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Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory
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10.1063/1.2210930
/content/aip/journal/jcp/125/2/10.1063/1.2210930
http://aip.metastore.ingenta.com/content/aip/journal/jcp/125/2/10.1063/1.2210930

Figures

Image of FIG. 1.
FIG. 1.

Configuration of LC cells. Shown above left is a top view of the interdigitated electrode array (IDEA) that is patterned onto ZnSe. Top right illustration shows the cross section of the microfabricated IDEA on ZnSe substrate. Electrode bands are wide and apart. As illustrated on the bottom, infrared radiation is incident in transmission geometry and the optical polarizations (parallel and perpendicular) are defined with respect to the electrode bands. The structure of the LC molecule 5CB is shown.

Image of FIG. 2.
FIG. 2.

XPS analysis of the polyimide-coated ZnSe surface: (a) Survey spectrum after the photolithographic microfabrication of the Au IDEAs. Only O , N , and C signals are detected; no Zn or Se peaks are visible, indicating that the polyimide layer masks the underlying ZnSe substrate. Imaging spectra of the polyimide-coated ZnSe and the Au IDEA microfabricated on the surface are shown in (b) Au and (c) N spectra. The intensities in (b) and (c) are complementary, thus indicating that the polyimide layer underlying the Au electrodes does not delaminate during the microfabrication process.

Image of FIG. 3.
FIG. 3.

Representative AFM micrographs of (a) unmodified, (b) polyimide-coated, and (c) polished ZnSe surfaces. The micrographs image and areas of the substrates, and are representative of the respective surfaces. The random grooves in the as-received ZnSe are clearly visible in (a), whereas these features are largely softened after coating with polyimide, as seen in (b). The contrast range for (a) is , while that for (b) is , further emphasizing the flattening effect of the polyimide layer. Surface roughness analysis of the AFM micrographs show the root mean square (rms) roughness of the unmodified ZnSe substrate (a) to be around , while the surface roughness of (b) is , and that of (c) is also . In (c), the unidirectional polishing did not decrease the surface’s roughness, but rather, produces an anisotropically grooved surface. The IDEA electrode bands were microfabricated so as to be parallel to the grooves.

Image of FIG. 4.
FIG. 4.

Dichroic spectra of the (a) 40 and (b) thick 5CB films in polyimide cells, both in planar alignment with the director parallel to the electrode bands. Intense positive features of the and in-plane aromatic stretch modes suggest that the 5CB director predominantly lies parallel to the electrode digits.

Image of FIG. 5.
FIG. 5.

Stacked difference spectra at incrementally increasing potentials for the (a) 40 and (b) thick films in polyimide cells. The LC cell at is used as the background spectrum. The top panels show spectra with the perpendicular IR polarization, and the bottom panels show spectra in the parallel IR polarization. The modes (indicative of the 5CB director) approximately mirror each other in the two optical polarizations and suggest that the 5CB molecules twist to orient with the applied field.

Image of FIG. 6.
FIG. 6.

Peak areas of the stretch as a function of applied potential for the (a) 40 and (b) thick films in polyimide cells. These plots quantify the spectra shown in Fig. 5. The voltage-dependent absorbance changes of are approximately commensurate in the two optical polarizations. indicates the critical field at which the LC molecules start reorienting.

Image of FIG. 7.
FIG. 7.

(Color) Normalized time-domain data extracted from TRS data at the stretch for the 40, 50, and thick films in polyimide and polished EO cells in the two IR polarizations. The data show the electro-optical responses of 5CB to electric field pulses. The denotes data taken from the polished EO cell of a thick film.

Image of FIG. 8.
FIG. 8.

Rate constants of absorbance changes at 1495, 1607, and for the orientation and relaxation processes of the LC films examined in Fig. 7. The denotes data taken from the polished EO cell of a thick film. The rates shown are for spectra taken under the application of electric field pulses.

Image of FIG. 9.
FIG. 9.

Dichroic spectra of the (a) 50 and (b) thick 5CB films in the polished EO cells. The positive features of the and in-plane aromatic stretch modes suggest that the director of the LC film also lies parallel to the electrode digits, as in the case of the films formed on polyimide-coated substrates.

Image of FIG. 10.
FIG. 10.

Stacked difference spectra for the (a) 50 and (b) thick films in the polished cells. The top panels show data taken with perpendicular infrared polarization and the bottom panels show those taken with parallel polarization.

Image of FIG. 11.
FIG. 11.

Peak areas of the stretch plotted as a function of applied voltages for the (a) 50 and (b) thick films. The data points quantify the intensity differences observed in the data in Fig. 10. indicates the critical field at which the LC molecules start aligning with the applied field.

Image of FIG. 12.
FIG. 12.

Orientation (a) and relaxation (b) rate constants for a series of variously anchored 5CB thin films: 40 and thick films using a polyimide modified IDEA cell, and 50 and thick films aligned using ZnSe surface textured by polishing. The orientation rate data are plotted as a function of the ratio of the applied voltage to the experimentally determined critical voltage . The relaxation rate data are plotted as a direct function of the applied voltage . The rate constants were calculated from time domain FTIR data for the mode at . The plots show fits made according to specific functional forms: for (a) the data were fitted using a parabolic functional form constrained such that for , while for (b) the values of were fitted as a linear function of the voltage used to perturb the quiescent nematic order.

Image of FIG. 13.
FIG. 13.

Plot of orientation rate constants calculated from mode absorbance changes for various films vs the projection of the mean nematic alignment, , along the direction of the applied electric field. Values of were calculated from experimentally determined values of the dichroic ratio via the relation . Rate data were measured at three potentials (8, 10, and ) lying far above the critical voltage . The plot shows data for four main EO cells examined in this work along with a separate data point for an additional ZnSe aligned system examined in an earlier study, one similar to but somewhat more weakly aligned than the main EO device examined in this work. Two linear regression fits were carried out on the data: one includes the , PI aligned data points (dotted line), while the other excludes these same data (solid line). As is evident in the graph, has an essentially linear dependence on , a parameter directly linked to the initial anisotropy and alignment of the LC film.

Image of FIG. 14.
FIG. 14.

Representative polarized FTIR spectra of the polyimide-coated ZnSe surface.

Image of FIG. 15.
FIG. 15.

(Color) Three-dimensional time-resolved spectra of 5CB for (a) perpendicular and (b) parallel IR polarizations.

Tables

Generic image for table
Table I.

Vibrational mode assignments for 5CB and the angles of the transition moments relative to the long axis of the molecule.

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/content/aip/journal/jcp/125/2/10.1063/1.2210930
2006-07-14
2014-04-24
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Anchoring and electro-optical dynamics of thin liquid crystalline films in a polyimide cell: Experiment and theory
http://aip.metastore.ingenta.com/content/aip/journal/jcp/125/2/10.1063/1.2210930
10.1063/1.2210930
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