Soft X-Ray Emission Studies of the Electronic Structure in Silicon Nanoclusters
- PDF / 417,900 Bytes
- 5 Pages / 412.92 x 646.2 pts Page_size
- 60 Downloads / 130 Views
OF THE ELECTRONIC STRUCTURE IN
T. VAN BUUREN*, L. N. DINH*, L.L.CHASE*, W. J. SIEKHAUS*, I. JIMENEZ*, L.J. TERMINELLO*, M. GRUSH **, T.A. CALLCOTI'**, J.A. CARLISLE t * Chemistry and Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA, 94556 ** Department of Physics, University of Tennessee, Knoxville, TN 37996 t Department of Physics, Virginia Commonwealth University, Richmond, VA 23284-2000 ABSTRACT Density of states changes in the valence and conduction band of silicon nanoclusters were monitored using soft x-ray emission and absorption spectroscopy as a function of cluster size. A progressive increase in the valence band edge toward lower energy is found for clusters with decreasing diameters. A similar but smaller shift is observed in the near-edge x-ray absorption data of the silicon nanoclusters. INTRODUCTION There are two main models for explaining the visible luminescence found in porous silicon. The first model " quantum confinement " is based on the idea that the restricted size of the nanometer scale silicon particles alters the band structure relative to bulk silicon [1]. In the competing model a surface layer or surface defect is responsible for the visible luminescence properties [2,3]. Recently it has been proposed that a combination of both models is needed to describe the optical properties found in porous silicon [4]. In order to shed more light on this matter, it seems useful to first examine more controllable systems, such as silicon nanocrystals with well defined sizes. In this paper, we show how Si nanoclusters with a narrow size dispersion can be synthesized in a well-controlled environment. The electronic structure of the silicon nanoclusters is then investigated using soft x-ray emission (SXE) and x-ray absorption spectroscopies. A prediction of the quantum confinement model is that the energies of the valence band (VB) and conduction band (CB) are shifted relative to the bands of bulk silicon. In order to overcome the charging and surface sensitivity problems of the electron spectroscopies we have used a method based on soft x-ray emission. SXE is explicitly a bulk-sensitive probe of the electronic structure. Photons penetrate many atomic layers deep into the material, as compared to a few atomic layers for electrons. For the determination of the occupied valence electronic states we have measured the spectral distribution of the energy-resolved soft x-ray fluorescence (SXF) radiation generated in the transitions of the Si valence band electrons to the Si 2 p 312core level. The core hole is selectively excited using monochromatized synchrotron radiation. Due to the photon in, photon out process the experiment is not affected by sample charging. With selective excitation it is furthermore feasible to separate the emission from Si atoms with and without oxygen neighbors, due to the chemical shift induced by the oxygen atoms. Near-edge x-ray absorption fine structure spectroscopy (NEXAFS) measured with total photon yield can give complimentary information abo
Data Loading...
