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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 41 (2008) 152007 (4pp)

doi:10.1088/0022-3727/41/15/152007

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Arrays of aligned, single crystalline silver nanorods for trace amount detection Qin Zhou, Zhengcao Li, Ye Yang and Zhengjun Zhang1 Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China E-mail: [email protected]

Received 20 April 2008, in final form 20 June 2008 Published 16 July 2008 Online at stacks.iop.org/JPhysD/41/152007 Abstract Arrays of vertically aligned, single crystalline silver nanorods were deposited on silicon substrates via the glancing angle deposition technique using an e-beam system. The single crystalline Ag nanorods are several tens of nanometres in diameter and several hundred nanometres in length and could serve as excellent surface-enhanced Raman scattering substrates. Using these nanorods, Rhodamine 6G molecules can be detected to a concentration limit of 10−14 mol L−1 , showing the possibility of applications in the trace amount detection of organics. (Some figures in this article are in colour only in the electronic version)

media and optoelectronic devices [9–14]. For example, arrays of Ag nanorods were found to be good SERS substrates for the detection of trans-1,2-bis(4-pyridyl)ethane molecules, with a SERS enhancement factor greater than 108 [2]. It is therefore of considerable interest to investigate the growth of metal nanostructures by the GLAD technique. In this letter, we report on our efforts to grow arrays of aligned, single crystalline Ag nanorods on planar Si substrates by the GLAD technique, at substrate temperatures lower than room temperature, and the excellent performance of the silver nanorods as the SERS substrate in detecting Rhodamine 6G molecules at an ultrahigh sensitivity, i.e. to a concentration of 10−14 mol L−1 . The substrates used in this study were pristine Si wafers with (0 0 1) orientation. These were first cleaned in acetone, ethanol and de-ionized water baths in a sequence supersonically and were fixed on the GLAD substrate in an e-beam deposition system. The system was pumped down to a vacuum level of 3 × 10−5 Pa, and then the Ag thin film was deposited on the substrate at a deposition rate of 0.5 nm s−1 , with the thickness monitored by a quartz crystal microbalance. To produce films of aligned Ag nanorods, the incident beam of the Ag flux was set at ∼85◦ from the normal of the silicon substrate, at different substrate temperatures. The

The surface-enhanced Raman scattering (SERS) technique has attracted enormous attention recently due to its excellent performance and potential applications in the trace amount detection of chemical and biological molecules [1–3]. As a nondestructive analytical method, it could be applied in various fields, e.g. environmental pollutant detection, chemical and biological sensors. The detection sensitivity of this technique depends considerably on the surface property of the SERS substrate. For instance, using ordered arrays of gold particles prepared through a porous alumina template as the SERS substrate, Rhodamine 6G (R6G) molecules were detected to a concentration limit of 10−12 M [4]; arrays of silicon nanorods coated with Ag thin films served as good SERS substrates for R6G molecule detection, etc [2, 5]. Thus, the preparation of SERS substrates with the preferred surface property is of great significance. Among the approaches so far available to prepare nanostructured materials, the glancing angle deposition (GLAD) technique is a simple but powerful means which is capable of producing thin films with pre-designed nanostructures such as nanopillars, slanted posts, zigzag columns and spirals [6–8]. These nanostructures can be used in a variety of fields, e.g. photonic crystals, magnetic storage 1

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J. Phys. D: Appl. Phys. 41 (2008) 152007

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Figure 2. An XRD pattern of the Ag film consisting of well-separated, single crystalline nanorods shown by figure 1(c).

this temperature, Ag nanorods were formed in two films with a length of 500 nm; yet they were not well separated—most nanorods were joined together. A major difference between the two is the growth direction of the joined nanorods, i.e. without rotation the nanorods were grown at a glancing angle on the substrate, while with substrate rotation the nanorods were grown vertically aligned. Another noticeable difference is the size of the nanorods, i.e. nanorods grown with substrate rotation have a slightly larger diameter. Figures 1(c) and (d) show, respectively, the surface morphology of Ag thin films deposited at −40 ◦ C, without substrate rotation and with rotation at a speed of 0.2 rpm. On comparing with figures 1(a) and (b), it is noticed that the decrease in the deposition temperature led to the formation of well-separated Ag nanorods in the two films, while the rotation of the substrate still determined the growth direction and diameter of the nanorods, as observed from figures 1(a) and (b). The Ag nanorods grown at this temperature are 20–30 nm in diameter, ∼800 nm in length and are well separated. Therefore, by adjusting the temperature and movement of the substrate one can grow well-separated and aligned Ag nanorods on planar silicon substrates. Figures 1(e) and (f ) show, respectively, a bright-field TEM and an HRTEM image of Ag nanorods shown by figure 1(c); the inset of figure 1(f ) shows the corresponding SAD pattern. The images and the SAD pattern were taken with a JEM-2011F working at 200 kV. One sees from the figures that the Ag nanorod is ∼30 nm in diameter and is single crystalline. By indexing the SAD pattern it is noticed that during growth the {1 1 1} plane of the nanorod was parallel to the substrate surface, with its axis along the 1 1 0 direction. This was confirmed by XRD analysis. Figure 2 shows an XRD pattern of the Ag nanorods shown by figure 1(c). The pattern was taken with a Rigaku x-ray diffractometer using the Cu kα line, working at the θ –2θ coupled scan mode. From the figure one observes a very strong (1 1 1) texture, indicating that the {1 1 1} plane of the Ag nanorods was parallel to the substrate surface. These suggest that one can produce arrays of aligned, single crystalline Ag nanorods by the GLAD technique even at a low substrate temperature, i.e. −40 ◦ C.

Figure 1. Growth morphology of Ag thin films by GLAD at various conditions. (a) at 120 ◦ C without substrate rotation, (b) at 120 ◦ C and substrate rotation at 0.2 rpm; (c) at −40 ◦ C without substrate rotation; (d) at −40 ◦ C and substrate rotation at 0.2 rpm. (e) and (f ) show, respectively, a bright-field TEM and an HRTEM image of the nanorods shown by figure 1(c); the inset of (f ) shows the corresponding SAD pattern.

morphology and structure of the Ag thin films were observed and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM, selected area diffraction (SAD) and x-ray diffraction (XRD), respectively. The performance of the nanostructured Ag films as SERS substrates was evaluated with a microRaman spectrometer using R6G as the model molecule. It is well known that the major factors influencing the growth morphology of the films by GLAD are the incident direction of the depositing beam flux, the temperature and the movement of the substrate and the deposition rate. When fixing the incident Ag flux at ∼85◦ from the normal of the substrate and the deposition rate at ∼0.5 nm s−1 , the growth morphology of the Ag films was considerably dependent on the temperature and movement of the substrate. Figure 1 shows the growth morphology of Ag thin films versus the temperature and movement of the substrate. The SEM micrographs were taken with an FEI SEM (QUANTA 200FEG) working at 20 kV. Figures 1(a) and (b) show typical SEM images of the surface morphology of Ag thin films deposited at 120 ◦ C, without substrate rotation and with substrate rotation at a speed of 0.2 rpm, respectively. One sees from the images that at 2

J. Phys. D: Appl. Phys. 41 (2008) 152007

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Figure 3. Raman spectra of R6G on Ag thin films consisting of (a) joined nanorods shown by figures 1(a) and 1(b), and (b) separated Ag nanorods shown by figures 1(c) and 1(d), respectively, at a concentration of 1 × 10−6 mol L−1 .

Figure 4. (a) Raman spectra of R6G at concentrations ranging from 1 × 10−8 to 1 × 10−16 mol L−1 and (b) the Raman spectrum of R6G at a concentration of 1 × 10−14 mol L−1 , on the Ag thin film consisting of well-separated, single crystalline Ag nanorods.

By using Rhodamine 6G as the model molecule, we examined the performance of Ag thin films shown by figures 1(a)–(d) as the SERS substrate. These samples were dipped in a 1 × 10−6 mol L−1 solution of R6G in water for 30 min and dried with a continuous gentle nitrogen blow. Figures 3(a) and (b) show Raman spectra of R6G obtained on the four nanostructured Ag films by a Reinshaw 100 Raman spectrometer using a 514 nm Ar+ laser as the excitation source. It is observed that using the Ag thin films as the SERS substrate, all spectra clearly exhibit characteristic peaks of R6G molecules at 612 nm, 774 nm, 1180 nm, 1311 nm, 1361 nm, 1511 nm, 1575 nm and 1648 nm, respectively [2, 5]. However, the intensity of the Raman peaks was dependent on the morphology of the films. It is noticed that on Ag films consisting of well-separated nanorods, see figure 3(b), the Raman peaks of R6G are much stronger than that on films of joined nanorods, see figure 3(a). This suggests that arrays of aligned but well-separated Ag nanorods could work as excellent SERS substrates. Using arrays of aligned Ag nanorods shown by figures 1(c) and (d) as SERS substrates, we examined the detection limit of R6G molecules in water by the SERS method. Figure 4(a) shows the Raman spectra of R6G obtained on Ag nanorods shown by figure 1(c), as a function of the concentration of R6G in water ranging from 1 × 10−8 to 1 × 10−16 mol L−1 . Similar results were also obtained for Ag nanorods shown by

figure 1(d). The Raman spectra were obtained by one scan with an accumulation time of 10 s, at a laser power of 1% to avoid decomposition of R6G. One sees that characteristic peaks of R6G were observed at all concentrations. To clearly show this, we plot the Raman spectrum at 10−14 mol L−1 in figure 4(b). It is noticed that although the intensity of the peaks is almost two orders lower than that at 10−6 mol L−1 , the spectrum clearly shows the characteristic peaks of R6G [2, 5]. These suggest that Ag films consisting of aligned and wellseparated single crystalline Ag nanorods could serve as an excellent SERS substrate for trace amount detection of R6G molecules. However, in the Raman spectrum at 10−16 mol L−1 in figure 4(a), some of the peaks of R6G cannot be observed. This suggests that 10−14 mol L−1 is the detection limit of this method in the authors’ work. In summary, we have studied the effect of the temperature and movement of the substrate on the surface morphology of Ag films deposited by the GLAD technique and fabricated large scale arrays of aligned and well-separated single crystalline Ag nanorods on the planar silicon substrate. With these films R6G molecules were detected even at a concentration of 10−14 mol L−1 by the SERS method. This study provides an excellent SERS substrate for the trace amount detection of organics and a powerful method to prepare SERS substrates. 3

J. Phys. D: Appl. Phys. 41 (2008) 152007

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Acknowledgments The authors are grateful for the financial support by the National Natural Science Foundation of China (10675070) and the National Basic Research Program of China (973 program, 2007CB936601).

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