Quantum capacitance and density of states of graphene

We report on measurements of the quantum capacitance in graphene as a function of charge carrier density. A resonant LC-circuit giving high sensitivity to small capacitance changes is employed. The density of states, which is directly proportional to the quantum capacitance, is found to be significantly larger than zero at and around the charge neutrality point. This finding is interpreted to be a result of potential fluctuations with amplitudes of the order of 100 meV in good agreement with scanning single-electron transistor measurements on bulk graphene and transport studies on nanoribbons

Electrochemical studies of capacitance in cerium oxide thin films and its relationship to anionic and electronic defect densities

Small polaron carrier density in epitaxial, doped CeO_2 thin films under low oxygen partial pressure was determined from electrochemically-measured capacitance after accounting for interfacial effects and shown to agree well with bulk values

Do Capacitance Measurements Reveal Light-Induced Bulk Dielectric Changes in Photovoltaic Perovskites?

Several studies have identified complex interactions between photogenerated carriers and the crystal lattice in perovskite materials for photovoltaics, which are regarded to have an inherently soft character. Light is known to induce phase segregation in mixed halide perovskites, enhance piezoelectricity, and change dipole moments of the unit cell. Therefore, it is appealing to consider photogenerated variations in the bulk polarizability and dielectric properties. Light-induced bulk polarization changes should be observable by capacitance measurements, preferentially at intermediate frequencies where the geometrical capacitance dominates. However, capacitance spectra are influenced by several capacitive and resistive mechanisms, which are also modulated by light. Capacitances may arise from dielectric bulk polarization, space-charge depletion zones, chemical electronic bulk storage, and interfacial accumulation mechanisms. This variety of capacitive mechanisms may induce wrong interpretations and produce misleading outcomes when uncritically connected to the bulk polarization response. It is shown here how capacitance–voltage analyses performed at a given frequency are influenced by overlapping effects that mask the actual value of the geometrical capacitance. Careful analyses are then needed before attributing the light-induced modulation of the measured capacitance to hybrid perovskite dielectric changes

Time-resolved PhotoEmission Spectroscopy on a Metal/Ferroelectric Heterostructure

In thin film ferroelectric capacitor the chemical and electronic structure of the electrode/FE interface can play a crucial role in determining the kinetics of polarization switching. We investigate the electronic structure of a Pt/BaTiO3/SrTiO3:Nb capacitor using time-resolved photoemission spectroscopy. The chemical, electronic and depth sensitivity of core level photoemission is used to probe the transient response of different parts of the upper electrode/ferroelectric interface to voltage pulse induced polarization reversal. The linear response of the electronic structure agrees quantitatively with a simple RC circuit model. The non-linear response due to the polarization switch is demonstrated by the time-resolved response of the characteristic core levels of the electrode and the ferroelectric. Adjustment of the RC circuit model allows a first estimation of the Pt/BTO interface capacitance. The experiment shows the interface capacitance is at least 100 times higher than the bulk capacitance of the BTO film, in qualitative agreement with theoretical predictions from the literature.Comment: 7 pages, 10 figures. Submitted to Phys. Rev.

Deep level transient spectroscopy (DLTS) study of P3HT:PCBM organic solar cells

The electronic structure of an organic photovoltaic bulk heterojunction cell strongly deviates from the typical textbook examples of a single sided junction used to explain electrical characterisation of defects in semiconductors. Therefore it is not so straightforward to assign the capacitance of this device or the charge in it to the presence of a depleted layer within this structure. However, conventional electronic spectroscopic techniques could give useful information to understand the electronic behaviour of the device. Therefore, in this work capacitance and charge DLTS have been performed on P3HT:PCBM solar cells. At 1MHz only negligible variation in the capacitance as a function of temperature and bias has been observed. As a result no spectrum could be recorded using a standard DLTS setup, registering the capacitance at this high frequency. To avoid this parasitic effect low frequency capacitance DLTS (40 kHz) has been performed, showing an anomalous signal with negative amplitude and an activation energy of 160meV, and a complementary positive signal could be observed altering the biases. Charge DLTS clearly revealed that both signals transients, conventional and with altered bias have the same time constants. A recent study has shown that such behaviour cannot be explained by the thermodynamic properties of capture and emission of carriers by a defect in bulk semiconductor. The validity of alternative explanations, including interface states, non-ideal ohmic contacts and effects of carrier hopping on charge mobility, will discussed

Analytical interfacial layer model for the capacitance and electrokinetics of charged aqueous interfaces

We construct an analytical model to account for the influence of the subnanometer-wide interfacial layer on the differential capacitance and the electro-osmotic mobility of solid–electrolyte interfaces. The interfacial layer is incorporated into the Poisson–Boltzmann and Stokes equations using a box model for the dielectric properties, the viscosity, and the ionic potential of mean force. We calculate the differential capacitance and the electro-osmotic mobility as a function of the surface charge density and the salt concentration, both with and without steric interactions between the ions. We compare the results from our theoretical model with experimental data on a variety of systems (graphite and metallic silver for capacitance and titanium oxide and silver iodide for electro-osmotic data). The differential capacitance of silver as a function of salinity and surface charge density is well reproduced by our theory, using either the width of the interfacial layer or the ionic potential of mean force as the only fitting parameter. The differential capacitance of graphite, however, needs an additional carbon capacitance to explain the experimental data. Our theory yields a power-law dependence of the electro-osmotic mobility on the surface charge density for high surface charges, reproducing the experimental data using both the interfacial parameters extracted from molecular dynamics simulations and fitted interfacial parameters. Finally, we examine different types of hydrodynamic boundary conditions for the power-law behavior of the electro-osmotic mobility, showing that a finite-viscosity layer explains the experimental data better than the usual hydrodynamic slip boundary condition. Our analytical model thus allows us to extract the properties of the subnanometer-wide interfacial layer by fitting to macroscopic experimental data