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Single Photon Detection in a Large array of Quantum Capacitance Detectors
No abstract provided
Single Photon Detection in a Large array of Quantum Capacitance Detectors
No abstract provided
Contribution of dielectric screening to the total capacitance of few-layer graphene electrodes
We apply joint density functional theory (JDFT), which treats the electrode/electrolyte interface self-consistently, to an electric double-layer capacitor (EDLC) based on few-layer graphene electrodes. The JDFT approach allows us to quantify a third contribution to the total capacitance beyond quantum capacitance (C Q ) and EDL capacitance (C EDL ). This contribution arises from the dielectric screening of the electric field by the surface of the few-layer graphene electrode, and we therefore term it the dielectric capacitance (C Dielec ). We find that C Dielec becomes significant in affecting the total capacitance when the number of graphene layers in the electrode is more than three. In conclusion, our investigation sheds new light on the significance of the electrode dielectric screening on the capacitance of few-layer graphene electrodes.
Quantum mechanics capacitance molecular mechanics modeling of core-electron binding energies of methanol and methyl nitrite on Ag(111) surface
We study a newly devised quantum mechanics capacitance molecular mechanics (QMCMM) method for the calculation of core-electron binding energies in the case of molecules adsorbed on metal surfaces. This yet untested methodology is applied to systems with monolayer of methanol/methyl nitrite on an Ag(111) surface at 100 K temperature. It was found out that the studied C, N, and O 1s core-hole energies converge very slowly as a function of the radius of the metallic cluster, which was ascribed to build up of positive charge on the edge of the Ag slab. Further analysis revealed that an extrapolation process can be used to obtain binding energies that deviated less than 0.5 eV against experiments, except in the case of methanol O 1s where the difference was as large as 1.8 eV. Additional QM-cluster calculations suggest that the latter error can be connected to the lack of charge transfer over the QM–CMM boundary. Thus, the results indicate that the QMCMM and QM-cluster methods can complement each other in a holistic picture of molecule-adsorbate core-ionization studies, where all types of intermolecular interactions are considered.
Capacitive Energy Extraction by Few-Layer Graphene Electrodes
Capacitive double-layer expansion is a promising technology to harvest energy arising from the salinity difference between freshwater and seawater. Its optimal performance requires a careful selection of the operation potentials and electrode materials. While carbonaceous materials such as graphene and various forms of activated carbons are routinely used as the electrodes, there is little knowledge on how the quantum capacitance and the electric double-layer (EDL) capacitance, which are on the same order of magnitude, affect the capacitive performance. Toward understanding that from a theoretical perspective, here we study the capacitive energy extraction with graphene electrodes as a function of the number of graphene layers. The classical density functional theory is joined with the electronic density functional theory to obtain the EDL and the quantum capacitance, respectively. The theoretical results show that the quantum capacitance contribution plays a dominant role in extracting energy using the single-layer graphene, but its effect diminishes as the number of graphene layers increases. The overall extracted energy is dominated by the EDL contribution beyond about four graphene layers. Electrodes with more graphene layers are able to extract more energy at low charging potential. Here, because many porous carbons have nanopores with stacked graphene layers, our theoretical predictions are useful to identify optimal operation parameters for capacitive energy extraction with porous electrodes of different wall thickness.
Origins and Implications of Interfacial Capacitance Enhancements in C 60 -Modified Graphene Supercapacitors
Understanding and controlling the electrical response at a complex electrode–electrolyte interface is key to the development of next-generation supercapacitors and other electrochemical devices. In this work, we apply a theoretical framework based on the effective screening medium and reference interaction site model to explore the role of electrical double-layer (EDL) formation and its interplay with quantum capacitance in graphene-based supercapacitors. In addition to pristine graphene, we investigate a novel C60-modified graphene supercapacitor material, which promises higher charge-storage capacity. Beyond the expected enhancement in the quantum capacitance, we find that the introduction of C60 molecules significantly alters the EDL response. These changes in EDL are traced to the interplay between surface morphology and charge localization character and ultimately dominate the overall capacitive improvement in the hybrid system. Our study highlights a complex interplay among surface morphology, electronic structure, and interfacial capacitance, suggesting general improvement strategies for optimizing carbon-based supercapacitor materials.
Anticorrelation of compressibility and resistivity in a microwave-irradiated two-dimensional electron system
Here, we report our direct study of the compressibility on an ultrahigh mobility two-dimensional electron system ( μ e ~ 1 × 10 7 cm 2 /V s) in GaAs/AlGaAs quantum wells under microwave irradiation. The field penetration current results show that the quantum capacitance oscillates with microwave-induced resistance oscillations, yet, the trend is opposite with respect to the compressibility of equilibrium states in previous theoretical explanations. The anomalous phenomena provide a platform for study on the nonequilibrium system under microwaves. Moreover, the quantum capacitance indication for a multiphoton process around j = 1 / 2 is detected under an intensive microwave below 30 GHz.
Quantum Effects on the Capacitance of Graphene-Based Electrodes
We recently measured quantum capacitance for electric double layers (EDL) at electrolyte/graphene interfaces. However, the importance of quantum capacitance in realistic carbon electrodes is not clear. Toward understanding that from a theoretical perspective, here we studied the quantum capacitance and total capacitance of graphene electrodes as a function of the number of graphene layers. The quantum capacitance was obtained from electronic density functional theory based on fixed band approximation with an implicit solvation model, while the EDL capacitances were from classical density functional theory. We found that quantum capacitance plays a dominant role in total capacitance of the single-layer graphene both in aqueous and ionic-liquid electrolytes but the contribution decreases as the number of graphene layers increases. Moreover, the total integral capacitance roughly levels off and is dominated by the EDL capacitance beyond about four graphene layers. Finally, because many porous carbons have nanopores with stacked graphene layers at the surface, this research provides a good estimate of the effect of quantum capacitance on their electrochemical performance.
Anomalous capacitance of quantum well double-barrier diodes
The S-parameters of several different quantum well double barrier diodes have been measured. A technique has been developed for measuring whisker contacted diodes with an HP 8510B automatic network analyzer. Special coaxial mounts using K-connectors were designed to enable measurements up to 20 GHz. The voltage-dependent conductance and capacitance were derived from the measured reflection coefficient of each device. The C/V characteristics were observed to exhibit an anomalous increase at voltages corresponding to the negative differential resistance region (NDR). These are the first reported S-parameter measurements in the negative differential resistance region of quantum well double barrier diodes. A theory is presented that explains, in part, the observed results.
Capacitive charge storage at an electrified interface investigated via direct first-principles simulations [Direct Simulation of Capacitive Charging of Graphene and Implications for Supercapacitor Design]
Understanding the impact of interfacial electric fields on electronic structure is crucial to improving the performance of materials in applications based on charged interfaces. Supercapacitors store energy directly in the strong interfacial field between a solid electrode and a liquid electrolyte; however, the complex interplay between the two is often poorly understood, particularly for emerging low-dimensional electrode materials that possess unconventional electronic structure. Typical descriptions tend to neglect the specific electrode-electrolyte interaction, approximating the intrinsic “quantum capacitance” of the electrode in terms of a fixed electronic density of states. Instead, we introduce a more accurate first-principles approach for directly simulating charge storage in model capacitors using the effective screening medium method, which implicitly accounts for the presence of the interfacial electric field. Applying this approach to graphene supercapacitor electrodes, we find that results differ significantly from the predictions of fixed-band models, leading to improved consistency with experimentally reported capacitive behavior. The differences are traced to two key factors: the inhomogeneous distribution of stored charge due to poor electronic screening and interfacial contributions from the specific interaction with the electrolyte. Lastly, our results are used to revise the conventional definition of quantum capacitance and to provide general strategies for improving electrochemical charge storage, particularly in graphene and similar low-dimensional materials.
Adiabatic two-qubit gates in capacitively coupled quantum dot hybrid qubits
Abstract The ability to tune qubits to flat points in their energy dispersions (“sweet spots”) is an important tool for mitigating the effects of charge noise and dephasing in solid-state devices. However, the number of derivatives that must be simultaneously set to zero grows exponentially with the number of coupled qubits, making the task untenable for as few as two qubits. This is a particular problem for adiabatic gates, due to their slower speeds. Here, we propose an adiabatic two-qubit gate for quantum dot hybrid qubits, based on the tunable, electrostatic coupling between distinct charge configurations. We confirm the absence of a conventional sweet spot, but show that controlled-Z (CZ) gates can nonetheless be optimized to have fidelities of ~99% for a typical level of quasistatic charge noise ( σ ε ≃ 1 μeV). We then develop the concept of a dynamical sweet spot (DSS), for which the time-averaged energy derivatives are set to zero, and identify a simple pulse sequence that achieves an approximate DSS for a CZ gate, with a 5× improvement in the fidelity. We observe that the results depend on the number of tunable parameters in the pulse sequence, and speculate that a more elaborate sequence could potentially attain a true DSS.
Coupling capacitance between double quantum dots tunable by the number of electrons in Si quantum dots
Tunability of capacitive coupling in the Si double-quantum-dot system is discussed by changing the number of electrons in quantum dots (QDs), in which the QDs are fabricated using pattern-dependent oxidation (PADOX) of a Si nanowire and multi-fine-gate structure. A single QD formed by PADOX is divided into multiple QDs by additional oxidation through the gap between the fine gates. When the number of electrons occupying the QDs is large, the coupling capacitance increases gradually and almost monotonically with the number of electrons. This phenomenon is attributed to the gradual growth in the effective QD size due to the increase in the number of electrons in the QDs. On the other hand, when the number of electrons changes in the few-electron regime, the coupling capacitance irregularly changes. This irregularity can be observed even up to 40 electrons. This behavior is attributable the rough structure of Si nano-dots made by PADOX. This roughness is thought to induce complicated change in the electron wave function when an electron is added to or subtracted from a QD.
Full-range electrical characteristics of WS{sub 2} transistors
We fabricated transistors formed by few layers to bulk single crystal WS{sub 2} to quantify the factors governing charge transport. We established a capacitor network to analyze the full-range electrical characteristics of the channel, highlighting the role of quantum capacitance and interface trap density. We find that the transfer characteristics are mainly determined by the interplay between quantum and oxide capacitances. In the OFF-state, the interface trap density (<10{sup 12} cm{sup –2}) is a limiting factor for the subthreshold swing. Furthermore, the superior crystalline quality and the low interface trap density enabled the subthreshold swing to approach the theoretical limit on a back-gated device on SiO{sub 2}/Si substrate.
Transistor roadmap projection using predictive full-band atomistic modeling
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.
Side-gate modulation effects on high-quality BN-Graphene-BN nanoribbon capacitors
High-quality BN-Graphene-BN nanoribbon capacitors with double side-gates of graphene have been experimentally realized. The double side-gates can effectively modulate the electronic properties of graphene nanoribbon capacitors. By applying anti-symmetric side-gate voltages, we observed significant upward shifting and flattening of the V-shaped capacitance curve near the charge neutrality point. Symmetric side-gate voltages, however, only resulted in tilted upward shifting along the opposite direction of applied gate voltages. These modulation effects followed the behavior of graphene nanoribbons predicted theoretically for metallic side-gate modulation. The negative quantum capacitance phenomenon predicted by numerical simulations for graphene nanoribbons modulated by graphene side-gates was not observed, possibly due to the weakened interactions between the graphene nanoribbon and side-gate electrodes caused by the Ga{sup +} beam etching process.
Electronic properties of a Weyl semimetal in crossed magnetic and electric fields
The study of Weyl semimetals is one of the most challenging problems of condensed matter physics. These materials exhibit interesting properties in a magnetic field. In this work, we investigate the Landau bands and the density of states (DOS) oscillations in a Weyl semimetal in crossed magnetic and electric fields. An expression is obtained for the energy spectrum of the system using the following three different methods: an algebraic approach, a Lorentz shift-based approach, and a quasi-classical approach. It is interesting that the energy spectrum calculated in terms of the quasi-classical approach coincides with the spectrum obtained using the microscopic approaches. An electric field is shown to change the Landau bands radically. In addition, the classical motion of a three-dimensional Dirac fermion in crossed fields is studied. In the case of a Dirac spectrum, the longitudinal (with respect to magnetic field) component of momentum (p{sub z} ǁ H) is shown to be an oscillating function of the magnetic field. When the electric field is v⊥H/c, the Landau levels collapse and the motion becomes fully linear in an unusual manner. In this case, the wavefunction of bulk states vanishes and only states with p{sub z} = 0 are retained. An electric field affects the character of DOS oscillations. An analytical expression is obtained for the quantum capacitance in crossed fields in the cases of strong and weak electric fields. Thus, an electric field is an additional parameter for adjusting the diamagnetic properties of Weyl semimetals.