Showing papers on "Quantum capacitance published in 2014"
TL;DR: The electrical double-layer capacitance in one to five-layer graphene is measured and it is found that the capacitances are suppressed near neutrality, and are anomalously enhanced for thicknesses below a few layers.
Abstract: Experimental electrical double-layer capacitances of porous carbon electrodes fall below ideal values, thus limiting the practical energy densities of carbon-based electrical double-layer capacitors. Here we investigate the origin of this behaviour by measuring the electrical double-layer capacitance in one to five-layer graphene. We find that the capacitances are suppressed near neutrality, and are anomalously enhanced for thicknesses below a few layers. We attribute the first effect to quantum capacitance effects near the point of zero charge, and the second to correlations between electrons in the graphene sheet and ions in the electrolyte. The large capacitance values imply gravimetric energy storage densities in the single-layer graphene limit that are comparable to those of batteries. We anticipate that these results shed light on developing new theoretical models in understanding the electrical double-layer capacitance of carbon electrodes, and on opening up new strategies for improving the energy density of carbon-based capacitors.
583 citations
TL;DR: In this article, the present status of various aspects of this important class of materials is discussed and a review of the recent literature on this subject citing all the major references is provided.
347 citations
TL;DR: In this paper, a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes is presented, where a bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach.
Abstract: We present a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach, showing good electronic quality with mobilities ranging from 17 000 cm2 V−1 s−1 at room temperature to 49 000 cm2 V−1 s−1 at 4.2 K, and entering the quantum Hall regime below 0.5 T. This method provides a strong and useful tool for the fabrication of future high quality layered crystal devices.
333 citations
TL;DR: This work fabricates the thinnest possible nanocapacitor system, essentially consisting of only monolayer materials: h-BN with graphene electrodes, and finds a significant increase in capacitance below a thickness of ∼5 nm, more than 100% of what is predicted by classical electrostatics.
Abstract: Conventional wisdom suggests that decreasing dimensions of dielectric materials (e.g., thickness of a film) should yield increasing capacitance. However, the quantum capacitance and the so-called “dead-layer” effect often conspire to decrease the capacitance of extremely small nanostructures, which is in sharp contrast to what is expected from classical electrostatics. Very recently, first-principles studies have predicted that a nanocapacitor made of graphene and hexagonal boron nitride (h-BN) films can achieve superior capacitor properties. In this work, we fabricate the thinnest possible nanocapacitor system, essentially consisting of only monolayer materials: h-BN with graphene electrodes. We experimentally demonstrate an increase of the h-BN films’ permittivity in different stack structures combined with graphene. We find a significant increase in capacitance below a thickness of ∼5 nm, more than 100% of what is predicted by classical electrostatics. Detailed quantum mechanical calculations suggest t...
124 citations
TL;DR: In this paper, the authors demonstrate that the low theoretical quantum capacitance of graphene-based electrodes can be significantly improved by altering local structural and morphological features such as point defects, dopants, strain, and surface rippling.
Abstract: Using density-functional theory calculations on a variety of model surfaces, we demonstrate that the low theoretical quantum capacitance of graphene-based electrodes can be significantly improved by altering local structural and morphological features. Common point defects, dopants, strain, and surface rippling are considered, as well as differences between locally single-layer and multilayer configurations. Local curvature is particularly effective at improving quantum capacitance, as is the inclusion of certain point defects and substitutional dopants at sufficiently high concentrations. We also show that single-layer graphene exhibits poor screening behavior of the double-layer potential when compared with multilayer samples, which suggests that higher area-specific capacitance can be obtained with samples a few layers thick. Overall, our results demonstrate the viability of local structural engineering as a tool to optimize graphene derivatives for use as supercapacitor electrodes, potentially increas...
121 citations
TL;DR: D density functional theory calculations are used to investigate the interactions of Li with a wide variety of sp(2) C substrates, including pristine, defective, and strained graphene, planar C clusters, nanotubes, C edges, and multilayer stacks, and suggest specific guidelines for designing more effective C-based anodes.
Abstract: Many key performance characteristics of carbon-based lithium-ion battery anodes are largely determined by the strength of binding between lithium (Li) and $s{p}^{2}$ carbon (C), which can vary significantly with subtle changes in substrate structure, chemistry, and morphology. Here, we use density functional theory calculations to investigate the interactions of Li with a wide variety of $s{p}^{2}$ C substrates, including pristine, defective, and strained graphene, planar C clusters, nanotubes, C edges, and multilayer stacks. In almost all cases, we find a universal linear relation between the Li-C binding energy and the work required to fill previously unoccupied electronic states within the substrate. This suggests that Li capacity is predominantly determined by two key factors---namely, intrinsic quantum capacitance limitations and the absolute placement of the Fermi level. This simple descriptor allows for straightforward prediction of the Li-C binding energy and related battery characteristics in candidate C materials based solely on the substrate electronic structure. It further suggests specific guidelines for designing more effective C-based anodes. The method should be broadly applicable to charge-transfer adsorption on planar substrates, and provides a phenomenological connection to established principles in supercapacitor and catalyst design.
95 citations
TL;DR: In this article, the authors investigated the impact of point-like topological defects in graphene on the electronic structure and quantum capacitance, and showed that the presence of defects, such as Stone Wales, di-vacancies, and di-interstitials, can substantially enhance the capacitance when compared to pristine graphene.
73 citations
TL;DR: The theory that surface groups and defects in the graphitic structure act as dopants that allow facile movement of ions into pores, improve screening in the superionic state, and affect the quantum capacitance contribution from the carbon structure is corroborated.
Abstract: This article reports on changes in electric double layer charge storage capacity as a function of surface chemistry and graphitic structure of porous carbon electrodes. By subjecting 20 nm to 2.0 μm sized carbide-derived carbons (CDCs) synthesized at 800 °C to high-temperature vacuum annealing at 700–1800 °C, we produce three-dimensional internal surface architectures with similar pore sizes and volumes but divergent surface chemistry and wall graphitization. Annealing increases carbon ordering and selectively removes functional groups, and both transformations affect conductivity and wettability. Contrary to an expected increase in gravimetric capacitance, we demonstrate no increases in charge storage despite increased conductivity and pore accessibility. At the same time, annealing improves the charge/discharge rates in EMIm-TFSI ionic liquid electrolyte. The annealing process eliminates faradaic reactions that limit the voltage window, but potentially accelerates catalytic breakdown of the ions themselves. We therefore corroborate the theory that surface groups and defects in the graphitic structure act as dopants that allow facile movement of ions into pores, improve screening in the superionic state, and affect the quantum capacitance contribution from the carbon structure.
58 citations
TL;DR: The Fermi level or electrochemical signature of a molecular film containing accessible orbital states is ultimately governed by two measurable series energetic components, an energy loss term related to the charging of appropriately addressable molecular orbitals, and an energy storage or Electrochemical capacitance component.
Abstract: The Fermi level or electrochemical signature of a molecular film containing accessible orbital states is ultimately governed by two measurable series energetic components, an energy loss term related to the charging of appropriately addressable molecular orbitals (resonant or charge transfer resistance), and an energy storage or electrochemical capacitance component. The latter conservative term is further divisible into two series contributions, one being a classic electrostatic term and the other arising from the involvement and charging of quantized molecular orbital states. These can be tuned in and out of resonance with underlying electrode states with an efficiency that governs electron transfer kinetics and an energetic spread dependent on solution dielectric. These features are experimentally resolved by an impedance derived capacitance analysis, a methodology which ultimately enables a convenient spectroscopic mapping of electron transfer efficacy, and of density of states within molecular films.
56 citations
TL;DR: The temperature dependence of the carrier mobility in the high density regime indicates that short-range scatterers limit charge transport at low temperatures, and quantum capacitance can be roughly estimated to be on the order of 1 μF/cm2 for all devices studied.
Abstract: Charge transport in MoS2 in the low carrier density regime is dominated by trap states and band edge disorder. The intrinsic transport properties of MoS2 emerge in the high density regime where conduction occurs via extended states. Here, we investigate the transport properties of mechanically exfoliated mono-, bi-, and trilayer MoS2 sheets over a wide range of carrier densities realized by a combination of ion gel top gate and SiO2 back gate which allows us to achieve high charge carrier (>10^13) density. We discuss the gating properties of the devices as a function of layer thickness and demonstrate resistivities of as low as 1 k{\Omega} for monolayer and 420{\Omega} for bilayer devices at 10 K. We show that from the capacitive coupling of the two gates, quantum capacitance can be roughly estimated to be on the order of 1 {\mu}F/cm^2 for all devices studied. Temperature dependence of the carrier mobility in the high density regime indicates that short-range scatterers limit charge transport at low temperatures.
50 citations
TL;DR: In this article, a very thin metal or metal-like layer (a quantum metal) between the ferroelectric (FE) and the semiconductor channel is proposed to attenuate the polarization charge of the FE, applying an appropriate charge to the semiconductors, while at the same time presenting a relatively constant capacitance to the FE layer, as is needed to stabilize the negative capacitance regime.
Abstract: It has recently been suggested that ferroelectric (FE) negative capacitance effects can be used to achieve steep subthreshold slope field-effect transistors, which are greatly desired for reducing energy consumption in modern digital electronics. Here, we propose that this concept can be improved by the introduction of a very thin metal or metal-like layer (a quantum metal) between the FE and the semiconductor channel. We show how to design this layer so that it attenuates the polarization charge of the FE, applying an appropriate charge to the semiconductor, while at the same time presenting a relatively constant capacitance to the FE layer, as is needed to stabilize the negative capacitance regime. For homogeneous polarization, we estimate that this device (a QMFeFET) can have extremely steep subthreshold characteristics (2 mV/decade over 11 decades) and that its energy and delay performance are advantageous.
TL;DR: In this article, the influence of edge-passivated zigzag graphene nanoribbons (ZGNRs) on both the quantum and electric double layer (EDL) capacitances of supercapacitors was investigated.
Abstract: The inherently large surface area and electrical conductivity of graphene-like electrodes have motivated extensive research for their use in supercapacitors. Although these properties are beneficial for the electric double layer (EDL) capacitance, the full utilization of graphene is curtailed by its intrinsically limited quantum capacitance due to the low density of electronic states near the neutrality point. While recent work has demonstrated that modifications to graphene can generally mitigate this limitation, a comprehensive analysis of the impact of graphene edges, which can be created during synthesis and post-treatment, has yet to be reported. Using a theoretical approach, we investigate the influence of graphene edges on both the quantum and EDL capacitances using edge-passivated zigzag graphene nanoribbons (ZGNRs) in [BMIM][PF6] ionic liquid as model systems. Our findings show that the presence of edges improves the quantum capacitance by increasing the electronic density of states, which is fur...
TL;DR: In this paper, a wireless vapor sensor based on the quantum capacitance effect in graphene is demonstrated, which consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit.
Abstract: A wireless vapor sensor based on the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 kHz/%RH ± 0.3 kHz/%RH. The capacitance values extracted from the wireless measurements agree with those determined from capacitance-voltage measurements, providing strong evidence that the sensing arises from the variable quantum capacitance in graphene. These results represent a new sensor transduction mechanism and pave the way for graphene quantum capacitance sensors to be studied for a wide range of chemical and biological sensing applications.
TL;DR: In this article, a model for graphene electrolyte-gated field effect transistors (EGFETs) is presented that incorporates the effects of the graphene-electrolyte interface and quantum capacitance of graphene.
Abstract: This paper presents a model for graphene electrolyte-gated field-effect transistors (EGFETs) that incorporates the effects of the graphene-electrolyte interface and quantum capacitance of graphene. The model is validated using experimental data collected from fabricated graphene EGFETs and is employed to extract device parameters such as mobility, minimum carrier concentration, interface capacitance, contact resistance, and effective charged impurity concentration. The proposed graphene EGFET model accurately determines a number of properties necessary for circuit design, such as current-voltage characteristics, transconductance, output resistance, and intrinsic gain. The model can also be used to optimize the design of EGFETs. For example, simulated and experimental results show that avoiding the practice of partial channel passivation enhances the transconductance of graphene EGFETs.
TL;DR: In this paper, a dual-gate G-FET with an electrolyte phase was investigated, where the electrolyte was used to gate the graphene channel and the gate induced band filling potential was determined.
Abstract: We report here an investigation of graphene field-effect transistors (G-FETs) in which the graphene channel is in contact with an electrolyte phase. The electrolyte and the ultrathin nature of graphene allow direct measurement of the channel electrochemical potential versus a reference electrode also in contact with the electrolyte. In addition, the electrolyte can be used to gate the graphene; i.e., a dual-gate structure is realized. We employ this electrolyte modified G-FET architecture to (1) track the Fermi level of the graphene channel as a function of gate bias, (2) determine the density of states (i.e., the quantum capacitance CQ) of graphene, and (3) separate the gate induced band filling potential δ from the electrochemical double-layer charging potential ΔϕEDL. Additionally, we are able to determine the electric double-layer capacitance CEDL for the graphene/electrolyte interface, which is ∼5 μF/cm2, the same order of magnitude as CQ. Overall, the electrolyte modified G-FETs provide an excellent...
TL;DR: The findings indicate that metal-doping of graphene-like electrodes can be a promising route toward increasing the interfacial capacitance of electrochemical double layer capacitors, primarily by enhancing the quantum capacitance.
Abstract: Chemically doped graphene-based materials have recently been explored as a means to improve the performance of supercapacitors. In this work, we investigate the effects of 3d transition metals bound to vacancy sites in graphene with [BMIM][PF6] ionic liquid on the interfacial capacitance; these results are compared to the pristine graphene case with particular attention to the relative contributions of the quantum and electric double layer capacitances. Our study highlights that the presence of metal-vacancy complexes significantly increases the availability of electronic states near the charge neutrality point, thereby enhancing the quantum capacitance drastically. In addition, the use of metal-doped graphene electrodes is found to only marginally influence the microstructure and capacitance of the electric double layer. Our findings indicate that metal-doping of graphene-like electrodes can be a promising route toward increasing the interfacial capacitance of electrochemical double layer capacitors, pri...
TL;DR: In this article, the transport properties of monolayer graphene with a laterally modulated potential profile were investigated, employing striped top gate electrodes with spacings of 100 to 200 nm.
Abstract: We report on transport properties of monolayer graphene with a laterally modulated potential profile, employing striped top gate electrodes with spacings of 100 to 200 nm. Tuning of top and back gate voltages gives rise to local charge carrier density disparities, enabling the investigation of transport properties either in the unipolar ($n{n}^{\ensuremath{'}}$) or the bipolar ($n{p}^{\ensuremath{'}}$) regime. In the latter, pronounced single- and multibarrier Fabry-P\'erot (FP) resonances occur. We present measurements of different devices with different numbers of top gate stripes and spacings. The data are highly consistent with a phase coherent ballistic tight-binding calculation and quantum capacitance model, whereas a superlattice effect and modification of band structure can be excluded.
TL;DR: In this paper, an atomistic simulation based on the nonequilibrium Green's function (NEGF) formalism is employed to evaluate the performance of field effect tunneling transistors based on vertical graphene-hBN heterostructure (VTGFET) and vertical graphene nanoribbon (GNR)-hBN (VTGNRFET) for future electronic applications.
Abstract: In this paper, for the first time device characteristics of field-effect tunneling transistors based on vertical graphene-hBN heterostructure (VTGFET) and vertical graphene nanoribbon (GNR)-hBN heterostructure (VTGNRFET) are theoretically investigated and compared. An atomistic simulation based on the nonequilibrium Green's function (NEGF) formalism is employed. The results indicate that due to the presence of an energy gap in GNRs, the ION/IOFF ratio of VTGNRFET can be much larger than that of VTGFET, which renders VTGNRFETs as promising candidates for future electronic applications. Furthermore, it can be inferred from the results that due to smaller density of states and as a result smaller quantum capacitance of GNRs in comparison with that of graphene, better switching and frequency response can be achieved for VTGNRFETs.
TL;DR: Graphene-based field-effect transistors combined with supported lipid bilayers are reported as a platform for measuring, for the first time, individual ion channel activity.
Abstract: The interaction of cell and organelle membranes (lipid bilayers) with nanoelectronics can enable new technologies to sense and measure electrophysiology in qualitatively new ways. To date, a variety of sensing devices have been demonstrated to measure membrane currents through macroscopic numbers of ion channels. However, nanoelectronic based sensing of single ion channel currents has been a challenge. Here, we report graphene-based field-effect transistors combined with supported lipid bilayers as a platform for measuring, for the first time, individual ion channel activity. We show that the supported lipid bilayers uniformly coat the single layer graphene surface, acting as a biomimetic barrier that insulates (both electrically and chemically) the graphene from the electrolyte environment. Upon introduction of pore-forming membrane proteins such as alamethicin and gramicidin A, current pulses are observed through the lipid bilayers from the graphene to the electrolyte, which charge the quantum capacitance of the graphene. This approach combines nanotechnology with electrophysiology to demonstrate qualitatively new ways of measuring ion channel currents.
TL;DR: The electrical properties of pentacene field effect transistors prepared using graphene electrodes could be enhanced by employing the ITM to introduce a polymer layer that tuned the work function of graphene.
Abstract: The polymer-supported transfer of chemical vapor deposition (CVD)-grown graphene provides large-area and high-quality graphene on a target substrate; however, the polymer and organic solvent residues left by the transfer process hinder the application of CVD-grown graphene in electronic and photonic devices. Here, we describe an inverse transfer method (ITM) that permits the simultaneous transfer and doping of graphene without generating undesirable residues by using polymers with different functional groups. Unlike conventional wet transfer methods, the polymer supporting layer used in the ITM serves as a graphene doping layer placed at the interface between the graphene and the substrate. Polymers bearing functional groups can induce n-doping or p-doping into the graphene depending on the electron-donating or -withdrawing characteristics of functional groups. Theoretical models of dipole layer-induced graphene doping offered insights into the experimentally measured change in the work function and the D...
TL;DR: The results indicate that the variation in both cutoff frequency and Ion/Ioff ratio versus applied tensile strain inversely corresponds to that of the band gap and effective mass.
Abstract: The effects of uniaxial tensile strain on the ultimate performance of a dual-gated graphene nanoribbon field-effect transistor (GNR-FET) are studied using a fully analytical model based on effective mass approximation and semiclassical ballistic transport. The model incorporates the effects of edge bond relaxation and third nearest neighbor (3NN) interaction. To calculate the performance metrics of GNR-FETs, analytical expressions are used for the charge density, quantum capacitance, and drain current as functions of both gate and drain voltages. It is found that the current under a fixed bias can change several times with applied uniaxial strain and these changes are strongly related to strain-induced changes in both band gap and effective mass of the GNR. Intrinsic switching delay time, cutoff frequency, and Ion/Ioff ratio are also calculated for various uniaxial strain values. The results indicate that the variation in both cutoff frequency and Ion/Ioff ratio versus applied tensile strain inversely corresponds to that of the band gap and effective mass. Although a significant high frequency and switching performance can be achieved by uniaxial strain engineering, tradeoff issues should be carefully considered.
TL;DR: In this article, the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barrier on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices is discussed.
Abstract: Recently, it has been shown that oxide barriers in graphene-based non-local spin-valve structures can be the bottleneck for spin transport. The barriers may cause spin dephasing during or right after electrical spin injection which limit spin transport parameters such as the spin lifetime of the whole device. An important task is to evaluate the quality of the oxide barriers of both spin injection and detection contacts in a fabricated device. To address this issue, we discuss the influence of spatially inhomogeneous oxide barriers and especially conducting pinholes within the barrier on the background signal in non-local measurements of graphene/MgO/Co spin-valve devices. By both simulations and reference measurements on devices with non-ferromagnetic electrodes, we demonstrate that the background signal can be caused by inhomogeneous current flow through the oxide barriers. As a main result, we demonstrate the existence of charge accumulation next to the actual spin accumulation signal in non-local voltage measurements, which can be explained by a redistribution of charge carriers by a perpendicular magnetic field similar to the classical Hall effect. Furthermore, we present systematic studies on the phase of the low frequency non-local ac voltage signal which is measured in non-local spin measurements when applying ac lock-in techniques. This phase has so far widely been neglected in the analysis of non-local spin transport. We demonstrate that this phase is another hallmark of the homogeneity of the MgO spin injection and detection barriers. We link backgate dependent changes of the phase to the interplay between the capacitance of the oxide barrier to the quantum capacitance of graphene.
TL;DR: In this article, a first-principles approach based on the effective screening medium framework is introduced to directly simulate the charge storage behavior of single and multi-layered graphene in a way that more closely approximates operating devices.
Abstract: Supercapacitors store energy via the formation of an electric double layer, which generates a strong electric field at the electrode-electrolyte interface. Unlike conventional metallic electrodes, graphene-derived materials suffer from a low electronic density of states (i.e., quantum capacitance), which limits their ability to redistribute charge and efficiently screen this field. To explore these effects, we introduce a first-principles approach based on the effective screening medium framework, which is used to directly simulate the charge storage behavior of single- and multi-layered graphene in a way that more closely approximates operating devices. We demonstrate that the presence of the interfacial field significantly alters the capacitance in electrodes thinner than a few graphene layers, deriving in large part from intrinsic space-charge screening limitations. The capacitance is also found to be highly sensitive to the gap between the electrode and the solvent (contact layer), which offers possibilities for tuning the interfacial capacitance of the electrode by proper engineering of the electrolyte. Our results offer an alternative interpretation of discrepancies between experimental measurements and fixed-band models, and provide specific implications for improving graphene-based devices.
TL;DR: In this article, the Ag-adatom-induced resonant peak as measured by quantum capacitance grows more intense at cryogenic temperatures, at higher impurity concentrations, and in stronger magnetic fields.
Abstract: We investigated Ag-adatom-induced resonant impurities of graphene by quantum capacitance measurement. Different from charged impurities and other conventional resonant impurities, Ag atoms form very weak covalent bonds with graphene. The Ag-adatom-induced resonant peak as measured by quantum capacitance grows more intense at cryogenic temperatures, at higher impurity concentrations, and in stronger magnetic fields, in accordance with our theoretical calculations. The appearance of resonant states and the split of the zeroth Landau level for Ag-adsorbed graphene are manifestations of the formation of a flat impurity band near the Dirac point.
TL;DR: In this manuscript, the effect on the quantum capacitance of noncovalent basal plane functionalization using 1-pyrenebutanoic acid succimidyl ester and glucose oxidase is reported and it is found that functionalized samples tested in air have increased maximum capacitance compared to vacuum but similar to air, and quantum capacitor "tuning" that is greater than that in vacuum and ambient atmosphere.
Abstract: The concentration-dependent density of states in graphene allows the capacitance in metal–oxide–graphene structures to be tunable with the carrier concentration. This feature allows graphene to act as a variable capacitor (varactor) that can be utilized for wireless sensing applications. Surface functionalization can be used to make graphene sensitive to a particular species. In this manuscript, the effect on the quantum capacitance of noncovalent basal plane functionalization using 1-pyrenebutanoic acid succimidyl ester and glucose oxidase is reported. It is found that functionalized samples tested in air have (1) a Dirac point similar to vacuum conditions, (2) increased maximum capacitance compared to vacuum but similar to air, (3) and quantum capacitance “tuning” that is greater than that in vacuum and ambient atmosphere. These trends are attributed to reduced surface doping and random potential fluctuations as a result of the surface functionalization due to the displacement of H2O on the graphene sur...
Abstract: We demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100 atm during post-deposition annealing (HP-PDA). Consequently, the quantum capacitance measurement for the monolayer graphene reveals the largest Fermi energy modulation (EF = ~0.52 eV, i.e., the carrier density of ~2*10^13 cm^-2) in the solid-state topgate insulators reported so far. HP-PDA is the robust method to improve the electrical quality of high-k insulators on graphene.
TL;DR: In this article, the authors demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100' atm during post-deposition annealing (HP-PDA).
Abstract: We demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100 atm during post-deposition annealing (HP-PDA). Consequently, the quantum capacitance measurement for the monolayer graphene reveals the largest Fermi energy modulation (EF = ∼0.52 eV, i.e., the carrier density of ∼2 × 1013 cm−2) in the solid-state topgate insulators reported so far. HP-PDA is the robust method to improve the electrical quality of high-k insulators on graphene.
TL;DR: In this paper, the authors report on DC and microwave electrical transport measurements in silicon-on-insulator nano-transistors at low and room temperature, and attribute this to Coulomb blockade resulting from barriers formed at the spacer-gate interfaces.
Abstract: We report on DC and microwave electrical transport measurements in silicon-on-insulator nano-transistors at low and room temperature. At low source-drain voltage, the DC current and radio frequency response show signs of conductance quantization. We attribute this to Coulomb blockade resulting from barriers formed at the spacer-gate interfaces. We show that at high bias transport occurs thermionically over the highest barrier: Transconductance traces obtained from microwave scattering-parameter measurements at liquid helium and room temperature are accurately fitted by a thermionic model. From the fits we deduce the ratio of gate capacitance and quantum capacitance, as well as the electron temperature.
TL;DR: In this article, a large area graphene ion sensitive field effect transistors (ISFETs) with tantalum pentoxide sensing layers were fabricated and characterized, and demonstrated pH sensitivities approaching the Nernstian limit.
Abstract: We have fabricated and characterized large area graphene ion sensitive field effect transistors (ISFETs) with tantalum pentoxide sensing layers and demonstrated pH sensitivities approaching the Nernstian limit. Low temperature atomic layer deposition was used to deposit tantalum pentoxide atop large area graphene ISFETs. The charge neutrality point of graphene, inferred from quantum capacitance or channel conductance, was used to monitor surface potential in the presence of an electrolyte with varying pH. Bare graphene ISFETs exhibit negligible response, while graphene ISFETs with tantalum pentoxide sensing layers show increased sensitivity reaching up to 55 mV/pH over pH 3 through pH 8. Applying the Bergveld model, which accounts for site binding and a Guoy-Chapman-Stern picture of the surface-electrolyte interface, the increased pH sensitivity can be attributed to an increased buffer capacity reaching up to 1014 sites/cm2. ISFET response was found to be stable to better than 0.05 pH units over the course of two weeks.
TL;DR: In this article, a full band atomistic quantum transport tool is used to predict the performance of double gate metaloxide-semiconductor field effect transistors (MOSFETs) over the next 15 years for International Technology Roadmap for Semiconductors (ITRS).
Abstract: In this letter, a full band atomistic quantum transport tool is used to predict the performance of double gate metal-oxide-semiconductor field-effect transistors (MOSFETs) over the next 15 years for International Technology Roadmap for Semiconductors (ITRS). As MOSFET channel lengths scale below 20 nm, the number of atoms in the device cross-sections becomes finite. At this scale, quantum mechanical effects play an important role in determining the device characteristics. These quantum effects can be captured with the quantum transport tool. Critical results show the ON-current degradation as a result of geometry scaling, which is in contrast to previous ITRS compact model calculations. Geometric scaling has significant effects on the ON-current by increasing source-to-drain (S/D) tunneling and altering the electronic band structure. By shortening the device gate length from 20 nm to 5.1 nm, the ratio of S/D tunneling current to the overall subthreshold OFF-current increases from 18% to 98%. Despite this ON-current degradation by scaling, the intrinsic device speed is projected to increase at a rate of at least 8% per year as a result of the reduction of the quantum capacitance.