Semiconducting behavior of Ag2Te thin films and the dependence of band gap on thickness

Abstract: In this paper, we report a solution-phase synthesis of silver telluride and bismuth telluride nanowire heterostructure using tellurium (Te) nanowire as sacrificial template and site-selective conversion strategy. High-resolution transmission electron microscope studies confirm sharp interface with possible epitaxial growth between silver telluride and bismuth telluride regions. Through tuning the precursor amount (bismuth and silver precursors) during the reaction, the composition between silver telluride and bismuth telluride can be adjusted. Moreover, the mass produced powder of nanowire heterostructure is consolidated into nanocomposite pellets, and thermoelectric properties of the nanocomposite pellets are investigated between 300 and 400 K. Results show that our materials are p-type with reduced lattice thermal conductivity and a ZT of ∼0.41 at 400 K, which is the best reported value for p-type silver telluride.

Abstract: Thin films of silver selenide (Ag2Se) between thicknesses of about 700 and 2200 A have been prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. After vacuum annealing the films (at about 373 K for 3 h) electrical resistivity measurements on these films have been carried out in vacuum. From the increase in the rate of decrease of resistance with temperature, the phase transition temperatures (orthorhombic to body‐centered cubic) of the different films have been located. It is found that the phase transition temperature of the thin films is a function of thickness increasing with a decrease in the thickness. This observation has been explained by a recently developed theory [V. Damodara Das and D. Karunakaran, J. Phys. Chem Solids 46, 551 (1985)] of phase transitions in thin films modified further. Also, an order‐of‐magnitude value of the difference in the function of specific surface and interfacial energies of the two phases has also been determined using the theory.

Abstract: It is observed from the studies of electrical resistivity of films of SnSe of different thicknesses and substrate temperatures that resistivity decreases with increase in thickness and substrate temperature. The study of variation in band gap with thickness shows a linear relationship between band gap and 1/t2. Finally, band gap – substrate temperature studies show that the band gap increases with substrate temperature.

Abstract: Monoclinic molybdenum trioxide (β-MoO3) nanostructures (shaped like nanoribbons: NRs) were grown on Si(1 0 0), Si(5 5 1 2) and fluorine-doped tin oxide (FTO) by molecular beam epitaxy (MBE) technique under ultra-high vacuum (UHV) conditions. The dependence of substrate conditions and the effective thickness of MoO3 films on the morphology of nanostructures and their structural aspects were reported. The electron microscopy measurements show that the length and the aspect ratio of nanostructures increased by, 260% without any significant change in the width for a change in effective thickness from 5 nm to 30 nm. NRs are grown along 〈0 1 1〉 for all the effective thickness of MoO3 films. Similarly, when we increased the film thickness from 5 nm to 30 nm, the optical band gap decreased from 3.38 ± 0.01 eV to 3.17 ± 0.01 eV and the local work function increased from 5.397 ± 0.025 eV to 5.757 ± 0.030 eV. Field emission turn-on field decreased from 3.58 V/μm for 10 μA/cm2 to 2.5 V/μm and field enhancement factor increased from 1.1 × 104 to 5.9 × 104 for effective thickness variation of 5–30 nm β-MoO3 structures. The β-MoO3 nanostructures found to be much better than the α-MoO3 nanostructures due to low work function, low turn on field and high field enhancement factor, and are expected to be useful applications.

Abstract: The barrier theory of the infrared photoconductivity of PbS films is discussed, according to which the high resistance of the films arises from $n\ensuremath{-}p\ensuremath{-}n$ barriers at the surfaces between the crystallites forming the films, the barriers being formed in the oxidizing process used in preparing the films. Under the action of light, electron hole pairs are formed, these carriers become trapped in the $n$- and $p$-type regions, respectively, the resulting charge density lowers the barriers, and hence the conductivity is increased. This theory is worked out quantitatively, and compared with experimental results of Mahlman on films of the type actually used as sensitive photoconductors. The theory shows good qualitative and quantitative agreement with experiment in numerous respects, including the explanation of the dark conductivity of the films and its dependence on temperature, the photoconductivity as a function of irradiance and temperature, the time constants involved in the rise or decay of the photoconductivity, and the short-wave limit of the photoconductivity. In working out the theory of the barrier model, we use the properties of the bulk material as determined by Petritz and Scanlon, and the properties of the films are found to be consistent with our knowledge of the behavior of the bulk material.

Abstract: The Seebeck coefficient α, electrical conductivity σ, and thermal conductivity K data are given for a number of Ag2Te specimens measured at room temperature. The maximum value observed for the figure of merit α2σ/K was 1.3×10−3°C−1. The relationship between K and σ was linear, and Kel could be expressed approximately by (π2/3) (k/e)2σT. Kph was found to be 0.72×10−2 w cm−1°C−1. There was no evidence of an ambipolar diffusion contribution to K. It is suggested that the small energy gap (0.02 ev) previously determined in this compound could account for the absence of a measurable ambipolar diffusion effect, the degeneracylike behavior of Kel, and the relatively low values of the Seebeck coefficient.

Abstract: Structural transformation in thin films of binary alloys of 1st and 6th group elements, prepared by vacuum evaporation, has been studied by the author in detail by electron diffraction. The alloys chosen for the investigation were Ag2Te, Ag2Se, Ag2S, Cu2Te and Cu2Se. The study of thin films of these alloys revealed that there occurs a transformation in structure and on heating, the temperatures of transformation are higher compared to the alloys in the bulk state. It has also been observed that the temperatures of structural transformation during heating and cooling are different. No quantitative explanation of the result has been given and it is expected that detailed discussion of the results will be published later.