Nov 6, 2013 - aspect, conventional metal film is a poor platform for dielectric constant ... confirms that our SWNT films can be regarded as a good conducting sheet ..... (4) Seo, M. A.; Park, H. R.; Koo, S. M.; Park, D. J.; Kang, J. H.;. Suwal ...
Letter pubs.acs.org/JPCL
Dielectric Constant Engineering of Single-Walled Carbon Nanotube Films for Metamaterials and Plasmonic Devices J. T. Hong,† D. J. Park,† J. H. Yim, J. K. Park, Ji-Yong Park, Soonil Lee, and Y. H. Ahn* Department of Physics and Division of Energy Systems Research, Ajou University, Suwon 443-749, Korea S Supporting Information *
ABSTRACT: We demonstrate the fabrication of plasmonic and metamaterials devices, operating in the terahertz frequency range, by using highly conductive, single-walled carbon nanotube (SWNT) network films. We fabricated various patterns on the SWNT films using photolithography or laser machining techniques, whose resonance behaviors are determined by geometric parameters such as the periodicity of the array patterns or the shape of the individual elements. The excellent mechanical properties of SWNT films enabled us to fabricate free-standing and highly flexible devices. More importantly, using postprocessings such as chemical treatments and nanoparticle coatings, we were able to engineer the dielectric constants of the SWNT films, such as enhancing or degrading the conductive properties. As a result of the postprocessings, the resonance peak of the plasmonic devices was suppressed or retrieved, which is not achievable in conventional metal films. In particular, we were able to control the metamaterials resonances, implying the possibility of fabricating tunable optoelectronic devices without changing the device structures. SECTION: Plasmonics, Optical Materials, and Hard Matter
P
by manipulating their internal electronic structures. In this aspect, conventional metal film is a poor platform for dielectric constant engineering because its carrier density is already high and it allows only very limited modification of the optical constants. On the contrary, highly conductive, single-walled nanotube (SWNT) network films may be one of the alternative platforms for future THz optical and optoelectronic devices, replacing conventional metal films. A recent report revealed that SWNT films show high shielding effectiveness accompanied by a high dielectric constant close to that of normal metal in the THz frequency region.19−21 In particular, the solution-based processes of the SWNT film fabrication enable us to achieve large-scale, mass production of optically thick films without requiring a large expensive vacuum system. More importantly, the carrier density of SWNT films is easily controlled by postprocessings such as chemical treatment, molecular functionalization, and nanoparticle coating, which suggests the feasibility of dielectric constant engineering. We demonstrate the fabrication of plasmonic and metamaterial patterns on highly conductive SWNT films, either freestanding or supported on various substrates. The devices exhibited resonance behaviors in the THz transmission spectra,
lasmonic and metamaterials have attracted much interest due to their potential in visible to microwave-range optoelectronic applications such as superlenses,1 active plasmonic devices,2,3 subdiffraction focusing,4,5 and invisibility cloaks.6,7 The extraordinary phenomena in the electromagnetic waves interacting with these artificial materials are caused by the resonance excitation of the optical near-field determined by various patterns fabricated on thin metal films.8−10 For example, plasmonic crystals excite surface plasmon polariton in the near-field, induced by evanescent coupling of the diffracted wave.11,12 In metamaterials, the functioning frequency region is governed by the resonance surface current generation, where these resonances are determined by specific geometrical patterns with ring resonators and capacitors, fabricated on metal film.13−15 There have been increasing demands for fabricating devices capable of manipulating electromagnetic waves through structures. Current technologies primarily focus on controlling resonances by introducing various patterns on conventional metal films with fixed optical constants of the metal materials. Recently, a series of efforts have attempted to control the surrounding media by, for instance, manipulating the dielectric constant of the substrate rather than of the metal film itself through light illumination,16 heating,17 and electrical bias,18 which operates in the terahertz (THz) frequency range. Additional freedom can be provided if the optical and AC electrical properties of the conducting films can be engineered © 2013 American Chemical Society
Received: September 17, 2013 Accepted: November 6, 2013 Published: November 6, 2013 3950
dx.doi.org/10.1021/jz4020053 | J. Phys. Chem. Lett. 2013, 4, 3950−3957
The Journal of Physical Chemistry Letters
Letter
Figure 1. (a) Photograph of free-standing, 20 μm thick SWNT films. (b) Time trace of THz transmission through 2.5 μm thick SWNT film (red curve) together with reference trace. (c) Real part (red curve) and imaginary part (blue curve) of the refractive index extracted from panel b. (d) Real part (red curve) and imaginary part (blue curve) of the dielectric constant obtained from the refractive index.
incident fields. This observation confirms that it is possible to fabricate very thick films while preserving the high conductivity by using this method. The complex refractive index and dielectric constants of the free-standing, thick SWNT films can be extracted from the amplitude and phase information in the time-domain spectroscopy.29 Figure 1c depicts the real and imaginary parts of the refractive index obtained by this method, and Figure 1d shows the dielectric constants. Both the real and imaginary parts of the dielectric constants show a frequency response consistent with the classical Drude model for conductive metals.21 The plasma frequency and the scattering rate were obtained by fitting the dielectric constant shown in Figure 1d with the simple Drude model equation εr(ω) = ε∞− ωp2τ2/(1+ ω2τ2), where ε∞ is the background dielectric constant, τ is the scattering time, and ωp is the plasma frequency. A plasma frequency of 77.1 THz and a scattering time of 1.24 ps were obtained from the results of fitting, which confirms that our SWNT films can be regarded as a good conducting sheet, mimicking good metal films in the THz region. Motivated by the fact that the SWNT network films exhibit highly conductive metallic properties, we fabricated plasmonic devices, consisting of 2D periodic arrays of square apertures on the highly conductive, free-standing SWNT films, as schematically shown in Figure 2a.30 By using laser-machining techniques,21 the square array patterns with different periods (Λ) of 150 and 200 μm were produced on free-standing 2.5 μm thick SWNT films, as shown in the microscopic images of Figure 2b. THz transmission spectra of these samples were obtained by a fast Fourier transform of the THz transmission amplitude recorded using a conventional THz-TDS method and plotted in Figure 2c. For Λ = 150 and 200 μm, the transmission peaked at 1.76 and 1.3 THz, respectively. The resonance frequency was consistent with the values determined by the periodicity of the patterns. Transmission dips were found together at 1.95 (for Λ = 150 μm) and 1.45 THz (for Λ
which were determined by the periodicity of a pattern array or the shape of individual pattern. Postprocessings such as chemical treatments and nanoparticle coatings were applied to the films, resulting in considerable changes in the dielectric constants. The resonance behaviors of the devices were modified accordingly, proving that device performance can be manipulated by dielectric constant engineering. To fabricate thick SWNT films as an alternative platform for plasmonic devices and metamaterials, we used a filtration method.22−24 This method is more efficient than spin-coating techniques25 because it achieves larger thicknesses, and it has advantages over the spraying method in its production of homogeneous and highly conductive films. This filtration method uses an SWNT dispersion solution that is vacuumfiltered on a cellulose membrane.21 The films, with thicknesses varying from 1 to 20 μm, can be prepared by varying the amount of solution. The mechanical strength of the film is high enough to allow free-standing films to be obtained simply by detaching the membrane substrate. Figure 1a shows a picture of a free-standing SWNT film with a thickness of 20 μm. Such a free-standing film is beneficial when fabricating plasmonic and metamaterial devices because it is free of undesired modifications in the device performance, which originates from index mismatching between the substrate and air.26−28 In addition, these films are free of the substrate multiple reflection effects that frequently obscure the transmittance spectra. To examine the optical properties of the free-standing SWNT films in terms of the dielectric constant, we measured the THz transmission amplitudes of the films by using a conventional THz time-domain spectroscopy (THz-TDS) setup exploiting an electro-optic sampling method.19 Figure 1b shows a time trace of the THz transmission amplitudes for a free-standing SWNT film with a thickness of 2.5 μm (red curve, 10 times magnification), together with a reference without the film (black curve). It is clearly observable that the transmitted THz amplitude decreases significantly, reaching
