CRYSTAL GROWTH & DESIGN
ZnE (E ) S, Se, Te) Nanowires Grown by the Solution-Liquid-Solid Mechanism: Importance of Reactant Decomposition Kinetics and the Solvent
2008 VOL. 8, NO. 9 3246–3252
Dayne D. Fanfair and Brian A. Korgel* Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062 ReceiVed December 4, 2007; ReVised Manuscript ReceiVed February 14, 2008
ABSTRACT: The synthesis of ZnE (E ) S, Se, Te) nanowires in solution via the solution-liquid-solid (SLS) mechanism is reported. Relatively low nanowire growth temperatures, between 340 and 350 °C, were made possible by using bismuth nanocrystals as seeds. Diethyl zinc and zinc(oleate)2 were studied as Zn reactants, and TOP:E complexes were explored as the chalcogen source. The influence of the solvent on the quality and yield of the nanowires was studied with reactions carried out in either the noncoordinating solvent squalane or the coordinating solvents, trioctylamine (TOA) or trioctylphosphine oxide (TOPO). The solvent and reactant chemistry dramatically affect the yield and quality of the nanowires, with Et2Zn being more reactive than Zn(oleate)2. The use of coordinating solvents provides a means to optimize nanowire growth. Introduction The bandgap energies of zinc chalcogenides fall in the visible range of the electromagnetic spectrum [i.e., ZnS (Eg ) 3.68 eV),1 ZnSe (Eg ) 2.7 eV),2 and ZnTe (Eg ) 2.26 eV)3], making them candidates for use in a variety of optoelectronic applications,4–9 including light-emitting diodes (LEDs), solar cells,2 flat panel displays, and lasers.10 Nanowires of these materials are particularly interesting because of their size- and shape-dependent optical, electronic, and mechanical properties. Furthermore, nanowires can be dispersed in various solvents and printed on plastic substrates for flexible, disposable electronic applications.11 Three different approaches have been relatively successful for making zinc chalcogenide nanowires: (i) “spontaneous” nanowire crystallization, (ii) templated growth, and (iii) metal particle-seeded growth. Under high-temperature gas-phase conditions and in solution, zinc chalcogenide nanowires have been produced by simply mixing zinc and chalcogen reactants and then heating. For example, ZnS12–17 nanowires have been obtained at high temperature by evaporation and condensation, and ZnSe6,7,18,19 and ZnS1,20,21 nanowires have been crystallized in solution by combining Zn and S or Se reactants. For this to occur, the zinc chalcogenides must have an anisotropic crystal structure like wurtzite that induces unidirectional crystallization and nanowire formation, similar to the well-known cases of ZnO nanowires,22–24 nanobelts,25,26 and nanoribbons.27 Although reasonably high yields of nanowires have been obtained using these methods, they offer little control over the average diameter and the wire diameters are typically large (>100 nm). Furthermore, zinc chalcogenides with zinc blende crystal structure do not spontaneously form nanowires using these approaches. Better diameter control has been achieved by crystallizing nanowires in mesoporous templates, such as ZnTe28,29 and ZnS30,31 nanowires. However, the nanowire yield using templates is relatively small, and the nanowires are often polycrystalline. Two synthetic approaches with the potential for high yields, good crystallinity, and narrow diameters and diameter distributions are vapor-liquid-solid (VLS) and solution-liquid* To whom correspondence should be addressed. Tel: 512-471-5633. Fax: 512-471-7060. E-mail:
[email protected].
solid (SLS) growth. In both approaches, the semiconductor of interest forms a liquid eutectic with a seed metal, which induces nanowire formation. VLS growth has been used to produce ZnS,10,31–42 ZnSe,4,5,8,9,43–49 ZnSe/CdSe50 superlattice, and ZnTe51–53 nanowires. VLS and SLS approaches are similar, except that SLS is carried out in solution, whereas VLS is carried out in the gas phase at relatively high temperatures. Therefore, low-melting metals like In, Sn, and Bi that form eutectics with various semiconductors at temperatures less than 350 °C must be used for SLS growth.54–62 For example, Buhro’s research group reported the bismuth-seeded SLS synthesis of InP, InAs, GaAs, CdSe, CdTe, and ZnTe nanowires.54 Buhro et. al. also recently reported bismuth nanocrystal-seeded SLS growth of ZnSe, ZnTe, and ZnSe-ZnTe heterostructured nanowires57 and branched ZnSe and heterobranched CdSe-ZnSe nanowires.56 Kuno et. al. also demonstrated the use of Au/Bi core/shell nanocrystals to seed the SLS growth of CdSe,58 CdTe,59 and PbSe60 nanowires. Our group recently demonstrated SLS growth of Ge nanowires using Bi nanocrystals.62 Herein, SLS synthesis of ZnS, ZnSe, and ZnTe nanowires seeded with Bi nanocrystals is reported. Two different Zn reactants, diethylzinc and zinc oleate, were used to make nanowires, in conjunction with the chalcogenide complexes, TOP:E (E ) S, Se, Te), reacted in three different solvents, squalane, trioctylphosphine oxide (TOPO), and trioctylamine (TOA). The solvent and reactant chemistry significantly influenced the quality of the nanowires, and despite their chemical similarity, the synthesis of ZnS, ZnSe, or ZnTe nanowires required substantially different reaction chemistries for the highest yields and best quality of each different material. Experimental Details Reagents. Tri-n-octylphosphine (97%, TOP), TOPO (99%, C24H51PO)bismuth(III) 2-ethylhexanoate, and selenium powder were used as received from STREM. Zinc oxide (99.999%, ZnO), diethylzinc solution (1 M in hexane, Et2Zn), oleic acid (g60%, OA), ethylenediamine, dioctylether (g90%), sodium borohydride (NaBH4), TOA (98%, C24H51N), squalane (C30H62), sulfur, and tellurium powder were used as received from Sigma-Aldrich. Stock solutions of tri-n-octylphosphine selenide (1 M, TOP-Se), trin-octylphosphine telluride (0.75 M, TOP-Te), and tri-n-octylphosphine sulfide (1 M, TOP-S) were prepared for the nanowire reactions in a
10.1021/cg701191k CCC: $40.75 2008 American Chemical Society Published on Web 07/23/2008
ZnE Nanowires Grown by the SLS Mechanism nitrogen-filled glovebox by dissolving either 400 mg of Se, 480 mg of Te, or 160 mg of S into 5 mL of TOP. Bismuth nanocrystals were prepared on a Schlenk line under nitrogen by room temperature reduction of bismuth(III) 2-ethylhexanoate with a mixture of NaBH4 and ethylenediamine in TOP as described previously.55 Nanowire Synthesis. Nanowires were synthesized under nitrogen in a four-neck flat-bottom flask on a Schlenk line. The flask was charged with solventseither 3 mL of squalane or TOA or 3 g of TOPOsas detailed below and dried and degassed for 1 h by heating to 100 °C and alternating vacuum and purge cycles. The reaction flask was then backfilled with nitrogen and heated to the nanowire growth temperature stated below. For reactions using zinc(oleate)2 as the Zn reactant, 11.7 mg of ZnO was added to the reaction flask with the solvent before drying and degassing at 100 °C. A 200 µL amount of OA was then added to the reaction flask, degassed at 100 °C for an additional 30 min, and then heated to 350 °C and stirred for 30 min to form zinc(oleate)2, which was evidenced by a change from a milky white to optically clear solution. (Separately, 1H NMR spectroscopy was used to confirm the formation of zinc(oleate)2 [see the Supporting Information, Figure S1].) Meanwhile, reactant solutions of Bi nanocrystals and chalcogenide reactants, and Et2Zn (when it was used as the Zn reactant), were prepared separately in a nitrogen-filled glovebox with the concentrations detailed below. Once the reaction flask reached the nanowire growth temperature, the reactant solution was removed from the glovebox and rapidly injected into the reaction flask on the Schlenk line. The reaction temperature dropped by approximately 10 °C after injection. The reaction flask was reheated to the initial injection temperature and heated for 5 more min. ZnS Nanowire Synthesis Using Et2Zn. The reaction flask was charged with either 3 mL of squalane or TOA or 3 g of TOPO as the solvent. The reactant solution was 1.5 mg of Bi nanocrystals, 144 µL of Et2Zn, and 144 µL of TOP-S in 216 µL of toluene. Nanowire growth was carried out at 350 °C. ZnS Nanowire Synthesis Using Zn(oleate)2. The reaction flask was charged with 11.7 mg of ZnO and 3 mL of either squalane or TOA or 3 g of TOPO and degassed at 100 °C. A 200 µL amount of OA was added to the reaction flask and degassed at 100 °C for another 30 min. The reaction flask was heated to 350 °C and stirred for another 30 min. The reactant solution of 1.5 mg of Bi nanocrystals and 144 µL of TOP-S in 360 µL of toluene was prepared in the glovebox and then injected into the reaction flask on the Schlenk line. Nanowire growth was carried out at 350 °C. ZnSe Nanowire Synthesis Using Et2Zn. Three milliliters of either squalane or TOA or 3 g of TOPO was used as the solvent. The reactant solution was 1.5 mg of Bi nanocrystals, 144 µL of Et2Zn, and 144 µL of TOP-Se in 216 µL of toluene. Nanowire growth was carried out at 350 °C. ZnSe Nanowire Synthesis Using Zn(oleate)2. The reaction flask was charged with 11.7 mg of ZnO and 3 mL of either squalane or TOA or 3 g of TOPO and degassed at 100 °C. A 200 µL amount of OA was added to the reaction flask and degassed at 100 °C for an additional 30 min. The reaction flask was heated to 350 °C and stirred for another 30 min. The reaction solution of 1.5 mg of Bi nanocrystals and 144 µL of TOP-Se in 360 µL of toluene was prepared in the glovebox and then injected into the reaction flask on the Schlenk line. Nanowire growth was carried out at 350 °C. ZnTe Nanowire Synthesis Using Et2Zn. Three milliliters of either squalane or TOA or 3 g of TOPO was used as the solvent. The reactant solution was 1.5 mg of Bi nanocrystals, 144 µL of Et2Zn, and 191 µL of TOP-Te in 169 µL of toluene. Nanowire growth was carried out at 340 °C. ZnTe Nanowire Synthesis Using Zn(oleate)2. The reaction flask was charged with 11.7 mg of ZnO and 3 mL of either squalane or TOA or 3 g of TOPO and then degassed at 100 °C. A 200 µL amount of OA was added to the reaction flask and degassed at 100 °C for another 30 min. The reaction flask was heated to 350 °C and stirred for another 30 min. The reaction solution of 1.5 mg of Bi nanocrystals and 191 µL of TOP-Te in 360 µL of toluene was prepared in the glovebox and injected into the reaction flask on the Schlenk line. Nanowire growth was carried out at 340 °C. ZnE Nanowire Purification. After the reaction flask was removed from the heating mantle, it was allowed to cool to 60 °C. Ten milliliters of toluene was then added to the crude nanowire product. The nanowires were isolated as a precipitate by centrifuging this solution at 8000 rpm
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Figure 1. SEM images of Bi-seeded ZnS nanowires grown using Et2Zn in (A) squalane, (C) TOPO, and (E) TOA and zinc(oleate)2 in (B) squalane, (D) TOPO, and (F) TOA at 350 °C. for 10 min. The supernatant was discarded. The precipitate was redispersed in chloroform and further purified by adding ethanol and centrifuging again to remove remaining organic and molecular byproducts. Instrumentation and Characterization. Nanowires were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). For SEM, nanowires were drop-cast from chloroform dispersions onto glassy-carbon substrates, and imaging was performed using a LEO 1530 field emission SEM at 3 kV accelerating voltage with digital image acquisition with an Inlens detector and LEO 32 software system. For TEM, nanowires were deposited by evaporating the solvent from chloroform dispersions onto 200-mesh lacey carbon-coated Cu grids (Electron Microscopy Sciences). The nanowires were imaged with a JEOL 2010F field emission TEM operated at 200 kV accelerating voltage. Images were digitally acquired with a Gatan multipode scanning CCD camera. XRD was obtained with 0.5-1.0 mg of nanowires on quartz slides with a Bruker-Nonius D8 Advance θ-2θ powder diffractometer using Cu KR radiation (λ ) 1.5418 Å) and collection on a scintillation detector for 16 h with an incremental angle of 0.02° at a scan rate of 12°/min. 1H NMR spectra of zinc(oleate)2 and oleic acid dissolved in CDCl3 were measured using a 500 MHz Varian Inova 500 spectrometer (see the Supporting Information for NMR characterization data).
Results and Discussion ZnS Nanowires. Figure 1 shows SEM images of ZnS precipitated in either squalane, TOPO, or TOA using either Et2Zn or Zn(oleate)2 with TOP:S as reactants in the presence of Bi nanocrystals at 350 °C. Only large nanoparticles were produced when Zn(oleate)2 was used as the Zn reactant (Figures 1B,D,F). In contrast, Et2Zn gave ZnS nanowires in squalane (Figure 1A and Figure 2) and TOPO (Figure 1C and Figure 3). In TOA, Et2Zn gave only particulates (Figure 1E).
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Figure 2. TEM images of Bi-seeded ZnS nanowires produced using Et2Zn and TOP-S in (A) squalane and (B) TOPO at 350 °C. The inset in panel A is an FFT of the nanowire shown.
Figure 4. SEM images of Bi-seeded ZnSe nanowires grown using Et2Zn in (A) squalane, (C) TOPO, and (E)TOA and zinc(oleate)2 in (B) squalane, (D) TOPO, and (F) TOA at 350 °C.
Figure 3. XRD on Bi-seeded ZnS nanowires grown using Et2Zn and TOP-S in TOPO at 350 °C. The “*”, “W”, and “ZB” represent rhombohedral bismuth, wurtzite, and zinc-blende ZnS, respectively.
The ZnS nanowires obtained in squalane with Et2Zn and TOP:S were relatively short and had a tortuous morphology (Figures 1A and Figure S1 in the Supporting Information). The same reaction carried out in TOPO (Figures 1C and Figure S2 in the Supporting Information) gave a higher yield of ZnS nanowires and the nanowires were longer, but they also had a relatively tortuous morphology (see Figure S2 in the Supporting Information). A tortuous nanowire morphology occurs when the reactant decomposition is too slow to maintain a sufficient nanowire growth rate and leads to extended crystal defects that alter the nanowire growth direction.63 Figure 2 shows TEM images of a few kinked ZnS nanowires produced in squalane and TOPO. XRD of the nanowires (Figure 3) confirmed that they were composed of zinc blende ZnS (as discussed in more detail below), but sharp diffraction peaks corresponding to Bi were also present in the diffraction patterns. The primary growth direction of the nanowires was the direction. The narrow diffraction lines indicate that a large amount of the Bi seed particles aggregate into large particulates, which occurs when the nanowire growth rate is relatively slow. ZnSe Nanowires. Reactions between Et2Zn and TOP:Se in squalane or TOPO in the presence of Bi nanocrystals at 350 °C gave ZnSe nanowires (Figures 4A,C); however, the yields were
Figure 5. XRD on Bi-seeded ZnSe nanowires synthesized by reaction 9 with zinc(oleate)2 and TOP-Se in the presence of Bi in TOPO at 350 °C. Peaks labeled with “9” and “b” index to zincite (hexagonal ZnO) and rhombohedral Bi, respectively.
relatively low. The same reaction in TOA gave only particulates (Figure 4E). Reactions between Zn(oleate)2 and TOP:Se in all
ZnE Nanowires Grown by the SLS Mechanism
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Figure 6. (A-C) TEM images and (D) EDS data of Bi-seeded ZnSe nanowires produced using zinc(oleate)2 and TOP-Se in TOA at 350 °C. The FFT in the inset in panel A is of the Bi seed nanocrystal. The spots index to rhombohedral Bi (JCPDS #44-1246). The FFT in the inset in panel B is of the nanowire shown.
Figure 8. (A-E) TEM images of Bi-seeded ZnTe nanowires produced using Et2Zn in TOPO at 340 °C.
Figure 7. SEM images of Bi-seeded ZnTe nanowires grown using Et2Zn in (A) squalane, (C) TOPO, and (E) TOA and zinc(oleate)2 in (B) squalane, (D) TOPO, and (F) TOA at 340 °C.
three solvents gave ZnSe nanowires (Figure 4B,D,F), but the yield and quality of the nanowires was better in TOPO and TOA than squalane. The reactions in squalane produced a large amount of particulate byproduct in addition to nanowires (see the SEM image in Figure 4B, for example). The best nanowires were obtained from reactions between Zn(oleate)2 and TOP:Se in TOPO (see Figure S3 in the Supporting Information). These nanowires are straight and of good quality. Figure 5 shows an XRD pattern obtained from the nanowires, confirming that they are composed of zinc blende ZnSe. The ZnSe nanowires produced from zinc(oleate)2 and TOP: Se in TOA tended to have short branches extending from their sidewall surfaces as shown in Figure 6C. Bi nanocrystals are observed only at the tips of the nanowires (Figure 6A) and not at the tips of the sidewall branches (Figure 6C). Most of the nanowires grow in the κ direction; however, a large proportion of ZnSe nanowires grown in TOA also grew in the direction [of 75 nanowires observed by TEM, 57% grew in the direction (as in Figure 6B) and 43% in the direction (as in Figure 6C)]. The nanowires exhibited large numbers of {111} twins, similar to SLS-grown nanowires of zinc blende III-V materials like GaP and GaAs.64,65 These twins cross-section the nanowires growing in the direction and span the length of the nanowires that grow in the direction, as in the TEM images in Figure 6B,C. The nanowires growing in the direction were primarily the ones that had sidewall branching, and the twinning defects appear to be intimately related to the sidewall branch growth. The sidewall
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Figure 9. (A) TEM image of a Bi2Te3 nanocrystal at the enlarged tip of a ZnTe nanowire. (B) FFT and (C) EDS data of the nanocrystal tip in panel A.
branches were not observed on ZnSe nanowires grown in TOPO. It appears that TOPO coordinates stronger to the ZnSe surface than TOA and prevents homogeneous sidewall ZnSe deposition and growth. ZnTe Nanowires. Reactions between Et2Zn and TOP:Te in the presence of Bi nanocrystals at 340 °C in all three solventsssqualane, TOA, and TOPOsproduced ZnTe nanowires. Figure 7 shows SEM images of the ZnTe products obtained from these reactions. The best results were achieved in TOPO as in Figure 7C (also see Figure S4 in the Supporting Information). The nanowires are relatively straight, although some nanowires are noticeably tangled and there is some particulate formation. The reactions in squalane yielded relatively straight ZnTe nanowires, but a large fraction of the reaction products were particulates (Figure 7A). The reactions in TOA produced ZnTe nanowires (Figure 7E) but also with relatively low yields. Reactions with Zn(oleate)2 and TOP:Te in either squalane or TOA (Figure 7B,F) at 340 °C in the presence of Bi nanocrystals also produced ZnTe nanowires. The reactions between Zn(oleate)2 and TOP:Te in TOPO gave only particulates (Figure 7D). The reactions in squalane yielded a significant amount of ZnTe nanowires, but in comparison, the reactions between Et2Zn and TOP:Te in TOPO gave higher yields and better quality ZnTe nanowires. Considering that Et2Zn was such
Figure 10. XRD on Bi-seeded ZnTe nanowires produced using Et2Zn and TOP-Te in TOPO. The peaks labeled with a “9” index to rhombohedral Bi2Te3 (JCPDS #00-015-0863). “W” and “ZB” represent wurtzite and zinc-blende ZnTe, respectively.
Figure 11. Five regimes of nanowire growth as a function of ZnE (E ) S, Se, Te) monomer supply rate. The blue sphere represents a Bi seed nanocrystal. The ZnE monomer supply rate increases from left to right.
an effective reactant in TOPO, it is somewhat surprising that Zn(oleate)2 and TOP:Te reacted in TOPO only gave particulates. Figure 8 shows some high-resolution TEM images of the ZnTe nanowires produced from reactions between Et2Zn and TOP:Te in TOPO. Many of the ZnTe nanowires had follicleshaped tips like those in Figures 8C-E and 9A. The length of the nanowires is consistently narrow and only tapers at the tips, growing to 2-8 times larger in diameter. Nanobeam EDS (Figure 9C) of the nanowire tips revealed Bi and Te (and no zinc) with a Bi:Te mole ratio of 0.6, consistent with Bi2Te3.
Table 1. Summary of the Reaction Products Obtained for All of the Bi-Seeded ZnE (E ) S, Se, Te) Nanowire Synthesesa ZnS squalane TOPO TOA
ZnSe
ZnTe
Et2Zn
Zn(oleate)2
Et2Zn
Zn(Oleate)2
Et2Zn
high yield, short (
