Nucleation−Dissolution−Recrystallization: A New Growth Mechanism

CRYSTAL GROWTH & DESIGN

Nucleation-Dissolution-Recrystallization: A New Growth Mechanism for t-Selenium Nanotubes Guangcheng Xi, Kan Xiong, Qingbiao Zhao, Rui Zhang, Houbo Zhang, and Yitai Qian* Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, UniVersity of Science & Technology of China, Hefei, Anhui 230026, P. R. China

2006 VOL. 6, NO. 2 577-582

ReceiVed August 27, 2005; ReVised Manuscript ReceiVed October 29, 2005

ABSTRACT: We report a hydrothermal reaction to make t-selenium nanotubes, in the absence of a surfactant or polymer to direct nanoparticle growth, and without externally added forces (such as ultrasonic). A series of electron microscopy characterization results suggest that the growth of t-selenium nanotubes is governed by a nucleation-dissolution-recrystallization growth mechanism. In this mechanism, t-selenium nanoparticles were initially formed in the hydrothermal system, then the t-selenium nanoparticles started to dissolve into the solution and grow onto large nanoparticles of selenium, and spherelike microparticles were obtained. The spherelike microparticles then gradually dissolved to generate selenium atoms in the solution; these selenium atoms were renewedly transferred onto the surfaces of the spherelike microparticles and were recrystallized. Along with the dissolution-recrystallization process, the spherelike microparticles gradually evolved into novel groovelike nanostructures. The nanogrooves could grow along the circumferential direction and the tuber axis direction until all spherelike microparticles had been completely consumed, eventually growing into t-selenium nanotubes. Studies found that this growth mechanism is strongly affected by temperature and concentrations of NaOH. By adjusting temperature and concentrations of NaOH, t-selenium nanotubes, nanowires, microrods, porous microtubes, and polyhedrons can be synthesized, respectively. 1. Introduction It is now commonly understood that the properties of nanophase inorganic materials strongly depend on the shapes and sizes of the particles, which are thus a key factor to their ultimate performance and applications. Although, in the past decade, a variety of techniques have been applied to fabricate nanostructures of a broad class of inorganic materials, ranging from ceramic dielectrics,1,2 semiconductors,3,4 metals,5-7 metal oxides,8-10 sulfides,11,12 and metal hydroxides13-15 to fullerene carbons,16-18 the challenge of synthesis of nano-building blocks with controlled shape and size still exists. Understanding the shape-guiding process and the growth mechanism of nanocrystals is important in the shape- and size-controlled synthesis of nanocrystals and will make it possible to program a system to yield nano-building blocks with a desired shape and size.19 Elemental selenium is an indirect gap semiconductor with excellent photoelectrical and semiconducting properties.20 It has a relatively low melting point (∼490 K), high photoconducting (∼8 × 104 S cm-1),21 and low photomelting temperature (∼77 K).22 Selenium has been widely used in the field of rectifiers, solar cells, photographic exposure meters, and xerography.23 As those properties within the nanometer regime might be associated with their morphologies,24,25 thus if selenium was obtained in nanostructures (such as nanorods, nanowires, and nanotubes), they might act as highly functionalized materials. Heretofore, a lot of effort has been devoted to the synthesis of t-selenium 1D nanostructures. Gates et al. prepared t-selenium nanowires by a solution phase approach in which colloidal particles of amorphous R-Se were converted into t-Se wires with diameters of 10-30 nm and lengths up to hundreds of microns.26 Jiang et al. reported the preparation of selenium nanorods of diameters ranging from 20 nm to several hundred nanometers by laser ablation of selenium powder.27 Gautam et al. have prepared t-selenium nanorods and nanowires by a reaction of selenium powder with NaBH4 in solution or the thermal decomposition * Corresponding author. Phone: +86-0551-360-1589. Fax: +86-551360-7402. E-mail: [email protected].

of [(CH3)4N]4Ge4Se10.28 Cao et al. synthesized t-selenium nanoribbons via a vapor-liquid-solid growth route.29 Ma et al. synthesized t-selenium nanobelts and nanowires in micellar solutions of nonionic surfactants.30 With the assistance of silicon powder, Ren et al. obtained t-selenium nanowires via a vapor growth process.31 A sonochemical method has also been used to fabricate t-selenium nanowires.32,33 Recently, Zhang et al. first reported fabrication of t-selenium nanotubes by a hydrothermal-ultrasonic route.34 Later, Ma et al. synthesized tselenium nanotubes in micelles of a nonionic surfactant.35 More recently, Li et al. synthesized t-selenium nanotubes by a sonochemical process.36 Their studies showed that the growth processes of t-selenium nanotubes are not alike under different prepared conditions. So it is meaningful to investigate the growth process of t-selenium nanotubes prepared under different experiment conditions, especially in a more natural conditions, which can provide important information to the fields of crystal growth and design. In this paper, a hydrothermal synthesis of t-selenium nanotubes was developed. To study the growth mechanism of t-selenium nanotubes in more natural conditions, no surfactants or polymers templates or outside forces (such as ultrasonic) were introduced into our synthetic system. We reexamine the reaction of Na2SeO3 and NaCHO2, which makes it possible to examine the t-selenium crystals in different growth stages. A series of electron microscopy characterization results suggests that the growth of t-selenium nanotubes prepared under the present experimental condition is governed by a nucleation-dissolution-recrystallization growth mechanism. 2. Experimental Section Preparation of t-Selenium Nanotubes. All the reagents used in the experiment were analytical pure and were purchased from Shanghai Chemical Reagent Company and were used without further purification. In a typical procedure, 0.0005 mol of sodium selenite (Na2SeO3), 10 mL of sodium hydroxide (NaOH) (2.4 M) and 0.002 mol of sodium formate (NaCHO2) were added into a Teflon-lined stainless steel autoclave of 60-mL capacity, which gave a final concentrations of

10.1021/cg050444c CCC: $33.50 © 2006 American Chemical Society Published on Web 01/10/2006

578 Crystal Growth & Design, Vol. 6, No. 2, 2006

Xi et al.

Figure 1. (a) XRD pattern of the nanotubes. (b) XRD pattern of the selenium nanoparticles. 0.0091 mol‚L-1 SeO32-, 0.45 mol‚L-1 OH-, and 0.0364 mol‚L-1 CHO2-. The autoclave was sealed and maintained at 100 °C for 25 h. After that, the autoclave was allowed to cool to room temperature naturally. It was found that a large quantity of dark-gray particles floated on the top of the solution. The dark-gray particles were filtered off, washed several times with distilled water and absolute ethanol to remove impurities, and then dried in a vacuum at 50 °C for 4 h. On the basis of the quantitative analysis of the components of the reaction mixture, the chemical reaction can be formulated as

SeO32- + 2CHO2- + H2O f Se +2CO2 + 4OH- (*) Characterizations. X-ray powder diffraction (XRD) pattern of the products was recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). X-ray photoelectron spectrum (XPS) was carried out on a VGESCALAB MK II X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-ray as the excitation source. The Raman spectrum was produced at room temperature on a Spex 1403 Raman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. The scanning electron microscope (SEM) images of the products were recorded on a Hitachi X-650 microscope and a JEOL6300F microscope. The transmission electron microscope (TEM) images, selected area electron diffraction (SAED) pattern, and highresolution transmission electron microscope (HRTEM) image were recorded on a JEOL 2010 microscope. The samples used for SEM, TEM, and HRTEM characterization were dispersed in absolute ethanol and were ultrasonicated before observation.

3. Results and Discussion Powder XRD, XPS, Raman Spectrum, Morphology, and Crystalline Orientation. The phase of the as-obtained products was examined by XRD (Figure 1a). All of the reflections in Figure 1a can be readily indexed to a trigonal phase of selenium (space group: P3121 (152), with infinite, helical chains of selenium atoms packed parallel to each other along the c-axis) with lattice constants of a ) 0.4365 nm and c ) 0.4948 nm, which are in agreement with the values reported in literature (JCPDS 73-0465). No other crystalline impurities were detected by XRD, which indicate that the pure t-selenium can be obtained via the hydrothermal condition. The sample was further characterized by XPS and Raman spectroscopy. Figure 2a shows the XPS spectrum of the assynthesized sample. The binding energy at 54.90 eV, corresponding to Se3d, is the characteristic peak for elemental selenium.37 Figure 2b presents Raman spectrum of the sample.

Figure 2. (a) XPS spectrum of the as-synthesized t-selenium nanotubes. (b) Raman spectrum of the t-selenium nanotubes.

The resonance peak at 237 cm-1 is a characteristic signature of t-selenium, which can be assigned to the vibration of selenium helical chains. In comparison, the characteristic Raman resonance absorption band for monoclinic selenium and R-Se are at 256 and 264 cm-1, respectively.38 Combining the results of XRD with those of XPS and Raman spectroscopy, it can be concluded that high-purity of t-selenium was obtained by the synthetic method. Figure 3a is a SEM image of the sample. One can see that the panoramic morphology of the sample consists of needlelike crystals that are usually 100-700 nm in diameter and 10-40 µm in length. Figure 3b shows a magnified view of the needlelike crystals, revealing that there are openings at the end of the needlelike crystals. TEM image of a single selenium nanotube is shown in Figure 3c, which clearly indicates that the wall thickness of the nanotube ranges from 50 to 70 nm. The SAED pattern of the nanotube (inserted in Figure 3c) is single-crystalline and indicates that the tube grew along the [001] direction. An HRTEM image taken from this nanotube is shown in Figure 3d. As can be seen from the image, this particular nanotube is a structurally uniform single crystal. The observed interplanar spacing is about 0.49 nm, which corresponds to the separation between (001) lattice planes of t-selenium. HRTEM and SAED analysis on more nanotubes indicates that they are single-crystalline and all along the [001] direction.

Nucleation-Dissolution-Recrystallization

Crystal Growth & Design, Vol. 6, No. 2, 2006 579

Figure 3. (a) Low magnification SEM images of the t-selenium nanotubes. (b) High magnification SEM images of the t-selenium nanotubes. (c) TEM image and SAED pattern (inset) of an individual t-selenium nanotube. (d) HRTEM image recorded from the wall of the t-selenium nanotube.

Morphology Evolutional Process of the t-Selenium Nanotubes. To substantially understand the growth mechanism of the t-selenium nanotubes under the present experimental condition, we have systematically surveyed the growth process by analyzing the samples at different growth stages. Figure 4 gives the electron microscopy images of seven samples that were taken from the different stages of the hydrothermal reaction: 2-25 h. These images indicate the evolution of selenium into nanostructures having different morphologies as the hydrothermal reaction was kept at 100 °C. After hydrothermal reaction for 2 h, the initial product was selenium nanoparticles (Figure 4a): most of them were 10 nm in diameter. SAED (inserted in Figure 4a) and XRD (Figure 1b) patterns of the nanoparticles can be indexed as a trigonal phase of selenium, indicating the formation of t-selenium nanocrystals through the reduction of Na2SeO3 by NaCHO2, which is analogous to the phenomenon Zhang et al. described.26 After 4 h of the hydrothermal reaction, most of the t-selenium nanoparticles disappeared and many spherelike microparticles with diameter of 1-3 µm appeared (Figure 4b). After 8 h of the hydrothermal reaction, some rodlike nanostructures (indicated by arrows in Figure 4c) growing from the surface of the spherelike microparticles can be observed. High-magnification image of the rodlike nanostructures (Figure 4d) reveals that they are not solid nanorods but have some groovelike nanostructures. The length of these groovelike nanostructures ranged from 0.3 to 2 µm, and the diameter is about 250 nm. After 14 h reaction, the product mainly consists of two different forms of selenium nanostructures, nanogrooves (∼40%) and spherelike particles (∼40%) (Figure 4e), indicating many spherelike particles have developed into groovelike nanostructures. Besides the particles and the nanogrooves, there are small quantities (∼20%) of intermediary nanotubes in the sample, which have unclosed segments at the ends (Figure 4f). After 17 h reaction, the intermediary nanotubes developed into relatively complete nanotubes, which still had prongs at the ends (Figure 4g). When the reaction was lengthened to 20 h, most of the groovelike nanostructures were developed into more complete nanotubes (Figure 4 h), and about 5% selenium particles remained; however, the nanotubes still have small prongs at the ends. Finally, as the hydrothermal reaction proceeded long enough (25 h), most of the products are perfect

Figure 4. SEM images of the products prepared at various time: (a) 2 h, (b) 4 h, (c and d) 8 h, (e and f) 14 h, (g) 17 h, (h) 20 h, and (i) 25 h. (j and k) T-selenium nanowires synthesized with PVP as surfactant. (l) Surface morphology of the t-selenium microspheres.

Scheme 1. (a) Formation of t-Selenium Nanoparticles. (b) Formation of t-Selenium Microspheres. (c) Formation of Groovelike Nanostructures on the Surface of the Microspheres. (d) Formation of t-Selenium Nanotubes

nanotubes, and almost no selenium particles were observed (Figure 4i). The process of the shape transition from nanoparticles to nanotubes is summarized in Scheme 1. Growth Mechanism of the t-Selenium Nanotubes. Recently, the ultrasonic-assisted oriented attachment growth mechanism34,36 and the surfactant-directed growth mechanism35 were used to account for the formation of t-selenium nanotubes. However, we do not think the mechanisms work in the formation of the present t-selenium nanotubes because the synthetic route we adopted has neither an ultrasonic procedure nor surfactants. From the above experimental observations, we believe that formation of the t-selenium nanotubes can be rationally expressed as a kinetically controlled nucleation-dissolutionrecrystallization mechanism. First, when the reduction reaction

580 Crystal Growth & Design, Vol. 6, No. 2, 2006

was carried out in the hydrothermal system at 100 °C, it directly gave t-selenium nanocrystals, which were formed in the solution through a homogeneous nucleation process (Figure 4a). This phenomenon observed by us is similar to the observation of Zhang et al.34 but different from that observed by Ma et al.35 and Li et al.36 In their experiments, red R-Se immediately appeared in solution when HAc reacted with Na2SeSO3 or Na2SeO3 reacted with N2H4. We think that two factors may result in the formation of t-selenium nanocrystals in our experiment: (1) from the chemical reaction (*), it can be seen that an increase of pH restrains the reaction rate by suppressing the reduction of Se precursors; and (2) the reducibility of NaCHO2 is moderate. The two factors result in the slow rate of the reaction. As a parallel experiment, when the reducing agent NaCHO2 was replaced by N2H4 with stronger redox, red R-Se immediately appeared in solution. So we believe that the phenomenon of crystalline t-selenium nanoparticles instead of amorphous R-Se may be attributed to the slow nucleation rate of the selenium. When this colloidal dispersion was constantly hydrothermally treated at 100 °C, the small t-selenium nanoparticles started to dissolve into the solution and grow onto large nanoparticles of selenium via a process known as Ostwald ripening.39 Because there were no surfactants or polymers in the solution, the large selenium nanoparticles were able to grow into spherelike microparticles as shown in Figure 4b.40 In contrast, when polymer, such as poly (vinyl pyrrolidone) (PVP) was introduced into the hydrothermal system, no spherelike microparticles were obtained, but t-selenium nanowires were synthesized (Figure 4j,k). Second, under the present hydrothermal conditions, the spherelike microparticles were gradually dissolved to generate selenium atoms in the solution. There are numerous small protuberances in the surfaces of the microparticles (Figure 4l), which provide many high-energy sites for nanocrystalline growth.41 So, when the selenium atoms contained in the solution are transferred onto the surfaces of the spherelike microparticles, they will spontaneously nucleate onto the small protuberances. Furthermore, due to anisotropic crystal structure, there was an intrinsic tendency for the nucleation growth along the 1D direction.42 Nevertheless, from Figure 4d, it can be seen that the 1D nanostructures are neither solid nanowires nor hollow nanotubes; they are groovelike nanostructures. How do the nanogrooves form? Since the dissolution speed of crystalline t-selenium in solution progressed very slowly,43 it could not provide enough selenium atoms for the growth of solid rodlike crystals. This would lead to undersaturation in the central part of the growing regions of the surface of the spherelike microparticles.44,45 More important, the continuous feeding of selenium atoms on the surface of spherelike microparticle could diffuse into two directions: circumferential diffusion and diffusion parallel to the tuber axis ([001]), which will induce selenium crystals growth along the circumferential direction and the tuber axis direction. Because of the unusual anisotropic crystal structure of selenium (The t-selenium structure consists of a spiral chain of atoms with three atoms per turn. The structure consists of helical chains of covalently bound atoms parallel to the c-axis, which are in turn bound together through van der Waals interactions. This difference is very large and indicates a much greater cohesion between atoms in the same spiral than between those in different spirals.),42 it is rational to speculate that the growth rate of the latter is faster than that of the former. If the growth rate of the former is faster than the latter, nanotubes could form than nanogrooves on the surfaces of the spherelike microparticles. Combining the difference of

Xi et al. Scheme 2. (a) Growth along Tuber Axis Direction. (b) Growth along Circumferential Direction

the growth rate with the undersaturation in the central part of the growing regions, groovelike nanostructures can be grown. The formation process of the nanogroove is summarized in Scheme 2. Following the incessant dissolution of the spherelike tselenium microparticles, the groovelike nanostructures gradually developed into incomplete nanotubes with unclosed segments at the ends (Figure 4f,g,h), which further demonstrated that the tuber axis directional growth rate is faster than the circumferential directional growth rate. The growth of the incomplete nanotubes continued by consuming selenium atoms that were resulted from the dissolution of the microparticles. Furthermore, we noted that the two growth rates would tend to balance following the reduction of the selenium atoms in solution. Eventually, complete t-selenium nanotubes formed (Figure 4i). Controlling experiments have been done to test the nucleationdissolution-recrystallization process. Particularly, the t-selenium nanoparticles generated at the initial step of the hydrothermal reduction reaction were precipitated out of the growth solution and then one more time out of distilled water to remove unreacted precursors (SeO32-) and excess reducing agent (CHO2-). The nanoparticles were then dispersed in 55 mL of distilled water and transferred to an autoclave to continue the hydrothermal process at 100 °C. After 23 h, t-selenium nanotubes were obtained as the final products. It was also found that when the t-selenium micropheres replaced the t-selenium nanoparticles as the precursors for the hydrothermal process, the same result was obtained. Furthermore, no nanoparticles were found during the dissolution-recrystallization process, which demonstrated that the oriented attachment growth mechanism is not applicable in the present hydrothermal process.46-48 On the basis of the experimental phenomena, it can be concluded that the nucleation-dissolution-recrystallization process proposed here is rational. In previous related literature, using commercial t-selenium powders as raw material, t-selenium microtubes have been synthesized via a dissolution-recrystallization process.42 However, t-selenium nanotubes are not obtained via the direct dissolution-recystallization process. In present study, it is believed that the formation nanogrooves on the surface of the microspheres and diffusion of materials from inside the spheres is a key step to grow t-selenium nanotubes. In addition, the small protuberances in the surfaces of the microparticles (Figure 4l), which provide many high-energy sites for nanocrystalline growth,41 may help in the formation of groovelike nanostructures. As it is difficult to monitor the reaction once the system is sealed, quantities of work are needed to investigate the kinetics of the reaction. Influence of Reaction Temperature. Experimental studies found that the final morphologies of the products are strongly affected by the reaction temperature. At 120-150 °C, the products are mixture of t-selenium nanotubes and nanowires (Figure 5a). At 160 °C, a large quantity of t-selenium nanowires was obtained (Figure 5b). As the reaction temperature was further increased to 180 °C, the reaction only gave t-selenium

Nucleation-Dissolution-Recrystallization

Crystal Growth & Design, Vol. 6, No. 2, 2006 581

Reactions with more large quantities of NaOH (40 mL of 1 M NaOH) were also run and showed an abundance of large polyhedrons (Figure 6c). Because selenium is more easily dissolved in alkaline solution, following the increase of the NaOH quantity, the solubility of selenium is increased. We speculate that the increase of the solubility lead to the appearance of the porous microtubes and polyhedrons. Control reactions in which no NaOH was added gave R-Se initially, which then gradually developed into t-Se microrods and a small quantity of nanorods (Figure 6d). 4. Conclusion

Figure 5. TEM and SEM images of the products prepared at different temperature: (a) 120-150 °C, (b) 160 °C, (c) 180 °C, (d) after 6 h reaction at 180 °C.

In summary, with NaCHO2 as reductant, t-selenium nanotubes were successfully synthesized via a simple hydrothermal reduction process. The method is significant in that it does not require a surfactant or polymer template, or ultrasound, which would allow researchers to investigate the growth mechanism of the selenium nanotubes in more natural conditions. The electron microscopy investigation indicated that the growth process of the nanotubes was governed by a nucleation-dissolutionrecrystallization growth mechanism. Acknowledgment. This work was supported by National Science Foundation of China and the 973 Project of China. References

Figure 6. SEM images of the products prepared at different concentrations of NaOH: (a and b) 20 mL, (c) 40 mL, (d) without NaOH.

rodlike microcrystals (Figure 5c). Figure 5d shows the image of the products obtained after 6 h reaction at 180 °C. From the image, it can be seen that the rodlike crystal grown on the surface of the spherelike t-selenium microcrystal is not groovelike but solid. We believe that these experimental phenomena can be rationally expressed as a concentration effect of selenium in solution because following the increase of reaction temperature the solubility of selenium is increased. The increase of solubility of selenium will result in the increase of selenium atoms concentration in solution. Because of the increased concentration of selenium atoms in solution, the undersaturation in the central part of the growing regions of the spherelike particles will decrease or disappear, so there are enough selenium atoms for the growth of the growing solid rodlike crystals. Influences of the Concentrations of NaOH. The quantity of NaOH was also found to play an important factor in determining the morphology of the final product. Increasing the amount of NaOH (20 mL) in the reaction mixture leads to porous t-Se microtubes appearing (Figure 6a). The microtubes are 3-5 µm in diameter and 15-20 µm in length. Figure 6b gives the SEM image of the tips of the microtubes, which clearly demonstrate that the inner portion of the microtubes is porous. It was also observed that the aggregations of the porous microtubes are often radical, which indicate that the porous microtures are developed from the same t-Se microsphere.

(1) Ishikawa, T.; Yamaoka, H.; Y. Harada, Y.; Fujii, T.; Nagasawa, T. Nature 2002, 416, 64. (2) Liu, C.; Zou, B.; Rondinone, A. J.; Zhang, Z. J. J. Am. Chem. Soc. 2001, 123, 4344. (3) Penn, R. L.; Banfield, J. F. Science 1998, 281, 969. (4) (a) Yu, W. W.; Wang, Y. A.; Peng, X. G. Chem. Mater. 2003, 15, 4300. (b) Battaglia, D.; Li, J. J.; Wang, Y. J.; Peng, X. G. Angew. Chem., Int. Ed. 2003, 42, 5035. (5) Salem, A. K.; Chen, M.; Hayden, J.; Leong, K. W.; Searson, P. C. Nano. Lett. 2004, 4, 1163. (6) Sun, Y. G.; Tao, Z. L.; Chen, J.; Herricks, T.; Xia, Y. N. J. Am. Chem. Soc. 2004, 126, 5940. (7) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (8) Kong, X. Y.; Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 570. (9) Wang, X. D.; Ding, Y.; Summers, C. J.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 8773. (10) Kong, X. Y.; Ding, Y.; Yang, R.; Wang, Z. L. Science 2004, 303, 1348. (11) Zhao, Y.; Zhang, Y.; Zhu, H.; Hadjipanayis, G. C.; Xiao, J. Q. J. Am. Chem. Soc. 2004, 12, 6874. (12) Leontidis, E.; Orphanou, M.; Kyprianidou-Leodidou, T.; Krumeich, F.; Caseri, W. Nano Lett. 2003, 3, 569. (13) Li, Y.; Sui, M.; Ding, Y.; Zhang, G.; Zhuang, J.; Wang, C. AdV. Mater. 2000, 12, 818. (14) Salvadori, B.; Dei, L. Langmuir 2001, 17, 2371. (15) Sampanthar, J. T.; Zeng, H. C. J. Am. Chem. Soc. 2002, 124, 6668. (16) Iijima, S. Nature 1991, 354, 56. (17) Ebbesen, T. W.; Ajayan, P. M. Nature 1992, 358, 220. (18) Thess, A.; Lee, R.; Nikolaev, P.; Dai, H.; Petit, P.; Robert, J.; Xu, C.; Lee, Y. H.; Kim, S. G.; Rinzler, A. G.; Colbert, D. T.; Scuseria, G. E.; Tomanek, D.; Fischer, J. E.; Smalley, R. E. Science 1996, 273, 483. (19) Lee, S.-M.; Cho, S.-N.; Cheon, J. AdV. Mater. 2003, 15, 441. (20) Ibragimov, N. I.; Abutalibova, Z. M.; Agaev, V. G. Thin Solid Films 2000, 359, 125. (21) Berger, L. I. Semiconductor Materials; CRC Press: Boca Raton, FL, 1997. (22) Poborchii, V. V.; Kolobov, A. V.; Tanaka, K. Appl. Phys. Lett. 1999, 74, 215. (23) Johnson, J. A.; Saboungi, M. L.; Thiyagarajan, P.; Csencsits, R.; Meisel, D. J. Phys. Chem. B 1999, 103, 59. (24) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (25) Link, S.; Sayed, M. A. E. L. J. Phys. Chem. B 1999, 103, 8410. (26) Gates, B.; Yin, Y. D.; Xia, Y. N. J. Am. Chem. Soc. 2000, 122, 12582.

582 Crystal Growth & Design, Vol. 6, No. 2, 2006 (27) Jiang, Z.; Xie, Z.; Xie, S.; Zhang, X.; Huang, R.; Zheng, L. Chem. Phys. Lett. 2003, 368, 425. (28) Gautam, U. K.; Natha, M.; Rao, C. N. R. J. Mater. Chem. 2003, 13, 2845. (29) Cao, X. B.; Xie, Y.; Zhang, S. Y.; Li, F. Q. AdV. Mater. 2004, 16, 649. (30) Ma, Y. R.; Qi, L. M.; Shen, W.; Ma, J. M. J. Phys. Chem. B 2005, 21, 6161. (31) Ren, L.; Zhang, H. Z.; Tan, P. H.; Chen, Y. F.; Zhang, Z. S.; Chang, Y. Q.; Xu, J.; Yang, F. H.; Yu, D. P. J. Phys. Chem. B 2004, 108, 4627. (32) Mayers, B. T.; Liu, K.; Sunderland, D.; Xia, Y. N. Chem. Mater. 2003, 15, 3852. (33) Gates, B.; Mayers, B.; Grossman, A.; Xia, Y. N. AdV. Mater. 2002, 14, 1749. (34) Zhang, H.; Yang, D.; Ji, Y. J.; Ma, X. Y.; Xu, J.; Que, D L. J. Phys. Chem. B 2004, 108, 1179. (35) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Cheng, H. M. AdV. Mater. 2004, 16, 1023. (36) Li, X. M.; Li, S. Q.; Zhou, W. W.; Chu, H. B.; Chen, W.; Li, I. L.; Tang, Z. K. Cryst. Growth Des. 2005, 5, 911.

Xi et al. (37) Gao, X.; Zhang, J.; Zhang, L. AdV. Mater. 2002, 14, 290. (38) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. AdV. Funct. Mater. 2002, 12, 219. (39) Roosen, A. R.; Carter, W. C. Physica A 1998, 261, 232. (40) Zhang, Z.; Zhao, B.; Hu, L. J. Solid State Chem. 1996, 121, 105. (41) Nucci, J. A.; Keller, R. D.; Field, P.; Shacham-Diamand, Y. Appl. Phys. Lett. 1997, 70, 1242. (42) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. Y. J. Mater. Chem. 2002, 12, 2755. (43) Dean, J. A. Lange’s Handbook of Chemistry; Dean, J. A., Ed.; McGraw-Hill: New York, 1985. (44) Tang, Q.; Liu, Z. P.; Li, S.; Zhang, S. Y.; Liu, X. M.; Qian, Y. T. J. Cryst. Growth 2003, 259, 208. (45) Mayers, B.; Xia, Y. N. AdV. Mater. 2002, 14, 279. (46) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (47) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (48) Korgel, B. A.; Fitzmaurice, D. AdV. Mater. 1998, 10, 661.

CG050444C