Leu-M

In recent years, much effort have been dedicated to achieve thin, lightweight and even flexible energy-storage devices for wearable electronics. Here we demonstrate a novel kind of ultrathin all-solid-state supercapacitor configuration with an extremely simple process using two slightly separated polyaniline-based electrodes well solidified in the H(2)SO(4)-polyvinyl alcohol gel electrolyte. The thickness of the entire device is much comparable to that of a piece of commercial standard A4 print paper. Under its highly flexible (twisting) state, the integrate device shows a high specific capacitance of 350 F/g for the electrode materials, well cycle stability after 1000 cycles and a leakage current of as small as 17.2 μA. Furthermore, due to its polymer-based component structure, it has a specific capacitance of as high as 31.4 F/g for the entire device, which is more than 6 times that of current high-level commercial supercapacitor products. These highly flexible and all-solid-state paperlike polymer supercapacitors may bring new design opportunities of device configuration for energy-storage devices in the future wearable electronic area.

Highly Flexible and All-Solid-State Paperlike Polymer Supercapacitors

This review is an update of a previous review (A. J. Minnich, et al., Energy Environ. Sci., 2009, 2, 466) published two years ago by some of the co-authors, focusing on progress made in thermoelectrics over the past two years on charge and heat carrier transport, strategies to improve the thermoelectric figure of merit, with new discussions on device physics and applications, and assessing challenges on these topics. Understanding of phonon transport in bulk materials has advanced significantly as the first-principles calculations are applied to thermoelectric materials, and experimental tools are being developed. Some new strategies have been developed to improve electron transport in thermoelectric materials. Fundamental questions on phonon and electron transport across interfaces and in thermoelectric materials remain. With thermoelectric materials reaching high ZT values well above one, the field is ready to take a step forward and go beyond the materials' figure of merit. Developing device contacts and module fabrication techniques, developing a platform for efficiency measurements, and identifying applications are becoming increasingly important for the future of thermoelectrics.

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Perspectives on thermoelectrics: from fundamentals to device applications

One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression in which the intercept gives the band-gap and the fitting exponent identifies the electronic transition as direct or indirect (see Tauc et al., Phys. Status Solidi 15, 627 (1966); these are often called “Tauc” plots). While the technique is powerful and simple, the accuracy of the fitted band-gap result is seldom stated or known. We tackle this question by refitting a large number of Tauc plots from the literature and look for trends. Nominally pure zinc oxide (ZnO) was chosen as a material with limited intrinsic deviation from stoichiometry and which has been widely studied. Our examination of the band gap values and their distribution leads to a discussion of some experimental factors that can bias the data and lead to either smaller or larger apparent values than would be expected. Finally, an easily evaluated figure-of-merit is defined that may help guide more accurate Tauc fitting. For samples with relatively sharper Tauc plot shapes, the population yields Eg(ZnO) as 3.276 ± 0.033 eV, in good agreement with data for single crystalline material.

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system

The concept of energy filtering of the carriers to control the thermoelectric properties of PbTe is experimentally applied in this present work. The energy barriers at the grain interfaces of the nanocomposites and the embedded Ag-rich nanodots within the grains are supposed to control the energy dependency of carrier scattering: that is what we mean by energy filtering of carriers. As a case study, vertical Bridgman grown bulk PbTe:undoped, PbTe:Ag crystals and nanocomposites of PbTe:Ag are used as samples. Thermoelectric properties of all the samples have been evaluated through temperature dependent electrical conductivity, Seebeck coefficient and room temperature Hall and thermal conductivity measurements. It is found that the PbTe:Ag nanocomposites has the highest power factor of 18.78×10−4 W m−1 K−2 with a room temperature thermal conductivity of 1.69 W m−1 K−1. The crystal structures of these samples show the effective potential barrier at the grain boundaries and embedded nanodots within the grains...

Embedded Ag-rich nanodots in PbTe: Enhancement of thermoelectric properties through energy filtering of the carriers

Zinc oxide (ZnO) is one of the most promising semiconducting metal oxides, particularly for gas sensing applications. Impurity doped ZnO has offered much improved sensing performance, as compared to its undoped counterpart. In this work, undoped as well as indium doped nanocrystalline ZnO thin films are synthesized by a low cost chemical solution deposition route. X-ray diffraction patterns of the synthesized films reveal preferential orientation along the (002) plane. The surface and cross section morphology has clearly changed with the variation of indium content. Optical transmittance values increase with the increase in indium concentration and the band gap energy is found in the range 3.210 eV to 3.221 eV. PL spectra reveal three characteristic peaks, owing to band to band transition and defect level emissions. The effect of indium doping on the electrical parameters of ZnO is analyzed through Hall effect measurements at room temperature. The gas sensing characteristics of these sensors offer good reproducibility and stability towards various reducing gases, with an enhancement of response% and lower detection limit as compared to undoped ZnO. A doping level of 3 wt% of indium in ZnO is found to give optimum response and the lowest detection limit of hydrogen of 1 ppm or even lower. However, further increase in the doping concentration resulted in reduced sensing performance. This is attributed to the gas sensing mechanism related to the substitution of In3+ ions at Zn2+ ion sites enhancing the number of free charge carriers at the optimum level of indium. Through exploring this gas sensing mechanism, it is argued that the sensor performance can be dramatically improved by tailoring the indium concentration in ZnO for its practical application in various sectors as an effective gas sensor.

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

InP quantum dots (QDs) were grown on catalyst-free Si substrates by MOCVD to study the behavior of growth of low dimensional III–V structures on Si substrates. It is found that at temperature 575 °C, uniform QDs with diameter 20–50 nm and height 6–8 nm were obtained, whereas at 600 °C, InP nanoislands with wetting layers were formed instead of QDs. From the photoluminescence measurements, blue shift of the band gap is observed with a value of 1.395 eV. The densities of the QDs were found to be 7–8 × 1013 m−2. X-ray photoelectron spectroscopy establishes the presence of InP rather than indium droplet. X-ray diffraction spectra show different surface planes of the QDs. The effect of growth temperature has been discussed.

Pallab Banerji

Papers

Embedded Ag-rich nanodots in PbTe: Enhancement of thermoelectric properties through energy filtering of the carriers

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Catalyst-free direct growth of InP quantum dots on Si by MOCVD: a step toward monolithic integration

Role of oxygen vacancy in optical and gas sensing characteristics of ZnO thin films

Studies on resistive switching characteristics of aluminum/graphene oxide/semiconductor nonvolatile memory cells