Article pubs.acs.org/cm
Colloidal Synthesis of InSb Nanocrystals with Controlled Polymorphism Using Indium and Antimony Amides Maksym Yarema†,‡ and Maksym V. Kovalenko*,†,‡ †
Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland EMPA-Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland
‡
S Supporting Information *
ABSTRACT: We report a new synthetic pathway for growing monodisperse colloidal indium antimonide nanocrystals. We propose that highly reactive element-nitrogen bonded precursors, such as In and Sb amides, may provide required nucleation and growth kinetics for the formation of uniform colloidal nanocrystals of InSb. Size-dependent absorption and emission spectra of InSb nanocrystals in the near-infrared region of 1500−2000 nm point to semiconductor behavior with quantum confinement. Furthermore, we demonstrate zinc blende/wurzite polymorphism and polytypism of InSb nanocrystals, conveniently controlled by the In/Sb molar ratio of precursors. This amide-based synthesis route may open new opportunities for designing near- and mid-IR active III−V semiconductor nanostructures. KEYWORDS: nanocrystals, quantum dots, colloidal synthesis, infrared materials, III−V semiconductors
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INTRODUCTION Synthesis of high-quality III−V colloidal nanocrystals (NCs) represents one of the major challenges in the chemistry of colloidal quantum dots.1 In the family of indium pnictides, significant progress has been achieved only for InP NCs emitting in the visible spectral region and for InAs NCs active in the near-infrared (near-IR).2 Apart from one very recent report on colloidal InSb quantum dots,3 this material remains largely unexplored. Esteemed physical characteristiscs of the bulk InSb, such as extremely high room-temperature electron mobilities up to 78 000 cm2/V·s, narrow direct band-gap energies of 0.18 eV (300 K),4 and a very large excitonic Bohr radius of ∼70 nm,5 make the solution-processable quantum dots of InSb highly promising for nanoelectronic and infrared devices. InSb colloidal quantum dots can be considered as a viable low-toxic alternative to Pb and Hg chalcogenide NCs, especially at infrared wavelengths beyond 2000 nm. To date, only HgTe NCs and PbSe NCs were reported to exhibit photoluminescence beyond 2000 nm and up to the mid-IR wavelengths of 5 μm (HgTe).6 The history of HgTe- and PbSe-based NCs is quite illustrative: it took a decade to develop the synthesis methodologies that fulfill multiple requirements, such as chemical stability, optimal surface chemistry, and integration into solid-state devices.6a,b,7 Yet, for InSb, the state of the art is still at the very early stage of finding the appropriate liquidphase reaction conditions. Herein, we report our findings toward the optimal choice of precursors and demonstrate monodisperse colloidal InSb NCs with controlled poly© 2013 American Chemical Society
morphism using element-nitrogen bonded precursors, such as In and Sb amides (Figure 1).
Figure 1. Schematic illustration of synthetic pathways to colloidal InSb nanocrystals and nanorods. TOA, trioctylamine; TOP, trioctylphosphine.
The lack of robust liquid-phase syntheses of III−V nanomaterials can be primarily attributed to the strongly covalent character of bonding in respective tetrahedrally bonded III−V compounds, most pronounced for GaAs.1 As a consequence, slow crystallization requires higher reaction temperatures or longer growth times, both causing significant size broadening of the resulting III−V NCs.1,8 In this regard, many III−V compounds are reminiscent of tetrahedral covalent lattices of group-IV elements (C, Si, Ge), all requiring high Received: January 25, 2013 Revised: March 31, 2013 Published: April 3, 2013 1788
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Figure 2. (A−C) Low- and high-resolution TEM images of dot-shaped InSb NCs prepared in TOP and the corresponding size distribution histogram. Interplanar distance in (B) corresponds to the theoretical value for {321}-oriented bulk InSb. (D, E) Typical low-resolution TEM image and size distribution histogram of rod-shaped InSb NCs grown in TOP. (F−I) Dot-shaped InSb NCs grown in TOA with the mean size controlled by the growth time (5−15 min). adecane (anhydrous, ≥99%), oleic acid (tech. 90%), diethyl ether (anhydrous, ≥99.7%), and tetrachloroethylene (anhydrous, ≥99%) were purchased from Sigma-Aldrich. Trioctylphosphine (TOP, 97%) was obtained from Strem. Oleic acid was dried under vacuum at 110 °C for 1 h; Sb[NMe2]3 was redistilled at 60 °C (0.7 Torr). Other reagents were used as received. Indium and antimony precursors as well as TOP and all anhydrous solvents were handled air-free at all times (glovebox and Schlenk line). Synthesis of In[N(SiMe3)2]3. Indium tris[bis(trimethylsilyl)amide] was prepared according to Bürger et al.14 InCl3 (6.7 mmol) was dissolved in refluxing diethyl ether (80 mL), followed by the dropwise addition of 20 mmol of LiN(SiMe3)2 dissolved in 40 mL of diethyl ether. The reaction mixture slowly turned turbid due to the formation of insoluble LiCl. Solution was left refluxing for 24 h, and then filtered and vacuum-dried. The yield of pale yellow In[(NSiMe3)2]3 is 80−90%. To further purify, In[N(SiMe3)2]3 was recrystallized three times from diethyl ether at −10 °C, yielding a white crystalline product. TOP-Based Synthesis of InSb NCs. To prepare 8 nm InSb NCs, In[N(SiMe3)2]3 (0.3 g, 0.5 mmol) was mixed with Sb[NMe2]3 (50 μL, 0.25 mmol) and 5 mL of TOP in a glovebox. This slurry was then transferred to the Schlenk line with the syringe and carefully heated to 200 °C at a rate of 10−15°/min. After 5 min at 200 °C, the mixture was cooled down and purified by a standard solvent/nonsolvent precipitation procedure, using tetrachloroethylene and acetone as solvent and nonsolvent, respectively. Oleic acid (0.2 mL) was added to the crude solution to replace weakly binding TOP molecules and to provide long-term colloidal stability. For obtaining InSb NCs with a rodlike morphology, In NCs, synthesized according to ref 16, were introduced into an identical In−Sb precursor mixture as described above. The mixture was rapidly heated to 160 °C and then kept at this temperature for 10−20 min, followed by the same purification procedure as that for InSb dots. TOA-Based Synthesis of InSb NCs. TOA (10 mL) was dried under vacuum at 110 °C for 1 h, then backfilled with nitrogen and heated to 250 °C. At this temperature, a mixture of In[N(SiMe3)2]3 (75 mg, 0.12 mmol) and Sb[NMe2]3 (12.5 μL, 0.06 mmol) in hexadecane (6 mL) was swiftly added via syringe. After hot injection, the reaction mixture was allowed to cool down to 200 °C and kept for 5−15 min, followed by the same purification procedure as that described above for TOP-based synthesis. Optionally, additional injections of antimony precursor solution (containing 0.06 mmol of Sb[NMe2]3 in 1 mL of hexadecane) can be carried out 3 min after the first injection in order to equate the molar ratio between In and Sb.
temperatures for crystallization. Another difficulty is the rather poor choice of suitable precursors, further limiting the experimental access to controlling the nucleation and the growth. In a single source precursor, both elements can be incorporated in a desired 1:1 ratio.9 However, this approach had so far yielded very limited success for III−V NCs.9b,c,f Furthermore, the reaction kinetics is expected to considerably differ when two separated monatomic precursors are used. In fact, most developed state-of-the-art methodologies employ highly reactive organometallic precursors, such as trimethylsilyl (TMS) derivatives, P(TMS)3, and As(TMS)3.2c,e,8,10 The P(TMS)3-based synthesis made InP NCs a viable alternative to Cd-chalcogenides in terms of bright and tunable emission in the visible spectral region.2c,e,10b,c The analogous As(TMS)3 precursor had been used for high-quality InAs NCs, luminescent in the near-IR spectral region.2c,10d,e The Sb(TMS)3 is by far less stable than P and As analogues, and had very limited utility in the synthesis of InSb.10f,11 Similar stability trends have been observed for EH3 or Na(K)3E as pnictogen precursors.1,2d,12 We, therefore, conclude that inappropriate reactivity of antimony precursors remained the major obstacle toward synthesis of monodisperse InSb NCs.10f,g,13 To find an antimony source with the reactivity in-between polar and unreactive SbCl3 or Sb(RCOO)3 from one side and covalent, but unstable, Sb(TMS)3 and SbH3 from the other side, we explored alkylamides of Sb. In particular, tris(dimethylamido)antimony, Sb[NMe2]3, is an attractive candidate owing to its commercial availability from common vendors as a precursor for metal−organic chemical vapor deposition (MOCVD). For instance, Sb[NMe2]3 had been previously implemented in CVD grown of epitaxial InSb at temperatures as low as 285 °C.15 For a successful synthesis of InSb NCs, we combined this precursor with indium tris(silylamide) (Figure 1).
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EXPERIMENTAL SECTION
Chemicals and Solvents. Indium(III) chloride (99.99%), lithium bis(trimethylsilyl)amide (LiN(SiMe3)2, 97%), tris(dimethylamino)antimony (Sb[NMe2]3, 99.99%), trioctylamine (TOA, 98%), hex1789
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Characterization. Transmission electron microscopy (TEM) images and electron diffraction patterns were acquired using a Philips CM30 TEM microscope operated at 300 keV; EDX analysis was performed using an Hitachi S-4800 SEM microscope, equipped with a silicon EDX detector. X-ray diffraction patterns were collected by a Stoe Stadi P powder X-ray diffractometer. UV/vis/IR absorption spectra were collected using a Jasco V670 spectrometer. Photoluminescence spectra were acquired at room temperature with a Fluorolog-3 spectrometer (Horiba Jobin-Yvon) equipped with a 808 nm laser diode as excitation source and a liquid-nitrogen-cooled InGaAs photodetector.
considerably narrower than those in the TOP-based protocol. In a typical TOA-based synthesis, In[N(SiMe3)2]3 and Sb[NMe2]3 precursors dissolved in anhydrous hexadecane were swiftly injected into TOA at 250 °C. The temperature was maintained at 200 °C for 3−20 min. It should be noted that TEM images of the largest InSb NCs (Figure 2G,H) contained areas with nonuniform contrast, which can be attributed to inhomogeneous diffraction contrast due to polycrystallinity. TOA-synthesized InSb NCs are equally susceptible to oxidation, as in the case of TOP-prepared NCs. Polymorphism and Polytypism. In II−VI compound NCs, particularly in CdSe-based NCs, zinc blende/wurzite (ZB/WZ) polymorphism is well-documented and has been carefully studied due to the crucial role it plays in controlling the shape and optical and electronic characteristics of NCs.18 In the case of CdSe, the crystal structure is primarily governed by the growth temperature18e,19 and through the effect of capping ligands.18c,20 Another example is copper−indium sulfide NCs, crystallizing in ZB and WZ crystal structures depending on the surfactant used.21 In bulk InSb, the solely cubic ZB phase is stable at ambient conditions. However, Semiletov and Rozsival reported hexagonal WZ-InSb,22 studying InSb thin films by electron diffraction. We observed pure phase ZB and WZ-InSb NCs (polymorphism, Figure 3A,C,D,F) as well as mixed-phase InSb NCs
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RESULTS AND DISCUSSION Our initial tests also showed that indium carboxylates, In(RCOO)3, lack reactivity when combined with Sb[NMe2]3, even though they proved to be optimal for InP and InAs NCs.2c,d We, therefore, explored covalently bonded compounds, trimethylindium, InMe3, and indium tris[bis(trimethylsilyl)amide], In[N(SiMe3)2]3. The former had been used for a synthesis of colloidal InP NCs10b and for CVD growth of epitaxial InSb,15 whereas the latter has proven its utility in the synthesis of monodisperse metallic indium NCs.16 We found InMe3 to be chemically stable at moderate temperatures, whereas, above 200 °C, the fast reaction between InMe3 and Sb[NMe2]3 uncontrollably leads to polycrystalline, polydisperse, and aggregated InSb particles (Figure S1, Supporting Information). The InMe3−Sb[NMe2]3 system was only slightly improved toward nonaggregated 10−20 nm particles by adding indium stearate as a capping agent, still with poor morphological control (Figure S1, Supporting Information). In contrast, the reaction between In[N(SiMe3)2]3 and Sb[NMe2]3 in weakly coordinating solvents, such as trioctylphosphine or trioctylamine (TOP and TOA, respectively), yielded uniform, sub-10 nm InSb NCs (Figure 2). TOP-Based Synthesis. In TOP, reaction between In[N(SiMe3)2]3 and Sb[NMe2]3 takes place already at 130 °C with, however, a very low reaction rate and low reaction yield. At 200 °C, this reaction was complete within a few minutes (Figure 2A−C). The In-to-Sb molar ratio was used to conveniently control the mean size (Figure S2, Supporting Information). The diameter of InSb NCs can be additionally altered by the growth time (2−10 min) or by the growth temperature (180− 220 °C). InSb NCs exhibit a spherical morphology and high crystallinity (Figure 2A, B). The size distribution was typically in a range of 10−15% (Figure 2C). After purification, InSb NCs form long-term stable colloids in nonpolar solvents, such as hexane or chloroform. The typical yield of InSb NCs is 30− 40%. Under ambient conditions, each nanoparticle is covered by a native oxide shell with the average thickness of 1.2 nm (estimated by TEM, e.g., Figure 2B). After initial oxidation, however, InSb NCs remained stable for months. The TOP-based synthesis can be also adjusted for the anisotropic growth of InSb NCs (Figure 2D, Figure S3, Supporting Information). For that purpose, ex situ synthesized In NCs16 were added to the mixture of In and Sb amides in TOP directly, followed by the heating at 160 °C for 15 min (Figure 2D,E). The resulting 15 × 40 nm rods are tipped with an indium particle at one side (Figure S3, Supporting Information), suggesting the solution−liquid−solid (SLS) mechanism of anisotropic growth.17 TOA-Based Synthesis. The reaction in TOA was carried out at higher temperatures of 200−300 °C. The size of InSb NCs was varied by the growth time (Figure 2F−I) and by the In-to-Sb molar ratio of precursors, while size distributions were
Figure 3. Polymorphism and polytypism of InSb NCs. (A−C) XRD patterns of hexagonal wurtzite-type (WZ), cubic zinc-blende-type (ZB), and mixed WZ−ZB InSb NCs and (D−F) corresponding highresolution TEM images. (G, H) Influence of the In-to-Sb molar ratio of precursors on the crystal structure and size of InSb NCs (illustrated for a case of TOA-based synthesis). 1790
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Figure 4. Size-dependent optical properties of wurtzite-type InSb NCs. (A) Size tunable absorbance spectra and corresponding size distributions estimated from TEM images (inset). (B) Size-dependent band-gap energies of InSb NCs, presented along with theoretical calculations of Efros and Rosen.5 Crosses point to the overall diameter of InSb NCs; circles indicate the actual size of the InSb core after surface oxidation. Same colors in (A) and (B) point to the same sample. (C) Absorption and emission spectra of ∼8.5 nm InSb NCs.
(polytypism, Figure 3B,E). The actual structure-directing factor is the In-to-Sb molar ratio of amide precursors in the reaction mixture. Notably, both polymorphs can be obtained at the same growth temperature in both solvents (TOP or TOA). Although the In-to-Sb precursor ratio has also a pronounced effect on the NC size (Figure 3H), we have not observed any clear evidence of the size dependence in polymorphism. For instance, Figure 3A,C illustrates pure-phase WZ- and ZB-InSb NCs of approximately the same mean crystallite size of 8−9 nm, but grown in two different solvents. The general trend can be best illustrated for NCs grown in TOA at 230 °C for 5 min (Figure 3G, Figure S4, Supporting Information). If the excess of In[N(SiMe3)2]3 is introduced, WZ-type InSb NCs were formed, whereas, at a 1:1 In/Sb molar ratio or at antimonyrich conditions, ZB-type InSb NCs can be synthesized. A further increase of the In/Sb ratio leads to the formation of In metallic phase (Figure S4, Supporting Information). Polytypism, that is, the coexistence of two structures within the same crystallite, is observed at a small excess of In precursor (Figure 3B,E, Figure S4, Supporting Information). In TOP, 2-fold excess of In precursor leads to WZ-InSb NCs (5 min growth at 200 °C, Figure S5, Supporting Information), whereas a 1:1 ratio results in ZB-InSb NCs (1 h growth, Figure 3C). Note that 5 min of growth at a 1:1 ratio in TOP leads to very small NCs with an unresolvable crystal structure. Detailed analysis of XRD patterns by Rietveld refinement confirmed pure-phase ZB and WZ structures (Figure S6, Supporting Information) for 1:1 and 2:1 In/Sb ratios, respectively. Highresolution TEM images (Figure 3D−F) are also fully consistent with conclusions derived from XRD patterns. In contrast to dot-shaped InSb NCs, InSb nanorods exhibit exclusively a ZB crystal structure (Figure S7, Supporting Information). The ZBtype InSb NCs appear to be fully stoichiometric, within the experimental accuracy of EDX analysis (Figure S8, Supporting Information). At the same time, WZ-type InSb NCs contain up to 60 at % of In, which can be attributed to the In surface termination, a common cause for metal excess in sub-10 nm quantum dots, or partial decomposition of the excess indium silylamide at the NC surface. Optical Properties. Because of their much narrower size distributions, we studied optical properties of WZ-type InSb NCs. Purified samples were dispersed in near-IR transparent
tetrachloroethylene. We observed size-tunable absorption by InSb NCs with a single peak/shoulder that can be attributed to the first excitonic transition. The wavelengths of this transition could be tuned from 1250 to 1750 nm (1.0−0.7 eV) by adjusting the average size of InSb NCs from 5.6 to 8.6 nm (Figure 4A,B). The size distributions were in the range of 10− 15%, and no size-selection procedures were applied. Overall, the size dependence and width of absorption features are very similar to those of as-synthesized samples of ZB-InSb NCs with similar size distributions of ∼15%, recently reported by Liu at al. in the Supporting Information of ref 3. To the best of our knowledge, there are no theoretical studies on the InSb band structure with a WZ crystal structure. We, therefore, compared our results to a single available theoretical work on ZB-InSb quantum dots by Rosen and Efros, where an eight-band Pidgeon and Brown model was implemented, which takes into account coupling between valence and conduction bands.5 After subtracting the thickness of the oxidized shell, the corrected sizes indicated in Figure 4B indeed show a similar size dependence of band-gap energy. It should be noted that nearly indistinguishable sizing curves for ZB and WZ polymorphs are systematically reported for CdSe NCs.23 InSb NCs also exhibit band-edge photoluminescence, as exemplarily shown for the largest studied size of 8.5 nm with the emission peak at 1900 nm. The emission intensity is, however, very weak, presumably due to surface oxidation of InSb NCs.
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CONCLUSIONS In summary, we report the convenient and scalable synthesis of colloidal InSb NCs using element-nitrogen bonded precursors of indium and antimony. We find that sub-10 nm InSb NCs exhibit ZB/WZ polymorphism and polytypism, which can be deliberately controlled by the In-to-Sb precursor ratio. Furthermore, obtained InSb NCs exhibit clear semiconductor behavior with strong quantum confinement, such as size-tunable absorption and emission spectra in the near-IR spectral region. We also report monodisperse InSb nanorods for the first time. Further work is needed to extend the optical response of InSb NCs to mid-IR wavelengths and to improve the chemical stability of InSb NCs by overcoating with wider band-gap, stable semiconductors, such as CdTe, CdSe, and GaAs. 1791
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ASSOCIATED CONTENT
S Supporting Information *
Additional TEM images, EDX spectra, and Rietveld-refined XRD patterns (PDF). 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]. Author Contributions
The manuscript was written through contributions of all authors. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Swiss National Science Foundation (SNF, project Nr. 200021_140245), by the Marie Curie Co-fund (Empa postdoc program), and by ETH Zürich. The electron microscopy investigations were performed at the electron microscopy center of ETH Zürich (EMEZ) and at the EMPA Electron Microscopy Center. The authors are grateful to Matt Conley and Maryna Bodnarchuk for synthesizing indium silylamide and to Peter Reiss, Rolf Erni, Daniel Schreier, and Michael Stiefel for helpful discussions.
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REFERENCES
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