Role of Fatty Acid in Controlling Nucleation and Growth of CdS Nanocrystals in Solution Niladri S. Karan, Aritra Mandal, Subhendu K. Panda, and Narayan Pradhan* Department of Materials Science and Centre for AdVanced Materials, Indian Association for the CultiVation of Science, JadaVpur, Kolkata 700032, India ReceiVed: March 19, 2010 We report here the role of free fatty acid in the synthesis of CdS nanocrystals following a well-developed colloidal synthetic procedure using cadmium carboxylate as cadmium and elemental sulfur as sulfur precursor. It is found that fatty acid enhances the reactivity of sulfur precursor, which accelerates the rate of both nucleation and growth of CdS nanocrystals. By controlling this reactivity, high-quality CdS nanocrystals are synthesized at reduced temperature (∼150 °C). This well-known colloidal synthetic method is also found to be a biphasic reaction system where sulfur approaches from the gas phase as H 2 S and “Cd” from the solution phase. 1. Introduction Fluorescing semiconductor nanocrystals, commonly known as quantum dots (QDs), are of great interest for their applications in different fields like light emitting diodes, solar cells, bioimaging and display devices, etc. 1–5 Synthesis of different types of high-quality Group II-VI 6–10 and III-V 11–13 semicon- ductor QDs is extensively studied and reported in the literature. The most common QDs of this category are CdS and CdSe nanocrystals which are believed to have well-developed syn- thetic schemes and thoroughly investigated growth mecha- nism. 6,8,14–17 Nucleation, growth and morphology of these nanocrystals are controlled with proper manipulation of reaction parameters 8,17 such as strength of ligand binding, 18,19 ligand dynamics, 20 design of the nature of precursors, 10 and studying the crystal growth in the colloidal dispersed medium. 8,14,21,22 These high-temperature injection and growth techniques for synthesis of such high-quality nanocrystals are well-documented in the literature where one can successfully achieve tunable and pure excitonic emissions with the high (>50%) quantum efficiency needed for different practical applications. 8 But when the chemistry of formation and control of growth kinetics using different precursors are questioned, many unsolved reaction paths appeared. One example is the role of free fatty acid for the most common synthesis of CdSe and CdS nanocrystals. The report on CdSe synthesis suggests that more free acid suppresses the decomposition of Cd-precursor 20 and slows down the growth whereas for CdS synthesis, contradictory results are obtained 17 even if both syntheses have identical Cd-precursor. This, of course, is not the only factor that controls the growth kinetics but certainly this question casts doubts about the hidden reaction mechanism for formation and growth of colloidal nanocrystals in solution. We have investigated here the role of free acid in the most common method of synthesis of high-quality CdS nanocrystals using elemental sulfur and Cd-carboxylate precursors in 1-oc- tadecene (ODE) solvent at reduced temperature and found that the reaction is actually a biphasic system, not identified earlier, where sulfur approaches from the gas phase and cadmium from the solution phase. The rate of the reaction is controlled by the amount of free fatty acid that in fact controls the sulfur source concentration in the reaction system. This mechanism also helped us to explain the synthesis of such high-quality CdS nanocrystals at reduced temperature (∼150 °C) and in a moderate time scale. 2. Experimental Section 2.1. Chemicals. Cadmium oxide (CdO, >99%), oleic acid (OA, 90%), 1-octadecene (ODE, tech.), 1-hexadecylamine (HDA, tech., 90%), tributyl phosphine (TBP), stearic acid (SA, 95%), and sulfur powder (99.98%) were purchased form Aldrich. Selenium powder (200 mesh, 99.999% metal basis) was purchased from Alfa Aesar. All these chemicals were used as received without any further purification. 2.2. Preparation of Stock Solutions. Sulfur stock solution was prepared by dissolving 0.016 g (0.5 mmol) sulfur powder in 10 mL of ODE. This mixture was heated and purged with argon to obtain a clear solution. TBPSe stock solution was prepared by taking 1.8 g (22.8 mmol) of Se powder in 8 g of TBP in a vial inside a glovebox. This mixture was shaken to obtain a clear solution. These two solutions were considered as stock and used for further reactions. 2.3. Synthesis of Cadmium Sulfide Nanocrystals. In a typical synthesis, 0.0128 g (0.1 mmol) of CdO, 0.141 g (0.5 mmol) (or required amount) of OA, and 4 g of ODE were loaded in a 25 mL three-necked flask. This mixture was purged with argon for 10 min. After that the mixture was heated to obtain a clear solution. When the clear solution was obtained the flask was cooled to the required injection temperature (150 or 180 °C). One milliliter of the S stock solution was sharply injected into the above reaction flask. The reaction was continued until the desired size was obtained, which was monitored by taking the UV-vis spectra of the aliquots at regular intervals. CdS in the presence of different fatty acid concentrations was synthesized by changing the molar amount of the corresponding acid. Similar results were also observed when purified cadmium oleate prepared from CdO and OA was used for the synthesis of CdS in place of CdO and OA as the precursor. CdSe nanoparticles were also synthesized with TBPSe as Se source. The higher amount of fatty acid (stearic acid) reduced the growth rate unlike in the CdS system. 2.4. Evolution of H 2 S in the Presence of Free Acid. The formation of H 2 S gas was confirmed by common laboratory test. The reagents that are used for CdS synthesis were taken in a * To whom correspondence should be addressed. E-mail: camnp@ iacs.res.in. J. Phys. Chem. C 2010, 114, 8873–8876 8873 10.1021/jp1024944  2010 American Chemical Society Published on Web 04/15/2010