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, 2002, 44, . 6 Orientation of discotic and ferroelectric liquid crystals in macroporous silicon matrix T.S. Perova, E.V. Astrova, S.E. Tsvetkov, A.G. Tkachenko, J.K. Vij, S. Kumar Department of Electronic & Electrical Engineering, Trinity College, Dublin 2, Ireland Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St.Petersburg, Russia Institute for Liquid Crystal Research, Bangalor, India E-mail: perovat@tcd.ie ( 10 2001 .

18 2001 .) Macroporous silicon with deep regular channels 3-4.5 m in diameter was infiltrated with discotic and ferroelectric liquid crystals (LCs) at the temperatute of the isotropic phase and then the system was slowly cooled down to room temperature, with the liquid crystalline mesophase formed. The orientation of the LC molecules in the porous matrix was studied by FTIR spectroscopy. The alignment of LCs was ascertained by comparing the behavior of various vibrational bands of a liquid crystal introduced into the porous matrix with that for LC inside the bulk cells of planar and homeotropic alignment. The molecules of the discotic LC show a planar orientation of their columns axis with respect to the surface of the macroporous silicon wafer, i. e., are perpendicular to the channel axis. The long molecular axis of the ferroelectric LC is aligned with the pore walls, having homeotropic orientation with respect to the wafer surface. In a macroporous silicon matrix, both kinds of LCs show unexpected enhancement of the low-frequency vibrational bands.

The financial support of Enterprise Ireland through the International Collaboration Program IC/2001/042, Russian State Program Nanostructures in Physics and St.Petersburg Science Center Program Low Dimensional Structures are gratefully acknowledged.

Over the last decade, considerable scientific effort has gap [911]. Infiltration of porous systems of this kind been focused on studying liquid crystals within a con- with liquid crystals allows the control over the position fined geometry. Liquid crystals are a crucially important of their photonic band gap, relying upon the fact that the enabling technology for the manufacture of displays. The refractive index of LC changes with temperature [12] or understanding of the layer structure, phase transitions, external electric field [13]. It is important to find out how different kinds of liquid crystal molecules will behave in a order parameter fluctuations, liquid-solid interface, and macro-porous silicon (ma-PS) matrix. In this study, Fourier dynamics of collective modes and molecular motions will Transform Infrared Spectroscopy (FTIR) was used as a tool be significantly advanced if results on samples restricted to investigate the alignment of liquid crystals, both discotic to confined geometries are compared with those in the and ferroelectric, infiltrated into ma-PS.

bulk [14].

Porous systems are suitable hosts for the liquid crystal molecules. These systems are characterized by a ligh 1. Experimental surface-to-volume ratio, and therefore are very sensitive to any interactions between the infiltrated molecules and the Two different types of liquid crystal materials were used surrounding walls. A possible interaction of this kind is the in this study: (i) commercial ferroelectric liquid crystal aligning power of the walls [5], which can give rise to such (FLC) mixture SCE-8; (ii) triphenylene-based discotic liquid sub-phases as a quasi-nematic layer [6].

crystal (DLC) H5TNO2 [14]. These LCs were chosen Polarized infrared spectroscopy with normal and oblique for experimental convenience, both having a mesophase incidence of light is one of the most powerful techniques at room temperature. The structural formulae and phase that can be used to investigate the orientation of liquid sequences of these compounds are shown in Fig. 1, a. These crystal (LC) molecules with respect to a substrate [7,8]. The liquid crystals were infiltrated into macroporous silicon application of this technique to confined LCs is frequently (shown in Fig. 1, b) by means of the capillary effect at restricted by the nature of the host material (e. g., Vocor temperatures approximately 10C above the temperature of glass), which is usually opaque to light in the IR region.

transition to the isotropic phase.

However, this problem can be overcomed by using porous The macroporous silicon used in this study (Fig. 1, b) has silicon for LC infiltration. In this case, the size and depth a system of regular cylindrical pores of micrometer diameter of pores and the porosity can be varied widely.

and high aspect ratio. The starting material was a singleRegular porous structures, such as microporous superlat- crystal (100)-oriented Czochralski-grown n-type silicon with tices and macroporous silicon have attracted considerable resistivity = 15 cm. A standard photolithographic scientific interest because of their having a photonic band process was employed to form pits spaced 12 m apart 1146 T.S. Perova, E.V. Astrova, S.E. Tsvetkov, A.G. Tkachenko, J.K. Vij, S. Kumar Figure 1. Material properties and experimental setup. a structural formulas and phase sequences of ferroelectric liquid crystalline mixture SCE8 and tryphenylene-based discotic liquid crystal H5TNO2 (R = OC5H11); b SEM image of macroporous silicon matrix (cross section side and top view) used to infiltrate with the liquid crystals; c schematic view of FTIR experiment shows the sample rotation angle and macroporous Si sample coordinate frame; d planar and homeotropic alignment of discotic liquid crystal in ZnSe sell; e planar and homeotropic alignment of ferroelectric liquid crystal in ZnSe cell.

, 2002, 44, . Orientation of discotic and ferroelectric liquid crystals in macroporous silicon matrix Assignment of infrared vibrational bands on the polished surface of the silicon wafer. Deep pores were etched electrochemically in a 2.5% aqueous-ethanol Band assignment and orientation solution of HF for 300 to 450 min under backside illuof the transition dipole moment H5TNO2 SCEmination [15] at a voltage of 5 V and a constant current with respect to the core density j = 3 mA/cm2. The pore depth and diameter d CH2 symmetric stretching 2861 were, respectively, 200-250 and d = 3-4.5 m, with these asymmetric stretching 2934 parameters corresponding to a porosity of 5.7-12.8% for CH3 symmetric stretching 2873 our triangular lattice.

asymmetric stretching 2957 FTIR measurements were performed using a Biorad N=O asymmetric stretching 1530 FTS60A spectrometer fitted with a liquid-nitrogen-cooled CC aromatic stretching 1507 MCT detector. A schematic of the FTIR experiment is shown in Fig. 1, c. Scans were performed between 1616 450-4000 cm-1 with a resolution ranging from 2 to 8 cm-1.

A total of 64 scans were co-added to improve the signal-to- NO bending 845 noise ratio. IR spectra were initially recorded for an empty CH aromatic out-of-plane 890 macroporous silicon wafer and then for a matrix infiltrated deformation 831 with the liquid crystal. The difference between these two spectra was taken to be the spectrum of a liquid crystal infiltrated into the porous matrix. The alignment of the CC aromatic out-of-plane LCs within the porous silicon matrix was deduced from a deformation comparison of the relative intensities and positions of the difNO2 wagging 750 ferent vibrational bands in the obtained spectra with those of CH rocking 733 the bulk liquid crystal. For this purpose, a number of liquidcrystal sells with different types of alignment (homeotropic and planar, Fig. 1, d, e) were prepared as follows.

A planar cell with H5TNO2 was fabricated with with that in spectra of planar and homeotropic bulk LC ZnSe windows (with a spacer of thickness 12 m).


A homeotropic cell was obtained when two ZnSe winThe alignment of liquid crystals is usually considered dows were coated with nylon 6/6 (see [7] for more with respect to the orientation of the long molecular axis details). A planarly aligned cell with SCE8 was obtained (in the case of rod-like molecules) and with respect to the using two ZnSe windows coated with the commercial column axis n (which is, in general, perpendicular to the (Nissan Chemical Industries, Ltd.) orientant RN-core plane) for discotic liquid crystals. Alignment will be (a 0.4% solution in a mixture of 10% butyl cellosolve and planar if the long molecular axis (column axis) is oriented 90% N-methyl pyrrolidone) and rubbed with velvet in a one parallel to the substrate plane and homeotropic in the direction. The thickness of this cell, found from infrared case of perpendicular orientation. Therefore, conclusions fringes, was 9 m. A homeotropically aligned SCE8 cell, about the alignment can be drawn from the behavior of of aproximate thickness 9 m, was prepared using ZnSe bands associated with vibrations having the transition dipole windows coated with a carboxylato chromium complex moment parallel ( ) and perpendicular () to the rigid part (chromolane) (see Ref. [16] for more details). All these cells or core of molecules. These bands are listed in Table for were filled by means of the capillary effect at a temperature both kinds of liquid crystals under study. It should be noted of 10C above the transition to the isotropic phase.

that the spectral position of the majority of the vibrational bands of liquid crystals in a porous matrix is the same as that in the bulk LC cells.

2. Results and discussion 2.1. Di scot i c l i qui d cryst al. There are two methods for determining the type and the extent of alignment Complete information of the structure and orientation of for discotic liquid crystals: (i) from the ratio of the peak LC molecules can be obtained by studying a combination of intensities for a particular vibrational band in isotropic and in polarized and oblique infrared transmission [7,8]. However, the discotic phase (RDI = AiD/AiI) and (ii) from the ratio of our investigations show that additional interference effects the peak intensities for the same band in the discotic phase appear in the frequency range 700-1500 cm-1 for tilted at normal and oblique incidence of light (R = Ai /Ai), macroporous silicon samples. This makes the analysis of the which allows the intensity of the vibrational band to be IR spectra of the liquid crystal itself impossible at oblique determined for vibrations with transition dipole moments incidence of light. Therefore, the alignment of LCs was parallel (A ) and perpendicular (A) to the substrate plane founded by comparing the behavior of various vibrational (see Ref. [7] for more details). Neither of these techniques is bands in the spectra obtained at normal incidence of the suitable for our purposes for the technical reasons described infrared beam on luquid crystals contained in porous matrix above.

, 2002, 44, . 1148 T.S. Perova, E.V. Astrova, S.E. Tsvetkov, A.G. Tkachenko, J.K. Vij, S. Kumar CH aromatic out-of-plane deformation at 830 cm-1 is purely perpendicular to the core ( j-band). These vibrational bands are shown in Figs. 2, a and b for the H5TNOdiscotic liquid crystal in the porous matrix and for both homeotropic and planar alignment of molecules in a liquidcrystal cell.

Qualitative information of the type of alignment in the porous matrix can be obtained by comparing the spectra of different samples in the region 800-890 cm-1. Although the vibrational band in this region is rather complicated, it can be clearly seen that the main maximum of this band is shifted to 830 cm-1 for the cell with planar alignment and for the porous matrix, whereas for the cell with homeotropic alignment the peak lies at 820 cm-1.

In order to obtain more precise information on the alignment of H5TNO2 molecules in a porous silicon matrix, we estimated the dichroic ratio for three samples, using Eq. (1). For this purpose, we fitted the composite band in the region 800-890 cm-1 with four (for the bulk planar cell) or five (for the bulk homeotropic cell) Voight functions in order to determine the intensity of the band at 830 cm-1. The results of this fit are shown in Figure 2. FTIR spectra of H5TNO2 discotic liquid crystal infiltrated into porous silicon (solid line) and embedded in ZnSe cells with planar (dotted line) and homeotropic (dashed line) alignments. a CC streching band, parallel to the core, b CH aromatic out-of-plane deformation band, perpendicular to the core. (Note the different scales for the heavy line in (b) and the fact that the absorption of all the samples is reduced in accordance with the intensity of the alkyl chain vibrations in the region 2800-3200 cm-1).

We introduce here a third method for determining the type of alignment by measuring the ratio intensities of two different vibrational bands, i and j, observed in the mesophase (a type of dichroic ratio) Ri j = Ai/Aj, (1) where Ai is the intensity of the band associated with vibrations parallel to the LC molecule core and Aj is that associated with vibrations perpendicular to the core.

Particular care should be taken in choosing the appropriate set of bands for calculating the dichroic ratio by means of Eq. (1). These bands should correspond to vibrations with transition dipole moments at a right angle ( 90) with respect to each other. Our previous investigation [7] performed for a number of triphenylene derivatives (H5T, Figure 3. Example of the fitting procedure for the low-frequency H7T and H7TNO2 discotic liquid crystals) demonstrated band for H5TNO2 contained in planar (a) and homeotropic (b) that the aromatic CC stretching vibrations at 1510 and cells. Note that the thickness of the homeotropic cell is 2.45 times 1610 cm-1 are parallel to the core (i-band) while the that of the planar cell.

, 2002, 44, . Orientation of discotic and ferroelectric liquid crystals in macroporous silicon matrix beam. This fraction was evaluated by multiplying the total volume (V ) by the porosity (p = 9.5%). The comparison enabled us to conclude that strong intensity enhancement is observed for the low-frequency vibrational bands. This conclusion is further supported by a comparison of the intensities of the vibrational bands for the alkyl chain in the region 2800-3200 cm-1. It is accepted that the alkyl chain is rather disordered for discotic liquid crystals, and its absorption intensity in this range for different cells depends Figure 4. Side view of the orientation of ferroelectric (a) and only on thickness, being independent of the alignment.

descotic (b) liquid crystals in the macroporous silicon matrix.

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