© Copyright 1996 American Chemical Society
JANUARY 24, 1996 VOLUME 12, NUMBER 2
Letters Observation of Polymer Birefringence in Near-Field Optical Microscopy H. Ade,*,† R. Toledo-Crow,‡ M. Vaez-Iravani,‡ and R. J. Spontak§ Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, Center for Imaging Science, Rochester Institute of Technology, P.O. Box 9887, Rochester, New York 14623-0887, and Department of Materials Science, North Carolina State University, Raleigh, North Carolina 27695 Received February 6, 1995. In Final Form: May 8, 1995X Birefringence, arising from locally preferred orientation of molecules or functional groups in polymeric and related materials has been observed in a near-field scanning optical microscope (NSOM) at suboptical spatial resolution. Our observations were successfully correlated to existing structural models of the samples, demonstrating the utility of birefringence observations in NSOM. We present data acquired from thin sections of a partially ordered polymer fiber (Kevlar) and a polymer dispersed small-moleculeliquid-crystal composite.
Observation of birefringence in conventional, or farfield, polarization light microscopy is an enormously useful and widespread form of characterization. It is routinely utilized in materials and polymer research and many other fields of science, including geology, pharmacology, and biology. Birefringence provides a unique contrast mechanism, as well as information regarding sample anisotropy, for a multitude of materials. Dynamical changes in birefringence as a function of temperature, pressure, or concentration are utilized, for instance, to discern phase boundaries in liquid crystalline systems. Such observations in far-field polarization microscopy are, however, limited in spatial resolution by diffraction, which is a shortcoming shared with other forms of far-field visible light microscopy. The diffraction limit to resolution can be overcome by resorting to near-field scanning optical microscopy (NSOM),1,2 utilizing a variety of contrast * To whom correspondence should be addressed. † Department of Physics, North Carolina State University. ‡ Center for Imaging Science, Rochester Institute of Technology. § Department of Materials Science, North Carolina State University. X Abstract published in Advance ACS Abstracts, July 15, 1995. (1) Pohl, D.; Denk, W.; Lanz, M. Appl. Phys. Lett. 1984, 44, 651. (2) Betzig, E.; Trautman, J. K. Science 1992, 257, 189.
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mechanisms based on fluorescence and luminescence,3-6 interference,7 and polarization,8 including the observation of Faraday rotation in magneto-optical bits.9 In this paper we exploit an aspect of polarization different from Faraday rotation. We demonstrate that the utility of birefringence observation can be preserved in a near-field microscope, and that it can be utilized for the investigation of surfaces and thin films of polymeric and related materials. Two classes of material have been studied. One is poly(p-phenyleneterephthalamide) (PPTA) fibers (Kevlar). The other system consisted of a polymerdispersed small-molecule-liquid-crystal (PDLC) composite. The apparatus utilized for this work is a forceregulated, near-field linear polarizing optical microscope10 (3) Betzig, E.; Chichester, R. J. Science 1993, 262, 1422. (4) Hess, H. F.; Betzig, E.; Harris, R. D.; Pfeiffer, L. N.; West, K. W. Science 1994, 264, 1740. (5) Xie, X. S.; Dunn, R. C. Science 1994, 265, 362. (6) Ambrose, W. P.; Goodwin, P. M.; Martin, J. C.; Keller, R. A. Science 1994, 265, 364. (7) Vaez-Iravani, M.; Toledo-Crow, R. Appl. Phys. Lett. 1993, 62, 1044-1046. (8) Betzig, E.; Trautman, J. K.; Weiner, J. S.; Harris, T. D.; Wolfe, R. Appl. Opt. 1992, 22, 4563. (9) Betzig, E.; et al. Appl. Phys. Lett. 1992, 61, 142-144. (10) Vaez-Iravani, M.; Toledo-Crow, R. Appl. Phys. Lett. 1993, 63, 138.
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Figure 1. Schematic of NSOM setup. In addition to the now common near-field microscopy components, the NSOM utilized here employs pre-postmodulation via an electro-optical modulator (Pockels cell) that is driven at a frequency fm to acquire two complementary images simultaneously.
(see below). We believe this system is ideally suited to the investigation of polymeric samples, due to three significant attributes, namely, its (i) capability to provide high resolution and sensitivity in images of birefringence effects, independent of the variation of other optical characteristics such as transmissivity or reflectivity, (ii) ability to preserve the sign of the birefringence signal, so as to determine the relative orientation of the optical anisotropy parameters of the structures under study, and (iii) ability to correlate optical information with the simultaneously generated topographic map of the sample, to distinguish inherent birefringence from topographically induced and correlated birefringence, as well as to determine the origin of the observed birefringence (surface or subsurface). To appreciate the significance of the attributes other than high resolution and sensitivity, we note that the normal mode of performing polarizing microscopy (viewing the sample between crossed polarizers) results in a squarelaw dependence of the signal on the birefringence. In this process, besides reducing sensitivity, one loses the sign of the birefringence.10 Thus, no direct conclusions can be drawn about the relative structural values of the system under examination. In addition, complementary highresolution techniques for investigating such polymeric structures, such as linear dichroic X-ray microscopy,11 do not offer a simultaneous topographic imaging capability, an issue which can be of great significance in image interpretation. We also note here that the polarization of light as it emerges from the aperture in an NSOM has been shown to be maintained to a very high degree.8-10 On the basis of theoretical considerations, however, one would expect the output light to include a certain amount of other orthogonal polarizations, which increases as the aperture diameter becomes very small.12 In our experiments, we did not observe any significant anomalies associated with this effect. The system design is shown in Figure 1.10 At the heart of the system is a pulled length of polarization preserving fiber, whose curved surface has been covered with a layer of aluminum, a process that leaves a small aperture at the very tip, and renders the rest of the structure opaque. The aperture size depends on the exact fabrication parameters and can be as small as about 20 nm. The system uses a He-Ne laser (at 633 nm), and prepostmodulation via an electro-optical modulator (Pockels cell) that is driven at frequency fm to acquire two complementary images simultaneously. This is achieved (11) Ade, H.; Hsiao, B. Science 1993, 262, 1427. (12) Bouwkamp, C. J. Philips Res. Rep. 1950, 5, 321.
Figure 2. Micrographs of several embedded and sectioned (0.2 µm thick) Kevlar 29 fibers: (A) first harmonic image; (B) second harmonic image; (C) force image; (D) the rectified ratio (i.e., absolute value of first/second harmonic ratio) image.
by lock-in detection of the signal from the detector at frequency fm (first harmonic) and 2fm (second harmonic). The polarizer and analyzer are at 90° with respect to each other. In the simplest case, the first harmonic image is proportional to sin(δφ), where δφ ) 2π∆nt is the phase change produced by birefringence, t is the wavelength normalized sampling depth, and ∆n is the birefringence, i.e., the difference in the index of refraction along the fast and slow axes. The second harmonic signal is proportional to cos(δφ). Both images contain intensity changes which depend on the sample transmission or reflectivity. Judicious adjustment of the Pockels cell allows these variations to be matched in intensity. Consequently, a pure, linear representation of the sample birefringence can be obtained in the limit of small phase changes, by dividing the first harmonic image by the second harmonic image. The resulting “tangent” image is therefore independent of the sample transmissivity or reflectivity (only scanning microscopes can be modified and operated in this fashion). The system has the inherent capability of performing simultaneous force microscopy of the sample and has been described elsewhere in detail.13 Of the various commercial grades of Kevlar fibers available, we investigated Kevlar-29 which has the smallest degree of crystallinity and thus shows the least orientational order along the fiber axis.14 All Kevlar fibers exhibit a radially symmetric structure, in which the degree of radial order varies with different fiber grades.14,15 A model of a radially symmetric strucure imaged with our microscope predicts that the first harmonic signal is proportional to [sin(δφ) sin(2γ)], whereas the second harmonic signal is proportional to [cos(δφ) sin2(2γ) + cos2(2γ)], where γ is the angle between the instantaneous (13) Yang, P. C.; Chen, Y.; Vaez-Iravani, M. J. Appl. Phy. 1992, 71, 2499. (14) Yang, H. H. Kevlar Aramid Fiber; John Wiley & Sons: New York, 1993 and references therein. Yang, H. H. Aromatic High Strength Fibers; Wiley-Interscience Publishers: New York, 1989, and references therein. Dobb, M. D.; Johnson, D. J.; Saville, B. P. J. Polym. Sci. Phys. Ed. 1977, 15, 2201. (15) Ade, H.; Smith, A. P.; Subramoney, S.; Hsiao, B. In preparation.
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Figure 3. Micrographs of the lower right fiber of Figure 2 shown at higher magnification: (A) NSOM image with nearly crossed polarizers and no Pockels cell (DC); (B) ratio (i.e., first/ second harmonic) image; (C) rectified (absolute value) ratio image; (D) far-field scanning optical image with nearly crossed polarizers.
angular position and the polarizer axis and δφ is the phase shift due to birefringence. For a finite and constant value of birefringence, the first harmonic signal should therefore result in a butterfly pattern, with two opposite quadrants above (positive) and the other two opposite quadrants below (negative) the background (no signal). The Kevlar29 fibers analyzed using NSOM are presented in Figure 2. In the first harmonic image shown in Figure 2A, besides small distortions due to imperfect radial symmetry of the fibers, we observe the butterfly pattern as predicted. Owing to the radial nature of the sample, the observed butterfly pattern did furthermore not (and should not) change its orientation nor intensity upon a relative rotation of the sample with respect to the polarized light. In addition to the butterfly pattern, several semiregular parallel features are visible. A closer examination of the force image (Figure 2C), particularly at higher magnification (not shown here), shows similar features in topography with low contrast. These are modifications of the sample surface due to the sectioning process. The same kind of structure is observed in the transmission electron microscope (TEM)16 and X-ray microscopy,11,15 but only in the force-regulated near-field microscope, owing to the simultaneous topographic imaging capability, are these structures directly identified as artifacts that manifest themselves as optical transmission, as well as birefringence, features in the optical images. On the basis of the polarity and contrast in the first harmonic signal, and a radial model, we can deduce the important characteristic that the slow (larger n) and fast axes lie along the radial and tangential directions, respectively. This is consistent with existing data from Kevlar polymorphs17,18 and the average aromatic ring planes and carbonyl groups pointing radially outward. In addition, all Kevlar fibers exhibited skin/core structural differences. In the shot noise limit, the theoretical sensitivity of the system employed (receiving about 1 nW (16) Hamza, A. A.; Sikorski, J. J. Microsc. 1978, 113, 15. (17) Heuse, O.; Adolf, F. P. J. Forensic Sci. Soc. 1982, 22, 103. (18) Subramoney, S. Personal communication.
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of light, and using 100 Hz bandwidth) is calculated at ∆n ) 2 × 10-4 for a sample depth of 40 nm, roughly corresponding to the transverse spatial resolution. Thus even for a weakly birefringent sample, such as thin sections of Kevlar-29, the quality of the images is excellent. The complementary signal to the one used in Figure 2A is utilized to acquire the micrograph shown in Figure 2B. Additional micrographs obtained include a force image Figure 2C, and the rectified ratio (i.e., first/second harmonic) image, 2D. In parts B and D of Figure 2, one can also see the series of parallel lines running across the Kevlar samples as discussed above for Figure 2A. We have chosen to present the rectified ratio (absolute value of first harmonic/second harmonic) image in Figure 2D, as this fully positive signal is analogous to what is observed in far-field systems between crossed polarizers. The relative merits of our approach can, perhaps, be best demonstrated by a series of four images taken of the same single fiber (Figure 3). Here, we present the nearfield image of the sample between nearly crossed polarizers (Figure 3A), the near-field ratio image (linear) (Figure 3B), the rectified version of the ratio image (Figure 3C), and, finally, the far-field scanning optical image of the fiber taken between nearly crossed polarizers (Figure 3D). We note, firstly, that there is little variation of transmissivity in this image. Thus, the ratio image, Figure 3C, greatly resembles the first harmonic image (not shown here, but similar to Figure 2A). Secondly, although the resolution of the far-field image (Figure 3D) at about 0.5 µm is not the best that far-field systems are capable of, the superior resolution of our system is apparent. We also note that the far-field sensitivity is considerably better than that of the near-field, nearly cross-polar, version (Figure 3A) which is a direct consequence of the much greater light level available in the far-field system. On the other hand, the sensitivity in the near-field, linear, image (Figure 3C) is comparable to that in the far-field image, which demonstrates the importance of linearizing the weak birefringence signal in near-field imaging. We should mention here that, based on theoretical considerations, the DC image obtained between crossed polarizers should show a Maltese cross pattern for a Kevlar fiber cut perpendicularly to its axis. However, since the sample was cut obliquely, this situation was somewhat altered. This is shown in parts A and D of Figure 3 where a Maltese cross pattern was obtained by placing the analyzer a few degrees away from exact perpendicularity with respect to the polarizer. The second class of material investigated is a polymer dispersed liquid crystal (PDLC) composite. The system is an emulsion of nematic small-molecule-liquid crystal domains in a matrix of a glassy polymer. We investigated both cast and sectioned thin films. Figure 4 shows the resulting micrographs of a sectioned thin film about 1-2 µm thick, including the first harmonic (A), second harmonic (B), and the rectified first/second harmonic image (C). The patterns such as the ones shown with the arrows in Figure 4B are due to transmissivity variations, and not birefringence. Accordingly, they are canceled out in the ratio image (Figure 4C), demonstrating our system’s ability to obtain pure birefringence images. Only a few features exhibit significant topography in the higher magnification force image (Figure 4E). Feature a is identified as a crater, while feature b is identified as a bump. Both features can be correlated to features in Figure 4D and exhibit unique polarization contrast. In addition, several subsurface LC domains, only hinted at in the force image, are observed with a Maltese cross pattern. This pattern is consistent with the orientation of the liquid crystal in the droplet in which the director
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Figure 4. Micrograph of a polymer-dispersed small-molecule-liquid-crystal (PDLC) composite. First harmonic (A), second harmonic (B), and the rectified first/second harmonic ratio image (C) of a sectioned thin film about 1 µm thick. Figure 4D shows the rectified first harmonic image of a portion of the sample, and Figure 4E shows the corresponding topography image, derived from the force feedback, at higher magnification. In these images, the polarizer was set at 50° with respect to the horizontal.
lies parallel to the polymer walls, and two point disclinations at opposite ends of the droplet define an axis of cylindrical symmetry.19 As a result of this geometry, and in contrast to the cross-sectioned Kevlar fibers, the intensity and definition of the patterns could be altered, in linear polarization imaging, by rotating the sample with respect to the polarizer/analyzer axes. Since these droplets are below the surface, they are not fully in the near-field. As a result, the resolution of most features in this sample is not as high as the system is capable of for features closer to the surface (within about 40 nm). Conversely, our imaging method allows differentiation of surface and subsurface features as correlated to topography. Of particular interest in these images are the halos around all of the LC domains or previous domains, such as the (19) Drzaic, P. S. J. Appl. Phys. 1986, 80, 2142.
crater. This apparent feature suggests that the initially random poly(vinyl alcohol) matrix has undergone conformational ordering in the vicinity of the polymer/LC interface, which is consistent with a variety of bulk property measurements.20 Acknowledgment. We thank S. Subramoney and B. Hsiao (DuPont) for the Kevlar samples, P. Drzaic (Raychem Corp.) for the PDLC samples, and S. Roy for his help with sample preparation. We are also grateful to P. Moyers for his help during the initial and preliminary phases of this work and A. P. Smith with the manuscript preparation. All authors acknowledge support from the National Science Foundation, under Grant DMR-9315676. LA9500890 (20) Roy, S. C.; Spontak, R. J. In preparation.
