Article pubs.acs.org/cm
Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
Formation of Size-Tunable and Nearly Monodisperse InP Nanocrystals: Chemical Reactions and Controlled Synthesis Zheheng Xu, Yang Li, Jiongzhao Li, Chaodan Pu, Jianhai Zhou, Liulin Lv, and Xiaogang Peng* Center for Chemistry of Novel & High-Performance Materials, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China
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S Supporting Information *
ABSTRACT: Formation of InP quantum dots (QDs) in a noncoordinating solvent is divided into four stages for studying the chemical reactions. By introducing tertiary phosphines, such as trioctylphosphine (TOP), in the first stage, the four stages are all altered significantly, which enables the formation of InP QDs with high optical quality, that is, with a well-defined first-exciton absorption peak and a high-energy absorption shoulder in their ultraviolet−visible spectra. The first stage is the formation of a less sterically hindered complex with three monodentate carboxylates and one TOP ligand [In(TOP)(St)3] by reacting indium stearate [In(St)3] with TOP, which is soluble and reactive at room temperature. The second stage is the formation of InP clusters with near-unity yield and very small size by reacting In(TOP)(St)3 with tris(trimethylsilyl)phosphine [(TMS)3P] at ambient temperatures (20−50 °C). During the third stage, tiny InP clusters formed with the In(TOP)(St)3 precursor enable the formation of nearly monodisperse InP QDs by a hot-injection approach. In the following fourth stage, the InP QDs formed in the third stage grow further in the same pot by the secondary injections of the InP clustersthe ones formed at ambient temperatures with the In(TOP)(St)3 precursor for efficient “self-focusing of size distribution”to finally obtain high-quality InP QDs with their absorption peak covering most part of the visible window (between ∼480 and 660 nm).
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INTRODUCTION Colloidal semiconductor nanocrystals with their sizes in the quantum confinement regime [quantum dots (QDs)] are rapidly becoming a visible player as emitters in industries such as display, bio-medical labeling, and solid-state-lighting.1−5 While their size-tunable emission, intrinsically high color purity, and outstanding stability render them as ideal emitters, CdSe-based QDs are the only system that can be synthesized with nearly ideal optical properties in the entire quantumconfined size regime.6,7 Despite their poor stability, QDs of lead-halide perovskites have been recently developed and demonstrate high optical quality.8 However, both Cd and Pb are considerable environmental concerns for the general public, and InP QDs are regarded as the most promising Cd/Pb-free alternatives.9 Synthetic chemistry of InP as well as other III−V QDs has attracted much attention in the past 25 years.10−26 While monodisperse CdSe QDs can be readily synthesized with tunable sizes with their first-excitonic absorption peak to cover most part of the visible window (roughly between 450 and 650 nm),27,28 both size and size distribution of InP QDs are difficult to be controlled.10−14,17−19,21,24,25 In the past 25 years, any progress on the synthesis of II−VI QDs would be more or less immediately extended to the III−V ones, but those efforts usually failed badly. One hypothesis for such drastic differences between the growth of CdSe and InP QDs is the more covalent nature of III−V compounds,11 which is an intrinsic challenge and does not have an easy solution. © XXXX American Chemical Society
The second hypothesis is the poorly balanced reactivity of the P and In precursors,14,17,19,29 which are commonly tris(trimethylsilyl)phosphine [(TMS)3P] and indium carboxylates, respectively. Along this hypothesis, significant efforts have been devoted to reduce the reactivity of the P precursors,17−19,21,22,29 all of which have demonstrated limited success. In 2007, we suggested activation of indium carboxylates by fatty amines to extend the size range of InP QDs,14 which was later proven to be a result of surface activation by the residual acetates.24 Recently, we found that crowded surface passivation by indium alkanoates ligandsthe most common ligands for InP QDsplays an important role to retard the growth of InP QDs.24 By applying ligands with a large footprint and a small hydrocarbon tail, available sizes of InP QDs could be extended further, although sharpness of the absorption spectra indicator of their size monodispersitywas still significantly worse than that of CdSe QDs. In the literature, synthesis of InP QDs in the presence of zinc alkanoates has been studied by several groups,15,25,26,30−32 which might also be associated with relief of crowded surface passivation through replacing indium alkanoates by zinc ones. In any case, though InP QDs might be doped by zinc ions during the growth and diminish Received: June 12, 2019 Revised: July 2, 2019 Published: July 2, 2019 A
DOI: 10.1021/acs.chemmater.9b02292 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials their emissive properties,33 this approach has yielded InP QDs with the sharpest absorption spectra so far.25,26 In 2008, we reported a “self-focusing of size distribution” route for the synthesis of good quality InAs QDs within a decent size range.16 Results confirmed that all As precursors, that is, tris(trimethylsilyl)arsenide [(TMS)3As], would react with indium carboxylates immediately at relatively low temperatures to form small clusters. Upon increasing the temperature of the reaction mixture, the clusters would undergo “self-focusing of size distribution,” some of which relatively small oneswould dissolve completely and feed the monomers to the relatively large ones in the solution for their growth in size.34 Such cluster-based routes have recently been extended from InAs QDs to InP ones, which have achieved some advancements.20,25,26,31,35 In this work, we first re-examine the successful “self-focusing of size distribution” route for InAs QDs described above16 and show that without tertiary phosphines in the reaction solution, it is impossible to obtain nearly monodisperse InAs QDs. This motivates us to systematically study the interaction between indium stearate [In(St)3] and trioctylphosphine (TOP), which suggests the formation of a one-to-one complex [In(TOP)(St)3]. This complex is found to substantially affect the formation of InP clusters, which in turn impacts both nucleation and growth of InP QDs drastically. Finally, a new synthetic scheme is developed for the synthesis of nearly monodisperse InP QDs with a clearly defined first-exciton absorption peak between 480 and 650 nm.
Figure 1. (a) Visual illustration of the interaction between indium stearate and TOP at room temperature. (b) Solution-phase FTIR spectra and (c) 31P NMR spectra of the mixtures of indium stearate and TOP in n-dodecane.
Liquid-phase Fourier transform infrared (FTIR) measurements (Figure 1b) reveal that upon addition of TOP into a diluted solution of In(St)3 in n-dodecane, the asymmetric stretching mode of the COO− group changes continuously until 1 equivalent of TOP is added. In the FTIR spectrum of In(St)3 in n-dodecane, the asymmetric stretching vibration exhibits multiple peaks at about 1585 and 1544 cm−1. When the concentration of TOP is equal to (and beyond) 1 equivalent of In(St)3, the asymmetric vibration shifts to a high frequency, with the main peak at ∼1604 cm−1. Such a frequency shift is assigned as a signature of significantly weakened bonding between the carboxylate groups and the metal ion, that is, likely from bidentate to monodentate bonding.44,45 31 P nuclear magnetic resonance (NMR) spectra also reveal a strong coordination between TOP and In(St)3 (Figure 1c). When the TOP concentration is less than 1 equivalent of In(St)3, the 31P NMR peak shifts significantly from −30.819 ppm for free TOP to −6.7 ppm, similar to that observed for a complex of cadmium oleate and tributylphosphine.46 A single and sharp peak at −6.7 ppm suggests that almost all TOP are firmly coordinated with indium ions at a low concentration of TOP. When TOP in the solution exceeds 1 equivalent, both peaks are observed, with the peak at −30.819 ppm being broad. The broad peak usually indicates either complex chemical environments for TOP molecules or aggregation,47 which needs to be studied further to fully identify its nature. Overall, the results in Figure 1a−c suggest the formation of In-TOP complex between indium fatty acid salts and TOP. The In and TOP ratio in the complex is likely one-to-one. After formation of the complex, bonding between carboxylate groups and In ions becomes much weaker and likely switches from bidentate to monodentate (Figure S2, Supporting Information). Considering the charge balance and no formation of either free acid or other carbonyl compounds (Figure 1b), we would tentatively label the complex as In(TOP)(St)3 (eq 1).
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RESULTS AND DISCUSSION Interaction between Tri-n-octylphosphine and Indium Stearate. Tertiary organophosphines, typically TOP and other phosphorous-containing organic compounds, have been widely applied in the synthesis of colloidal QDs since early 1990s.36−39 Secondary (and possibly tertiary) organophosphines were found to play a key role in the formation of monodisperse CdS QDs.40 To the best of our knowledge, it is unclear whether organophophines would play a role in the synthesis of III−V QDs. In 2008, we reported the synthesis of good-quality InAs QDs using InAs magic-size clusters, for which the clusters were preformed by reacting indium carboxylates and (TMS)3As.16 Evidently, exclusion of tertiary organophosphines from this established approach would not be able to yield magic-size clusters, and the size distribution of the resulting InAs QDs would be substantially worsened (Figure S1, Supporting Information). These results suggest that tertiary organophosphines might play a decisive role in the formation of high-quality III−V QDs. From a structural viewpoint, (TMS)3P and TOP adopt a similar steric structure and are isoelectronic for the phosphorus atom in both molecules. Thus, one would expect TOP (or other types of tertiary organophosphines) to have some strong interaction with indium carboxylates. In fact, for the synthesis of metal phosphide nanocrystals, tertiary organophosphines might become phosphorus precursors when very active organometallic precursors were applied.41−43 Similar to all types of indium fatty acid salts, solubility of indium stearate [In(St)3] in typical nonpolar solvents, such as 1-octadecene (ODE), at room temperature is low. However, addition of about 1 equivalent TOP would make In(St)3 highly soluble (Figure 1a).
In(St)3 + TOP → In(TOP)(St)3 B
(1) DOI: 10.1021/acs.chemmater.9b02292 Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 2. (a) General scheme for the synthesis of TOP-Cluster (top route) and n-Cluster (bottom route). UV−vis spectra of TOP-Cluster (b) and n-Cluster (c) synthesized at different temperatures. (d) Temporal evolution of in situ FTIR spectra during the reaction between (TMS)3P and In(TOP)(St)3 at 20−30 °C. (e) Concentrations of TMS-St determined by FTIR absorbance at 1724 cm−1 during the formation of two types of clusters at 20−30 °C with identical concentrations of all chemicals except TOP.
at ∼430 nm, indicating a significant increase in the size of the clusters. All TOP-Clusters are found to be reproducible and stable by storing them below the freezing temperature of ODE (
