EFFECT OF GRANULAR SILVER FILMS MORPHOLOGY ON THE MOLECULES ORIENTATION AND ION CONTAMINATION OF NEMATIC

The structure of granular silver films at the interface of liquid crystal (LC) cells and their influence on LC molecule orientation and ionic contamination are examined. Granular silver films were deposited on a glass substrate covered with an ITO electrode and a-C:H thin films. The morphology structure of the silver films was changed after their annealing at 200 °C. The silver granules became spheroidal with an average diameter of –30 nm and the channel area between them increased. The change in the structure of the Ag films led to an increase in the phase retardation and a decrease in the pretilt angle of the LC director from 51° to 7°. The density of ionic impurities in the LC cell with the annealed silver film was three times more than in the LC cell with the unannealed film. The impact of the alignment of the LC molecules at the surface of the granular silver films on the intensity of the plasmonic peak and its red shift in the absorption spectra is shown.


Introduction
Currently, the study of metallic nanostructures and liquid crystals (LC) interaction is relevant. It is important from a fundamental point of view and attracts scientist's attention due to the possibilities of practical use of such systems. Periodic metal fi lms and liquid crystals allow to create new types of photonic devices. Plasmonic properties of metal fi lms are sensitive to a change of the refractive index of surrounding media. LC refractive index depends on LC molecule orientation and can be controlled by an external action. Th e combination of periodic metal surfaces with LCs provides the possibility of tuning their plasmonic properties by an external electric fi eld [1][2][3]. Metal/ LC interfaces are employed as tunable microwave and IR devices [4][5][6].
Th e orientation of LC molecules depends on Van der Waalse intermolecular forces and an anisotropic elastic interaction at interfaces with a solid surface [7]. Modern techniques of preparing thin solid fi lms allow to control a pretilt angle and LC molecules alignment by changing a surface morphology and physicochemical properties [8][9][10][11][12][13]. Previous studies established that the orientation of LC molecules on metals surfaces depends on the properties of these materials, thickness of metal fi lms and morphology of metal fi lm surfaces. Au, Cr and Al fi lms were used to produce a planar orientation of LC molecules. Th e use of smooth silver fi lm provides a vertical LC orientation, while relief surfaces resulted in a planar or a tilted orientation of LC molecules [1; 14-18].
Liquid crystals are not an ideal dielectric and contain ionic impurities. Nanoparticles added in LC bulk and orienting layers on an interface aff ect the ionic density. Th e balance between the processes of ion adsorption and desorption on nanoparticles and thin fi lm surfaces determine the ionic density in LC bulk [19][20][21]. Doping metal nanoparticles in LC can enhance LC conductivity [22][23][24][25][26][27]. In the case with LC display the increase of ionic conductivity is a drawback. However, LC conductivity enhancement may be used in non-display applications, such as diff use light scattering devices [28], wave-front correctors [29] and generator of low-frequency oscillations [30].
Th e impact of granular silver fi lms morphology on the orientation of molecules and ion contamination of nematic liquid crystal are discussed in this paper. We have studied how the morphology changing of granular silver fi lms as a result of their annealing impacted on liquid crystal alignment at the interface. SEM images of the structures of granular silver fi lms before and aft er annealing were compared. We measured the transmittance versus voltage curves of the LC cells with unannealed and annealed Ag fi lms to compare phase retardations and the pretilt angle of director. Th e diff use currents were measured in LC cells with unannealed and annealed Ag fi lms at the interface to determine ionic densities. Th e impact of interaction of granular silver fi lms with liquid crystal on their absorption spectra is discussed also.

Experimental details
We have examined granular silver fi lms and plane-parallel LC cells with these fi lms at the interface. Granular silver fi lms were produced by thermal evaporation in a PVD 75 vacuum chamber (Kurt J. Lesker) at a residual pressure of ∼1,310 -5 Pa and room temperature. Th e silver fi lms thickness was about 40 Е [31]. Th ese fi lms were deposited on glass substrates covered with ITO transparent electrode and a-C:H thin fi lm. ITO transparent electrodes were obtained by cathode sputtering. Th e a-C:H insulating layers were deposited in glow discharge plasma from toluene vapor at a pressure of 2,710 -2 Pa and a room temperature. Such a-C:H layers can be used as LC alignment layers [13]. To change the structure morphology, the granular silver fi lms were annealed at 200 C for 20 minutes in vacuum aft er deposition. ITO, a-C-H and granular silver fi lms were deposited on the LC cell surface of substrate in series. Plane-parallel LC cells were assembled from the two substrates separated by spacers. Th e second LC cell substrate with ITO electrode was coated with a polyimide layer to align LC molecules parallel to the interface. Th is layer was obtained by spin coating of the solution, followed by annealing at a temperature of 180 °C. Th en the polyimide surface was rubbed in one direction. An electrically controlled LC cell with a gap of 12,5 ± 0,5 m was fi lled with a nematic liquid crystal under the action of capillary forces. Th e nematic LC ZhK-1282 (NIOPIK, Moscow) with optical anisotropy n = 0,17 at 632,8 nm wavelength, phase transition temperature 63 °C, and positive dielectric anisotropy = 9.9 at a frequency of 1 kHz was used for cells fabrication. Th e homogeneity of LC orientation had verifi ed before experiments using polarized optical microspore.
Th e morphology features of the granular silver fi lms were investigated by a Merlin scanning electron microscope (SEM) (Carl Zeiss). Th e sizes and areas of silver nanoparticles were analyzed by statistical image processing using the Toup View computer program. Spectral studies of the Ag / a-C:H / ITO structures on a glass substrate and in the LC cells were performed with an SF-56 spectrophotometer (LOMO). Nonpolarized light beam with a wave vector always perpendicular to the substrate was used to measure optical density.
To measure curves of the transmittance vs. voltage (T-V) of LC cells we used the electro optic setup comprising a laser LED with a wavelength of 650 nm, a photo detector, a voltage generator, an oscilloscope and computer. LC cells were placed between crossed polarizers in such a way that the angle between the directions of the long axes of LC molecules and polarized light vector to be 45. Sinusoidal voltage with 1 kHz frequency was applied to the cells to measure T-V curves at a wavelength of 650 nm. Pretilt angle in LC cells are estimated using equation [32]: R 0 is the phase retardation at initial time, n o and n e are ordinary and extraordinary refractive index,  -wavelength.
Th e contact of the liquid crystal with the granular silver fi lm results in ionic contamination. To characterize this process a diff use current leakage was measured aft er removing DC voltage applied to the cell. We used the circuitry in which a LC cell was connected in series with a resistor and voltage generator. Th e drop of voltage at probing resistor was measured with an oscilloscope connected in parallel to the resistor. Th e current in the circuit was calculated by the Ohm law. For the experiments we used a 10 k probing resistor. Th e density of the ions was evaluated as 3. Results and discussion SEM images of the structures of granular silver fi lms before (a) and aft er annealing (b) are depicted in Figure 1.
Th e granules of silver fi lms are separated by channels with a-C:H. Flat granules of Ag became more relief aft er annealing and had an almost spheroid shape with an average diameter of 30 nm.
Th e fi lling factor was determined as the ratio of the sum of nanoparticle areas to the substrate area. Th e estimation showed that this factor decreased from 53 % to 42 % aft er annealing of silver fi lms. Our study has indicated the decrease of nanoparticle dimension in the plane of the substrate and the increase of the channel area between the granules [34].
LC molecules orientation on the surfaces consisted of several materials depend on the ratio of areas, the anchoring strength and the relief of materials surface [18]. On the smooth Ag fi lm surface LC molecules tend to align perpendicular [15]. In Figure 2 the 107 T-V dependences are shown at a wavelength of 650 nm for the planar-oriented LC cells using a-CH thin fi lms without Ag (a), with unannealed (b) and annealed (c) granular silver fi lms at the one interface. T-V curves were obtained applying sinusoidal voltage at a frequency of 1 kHz. Th e T-V curves were used to calculate the phase retardation and a pretilt angle of the LC cells as was described in [31].   Figure 3 shows the phase retardation curves for the studied LC cells. Th e phase retardation curve of the hybrid LC cell with an annealed silver fi lm at the interface diff ered from the curve for planar oriented LC cell with the a-C:H layers without silver fi lm in the voltage range below 4 V. Th at is due to diff erent threshold voltages and phase retardation in initial time.
In case of the LC cell without Ag fi lms, the phase retardation was 6,6 and the pretilt angle was equaled to 3. Th e phase retardations were 2,3 and 6,4 in the LC cells with the unannealed and annealed fi lms respectively. Th e calculation of the pretilt angle from equation (1) showed that it is equal to 51 in the LC cell with an unannealed Ag fi lm. Th is means that the LC cell has a hybrid orientation of the LC, since the polyimide layer deposited on the opposite substrate had a pretilt angle close to zero. Th us, at the interface with the unannealed granular silver fi lm, the LC molecules align with a pretilt angle more than 51.
A pretilt angle of the LC cell with the annealed Ag fi lm was near 7. Th is means that a change in the structure of the Ag fi lm as a result of annealing causes a planar orientation of LC molecules with a small pretilt angle at the interface. Annealing the granular fi lms led to the diff usion of Ag atoms and small clusters in the channels. Th e shape and size of individual granules varied and the channels area increased (fi g. 1b) as the fi lling factor decreased on 11%. Th us, it can be concluded that the change of LC orientation as a result of the fi lms annealing can be explained by three factors -increasing the channels area, removing silver atoms from the channels and changing the surface relief.
We have studied changes in the ion density in the LC cells with the granular silver fi lms. fi g. 4 shows the dependence of the diff usion current of ions (I) on the time (t) aft er the removal of the rectangular pulse amplitude of 20 V with the duration of 300 ms applied to the LC cells without Ag fi lms (a) and with unannealed (b) and annealed (c) Ag fi lms.
Th e ion density in the LC cell with the annealed Ag fi lm estimated by integrating the curves c in fi g. 4 was 1,810 16 cm -3 . In the LC cell with the unannealed Ag fi lm the ion density was less and equaled 5,510 15 cm -3 . In the cell without Ag fi lms ion density was equaled to 0,2310 15 cm -3 . Th e presence of granular silver fi lm at the interface of the LC cells led to the generation of additional ions. Additional ions in the LC cells with such structures may also arise as a result of a disengagement of ions from the surface of the granular Ag fi lm. Th us, the morphology change in the granular silver fi lms aft er annealing aff ected both the orientation of the LC molecules and the ion density in the LC cell. Increasing the charge carriers in the LC cell with the annealed Ag fi lm can result from the enhancement of electron emission from the surface of Ag/ a-C:H/ITO thin multilayer fi lms by applying constant electric fi eld. We suppose that the presence of Ag nanoparticles on the surface of multilayer structure may result in the reduction of electron work function that facilitates electron emission.
To show the eff ect of the interaction of granular silver fi lms with liquid crystals, we compared the extinction spectra of these fi lms on glass substrates in structures with a-C:H/ ITO and in the LC cells. Th e measured optical density D depends on the extinction cross section, which is the sum of the absorption and scattering cross sections. In our case, the scattering can be neglected, since the size of the silver granules is much less than 100 nm [35]. Only one peak in the absorption spectra was observed due to the use of non-polarized light beam with a wave vector always perpendicular to the substrate. Th e plasmonic peak in the spectra of metal nanoparticles depends on their form and size [35; 36] as well as the distance between nanoparticles [37]. Th e extinction spectra of the granulated silver fi lm, which characterize its plasmon properties, are shown in fi g. 5. Th e comparison of the extinction spectra 1 and 2 in fi g. 5 has shown the blue shift of the absorption band peak aft er annealing. Th e value of blue shift was about 16 nm. We can conclude that both the decrease of the granules dimension and increase of the distance between them lead to the blue shift aft er annealing as other conditions do not change.
Th e liquid crystal is transparent in the visible region [38]. Th us, observed in the absorption spectra 3 and 4 in fi g. 5 peaks are associated only with the localized surface plasmons in granular silver fi lms. Th e red shift of plasmonic peak was observed in the LC cells spectra for both annealed and unannealed Ag fi lms at the interface. Th e oscillations in the spectral curves appeared due to the interference in LC layer. Th e red shift values in the absorption spectra of granular silver fi lms in LC cells were approximately the same for annealed and unannealed Ag fi lms and equal to 45 nm comparing with the fi lms on a glass substrate. Moreover, the peak intensity increased almost about 1.6 times for the annealed Ag fi lm and 1,4 times for the unannealed fi lm. Th e data of a wavelength and optical density of the localized plasmon excitation peak of the LC cells are listed in Table 1. Th e peak intensity was bigger for the annealed Ag fi lm at the interface with LC (fi g. 5, curve 4). Th e absorption cross-section of metal nanoparticles depends on refractive index of surrounding media. Its increasing results in the red shift of the plasmonic peak and infl uences on the maximum absorption [35; 36]. Th e changes observed in the spectra 3 and 4 in fi g. 5 are related to the change in the refraction index of LC at the interface with the Ag fi lms.

Conclusion
We have studied the change in the structure and plasmonic properties of granular silver fi lms deposited by thermal evaporation in vacuum and then annealed at 200 C. Th ese Ag fi lms were formed on a surface of a-C:H thin fi lms deposited in glow discharge plasma which are aligned LC molecules planar. It has been shown by statistical analyses of SEM images that annealing the granular silver fi lm on glass substrate covered with a-C:H/ITO fi lms leads to the spheroid form of Ag nanoparticles with the average diameter of 30 nm and increase of the area of channels between them. It has been shown the impact of granular Ag fi lms at the interface of LC cells on the molecules orientation by the studies of the transmission of LC cells in dependence on the applied voltage and the calculations of the phase retardation and the pretilt angle of the molecules. Th e pretilt angel of LC molecules was about 51 before annealing fi lm and decreased to 7 aft er that. It has been founded the density of ion impurities in the LC cell with the annealed granular silver fi lm at the interface in LC cells was three times higher than in the case of the unannealed fi lm by measurement of the diff use current of charges. We have studied the extinction spectra of the granulated silver fi lm and observed that the intensity of plasmonic peak and its red shift in the absorption spectra of granular silver fi lms at the interface in the LC cell depend on the LC molecules alignment on nanoparticles surface. Th e obtained results contribute to gain a more thorough understanding of the interaction between metallic nanostructured surfaces and liquid crystals for the development of optical device technology based on them.