Spectroscopy Studies of InP Nanocrystals Synthesized Through a Fast Reaction
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Spectroscopy Studies of InP Nanocrystals Synthesized Through a Fast Reaction Madalina Furis, David J. MacRae1, D. W. Lucey1 Yudhisthira Sahoo 1, Alexander N. Cartwright, Paras N. Prasad1 Department of Electrical Engineering, University at Buffalo, Buffalo, NY, 14260, USA 1 Institute for Lasers, Photonics and Biophotonics, University at Buffalo, Buffalo, NY, 14260, USA ABSTRACT We present spectroscopic characterization of InP nanocrystals grown through a fast reaction in a non-coordinating solvent. The photoluminescence (PL) spectra collected from these nanocrystals exhibit a sharp feature associated with the band-edge emission and a broad infrared feature associated with deep level surface trap emission. The emission efficiencies of the asgrown nanocrystals vary between 0.3% and 1% from sample to sample. After undergoing an HF etching process, the emission efficiency increases to 18% and the emission associated with surface states is eliminated from the PL spectrum. Time-resolved photoluminescence (TRPL) experiments conducted at room temperature on the as-grown and HF-etched nanocrystals show that before etching the PL intensity decay is multi-exponential, with a fast (3ns) component independent of wavelength, associated with the non-radiative recombination processes. The etching process effectively eliminates the non-radiative component and the post-etching PL decay can be fitted with a single exponential decay characterized by long (45ns) lifetimes. We tentatively associate these long lifetimes with the recombination of carriers from spin-forbidden states. This assignment is supported by the observation of a significant redshift of the feature associated with band-edge recombination in the PL spectrum with respect to the lowest energy feature in the photoluminescence excitation (PLE) spectrum. INTRODUCTION Semiconductor nanocrystals are very important for potential photonic and optoelectronic applications due to the possibility of tuning the wavelength of the emission by varying the nanocrystal size [1] and because of the high emission efficiencies measured in most II-VI and III-V nanocrystals [2,3]. Historically, II-VI nanocrystals were the first ones to be synthesized due to the less restrictive conditions imposed on the growth process and are now the only ones readily available in commercial quantities. By comparison, the synthesis of III-V nanocrystals is more challenging since the reaction must take place under very strict vacuum and humidity conditions. The traditional method of preparing II-VI as well as III-V nanocrystals consists of heating the cation and anion precursors in the presence of a coordinating solvent, such as trioctylphosphine oxide (TOPO) or dodecylamine (DDA) at high temperatures (~ 150-200oC for II-VI nanocrystals and 300oC for III-V nanocrystals) for several (2-5) days [4-8]. The role of the coordinating solvent is to arrest the growth of nanocrystals while they are still only a few nanometers in size and passivate the dangling bonds on the surface. The products of these reaction
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