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Colloidal Chemistry in Molten Salts: Synthesis of luminescent In GaP and In GaAs Quantum Dots 1-x

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Vishwas Srivastava, Vladislav Kamysbayev, Liang Hong, Eleanor Dunietz, Robert F Klie, and Dmitri V. Talapin J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06971 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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Journal of the American Chemical Society

Colloidal Chemistry in Molten Salts: Synthesis of luminescent In1-xGaxP and In1-xGaxAs Quantum Dots

Vishwas Srivastava,† Vladislav Kamysbayev,† Liang Hong,‡ Eleanor Dunietz,† Robert F. Klie‡ and Dmitri V. Talapin† § *

†

Department of Chemistry and James Franck Institute, University of Chicago, Illinois, 60637, USA ‡

Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, USA

§

Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, 60439, USA

E-mail: [email protected]

Abstract Control of composition, stoichiometry and defects in colloidal quantum dots (QDs) of III-V semiconductors has proven to be difficult due to their covalent character. Whereas the synthesis of colloidal indium pnictides such as InP, InAs and InSb has made significant progress, gallium containing colloidal III-V QDs still remain largely elusive. Gallium pnictides represent an important class of semiconductors due to their excellent optoelectronic properties in the bulk, however, the difficulty with the synthesis of gallium containing colloidal III-V QDs has largely prohibited their exploration as solution-processed semiconductors. Here we introduce molten inorganic salts as high temperature solvents for the synthesis and manipulation of III-V QDs. We demonstrate cation exchange reactions on pre-synthesized InP and InAs QDs to form In1-xGaxP and In1-xGaxAs QDs at temperatures above 380°C. This approach produces novel ternary alloy

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QDs with controllable compositions that show size- and composition-dependent absorption and emission features. Emission quantum yields of up to ~50% can be obtained for In1-xGaxP/ZnS core shell QDs. A comparison of the optical properties of InP/ZnS core-shells with In1xGaxP/ZnS

core shells reveals that Ga incorporation leads to significant improvement in the

optical properties of III-V/II-VI core-shell emitters which is of great importance for quantum dot based lighting and display applications. This work also demonstrates the potential of molten inorganic salts as versatile solvents for the synthesis and processing of colloidal nanomaterials at temperatures inaccessible for traditional solvents.

Introduction III-V compounds (GaAs, GaN, InGaP, InGaAs, etc.) represent one of the most important classes of semiconductors with direct band gaps and excellent electronic properties.1 One of the salient features of these compounds is the ability to make alloys with desired compositions that not only allows the band gaps to be precisely tuned but also enables epitaxial growth of complex multilayer device architectures. For instance, epitaxially matched InGaP/GaAs stacks are used for high efficiency multi-junction solar cells.2-3 Similarly, InGaAs is grown on an epitaxially matched InP substrate for near-infrared (NIR) detector applications.4 Colloidal quantum dots (QDs) of III-V compounds have been used for a plethora of applications such color converters in liquid crystal displays,5 LEDs,6-7 thin-film transistors8 and bioimaging910

. In recent years, InP is replacing CdSe as the material of choice for commercial quantum dot

displays due to its lower toxicity.11-12 The other members of the III-V family such as InAs and


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InSb have also seen a significant surge of interest as infrared active materials for bioimaging, night vision and telecommunication.13-14 Even though the synthetic chemistry of colloidal III-V semiconductors has seen significant progress,15-17 Ga based III-V QDs remain under-developed. For example, colloidal GaAs QDs are still difficult to synthesize due to the formation of crystalline defects when they are synthesized at temperatures accessible for traditional colloidal chemistry, i.e., below 400°C.18 The covalent character of Ga pnictides and the high oxophilicity of Ga(+III) makes solution synthesis of Ga-containing III-V compounds challenging.18-21 Ga containing ternary III-V QDs, e.g., In1-xGaxP and In1-xGaxAs, are technologically important compounds due to the flexibility they offer in terms of band gap and lattice constant engineering. For example, In1-xGaxP nanoparticles emitting green light are expected to be superior to their InP counterparts due to their larger size and correspondingly larger absorption cross-sections and smaller surface to volume ratio. Moreover, incorporation of Ga into the InP lattice reduces the lattice mismatch with wider gap shell materials such as ZnS, making the material less strained.22 The composition-graded core shell structures have shown to be important for slowing down Auger recombination rates.23 However, it is impossible to grow a graded wide-gap shell for III-V/II-VI system without disruption in the local electron count. Core-shell nanocrystals with III-V core and compositionally graded III-V shell can confine the electron and hole wavefunctions to the core while minimizing the interfacial strain and avoiding formation of in-gap states caused by disrupted electron count at the interface.24 Therefore, efficient incorporation of III-V QDs into commercial technologies requires new methods to engineer their composition. Attempts to synthesize III-V QDs with alloy compositions such as In1-xGaxP or In1-xGaxAs via direct solution routes have so far seen only limited success.20,

25-26

Crystalline In1-xGaxP 3

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nanocrystals could only be obtained upon annealing the reaction product at 400°C.20 Cation exchange reactions have proven to be a successful strategy to obtain alloy compositions in ionic nanocrystals,27-28 but similar attempts on covalently bonded III-V nanocrystals have only resulted in surface exchanges even at elevated temperatures.22, 24 The use of nanocrystals of more ionic compounds such as Cu3-xP or Cd3As2 as sacrificial templates for the synthesis of InP or GaAs nanocrystals has also been explored.29-30 While the transformation of CdSe to Ag2Se can be spontaneously achieved at room temperature,28 the transformation of Cd3As2 (II-V compound) to GaAs is only possible at 300°C, with a significant amount of Cd remaining in the sample.30 This can be attributed to the large difference in the diffusivity of cations in covalent III-V semiconductors as compared to the more ionic II-VI semiconductors.31 For these reasons, even thermodynamically favorable cation exchange reactions in III-V nanocrystals require high temperatures to overcome the diffusion barriers. Our group recently reported that stable colloidal dispersions of inorganic nanoparticles can be formed in molten inorganic salts.32 Many molten salts are not only stable up to very high temperatures, they also offer an extremely inert environment for air sensitive reactions.33 Molten salt fluxes have been previously employed for the synthesis of a variety of nanostructured covalent compounds.33-36 The ability to disperse colloidal nanocrystals in molten salts offers us the unique possibility of performing precise chemical manipulations on them at high temperatures. In this report, we demonstrate the synthesis of ternary In1-xGaxP and In1-xGaxAs QDs via cation exchange reactions performed on pre-synthesized InP and InAs QDs dispersed in molten salts as an example (Figure 1a). The resultant ternary III-V QDs show absorption and emission features that are blue shifted in comparison to the starting materials, in line with the expected change in band gap upon homogenous alloying (Figure 1b). Bright luminescence with


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quantum yields of up to 50% was obtained without much optimization from core-shell In1-xGaxP /ZnS nanocrystals making them potential candidates for next generation Cd-free display technology.

Results and Discussions Colloidal InP and InAs QDs can be dispersed in a variety of inorganic salt eutectics (See Table S1 and Figure S1 for pictures of QDs in molten salts) such as NaSCN/KSCN (26.3:73.7 mol %, m.p. 137°C) or CsBr:LiBr:KBr (25:56.1:18.9 mol%, m.p. 236°C). The native organic ligands were first removed from the QD surface by either using HBF4 as the stripping agent or by decorating the QDs surface with short inorganic S2- ligands.37 Dried powders of organic-ligandfree QDs were stirred in the molten salt at temperatures slightly above their melting point for several hours to obtain stable dispersions (Figure 1c). The detailed procedures can be found in the experimental section. The dispersions of InP and InAs QDs in molten CsBr:LiBr:KBr were stable at temperatures well above 400°C. At these temperatures, traditional organic solvents and surface ligands either boil or decompose.38 The stability of QDs in molten salts has been attributed to the ability of the salt anions and cations to form strongly ordered templates around the nanocrystal surface (Figure 1a).32 Halide ions are well-known to bind to the surface of III-V nanocrystals which is essential to induce enhanced ordering of the ions around each nanocrystal.39-40



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Figure 1. (a) A schematic showing the cation exchange process in molten inorganic salts. The templating of molten salt ions around the QD surface is responsible for stabilization of QDs in molten salt. The green and grey circles represent the anions and cations of molten salt, respectively. Addition of GaI3 to the molten salt dispersion of InP or InAs QDs leads to the cation exchange towards In1-xGaxP or In1-xGaxAs QDs. (b) The lattice constants and bulk band gaps (at 300 K) for alloys of InP and GaP, InAs and GaAs and ZnS and ZnSe (Adapted from ref41). ZnS15, 24 and ZnSe15, 42 are typically used as the wide gap shell materials for InP QDs. (c) Pictures of InP QD dispersions in molten CsBr:LiBr:KBr eutectic.

To perform cation exchange reactions on III-V QDs, we chose the CsBr:LiBr:KBr eutectic due to its high temperature stability, low vapor pressure, high solubility of group III halides and inertness to InP and InAs QDs.43 QDs capped with sulfide ligands showed better stability in this eutectic mixture and also did not show Ostwald ripening in these salts. We added a desired amount of GaI3 salt (m.p. = 212°C) to the QD/molten salt solution and stirred it for 2 h at ~250°C to allow homogenization. The mixture was then transferred to a furnace and heated to 380°C-


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450°C for one hour. According to Pearson’s HSAB principle, the softer In3+ should have a higher preference than Ga3+ for the soft iodide ion favoring the exchange. The reaction mixture was cooled down to room temperature and the salt was removed by repeated washing using formamide (FA) in inert atmosphere. The QDs were separated as powder by centrifugation, dispersed

in

FA

using

sulfide

ligands,

and

transferred

to

toluene

phase

using

didodecyldimethylammonium bromide (DDAB) as the phase transfer agent. The resulting colloidal solution in toluene was stable for months in an inert atmosphere. Gallium pnictides are thermodynamically more stable than their corresponding indium pnictides. For example, the standard heats of formation of GaP and InP are -103.2 kJ/mol and -70.2 kJ/mol, respectively (-87.7 kJ/mol and -60 kJ/mol for GaAs and InAs).44 Therefore, the cation exchange is expected to be only diffusion limited and can be accelerated by increasing the reaction temperature. The extent of Ga incorporation in the QDs could indeed be controlled by the temperature at which the exchange was performed. Figure 2a shows powder X-ray diffraction (XRD) patterns of In1-xGaxP nanocrystals with varying compositions obtained from cation exchange at temperatures ranging from 380°C- 430°C. A consistent shift of all X-ray reflections to higher 2θ values was observed with increasing temperature which suggests increasing Ga incorporation into the lattice with temperature. We obtained similar results for In1-xGaxAs QDs (Figure 2b); however, Ga incorporation into InAs lattice required higher temperatures, 400°C 450°C. No change in XRD patterns was observed when the particles were annealed in the absence of GaI3 (Figure S2). The full width at half maximum (fwhm) of the (111) diffraction peak did not appreciably change when the cation exchange reactions were performed at temperatures below 450°C indicating that the QDs did not grow or etch significantly. Although we could drive the composition almost completely to the GaAs phase when the exchange was



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performed at 500°C, it was accompanied by significant narrowing of the diffraction peaks, indicative of an increase in the particle size (Figure S3). The composition of the alloy was estimated from the lattice parameters (aInP= 0.586 nm and aGaP= 0.545 nm) using Vegard’s law (Figures 2a and 2b). ICP-OES analysis of the QDs was in a good agreement with the compositions estimated from the X-ray diffractions patterns (Tables S2 and S3). Further insight into the nature of alloying could be obtained from the Raman spectroscopy. Figures 2c and 2d show the Raman spectra for different alloy compositions of In1-xGaxP and In1-xGaxAs QDs. A continuous one-mode shift in the TO and LO phonon modes of the parent InP and InAs phase could be seen for In1-xGaxP and In1-xGaxAs QDs with increasing Ga incorporation indicating that the alloy QDs did not have phase segregated domains of InP and GaP. We noticed that the TO and LO phonon features for the alloys with higher Ga component showed significant broadening, indicating the lack of a long range order between In and Ga sub-lattices in the alloy QDs. Similar Raman spectra have been reported for InGaP nanowires grown using the Solution-Liquid-Solid (SLS) method.45


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Figure 2. (a,b) X-Ray diffraction patterns of In1-xGaxP and In1-xGaxAs alloy QDs obtained by cation exchange at different temperatures. The vertical lines show the positions and intensities of X-ray reflections of bulk InP, GaP, InAs and GaAs. (c,d) Raman spectra of In1-xGaxP and In1-xGaxAs alloy QDs obtained by cation exchange at different temperatures. The vertical lines show the corresponding TO and LO phonon modes of bulk InP, GaP, InAs and GaAs.

We characterized the morphology of the alloy In1-xGaxP and In1-xGaxAs QDs using electron microscopy. Figure 3a shows a representative scanning transmission electron microscope (STEM) image of In1-xGaxP QDs. A high resolution image with clear lattice fringes is shown in the inset. The homogeneity of structural alloying is further supported by the energy dispersive X-ray (EDX) elemental mapping of In1-xGaxP QDs (Figures 3b, S4). We could detect the presence of both indium and gallium in each individual QD without any apparent phase



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segregation. Elemental line scans for individual QDs are shown in Figure S5. Figure S6 shows a STEM-EDS map of an individual In1-xGaxAs QD showing the presence of both In and Ga atoms.

Figure 3. (a) STEM image of In1-xGaxP QDs. Inset shows a high resolution STEM image with clearly resolved lattice fringes. (b) Energy dispersive X-ray map and the corresponding STEM image showing a homogenous distribution of indium and gallium in every QD. For Ga and P, the Kα edge was measured whereas for In Lα edge was measured. (c,d) TEM image of InP QDs and the In0.6Ga0.4P alloy QDs. (e) Experimental Small-angle X-ray Scattering curves (open squares) and the fits (black lines) for InP and In0.6Ga0.4P QDs. Inset shows the size distributions extracted from the fits.

We did not observe a significant change in the size of QDs upon cation exchange. Figures 3c and 3d show TEM images of the starting InP QDs and the resultant In0.6Ga0.4P QDs obtained after cation-exchange at 410°C for comparison. Small angle X-ray scattering (SAXS) analysis was employed to quantitatively study the change in the size and size distribution of the QDs upon cation exchange (Figure 3e). The average size of In0.6Ga0.4P QDs shrunk by ~3% which is 10 ACS Paragon Plus Environment

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consistent with the change in volume expected due to the smaller unit cell of GaP compared to InP (aInP= 0.586 nm and aGaP= 0.545 nm ). The size distribution of 11 % for the initial InP dots increased to 13.5 % after the cation exchange. Similar results were obtained when cation exchange was performed on InAs QDs. TEM images of In0.5Ga0.5As QDs and the starting InAs QDs are shown in Figure S7. High resolution TEM images of In1-xGaxP and In1-xGaxAs QDs are shown in Figures S8 and S9. The Fourier transform of HRTEM image of an individual alloy QD further supported the shrinkage of unit cell from a = 5.9 Å in InP to a = 5.6 Å in In0.5Ga0.5P QDs (Figure S10). Next, we studied the effect of Ga incorporation on the optical properties of InP QDs. InP QDs shelled with a wide band gap II-VI semiconductor such as ZnSe or ZnS are currently used as the red and green emitters in displays and TVs, however, their photostability, color purity and emission quantum yields are lagging behind the characteristics of the best CdSe-based quantum dots. Alloyed In1-xGaxP QDs are expected to show continuously blue shifted absorption with increasing Ga content as compared to InP QDs of the same size owing to the larger band gap of GaP (Eg,bulk = 2.3 eV(indirect)/2.77 eV(direct)) as compared to InP (Eg,bulk = 1.34eV, See Figure 1c).

Figure 4a shows the absorption spectra of In1-xGaxP QDs synthesized at different

temperatures. The excitonic features were continuously blue shifted for In1-xGaxP alloy QDs with increasing x-values. No blue shift of absorption was observed when InP QDs were annealed in molten salts without GaI3 (Figure S11). Similar blue shifts were observed upon alloying in InP QDs of all sizes (Figure 4b and Figure S12). The extent of blue shift was found to vary for different QD sizes and was not linearly correlated to the % of Ga in the lattice, which can be attributed to the band-bowing effect in ternary In1-xGaxP.46



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Figure 4. (a) Absorption spectra of initial InP QDs (black), and In1-xGaxP QDs synthesized from InP QDs at different temperatures. (b) Absorption Spectra InP QDs with different sizes (dotted lines) and In1-xGaxP QDs (solid lines) obtained after cation exchange reactions at 400°C. (c) Absorption and Emission spectra of representative size-selected In1-xGaxP QDs showing Stokes shifts of the emission bands. The full-width half maximum of the emission bands was 48 nm (green), 50 nm (orange) and 51 nm (red). (d) Comparison of Molar extinction coefficients (per particle) of InP and In0.6Ga0.4P QDs with similar emission spectra shown in the inset.

The absorption features for the alloy QDs are generally slightly broader as compared to the starting materials which can be attributed to two factors (1) there may be some heterogeneity in the distribution of Ga in the ensemble and (2) a slight change in size distribution is also observed after cation exchange. We believe that both these factors can be mitigated by mechanical stirring of the reaction mixture during the high temperature cation exchange. Further optimization and scale up of this process should also result in better size dispersions. It should also be noted here that broad absorption features have previously been observed for other QDs with ternary 12 ACS Paragon Plus Environment

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compositions.25, 47 We resorted to mild size selective precipitation to partially eliminate the effect of ensemble heterogeneity on the optical properties of the alloy QDs. Size selective precipitation allowed us to separate the particles into smaller batches with tighter size distributions and narrower excitonic features. The absorption spectra of a size-selected fraction of In0.6Ga0.4P QDs and InP QDs with very similar mean size are shown in Figure S13 for comparison. The absorption features were slightly broader for the alloy QDs as compared to the binary phase which can be attributed to either heterogeneity in Ga incorporation or intrinsic differences in the exciton fine structure and electron phonon coupling parameter. As-synthesized In1-xGa1-xP QDs also showed band-edge emission which was blue shifted as compared to the starting InP QDs (Figure S14). Figure 4c shows representative emission spectra of alloy In1-xGa1-xP QDs with their corresponding absorption spectra. The full width at half maximum (fwhm) for the PL band was near 50 nm for all size ranges which is slightly larger than what can now be achieved for state of the art InP QDs.48 It must be noted here that the PL fwhm for starting InP QDs were also ~50 nm.

The Stokes shift for In1-xGa1-xP QDs is

comparable to that obtained for InP particles emitting at similar wavelengths. We also estimated the molar extinction coefficient of In0.6Ga0.4P alloy QDs and compared it to InP QDs with a similar emission spectrum as that of the alloy QDs (Figure 4d, See Table S4 for concentration estimation). The extinction coefficient per particle for the alloy QDs was found to be significantly higher than that of InP QDs in the blue spectral range. This is expected since the extinction coefficients of QDs scale linearly with the number of unit cells which is higher for InGaP than InP for the same QD size.49-50 The absorption cross-section at 450 nm for the alloy QDs with emission maxima centered at 576 nm was found to be 1.5 times that of InP QDs



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emitting at the same wavelength. This is of great significance for display applications where blue light is used to excite green and red emitting QD phosphors. The emission quantum yields of In1-xGa1-xP QDs could be significantly enhanced upon shell growth. Core-only alloy QDs showed emission quantum yields in the range of 1-5% which increased to 46% upon ZnS shell growth (Figure 5a). Shell growth could be tracked by monitoring absorption below 350 nm (Figure S15). A type-I band alignment is expected for these core-shell QDs which is evidenced by the lack of substantial red-shift upon shell growth. We could prepare In1-xGa1-xP /ZnS core shell QDs emitting in the range of ~495 nm to ~640 nm with quantum yields in the range of 30-40% routinely observed across the size range (Figure 5b). The incorporation of 50% Ga in InP QDs should reduce the lattice mismatch with the ZnS shell from 7.5% to 4% (Figure 1b) which can substantially alleviate interfacial strain between the core and the shell. The X-ray diffraction pattern of In0.6Ga0.4P/ZnS QDs is shown in Figure 5c. HRTEM images showing lattices fringes are given in the inset (See Figure S16 for a large area TEM image). To understand the effect of Ga incorporation on the homogeneous width of the excitonic transitions of QDs, we measured photoluminescence excitation (PLE) spectra of In0.5Ga0.5P/ZnS and InP/ZnS core shell nanoparticles of similar sizes (Figure 5d). PLE spectra allow us to deconvolute the homogenous and inhomogenous contributions to the absorption spectra of QD ensembles. We collected PL in a narrow (2 nm) band while scanning the excitation wavelength to reconstruct the absorption spectrum of QD species emitting at a particular wavelength.51 This way we observed significantly narrower PLE spectra for In0.5Ga0.5P /ZnS QDs in comparison to InP/ZnS QDs of similar sizes indicating that homogenous linewidths of the core shell QDs with alloy cores may be fundamentally narrower than InP/ZnS core shell QDs. The broadening of 14 ACS Paragon Plus Environment

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PLE spectra for InP/ZnS core shells can be attributed to the lattice strain in these systems, which is minimized in our alloy system. Additionally, a graded composition of the In1-xGaxP core could also explain this observation. The tighter GaP lattice may minimize the diffusion of Zn2+ into the QD core thereby reducing disorder related Stokes shift. A series of PLE spectra collected at different positions of the emission band showed a significant distribution of transition energies in the ensemble indicative of heterogeneity in the size dispersion (Figure S17).

Figure 5. (a) Absorption and PL spectra of In0.5Ga0.5P QD core (black solid and dashed curves) and In0.5Ga0.5P/ZnS core-shell QDs (red solid and dashed curves). The PL spectra depict the relative change in PL intensity upon shell growth. xGaxP/ZnS

(b) Representative PL spectra of In1-

core shell QDs extending the range between ~ 490 nm and 640 nm showing the range

of emission wavelengths. Inset shows photographs of dilute solutions of QDs illuminated by UVlamp (c) X-ray diffraction pattern of In0.6Ga0.4P/ZnS QDs. Inset shows HR-TEM image of individual In0.6Ga0.4P/ZnS QDs showing clear lattice fringes for the core-shell particles. (d) A comparison of photoluminescence excitation (PLE) spectra for InP/ZnS QDs (blue) and In0.5Ga0.5P/ZnS QDs (red). The corresponding PL spectra are shown in dashed lines. PLE was measured at the corresponding emission maxima with a slit width of 2 nm. 15 ACS Paragon Plus Environment


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For the technological implementation of QDs in LEDs, display panels and solar concentrators, the retention of luminescence efficiencies at elevated temperatures (up to 150°C for LEDs with on-chip color conversion) is an important requirement.42,

52-54

The loss in luminescence

efficiency at higher temperatures is typically attributed to thermally activated trapping of photoexcited carriers.53 Although luminescence retention at high temperatures is significantly enhanced upon shell growth, factors like core size, synthesis temperature and interfacial strain are known to play an important role in determining the thermal stability of photoluminescence.54 We examined the effect of Ga incorporation on the thermal stability of photoluminescence in InP/ZnS QDs. InP/ZnS QDs and the In0.6Ga0.4P/ZnS alloy QDs with similar core sizes were integrated in a crosslinked poly-(laurylmethacrylate) matrix (Figure S18) and their PL was measured at elevated temperatures upon excitation with a 473 nm laser (Figures 6a and 6b) (See supporting information for experimental details). Whereas the PL of InP/ZnS QDs decreased drastically above 100°C and reduced to ~30% of the initial intensity upon heating to 150°C, consistent with previous reports42, the PL of our unoptimized In0.6Ga0.4P/ZnS QDs only decreased to ~60% of the initial intensity upon heating to 150°C (Figure 6c). The PL intensity recovered to almost their initial value upon cooling back to room temperature in both cases indicating the better thermal stability of the alloy QDs at elevated temperatures (Figure S19). Similar results were obtained when temperature dependent PL studies were performed in solution (Figures 6d and S20). Interestingly In0.6Ga0.4P/ZnS QDs retained photoluminescence even at temperatures up to 280°C! (See inset in Figure 6d) We attribute this significantly better performance of the alloy QDs to the reduced lattice mismatch and the resultant low strain at the core-shell interface.


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Figure 6. Temperature dependent PL spectra of (a) In0.6Ga0.4P/ZnS QDs and (b) InP/ZnS QDs integrated in a matrix of cross-linked poly (lauryl methacrylate). (c) Change in integrated PL area with increase in temperature for In0.6Ga0.4P/ZnS QDs and InP/ZnS QDs immobilized in a polymer matrix. Insets show photographs of QD-polymer composites (d) Change in integrated PL area with increase in temperature for In0.6Ga0.4P/ZnS QDs and InP/ZnS QD solutions in decane. Inset shows a photograph of the hot (280°C) reaction flask containing In0.6Ga0.4P/ZnS QDs illuminated with a 405 nm laser pointer.

The optical properties of In1-xGaxAs alloy QDs were qualitatively similar to In1-xGaxP QDs. A continuous blue shift of the absorption edge was observed with increasing Ga content in the alloy QDs (Figure 7a). The excitonic features of the alloys were broader as compared to the initial InAs QDs. Strong band-edge luminescence could be seen upon growing a shell of CdS or ZnSe 17 ACS Paragon Plus Environment


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on In1-xGaxAs (Figure 7b and S21). Shell growth was accompanied by a slight red shift of the excitonic band, likely due to the leakage of the electron wavefunction into the shell. The highest quantum yield of 9.8% was obtained for In0.5Ga0.5As/CdS QDs with emission centered at ~860 nm (Figure 7b). PL intensity was weaker for the alloy QDs with ZnSe shells, likely due to a larger lattice mismatch with the ZnSe shell and unoptimized shell growth conditions. The emission wavelength could be tuned in the biological tissue transparent window of ~750 nm 950 nm by simply varying the alloy composition (Figure S22). InAs QDs are being actively pursued as NIR emitting probes for in-vivo biological imaging.13, 17 A potential disadvantage of InAs QDs for these applications is that InAs QDs have to be smaller than ~3 nm in size to emit in the window of ~700nm - 950nm which makes them rather unstable. Alloying of Ga in InAs QDs affords larger sized QDs emitting in this region. We estimated the size of In0.5Ga0.5As QDs with an excitonic transition around ~800 nm to be ~ 4 nm (Figure S23). We also measured 3D PL contour maps on luminescent In0.5Ga0.5As/CdS QDs to estimate the effect of size heterogeneity on their absorption and emission properties (Figure S24). The diagonal elongation of spectra features in these PL intensity maps shows that the sample consists of an ensemble of QDs with a range of transition energies. Slicing of these maps gave a series of PL excitation spectra which showed narrow excitonic transitions (Figure S25). Similarly PL line narrowing experiments allowed us to selectively excite only a fraction of QDs in the ensemble which showed narrow PL spectra (Figure S26). Both these experiments demonstrate that the broad emission linewidths in the alloy QDs are not inherent to the material itself but is a manifestation of residual particle size distribution and can be improved by further optimization of the synthesis conditions.


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Figure 7. (a) Absorption spectra of initial InAs QDs, and In1-xGaxAs QDs synthesized from InAs QDs at different temperatures. (b) Absorption spectra (solid lines) of In0.5Ga0.5As and In0.5Ga0.5As/CdS core-shell QDs and PL spectrum (dashed) of In0.5Ga0.5As/CdS core-shell QDs.

Conclusions In summary, we present a novel approach to high temperature colloidal chemistry by using molten inorganic salts as solvents. We show that InP and InAs QDs can be transformed into In1xGaxP

and In1-xGaxAs QDs by performing cation exchange reactions in molten salts at high

temperatures without a significant change in their morphology. To the best of our knowledge, this work is the first report of cation exchange reactions in covalent III-V compounds. Both In1xGaxP

and In1-xGaxAs QDs show blue shifted absorption compared to the starting materials and

strong band edge emission can be observed from the alloy QDs. Nanocrystal phosphors based on InP/ZnE (E=S/Se) core shell structures are not yet on par with CdSe based core/shells in terms of both emission linewidth and quantum yields. This has been attributed to the incorporation of Zn into the InP lattice and significant interfacial strain between the III-V and II-VI core.55 Creation of a graded III-V/II-VI interface can be useful to alleviate these issues. We show that the ternary In1-xGaxP QDs shelled with ZnS are potentially superior to InP/ZnS QDs in terms of their their 19 ACS Paragon Plus Environment


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optical properties. In1-xGaxAs based emitters should be evaluated as potential candidates for bioimaging applications in the important NIR range of 750 nm -1000 nm. We believe that this work not only shows the potential of molten salts as a novel platform for the synthesis of strongly covalent systems in a homogenous colloidal form but also demonstrates the advantage of cation exchange reactions as a lever for band-gap engineering of technologically relevant semiconductor QDs. Experimental Section All manipulations with molten salts were performed in a nitrogen filled glove box with moisture and oxygen levels under 0.1ppm. Ultra-dry salts were used for all experiments. The synthesis of InP and InAs QDs was performed based on reported protocols.15-16, 56 Details of these syntheses are given in supporting information. Details of characterization techniques are also provided in the supporting information. Ligand Exchange on InP and InAs QDs. The purified InP and InAs QDs (~0.3 mmol QDs) were transferred to the polar formamide (FA) phase using (NH4)2S as inorganic capping ligands. 150 µL volume of 40-48% aq. (NH4)2S solution in 10 mL FA and QDs suspended in toluene were stirred together for 20 min to completely transfer the particles to the polar FA phase. The organic phase was removed and fresh toluene was added and the biphasic mixture was stirred for another 15mins. This process was repeated thrice to completely remove all the organic ligands. The particles were colloidally stable in FA. The particles could be precipitated using excess CH3CN and dried as powders. These powders were further used for dispersion in the molten salts.


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Alternatively, the particles could be transferred from the FA phase to toluene using didodecyldimethylammonium bromide (DDAB) as the phase transfer agent. This ligand decomposes cleanly into gaseous products via Hoffman elimination leaving no organics behind.38 The toluene phase containing QDs was transferred to a centrifuge tube and precipitated with ethanol to get rid of excess ligands and re-dispersed in 2-3 mL of toluene. This solution was used for dispersion in molten salts. Absorption spectra before and after the ligand exchange were measured in toluene. Bare InP QDs were prepared by stripping with HBF4 using previously reported protocols.37 Bare InP QDs were used as powders for dispersion in the molten salts.

Dispersion of InP and InAs QDs in Molten Salt Matrix and Cation Exchange into In1-xGaxP and In1-xGaxAs QDs. A eutectic mixture of CsBr:LiBr:KBr (25:56.1:18.9 mol % , (melting point 236° C)) was taken in a vial and heated to 250° C under inert atmosphere until a complete liquid phase was formed . The molten salt was cooled to r.t. and grinded into a fine powder. ~0.3 mmol InP/InAs QD powders capped with S2- ligands were then added as powder or as a toluene solution (see section above) to the finally grinded eutectic mixture and heated to 275° C under stirring for a few hours until a stable solution was obtained. Similar protocols were used for the dispersion of QDs in other molten salts. For cation exchange, ~4-8 molar equivalents of GaI3 (0.5g -1 g GaI3) was added to the QD/molten salt dispersion as a source of Ga3+ cations. GaCl3 or GaBr3 could also be added as a source of Ga3+, however best results were obtained with GaI3 due to its higher boiling point. The mixture was then further heated at 300° C for 1 h to completely homogenize the QDs



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and dopant salt. The mixture was cooled to room temperature and then transferred to a furnace where it was further heated at a desired temperature (380°C -500°C) for 1 h in N2 atmosphere. The mixture was cooled to room temperature and salt matrix was dissolved using excess FA. The cation exchanged QDs were centrifuged. The QDs were washed twice with FA to completely remove the salt matrix. Finally, the QDs were re-dispersed in FA using (NH4)2S (~100 µL in 10 mL FA) and transferred to toluene using DDA+ as the counter-ion and used for further characterization. This surface capping procedure is similar to the one described above. Size selection of the crude solution in toluene into a desired number of fractions was performed by sequential precipitation with an appropriate amount of ethanol. ZnS Shell growth on In1-xGaxP QDs In a 50 mL 3-neck flask, 0.4 mmol of Zn (OAc)2 and 1 mmol of Oleic acid were mixed in 6 mL ODE. The solution was heated to 120°C for an hour and cooled to r.t. A toluene solution of the cation exchanged and size selected In1-xGaxP/S2-/DDA+ QDs was injected into this solution. Toluene was first evaporated under vacuum at 60°C and the solution was then heated to 280°C under N2. 0.3 mmol S in 3 mL TOP was then injected into this solution at the rate of 1 mL/h using a syringe pump. The reaction temperature was ramped to 300°C, 30 min after the syringe pump injection began. The reaction mixture was cooled to r.t. after the reaction was completed and the core-shell QDs were washed using toluene/ethanol as the solvent and non-solvent. Shell growth on In1-xGaxAs QDs The protocol for shell growth on In1-xGaxAs QDs was similar to the protocol for ZnS shell growth on In1-xGaxP QDs. For CdS shell growth, pre-synthesized Cd-oleate was used as the Cd


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precursor. The reaction temperature was held at 240°C. For ZnSe shell growth, TOP:S was replaced with TOP:Se , rest of the protocol was similar to ZnS shell growth. Preparation of QD/Polymer composites The polymer composites containing InP/ZnS and In1-xGaxP/ZnS QDs were prepared according to a reported protocol.52 A stock solution of the monomer was prepared by mixing lauryl methacrylate (80 wt %) and cross-linker ethylene glycol dimethyacrylate (20 wt %) at room temperature. A toluene solution of QDs was precipitated using ethanol and re-dispersed in this mixture in a low concentration. For polymerization, 0.3 wt% of AIBN (Azobisisobutyronitrile) was added as a thermal initiator to this mixture and the combination was heated to 70°C overnight in a 4mL vial under inert atmosphere.

ASSOCIATED CONTENT Supporting Information Experimental Details, Characterization techniques, Estimation of alloy composition, Quantum Yield and Molar Extinction coefficient Estimation, TEM images, ICP-OES analysis, spectral data, photographs. This material is available free of charge via the Internet at http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author [email protected] Notes The authors declare no competing financial interests.



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ACKNOWLEDGMENTS We would like to thank Dr. Justin Jureller for helping with high-temperature PL measurements and Dr. Igor Coropceanu for help with Python codes. We also thank Eric Janke, Margaret Hudson and S. Nitya Sai Reddy for helpful discussions and comments on the manuscript. This work was supported by the National Science Foundation under award number DMR-1611371, by the Department of Defense (DOD) Air Force Office of Scientific Research under grant number FA9550-15-1-0099, and by University of Chicago Materials Research Science and Engineering Center, which is funded by NSF under award number DMR-1420709. E. D. was supported by the Arnold and Mabel Beckman foundation through Beckman Scholars Program.

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