Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals

1. NaNaX4“NanosciencewithNanocrystals”, Munich-Tutzing, April11- 15, 2010 Equilibrium size maps and novel aspects of the growth kinetics of CdSe nanocrystals E. Slejko*, A.Radivo**, F. Antoniolli** and V. Lughi* *University of Trieste, Department of Materials and Natural Resources, via Valerio, 6a - 34127 Trieste, Italy – vlughi@units.it ** MaXun srl, Trieste, Italy Background • Objectives: • Study the growthkineticsofCdSe nanocrystals, as a functionof the synthesisparameters in orderto: • Betterunderstand the mechanismsgoverning the synthesis • Provide a supporttoolforsynthesis design Experimental Despite the vast literature on nucleation and growth dynamics of II-VI semiconductor nanocrystals synthesized by hot injection methods, uniform guidelines for a reliable nanocrystal (NC) synthesis have not been developed yet, and some apparently contrasting observations have yet to be accounted for. Within this context, it is of particular interest to study the “equilibrium” size that the NCs reach upon completion of the chemical reactions involved, as this would enable control of the NC size without sacrificing the efficiency of the synthesis. Synthesis: Hot injection in octadecene (precursors: Cd oleate andTOPSe). Systematic variation of cadmium/oleic acid (OA) ratios (from 1:19 to 1:5) and growth temperature (210 – 270 °C). Injection temperature: 225°C Characterization: Sampling of nanocrystal suspension during synthesis, followed by UV-VIS spectroscopy and photoluminescence Data Analysis Results and Discussion Particle size distribution • The size of the nanocrystals is estimated from the absorption peak position via the calibration performed by Yu et al., 2003. • As previously observed, the size rapidly increases after injection, then reaching a plateau (figure below) • An empirical model based on classic AJMK kinetics fits the data (figure below) and is used to evaluate: • The initial nuclei size, d0, before growth starts • The final, pre-ripening size, d • The kinetic parameter k, associated to the growth rate Size of nuclei, d0 • As expected, the initial particle size , d0, is not affected by the growth temperature • d0 is nearly constant for low values of the Cd:OA ratio • d0 is significantly larger when Cd:OA = 1:5 Cd:OA = 1:5 1:6.5 Initialsizeof nuclei, d0 [nm] FWHM of the PL peak[nm] • The full width at half maximum of the NC’s photoluminescence peak is taken as a measure of the particle size distribution • The size distribution shows a marked minimum when NCs are grown at around 255°C • Narrowest size distribution is obtained for Cd:OA ratios of 1:10 or less 1:15 1:10 1:19 Fig. 2 Fig. 3 [Cd]:[OA] ratio Temperature [K] d Final nanocrystal size, d • We define the final size, d, as the size that the nanocrystals approach when the chemical reactions have reached equilibrium, before the onset of Ostwald ripening. • A size range between 3.6 and 4.6 nm is spanned when growth is performed between 210 and 270 °C and a Cd:OA ratio between 0.05 and 0.2 is used • dis normally larger for higher growth temperatures. This effect is more marked when the Cd:OA ratio is high. At 210°C the final size is markedly lower throughout the explored range of Cd:OA ratios • dreaches a minimum for Cd:OA ratios between 0.05 and 0.1. Two competing mechanisms concur to this peculiar behavior: On one hand an increase in oleic acid is known to limit nucleation thus making more precursor available for the growth phase, while on the other hand an increase of oleic acid enhances the solubility of Cd and Se, i.e. reduces the particle size Growth temperature 270°C 255°C 235°C Final (equilibrium) size, d [nm] d0 210°C Injection ofprecursor Size [nm] Conclusions [Cd]:[OA] Growth rate • A rough map of the NC size at equilibrium – i.e. for the maximum synthesis efficiency – is provided as a function of the growth temperature and the ratio [cadmium]:[oleic acid]. This lays the basis for the development of a useful tool for the design and use of reliable synthesis protocols. • Larger NC size than that obtained with the synthesis protocols presented here could be reached by expanding the Cd:OA ratio in both directions, or by increasing the temperature while keeping high Cd:OA ratios; Smaller NC size cannot be obtained by expanding the range of Cd:OA ratios, while it could be obtained by lowering the temperature • The growth kinetics are thermally activated with an activation energy of 0.35 eV. Knowledge of the activation energies associated to the single mechanisms that concur to growth would enable identification of the limiting growth step • The parameter k can be directly associated to the growth rate. It is a measure of how fast the particle size reaches a plateau • k does not depend on the Cd:OA ratio (left) • k increases with temperature (right) • k follows Arrhenius’ law • as shown by the linear dependence of ln(k) on 1/T (right), thus demonstrating a thermally activated behavior with a thermal activation energy, Ga, of 0.35 Ev • The lack of dependence of the growth rate on the content of oleic acid suggests that the growth in this case is limited by surfaces mechanisms rather than by diffusion in the solution Growth temperature 270°C Time [s] 255°C ln(k) Growth rate parameter, k [1/s] kB: Boltzmann constant 235°C 210°C [Cd]:[OA] 1/T [1/K] References D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, H. Weller; “Highly Luminescent MonodisperseCdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine-Trioctylphosphine Oxide-Trioctylphospine Mixture”; Nano Letters; Volume 1; Number 4; 207; 2001. W. W. Yu, L. Qu, W. Guo, and X. 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