Phonon Anharmonicity and Phase Transitions in Bulk and Nanoparticle ZnSe under High Pressure – Bernard Weinstein

Date: Mon. April 24th, 2006, 12:30 pm-1:30 pm
Location: Rockefeller 221

Resonant multi-phonon interactions strongly modify the life-times of the TO(Gamma) and LO(Gamma) normal modes in many bulk semiconductors.[1] The optically active confined and surface/interface modes in nanoparticles[2] are subject to enhanced anharmonic coupling because of the loss of q-conservation, the mixing of LO and TO polarities, and the presence of surfactant. Raman scattering experiments, carried out under variable pressure conditions, are a revealing probe of these phenomena due to the ability to tune the vibrational modes in and out of resonant decay channels, and to cycle through structural phase transitions [3] that often modify volume and/or surface disorder. Recent work at Buffalo has shown that unusually strong changes occur in the Raman spectra of bulk and nanorod ZnSe under applied hydrostatic pressure. In the TO(Gamma) region, a broad band (~60cm-1 wide) appears and grows in strengthfor pressures above 11GPa. We attribute this band to disorder and phonon mixing arising from antecedent behavior connected with nucleation of the high-pressure phase. On decreasing pressure after the forward and reverse transitions, similar Raman features are seen, including the broad band in the TO(Gamma) region. Below 1 GPa the zincblende TO(Gamma) and LO(Gamma) peaks return. However, a new sharp peak at 235cm-1 indicates that micro-domains of a metastable phase are retained. Below 3GPa, the Raman spectra of an ensemble of colloidal ZnSe nanorods exhibits two distinct peaks slightly higher and lower than the bulk TO(Gamma) and LO(Gamma) frequencies. Above 3GPa the two peaks broaden into a single band. This feature resembles the broad band noted above for bulk ZnSe, and may be linked to mode-mixing, disorder, and antecedent phase-transition behavior — all with pressure dependencies changed from those in the bulk by the nanorod size- and shape- variations, and surfactant related effects. [1] B. Weinstein, Solid State Commun. 20, 999 (1976); C. Ulrich et. al., Phys. Rev. Lett. 78, 1283 (1997) ; J. Serrano, et. al., Phys. Rev. B69, 014301 (2004) ; R. E. Tallman, et. al, Phys. Stat. Sol. (b) 241, 3143 (2005). [2] C. Trallero-Giner, e.t.al., Phys. Rev. B57, 4664 (1998); F. Comas and C. Trallero-Giner, Phys. Rev. B67, 115301 (2003). [3] S. Tolbert and A. Alivisatos, Annu. Rev. Phys. Chem. 46, 595 (1995); K. Jacobs and A. Alivisatos, Revs. in Miner. and Geochem. 44, 59 (2001).