Fabrication of Highly Conductive Pd Nanowires using PDMS Transfer Method on DNA Scaffolds through Ethanol-Reduction

Transcription

1 CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 26, NUMBER 5 OCTOBER 27, 2013 ARTICLE Fabrication of Highly Conductive Pd Nanowires using PDMS Transfer Method on DNA Scaffolds through Ethanol-Reduction Hong-bing Cai a, Zhen-xing Wang b, Jin-yang Liu c, Kun Zhang a, Xiao-ping Wang a,c a. Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei , China b. National Center for Nanoscience and Technology, Beijing , China c. Department of Physics, University of Science and Technology of China, Hefei , China (Dated: Received on May 10, 2013; Accepted on May 20, 2013) We have developed a simple, productive, and effective poly(dimethysiloxane) (PDMS) transfer method to fabricate highly conductive Pd nanowires following DNA scaffolds on various substrates, based on ethanol-reduction at low temperature. Pd nanoparticles were selectively deposited and confined onto the DNA templates on a PDMS sheet to form Pd nanowires and then the nanowires were transferred to other various substrates with a low occurrence of parasitic nanoparticles. The structure, morphology and the conductance of Pd nanowires were characterized with transmission electron microscopy, field emission scanning electron microscopy, and electrical transport measurement, respectively. Moreover, the growth process of the Pd nanowires was investigated by varying the incubation time and reaction temperature. The present strategy provides a new method to fabricate extremely dense, highly conductive, and well aligned Pd nanowires on various substrates, which make it promising for building nanosensors and nanoelectronic devices. Key words: Pd nanowires, DNA template, Poly(dimethysiloxane) transfer, Ethanol reduction I. INTRODUCTION One of the main goals of nanotechnology is to produce and manipulate well defined structures at the nanoscale level with high accuracy. However, it is well known that conventional top-down approaches are beset by various difficulties, including experimental difficulties, high cost issue and complicate processes etc. This leads to great interest in the development of new strategies based on bottom-up approaches, among which DNA has been extensively exploited as the template to fabricate various nanostructures [1 6]. To realize the large-scale functional circuits, various geometrical shapes of DNA such as the Y type [7], squares [8], and cubes have been designed and synthesized [9], which are particularly useful in the construction of complex functional circuits. However, these geometrical patterns cannot be directly applied in the functional circuits because of the poor conductivity of DNA [10]. From the technological standpoint, one strategy to overcome this electrical inactivity of DNA is using the molecules as templates for the fabrication of semiconducting and/or metallic nanowires and/or nanopatterns. So far, amounts of semiconducting or metallic quasi-one-dimensional nanostructures have been fabricated on DNA scaffolds, including CdS [3, 11, 12], CdSe [13], CuS [14], copper [6], silver [15], gold [2, 16], and platinum [17]. Due to the potential applications in H 2 sensors [18, 19], catalysts [20 22], and highly conductive connection, Pd nanowires have attracted a great deal of attentions and been extensively investigated [1, 23 25]. Yet, due to the fast kinetics of the growth reaction in the previous reported synthesis methods, it is difficult to produce nanowires with homogeneous morphologies and high density. Besides, many parasitic nanoparticles formed in a random area are inevitable. Recently, Nguyen et al. reported a method to increase the quality of the nanowires by reducing PdO- DNA complexes with H 2 [25]. Unfortunately, the maximum continuity length of the nanowires was only 3 µm. In this work, we proposed and developed a new PDMS (poly(dimethysiloxane)) transfer method based on ethanol-reduction at low temperature and fabricated well-aligned, high density, long continuity, homogenous, and highly conductive Pd nanowires with DNA templates. II. EXPERIMENTS Authors to whom correspondence should be addressed. xpwang@ustc.edu.cn, wangzx@nanoctr.cn 0.3 mg/ml double stranded λ-dna solution stored in 10 mmol/l Tris-HCl, 1 mmol/l EDTA buffer, with 607 c 2013 Chinese Physical Society

2 608 Chin. J. Chem. Phys., Vol. 26, No. 5 Hong-bing Cai et al. Pipet tip Reaction solution DNA combed on PDMS Pd nanowires on PDMS Reverse Pd nanowires on silica Peel PDMS off silica Press it on silica FIG. 1 Schematic procedures of fabricating Pd nanowires on DNA scaffolds by a PDMS transfer method. a size of base pairs and a molecular weight of daltons, was purchased from Fermentas. Poly (dimethylsiloxane). Palladium chloride (PdCl 2, AR) was purchased from Sinopharm chemical Reagent Co, Ltd. (SCRC). Millipore water, with a conductivity of 18 MΩ cm, was used in all the experiments. Sylgard 184 silicone elastomer was used for preparation of PDMS. The mixture gel was first prepared by mixing the base with the curing agent at a volume ratio of 10:1. Then the mixture was poured onto a fresh clean silicon substrate in a glass container and was baked on hot plate at 150 C for 10 min. The PDMS was uncovered from the silicon and the side contacted with the silicon was used. The procedures for synthesis of DNA-templated Pd nanowires on a substrate are as the following. Firstly, a droplet solution of 7 µl 10 ng/µl λ-dna was deposited on a PDMS sheet, and then this droplet was driven and combed by a pipet tip at a moving speed of about 1 mm/s along one certain direction. The process was repeated several times to extensively align DNA on the whole PDMS surface. Secondly, the DNA-PDMS sheet was submerged in the reaction solution containing 2 ml ethanol, 1.76 ml Millipore water, and 240 µl 10 mmol/l PdCl 2 in a small weighing bottle with a lid. The bottle was immediately put into a refrigerator at 4 C for 24 h to form Pd nanowires on PDMS. And then, the bottle stood for 6 h at room temperature for further growth. Finally, the PDMS sheet was taken out from the reaction solution and the Pd nanowires on PDMS were transferred onto other substrates for characterization. The samples were characterized by field emission scanning electron microscope (FESEM, Raith e Line, Germany). High resolution transmission microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDX) were performed using JEM-2010 (Joel, Japan). The composition of the nanowires was also analyzed by the X-ray photoelectron spectroscopy (XPS, VG ES- CALAB MKII system). Transport measurements were performed by semiconductor characterization system, Keithley 4200, at room temperature. FIG. 2 FESEM images of Pd nanowires on a silica substrate, fabricated by the PDMS transfer method with ethanol reducing Pd(II) for 24 h at 4 C and further incubated at room temperature for 6 h. Pb nanowires with the length more than 9 µm, Pb nanowires with the diameter of 83.7 nm. III. RESULTS AND DISCUSSION Figure 1 shows the schematic procedures of our proposed method, in which the metallization of DNA is achieved in two distinct steps. First, DNA strands combed onto a freshly prepared PDMS sheet, which are gently incubated with a solution containing palladium chloride and ethanol at 4 C. In this process, Pd nanowires array are attained on the PDMS sheet. Secondly, Pd nanowires on PDMS substrate are transferred onto other substrates, such as Si, SiO 2, carbon film, gold film, and silicon nitride. The process is simple, productive, high effective, and environmentally friendly. Figure 2 illustrates the FESEM images of typical Pd nanowires on a silica substrate, fabricated by the PDMS transfer method with ethanol reducing palladium chloride for 24 h at 4 C and for another 6 h at room temperature. Compared to previous reports [1, 23, 25], our method has several apparent features. First, the nanowires fabricated by this method have higher quality. As shown in Fig.2, we also find that the maximum continuity length of the nanowire can be up to 14 µm, far longer than that reported by Nguyen et al. [25]. The mean length of prepared Pd nanowires estimated from total 256 nanowires reaches 3.9±2.3 µm, while the mean width of the nanowires approaches 83.0±13.0 nm determined from total 65 nanowires. One c 2013 Chinese Physical Society

3 Chin. J. Chem. Phys., Vol. 26, No. 5 Pd Nanowires using PDMS Transfer Method 609 Pd Cu C Cu Pd Energy / ev FIG. 3 FESEM images of Pd nanowires, transferred from PDMS sheets to gold film, carbon film, (c) silicon nitride, and (d) silicon dioxide. nanowire with the width of 84 nm is shown in Fig.2. Secondly, most of the Pd particles are selectively deposited and confined on the DNA backbone, and only very few parasitic Pd particles occur on the substrate. This can be owed to the hydrophobic and inert PDMS surface [26]. Thirdly, the Pd nanowires fabricated on PDMS sheet can be readily transferred onto various other substrates, as shown in Fig.3. The merit undoubtedly expands the potential application of nanowires for the fabrication of nanodevices. Lastly, as compared to previously reported method [25], we emphasize that our method is simple and has high yield. No special surface modification is required, ethanol is a convenient reagent and 4 C is a general reaction temperature. Moreover, large numbers of nanowires could be readily fabricated in a large area. The chemical composition of the nanowires is determined by both EDX and XPS. As shown in Fig.4, the EDX result indicates that the nanowires are composed of Pd element. The appearance of C and Cu is attributed to the carbon coating Cu grid. It should be noted that Cl element is absent in EDX, implying that Pd(II) had been completely reduced to Pd(0) by ethanol. XPS spectra, shown in Fig.4, further confirmed this result. Two peaks around and ev correspond to 3d5/2 and 3d3/2 of Pd(0) respectively, consistent with the previous reports [27]. Both results of EDX and XPS indicate that the nanowires are composed of Pd(0). TEM images of Pd nanowires are shown in Fig.4(c). It is found that Pd nanowires are composed of large numbers of dense Pd nanoparticles and are quite well aligned on the carbon film. The high magnification TEM image of a Pd nanoparticle is shown in Fig.4(d). This image shows that the particle is crystalline and the d-spacing of 2.32 A can be observed clearly. From the selected area electron diffraction (SAED) image shown in the inset of Fig.4(d), a series of d-spacing parame FIG. 4 Characterization of Pd nanowires. EDX spectrum, XPS, (c) TEM image of synthesized Pd nanowires, and (d) HRTEM image of one Pd nanoparticle in Pd nanowires and inset is the SAED image of Pd nanowires. ters, 2.32, 1.93, 1.37, and 1.16 A corresponding to the (111), (200), (220), and (311) planes of Pd crystal can be indexed, indicating the nanowire has face-centered cubic lattice structure. For the characterization of electrical properties of nanowire, we transferred the Pd nanowires onto a Si substrate covered with 100 nm thick SiO2 layer and the electrodes composed of Cr/Au (8 nm/80 nm) were fabricated using electron beam lithography. Figure 5 shows a typical I-V curve of a single Pd nanowire with the resistance of about 423 Ω. Figure 5 is the FESEM image of Pd nanowire under the metal electrodes, showing the width and the length is about 75 and 1500 nm, respectively. Therefore, the resistivity of 1.59 µω m of the Pd nanowires can been calculated from I-V curve. Note that this value is just one order of magnitude higher than that of bulk Pd (0.105 µω m), indicating c 2013 Chinese Physical Society

4 610 Chin. J. Chem. Phys., Vol. 26, No. 5 Hong-bing Cai et al. FIG. 5 I-V curve of single Pd nanowire, the nanowire between two electrodes. Pd nanowires possess very good conductivity [23, 25]. To investigate the growth process of the nanowires, we fabricated Pd nanowires with different incubation time of 1, 4, and 24 h at 4 C. For 1 h incubation time, the width and length of the nanowires is only 32 nm and hundreds of nanometers respectively, as shown in Fig.6. As the incubation time increases up to 4 h, the width of the nanowires exceeds 50 nm and the maximum length reaches over 5 µm (Fig.6). When the time approaches 24 h, the width of nanowire increases up to 89 nm (Fig.6(c)). We further find that the Pd nanoparticles will become more densely aligning on the Pd nanowires with further 6 h incubation time at room temperature, while the length and the width of the nanowire have no obvious variation. Additionally, the reaction temperature also plays an important role in the whole process of nanowire fabrication. The Pd nanowires incubated at 4 C show denser and more compactly morphologies as compared to that at 25 C for the same incubation time. (c) FIG. 6 FESEM images of Pd nanowires for different incubation time of 1 h, 4 h, and (c) 24 h at 4 C. shows that the Pd nanowire has high conductance. The proposed method can expand to be used to fabricate other nanomaterial and is promising for the wide applications. V. ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology of China (No.2011CB921403), the National Natural Science Foundation of China (No , No , and No ), and Chinese Academy of Sciences (No.XDB ). IV. CONCLUSION Highly dense, well-aligned and highly conductive Pd nanowires are fabricated by PDMS transfer method using DNA scaffolds based on ethanol-reduction at low temperature. The mean length and average width of the nanowires can reach 3.9±2.3 µm and 83.6±13.0 nm respectively, while the maximum length can exceed 14 µm. This method can also suppress the parasitic nanoparticles due to the hydrophobic and inert PDMS surface. The fabricated Pd nanowires can be readily transferred onto various substrates. I-V measurement [1] J. Richter, R. Seidel, R. Kirsch, M. Mertig, W. Pompe, J. Plaschke, and H. K. Schackert, Adv. Mater. 12, 507 (2000). [2] G. Braun, K. Inagaki, R. A. Estabrook, D. K. Wood, E. Levy, A. N. Cleland, G. F. Strouse, and N. O. Reich, Langmuir 21, (2005). [3] T. Hirai, H. Okubo, and I. Komasawa, J. Phys. Chem. B 103, 4228 (1999). [4] Z. Z. Huang, F. Pu, Y. H. Lin, J. S. Ren, and X. G. Qu, Chem. Commun (2011). c 2013 Chinese Physical Society

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