Graphite Thin Films Research Articles

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Synthesis of a flexible self-standing graphene-like graphite thin film and its application to anode materials for thin-film all-solid-state lithium-ion batteries

Roll-to-Roll Deposition of Thin Graphitic Films and Dependence on Discharge Modes in Radio Frequency Capacitively Coupled Plasma

We report the deposition of nanocrystalline graph- itic films on Cu foil in a roll-to-roll (R2R) plasma-enhanced chemical vapor deposition (PECVD) system. The influence of discharge modes on the graphitic film synthesis is studied in radio frequency (RF) capacitively coupled Ar/H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> plasma. First, electrical and spectroscopic characterizations supported with modeling are carried out to study the properties of hybrid and gamma modes in Ar plasma at an excitation frequency of 80 kHz. A hybrid mode, which exhibits both alpha and gamma characteristics, exists at 950 Pa for Ar plasma at powers up to 1100 W. However, Ar plasma transitions at 1380 Pa from hybrid to gamma mode when the power is greater than 750 W. Second, the quality of the nanocrystalline graphitic films, which is assessed with Raman spectroscopy, is correlated with the characteristics of hybrid and gamma modes in Ar/H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> plasma. The hybrid mode in Ar/H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> plasma has relatively less ion bombardment. However, the gamma mode in Ar/H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /CH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> plasma provides abundant intermediate carbon species and heats the Cu substrate directly to around 1193 K. Hence, thin films of nanocrystalline graphite with different qualities can be produced at a web feed rate of 30 mm/min without supplemental heating by adjusting the discharge modes.

Dissolving Diamond: Kinetics of the Dissolution of (100) and (110) Single Crystals in Nickel and Cobalt Films

We report a study of the kinetics of dissolution of (100) and (110) single-crystal diamond plates (“D(100)” and “D(110)”) in thin films of nickel (Ni) and cobalt (Co). This dissolution occurs at the metal–D(100) or metal–D(110) interface and was studied in the presence and also in the absence of water vapor at temperatures near 1000 °C. The single-crystal D(100) dissolves in Ni, and also in Co, in the temperature range 900–1050 °C. The dissolution is too slow to measure below 900 °C. In an argon (Ar) atmosphere (under an Ar(g) flow at 1000 sccm and 1 atm pressure, with no water vapor present in the reaction chamber) and at any temperature in the range 900–1050 °C, the metal film is rapidly saturated with dissolved carbon (C), thin graphite films form on the free metal surface and at the metal–D interface during heating at or above 650 °C, and the dissolution of the diamond then stops. For addition of water vapor, its partial pressure was controlled by using a water bubbler immersed in a constant temperature bath and Ar(g) was used as the carrier gas. We discovered two different regimes (I and II) for the kinetics of dissolution of D(100) and D(110), in which the rate-determining step was the removal of carbon atoms on the open metal surface (regime I, lower partial pressure of water vapor) or dissolution of diamond at the metal–diamond interface (regime II, higher partial pressure of water vapor) that yielded different Arrhenius parameters. Time-of-flight-secondary ion mass spectrometry depth profiles show the concentration gradient of C from a certain depth into the metal film surface down to the M–D(100) interface, and residual gas analyzer measurements show that the gas products formed in the presence of water vapor on the metal surface are CO and H2. It was found that the rate of dissolution of diamond in Co was higher than that in Ni for both D(100) and D(110) and for both regimes I and II, and possible reasons are suggested. We also found that D(111) could not be dissolved at the Ni/D(111) and Co/D(111) interface in the presence of water vapor (over the same range of sample temperatures). The reaction paths for dissolution of C at the M–D(100) or M–D(110) interface and for removal of C from the free surfaces of Ni and Co were assessed through density functional theory modeling at 1273 K.

Thermal noise of hot carriers under natural sunlight

The efficiency of hot carrier solar cells strongly depends on the attainable carrier temperature in the absorber material. Measurements of hot carrier temperature under natural concentrated sunlight were performed at thin films of graphite, bismuth and PbTe, cooled through AlN ceramic substrate. The carrier temperature was assessed by magnitude of thermal noise voltage of charge carriers in the frequency range between 300 and 20,000 Hz according to Nyquist formula. Noise measurement equipment was calibrated in a range of sample impedances by using heated metal film resistors. We found that the carrier temperature in graphite films can exceed 600 °C under up to 15,000× suns and is much higher than the estimated lattice temperature in the film under the given cooling conditions. Secondary electron micrographs illustrate a good mechanical contact between the graphite film and the AlN substrate. We found also distinct heating of charge carriers in PbTe up to about 100 °C. Contrary to that, no measurable carrier heating above the lattice temperature was detected in bismuth films. Johnson noise thermometry demonstrates potential in identification of attractive candidate materials for absorbers in hot carrier solar cells.

Design of an ultrafast electron diffractometer with multiple operation modes* *Project supported by the Director Fund of WNLO (Grant No. WNLOZZYJ1501), the Fundamental Research Funds for the Central Universities, HUST (Grant No. 2017KFXKJC001), the Innovation Fund of WNLO, and the Fund of State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics (Grant No. T152012).

Directly resolving structural changes in material on the atomic scales of time and space is desired in studies of many disciplines. Ultrafast electron diffraction (UED), which combines the temporal resolution of femtosecond-pulse laser and the spatial sensitivity of electron diffraction, is an advancing methodology serving such a goal. Here we present the design of a UED apparatus with multiple operation modes for observation of collective atomic motions in solid material of various morphologies. This multi-mode UED employs a pulsed electron beam with propagation trajectory of parallel and convergent incidences, and diffraction configurations of transmission and reflection, as well utilities of preparation and characterization of cleaned surface and adsorbates. We recorded the process of electron–phonon coupling in single crystal molybdenum ditelluride following excitation of femtosecond laser pulses, and diffraction patterns of polycrystalline graphite thin film under different settings of electron optics, to demonstrate the temporal characteristics and tunable probe spot of the built UED apparatus, respectively.

Exchange interaction, disorder, and stacking faults in rhombohedral graphene multilayers

We apply the mean-field Hartree Fock theory of gapped electronic states at charge neutrality in bilayer graphene to thin films of rhombohedral graphite with up to thirty layers. For the ground state, the order parameter (the separation of bands at the valley center) saturates to a constant non-zero value as the layer number increases, whereas the band gap decreases with layer number. We consider chiral symmetry breaking disorder in the form of random layer potentials and chiral preserving disorder in the form of random values of the interlayer coupling. The former reduces the magnitude of the mean band gap whereas the latter has a negligible effect, which is due to self-averaging within a film with a large number of layers. We determine the ground state in the presence of an individual stacking fault which results in two pairs of low-energy bands and we identify two separate order parameters. One of them determines the band gap at zero temperature, the other determines the critical temperature leading, overall, to a temperature dependence of the band gap that is distinct to that of pristine rhombohedral graphite. In the presence of stacking faults, each individual rhombohedral section with $m$ layers contributes a pair of low-energy flat bands producing a peak in the Berry curvature located at a characteristic $m$-dependent wave vector. The Chern number per spin-valley flavor for the filled valence bands in the ground state is equal in magnitude to the total number of layers divided by two, the same value as for pristine rhombohedral graphite.

Current Tunneling Characterization of Oxidized Black Phosphorus by Graphite Thin Film Electrodes

Current Tunneling Characterization of Oxidized Black Phosphorus by Graphite Thin Film Electrodes

Modified-edge-support heat treatment method of polyimide for crystalline, large-area, and self-standing ultrathin graphite films

Using a newly developed edge-support heat treatment method of polyimide, self-standing graphite thin films (GTFs) with a frame were prepared. The graphite basal plane in the GTF was oriented in the direction of the film surface, resulting in GTFs with high quality, large area, and a uniform thickness of 50–120 nm. The thickness distribution (3σ) with an area of 25 mm × 25 mm and a thickness of 54 nm sample was 5.17 nm (measurement area 7.8 mm × 10.4 mm). The electric conductivity of a similarly prepared sample was 1.81 × 104 S/cm. The Young's modulus and ultimate tensile strength of a 60-nm-thick GTF were 1.02 × 102 and 5.34 GPa, respectively. Approximately 50 nm thick GTF samples were thinned by reactive ion etching using oxygen to fabricate a thickness between 10 and 20 nm. A film with an area of 10 mm × 10 mm and thickness of 16.7 nm exhibited a 3σ value of 1.80 nm which means that the thickness difference corresponds to 5 layers of graphite. The conductivity of a 16.2-nm-thick GTF was 1.79 × 104 S/cm. These results indicate that the etching proceeded uniformly. The proposed top-down method is an industrially superior method that overcomes the drawbacks of conventional bottom-up methods.

Tunable Terahertz Plasmons in Graphite Thin Films.

Tunable terahertz plasmons are essential for reconfigurable photonics, which have been demonstrated in graphene through gating, though with relatively weak responses. Here we demonstrate strong terahertz plasmons in graphite thin films via infrared spectroscopy, with dramatic tunability by even a moderate temperature change or an insitu bias voltage. Meanwhile, through magnetoplasmon studies, we reveal that massive electrons and massless Dirac holes make comparable contributions to the plasmon response. Our study not only sets up a platform for further exploration of two-component plasmas, but also opens an avenue for terahertz modulation through electrical bias or all-optical means.

Intercalation of Dyes in Graphene Oxide Thin Films and Membranes.

Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

First-principles study of ultrafast and nonlinear optical properties of graphite thin films

We theoretically investigate ultrafast and nonlinear optical properties of graphite thin films based on first-principles time-dependent density functional theory. We first calculate electron dynamics in a unit cell of graphite under a strong pulsed electric field and explore the transient optical properties of graphite. The optical response of graphite shows a sudden change from the conducting to the insulating phase at a certain intensity range of the applied electric field. It also appears to be a saturable absorption (SA) in the energy transfer from the electric field to electrons. We next investigate a light propagation in graphite thin films by solving the coupled dynamics of the electrons and the electromagnetic fields simultaneously. It is observed that the SA manifests in the propagation with small attenuation in the spatial region where the electric field amplitude is about $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{4.pt}{0ex}}\text{to}\phantom{\rule{4.pt}{0ex}}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\AA{}$.

Production of thin graphite films on a dielectric substrate by heteroepitaxial synthesis

Problem statement. Development of methods for producing large-area isolated graphene under controlled conditions is an important task, which is primarily interested in the unique physical and chemical properties of graphene: high electrical and thermal conductivity, dependence of electronic characteristics on the presence of attached different radicals on the graphene surface, adjustable band gap, and high carrier mobility. Goal. It is necessary to develop regimes for producing thin graphite films on a dielectric substrate by annealing the structure (0001)Al2O3/(111)Ni/ta-C with a minimum density of defects in the crystal structure by the heteroepitaxial synthesis method. Results. The method of obtaining thin graphite films on a dielectric substrate by annealing the structure (0001)Al2O3/(111)Ni/ta-C has been tested. The method is based on the catalytic decomposition of hydrocarbons on the single crystal surface of a metal catalyst, diffusion and crystallization of carbon on the reverse side of the metal film. The surface of the obtained carbon films with a thickness significantly exceeding one atomic layer is uniform within the 5x5 mm samples. Varying the annealing temperature and time, as well as the initial amount of carbon, will further allow to control the amount of carbon involved in the formation of atomic layers. Practical significance. The described method is promising for developing a scalable technological process for obtaining largearea graphene on a dielectric substrate.

Electrical conductivity of MoS2 thin films and graphite under continuous injection to protons

The electrical conductivity of thin films of MoS2 and graphite has been studied under conditions of continuous injection of low-energy protons (∼ 1-4 keV). The effect of a sharp increase (by 3-4 orders of magnitude) in the electrical conductivity of substances with a layered structure (MoS2 and graphite) at T ∼ 293 K has been established. The observed effect increases (from 2 to 10 times) as the temperature decreases from room temperature to liquid nitrogen temperature (T ∼ 77 K) and with increasing number of protons injected into the sample. The established temperature variation of electrical resistance is typical for materials with a metallic type of conductivity. It is shown that a thin surface layer, onto which protons penetrate, is responsible for the change in the electrical conductivity of MoS2 and graphite films.

Chemiresistive detection of silver ions in aqueous media

Silver is used as a water disinfectant in hospital settings as well as in purifiers for potable water. Although there are no strict regulations on the concentration of silver in water, adverse effects such as argyria and respiratory tract irritation have been correlated to excess silver consumption. Based on this, the levels of silver in water are recommended to be maintained below 100 ppb to ensure safety for human consumption. In this work, we present a silver sensor for use in aqueous media that utilizes bathocuproine, a silver selective chromophore, adsorbed onto few-layer graphene (FLG) flake networks for the chemiresistive detection of silver. Complexation of silver to bathocuproine modulates the conductivity of the FLG film, which can be probed by applying a small voltage bias. The decrease in resistance of the film correlates with the concentration of silver in solution between 3 ppb and 1 ppm. Exposing the sensor to a lower pH resets the sensor, allowing it to be reused and reset multiple times. This sensor demonstrates a new pathway to chemiresistive cation sensing using known selective complexing agents adsorbed onto graphitic thin films. This concept can be expanded to the detection of other relevant analytes in domestic, industrial and environmental water sources.

Coupling between structural properties and charge transport in nano-crystalline and amorphous graphitic carbon films, deposited by electron-beam evaporation

Nano-crystalline and amorphous films of graphitized carbon were deposited by electron-beam evaporation of bulk graphite. Structural properties and the size of graphite nanoclusters (L ≈ 1.2–5 nm) in the films were determined from the analysis of their Raman spectra. Electrical properties of the bulk nano-crystalline graphite reference sample and the deposited graphitic carbon films were measured by means of the Hall effect technique within the temperature range from 290 to 420 K. The electrical conductivity σ and Hall mobility μH of all samples exhibited exponential temperature dependences, indicating on the non-metallic behavior. Electrical properties of the amorphous graphitic carbon thin films, deposited at low substrate temperatures (620 and 750 K) were analyzed in the scope of the hopping charge transport mechanism via localized states. We have shown that the charge transport in the bulk and thin film (920 K) nano-crystalline graphite samples is carried out via the tunneling and thermionic emission over potential barriers at the grain boundaries.This paper contributes towards better understanding of coupling between structural and electrical properties of graphitic carbon thin films.

Synthesis and Optical Characterization of Carbon Nanofibers

Radio frequency plasma enhanced chemical vapor deposition technique has been used to synthesized graphitic carbon nanofibers thin films. Ni catalyst in thin film form used for the synthesis of carbon nanofibers. The deposition temperature of the substrate has been varied from 500 – 600 0C. The morphology of the CNF thin films changed with the variation of substrate temperature. The graphitic phase of the synthesized carbon nanofibers has confirmed by X-ray diffraction patterns analyses. Field emission scanning electron microscopic studies showed fibrous structure in the films. The length of the carbon nanofibers few micrometers and the diameter range 300-400 nm. The different vibrational modes of carbon nanofibers analyzed using Fourier transformed infrared spectroscopy measurements. Photoluminescence of the carbon nanofibers have also been studied which showed a strong emission peak at 468 nm.

Graphite/n-InSe photosensitive junctions fabricated by transferring dry drawn graphite film onto as-cleaved surface of InSe

We report on reproducible fabrication and characterization of new photosensitive Schottky-type junctions prepared by an extremely simple technique when a substrate-free dry drawn graphite film is transferred from water surface onto as-cleaved van der Waals surface (0001) of n-lnSe substrate. Electrical and photoelectric characterization of the obtained junctions is investigated using current-voltage, capacitance-voltage and photosensitivity spectra measurements. It is revealed that the thin graphite film transferred onto the n-lnSe (0001) substrate creates a Schottky junction with a potential barrier of 0.49 eV and rectification ratio of RR ~ 200 at 0.9 V. An abrupt rectifying electrical junction graphite/n-lnSe is formed due to van der Waals attraction forces between the thin graphite film and atomically smooth n-lnSe (0001) surface. The graphite/n-lnSe junctions generated short-circuit current density of Jsc = 0.29 mA/cm2, open-circuit voltage of Voc = 0.31 V and filling factor of FF = 0.33 under 80 mW/cm2 white light illumination. The junctions exhibit photosensitivity in the near-infrared and visible spectral ranges, making them promising for photodetector applications. Their energy band diagram is built up.

Direct writing of graphite thin film by laser-assisted chemical vapor deposition

This study reports the direct writing of low-resistance graphite film strip of 25 μm width and ∼65 nm height on nickel substrates by laser-assisted chemical vapor deposition (LCVD). Raman spectra of the LCVD graphite showed no D-band, confirming that it had little defects. The G-band was nearly constant but the 2D-band showed variations in shape and intensity. A detailed analysis of the 2D-band revealed that the LCVD graphite is a mixture of highly ordered graphite and turbostratic graphite. An observation of the stacking feature by transmission electron microscope further confirmed that the layers close to substrate was closely packed, whereas the layer interspacing gradually increased with distance, transforming into turbostratic graphite. The resistivity of LCVD graphite estimated from the sheet resistance measured by the 4-point method was 2.45 × 10−5 Ω m.

Preparation and characterization of ion beam sputtered graphitic carbon nitride thin film

Ion beam sputtering is a versatile technique to create nanostructured thin films with precise control over process parameters. We report the properties of graphitic carbon nitride (g-C3N4) thin films deposited by ion beam sputtering. Ion beam analysis methods including Rutherford Backscattering Spectrometry (RBS), Nuclear Reaction Analysis (NRA) and Particle Induced X-ray Emission (PIXE) were used for quantitative determination of composition, thickness, elemental depth profile and trace, major elements and impurities. It was found that by directly using compressed g-C3N4 powders as the sputter target material and tuning the process parameters, g-C3N4 thin films with well controlled deposition rate were obtained. Post-annealing above 250 °C under argon atmosphere for 30 min was needed to recrystallize the g-C3N4 nanostructure. Our results suggest that ion beam sputtering technique is a promising method to obtain uniform and high-quality g-C3N4 thin films.

Growth of Thin Graphite Films on a Dielectric Substrate using Heteroepitaxial Synthesis

A technique for growing thin graphite films on a dielectric substrate by annealing the Al2O3(0001)/Ni(111)/ta-C structure has been optimized. This technique is based on catalytic decomposition of hydrocarbons on the surface of a single-crystal catalyst metal film on a dielectric substrate and subsequent diffusion and crystallization of carbon between the metal film and the substrate. A thin graphite film with a low density of crystal-structure defects is obtained on the dielectric substrate after chemical etching of the metal film.