Nanographite Films Research Articles

Room-temperature superconductivity in carbons – a mini review

The search for room-temperature superconductivity in carbons is gathering momentum because it has a long history, impressive track record, clear advancement route, and theoretical backup. The report of a suspected Josephson current in Al-C-Al sandwiches at room temperature, published in Nature 50 years ago, led to the report of a voltage in nano-graphite films believed to mark the reverse Josephson effect. The report on oxygen-doped diamonds led to full-blown superconductivity in boron-doped diamonds. The reports on highly oriented pyrolytic graphite (HOPG) and water-treated graphite powder led to the claim of a current running by itself for 50 days in a ring of HOPG soaked with alkane at room temperature. It also led to a claim of a current running for 1,000 seconds in a ring of graphene soaked in hexane. The carbons are amicable to intercalation chemistry, electrostatic carrier doping, and surface-proving techniques to promote an increase in carrier density, metallization, and applicability of the Ashcroft proposal. Consequently, the high Debye temperatures (1,670, 2,230 and 2,500 K in graphene, diamond and HOPG) may be sufficient to underwrite superconductivity at T∼300 K in carbons, provided the phonon-exchange factor, λ, is around 3 to mark strong enough interactions between electrons and phonons.

Giant hot electron thermalization via stacking of graphene layers

The capability of graphene to generate hot electrons is predicted to be effective in converting low energy photons into electrical currents for the mid-infrared photodetection [1,2]. However, the quantum yield of such hot electrons is not sufficient due to the limited thickness of two-dimensional graphene [3-5]. Therefore, it raises the question whether the electron thermalization is efficient enough to generate a large number of hot electrons in graphitic materials as a detectable photocurrent. Here, an experimental demonstration of the sufficient hot electron generation in Bernal stacking sequence nano-graphite films is presented. A comprehensive layer number dependence (1–120-layers graphene) study verifies the strong hot electron scattering correlations, exhibiting intriguing two-dimensional properties into their bulk counterparts. Consequently, the spectral coverage of hot electrons promoted from mid-infrared (4 μm) to near-infrared (1.2–1.6 μm) energy level is achieved, leading to a 109 eV−1 cm−2 populated hot electron density for the mid-infrared photodetection. In addition, the consistently increased number of photo-excited electrons via stacking of graphene layers, results in a gradual evolution of subsequent electron thermalization. The proposed scheme for exploring the thickness dependence electron thermalization property of the graphitic material paves the way to design ultrafast and sensitive mid-infrared photodetecters.

A sensing strategy based on aptamers alkylated with melphalan and graphite nanocrystals in a bed of tetrahedral amorphous carbon for electrochemical detection of lead ions in human urine

We report a voltammetric aptasensor based on the graphite nanocrystals in a bed of tetrahedral amorphous carbon (GNC-TAC) and guanine (G)-rich aptamers alkylated with melphalan anticancer for determination of ultratrace Pb2+ in the clinical samples. In this work, an adhesive and stable layer of GNC-TAC was formed on the cellulose paper by the ion beam sputtering deposition (IBSD) method. This porous conductive nanographite film plays a key role in providing a large area for adsorption of ferrocene carboxylic acid (FC-COOH) anticancer as a redox indicator. Then, melphalan that attaches the alkyl groups to the guanine bases of DNA aptamer was bonded covalently to the carboxylic groups of FC-COOH via the aid of EDC/NHS. The introduction of Pb2+ ions to the aptamer/melphalan/FC-COOH/GNC-TAC/paper electrode leads to the formation of a G-quadruplex (G4)/Pb2+ complex and dissociation of this complex from the modified electrode surface. The target-induced aptamer release from melphalan/FC-COOH/GNC-TAC/paper electrode makes the sensing interface smaller which results in an increase in the current signal. Based on this signal-on mechanism, the sensitive detection of Pb2+ ions in the dynamic linear range (DLR) of 0.5–700.0 μg L−1 is realized (limit of detection (LOD): 0.15 μg L−1). The present strategy shows remarkable advantages including high selectivity and wide detectable range. More importantly, the good consistency of the results obtained with the aptasensor and those by the quality control material (QCM) analysis and atomic absorption spectroscopy equipped with graphite furnace (GF-AAS) technique showed that the proposed strategy has a high potential for Pb2+ detection in the clinical samples.

Auger scattering dynamic of photo-excited hot carriers in nano-graphite film

Charge carrier scattering channels in graphite bridging its valence and conduction band offer an efficient Auger recombination dynamic to promote low energy charge carriers to higher energy states. It is of importance to answer the question whether a large number of charge carriers can be promoted to higher energy states to enhance the quantum efficiency of photodetectors. Here, we present an experimental demonstration of an effective Auger recombination process in the photo-excited nano-graphite film. The time-resolved hot carrier thermalization was analyzed based on the energy dissipation via the Auger scattering channels. We split the Auger recombination occurrence centered at 0.40 eV energy state into scattering and recombination parts, for characterizing the scattering rate in the conduction band and the recombination rate toward the valence band. The scattering time with respect to the energy state was extracted as 8 ps · eV−1, while the recombination time with respect to the energy state was extracted as 24 ps · eV−1. Our study indicates a 300 fs delay between the hot carrier recombination and generation, leading to a 105 ps−1 · cm−3 Auger scattering efficiency. The observed duration for the Auger recombination to generate hot carriers is prolonged for 1 ps, due to the hot carriers energy relaxation bottleneck with optical-phonons in the nano-graphite. The presented analytic expression gives valuable insights into the Auger recombination dynamic to estimate its most efficient energy regime for mid-infrared photodetection.

Josephson Effect in Nanographite Films

Josephson Effect in Nanographite Films

Nano-graphite field-emission cathode for space electric propulsion systems

Improving the thruster efficiency is a crucial challenge for the development of space electric propulsion systems, especially advanced air-breathing thrusters utilizing the surrounding rarefied atmosphere as fuel. A significant reduction in thruster power consumption can be achieved by using field emission (FE) cathodes that do not require heating and have the highest energy efficiency. In this work, we study FE from nano-graphite thin films, consisting of carbon nanostructures with a high aspect ratio, and demonstrate their suitability for use in the space electric propulsion systems. The films shown appropriate FE characteristics in a wide range of gas pressures at high current loads in constant and pulsed operation modes. Based on the obtained experimental results, nano-graphite cathodes were employed for the design of an electron gun with increased reliability and minimized energy losses associated with electron extraction. The possibility of using such a gun in a specific air-breathing satellite operating in low Earth orbits is demonstrated.

Electrophoretic Deposition and Structural Analysis of Nanographite Films

Electrophoretic Deposition and Structural Analysis of Nanographite Films

Josephson effect in nanographite films

In nanographite films, for the first time the Josephson current at room temperature has been obtained when measuring the current-voltage characteristics. Sach measurement confirms the earlier observations in nanographite film the effects of weak superconductivity: zeroing of the temperature dependence at 650 K of constant voltage on the sample when it exposed to an alternating microwave voltage due to the inverse Josephson effect, as well as the observation of local areas with the structure of magnetic vortices in a magnetic force microscope at room temperature. The resulting critical current value of 0.8 μA is significantly lower than expected values for the superconducting gap, as well as for pinning at the Bean--Levingston barrier. The measures for increasing the critical current is proposed. Keywords: Josephson effect, nanographite film, superconductivity, room temperature.

Эффект Джозефсона в нанографитовых пленках

In nanographite films, for the first time the Josephson current at room temperature has been obtained when measuring the current–voltage characteristics. Sach measurement confirms the earlier observations in nanographite film the effects of weak superconductivity: zeroing of the temperature dependence at 650 K of constant voltage on the sample when it exposed to an alternating microwave voltage due to the inverse Josephson effect, as well as the observation of local areas with the structure of magnetic vortices in a magnetic force microscope at room temperature. The resulting critical current value of 0.8 µA is significantly lower than expected values ​​for the superconducting gap, as well as for pinning at the Bean–Levingston barrier. The measures for increasing the critical current is proposed. .

Возможности увеличения мощности нанографитового токоограничителя

The article discusses the possibilities of increasing the switching currents of the previously proposed contactless nanographite current limiter. It is shown that an increase in the length of the contact pads to 1 m will reduce the electrical resistance of the film by a factor of 10,000 – to ~ 0.1 Ohm, and the switching current will increase to 1 kA, while the power dissipated by a contactless nanographite current limiter will increase to 100 kW. The possibility of heat removal by means of water cooling using a chiller of appropriate capacity is being considered. The analysis shows that these measures will make it possible to create a competitive current limiter for smart grids based on nanographite film.

Traces of superconducting correlations in nanographite films

The review of structural, electronic and magnetic properties of nanographite films is presented. The superconducting correlations such as AC-to-DC conversion associated with the reversed Josephson Effect, pinning of vortices on columnar topological structure of film surface observed in atomic force and magnetic force microscope, non-zero current at zero voltage in scanning tunneling microscope in local area of nanographite film surface have been found. These results are broadly in line with other our observations on abrupt resistivity jump accompanied by light emission having potential applications as switchers and compact light emitter. Further experiments on studies of local conductivity related to ion irradiation hoping to find a zero resistance state are proposed.

NANO GRAPHITE FILM-CONTROLLED ELECTRICAL SWITCH

The effect of switching electrical resistivity in nanographite films is described. In contrast to the cases published earlier, switching in nanographite films occurs from a stable state with high conductivity to a metastable state with low conductivity. The critical switching current varies in the range of 10–500 mA and can be increased to values of 100 A or more, suitable for using nanographite switches in intelligent electric networks as contactless current limiters and circuit breakers. Possible mechanisms of the switching phenomenon in nanographite films are discussed.

Effect of nanometric and micronic particles size on physical and electrical properties of graphite thick film

Precise control and design of the microstructure of active layers in thick films is essential as it has a profound impact on the overall thick film properties. This includes the particle size effect of the starting material which forms the active layer of the thick film. Given the fact that nanosized particles have distinct properties from their bulk counterpart, the effect of the size of graphite starting material towards the formation of thick film graphite and its properties is worthwhile studying. Herein, we report on the study of effect of different particle size of graphite starting material towards the thermal stability and rheological properties of nanographite and micrographite thick film paste, and electrical conductivity of graphite thick film. Graphite starting material with the average particle size of <20 μm was used as source of micron size particles while the same material was milled for 6 h to generate nanosized particles in the range from 13.3 mm to 116.5 nm. Organic binder solution comprising linseed oil, m-xylene and α-terpineol was mixed with the nano and micron-sized particles of graphite powder respectively at 40°C for 24 h. Thermal analysis results showed that thick film paste obtained from nanosized graphite starting material (A) decomposed at lower temperature than the paste prepared from micron-sized graphite starting material (B). Both graphite paste samples with different particle size were printed onto alumina substrates using tape casting method followed by subsequent heating at 300°C and 350°C. Viscosity measurements revealed that the viscosity of paste obtained from nanosized graphite was significantly higher than the paste obtained from micron-sized graphite. The electrical resistance of thick film obtained from micron-sized graphite was lower than the resistance of thick film obtained from nanosized graphite, suggesting that the defects introduced to the particles from the milling process could have impeded the movement of electrons. In addition, the tendency of nanosized particles to react with other molecules, namely oxygen is much greater due to increased surface area, which in turn has also contributed to the higher value of electrical resistance.

Preparation, Characterization, and Performance Control of Nanographitic Films.

Using methane as a carbon source, low-dimensional carbon nanomaterials were obtained in this work. The films were deposited directly on glass substrates by radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The configuration and compositions of this nanographite films were identified by X-ray photoelectron spectroscopy (XPS) as carbon in sp2 bonding form. Raman spectral characterization verified the configuration of the films to be hexatomic ring of carbon atoms. As a result, they were found to be nanographite films (NGFs). Also, the atomic force microscopy (AFM) topography and Raman spectra of different areas demonstrated the diversity of the films at the nano scale. The high light-transmitting and electron mobility indicated that the NGFs possessed excellent optic-electronic properties and could be used as good photoelectrical function materials. Furthermore, the physical and chemical growth mechanism of NGFs were analyzed by PECVD. NGFs could be obtained in a controlled process by modulating the growth conditions. In this work, the complicated transfer process commonly used for optoelectronic devices could be avoided. Also, by growing the films directly on a glass substrate, the quality degradation of the film was not a problem. This work can further promote the development of next-generation electronic or optoelectronic function materials, especially for their application in transparent conductive electrode fields.

Thickness-controlled direct growth of nanographene and nanographite film on non-catalytic substrates

Metal-catalyzed chemical vapor deposition (CVD) has been broadly employed for large-scale production of high-quality graphene. However, a following transfer process to targeted substrates is needed, which is incompatible with current silicon technology. We here report a new CVD approach to form nanographene and nanographite films with accurate thickness control directly on non-catalytic substrates such as silicon dioxide and quartz at 800 °C. The growth time is as short as a few seconds. The approach includes using 9-bis(diethylamino)silylanthracene as the carbon source and an atomic layer deposition (ALD) controlling system. The structure of the formed nanographene and nanographite films were characterized using atomic force microscopy, high resolution transmission electron microscopy, Raman scattering, and x-ray photoemission spectroscopy. The nanographite film exhibits a transmittance higher than 80% at 550 nm and a sheet electrical resistance of 2000 ohms per square at room temperature. A negative temperature-dependence of the resistance of the nanographite film is also observed. Moreover, the thickness of the films can be precisely controlled via the deposition cycles using an ALD system, which promotes great application potential for optoelectronic and thermoelectronic-devices.

Electrochemical characterization of mesoporous nanographite films

The mesoporous nanographite (NG) film material consisting of tiny flake-like graphite crystallites of nanometer-scale thickness is deposited onto carbon fibers with diameter of few micrometers using adapted plasma enhanced chemical vapor deposition (CVD) method. Macroscopic morphology parameters of NG material depend on deposition time duration so, that thickness of the NG coating is increased from 0.5 to 2 μm, whereas the average sizes of the individual crystallites and the pores between them are decreased from 0.5 to 0.1 nm, with deposition time extension from 15 to 60 min, respectively. The high resolution electron microscopy examinations indicate that the NG flakes consist of parallel graphene layers separated by about 0.34 nm and those layers are paired at the edges of the flakes producing arced structures. The electrochemical capacitance measurements were served for characterization of the mesoporous nanographite coating; the specific capacitance arising from the electrostatic charge accumulated at the electrode–electrolyte interface was estimated using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrical impedance spectroscopy. The effects of the deposition time duration and other parameters of CVD process on electrochemical performance and structural properties of the prepared NG films are to be discussed.

Nanographite Films: Structure and Properties

Nanographite film structures of different morphology have been grown on various substrates using the activated carbon fibers (ACF) as a source of nanographites. As was revealed from the data of Raman spectroscopy, the fabricated films consisted mainly of the same structural blocks as the initial ACF. Scanning electron microscopy was used to study the films morphology. The presence of lengthy zigzag edges in nanographites, which is prerequisite for their nontrivial electronic structure and magnetic characteristics, has been established. The X-ray photoelectron spectroscopy data show the appearance of expressed maxima in the C1s spectra of the films in the usually observable "diffuse" structure of π→π* shake-up satellites, π and π + σ plasmons.

Atomic layer deposition of TiO2and Al2O3on nanographite films: structure and field emission properties

Atomic layer deposition (ALD) of metal oxides (MO) was used to modify the properties of nanographite (NG) films produced by direct current plasma–enhanced chemical vapor deposition technique. NG films consist of a few layers of graphene flakes (nanowalls) and nanoscrolls homogeneously distributed over a silicon substrate with a predominantly vertical orientation of graphene sheets to the substrate surface. TiO2 and Al2O3 layers, with thicknesses in the range of 50 to 250 nm, were deposited on NG films by ALD. The obtained NG-MO composite materials were characterized by scanning electron microscopy, energy dispersive x-ray analysis, and Raman spectroscopy. It was found that ALD forms a uniform coating on graphene flakes, while on the surface of needle-like nanoscrolls it forms spherical nanoparticles. Field emission properties of the films were measured in a flat vacuum diode configuration. Analysis based on obtained current–voltage characteristics and electrostatic calculations show that emission from NG-TiO2 films is determined by the nanoscrolls protruding from the TiO2 coverage. The TiO2 layers with thicknesses of <200 nm almost do not affect the overall field emission characteristics of the films. At the same time, these layers are able to stabilize the NG films’ surface and can lead to an improvement of the NG cold cathode performance in vacuum electronics.

Field emission spectroscopy evidence for dual-barrier electron tunnelling in nanographite

Nanocarbon films with upstanding flake-like graphite crystallites of nanometre thickness were fabricated by carbon condensation from a methane–hydrogen gas mixture activated by a direct-current discharge. The nanographite (NG) crystallites are composed of a few graphene layers. The adjacent atomic layers are connected partially at the edges of the crystallites to form strongly curved graphene structures. The extraordinary field emission (FE) properties were revealed for the NG films with an average current density of a few mA/cm2, reproducibly obtained at a macroscopic applied field of about 1 V/μm. The integral FE current–voltage curves and electron spectra (FEES) of NG cathodes with multiple emitters were measured in a triode configuration. Most remarkably, above a threshold field, two peaks were revealed in FEES with different field-dependent shifts to lower energies. This behaviour evidences electron emission through a dual potential barrier, corresponding to carbon–carbon heterostructure formed as a result of the graphene bending.

CVD nanographite films covered by ALD metal oxides: structural and field emission properties

AbstractNanographite films produced by direct current plasma enhanced chemical vapour deposition technique were covered by metal oxides TiO2 and Al2O3 using atomic layer deposition. Nanographite films are composed of few‐layer graphene nanowalls and nanoscrolls, which are mostly oriented perpendicular to the silicon substrate surface. The nanographite and nanographite‐metal oxide composite film materials, with oxide layer thickness between 50 nm and 250 nm, were characterized by scanning electron microscopy, energy dispersive X‐ray analysis, thermal gravimetric analysis, and Raman spectroscopy. We demonstrate that the metal oxide deposit homogenously covers graphene nanowalls and forms individual spherical particles on the surface of needle‐like nanoscrolls. Field emission properties of the samples were measured in a flat vacuum diode configuration. It was found that the metal oxide layers thinner than 200 nm do not significantly affect the field emission characteristics of the nanographite. At the same time, metal oxides form a passivation shell which may improve the nanographite film performance as cold cathode in vacuum electronic devices. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)