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J. Phys. Chem. C 2007, 111, 4180-4184
Spectral Sensitivity of Uniform Arrays of Gold Nanorods to Dielectric Environment Kosei Ueno,†,‡ Saulius Juodkazis,†,‡ Masahiro Mino,†,‡ Vygantas Mizeikis,‡ and Hiroaki Misawa*,†,‡ Research Institute for Electronic Science (RIES), Hokkaido UniVersity, Sapporo 001-0021, Japan and Core Research for EVolution Science & Technology (CREST), Japan Science and Technology Agency (JST), Japan ReceiVed: December 1, 2006; In Final Form: January 22, 2007
Uniform, ordered arrays of gold nanorods on glass substrates were fabricated with high accuracy using electron beam lithography and lift-off techniques. These structures exhibit longitudinal mode of localized surface plasmon (LSP) resonance with spectral position highly sensitive to the refractive index of local dielectric environment of the nanorods. The LSP resonance was found to red-shift nearly linearly, both with refractive index and aspect ratio of the nanorods, to the extent that enables optical detection of the refractive index changes of the order of 10-2. The entire spectral range of the detection can be defined by tailoring size and aspect ratio of the nanorods during the fabrication and typically falls into the optical communications region. These structures are therefore promising as optical sensors enabling simple and efficient monitoring of chemical and biological processes.
Introduction Plasmonics offers a unique opportunity of tailoring propagation and localization of optical near-field at the surface of noble metals. Optical surface plasmon resonances (SPR) associated with propagating or localized surface plasmon modes, allow concentration of incident optical fields into nanometric regions with high local intensity. This has enabled novel applications of plasmonics in near-field optics,1-4 nanolithography,5,6 and photonics.7 Sensitivity of SPR to the dielectric environment at the metals’ surface has also become widely exploited. In a typical sensor application, analyte molecules adsorbed to a metallic surface modify dielectric permittivity of the region occupied by the surface plasmon near-field, leading to changes in the reflectivity. This approach has been already applied in various chemical sensors such as affinity biological sensor.8-11 To launch the surface plasmon-polariton (SPP) modes propagating at the metals’ surface, a prism or diffraction grating (Otto and Kretschmann configuration)12,13 is used to ensure the k-vector matching. For practical applications, experimental configurations that relax the k-vector matching requirement are more desirable. Nanometric-sized particles of noble metals, such as gold (Au) or silver (Ag) support localized surface plasmons (LSP) for a broad range of incidence directions. Field localization and intensity enhancement achievable with LSP are comparable or higher than those of SPP, and can be tailored by varying shapes and sizes of the nanoparticles. LSP resonances, which cause scattering bands typically at visible and nearinfrared (NIR) wavelengths, are also sensitive to the local dielectric environment and have been exploited in high-fidelity chemical sensors. Spectral shift of the LSP scattering bands,14-16 or surface- enhanced Raman scattering (SERS)17-19 process can be used to monitor local dielectric environment and the associated molecular environment. * Corresponding author. E-mail:
[email protected]. Tel: +(81) 11 706-9358. Fax: +(81) 11 706-9359. † Hokkaido University. ‡ Japan Science and Technology Agency.
So far, most explorations of environmental sensors based on LSP effects were focused on chemically synthesized nanoparticles having spherical and ellipsoidal shapes.20-22 For example, Okamoto et al. have demonstrated the optical sensor functionality of a colloidal gold monolayer on a glass slide,23 which exploited red-shift of the LSP band with dielectric permittivity of the surroundings. However, chemical techniques usually produce nanoparticles having significant variations in their size, shape, and orientation. Since LSP bands are intrinsically very sensitive to these factors, performance of optical sensors will be degraded due to the inhomogeneous broadening. Arrays of nanoparticles fabricated on supporting substrates by lithographic techniques are highly homogeneous and show high promise for applications in optical sensing. Malinsky et al. have demonstrated extreme spectral sensitivity of Ag nanoparticles fabricated by nanosphere lithography to the properties of alkanethiol selfassembled monolayers, which exhibited red-shift by 3 nm with addition of every carbon atom to the alkane chain.24 This result shows that accurate, well-controlled fabrication may lead to the creation of high-sensitivity DNA and protein optical detectors. In a recent study, we have used electron-beam lithography (EBL) and lift-off techniques to fabricate arrays of highly homogeneous rectangular Au nanorods on glass substrates.25 LSP bands in these structures were found to be spectrally sensitive to atomic-scale variations in the ensemble-averaged nanorod length.26 It can be expected, that such nanorod structures will be also highly sensitive to their dielectric environment and therefore potentially suitable for sensor applications. Here, we investigate LSP extinction spectra of Au nanorod structures with various average rod dimensions and aspect ratios, immersed in liquids having different refractive index. These structures were found to show excellent sensitivity to their dielectric surroundings. Moreover, precise control over the size and aspect ratio of the nanorods during the fabrication allows convenient tuning of the LSP extinction band in the visible and NIR spectral regions.
10.1021/jp068243m CCC: $37.00 © 2007 American Chemical Society Published on Web 02/24/2007
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Figure 1. (a) SEM image of gold nanorods having size of (45 × 224 × 40) nm3, aspect ratio R ) 5; (b) high-magnification SEM image of nanorods with size of (34 × 300 × 40) nm3, R ) 9; (c) schematic drawing of the nanorod on glass substrate with an explanation of nanorod dimensions and directions of linear polarization required for the excitation of L and T modes of the LSP; (d) illustration of a nanorod immersed in a liquid.
Experimental Details Planar patterns of Au nanorods were defined on glass substrates (Matsunami Co., Japan) having an area of (24 × 24) mm2 using a high-resolution EBL system (ELS-7700H, Elionix Co., Ltd., Japan) at a 100 kV accelerating voltage. The structures occupied an area of (30 × 30) µm2 and comprised from 3000 to 4000 nanorods. A copolymer resist (ZEP-520a, Zeon Co., Ltd., Tokyo, Japan) diluted by ZEP thinner (1:1) was spin-coated on the substrates at 1000 rpm for 10 s and 4000 rpm for 90 s. After the prebake on a hot plate for 3 min at 180 °C, the EBL was carried out at an exposure dose of 1.2 µC/cm2 and an electrical current of 5 pA. After the development in a standard developer (Zeon Co., Ltd., Japan), a 2 nm chromium and 40 nm gold bilayer was deposited by sputtering (ULVAC, MPS4000, Japan).25 Then, lift-off was carried out by immersing in an acetone solution (Wako Pure Chemical Industrials Ltd., GR Grade) in an ultrasonic bath for 2 min and in a resist remover (Zeon Co., Ltd., Tokyo, Japan) for 5 min. Scanning electron microscopy (SEM) images of the sample with nanorod dimensions of (34 × 300 × 40) nm3 (w × l × h) and spacing of 200 nm are shown in Figure 1(a,b). Figure 1(c) explains schematically parameters of the nanorods. Their aspect ratio is defined as the length-to-width ratio R ) l/w. Nanorods with aspect ratio ranging from 1 to 9, as well as having the same aspect ratio but different volumes, were fabricated for this study. In the sample shown in the SEM images above, the nanorods have design value of R ) 9. The aspect ratio of nanorods resulting from the fabrication might be slightly different due to imperfections introduced during the pattern definition and lift-off steps. SEM resolution, limited to about 10 nm, does not allow easy assessment of the actual aspect ratios. Therefore, in the following we relate our observations to the design parameters of the nanorods. For aspect ratios R > 1, elongated shape of the nanorod allows the distinction of longitudinal (L) and transverse (T) LSP modes, which can be excited by radiation incident normally on the substrate as shown in Figure 1(d) and polarized parallel to the longer and shorter axes of the nanorod, respectively (see Figure 1(c)). LSP extinction spectra were measured by a Fourier-transform infrared (FTIR) spectrometer with a microscope attachment
Figure 2. (a) LSP extinction spectra for L-modes in nanorods with different aspect ratios: R ) 1 (black), 5 (red), and 9 (blue), in air (solid lines), and in propylene carbonate solution with refractive index nm ) 1.42 (dashed-lines). (b) Extinction spectra of gold nanoblocks with aspect ratio R ) 5 immersed in liquids having different values of the refractive index nm given by the numbers.
(FT-IR, IRT-3000) in the wavelength range of 670-3000 nm. In the measurements, an area of (20 × 20) µm2 on the sample was probed. Transparent liquids having different values of refractive index nm were utilized to emulate different dielectric environments for the nanorods. During the measurements the samples were immersed in: water (nm ) 1.33), propanol (nm ) 1.38), propylencarbonate (nm ) 1.42), cyclohexanone (nm ) 1.45), toluene (nm ) 1.48), and immersion oil (nm ) 1.73) as shown schematically in Figure 1(d). Results and Discussions First, we examine general optical properties and structural quality of samples having the same rod volume but different aspect ratios. The measured extinction spectra for L-modes in samples with R ) 1, 5, and 9 are shown in Figure 2(a). Each spectrum is dominated by LSP resonant scattering peak. The peaks exhibit a clear red-shift with aspect ratio. Structural quality of the samples can be judged from their extinction spectra by estimating the free-carrier dephasing time T2, which describes exponential relaxation of the optically driven electronic component of the LSP state. The dephasing time is given as T2 ) 2p/Γ, where Γ is spectral half-width of the extinction band.27 Dephasing time in bulk gold, T2 ) 9.3 ( 0.9 fs, was determined earlier.28 In our nanorods, T2 ) 7-8 fs was found, independent of the aspect ratio. A dephasing slightly faster than that in the bulk can be explained by the small width and height of nanorods, which are comparable with the mean free path of electrons in gold of about 37 nm. Thus, a small additional contribution to the dephasing may come from the
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Ueno et al. of the LSP peak, λmax, and the refractive index nm. Figure 3(a) shows the measured peak wavelength of the LSP resonance versus the refractive index of the environment for structures having different nanorod aspect ratios R. For each value of R, spectral position of the LSP peak is linearly proportional to the refractive index nm. It can be also noticed from Figure 3(a) that slopes of the dependencies become steeper with R. Closer inspection of the data reveals that for a fixed refractive index nm, LSP peaks red-shift nearly linearly with R as well. This allows for a fairly accurate linear representation of the dependency λmax(nm, R). The dashed lines in Figure 3(a) are linear fits to all data sets using the expression
λmax (nm, R) ) 502.4 + 122.6(nm) + 65.6(R) + 32(nm)(R) (nanometers) (1)
Figure 3. (a) LSP peak spectral position λmax versus the refractive index nm for different aspect ratios R. The solid lines are individual linear least-square fits for each R and play the role of guides for the eye to stress the linearity, and the green dashed lines are fits to all datasets by a single expression given in eq 1. (b) The same dependencies calculated from Gans theory (solid lines) and their linear fits by a single expression λmax (nm, R) ) 396.1 + 23.95(nm) - 28.18(R) + 89.2(nm)(R).
carrier scattering on the rough surface of the nanorods. We have also found that in other similar structures (i.e., in checkerboard patterns of nanorods connected at the corners) prepared and investigated using the same methods, dephasing times slightly longer than that in bulk gold were found. This finding illustrates further that our fabrication technique provides high-quality nanoparticles and will be reported in detail in the future. Next, we turn our attention to the role of dielectric environment for the LPS extinction spectra. Figure 2(a) shows extinction spectra of the samples immersed in a propylene carbonate solution with refractive index nm ) 1.42. Although shapes of these spectra are nearly identical to those in air, the immersion results in a pronounced red-shift of the extinction peaks; their absolute magnitudes increase with aspect ratio. To follow this dependency in finer steps of nm, extinction spectra of the samples were also measured in other liquids (see Experimental). Figure 2(b) summarizes the data for nanorods having an intermediate aspect ratio of R ) 5. Amplitudes of the extinction peaks depended on the liquid used, but were close to each other within about 3% range. These spectra also exhibit a red-shift proportional to the refractive index as will be demonstrated below. It is helpful to note that data in Figure 2 already indicates high spectral sensitivity of the nanorod structures to their dielectric environment and basically proves their applicability as optical sensors. For convenience, one would prefer a sensor with a simple and predictable relationship between the position
As can be seen, this expression reproduces slopes of the experimental dependencies very well, thus confirming our earlier conclusion about linearity of the λmax(nm)|R)const dependence. The coefficient before nm in the fourth term of eq 1 reaches 288 for R ) 9, indicating that refractive index change of the order of 10-2 leads to the LSP resonance peak shift by about 3 nm. In practice, shifts of similar magnitude are easily detectable. A small vertical displacement between the experimental data and the fits for some values of R is also evident from the figure. This difference can be explained by the slight deviations between the designed and actual ensemble-averaged aspect ratios in some samples. We do not attempt to eliminate these small aspect ratiorelated discrepancies, as the linearity of the λmax(nm)|R)const dependence is the most important for optical sensor applications. Equation 1 also demonstrates that precise control of the aspect ratio exercised during the fabrication allows tuning of the basic LSP resonance wavelength in the technically convenient optical telecommunications range (1.3-1.5 µm). Linearity of the dependencies allows easy prediction of the parameters of the structures by linear extrapolation, thus aiding the design of nanorod-based optical sensors. The observed spectral behavior of LSP scattering peaks in our samples can be compared with predictions of Gans theory,29 which has been widely used for the interpretation of LSP scattering spectra of noble metal nanorods obtained by electrochemical techniques.30 Generally, this theory applies to ensembles of randomly oriented, ellipsoidal nanoparticles located in isotropic dielectric surroundings and having the length of their major axis less than one tenth of the irradiating wavelength. One can notice that our structures meet none of these conditions. Nevertheless, as will be shown below, their LSP spectra agree qualitatively with the aforementioned theory. Gans theory allows direct deduction of the peak wavelength λmax without need to obtain the LSP scattering spectra. In a quasi-static dipole approximation, LSP resonance condition for a small metal particle of arbitrary shape is
m + L(1(λ) - m) ) 0
(2)
where m ) n2m is the dielectric permittivity of a nondispersive surrounding medium, 1 is the real part of the dielectric permittivity of gold, and L is the depolarization factor. For L-modes of ellipsoidal nanoparticles,30 the depolarization factor is
L)
1+e 1 - e2 1 -1 ln 2 2e 1 -e e
[
(
) ]
e)
x
1-
1 R2
(3)
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Figure 4. Spectral shift of the LSP peak wavelength ∆λmax due to the refractive index change from nm ) 1 to 1.73 for nanorods having the same aspect ratio R ) 5 but different volumes. The volume is normalized to that of a nanorod having size of (45 × 224 × 40) nm3. The solid line is the least-square linear fit.
At infrared wavelengths, dielectric permittivity of gold is very well described by the Drude function
1(λ) ) ∞ -
λ2 λ2p
(4)
where ∞ is the high-frequency contribution to dielectric permittivity due to interband transitions in gold, and λp is the wavelength corresponding to plasma frequency, ωp. The dependence of LSP resonance wavelength on the refractive index of the environment can be found by substituting eq 4 into eq 1:
x (
λmax (nm, R) ) λ2p
∞ +
)
1 - 1 n2m L(R)
(5)
where the function L(R) is described according to eq 3. Figure 3(b) shows plots deduced from the above expression for the ranges of refractive index and aspect ratios used in the experiments. In these circumstances the λmax(nm, R) dependency can be also approximated by linear scaling against both arguments. This is illustrated by the linear fits included in the same figure (the expression used for the fitting similar to eq 1 is given in the caption). The data in panels (a) and (b) exhibit qualitatively similar behavior. However, Gans theory predicts LSP peaks at shorter wavelengths, and steeper slopes of λmax(nm)|R)const than were observed experimentally. The above discrepancies are not surprising, bearing in mind that nanorods investigated in this work are identically oriented, have well-defined tetragonal shapes, are surrounded by nonhomogeneous dielectric environments (glass substrate, and air or liquid), and some of them have lengths exceeding one tenth of the irradiation wavelength. Some studies have succeeded in adapting Gans theory to ensembles of rectangular randomly oriented30 or ordered nanorods25 for example by assigning empirical dependence to the refractive index nm on the aspect ratio nm(R) in eqs 2 and 5. In electrochemically prepared random ensembles of gold nanorods, this assignment is justified by different arrangements of surfactant molecules (having different microscopic dielectric parameters) on the nanorods of different aspect ratios.30 For our nanorods, prepared using a different technique and surrounded by homogeneous air or liquid environment (from five sides out of six), such assignment can hardly be justified. Most importantly, the very idea of treating the refractive index as an adjustable parameter is alien to the index sensing applications. Therefore we do not attempt to achieve quantitative agreement between the Gans theory and
the experimental data. The empirical relation (eq 1), which allows determination of the refractive index, is the main result of this study. Finally, we have investigated the sensitivity of nanorod structures for different nanoblock volumes. Figure 4 compares spectral shifts of the LSP peak wavelength, ∆λmax, induced by the refractive index change from nm ) 1 to 1.73 for several structures having the same aspect ratio R ) 5 but different nanorod volumes. As can be seen, the spectral shift remains nearly constant when the average nanorod volume increases by more than three times. This finding indicates that increase in the radiative losses in nanorods of larger volume remains negligible and does not affect spectral shape and position of the LSP resonance. Hence, spectral LSP response of gold nanorod arrays is not very sensitive to the nanorod volume. Conclusions Gold nanorod structures fabricated using electron beam lithography and lift-off techniques with uniform orientation and well-controlled dimensions and aspect ratios were found to exhibit longitudinal LSP resonances with central wavelength linearly dependent on the refractive index of their dielectric environment. This dependence is sensitive enough to allow practical detection of the index change of the order of 10-2. The fabrication method allows tailoring of the nanoblock dimensions, aspect ratios and volumes in a wide range. Optical characterization of the fabricated samples demonstrates that such tailoring allows tuning of the LSP longitudinal resonance to the desired spectral position within the optical telecommunications spectral range, convenient for practical applications. These results provide guidelines for the creation of simple and efficient chemical and biological sensors based on optical monitoring of the LSP resonance of noble metal nanoparticles. Acknowledgment. H.M. is grateful for a Grant-in-Aids from the Ministry of Education, Science, Sports, and Culture, Japan (No. 17360110) for partial financial support of the research. References and Notes (1) Ashino, M.; Ohtsu, M. Appl. Phys. Lett. 1998, 72, 1299. (2) Krenn, J. R.; Dereux, A.; Weeber, J. C.; Bourillot, E.; Lacroute, Y.; Goudonnet, J. P.; Schider, G.; Gotschy, W.; Leitner, A.; Aussenegg, F. R.; Girard, C. Phys. ReV. Lett. 1999, 82, 2590. (3) Imura, K.; Nagahara, T.; Okamoto, H. J. Phys. Chem. B. 2004, 108, 16344. (4) Kim, Z. H.; Leone, S. R. J. Phys. Chem. B. 2006, 110, 19804. (5) Srituravanich, W.; Fang, N.; Sun, C.; Luo, Q.; Zhang, X. Nano Lett. 2004, 4, 1085. (6) Luo, X.; Ishihara, T. Appl. Phys. Lett. 2004, 84, 4780. (7) Ozbay, E. Science 2006, 311, 189. (8) Peterlinz, K. A.; Georgiadis, R. M.; Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 3401. (9) Thiel, A. J.; Frutos, A. G.; Jordan, C. E.; Corn, R. M.; Smith, L. M. Anal. Chem. 1997, 69, 4948. (10) Yamaguchi, A.; Juodkazis, S.; Matsuo S.; Misawa, H. Chem. Lett. 2002, 190. (11) Kyo, M.; Yamamoto, T.; Motohashi, H.; Kamiya, T.; Kuroita, T.; Tanaka, T.; Engel, J. D.; Kawakami, B.; Yamamoto, M. Genes Cells 2004, 9, 153. (12) Otto, A. Z. Physik 1968, 216, 398. (13) Kretschmann, E. Z. Phys. 1971, 241, 313. (14) Sun, Y.; Xia, Y. Anal. Chem. 2002, 74, 5297. (15) Evanoff, D. D.; White, R. L.; Chumanov, G. J. Phys. Chem. B. 2004, 108, 1522. (16) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. J. Phys. Chem. B. 2004, 108, 13963. (17) Vo-Dinh, T. Sens. Actuators, B 1995, 29, 183. (18) Shafer-Peltier, K. E.; Haynes, C. L.; Glucksberg, M. R.; Van Duyne, R. P. J. Am. Chem. Soc. 2003, 125, 588. (19) Wen, X.; Xie, Y. T.; Mak, M. W. C.; Cheung, K. Y.; Li, X. Y.; Renneberg, R.; Yang, S. Langmuir 2006, 22, 4836.
4184 J. Phys. Chem. C, Vol. 111, No. 11, 2007 (20) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316, (2002). (21) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B. 2003, 107, 668. (22) Hutter, E.; Fendler, J. H. AdV. Mater 2004, 16, 1685. (23) Okamoto, T.; Yamaguchi, I.; Kobayashi, T. Opt. Lett. 2000, 25, 372. (24) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van, Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471.
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