Search

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF3 ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 μF·cm-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF3 substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

Solid electrolytes with disordered structures, both crystalline and glassy, as well as supercooled ionic melts, exhibit surprisingly similar features in their conductivity spectra, σ(ν). This finding suggests that the dynamics of the mobile ions in the different systems should be governed by similar rules. Examples are given in this study, including new results on γ-RbAg4I5, β-AgI, and several glassy electrolytes. In spite of their overall similarity, however, the spectra also display characteristic differences in their shapes and in their scaling behaviour, the latter feature causing, e.g., Arrhenius or non-Arrhenius temperature dependences of the dc conductivity. The observed characteristics of the spectra, both the common and the more specific ones, are well reproduced with the help of two coupled rate equations describing the evolution of the ion dynamics with time. This treatment is based on the jump relaxation model, and is called the concept of mismatch and relaxation (CMR).

AgI: AgPO3 glasses are an interesting model system in which to address the relationships between carrier density and mobility, transport mechanism and medium-range order in fast ion-conducting glasses. In this paper we discuss an anomaly observed in the dc conductivity behaviour below room temperature. This anomaly is related to the opening of percolative channels for AgI contents higher than 30 mol % . Solid-state 109 Ag nuclear magnetic resonance (NMR) measurements on the 1:1 AgI: AgPC3 glass allow us to determine the change in the number of mobile carriers over all the temperature region where the anomaly is observed.

Silver iodide-silver molybdate glasses, (AgI)(x)(Ag2MoO4)(1 - x), with x = 0.75 and x = 0.67, exhibit high ionic conductivity, comparable to that of some ionic liquids at room temperature, and low values of the calorimetric glass transition temperature. The behaviour of the attenuation of acoustic waves as a function of temperature in silver iodomolybdate glasses below T-g is peculiar, arising from two different and almost overlapping relaxational contributions. In this work we investigate the frequency dependent conductivity and the dielectric properties of silver iodide-silver molybdate glasses in the mHz-GHz frequency range at temperatures between 300 K and 70 K, aiming in particular to ascertain whether a correlation can be found between the features of the dielectric response and the anomalous mechanical response of these fast ion conducting glasses. (C) 2013 Elsevier B.V. All rights reserved.

To determine the effects of pressure on ionic conduction mechanisms the a.c. conductivity of AgPO3 and (Ag2S)0.3(AgPO3)0.7 superionic glasses has been measured under hydrostatic pressure at selected temperatures, in the radio-frequency region. The experimental data show a monotonic decrease in conductivity at increasing pressure up to 2 kbar, while the onset of the dispersive a.c. conductivity depends on the pressure at fixed temperature. The data have been analyzed and compared with the temperature dependence of the a.c. conductivity obtained previously.

International audience; The thermal expansion of the ionically conducting silver iodide - silver molybdate glass (AgI)0.67(Ag2MoO4)0.33 has been investigated from low temperatures up to about the glass transition region using a capacitance dilatometer with a quasi-adiabatic method. The results do not show any anomalous behaviour in a temperature range where the existence of relaxation processes with peculiar features was previously revealed using mechanical spectroscopy, but rather a monotonic increase of positive values of the thermal expansion coefficient.

The thermal expansion of the ionically conducting silver iodide-silver molybdate glass (AgI)(0.67)(Ag2MoO4)(0.33) was investigated from low temperatures up to about the glass transition region using a capacitance dilatometer via a quasi-adiabatic method. The results do not show any anomalous behaviour in a temperature range where the existence of relaxation processes with peculiar features was previously revealed using mechanical spectroscopy, but rather a monotonic increase of positive values of the thermal expansion coefficient.

Dielectric measurements on ion conducting silver metaphosphate glasses, pure and doped with silver sulphide or silver sulphate, have been performed in the microwave frequency region up to 60 GHz in a wide temperature range between 77 and 413 K. With the current extension of previous measurements, a frequency range larger than 10 decades has been investigated and several distinct conductivity regimes have been observed. Above the low frequency dc conductivity value, the progressive increase of conductivity with frequency has been found to obey first a power law with exponent p ∼2/3 and later another power law with exponent q ∼4/3, nearly independent of temperature and of composition. In the low temperature region a nearly constant loss contribution to the conductivity has been also detected.

Dielectric data of mixed alkali LixRb1−xPO3 glasses have been analysed in the modulus formalism. The imaginary part of the complex modulus was fitted with a KWW function. The stretching parameter β is larger in the mixed glasses as compared to the single alkali glasses, which indicates that the mixed alkali glasses behave as single alkali glasses of effectively lower concentrations. The finding is consistent with the random ion distribution model, which assumes that the two kinds of alkali ions are moving in distinctly different conduction pathways.

The dynamical response of ionic glasses can be accurately characterized, in terms of frequency dependent conductivity, by dielectric spectroscopy techniques. Performing measurements on superionic vitreous conductors in the microwave frequency region, it has been discovered that the dispersive behavior of the conductivity obeys a double power law. Model considerations associate the two power law contributions at low and high frequencies respectively with translational and localized ionic hopping processes, characterized by different activation energies. In the present study on pure and doped silver phosphate glasses, experimental evidence is found that the power law exponents assume constant values, independent of the exact glass composition, and remarkably similar to those obtained for other superionic glasses.

Results of X-ray diffraction studies on glasses in the AgI-Ag 2 O-V 2 O 5 system are presented. In these glasses, the vanadium atom adopted five-fold coordination of oxygen atoms more frequently than four-fold and six-fold and the V-O distances lay between 1.76 and 1.79 A. The VO x polyhedra in the glasses shared corners, the V-V distances in the glasses lay between 3.30–3.41 A. Short and long Ag-O interactions were observed. An Ag-I interaction between 2.829–2.876 A was observed, which is characteristic of all AgI based glasses. The I-I distance decreased as the iodine content increased, having a value close to that in α-AgI at a high AgI content and approaching the value of β-AgI with decreasing AgI content.

The structure of the (Ag2SO4)x(AgPO3)1−x glasses is investigated with 31P NMR magic angle spinning techniques. For x 0.1. The structural description qualitatively agrees with conductivity, acoustic attenuation, and glass transition temperature (Tg) data. In particular, a Debye-like peak associated with the Ag+ jumping is seen at 168 K in the radiofrequency acoustic absorption of the x=0.3 glass. This suggests that silver is very mobile near the interstitial sulphates, but that its long-range diffusion is controlled by slower processes.

The modulus formalism has been used in the present study for a direct comparison between mechanical and electric relaxation responses in a superionic glass. The values of complex modulus have been calculated from the results of dielectric and mechanical measurements on the (Ag2S)0.3(AgPO3)0.7 glass at the same frequency, 10 MHz, as a function of temperature in the range 20–400 K. A peak in the imaginary part of the response function characterizes the dielectric as well as the mechanical relaxation data. The mechanical relaxation peak is larger than the electric relaxation peak. Assuming a phenomenological Cole–Davidson stretched response function, the resulting low value of the stretching exponent indicates a broad distribution of relaxation times. This could indicate that more microscopic processes with typical energies lower than the E dc, which characterizes ionic long-range diffusion, participate in both macroscopic dynamic responses.