© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 45, NOVEMBER 6, 1997
LETTERS Dichroic Thin Layer Films Prepared from Alkanethiol-Coated Gold Nanoparticles Alice H. Lu, Grace H. Lu, Ann M. Kessinger, and Colby A. Foss, Jr.* Department of Chemistry, Georgetown UniVersity, Washington, District of Columbia 20057 ReceiVed: July 23, 1997; In Final Form: September 16, 1997X
A simple method for preparing thin dichroic metal composite films is described. Alkanethiol-derivatized gold particles are incorporated into poly(tetrafluoroethylene) (PTFE) matrixes that have been friction-transferred onto glass slides. Exposure of these gold particle/PTFE composites to flame (ca. 1700 °C) leads to strong dichroic properties in the visible spectrum. Extinction spectra obtained with the incident electric field polarized perpendicular to the direction of friction orientation show a plasmon extinction band with a maximum at ca. 545 nm. Spectra collected with the incident field polarized parallel to the friction orientation direction show a strong extinction band with a plateau from λ ) 550 to 850 nm. The experimental polarization spectra are in general accord with the predictions of the Bruggeman effective medium approximation (EMA). EMA theory modeling suggests that the gold structures are not surrounded primarily by air, but rather are coated with, or imbedded within, a material that itself is highly absorbing.
Introduction Polymers oriented by stretching or rubbing have found useful applications in liquid crystal displays,1 in optical polarizers,2,3 and also in fundamental studies of molecular solute-polymer host interactions4,5 and surface second-harmonic generation.6 A few years ago, Wittmann and Smith developed the so-called friction transfer method,7 wherein a block of poly(tetrafuoroethylene) (PTFE) is translated across the surface of a glass slide. The result is a glass surface coated with highly oriented chains of PTFE, which can in turn orient a wide variety of substances, from small molecules to polymers7,8 and liquid crystals.9 The versatility of the friction-transfer method led us to consider its application to the preparation of optically anisotropic nanometal composite films for use in surface-enhanced spectroscopy10 and second-harmonic generation studies. While the orientation of metal crystallites has been achieved previously by vacuum deposition and epitaxial growth of these materials on stretch-oriented polymers,11-13 we have been interested in * Corresponding author. X Abstract published in AdVance ACS Abstracts, October 15, 1997.
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synthetic methods that do not require vacuum deposition. In this paper, we describe a method for preparing nanometal composite films that is based on PTFE friction transfer. Instead of employing vacuum deposition methods, we introduce metal onto the polymer substrate in the form of alkanethiol-coated gold nanoparticles in organic solvents,14 which have a much greater affinity for PTFE surfaces than aqueous colloids. Subsequent rubbing and flame treatment lead to strong linear dichroism in the visible spectrum. Experimental Section Microscope slides (Kimble 75 mm × 25 mm) were etched in 15% hydrofluoric acid (prepared from 48% HF, Mallinckrodt Analytical Reagent) for 5-10 min and allowed to dry at room temperature. A PTFE layer was applied by manually streaking the glass surfaces at room temperature with the edge of a Teflon block (5 cm × 5 cm × 2.5 cm, Read Plastics, Inc., Rockville, MD). The friction transfer of PTFE involved three or four successive motions of the polymer block across the glass surface, after which a dull surface layer was plainly visible. © 1997 American Chemical Society
9140 J. Phys. Chem. B, Vol. 101, No. 45, 1997
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Figure 1. Extinction spectra of alkanethiol-coated gold particles on friction-transferred PTFE/glass substrate prior to flame treatment. Solid curve: incident field polarized perpendicular to friction orientation direction. Dotted curve: incident field polarized parallel to friction orientation direction.
Figure 2. Extinction spectra of alkanethiol-coated gold particles on friction-transferred PTFE/ glass substrate after flame treatment. Solid curve: incident field polarized perpendicular to friction orientation direction. Dotted curve: incident field polarized parallel to friction orientation direction. Feature near 900 nm is a detector change artifact.
Thiol-coated nanoscopic gold particles were prepared according to the procedures developed by Brust et al.14 and Leff et al.15 Chloroauric acid (HAuCl4‚ 3H2O, Sigma) (0.118 g) was dissolved in 10 mL of deionized water. This solution was shaken in a separatory funnel with ca. 6.5 mL of a 0.027 g/mL solution of tetra-n-octylammonium bromide (Alfa 98%) in toluene (Fisher Reagent). Once the chloroauric anion was completely transferred into the toluene phase, this solution was isolated and combined with 4.2 mg of dodecanethiol (Fluka, 97%). The solution containing tetra-n-octylammonium tetrachloroaurate and dodecanethiol was then stirred for 3 h with 4.2 mL of a 0.0149 g/mL aqueous solution of sodium borohydride (Aldrich 98%). The toluene phase turned from orange to red as the nanoscopic gold particles formed. After the aqueous phase was removed, the thiol-coated Au particles were precipitated from the toluene phase by addition of ca. 70 mL of ethanol. This mixture was cooled overnight in a freezer at ca. - 20 °C. The gold particles, which in precipitate form appear as a blueblack powder, were collected by filtration onto a Whatman No. 1 filter paper. The thiol-coated gold particles were recovered from the filter paper by placing the paper into a test tube containing methylene chloride. A few drops of this dark red solution were then placed on the PTFE-coated surface of the microscope slide. After the solvent evaporated, the surface was rubbed again with the Teflon block, in the same manner and direction as done prior to the gold particle addition. The Au particle/PTFE coated slides were then grasped with a pair of pliers and exposed to a Fisher burner flame (ca. 1700 °C16) for a few seconds until the glass glowed a dull pink. From a visual inspection of the glass slides with a Polaroid film, the surface layer appeared blue-green for incident fields polarized parallel to the direction of PTFE friction orientation and pink in the perpendicular polarization. UV/visible spectra were obtained using a Hitachi U3501 spectrometer equipped with a model 210-2130 polarizer. The efficiency of this polarizer decreases markedly above λ ≈ 800 nm, and thus we do not consider the spectral data above this wavelength to be quantitatively meaningful. All spectra were collected with the propagation vector normal to the glass slide surface. The Au particle/PTFE surfaces (post-flame treatment) were also characterized using a Hitachi S-570 scanning electron microscope.
The plasmon resonance band measured with the incident light polarized parallel to the direction of friction orientation is slightly red-shifted and increased in intensity relative to the spectrum collected with the light polarized perpendicular to the friction orientation direction. After flame treatment, the UV/ visible spectra show a profound dependence on incident polarization. Figure 2 depicts the spectra of the gold particle/ PTFE/glass sample collected with the incident light polarized perpendicular (solid line) and parallel (dashed line) to the direction of friction orientation. The perpendicular polarization spectrum has a profile similar to that of bulk solution gold colloids. The parallel polarization spectrum shows a broad extinction band that is dramatically red-shifted relative to the perpendicular polarization band and exhibits a plateau between 600 and 850 nm. From a comparison of Figures 1 and 2, it is clear that the total extinction intensity has dropped by more than 50%, indicating that some gold has been vaporized during the flame treatment. Figure 3 is a scanning electron micrograph of a microscope slide coated with gold particles according to the PTFE frictionorientation/alkanethiol gold particle preparation described above. Large randomly coiled fibrils that appear white in the SEM image are likely the ashed remnants of the PTFE matrix (such structures were not apparent on gold-sputtered microscope slides that served as TEM control samples). The gold structures appear as chains of spherical and rodlike particles with a definite gross alignment. While the SEM magnification precludes precise particle size determination, the individual particles appear to be 30-60 nm in diameter or axial length. The elongated structures appear to be the result of heat-induced particle agglomeration. However, we should note that the gold particle/ glass slide samples were sputtered with ca. 5 nm of gold to facilitate SEM imaging. Exposure of the Au particle/PTFE layer directly to a 1700 °C flame certainly destroys both the PTFE component of the composite as well as the alkanethiol surface chains. It would appear that friction orientation of the PTFE matrix induces a chainlike ordering in the thiol-coated gold particles, but they remain in electrical isolation until exposure to the flame (hence, the weak dichroism seen in Figure 1). The chainlike ordering persists during the flame treatment, and destruction of the alkanethiol coating allows the gold particles to interact and even coalesce to some extent. The polarization spectra can be modeled using the simple effective medium approximation of Bruggeman,17 which is most appropriate in this case because of the random positions of the
Results and Discussion Figure 1 shows the UV/visible polarization spectra of a gold particle/PTFE-coated glass slide prior to the flame treatment.
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J. Phys. Chem. B, Vol. 101, No. 45, 1997 9141
Figure 4. UV/visible spectra calculated using Bruggeman effective medium theory. The metal volume fraction is 0.05 and the optical path length 0.02 µm. Solid curves: spectrum corresponding to incident field polarized along the short axis of an ellipsoid of revolution with κ⊥ ) 1.5. Dotted curves: spectrum corresponding to incident field polarized along the axis of revolution with κ| ) 4. (A) The insulating component is assumed to have refractive index n ) 1.00 and absorption coefficient k ) 0. (B) The insulating component is assumed to be an absorbing medium with a single resonance at λ ) 300 nm (see text for details).
Figure 3. Scanning electron micrograph of gold particle/PTFE/glass slide sample after flame treatment. White fibrils are probably remnants of PTFE after flame treatment. Scale bar corresponds to 2 µm.
particles and broad distribution of interparticle spacings. The effective dielectric function , which is related to the optical refractive index n and absorption coefficient k via ) (n + ik)2 can be calculated from the corresponding dielectric functions of the metal m and insulating o components of the composite by solution of the expression17,18
m - o - + fo )0 fm m + κ o + κ
(1)
where fm and fo are the volume fractions of the metal and insulating components, respectively. The factor κ depends on particle shape and orientation in the incident field. For example, a sphere has a κ-value of 2, while a cylinder or ellipsoid whose aspect ratio is 3:1 will have a κ-value of 4 when the incident electric field is polarized along the particle axis. For an electric field incident along the radius of the same ellipsoidal structure, the κ-value is 1.5.19 Using eq 1 and the gold optical data of Johnson and Christy,20 we calculated composite spectra for different values of κ corresponding to the incident light polarized both parallel and perpendicular to the axis of friction orientation in our Au particle/PTFE/glass systems. For the insulating component we have considered a few possibilities such as air (n ≈ 1, k ≈ 0), or a nonabsorbing material (n ) 1.33, k ) 0). For both of these cases, we find that the plasmon resonance extinction in parallel polarization is more intense than in the perpendicular polariza-
tion over the entire visible spectrum (see Figure 4A). This is in stark contrast to the experimental results, where the extinction in the parallel polarization is only slightly greater than in the perpendicular case. The intensity discrepancy is minimized when the insulating component is assumed to have a complex refractive index. Figure 4B shows polarization spectra simulated assuming an effective κ of 4 for the parallel polarization and 1.5 for the perpendicular case. The insulating component is assumed to possess the wavelength-dependent complex refractive index of a single Lorentz oscillator,21 with a resonance at λ ) 300 nm, a damping factor of 1 × 1014 s-1, and an electron concentration of 1026 m-3. The simulations discussed above are simplistic for a variety of reasons, including the reality that the Au particle/PTFE/glass systems possess a large distribution of particle sizes and shapes. Also, the treatment of the host medium as a Lorentz oscillator is arbitrary. A host medium composed of graphite particles (a likely flame product) may account for the unexpectedly low gold plasmon resonance extinction values in the parallel polarization spectra. Modeling the spectra using available data for graphite requires some speculation as to the relative contributions of the so-called ordinary- and extraordinary-ray properties22 and is thus beyond the scope of this communication. We will address these issues in a subsequent paper.23 However, it is clear that the dichroic properties of the composite films prepared via friction orientation of PTFE arise from substrateinduced ordering of the gold particles. Using effective medium theory as a guide, we can anticipate that optimization of the procedures outlined here will allow for the inexpensive preparation of metal line polarizers, where κ approaches unity and infinity in perpendicular and parallel polarization, respectively.24 Conclusions We have prepared optically anisotropic nanometal composite films by adsorbing alkanethiol-coated gold particles on glass
9142 J. Phys. Chem. B, Vol. 101, No. 45, 1997 slides modified by PTFE friction transfer. After initial deposition and rubbing, the gold particle/PTFE/glass samples are only weakly dichroic, indicating that the metal particles remain electrically isolated. Subsequent flame treatment appears to destroy the alkanethiol coating as well as the PTFE matrix, thus allowing for electrodynamic interaction between the gold particles. With regard to extinction band position and shape, spectra calculated using Bruggeman theory are in reasonable agreement with experiment. However, the theoretical spectra do not exhibit the same relative extinction intensities for parallel and perpendicular polarization seen in experiment. Acknowledgment. This material is based on work supported by the National Science Foundation under Grant No. DMR 9625151. This work was also supported in part by the Lombardi Cancer Center Microscopy and Imaging Shared Resource, U.S. Public Health Service Grant 2P30-CA-51008. C.F. also thanks Dr. Maozheng Dai of Lombardi Cancer Center for electron microscopy assistance and Prof. Steven Pollack of Howard University for useful discussions on related oriented polymer systems.
Letters (10) Al-Rawashdeh, N.; Foss, C. A., Jr. Nanostruct. Mater. 1997, 9, 383. (11) Jandt, K. D.; Buhk, M.; Petermann, J. J. Mater. Sci. 1996, 31, 1779. (12) Jung, M.; Baston, U.; Steiner, P.; Petermann, J. J. Mater. Sci. 1991, 26, 5467. (13) Petermann, J.; Broza, G. J. Mater. Sci. 1987, 22, 1108. (14) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (15) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (16) Barr, W. E.; Anhorn, V. J. Scientific and Industrial Glass Blowing and Laboratory Techniques; Instrument Publishing Co.: Pittsburgh, PA, 1959. (17) Aspnes, D. E. Am. J. Phys. 1982, 50, 704. (18) When the dielectric functions are complex, eq 1 has four roots. We select the root for which the absorption coefficient k is always greater than zero. From we find n and k and calculate the absorbance using the expression:
{
A ) 2 log 1 -
(n + 1)2 + k2
+
4πkd λ
where d is the optical path length and λ is the incident wavelength. (19) The screening factor κ is related to the depolarization factor q via κ ) l-1 - 1. For ellipsoids of revolution with semimajor axis a and semiminor axis b, the depolarization factors for fields incident along a given axis can be approximated by
References and Notes la ) (1) Yokoyama, H. Mol. Cryst. Liq. Cryst. 1988, 165, 265. (2) Dirix, Y.; Tervoort, T. A.; Bastiaansen, C. Macromolecules 1997, 30, 2175. (3) Dirix, Y.; Tervoort, T. A.; Bastiaansen, C. Macromolecules 1995, 28, 486. (4) Thulstrup, E. W.; Michl, J. Spectroscopy with Polarized Light; VCH Publishers: New York, 1986. (5) Parikh, D.; Phillips, P. J. J. Chem. Phys. 1985, 83, 1948. (6) Shirota, K.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Shiibashi, T. Jpn. J. Appl. Phys. 1995, 43, L316. (7) Wittmann, J. C.; Smith, P. Nature, 1991, 352, 414. (8) Gill, R. E.; Hadziioannou, G.; Lang, P.; Garnier, F.; Wittmann, J. C. AdV. Mater. 1997, 9, 331. (9) Hubert, P.; Dreyfus, H.; Guillon, D.; Galerne, Y. J. Phys. II 1995, 5, 1371.
}
(n - 1)2 + k2
1/a 1/a + 2/b
lb )
1/b 1/a + 2/b
These expressions lead to an approximate relationship between the screening factors for electric fields incident parallel (κ|) and perpendicular (κ⊥) to the particle axes: κ⊥ ) 2/κ| + 1. For a discussion of the rigorous and approximate forms of l, see: van de Hulst, H. C. Light Scattering by Small Particles; Dover: New York, 1980. (20) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6, 4370. (21) See, for example, Hummel, R. E. Electronic Properties of Materials; Springer-Verlag: New York, 1993. (22) Borghesi, A.; Guizzetti, G. In Handbook of Optical Constants of Solids II; Palik, E., Ed. Academic Press: New York, 1991. (23) Foss, C. A., Jr. Manuscript in preparation. (24) Aspnes, D. E.; Heller, A.; Porter, J. D. J. Appl. Phys. 1986, 60, 3028.
