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Communication
Two Step “Seed-Mediated” Synthetic Approach to Colloidal Indium Phosphide Quantum Dots with High-Purity Photo- and Electroluminescence Parthiban Ramasamy, Keum-Jin Ko, Jae-Wook Kang, and Jong-Soo Lee Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02049 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018
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Chemistry of Materials
Two Step “Seed-Mediated” Synthetic Approach to Colloidal Indium Phosphide Quantum Dots with High-Purity Photo- and Electroluminescence Parthiban Ramasamya‡, Keum-Jin Kob‡, Jae-Wook Kangb and Jong-Soo Leea* a
Department of Energy Science and Engineering, DGIST, Daegu 42988, Republic of Korea. Department of Flexible and Printable Electronics, Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju 54896, Republic of Korea.
b
ABSTRACT: Synthesis of size-tunable indium phosphide (InP) quantum dots (QDs) with narrow size distribution has been long known to be more challenging due to the difficulties in controlling QDs nucleation and growth processes via precursor conversion kinetics. Here we present a “seed-mediated” two-step synthesis method for highly monodisperse InP QDs with sizes ranging from 1.9 to 4.5 nm. We further show the synthesis highly luminescent InP-based core-shell-shell QDs with tunable emission across the visible spectrum (515 - 640 nm), having a full width at half-maximum (FWHM) of less than 40 nm for all the wavelengths. We then demonstrate the fabrication of green and red QDs light emitting diodes (QLEDs), and show that our devices exhibit superior color purity than that of InP QDs-based LEDs reported previously. This study advances the chemical research community toward the goal of realizing high color pure emissions from cadmium-free QDs for display applications.
InP QDs have attracted a great deal of attention in next generation display technologies, owing to their low intrinsic toxicity and tunable emission from visible to near-infrared region. Despite recent progress in improving the optical properties of InP-based QDs, current synthesis methods have often failed to produce InP QDs in a range of sizes with narrow size distributions, a prerequisite for realizing high color pure emissions.1-11 This is due to the challenges associated with separating the nucleation and growth events during the synthesis of InP QDs.12-15 Tris(trimethylsilyl) phosphine [P(SiMe3)3] is the most commonly employed phosphorous precursor for InP QDs synthesis. P(SiMe3)3 is extremely reactive and consumed completely within a few seconds of injection in such a way that the growth of InP QDs precludes the size-selective growth, leading to broader size distribution via Ostwald ripening (Scheme. S1). The rapid depletion of the phosphorous precursor also makes it difficult to synthesize InP QDs with first excitonic peak above 600 nm (Figure. S1 and S2). To overcome this problem, several attempts have been made to lower the reactivity of phosphorous precursors. Harris et al. replaced Si atom in P(SiMe3)3 with Ge to synthesize less reactive P(GeMe3)3.16 Despite slowing down the precursor conversion rate by four times, there was no significant change observed in the size and size distribution of the synthesized InP QDs. Other attempts by Joung et al.17 and Gary et al.13 to control the precursor reactivity, resulted in slight increase in the average size of the InP QDs, however, the monodispersity could not be improved. Later, Gary et al. showed that the InP QDs synthesis proceeds via formation of magic-sized clusters (MSCs) followed by subsequent heterogeneous growth from the MSCs to QDs.18 Recently, Xie et al. also confirmed the presence of MSCs during the InP QDs growth.19 These findings are contradictory to the classical nucleation theory which
has been quite successful in explaining the growth mechanism of II-VI and IV-VI QDs, and suggest that simple change in precursor reactivity alone is not sufficient enough to control the size and size distribution of InP QDs. Last year, our group presented an effective successive ion layer adsorption and reaction (SILAR) method to tune the size of InP QDs.1 Although, this method produced InP QDs absorbing more than 600 nm, the corresponding core-shell QDs had broader emission FWHM (~ 44 nm) values at higher wavelengths (>550 nm). In addition, the method was laborious and took nearly seven hours to synthesize larger QDs. Recently, Franke et al. successfully demonstrated the possibility to tune the size of InAs QDs with narrow size distribution by continuous injection synthesis and later tried to replicate the results for InP QDs but ended up with limited success.20
Scheme 1. Schematic of “seed-mediated” synthesis of larger InP QDs and InP/ZnSe/ZnS QDs.
In this communication, we present a two-step approach to synthesize highly monodisperse InP QDs (
