An integrated capacitance bridge for high-resolution, wide temperature range quantum capacitance measurements

We have developed a highly-sensitive integrated capacitance bridge for quantum capacitance measurements. Our bridge, based on a GaAs HEMT amplifier, delivers attofarad (aF) resolution using a small AC excitation at or below kT over a broad temperature range (4K-300K). We have achieved a resolution at room temperature of 10aF per root Hz for a 10mV AC excitation at 17.5 kHz, with improved resolution at cryogenic temperatures, for the same excitation amplitude. We demonstrate the performance of our capacitance bridge by measuring the quantum capacitance of top-gated graphene devices and comparing against results obtained with the highest resolution commercially-available capacitance measurement bridge. Under identical test conditions, our bridge exceeds the resolution of the commercial tool by up to several orders of magnitude.Comment: (1)AH and JAS contributed equally to this work. 6 pages, 5 figure

Impregnation isotherms of hydroxybenzoic acid on PMMA in supercritical carbon dioxide

International audienceThe use of supercritical fluid and specifically carbon dioxide as carrier to infiltrate various matrices is gaining importance. This study focuses on the impregnation of poly(methyl methacrylate) by a model drug, o-hydroxybenzoic acid (o-HBA), using supercritical carbon dioxide (SCCO2) as carrier and swelling agent, with special emphasis to equilibrium data. A stirred batch reactor equipped with sampling device was employed in order to determine the sorption isotherms with a concentration parameter independent of pressure. The impregnated amount of HBA/gpolymer was determined at 40 °C and 12, 16 and 20 MPa for concentrations of HBA in SCCO2 ranging from 0.05 to 0.7 mg/g. Reproducibility of the impregnated amount was below 3.2%. At the highest investigated concentrations, the uptakes were of 27 mg/gpolymer when experiments were performed at 12 or 16 MPa, and of 20 mg/gpolymer for the investigations carried at 20 MPa. Experimental data were further correlated by four isotherm types: Langmuir, Freundlich, Sips and Toth. Only the Toth model was found to fit satisfactorily the whole range of concentration, for the three pressures