Method for fabricating pixelated, multicolor polarizing films Alethia G. de Leo´n, Yvo Dirix, Yannick Staedler, Kirill Feldman, Georg Ha¨hner, Walter R. Caseri, and Paul Smith
Pixelated, multicolor polarizing filters— of potential use in full-color displays—were produced by what we believe to be a novel method, i.e., masked evaporation of silver and gold onto glass substrates partially covered with separated sub-micrometer-wide strips of oriented poly共tetrafluoroethylene兲 共PTFE兲, prepared by friction deposition. The evaporated metal films preferentially nucleated at the glass surface and, consequently, formed parallel arrays in between the PTFE strips. The structures thus produced feature a strong angle-dependent absorption of polarized visible light, allowing for optical switching between red and blue and between green and yellow. © 2000 Optical Society of America OCIS codes: 160.4670, 230.5440, 310.1860, 310.6860, 160.3900, 160.5470.
1. Introduction
Materials that act as color polarizing filters were described more than 100 years ago in widely forgotten reports1– 4 and have attracted interest again only in recent years.5–10 These materials typically are oriented nanocomposite fibers or films consisting of a polymeric matrix, for example, natural fibers, polyethylene, or poly共vinylalcohol兲, that contain uniaxially oriented arrays of nanoparticles of metals, e.g., silver, gold, or tellurium. The systems described display a transmitted color in linearly polarized light that depends on the angle between the polarization direction of the incident light and the orientation direction of the metal particle arrays. The latter phenomenon presumably finds its origin in the fact that the plasmon resonance frequency depends on the angle between the arrays and the polarization direction of the incident light.11 The colors that were observed varied with the size of the metallic particles in the nanocomposite9 and the selected metal.1,3 Similarly, thin structures of parallel metal whiskers or needles have been shown to act as polarizers,12–14 predominantly in the infrared regime. The latter are produced by oblique deposition of
The authors are with the Department of Materials, Eidgeno¨ssische Technische Hochschule Zu¨rich, CH-8092 Zu¨rich, Switzerland. W. Caseri’s e-mail address is
[email protected]. Received 4 November 1999; revised manuscript received 19 June 2000. 0003-6935兾00兾264847-05$15.00兾0 © 2000 Optical Society of America
metal vapors. In this process a beam of evaporated metal atoms is directed toward a substrate under a large angle of incidence, resulting in linear, uniaxially oriented metal structures on the surface. Like the oriented nanocomposites described above, these metal strips or needles polarize light in the nearinfrared or the visible 共VIS兲 wavelength regions. In particular, silver,12–14 gold, and copper12 have been employed for this purpose. Other polarizing systems have been prepared by means of sputtering silver15 or gold16 on glass, which was afterward drawn at high temperatures. Recently, the usefulness of color polarizing films of oriented polymer–silver nanocomposites was demonstrated in combination with a twisted nematic optical cell, resulting in a bicolored liquid-crystal display.8,10 For multicolored displays pixelation of the color filter naturally is required. However, this is virtually impossible or impractical to obtain with the described drawing procedures and the other above-mentioned methods. Here we report on a new, to our knowledge, simple technique for the preparation of pixelated, multicolor polarizing filters based on masked metal evaporation onto glass substrates partially coated with oriented poly共tetrafluoroethylene兲 共PTFE兲 strips. 2. Preparation and Characterization of the Substrates and the Metal Films
Microscope slides were used as glass substrates, which were cleaned with dichloromethane, acetone, and methanol prior to use. Oriented strips of PTFE were deposited with a friction deposition device 共Tri10 September 2000 兾 Vol. 39, No. 26 兾 APPLIED OPTICS
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Fig. 1. Atomic force microscopy image of a glass slide covered with oriented strips of PTFE prepared by friction deposition.
botrak, Daca Instruments, Santa Barbara, California兲 in which a PTFE rod was moved across the glass surface at a temperature of 350 °C, a velocity of 10 mm兾s, and a pressure of 0.35 MPa 共T was varied between 200 and 375 °C, but the most pronounced optical effects described below were observed for T ⫽ 350 °C兲.17,18 A typical array of oriented PTFE strips produced under these experimental conditions, as observed in a Nanoscope IIIa atomic force microscope 共Digital Instruments, Santa Barbara, California兲, is displayed in Fig. 1. Approximately 45% of the glass surface was covered with PTFE strips with an aver-
age width of ⬃130 nm and a height of ⬃20 nm. The average width of the uncoated glass areas separating the PTFE was ⬃180 nm. Silver or gold 共Balzers, Liechtenstein兲 was thermally evaporated onto the PTFE-coated glass substrates at a rate of 0.1 nm兾s with a Bal-Tec Med 20 coating system. A tungsten filament carrying a current of approximately 25 A was employed to heat the metal in the vacuum chamber 共approximately 2 ⫻ 10⫺5 mbar兲. The thickness of the metal layers was measured with a quartz-crystal thickness monitor and was between 0.5 and 11 nm. The optical effects
Fig. 2. SEM micrograph of a 7-nm silver layer thermally evaporated onto a PTFE friction-coated glass substrate; the sliding direction of the PTFE rod was horizontal.
Fig. 3. SEM micrograph of a 9-nm gold layer thermally evaporated onto a PTFE friction-coated glass substrate; the sliding direction of the PTFE rod was horizontal.
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Fig. 4. Polarized UV–VIS spectra of oriented metal–PTFE structures coated with silver. indicates the angle between the polarization direction of light and the orientation direction of the PTFE layer.
described below were most pronounced for 7-nm silver and 9-nm gold layer thicknesses, respectively. A representative example of a SEM image 共Hitachi Model S-900兲 of the resulting silver structures is shown in Fig. 2, which was taken with the backscattered electrons. The white areas in the image represent silver clusters, which are arranged predominantly in arrays oriented parallel to the sliding direction of the PTFE rod. Each metal strip is composed of irregularly shaped silver grains with a diameter ranging between approximately 20 and 40 nm. SEM images of gold-coated slides disclosed parallel metal lines of dimensions and spacings comparable with those of the silver samples 共Fig. 3兲. However, compared with the silver samples, the gold strips appeared to be more continuous, owing to the smaller grain size of the gold particles. Between the oriented structures some randomly distributed, spherical gold particles of 10 –20-nm diameter were also observed.
Fig. 5. Polarized UV–VIS spectra of oriented metal–PTFE structures coated with gold. indicates the angle between the polarization direction of light and the orientation direction of the PTFE layer.
Fig. 6. Schematic representation of the preparation of 250-mpixelated structures by sequential evaporation of metals through masks.
3. Optical Properties
When examined in transmitted light, the silvercoated samples appear blue when the polarization direction of the incident light is oriented parallel to the metal strips and red for perpendicular orientation. These color changes were quantified by absorption spectra recorded with polarized light in the VIS wavelength range 共Perkin-Elmer, Lambda 900 spectrophotometer兲. Figure 4 shows spectra at various angles, , between the polarization direction of light and the sliding direction of the PTFE rod, i.e., the long axis of the silver arrays. An absorption maximum in perpendicular polarized light was located at 533 nm, and an isosbestic point was observed at 721 nm. The same phenomenon was observed with the samples containing gold. Similar to the above silverbased films, the most pronounced polarization effects in the visible range were observed at an average gold thickness of ⬃9 nm. SEM images disclosed parallel metal arrays of dimensions and spacings comparable with those of the silver structures; however, the gold strips appeared to be more continuous because of the smaller grain size of the gold particles. When observed through an absorbing polarizer, these samples appeared yellow when the polarization direction of the incident light is oriented parallel to the direction of the metal strips and green for perpendicular orientation. Polarized UV–VIS spectra 共Fig. 5兲 showed one distinct absorption maximum in the visible wavelength range for ⫽ 90° at 627 nm and an isosbestic point at 784 nm. The above simple evaporation technique permits, without difficulty, the preparation of pixelated struc10 September 2000 兾 Vol. 39, No. 26 兾 APPLIED OPTICS
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Fig. 7. Polarized optical photograph of a color polarizing filter based on a pixelated oriented silver– gold–PTFE structure. Arrows indicate direction of polarizer.
tures with differently colored pixels. With the help of masks, which were placed on the PTFE-coated glass substrates 共Fig. 6兲, sequential deposition of silver and gold on different areas resulted in pixelated structures that allowed for two dichroic color changes in one optical film 共blue–red for silver and yellow– green for gold兲. Figure 7, finally, presents an optical photomicrograph 共Leica DMRX microscope兲 of the resulting multicolor polarizing filter with a pixel size of 250 m, viewed with the polarization axis of the transmitted light parallel 共a兲 and perpendicular 共b兲 to the direction of friction deposition. The dichroic spectra of the silver- and the goldcoated samples show qualitative similarities to those obtained with oriented silver aggregates embedded in polymers5,8 –10 or elongated gold particles aligned on surfaces.6 These systems are also characterized by isosbestic points and bathochromic shifts of the absorption maxima with decreasing angle . In summary, a versatile method is offered for the fabrication of pixelated, multicolor polarizing filters. It should be noted that the present method is based on self-organization of metals into oriented arrays of particular widths. The latter, which here is due to oriented, separated PTFE strips deposited onto glass, of course, may also be induced by, for example, substrates of oriented copolymers composed of alternating polar and nonpolar lamellar structures. 4850
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The authors thank Paul Walther from the Laboratory of Electron Microscopy I of the Eidgeno¨ssische Technische Hochschule 共ETH兲 Zu¨rich and Jeroen Visjager for valuable assistance, Werner Isler from the Swiss Federal Laboratories of Materials Testing and Research 共EMPA兲 for his help in the preparation of some figures, and the ETH Templated Materials program 共TEMA兲 for financial support. References ¨ ber Pleochroismus pflanzlicher und thierischer 1. H. Ambronn, U Fasern, die mit Silber- und Goldsalzen gefa¨rbt sind,” Kgl. Sa¨chs. Ges. Wiss. 8, 613– 628 共1896兲. ¨ ber Pleochroismus doppel2. H. Ambronn and R. Zsigmondy, U brechender Gelatine nach Fa¨rbung mit Gold- und Silberlo¨sungen,” Ber. Sa¨chs. Ges. Wiss. 51, 13–15 共1899兲. 3. A. Frey-Wyssling, “Ro¨ntgenometrische Vermessung der submikroskopischen Ra¨ume in Geru¨stsubstanzen,” Protoplasma 27, 372– 411 共1937兲. 4. E. H. Land, “Some aspects of the development of sheet polarizers,” J. Opt. Soc. Am. 41, 957–963 共1952兲. 5. W. Heffels, J. Friedrich, C. Darribe`re, J. Teisen, K. Interewicz, C. Bastiaansen, W. Caseri, and P. Smith, “Polymers and metals: nanocomposites and complex salts with metallic chain structure,” Recent Res. Devel. Macromol. Res. 2, 143–156 共1997兲. 6. A. H. Lu, G. H. Lu, A. M. Kessinger, and C. A. Foss, “Dichroic thin layer films prepared from alkanethiol-coated gold nanoparticles,” J. Phys. Chem. B 101, 9139 –9142 共1997兲. 7. N. A. F. Al-Rawashdeh, M. L. Sandrock, C. J. Seugling, and C. A. Foss, “Visible region polarization spectroscopic studies of
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