Published: February 08, 2011 r2011 American Chemical Society 5102 dx.doi.org/10.1021/jp109351t | J. Phys. Chem. C 2011, 115, 5102–5107 ARTICLE pubs.acs.org/JPCC Optical Properties of Femtosecond Laser-Synthesized Silicon Nanoparticles in Deionized Water R. Intartaglia,* ,† K. Bagga, † F. Brandi, † G. Das, † A. Genovese, † E. Di Fabrizio, †,‡ and A. Diaspro † † Italian Institute of Technology (IIT), via Morego 30, 16152 Genoa, Italy ‡ BIONEM lab, University Magna Graecia, Viale Europa Catanzaro, Italy ABSTRACT: Silicon nanoparticles were prepared by ultrafast laser ablation of a silicon target in deionized water. The nanoparticles were characterized by using optical absorption, Raman spectroscopy, and transmission electron microscopy. The mean size is found to vary from 60 to 2.5 nm in the absence of any reducing chemical reagents, decreasing the pulse energy value. High-resolution transmission electron microscopy to- gether with Raman spectroscopy confirms the crystalline structure of the generated silicon nanoparticles. The energy confinement of carriers which is evaluated from optical experi- ments varies from 90 to 550 meV when the mean nanoparticles size decreases from 60 to 2.5 nm. In particular, the evaluated nanoparticle sizes from optical analysis and the LCAO theoretical model are found in agreement with transmission electron microscopy and Raman measurements for the silicon nanoparticles with a size less than 6 nm. Finally, we present stability studies which show that the smallest nanoparticles aggregate over time. 1. INTRODUCTION Silicon nanoparticles (Si-NPs) present a growing interest due to their particular size-dependent optical properties leading to important applications such as light-emitting devices, 1 energy source, 2 and in biomedicine. 3-7 The integration of ultrathin films of Si-NPs on silicon solar cells is found to enhance the power performance of polycrystalline cells by 60% in the UV range. Recently, Si-NPs have been shown to be able to generate singlet oxygen under irradiation, making them promising candidates for photodynamic therapy. A variety of chemical 8-11 and physical 12-17 methods have been employed to prepare Si-NPs. Among them, wet chemistry routes are attractive because particle size and surface properties can be controlled simultaneously. Nevertheless, the obtained nanoparticles are contaminated with the residual byprod- ucts such as ions and reducing agents, which is not suitable for biological application of nanoparticles. Laser ablation synthesis in liquid environment provides the advantage to reduce the risk of contamination. Biocompatibility improvement of the laser-produced nanoparticles is predicted due to their restricted surface contamina- tion since the synthesis can be carried out in water or in solution of a biocompatible ligand, which is a key to the subsequent successful functionalization of the nanoparticle surface. 18 There are many irradiation parameters which should be taken into account for controlling the size and shape of nanoparticles. Some of these parameters include laser wavelength, pulse energy, pulse duration, repetition rate, and liquid environments. 19-22 In particular, the laser pulse duration is found to affect directly the ablation, nucleation, growth, and aggregation mechanisms. Long laser pulses (nanosecond) release energy slowly on a time scale comparable to the thermal relaxation processes of the target, while femtosecond laser pulses release energy to electrons in the target on a time scale much faster than electron-phonon thermalization processes. Local heating on the target in this way can be reduced in the case of femtosecond pulses. Moreover, temporal overlap between laser pulse duration and the time of material evaporation induces thermodynamic instability of the plasma during this expansion. 23 Consequently, some differences are observed in the generated nanoparticles produced by means of laser with different pulse durations. 22 Only few works have been reported on generation of Si-NPs in liquid environment. Nanosecond laser ablation of silicon shows generation of nano- particles which stabilize into clusters due to the agglomeration effect; i.e., Si-NPs are held together by an irregular network. 24-26 Strong agglomeration of the produced nanoparticles is a major barrier to most of the applications requiring nonagglomerated substrate-free nanoparticles. Recently, isolated Si-NPs have also been produced using a UV femtosecond pulse laser source. 27 The present paper focuses on the infrared femtosecond laser ablation of silicon in deionized water aiming to clarify the possibility to get isolated Si-NPs with controllable sizes. We report the production of the Si-NPs with a mean size ranging from 60 to 2.5 nm in the absence of any reducing chemical reagents, decreasing the pulse energy value. High-resolution transmission electron microscopy (HR-TEM) together with Raman spectroscopy confirm the crystalline structure of the generated Si-NPs. The energy confinement of carriers which is Special Issue: Laser Ablation and Nanoparticle Generation in Liquids Received: September 29, 2010 Revised: December 14, 2010