Article pubs.acs.org/cm
Zinc−Phosphorus Complex Working as an Atomic Valve for Colloidal Growth of Monodisperse Indium Phosphide Quantum Dots Sungjun Koh,† Taedaehyeong Eom,‡ Whi Dong Kim,† Kangha Lee,† Dongkyu Lee,† Young Kuk Lee,§ Hyungjun Kim,*,‡ Wan Ki Bae,*,∥ and Doh C. Lee*,† †
Department of Chemical and Biomolecular Engineering, KAIST Institute for the Nanocentury and ‡Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea § Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea ∥ Photoelectronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea S Supporting Information *
ABSTRACT: Growth of monodisperse indium phosphide (InP) quantum dots (QDs) represents a pressing demand in display applications, as size uniformity is related to color purity in display products. Here, we report the colloidal synthesis of InP QDs in the presence of Zn precursors in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. Nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry analyses on aliquots taken during the synthesis allow us to monitor the appearance of metal− phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn−P bonding appears. The Zn−P intermediate complex with P(SiMe3)3 exhibits lower reactivity than that of the In−P complex, which is corroborated by our prediction based on density functional theory and electrostatic potential charge analysis. The formation of a stable Zn−P intermediate complex results in lower reactivity, which enables slow growth of QDs and lowers the extreme reactivity of P(SiMe3)3, hence monodisperse QDs. Insights from experimental and theoretical studies advance mechanistic understanding and control of nucleation and growth of InP QDs, which are key to the preparation of monodisperse InP-based QDs in meeting the demand of the display market.
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INTRODUCTION The past two decades have witnessed remarkable progress in the chemical synthesis of semiconductor nanocrystals (NCs).1 Understanding of growth kinetics enabled progress in shape control2 and surface chemistry,3 which dictate the exciton dynamics4 of QDs and thus the performance of light-emitting applications.5−7 One of the most important advances is the control of colloidal growth of NCs via arrested precipitation.8 In particular, semiconductor NCs in the quantum confinement regime, typically referred to as quantum dots (QDs), have received a tremendous amount of attention by virtue of their size-dependent and quantized optical transitions. Colloidal growth of monodisperse QDs results in narrow emission bandwidth, which would translate into high color purity.9,10 In the wake of the deployment of QDs in commercial products, demand for nontoxic, environment-friendly materials have effectively turned the focus from cadmium chalcogenide, the most studied materials,11,12 to other semiconductors, such as indium phosphide (InP) and copper indium selenide.13−15 With a bulk band gap of 1.35 eV and relatively devoid of © 2017 American Chemical Society
impurity states, InP is deemed to be best suited for emission over the entire visible region if under a proper quantum confinement regime.16 In fact, InP QDs have been utilized in a series of recently commercialized display products, which echo the need for high color-purity luminescence.17 From the context of display applications, especially for liquid crystal displays (LCDs)18 or light emitting diodes (LEDs),19 the emission color purity of materials is a principal prerequisite because it has become important for display products to satisfy a bundle of parameters, especially the color rendering index (CRI) proposed by the International Telecommunication Union (ITU) in 2012.20 The utilization of InP QDs in a wider variety of display devices is challenging because of the broad emission line width resulting from the relatively poor size distribution of InP QDs synthesized via previously published protocols.21 Compared to cadmium chalcogenide NCs, Received: April 22, 2017 Revised: July 10, 2017 Published: July 10, 2017 6346
DOI: 10.1021/acs.chemmater.7b01648 Chem. Mater. 2017, 29, 6346−6355
Article
Chemistry of Materials
aliquots during progression of the synthesis. We analyzed ultraviolet−visible (UV−vis) absorption spectra of the aliquots to characterize how the absorption features evolve in the cases of InP and In(Zn)P QDs. To elucidate the molecular origin of differences between the two growth cases, we used 1H nuclear magnetic resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), proton-decoupled 31P (31P{1H}) NMR spectroscopy, and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). Directly observing the formation of metal−phosphorus molecular intermediates, In−P and Zn−P complexes, at room temperature in XPS spectra, and depletion at different reaction temperatures in MALDI-TOF mass spectra, would allow for confirmation of the difference in reactivity between these two complexes. On top of the experimental results, we used density functional theory (DFT) calculations and electrostatic potential (ESP) charge analysis to elucidate the formation of the Zn−P and In−P complexes from the molecular thermodynamic viewpoint. To examine the effect of the reactivity difference on resultant QDs, we synthesized In(Zn)P QDs based on In−P or Zn−P complexes and compared optical properties of consequential QDs using PL emission spectroscopy and UV− vis spectroscopy. Furthermore, by comparing the influence of these complexes on the amount of zinc in QDs utilizing elemental analysis and inductively coupled plasma mass spectrometry (ICP-MS), we can provide insight into the pathway of zinc incorporation in the synthesis of In(Zn)P QDs.
bonding in III−V semiconductors is more covalent than ionic; therefore, the synthesis of InP QDs requires high reaction temperature and highly reactive precursors.22 For example, P(SiMe3)3, one of the most widely used phosphorus precursors, is depleted quickly in the nucleation stage in such a way that the growth of InP QDs simply forgoes the size-focusing regime,23 leading to size-broadening via Ostwald ripening.21,24 To overcome this chronic obstacle in the synthesis of monodisperse InP QDs, several attempts have been made to lower the reactivity of the precursors.25−27 For instance, Harris et al. synthesized P(GeMe3)3, which is far less reactive than P(SiMe3)3, and utilized it to synthesize InP QDs only to observe marginal narrowing of the size distribution of the QDs.26 Franke et al. reported that the size and the size distribution of III−V semiconductor QDs could not be controlled by simply manipulating the precursor conversion rate and claimed the need of a new model for the synthesis of III−V semiconductor QDs.28 Gary et al. revealed that the synthesis of InP QDs proceeds via heterogeneous growth from magic-sized clusters (MSCs), basically echoing the sentiment that a narrow size distribution of InP QDs demands something beyond a simple precursor change.29 Later, Gary et al. verified the existence of these intermediate clusters using X-ray crystallography,30 and Xie et al. confirmed their kinetic persistence using mass spectrometry.31 Alloying and doping of InP QDs with other cations, e.g., Zn2+, have offered a somewhat different angle of attack with regard to size uniformity and thus the emission color purity.32,33 These approaches result in both enhanced color purity and increased photoluminescence (PL) quantum yield (QY) in comparison to those of bare InP QDs synthesized in nearly identical reaction conditions.32,34,35 In this sense, the addition of zinc sources in the synthesis of InP QDs has been recognized by several accounts to elevate the color purity of the QDs.36−39 For instance, Yang et al. observed that zinc sources in the synthesis of Zn-incorporated InP (reffered to as In(Zn)P hereafter) QDs affect growth by both stabilizing the surface of particles and reducing the critical size of nuclei, enabling slower growth and eventually yielding a narrower size distribution.40 Thuy et al. compared luminescence properties of bare InP QDs with ∼90 nm of full-width at half-maximum (fwhm) and In(Zn)P QDs with ∼50 nm of fwhm and attributed the enhanced PL QY to higher crystallinity of In(Zn)P QDs.32 In addition, the effect of zinc additives on sharpening absorption features of InP QDs has recently been observed in the synthesis using P(NMe2)3 or P(NEt2)3, hypothesized to result from trap and defect passivation by zinc.41 However, these speculations have effectively ruled out the possible role of zinc in precursor conversion chemistry that should affect the growth kinetics of In(Zn)P QDs, which has been well-characterized in the case of the synthesis of Zn3P2 QDs using P(SiMe3)3.42,43 Moreover, the actual incorporation of zinc into InP NCs has yet to be characterized on a molecular scale to validate these speculations. Therefore, to exploit the effect of zinc on the color purity of In(Zn)P QDs to obtain QDs with narrow emission line width and to potentially expand the library of cations beyond zinc for further optimization of the synthesis protocol, the mechanistic role of zinc additives needs to be elucidated. Herein, we examine the effect of zinc in In(Zn)P QD synthesis with respect to growth kinetics and improved color purity. To investigate the effect, we employed a simple heat-up method to synthesize InP and In(Zn)P QDs and removed
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EXPERIMENTAL SECTION
Chemicals. Indium(III) acetate (≥99.99%), zinc(II) stearate (purum grade), 1-octadecene (≥90%), dithranol (matrix substance for MALDI-MS, ≥98%), toluene (anhydrous, ≥99.8%), and trimethyl phosphate (≥99%) were purchased from Sigma-Aldrich Co. Myristic acid (≥99%) was purchased from Tokyo Chemical Industry Co., Ltd. Tris(trimethylsilyl)phosphine (≥98%, 10 wt % in hexanes) was purchased from Strem Chemicals, Inc. Tris(trimethylsilyl)phosphine (≥95%) was purchased from JSI Silicone Co., Ltd. Benzene-d6 (≥99.5%) was purchased from Cambridge Isotope Laboratories, Inc. All of the purchased chemicals were used without further purification. Synthesis of InP QDs. First, 58.4 mg of indium acetate (0.2 mmol), 137 mg of myristic acid (0.6 mmol), and 10 mL of 1-octadecene were added to a three-neck round-bottom flask (50 mL, 14/20) connected to a reflux condenser and equipped with rubber septa and thermocouple. After the flask was placed on a heating mantle, the solution was degassed under reduced pressure (
