Ambipolar Electric Field Effect Research Articles

Graphene and graphene‐like structure from biomass for Electrochemical Energy Storage application‐ A Review

AbstractGraphene, an SP2 hybridized 2‐D atomic crystal, has been one of the most significant materials of the current century. Researchers in the recent years find graphene to be highly appealing for its superlative electrochemical energy storage applications and some of its unique features such as fractional quantum hall effect which is even exhibited in normal temperatures such as room temperature, ballistic conduction of charge carriers along with the exhibition of ambipolar electric field effect and modifiable bandgap. Enhanced electrochemical performances have been achieved by coalescing graphene with other electrochemical materials. This review focuses on various bio‐precursors involved in the synthesis of graphene and its diverse production methods along with their merits and demerits. It also summarizes the ensuing features and macro‐structural complexity of the graphene‐derived structural composites as well as its applications as competent electrode material in energy storage devices. It specifically endeavors in providing a comprehensible idea on the availability of a variety of biomass sources and its synthesis techniques for a better and optimized mass production of graphene with uncompromising purity as well as the perspectives of the graphene‐based composites that pave path for its improved application in advanced storage devices. This review article casts light upon the enormous opportunities of graphene production with easy synthesizing techniques that could aid in the fabrication of high performance of supercapacitors.

A Double Hemispherical Probe for Characterizing and Minimizing the Self‐Wake Effects on Probe Measurements

AbstractSpace‐borne Langmuir probes generally have a self‐wake behind themselves due to supersonic relative velocities (Mach number M > 1) between the spacecraft and ambient plasma ions. Such a wake may create difficulties for correctly measuring plasma characteristics. Here we present a new technique—the Double Hemispherical Probe (DHP) to characterize and minimize the self‐wake effects on probe measurements. The DHP consists of two hemispheres that are simultaneously swept with a bias voltage to obtain two independent current‐voltage (I‐V) curves, which are used to identify the wake effects of the probe. A laboratory DHP model is inserted in plasma flows (M > 10) created in the Colorado Solar Wind Experiment (CSWE) chamber. In this case, the ion current is the ram current collected by the upstream hemisphere of the DHP, leaving an ion wake behind the downstream hemisphere. A wide range of the Debye ratio RD (ratio of the probe radius to electron Debye length) is tested, and it is found that (1) when RD < 1, the electron currents collected by the two hemispheres are similar, indicating a uniform electron density around the probe, and (2) when RD > 1, the electron current collected by the downstream hemisphere becomes lower than the upstream, indicating a reduced electron density in the probe's wake due to the ambipolar electric field effect. In this case, the electron density measured by traditional single Langmuir probes will be underestimated. With the DHP, we can use the upstream hemisphere measurements to minimize the self‐wake effects.

Highly Selective Gas Sensors Based on Graphene Nanoribbons Grown by Chemical Vapor Deposition.

Despite the recent advances in bottom-up synthesis of different kinds of atomically precise graphene nanoribbons (GNRs) with very diverse physical properties, the translation of these GNRs into electronic devices remains challenging. Among other factors, the electronic characterization of GNRs is hampered by their complex synthesis that often requires custom-made organic precursors and the need for their transfer to dielectric substrates compatible with the conventional device fabrication procedures. In this paper, we demonstrate that uniform electrically conductive GNR films can be grown on arbitrary high-temperature-resistant substrates, such as metals, Si/SiO2, or silica glasses, by a simple chemical vapor deposition (CVD) approach based on thermal decomposition of commercially available perylenetetracarboxylic dianhydride molecules. The results of spectroscopic and microscopic characterization of the CVD-grown films were consistent with the formation of oxygen-terminated N = 5 armchair GNRs. The CVD-grown nanoribbon films exhibited an ambipolar electric field effect and low on-off ratios, which were in agreement with the predicted metallic properties of N = 5 armchair GNRs, and remarkable gas sensing properties to a variety of volatile organic compounds (VOCs). We fabricated a GNR-based electronic nose system that could reliably recognize VOCs from different chemical classes including alcohols (methanol, ethanol, and isopropanol) and amines (n-butylamine, diethylamine, and triethylamine). The simplicity of the described CVD approach and its compatibility with the conventional device fabrication procedures, as well as the demonstrated sensitivity of the GNR devices to a variety of VOCs, warrant further investigation of CVD-grown nanoribbons for sensing applications.

Carbon nanomaterial-based electrochemical biosensors for label-free sensing of environmental pollutants

Carbon allotropes such as graphene and carbon nanotubes, have been incorporated in electrochemical biosensors for highly sensitive and selective detection of various analytes. The superior physical and electrical properties like high carrier mobility, ambipolar electric field effect, high surface area, flexibility and their compatibility with microfabrication techniques makes these carbon nanomaterials easy to integrate in field-effect transistor (FET)/chemiresistor type configuration which is suitable for portable and point-of-use/field-deployable sensors. This review covers the synthesis of carbon nanostructures (graphene and CNTs) and their integration into devices using various fabrication methods. Finally, we discuss the recent reports showing different sensing platforms that incorporate biomolecules like enzymes, antibodies and aptamers as recognition elements for fabrication of simple, low cost, compact biosensors that can be used for on-site, rapid environmental monitoring of environmental pollutants like pathogens, heavy metals, pesticides and explosives.

Ambipolar Surface State Thermoelectric Power of Topological Insulator Bi2Se3

We measure gate-tuned thermoelectric power of mechanically exfoliated Bi2Se3 thin films in the topological insulator regime. The sign of the thermoelectric power changes across the charge neutrality point as the majority carrier type switches from electron to hole, consistent with the ambipolar electric field effect observed in conductivity and Hall effect measurements. Near the charge neutrality point and at low temperatures, the gate-dependent thermoelectric power follows the semiclassical Mott relation using the expected surface state density of states but is larger than expected at high electron doping, possibly reflecting a large density of states in the bulk gap. The thermoelectric power factor shows significant enhancement near the electron-hole puddle carrier density ∼0.5 × 10(12) cm(-2) per surface at all temperatures. Together with the expected reduction of lattice thermal conductivity in low-dimensional structures, the results demonstrate that nanostructuring and Fermi level tuning of three-dimensional topological insulators can be promising routes to realize efficient thermoelectric devices.

Experimental evidences of topological surface states of β-Ag2Te

We present evidence of topological surface states in β-Ag2Te through first-principles calculations, periodic quantum interference effect and ambipolar electric field effect in single crystalline nanoribbon. Our first-principles calculations show that β-Ag2Te is a topological insulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality β-Ag2Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of β-Ag2Te nanoribbon and therefore the existence of topological surface states, which is further supported by the ambipolar electric field effect for devices fabricated by β-Ag2Te nanoribbons. The experimental evidences of topological surface states and the theoretically predicted anisotropic Dirac cone of β-Ag2Te suggest that the material may be a promising candidate of topological insulator for fundamental study and future spintronic devices.

Indications of surface-dominated transport in single crystalline nanoflake devices of topological insulator Bi1.5Sb0.5Te1.8Se1.2

We report experimental evidence of surface dominated transport in single crystalline nanoflake devices of topological insulator Bi1.5Sb0.5Te1.8Se1.2. The resistivity measurements show dramatic difference between the nanoflake devices and bulk single crystal. The resistivity and Hall analysis based on a two-channel model indicates that ~99% surface transport contribution can be realized in 200 nm thick BSTS nanoflake devices. Using standard bottom gate with SiO2 as a dielectric layer, pronounced ambipolar electric field effect was observed in devices fabricated with flakes of 100 - 200 nm thick. Moreover, angle-dependent magneto-resistances of a nanoflake device with thickness of 596 nm are fitted to a universal curve for the perpendicular component of the applied magnetic field. The value of phase coherence length obtained from 2D weak antilocalization fitting further confirmed the surface dominated transport. Our results open a path for realization of novel electric and spintronic devices based on the topological helical surface states.

Field effect transistors and RC filters from pencil-trace on paper

We report the fabrication of Resistor-Capacitor (RC) filters and field effect transistors (FETs) based on pencil drawings on paper, which contain turbostratic graphite crystallites as evidenced from Raman analysis. Pencil drawings have been employed as resistor and an ion gel, 1-butyl-3-methylimidazolium octyl sulfate mixed with polydimethylsiloxane (PDMS) as dielectric, for the fabrication of RC filters with a cut-off frequency of 9 kHz. With ion gel as gate dielectric, an ambipolar electric field effect has been obtained from the pencil-trace at low operating voltages. The carrier mobilities were found to be ∼106 and 59 cm(2) V(-1) s(-1) for holes and electrons, respectively. The mobility value showed only 15% variation among the devices tested, truly remarkable given the simplicity of the fabrication process.

Synthesis and Selected Properties of Graphene and Graphene Mimics

Graphene has generated great excitement in the last few years because of its novel properties with potential applications. Graphene exhibits an ambipolar electric field effect, ballistic conduction of charge carriers, and the quantum Hall effect at room temperature. Some of the other interesting characteristics of graphene include high transparency toward visible light, high elasticity and thermal conductivity, unusual magnetic properties, and charge transfer interactions with molecules. In this Account, we present the highlights of some of our research on the synthesis of graphene and its properties. Since the isolation and characterization of graphene by micromechanical cleavage from graphite, several strategies have been developed for the synthesis of graphene with either a single or just a few layers. The most significant contribution from our laboratory is the synthesis of two to four layer graphene by arc-discharge of graphite in a hydrogen atmosphere. Besides providing clean graphene surfaces, this method allows for doping with boron and nitrogen. UV and laser irradiation of graphene oxide provides fairly good graphene samples, and laser unzipping of nanotubes produces graphene nanoribbons. We have exploited Raman spectroscopy to investigate the charge-transfer interactions of graphene with electron-donor and -acceptor molecules, as well as with nanoparticles of noble metals. Graphene quenches the fluorescence of aromatics because of electron transfer or energy transfer. Notable potential applications of the properties of graphene are low turn-on field emission and radiation detection. High-temperature ferromagnetism is another intriguing feature of graphene. Although incorporation of graphene improves the mechanical properties of polymers, its incorporation with nanodiamond or carbon nanotubes exhibits extraordinary synergy. The potential of graphene and its analogues as adsorbents and chemical storage materials for H(2) and CO(2) is noteworthy.

Synthetic Aspects and Selected Properties of Graphene

Graphene has generated great sensation owing to its fascinating properties with possible potential applications. This two-dimensional material exhibits half-integer quantum Hall effect and an ambipolar electric field effect, along with ballistic conduction of charge carriers. In this article, we provide a overview on some aspects of graphene devoting the special attention to synthesis, functionalization, self-assembly, surface properties, gas adsorption and fluorescence quenching ability of graphene. Graphenes with varying number of layers can be synthesized by using different strategies. Graphene can be functionalized by different means in order to disperse it in various solvents. We also present the self-assembly of graphene at the liquid-liquid interface besides its surface properties including adsorption of hydrogen, carbon dioxide and methane. The remarkable property of graphene of quenching fluorescence of aromatic molecules is shown to be associated with photo-induced electron transfer.

Conversion of Self-Assembled Monolayers into Nanocrystalline Graphene: Structure and Electric Transport

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tunable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfill this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well-defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility, and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects toward the engineering of free-standing 2D carbon materials with tunable properties on various solid substrates and on holey substrates as suspended membranes.

Electronic transport in monolayer graphene nanoribbons produced by chemical unzipping of carbon nanotubes

We report on the structural and electrical properties of graphene nanoribbons (GNRs) produced by the oxidative unzipping of carbon nanotubes. GNRs were reduced by hydrazine at 95 °C and further annealed in Ar/H2 at 900 °C; monolayer ribbons were selected for the fabrication of electronic devices. GNR devices on Si/SiO2 substrates exhibit an ambipolar electric field effect typical for graphene. The conductivity of monolayer GNRs (∼35 S/cm) and mobility of charge carriers (0.5–3 cm2/V s) are less than the conductivity and mobility of pristine graphene, which could be explained by oxidative damage caused by the harsh H2SO4/KMnO4 used to make GNRs. The resistance of GNR devices increases by about three orders of magnitude upon cooling from 300 to 20 K. The resistance/temperature data is consistent with the variable range hopping mechanism, which, along with the microscopy data, suggests that the GNRs have a nonuniform structure.

Charge Carriers in Few-Layer Graphene Films

The nature of the charge carriers in 2D few-layer graphites (FLGs) has been recently questioned by transport measurements [K. S. Novoselov, Science 306, 666 (2004)10.1126/science.1102896] and a strong ambipolar electric field effect has been revealed. Our density functional calculations demonstrate that the electronic band dispersion near the Fermi level, and consequently the nature of the charge carriers, is highly sensitive to the number of layers and the stacking geometry. We show that the experimentally observed ambipolar transport is only possible for an FLG with a Bernal-like stacking pattern, whereas simple-carrier or semiconducting behavior is predicted for other geometries.

Electric Field Effect in Atomically Thin Carbon Films

We describe monocrystalline graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are found to be a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar electric field effect such that electrons and holes in concentrations up to 10(13) per square centimeter and with room-temperature mobilities of approximately 10,000 square centimeters per volt-second can be induced by applying gate voltage.

No Sign Dependence of Ambipolar Radial Electric Field Effect on Resistive Fluid Turbulence

No Sign Dependence of Ambipolar Radial Electric Field Effect on Resistive Fluid Turbulence