pits are initiated on pristine graphene surfaces from lattice vacancies continually formed by N atoms. A similar wide size distribution of shallow hexagonal pits is observed in an n-layer graphene sample after N-atom etching.

3D-Hydrogen analysis of ferromagnetic microstructures in proton irradiated graphite

Recently, magnetic order in highly oriented pyrolytic graphite (HOPG) induced by proton broad- and microbeam irradiation was discovered. Theoretical models propose that hydrogen could play a major role in the magnetism mechanism. We analysed the hydrogen distribution of pristine as well as irradiated HOPG samples, which were implanted to μm-sized spots as well as extended areas with various doses of 2.25MeV protons at the Leipzig microprobe LIPSION. For this we used the sensitive 3D hydrogen microscopy system at the Munich microprobe SNAKE. The background hydrogen level in pristine HOPG is determined to be less than 0.3 at-ppm. About 4.8×1015H-atoms/cm2 are observed in the near-surface region (4μm depth resolution). The depth profiles of the implants show hydrogen located within a confined peak at the end of range, in agreement with SRIM Monte Carlo simulations, and no evidence of diffusion broadening along the c-axis. At the sample with microspots, up to 40at.% of the implanted hydrogen is not detected, providing support for lateral hydrogen diffusion.

The intrinsic temperature effect of the Raman spectra of graphite

The Raman spectra of ion-implanted highly oriented pyrolytic graphite (HOPG) are reported, in which an additional mode at 1083 cm−1 and three doublet structures in the positions of ∼1350, ∼2450, and ∼2710 cm−1 are revealed. Noticeable frequency shifts are observed for all the Raman bands between the spectra excited with different laser powers, which are interpreted as the pure temperature effect and a downshift in the C–C stretching frequency induced by the thermal expansion. Moreover, the pure temperature effect (dω/dT)V without anharmonic contribution is achieved in pristine HOPG. The results suggest that the pure temperature effect without anharmonic contribution plays an important role in the frequency shifts with temperature.

Preparation, structure, and electrical conductivity of vanadium fluoridegraphite intercalation compound

Graphite intercalation compounds (GIC) of vanadium fluoride have been prepared in a fluorine atmosphere. The GICs prepared from highly oriented pyrolytic graphite (HOPG) were stage 1–8 compounds with composition, C 8.4–79.5VF 5.8–6.0. The apparent size of intercalated VF 6 −(d i) decreased from 5.33 Å to 4.15 Å along the c axis with increasing x in C xVF 6. Various intercalated structures of VF 6 − between the carbon layers have been proposed for this change in d i values. The compounds with small d i around 4.2 Å show high stability in the air, which is due to the nestling of the VF 6 − anion between the carbon layers. Electron diffraction measurements have indicated that a well-nestled stage 2 GIC has high regularity in orientation of intercalated VF 6 − anions, which make two large unit cells of the hexagonal system ( a 0:8.868 A ̊ ) with the different vectors by ±14° from that of graphite lattice. The 19F-NMR spectra and X-ray diffraction data in the low temperature region suggest a reversible phase transition. The highest electrical conductivity was 1.97 × 10 5 S cm −1, which is 12 times that of pristine HOPG.

Magnetostriction of pristine graphite and SbCl 5-graphite intercalation compounds

Magnetostrictive strain in pristine highly oriented pyrolytic graphite (HOPG) and SbCl 5-graphite intercalation compounds (GIC's) has been studied in steady magnetic fields up to 10 tesla, including the quantum oscillation effect. Fourier spectra of the magneto-oscillations are stage dependent and in agreement with those reported by Takahashi et al. For pristine HOPG, the monotonie component of the longitudinal magnetostrictive strain (LMS) parallel to the c-axis is found to be negative and to increase in magnitude with increasing width at half maximum intensity of the (002) line of X-ray diffraction. This result is explained qualitatively within the framework of the Keyes theory by considering the reduction of the static shear modulus of graphite due to the displacement of basal-plane dislocations. The stress coefficient of the majority hole Fermi surface is determined as δ In S h δσ [0001] = 0.034 ± 0.006 kbar − by using magnetostriction and torque de Haas-van Alphen measurements.