Graphite Thin Films Research Articles

The fabrication of graphitic thin films with highly dispersed noble metal nanoparticles by direct carbonization of block copolymer inverse micelle templates

Ordered arrays of Au or Ag nanoparticles supported on two-dimensional graphitic carbon films were prepared by direct carbonization of stabilized asymmetric polystyrene- block-poly(4-vinyl pyridine) (PS- b-P4VP) inverse micellar films loaded with metal precursors. Crosslinked PS- b-P4VP thin film templates with metal precursors selectively distributed in P4VP domains were converted to carbonaceous thin films having well-defined, highly dispersed metal nanoparticle (NP) arrays by ultraviolet (UV) irradiation under vacuum and subsequent carbonization. Mesoporous carbon films were also obtained after extracting the metal NPs by sonication in selected solvents. PS- b-P4VP was employed not only as carbon source, but also as template for introducing metal NPs in a nanopatterned configuration. The characteristic features and properties of thus generated hybrid carbon nanostructures were investigated by microscopy, UV–visible spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction measurement, and Raman spectroscopy.

Inkjet‐Printed Organic Electrodes for Bottom‐Contact Organic Field‐Effect Transistors

AbstractA graphite thin film was investigated as the drain and source electrodes for bottom‐contact organic field‐effect transistors (BC OFETs). Highly conducting electrodes (102 S cm−1) at room temperature were obtained from pyrolyzed poly(l,3,4‐oxadiazole) (PPOD) thin films that were prepatterned with a low‐cost inkjet printing method. Compared to the devices with traditional Au electrodes, the BC OFETs showed rather high performances when using these source/drain electrodes without any further modification. Being based on a graphite‐like material these electrodes possess excellent compatibility and proper energy matching with both p‐ and n‐type organic semiconductors, which results in an improved electrode/organic‐layer contact and homogeneous morphology of the organic semiconductors in the conducting channel, and finally a significant reduction of the contact resistance and enhancement of the charge‐carrier mobility of the devices is displayed. This work demonstrates that with the advantages of low‐cost, high‐performance, and printability, PPOD could serve as an excellent electrode material for BC OFETs.

Fabrication Method of Graphite Thin Films from Organic Solutions

Fabrication Method of Graphite Thin Films from Organic Solutions

Graphite Thin Film Deposition using Laser Induced Plasma

The Excimer KrF laser (of wave length 248nm, pulse energy of 13-50mJ and pulse width of 20ns) has been used to ablate graphite solid target. Thin films of graphite material have been grown on silicon (Si) substrate at different temperatures (25°C & 300°C). The techniques x-ray diffraction (XRD) and scanning electron microscopy has been used to study the structure and surface morphology of the deposited thin films. The whole experiment has been performed in the stainless steel chamber under pressure 10-4 torr and each thin film has been deposited for 10,000 laser shots. The graphite thin film deposited at higher substrate temperature has smooth structure and the film is uniform.

First-principles Calculations under an External Electric Field: Electrochemical Reactions and Control of Electronic Structures

Electronic structure study under an external electric field is one of the important subjects in the electronic device development and sustainable energy technologies. We developed a calculation method, called effective screening medium (ESM) method, to include the external electric field or bias potential within the density functional theory. In this article, the details of the ESM method are explained and two recent researches are reviewed. First, a simulation of the electrochemical reaction on a metal-water interface is reviewed. We have elucidated an electronic state modulation of metal surface during the reaction. Second, a study about the electronic structure of an ABC-stacked graphite thin film with or without an external electric field is reviewed. We have found that ferrimagnetic spin polarization occurs on the both surface sides of the thin film. The polarization is associated with the peculiar surface localized state that possesses the same characteristics as the edge-localized states of graphene nanoribbons. The spin polarization is, therefore, controllable by applying an external electric field.

The role of hydrogen in room-temperature ferromagnetism at graphite surfaces

We present an x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon π-states, hydrogen-mediated electronic states also exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top ≈10 nm of the irradiated sample where the average magnetization reaches ≃15 emu g− 1 at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin to that found in proton-bombarded graphite. Our findings also show that the magnetic properties of graphite surfaces, thin films or two-dimensional graphene samples can be reliably studied using soft x-ray dichroism. Fundamental new insights into the magnetic properties of carbon-based systems can thus be obtained.

Evaluation of morphology and deposits on worn polyimide/graphite composite surfaces by contact-mode AFM

Sintered polyimide/graphite composite cylinders were worn in a macroscale line-on-plate contact at different temperatures, revealing instable and relatively high coefficients of friction at 23 < T < 100 °C, decreasing coefficients of friction at 100 < T < 180 °C and constant values for coefficients of friction at 180 < T < 260 °C. The tendencies correspond with a transition from low wear rates at 23 < T < 100 °C into high and stable wear rates at 180 < T < 260 °C. These observations suggest that the lubricating effect of graphite does not prevail at all temperatures. Contact-mode atomic force microscopy of the worn surfaces was used for getting insight in the sliding processes at a micro- to nanoscale. The friction signals provide insight in the local distribution of the composite phases over the surface. Selection criterions for surface patterns representing optical interferences or shear banding patterns, were based on fast Fourier transform images. At low temperatures, severe wear grooves and shear bands within the graphite phase indicate the abrasiveness of graphite additives together with inhomogeneous graphitic transfer onto the polyimide surface areas. At intermediate temperatures, a very thin and discontinuous graphite film forms while the imide zones show a finely structured surface with homogeneous mixture of polyimide grains and graphitic particles. At high temperatures, the surfaces are relatively smooth and homogeneous with a thin graphite film covering the entire surface. The relation between coefficients of friction and surface morphology is most significantly quantified in the evolution of roughness parameters determined from AFM scans.

Epitaxial growth of graphitic carbon on C-face SiC and sapphire by chemical vapor deposition (CVD)

Epitaxial, graphitic carbon thin films were directly grown on C-face/(0 0 0 1¯) SiC and (0 0 0 1) sapphire by chemical vapor deposition (CVD), using propane as a carbon source and without any catalytic metal on the substrate surface. Raman spectroscopy shows the signature of multilayer graphene/graphite growth on both the SiC and sapphire. Raman 2D-peaks have Lorentzian lineshapes with FWHM of ∼60 cm −1 and the ratio of the D-peak to G-peak intensity ( I D/ I G) linearly decreases (down to 0.06) as growth temperature is increased. The epitaxial relationship between film and substrates were determined by X-ray diffraction. On both substrates, graphitic layers are oriented parallel to the substrate, but exhibit significant rotational disorder about the surface normal, and predominantly rhombohedral stacking. Film thicknesses were determined to be a function of growth time, growth temperature, and propane flow rate.

Optical reflection and transmission properties of exfoliated graphite from a graphenemonolayer to several hundred graphene layers

The optical reflection contrast and optical transmission contrast of graphitic films onglass ranging in thickness from a monolayer to the limit of bulk graphite havebeen experimentally measured. For samples with more than 10 graphene layerswhere optical contrast quantization becomes difficult to observe, atomic forcemicroscopy was used to measure the sample thickness. The visible optical reflectionand transmission of thin graphitic films is found to depend strongly on the realcomponent of the optical conductance per graphene layer, and comparatively weaklyon the imaginary component of optical conductance. This observation in partexplains the significant variation in the refractive index of graphene and graphitereported in the literature to date. Spectroscopic measurements reveal a strongdispersion in the optical conductance of even a 10 layer film, consistent with animaginary conductance arising from virtual transitions at the band edges of theπ andσ bands atthe M and Γ points, respectively.

Phase control of magnetic state of graphite thin films by electric field

Based on first-principle total-energy calculations, we have found that by applying an external electric field it is possible to control the magnetic state of graphite thin film with the rhombohedral stacking arrangement. When exposed to a moderate electric field normal to the film, the surface of a thin film of rhombohedral graphite undergoes a magnetic phase transition from the antiferromagnetic state to the ferromagnetic state. The polarized electron spin is primarily distributed in the bottommost layer of the film, which forms the interface with the negative electrode. The amount of polarized electron spin is calculated to be 0.067 μB/nm2. The ferromagnetic ordering with the characteristic distribution of the polarized electron spin opens the possibility of using graphite thin films in electronic devices with spin degree of freedom.

Electron Correlation and Shake up Effects on Energy Bands of Low Dimensional Carbon Systems

The electron correlation and shake up effects on energy band structures in the diamond and tetrahedral amorphous carbon films (both on the substrate of NiTi alloy), the graphite (thin film) and precipitates of the titanium carbide (in the NiTi matrix) were studied by the Auger electron spectroscopy. The description of carbon spectrum is based on the convolution of two-conduction particles and one-core particle spectral densities. It was concluded that the comparisons of the experimental and theoretical Auger spectra were determined the quantitative assessment of the electron correlation and shake up effects in the carbon systems. We also discuss the electron correlations in a quantum wire/dot or bulk material.

Anomalous Anderson Weak Localization of Graphite Thin Film Fabricated by Plasma-Enhanced Chemical Vapor Deposition

The conductivity and magnetoresistance of graphite thin film (GTF), fabricated by plasma-enhanced chemical vapor deposition (PECVD), were investigated. Below 100 K, the conductivity was proportional to log T , and the transverse magnetoresistance (MR) was negative, whereas the longitudinal MR was small, but positive. The conductivity and magnetoresistance results are well-described by weak-localization theory, applied to a two-dimensional disordered system. The carrier dephasing length ( L e ), which was estimated from the transverse MR, varied as T - p ( p = 0.05–0.11). This p value is an order of magnitude smaller than that of electron–electron interaction derived from Fermi liquid theory ( p = 1). This small p value strongly suggests that the electronic state of GTF is different from that of typical graphite.

Focused-ion-beam-assisted selective control of graphene layers: acquisition of clean-cutultra thin graphitic film

A focused-ion beam (FIB) and a nanomanipulator provide a novel way to selectivelycontrol and obtain a few layers of graphene. Because of its weak van der Waals force in theinterlayer of graphite, the nanomanipulator could easily exfoliate a graphitic thinlayer with no wrinkles on the surface from a highly oriented pyrolitic graphite(HOPG) by applying a shear force which exceeds the static interlayer shear force.Subsequently, a few layers of graphene were successfully obtained by applying a uniformshear force from a detached graphitic thin layer that had been transferred to apre-determined site on an oxide wafer. The required shear force for clean cleavage of agraphitic thin layer was then estimated based upon experimental data. Ramanscattering analysis was used to confirm the number of placed graphene layers and theplacement of a few layers of graphene was projected to have about five atomic layers.

Generation of electron beams carrying orbital angular momentum

All forms of waves can contain phase singularities. In the case of optical waves, a light beam with a phase singularity carries orbital angular momentum, and such beams have found a range of applications in optical manipulation, quantum information and astronomy. Here we report the generation of an electron beam with a phase singularity propagating in free space, which we achieve by passing a plane electron wave through a spiral phase plate constructed naturally from a stack of graphite thin films. The interference pattern between the final beam and a plane electron wave in a transmission electron microscope shows the 'Y'-like defect pattern characteristic of a beam carrying a phase singularity with a topological charge equal to one. This fundamentally new electron degree of freedom could find application in a number of research areas, as is the case for polarized electron beams.

Large-scale graphitic thin films synthesized on Ni and transferred to insulators: Structural and electronic properties

We present a comprehensive study of the structural and electronic properties of ultrathin films containing graphene layers synthesized by chemical vapor deposition based surface segregation on polycrystalline Ni foils then transferred onto insulating SiO2/Si substrates. Films of size up to several mm’s have been synthesized. Structural characterizations by atomic force microscopy, scanning tunneling microscopy, cross-sectional transmission electron microscopy (XTEM), and Raman spectroscopy confirm that such large-scale graphitic thin films (GTF) contain both thick graphite regions and thin regions of few-layer graphene. The films also contain many wrinkles, with sharply-bent tips and dislocations revealed by XTEM, yielding insights on the growth and buckling processes of the GTF. Measurements on mm-scale back-gated transistor devices fabricated from the transferred GTF show ambipolar field effect with resistance modulation ∼50% and carrier mobilities reaching ∼2000 cm2/V s. We also demonstrate quantum transport of carriers with phase coherence length over 0.2 μm from the observation of two-dimensional weak localization in low temperature magnetotransport measurements. Our results show that despite the nonuniformity and surface roughness, such large-scale, flexible thin films can have electronic properties promising for device applications.

Microwave plasma chemical vapor deposition of graphitic carbon thin films

Thin layers of graphitic carbon were produced from solid organic precursors by a one-step microwave plasma chemical vapor deposition method. Low-pressure Ar-plasma and strong electromagnetic radiation led to the rapid evaporation and pyrolysis of organic precursors at relatively low temperatures, yielding uniform films of nanometer-sized graphitized carbon particles. The structure and morphology of the carbon films were examined using scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. A direct correlation between electrical conductivity of graphitic thin films and their structure was established.

Capacitance-voltage and current-voltage characteristics of graphite oxide thin films patterned by ultraviolet photolithography

Electrical characteristics of graphite oxide (GO) thin films deposited on a p-type silicon substrate were investigated to explore its potential application as a dielectric material in organic field effect transistors. Channel current in the GO films exhibited linear response with the applied bias in the positive voltage regime and increased exponentially for negative source-drain bias. This rectifying behavior arises due to the Coulombic interaction between the electrons emitted from the metal contact and the space charge region in the GO film. A clockwise hysteresis loop was observed in the capacitance-voltage characteristics due to the presence of traps at the interface.

Theoretical study of superconducting proximity effect in single and multi-layered graphene

Theoretical analysis of superconducting current in graphite thin films (or graphene) in proximity to superconductors is presented, especially paying attention to the band structure. We derive the general formula to calculate the free energy of the superconductor–graphite film–superconductor junction, which enable us to calculate the critical current of the junction. We introduce two models for the band structures: (1) Fermi point type (characteristic to monolayer case) and (2) zero-gap semiconductor type (characteristic to bilayer case). Then we calculate the superconducting critical current as a function of junction length and temperature.

Transparent and flexible polymerized graphite oxide thin film with frequency-dependent dielectric constant

Here we report on the preparation of transparent and flexible polymerized graphite oxide, which is composed of carbons with sp3-hybridized orbitals and a nonplanar ring structure, and which demonstrates dispersion in its dielectric constant at room temperature. This frequency dependence renders the material suitable for creating miniaturized, flexible, and transparent variable capacitors, allowing for smaller and simpler integrated electronic devices. We discuss this polarizability in terms of space charge effects.

Cross-sectional transmission electron microscopy of thin graphite films grown by chemical vapor deposition

Graphene has been the subject of an extraordinary upsurge of interest due to its intriguing properties and potential applications. Recent work has shown that excellent electronic properties are exhibited by large-scale ultrathin graphite films, grown by chemical vapor deposition on a polycrystalline metal and transferred to a device-compatible surface. The properties of such multilayered graphene films could depend strongly on film thickness and uniformity. Unlike the other common methods for analysis in the literature, cross-sectional transmission electron microscopy (TEM) would provide direct, straightforward analysis of film thickness and quality, as well as provide a means to investigate specific defect structures (e.g. wrinkles). However, this approach has not often been pursued due to the sensitivity of graphite to the electron and ion beam damage in such a procedure. Here, an approach to creating cross-sectional TEM samples using a focused ion beam lift-out method is presented, along with the resulting TEM images of thin graphite films and several wrinkle defects. Samples removed from as-grown films on polycrystalline nickel and films removed from measured devices are presented. The benefits and limitations to this approach are discussed.