Theoretical Metastability of Semiconductor Crystallites in High-Pressure Phases, with Application to β-Tin Structure Silicon

Abstract

We examine prospects for metastability of six-coordinate high-pressure semiconductor phases at ambient temperature and pressure (STP). We investigate a simple “thermodynamic”, coherent transformation model for nanocrystals of size >2 nm and, as a quantitative example, apply it to the silicon β-tin to diamond structural phase transition. The unit cell transformation path is taken from a calculation by Mizushima et al. (Mizushima, K.; Yip, S.; Kaxiras, E. Phys. Rev. 1994, B50, 14952). Surface energies and the initial crystallite shape are included in an absolute rate model. The model assumes that the crystallite shape substantially changes to accommodate the unit cell c/a variation from 1.414 (diamond) to 0.55 (β-tin). The model predicts that the β-tin nanocrystal lifetime increases rapidly with increasing size. Near-round β-tin nanocrystals are more stable and have slower transformation rates than oblate spheroid nanocrystals with larger surface energy. For size >2 nm, both near-round and oblate β-tin nanocrystals are metastable with half-lives of years or more. An alternate, classical nucleation model is considered for surface nucleation in larger microcrystals. In this model the β-tin nanocrystal lifetime decreases with increasing size. Yet micron-size and smaller crystallites are metastable as well. Defect and strain-free β-tin microcrystals appear to be metastable at STP. More generally, stabilization of high-pressure semiconductor phases at STP should be more widespread in nanocrystals than in bulk crystals, because of (1) the relative ease in annealing out defects, strain, and impurities in nanocrystals and (2) the use of surface passivation in lowering the surface energy of the high-pressure phase.

Increased stability of nanocrystals of Gd2(Ti0.65Zr0.35)2O7 pyrochlore at high pressure

“…The size effect on the formation of defects, shape and phase stability of nano-sized materials has been widely investigated both theoretically and experimentally [11][12][13][14][15]23,24]. The size dependence from the prospective of thermodynamics and kinetics as a function of pressure has been previously investigated for some semiconductors [14,24] and even a single "nucleation" per nanocrystal was observed in very fine nano-sized material [14].…”

“…The size dependence from the prospective of thermodynamics and kinetics as a function of pressure has been previously investigated for some semiconductors [14,24] and even a single "nucleation" per nanocrystal was observed in very fine nano-sized material [14]. In other words, very small nanocrystals tend to be defect free.…”

Synthesis and studies on structural and optical properties of zinc oxide and manganese-doped zinc oxide nanoparticles

“…Zinc oxide has a wide band gap large excitation energy of 60 meV, high chemical stability, good piezo-electric properties, nontoxicity and biocompatibility. To synthesize ZnO nanoparticles with transition metals, several different methods have been employed, such as wet chemical methods like sol-gel [32], co-precipitation [33], combustion [34] etc. These nanoparticles have been used in a wide range of applications, such as sensors, varistors, piezoelectric, transducers, surface acoustic wave devices, phosphors, transparent conducting oxides, optoelectronic devices, ferromagnetic devices, and heterogeneous photo catalysts [35][36][37][38][39][40].…”

Electrodeposited Quantum Dots: Metastable Rocksalt CdSe Nanocrystals on {111} Gold Alloys

“…In some cases, these thermodynamically unstable forms may be kinetically stable in ambient conditions, a state known as metastability. [1] Bulk CdS and CdSe are known to undergo a phase transition from the tetrahedral Wurtzite (WZ) structure to an octahedral rocksalt (RS) form under high pressure. The lattice parameters of the RS form depend on the pressure, but are typically 5 % smaller than those of the WZ or Zincblende (ZB) (also tetrahedral) structures.…”

“…CdSe nanocrystals were shown to undergo the WZ to RS transition at higher pressures than bulk CdSe and the hysteresis range was much larger. [3,4] Tolbert and Alivisatos [4] and Brus et al [1] discussed the differences between solid±solid phase changes in bulk and nanocrystalline materials in terms of two main factors. 1) The absence of defects in nanocrystals (phase change in bulk material is usually initiated at defects), which introduces a kinetic barrier to the phase change.…”