A Transmetalation Route for Colloidal GaAs Nanocrystals and

Aubuchon , S. R.; McPhail , A. T.; Wells , R. L.; Giambra , J. A.; Bowser , J. R. Chem. Mater. 1994, 6 ..... Joel Q. Grim , Liberato Manna , Iwan More...
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A Transmetalation Route for Colloidal GaAs Nanocrystals and Additional III−V Semiconductor Materials Jannika Lauth,*,† Tim Strupeit,† Andreas Kornowski, and Horst Weller* Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, D-20146 Hamburg, Germany S Supporting Information *

ABSTRACT: We present a simple solution processed synthesis route for GaAs nanocrystals (NCs) with narrow size distribution and high crystallinity using wet chemical methods and commercially available inexpensive precursors with reduced toxicity. The reaction pathway can be described in three steps, starting with a transmetalation reaction between the gallium(III) halide precursor GaCl3 and the reduction agent n-butyllithium. At elevated temperatures elemental gallium is released in this process and enables the formation of GaAs NCs with magnesium arsenide (Mg3As2) as the arsenic source. We obtained a variety of different III−V semiconductor NCs including GaAs, InP, InAs, and GaP using this transmetalation reaction pathway. KEYWORDS: gallium arsenide, indium phosphide, III−V nanocrystals, colloidal nanoparticles, quantum dots

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INTRODUCTION GaAs. III−V semiconductor compounds, GaAs in particular, can be classified as basic materials in fabricating modern optoelectronic devices and solar cells.1−3 GaAs is a direct band gap material (1.42 eV at 300 K),4,5 showing superior electronic properties to commonly used silicon with an indirect band gap (1.12 eV at 300 K).4 GaAs has a higher electron mobility allowing devices to be operated at frequencies above 250 GHz. Additionally its band gap is much closer to the optimum for reaching best conversion efficiencies at AM 1.5−1 calculated to be 1.34 eV.6,7 However, the use of GaAs has been restricted as there are difficulties in growing and optimizing the material. This, up to now, has made GaAs based solar cells too expensive to be used as standard solar cell material. As well as in solid state research, colloidal GaAs and different other III−V semiconductor NCs have attracted much attention and significant technological interest in the field of NC research in the past decade because of their exceptional properties associated with their intrinsic properties and the quantum confinement. As a result of its large exciton diameter of 19 nm, quantum confinement effects in GaAs should already occur at rather big crystallite sizes.8,9 Nevertheless, difficulties in establishing applicable and feasible synthesis routes have strongly restricted the exploration and utilization of size dependent properties in nanocrystalline III−V materials up to now.8 The relatively high covalent character in the tetrahedral bonded atomic lattice of III−V semiconductors causes problems in effective NC syntheses. The ionic precursor reaction routes used to obtain monodisperse, well-defined II−VI NCs like CdSe10−12 fail for most III−V semiconductor syntheses. This is because bare atoms and ions of the third and fifth main group elements are chemically unstable, so that precursors for III−V syntheses often have to be strongly complexed leading to high temperatures to bring up the activation energy for reaction. A separation of nucleation © 2012 American Chemical Society

and growth processes, as in the case of II−VI NC syntheses,13,14 has so far not been achieved for III−V NC syntheses which, therefore, lead to a broad size distribution of NC compositions.15 However, there are few methods to obtain nanocrystalline GaAs material. Non-wet-chemical ways like metal organic chemical vapor deposition (MOCVD) may result in very uniform and clean GaAs structures without crystal defects,16,17 but highly toxic precursors like arsine and pyrophoric substances like trimethylgallium are used for these assemblies, so that many attempts have been made to find a less toxic and more cost-effective wet-chemical route for high quality NCs. The few existing wet-chemical ways are mainly based on the dehalosylation reaction established by Wells et al. Herein a gallium or indium salt was reacted with highly toxic tris(trimethyl)silylarsine ((TMS)3As) in a high boiling solvent.18 A modification of the dehalosylation reaction by Kher and Wells resulted in nanocrystalline GaAs structures.19 Olshavsky et al. and Uchida et al. synthesized GaAs NCs with (TMS)3As in quinoline.9,20,21 Butler et al. prepared GaAs nanocrystalline material based on the method of Wells et al. and discussed optical properties of the obtained material.22 Janik et al. presented a way to obtain GaAs by using a single source precursor [H2GaAs(SiMe3)]3. Subsequent pyrolysis resulted in the formation of GaAs.23 Malik et al. showed absorption measurements of GaAs NCs synthesized from GaCl3 and As(NMe2)3.24 Special Issue: Synthetic and Mechanistic Advances in Nanocrystal Growth Received: June 22, 2012 Revised: August 19, 2012 Published: September 5, 2012 1377

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GaAs NCs obtained by the described routes lack monodispersity and crystallinity. We therefore present a novel simple synthesis for colloidal GaAs NCs which are highly crystalline and have a narrow size distribution by using a wet chemical approach and reagents with reduced toxicity. Herein different gallium(III) halides are transmetalated with nbutyllithium and subsequently reduced to elemental gallium. The elemental gallium reacts with Mg3As2 to form rather monodisperse and crystalline GaAs NCs. The implementation of the transmetalation reaction pathway is used to obtain a variety of the most conventional III−V semiconductor NCs like GaAs, InP, InAs, and GaP. InP. InP is the most extensively studied III−V semiconductor NC system, and numerous wet-chemical ways have been established to obtain InP NCs.25−29 Most syntheses involve the use of tris(trimethyl)silylphosphine ((TMS)3P) as the phosphorus source, which is highly toxic and very expensive. Allen et al. described mechanistic insights into the formation of InP NCs with (TMS)3P in 2010.30 In 2008 Liu et al. synthesized InP NCs in a coreduction step of In(OAc)3 and PCl3 with superhydride (LiBH(C2H5)3).31 All described InP NC syntheses yielded relatively small NCs with average sizes from 2 to 8 nm. When trying to increase the particles’ diameter, reaction times usually had to be extended to several days at high temperatures and resulted in a broadening of the size distribution. The transmetalation reaction pathway presented within this paper offers an easy, inexpensive and less toxic way to synthesize high quality InP NCs in terms of crystallinity and size distribution. Different indium(III) halides are used as precursors and tri-n-octylphosphine (TOP) serves as solvent, stabilizing agent, and phosphorus source. The properties of TOP as phosphorus source have been described and summarized by Henkes and Schaak.32 Their results show that miscellaneous nanocrystalline metal phosphides can be straightforwardly synthesized by using TOP as reagent. The assumed reaction relies on results made by Chen et al. who describe a catalytic cleavage of the P−C bond in TOP by elemental iron NCs.33 Phosphorus is liberated during this process and diffuses into the iron NCs that convert to iron phosphides. The same reaction pathway is most likely for the formation of a series of metal phosphides including the InP and GaP NCs described within this article. The InP NCs obtained by our transmetalation method exhibit a small size distribution for bigger particle sizes of 8−9 nm. However, to extract the optical features of the material, the NCs have to be further size selectively precipitated. InAs. InAs has great potential in nanoscaled integrated electronic circuits and telecommunication applications in the near-infrared (NIR) region.34−36 The first InAs synthesis was described by Wells et al. who used indium(III) halides and tris(trimethylsilyl)arsine ((TMS3)As) as precursors.37 In 1996 Guzelian et al. obtained broadly size distributed InAs NCs capped with TOP and showed absorption measurements.38 Our group was able to obtain narrow size distributed samples of InAs NCs, showing ordered 2D-arrangement, by size selective precipitation methods.39 Xie and Peng40 and Battaglia and Peng25 improved the InAs NC synthesis in terms of size distribution, crystallinity, and shortening of reaction time by introducing fatty acids as capping agents. In 2009 a synthesis method to obtain InAs NCs by using in situ generated arsine (AsH3) as arsenic precursor was described by J. Zhang and D. Zhang.41

In general InAs NCs are prone to oxidation. This can be prevented growing an epitaxial layer of a different wide band gap material on the InAs core, obtaining core shell materials that emit in the NIR.36,40,42,43 All syntheses presented use highly toxic and volatile (TMS3)As or AsH3 as arsenic source for the reaction. The transmetalation reaction pathway therefore presents a simple way to obtain InAs NCs in satisfactory quality with costeffective methods and precursors with reduced toxicity. GaP. GaP is an indirect band gap semiconductor (2.2 eV at 300 K).4 There are few wet-chemical routes to obtain nanocrystalline GaP material. In 1994 Kher and Wells described one of the first syntheses of GaP applying their dehalosylation reaction pathway.19 Micic et al. used organometallic precursors to synthesize nanocrystalline GaP structures,44−46 and Janik et al. presented a way to obtain GaP decomposing a single source precursor [H2GaP(SiMe3)]3.23 Pan et al. described a low temperature GaP NC synthesis route under atmospheric pressure.47 Kim et al. presented a GaP NC synthesis by thermal decomposition of Ga(PtBu2)3 in trioctylamine and hexadecylamine and obtained monodisperse GaP NCs and elongated GaP rods.48 Gao et al. analyzed the growth process and stability of GaP NCs prepared under solvothermal conditions49 and in 2002 presented an aqueous synthesis method for the material.50 In 2007, Hwang et al. used a sodium naphthalenide reduction method to obtain GaP NCs.51 Most of the syntheses described lead to GaP NCs with a broad size distribution and to GaP agglomerates. We present first results on a GaP NC synthesis using the transmetalation reaction pathway and TOP as reagent, stabilizer, and solvent.

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EXPERIMENTAL SECTION

Synthesis. Materials. All chemicals were used as received without further purification. Gallium(III) fluoride (GaF3), gallium(III) chloride (GaCl3), gallium(III) bromide (GaBr3), gallium(III) iodide (GaI3), indium(III) fluoride (InF3), indium(III) chloride (InCl3), indium(III) bromide (InBr3), indium(III) iodide (InI3), 1-hexadecylamine (HDA), and n-butyllithium (1.6 M in hexane) were purchased from SigmaAldrich, and magnesium arsenide (Mg3As2) was purchased from Alfa Aesar. All these chemicals were stored under inert gas atmosphere (N2) inside a glovebox. The solvents and stabilizing agents 1octadecene (ODE), tri-n-octylphosphine (TOP), and tri-n-octylphosphine oxide (TOPO) were purchased from Merck and respectively Alfa Aesar. General Procedures. All syntheses were carried out under inert gas conditions (N2) using Schlenk techniques or working inside a glovebox. The precipitation of the NCs and all characterization steps were performed under air. For all syntheses a 25 mL three-necked flask was placed in a heating mantle and equipped with a condenser, a thermocouple, and a septum. The particular reaction components were kept under an oil pump vacuum at 120 °C for one hour to remove residual oxygen and water before they were set under nitrogen and the reaction was started. GaAs NC Synthesis. In a standard GaAs NC synthesis, 0.13 mmol of GaCl3 (22 mg) and 0.14 mmol of Mg3As2 (31 mg) were mixed with 1 g of TOPO as the stabilizing agent and 10 mL of ODE as solvent. The reaction mixture was set under nitrogen and heated to 315 °C, then 0.4 mmol of n-butyllithium (0.25 mL of a solution of 1.6 M nbutyllithium in hexane) diluted in 4 mL ODE was added dropwise to the reaction mixture with a syringe pump at a rate of 10 mL/h. The solution rapidly turned greyish during addition of the nbutyllithium. Within 30 min the color changed to brownish red. Upon further progression of the reaction the solution got turbid and turned dark brown. After 90 min the mixture was cooled to 280 °C and kept at this temperature for an additional hour. Subsequently it was cooled to 100 °C, and 10 mL of toluene were added before the crude product 1378

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was centrifuged at 4500 rpm for 10 min to obtain a clear deep brownish supernatant which was separated from the sediment. The NCs were precipitated from the supernatant by adding 10 mL of acetone or methanol and redispersed in toluene. This procedure was performed three to five times. InP NC Synthesis. The above-described procedure was followed, with the exception that 0.13 mmol of InF3 (22 mg) and 22 mmol of TOP (10 mL) were mixed with 1 g of TOPO. InP NCs were obtained at a reaction temperature of 300 °C by the addition of 0.4 mmol of nbutyllithium diluted in 4 mL of ODE at a rate of 5 mL/h. The reaction mixture rapidly turned reddish brown and metallic turbid. The metallic character disappeared after 30 min at 300 °C, and the clear reaction mixture turned dark brown. After an additional hour of refluxing at 300 °C the reaction mixture was cooled to room temperature. InAs NC Synthesis. A total of 0.14 mmol (30 mg) of InCl3, 0.13 mmol (28 mg) of Mg3As2, 5 g of TOPO, and 5 g of HDA as stabilizing agents were mixed and heated to 330 °C for reaction. After reaching this temperature, 0.4 mmol of n-butyllithium diluted in 4 mL of ODE were added at a rate of 5.33 mL/h. Within the first 30 min the reaction mixture turned from dark brown to black. Afterward the solution turned gray and slightly turbid in the end. For a better stabilization of the NCs, 5 mL of TOP were added to the reaction before it was kept at 280 °C for one additional hour. GaP NC Synthesis. A total of 0.12 mmol (56 mg) of GaI3, 22 mmol of TOP (10 mL), and 1 g of TOPO yielded GaP NCs at a reaction temperature of 300 °C. At this temperature, 0.4 mmol of nbutyllithium diluted in 1 mL of ODE were quickly injected into the reaction mixture, which immediately turned bright orange and kept this color within four hours of reaction length. By way of derogation from the standard synthesis procedure the NCs were precipitated directly from the reaction mixture by addition of 10 mL of acetone to the clear solution and centrifugation at 10 000 rpm for 20 min. The NCs were redispersed in toluene and characterized directly from this solution. Structural Characterization. The samples were characterized by high-resolution transmission electron microscopy (HR-TEM) with a Philips CM 300 UT at an acceleration voltage of 200 kV. Energy dispersive X-ray spectroscopy (EDS) was performed with an EDAX DX-4 system installed at the Philips CM 300 UT. For TEM analysis the toluene solution of the NC samples was dropped on a carbon coated copper grid, and excess of solvent was removed with a filter paper. The sample preparation for powder X-ray diffraction (XRD) measurements involved dropping of the NC dispersion in chloroform on a single crystal Si support plate and evaporating the solvent. The Xray diffractograms were measured on a Philips X’Pert diffractometer with Bragg−Brentano geometry and Cu Kα radiation. UV−vis absorption spectroscopy was carried out with a Varian Cary 50Bio UV/vis spectrometer.

The degree of substituted halide ligands in the particular metal salt depends on the molar ratio of n-butyllithium to the corresponding halide used for reaction. All three transition states are likely to coexist, although they are not isolated during reaction. The metal alkyl compounds decompose to form elemental gallium which subsequently reacts with Mg3As2 to form GaAs NCs as described in Scheme 1. Scheme 1. Transmetalation Reaction of GaCl3 Resulting in Elemental Gallium at Elevated Temperatures and in GaAs NCs after Reaction with Mg3As2

Figure 1 shows TEM images of crystalline and rather monodisperse GaAs NCs obtained in a typical synthesis with GaCl3 as precursor. The particles have an average size of 8.4 nm (±1.7 nm) in diameter. The crystal phase of the NCs was characterized by selected area electron diffraction (SAED) shown in Figure 1B and powder XRD (Figure 2A). The SAED shows the characteristic pattern of the cubic phase (F43̅ m) of GaAs with the corresponding lattice spacing of 3.27 Å [111], 2.00 Å [220], and 1.71 Å [311]. EDS confirms the composition of the material is gallium and arsenic nearly 1:1 with a slight gallium excess (see Supporting Information, Figure S1). The samples for powder XRD measurements were precipitated several times to remove residues of unconverted Mg3As2 and byproduct like MgO, which forms in the course of the NC purification from elemental magnesium. A small amount of elemental magnesium is formed during the reaction, because of the reductive conditions supplied by n-butyllithium addition. All reflexes correspond to the common face-centered cubic (fcc) structure of GaAs (JCPDS No. 80-16). Distinctive absorption edges in the absorption spectrum of a typical GaAs NC synthesis shown in Figure 2B are absent. This result is in qualitative agreement with observations made by Butler et al. who describe that absorption features in GaAs are only observable for NCs with sizes less than or equal to 4 nm and who attribute these features to discrete electron transitions in the material.22 When exceeding sizes of 4 nm, GaAs NCs lack spectral features like those similarly examined by Byrne et al.54and Sandroff et al.55 A possible reason for the absence of absorption features in GaAs NCs might be the low exciton binding energy in the material (0.007 eV).22,56 This would mean that absorption features can only be observable at temperatures of 80 K and lower. Molecular Gallium to Arsenic Ratio in the Reaction. An important factor in the GaAs NC synthesis is the molar ratio of the gallium and arsenic precursors. Best crystalline GaAs NCs with smallest size distribution were obtained with a gallium to arsenic ratio of 1:2. A stochiometric gallium to arsenic ratio of 1:1 led to a broader size distribution of the NCs with additional gallium spheres in the reaction mixture. A gallium excess (Ga:As ratio of 2:1) led to a sediment of bulk material composed of GaAs and undefined byproduct.

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RESULTS AND DISCUSSION Three-step Formation of III−V NCs. The general reaction path for the synthesis of III−V NCs is discussed exemplarily for GaAs. The formation of GaAs NCs using n-butyllithium as a reduction reagent occurs in three steps. Every NC reaction starts with a transmetalation taking place, when GaCl3 reacts with n-butyllithium at elevated reaction temperatures. Herein, three intermediate products have been reported (see eqs 1−3). Note that diverse fragmentation modes are possible for RnGaCl3−n compounds.52,53 GaCl3 + RLi → RGaCl 2 + LiCl

(1)

GaCl3 + 2RLi → R 2GaCl + 2LiCl

(2)

GaCl3 + 3RLi → R3Ga + 3LiCl

(3) 1379

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Figure 1. TEM image of crystalline GaAs particles and inset of a single NC (A), corresponding SAED (B), and a histogram of NC size distribution.

the dependency of the sizes, size distribution, and morphology of the NCs on the different precursors. In good correlation with the Hard and Soft Acids and Bases (HSAB) principle introduced by Pearson, the leaving group ability of the gallium(III) halides enhances from fluoride over chloride and bromide to iodide; the bigger the halide, the better its ability to stabilize a negative charge.57,58 This ability leads to the assumption that elemental gallium available for the NC reaction is formed the slowest from GaF3 and the fastest from GaI3. Figure 3 shows the syntheses results with GaCl3

Figure 3. GaCl3 (A), GaBr3 (B), and GaI3 (C) as precursor for the reaction.

Figure 2. (A) Powder XRD pattern of GaAs NCs, vertical lines below indicating the corresponding bulk GaAs reflexes, (B) UV−vis absorption spectrum of a typical GaAs NC synthesis taken after 1 h of reaction time.

(A), GaBr3 (B), and GaI3 (C) as gallium precursors. The formation of GaAs NCs failed using GaF3 as a precursor. The average size of GaAs NCs in Figure 3A is 8.4 nm (±1.7 nm) in diameter. GaAs NCs obtained with GaBr3 as precursor are slightly bigger, showing an average size of 11.6 nm (±2.8 nm, see Supporting Information, Figure S2). Polycrystalline structures of different morphologies like elemental gallium spheres, GaAs nanoneedles, and NCs were obtained by using GaI3 as precursor (see Figure 3C). GaAs Synthesis Reaction Temperature. The influence of the reaction temperature on GaAs NC syntheses was tested for 250, 270, 300, and 315 °C. The best results were achieved at a reaction temperature of 315 °C. Below 300 °C the reaction mixtures turned turbid gray, indicating the formation of

Only if the formation rate of elemental gallium was adjusted with the reactivity of the arsenic source Mg3As2 at temperatures of 300 °C NCs were obtained. If the concentration of elemental gallium in solution was too high, the arsenic source was not able to react with all the gallium available for reaction, which resulted in the formation of elemental gallium spheres. If the release of elemental gallium in solution was too low, the reaction yielded amorphous GaAs agglomerates. Reactivity of Gallium(III) Halides and HSAB Principle. Using the different gallium(III) halides GaF3, GaCl3, GaBr3, and GaI3 as precursors for the GaAs synthesis clearly showed 1380

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(C) fraction 2 with NCs with an average size of 6.3 nm (±1.8 nm, Supporting Information, Figure S9). The associated absorption spectra in Figure 6A indicate that, by smaller crystallite sizes (fraction 2), the solution shows an

elemental, macro-sized gallium with diameters of several hundred nanometers (see Supporting Information, Figure S3). The transmetalation reaction and the subsequent formation of NCs from elemental gallium or indium formed within this process was tested for a variety of different III−V semiconductor NCs. InP NCs. In Figure 4A a TEM image of InP NCs obtained in a typical synthesis with InF3 as precursor is shown. The NCs

Figure 6. (A) UV−vis absorption spectra of a typical InP NC synthesis from size selective precipitation, absorption edge of the monodisperse fraction 2 lying at 680 nm, (B) absorption and fluorescence spectrum of HF-etched InP NCs, emission maximum lying at 639 nm.

absorption edge at 680 nm, whereas the bigger NCs in fraction 1 and the crude product show no distinct absorption features. In another experiment we investigated the fluorescence of the so-prepared InP NCs. The unmodified InP NCs lack emission features due to surface phosphorus vacancies, acting as electron traps. This can be prevented by etching of the colloidal InP with fluorides as shown by Micic et al.60 and Talapin et al.61 Figure 6B depicts the absorption and fluorescence spectra of asprepared InP NCs with a size distribution of 9.2 nm (±3.8 nm) after etching with HF. The emission maximum of the NCs lies at 639 nm, and they show a quantum efficiency of 10% in comparison to Rhodamin 6G. InAs NCs. InAs NCs obtained in a typical synthesis with InCl3 as precursor are crystalline (Figure 7A) and have an

Figure 4. (A) TEM image of crystalline InP NCs with InF3 as precursor and single NC inset, (B) associated XRD pattern, vertical lines below indicating the corresponding bulk InP reflexes.

have a narrow size distribution with an average size of 8.2 nm (±2.7 nm) in diameter. Figure 4B depicts the corresponding powder XRD pattern of the InP NCs. All reflexes are related to fcc bulk crystal patterns for InP (JCPDS No. 73-1983). Depending on the indium(III) halide used for the reaction, either InP NCs with average sizes of 8.2 nm (±2.7 nm) in diameter for InF3 and 9.2 nm (±3.8 nm) for InCl3 (see Supporting Information, Figures S4 and S5) or InP nanoneedles and elongated structures as recently described by Strupeit et al.59 for InBr3 and InI3 were obtained (see Supporting Information, Figures S6 and S7). InF3 seems to show the lowest reactivity for the reaction, which means that every elemental indium seed formed reacts with TOP, which is available for reaction in high excess in the mixture. This leads to rather monodisperse InP NCs in the case of InF3 and InCl3, whereas a high rate of indium release from InBr3 and InI3 results in less uniform NCs and nanoneedles. To the best of our knowledge the InP NCs with average sizes of 8.2 nm in the case of InF3 and 9.2 nm for InCl3 are the biggest NCs with high crystallinity, obtained by relatively short reaction times (usually 90 min) and with low toxic and inexpensive precursors. Size selective precipitation of a typical InP NC synthesis from InCl3 as precursor is depicted in Figure 5 (A) showing the crude product, (B) fraction 1 with NCs having an average size of 8.7 nm (±1.9 nm, Supporting Information, Figure S8), and

Figure 7. (A) InAs NCs obtained with InCl3 as precursor and single NC inset, (B) associated XRD pattern.

average size of 15.7 nm (±4.3 nm, Figure S10 in Supporting Information). Figure 7B shows the XRD pattern of the obtained NCs with slightly broadened reflexes and vertical lines indicating the associated InAs bulk crystal pattern (JCPDS No. 73-1984). Relatively big InAs NCs (15.7 nm ±4.3 nm in diameter) were obtained using the transmetalation reaction pathway with less toxic Mg3As2 as arsenic source. GaP NCs. First results in a GaP NC synthesis using the transmetalation reaction pathway are presented. The GaP NCs obtained are small and crystalline but tend to agglomerate to form bigger composites (see Figure 8A,B). EDS measurements confirm a slight excess of gallium to phosphor (53:47 atomic percentage, see Supporting Information, Figure S11).

Figure 5. Size selective precipitation of InP NCs: (A) crude product, (B) fraction 1, and (C) fraction 2. 1381

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support by the State Excellence Initiative for the Nanotechnology in Medicine cluster is gratefully acknowledged.

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Figure 8. (A) GaP NCs obtained with GaI3 as precursor and (B) high magnification of a GaP NC assembly.

Precipitation steps had to be performed with high centrifugation speed to obtain the GaP NCs, which are air sensitive and tend to decompose after several days under ambient conditions. Two possible reasons for the shape of the NCs can be oriented attachment of smaller GaP NCs to form hollow ringlike structures or a nanoscale Kirkendall effect taking place. Further work will be directed to understand and explain these observations.

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CONCLUSION A novel simple wet chemical way to obtain rather monodisperse GaAs NCs (8.4 nm) in a three-step formation has been presented. The described transmetalation reaction pathway offers an effective way to obtain high qualitiy GaAs and InP NCs in terms of size distribution and crystallinity. InP NCs (8.2 and 9.2 nm) have been synthesized that, to the best of our knowledge, show the best size distribution for bigger sized InP NCs up to now. The principle of the transmetalation reaction pathway has been extended to InAs NCs, which tend to show bigger average sizes (15.7 nm) than most commonly synthesized InAs NCs from (TMS)3As as precursor. First experiments in obtaining GaP NCs have been made, showing the universal use of the transmetalation reaction. The precursors used for reaction are low-cost reactants with a moderate toxicity, offering a cost-effective way to obtain the desired materials. The presented solution-processed synthesis route significantly broadens the scope of possible applications for GaAs and other III−V semiconductor NCs.

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ASSOCIATED CONTENT

S Supporting Information *

EDS measurements of GaAs NCs, size distribution histograms of different III−V NCs syntheses, and representative TEM images of different GaAs and InP NCs syntheses with varying gallium(III) and indium(III) halides. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; horst.weller@ chemie.uni-hamburg.de. Author Contributions †

These authors contributed equally to this work. 1382

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