Graphite Thin Films Research Articles

Comprehensive analysis of charge carriers dynamics through the honeycomb structure of graphite thin films and polymer graphite with applications in cold field emission and scanning tunneling microscopy

Polymer graphite electron sources have performed satisfactorily as field emission emitters and scanning tunneling microscopy probes in the past few years. However, the emission process was characterized by limited total emission currents. This paper introduces the elemental, vibrational, electronic structure, and optical analysis of polymer graphite and glass-graphite composite field emission cathodes to study these limitations. Moreover, the field emission characteristics are studied including the changes in the potential energy barrier of the used materials and structures. Among the studied structures, the cathodes prepared from graphite thin films deposited on a micropointed glass substrate (film-GMF) showed superior performance as random field emission arrays. This includes obtaining much higher emission current values ≈ 20 times) and lower threshold voltages ≈ 1/2) compared to the results obtained from polymer graphite samples. The enhancement factor in such emitters is believed to be the three-dimensional honeycomb structure of graphite. Moreover, the study includes applying graphite coatings to tungsten nano-field emission cathodes and scanning tunneling microscopy probes, which improves the performance of such cathodes/probes in both microscopic techniques.

Graphite sheet-supported bimetallic RhNi thin film alloys for enhanced and durable hydrogen evolution in acidic environments

Noble metal-based materials are excellent catalysts for hydrogen production from water electrolysis, but their high cost and limited availability hinder widespread use. However, precious metals can be effectively utilized in catalysts by forming alloys with low-cost 3d transition metals, thereby reducing the need for precious metals, while enhancing the performance. In this study, aerosol-assisted chemical vapor deposition was employed to deposit bimetallic rhodium-nickel (RhNi) alloy thin films on a graphite sheet for hydrogen evolution reaction (HER) catalysis in a 0.5 M H2SO4 solution. Thin-film morphological patterns were significantly changed as the deposition time varied from 45 to 135 min. The RhNi alloy fabricated for 90 min outperformed its counterparts, displaying lower overpotential of 40 and 197 mV at 10 and 1000 mA cm−2, small Tafel kinetics (54 mV dec−1), high conductivity (0.57 Ω) and large electrochemical surface area (3312 cm2), and extended stability (48 h) compared to pure Rh metal and other reported catalysts. The superior HER performance arises from the synergistic effects between Rh and Ni metals, coupled with a uniform layer of spherical particles that enhance conductive properties and reaction rates by providing maximized active sites on graphite surface. This work provides valuable guidelines for efficiently preparing highly active and durable thin-film electrocatalyst within short time span and applying directly in fuel cell for hydrogen production.

On the mechanism of piezoresistance in nanocrystalline graphite.

Strain sensors are sensitive to mechanical deformations and enable the detection of strain also within integrated electronics. For flexible displays, the use of a seamlessly integrated strain sensor would be beneficial, and graphene is already in use as a transparent and flexible conductor. However, graphene intrinsically lacks a strong response, and only by engineering defects, such as grain boundaries, one can induce piezoresistivity. Nanocrystalline graphene (NCG), a derivative form of graphene, exhibits a high density of defects in the form of grain boundaries. It holds an advantage over graphene in easily achieving wafer-scale growth with controlled thickness. In this study, we explore the piezoresistivity in thin films of nanocrystalline graphite. Simultaneous measurements of sheet resistance and externally applied strain on NCG placed on polyethylene terephthalate (PET) substrates provide intriguing insights into the underlying mechanism. Raman measurements, in conjunction with strain applied to NCG grown on flexible glass, indicate that the strain is concentrated at the grain boundaries for smaller strain values. For larger strains, mechanisms such as grain rotation and the formation of nanocracks might contribute to the piezoresistive behavior in nanocrystalline graphene.

Anisotropic Klemens model for the thermal conductivity tensor and its size effect

With the constraint from Onsager reciprocity relations, here we generalize the Klemens model for intrinsic phonon-phonon scattering from isotropic to anisotropic. Combined with the anisotropic Debye dispersion, this anisotropic Klemens model leads to analytical expressions for heat transfer along both ab-plane (κab) and c-axis (κc), suitable for both layered and chainlike materials with any anisotropy ratio of the dispersion and the scattering. This combination of dispersion and scattering is further applied to the equation of phonon radiative transfer to model heat conduction across anisotropic thin films. The model is compared to experimental κab and κc of bulk graphite at high temperatures (T≥0.1×max(θD,ab,θD,c), where θD,ab and θD,c are the two Debye temperatures), leading to best-fit scattering parameters that capture the harmonic and anharmonic nature of the intra- and inter-layer bonding of graphite. With no further adjusting of these parameters, the anisotropic scattering model performs at least 9 times better than the isotropic one at explaining the experimental κc of graphite thin films. This size effect is further justified by comparing the accumulation function of κc.

Electronic properties of individual CsPbI2Br nanocrystals investigated by LT-STM

Solution-processed metal halide perovskite nanocrystals show promise for various potential optoelectronic applications and the exploration of the fundamental physics underlying them. However, the electronic properties of individual nanocrystals have not been thoroughly studied. Here, we applied low-temperature scanning tunneling microscopy to investigate the properties of metal halide perovskite CsPbI2Br nanocrystals with a diameter in the range of 10–20 nm. Sub-monolayer dispersions of the nanocrystal on highly oriented pyrolytic graphite and gold thin film substrates were achieved. Using scanning tunneling microscopy, we resolved topographies of individual nanocrystals on the gold film, and their electronic properties were probed by scanning tunneling spectroscopy. In our experiment, no obvious dependence of the extracted energy gap on the nanocrystal size and shape was found, which is consistent with the reported small exciton Bohr radius in metal halide perovskite materials. Additionally, we observed that the energy gaps of some nanocrystals are smaller than that of the bulk, suggesting the influence of factors such as deep-level defects/traps, ion migration, etc. on the electronic structure.

Triazine-Based Graphitic Carbon Nitride Thin Film as a Homogeneous Interphase for Lithium Storage.

Triazine-based graphitic carbon nitride is a semiconductor material constituted of cross-linked triazine units, which differs from widely reported heptazine-based carbon nitrides. Its triazine-based structure gives rise to significantly different physical chemical properties from the latter. However, it is still a great challenge to experimentally synthesize this material. Here, we propose a synthesis strategy via vapor-metal interfacial condensation on a planar copper substrate to realize homogeneous growth of triazine-based graphitic carbon nitride films over large surfaces. The triazine-based motifs are clearly shown in transmission electron microscopy with high in-plane crystallinity. An AB-stacking arrangement of the layers is orientationlly parallel to the substrate surface. Eventually, the as-prepared films show dense electrochemical lithium deposition attributed to homogeneous charge transport within this thin film interphase, making it a promising solution for energy storage.

Facile fabrication of conductive graphitic carbon films as transparent electrodes in organic solar cells by ion beam irradiation of polystyrene films and carbonization

Conductive graphitic carbon films were fabricated from insulating polystyrene (PS) by ion beam irradiation and subsequent thermal treatment, and used as a transparent anode for organic solar cells (OSCs). Analyses of optical and chemical properties revealed that the thermally resistant oxidized carbon clusters in the PS precursors (spin-coated on quartz) were efficiently generated by ion beam irradiation at a fluence of 2 x 1016 ions/cm2, and transformed to graphitic carbon thin films through high-temperature treatment. The graphitic carbon films exhibited thickness-dependent conductivity (the highest conductivity of 374 S/cm), while their work function (5.20 eV) and surface roughness (0.560–0.596 nm) were unaffected by the thickness of the films. Furthermore, the introduction of the 10.2 nm-thick graphitic carbon films into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)-based OSCs resulted in a power conversion efficiency (PCE) of 1.40%, demonstrating the feasibility of using the graphitic carbon films as transparent electrodes.

Tunable broadband terahertz absorption and shielding of bioderived graphitic carbon microspheres

The rapid advancement of Terahertz (THz) science and technology has led to a growing interest in electromagnetic interference (EMI) shielding in the THz frequency region. There is an urgent need for broadband and tunable devices capable of absorbing terahertz (THz) waves to effectively eliminate undesired THz interference. In this regard, this work demonstrates the facile synthesis of graphitic carbon microspheres from turpentine oil (a natural carbon source) and its broadband absorption and shielding properties in the THz frequency region from 0.2 THz to 1.6 THz. Advanced characterization techniques like XRD, FESEM, RAMAN, and XPS have been employed to explore the structural, microstructural, and elemental properties of graphitic carbon microspheres (GCMs). Herein, an effort has been made to discuss the underlying mechanism for THz absorption by GCMs. The minimum shielding efficacy reaches −27.5 dB covering the broad frequency region from 0.2 to 1.6 THz with a matching thickness of ∼3.75 μm, signifying that more than 99% of the THz radiations can be shielded. Also, GCMs are exhibiting exceptional terahertz absorption (∼86%) spanning from 0.2 to 1.6 THz for the same sample thickness. Our findings reveal valuable insights into the THz shielding properties of graphitic carbon thin films without any complex element doping or functionalization, shedding light on their suitability for future THz-based devices.

Analysis of transformation of a thin graphite-like carbon layer on the detonation nanodiamond surface into diamond

The article considers the process of obtaining nanostructured diamond materials due to phase transformation of thin films based on carbon non-diamond forms on the nanodiamond surface into diamond. For the synthesis of nanostructured diamond polycrystalline superhard material, various variants of the initial mixture based on nanodiamond with non-diamond carbon on the surface are proposed (surface-graphitized nanodiamond, surface-graphitized nanodiamond with addition of purified nanodiamond, detonation diamond-containing charge with a surface layer of “amorphous” carbon, including purified nanodiamond additives). The influence of the structure of a thin non-diamond layer on the parameters of graphite to diamond transition is revealed. For a carbon film with a disordered structure (the so-called “amorphous” carbon), the transition pressure to diamond will be about 10–15 GPa, which is significantly higher than the phase transformation pressure for thin graphite films in this temperature range. It is shown that the increase in the pressure of transformation of a thin layer of “amorphous” carbon is caused by its lower surface energy compared to the surface energy of graphite. It has been established that the region of transformation of a thin graphite-like film with a thickness of about 1 nm, formed on the 2–10 nm nanodiamond surface into diamond will be below the graphite-diamond equilibrium line in the temperature range of 1000– 2500 °C. Additional introduction of purified nanodiamond particles leads to a decrease in the pressure of transformation of thin layers of non-diamond carbon into a diamond structure from 10–15 to 2–7 GPa, which is due to the effect of the surface of catalytically active diamond nanoparticles on the thermodynamic stimulus of phase transformation.

Mixed-dimensional moiré systems of twisted graphitic thin films.

Moiré patterns formed by stacking atomically thin van der Waals crystals with a relative twist angle can give rise to notable new physical properties1,2. The study of moiré materials has so far been limited to structures comprising no more than a few van der Waals sheets, because a moiré pattern localized to a single two-dimensional interface is generally assumed to be incapable of appreciably modifying the properties of a bulk three-dimensional crystal. Here, we perform transport measurements of dual-gated devices constructed by slightly rotating a monolayer graphene sheet atop a thin bulk graphite crystal. We find that the moiré potential transforms the electronic properties of the entire bulk graphitic thin film. At zero and in small magnetic fields, transport is mediated by a combination of gate-tuneable moiré and graphite surface states, as well as coexisting semimetallic bulk states that do not respond to gating. At high field, the moiré potential hybridizes with the graphitic bulk states due to the unique properties of the two lowest Landau bands of graphite. These Landau bands facilitate the formation of a single quasi-two-dimensional hybrid structure in which the moiré and bulk graphite states are inextricably mixed. Our results establish twisted graphene-graphite as the first in a new class of mixed-dimensional moiré materials.

Basal-plane heat transport in graphite thin films

While the phonon hydrodynamic regime has recently been highlighted experimentally in graphite films, the understanding and modeling of heat transport along their basal plane remain elusive. From first-principles-based modeling, we predict a significant influence of the surface roughness on basal-plane thermal conductivity due to the collective phonon drift. The occurrence condition of the phonon Knudsen minimum is also shown to strongly depend on the surface roughness. We summarize the basal-plane heat transport along graphite thin films into coherent and incoherent regimes and also present a speculation for the recently observed anomalous thickness-dependent thermal conductivity. Our work provides guidance for the experimental explorations of phonon hydrodynamics in graphitic micro- and nanostructures in the future.

Electronically controlled infrared radiation of graphite thin films with crumpled surfaces

Electronically controlled infrared radiation of graphite thin films with crumpled surfaces

Flat Bands for Electrons in Rhombohedral Graphene Multilayers with a Twin Boundary

AbstractTopologically protected flat surface bands make thin films of rhombohedral graphite an appealing platform for searching for strongly correlated states of 2D electrons. In this work, rhombohedral graphite is studied with a twin boundary stacking fault and the semimetallic and topological properties of low‐energy bands, localized at the surfaces and at the twinned interface, are analyzed. An effective 4‐band low energy model is derived, where the full set of Slonczewski–Weiss–McClure (SWMcC) parameters is implemented, and the conditions for the bands are found to be localized at the twin boundary, protected from the environment‐induced disorder. This protection together with a high density of states at the charge neutrality point, in some cases—due to a Lifshitz transition, makes this system a promising candidate for hosting strongly‐correlated effects.

Systematic Investigations on the Thermal Management Performance of Graphite Thin Films for High-Electron-Mobility Transistors

Self-heating is a severe problem for high-electron-mobility transistors (HEMTs). Graphene and thin graphite films are attractive candidates for heat spreader materials at the transistor level due to their ultrahigh in-plane thermal conductivity. However, a realistic and systematic simulation study of how they can improve the thermal management of HEMTs has not been carried out yet and is needed to push the method into practical use and realize industrialization. This study systematically investigated the effect of graphene and thin graphite films, and the most practical boundary condition in the simulations was primarily selected with comparisons. Then, the thermal properties consisting of thermal conductivity and interfacial thermal resistance (ITR) between graphite and other materials were fully investigated, which may bottleneck the actual improvement of the thermal management in applications. Our steady-state results indicated that the ITR between graphite and SiO2 determined the whole improvement in thermal management performance through the added graphite heat bridge. Subsequently, the transient thermal analysis was conducted on the HEMTs to explore the effect of graphene and thin graphite films under repeated joule heating pulses on a microsecond time scale, and we obtained the effect of graphene and thin graphite films on each heating and cooling process. Finally, based on all the results and analysis mentioned above, we can confirm that the graphite-based heat-spreader structures can still offer efficient heat dissipation under near-real conditions, thereby indicating ways of the optimization of HEMTs heat dissipation.

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

The Effect of UV–Ozone Treatment on Structural, Optical, and Dielectric Properties of Thermally Evaporated Graphitic Carbon Nitride Thin Film

The Effect of UV–Ozone Treatment on Structural, Optical, and Dielectric Properties of Thermally Evaporated Graphitic Carbon Nitride Thin Film

INFLUENCE OF GRAPHITE FILM THICKNESS ON ELECTRICAL AND PHOTOELECTRIC PROPERTIES OF GRAPHITE/n-Si SCHOTTKY-TYPE HETEROJUNCTION

Graphite /n-Si Schottky diodes were fabricated by electron beam evaporation of graphite thin films on n-type silicon substrates. The influence of the thickness of graphite films on the photoelectric and electrical properties of these diodes has been studied. It is determined that our Schottky diodes can be used as photodiodes and solar cells. The temperature dependences of shunt and series resistances of diodes were also investigated. In the case of forward and reverse bias, the dominant mechanisms of current transfer through the studied diodes were determined. The responsivity and detectivity of graphite/n-Si Schottky diodes were also calculated. The studied heterojunctions have pronounced diode characteristics with a rectification coefficient for a structure with a thinner film RR ≈ 5·102, and for a structure with a thicker film RR ≈ 102.

Graphitic carbon nitride thin films for light-induced photocatalysis in a slit geometry microreactor

A combination of a photocatalysis and a micro-flow technology provides a highly capable tool for the design and utilisation of functional thin films. Here, they are represented by graphitic carbon nitride films, potentially used for photo-oxidations in environmental chemistry or for hydrogen evolution by water splitting. The optimised synthesis involves a study of the role of monomers, solvents, calcination temperatures, particle size, etc., in respect of the achieved photocatalytic activity. Films were prepared from the mixture of milled powder catalysts and a binder, dried, and subjected to post-processing treatments (post annealing, plasma application). The intended functionality was assessed in two types of reactions, photo-oxidation of a fluorescent dye, Rhodamine B, and photo-oxidation of a common antibiotic, Tetracycline. The performance was compared for the bulk catalyst, and for the thin film. In either case, the same part of light spectrum, concentrations, and other conditions were applied. However, for the thin films, a special experimental setup was utilised. It had the appearance of a micro-flow apparatus designed as a slit geometry photo-microreactor. Comparison of the two different reaction systems revealed the advantages of the thin film catalyst, which was immobilised directly on the photo-microreactor wall. The specific rate constant was more than 50 times higher in this arrangement for both model reactants. In addition, thin films of g-C3N4 were stable and well adherent during the microreactor tests with the repetitive reaction cycles. Plasma treatment considerably increased the hydrophilicity of the dried thin films, which resulted in their substantially higher photoactivity.

Ultrafast charge-transfer at interfaces between 2D graphitic carbon nitride thin film and carbon fiber towards enhanced photocatalytic hydrogen evolution

Homogeneous deposition of graphitic carbon nitride (g-C3N4) onto substrates on a large scale is of great interest in recyclable photocatalysis. However, g-C3N4 films prepared via conventional thermal condensation are thick and have problem of bad interface contacts with substrate, resulting in a poor photocatalytic performance. Herein, a thermal vapor condensation approach is developed for the growth of homogeneous g-C3N4 thin film on large area carbon fiber cloths (CFC). CFC serves as the template to guide g-C3N4 growth, resulting in a conformal thin overlayer of g-C3N4 on CFC surface. Moreover, a robust interaction between g-C3N4 and carbon fibers is achieved. A close stacking of g-C3N4 interlayers coupled with an efficient electron-withdrawing ability of carbon fiber expedites charge transfer and suppresses charge recombination. Consequently, the film exhibits an enhanced photocatalytic H2 evolution compared with g-C3N4 powder. A solid–gas bi-phase photocatalytic system is also constructed for manifesting remarkable superiority in gas evolution than solid–gas-liquid tri-phase system. Moreover, the sample is highly durable and recyclable, ensuring a robust, economical and environmentally friendly photocatalyst films.

Structural, Electrical and Optical Properties of Graphite Films are Drawn with Pencils of Different Hardness

The paper presents the results of studying the structural, optical and electrical properties of thin films of graphite depending on the hardness of the rods (2H, H, HB, B and 2B) obtained by the "Pencil-on-semiconductor" method. Such studies are of great importance for the further development of highly efficient devices based on heterojunctions for electronics and optoelectronics. Typical images of the surface formed by reflected electrons (BSE) were obtained using a scanning electron microscope and shown at three magnifications (100x, 500x and 1000x). Since the cores of the studied pencils consist of mixtures of clay and graphite, a more detailed analysis of the elements that make up the cores was conducted. EDS analysis showed that the main components of the studied rods are purified graphite powder, as well as O, Al and Si, which are part of kaolin whose formula is H4Al2Si2O9, or Al2O3 • 2SiO2 • 2H2O - the main component of ordinary clay. The elemental composition of the microvolume of the studied samples was also determined. Regardless of the error that occurs when determining the composition of C and O (~ 12%), it can be argued that there is still a regularity between the graphite content and the hardness of the pencil. That is, the higher the graphite content, the softer the rod. The thickness of the graphite films was measured using the MII-4 interferometer according to the standard method. The average thickness of all investigated films was ~ 150 nm since the thickness of the films obtained by this method is mainly determined by the roughness of the surface of the salt substrate. Drawn graphite films have a higher resistivity than bulk samples (pencil rods) from which they were made. The resistance of the films increases with an increase in the hardness of pencils, due to an increase in the number of clay impurities in graphite, which is a dielectric. It was found that an increase in stick hardness leads to an increase in transmission.