Synthesis, Assembly, and Optical Properties of Shape- and Phase

Langmuir 2007, 23, 765-770

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Synthesis, Assembly, and Optical Properties of Shape- and Phase-Controlled ZnSe Nanostructures Asit Baran Panda,† Somobrata Acharya,‡ Shlomo Efrima,† and Yuval Golan*,‡ Department of Chemistry, Department of Materials Engineering, and Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion UniVersity, Beer-SheVa, Israel 84105 ReceiVed June 6, 2006. In Final Form: September 5, 2006 Shape-, size-, and phase-controlled ZnSe nanostructures were synthesized by thermolysis of zinc acetate and selenourea using liganding solvents of octadecylamine (ODA) and trioctylphosphineoxide (TOPO) at different molar ratios. Materials synthesized in pure ODA resulted in uniform ultranarrow nanorods and nanowires of 1.3 nm in diameter. Morphological change from nanowire to spherical particle of larger diameter occurs with increasing TOPO/ODA ratio. Variation of the TOPO content in the mixed solvent also allows control of the crystallographic phase of ZnSe (wurtzite or zinc blende). The conditions and mechanisms of shape and phase control are discussed. Ultra-high-density networks of the ordered wires are achieved using the Langmuir-Blodgett technique in a single step as an essential stage on the route to ultra-high-density semiconductor nanocircuit fabrication.

1. Introduction One-dimensional (1D) semiconductor nanostructures and their assembly are of fundamental importance due to their unique dimension-dependent optical and mechanical properties and their potential applications as building blocks in nanoelectronics, nanooptronics, nanosensors, and actuators.1-3 However, the development of 1D materials is relatively slow in comparison to that of 3D materials due to the difficulties associated with controlled dimensionality, morphology, phase purity, and chemical composition. The advantages of colloidal methods based on manipulation via changing the ligand molecules used in the synthesis allow the design of 1D nanomaterials on the molecular scale and the elucidation of the underlying mechanism in which ligand molecules control the growth of nanocrystals.4-13 ZnSe, an important II-VI semiconductor, has attracted considerable attention for applications in light-emitting diodes, photodetectors, and full-color displays.14-17 Additionally, ZnSe * Corresponding author. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Materials Engineering, and Ilse Katz Center for Meso and Nanoscale Science and Technology. (1) Wang, J. F.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455. (2) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science 2001, 292, 2060. (3) Friedman, R. S.; McAlpine, M. C.; Ricketts, D. S.; Ham, D.; Lieber, C. M. Nature (London) 2005, 434, 1085. (4) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (5) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature (London) 2000, 404, 59. (6) Ahrenkiel, S. P.; Micic, O. I.; Miedaner, A.; Curis, C. J.; Nedeljkovic, J. M.; Nozik, A. J. Nano Lett. 2003, 3, 833. (7) Yang, J.; Xue, C.; Yu, S.-H.; Zeng, J.-H.; Quian, Y.-T. Angew. Chem., Int. Ed. 2002, 41, 4697. (8) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (9) Cheon J.; Kang N.-J.; Lee, S.-M.; Lee, J.-H.; Yoon, J.-H.; Oh, S. J. J. Am. Chem. Soc. 2004, 126, 1950. (10) Qian, C.; Kim, F.; Ma, L.; Tsui, F.; Yang, P.; Liu, J. J. Am. Chem. Soc. 2004, 126, 1195. (11) Kim, Y. H.; Jun, Y. W.; Jun, B. H.; Lee, S. M.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 13656. (12) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (13) Yu, W. W.; Wang, Y. A.; Peng, X. Chem. Mater. 2003, 15, 4300. (14) Matasuoka, T. AdV. Mater. 1996, 8, 469. (15) Wang, J.; Hutchings, D. C.; Miller, A.; Van Stryland, E. W.; Welford, K. R.; Muirhead, I. T.; Lewis, K. L. J. Appl. Phys. 1993, 73, 4746.

has significantly large exciton binding energy (21 meV)18 in comparison to that of GaAs (4.2 meV),19 which makes it an ideal candidate for efficient room-temperature exciton devices with improved temperature characteristics.20 Several novel applications have been presented which require size, shape, and phase control of ZnSe nanostructured materials.21-24 ZnSe has been studied to a lesser extent compared to its “closest of kin”, CdS, CdSe, and ZnS nanoparticles. Several pioneering reports have described various synthesis routes for ZnSe nanoparticle preparation, focusing on their structural characterization and occasionally on their photoluminescence (PL) properties.25-35 However, most of the reports yielded the cubic modification of ZnSe, namely, the zinc blende (ZB) structure, with very few in the hexagonal modification, known as the wurtzite (WZ) structure.25-28 Following synthesis, the significant challenge remains to achieve controlled nanoscale assembly of (16) Hong, S. K.; Kurts, E.; Chang, J. H.; Hanada, T.; Oku, M.; Yao, T. Appl. Phys. Lett. 2001, 78, 165. (17) Jeon, H.; Ding, J.; Patterson, W.; Nurmikko, A. V.; Xie, W.; Grillo, D. C.; Kobayashi, M.; Gunshor, R. L. Appl. Phys. Lett. 1991, 59, 3619. (18) Zhu, Z. M.; Liu, N. Z.; Li, G. H.; Han, H. X.; Wang, Z. P.; Wang, S. Z.; He, L.; Ji, R. B.; Wu, Y. J. Infrared Millimeter WaVes 1999, 18, 13. (19) Wang, S. Z.; Yoon, S. F.; He, L.; Shen, X. C. J. Appl. Phys. 2001, 90, 2314. (20) Rujkorakarmn, R.; Nelson, A. J. J. Appl. Phys. 2000, 87, 8557. (21) Lieber, C. M. Nano Lett. 2002, 2, 82. (22) Gudiksen, M. S.; Lauhon, L. J.; Wang, J. F.; Smith, D. C.; Lieber, C. M. Nature (London) 2002, 415, 617. (23) Wu, Y.; Fan, R.; Yang, P. Nano Lett. 2002, 2, 83. (24) Jiang, Y.; Meng, X. M.; Liu, J.; Hong, Z. R.; Lee, C. S.; Lee, S. T.; AdV. Mater. 2003, 15, 1195. (25) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. B 1998, 102, 3655. (26) Revaprasadu, M.; Malik, M. A.; O’Brien, P.; Zulu, M. M.; Wakefield, G. J. Mater. Chem. 1998, 8, 1885. (27) Smith, C. A.; Lee, H. W. H.; Leppert, V. J.; Risbud, S. H. Appl. Phys. Lett. 1999, 75, 1688. (28) Cozzoli, P. D.; Manna, L.; Curri, M. L.; Kudera, S.; Giannini, C.; Striccoli, M.; Agostiano, A. Chem. Mater. 2005, 17, 1296. (29) Zhu, Y. C.; Bando, Y. Chem. Phys. Lett. 2003, 377, 367. (30) Dong, Y.; Peng, Q.; Li, Y. Inorg. Chem. Commun. 2004, 7, 370. (31) Jiang, Y.; Meng, X. M.; Yiu, W. C.; Liu, J.; Ding, J. X.; Lee, C. S.; Lee, S. T. J. Phys. Chem. B 2004, 108, 2784. (32) Solanki, R.; Huo J.; Freeouf, J. L.; Miner, B. Appl. Phys. Lett. 2002, 81, 3864. (33) Duan, X. F.; Lieber, C. M. AdV. Mater. 2000, 12, 298. (34) Wang, W.; Geng, Y.; Yan, P.; Liu, F.; Xie, Y.; Qian, Y. Inorg. Chem. Commun. 1999, 2, 83. (35) Govindaraj, A.; Deepak, F. L.; Gunari, N. A.; Rao, C. N. R. Isr. J. Chem. 2001, 41, 23.

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the ZnSe nanostructures in two dimensions to meet desired functionality. Here, we report on a one-step, benchtop surfactant-mediated synthesis of shape-, size-, and phase -controlled ZnSe nanostructures that assemble spontaneously into ordered as well as into randomly distributed rods/wires and spherical particles. Relatively innocuous zinc acetate and selenourea precursors in liganding solvents of octadecylamine (ODA) and/or trioctylphosphine oxide (TOPO) at 70-220 °C are used for synthesis of ZnSe. By varying the ratio of ODA to TOPO, we achieve control between the WZ and ZB structures along with different particle geometries (wires, rods, and spherical particles). The as-synthesized ZnSe nanocrystals exhibit distinguishable shapedependent optical properties with prominent quantum confinement. We evaluate the formation mechanism of different ZnSe nanostructures by the interrupted growth approach for monitoring the resulting materials during synthesis. The interfacial behavior of the nanoparticles at the air-water interface when forming Langmuir and Langmuir-Blodgett (LB) films is reported. Highly ordered 2D arrays of ZnSe nanowires are shown to be useful for nonlithographic, bottom-up fabrication of ultra-high-density nanoelectronic circuits. 2. Experimental Section Chemicals. Highly pure zinc acetate (99.99%) and selenourea (>99.9%) were purchased from Aldrich and used without further purification. Octadecylamine (ODA) (99%) and trioctylphosphine oxide (TOPO) (99%) were purchased from Fluka and used as received. N,N-dimethylformamide (DMF), methanol, dichloromethane, and toluene were of analytical reagent grade. Synthesis. Typically, all syntheses were carried out at or below 220 °C in an oil bath with nitrogen purging. Solutions of zinc acetate (0.075 g, 3.41 × 10-4 mol) were prepared in molten ODA and/or in TOPO (2.96 × 10-3 mol) at 140 °C in a two-necked flask with a reflux condenser circulating warm water (45 °C) in one neck and N2 gas purging through other neck. A solution of selenourea (0.075 g, 3.41 × 10-4 mol) was prepared simultaneously in molten ODA (0.2 g, 7.42 × 10-4 mol) at temperature 140 °C in a test tube with N2 bubbling for 5 min. A few drops of DMF were added to this solution at 140 °C to make selenourea soluble in molten ODA. This selenourea solution was then rapidly added to the zinc acetate solution with continuous stirring, and the second neck was closed by a septum. As a precaution, N2 was purged throughout the reaction by a needle inserted through the septum, because the flashpoint of the ODA and TOPO was below the reaction temperature. Immediately after the addition of selenourea to zinc acetate, a clear solution appeared which persisted for a few minutes and then turned into a light yellow turbid solution, indicating the formation of ZnSe. After annealing for 1-6 h, the temperature of the solution was cooled to 70 °C, and the ZnSe particles were harvested by flocculating the sample with methanol, centrifuging, and redispersing in nonpolar solvents such as toluene, chloroform, or dichloromethane. All the manipulations over size, shape, and phase control were done by varying the ratio of ODA and TOPO from 5:0 to 1:4, respectively, in the mixture. Synthesis of Ordered Rods and Wires. The selenourea solution was rapidly added to the zinc acetate in pure ODA solution with continuous stirring at 140 °C, and the growth process is carried out at this temperature. For synthesis of rods, the reaction mixture was immediately cooled to 70 °C after 15 min. Quality optimization of the rods may require slight variation of this reaction time, temperature, and concentration. Parallel wires are synthesized under similar conditions after annealing for 1-2 h. A synthetic protocol employing microwave radiation with the same precursors has been recently reported by one of us.36 Synthesis of Random Rods and Wires. The selenourea solution was rapidly added to a solution of zinc acetate in a mixture of ODA (36) Panda, A. B.; Glaspell, G.; El-Shall, M. S. J. Am. Chem. Soc. 2006, 128, 2790.

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Figure 1. (a) UV-vis absorption and (b) PL spectra of spherical ZnSe particles formed at 220 °C with reaction times of 30 min (full line), 45 min (dashed line), 60 min (dotted line), 120 min (dasheddotted line), and 240 min (dashed-dotted-dotted line). (c,d) TEM micrographs of the ZnSe particles collected at different reaction times of 30 and 240 min. The inset shows the corresponding HRTEM images of the particles (the scale bar is 2 nm). and TOPO with continuous stirring at 140 °C. After 5 min, the temperature of the mixed solution was increased to 220 °C, and the growth process was carried out for 5-6 h. The ratio of ODA and TOPO in the final solution was varied from 4:1 to 2:3. Synthesis of Spherical Particles. The selenourea solution was rapidly added to the solution of zinc acetate in ODA and TOPO mixture in such a way that the ratio of ODA and TOPO in the final solution is 1:4, with continuous stirring at 140 °C. After 5 min, the temperature of the solution was increased to 220 °C, and the growth process is carried out for 4-5 h at this temperature. 2D Assembly Formation. A modular dual trough from Labcon was used for surface pressure-area (Π-A) measurements and LB deposition. Ultrapure water (resistivity 18 MΩ-cm) from Barnsted E-pure water purifier system was used for the aqueous subphase. Π-A isotherms were measured 1 h (or later) after spreading to ensure evaporation of the solvent (toluene) and equilibration of the film. Typically, the compression and expansion rates were 2 mm2/ min. The deposition of the LB films was carried out for at least 1 h after reaching desired surface pressure. The deposition was performed onto either a hydrophilic or a hydrophobic slide or onto lacey carbon-coated TEM grid (01883-F, 300 mesh, Ted Pella) at a speed of 2 mm/min for both up and down strokes. Sufficient time (∼20 min) was allowed for drying of the film between successive cycles. Characterization. Synthesized ZnSe nanomaterials were characterized using TEM (JEOL model 2010 HR-TEM microscope system), XRD (Rigaku), and EDS (Oxford linked ISIS 6498 version 1.4 spectrometer). Samples for TEM were prepared on lacey carboncoated, 300-mesh copper grid by putting a drop of colloid suspension from dichloromethane or via LB deposition during assembly formation. UV-vis and photoluminescence (PL) spectra of ZnSe nanoparticles dispersed in dichloromethane were recorded with a JASCO V-560 UV-vis diode array spectrophotometer and Jobin Yvon Fluorolog-3, respectively.

3. Results and Discussion 3.1. Shape-Controlled Synthesis of ZnSe Nanostructures and Their Optical Properties. 3.1.1. ZnSe Nanospheres. Spherical ZnSe particles were synthesized using a 4:1 ratio of ODA to TOPO at 220 °C for 5-6 h. The entire growth process was monitored by withdrawal of material at different times during the reaction period and studied by UV-vis absorption (Figure 1a), photoluminescence (PL, Figure 1b), and transmission electron

Synthesis and Assembly of ZnSe Nanostructures

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Figure 3. TEM images monitoring 1D growth of ZnSe nanocrystals in pure ODA for (a) 8 min, (b) 10 min, and (c) 15 min.

Figure 2. TEM micrographs at different magnifications. (a,b) Highly oriented ZnSe rods. (c,d) Ordered ZnSe wires. Note the lattice fringes in (d) which correspond to the basal planes of wurtzite ZnSe.

microscopy (TEM, Figure 1c,d). The diameter of spherical particles can be tuned in the range 2-5 nm. The monodispersity of the particles is manifested from the absorption spectrum (Figure 1a) by the appearance of a clear absorption peak and narrow emission band (∼30-40 nm fwhm).37,38 The size selectivity is reflected from the PL spectra with a consistent red shift from 440 to 480 nm with increasing reaction time (particle diameter). The measured quantum efficiency of these ZnSe particles is 22%. The use of pure TOPO as solvent also results in spherical particles but requires higher reaction temperatures (g300 °C). The HRTEM images (inset in Figure 1c,d) show well-defined lattice planes reflecting a high degree of crystallinity within each single crystal. The interplanar distances of 0.33 ( 0.03 and 0.28 ( 0.05 nm are consistent with ZnSe ZB structure with 0.327 nm for the (111) planes and 0.283 nm for the (200) planes, respectively (JCPDS-37-1463). 3.1.2. ZnSe Ordered Rods and Wires. Ultranarrow ordered rods and wires were synthesized using pure ODA as solvent.39,40 The growth carried out for 15 min at 70 °C resulted in uniform ultranarrow (1.3 nm wide) rods of 4.5 nm in length with an aspect ratio of ∼4 nm (Figure 2a,b). The longer reaction times produced wires (Figure 2c,d) of very narrow width (same as for the rods) with lengths exceeding 200 nm (an aspect ratio of ∼150). Most interestingly, the nanorods self-assembled into large regions (extending over hundreds of nanometers) into ordered 2D superstructures (Figure 2a,b). The wires also self-assembled into large ordered domains exceeding typically 500 × 200 nm in which they are strictly parallel to each other (Figure 2c,d). The use of shorter hydrocarbon chain amines such as hexadecylamine (HDA) and tetradecylamine (TDA) as the solvent reduces the interparticle spacing systematically. The organization of the nanoparticles into 2D superstructures is consistent with previous reports that showed nanoparticle organization in smectic or nematic arrangements.41-45 The shape of the rods and wires and their uniformity are major controlling factors in determining the superstructure, whereas the length of the alkyl chain of the liganding agent (the primary alkylamine solvent) affects the interparticle separation. The effect of the alkylamine surfactant is noticeable in many ways: (i) It (37) Murry, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (38) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5854. (39) Panda, A. B.; Acharya, S.; Efrima, S. AdV. Mater. 2005, 17, 2471. (40) Pradhan, N.; Efrima, S. J. Phys. Chem. B 2004, 108, 11964. (41) Li, M.; Schnablegger, H.; Mann, S. Nature (London) 1999, 402, 393. (42) Nikoobakht, B.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 8635. (43) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (44) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (45) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969.

Figure 4. UV-vis absorption spectra (inset) and PL spectra (excited at 340 nm) of highly oriented ZnSe rods (full lines) and wires (dashed-dotted lines) synthesized in pure ODA.

is responsible for their uniform anisotropic growth into rods and wires; (ii) it stabilizes the nanoparticles; (iii) it defines the lateral spacing between the nanoparticles. The HRTEM images show well-defined lattice planes within each single crystal (Figure 2b,d) with an interplaner distance of 0.32 ( 0.07 nm for rods and 0.33 ( 0.02 nm for wires, respectively, consistent with 0.325 nm for the (00.2) planes of WZ or/and 0.327 nm for the (111) planes of ZB (JCPDS files #15-0105, 37-1463, respectively). For the wires, the c-axis is clearly found to be parallel to the major axis. This implies a chemical bipolarity, with one end composed of a plane of selenium ions and the other of a plane of zinc ions. The alkylamine molecules are most likely to adsorb very differently to the terminal metal end and to the mixed planes on the sides of the rods/wires, blocking the 2D growth and inducing 1D growth from the selenide terminal. To monitor the 1D growth, we collected samples at different growth times as shown in Figure 3. Starting from the 1.8 nm spherical particles (after 8 min), fully developed rods are observed after 15 min with an intermediate stage of elongated spherical particles observed after 10 min. An increase in reaction time turns rods into wires while retaining the same width. The mechanism of rod to wire transition has been described by our group previously40 and is in line with the oriented head-to-tail attachment mechanism recently proposed by Penn et al.45 Interestingly, we do not observe any crossed-wire configuration in the present case. The higher widths reported in ref 15 (6 nm compared to the ultranarrow 1.3 nm rods in our case) may be responsible for the alternation of rods in two crossed directions. The PL and UV-vis spectra of ZnSe ordered rods and wires dispersed in dichloromethane are shown in Figure 4. The absorbance threshold at ∼410 nm is blue-shifted compared to bulk ZnSe (Figure 4, inset). There are two unusual sharp absorption peaks in the higher-energy region (328 and 344 nm corresponding to 3.79 and 3.61 eV, respectively). The spectra for rods and wires are nearly similar, suggesting that the narrow width is the major controlling factor for the confinement effect. Our spectral features differ significantly from most reported spectra. 26,27 Smith et al. reported ZnSe nanospheres in glass matrix with two additional spectral features at shorter wavelengths which resemble ours and were attributed to spin-orbit coupling and quantum confinement.27 However, our spectral features are much sharper and more blue-shifted. This difference is most

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Figure 5. TEM and HRTEM (inset) images of randomly oriented ZnSe rods and wires synthesized in mixed solvents of at different molar ratios of ODA to TOPO: (a) 2:3 (aspect ratio 2-4), (b) 3:2 (aspect ratio 4-10), (c) 4:1 (aspect ratio 75-100).

probably due to the ultranarrow width and higher uniformity of our samples. The room-temperature PL spectra of ZnSe nanorods and nanowires show a strong emission band at around 436 nm and two low-energy emission bands at 500 and 546 nm, respectively (Figure 4). The emission band at 436 nm was attributed to the band-edge emission of ZnSe,46 while the emission bands at 500 and 546 nm have been associated with the vacancies of Zn in ZnSe47 or impurities.46-49 3.1.3. ZnSe Random Rods and Wires. Random ZnSe rods and wires with different widths and aspect ratios were synthesized in different molar ratios of TOPO and ODA at 220 °C for 5-6 h. The molar ratio of 3:2 TOPO/ODA resulted in randomly oriented rods of ∼4 nm width and 7-15 nm length (aspect ratio ∼2-4), with elongated spherical particle geometry (Figure 5a). Increasing molar ratio of ODA to 2:3 (TOPO/ODA) in the mixed solvent resulted in particles of similar width but with higher aspect ratios of ∼4-10 (Figure 5b). Further increase of ODA percentage to 1:4 (TOPO/ODA) in the mixture resulted in wires (1.3-nm wide) with aspect ratios of 75-100 (Figure 5c). Also here, the growth process was monitored by withdrawal of samples at different reaction stages that were studied by TEM. At a ratio of 3:2 TOPO/ODA (the mixed solvent with the lowest ODA content), initially 2-nm spherical particles were observed, which turned to 4-nm spherical particles by isotropic growth. Increase of ODA concentration in the mixture enhanced anisotropic growth into elongated particles. This suggests that the presence of different ratios of the two stabilizing agents with different chain lengths is the major controlling factor in the growth process. The PL and UV-vis absorption spectra of ZnSe random rods and wires in dichloromethane are shown in Figure 6. The rods with the same width (4 nm) but with different aspect ratios give almost identical spectra. However, the wires with lesser width (1.3 nm) behaved differently from the rods. As expected, the emission spectra of random rods (4 nm width) are red-shifted to 450 nm in comparison to the wires (1.3-nm width) at 436 nm in PL. Note that, as expected, the optical spectra of 1.3-nm-wide ordered rods (Figure 4) and wires are identical. Interestingly, the random rods also show two absorbance peaks in the high-energy region at 328 and 344 nm (3.79 and 3.61 eV, respectively), but the peak widths are significantly broader compared to the ordered rods. The results again suggest that the width of the particles below the Bohr radius (4.5 nm for ZnSe) is the major controlling factor in determining the optical properties and degree of quantum confinement. 44 (46) Garcia, J. A.; Remon, A.; Zubiaga, A.; Sanjose, V. M.; Tomas, C. M. Phys. Status Solidi A 2002, 194, 338. (47) Fujita, S.; Mimoto, H.; Naguchi, T. J. Appl. Phys. 1979, 50, 107. (48) Sankar, N.; Ramachandran, K. J. Cryst. Growth 2003, 247, 157. (49) Bukaluk, A.; Trzcinski, M.; Firszt, F.; Legowski, S.; Meczynska, H. Surf. Sci. 2002, 507-510, 175.

Figure 6. UV-vis absorption (inset) and PL spectra of randomly oriented ZnSe rods with aspect ratio 2-4 (dotted line), with aspect ratio 4-10 (dashed-dotted-dotted line); and wires with aspect ratio 75-100 (full line).

Figure 7. Powder XRD patterns of (a) ordered rod and wires, (b) random wires, (c) random rods with aspect ratio 2-4, (d) random rods with aspect ratio 4-10, and (e) spherical particles.

3.2. Phase Control of ZnSe Nanocrystals. The WZ ZnSe structure is thermodynamically metastable at low temperatures in general. We achieve phase-controlled synthesis of WZ ZnSe at relatively low temperatures (140 °C) using nonhazardous zinc acetate and selenourea precursors in pure ODA solvent. Figure 7a shows typical powder XRD patterns of ordered ZnSe rods and wires, which confirm the crystallinity of both the rods and the wires. The peaks [note the characteristic triplet of 10.0, 00.2, and 10.1 WZ diffraction peaks] match well to the Bragg reflections corresponding to the WZ structure of ZnSe (P63mc, a ) 3.993Å, c ) 6.55Å, JCPDS file #15-0105). The selected area electron diffraction (SAED) patterns confirm the WZ structure with all expected diffraction rings (Supporting Information, Figure 1a). From the XRD and SAED, it is confirmed that the d spacings

Synthesis and Assembly of ZnSe Nanostructures

(0.32 ( 0.07 and 0.33 ( 0.02 nm for rods and wires, respectively) observed in HRTEM correspond to the (00.2) planes of WZ structure (Figure 2c,d). The stronger and narrower 00.2 peak in Figure 7a indicates that the nanocrystals are elongated along the c-axis. Notably, the increase of the reaction temperature or of reaction time had no effect on the crystallographic phase. The powder XRD patterns of spherical particles synthesized in mixed solvent of TOPO and ODA in a ratio of 4:1 (Figure 7e) confirm the crystallinity of the nanostructures obtained. The peaks match well to the ZB structure of ZnSe (P63mc, a ) 5.668Å, JCPDS file #37-1463). The absence of the 200 reflection is not surprising due to the very weak intensity (