Effect of nanoscale confinement on the β−α phase transition in Ag2Se

Abstract

The confinement of silver selenide was investigated using mesoporous silica. Results from x-ray diffraction and electron microscopy show that the confined material still exhibits a β to α transition similar to the one that takes place in the bulk crystalline state but with a transition temperature that depends significantly on the confinement conditions. Decreasing the pore size leads to an increase of the transition temperature, opposite to the behavior of the melting point observed in several metallic and organic materials. In the free particles, on the other hand, no size dependence is observed with particle sizes down to 4 nm.A recent review by Alcoutlabi and McKenna 1 has summarized the state of the art of nanoscale confinement effects on the first-order thermodynamics in confined geometries and shown that the Gibbs-Thomson equation is suitable for describing the changes in melting phenomena observed in the literature due to nanometer scale confinement. They also discussed the more challenging issue of the glass transition for confined systems under different geometrical conditions. However, to our knowledge the problem of the changes in solid-solid phase transitions due to confinement has not been addressed. Some silver ionic salts as well as silver chalcogenides undergo a relatively low-temperature solid-solid transition with dramatic differences between the two phases, e.g., transition from an ordered to a disordered phase with changes in the nature of the conduction mechanism in each phase. 2,3 The structure and spectroscopic properties of various Se based compounds confined in cages of zeolites have been studied, 4 but no attempts have been made to investigate changes induced by the confinement on firstorder transitions. Silver selenide, Ag 2 Se, is such an example and here we focus uniquely on this compound. At room temperature, Ag 2 Se is a narrow band-gap semiconductor with high carrier mobility and without magnetoresistance (MR) and undergoes a phase transition in the solid state. 5,6 The lowtemperature phase, designated as β−Ag 2 Se with an orthorhombic structure, has been widely used as a photosensitizer in photographic films and thermochromic materials. The transition to the hightemperature phase α−Ag 2 Se takes place accompanied by a decrease in volume and has been reported with values of the transition temperature T βα ranging from 127 °C to 143 °C. 7,8 The high-temperature phase is a superionic conductor with mobile Ag + ions used as the solid electrolyte in photochargeable secondary batteries. 9 In the liquid state, it has been shown that the temperature dependence of the conductivity is negative, which is unusual for liquids with a conductivity in the range typical for liquid semiconductors in the narrow definition of Mott. 10,11 Moreover, β−Ag 2 Se has been recently shown to exhibit a significant magnetoresistance (MR) by manipulating small departures from stoichiometry. 12 This large magnetic response, a near-linear increase of the resistance with the applied magnetic field witho...

Solid-Phase Flexibility in Ag2Se Semiconductor Nanocrystals

“…Similar increases have been reported for Ag 2 S or Ag 2 Se nanocrystals that were embedded in a matrix of GeS 2 or mesoporous silica, respectively. In general, the increase should depend on the lattice mismatch of the two materials, the size of the core, and the thickness of the shell.…”