Polyol Synthesis of Silver Nanowires: An Extensive Parametric Study

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Polyol Synthesis of Silver Nanowires: An Extensive Parametric Study Sahin Coskun, Burcu Aksoy, and Husnu Emrah Unalan* Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara, Turkey 06800

bS Supporting Information ABSTRACT: Silver nanowires have been synthesized by a polyol process. A detailed parametric study determining the relationship between final morphology of the products and temperature, injection rate, molar ratio of poly(vinylpyrrolidone) to silver nitrate, sodium chloride amount, and stirring rate is presented. The as-synthesized silver nanowires are analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The dependency of nanowire morphology and aspect ratio on synthesis parameters is shown via SEM images. Furthermore, the nanowire diameter is found to decrease with stirring rate, poly(vinylpyrrolidone) to silver nitrate molar ratio, and temperature and to increase with injection rate. The lack of sodium chloride and its presence in excess amounts resulted in the formation of particles with different sizes in addition to nanowires. In addition, poly(vinylpyrrolidone) to silver nitrate molar ratio is found to affect the morphology of the resulting nanostructure, leading to formation of particles at high and low ends. The results reported in this paper provide a basis for optimizing silver nanowire growth through the self-seeding polyol method.

’ INTRODUCTION During the past decades, one-dimensional (1D) metallic nanostructures have received a lot of attention due to their different electrical, optical, magnetic, and thermal properties as compared to their bulk counterparts. Moreover, these 1D metallic nanostructures have found possible applications in various devices.1
13 Since bulk silver (Ag) has the highest electrical and thermal conductivity among all metals, its 1D structure has received considerable attention. Ag nanowires have been used in various applications such as catalysis,14 surface-enhanced Raman scattering (SERS),15,16 photonic crystals,17 microelectronics,18 and biological nanosensors.19 Up to now, several methods for the synthesis of Ag nanowires have been developed. These methods include chemical synthesis,20,21 electrochemical technique,22,23 hydrothermal method,24,25 ultraviolet irradiation photodetection technique,26,27 DNA template,28,29 porous materials template,30,31 and polyol process.32
34 When these methods are compared in terms of cost, yield, and simplicity, the polyol process seems to be the most promising one.35 The polyol process was first introduced by Fievet and co-workers36 for the synthesis of colloidal particles of metals and alloys such as silver (Ag), gold (Au), copper (Cu), cobalt (Co), palladium (Pd), iridium (Ir), platinum (Pt), ruthenium (Ru), cobalt
nickel alloy (CoNi), and iron
nickel alloy (FeNi). The polyol process is based on the reduction of an inorganic salt by a polyol at an elevated temperature. A surfactant is used to prevent agglomeration of the colloidal particles. Afterward, Xia and co-workers37 developed a suitable polyol process for the shape-controlled synthesis of Ag nanowires. In this method, ethylene glycol (EG) was used as both solvent and reducing agent, poly(vinylpyrrolidone) (PVP) was used as stabilizing agent, and silver nitrate (AgNO3) was used as r 2011 American Chemical Society

Ag source. Since then, many research groups have explored different approaches in order to improve the polyol process. There are also several reports in literature that focus on the effect of different parameters on the polyol synthesis of Ag nanowires.38
43 However, in these studies only a few parameters of the polyol process have been investigated, but not in extensive detail. In this work, we demonstrate an extensive parametric study on self-seeding polyol synthesis of Ag nanowires. We examined effects of temperature, injection rate, polymer to AgNO3 ratio, and sodium chloride (NaCl) amount. Moreover, we investigated a novel parameter that has not been reported yet: the effect of solution stirring rate.

’ EXPERIMENTAL PROCEDURES All glassware used in the experiments were cleaned with deionized water, basic solution (pH 11), acidic solution (pH 2), acetone (99.8%), isopropyl alcohol (99.8%), and finally deionized water (18.3 MΩ). All chemicals were purchased from Sigma
Aldrich and used without further purification. In a typical experiment, 10 mL of 0.45 M EG solution of PVP (monomer-based calculation MW = 55 000) was prepared and 7 mg of NaCl (99.5%) was added into the solution. After that, the solution was heated at 170 °C in a two-necked round flask. In the meantime, a 0.12 M AgNO3 (99.5%) solution in 5 mL of EG was prepared and added dropwise into the PVP solution by an injection pump (Top-5300 model syringe pump) at a rate of 5 mL/h. The solution was stirred at a rate of 1000 rpm by a magnetic stirrer during the whole process. During the nanowire synthesis, with the introduction of Ag+ ions into the Received: July 11, 2011 Revised: August 22, 2011 Published: August 23, 2011 4963

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Figure 1. (a) SEM images of nanowires. Inset shows their pentagonal cross section. (b) XRD pattern for the nanowires. (c) Low- and (d) highresolution TEM images of nanowires.

solution, Ag nanoparticles start to form via homogeneous nucleation. Chemical adsorption of PVP molecules onto the surfaces of as-formed Ag nanoparticles is the reason for them to remain at nanoscale.45 As the process continues, some of the Ag nanoparticles start to dissolve and grow as larger nanoparticles via the mechanism known as Oswald ripening.46 With the passivation of some facets of these particles by PVP, some nanoparticles can grow into multitwin particles. PVP is believed to passivate (100) faces of these multitwin particles and leave (111) planes active for anisotropic growth at [110] direction. As the addition of Ag+ ions continue, multitwin particles grow into Ag nanowires. Final color indicates the presence of nanowires. Upon the completion of dropwise addition, the nanowire solution was annealed for 30 min at 170 °C and finally air-cooled to room temperature. Then, in order to separate polymer from the Ag nanowires, the solution was diluted with acetone (in a ratio of 1:5) and centrifuged two times at 6000 rpm for 20 min. After that, the nanowires were dispersed in ethanol and again centrifuged at 6000 rpm for 20 min. The final product was dispersed in ethanol for further characterization. For the length and diameter analysis, a diluted ethanolic nanowire solution was spraycoated onto silicon substrates. Obtained SEM images from these substrates were then processed through image analysis software. A representative SEM image used for the length and diameter calculations is given in Figure S1 (Supporting Information). Apart from the parameters given above, a parametric study on the effect of temperature (in a range of 110
200 °C), AgNO3 solution injection rate (in a range of 1
300 mL/h), PVP:AgNO3 molar ratio (in a range of 3:1
11:1), NaCl addition (in a range of 0
85.5 μM), and stirring rate throughout the reaction duration (in a range of 0
1000 rpm) was conducted. Ag nanowires were characterized by SEM, TEM and XRD. The SEM studies were performed on a FEI Nova Nano SEM 430 microscope operated at 10 kV. For the preparation of SEM samples, an ethanolic solution of nanowires was simply sprinkled onto SEM stubs with carbon tapes and allowed to dry. No coating was utilized for analysis. Highresolution transmission electron microscopy (HRTEM) was performed on a JEOL TEM 2100F microscope, operated at 200 kV. For the preparation of TEM samples, ethanolic solution of nanowires were dropcast onto lacey carbon-coated copper grids and allowed to dry. The crystal structure of the Ag nanowires was investigated by XRD on a

Figure 2. SEM images of Ag nanowires grown through the polyol process conducted at (a) 110, (b) 130, (c) 150, (d) 170, (e) 190, and (f) 200 °C. All scales are the same. Changes in (g) nanowire diameter and (h) length with reaction temperature are also shown. Lines are drawn for illustration. Rigaku D/Max-2000 pc diffractometer with Cu Kα radiation operating at 40 kV.

’ RESULTS AND DISCUSSION A SEM image of the Ag nanowires synthesized by the polyol process is shown in Figure 1a. The presence of nanosized cubic and pyramidal byproduct particles is also evident in the figure. With the described growth parameters, diameter and length of the nanowires are found to be 62 nm and 8.4 μm, respectively. The inset shows the pentagonal cross section of the Ag nanowires. The XRD pattern of the Ag nanowires is shown in Figure 1b. The peaks at 38° and 44° correspond to (111) and (200) planes of Ag, respectively (JCPDS card 04-0783). No impurities are detected from this pattern within the resolution limit of XRD. Low- and high-resolution TEM images of the Ag nanowires are provided in Figure 1 panels c and d, respectively. The diameter of the nanowires becomes more evident in those 4964

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conversion (shown in eq 1) occurs above 150 °C with the help of oxygen.47 2HOCH2CH2OH þ O2 f 2HOCH2CHO þ 2H2 O

Figure 3. SEM images of Ag nanowires synthesized with different injection rates of (a) 1, (b) 3, (c) 5, (d) 6, (e) 8, and (f) 300 mL/h. All scales are the same. Changes in (g) nanowire diameter and (h) length with injection rate are also shown.

images, and TEM images indicate the presence of a residual PVP layer on top of the nanowires following purification. In the following experiments, we kept all the parameters constant except the one being investigated. The parameters used in the experiments are 5 mL/h injection rate, 170 °C growth temperature, 7.5:1 PVP:AgNO3 molar ratio, 12 μM NaCl addition, and 1000 rpm stirring rate. Effect of Temperature. The temperature at which the reaction is conducted is an important parameter for Ag nanowire synthesis via polyol process. SEM images of the obtained Ag nanostructures following growth at 110, 130, 150, 170, 190, and 200 °C are shown in Figure 2 panels a
f, respectively. The change in nanowire length and diameter with respect to the growth temperature is plotted in Figure 2 panels g and h, respectively. It is found that, below a critical temperature, high aspect ratio Ag nanowire synthesis is not possible. High temperatures are also crucial for the conversion of ethylene glycol to glycolaldehyde, which reduces Ag+ ions to Ag atoms. This

ð1Þ

In another study, Sun et al.42 claimed that the importance of temperature can be attributed to the deficiency of thermal energy for the formation of specific faces. For instance, in our case following growth at 110 °C (Figure 2a), instead of rod or nanowire structures, micrometer-sized Ag structures are formed. As the temperature increases to 130 and 150 °C, twin particles start to form and their tendency toward growing into rod-shaped structure increases. Ag nanorods with low aspect ratio, formed at 130 and 150 °C, are shown in Figure 2 panels b and c, respectively. Average length of the nanowires above 130 °C is found to increase with temperature. Upon the increase in reaction temperature to 170 °C, the available thermal energy becomes enough for the formation of multitwin particles, leading to the formation of Ag nanowires with high yield. Further increase in the reaction temperature (190 and 200 °C) increases the number of multitwin particles formed in the early steps of reaction due to availability of excess thermal energy for the formation of twinning. As a result, initially formed Ag nanowires could grow up to 50 μm. However, most of the others have low aspect ratios due to limited amount of Ag atoms against extreme number of Ag nanowires. Hence, they cannot find enough Ag atoms for further growth. Figure S2a (Supporting Information) shows a SEM image of the nanowires synthesized at 190 °C deposited onto silicon substrates with low density for length analysis. Corresponding length distribution is given in Figure S2b (Supporting Information). Effect of Injection Rate. SEM images of the Ag nanowires synthesized with injection rates of 1, 3, 5, 6, 8, 300 mL/h while maintaining the other parameters constant, are shown in Figure 3 panels a
f, respectively. Ag nanowires can be clearly seen in all images. When the injection rate is very low or very high, formation of Ag nanowires as well as micrometer-sized Ag particles is observed. At high injection rates, extensive numbers of Ag clusters with very small diameters are formed because of rapid supersaturation of Ag. Instead of forming multitwin particles, these small clusters dissolve and coalescence with bigger particles via the Oswalt ripening process. Hence, we observe formation of micrometer-sized Ag particles with an increase in the injection rate. Moreover, we conducted an experiment to investigate the importance of dropwise addition of Ag+ ions into the PVP/EG solution at 300 mL/h, the result of which is shown as a SEM image in Figure 3f. This injection rate corresponds to addition of all the AgNO3/EG solution into PVP/EG solution in a minute. SEM image reveals micrometer-sized Ag particles with different shapes for this particular case. Although Ag nanowires are formed even at this injection rate, they are short and their yields are very low. At this injection rate, the maximum number of Ag clusters instantly form. This increases the number of dissolving clusters, eventually leading to the formation of micrometer-sized Ag particles. On the other hand, when the injection rate is considerably lower, due to deficiency of Ag+ ions, large clusters and Ag nanoparticles start to dissolve and coalescence to form bigger particles. Therefore, the amount of micrometer-sized Ag particles is found to be high at low injection rates. As seen from Figure 3c, 5 mL/h corresponds to optimum Ag+ injection rate into the solution, and at this rate, only nanoparticles of Ag in addition to Ag nanowires are observed. Plots for the nanowire diameter and 4965

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Figure 4. SEM images of Ag nanowires synthesized at different PVP: AgNO3 molar ratios of (a) 3:1, (b) 4.5:1, (c) 6:1, (d) 7.5:1, (e) 9:1, and (f) 11:1. All scales are the same. Changes in (g) nanowire diameter and (h) length with PVP:AgNO3 molar ratio are also shown.

length with respect to injection rate are given in Figure 3 panels g and h, respectively. When the injection rate is considerably low (1 mL/h), deficiency of Ag+ concentration leads to the formation of multitwin particles with small diameters. As a result, these multitwin particles grow as thin Ag nanowires. Injection rates of 3, 5, and 6 mL/h result in the formation of Ag nanowires with diameters similar to each other. Further increase in the injection rate increases Ag+ concentration and formation of larger multitwin particles at the early stages of the reaction, leading to the formation of large-diameter Ag nanowires. Slow injection rate also decreases the number of initially formed multitwin particles. Therefore, these multitwin particles will grow as long nanowires. Further increase in the injection rate up to 5 mL/h decreases the length of the formed nanowires. Beyond this point, formation of the micrometer-sized Ag particles decreases the number of Ag nanowires and increases their length. Effect of PVP:AgNO3 Molar Ratio. The final morphologies of Ag nanostructures at the end of the polyol process are strongly

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Figure 5. SEM images of the polyol process products produced with different NaCl amounts of (a) 0, (b) 8.5, (c) 12, (d) 17.1, (e) 25.6, and (f) 85.5 μM. All scales are the same. (g) XRD pattern of AgCl particles (JCPDS card 31-1238).

dependent on the PVP:AgNO3 molar ratio. In Figure 4 panels a
f, SEM images of the nanowires synthesized with different PVP:AgNO3 molar ratios of 3:1, 4.5:1, 6:1, 7.5:1, 9:1, and 11:1 are shown, respectively. When the PVP:AgNO3 molar ratio is 3:1 or 4.5:1, the passivation of {100} faces of multitwin particles is insufficient and Ag nanostructure growth occurs on both {111} and {100} faces. Under these conditions, Ag nanowires synthesized at low PVP:AgNO3 molar ratios have large diameters. Moreover, the multitwin particles that could not grow into nanowires agglomerate and form large amounts of micrometer-sized Ag particles. A TEM image of micrometer-sized Ag particle is provided in Figure S3 (Supporting Information). As the PVP: AgNO3 molar ratio increases, the diameter of the nanowires decreases gradually, as shown in Figure 4g. On the other hand, with increasing PVP:AgNO3 molar ratio, micrometer-sized Ag particle formation is observed, as shown in Figure 4 panels e and f. 4966

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Crystal Growth & Design This can be attributed to excess PVP molecules covering all surfaces of Ag nanoparticles, including the ones that must be left active. These excess PVP molecules are also responsible for blocking anisotropic growth (1D) of Ag nanowires. As shown in Figure 4h, the length of Ag nanowires decreases with increasing PVP:AgNO3 molar ratio. Optimum PVP:AgNO3 molar ratio is then determined as 7.5:1, as inferred from Figure 4d. Under these conditions, only {100} faces of multitwinned particles are passivated, and as the Ag atoms join active (111) planes, longitudinal growth in the [110] direction becomes favorable. Effect of NaCl Amount. There are a few reports on the effect of chloride ions during the polyol process. It was claimed by Wiley et al.43 that the chloride ions stabilize Ag nanoparticles against aggregation. This stabilization may also prevent the growth of these nanoparticles. As a result, nanoparticles that can grow will dissolve via Oswalt ripening. In another study by Im et al.,44 the effect of hydrochloric acid (HCl) in the polyol synthesis of Ag nanocubes was investigated. It was stated that dissolved Cl
adsorbs on the 100 facets of twinned particles and inhibits the etching process of nitric acid (HNO3), which is formed during the reaction. Moreover, the silver chloride (AgCl) precipitate that forms in the early stages of the reaction serves as a seed for multitwinned particles. Wang et al.25 asserted that the formed AgCl nanoparticles can be reduced slowly and decreased reaction rate makes anisotropic growth of Ag nanowires favorable. We have investigated the effect of NaCl in the polyol process in an extensive range. SEM images of the Ag structures synthesized via the polyol process with NaCl amounts of 0, 8.5, 12, 17.1, 25.6, and 85.5 μM are shown in Figure 5 panels a
f, respectively. Figure 5a shows a SEM image of Ag nanoparticles synthesized without the addition of NaCl. As shown in the figure, the absence of Cl
ions in the solution results in the formation of only Ag nanoparticles without nanowires. This result may arise from fast reduction of Ag+ ions to Ag0 atoms since Ag is only present in ionic form in the reaction. On the other hand, addition of NaCl leads to the presence of Ag in a compound form (AgCl) and slows down the reduction of Ag+ ions. These slow reaction conditions enable Ag nanoparticles to grow as multitwin particles, which would then grow in the form of nanowires. In order to investigate the effect of slow reduction, we conducted an experiment at a very slow injection rate (0.5 mL/h) in the absence of NaCl. SEM images of the reaction products, given in Figure S4 panels a and b (Supporting Information), revealed the formation of wirelike structures. Therefore, slow reaction kinetics could lead to the formation of 1D wirelike structures and this is analogous with the effect of NaCl. Ag nanowires form if NaCl is used during the synthesis. In Figure 5b, a SEM image of Ag nanowires synthesized with 8.5 μM NaCl addition is shown. Besides nanowires, micrometersized Ag particles are also formed. This could be attributed to the reaction rate being still fast due to insufficient formation of AgCl compound. So, the reduction of Ag+ ions into Ag0 atoms occurs faster. Hence, some Ag nanoparticles grow as multitwin particles and others as micrometer-sized particles. We have determined that the optimum amount of NaCl that needs to be added during the polyol process is 12 μM for the formation of Ag nanowires without the micrometer-sized particles, revealed by the SEM image in Figure 5c. Further increase in NaCl amount to 17.1 and 25.6 μM again started the formation of micrometer-sized Ag particles, as shown in the SEM images provided in Figure 5 panels d and e. Excess amount of NaCl, 85.5 μM, allows the precipitation of AgCl particles that are micrometers in size, as

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Figure 6. SEM images of Ag nanowires synthesized with stirring rates of (a) 0, (b) 300, (c) 700, and (d) 1000 rpm. All scales are the same. Effects of stirring rate on (e) diameter and (f) length of the nanowires are also shown. Lines are a visual aid.

shown in the SEM image in Figure 5f. Formation of AgCl compound is attributed to the saturation of Cl
in the growth solution. Hence, the reaction rate slows down, leading to the formation of AgCl particles without any nanowires. Figure 5g shows the XRD pattern of the AgCl particles. All the diffraction peaks can be indexed to face-centered cubic AgCl. Although the presence of NaCl in self-seeding polyol synthesis of Ag nanowires is crucial, its amount has not shown any remarkable effect on the diameter and length of the Ag nanowires. We believe that the role of NaCl during polyol synthesis of Ag nanowires is as follows: (i) AgCl is formed by utilization of its Cl
ions, (ii) the amount of free Ag+ ions in solution is decreased, and (iii) the kinetics of the reduction process of AgNO3 are slowed down. Effect of Stirring Rate. We have found that the solution stirring rate also affects the polyol process. SEM images of Ag nanowires synthesized at stirring rates of 0, 300, 700, and 1000 rpm are shown in Figure 6 panels a
d, respectively. Variation of Ag nanowire diameter with solution stirring rate is provided in Figure 6e. Ag nanowire formation is observed with all stirring rates, even in the stagnant solution. However, the nanowires synthesized in the stagnant solution have an average diameter of 110 nm. Formation of micrometer-sized particles is also evident from the SEM image. In the stagnant growth solution, dropwise addition of Ag+ ions into the solution increases the local concentration of Ag+ ions. This leads to the formation of excessive Ag particles. As the stirring rate increases, diameter and length of Ag nanowires decrease, as shown in Figure 6 panels e and f, respectively. As inferred from the standard deviations, 4967

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Crystal Growth & Design stirring also homogenizes the diameter and length of the synthesized nanowires. Stirring homogenizes Ag+ ion concentration throughout the whole solution. Increased stirring rate decreases the possibility of Ag atoms and multitwin particles coming together. Therefore, these multitwin particles grow as thinner and shorter nanowires. Polyol process, as reported in this work, allows control over the length and diameter of Ag nanowires. This is of prime importance because Ag nanowires are primarily utilized in electrical and thermal applications. Electrical properties of the nanowires, such as their resistance, will be determined by their length and diameter. Similarly, the diameter and length of the nanowires will dictate electron
phonon coupling and therefore their thermal conductivity. Summary of the reaction conditions and the corresponding nanowire properties (length and diameter with standard deviations) are tabulated and provided in Table S1 (Supporting Information).

’ CONCLUSIONS The process parameters such as temperature, injection and stirring molar rates, PVP:AgNO3 ratio, and NaCl amount for the synthesis of Ag nanowires by the polyol process are studied in detail. Temperature has a profound effect on nanowire formation. Below a certain temperature, high aspect ratio Ag nanowire formation is not possible. As reaction temperature increases, anisotropic growth becomes favorable. Optimum Ag nanowire growth temperature for the polyol synthesis is found to be 170 °C. Beyond that temperature, the length distribution of nanowires widens. Injection rate affects the final morphology of Ag nanostructures. Both slow and fast injection rates result in the formation of micrometer-sized Ag particles in addition to nanowires. Deficient and excess amounts of PVP over AgNO3 result in the formation of undesired Ag structures in addition to nanowires. The presence of NaCl in the polyol process is also critical for the formation of Ag nanowires. Absence of NaCl results in the formation of only Ag particles. We have determined the amount of NaCl that needs to be added to growth solution so that most of the byproducts can be eliminated. Beyond its optimum value (12 μM), due to oversaturation, micrometersized Ag particles start to grow dominantly, suppressing Ag nanowire formation. Stirring rate affects the diameter and length of Ag nanowires. As stirring is done faster, the diameter and length of Ag nanowires decrease. Finally, our work demonstrates the optimum conditions to synthesize Ag nanowires with desired aspect ratios via this simple, solution-based selfseeding polyol process. ’ ASSOCIATED CONTENT

bS

Supporting Information. Four figures showing a representative SEM image used in dimensional analysis, SEM image and length distribution of Ag nanowires synthesized at 190 °C, TEM image of micrometer-sized Ag particle, SEM images of Ag structures synthesized with an injection rate of 0.5 mL/h in the absence of NaCl, and one table listing Ag nanowire diameters and lengths with standard deviations. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail [email protected]; tel+ 90 312 210 5939; fax +90 312 210 25 18.

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’ ACKNOWLEDGMENT This work was supported by The Scientific and Technological Research Council of Turkey (TUBITAK, Grant 109M487) and the Distinguished Young Scientist Award of the Turkish Academy of Sciences (TUBA). S.C. acknowledges support from METUOYP Project 439, and Assistant Professor Dr. Y. Eren Kalay is greatly thanked for his help with TEM analysis. ’ REFERENCES (1) Pascual, J. I.; Mendez, J.; Herrero, J. G.; Baro, M. A.; Garcia, N.; Landman, U.; Luedtke, W. D.; Bogachek, E. N.; Cheng, H. P. Science 1995, 267, 1793. (2) Walter, E. C.; Ng, K.; Zach, M. P.; Penner, R. M; Favier, F. Microelectron. Eng. 2002, 61, 555. (3) Wu, Y; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430, 704. (4) Strevens, A. E.; Drury, A.; Lipson, S. M.; Kroell, M.; Blau, W. J.; Hoerhold, H. H. Appl. Phys. Lett. 2005, 86, No. 143503. (5) Murphy, C. J.; Sau, T. K.; Gole, A.; Orendorff, C. J. MRS Bull. 2005, 30, 349. (6) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (7) Tao, A. R; Yang, P. J. Phys. Chem. B. 2005, 109, 15687. (8) Whitney, T. M.; Jiang, J. S.; Searson, P. C.; Chien, C. L. Science 1993, 261, 1316. (9) Ciraci, S.; Buldum, A.; Batra, I. P. J. Phys.: Condens. Matter 2001, 13, R537–R568. (10) Wu, B.; Heidelberg, A; Boland, J. J. Nat. Mater. 2005, 4, 525. (11) Atashbar, M. Z.; Singamaneni, S. Sens. Actuators, B 2005, 111, 13. (12) Kovtyukhova, N.; I. Mallouk, T. E. Chem.—Eur. J. 2002, 8, 4354. (13) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 8, 20. (14) Chimentao, R. J.; Kirm, I; Medina, F.; Rodriguez, X.; Cesteros, Y.; Salagre, P.; Sueiras, J. E. Chem. Commun. 2004, 846. (15) Rycenga, M.; Kim, M. H.; Camargo, P. H. C.; Cobley, C.; Li, Z. Y.; Xia, Y. N. J. Phys.Chem. A 2009, 113, 3932. (16) Zhang, J. T.; Li, X. L.; Sun, X.; Li, Y. D. J. Phys. Chem. B 2005, 109, 12544. (17) Hu, X.; Chan, C. T. Appl. Phys. Lett. 2004, 85, 1520. (18) Jiang, H. J.; Moon, K. S.; Hua, F.; Wong, C. P. Chem. Mater. 2007, 19, 4482. (19) Paul, A. Nat. Biotechnol. 2004, 22, 47. (20) Kim, S. H.; Choi, B. S.; Kang, K.; Choi, Y. S.; Yang, S. I. J. Alloys Compd. 2007, 433, 261. (21) Grijalva, A. S.; Urbina, R. H.; Silva, J. F. R.; Borja, M. A.; Barraza, F. F. C.; Amarillas, A. P. Physica E (Amsterdam, Neth.) 2005, 25, 438. (22) Mazur, M. Electrochem. Commun. 2004, 6, 400. (23) Huang, L. M.; Wang, H. T.; Wang, Z. B.; Mitra, A; Bozhilov, K. N.; Yan, Y. S. Adv. Mater. 2002, 14, 61. (24) Xu, J.; Hu, J.; Peng, C. J.; Liu, H. L.; Hu, Y. J. Colloid Interface Sci. 2006, 298, 689. (25) Wang, Z.; Liu, J.; Chen, X.; Wan, J.; Qian, Y Chem.—Eur. J. 2005, 11, 160. (26) Zhou, Y.; Yu, S.; Wang, C.; Li, X.; Zhu, Y.; Chen, Z. Adv. Mater. 1999, 11, 850. (27) Zou, K.; Zhang, X. H.; Duan, X. F.; Meng, X. M.; Wu, S. K. J. Cryst. Growth 2004, 273, 285. (28) Braun, E.; Eichen, Y.; Sivan, U.; Yoseph, G. B. Nature 1998, 391, 775. (29) Keren, K.; Krueger, M.; Gilad, R.; Ben-Yoseph, G.; Sivan, U.; Braun, E. Science 2002, 297, 72. (30) Berchmans, S.; Nirmal, R. G.; Prabaharan, G.; Madhu, S.; Yegnaraman, V. J. Colloid Interface Sci. 2006, 303, 604. (31) Kong, L. B.; Lu, M.; Li, M. K.; Li, H. L.; Guo, X. Y. J. Mater. Sci. Lett. 2003, 22, 701. (32) Piquemal, J. Y.; Viau, G.; Beaunier, P.; Verduraz, F. B; Fievet, F. Mater. Res. Bull. 2003, 38, 389. 4968

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Crystal Growth & Design

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dx.doi.org/10.1021/cg200874g |Cryst. Growth Des. 2011, 11, 4963–4969