Colloidal Semiconductor Quantum Dots with Tunable Surface Composition Helen Hsiu-Ying Wei, † Christopher M. Evans, † Brett D. Swartz, † Amanda J. Neukirch, ‡ Jeremy Young, ‡ Oleg V. Prezhdo, †,‡ and Todd D. Krauss* ,†,§ † Departments of Chemistry and ‡ Physics and Astronomy and the § Institute of Optics, University of Rochester, Rochester, New York 14627, United States * S Supporting Information ABSTRACT: Colloidal CdS quantum dots (QDs) were synthesized with tunable surface composition. Surface stoichiometry was controlled by applying reactive secondary phosphine sulfide precursors in a layer-by-layer approach. The surface composition was observed to greatly affect photoluminescence properties. Band edge emission was quenched in sulfur terminated CdS QDs and fully recovered when QDs were cadmium terminated. Calculations suggest that electronic states inside the band gap arising from surface sulfur atoms could trap charges, thus inhibiting radiative recombination and facilitating nonradiative relaxation. KEYWORDS: Semiconductor nanocrystals, photoluminescence, synthesis, surface composition, nonradiative relaxation Colloidal semiconductor nanocrystals (or quantum dots, QDs) have attracted attention for several decades due to their size dependent optical properties based on quantum confinement in all three dimensions, 1 and can be potentially applied to improve technology in several fields (biological imaging or labeling, 2 solar cells, 3 and light-emitting diodes 4 ). Within numerous combinations of compound semiconductors, CdSe and CdS QDs have been widely investigated due to their facile synthesis and optical activity in the visible range of the electromagnetic spectrum. Synthesis of high quality QDs is a key factor for fundamental studies of physical properties and for developing emerging applications of QDs. However, a deep fundamental understanding of the CdSe or CdS QD synthesis method is still open to question. Conventional colloidal II-VI (and IV-VI) QD synthesis methods based on tertiary phosphine chalcogenides and metal salts can produce very high quality QDs 5 but also suffer from well-known irreproducibilities or inconsistencies. For example, it is hard to achieve batch-to-batch a consistent fluorescence efficiency or an exact size (for a given growth time), and the synthetic conversion yield can be very low (<2%). 6 These inconsistencies may arise from the fact that traditional QD syntheses are based on tertiary phosphine chalcogenide molecular precursors, which at temperatures under 200 °C were found to be largely unreactive and thus not responsible for QD formation. 7 Rather, secondary phosphines, which are often impurities in commercial tertiary phosphines, were shown to be the more reactive species (with metal salts) driving QD formation. For example, adding secondary phosphines to highly puri fied tertiary phosphine selenides (trioctylphosphine selenide, TOP-Se) accelerates the rate of QD formation and leads to a quantitative increase of conversion yields for PbSe. 8 In fact, chemical synthetic yields of CdSe and PbSe QDs approached 100% when pure secondary phosphine selenide precursors were used. 7 Sulfur, lying above selenium in the periodic table, is expected to exhibit similar chemical reactivity for the formation of QDs. Thus, secondary phosphine sulfides (such as diphenylphos- phine sulfide, DPP-S) are also expected to display very high reactivity with metal carboxylates in QD syntheses. Indeed, using NMR spectroscopy, we have verified the complete consumption of DPP-S in the presence of stoichiometric amounts of Cd-oleate (Supporting Information Figure S1). Here, we report the synthesis of CdS QDs using DPP-S and Cd-stearate in tetradecane. By taking advantage of the highly reactive DPP-S precursor, high-quality CdS QDs were synthesized with controllable size (2.8-5.2 nm in diameter), a narrow size distribution (±11%), and under a relatively low nucleation temperature (160 °C), which is significantly lower than most conventional CdS QD syntheses. Importantly, the complete conversion of S in DPP-S to CdS allowed for unprecedented control over the CdS QD surface composition using a SILAR (successive ionic layer adsorption and reaction) 9 type process. CdS QD surface composition could be tuned from essentially all Cd to all S termination, as confirmed by X- ray photoelectron spectroscopy (XPS). The chemical compo- sition of the surface was observed to have a dramatic effect on the band edge photoluminescence (PL) of the QDs. Terminating the CdS QD core with sulfur completely quenched PL emission, while capping with cadmium restored the full magnitude of the PL intensity. Additionally, it was found that the PL intensity scaled directly with the relative percentage of Cd or S atoms on the surface. Future use of Received: April 5, 2012 Revised: July 12, 2012 Published: August 27, 2012 Letter pubs.acs.org/NanoLett © 2012 American Chemical Society 4465 dx.doi.org/10.1021/nl3012962 | Nano Lett. 2012, 12, 4465-4471 Downloaded via UNIV OF SOUTHERN CALIFORNIA on November 21, 2019 at 22:55:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.