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Synthesis of Single Crystalline Tin Nanorods and Their Application as Nanosoldering Materials Qingzhou Cui,† Karunaharan Rajathurai,† Wenzhao Jia,‡ Xiaopeng Li,† Fan Gao,† Yu Lei,‡ and Zhiyong Gu*,† Department of Chemical Engineering, UniVersity of MassachusettssLowell, One UniVersity AVe., Lowell, Massachusetts 01854, United States, and Department of Chemical, Materials and Biomolecular Engineering, UniVersity of Connecticut, Storrs, Connecticut 06269, United States ReceiVed: June 28, 2010; ReVised Manuscript ReceiVed: October 8, 2010
Nanosoldering is a promising technique for nanoscale joining and interconnect formation for many newly emerging nanobuilding blocks and nanofabrication processes. In this study, single crystalline tin nanorods are synthesized by a simple, one-pot chemical reduction method assisted by sodium dodecyl sulfate surfactant. The effect of surfactant concentration on the shape variation of tin nanostructures is investigated. The microstructure and the melting behavior of the tin nanorods are characterized. Interconnects of the tin nanorods, either in the form of randomly dispersed networks or dielectrophoretically assembled structures are formed on interdigitated gold electrodes. The I-V electrical properties showed that the contact resistance between nanorods can be significantly lowered by the soldering process. The results show that the tin nanorods are a promising candidate for nanoscale soldering applications. 1. Introduction With the burst of nanotechnology in the last two decades, researchers’ ability to manipulate nanostructures has improved significantly. Synthesis and novel property characterization of various nanomaterials, including carbon nanotubes, semiconducting or metallic nanowires, quantum dots, and shapecontrolled nanoparticles, have led to many new applications in functional nanodevices, composite materials, nanoelectronics, information and energy storage, drug delivery, etc.1-8 Many of these applications have been successfully demonstrated at the laboratory level through self-assembly and directed assembly.9-12 However, broader applications of nanostructures in industrial manufacturing are still far from mature. One major barrier to large scale manufacturing is interconnect formation of the nanobuilding blocks.13 Especially for those applications in nanoelectronics, nanoelectromechanical systems (NEMS), and nanophotonics, forming robust interconnects with good mechanical, thermal, and electrical properties is one of the key requirements for functional device fabrication. Among the many methods for robust interconnect formation, soldering has shown great promise, and nanoscale soldering through solder nanoparticles has been demonstrated to be able to offer excellent fine patterning with superior process capability as reviewed before.13 The effort to search for cost-effective, low melting-point solder nanoparticles has been under intensive investigations. Tin nanoparticles, with regard to their cost, melting point, soldering ability, wetting, and mechanical properties, are a promising candidate for nanoscale soldering. However, how to control their size/shape to get uniform nanoparticles is still an unsolved problem.14,15 Meanwhile, shape-controlled metal nanostructures have been synthesized successfully from metals including gold, * To whom correspondence should be addressed. E-mail: Zhiyong_Gu@ uml.edu. Phone: 01-978-934-3540. Fax: 01-978-934-3047. † University of MassachusettssLowell. ‡ University of Connecticut.
silver, etc., and those uniform nanoparticles have opened many new possibilities to design ideal nano-building blocks for future nanofabrication and manufacturing.6,16-19 In the present study, we report a facile synthesis strategy for single crystalline tin nanorods with [200] crystalline plane. The synthesis is a controlled nanoparticle growth process mediated by an anionic surfactantssodium dodecyl sulfate (SDS). The shape variation of the tin nanostructures is investigated in this report. Melting behavior and morphology evolvement of the tin nanorods are studied by differential scanning calorimetry (DSC) and electron microscope techniques. These nanorods are assembled onto interdigitated gold electrodes by a dielectrophoresis process and interconnection is formed between nanorods. I-V property of the tin solder interconnects formed is measured. From these results, the feasibility for the tin nanorods as nanosoldering materials is demonstrated. 2. Experimental Section 2.1. Tin Nanostructure Fabrication. The synthesis of tin nanoparticles was conducted in a three-neck 200-mL flask. Water (40 mL, Barnstead Nanopure Water Purification System, g17.5 MΩ cm-1) was added into the flask, and then SDS stock solution was added as surfactant. Tin sulfate (40 mg, ACS grade, purchased from Acros) was added and stirred for 10 min for complete dissolution. Then 24 mg of sodium borohydride (Fishier Scientific) was added, and the solution was stirred at 350 rpm with a magnetic stirring bar (Fisher Scientific, catalog no. 14-513-65SIX) for 30 min at room temperature. The resulted samples were centrifuged and cleaned with nanopure water 5 times and ethanol (Fishier Scientific) 3 times through dispersion and centrifuge cycles. The washed samples were dried in vacuum oven for 24 h and kept in a glovebox before DSC characterization. 2.2. Electrode Fabrication and Dielectrophoretic (DEP) Assembly. Patterned gold electrodes were fabricated through a standard photolithography/lift-off process. The predesigned electrode pattern was first transferred from a photo mask to a
10.1021/jp105969x 2010 American Chemical Society Published on Web 12/01/2010
Single Crystalline Tin Nanorods for Nanosoldering spin-coated negative photoresist layer (Futurrex NR9-1500PY) on a 4-in. Si/SiO2 wafer by exposure to UV light. After the development process of dissolving photoresist of unexposed area, a 25-nm chromium adhesion layer and 200 nm of gold were deposited on the wafer by thermal evaporation (NTE-3000 thermal evaporation system). Then the pattern was immersed in acetone to remove the rest of photoresist, leaving only the desired gold electrodes on the substrate. To conduct DEP assembly, the patterned gold electrodes were connected to a function generator (Tektronix FG502) with output signal of a sine wave (6 MHz in frequency and peak-to-peak voltage of 6 V). Then a drop of tin nanorod suspension was transferred onto the electrode substrate. While the solvent was evaporated, the nanorods were trapped in the gap of two parallel electrodes. A source meter (Keithley 2400) was utilized to measure the electrical properties of the tin nanorods assembled on the gold electrode. Because of the nonlinearity of the system before soldering, especially the contact resistance, the electrical properties from the current-voltage (I-V) curves indicated the conductivity change when an external voltage was applied. 2.3. Material Characterization and Other Instruments. Scanning electron microscopy (SEM) was performed on a JEOL 7401F field emission-scanning electron microscope operated at 10 kV. The sample was made by drop-casting nanoparticle dispersion (in ethanol) onto a conductive silicon wafer, which was attached onto sample stub by carbon tape. To enhance the SEM sample conductivity for better imaging, Electrodag 502 (Ted Pella, Inc.) was applied between the wafer and sample stub. The FE-SEM was also equipped with an electron dispersive X-ray spectrometer (EDS), which was used for element analysis and element distribution mapping analysis. Transmission electron microscopy (TEM) was performed with a Philips EM400 transmission electron microscope operated at 100 kV. Selected area electron diffraction (SAED) was performed with a Topcon transmission electron microscope operated at 200 kV. Samples were made by drop-casting nanoparticle dispersion onto copper grids coated with Formvar and carbon film (200 mesh, SPI, West Chester, PA). Differential scanning calorimetry (DSC) analysis was performed on a TA Instruments Q100. Typically, samples about 3 mg were weighed and put in an aluminum pan for measurements. During the process, the DSC samples were heated at a rate of 10 °C/min in N2 stream. The crystal structure of the samples were characterized by X-ray diffraction (XRD) using a BRUKER AXS D5005 X-ray diffractometer (Cu KR radiation, λ ) 1.540598 Å). The samples for XRD were prepared by dropping the as-prepared products on the glass slide. 3. Results and Discussions 3.1. Tin Nanorods and Effect of Surfactant on Nanoparticle Shapes. It is shown in Figure 1a (SEM image) and Figure 1b (TEM image) that tin nanorods are formed in aqueous solution by reducing tin sulfate in the presence of 8 mM SDS. SAED pattern on a single nanorod (Figure 1c) shows the tin nanorod has a single crystalline structure. The EDS in Figure 1d demonstrates that the nanorods are mainly composed of metal tin. The silicon (Si) peak is from the Si substrate, while the carbon (C) peak is probably from the surfactant adsorption on the nanorod surface. The O peak may come from the slight surface oxidation of the tin nanorods. For the uniform nanorod sample, around 1.2 mg of tin nanorods were obtained from each synthesis process. The final nanorods were dispersed into 1.5 mL of ethanol solution, which has a nanorod concentration of about 3 × 1010 nanorods/mL solution. For nanorod characterization and assembly, appropriate dilutions are needed for the samples.
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Figure 1. (a) SEM, (b) TEM, (c) SAED, and (d) EDS for tin nanorods synthesized at SDS concentration of 8 mM.
Figure 2. Tin nanostructures formed at different SDS concentrations of (a) 0.5, (b) 4, (c) 10, and (d) 15 mM.
SDS concentration is critical in controlling nanoparticle shapes. At low SDS concentrations of 0.5 and 4 mM, polydispersed spherical tin nanoparticles are formed as shown in parts a and b of Figure 2, respectively. The spherical nanoparticles turn smaller when SDS concentration increases from 0.5 to 4 mM. With a further increase in SDS concentration, high-yield, uniform tin nanorods start to form in solution at SDS concentration of 8 (Figure 1) and 10 mM (Figure 2c). When SDS concentration further increases to 15 mM, the nanorods turn into a less uniform mixture composing mainly of nanorods and nanoparticles (Figure 2d). Compared to others’ study on tin and tin alloy nanoparticles,14,15 the current work leads to production of more uniform tin nanorods, which are critical in patterning and aligning nanostructures. In addition, the nanorods are stable over weeks if being kept in ethanol solvent; no fusion is observed for the nanorods at ambient temperature. Overall, the new synthetic route is a simple, one-step, easy to scale up, and roomtemperature method without any organic solvent involved.
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Figure 3. XRD results from the tin spherical nanoparticles and nanorods.
Therefore, it shows great potential for massive production and industrial-scale applications. It is believed that the surface capping of SDS on metallic nanoparticles is the main reason to stabilize the tin nanoparticles.17,20-22 Zero valent tin atoms are generated from the reduction reaction, and these tin atoms form nuclei and nanocrystals. During this process, the solution color turns darkbrown. The existence of the small nuclei/nanocrystals is confirmed by a TEM image which was taken on a colored solution right after the reaction started (Figure S1 of Supporting Information). When the SDS concentration is low and not enough to completely cover the surface of nanoparticles, the nuclei and nanocrystals tend to merge into larger particles via agglomeration. Only through this agglomeration mechanism can the nanoparticles thus formed be completely covered by SDS. Therefore, large nanoparticles are observed at SDS concentration of 0.5 mM as shown in Figure 2a. With increasing SDS concentration, the tin nanoparticles turn into smaller nanoparticles, which are a thermodynamically more favored state because more SDS is available for surface coverage of smaller nanoparticles with larger interface area. Therefore, the tin nanoparticles turn smaller when SDS concentration increases from 0.5 to 4 mM as shown in Figure 2. A similar nanoparticle size development trend at different surfactant concentrations has also been observed by Kundu and Liang during photoinduced Pt nanoparticle synthesis.23 Tin nanorods are also formed through the nanoparticle agglomeration mechanism mediated by SDS. However, the agglomeration takes place in a regime with higher SDS concentrations, at which condition excessive SDS is present in solution. This excessive SDS can serve as a mediate reagent for crystal facet reorientation and development due to free SDS exchange between solution and nanoparticle surface. Only the lowest energy state with stable surface capping can survive from the growth and reorganization process. The strong tendency for SDS reorganization/reorientation to achieve the lowest surface energy for nanoparticles has been reported by numerous researchers.24,25 These spherical nanoparticles can evolve into rod shape nanoparticles due to preferential SDS absorption on a certain crystalline surface. As a result, the nanostructures from the process are predominantly associated with a certain crystalline plane. XRD results as shown in Figure 3 show that both spherical nanoparticles and nanorods display high crystallinity with different crystalline planes. The diffraction peaks for the spherical nanoparticles at 2θ values of 30.7, 32, 44, and 45 can be indexed as the [200], [101], [220], and [221] crystal planes of tetragonal tin.26 For the tin nanorods, the diffraction peak at
Cui et al. a 2θ value of 30.7 is more pronounced while other peaks diminished. The overwhelming intensity of [200] plane represents that the tin nanorods are single crystalline Sn with only [200] surface plane. It is believed that the growth rate on the [200] plane slows down and growth in other planes such as the [101] and [220] is more aggressive during tin nanorods growth process due to strong SDS absorption tendency on [200] plane. As a result, the [200] plane survives from the selective SDS absorption and crystal growth process, which leads to formation of elongated rod shape particles. At low SDS concentrations, nanoparticles with all possible planes, however, are formed because most SDS is absorbed on particle surface during nucleation and nanocrystal formation, and nanoparticle evolution is very slow in absence of free SDS in solution. Thus, the existence of excessive SDS in solution is crucial for the nanoparticle evolution from spherical to rod shape. In summary, selective adsorption of SDS molecules on different crystal planes is a key parameter in controlling the nanoparticle growth.17,27,28 In the present study, SDS preferentially adsorbs onto the facet of [200], thereby leading to favored growth of the [200] plane into an elongated rod-shaped particle with only a [200] plane. Such shaped-controlled growth due to preferential adsorption has been observed in the synthesis of many other nanomaterials.29-33 Although we have postulated that the thermodynamic process lead to the formation of the tin nanorods and nanoparticles, it has to be admitted that surfactant-mediated nanoparticle growth is a complex process. Because of the limitation of in situ analysis tools, it is unlikely that one universal mechanism could be agreed upon by all researchers among many possible mechanisms.34 It is highly likely that a simple thermodynamic model can not describe the full picture. Other control parameters such as growth kinetics, capping agent, and reactant concentration are all likely to affect the nucleation and growth process. 3.2. Melting Behavior and Solder Reflow Properties. Melting temperature is an important property for solder materials and is critical in designing electronics assembly and manufacturing processes. DSC results as shown in Figure 4a indicate that the melting point for the tin nanorods is 230.7 °C, which is slightly smaller than but close to that of bulk tin metal (232 °C). Therefore, a slight melting point depression effect is observed on the tin nanorods. It is believed that there is always a thin oxide layer on tin nanoparticle surface which is formed during synthesis and posttreatment including washing and storage. For nanoparticle-based soldering materials, incomplete melting of individual nanoparticle caused by the surface oxide layer induces large interconnect resistance and makes the tin nanorods less appealing as a nanosoldering material. To decrease the effect of surface oxidation, an oxygen-free environment was used to study the thermal properties for the single crystalline tin nanorods. In Figure 4, we also compare the reflow behavior of the tin nanorods in both air and nitrogen environments. As shown in the figure, the tin nanorods keep intact in reflowed air at 300 °C. This is because that at elevated temperatures the nanoparticles can be easily oxidized, and this increased oxidation process leads to more dense oxide layer on the nanoparticle surface, which impedes tin from complete melting. Therefore, almost all tin nanorods keep their original shape as shown in Figure 4b. However, in an oxygen-free environment of nitrogen, the tin melting inside the nanorods can break the natural thin oxide layer easily, and thus tin nanorods can form spherical shapes more easily and many neighboring tin nanorods merged into larger solder balls as shown in Figure 4c. These results are consistent with our previous observation on the melting behavior
Single Crystalline Tin Nanorods for Nanosoldering
Figure 4. (a) DSC for the tin nanorods shows that the melting point for the tin nanorods is close to that of bulk metal. (b) Rod shape was kept when tin nanorods were reflowed in air. (c) Most tin nanorods melt and merged into large solder balls when reflowed in nitrogen (inserted image show individual solder ball formation).
of metallic tin and tin/silver nanowires with large aspect ratio, which were synthesized from hard template-based eletrodeposition method.35 From these results, the oxidation process can be significantly suppressed and solder melting and large solder ball formation are observed in presence of nitrogen gas. 3.3. Assembly of Tin Solder Nanorods and I-V Properties of Interconnects. To demonstrate the feasibility for interconnect formation from the single crystalline tin nanorods, we deposit the tin nanorods onto patterned gold electrode (bare electrode pattern shown in Figure S2 of Supporting Information). After solvent evaporation, the tin nanorod assemblies are heated to different temperatures to form interconnects among nanorods and between nanorod/gold electrode, and then the soldered nanorods are evaluated for their electrical behavior. At temperatures below the tin melting point (∼231 °C), the electrode pattern with tin nanorods displays curved nonlinear I-V responses (as shown Figure S3 of Supporting Information). The S-shaped nonlinear I-V responses are believed to be caused by interconnect resistance between nanorod/nanorod and nano-
J. Phys. Chem. C, Vol. 114, No. 50, 2010 21941 rod/electrode interface. From the results, there is a slight resistance decrease with increasing temperatures. This is because nanopartice sintering at elevated temperatures could lower the contact resistance between nanorods. As shown in Figure S4 of Supporting Information, compared to the small current responses at temperatures e200 °C (mostly sintering), the randomly formed tin nanorod patterns display significantly increased current responses when the heating temperatures are above the tin melting point. Furthermore, the contact resistance between nanorods turns into linear ohmic contact after solder melting. However, when the heating temperature is further increased to 350 °C, the resistance starts to increase again. SEM image shows that at the temperature of 200 °C most tin nanorods still keep their original shape (shown in Figure S4 of Supporting Information). This reveals that efficient interconnect formation (with low electrical resistance) can not be formed among the nanorods simply by sintering the sample at temperature below the tin melting point, and the natural surface oxide layer prevents electrons from hoping onto different nanoparticles. At temperature of 250 °C, the resistance is much smaller and SEM image shows that most nanorods melt and many of them form larger solder ball structures which are connected with each other (Figure S4 of Supporting Information). The formation of effective interconnection leads to increased current response under the voltage conditions. When the electrodes with nanorods are further heated to 350 °C, current decreases significantly as shown in Figure S3 of Supporting Information. Careful inspection of the gold pattern surface using SEM shows that the gold electrodes are damaged with separated metal islands formed (Figure S4c of Supporting Information). It is believed that at elevated temperatures tin has completely melted and formed alloy with gold which also damaged the gold pattern. The tendency for tin to form alloy with gold has been commonly known in the industry of microelectronics. The result implies that gold alone as a substrate may not be used as an ideal substrate for tin based soldering at temperatures close to or higher than 350 °C. In industrial practice, a barrier layer of nickel is normally used for soldering at such conditions. To realize soldering for nanodevices with the tin nanorods, positioning and aligning solder nanorods is a critical step. In reality, patterning or positioning nanoparticles has been always challenging and it is the essential part of nanomaterial assembly. To align and assemble nanoscale structures, researchers have used many methods including DEP,36 microfluidics,37 Langmiur-Blodgett (LB),38 optical manipulation,39 and magnetic method.40 Limitation and challenges coexist with those methods. Among those methods, the DEP method is well-known for its ability in controlling orientation and simplicity. Herein we demonstrate that we can use the tin nanorods to directly bridge two gold electrodes (with 2-3 µm gap) on the gold pattern through the DEP method. As shown in Figure 5a, tin nanorods can be well aligned to bridge the two gold electrodes.13 I-V curves are measured for the pattern before and after tin reflow at 300 °C. Before solder melting, the resistance of the patterned electrodes is large and nonlinear due to the absence of effective interconnects between the nanorods. After solder reflow at 300 °C, effective interconnect forms with tin nanorods merging and solder ball formation (as show in Figure 5b) and therefore leads to lower resistance. As shown in Figure 5a, the resistance decreased significantly (from ∼106 to ∼104 ohm) after tin solder reflow. The assembling method based on DEP alignment is demonstrated to be able to provide an efficient platform for further nanocomponent assembly and interconnect formation. The results also demonstrate that the tin nanorods are not only a feasible alternative to the currently precious metal nanoparticles based nanoink and nano-
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Cui et al. Supporting Information Available: TEM image of nuclei and nanocrystals formed during the reduction process, the patterned gold electrode, I-V responses from the patterned electrode bridged with tin nanorods, SEM images for the tin nanorods coated gold pattern electrode at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 5. (a) Tin nanorods are assembled by the DEP method onto interdigitated gold electrodes. (b) Interconnects are formed to bridge the gold electrode when the tin nanorods are heated and reflowed at 300 °C. (c) Resistance change from 25 °C (before solder reflow) to 300 °C (after solder reflow).
paste, they can also be directly used as a promising material for nanoscale assembly and interconnect formation. 4. Conclusions In conclusion, we have reported a simple surfactant-assisted method to synthesize single crystalline tin nanorods in the present study. These tin nanorods have been demonstrated to be able to completely melt and therefore show great promise to be used as nanosoldering materials including nanoink and nanopaste. Preliminary work on DEP assembly and I-V measurements of the interconnects shows that these tin nanorods can accommodate to the ultra fine features of current nanofabrication process and are promising for nanoscale soldering. Acknowledgment. This work is supported by a faculty startup fund from the University of MassachusettssLowell and partially supported by the 3M Nontenured Faculty Grant (Z.G.).
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