Chemical Nanopatterns via Nanoimprint ... - Wiley Online Library

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Chemical Nanopatterns via Nanoimprint Lithography for Simultaneous Control over Azimuthal and Polar Alignment of Liquid Crystals** By Sunggook Park,* Celestino Padeste, Helmut Schift, Jens Gobrecht, and Toralf Scharf We report on the use of chemical patterns to align a nematic liquid crystal (LC) in LC cells. Chemical patterns on the micro- and nanoscale, down to 50 nm in feature size, were fabricated by combining nanoimprint lithography (NIL) with subsequent reactive-ion etching (RIE), chemical modification with a fluorinated silane in the gas phase, and lift-off. Simultaneous control over both polar and azimuthal orientation of the LCs is possible by using the chemical nanopatterns as LC alignment layers. The polar orientation depends on the ratio of the homeotropic/planar surface potential areas, while the LC azimuthally orients along the direction of the silane patterns. All LC applications utilize the simple principle that LCs can easily be aligned by proper treatment of the contact surfaces. Consequently, better control over the LC alignment is one of the key aspects for the next generation of LC displays.[1] Rubbing processes are usually employed to produce linear gratings in the contact surfaces in the production of LC displays. However, this method fails to achieve simultaneous control over the azimuthal and polar orientations of supported LCs. As a new approach to achieve this, deliberate introduction of nanostructures based on the local variation of chemistry (chemical patterns) allows the creation of more controlled surface-alignment conditions and, thus, new functions of the LCs that interact with the structures.[2±10] The concept of simultaneous alignment of the azimuthal and polar orientations of LCs by chemical nanopatterns has been suggested in a few studies.[2±6] However, a systematic investigation has not been performed yet, mostly due to the lack of simple and reproducible methods to fabricate large-area chemical nanopatterns.

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[*] Prof. S. Park,[+] Dr. C. Padeste, Dr. H. Schift, Prof. J. Gobrecht Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut CH-5232 Villigen PSI (Switzerland) E-mail: [email protected] Dr. T. Scharf Institute for Microtechnology Rue A.-L. Breguet 2, CH-2000 Neuchâtel (Switzerland) [+] Present address: Mechanical Engineering Department, Louisiana State University Baton Rouge, LA 70803, USA. [**] We thank Konrad Vogelsang (Paul Scherrer Institut) for the technical support and Dr. Naci Basturk and Dr. Joachim Grupp (ASULAB) for fruitful discussion and providing glass substrates. This work is supported by Swiss CTI TOP NANO21 program, project NANOLIC. The bulk of this work was performed at PSI.

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The alignment of LCs by inhomogeneous surfaces was first proposed by Meyer et al., suggesting that controlled inhomogeneity of a surface provided a means of measuring the basic parameters that determine LC alignment.[3,9] The experimental evidence was first reported by Ong and Meyer, who used surfaces consisting of small metal patches that favored vertical (homeotropic) alignment surrounded by an obliquely evaporated SiO2 matrix that favored planar alignment.[4] In their experiments, the increase of the mean tilt angle (i.e., the polar orientation) of a nematic LC, 4-cyano-4¢-hexylibiphenyl (also referred to as 6CB) from 0 to 90 was observed as the metal coverage increased. Though those surfaces are comparable to inhomogeneously distributed chemical patterns, the results are indicative of controllability over the polar orientation of LCs by means of deliberate patterning of chemical nanostructures with different surface chemistries. The azimuthal orientation of LCs, on the other hand, seems to be generated by the presence of boundary lines between the two different surface-potential areas in chemical patterns, as observed by Lee and Clark.[9] A theoretical study of an alternating striped pattern of planar and homeotropic aligning potentials is in good agreement with the aforementioned experimental observation that pretilt of the LCs exists in the direction of the chemical gratings.[5,6] Furthermore, the alignment of bulk LC directors orthogonal to the grating direction when the period of the alternating potentials is close to zero is predicted by their calculation. This implies use of extremely small chemical patterns for the generation of LC biaxiality. To date, methods to fabricate high-resolution chemical patterns, such as microcontact printing and dip-pen lithography, have failed to demonstrate such interesting potential applications of chemical patterns to LC cells. Microcontact printing is suitable for large-area patterning but the lateral resolution is practically limited to a few hundred nanometers because the stamp is flexible and diffusion of the printed agent can occur during printing.[11] In contrast, dip-pen lithography is reportedly capable of patterning structures with feature sizes less than 50 nm, but has difficulties in large-area structuring and mass production.[12] The method of fabricating chemical nanopatterns which we present here is based on the pattern-transfer ability of NIL, which has shown resolution below 10 nm in the generation of metal lift-off.[13,14] It is advantageous over other methods in that mass production of large-area patterns with excellent lateral resolution is possible. The process is schematically shown in Figure 1a. A solid stamp with nanostructured relief is pressed into a thin film of poly(methyl methacrylate) (PMMA) that has been heated to above its glass-transition temperature. After conformal molding, the residual resist on the bottom of trench is removed by O2 RIE for window opening. For selective chemical modification of the substrate surface, a fluorinated silane, TFS, is deposited from the gas phase and covalently binds to the SiO2 areas. Finally, by lift-off of the residual PMMA, alternating chemical patterns of TFS and SiO2 are obtained. Most samples prepared for this study had structural areas of 9 mm 15 mm on 20 mm 20 mm glass substrates coated with

DOI: 10.1002/adma.200400989

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(b)

Figure 1. a) Process scheme for achieving chemical patterns via NIL and subsequent lift-off; ITO: indium tin oxide; TFS: (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane. b) Configuration of LC cells fabricated with chemical patterns.

ITO and sputtered SiO2. We have even successfully produced structures on 100 mm 100 mm substrates by NIL with good replication fidelity. Figure 2 shows atomic/lateral force microscopy (AFM/LFM) images, and the corresponding cross-sectional profiles of the smallest TFS chemical patterns we have produced to date. The stamp used for this fabrication is a linear grating with a 400 nm period, a trench width of ~ 70 nm, and slightly tapered sidewall profiles, which resulted in TFS lines 50±60 nm in width. It appears that the TFS line width is determined mainly by the stamp structures and RIE process, which may broaden the feature size. The ultimate lateral resolution of chemical patterns produced via NIL is not yet known. However, the sub-10 nm capability of NIL leads to the expectation that it allows a similar lateral resolution in the fabrication of chemical patterns.

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Figure 2. AFM/LFM images of 50 nm TFS lines and their corresponding profiles.

The LFM image reveals lower friction in the TFS-deposited areas, which is consistent with the low-surface-energy nature of TFS-coated surfaces. Topological contrast in the range 1±2 nm is also produced, where the lower trenches correspond to the TFS-coated areas. This is due to the incomplete removal of the residual PMMA under the lift-off conditions used in this study.[14] However, the presence of residual PMMA seems to have very little effect on the LC alignment characteristics, because both PMMA and uncoated SiO2 surfaces give rise to degenerated planar alignment. The alignment of a nematic LC mixture, E7, on the chemical patterns was investigated in LC cells with two glass substrates, one of which contained chemical patterns fabricated via NIL (Fig. 1b). The other surface was treated with a polyimide film that gave rise to homeotropic alignment of E7. Figure 3a shows optical textures of the cells with the chemical micropatterns of various dimensions observed under cross polarization as well as a scheme of the possible LC alignment on the surface. Black areas, which occur when the LCs align homeotropically on both surfaces in a cell, correspond to TFScoated surfaces. LCs on SiO2 areas larger than tens of square micrometers show the typical characteristics of a Schlieren texture of degenerated planar nematic LCs, where a network of black brushes emerging from point defects is observed. In addition, a clear phase separation with bright boundary lines between areas of TFS and SiO2 is seen when the boundary lines were oriented at ±45 with respect to the polarizer or analyzer. Those bright lines disappeared when the grating direction was parallel to the polarizer or analyzer. This is an indication of a new alignment of LCs at the boundaries, with their effective optical axes parallel to the boundaries. Such a preferred orientation at the boundaries can be explained by the fact that the boundaries on a patterned surface act as a system of lines to which molecular orientation can locally adapt due to the elastic anisotropy of the LCs. This interpretation agrees well with the LC behavior on microtextured surfaces reported by Lee et al.[9] As the periods of the chemical patterns decreased, the LCs on SiO2 areas did not show the degenerated texture any more but revealed a preferential orientation along the boundaries, with point defects located at the boundary lines only (Fig. 3a). It seems that the strong confinement of LCs between two narrow boundaries forced the LC molecules to orient along the boundaries. However, even though the same azimuthal orientation of the LCs is now adapted on both the boundary lines and SiO2 areas, the boundary lines can still be distinguished from the LC textures on the SiO2 areas. This is attributed to the generation of a pretilt angle at the boundaries and the presence of a splay rotation upon crossing the boundary lines, which will be discussed in the following with regard to chemical nanopatterns. When the period of the pattern reaches the submicrometer scale, the homeotropic/planar textures of LC alignment can no longer be distinguished optically and only one homogeneous texture is observed. As an example, Figure 3b shows optical micrographs under cross polarization for the LC cell

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polar alignment. Figure 4 shows the optical axes measured from the conoscopic images for chemical patterns with the same period (400 nm) but different silane coverage ratios (SCRs; defined as the TFS linewidth divided by the grating period). Deviation of the cross from the center in the cono-

Figure 4. Angles of optical axes with respect to surface plane as a function of SCR for E7 cells fabricated with chemical nanopatterns of 400 nm period. Insets: conoscopic interference images.

Figure 3. Micrographs of LC cells under cross polarization with a) TFS chemical micropatterns of various sizes. The scheme shows the different orientation of LC directors on the TFS and SiO2 surfaces and the presumed alignment at the boundaries. Analyzer (A) and polarizer (P) directions are indicated. b) Nanopatterns of 400 nm period and 50 nm TFS line width (silane coverage ratio = 0.12). The arrows indicate the direction of the TFS lines. Square-like patterns of side 160 lm stem from the stitching fields of electron-beam lithography during stamp fabrication.

fabricated with the chemical pattern corresponding to the surface shown in Figure 2. First of all, the LC texture is generally homogeneous over the structured area, which is also indicative of the homogeneity of the line widths of homeotropic/planar patches fabricated via NIL. The slight contrast difference that occurs every 160 lm is caused by the stitching of the exposure fields during fabrication of the stamp by electronbeam lithography. The highest transmission of the light occurs when the silane lines are at ±45 to the polarizer or analyzer. However, a complete extinction of light does not occur when the silane lines are parallel to the polarizer or analyzer. This is an indication that a splay rotation in the substrate plane (saddle-splay) occurs upon crossing the SiO2±TFS boundary lines, which is often observed when a periodic distortion exists at the surface.[9,15] According to Lee et al., a small extent of the in-plane splay rotation is a result of saddle splay elasticity and the increasing splay in the plane of the cell thickness at the boundary lines.[9] Conoscopic interference images are an identification tool for optical axes of LC cells and can provide direct evidence of 1400


scopic image is an indication of the presence of a pretilt angle, the extent of which is found to vary as a function of SCR. Namely, at higher SCRs, the cross is observed to be closer to the center of the conoscopic images. This is understandable, considering the increase in the areas of homeotropically aligning surface potential with increasing SCR. It should be noted that, due to the hybrid alignment in the LC cells, the director configuration is not uniform over the cell thickness and the conoscopic cross will therefore not directly correspond to the pretilt angle at the surface. The measured angles are the average of the LC directors over the cell thickness and, thus, the LC pretilt angle at the surface is expected to be much higher. This result implies that the polar orientation of LCs at the surface can be tuned between 0 and 90 by chemical patterns of different homeotropic/planar surface potential areas. To our knowledge, such a wide range of control over LC polar orientation is unique, considering that pretilt angles generated by rubbed surfaces are in the range of a few degrees from the surface plane. Linearly polymerized photopolymers reportedly also allow the creation of large pretilt angles,[16] but there are difficulties in achieving precise control of the process. The advantage of our method is the stable implementation of very large pretilt angles that can be chosen by using stamps of different line widths. The azimuthal orientation of the LCs on the chemical patterns is obtained by considering the direction of the deviation of the cross in the conoscopic images with respect to that of the silane lines. Despite the presence of the in-plane splay rotation at the boundaries, the deviation is found to always occur along the lines, demonstrating the average azimuthal orientation of the LCs over the cell thickness along the www.advmat.de

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Experimental Stamp Fabrication: The original master, which contained linear gratings of 400 nm period and 200 nm line width, was fabricated using a Leica Lion LV1 electron-beam writer with subsequent RIE of Si. To obtain structures of various line widths, the stamp copy process of NIL was employed. First the master was imprinted into a thin film of PMMA (~ 125 nm, molecular weight = 25 kg mol±1), spin-coated on Si, and baked on a hot plate for 1 min at 170 C. A combination of isotropic and anisotropic etching of PMMA and Si was used to reduce the line widths. By this method, structures with the same period of 400 nm, but four different trench widths (ca. 70 nm, 120 nm, 200 nm, and 300 nm, as measured by AFM) were fabricated and served as stamps for the production of chemical patterns. The structural area of each stamp was 9 mm 15 mm and the depths of the structures were in the range 100± 120 nm. Stamps with microstructures were produced with conventional photolithography and etched into Si using RIE. A TFS anti-adhesive coating was applied on all stamps from the gas phase. Chemical Patterns via NIL: A thin film of PMMA was spin-coated on glass substrates coated with ITO and sputtered SiO2, followed by baking on a hot plate for 1 min at 170 C. The PMMA film thickness was ~ 125 nm. The stamps were imprinted into the PMMA film for 15 min at 180 C under an applied pressure of 50 bar (1 bar = 105 Pa). The PMMA layer that remained at the bottom of the embossed structures was removed by homogeneously thinning the polymer by O2 RIE. TFS was then deposited from the gas phase in a vacuum chamber, followed by lift-off of the residual PMMA in acetone for 1 min to complete the formation of TFS patterned surfaces. Surface Characterization: Prior to fabrication of the LC cells, the chemical patterns were characterized by AFM/LFM (Digital Instrument Nanoscope III/Dimension 3100) under ambient conditions. Topology and friction images were obtained simultaneously in contact mode using a Si3N4 cantilever with a force constant of 0.5 N m±1 at a scan rate of 5 lm s±1. LC Cell Fabrication: The LC cells were fabricated by pairing two glass substrates (Fig. 1b). One substrate contained chemical patterns prepared as described above, and the other was treated with an alignment polymer, RN 783 (Nissan), which yielded homeotropic alignment of LCs at the surface. Glass fibers of 4.5 lm diameter were used as spacers between the two substrates. The cells were then sealed using a UV-curable glue everywhere, except two edges. A drop of a nematic LC, E7, which is a mixture of alkyl- and alkoxycyanobiphen-

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yls (Merck, nematic range ±10 to 60 C), was introduced at an opened edge while the cell was heated on a hot plate at 100 C. The cavity between the two substrates was filled with E7 in its isotropic phase by capillary action. Once filled, the cell was removed from the hot plate and cooled to room temperature. Analysis of LC Textures: The LC textures in the cells were characterized using a polarization microscope (Leica DMRXP) equipped with a homemade three-axis rotational stage, which allowed exact conoscopic determination measurements of the position of the optical axis. A 40 microscope objective (numerical aperture 0.6) and the S15/0.5 condenser head (numerical aperture 0.5) were used to make conoscopic images.

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boundary lines. This is in agreement with the LC orientation observed in chemical micropatterns by Lee et al.[9] Such a preferred orientation appears as LC elastic distortion, which occurs due to the presence of the boundaries between the two areas becoming dominant over surface-aligning potentials.[5,6] The azimuthal orientation orthogonal to the silane lines, which was predicted theoretically, was not observed for chemical patterns used in this study. However, chemical patterns with even smaller period might lead to such an orientation. In summary, this paper reports a reliable method for the fabrication of large-area and high-resolution chemical patterns via NIL with TFS line widths as small as 50 nm, which is proven to be suitable for studies of LC alignment. It was found that the ratio of areas with different surface-aligning potentials plays an important role in the polar orientation of LCs, while the azimuthal orientation is determined by the direction of the chemical patterns. The results imply that chemical patterns with different chemistry and more complex structures can be applied to control all other LC properties, including anchoring conditions and switching behaviors.

Received: June 22, 2004 Final version: March 9, 2005

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[1] M. F. Toney, T. P. Russell, J. A. Logan, H. Kikuchi, J. M. Sands, S. K. Kumar, Nature 1995, 374, 709. [2] D. W. Berreman, J. Phys. 1979, 40, C3-58. [3] R. B. Meyer, presented at the 7th Int. Liquid Crystal Conf., Bordeaux, France, July 1978. [4] H. L. Ong, A. J. Hurd, R. B. Meyer, J. Appl. Phys. 1985, 57, 186. [5] T. Z. Qian, P. Sheng, Phys. Rev. Lett. 1996, 77, 4564. [6] T. Z. Qian, P. Sheng, Phys. Rev. E 1997, 55, 7111. [7] V. K. Gupta, N. L. Abbott, Langmuir 1999, 15, 7213. [8] V. K. Gupta, N. L. Abbott, Science 1997, 276, 1533. [9] B. Lee, N. A. Clark, Science 2001, 291, 2576. [10] S. R. Kim, A. I. Teixeira, P. F. Nealey, A. E. Wendt, N. L. Abbott, Adv. Mater. 2002, 14, 1468. [11] Y. Xia, J. A. Rogers, K. E. Paul, G. M. Whitesides, Chem. Rev. 1999, 99, 1823. [12] M. Zhang, D. Bullen, S. W. Chung, S. Hong, K. S. Ryu, Z. Fan, C. A. Mirkin, C. Liu, Nanotechnology 2002, 13, 212. [13] H. Schift, L. J. Heyderman, C. Padeste, J. Gobrecht, Microelectron. Eng. 2002, 61±62, 423. [14] S. Park, C. Padeste, H. Schift, J. Gobrecht, Microelectron. Eng. 2003, 67±68, 252. [15] A. Sparavigna, L. Komitov, O. D. Lavrentovich, A, Strigazzi, J. Phys. II 1992, 2, 1881. [16] M. Schadt, K. Schmitt, V. Kozinkov, V. Chigrinov, Jpn. J. Appl. Phys., Part 1 1992, 31, 2155.

Single-Crystalline a-Al2O3 Nanotubes Converted from Al4O4C Nanowires** By Yubao Li,* Yoshio Bando, and Dmitri Golberg Tubular nanostructures are expected to be highly valuable for many applications due to their multifunctionality. Along with the intense study of graphitic carbon nanotubes, which have been the focus of much research during the last decade,

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[*] Dr. Y. B. Li, Prof. Y. Bando, Dr. D. Golberg National Institute for Materials Science Advanced Materials Laboratory Namiki 1-1, Tsukuba, Ibaraki 305-0044 (Japan) E-mail: [email protected] [**] Supporting Information is available online from Wiley InterScience or from the author.

DOI: 10.1002/adma.200401384


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