Simultaneous control of the stoichiometry, microstructure, and compositional homogeneity is a prerequisite for understanding the properties of Ag2Se. These are difficult to attain because of the highly mobile Ag+ ions above the superionic phase transition at 407 K. Here we report on a novel synthesis of well crystallized orthorhombic Ag2Se carried out at room temperature, which requires no expensive instrumentation, and yields a single-phase material in a very short time. Our facile reaction process is a self-sustaining room temperature synthesis driven by the dissociative adsorption of Se by Ag and promoted by stirring and intermittent grinding under ambient conditions. Systematic experimental and theoretical studies of chemical reactions between Ag and Q (Te, Se, and S) revealed that the reaction mechanism between Ag and Q is in line with the Hard Soft Acid Base (HSAB) scheme (rate order Ag2Te > Ag2Se > Ag2S). The low carrier concentration achieved ∼1018 cm−3 and the optimized weighted majority-to-minority carrier mobility ratio observed in the samples as corroborated by the state-of-the-art thermoelectric performance of ZT ∼1.2 at 390 K attest to the superiority of the synthesis route in yielding highly stoichiometric Ag2Se samples.

Facile room temperature solventless synthesis of high thermoelectric performance Ag2Se: Via a dissociative adsorption reaction

Thickness-dependent properties of sprayed cobalt oxide thin films

Ag2Se is considered as an attractive candidate for use in room-temperature thermoelectric applications owing to its unique transport properties, such as glass-like thermal conductivity and good electrical conductivity. However, understanding the correlation between composition (Ag/Se ratio), defect structure, and transport properties is an important prerequisite to optimize its figure of merit (ZT). Using in-depth microscopic analysis, this study reveals the coexistence of a metastable and the main orthorhombic crystal structure in stoichiometric Ag2Se. The formation of the metastable structure was found to be detrimental to the transport properties of bulk Ag2Se. We were able to successfully inhibit its formation and stabilize the main orthorhombic structure via small anion (Se and S) excess. The compositions Ag2SeChy (y ≤ 0.01; Ch = Se, S) yielded 40–70% rise in carrier mobility with a value of 2510 cm2 V−1 s−1 at 300 K and extremely low lattice-thermal-conductivity (0.2–0.1 W m−1 K−1 over 300–375 K). This combination of transport properties yielded a room-temperature power factor of 3.2 mW m−1 K−2 and a nearly flat ZT value of ∼1.0 over the 300–375 K temperature range. Additionally, a record-high conversion efficiency (ηmax) of 3.7% was theoretically obtained for single-leg Ag2Se for a small temperature gradient of ∼80 K.

Structural stability enables high thermoelectric performance in room temperature Ag2Se

A novel chemical method has been developed for the preparation of crystalline nanotubes of silver selenide (Ag2Se). The method is based on the template-engaged synthesis in which the trigonal Se nanotubes as-prepared were used as template reagent. The products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy-dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), ultraviolet (UV) analysis, photoluminescence (PL) analysis, differential scanning calorimetric (DSC) analysis, and thermogravimetric (TG) analysis. Based on a series of experiments and characterizations, the formation mechanism of the Ag2Se nanotubes was proposed. Furthermore, the useful electrochemical behavior of the Ag2Se nanotubes was studied by the voltammetric technique.

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Synthesis and electrochemical behavior of crystalline Ag2Se nanotubes

A series of dichalcogenophosphinato [Ag(Se2PiPr2)] (1), [Ag(SSePiPr2)]4 (2), and imidodichalcogenodiphosphinatosilver(I) [Ag{iPr2P(S)NP(Se)iPr2}]3 (3) [Ag{(SePiPr2)2N}]3 (4) complexes were synthesised; (2) and (3) were characterised by X-ray single crystallography. Complex (2) has a tetrahedral arrangement of four silver atoms with four thioselenophosphinate ligands along the edges, (3) is a trimer. The compounds (1), (2), (3) and (4) were employed as single-source precursors (SSPs) in aerosol assisted chemical vapour deposition (AACVD) and low pressure (LP) CVD. All precursors gave silver selenide (Ag2Se) films by AACVD, whereas only (3) and (4) deposited Ag2Se films by LPCVD. The as-obtained films were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) methods.

D Karunakaran

Papers

Variations of energy gap, resistivity, and temperature coefficient of resistivity in annealed beta -Ag2Se thin films.