High-resolution electron-microscopic studies of the polymorphs in Ag_2±δSe films

Abstract

The polymorphs that appear in the low-temperature phase of silver selenide have been studied by highresolution electron microscopy. The specimen films are intentionally prepared with excess silver or selenium over stoichiometric composition by flash evaporation, asdepositing carbon films on both sides of the specimen films to protect them from selenium sublimation and to maintain the composition throughout the heat treatment. It is shown that four different types of low-temperature phase exist: tetragonal (a = 6.98, c = 4.96A) for a metastable phase only formed with a small grain size of less than 50 nm; face-centred cubic (a = 10.9 ~) for a non-stoichiometric phase with excess silver; monoclinic (a = 7.05, b = 8.17, c = 4.34~, o~ = 101.0 °) for a non-stoichiometric phase with excess selenium; and orthorhombic (a = 7.05, b = 7.82, c = 4.34A) for the stoichiometric stable phase. The topotactic relations between the orthorhombic and monoclinic types are found to be fully coherent, having the same a and c lattice parameters.

Ligand-Mediated Phase Control in Colloidal AgInSe2 Nanocrystals

“…9,43 However, an additional metastable tetragonal polymorph is known to form within polycrystalline thin films and for Ag2Se nanocrystals. [9][10][11][12][13] To the best of our knowledge, the crystal structure of this tetragonal phase of Ag2Se has not yet been unambiguously determined, in large part due to its instability as a bulk material under any known conditions. Even so, Wang et al conducted a thorough investigation of the phase transitions that occur between the tetragonal, orthorhombic, and cubic phases of Ag2Se nanocrystals by variable-temperature powder XRD measurements.…”

“…[5][6][7][8] Furthermore, entirely new crystal phases can arise on the nanoscale that have no known counterparts in bulk. [9][10][11][12][13] Because the physical properties of a material are linked to its crystalline structure, the ability to isolate new or difficult-to-access metastable structures on the nanoscale holds promise for the discovery of novel functional materials with properties different from, and possibly superior to, the properties of more thermodynamically stable materials. [14][15][16][17][18] To synthetically target such materials, it is important to consider that a metastable state is only isolable if, under some set of conditions, that state represents a thermodynamic minimum.…”

Polymorphic Metastability in Colloidal Semiconductor Nanocrystals

“…Reductions in free energy differences between polymorphs and the fast reaction kinetics of nanocrystal nucleation and growth make it possible to synthesize colloidal nanocrystals with metastable crystal structures that in bulk are only accessible at much higher temperatures than those required for the analogous nanocrystal syntheses [12] . In addition to nanocrystalline analogues of bulk materials, entirely new crystal phases can also arise on the nanoscale that have no bulk counterparts [13–23] . Therefore, colloidal nanocrystal chemistry presents both thermodynamic and kinetic advantages in the preparation of metastable materials, and promises to be fruitful for the discovery of new metastable materials with unique properties.…”

Thermal stability of silver selenide thin films on silicon formed from the solid state reaction of Ag and Se films

“…The low temperature polymorph is a semiconductor with a narrow band gap of 70 meV at absolute zero [5]. Orthorhombic, tetragonal and triclinic structures have been reported for the low temperature phase [6][7][8][9]. Above the transition temperature, the compound is characterized by a cubic structure and high ionic conductivity [5].…”