Showing papers on "Quantum capacitance published in 2013"

How do the electrical properties of graphene change with its functionalization

Abstract: Functionalization of graphene is essential to interface it with other moieties to expand the scope of its electrical/electronic applications. However, chemical functionalization and/or molecular interactions on graphene sensitively modulate its electrical properties. To evaluate and take advantage of the properties of functionalized graphene, it is important to understand how its electrical attributes (such as carrier scattering, carrier concentration, charge polarity, quantum-capacitance enhanced doping, energy levels, transport mechanisms, and orbital hybridization of energy-bands) are influenced by a change in carbon's structural conformation, hybridization state, chemical potential, local energy levels, and dopant/interface coupling induced via functionalization or molecular interactions. Here, a detailed and integrated model describes factors influencing these electrical characteristics of functionalized graphene (covalent bonds, adsorption, π-π bonds, and lattice incorporation). The electrical properties are governed via three mechanisms: (a) conversion of carbon's hybridized state, (b) dipole interactions enhanced via quantum capacitance, and (c) orbital hybridization with an interfacing molecule. A few graphenic materials are also identified where further studies are essential to understand the effect of their functionalization.

Interaction phenomena in graphene seen through quantum capacitance

Abstract: Capacitance measurements provide a powerful means of probing the density of states. The technique has proved particularly successful in studying 2D electron systems, revealing a number of interesting many-body effects. Here, we use large-area high-quality graphene capacitors to study behavior of the density of states in this material in zero and high magnetic fields. Clear renormalization of the linear spectrum due to electron–electron interactions is observed in zero field. Quantizing fields lead to splitting of the spin- and valley-degenerate Landau levels into quartets separated by interaction-enhanced energy gaps. These many-body states exhibit negative compressibility but the compressibility returns to positive in ultrahigh B. The reentrant behavior is attributed to a competition between field-enhanced interactions and nascent fractional states.

Electric double-layer capacitance between an ionic liquid and few-layer graphene

Abstract: Ionic-liquid gates have a high carrier density due to their atomically thin electric double layer (EDL) and extremely large geometrical capacitance C-g. However, a high carrier density in graphene has not been achieved even with ionic-liquid gates because the EDL capacitance C-EDL between the ionic liquid and graphene involves the series connection of C-g and the quantum capacitance C-q, which is proportional to the density of states. We investigated the variables that determine C-EDL at the molecular level by varying the number of graphene layers n and thereby optimising C-q. The C-EDL value is governed by C-q at n, 4, and by C-g at n > 4. This transition with n indicates a composite nature for C-EDL. Our finding clarifies a universal principle that determines capacitance on a microscopic scale, and provides nanotechnological perspectives on charge accumulation and energy storage using an ultimately thin capacitor.

Semiclassical spatially dispersive intraband conductivity tensor and quantum capacitance of graphene

Abstract: Analytical expressions are presented for the intraband conductivity tensor of graphene that includes spatial dispersion for arbitrarily wave-vector values and the presence of a nonzero Fermi energy. The conductivity tensor elements are derived from the semiclassical Boltzmann transport equation under both the relaxation-time approximation and the Bhatnagar-Gross-Krook model (which allows for an extra degree of freedom to enforce number conservation). The derived expressions are based on linear electron dispersion near the Dirac points, and extend previous results that assumed small wave-vector values; these are shown to be inadequate for the very slow waves expected on graphene nanoribbons. The new expressions are also compared to results obtained by numericalintegrationoverthefirstBrillouinzoneusingtheexact(tight-binding)electrondispersionrelation.Very good agreement is found between the new analytical expressions and the exact numerical results. Furthermore, a comparison with the longitudinal random-phase conductivity is also made. It is shown analytically that these new expressions lead to the correct value of the quantum capacitance of a graphene sheet and that ignoring spatial dispersion leads to serious errors in the propagation properties of fundamental modes on graphene nanoribbons.

Quantum capacitance measurements of electron-hole asymmetry and next-nearest-neighbor hopping in graphene

Abstract: the highest theoretical values. Here, we report dedicated measurements of the density of states in graphene by using high-quality capacitance devices. The density of states exhibits a pronounced electron-hole asymmetry that increases linearly with energy. This behavior yields t � ≈− 0.3 eV ±15%, in agreement with the high end of

Impact of Oxide Thickness on Gate Capacitance—A Comprehensive Analysis on MOSFET, Nanowire FET, and CNTFET Devices

Abstract: Carbon nanotube-based FET devices are getting more and more importance today because of their high channel mobility and improved gate capacitance against gate voltage. This paper compares and analyzes the effect of variation of oxide thickness on gate capacitance for single gate MOSFET, double gate MOSFET, silicon nanowire FET, and CNTFET devices through an exhaustive simulation. It is seen that in nanometer regime quantum capacitance is the deciding factor in calculating the gate capacitance of a FET device. CNTFET and silicon nanowire FET have a favorable characteristics of decreasing gate capacitance with the decrease in oxide thickness in deep nanometer regime, which is not possible to get in a single gate or a double gate MOSFET. This decrease in gate capacitance is observed at a gate voltage of 0.5 V and above which leads to reduced propagation delay and lower leakage compared to MOSFET devices.

Very Large Current Modulation in Vertical Heterostructure Graphene/hBN Transistors

Abstract: In this paper, we investigate the electrical behavior of transistors based on a vertical graphene-hexagonal boron nitride (hBN) heterostructure, using atomistic multiphysics simulations based on density-functional theory and non-equilibrium Green's function formalism. We show that the hBN current-blocking layer is effective and allows modulation of the current by five orders of magnitude, confirming experimental results. We also highlight - through accurate numerical calculations and simplified analytical modeling - some intrinsic limitations of vertical heterostructure transistors. We show that the overlap between gate contacts and source/drain leads screens the electric field induced by the gates and is responsible for the excessive degradation of the sub-threshold swing, the ION/IOFF ratio, and the cut-off frequency.

Curvature effects on the interfacial capacitance of carbon nanotubes in an ionic liquid

Abstract: Carbon nanotube (CNT) electrodes in supercapacitors have recently demonstrated enhanced performance compared to conventional carbon-based electrodes; however, the underlying relationships between electrode curvature and capacitance remain unclear. Using computer simulations, we evaluate the capacitive performance of metallic (6,6), (10,10), and (16,16) CNTs in [BMIM][PF6] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (CD) at the CNT/IL interface and the electrode quantum capacitance (CQ). Our classical molecular dynamics simulations reveal that CD improves with increasing electrode curvature, which we discuss in terms of how the curvature affects both the electric field strength and EDL microstructure. In addition, the CQ of the CNTs is constant near the Fermi level and increases with curvature, as also demonstrated by density functional theory calculations. Our study shows that the electrode curvature effect on the total interfac...

Analogs of basic electronic circuit elements in a free-space atom chip.

Abstract: Using a thermal sample of laser-cooled rubidium atoms, we have constructed a neutral-atom circuit analogous to an electronic capacitor discharged through a resistor. The atoms are confined using what we call a free-space atom chip, an optical dipole trap created using a generalized phase-contrast imaging technique. We have also calculated theoretical values for the capacitance and resistance, which agree with our experiments, as well as theoretical value for an atomic analog of electrical inductance. We show that atomic capacitance is analogous to the quantum capacitance, the atomic resistance is analogous to the ballistic, or Sharvin resistance and the atomic inductance is analogous to kinetic inductance.

Estimation of residual carrier density near the Dirac point in graphene through quantum capacitance measurement

Abstract: We discuss the residual carrier density (n*) near the Dirac point (DP) in graphene estimated by quantum capacitance (CQ) and conductivity (σ) measurements. The CQ at the DP has a finite value and is independent of the temperature. A similar behavior is also observed for the conductivity at the DP, because their origin is residual carriers induced externally by charged impurities. The n* extracted from CQ, however, is often smaller than that from σ, suggesting that the mobility in the puddle region is lower than that in the linear region. The CQ measurement should be employed for estimating n* quantitatively.

A Dual-Gate Graphene FET Model for Circuit Simulation—SPICE Implementation

Abstract: This paper presents a SPICE compatible model of a dual-gate bilayer graphene field-effect transistor. The model describes the functionality of the transistor in all the regions of operation for both hole and electron conduction. We present closed-form analytical equations that define the boundary points between the regions to ensure Jacobian continuity for efficient circuit simulator implementation. A saturation displacement current is proposed to model the drain current when the channel becomes ambipolar. The model proposes a quantum capacitance that varies with the surface potential. The model has been implemented in Berkeley SPICE-3, and it shows a good agreement against experimental data with the normalized root-mean-square error less than $10\%$ .

Relative contributions of quantum and double layer capacitance to the supercapacitor performance of carbon nanotubes in an ionic liquid

Abstract: Motivated by promising demonstrations of carbon nanotube (CNT) electrodes in supercapacitors, we evaluate the capacitive performance of a (6,6) CNT in [BMIM][PF6] ionic liquid (IL), with particular attention to the relative contributions of the electric double layer (EDL) capacitance (CD) at the CNT/IL interface and the quantum capacitance (CQ) of the CNT. Our classical molecular dynamics simulations reveal that the use of the CNT improves CD when compared to planar graphene, which we discuss in terms of how the electrode curvature affects both the electric field strength and IL packing density. In addition, according to density functional theory calculations, the CQ of the CNT is constant and significantly larger than that of graphene near the Fermi level, which is a consequence of the larger number of available electron states in the CNT. Our study also shows that the relative performance of the CNT- and graphene-based electrodes can be a strong function of applied voltage, which we attribute to the shifting contributions of CQ and CD.

The density of states of graphene underneath a metal electrode and its correlation with the contact resistivity

Abstract: The density of states (DOS) of graphene underneath a metal is estimated through a quantum capacitance measurement of the metal/graphene/SiO2/n+-Si contact structure fabricated by a resist-free metal deposition process. Graphene underneath Au maintains a linear DOS–energy relationship except near the Dirac point, whereas the DOS of graphene underneath Ni is broken and largely enhanced around the Dirac point, resulting in only a slight modulation of the Fermi energy. Moreover, the DOS of graphene in the contact structure is correlated with the contact resistivity measured using devices fabricated by the resist-free process.

A manufacturable process integration approach for graphene devices

Abstract: In this work, we propose an integration approach for double gate graphene field effect transis- tors. The approach includes a number of process steps that are key for future integration of graphene in microelectronics: bottom gates with ultra-thin (2 nm) high-quality thermally grown SiO2 dielectrics, shallow trench isolation between devices and atomic layer deposited Al2O3 top gate dielectrics. The com- plete process flow is demonstrated with fully functional GFET transistors and can be extended to wafer scale processing. We assess, through simulation, the effects of the quantum capacitance and band bend- ing in the silicon substrate on the effective electric fields in the top and bottom gate oxide. The proposed process technology is suitable for other graphene-based devices such as graphene-based hot electron transistors and photodetectors.

Theory of carrier density in multigated doped graphene sheets with quantum correction

Abstract: The quantum capacitance model is applied to obtain an exact solution for the space-resolved carrier density in a multigated doped graphene sheet at zero temperature, with quantum correction arising from the finite electron capacity of the graphene itself taken into account. The exact solution is demonstrated to be equivalent to the self-consistent Poisson-Dirac iteration method by showing an illustrative example, where multiple gates with irregular shapes and a nonuniform dopant concentration are considered. The solution therefore provides a fast and accurate way to compute spatially varying carrier density, on-site electric potential energy, as well as quantum capacitance for bulk graphene, allowing for any kind of gating geometry with any number of gates and any types of intrinsic doping.

Number of Conducting Channels for Armchair and Zig-Zag Graphene Nanoribbon Interconnects

Abstract: Nanowire-based circuits are candidates for future high-speed electronics. Signal propagation in nanowires can be studied by combining the semiclassical Boltzmann transport theory to the classical transmission line theory. In this paper, we apply this approach to model the signal propagation in graphene nanoribbon (GNR) interconnects. We express the kinetic inductance and the quantum capacitance in terms of the number of effective conducting channels. We study in detail the behavior of the number of effective conducting channels for both the armchair and zig-zag GNRs as their widths vary. This number is computed rigorously, taking into account the actual distribution of the energy spectrum and of the velocity of the conduction electrons. We found that the expressions for the number of conducting channels proposed in the literature give a significant overestimation of its values.

Photon shot noise limited detection of terahertz radiation using a quantum capacitance detector

Abstract: We observed a sweep rate dependence of the quantum capacitance in a single Cooper-Pair box used as the readout of a Quantum Capacitance Detector. A model was developed that fits the data over five orders of magnitude in sweep rate and optical signal power and provides a natural calibration of the absorbed power. We are thereby able to measure the noise equivalent power of the detector as a function of absorbed power. We find that it is shot-noise-limited in detecting 1.5 THz photons with absorbed power ranging from 1 × 10−22 W to 1 × 10−17 W.

Graphene Base Transistors: A Simulation Study of DC and Small-Signal Operation

Abstract: A simulation study aimed at investigating the main features in dc and small-signal operating conditions of the hot-electron graphene base transistor (GBT) for analog terahertz operation is presented. Intrinsic silicon is used as reference material. The numerical model is based on a self-consistent Schrodinger-Poisson solution, using a 1-D transport approximation and accounting for multiple-valley and nonparabolicity band effects. Some limitations in the extension of the saturation region and in the output conductance related to the finite quantum capacitance of graphene and to space charge effects are discussed. A small-signal model is developed that catches the essential physics behind the voltage gain and the cutoff frequency, which shows that the graphene quantum capacitance severely limits the former but not the latter. According to simulations carried out within the ballistic transport approximation, a 20-nm-long GBT can achieve at the same time a voltage gain larger than 10 and a cutoff frequency largely above 1 THz within a reasonably wide bias range.

Effect of dielectric response on the quantum capacitance of graphene in a strong magnetic field

Abstract: The quantum capacitance of graphene can be negative when the graphene is placed in a strong magnetic field, which is a clear experimental signature of positional correlations between electrons. Here we show that the quantum capacitance of graphene is also strongly affected by its dielectric polarizability, which in a magnetic field is wave-vector dependent. We study this effect both theoretically and experimentally. We develop a theory and numerical procedure for accounting for the graphene dielectric response, and we present measurements of the quantum capacitance of high-quality graphene capacitors on boron nitride. Theory and experiment are found to be in good agreement.

Covalent Functionalization of Dipole‐Modulating Molecules on Trilayer Graphene: An Avenue for Graphene‐Interfaced Molecular Machines

Abstract: The molecular dipole moment plays a significant role in governing important phenomena like molecular interactions, molecular configuration, and charge transfer, which are important in several electronic, electrochemical, and optoelectronic systems. Here, the effect of the change in the dipole moment of a tethered molecule on the carrier properties of (functionalized) trilayer graphene—a stack of three layers of sp2-hybridized carbon atoms—is demonstrated. It is shown that, due to the high carrier confinement and large quantum capacitance, the trans-to-cis isomerisation of ‘covalently attached’ azobenzene molecules, with a change in dipole moment of 3D, leads to the generation of a high effective gating voltage. Consequently, 6 units of holes are produced per azobenzene molecule (hole density increases by 440 000 holes μm−2). Based on Raman and X-ray photoelectron spectroscopy data, a model is outlined for outer-layer, azobenzene-functionalized trilayer graphene with current modulation in the inner sp2 matrix. Here, 0.097 V are applied by the isomerisation of the functionalized azobenzene. Further, the large measured quantum capacitance of 72.5 μF cm−2 justifies the large Dirac point in the heavily doped system. The mechanism defining the effect of dipole modulation of covalently tethered molecules on graphene will enable future sensors and molecular-machine interfaces with graphene.

Analytical Gate Capacitance Modeling of III–V Nanowire Transistors

Abstract: In this paper, we propose a physically based analytical model for the gate capacitance (CG) of III-V nanowire (NW) transistors. The model explicitly accounts for different terms that contribute to CG: the insulator capacitance, the finite density of states, and the charge distribution in the NW. It considers the 2-D quantum confinement of the carriers, the wavefunction penetration into the gate insulator, Fermi-Dirac statistics and the conduction band nonparabolicity, providing analytical expressions for all the capacitance contributions. Furthermore, the behavior and role of the density of states and the charge distribution in the NW are discussed for several materials and the influence of the wavefunction penetration into the gate insulator is also studied. We show that our analytical model is in very good agreement with the numerical solution for different device sizes and materials.

Density of States and Its Local Fluctuations Determined by Capacitance of Strongly Disordered Graphene

Abstract: We demonstrate that fluctuations of the local density of states (LDOS) in strongly disordered graphene play an important role in determining the quantum capacitance of the top-gate graphene devices. Depending on the strength of the disorder induced by metal-cluster decoration, the measured quantum capacitance of disordered graphene can dramatically decrease in comparison with pristine graphene. This is opposite to the common belief that quantum capacitance should increase with disorder. To explain this counterintuitive behavior, we present a two-parameter model which incorporates both the non-universal power law behavior for the ADOS and a lognormal distribution of LDOS. We find excellent quantitative agreements between the model and measured quantum capacitance for three disordered samples in a wide range of Fermi energies. Thus, by measuring the quantum capacitance, we can simultaneously determine the ADOS and its fluctuations. It is the LDOS fluctuations that cause the dramatic reduction of the quantum capacitance.

Quantum capacitance of the armchair-edge graphene nanoribbon

Abstract: The quantum capacitance, an important parameter in the design of nanoscale devices, is derived for armchair-edge single-layer graphene nanoribbon with semiconducting property. The quantum capacitance oscillations are found and these capacitance oscillations originate from the lateral quantum confinement in graphene nanoribbon. Detailed studies of the capacitance oscillations demonstrate that the local channel electrostatic potential at the capacitance peak, the height and the number of the capacitance peak strongly depend on the width, especially a few nanometres, of the armchair-edge graphene nanoribbon. It implies that the capacitance oscillations observed in the experiments can be utilized to measure the width of graphene nanoribbon. The results also show that the capacitance oscillations are not seen when the width is larger than 30 nm.

Negative Quantum Capacitance Induced by Midgap States in Single-layer Graphene

Abstract: We demonstrate that single-layer graphene (SLG) decorated with a high density of Ag adatoms displays the unconventional phenomenon of negative quantum capacitance. The Ag adatoms act as resonant impurities and form nearly dispersionless resonant impurity bands near the charge neutrality point (CNP). Resonant impurities quench the kinetic energy and drive the electrons to the Coulomb energy dominated regime with negative compressibility. In the absence of a magnetic field, negative quantum capacitance is observed near the CNP. In the quantum Hall regime, negative quantum capacitance behavior at several Landau level positions is displayed, which is associated with the quenching of kinetic energy by the formation of Landau levels. The negative quantum capacitance effect near the CNP is further enhanced in the presence of Landau levels due to the magnetic-field-enhanced Coulomb interactions.

Electron-electron interactions in monolayer graphene quantum capacitors

Abstract: We demonstrate the effects of electron-electron (e-e) interactions in monolayer graphene quantum capacitors. Ultrathin yttrium oxide showed excellent performance as the dielectric layer in top-gate device geometry. The structure and dielectric constant of the yttrium oxide layers have been carefully studied. The inverse compressibility retrieved from the quantum capacitance agreed fairly well with the theoretical predictions for the e-e interactions in monolayer graphene at different temperatures. We found that electron-hole puddles played a significant role in the low-density carrier region in graphene. By considering the temperature-dependent charge fluctuation, we established a model to explain the round-off effect originating from the e-e interactions in monolayer graphene near the Dirac point.

Negative Quantum Capacitance Induced by Midgap States in Single-layer

Abstract: We demonstrate that single-layer graphene (SLG) decorated with a high density of Ag adatoms displays the unconventional phenomenon of negative quantum capacitance. The Ag adatoms act as resonant impurities and form nearly dispersionless resonant impurity bands near the charge neutrality point (CNP). Resonant impurities quench the kinetic energy and drive the electrons to the Coulomb energy dominated regime with negative compressibility. In the absence of a magnetic field, negative quantum capacitance is observed near the CNP. In the quantum Hall regime, negative quantum capacitance behavior at several Landau level positions is displayed, which is associated with the quenching of kinetic energy by the formation of Landau levels. The negative quantum capacitance effect near the CNP is further enhanced in the presence of Landau levels due to the magnetic-field-enhanced Coulomb interactions.

Strained-Si/strained-Ge type-II staggered heterojunction gate-normal-tunneling field-effect transistor

Abstract: A SiGe-based n-channel tunnel field-effect transistor design employing a strained-Si/strained-Ge staggered-gap heterojunction with a small effective band-gap (122 meV) at the interface is investigated via numerical simulations using a semi-classical quantum correction obtained from the density-gradient model. A gate-normal tunneling geometry is used to increase tunneling area and reduce subthreshold swing. The strain leads to degeneracy breaking among the silicon conduction band valleys, reducing the density of states and associated quantum capacitance with better gate-to-tunnel barrier coupling. Performance evaluation using a figure-of-merit “I60,” where the drain current corresponds to a subthreshold slope of 60 mV/decade, suggests that the device has the potential to be competitive with modern metal-oxide-semiconductor field-effect transistors.