Photo-oxidation of Ge Nanocrystals: Kinetic Measurements by In Situ Raman Spectroscopy
- PDF / 420,067 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 27 Downloads / 237 Views
0958-L03-08
Photo-Oxidation of Ge Nanocrystals: Kinetic Measurements by In Situ Raman Spectroscopy I. D. Sharp1,2, Q. Xu1,2, C. W. Yuan1,2, J. W. Beeman1, J. W. Ager III1, D. C. Chrzan1,2, and E. E. Haller1,2 1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720 2 Materials Science and Engineering, University of California, Berkeley, Berkeley, CA, 94720 ABSTRACT Ge nanocrystals are formed in silica by ion beam synthesis and are subsequently exposed by selective HF etching of the silica. Under ambient conditions, the exposed nanocrystals are stable after formation of a protective native oxide shell of no more than a few monolayers. However, under visible laser illumination at room temperature and in the presence of O2, the nanocrystals rapidly oxidize. The oxidation rate was monitored by measuring the Raman spectra of the Ge nanocrystals in-situ. The intensity ratio of the anti-Stokes to the Stokes line indicated that no significant laser-induced heating of illuminated nanocrystals occurs. Therefore, the oxidation reaction rate enhancement is due to a photo-chemical process. The oxidation rate varies nearly linearly with the logarithm of the laser intensity, and at constant laser intensity the rate increases with increasing photon energy. These kinetic measurements, along with the power dependencies, are described quantitatively by an electron active oxidation mechanism involving tunneling of optically excited electrons through the forming oxide skin and subsequent transport of oxygen ions to the Ge nanocrystal surface. INTRODUCTION Oxidation phenomena can have a significant impact on the structural and electronic properties of nanoscale materials because of their large surface to volume ratios. For example, oxidation of Si nanocrystals is commonly used to passivate surface states and enhance optical activity [1]. Furthermore, size-dependent self-limiting oxidation has been observed and is used to control the size distributions of nanomaterials [2]. Although nanocrystals are often embedded in a host matrix, such as silica, a number of synthesis routes lead to the formation of exposed nanocrystals. Therefore, it is of significant importance to understand oxidation processes that occur upon exposure of nanocrystals to ambient atmospheric conditions. In this work, we show that exposed Ge nanocrystals can be rapidly photo-oxidized. This oxidation process is monitored by in situ Raman spectroscopy and a quantitative mechanistic model is proposed. EXPERIMENTAL DETAILS Ge nanocrystals were formed by ion implantation of 74Ge+ at 150 keV to 3×1016 cm-2 into 500 nm thick SiO2 layers on Si substrates followed by thermal annealing at 950 °C for 1 h under Ar. Transmission electron microscopy (TEM) shows nanocrystals located in the near surface region of the oxide film and with an average diameter of 6.3 nm. High resolution TEM (HRTEM) images (not shown) demonstrate that silica-embedded Ge nanocrystals are spherical with sharp interfaces to the surrounding matrix [3].
Intensity (a.u.)
Fig. 1
Data Loading...
