The Synthesis of Colloidal Metal Sulfide Nanocrystals

Abstract

As nanotechnology becomes ever more pervasive in everyday life, the ability to control the bottom-up synthesis of nanomaterials is of great technological interest. In particular, colloidal semiconducting nanocrystals, or quantum dots, are beginning to find applications in biological imaging, solar cells, and as down-converters for LED-driven light bulbs and displays. Herein we report various experimental endeavors that explore the role of the precursors used to make these nanocrystals, with a special interest in the kinetics of their reactivity. In doing so, we highlight the influence that precursor conversion rate has on the size of nanocrystals, which is essential to optimizing their performance in optoelectronic devices, catalysis, and imaging applications. After surveying the history, applications, and theoretical models describing the synthesis of semiconductor nanocrystals, we focus on phosphine-based precursors. We describe the synthesis of cadmium bis(diphenyldithiophosphinate) (Cd(S₂PPh₂)₂) from secondary phosphine sulfides and its conversion to cadmium sulfide nanocrystals. Heating Cd(S₂PPh₂)₂ and cadmium tetradecanoate to 240 °C results in complete conversion of Cd(S₂PPh₂)₂ to cadmium sulfide nanocrystals with tetradecanoate surface termination. The nanocrystals have a narrow size distribution that is evident from the line width of the lowest energy absorption feature and display bright photoluminescence. Monitoring the reaction with ³¹P NMR, UV-Visible, and infrared absorption spectroscopies shows that the production of cadmium diphenylphosphinate (Cd(O₂PPh₂)₂) and tetradecanoic anhydride co-products is coupled with the formation of cadmium sulfide. From these measurements we propose a balanced chemical equation for the conversion reaction and use it to optimize a synthesis that affords CdS nanocrystals in quantitative yield. Interestingly, the final diameter is insensitive to the reaction conditions, including the total concentration of precursors, which we attribute to a first-order rate of precursor conversion. Using CdS nanocrystals synthesized from Cd(S₂PPh₂)₂ as a model system, we demonstrate that metal carboxylate complexes (L− Cd(O₂CR)₂, R = oleyl, tetradecyl) are readily displaced from carboxylate-terminated nanocrystals. Removal of up to 90% of surface-bound Cd(O₂CR)₂ from the CdS nanocrystals is possible with N,N,N’,N’-tetramethylethylenediamine (TMEDA), decreasing the photoluminescence quantum yield (PLQY) from 20% to <1% and broadening the 1Sₑ-2S_(3/2)h absorption feature. These changes are partially reversed upon rebinding of M(O₂CR)₂ at room temperature (∼60%) and fully reversed at elevated temperature. A model is proposed in which electron-accepting M(O₂CR)₂ complexes (Z-type ligands) reversibly bind to nanocrystals, leading to a range of stoichiometries for a given core size. The results demonstrate that nanocrystals lack a single chemical formula, and are instead dynamic structures with concentration-dependent compositions. Following the precursor reactivity and rate study undertaken on Cd(S₂PPh₂)₂, we establish a novel method of controlling the number and size of nanocrystals produced from a reaction through the use of a precursor library. We report a library of thioureas whose substitution pattern tunes their conversion reactivity over more than five orders of magnitude and demonstrate that faster thiourea conversion kinetics increases the extent of crystal nucleation. Tunable kinetics thereby allows the nanocrystal concentration to be adjusted and a desired crystal size to be prepared at full conversion. Controlled precursor reactivity and quantitative conversion improve the batch-to-batch consistency of the final nanocrystal size at industrially relevant reaction scales and open up new synthetic routes towards commercially-relevant core/shell heterostuctures. The ability to tune reaction rate independent of the reaction conditions for the first time also enables new studies of the underlying mechanisms of nanocrystal synthesis.