Fabrication of DNA-Templated Te and Bi2Te3 Nanowires by Galvanic

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Fabrication of DNA-Templated Te and Bi2Te3 Nanowires by Galvanic Displacement Jianfei Liu,† Bibek Uprety,† Shailendra Gyawali,† Adam T. Woolley,‡ Nosang V. Myung,§ and John N. Harb*,† †

Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602, United States Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, United States § Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, United States ‡

S Supporting Information *

ABSTRACT: This paper demonstrates the use of galvanic displacement to form continuous tellurium-based nanowires on DNA templates, enabling the conversion of metals, which can be deposited site-specifically, into other materials needed for device fabrication. Specifically, galvanic displacement reaction of copper and nickel nanowires is used to fabricate tellurium and bismuth telluride nanowires on λ-DNA templates. The method is simple, rapid, highly selective, and applicable to a number of different materials. In this study, continuous Ni and Cu nanowires are formed on DNA templates by seeding with Ag followed by electroless plating of the desired metal. These wires are then displaced by a galvanic displacement reaction where either Te or Bi2Te3 is deposited from an acidic solution containing HTeO2+ ions or a combination of HTeO2+ and Bi3+ ions, and the metal wire is simultaneously dissolved due to oxidation. Both tellurium and bismuth telluride wires can be formed from nickel templates. In contrast, copper templates only form tellurium nanowires under the conditions considered. Therefore, the composition of the metal being displaced can be used to influence the chemistry of the resulting nanowire. Galvanic displacement of metals deposited on DNA templates has the potential to enable site-specific fabrication of a variety of materials and, thereby, make an important contribution to the advancement of useful devices via self-assembled nanotemplates.

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the DNA template into a metal ion solution for hours or days, and then to react the metal ions that had associated with the template with a chalcogenide source (e.g., H2S, S2−, Se2−) to produce semiconductor nanoparticles.22 In the work reported by Dong et al.,17 this process was repeated to form continuous nanowires in a few days. Unfortunately, the additional deposition time required to produce continuous wires also resulted in significant background deposition that is likely to be problematic for actual devices. The diameter of semiconductor nanowires, which may have a significant impact23 on their physical properties, was also difficult to control. Also, the process was limited to specific materials. Recently, using DNA assembly technology, Sharma et al. fabricated DNA-templated semiconductor quantum dot arrays,24 and Bui et al. assembled CdSe/ZnS core/shell streptavidin-conjugated quantum dots on linear DNA.25 However, these assembled quantum dots on DNA are not continuous. By contrast, one-dimensional (1D), continuous semiconductor nanowires show unique optical, electrical, and thermal transport properties, with potential application in nanophotonics, nanoelectronics, and energy

NA has shown considerable promise as a template for the fabrication of nanodevices due to its small diameter, abundance of functional groups, molecular recognition properties, and ability to self-assemble into different structures.1−3 In particular, DNA origami has been used to form a wide variety of two-dimensional (2D) patterns4−6 and three-dimensional (3D) structures7 for the construction of nanodevices. The ability to reliably and controllably deposit a number of different materials onto the DNA is essential for it to reach its full potential as a template for device fabrication. Several different types of metals have been successfully deposited on DNA templates,8−13 primarily by electroless deposition.14 Sitespecific metal deposition was recently demonstrated on DNA origami, enabling a new level of complexity for potential device fabrication.15,16 By contrast, the deposition of other materials, such as semiconductors, has been much more limited. This paper addresses the need for both flexible methods of fabrication and additional types of materials through the use of galvanic displacement to form tellurium and bismuth telluride nanowires on λ-DNA templates. Semiconductor materials that have been deposited on DNA templates for the formation of nanowires or nanochains include CdS,17,18 CuS,19 CdSe,20 and CdSe/ZnS.21 The principal method for deposition of these materials has been to immerse © 2013 American Chemical Society

Received: July 17, 2013 Published: July 31, 2013 11176

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conversion.26 The present study uses galvanic displacement to fabricate continuous semiconductor nanowires in an efficient and effective manner. The approach enables the fabrication of DNA-templated structures from materials that had not previously been used. Use of galvanic displacement reactions (GDR) is a simple, yet powerful and scalable technique for selectively changing the composition and/or morphology of nanostructures. Galvanic displacement takes place via a spontaneous electrochemical reaction driven by the difference in redox potentials between the solid material to be displaced and the ions of the source material, and is well suited to high-throughput processing.27,28 GDR has been used previously to create metal nanostructures with hollow interiors exhibiting a spectrum of geometries and multiwalled metal nanoshells,29,30 as well as to coat Si with metal films/nanoparticles. 31,32 It was recently used to synthesize, for the first time, high aspect ratio 1D tellurium and metal telluride (i.e., Bi2Te3) nanostructures with controlled composition.33,34 GDR has also been used to form metal/ semiconductor heterostructures (i.e., nanopeapods)35 and composition and diameter modulated nanowires (i.e., Te-rich BiTe/Bi-rich BiTe) from Ni-rich NiFe/Fe-rich NiFe segmented nanowires.36 This study seeks to leverage previous success with GDR by combining it with DNA templates in order to enable fabrication of a wide variety of patterns in an effective, efficient manner. Tellurium (Te) and bismuth telluride (Bi3Te2) are low band gap semiconductor materials of interest to the current work.37,38 Te and its alloys have potential application in electronics, piezoelectric devices, high-efficiency photoconductors, gas sensors and thermoelectrics because of their unique anisotropic crystal structures consisting of helical chains of covalently bound atoms.34,39,40 Likewise, Bismuth telluride (Bi2Te3) and its alloys are known as the best near-roomtemperature bulk thermoelectric materials.41,42 Compared to the bulk material, 1D nanostructures are predicted to have a significantly higher thermoelectric figure of merit33,43 due to decreased thermal conductivity through phonon scattering in the 1D nanostructures. Thus, the fabrication of 1D Te and Bi2Te3 nanostructures is of great interest for the thermoelectric industry.34 Additionally, these materials, as well as derivatives thereof that can be formed in situ, are also suitable for use in phase-change random access memory and as topological insulators.44,45 Although multiple methods are now used to synthesize phase-change memory (PCM) devices, electrochemical processes such as GDR are versatile, cost-effective, scalable, rapid, and can be performed at or near room temperature and at ambient pressure. In this paper, we demonstrate the fabrication of continuous Ni and Cu nanowires on λ-DNA templates through Ag seeding and electroless Ni and Cu plating, and the conversion of these nanowires to tellurium (Te) or bismuth telluride (Bi2Te3) through a galvanic displacement reaction. Characterization of the resulting nanowires was performed with use of atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and electrical resistance measurements.

Figure 1. AFM images of λ-DNA before (A) and after (B) Ag seeding on a silanized SiO2 surface. Height scale is 6 nm in (A) and 25 nm in (B).

bundles, as seen in Figure 1A; such bundles can be more than 20 μm long. After alignment, the DNA was seeded using a method similar to that reported previously in ref 9. During the seeding process, silver ions from solution attached to the negatively charged DNA scaffold through a combination of electrostatic interactions and complexation with DNA bases. The attached silver ions were then reduced to form a seed layer consisting of Ag nanoparticles. Figure 1B shows an AFM image of Ag-seeded λ-DNA after seeding twice. The DNA was selectively seeded, although the seeding was not continuous (zoom-in image in Figure 1B). The discontinuous seeds serve as catalysts for electroless plating to form continuous nanowires. After seeding twice to increase the seed density, the average seeded height was 12.5 nm with a standard deviation of 3.2 nm (n = 51). Compared with other Ag seeding methods in the literature,10,11,46−48 this Ag seeding method is relatively simple, requiring less than 20 min, and is effective in producing samples with relatively high seed density and good selectivity. Fabrication of Nickel Nanowires. An electroless nickel plating bath49 was used to fabricate the nickel wires. This plating solution (either with or without the reducing agent) will displace Si, even with the native SiO2 layer and the silane layer on the surface. Therefore, a Si wafer with a thermal SiO2 layer of 500 nm was used as the substrate for the preparation of DNA-templated Ni nanowires. Initial attempts at electroless plating of nickel on the Ag-seeded λ-DNA samples were performed at 82 °C, and it was found that most of the DNA was removed from the surface in the plating solution. DNA removal was likely due to the relatively high temperature and to hydrogen gas evolution during electroless plating.50 The temperature was subsequently lowered to 70 °C. At this temperature, the DNA remained on the surface, but the deposition rate slowed dramatically. Consequently, the concentration of the reducing agent (i.e., NaBH4) was increased by a factor of 3 to increase the rate of plating in order to compensate for the lower reaction rate at the lower temperature. The increase in the electroless plating rate with concentration is consistent with expectations from mixed potential theory as shown in Figure 2. The higher concentration of the reducing agent shifted the I−V curve for the anodic reaction (blue) up so that the current at a given potential was higher than that at the original concentration. This shift in the anodic curve resulted in an increase in the current at which the anodic and cathodic curves are equal (intersection of the curves). The voltage (V′) at the new intersection point is the potential at which electroless plating takes place. The corresponding current represents the electro-

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RESULTS AND DISCUSSION Seeding of DNA Templates. Figure 1A shows λ-DNA aligned on a SiO2 surface that had been passivated with octyldimethylmonochlorosilane (C8DMS). When aligned on a hydrophobic silane monolayer, some of the DNA formed 11177

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molecules owing to a higher density of nucleation sites and negative charge.19,51 The length of continuous nanowires was typically less than 3 μm for a plating time of 2 min. The continuous length was limited by larger spacing between seeds at some locations along the length of the template. An increase in the Ni plating time to 4 min resulted in wires with an average width of 100 nm and a standard deviation of 6 nm (Figure 3D−F). SEM images of the wires clearly show that they are composed of distinct grains. Many of the continuous nanowires plated for 4 min had a length greater than 10 μm, much longer than those plated for the shorter time. The longer plating time allowed bridging of gaps that still existed on the nanowires plated for only 2 min. The nearly uniform width along single DNA-templated Ni nanowires represents a significant improvement from the literature, in which the width of the same nanowire varied significantly at different sites.52 EDX analysis (Figure 3G) on samples plated for 4 min confirmed the presence of Ni. The Ag peak was not seen on EDX due to the small size of the Ag nanoparticles relative to the diagnostic volume. Figure 3A−F shows some background deposition on samples plated for both 2 and 4 min; however, the nanowires are easily distinguished. We attribute the background deposition to Ag seeds deposited on the surface from the seeding step as no background deposition was found in control plating experiments performed prior to seeding (Figure 3H). We also did the same DNA-templated seeding and electroless Ni plating on a thermally oxidized silicon wafer (500 nm SiO2 on a Si wafer) that had been passivated with octadecyldimethylmonochlorosilane (C18DMS; see Materials and Methods). The resulting Ni nanowires (Figures 3I,J, plated for 2 and 4 min, respectively) display morphologies very similar to these prepared on the C8DMS-passivated surface. We observed, however, that seeding of DNA was not as reproducible on the C18DMS-passivated surface. Consequently, the displacement experiments were performed on C8DMSpassivated surfaces. The conductivity measurements presented below were performed on C18DMS-passivated surfaces prior to the displacement experiments, but the results are similar to those expected for C8DMS-passivated surfaces owing to the similar nanowire morphologies observed for the two types of substrates (Figure 3). Conductivity Measurements. Conductivity tests were performed on the DNA-templated Ni nanowires (plated for 4 min) that had been fabricated on a C18DMS-passivated surface. Electron beam lithography was used to pattern gold electrodes onto the SiO2 surface after the Ni nanowires had been formed (Figure 4A). The gap between adjacent gold electrodes was ∼340 nm (Figure 4B). The conductivity of nanowires across two adjacent electrodes was measured by applying a voltage across the electrodes and measuring the resulting current. This two-point measurement is less desirable than a four-point measurement because of the influence of the contact resistance. However, our principal objective was to demonstrate the continuity of the wires prior to displacement, and the two electrode measurement was much simpler to perform. Measurements were made on electrode pairs that had between one and three continuous Ni nanowires across the gap between the electrodes. The relationship between the measured current and voltage was linear in all cases, indicating ohmic behavior (Figure 4C). The results from 17 electrode pairs (Figure 4D) show the expected general trend that the electrical resistance decreased with an increased number of continuous

Figure 2. Analysis of electroless plating rate with mixed-potential theory. C is the concentration of the reducing agent.

less deposition rate, which increased as a result of the change in NaBH4 concentration. Figure 3A−C shows the results after plating with this modified solution for 2 min. The average height and width of

Figure 3. (A−C) AFM (A) and SEM (B,C) images of Ag-seeded λDNA templates following Ni plating for 2 min at 70 °C on C8DMS passivated surface. AFM height scale: 80 nm. (D−F) SEM images of Ag seeded λ-DNA after 4 min of electroless Ni plating at 70 °C on C8DMS passivated surface. (G) EDX analysis performed on a Ni nanowire (plated for 4 min) using the spot scan. (H) AFM image of a control sample after 4 min of electroless Ni plating on a blank (nonseeded) C8DMS passivated surface. Height scale: 6 nm. (I,J) SEM images of Ag seeded λ-DNA after 2 and 4 min of electroless Ni plating at 70 °C on C18DMS passivated surface.

the nanowires formed were 41 nm and 38 nm, respectively, with narrowest nanowires having a width of 28 nm. The variation of the width among different nanowires maybe due to the fact that single DNA molecules and DNA bundles are seeded with different seed sizes and densities. Specifically, DNA bundles appear to seed more effectively than single DNA 11178

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Figure 4. Low magnification (A) and high magnification (B) images of Au pads on Ni nanowires fabricated by e-beam lithography for two-terminal conductivity tests. (C) I−V plot showing ohmic behavior. (D) Measured electrical resistances from 17 devices with one (red), two (blue), or three (green) continuous wires across the gap.

nanowires across the gap. Four out of the five electrode pairs with measured electrical resistances greater than 5000 Ω had just one continuous nanowire across the gap. The electrical resistance per wire was found to vary considerably. For example, the resistance across gaps with one continuous nanowire varied from 1300 Ω to12 700 Ω, due likely to differences in the morphology of the nanowires. In situations where multiple nanowires bridged the same gap, the resistance of each wire was assumed to be the same and individual wire resistances were estimated by assuming that the wires represented equivalent resistors in parallel. The estimated single-wire resistance (from a total of 33 nanowires) ranged from 670 Ω to 31 400 Ω, with a median value of 2,500 Ω. These values correspond to Ni nanowire resistivities ranging from 1.0 × 10−5 to 4.8 × 10−4 Ω m, approximately 2−4 orders of magnitude higher than the bulk resistivity of Ni (6.9 × 10−8 Ω m). The median value from the measurements was 3.9 × 10−5 Ω m. These measurements establish the continuity of the nanowires, which are used as templates for galvanic displacement, and provide initial data on their electrical properties. Galvanic Displacement of Nickel Nanowires. As mentioned above, the fabrication of 1D Te nanostructures is of great interest.34 Here, DNA-templated Ni nanowires were converted to Te nanostructures at room temperature through a galvanic displacement reaction. The galvanic displacement of Ni by Te was previously demonstrated by Rheem et al.34 for Ni nanowires with diameters of 70 nm, 120 nm, 220 nm that had been fabricated by template-directed electrodeposition where the metal is electrodeposited in the pores of a polycarbonate membrane followed by dissolution of the membrane. Ni on the surface of the nanowires was displaced first, and an incomplete layer of Te was formed. Then, the reactants diffused through pores in the surface layer and displaced the Ni on the inside of the nanowires. Ni ions (Ni2+) produced in the displacement reaction diffused out through the pores and were removed in the rinsing step after 2 h of displacement.34 The GDR was

carried out in a solution of 0.01M HTeO2+ in 1 M HNO3. The standard potentials for the half-cell reactions of interest are Ni → Ni 2 + + 2e−;

E° = − 0.250V vs SHE

(1)

HTeO2+ + 3H+ + 4e− → Te + 2H 2O; E° = 0.551V vs SHE

(2)

Figure 5 shows SEM images of DNA-templated nanowires before (Figure 5A, Ni plated for 4 min) and after (Figure 5B) 2

Figure 5. Ni nanowires of 100 nm in width before (A) and after (B) 2 h of Te displacement. The contrast and morphology of nanowires in these two images are different. (C) EDX analysis performed on the nanowire in image (B) using spot scan. (D) Ni nanowires of 100 nm in width after 4 h of Te displacement reaction. Ni nanowires of 50 nm in width before (E) and after (F) 2 h of Te displacement. 11179

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h of Te-displacement. The nanowire after Te displacement reaction did not look as granular as it did prior to the displacement reaction. The contrast of SEM images of the wires was also different after displacement, where the brightness of the nanowires relative to the background was less than that observed for the wires prior to displacement. EDX analysis performed on the Te nanowire in Figure5B confirmed the presence of Te and disappearance of Ni as a result of the reaction (Figure 5C). The width of the Te nanowire was about 100 nm, approximately the same as that of the Ni nanowire prior to displacement. Given that 1 mol of Te displaces 2 mol of Ni, and considering the molecular weights and bulk densities of Ni and Te, the minimum Te diameter expected from a 100 nm diameter Ni nanowire would be 125 nm if the Te were solid and nonporous. With the presence of pores, the diameter of Te nanowires should be greater than 125 nm. It is likely that the smaller Te nanowires observed after displacement were due to partial dissolution of the deposited Te in HNO3 as follows:53,54

Figure 6. SEM images of Bi2Te3 nanowires obtained after immersing DNA-templated Ni nanowires into a solution consisting of 0.01 M HTeO2+, 0.01 M Bi3+ and 1 M HNO3 for 2 h at room temperature. (A) Lower magnification, scale bar = 500 nm, and (B) higher magnification, scale bar =200 nm.

for bismuth ion concentrations of 0.006 M, 0.008 M, and 0.01 M, respectively. The expected molar ratio of Te/Bi for pure Bi2Te3 is 1.5 and, according to the literature, no Te/Bi binary phase with a Te/Bi molar ratio higher than 1.5 is known to exist.55 In work by Stacy et al.55 on the electrodeposition of Bi2Te3, Te/Bi ratios higher than 1.5 were attributed to the excess Te in the deposit. Similarly, the high Te/Bi ratios obtained at low concentrations of Bi3+ could also be due to the coexistence of Te and Bi2Te3in the deposit. However, no significant morphology differences between wires with different Te/Bi ratios (2.4, 2, and 1.5) were apparent (see images in the Supporting Information). In another study, Stacy56 reported that the formation of electrodeposited Bi2Te3 takes place in two steps. The first step is reaction 2 to form Te. In the second step, elemental Te reacts with Bi3+ to form Bi2Te3 according to the following reaction:

3Te + 7H+ + 4NO3− → 3HTeO2+ + 4NO + 2H 2O (3)

To further explore this issue, the displacement of 100 nm Ni nanowires was allowed to proceed for 4 h rather than 2. The width of the resulting Te nanowires was about 60 nm (Figure 5D), smaller than that observed for the 2 h displacement, and consistent with dissolution of the Te nanowires with time. Thus, the timing of the displacement reaction is important for controlling the size of Te nanowires. Te displacement of smaller Ni nanowires (plated for 2.5 min; 50 nm wide; Figure 5E) for 2 h led to Te nanowires of 50 nm in width (Figure 5F), which were quite porous. In all cases, Te deposition occurred only by displacement of the Ni template; no other Te deposition on the surface was observed. Consequently, this type of displacement reaction will help to enable deposition of materials onto DNA templates with high precision. Galvanic displacement was also used to convert DNAtemplated Ni nanowires to Bi2Te3. The overall displacement reaction is33

3Te + 2Bi 3 + + 6e− → Bi 2Te3;

(5)

This two-step mechanism is consistent with the results of this study. Reactions 5 and 2 are not only reactions in series, but also competing electrochemical reactions. With a higher concentration of Bi3+, the rate of Reaction 5 was accelerated relative to that of Reaction 2, so that all of the Te produced from the first step was converted to Bi2Te3. With a lower concentration of Bi3+, the rate of Reaction 5 was not adequate to convert all of the Te produced from the first step to Bi2Te3 before the Ni (electron source) was completely consumed. Therefore, the ratio of Te/Bi in the deposit at this condition was greater than 1.5 since it consisted of both Bi2Te3 and excess elemental Te produced from Reaction 2. Fabrication of Copper Nanowires. The fabrication of continuous Cu nanowires on DNA templates was also performed in this study by Ag seeding and electroless plating. Unseeded silicon wafers with native oxide upon which a C18DMS monolayer had been deposited were found to be stable in the Cu plating solution, as evidenced by the absence of plating observed during 4 min of exposure to the plating solution. Consequently, this surface was used for the preparation of the DNA-templated Cu nanowires (see Materials and Methods). When plated for 1 min (Figure 7A), no continuous nanowires were found on the surface. After 3 min of Cu plating (Figure 7B), continuous nanowires with an average width of about 500 nm were observed. A high magnification SEM image of a sample plated for 3 min (Figure 7C) clearly shows the morphology of the Cu nanowires. The average rate of growth between 1 and 3 minutes of plating was estimated from the amount of material deposited on the surface to be about 0.0055 mol/ (m2·min). By contrast, the rate of Cu growth on Ag at plating times less than one minute was significantly slower than this rate, which may be due to the

2Bi 3 + + 3HTeO2+ + 9Ni(s) + 9H+ → Bi 2Te3(s) + 9Ni 2 + + 6H 2O

E° = 0.45 V vs SHE

(4)

The displacement solution consisted of 0.01M HTeO2+ in 1 M HNO3 with different Bi(NO3)3 concentrations between 0.006−0.01 M. The Bi2Te3-displacement reaction was allowed to proceed at room temperature for 2 h. After displacement, the sample morphology was characterized by SEM, and EDX analysis was used to measure the nanowire composition. Large 500 nm wide DNA-templated Ni nanowires were used for the Bi2Te3 displacement reaction to facilitate measurement of the relatively small amounts of Bi deposited during displacement (see Reaction 4). These large Ni nanowires were seeded as described above and then plated with a different electroless Ni plating solution (see Materials and Methods section) that plated very quickly. Figure 6 shows the DNA-templated Ni nanowires after displacement with Bi2Te3 from a solution containing 0.01 M Bi3+. The morphology of the resulting Bi2Te3 nanowires was quite different from that of the Ni template, with the crystal structure of Bi2Te3 clearly shown. The concentration of Bi3+ in solution impacted the final composition of the wires. Specifically, the molar ratio of Te to Bi of the wires after displacement, obtained from EDX analysis, was 2.4, 2, and 1.5 11180

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Figure 8. (A) SEM image of a nanowire initially made of Cu, after 2 h of Bi2Te3 displacement reaction at room temperature. (B) EDX analysis performed on the nanowire in (A) using spot scan.

morphology of the nanowires formed by displacement after 2 h in a solution consisting of 0.01 M HTeO2+, 0.006 M Bi3+, and 1 M HNO3 at room temperature, which reflects the morphology of the initial Cu nanowire. EDX analysis (Figure 8B) performed on the nanowire after displacement confirmed the disappearance of Cu and the presence of Te. However, no evidence of Bi was found in the EDX results. From these data, we conclude that the wires were Te and not Bi2Te3. The lack of Bi in the deposit may be due to a slow rate of reaction between elemental Te and Bi3+ since the driving force for Reaction 5 is considerably less on Cu than on Ni. The equilibrium potential for Reaction 5, although greater than that for the Cu reaction, is considerably lower than that of the Te reaction (Reaction 2). Penner et al.57 observed that the formation of Bi2Te3 on highly oriented pyrolytic graphite (HOPG) electrodes in a system similar to ours started well below the equilibrium potential. Therefore, it is possible that the driving force for the Bi reaction was inadequate for Reaction 5 to occur at an appreciable rate. To conclude, we have developed a method for the formation of Ni and Cu nanowires on λ-DNA that are continuous and suitable for use as templates for galvanic displacement. The electrical resistivity of the DNA-templated Ni nanowires was measured in order to verify the continuity of the wires. The measured values were 2−4 orders of magnitude greater than the bulk resistivity of nickel, and clearly establish the existence of continuous conductive Ni nanowires. A method for the preparation of continuous DNA-templated Cu nanowires was also developed. These Cu nanowires were fabricated using Ag seeding and electroless Cu plating. The key contribution of this study was the successful fabrication of tellurium and bismuth telluride nanowires by galvanic displacement of DNAtemplated copper and nickel. These wires were formed by a galvanic reaction where either Te or Bi2Te3 was deposited from an acidic solution containing Te or a combination of Te and Bi in soluble form, and the metal wire was simultaneously dissolved due to oxidation. Both tellurium and bismuth telluride wires were formed from nickel templates. In contrast, only tellurium nanowires were formed from copper templates under the conditions considered. Therefore, the composition of the metal being displaced was observed to influence the composition of the resulting nanowire. Galvanic displacement of metals deposited on DNA templates has the potential to enable site-specific fabrication of a variety of materials and, thereby, make an important contribution to the advancement of useful devices via self-assembled nanotemplates.

Figure 7. SEM images of Ag seeded λ-DNA after 1 min (A) and 3 min (B,C) of electroless Cu plating at 65 °C. (D) Cu nanowires of sample C after 2 h of Te-displacement reaction. (E,F) EDX analysis performed on the nanowires in images C and D, respectively.

influence of the Ag seeds on Cu nucleation and initial growth. A comparison of the observed growth rate with the rate expected for Cu transport under diffusion control indicates that this system likely operates at or near the diffusion limit after 1 min of plating. As transport-limited deposition tends to be nonuniform, there is an opportunity to optimize the copper process to yield nanowires with increased continuity that are even better suited for nanodevices. Our purpose here, however, was to examine for the first time the use of DNA-templated Cu for fabrication of Te and Bi2Te3 nanowires by galvanic displacement. Displacement of Copper Nanowires. Comparison of the standard electrode potentials of Cu2+/Cu (Reaction 6) and HTeO2+/Te (Reaction 2) indicates that it should be possible to galvanically displace Cu to form Te. Cu 2 + + 2e− → Cu;

E° = 0.342 V vs SHE

(6)

To explore this possibility, the DNA-templated copper nanowires were exposed to the same solution used for Te displacement of nickel, 0.01 M of HTeO2+in 1 M HNO3 at room temperature. Figure 7D shows an SEM image of a displaced Cu nanowire after 2 h in the displacement solution. The morphology of the Te nanowire after displacement (Figure 7D) was quite different from that of the Cu template (Figure 7C). EDX analysis performed before the displacement showed a strong Cu peak (Figure 7E), which was replaced almost entirely by a Te-peak that appeared as a result of the reaction (Figure 7F). We note that the morphology of the Te formed by Cu displacement was also different from that formed by Ni displacement. This morphology difference could be due, at least in part, to the increased time in solution after completion of the displacement reaction, owing to the faster displacement reaction for Cu. It may also be the result of the different processes that control the displacement reaction in the two systems. The displacement of DNA-templated Cu nanowires to form Bi2Te3 nanowires was also attempted. Figure 8A shows the

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MATERIALS AND METHODS

Materials. λ-DNA (in 10 mM Tris-HCl, with 1 mM EDTA, pH 8.0; Cat. No.: LS01203) was from Worthington Biochemical Corporation. It was diluted to ∼65 ng/μL in TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH = 8.0). AgNO3, KNaC4H4O6·4H2O (99.0%), CuSO4·5H2O (99.1%), HCHO (37.4%), NH4OH (28%-30% 11181

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as NH3), H2O2 (30% solution), H2SO4 (98%), NiSO4·6H2O (98%) and boric acid (99.5%) were obtained from Mallinckrodt Baker, Inc. NH2(CH2)2NH2 and C6H5O7Na3·2H2O (99.0%) were purchased from EMD Chemicals. NaOH (97%) was obtained from Spectrum Chemical Mfg. Corp. NiCl2·6H2O was purchased from Fisher Scientific Inc. Alcohol (200 proof) was purchased from Decon. Labs, Inc. Octadecyldimethylchlorosilane C18DMS (95%), NaBH4 (98%), TeO2 (99+%), acetone (>99.5%, A.C.S. reagent), Bi(NO3)3· 5H2O (99.99% metals basis) and dimethylamine borane (CH3)2NH· BH3 (97%) were purchased from Sigma-Aldrich, Inc. C8DMS was purchased from Gelest, Inc. The water bath used for electroless Ni plating and electroless Cu plating in this research was purchased from VWR International (Type: 89032−216). The oven used in the study was from LAB-LINE (LR 19314). Plasma cleaning was performed with an instrument from Harrick Plasma, NY(PDC-32G, 100 W of input power, 18 W of power applied to RF coil, 8−12 MHz). All water (except for the water bath) was purified with use of a Barnstead EASYpure UV/UF system (Thermo Scientific) and had a resistivity of 18.3 MΩ cm. Silanization of Oxidized Silicon Wafers with C8DMS. This procedure was modified from that reported in the literature.58 Small sections of an oxidized silicon wafer (1 cm × 1 cm) were cleaned in a plasma cleaner for 30 s to remove any contaminants on the surface. Then, each piece of wafer was put into a glass vial (20 mL volume; about 25 mm in diameter and 50 mm in height) and sufficient C8DMS (liquid at room temperature) was added to cover the surface of each wafer. The cap was placed tightly on each glass vial after adding the silane. Afterward, the glass vials were put into an oven at 70 °C for 10 min. Subsequently, the vials were removed from the oven and allowed to cool down in air (took about 10−15 min). Finally, the wafers were taken out of the vials, rinsed with acetone and ethanol for 5 s each by spraying the solvent on the surface, and dried in a stream of filtered air. Silanization of Oxidized Silicon Surfaces with C18DMS. This procedure was modified from that reported in the literature.59 An oxidized (either thermally grown or native oxide) silicon wafer (n-type, (100), prime wafer; Silicon Wafer Enterprises, LLC) was cleaned using a piranha solution (a mixture of 98% sulfuric acid and 30% H2O2 at a volume ratio of 7:3) at 130 °C for 10 min. Then, the silicon wafer was rinsed with water. This rinsing process was done as follows: after disposing of the piranha solution, an excess of water was added into the container with the wafer; this rinsing step was repeated three times to remove any remaining piranha solution. Next, the wafer was taken out of the container and rinsed with water for more than 30 s by spraying water on the front and back sides of the wafer. After rinsing, the wafer was dried in a stream of filtered air (at ∼28% humidity). Subsequently, the cleaned silicon wafer was placed into a 140 mmdiameter (15 mm in height) glass container and put into an oven at 120 °C for 5 min to remove the water layer on the surface. Then, a small, open glass vial (15 mm in diameter, 15 mm in height) containing 180 mg of C18DMS was placed beside the Si wafer, and the system was covered with an inverted container (see Figure S3 in Supporting Information). The oven temperature was kept at 120 °C for another 2 h. In this manner, the oxide surface was exposed to vapor phase of C18DMS for 2 h, during which the C18DMS reacted with the cleaned oxidized wafer surface to form a monolayer. Afterward, the whole glass system was taken out of the oven, cooled to room temperature (by sitting on the counter), and rinsed by spraying successively with methanol, acetone, ethanol, and water for 5 s each. Finally, the surface was dried in a stream of filtered air. The silanized wafer was finally cut into pieces (1 cm × 1 cm), rinsed again with water and dried in a stream of filtered air. The silanized wafer was stored in a plastic Petri dish sealed by Parafilm to keep it clean prior to use. Alignment of λ-DNA. Before use, the silanized wafer (silanized with either C18DMS or C8DMS) was heated to 120 °C in an oven for 10 min to remove water molecules attached to the surface, and allowed to cool in air (about 10 to 15 min) prior to use. Afterward, λ-DNA (10 μL of ∼65 ng/μL) was transferred onto the silanized surface using a micropipet. The DNA solution formed a droplet on the hydrophobic surface. The DNA solution was aligned on the surface using a Kimwipe

paper to absorb and move the solution along one direction. The surface was then dried in a stream of filtered air and was ready for seeding.59 Ag Seeding of λ-DNA. After alignment of λ-DNA on the silanized surface, basic silver nitrate solution (40 μL, 0.1 M AgNO3, 0.33 M ammonium hydroxide) was put on the surface using a micropipet and allowed to interact with the DNA for 5 min. Subsequently, the solution on the surface was blown dry along one direction with a stream of filtered, dry air. Then, hydroquinone solution (40 μL, 0.05M) was put on the surface to reduce silver ions to metallic Ag nanoparticles. The reaction was allowed to take place for 2 min. The surface was then rinsed with water for 5 s and dried in a stream of filtered air. The seeding process was performed twice in order to provide an adequate seed density on the λ-DNA. Electroless Plating. The seeded sample was put into a plastic Petri dish and left uncovered. The electroless Ni plating solution consisted of 30g/L NiCl2·6H2O, either 0.7 or 2.1 g/L NaBH4, 60g/L NH2(CH2)2NH2, 40g/L NaOH, pH = 13. Immediately after the preparation of electroless Ni plating solution, 50 μL of this Ni plating solution was transferred onto the sample surface using a micropipet. The Petri dish with the sample (with Ni plating solution already on the surface) was immediately put into a temperature bath where the Petri dish floated on water that was at either 70 °C or 82 °C. The electroless Ni plating was allowed to proceed for 1−4 min. After plating, the sample was taken out of water bath, rinsed with water for 5 s, and dried in a stream of filtered air. For plating the 500 nm-wide Ni nanowires for the Bi2Te3 displacement reaction, a different electroless Ni plating solution was used, which consisted of 0.1M nickel sulfate, 0.01M dimethylamine borane, 0.2M sodium citrate, and 0.5M boric acid.60 The pH was adjusted to 7 and the plating temperature was 70 °C. Ni plating was allowed to proceed for 4 min. The electroless Cu plating solution consisted of 5g/L CuSO4·5H2O, 25g/L KNaC4H4O6· 4H2O,7g/L NaOH, 10 g/L 37% HCHO and was prepared and used in a similar way to the nickel baths, but at a temperature of 65 °C. The Cu plating time was 1−3 min. Galvanic Displacement Reaction. The 0.01 M HTeO2+ solution for the galvanic displacement reaction was prepared by dissolving 24 mg of TeO2 into 1 mL of concentrated nitric acid overnight (no volume change was observed in the dissolution process) followed by dilution with 14 mL of water. Solid bismuth nitrate was added to HTeO2+ solution to yield a Bi3+ concentration of 0.006 M, 0.008 M, and 0.01 M. For the displacement reaction, the sample of DNAtemplated Ni or Cu nanowires was put into a humid chamber in order to prevent the solution from evaporating over the extended duration of the process. Then, displacement solution (60 μL) was transferred onto the nanowire surface via a micropipet. The solution did not spread much on the hydrophobic surface. The displacement reaction was allowed to proceed for 2 h. The surface was then rinsed with water for 5 s and dried in a stream of filtered air. Conductivity Measurement. After fabrication of the Ni nanowires on a Si surface with 500 nm of thermal SiO2, electron beam lithography, electron beam evaporation, and liftoff were used to make 5 nm Cr/100 nm Au electrodes. Two-point conductivity measurements were taken at room temperature using a source-drain voltage sweep of ±300 mV. National Instruments LabVIEW software was used with Vera Sazonova’s MeaSureit 2.2 VI to perform the measurement. SEM Imaging. SEM images were taken in the high-vacuum mode on a Philips XL30 ESEM FEG. EDX analysis was also performed on this ESEM using spot scanning with a spot size of 6. AFM Imaging. The samples were imaged in air using tapping mode on a Digital Instruments Nanoscope IIIa MultiMode AFM (Veeco) with silicon AFM tips (AppNano FORTA tips from Nanoscience Instruments, Inc.).

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ASSOCIATED CONTENT

S Supporting Information *

SEM images of BixTey nanowires and diagram of experimental setup for surface preparation. This material is available free of charge via the Internet at http://pubs.acs.org. 11182

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding for the BYU portion of the work from the National Science Foundation (CBET-0708347) and Brigham Young University, and for the UCR portion of the work from the Pioneer Research Center Program through the National Research Foundation of Korea (2011-0013323) funded by the Ministry of Education, Science and Technology (MEST) and the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea. Gratitude is also expressed to E. Gates, Y. Geng, B. Davis, J. Hickey, and H. Conley for their help and valuable insights.

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