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Surface Plasmon Resonances of Cu Nanowire Arrays J. L. Duan,*,†,‡ T. W. Cornelius,§ J. Liu,*,† S. Karim,| H. J. Yao,†,‡ O. Picht,§ M. Rauber,§ S. Mu¨ller,§ and R. Neumann§ Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China, The Graduate UniVersity of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China, GSI Helmholtzzentrum fu¨r Schwerionenforschung GmbH, Planckstrasse 1, D-64291 Darmstadt, Germany, and Physics DiVision, PINSTECH, Nilore, Islamabad, Pakistan ReceiVed: March 31, 2009; ReVised Manuscript ReceiVed: June 9, 2009
Surface plasmon resonances of arrays of parallel copper nanowires, embedded in ion track-etched polycarbonate membranes, were investigated by systematic changes of nanowires’ topology and arrays area density. The extinction spectra exhibit two peaks which are attributed to interband transitions of Cu bulk metal and to a dipolar surface plasmon resonance, respectively. The resonances were investigated as a function of wire diameter and length, mean distance between adjacent wires, and angle of incidence of the light field with respect to the long wire axis. The dipolar peak shifts to larger wavelengths with increasing diameter and length, and diminishing mean distance between adjacent wires. Additionally, the shape effect on the dipolar peak is investigated. 1. Introduction One-dimensional nanostructures have attracted enormous attention since they open up very promising applications.1 In the recent past, considerable effort has been devoted to the study of localized surface plasmon resonance (LSPR) related optical properties of metal (such as Au, Ag, and Cu) nanostructures in the ultraviolet, visible, and near-infrared spectral ranges. Metallic objects with dimensions on the nanometer scale exhibit sizedependent light extinction that is attributed to localized surface plasmon polaritons, i.e., collective oscillations of conduction electrons, in this case driven by the incident light field.2 Furthermore, as is well-known, the shape of a nanoobject plays a significant role for the specific characteristics of LSPR. Previous studies have demonstrated that LSPR occur in a wide variety of structures, and the optical properties of metal nanospheres and nanorods have been extensively studied.3-7 Recently, optical properties of nanoobjects with specific geometrical forms, such as nanocages, nanostars, nanocrescents, nanorice and nanobars, L-shaped particles, nanodisks, nanorings, and triangular nanoparticles have been reported.8-15 However, up to now only limited quantitative data are available on LSPR properties of metal nanowires. Concerning their shape, nanowires can be considered as nanorods with a larger aspect ratio (i.e., length over diameter). The LSPR characteristics like dipolar peak position and resonant modes along both the transverse and longitudinal directions of nanorods with comparatively small diameter (e20 nm) can be theoretically described within the framework of Gans’ theory,6 which is a quasistatic approximation method. However, for nanowires with larger diameter and higher aspect ratio, a more pronounced phase shift of the electric field along the long wire axis is expected. Therefore, Gans’ theory is no longer valid for predicting LSPR peak positions * To whom correspondence should be addressed. E-mail: j.duan@impcas. ac.cn (J.L.D.) and
[email protected] (J.L.). † Institute of Modern Physics, Chinese Academy of Sciences. ‡ The Graduate University of Chinese Academy of Sciences. § GSI Helmholtzzentrum fu¨r Schwerionenforschung GmbH. | Physics Division, PINSTECH.
and higher mode resonances. The present work focuses on optical properties of Cu nanowire arrays in the UV/visible spectral range. Previous studies have demonstrated that, in an ensemble of randomly distributed cylindrical wires, various LSPR modes along the directions parallel and perpendicular to the long wire axis can be excited.6,16 In the following, employing an abbreviated way of speaking, we denote plasmon modes, oscillating parallel or normal to the long wire axis, and the corresponding directions of oscillation as longitudinal or transVerse, respectively. To identify whether a resonance originates from a longitudinal or a transverse mode, well-aligned parallel wires represent an ideal configuration. For instance, when the wave vector of the incident light is parallel to the long wire axis, only transverse modes can be excited. In this work, motivated by the above-mentioned aspects, Cu nanowire arrays have been electrochemically deposited within the parallel nanochannels of ion-track templates. This method allows the creation of arrays consisting of parallel wires. The wire parameters such as diameter, length, crystallinity, and area density can be controlled by the ion irradiation, and the subsequent processes of track etching and wire deposition.1,17-20 The method facilitates the investigation of the LSPR properties as a function of wire geometry and mean distance between adjacent wires. We demonstrate that the frequency of the transverse dipolar plasmon resonance of nanowire arrays shifts within the visible range and can be tuned by varying wire diameter, length, distance, and angle of light incidence. 2. Experimental Section The Cu nanowire arrays were produced as follows. Polycarbonate (PC) foils (Makrofol N, thickness 30 µm) were irradiated by Pb ions at the UNILAC linear accelerator of GSI (Darmstadt, Germany). The ions were accelerated to a specific kinetic energy of 11.4 MeV/nucleon and impinged on the foil normal to its surface. The ions deposit their energy successively in the foil along their paths and produce cylindrical damage zones, socalled latent tracks, with diameters of around 10 nm. The track
10.1021/jp902894r CCC: $40.75 2009 American Chemical Society Published on Web 07/09/2009
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Duan et al. of the incident light, i.e., the electric field vector was perpendicular to the long axis. In such a configuration, only transverse modes are excited. For the thinnest wires (d ) 20 nm), a peak at 570 nm is observed. With increasing wire diameter, this resonance dramatically shifts to the red. For instance, the peak is located at 730 nm for d ) 135 nm. Accompanying the redshift of this resonance, a second peak located at ∼540 nm becomes more and more pronounced and does not shift with increasing d. The light wavelength at which the dipole mode of the transverse LSPR occurs for cylindrical wires with d ) 20 nm, embedded in a polymer matrix, was estimated by means of Gans’ theory.6
Figure 1. Representative SEM picture of an array of Cu nanowires with diameter 75 nm and area density 5 × 108 wires/cm2. The wires are cylindrical, and have smooth and homogeneous contours. The inset shows a wire section imaged by TEM.
area density is determined by the ion fluence, which was chosen to be 1 × 108 and 5 × 108 ions/cm2. To transform the tracks to nanochannels, the samples were chemically etched in 5 M NaOH at 50 °C, and simultaneously exposed to an ultrasonic field. The channel diameters were controlled via the etching time. Prior to etching, each foil surface was irradiated with ultraviolet light for 2 h in order to sensitize the tracks for the subsequent etching process. The wire deposition was performed by using a common twoelectrode setup. For this purpose, a thin Au layer (thickness ∼50 nm) was sputtered on one side of the PC template and reinforced by an electrochemically deposited Cu layer (thickness ∼10 µm). During wire growth, this electrically conductive layer and a copper cone served as cathode and anode, respectively. The deposition was performed potentiostatically with -200 mV at 50 °C, using an electrolyte of 75 g/L of CuSO4 · 5H2O and 30 g/L of H2SO4. The deposition current was monitored by a picoampmeter (Keithley 6485), and the wire length was controlled via the deposition time. To study the wire morphology by scanning electron microscopy (SEM, Philips XL30) and transimission electron microscopy (TEM, JEOL JEM-3010), the polymer matrix was dissolved in dichloromethane (CH2Cl2). For UV/vis transmission spectroscopy, the wires were left embedded in the membrane, but the backing layer (Au and Cu) was removed. The surface plasmons were excited with unpolarized light, and the resulting extinction spectra were measured in standard transmission geometry, using an UV-vis spectrometer (UNICAM, UV4). The angle of light incidence was defined as the angle between the long wire axis and the direction of light propagation. A pristine PC membrane was used as a reference. 3. Results and Discussion The scanning electron micrograph in Figure 1 demonstrates the cylindrical shape of the wires having a uniform diameter d of about 75 nm as well as smooth and homogeneous contours. The morphological characteristics of the wires are further confirmed by TEM evaluation (inset of Figure 1). For a fixed wire length L ) 30 µm, extinction spectra of arrays of wires with d varying from 20 to 135 nm are displayed in Figure 2a. The area density is 1 × 108 wires/cm2 with the reasonable assumption that all channels were filled. In these measurements, the long wire axis was parallel to the direction
3/2 4πεm γ ) NV 3λ
( )
[
1 ε2 P2 2 1-P ε1 + εm + ε22 P
(
) ]
Here, γ is the extinction coefficient, λ is the wavelength of the incident light in vacuum, N is the number of nanowires, V denotes the volume of a single wire, εm is the dielectric constant of the surrounding medium, and P is a geometrical factor that takes into account the aspect ratio of the wires. The real and imaginary parts of the dielectric constant of Cu are represented by ε1 and ε2, respectively. For computations, the dielectric constant of bulk Cu was taken from ref 21. According to the manufacturer, εm amounts to 3 for the PC membrane being used in the present work. The factor P perpendicular to the long wire axis is about 0.5 for nanowires with aspect ratios larger than 20.6 For d ) 20 nm, the dipole resonance wavelength calculated by this method amounts to ∼563 nm, in good agreement with the experimental value of 570 nm. Thus, the first single peak is assigned to a dipole resonance oscillating along the transverse direction. The differences of computation and experiment may be caused by phase retardation or shift of the electric field along the long wire axis. This effect is neglected in Gans’ theory, which assumes a quasistatic electromagnetic field, leading to wrong results for wires with larger diameters and lengths. As expected, the dipolar red-shift with respect to wire diameter is similar to the findings for nanowire gratings with rectangular wire cross-section.7,22 The peak located at ∼540 nm may originate from higher order modes of the transverse LSPR, mutual electromagnetic coupling of the wires, or interband transitions.3,23,24 However, this peak does not shift when changing wire diameter, length, mean distance of adjacent wires, and incident angle of light, as shown in the following sections. Therefore, we conclude that this peak originates substantially from interband transitions of Cu bulk metal since the other two are very sensitive to the aforementioned parameters.24 Recently, similar interband transitions of copper nanoparticles fabricated by nanosphere lithography also have been observed.15 Extinction spectra of arrays of wires with d ) 75 nm and different lengths are presented in Figure 3a. The electric field vector is perpendicular to the long wire axis and, thus, only transverse LSPR modes can be excited. For the shortest wires (L ) 3.5 µm), the dipole resonance at 597 nm dominates the spectrum. This peak is red-shifted with increasing length and is located at 636 nm for L ) 30 µm. Due to phase shifts, it is reasonable to suggest that counter-oscillating dipoles are arranged in a sequence along the long wire axis, as schematically illustrated by Figure 3b. Each dipole could sense the coupling
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Figure 2. (a) Extinction spectra of arrays of Cu nanowires with diameters d ) 135, 102, 75, 50, and 20 nm (from top to bottom). The wire length and area density are 30 µm and 1 × 108 wires/cm2, respectively. (b) Extinction peak wavelengths of a dipole resonance as a function of wire diameter. The scale bars indicate more than five measurements.
Figure 3. (a) Extinction spectra of Cu nanowire arrays with wire length L ) 3.5, 15, and 30 µm (from bottom to top). The wire diameter and area density are 75 nm and 5 × 108 wires/cm2, respectively. (b) Schematic description of counter-oscillating dipoles along nanowires due to a phase shift of the incident field.
Figure 4. Extinction spectra of Cu nanowire arrays with area densities of 5 × 108 wires/cm2 (top) and 1 × 108 wires/cm2 (bottom). The wire diameter and length are 75 nm and 30 µm, respectively.
Figure 5. Extinction spectra of Cu nanowire arrays for different angles of light incidence. The wire diameter, length, and area density are 75 nm, 30 µm, and 5 × 108 wires/cm2, respectively.
field of the nearest surrounding dipoles. According to a theoretical prediction, such coupling may give rise to cumulative plasmon field enhancement.25 With increasing L, the enhancement effect increases. Simply, the coupling can also be understood as the neutralization of the polarization charges formed at the wire surface. A similar coupling occurs in the case of nanoparticle chains and consequently such coupling should ultimately induce a red-shift of the resonance.4,25-27 Previous studies have demonstrated that electromagnetic coupling between metal nanostructures can significantly influence their LSPR properties.3,4,23,26,28,29 It is reasonable to consider the coupling at distances smaller than the decay length of the electromagnetic field associated with the resonance mode. For
nanoparticles, the plasmon coupling range is approximately 2.5 times as large as the particle diameter.4,26 In the present work, the average distance between adjacent wires was varied by changing the ion fluence. With increasing area density, i.e., diminishing interwire distance, the dipole resonance is redshifted (Figure 4). Here, d and L amount to 75 nm and 30 µm, respectively, while the area density was varied from 1 × 108 to 5 × 108 wires/cm2. Thus, the mean distance between neighboring latent ion tracks (whose diameter is negligible) changed from 500 to 220 nm, and between the wire surfaces facing each other from 425 to 145 nm. The decay length of the electromagnetic field for nanoparticles with diameter 75 nm amounts to ∼190 nm according to refs 4 and 26. Thus, for the sample with the
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Figure 6. (a) Extinction spectra of a rhombic Cu nanowire array as a function of angle; (b) dipole peak position as a function of angle. Here, the angle is defined as the angle between the polarization direction of polarizer and the short axis of nanowires.
larger area density, adjacent wires are located within the decay range and, consequently, are coupled to each other. Therefore, the red-shift of the dipole resonance suggests a strong electromagnetic field enhancement within the gaps of neighboring wires. A recent investigation on the strongly interacting Ag nanowire arrays shows that the gap plasmon modes existing in the region defined by two neighboring nanowires are excellent candidates to guide signals over tens of micrometers at nearinfrared frequencies.30 The electromagnetic field-coupling is further probed by angledependent spectroscopy. Figure 5 illustrates the extinction spectra of Cu nanowire arrays for different angles of light incidence. Here, θ is defined as the angle between the direction of light incidence and the long wire axis. In this sample, d, L, and area density are 75 nm, 30 µm, and 5 × 108 wires/cm2, respectively. The dipole resonance is blue-shifted as the angle increases. This may be caused by weakening of LSPR coupling. It is well-known that the strongest coupling occurs when the two linked dipoles are in phase. With increasing angle, neighboring wires experience increasingly different phases of the light field. Therefore, the coupling strength is lowered and, consequently, the dipole resonance shifts to shorter wavelengths. Note that, if θ approaches 90°, the longitudinal mode along the wires length would show up in the far-infrared range or further, which is beyond our spectra. To investigate the cross-section shape effect on the LSPR, an array of rhombic Cu nanowires was fabricated by employing an ion track-etched single-crystal muscovite mica template. The thickness of the mica template was 20 µm. The ion irradiation fluence was 1 × 108 ions/cm2. After 2 min etching in 40% hydrofluoric acid (HF), all pores of mica template possess the rhombic shape. The lengths of the long axis and the short axis are 58 and 36 nm, respectively, as shown by the SEM micrograph inserted in Figure 6a. The formation mechanism of the rhombic pore in the mica template is found elsewhere.31 When measuring the spectra, a light polarizer was employed. The obtained spectra of various angles are displayed in Figure 6a. Here, the angle is defined as the angle between the polarization direction of the polarizer and the short axis of the nanowires. Namely, 0° means they are parallel and 90° means they are perpendicular. As shown in Figure 6a, the dipolar peak shifts from 568 to 610 nm with increasing the angle from 0° to 90°. When the angle increases further, the peak shifts back to short wavelength again. The peak position as a function of the angle is plotted in a polar graph (Figure 6b). Obviously, when the polarization direction is parallel to the short axis, the dipole
peak has the shortest wavelength. However, when the polarization direction is parallel to the long axis, the dipole peak has the longest wavelength. This finding can be explained by the size effect, which is similar to the diameter effect discussed in the section of cylindrical wires. Additionally, the peak has the strongest intensity when the polarization direction is parallel to the long axis, indicating the largest extinction cross section of light. In addition to the dipolar peak, as shown in Figure 6a, a broad peak emerging at about 470 nm is strongly polarization dependent. This peak could be attributed to higher order modes of polaritons because of the shape effect.32 4. Conclusions In summary, the surface plasmon resonances of Cu nanowire arrays were investigated by optical spectroscopy. The dipolar surface plasmon resonance is red-shifted with increasing wire diameter, increasing wire length, and decreasing mean wire distance. In addition, the frequency shift is also affected by the angle of incidence of the light field due to varying strength of the coupled field. Additionally, the cross section shape of wires has a significant influence on their optical properties. The extension of this study to nanowire arrays of other metals, such as gold and silver as well as their alloys, would be of interest because they possess different dielectric constants. Acknowledgment. One of the authors (J.L.D.) thanks the Chinese Academy of Sciences (CAS) for a scholarship, the West Light Foundation of CAS and NSFC (Grant Nos. 10805062 and 10775161) for the additional financial support, and the Materials Research Department of GSI (Darmstadt, Germany) for the possibility of performing the experiments presented in this work. References and Notes (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353–389. (2) Kriebig, U.; Vollmer, M. Optical properties of metal clusters; Springer-Verlag: Heidelberg, Germany, 1995; Vol. 25. (3) Atay, T.; Song, J.-H.; Nurmikko, A. V. Nano Lett. 2004, 4, 1627– 1631. (4) Wei, Q. H.; Su, K. H.; Durant, S.; Zhang, X. Nano Lett. 2004, 4, 1067–1071. (5) Krenn, J. R.; Schider, G.; Rechberger, W.; Lamprecht, B.; Leitner, A.; Aussenegg, F. R. Appl. Phys. Lett. 2000, 77, 3379–3381. (6) van der Zande, B. M. I.; Bo¨hmer, M. R.; Fokkink, L. G. J.; Scho¨nenberger, C. Langmuir 2000, 16, 451–458. (7) Xu, Q.; Bao, J.; Capasso, F.; Whitesides, G. M. Angew. Chem., Int. Ed. 2006, 45, 3631–3635.
Surface Plasmon Resonances of Cu Nanowire Arrays (8) Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A.; Warsen, A.; Li, Z. Y.; Zhang, H.; Xia, Y.; Li, X. Nano Lett. 2007, 7, 1318–1322. (9) Nehl, C. L.; Liao, H.; Hafner, J. H. Nano Lett. 2006, 6, 683–688. (10) Shumaker-Parry, J. S.; Rochholz, H.; Kreiter, M. AdV. Mater. 2005, 17, 2131–2134. (11) Wiley, B. J.; Chen, Y.; McLellan, J. M.; Xiong, Y.; Li, Z. Y.; Ginger, D.; Xia, Y. Nano Lett. 2007, 7, 1032–1036. (12) Sung, J.; Hicks, E. M.; Van Duyne, R. P.; Spears, K. G. J. Phys. Chem. C 2007, 111, 10368–10376. (13) Langhammer, C.; Schwind, M.; Kasemo, B.; Zoric´, I. Nano Lett. 2008, 8, 1461–1471. (14) Aizpurua, J.; Hanarp, P.; Sutherland, D. S.; Ka¨ll, M.; Bryant, G. W.; Garcı´a de Abajo, F. J. Phys. ReV. Lett. 2003, 90, 057401. (15) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2007, 7, 1947–1952. (16) Khlebtsov, B. N.; Khlebtsov, N. G. J. Phys. Chem. C 2007, 111, 11516–11527. (17) Duan, J. L.; Liu, J.; Yao, H. J.; Mo, D.; Hou, M. D.; Sun, Y. M.; Chen, Y. F.; Zhang, L. Mater. Sci. Eng., B 2008, 147, 57–62. (18) Liu, J.; Duan, J. L.; Toimil-Molares, M. E.; Karim, S.; Cornelius, T. W.; Dobrev, D.; Yao, H. J.; Sun, Y. M.; Hou, M. D.; Mo, D.; Wang, Z. G.; Neumann, R. Nanotechnology 2006, 17, 1922–1926. (19) Cornelius, T. W.; Bro¨tz, J.; Chtanko, N.; Dobrev, D.; Miehe, G.; Neumann, R.; Toimil Molares, M. E. Nanotechnology 2005, 16, S246S249.
J. Phys. Chem. C, Vol. 113, No. 31, 2009 13587 (20) Karim, S.; Toimil-Molares, M. E.; Balogh, A. G.; Ensinger, W.; Cornelius, T. W.; Khan, E. U.; Neumann, R. Nanotechnology 2006, 17, 5954–5959. (21) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: New York, 1985; pp 350356. (22) Schider, G.; Krenn, J. R.; Gotschy, W.; Lamprecht, B.; Ditlbacher, H.; Leitner, A.; Aussenegg, F. R. J. Appl. Phys. 2001, 90, 3825–3830. (23) Kottmann, J. P.; Martin, O. J. F. Opt. Express 2001, 8, 655–663. (24) Fong, C. Y.; Cohen, M. L.; Zucca, R. R. L.; Stokes, J.; Shen, Y. R. Phys. ReV. Lett. 1970, 25, 1486–1490. (25) Ghenuche, P.; Quidant, R.; Badenes, G. Opt. Lett. 2005, 30, 1882– 1884. (26) Su, K.-H.; Wei, Q.-H.; Zhang, X. Nano Lett. 2003, 3, 1087–1090. (27) Dmitriev, A.; Pakizeh, T.; Ka¨ll, M.; Sutherland, D. S. Small 2007, 3, 294–299. (28) Jain, P. K.; Huang, W.; El-Sayed, M. A. Nano Lett. 2007, 7, 2080– 2088. (29) Jain, P. K.; El-Sayed, M. A. Nano Lett. 2007, 7, 2854–2858. (30) Manjavacas, A.; Garcı´a de Abajo, F. J. Nano Lett. 2009, 9, 1285– 1289. (31) Khan, H. A.; Khan, N. A.; Spohr, R. Nucl. Instrum. Methods 1981, 189, 577–581. (32) Kottmann, J. P.; Martin, O. J. F. Opt. Express 2000, 6, 213–219.
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