Letter pubs.acs.org/JPCL
Luminescent InP Quantum Dots with Tunable Emission by PostSynthetic Modification with Lewis Acids Jennifer L. Stein,† Elizabeth A. Mader,‡ and Brandi M. Cossairt*,† †
Department of Chemistry, University of Washington, Box 351700, Bagley Hall, Seattle, Washington 98195-1700, United States Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
‡
S Supporting Information *
ABSTRACT: We demonstrate the ability of M2+ Lewis acids (M = Cd, Zn) to dramatically enhance the photoluminescence quantum yield (PL QY) of InP quantum dots. The addition of cadmium and zinc is additionally found to red- and blue-shift, respectively, the lowest energy absorption and emission of InP quantum dots while maintaining particle size. This treatment results in a facile strategy to post-synthetically tune the luminescence color in these materials. Optical and structural characterization (XRD, TEM, XAS, ICP) have led us to identify the primary mechanism of PL turn-on as surface passivation of phosphorus dangling bonds, affording PL QYs up to 49% without the growth of a type I shell or the addition of HF. This route to PL enhancement and color tuning may prove useful as a standalone treatment or as a complement to shelling strategies.
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growth from Zn2+ and S2− precursors on an InP core, an increased QY (ranging up to 20%) was seen after the initial addition of Zn-carboxylate.11,14−16 A similar observation was also noted in the stepwise addition of Cd2+ and S2− precursors for InP/CdS core−shell QDs with a 36% PL QY after the addition of cadmium acetylacetonate.19 Varied proposals have emerged to explain the improvement in PL QY, including surface passivation, etching in the case of zinc,15 or doping in the case of cadmium;19 however, a detailed understanding of this phenomenon is lacking, and more data are needed to gain insight into the mechanism of PL QY enhancement. We report the systematic investigation of the mechanism of PL turn-on by the postsynthetic addition of metal carboxylates, specifically zinc and cadmium, to InP QDs. Although cadmium is toxic, precedent in the literature for cadmium salts to passivate quantum dot surfaces and improve photoluminescence has made it of academic interest in this study and may be used to devise a general strategy with other soft Lewis acids.20 We hypothesize that exogenous Lewis acids bind to undercoordinated surface phosphorus atoms, leading to bonding interactions that lower the energy of the relevant orbitals below the valence band edge. We have observed PL enhancement of InP particles with QYs up to 49% with the addition of cadmium oleate and up to 19% with zinc oleate. An additional feature of these treatments is the ability to tune the absorbance and emission profiles of the QDs with no apparent change in particle size. The addition of zinc oleate tunably blue-shifts the optical features of the InP, while the addition of cadmium
n order to meet the rapidly increasing global energy demand, it is important not only to find new and renewable forms of energy but also to use energy more efficiently. One market that could make a significant impact on energy consumption is solid-state lighting (SSL). A switch from incandescent lighting to SSL could save 217 TWh by 2025 in the U.S. alone.1,2 Semiconductor quantum dots (QDs) are a promising class of chromophores for this application because their wavelength tunability and narrow luminescence line widths have the potential to give SSL the appropriate color rendering index and correlated color temperature to compete aesthetically with incandescent lighting.3 As such, CdSe QDs have dominated early exploration and commercial efforts in the area of downconversion fluorophores for SSL and displays; however, the toxicity of cadmium presents a marked barrier to wide-scale commercialization efforts and a significant environmental concern. Indium phosphide (InP) is an attractive alternative due to its lower toxicity, larger intrinsic extinction coefficient,4 larger Bohr exciton radius, wider emission range, and enhanced optical stability;5−7 however, current InP QD colloidal syntheses have photoluminescence quantum yields (PL QY) of
