Open Access Nano Express

The role of the surfaces in the photon absorption in Ge nanoclusters embedded in silica

Salvatore Cosentino1, Salvatore Mirabella1*, Maria Miritello1, Giuseppe Nicotra2, Roberto Lo Savio1, Francesca Simone1, Corrado Spinella2 and Antonio Terrasi1

Author Affiliations

1 MATIS-IMM-CNR and Dipartimento di Fisica e Astronomia, Università di Catania, Via Santa Sofia 64, 95123 Catania, Italy

2 IMM-CNR, VIII Strada 5, 95121 Catania, Italy

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Nanoscale Research Letters 2011, 6:135  doi:10.1186/1556-276X-6-135

Published: 11 February 2011

Abstract

The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics.

PACS: 81.07.Ta; 78.67.Hc; 68.65.-k