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Economic Synthesis of High Quality InP Nanocrystals Using Calcium Phosphide as the Phosphorus Precursor
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Scheme 1. Experimental Setup for the Synthesis of InP NCs
Liang Li,† Myriam Protière,‡ and Peter Reiss*,† CEA Grenoble, 17 rue des Martyrs, 38054 Grenoble, France ReceiVed December 13, 2007 ReVised Manuscript ReceiVed February 15, 2008
Semiconductor nanocrystals (NCs) are of great interest for both fundamental research and technical applications because of their unique size dependent properties.1 Compared to most of the II-VI and IV-VI NCs, III-V compounds and in particular group III nitrides and phosphides are generally referred to as “greener” NCs because the constituting elements are more environmentally friendly than Cd, Pb, Hg, or Te. Nevertheless, the studies and applications of III-V NCs are rather sparse as compared to their II-VI analogues, which is principally caused by significant difficulties in their synthesis. In initial synthetic routes,2 the hot-injection method established for cadmium chalcogenide NCs3 was adapted to InP, but longer reaction times (3–7 days) were necessary to yield particles of good crystallinity. Peng and co-workers later reported a new approach, using fatty acids and more recently a mixture of fatty acids and amines as stabilizers instead of TOPO/TOP (trioctylphosphine oxide/trioctylphosphine) and applying a noncoordinating solvent, ODE (1octadecene).4 The use of this medium provided a fast and controllable reaction, yielding high quality InP NCs. Similar results were obtained when organometallic In precursors were used in combination with ester type solvents.5 However, in all cases the use of the expensive and pyrophoric phosphorus precursor P(TMS)3 (tris(trimethylsilyl)phosphine) was mandatory, limiting the scale-up of the synthesis and by consequence the technological applications of InP NCs. PH3 (phosphine) is a widely used reagent in the semiconductor industry for making semiconductor phosphide films such as InP and GaP. In this communication, we present a new * Corresponding author. E-mail:
[email protected]. † DSM/INAC/SPrAM(UMR 5819 CEA-CNRS-UJF)/LEMOH. ‡ DRT/LITEN/DTNM/L2T.
(1) (a) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226–13239. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (c) Jaiswal, J. K.; Mattoussi, H.; Mauro, J. M.; Simon, S. M. Nat. Biotechnol. 2003, 21, 47. (2) (a) Mic´ic´, O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J. J. Phys. Chem. 1994, 98, 4966–4969. (b) Mic´ic´, O. I.; Cheong, H. M.; Fu, H.; Zunger, A.; Sprague, J. R.; Mascarenhas, A.; Nozik, A. J. J. Phys. Chem. B 1997, 101, 4904–4912. (c) Guzelian, A. A.; Katari, J. E. B.; Kadavanich, A. V.; Banin, U.; Hamad, K.; Juban, E.; Alivisatos, A. P.; Wolters, R. H.; Arnold, C. C.; Heath, J. R. J. Phys. Chem. 1996, 100, 7212–7219. (d) Talapin, D. V.; Gaponik, N.; Borchert, H.; Rogach, A. L.; Haase, M.; Weller, H. J. Phys. Chem. B 2002, 106, 12659–12663. (3) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706–8715. (4) (a) Battaglia, D.; Peng, X. G. Nano Lett. 2002, 2, 1027–1030. (b) Xie, R.; Battaglia, D.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 15432–15433. (5) Xu, S.; Kumar, S.; Nann, T. J. Am. Chem. Soc. 2006, 128, 1054– 1055.
synthetic scheme for the preparation of InP NCs based on in situ generated gaseous PH3 from Ca3P2 (calcium phophide). The use of this air-stable phosphorus source dramatically reduces the costs of the InP NC synthesis. Furthermore, if compared to initially proposed procedures,4a,5 larger-sized NCs, emitting in a spectral range of 570–720 nm, can be obtained by the proposed method. For a typical synthesis of InP NCs (“standard reaction”, Scheme 1), 0.1 mmol of indium acetate (In(Ac)3) were mixed with 0.3 mmol of myristic acid (MA) and 6.8 g of ODE in a 50 mL three neck flask (A) equipped with a condenser (E) under inert atmosphere. In a drybox, 13.5 mg (0.15 mmol) of finely ground Ca3P2 powder was loaded into another flask (B), which was then connected to flask A by means of column C, containing P2O5 for the elimination of traces of water in the subsequently produced PH3 gas. Next, the indium precursor was heated to 100–120 °C to obtain an optically clear solution, and both flasks were degassed for 1 h and backfilled with Ar. Reaction flask A was then heated to 250 °C, followed by the injection of 1.5 mL of 4 M HCl into flask B, initiating the production of the PH3 gas.6 Carried by a flow of Ar gas, PH3 gas was bubbled into flask A and reacted with the indium precursor to form InP NCs. In the first two minutes after HCl injection, the observed PH3 production was the strongest. With the goal to induce a short nucleation burst via precursor supersaturation, a stronger Ar flow was used during this period. The color of the reaction mixture quickly changed from colorless to deep red during the first 5–10 min of the reaction. The excitonic peak in the absorption spectra of the prepared InP NCs underwent a bathochromic shift from 570 nm (5 min) to around 600 nm (20 min). An extension of the reaction time did not result in a further spectral evolution because of the consumption of the Ca3P2 precursor. After cooling to room temperature, the NCs were isolated by adding 1 volume equivalent of acetone and 10 equivalents of a CHCl3/methanol (1/1) mixture, followed by centrifugation.4 The resulting precipitate could easily be redispersed in a number of solvents, including (6) CAUTION: PH3 is a toxic gas, and therefore all manipulations have to be carried out in a fume-hood. The use of a trap (D) filled with CuSO4(aq) solution on the outlet of the reaction flask is recommended, assuring the neutralization of unreacted PH3.
10.1021/cm7035579 CCC: $40.75 2008 American Chemical Society Published on Web 03/20/2008
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Figure 2. Left panel: TEM micrograph (JEOL 4000EX, 300 KV) of 3.0 nm (left) and 6.4 nm (middle, right) InP NCs.
Figure 1. UV–vis absorption spectra evidencing the influence of different parameters on the size evolution of InP NCs (reaction time: 30 min): (a) temperature; (b) In/P ratio; (c) In/MA ratio; and (d) overall concentration (*: the precursor ratio in this reaction was In/MA/P ) 1:3:4).
hexanes, toluene, or chloroform. No size sorting procedures were performed for any of the samples presented here. The synthesis method is based on a two-step reaction as summarized in eq 1, involving (i) the in situ generation of PH3 gas from the Ca3P2 precursor upon HCl addition and (ii) the reaction of PH3 gas with the indium precursor, indium myristate, resulting in the formation of InP NCs. (i)
Ca3P2 + 6HCl f 2PH3 + 3CaCl2 (ii)
PH3 + In···MA f f InP
(1)
In contrast to the single injection of P(TMS)3 in the conventional method, the remaining Ca3P2 powder continually and slowly released PH3. Such a continuous addition of precursors assures, similar as in experiments with multiple precursor injections, that NC growth takes place in the “size focusing” regime when attaining larger particles.7 The resulting size distribution of the obtained InP NCs was narrow as indicated by the well-defined excitonic peak in the absorption spectra (Figure 1). CdTe is another prominent example of semiconductor NCs produced by gas injection.8 In this case, H2Te gas is generated from Al2Te3 and bubbled into a solution containing the Cd precursor and stabilizer, using a similar experimental setup. The effects of different experimental parameters on the NCs’ growth were studied, including the reaction temperature, the ratios, and the concentrations of precursors. Figure 1a shows the UV–vis absorption spectra of NCs prepared in a temperature range of 210–290 °C, using otherwise the parameters of the standard reaction. It is evident that with increasing reaction temperature the NCs’ excitonic peak is (7) Peng, X.; Wickham, J.; Alivisatos, A. P. J. Am. Chem. Soc. 1998, 120, 5343–5344. (8) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Hoppe, K.; Shevchenko, E. V.; Kornowski, A.; Eychmüller, A.; Weller, H. J. Phys. Chem. B 2002, 106, 7177–7185.
being bathochromically shifted, which implies the formation of bigger InP QDs. However, for the highest (290 °C) and the lowest (210 °C) temperatures investigated, the excitonic peak becomes broad and featureless. The increase of the absorbance at the excitonic peak, combined with its bathochromic shift, observed in Figure 1b, shows that with increasing P/In ratio not only larger quantities of NCs are formed but also their average size increases. On the other hand, the relatively small difference between the peak position for a wide range of the P/In ratio (from 0.5:1 to 2:1) indicates that the growth rate is limited by the reactivity of the In precursor. The latter is influenced by the amount of the stabilizer (MA), and consequently an increase in the amount of MA leads to a decrease in the reaction yield and in the formation of NCs of a smaller size (Figure 1c). Finally, it has been found that a higher overall concentration of the precursors yields smaller NCs (Figure 1d). Consistent with classical nucleation theory, higher precursor concentrations cause the formation of a larger number of nuclei, resulting in a smaller average size of the resulting NCs.9 Using the initially reported P(TMS)3 based synthesis method in ODE,4a it is difficult to obtain NC sizes exceeding 3 nm (excitonic peak > 600 nm). Larger NCs with the emission shifted to the near-infrared region are of special interest for in vivo biological imaging.10 Figure 1b,d illustrates how the experimental parameters of our method can be adjusted to obtain bigger NCs with an excitonic peak in the range of 650–700 nm (photolumincescence [PL]: 675–720 nm). Although decreasing the precursor concentration shows some influence, as discussed before (Figure 1d), the biggest shift to longer wavelengths can be obtained by increasing the P/In ratio to more extreme values (Figure 1b). In this context it should be pointed out that the obtained emission range of 570–720 nm does not represent any intrinsic upper or lower limit related to the newly developed synthetic scheme. It can be hypothesized that both shorter and longer wavelengths can be accessible by the fine-tuning of the indium precursor’s reactivity, for example, via amine addition as described in ref 4b. Figure 2 shows TEM images of two different sized samples with mean diameters of 3.0 and 6.4 nm, exhibiting a size distribution of approximately 11% and 9%, respectively. The contrast of these images is much lower than in the case of cadmium chalcogenide NCs of comparable size, yet they confirm the low distribution in size and shape of the samples, as already indicated by their absorption spectra. (9) Mullin, J. W. Crystallization, 4th ed.; Elsevier Butterworth-Heinemann: Oxford, 2001. (10) Zimmer, J. P.; Kim, S.-W.; Ohnishi, S.; Tanaka, E.; Frangioni, J. V.; Bawendi, M. G. J. Am. Chem. Soc. 2006, 128, 2526–2527.
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Figure 3. Powder X-ray diffractogram (Philips X’Pert, Co source λ ) 1.789 Å, 50 kV/35 mA).
Figure 3 presents the powder X-ray diffractogram of a typical sample. The peaks can clearly be indexed as originating from the cubic zinc blende structure of InP, and no peaks characteristic for indium oxide are visible. Using Scherrer’s formula, the crystallite size was determined as 2.63 nm, which is close to the mean size determined by TEM (3.0 nm, Figure 2, left). Therefore, it can be concluded that the prepared NCs are of high crystalline quality and consist essentially of monocrystals. The PL quantum yield (QY) of the as prepared InP NCs was, similar as in previous reports,2,4,5 generally inferior to 0.1% (standard: Rhodamine 6G). In addition to the band edge emission (fwhm 50–65 nm), lower energy emission bands arising from trap states could be detected around 730 and 820 nm. Overcoating with a shell of a higher band gap semiconductor is a general way to improve the optical properties of InP NCs.11 InP/ZnS core/shell NCs were synthesized by slowly adding appropriate amounts of the airstable ZnS precursors (zinc stearate and zinc ethylxanthate) to the crude core NC dispersion.12,13 Interestingly, the addition of zinc stearate (without a sulfur source) and heating to 250 °C resulted in a strong PL increase, which we attribute to the passivation of surface P dangling bonds by Zn car(11) (a) Mic´ic´, O. I.; Smith, B. B.; Nozik, A. J. J. Phys. Chem. B 2000, 104, 12149–12156. (b) Protière, M.; Reiss, P. Chem. Commun. 2007, 241, 7–2419. (c) Haubold, S; Haase, M.; Kornowski, A.; Weller, H. ChemPhysChem 2001, 2, 331–334. (12) (a) Protière, M.; Reiss, P. Nanoscale Res. Lett. 2006, 1, 62–67. (b) Protière, M.; Reiss, P. Small 2007, 3 (3), 399–403. (13) The procedure for ZnS overcoating of InP NCs comprised the separated additions of (i) 0.5 mmol of zinc stearate in 4 mL of ODE at 250 °C, keeping this temperature for 4 h, and (ii) 0.125 mmol of zinc ethylxanthate in 2 mL of ODE at 220 °C with 30 min of heating. The first compound led to a PL increase by a factor of 5–10 the second one to a subsequent increase of 2–5 times.
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Figure 4. (a) PL spectra of the as-prepared InP NCs (×5) and of the InP/ ZnS NCs (QY 22%). (b) Normalized PL spectra of a size series of InP/ ZnS NCs (QYs 8–22%). Excitation wavelength: 450 nm.
boxylate. A further improvement of the PL intensity was achieved by adding zinc ethylxanthate, leading to the formation of the ZnS shell. The detailed mechanism of the shell overgrowth is under current investigation and will be reported in a forthcoming publication. The complete surface passivation procedure strongly enhanced the PL QY of the InP NCs up to 22% (Figures 4a,b), and the trap state related emission at longer wavelengths is nearly completely supressed. The peaks of low intensity at around 730 and 810 nm visible in the spectra of the core/shell samples are assigned to the partial surface oxidation of the core NCs upon exposure to air before the shell growth,11b which can be avoided by applying a one-pot core/shell synthesis procedure. To summarize, a novel and rapid method for the synthesis of high quality InP NCs was developed based on the use of in situ generated PH3 gas as the phosphorus precursor. With respect to the conventionally used, pyrophoric P(TMS)3 precursor, the cost of the air-stable PH3 source (Ca3P2) is around 3 orders of magnitude lower.14 Furthermore, the presented method gives access to larger sized InP NCs without sacrificing a narrow size distribution. Finally, by overcoating the obtained InP NCs with a ZnS shell, PL QYs in the range of 20% have been obtained, making the resulting core/shell systems interesting for applications in optoelectronics and biological labeling. Acknowledgment. Financial support from CEA (program “Technologies pour la Santé”, project TIMOMA 2) and from the Agence Nationale pour la Recherche (project SYNERGIE) is acknowledged. CM7035579 (14) Source: Sigma-Aldrich, 2007.
