In Situ Observation of the Nucleation and Growth of CdSe Nanocrystals Lianhua Qu, W. William Yu, and Xiaogang Peng* Department of Chemistry and Biochemistry, UniVersity of Arkansas, FayetteVille, Arkansas 72701 Received December 18, 2003; Revised Manuscript Received January 31, 2004 ABSTRACT An in situ method is introduced for the study of nucleation and growth of crystals using the size-dependent properties of a given system in the transition size regime from molecular species to bulk sized crystals. The real-time measurements for the first system studied, a CdSe one, were carried out by recording the absorption spectra during the reaction with a millisecond resolution. Introduction. Colloidal semiconductor nanocrystals are nanometer sized fragments of the corresponding bulk crystals synthesized in solutions. Because of their interesting size- dependent properties and flexible processing chemistry, colloidal semiconductor nanocrystals have been widely studied for fundamental purposes and industrial applications, such as LEDs, 1 lasers, 2 solar cells, 3 and biomedical label- ing. 4,5 The synthesis of colloidal semiconductor nanocrystals with the desired quality is the first step for realizing all these applications. Quantitative understanding of nanocrystals will certainly benefit the development of synthetic chemistry for colloidal nanocrystals. 6 In addition, it will also promote the understanding of general crystallization processes that is lacking in the literature, despite the human interests in crystallization over the past thousands of years. 7 In the nanometer regime, the transition size regime from single molecules to bulk sized crystals, the optical properties of semiconductor nanocrystals are strongly size dependent due to quantum confinement. 8 In principle, the size and concentration of the crystals or clusters at a given time can be respectively determined by the peak position and the absorbance of the absorption spectrum of the crystals/clusters. In fact, the size dependent optical properties of semiconductor nanocrystals have already been broadly exploited for moni- toring the growth reactions and studying the growth kinetics, 9-12 although those measurements have been per- formed ex situ. Those ex situ studies of such systems have revealed some unusual phenomena 10-14 and promoted the development of those greener synthetic schemes. 15-17 However, those ex situ studies are based on taking aliquots from high- temperature reactions and quenching the reaction for room-temperature measurements. This limits the time resolution to tens of seconds at most. Quantitative and full evaluation of a system is almost impossible because it requires too many tedious, precise, and difficult purification and characterization steps. For an accurate determination of the particle and monomer concentrations in the reaction flask, a significant amount of the reaction mixture must be taken for each aliquot. 11,17 Such excessive sampling undoubtedly disturbs the reaction system. Some of the above-mentioned issues may be addressable by applying the recently intro- duced microfluidic methods. 18 In addition to these experi- mental difficulties, there are always some doubts regarding the difference between the events that occurred at high temperatures and the measurements performed at room temperature. This report demonstrates a nondisturbing and relatively general strategy to study in situ the crystallization systems using the size dependent optical properties of crystals in the transition regime from molecular precursors to regular nanocrystals. A greener approach for the growth of high- quality CdSe nanocrystals was employed for the current study, because this specific greener scheme is a relatively easy one to handle and reproducibly yields highly emitting CdSe nanocrystals with very narrow size distributions. 13 Consequently, the optical data can be converted to the desired chemical kinetic information relatively easily. A typical reaction studied is as follows: CdO, 0.0127 g (0.1 mmol), and 0.1140 g (0.4 mmol) of stearic acid were loaded into a 25 mL four-neck flask and heated to 150 °C under Ar flow. After CdO was completely dissolved, the mixture was allowed to cool to room temperature. TOPO and hexadecylamine, 3.44 g for each, were added to the flask, and the mixture was heated to 280 °C under Ar flow to form an optically clear solution. At this temperature, the Se solution containing 0.079 g (1 mmol) of Se dissolved in 2.920 g of TOP prepared in a drybox was swiftly injected into the reaction flask. After the injection, the temperature was set * Corresponding author. Phone: 479-575-4612. Fax: 479-575-4049. E-mail: xpeng@uark.edu. NANO LETTERS 2004 Vol. 4, No. 3 465-469 10.1021/nl035211r CCC: $27.50 © 2004 American Chemical Society Published on Web 02/14/2004