NANO LETTERS
Electric Field Tuning of Plasmonic Response of Nanodot Array in Liquid Crystal Matrix
2005 Vol. 5, No. 10 1978-1981
Pavel A. Kossyrev,*,† Aijun Yin,† Sylvain G. Cloutier,† David A. Cardimona,§ Danhong Huang,§ Paul M. Alsing,§ and Jimmy M. Xu†,‡ DiVision of Engineering and Department of Physics, Brown UniVersity, 182 Hope Street, Box D, ProVidence, Rhode Island 02912, and Air Force Research Lab, Space Vehicles Directorate, Kirtland Air Force Base, New Mexico 87117 Received July 15, 2005; Revised Manuscript Received August 26, 2005
ABSTRACT In this work we demonstrate the feasibility of electric-field tuning of the plasmonic spectrum of a novel gold nanodot array in a liquid crystal matrix. As opposed to previously reported microscopically observed near-field spectral tuning of individual gold nanoparticles, this system exhibits macroscopic far-field spectral tuning. The nanodot−liquid crystal matrix also displays strong anisotropic absorption characteristics, which can be effectively described as a collective ensemble within a composite matrix in the lateral dimension and a group of noninteracting individual particles in the normal direction. The effective medium model and the Mie theory are employed to describe the experimental results.
Plasmonic resonances in metallic structures, manifesting in the optical transmission and absorption spectra, are interesting and useful.1,2 Driven by a growing interest in exploiting potential applications in near-field optical lithography, surface-enhanced Raman spectroscopy, and nanooptics, numerous experimental and modeling efforts have been undertaken recently.2-4 Many of these have explored the possibilities of tailoring the plasmon resonance modes through varying the structural features, sizes, and spacing of the metallic structures. It is also clear that in addition to the physical modifications of the features, varying the effective dielectric medium surrounding the metallic structure could change the plasmonic resonance modes.5 Although these two methods may be viewed as equivalent, one introduces a static change, whereas the other can be dynamic, continuous, reversible, and voltage-controlled, and find a broad range of potential applications. The feasibility of the latter is what this work seeks to explore and demonstrate. Voltage-controlled tuning of plasmonic response by adjusting the effective dielectric constant of the environment is implemented through a relatively simple strategy: embedding a lateral hexagonal array of metallic (gold) nanodots in a liquid crystal layer, which is encapsulated by glass substrates containing interdigitated electrodes. Liquid crystals are an outstanding example of an electrooptically active dielectric medium, which has found its major application in * Corresponding author. E-mail:
[email protected]. † Division of Engineering, Brown University. ‡ Department of Physics, Brown University. § Kirtland Air Force Base. 10.1021/nl0513535 CCC: $30.25 Published on Web 09/20/2005
© 2005 American Chemical Society
displays.6 Other applications utilize liquid crystals to produce switchable optical periodic structures7,8 and color tunable polymer gratings.9 Voltage-induced wavelength-selective absorption shift of surface plasmons in plain films was demonstrated by utilization of liquid crystals in the Kretschmann-Raether geometry.10,11 In another recent experiment, liquid crystals were used as the host fluid for colloidal gold nanoparticles, and a near-field variation of the single gold particle absorption peak wavelength was observed via a microscope.12 As we show in this letter, the nanodot arrays are indeed placed in a beneficial position between the plain films and single nanoparticles, where, in the former case, a bulky prism and an intricate setup and, in the latter, a microscope are required to observe color tuning due to the manipulation of liquid crystal alignment by an electric field. As opposed to previously reported microscopically observed near-field spectral tuning of individual gold nanoparticles, the color tuning of the novel nanodot arrays described here can be observed in the far-field without a complex arrangement, and potentially can be accomplished on a flexible substrate. We find that there is a pronounced anisotropy in the nanodot array absorption behavior; the array acts effectively as a composite matrix of gold nanodots and liquid crystal in the plane of substrate, and as a collection of individual nanoparticles in the normal direction. Finally, we note that the exploration of electric field tuning of plasmonic resonances of nanodot arrays in liquid crystals represents an experimental platform for continued theoretical
Figure 1. (a) SEM image of nanostructured array of gold nanodots. (b) Cross-sectional SEM image at 75° to the substrate normal. (c) Geometry of gold nanodot arrays in a liquid crystalline cell with a metallic Bragg grating as a top interdigitated electrode (not shown). The idealized direction of electric field between interdigitated electrodes, E, is indicated. The molecular orientation of liquid crystal is schematically depicted. Image is drawn not to scale. The expressions for average refractive indexes of liquid crystal host that we used in our calculations to simulate the absorption peaks due to the normal plasmon and the lateral modes are indicated, where ne and no are the extraordinary and ordinary refractive indexes of liquid crystal, respectively.
interest in nanoparticle scattering dependence on liquid crystalline alignment.13 The gold nanodot array formed on a glass substrate was fabricated by using a thin nanopore array aluminum oxide membrane as a stencil in e-beam evaporation. The highly ordered and uniform pores of the membrane are hexagonally spaced and are formed in the process of anodization of high purity aluminum under carefully controlled and by now wellestablished conditions.14 For better adhesion, a 5 nm layer of titanium was first evaporated through-membrane onto the glass substrate. A scanning electron microscope (SEM) image of the fabricated nanodot array is shown in Figure 1a, from which it is determined that the array has an average period of 105 nm and a dot diameter of 74 nm. A good periodicity of arrays is observed in domains of size ∼1 µm2. The crosssectional SEM analysis shows a semispherical dome shape of the nanodots with an average height of around 25 nm (see Figure 1b). The absorption spectra of the nanodot arrays (see Figure 2a) were collected with a Varian Cary-500i double-beam spectrophotometer with a resolution of 1 nm. By measuring the spectrum at various angles of light incidence with unpolarized light, the plasmon absorption peaks of the nanodot array are revealed. The short-wavelength absorption Nano Lett., Vol. 5, No. 10, 2005
Figure 2. (a) The absorption spectra of an array of gold nanodots on a glass substrate at various angles of light incidence. Normalization of all spectra to unity was performed at the wavelength of 200 nm. The lower spectrum is the absorption of an array of nanodots in a liquid crystal cell under normal incidence. (b) The experimental absorption spectra at 0 and 2 V/µm electric fields. The spectra are normalized to unity at the lateral absorption peak wavelength. The fits (field-off and -on states) are shown by the black lines and are based on the effective medium model (solid and dash lines, respectively) and the Mie theory (dash-dot and dot lines, respectively).
peak (at ≈520 nm) appearing at off-normal light incidence is due to normal-to-substrate plasma oscillations. In the normal-to-substrate direction, the nanodot monolayer can be viewed as isolated particles, and the spectral position of the absorption peak, determined by the nanodot size and the refractive index of the surrounding environment, is due to plasmon resonances of a collection of noninteracting individual nanodots.5 However, in the plane of the substrate, the spacing between particles is rather small compared with the wavelength of light; therefore, for the treatment of the optical response of nanodots to radiation with in-plane polarization, we utilize the Maxwell-Garnett effective medium theory5 to correctly predict the observed long-wavelength absorption peak (at ≈682 nm). In the formalism of the effective medium model, the absorption peak of the nanodot array is due to 1979
the collective dielectric response of the entire medium (nanodots/surrounding), where plasmons and electronic dipole coupling of the collective medium are involved. As can be seen from the angular dependence of absorption (Figure 2a), the amount of light coupled to the normal plasmon and lateral modes is mostly governed by the light incidence angle. However, since the nanodots are efficient scatterers, the coupling to the normal plasmons is finite, even at normal incidence with no normal polarization component. The structural specifics of the nanodot array in a liquid crystal sample, schematically shown in Figure 1c, are as follows. The cell thickness was set at 5 µm by inserting glass bead spacers between the top electrode plate and the substrate. The top electrode glass plate consists of chrome interdigitated electrodes forming a Bragg grating. The period of the electrodes is 10 µm, the electrode width is 5 µm, and the electrode thickness is 80 nm. The primary purpose of the interdigitated electrodes is to be able to apply an electric field in the plane of the cell. All further measurements were performed with illumination from the electrode side at normal light incidence. Upon filling the liquid crystal between the top and bottom plates, the two absorption peaks, corresponding to normal and lateral modes, red-shift from 520 and 682 nm in air to, respectively, 527 and 716 nm in liquid crystals, due to the higher refractive index of the liquid crystal medium (see Figure 2a). Also, compared to the absorption in air, the height of the short-wavelength peak is enhanced relative to the longwavelength peak in liquid crystal cell, which can be attributed to additional scattering and therefore coupling to normal plasmons because of the presence of liquid crystal and grating. The liquid crystals used in the experiments are the commercially available product E7 from Merck, which is an eutectic mixture of different low-molecular-weight cyanobiphenyl molecules and is in nematic phase at room temperature.15 In the field-off state, a vertical alignment of liquid crystalline molecules in the cell is established by treating the substrates with lecithin surfactant.16 This alignment can be effectively changed to the in-plane molecular orientation by application of an electric field, because of the positive dielectric anisotropy of the E7 compound (∆ ) 13.8). Commonly used in liquid crystal devices,17 an alternating electric field of square waveform (1 kHz) was applied to avoid electrolysis. Due to the molecular realignment, schematically shown in Figure 1c, the effective refractive index changes, producing a spectral shift in the plasmonic absorption of the nanodot array. Indeed, in the field-on state, with a ∼2 V/µm field, both the absorption due to the normal-to-substrate plasmons of individual nanodots and that due to the lateral response of the composite nanodot-liquid crystal film shift in their peak wavelengths (see Figure 2b). However, these shifts differ in details. Whereas the normal plasmon mode exhibits a small blue shift (527 f 524 nm), the lateral mode shows a red shift, which is a few times larger (716 f 727 nm), as shown in Figure 3. Meanwhile, the absorption minimum at ≈600 nm shifts to the red by ∼18 nm (see Figure 2b). In 1980
Figure 3. The position of normal and lateral absorption peaks as a function of electric field strength.
addition, the relative change of the absorption peak height is much greater for the normal than for the lateral mode. By assuming an idealized liquid crystalline molecular configuration in the field-off and -on states, depicted in Figure 1c, one may approximate the refractive indexes of the liquid crystal host as an average over the molecular orientations.17 The normal and the lateral modes see different refractive indexes in the field-off and -on states. Consider the lateral case first. Due to the small distance between nanodots compared to the wavelength of light, the nanodot array and the surrounding liquid crystal host can be regarded as a composite layer of the same thickness as the average nanodot height (25 nm). The dielectric function of such a film, , can be obtained from the Maxwel-Garnett effective medium theory5 using the known dielectric functions18,19 for the bulk gold, m, and the E7 liquid crystal compound and averaging over molecular configuration, d: - d m - d ) fm + κd m + κd where κ is a screening parameter that depends on the shape of the nanodots, fm is the volume fraction of nanodots, and the average liquid crystal refractive index for incoming unpolarized light is shown in Figure 1c for field-off and -on states. The screening parameter was obtained by following the formalism described in ref 20. We also accounted for the titanium layer by performing the same procedure with m now being the dielectric function of titanium.21 Finally, the transmission of a composite film with the top layer of liquid crystals and the bottom glass plate can be computed on the basis of the theory of wave propagation in a stratified medium.22 Despite the fact that the experimental curves are much broader than the modeled ones, as previously found in other systems such as alumina membranes containing gold particles23 and attributed to heterogeneity of particle aspect ratios,5,24 we obtain a good agreement in the long-wavelength absorption peak positions (lateral mode) and the corresponding spectral shift value for the field-off and -on states (see Nano Lett., Vol. 5, No. 10, 2005
Figure 2b). Besides, the model also confirms the lateral mode absorption peak position (≈682 nm) of the nanodot array before liquid crystal infiltration. In the normal-to-substrate direction, the nanodots can be considered as individual particles, and the well-established Mie theory5,25 can be applied to calculate their extinction cross-section, σ. For simplicity, we approximate the nanodots as spheres with a diameter of 20 nm (average height minus 5 nm Ti layer) and take into account that this diameter is much smaller than the wavelength of light so that only the dipole oscillation contribution is significant to the extinction cross-section: σ)
′′m 18π 3/2 d V λ (′m + 2d)2 + (′′m)2
where V is the particle volume, λ is the wavelength of light, and m′ and m′′ denote the real and imaginary parts, respectively. The field-off and -on dielectric functions of the liquid crystal host determined from its average refractive index over molecular distribution were taken in the vicinity of nanodots (see Figure 1c) assuming a near-field character of the nanodot plasmon resonances.4 The possible effects of the titanium layer and the glass substrate were neglected. From the extinction cross-section, the modeled shortwavelength absorption peak (normal mode) adequately describes the experimental results in field-off and -on states (see Figure 2b) and also matches the experimental absorption of the nanodots (at ≈520 nm) before liquid crystal infiltration. Qualitatively, the observed red shift of the longwavelength absorption peak and the blue shift of the shortwavelength peak can be understood by observing that, upon applying an electric field, the refractive index for the lateral mode increases, while that for the normal mode decreases, because the extraordinary refractive index is higher than the ordinary one (ne > no as, for example, ne ) 1.746 and no ) 1.521 at 589 nm) for E7 liquid crystal. The actual distribution of the liquid crystal molecular orientation at the nanodot surfaces is, naturally, more complex. Only in the regions between the interdigitated electrodes can the molecular orientation be considered substantially horizontal upon application of an electric field, which is the very effect employed for the formation of liquid crystal phase-gratings.26 The complex molecular distribution under the interdigitated electric field may contribute to the width broadening of the normal plasmon peak and also to the increase of its relative height, due to increased scattering on liquid crystalline interdigitated domains (see Figure 2b). Finally, we note that even with this proof-of-concept and unoptimized platform, the color tuning with voltage was apparent and directly observable by an ordinary CCD camera. However, it is not possible to differentiate the color tuning due to the nanoarrays from that simply due to diffraction on the grating. Thus, one should rely on the spectrally resolved measurements of Figure 2b to deduce the color tuning due
Nano Lett., Vol. 5, No. 10, 2005
to nanoarrays. Further optimization of the device and its geometry are necessary to achieve greater and directly discernible color tuning, which we hope will follow from this first demonstration. In particular, plain electrodes, liquid crystals with larger optical anisotropy (∆n ) ne - no), and improved substrate molecular anchoring can be explored. In summary, this work reports on voltage-controlled tuning of the plasmonic response of a simple arrayed gold nanodotliquid crystal composite. Anisotropic plasmonic absorption spectra and spectral tuning of nanodot arrays are revealed. The electric field-off and -on behaviors of absorption spectra can be explained in terms of the effective medium theory for the lateral mode and the Mie theory for the normal mode. This first demonstration of tunable plasmonic nanocomposite materials can be expected to lead to further exploration of color tunable nanophotonic devices based on surface plasmonics. Acknowledgment. We thank Martin Fay and Daniel Levner from Digital Light Circuits, Inc. for the interdigitated electrodes. Support from AFRL, DARPA, and ONR for parts of this work are gratefully acknowledged. References (1) Liz-Marzan, L. M. Mater. Today 2004, 7, 26-31. (2) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824830. (3) Srituravanich, W.; Fang, N.; Sun, C.; Luo, Q.; Zhang, X. Nano Lett. 2004, 4, 1085-1088. (4) Kneipp, K.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. J. Phys.: Condens. Matter 2002, 14, R597-R624. (5) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (6) Yeh, P.; Gu, C. Optics of liquid crystal displays; John Wiley and Sons: New York, 1999. (7) Kossyrev, P.; Sousa, M. E.; Crawford, G. AdV. Funct. Mater. 2004, 14, 1227-1232. (8) Escuti, M. J.; Qi, J.; Crawford, G. P. Opt. Lett. 2003, 28, 522-524. (9) Bowley, C. C.; Kossyrev, P. A.; Crawford, G. P.; Faris, S. Appl. Phys. Lett. 2001, 79, 9-11. (10) Wang, Y. Appl. Phys. Lett. 1995, 67, 2759-2761. (11) Wang, Y.; Russell, S. D.; Shimabukuro, R. L. J. Appl. Phys. 2005, 97, 023708. (12) Mu¨ller, J.; So¨nnichsen, C.; von Poschinger, H.; von Plessen, G.; Klar, T. A.; Feldmann, J. Appl. Phys. Lett. 2002, 81, 171-173. (13) Park, S. Y.; Stroud, D. Phys. ReV. Lett. 2005, 94, 217401. (14) Liang, J.; Chik, H.; Yin, A.; Xu, J. M. J. Appl. Phys. 2002, 91, 25442546. (15) Penterman, R.; Klink, S. L.; de Koning, H.; Nisato, G.; Broer, D. J. Nature 2002, 417, 55-58. (16) Cognard, J. Mol. Cryst. Liq. Cryst., Suppl. Ser. 1981, 1, 1-77. (17) Chigrinov, V. G. Liquid Crystal DeVices: Physics and Applications; Artech House: Boston, 1999. (18) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6, 4370-4379. (19) Li, J.; Wu, S.-T.; Brugioni, S.; Meucci, R.; Faetti, S. J. Appl. Phys. 2005, 97, 073501. (20) Foss, C. A.; Hornyak, G. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963-2971. (21) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1974, 9, 5056-5070. (22) Born, M.; Wolf, E. Principles of Optics; Cambridge University Press: Cambridge, 1999. (23) Hornyak, G. L.; Martin, C. R. Thin Solid Films 1997, 303, 84-88. (24) Hornyak, G. L.; Patrissi, C. J.; Martin, C. R. J. Phys. Chem. B 1997, 101, 1548. (25) Link, S.; El-Sayed, M. A. Int. ReV. Phys. Chem. 2000, 19, 409453. (26) Fujieda, I. Appl. Optics 2001, 40, 6252-6259.
NL0513535
1981
