Copyright © 2011 American Scientific Publishers All rights reserved Printed in the United States of America RESEARCH ARTICLE Advanced Science Letters Vol. 4, 1–5, 2011 Structural and Optical Properties of Oxygenated Silicon Quantum Dots Sudip Chakraborty 1 ∗ , Ch. Rajesh 2 , Shailaja Mahamuni 2 , and S. V. Ghaisas 1 1 Department of Electronic Science, University of Pune, Pune 411 007, India 2 Department of Physics, University of Pune, Pune 411 007, India Silicon quantum dots of size as small as 1 nm in diameter were prepared by wet chemical route. These clus- ters are found to be covered with oxygen and hydrocarbon molecules. The optical measurements reveal strong absorption around 4.67 eV and weak absorption at lower energies. The clusters show a broad luminescence around 3.87 eV. These quantum dots are modeled using ab-initio Car Parinello Molecular Dynamics and compu- tational studies explain the absorption spectrum of the clusters over observed energy range. In principle, these quantum dots can be useful as scintillating layer on crystalline silicon solar cells to enhance the photovoltaic efficiency. Keywords: Quantum Dots, Optical Properties, Molecular Dynamics. 1. INTRODUCTION Announcement 1 of visible radiation from porous silicon in 1990 generated a great interest in obtaining and studying the quantum confinement effects in indirect band gap semiconductors. In order to understand the luminescence behavior of quantum confined Si, it is necessary to evaluate the structure and chemical composition of nanocrystals. However, most of the investigations are focused 2 on the optical properties disregarding the effect of chemical struc- ture. Usually, Si nanocrystals passivated by hydrogen are studied theoretically while structure and chemical environment of realis- tic Si nanocrystals is extremely complicated. 2 3 Quantum confined Si showing sensitivity in the visible regime has great prospect in optoelectronic devices including photo- voltaic solar cells. The emerging concept of using nanocrystals as luminescence down converters to make use of UV light for photovoltaic conversion is very attractive in this sense. 4 In prin- ciple, Si nanocrystals can be used to absorb ultra-violet radiation efficiently and emit in ultra-violet through visible by manifesting the defects. 5 Specifically, it is demonstrated that the nanocrys- talline Si/SiO 2 interface defects can be manipulated to tune the emission wavelength. The experimentally determined optical transitions in Si QDs are explained using the ab-initio calculations, based on the Car Parrinello Molecular Dynamics (CPMD) and Pseudopotential Algorithm for Real Space Electronic Calculation (PARSEC). These studies shed light on the structure of Si–O nanocrystals. ∗ Author to whom correspondence should be addressed. 2. EXPERIMENTAL DETAILS Si QDs were synthesized by adapting the method suggested by Tilley et al. 6 In Ref. [6] 1-heptene capped Si nanoparticles were obatined. In order to obtain oxygen capping the ratio of silicon tetrachloride (SiCl 4  to lithium aluminum hydride (LiAlH 4  in THF was varied. Just by changing the ratio and keeping the entire process same as described in Ref. [6], oxygen passivated Si QDs were synthesized. With this method the size of the particle seems to be fixed. Variation in relative concentrations of the chemi- cals did not make any size variations. Phase as well as average size of QDs was determined by using X-ray diffraction (XRD). XRD measurements were performed on Bruker D8 advance pow- der X-ray diffractometer, with an incident radiation of CuK ( = 15402 Å). Transmission electron microscopic (TEM) mea- surements were carried out using a Philips CM200 microscope operating at 200 kV. The samples were deposited on the carbon grid for the TEM image and electron diffraction pattern. Optical absorption spectra were measured on JASCO V-670 spectropho- tometer. For photoluminescence (PL) studies, Perkin Elmer LS55 spectrophotometer was used. Fourier transform infrared spectra (FTIR) were recorded with the aid of JASCO FT/IR-6100. For FTIR measurements, the samples in powder form was mixed with the potassium bromide (KBr) powder and were made in the form of pellets. X-ray photoelectron spectroscopic (XPS) mea- surements were carried out on VSW Scientific Instruments elec- tron spectrometer. The base pressure of the spectrometer was about 3 × 10 -9 mbar. Experiments were performed using AlK radiation at analyzer pass energy of 40 eV. C 1s energy level served as an internal standard. Adv. Sci. Lett. Vol. 4, No. xx, 2011 1936-6612/2011/4/001/005 doi:10.1166/asl.2011.1909 1