Infrared ellipsometry of highly oriented pyrolytic graphite

We study the anisotropic response of highly oriented pyrolytic graphite (HOPG) in mid-infrared range, utilizing the potential of spectroscopic ellipsometry. We aim at the parameters of the infrared-active vibrations of E_1u and A_2u symmetry, and examine the phenomena related to the strong uniaxial anisotropy of HOPG. We explain the appearance of the weakly polar out-of-plane vibration in the elipsometric spectra taken on cleaved planes. We find a detectable Fano-type behavior of the in-plane phonon.     

超声剥离二次膨胀石墨制备石墨烯纳米片

以天然鳞片石墨为原料,氧化插层制备可膨胀石墨,微波热解膨胀后,对膨胀石墨进行二次氧化插层并微波膨胀,采用超声剥离法制备出包含大量少层数碳原子的石墨烯纳米片。采用傅里叶红外光谱(FT-IR)、X射线衍射(XRD)、扫描电镜(SEM)、原子力显微镜(AFM)和拉曼光谱(Raman)对其结构和形貌进行分析。结果表明,氧化插层增大了石墨层间距,膨胀石墨更易于进一步氧化插层引入含氧基团;在微波作用下,石墨内部含氧基团热分解放出气体,进一步增大石墨层间距,甚至部分剥离;对二次膨胀处理的石墨薄片进行超声剥离可得到石墨烯纳米片,大部分石墨烯层数低于5层。     

Field emission from few-layer graphene nanosheets produced by liquid phase exfoliation of graphite

Graphene nanosheets have been synthesized from commercial expandable graphite by heating in a microwave oven and dispersing in ethanol by ultrasonication. Scanning and transmission electron microscopy and electron energy-loss spectroscopy and atomic force microscope showed that the nanosheets were about 2 nm in thickness and 10 microm in diameter. The field emission of the graphene sheets has been investigated. An emission current density of 1 mA/cm2 has been achieved at an electric field of 3.7 V/microm with a turn-on field of 1.7 V/microm at 0.01 mA/cm2. The annealing of the samples at 400 degrees C in vacuum greatly improved the field emission performance.     

Self-assembly of vertically aligned nanorod supercrystals using highly oriented pyrolytic graphite

Supercrystallization of CdS nanorods (10 nm x 25 nm) into perpendicular superlattices was obtained by controlled evaporation of a nanorod solution trapped between a smooth substrate and a block of highly oriented pyrolytic graphite (HOPG). Hexagonal oriented domains 2 microm2 in size were routinely obtained on a variety of substrates without external electric fields.     

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.     

Randomly oriented graphene flakes film fabrication from graphite dispersed in N-methyl-pyrrolidone by using electrohydrodynamic atomization technique

In this work, we report the deposition of graphene flakes exfoliated through graphite dispersion in N-methyl-pyrrolidone using non-vacuum electrohydrodynamic atomization (EHDA) technique. Stable cone jet mode of EHDA is used to deposit graphene flakes on silicon substrate. The deposited graphene flakes film is characterized by Raman spectroscopy, microscopy, 3D-Nanomap, scanning electron microscope, and UV–visible spectroscopy. Through characterizations it is evident that a randomly oriented graphene flakes film has shown good transparency, conductivity and suitable work function. For electrical characterization of film, it is employed as cathode in a simple diode indium tin oxide/(poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate)/polydioctylfluorene-benzothiadiazole/graphene. It is observed that at voltage of 0.3 V, the current density in device is at low value of 2.67 A/cm² however as the voltage is increased to a value of 4 V the current density is increased by almost 100 times and reaches up to 2.65 × 10² A/cm². We believe that by further optimizing parameters of EHDA techniques for graphene deposition, more uniform and defect free graphene film can be obtained.     

High-yield production of graphene by liquid-phase exfoliation of graphite

Fully exploiting the properties of graphene will require a method for the mass production of this remarkable material. Two main routes are possible: large-scale growth or large-scale exfoliation. Here, we demonstrate graphene dispersions with concentrations up to approximately 0.01 mg ml(-1), produced by dispersion and exfoliation of graphite in organic solvents such as N-methyl-pyrrolidone. This is possible because the energy required to exfoliate graphene is balanced by the solvent-graphene interaction for solvents whose surface energies match that of graphene. We confirm the presence of individual graphene sheets by Raman spectroscopy, transmission electron microscopy and electron diffraction. Our method results in a monolayer yield of approximately 1 wt%, which could potentially be improved to 7-12 wt% with further processing. The absence of defects or oxides is confirmed by X-ray photoelectron, infrared and Raman spectroscopies. We are able to produce semi-transparent conducting films and conducting composites. Solution processing of graphene opens up a range of potential large-area applications, from device and sensor fabrication to liquid-phase chemistry.     

Effect of radiation damage on ion-induced electron emission from highly oriented pyrolytic graphite

The dependences of ion-electron emission yields γ for highly oriented pyrolytic graphite (HOPG) under 30 keV N₂⁺ ion irradiation on target temperature at different ion incidence angles (0-80°) have been measured. The fluences were 10¹⁸-10¹⁹ ion/cm² and irradiation temperatures were varied from room temperature to 400 °C. At normal ion incidence a step-like increase of yield, analogous to the γ-behaviour for polygranular graphites, has been found at an annealing temperature T_a. This effect and the changes of γ(T) with ion incidence angle are discussed in terms of the change of electron path length and the transparency of the lattice for ion beams with increasing degree of lattice order.     

Growth of self-assembled Mn, Sb and MnSb nanostructures on highly oriented pyrolytic graphite

The surface morphology of Mn, Sb and MnSb nanostructures grown on highly-oriented pyrolytic graphite (HOPG) has been studied in detail with scanning tunneling microscopy in ultrahigh vacuum. At the initial stage of growth, three dimensional (3D) clusters and islands of Mn, Sb and MnSb are formed at step edges and other defect sites. In middle stage, crystalline 2D islands and 1D nanorods of Sb can be observed together with crystalline 3D islands, whereas single- and double-layer cluster chains of Mn are obtained at room temperature (RT). MnSb crystallites form after deposition of both Mn and Sb on HOPG surface and annealing at T ~ 375 K. The ex-situ X-ray photoemission spectroscopy measurement revealed the formation of MnSb compound on graphite. Ferromagnetism was observed at RT for the 50-nm MnSb thin film using vibrating sample magnetometer.     

Effect of 305-MeV krypton ions on the surface of highly oriented pyrolytic graphite

The surface of highly oriented pyrolytic graphite (HOPG) irradiated by 305-MeV krypton ions was studied in a scanning tunneling microscope. It was found that inelastic energy losses of the krypton ions do not significantly affect the sputtering of carbon from the surface of crystalline HOPG grains. Similarly to the case of metals, preferential sputtering takes place in the grain boundaries. The inhomogeneous sputtering of polycrystalline conductors has to be taken into account in interpreting experimental data on the sputtering of such targets by high-energy heavy ions in the range of inelastic energy losses. The results can be also implemented in the technology of ion implantation into deep layers of solids.     

High-quality production of graphene by liquid-phase exfoliation of expanded graphite

As one of the most promising candidates, graphene exhibits a potential application in post-silicon nanoelectronics. However, it is a key issue to produce high-quality graphene in large scale. Here, a facile method is demonstrated to produce graphene dispersions by exfoliation of expanded graphite in the co-solvent with N,N-dimethylformamide (DMF) and water. We confirm that the optimal ratio of DMF to water for graphene exfoliation is 9:1 (v:v) by means of UV–Vis absorption spectra. This exfoliation results in large flakes ∼2 μm in diameter, which can potentially be improved by adjusting the sonication power. The relatively perfect hexagonal structure of graphene is confirmed by Raman spectroscopy and the as-prepared graphene nanosheet film the as-prepared graphene nanosheet film possesses good electrical conductivity (∼8.3 × 10³ S m⁻¹). DC electrical transport phenomena for the deposited film of graphene nanosheets are well described in terms of conduction models for non-crystal semiconductor. This convenient approach provides an extensive route to prepare high-quality graphene nanosheets.     

Epitaxial growth of p-sexiphenyl film on highly oriented pyrolytic graphite surface studied by scanning tunneling microscopy

The epitaxial growth of p-sexiphenyl (C₃₆H₂₆, 6P) on highly oriented pyrolytic graphite (HOPG) surface has been investigated by scanning tunneling microscopy (STM). 6P molecules prefer epitaxial growth with the long axis along the [110] direction (armchair direction) of the HOPG substrate, with the unit cell parameters b₁ = 0.67 ± 0.06 nm, b₂ = 5.97 ± 0.06 nm and angle of 88 ± 3° between them. The relation of the 6P overlayer lattice vectors with the HOPG substrate has also been deduced, i.e. the 5 × 1 supercell is in a point-on-point commensurate relation with respect to the HOPG substrate surface.     

Growth and characterization of stoichiometric BCN films on highly oriented pyrolytic graphite by radiofrequency plasma enhanced chemical vapor deposition

Hexagonal boron carbonitride (h-BCN) hybrid films have been synthesized on highly oriented pyrolytic graphite by radiofrequency plasma enhanced chemical vapor deposition using tris-(dimethylamino)borane as a single-source molecular precursor. The films were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopic measurements. XPS measurement showed that the B atoms were bonded to C and N atoms to form the sp²-B-C-N atomic hybrid chemical environment. The atomic composition estimated from the XPS of the typical sample was found to be almost B₁C₁N₁. NEXAFS spectra of the B K-edge and the N K-edge had the peaks due to the π* and σ* resonances of sp² hybrid orbitals implying the existence of the sp² hybrid configurations of h-BCN around the B atoms. The G band at 1592 and D band at 1352 cm^− 1 in the Raman spectra also suggested the presence of the graphite-like sp²-B-C-N atomic hybrid bonds. The films consisted of micrometer scale crystalline structure of around 10 µm thick has been confirmed by the field emission scanning electron microscopy.     

High-quality liquid phase-pulsed laser ablation graphene synthesis by flexible graphite exfoliation

This work reports a one-pot procedure of laser ablation on a graphite target in a liquid medium, based on the variation of different parameters such as target type, laser wavelength, and ablation medium,to obtain high-quality graphene nanosheets. The morphology of derived products was characterized by the field emission scanning electron microscopy(FE-SEM). Then, the morphology and structure of the optimized sample were characterized by transmission electron microscopy(TEM), X-ray diffraction(XRD), ultraviolet-visible-near infrared(UV–vis-NIR) spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy(XPS). By controlling the laser ablation parameters, we were able to prepare micrometer-sized few-layer graphene nanosheets with mainly less than ten layers. Such synthesized graphene nanosheets were grown at the surface of a flexible graphite target, indicating many potential applications in fundamental research, electrochemical and as hydrophobic surfaces.     

Improved exfoliation of surface-functionalized graphene oxide by epoxy monomer and enhanced mechanical properties of epoxy nanocomposites

Journal of Materials Science - The surface functionalization of graphene oxide (GO) provides an efficient way to improve the dispersion of GO in matrix and the interfacial properties of...     

Production of graphene by exfoliation of graphite in a volatile organic solvent

The production of unfunctionalized and nonoxidized graphene by exfoliation of graphite in a volatile solvent, 1-propanol, is reported. A stable homogeneous dispersion of graphene was obtained by mild sonication of graphite powder and subsequent centrifugation. The presence of a graphene monolayer was observed by atomic force microscopy and transmission electron microscopy. The solvent, 1-propanol, from the deposited dispersion was simply and quickly removed by air drying at room temperature, without the help of high temperature annealing or vacuum drying, which shortens production time and does not leave any residue of the solvent in the graphene sheets.     

Production of few-layer graphene by supercritical CO2 exfoliation of graphite

In this study, the authors report a supercritical CO₂ processing technique for intercalating and exfoliating layered graphite. Few-layer graphene is produced by immersing powdered natural graphite in supercritical CO₂ for 30 min followed by rapidly depressurizing the supercritical fluid to expand and exfoliate graphite. The graphene nanosheets are collected by discharging the expanding CO₂ gas directly into a solution containing dispersant sodium dodecyl sulfate (SDS) to avoid restacking. Atomic force microscopy (AFM) shows that the typical graphene sheet contains about 10 atomic layers. This technique offers a low-cost, simple approach to large-scale production of pure graphene sheets without the need for complicated processing steps or chemical treatment.