Independent Composition and Size Control for Highly Luminescent

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Independent Composition and Size Control for Highly Luminescent Indium-Rich Silver Indium Selenide Nanocrystals Olesya Yarema, Maksym Yarema, Deniz Bozyigit, Weyde M. M. Lin, and Vanessa C. Wood ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04636 • Publication Date (Web): 15 Sep 2015 Downloaded from http://pubs.acs.org on October 1, 2015

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Independent Composition and Size Control for Highly Luminescent Indium-Rich Silver Indium Selenide Nanocrystals Olesya Yarema, Maksym Yarema, Deniz Bozyigit, Weyde M.M. Lin, Vanessa C. Wood* Laboratory for Nanoelectronics, Department of Information Technology and Electrical Engineering, ETH Zurich, Gloriastr. 35, 8092 Zurich, Switzerland KEYWORDS: I-III-VI group semiconductors, ternary Ag-In-Se system, non-stoichiometric silver indium selenide, indium-rich AISe nanocrystals, colloidal quantum dots, size and composition control, tunable luminescence, high quantum yield.

ABSTRACT. Ternary I-III-VI nanocrystals, such as silver indium selenide (AISe), are candidates to replace cadmium and lead-based chalcogenide nanocrystals as efficient emitters in the visible and near IR, but, due to challenges in controlling the reactivities of the group I and III cations during synthesis, full compositional and size-dependent behavior of I-III-VI nanocrystals is not yet explored. We report an amide-promoted synthesis of AISe nanocrystals that enables independent control over nanocrystal size and composition. By systematically varying reaction time, amide concentration, and Ag- and In-precursor concentrations, we develop a predictive model for the synthesis, and show that AISe sizes can be tuned from 2.4 to 6.8 nm across a broad range of indium-rich compositions from AgIn11Se17 to AgInSe2. We perform structural and

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optical characterization for representative AISe compositions (Ag0.85In1.05Se2, Ag3In5Se9, AgIn3Se5, and AgIn11Se17) and relate the peaks in quantum yield to stoichiometries exhibiting defect ordering in the bulk. We optimize luminescence properties to achieve a record quantum yield of 73%. Finally, time-resolved photoluminescence measurements enable us to better understand the physics of donor-acceptor emission and the role of structure and composition in luminescence.


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Ternary I-III-VI semiconductor nanocrystals (NCs) are excellent choices to replace Cd- and Pb-containing colloidal NCs (or, quantum dots) in applications requiring visible and nearinfrared luminescence.1-2 After ten years of research, I-III-VI NCs now boast photoluminescence (PL) efficiencies of 86% for Cu-In-S,3 68% for Ag-In-S,4 60% for Cu-In-Se,5-6 and 40% for AgIn-Se systems.7 These high luminescent efficiencies along with long-term photostability, size and composition tunable electronic structure, reduced self-absorption, and lower toxicity put I-III-VI colloids well on their way to potential use in display, lighting, bio-medical applications.2, 8-9 At the same time, I-III-VI materials are more complex than binary chalcogenide NCs. While this complexity offers a fruitful playground for scientific investigation,1,

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it also makes

optimization of the luminescent properties of NCs far more difficult. First, composition dependencies of I-III-VI NCs properties are superimposed with size effects.2,

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Second, to

optimize the PL quantum yield (QY), the ratio between the metals is important. Indium-rich compositions of 1:1:2 phases (e.g., CuInS2 NCs) luminesce better than stoichiometric structures.14 Excess indium atoms occupy the positions of M(I) metals (i.e., antisite defects, In ), and each excess indium induces two copper (or silver) vacancies (2V , Figure S1). Donor acceptor pairs have a composition of (2V + In ), and the PL efficiency of I-III-VI materials is

proportional to their concentration.2 Furthermore, for certain compositions, donor-acceptor defect groups can self-order within the atomic lattice, forming ordered defect compounds (ODC).15 Cu3In5E9 (E = S, Se), one of the ODC compositions of the Cu-In-S and Cu-In-Se systems, exhibits peak luminescent efficiencies prior to ZnE shell coating.6,

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For Ag-In-S

materials, indium-rich AgIn5S8 ODC NCs also possess superior PL QYs with respect to AgInS2 composition.16


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Synthetically, Ag-In-Se (AISe) colloids are the least developed of highly luminescent, indiumcontaining I-III-VI NCs10 because of two main problems in their synthesis: (i) Ag(I) compounds easily reduce to metallic Ag(0), which eliminates many silver sources, and (ii) Ag+ is a soft acid while In3+ is a hard acid, which leads to imbalanced Ag and In reactivity during the nucleation as explained by the hard and soft acids and bases theory. Several solutions have been explored. Xie et al.17 introduced a way to selectively passivate the silver precursor during the reaction with the addition of a long-chain thiol, a soft Lewis base.7, 18 Alternatively, a mixed-metal, single-source selenocarboxylate, [P(C6H5)3]2AgIn[SeCO(C6H5)]4, can be used as a precursor to achieve a desired cation ratio.19-20 Finally, increasing the reactivity of Se precursor by employing weaklycoordinating oleylamine-Se complexes, selenourea, or bis(trimethylsilyl)selenide has been shown to improve composition control over AISe NCs.21-23 Despite this progress, the synthesis of AISe NCs remains underdeveloped, frequently including silver-rich or even binary Ag2Se intermediates.7, 18, 22 Furthermore, many protocols require growth times of 10 min to few hours to reach target compositions. These long times lead to broad size distributions and relatively large NC sizes.18-19, 22 The amide-promoted synthetic approach has been proven to simultaneously solve many of aforementioned issues.24 Amide-promoted synthesis differs from conventional hot-injection techniques by the presence of lithium amide (LiN(SiMe3)2), a common superbase in organic synthesis. The presence of amide speeds up the nucleation rate, so that the target composition is achieved within seconds. Recently, we showed the applicability of amide-promoted synthesis to cation-mixed Cu-In-Se materials,6 demonstrating that it is possible to maintain the same composition regardless of NC size and shape. For example, we achieved the same composition close to Cu3In5Se9 for growth times ranging from 15 s to 5 min, which allowed accurate size


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control from 2.7 to 7.9 nm. Additionally, the amide-promoted synthesis shows high yields, a potential for upscaling, and is particularly powerful for the smallest, highly luminescent (2 – 5 nm) sizes of colloidal NCs.25 Here we extend the use of amide-promoted synthesis to the Ag-In-Se ternary system. We demonstrate composition control for indium-rich AISe NCs and focus on several compositions to cover all the known ternary phases in the Ag2Se – In2Se3 diagram. Composition tunability is achieved by controlling the amount of LiN(SiMe3)2 salt or by the ratio between Ag and In precursor salts, while the size of AISe NCs is best-controlled by the growth time. Most importantly, we demonstrate independent tunability of AISe NC size and composition. This gives us the unprecedented possibility to not only study size-dependent optical properties of particular AISe stoichiometries, but also find a size and composition optima for luminescence properties.

RESULTS AND DISCUSSION Amide-promoted synthesis. The scheme of the synthesis is shown in Figure 1a. As for our previously published recipe for Cu-In-Se NCs,6 we employ an amide-promoted strategy for the synthesis of AISe NCs. In contrast to the Cu-In-Se synthesis which used chlorides, here we use iodides after determining that AgCl in trioctylphosphine (TOP) decomposes at T > 100°C. In a typical synthesis, AgI and InI3 are dissolved in TOP and heated to 260°C, at which point an injection mixture containing TOP solutions of Se and LiN(SiMe3)2 is swiftly added. The reaction solution turns turbid immediately after injection, indicating the formation of LiI containing byproducts, and its color gradually changes from yellow to orange and light brown indicating growth of AISe NCs. The reaction is typically stopped after 15-120 s and cooled to room


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temperature. AISe NCs are purified using the miscibility gap between TOP and methanol. LiI byproducts can be successfully washed out with alcohols during this purification protocol (Figure S2). Additional details of the synthesis are provided in the Methods. As we will describe in detail in the following sections, varying the amount of lithium amide as well as the ratio of metal iodides, enables us to control the composition of AISe NCs, covering all known ternary compounds in the Ag2Se – In2Se3 phase diagram (Figure S3).26 Figure 1 shows that across this wide compositional range, the NCs exhibit crystallinity, consistent morphology, and narrow size distributions. A representative high-resolution transmission electron microscope (TEM) measurement shows good crystallinity (Figure 1b). TEMs of 4 representative compositions with Ag-to-In atomic ratios of 0.8, 0.6, 0.3, and 0.1 (Figure 1c-f) reveal that all AISe NCs exhibit and narrow size distributions, and the same “nearly spherical” polygonal morphology seen in Figure 1b. The size distribution remains as low as 9-10% (Figure S4) for amide-promoted AISe syntheses up-scaled to yield > 1 gram of NC product. X-ray diffraction patterns of each composition are shown in Figures 1g,h. The peaks are broad because of the small AISe NC size (Figure S5) and the disorder in the cationic sublattice resulting from the silver vacancies (about 2.6 at.% of V for Ag:In = 0.8 and 14.7 at.% for Ag:In

= 0.1, Figure S6). However, we can observe that AISe NCs from the AgInSe2 phase (i.e., Ag:In = 0.8 or 0.6, Figure S3) crystallize in wurtzite/orthorhombic structures (Figure 1g), while indium-rich NCs crystallize in cubic/tetragonal structures (Figure 1h). It is not possible to further specify the phases even with comparative Rietveld refinement (Figures S7,8). Composition control. As shown in Figure 2a, to determine NC composition, we perform energy dispersive X-ray spectroscopy (EDX) and extract the Ag:In atomic ratio. First, we investigate the influence of the lithium amide salt. We keep growth time (tgrowth = 30 s) as well as


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the amount of and molar ratio between the metal iodides ( : = 0.75, = 0.19 mmol, = 0.25 mmol) constant. Systematically varying the LiN(SiMe3)2 concentration results in Ag:In atomic ratios in AISe NCs from 0.1 to 0.8 (black curve on Figure 2b). We find that for low amounts of lithium amide, the composition of AISe NCs is Ag-poor while, for high amide concentrations, AISe compositions keep the original AgI:InI3 molar ratio. The dependence of AISe NC composition on amide concentration (Figure 2b) has a sigmoid shape with two characteristic plateau regions separated by a transition (growth) region. We fit this sigmoid curve with a logistic growth function (Figure S9), and find that the inflection point ( , = 0.90 mmol) is close to the total concentration of iodide anions ( = + 3 × = 0.94 mmol). We can therefore identify the two flat parts of the growth curve as regions with a lack and excess of amide with respect to iodide anions. For a reaction with excess of LiN(SiMe3)2 (shaded area in Figure 2b), the composition of AISe NCs is solely determined by the ratio between initial iodides, providing a simple and reliable path for composition control. We explain these experimental results using Scheme 1. The formation of AISe NCs includes short-living metal-amide intermediates, as suggested previously for binary chalcogenide NCs.2425

First, silver(I) and indium(III) iodides react with LiN(SiMe3)2 with reaction rates rAg and rIn,

respectively. Metal-amide transients are then converted to ternary AISe NCs with a rate rAISe. The regime of low LiN(SiMe3)2 concentrations reveals the difference between rIn and rAg. We find indium-rich compositions of AISe NCs because the reaction between the amide and the indium salt is significantly faster than that between the amide and the silver salt (rIn >> rAg). This observation is consistent with the hard and soft acids and bases theory: as a hard base, lithium amide reacts preferentially with the hard In3+ acid than with the soft Ag+ acid. When lithium amide is added in excess, rAISe is limiting, and the final AISe NC composition reflects the initial


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ratio between AgI and InI3. Similarly to the synthesis of Cu-In-Se NCs6, the composition of AISe NCs remains constant for different growth times (Figures 2c and S10). As will be discussed in next section, longer growth times result in larger AISe NCs. Size control. As shown in the density plot in Figure 3a, AISe NC size can be tuned by different growth times as well as by the amount of introduced amide. We keep the molar ratio between AgI and InI3 at 0.75 so that the AISe size map can be directly compared to compositionrelated data in Figures 2a, 2c, and S10. Again, we designate two regions as the lack and excess of LiN(SiMe3)2 with respect to iodide anions. For a constant growth time (Figure 3b), larger AISe NCs are obtained when total amounts of iodide and amide anions are similar. This is reminiscent of the water-based synthesis of oxide nanocrystals, where NC size decreases upon increase of ionic strength of the solution.27 Performing the homogeneous nucleation far from the typical point of zero charge (PZC) leads to smaller NC sizes due to decrease of surface tension and lowering of the nucleation energy barrier.28 For our case, the equimolarity point between iodide and amide anions (the inflection point on sigmoid growth curve on Figures 2b and S9) could be viewed as analogous to the PZC. Our synthetic approach provides the rare opportunity to perform an accurate optimization of size and composition of I-III-VI NCs separately from each other. When the amount of amide is not equal to the total concentration of iodide anions (namide < niodides or namide > niodides), it is possible to obtain AISe NCs of constant compositions (Figure 2b) while their size is tuned from 2.4 to about 7 nm (Figures 3a, 3c and S11). Reactions with a surplus of amide – where, by changing reaction conditions such as the amount of metal iodides or the growth time, it is possible to reproducibly obtain both a target size and target composition for the AISe NCs –


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open great perspectives for the optimization of optical properties and performance of I-III-VI NC-based devices. As with other I-III-VI NCs, in addition to size effects, the optical properties of AISe NCs can be tuned by their composition.2 Figures 3d,e show PL peak positions corresponding to varying growth time with constant amide amount, and vice-versa. We find that with increasing growth time (and therefore NC size, Figure 3c) but stable AISe composition (Figure 2c), the PL peak monotonously shifts towards the infrared (Figure 3e). Varying the amide amount, the PL peak position changes substantially between 0.8 and 0.9 mmol of LiN(SiMe3)2 (Figure 3d), which cannot be solely attributed to the moderate size increase (Figure 3b) but rather to the change of AISe NC composition (Figure 2b). The same is apparent for other growth times (Figures S12 and 3a). Optical properties. Taking advantage of independent size and composition control for AISe NCs, we probe size dependencies of optical properties for three representative compositions: AgIn11Se17 (Ag:In = 0.1), AgIn3Se5 (Ag:In = 0.3), and Ag3In5Se9 (Ag:In = 0.6). Figures 4a-c show size-dependent emission spectra of specific AISe NC compositions. The PL peak wavelength can be tuned from 600 nm up to 1100 nm, covering an important part of the visible and near-infrared spectral regions.2, 8 Similarly to other colloidal semiconductors, PL peak shifts towards longer wavelengths as the AISe NC size increases.6 Comparing similar NC sizes, the emission spectrum red-shifts from AgIn11Se17 to AgIn3Se5 and Ag3In5Se9 (Figures 4d-f, S13). The width of the PL spectra, however, remains approximately 0.35 eV for all AISe compositions. Like other I-III-VI materials,1, 9 absorption spectra of AISe NCs are strongly shifted to higher energies (i.e., large Stokes shifts for all AISe NC compositions) and exhibit a broad shoulder. As shown in Figures 4d-f and Figure S14, the excitonic bandgap transition energies are estimated by


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using the inflection point on the first derivative (i.e., the local minimum on the second derivative) of the absorption coefficient curves.29 This allows us to identify and plot the sizedependence of the optical bandgaps for chosen compositions (Figure 4g). Sizing curves show the expected D–2 trends30 and can be fitted with = !"#$ + % × & . Bulk bandgaps, extracted from the fits (Figure 4g), are larger than the bulk bandgap for stoichiometric chalcopyrite AgInSe2 (1.2 eV)31 and increase from 1.48 eV for Ag3In5Se9 to 1.72 eV for AgIn3Se5 and 2.14 eV for AgIn11Se17 NCs. This observed trend of increasing bandgap with increasing indium content is more dramatic than but consistent with that reported for bulk Ag-In-Se materials31, 32 and has been explained for I-III-Se thin films by the presence of defects and p-d orbital hybridization.33 Optimizing and understanding the PL efficiency. To determine the optimal composition of AISe NCs for efficient luminescence, we design a series of syntheses, ensuring constant AISe NC size (ca. 3.0 nm), but variable composition (Figure 4h). PL QYs range between 15 and 25 % for all AISe compositions with Ag:In atomic ratios > 0.25, but drops notably for smaller Ag:In ratios. Two maxima in PL QY are apparent for NCs with Ag:In = 0.35 and 0.65. These compositions are very close to AgIn3Se5 and Ag3In5Se9 phases, which belong to the ordereddefect family of I-III-VI materials.15 Since the concentration of silver vacancies monotonously increases towards lower Ag:In atomic ratios (Figure S6), we conclude that, the PL QY of AISe NCs for the composition range between Ag:In = 0.35 and 0.65 is not determined by the defect concentration, but rather by the distribution (i.e., ordering) of defects within the crystal structure (Figure 4h). To improve PL efficiency, we grow a shell around the most luminescent NC cores with a Ag:In = 0.6.3, 7 We choose ZnSe as a shell material because of its wide bandgap, low toxicity,


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and smaller lattice mismatch in comparison with ZnS.6 AISe NCs are dissolved in TOP, and diethylzinc and TOP:Se are added dropwise at elevated temperatures.34 We perform the shell growth at two different temperatures, 50°C and 150°C (details in Methods). For both cases, the absorption and emission spectra shift towards higher energies (Figure 5a), indicating a cationexchange mechanism of shell formation.12 Accordingly, since smaller Zn atoms replace Ag and In, the size of AISe/ZnSe core/shell NCs slightly decrease (Figure 5b-d). Upon growth, Zn atoms substitute nearly equal amounts of Ag and In (Figure S15). As expected, higher shell growth temperature causes faster diffusion of cations, resulting in higher Zn contents.35 The PL QY increases to 53% for thin ZnSe shell and shows a record 73% for a thick ZnSe shell. To obtain addition insights into the role of NC composition on the luminescence properties, we measure the time-resolved photoluminescence of AISe NC cores of three different compositions – AgIn11Se17 (Ag:In = 0.1), AgIn3Se5 (Ag:In = 0.3), and Ag3In5Se9 (Ag:In = 0.6) – as well as Ag3In5Se9 NCs with thin and thick ZnSe shells (Figure 6a). The size of AISe-based NCs is kept between 3 and 4 nm, excluding possible size-dependent effects. The time-resolved PL curves are fitted with a biexponential decay function (black lines), where the short decay component is a stretched exponential (or Kohlrausch-Williams-Watts, KWW) decay with β as the stretching parameter (0 < β≤1):36-38 '()* = %+ ,

./012

+ %345 ,

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Independent Composition and Size Control for Highly Luminescent

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