WWW.DISSERS.RU


...
    !

Pages:     | 1 ||

Therefore, we compared the spectra of different samples by reducing their intensity in the range 2600 cm-1 to the same value. This normalization procedure allowed us Fig. 3, a, b. It can be seen that the CH aromatic outto conclude that there is a strong intensity enhancement of-plane deformation occurs at 831 cm-1 for both liquid ( 15 times) for the low-frequency vibrational bands.

crystal cells and for the porous matrix. The dichroic ratio Finally, it is obvious from our results that the alignment Ri j is 31 for the cell with homeotropic alignment, 7 for the of H5TNO2 in ma-PS is planar with respect to the wafer cell with the planar alignment, and is only 0.4 for H5T surface. However, as seen from Fig. 4, b, the alignment of NO2 infiltrated into the porous matrix. At first glance, discotic LCs is homeotropic with respect to the pore walls.

it seems that the alignment in the porous matrix is even This is in agreement with the results of previously published better than for the bulk cell with planar alignment, but work [7], where the high stability of homeotropic alignment a more detailed analysis of both types of alignment for has been found for discotic liquid crystals deposited on a number of triphenylene derivatives shows that this is untreated substrates.

not the case. The best planar alignment is observed for 2.2. Ferroel ect ri c l i qui d cryst al s. It should be hexapentyltryphenylene (H5T) [7]. Ri j was found to be in noted that for this type of molecelar shape, we have to the range from 3 to 5 for H5T.

take into account the polarization of light when measuring We conclude that the obtained small value of the dichroic the spectrum of the cell with planar alignment. We use the ratio is due either to the enhancement of the intensity of the following notation: P = 0 for the electric vector of the low-frequency vibrational bands or to the dampening of the incident light coinciding with the long molecular axis, and intensity of the high-frequency vibrational band. In order P = 90 otherwise. The IR spectra of SCE-8 introduced to obtain more precise data on this issue we compared into a porous silicon matrix show that the relative intensities the results obtained with all the three samples in terms of the absorption per unit volume. This was particularly of parallel (i) and perpendicular ( j) bands are in important for the porous matrix, in which the liquid crystal this case close to that observed for the bulk LC cell occupied only a part (VLC) of the volume probed by the IR with homeotropic alignment (Fig. 5). In particular, as seen Figure 5. FTIR spectra of SCE8 ferroelectric liquid crystal infiltrated into porous silicon (short dotted line) and contained in ZnSe cells with planar (heavy solid line) and homeotropic (thin solid line) alignments. (Note the different scales for the heavy line in (b) and the fact that the absorption of the FLC cells is decreases by a factor of 20 for both cells).

, 2002, 44, . 1150 T.S. Perova, E.V. Astrova, S.E. Tsvetkov, A.G. Tkachenko, J.K. Vij, S. Kumar from Fig. 5, a, b, the intensity of the CC aromatic stretching perpendicular to the core. This method is of particular vibration (i-band at 1513 cm-1) in the case of the planar interest for discotic liquid crystals, since it allows the cell is much higher with polarizer angle 0 (II polarization), alignment to be determined without heating a sample to when the electric vector of the incident light coincides with the temperature of the isotropic phase or using oblique the orientation of the transition dipole moment for this transmission IR spectroscopy.

molecular unit. At the same time, the intensity of the With this method, the alignments of two kinds of liquid j-bands, perpendicular to the core vibrations (e. g. CH crystals infiltrated into a macroporous silicon matrix have out-of-plane deformation at 828; CC aromatic out-ofbeen determined. The channel walls of macroporous silicon plane deformation at 765 and CH2 rocking vibrations affect the orientation of LC molecules so that the column at 722 cm-1) is smaller. The intensity ratio found for axis of the discotic LC is perpendicular to the walls.

these bands in the case of a planar cell at a polarizer The long molecular axis of the rod-like molecules of the angle of 0 Ri j = A1513/A765 = 4.3. For the homeotropic ferroelectric LC is aligned along the channel walls. A strong LC cell, the intensity of the perpendicular bands becomes intensity enhancement of the low-frequency vibrational higher, or at least comparable with the intensity of the bands was detected for the both kinds of LCs infiltrated parallel bands, owing to the difference in oscillator strength.

into the porous silicon matrix.

In this case, the intensity ratio Ri j = A1513/A765 = 1.1.

For the SCE8 infiltrated into the porous matrix this ratio References of Ri j = A1513/A765 = 0.4 is even smaller than that obtained for the homeotropic LC cell and indicates that the [1] T.Bellini, N.A. Clark, C.D. Muzny, Lei Wu, C.W. Garland, alignment of ferroelectric liquid crystal in confined porous D.W. Schaefer, B.J. Olivier. Phys. Rev. Lett. 69, 5, 788 (1992).

silicon matrix is definitely homeotropic with respect to the [2] N.A. Clark, T. Bellini, R.M. Malzbender, B.N. Thomas, substrates plane (Fig. 4, a). This result coincides with the A.G. Rappaport, C.D. Muzny, D.W. Schaefer, L. Hrubesh.

alignment obtained for nematic LCs deposited on untreated Phys. Rev. Lett. 71, 21, 3505 (1993).

surfaces of Anapore membranes [17] and infiltrated into [3] G.S. Iannacchione, G.P. Crawford, S. Zumer, J.W. Doane, both microporous [18] and macroporous [12] silicon. Such D. Finotello. Phys. Rev. Lett. 71, 16, 2595 (1993).

an orientation is expected if one considers the alignment [4] H. Xu, J.K. Vij, A. Rappaport, N. Clark. Phys. Rev. Lett. 79, of rod-like molecules with respect to the surface of the 2, 249 (1997).

pore, in which case the alignment is planar. The planar [5] P. Ziherl, A. Sarlah, S. Zumer. Phys. Rev. E58, 1, 602 (1998).

[6] P. Ziherl, S. Zumer. Phys. Rev. Lett. 78, 4, 682 (1997).

alignment is typically observed for rod-like molecules on [7] T.S. Perova, J.K. Vij, A. Kocot. Adv. Chem. Phys. 113, various untreated surfaces, including crystalline silicon [19].

(2000).

Only a special surface treatment or coating of the substrate [8] A. Kocot, J.K. Vij, T.S. Perova. Adv. Chem. Phys. 113, surface by a surfactant may give a homeotropic alignment (2000).

of LCs formed from rod-like molecules.

[9] O. Bisi, S. Ossicino, L. Pavesi. Surface Science Reports 38, In the course of these investigations, we found strong 13, 1 (2000).

intensity enhancement of the low-frequency vibrational [10] U. Gruning, V. Lehmann. Appl. Phys. Lett. 68, 6, 747 (1996).

bands in the region 600-900 cm-1 for both types of liquid [11] A. Chelnokov, K. Wang, S. Rowson, P. Garoche, J.-M. Lourcrystals in ma-PS. This enhancement becomes noticeable tioz. Appl. Phys. Lett. 77, 19, 2943 (2000).

when comparing the intensity ratios for the parallel and [12] S.W. Leonard, J.P. Mondia, H.M. van Driel, O. Toader, S. John, perpendicular bands. As already mentioned, the intensity K. Busch, A. Birner, U. Gosele, V. Lehmann. Phys. Rev. B61, ratio is much smaller than that obtained for both LC cells 4, R2389 (2000).

with homeotropic alignment. Moreover, this enhancement [13] M. Thonissen, M. Marso, R. Arens-Fisher, D. Hunkel, is observed for both parallel and perpendicular vibrational M. Kruger, V. Ganse, H. Luth, W. Theiss. J. Porous Materials 7, 1/3, 205 (2000).

bands shown in the low-frequency region. A similar effect [14] S. Kumar, M. Manickam, V.S.K. Balagurusamy, H. Schonherr.

has been observed by Alieva et al. [20] for the species in Liquid Crystals 26, 10, 1455 (1999).

the microcavity of a 1D photonic structure. It is worth [15] V. Lehmann, H. Foll. J. Electrochem. Soc. 137, 2, 653 (1990).

noting that the enhancement takes place as soon as the [16] N.M. Shtykov, J.K. Vij, M.I. Barnik, H.T. Nguyen. Crystallolight wavelength becomes close to the period of the artificial graphy Reports 45, 4, 682 (2000).

lattice (12 m). Although the frequency range is in our case [17] H. Binder, H. Schmiedel, G. Lantzsch. C. Cramer, G. Klose.

outside the main photonic band gap, which is expected in Liquid Crystals 21, 3, 415 (1996).

the lower frequency range, this observation seems to be [18] M.V. Wolkin, S. Chan, P.M. Fauchet. Phys. Stat. Sol. (a) 182, related to the properties of holey wave guides [21].

1, 573 (2000).

Thus, we conclude that traditional IR methods cannot [19] A. Sonin. The Surface Physics of Liquid Crystals. Gorestablish the alignment of liquid crystals infiltrated into a don&Breach Science Publishers, Amsterdam (1995). 180 p.

macroporous silicon matrix. We propose a new technique [20] E.V. Alieva, L.A. Kuzik, G. Mattei, J.E. Petrov, V.A. Yakovlev.

for determining the alignment, which is based on FTIR Phys. Stat. Sol. (a) 175, 1, 115 (1999).

[21] A.M. Zheltikov. Uspehi Fizicheskih Nauk 170, 11, investigation of LC in the mesophase, and implies a (2000) (in Russian).

comparison of the intensity of different vibrational bands, namely, those with transition dipole moment parallel and , 2002, 44, .

Pages:     | 1 ||



2011 www.dissers.ru -

, .
, , , , 1-2 .