Showing papers on "Quantum capacitance published in 2023"

Metalloids (B, Si) and non-metal (N, P, S) doped graphene nanosheet as a supercapacitor electrode: A density functional theory study

Abstract: Metalloids [Boron (B), Silicon (Si)] and Non -metals [Nitrogen (N), Phosphorus (P), Sulfur (S)] doped graphene nanosheets were considered for alternative lightweight electrode materials in energy storage device applications. The density functional theory studies were performed to investigate the structural stability, charge density distribution, excessive surface charge density, and quantum capacitance of doped graphene nanosheets. It is found that the structures are stable with negative formation energy, and the graphene nanosheet doped with P and S shows protrusion forming a pyramidal shape along the planar graphene nanosheet to maintain stability. Further, the doping of atoms into the graphene nanosheet introduced a minimal bandgap with a shift in the Fermi level, either into the conduction band or valence band, depending on the dopant type. This appearance of bandgap is attributed to the breaking of the symmetry in the graphene sublattice. Meanwhile, based on the type and concentration of dopant, localized and delocalized states are formed near the Fermi level. It is also found that the formation of a localized state near the Fermi level has profoundly improved the excessive surface charge density and quantum capacitance. The S – doped graphene nanosheet was observed to exhibit an excellent quantum capacitance of 38.88 μFcm−2 compared to all other structures and it can be used as a suitable light-weight electrode for asymmetric supercapacitors.

How Do Quantum Effects Influence the Capacitance and Carrier Density of Monolayer MoS2 Transistors?

Abstract: When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance (CG) becomes smaller than the oxide capacitance (Cox) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS2 as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS2 capacitance is smaller than its quantum capacitance, limiting the single-gated CG of an n-type channel to between 63% and 78% of Cox, for gate overdrive voltages between 0.5 and 1 V. Despite these limitations, for dual-gated devices, the on-state CG of monolayer MoS2 is 50% greater than that of silicon at 0.5 nm EOT and more than three times that of InGaAs at 1 nm EOT, indicating that such 2D semiconductors are promising for improved gate control of nanoscale transistors at future technology nodes.

Molecular understanding of the Helmholtz capacitance difference between Cu(100) and graphene electrodes.

Abstract: Unraveling the origin of Helmholtz capacitance is of paramount importance for understanding the interfacial structure and electrostatic potential distribution of electric double layers (EDL). In this work, we combined the methods of ab initio molecular dynamics and classical molecular dynamics and modeled electrified Cu(100)/electrolyte and graphene/electrolyte interfaces for comparison. It was proposed that the Helmholtz capacitance is composed of three parts connected in series: the usual solvent capacitance, water chemisorption induced capacitance, and Pauling repulsion caused gap capacitance. We found the Helmholtz capacitance of graphene is significantly lower than that of Cu(100), which was attributed to two intrinsic factors. One is that graphene has a wider gap layer at interface, and the other is that graphene is less active for water chemisorption. Finally, based on our findings, we provide suggestions for how to increase the EDL capacitance of graphene-based materials in future work, and we also suggest that the new understanding of the potential distribution across the Helmholtz layer may help explain some experimental phenomena of electrocatalysis.

Capacitive NO2 Detection Using CVD Graphene-Based Device

Abstract: A graphene-based capacitive NO2 sensing device was developed by utilizing the quantum capacitance effect. We have used a graphene field-effect transistor (G-FET) device whose geometrical capacitance is enhanced by incorporating an aluminum back-gate electrode with a naturally oxidized aluminum surface as an insulating layer. When the graphene, the top-side of the device, is exposed to NO2, the quantum capacitance of graphene and, thus, the measured capacitance of the device, changed in accordance with NO2 concentrations ranging from 1–100 parts per million (ppm). The operational principle of the proposed system is also explained with the changes in gate voltage-dependent capacitance of the G-FET exposed to various concentrations of NO2. Further analyses regarding carrier density changes and potential variances under various concentrations of NO2 are also presented to strengthen the argument. The results demonstrate the feasibility of capacitive NO2 sensing using graphene and the operational principle of capacitive NO2 sensing.

Enhanced quantum capacitance of MX4 (M=Fe, Co, Ni, Cu, and Zn; X=N, P) moieties embedded graphene: A DFT study

Abstract: Abstract 
In this work, density functional theory calculations are performed to study the impact of embedding transition metal-(N/P)4 moieties in graphene on its geometric structure, electronic properties, and quantum capacitance. Enhancement of quantum capacitance of transition metal doped nitrogen/phosphorus pyridinic graphenes is observed, which is directly related to the availability of states near the Fermi level. The findings show that electronic properties and hence quantum capacitance of graphene can be tuned by varying transition metal dopants and/or their coordination environment. Modified graphenes can suitably be chosen as positive or negative electrodes of asymmetric supercapacitors depending upon the values of quantum capacitance and stored charges. Furthermore, quantum capacitance can be enhanced by widening the working voltage window. The results can serve as guidelines for the design of graphene-based electrodes in supercapacitor applications.

A Phenomenological Model for Electrical Transport Characteristics of MSM Contacts Based on GNS

Abstract: Graphene nanoscroll, because of attractive electronic, mechanical, thermoelectric and optoelectronics properties, is a suitable candidate for transistor and sensor applications. In this research, the electrical transport characteristics of high-performance field effect transistors based on graphene nanoscroll are studied in the framework of analytical modeling. To this end, the characterization of the proposed device is investigated by applying the analytical models of carrier concentration, quantum capacitance, surface potential, threshold voltage, subthreshold slope and drain induced barrier lowering. The analytical modeling starts with deriving carrier concentration and surface potential is modeled by adopting the model of quantum capacitance. The effects of quantum capacitance, oxide thickness, channel length, doping concentration, temperature and voltage are also taken into account in the proposed analytical models. To investigate the performance of the device, the current-voltage characteristics are also determined with respect to the carrier density and its kinetic energy. According to the obtained results, the surface potential value of front gate is higher than that of back side. It is noteworthy that channel length affects the position of minimum surface potential. The surface potential increases by increasing the drain-source voltage. The minimum potential increases as the value of quantum capacitance increases. Additionally, the minimum potential is symmetric for the symmetric structure (Vfg = Vbg). In addition, the threshold voltage increases by increasing the carrier concentration, temperature and oxide thickness. It is observable that the subthreshold slope gets closer to the ideal value of 60 mV/dec as the channel length increases. As oxide thickness increases the subthreshold slope also increases. For thinner gate oxide, the gate capacitance is larger while the gate has better control over the channel. The analytical results demonstrate a rational agreement with existing data in terms of trends and values.

Quantum capacitance properties of the holes in planar germanium

Abstract: Quantum capacitance of two-dimensional (2D) systems contains useful physical information. Here, we report a high sensitivity quantum capacitance measurement with an improved radio frequency superheterodyne bridge technique for probing the electronic characteristic of Ge/SiGe 2D hole gas (2DHG) at low temperatures and under a perpendicular magnetic field B⊥. At low fields, a rapid decrease in quantum capacitance following [Formula: see text] dependence is observed, indicating an abrupt change in chemical potential near the gate boundary at high frequencies; at high fields, a series of capacitance oscillations are observed due to the Landau quantization and Zeeman splitting of the Ge/SiGe 2DHG, where gate-dependent effective [Formula: see text] factor under B⊥ is extracted. These results represent implementation of the high-precision capacitance measurement for exploring the physical properties of Ge/SiGe 2DHG.

Impact of Technology Scaling in DSM Region on Performance of Intercalation-doped MLGNR as VLSI Interconnects

Abstract: Intercalation-doped multilayer graphene nano-ribbons (ID-MLGNR) have been proposed as a potential contender for interconnect applications. In the present research work, the impact of technology scaling i.e. scaling in interconnect dimension is studied on intrinsic parameters of MLGNR based interconnects and consequently on its impedance behavior. The various analysis is carried out at sub-45-nm technology node (viz. 45 nm, 22 nm, 14 nm, 13 nm). It is found that scaling in width (W), thickness (T) and area (A) has an adverse effect on mean free path (MFP), Fermi-energy (Eg), number of conducting channels and layers ( ) the number of conducting (n). However, edge specularity and intercalation-doping in interconnect provide the key process parameters viz. specularity constant (p) and Fermi-energy (EF) to alleviate these adverse effects to a significant level. SPICE simulations reveal that the scaling in width increases the interconnect resistance. Similar trend is observed in inductance. It is noted that at a technology node of 13 nm, the percentage reduction in resistance of metallic armchair(AC)-GNR w.r.t. zigzag (ZZ)-GNR at EF= 0.2eV, 0.4eV & 0.6 eV are 26.2, 13.1 and 8.75 respectively, for local & intermediate level of interconnects. Similarly, respective values of percentage reduction in inductance are 26.28, 13.14, and 8.76. Further, interconnect scaling has a poor influence on the capacitance of AC-and ZZ-MLGNR.

How do Quantum Effects Influence the Capacitance and Carrier Density of Monolayer MoS$_2$ Transistors?

Abstract: When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance ($C_\textrm{G}$) becomes smaller than the oxide capacitance ($C_\textrm{ox}$) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS$_\textrm{2}$ as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS$_\textrm{2}$ capacitance is smaller than its quantum capacitance, limiting the single-gated $C_\textrm{G}$ of an n-type channel to between 63% and 78% of $C_\textrm{ox}$ for gate overdrive voltages between 0.5 and 1 V. Despite these limitations, for dual-gated devices, the on-state $C_\textrm{G}$ of monolayer MoS$_\textrm{2}$ is 50% greater than that of silicon at 0.5 nm EOT and more than three times that of InGaAs at 1 nm EOT, indicating that 2D semiconductors are promising for nanoscale devices at future technology nodes.

Impact of Large Gate Voltages and Ultrathin Polymer Electrolytes on Carrier Density in Electric-Double-Layer-Gated Two-Dimensional Crystal Transistors

Abstract: Electric-double-layer (EDL) gating can induce large capacitance densities (∼1–10 μF cm–2) in two-dimensional (2D) semiconductors; however, several properties of the electrolyte limit performance. One property is the electrochemical activity which limits the gate voltage (VG) that can be applied and therefore the maximum extent to which carriers can be modulated. A second property is electrolyte thickness, which sets the response speed of the EDL gate and therefore the time scale over which the channel can be doped. Typical thicknesses are on the order of micrometers, but thinner electrolytes (nanometers) are needed for very-large-scale-integration (VLSI) in terms of both physical thickness and the speed that accompanies scaling. In this study, finite element modeling of an EDL-gated field-effect transistor (FET) is used to self-consistently couple ion transport in the electrolyte to carrier transport in the semiconductor, in which density of states, and therefore quantum capacitance, is included. The model reveals that 50 to 65% of the applied potential drops across the semiconductor, leaving 35 to 50% to drop across the two EDLs. Accounting for the potential drop in the channel suggests that higher carrier densities can be achieved at larger applied VG without concern for inducing electrochemical reactions. This insight is tested experimentally via Hall measurements of graphene FETs for which VG is extended from ±3 to ±6 V. Doubling the gate voltage increases the sheet carrier density by an additional 2.3 × 1013 cm–2 for electrons and 1.4 × 1013 cm–2 for holes without inducing electrochemistry. To address the need for thickness scaling, the thickness of the solid polymer electrolyte, poly(ethylene oxide) (PEO):CsClO4, is decreased from 1 μm to 10 nm and used to EDL gate graphene FETs. Sheet carrier density measurements on graphene Hall bars prove that the carrier densities remain constant throughout the measured thickness range (10 nm–1 μm). The results indicate promise for overcoming the physical and electrical limitations to VLSI while taking advantage of the ultrahigh carrier densities induced by EDL gating.

Analytic modelling of Quantum Capacitance and Carrier Concentration for $\beta_{12}$-Borophene FET based Gas Sensor

Abstract: In this work, we investigate the physical and electronic properties of $\beta_{12}$-borophene FET-based gas sensor using a theoretical quantum capacitance model based on tight-binding approach. We study the impact of adsorbed NH$_3$, NO, NO$_2$ and CO gas molecule on its density of states, carrier concentration, quantum capacitance and I-V characteristics. We found a remarkable variation in the energy band structure and the density of states (DOS) of the $\beta_{12}$-borophene in the presence of the adsorbed gas molecule. The appearance of non-identical Van-Hove singularities in the DOS in the presence of adsorbed gas molecules strongly indicates the high sensitivity of $\beta_{12}$-borophene. We found a significant increase in the carrier concentration for NH$_3$ gas while it decreases for all other gases. Moreover, a drastic change in quantum capacitance and current-voltage relation is also observed in adsorbed gases. The different properties of the given gas molecules are compared with the pristine borophene and found to exhibit distinct wrinkles in each case, thereby indicating the strong selectivity of our proposed gas sensor. Though $\beta_{12}$ - borophene is found to be highly sensitive for all studied gases, the NO gas is found to be most sensitive compared to the others.

Quantum Capacitance and Fermi Level Change in Graphene nanoribbons due to Gas Sensing

Abstract: Here we used semiempirical computations to examine the property of nanoribbon of Graphene as a gas sensor with interaction of H2O gas molecule for both pure and defective GNRs which has been generated in Atomistix Toolkit (ATK) software. Density of States GNR before and after the interaction is shown in a (DOS) diagram with gas particles was discovered to be different which has been observed in MATLAB software. It's vital to look at the quantum capacitance when examining Graphene’s electrical properties. So, this study looked at change in quantum capacitance and Fermi Level of Graphene before and after gas sensing and the results were produced with necessary equations. Using a three-electrode electrochemical setup, we are able to directly quantify Graphene's quantum capacitance as a function of gate potential. If Graphene is used in a highly sensitive capacitive circuit, the change in Fermi energy was determined from experimental data of changed Density of States (DOS). Although this research has some limitations and future scopes, we can propose that the change in Fermi Energy level can be approximately 9.5 eV with respect to the quantum capacitance of fabricated Graphene interacting with H2O which is used as a MOSFET in this work.

Theoretical Studies on the Quantum Capacitance of Two-Dimensional Electrode Materials for Supercapacitors

Abstract: In recent years, supercapacitors have been widely used in the fields of energy, transportation, and industry. Among them, electrical double-layer capacitors (EDLCs) have attracted attention because of their dramatically high power density. With the rapid development of computational methods, theoretical studies on the physical and chemical properties of electrode materials have provided important support for the preparation of EDLCs with higher performance. Besides the widely studied double-layer capacitance (CD), quantum capacitance (CQ), which has long been ignored, is another important factor to improve the total capacitance (CT) of an electrode. In this paper, we survey the recent theoretical progress on the CQ of two-dimensional (2D) electrode materials in EDLCs and classify the electrode materials mainly into graphene-like 2D main group elements and compounds, transition metal carbides/nitrides (MXenes), and transition metal dichalcogenides (TMDs). In addition, we summarize the influence of different modification routes (including doping, metal-adsorption, vacancy, and surface functionalization) on the CQ characteristics in the voltage range of ±0.6 V. Finally, we discuss the current difficulties in the theoretical study of supercapacitor electrode materials and provide our outlook on the future development of EDLCs in the field of energy storage.

A Parameter Extraction Methodology for Graphene Field-Effect Transistors

Abstract: Graphene field-effect transistors (GFETs) have now been around for more than a decade and their transfer characteristics extensively used for device characterization. Model parameters, such as low-field charge-carrier mobility and device contact/series resistance, have often been the main interest. However, not until recently have the methods for device characterization themselves been the focus of research publications. In this article, I report on a structured methodology for extracting and validating the extracted GFET model parameter values based on the physics of FETs in general and of GFETs in particular. During the extraction process, the GFET resistance is divided into two parts, a constant part and a gate-voltage-dependent part, where the constant part often has been believed to represent the series/contact resistance. However, part of it depends on the channel length and contains first-order information about mobility degradation. Finally, I show that the main influence of the quantum capacitance can be captured by an equivalent oxide thickness (EOT) replacing the insulator thickness.

Quantum Capacitance and Fermi Level Change in Graphene nanoribbons due to Gas Sensing

Abstract: Here we used semiempirical computations to examine the property of nanoribbon of Graphene as a gas sensor with interaction of H2O gas molecule for both pure and defective GNRs which has been generated in Atomistix Toolkit (ATK) software. Density of States GNR before and after the interaction is shown in a (DOS) diagram with gas particles was discovered to be different which has been observed in MATLAB software. It's vital to look at the quantum capacitance when examining Graphene’s electrical properties. So, this study looked at change in quantum capacitance and Fermi Level of Graphene before and after gas sensing and the results were produced with necessary equations. Using a three-electrode electrochemical setup, we are able to directly quantify Graphene's quantum capacitance as a function of gate potential. If Graphene is used in a highly sensitive capacitive circuit, the change in Fermi energy was determined from experimental data of changed Density of States (DOS). Although this research has some limitations and future scopes, we can propose that the change in Fermi Energy level can be approximately 9.5 eV with respect to the quantum capacitance of fabricated Graphene interacting with H2O which is used as a MOSFET in this work.

A First-Principles Approach to Modeling Interfacial Capacitance in Graphene-Based Electrodes

Abstract: We present a first-principles computational model to calculate the interfacial capacitance of low-dimensional materials in contact with a bulk substrate. The model is based on density functional theory (DFT) calculations and incorporates key electrostatic and quantum mechanical components of electric field shielding in a nanoscopic interface. A material-agnostic formalism based on classical electromagnetic theory is introduced that allows the quantification of the electrostatic interfacial capacitance. The case studies investigated are the interfaces of monolayer graphene and bilayer graphene adsorbed on a silica substrate. Our model predicts the electrostatic capacitance in the studied interfaces to be field-independent, resulting in a reduction of the slope of the quantum capacitance with a shift in its minimum, aligning accurately and consistently with experimental measurements for both monolayer and bilayer graphene. The model provides an improved representation of the interfacial capacitance of low-dimensional materials, offering a better understanding of the electrochemical behavior of nanoscopic interfaces.

Discrete control of capacitance in quantum circuits

Abstract: Precise in-situ control of system parameters is indispensable for all quantum hardware applications. The capacitance in a circuit, however, is usually a simple consequence of electrostatics, and thus quite literally cast in stone. We here propose a way to control the charging energy of a given island by exploiting recently predicted Chern insulator physics in common Cooper-pair transistors, where the capacitance switches between discrete values given by the Chern number. We identify conditions for which the discrete control benefits from exponentially reduced noise sensitivity to implement protected tunable qubits.

Imaging the Quantum Capacitance of Strained MoS2 Monolayer by Electrostatic Force Microscopy

Abstract: We implemented radio frequency-assisted electrostatic force microscopy (RF-EFM) to investigate the electric field response of biaxially strained molybdenum disulfide (MoS2) monolayers (MLs), produced via hydrogen (H)-ion irradiation. MoS2 ML, a semiconducting transition metal dichalcogenide, has recently attracted significant attention due to its promising opto-electronic properties, further tunable by strain. Here, we take advantage of the RF assistance to distinguish the electrostatic response of atomic scale defects, such as sulfur vacancies or H-passivated sulfur vacancies, from that of the intrinsic quantum capacitance of the strained ML. In addition, measurements at fixed frequency (fRF = 300 MHz) elucidate the spatial variation of the quantum capacitance over mesoscopic length scales, due to the local modulation of the defect-driven n-type nature of the strained ML. Our finite-frequency capacitance imaging technique, which is non-invasive and nanoscale, opens up new possibilities for the investigation of frequency and spatial-dependent phenomena, such as the electron compressibility in quantum materials, which are difficult to measure by other methods.

Tuning Quantum Capacitance in 2D graphene electrodes: The Role of Defects and Charge Carriers Concentration

Abstract: Even though graphene has been intensively applied in electrochemical devices, the effects of oxidation and how the presence of graphene structural defects interfere with the monolayer graphene electrode-aqueous electrolyte interface...

The quantum geometric origin of capacitance in insulators

Abstract: In band insulators, where the Fermi surface is absent, adiabatic transport is allowed only due to the geometry of the Hilbert space. By driving the system at a small but finite frequency $\omega$, transport is still expected to depend sensitively on the quantum geometry. Here we show that this expectation is correct and can be made precise by expressing the Kubo formula for conductivity as the variation of the \emph{time-dependent polarization} with respect to the applied field. In particular, a little appreciated effect is that at linear order in frequency, the longitudinal conductivity results from an intrinsic capacitance, determined by the ratio of the quantum metric and the spectral gap. We demonstrate that this intrinsic capacitance has a measurable effect in a wide range of insulators with non-negligible metric, including the electron gas in a quantizing magnetic field, the gapped bands of hBN-aligned twisted bilayer graphene, and obstructed atomic insulators such as diamond whose large refractive index has a topological origin. We also discuss the influence of quantum geometry on the dielectric constant.