condensed matters. frunza 734 et al. 2 on the sign of lc dielectric anisotropy [10–12]. at...

CONDENSED MATTER

COVERING OBLIQUELY DEPOSITED SiOx WITH POLYVINYL CARBAZOLE CHANGES THE ORIENTATION PROPERTIES

S FRUNZA1 I ZGURA1 L FRUNZA1 O RASOGA1 P GHEORGHE2 A PETRIS2 V I VLAD2

1 National Institute of Materials Physics R-077125 Magurele Romania E-mail frunzainfimro irinazgurainfimro

2 National Institute for Laser Plasma and Radiation Physics R-077125 Magurele Romania

Received February 14 2014

Alignment layers of polyvinylcarbazole (PVK) were obtained by withdrawing from different solutions in toluene onto glass plates having SiOx layers obliquely evaporated in vacuum at 82o The alignment direction of nematic liquid crystal molecules with cyan-end group imposed by bare SiOx layers is changed when these layers are coated with PVK the same happens with over layers of polyvinylimidazole or of polyvinylcinnamate The effect is inhibited by doping PVK with fullerene C60 (01) when the liquid crystal orientation specific for bare SiOx obliquely evaporated substrates is obtained Other nematic (with methoxy-end group) does not show change of the orientation properties by covering the SiOx obliquely deposited layers with the mentioned polymer over layers

Key words SiOx obliquely deposited polyvinyl carbazole optical microscopy

1 INTRODUCTION

Since the discovery of the liquid crystal alignment by Mauguin a large variety of alignment techniques were developed to valorize the crucial role that liquid crystals (LCs) play in production of displaying devices (eg [1]) An equal motivation was the need to understand the alignment mechanisms since there is not yet a satisfactory knowledge of the whole field of these phenomena Non-stoichiometric SiOx films deposited on glass plates by evaporation in vacuum at large incident angles [2 3] align the liquid crystal molecules in the evaporation plane tilted against glass surface There is a very rich literature describing this alignment behavior either experimental works ([4ndash7]) or theoretic ones ([8]) It was shown that aligning micro relief could create nematic oriented structures which are defect less at the microscopic level [9] On SiOx films deposited at incident angles larger than 72deg the liquid crystal molecules align obliquely in the evaporation plane while SiOxSiO2 films deposited at angles less than 60deg induce an alignment normal to the evaporation plane or vertical to the glass plates depending

Rom Journ Phys Vol 59 Nos 7ndash8 P 733ndash744 Bucharest 2014

S Frunza et al 2 734

on the sign of LC dielectric anisotropy [10ndash12] At incidence angles greater than 60deg and less than 72deg a bistable orientation between planar and oblique orientation is induced for SiOx films of the thickness comparable with the molecular length [13]

A technique was developed which uses polyvinyl alcohol (PVA) to cover SiOx layers evaporated under incident angles higher than 80deg (eg [14ndash16]) PVA film deposited by dip coating from aqueous solution allows obtaining alignment angles for liquid crystal molecules from 27deg (for PVA concentration of 025) down to 7deg (for solution concentration of 165) For concentrations of 17 or higher the PVA film ceases to present alignment properties

On the other hand unidirectionally rubbed polymer layers have aligning properties [17] parallel to the rubbing direction rubbing the polymer films was thus imposed as the most widely used technique in the display devices There were also found polymer films which induce an alignment perpendicular to the rubbing direction for nematic liquid crystals with a cyan-end group for example polystyrene (PS) [18] polyvinylimidazole (PVI) [1920] or polyvinylcinnamate (PVCi) [19 21] Rubbing a layer of polyvinylcarbazole (PVK) (which is in addition an excellent film-forming material) induces homogeneous alignment of LCs with their director axes perpendicular to the rubbing direction [22]

In order to obtain new knowledge on the alignment mechanisms of liquid crystal molecules we combined the orienting properties of thin polymer layers and those of SiOx layers obliquely evaporated in vacuum Polymers with bulky or polar side chains as PVK PVI or PVCi were thus chosen and deposited over SiOx by dip or spin coating The withdrawing direction was chosen either parallel with the evaporation plane of SiOx or making an angle with this plane For the cells made of plates with SiOx evaporated at 82deg and coated with PVK from toluene solution with concentration between 025 and 150 we found that the final alignment of liquid crystal was perpendicular to the evaporation plane whereas uncoated SiOx evaporated at 82deg induced alignment in the evaporation plane The nematic LC used was a mixture of cyan-end group components (named E7) its behavior was compared with that of another nematic which has a methoxy-end group and imine structure Over layers of fullerene (C60) doped PVK [23] does not show this found perpendicular alignment

2 EXPERIMENTAL

21 MATERIALS

The liquid crystal (LC) used in this study was E7 (from Merck) a mixture of four cyanobiphenyl components This LC was selected among the nematics for

3 PVK changes the orienting upon obliquely deposited SiOx 735

having suitably low transition temperatures such as melting point at 263 K nematic-isotropic transition at 331 K and a glass transition at 211 K Behavior of E7 was compared with that of MBBA (N-(methoxybenzilidene)-4-buthylaniline) from Aldrich MBBA has a negative dielectric anisotropy The transition temperatures from solid to LC and from LC to isotropic liquid are 293 and 3169 K respectively

PVI was obtained from the corresponding monomer (Polysciences Inc) at the Institute of Macromolecular Chemistry ldquoPetru Ponirdquo (Iassy Romania) PVK has the molecular weight of ca 40000 while PVCi ca 200000 both were purchased from Polysciences Inc

Glass plates were soda-lime type either standard Cole-Parmer microscope slides or ordinary float plates

Other reactants and the solvents (toluene chloroform etc) were of very high purity and used as purchased

22 ALIGNMENT LAYERS

Non-stoichiometric silicon oxide (SiOx) layers were obtained on glass plates by obliquely evaporation in vacuum of SiO (Balzers) at incident angle of 82deg or 60deg in a B302 equipment (Hochvakuum Dresden) at 10-8 bar as already described [24ndash26] The SiOx deposited layers were subsequently coated with the polymeric layer mostly by dip coating but in some cases (mentioned in the following) by spin coating For a few experiments SiOx was subsequently covered with fullerene C60 [27 28] by spinning from a 01 toluene solution

PVK was deposited by dip coating toluene solutions of 025 050 075 and 1 were used the withdrawing speed was of 10 mmmin After deposition the glass plates were thermally treated 30 min at 363 K then 60 min at 453 K PVK was used as such or doped (01) with C60 fullerene

PVI was also deposited by dip coating (with a vertical withdrawing speed of 3 mmmin) from 1 or 15 aqueous solution The lifting set-up was used in a hut atmosphere The withdrawing direction was usually parallel with the incident plane of SiOx evaporation the sense was thus that the end of plate closer to the vapor source was the last to leave the solution Aqueous solutions of 05 PVI were used for spin coatings After PVI deposition the glass plates were backed at 373 K for 30 min

PVCi was spin coated from a solution 1 in chloroform In the following the samples were labeled by noting the type of polymer

deposited the SiOx evaporation angle the substrate nature for example PVKSiOx82G means a glass (G) plate containing a layer of SiOx deposited at 82o and an over layer of PVK the ldquoGrdquo support was usually dropped


PVK(SiOx82+C60) means that the first layer (of SiOx deposited at 82o) was covered with C60 layer the plates were dried and the layer of PVK was deposited The liquid crystal cells (see further) made from such plates have the notation of the corresponding plates

23 CHARACTERIZATION OF DEPOSITED LAYERS

Composition of glass deposited SiOx layers was investigated by X-ray photoelectron spectroscopy (XPS) in a SPECS Multimethod Surface Analysis System The photoelectron spectra were obtained using a Phoibos analyzer with monochromatic X-rays emitted by an anti-cathode of Al (14867 eV) operating in the constant energy mode with a pass energy of 20 eV and in ultra high vacuum conditions (310-9 mbar) The energy scale of the spectrometer has been calibrated using C1s (285 eV) line The spectra were processed using Spectral Data Processor v 23 (SDP) software

The thickness of the layers was evaluated by spectroscopic ellipsometry The layers for these measurements were deposited either onto the glass plates or especially onto silicon wafers by dip coating using the withdrawing system of a Langmuir Blodgett (LB) equipment (KSV Instruments) in order to have the best control of the withdrawing The ellipsometric angles ψ and ∆ were measured on each sample with a Woollam DUV-VIS-XNIR Variable Angle Spectroscopic Ellipsometer in the spectral interval 400-1000 nm at incidence angles of 60 65 and 75deg The experimental data were fitted with an adequate uniaxial optical model consisting of two Cauchy dispersions one perpendicular and the other parallel to the substrate

24 OBTAINING THE LIQUID CRYSTAL CELLS

The cells were made of rectangular glass plates provided or not with transparent electrodes of ITO which were subsequently covered with alignment layers The thickness of the cells was ensured with 15 microm strips of Mylar The assembled cells were immobilized with binder clips or with epoxy resin on the cell perimeter The filling with liquid crystal in the isotropic phase was made by capillarity to avoid the appearance of a privileged alignment related to the direction of the flow during the filling However this type of filling may produce point-like or linear discontinuities of the orientation when the anchoring to the alignment layer is weak When polymer layer was deposited by withdrawing from solution a three-region-structure of assembled cells was obtained as shown in Fig 1a The evaporation direction on the cell plates is shown in Fig 1b together with the alignment direction in the case of the cell deposited only with SiOx these cells are so-called symmetric ones


G

G

w ith dra wi ng fr om so lut ion

w ithdra w ing from so lut ion

SiO x

S iOx

L C ep o xi re sinep o xi re sinpo lym e r

p o lym e r

a)

b)

Fig 1 ndash Schemes of the plates in the LC cells a) A liquid crystal cell assembled from deposited glass plates The symbols are as follows G - glass SiOx - the SiOx deposited layer the spacers and the closing resin are shown as black stripes b) The evaporation direction of SiOx on the cell

plates The alignment direction of LC molecules is indicated by dotted-line arrow

Thus in the central region of the cell the alignment properties are defined by identical orientation at the two surfaces covered with SiOx and polymer layer The two lateral regions might have hybrid alignment properties determined by the possible different orientations at the surface layers a reference region covered only with SiOx and the studied region covered with a combined layer of SiOx and the polymer

After filling the alignment in the cells was checked either by direct visual inspection (between parallel or crossed polarizers in a parallel beam of light) or by conoscopy (between crossed polarizers in a convergent beam of light) The experimental setup includes as usually a polarizing microscope (Zeiss Amplival PolU or Leitz Ortoplan) with an adapted digital photo camera (Panasonic DMC-FZ8) or a video camera

3 RESULTS AND DISCUSSION


XPS investigation of SiOx layers was performed because it was found that SiOx layer horizontally aligns liquid crystals when the value of x is in the range from about 10 to about 15 but vertically aligns the liquid crystals when the value of x is in a range from about 15 to about 20 [29] Representative high resolution spectra are given in Fig 2 for silicium and oxygen Both spectra are decomposed into several components The highest Si2p peak belongs to Si4+ in SiO2 species [30 31] while the peak of medium intensity to SiO species The highest O1s peak shows O2- species [32] while the second one shows species OH- The various suboxides cannot be differentiated using this O1s photoelectron peak [32]


From the intensities of Si2p and O1s photoelectron peaks in combination with the different sensitivity coefficients of each element it was possible to determine the chemical composition of the surface Thus the estimated value for the SiO atomic ratio is 150 a limit value corresponding to the alignment imposed by SiOx layers deposited in vacuum under 60o incidence angles to MBBA and E7 molecules

96 98 100 102 104 106 1080

2000

4000

6000

8000

Binding Energy (eV)

Inte

nsity

(cou

nts)

a)

528 530 532 534 536 5380

2000

4000

6000

8000

10000

Binding Energy (eV)

Inte

nsity

(cou

nts)

b)

Fig 2 ndash High resolution photoelectron Si2p (a) and O1s (b) spectra (empty symbols) their deconvolution into Gaussian components (dashed lines) and their sum (continuous line) for the

sample SiOx82

Representative spectroellipsometric measurements of the sample PVKSi and their fit with the biaxial model are given in Fig 3 a rather good agreement can be observed The estimated values for the thickness of studied layers are presented in Table 1 SiOx layers have in average up to 100 nm (as expected from the deposition conditions) while PVK layers have a thickness of a few nm The parallel and perpendicular refraction indexes of the deposited layer as obtained by the applied model (not shown in Fig 3) are a bit smaller than those of a SiO2 glass [33] predictable on the basis of a smaller density of the ldquoporous-likerdquo structure of the deposited material Since the uniaxial model gave the best fitting results we can suppose that the SiOx layer is composed of SiO2 matrix with tilted and elongated nanopores inside

Refractive index of PVK in the film deposited onto silicon is also a bit smaller than that of a bulk material but roughly one can say its value agrees with literature data


4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)

Fig 3 ndash Ellipsometric data (empty symbols) and the corresponding fit (line) for the sample

PVKSi in the case of incidence angle of 75o Dotted lines belong to Si substrate

Table 1

Layer thickness of deposited layers

Sample Layer of interest

Layer thickness (nm)

Observations

SiOx82Si SiOx 634 Biaxial model SiOx82G SiOx 55 PVKSiOx82G PVK 4 PVKSi PVK 27 PVK deposited in LB cuvette

In the case of columnar structure the values are mediated

32 ALIGNMENT PROPERTIES OF SiOx COVERED WITH POLYMER LAYERS

Since it was known that SiOx evaporation at 60o incidence angle provides an alignment normal to the evaporation plane for cyanobiphenyls [34 35] and MBBA [25] we have focused mainly onto plates deposited with SiOx by evaporation at 82o and covered with polymer layers and obtained some reference cells

In the antisymmetric cells (see Fig 1b) made with plates deposited only with SiOx a homogeneous tilted alignment of liquid crystal molecules in the incident plane of evaporation can be obtained One of the polarizers has the axis parallel to the evaporation plane (Fig 4a) The conoscopic image of the same cell (Fig 4b) supports the tilted orientation of LC molecules in the evaporation plane

Representative images of the cells containing a PVK over layer made with plates deposited with SiOx82 and dipped into PVK solution (075) are shown in Figs 4cd The lateral light stripes (Fig 4c) correspond to zones where the glass plates are covered with SiOx without PVK over layer whereas the central zone has SiOx layer which is PVK covered The cell aspect indicates a twisted structure in the lateral stripes while in the central part the molecules are homogeneously


oriented perpendicular to the evaporation plane imposed by PVK over layer The alignment observed is quite stable (more than 2 months) PVK layers obtained from the other PVK concentrations behave similarly

The cells obtained from PVK solution doped with C60 show alignment tilted in the incidence plane as one can see from Figs 4ef

At this stage of investigations we can only speculate on the mechanism acting in the case of the surface alignment properties of our samples For the liquid crystal cells made with plates with SiOx evaporated at 60deg and coated with PVK by cooling the liquid crystal from the isotropic phase one ends in a state aligned perpendicularly to the evaporation plane as in the case of bare SiOx (not shown here)

a) b) c) d)

e) f) g) h)

Fig 4 ndash Up Images of cells made from SiOx82 plates (ab) or from PVKSiOx82 plates (cd) between crossed polarizers (ac) by conoscopy (b) or between parallel polarizers (d) Down

Images of a cell made with (PVK+C60)SiOx82 (ef) or PVK(SiOx+C60) plates (gh) between crossed polarizers (eg) or by conoscopy (fh) LC was E7 in all the cases

In Fig 5a the aspect between crossed polarizers of a representative LC cell

assembled from SiOx82 glass plates subsequently covered by a PVI layer from a 15 solution is shown As in the case of PVK over layered cells (Fig 4c) The lateral light stripes correspond to zones where one of the glass plates has the SiOx layer uncovered by dipping in PVI solution whereas the central zone has SiOx layer covered with PVI The cell aspect clearly indicates a twisted structure Taking into consideration that the alignment of LC molecules on SiOx is in the evaporation plane (parallel to the long side of the cell) the E7 molecules are aligned perpendicular to the evaporation plane on the PVI layer Therefore in the central zone of Fig 5a the LC molecules are uniformly aligned and perpendicular to the evaporation plane In Fig 5b the image between crossed polarizers of a cell with the SiOx82 glass plates withdrawn from PVI solution at 30o to the evaporation plane is presented It can be seen that the withdrawing direction does not influence


the obtained alignment imposed by the polymer layer This is definitely proved by Fig 5c where PVI layer was deposited (also from a 15 solution) onto SiOx by spin coating The alignment obtained in all these LC cells made with different layers on the plates is summarized in Table 2 One can see the change of the alignment of E7 molecules when passing from plates deposited only with SiOx layer to those with a polymer over layer It is noteworthy that fullerene C60 does not change the orientation plane when covers directly the SiOx layer In the case of PVK layer there are three situations PVK covering SiOx changes the orientation while PVK with C60 does not change the orientation plane as in the case of PVK covering (SiOx+C60) deposition For PVI layers onto SiOx the alignment remains normal to the evaporation plane It results from Table 2 that MBBA molecules are always in the evaporation plane no matter which is the treatment of SiOx layer

a)

b)

c)

Fig 5 ndash Images between crossed polarizers of a cell made from PVISiOx82 plates (ab) or with one PVISiOx82 plate and the other PVASiOx82 plate (c) The withdrawing direction is a) parallel b) at 30o to the evaporation plane The long side of the plate is parallel to the evaporation plane The

rubbing and evaporation directions for the cell in c) are at 90o LC was E7

Table 2

Alignment obtained in the investigated LC cells Layer(s) LC E7 MBBA SiOx82 SiOx82+C60 PVKSiOx82 perp PVISiOx82 perp PVCiSiOx82 perp PVASiOx82 PVK(SiOx82+C60) PVI(SiOx82+C60) perp (PVK+C60)SiOx82

The symbols mean - parallel perp - perpendicular to the evaporation plane

33 DISCUSSION

The mechanisms proposed to explain the alignment of LCs on substrate surfaces take into consideration the effect of the topography [5] steric hindrance


order electricity [36 37] van der Waals interactions [10 11] and even πminusπ electron coupling in the case of some polymer layers [38 39]

Alignment layers of the rubbed polymers either obtained from PVA polymethyl methacrylate (PMMA) or from polymers exposing aromatic rings in the lateral groups (PVI PVK PVCi) induce molecular ordering of LCs having a methoxy-end group (MBBA) following a direction parallel to the rubbing direction When LC has cyan-end groups LC molecules adopt an orientation parallel to the rubbing direction onto PVA PMMA layers while on the PVK PVI PVCi substrates the orientation is perpendicular to the rubbing direction [20] Birefringence measurements [17] polarized absorption spectra [39] NEXAFS [38] and surface specific sum frequency vibrational spectra [39] proved that the rubbing process effectively aligns polymer main chains and lateral groups Therefore the alignment of LCs by unidirectionally rubbed polymer layers is usually attributed to the interactions between LC molecules and polymer layer ordered by rubbing

The alignment of cyanobiphenyl LCs onto polymers having lateral groups PVI PVK PVCi [20] perpendicular to the rubbing direction might be related to the interaction of the cores of nematic molecules with polymer lateral groups [40] via πminusπ interactions as it was reported for PVCi [39]

Covering SiOx with polymer layers has shown that the alignment strongly depends on the polymer structure For PVA covering SiOx deposited at 82o the decreasing the tilt angle of pentyl cyanobiphenyl molecules with increasing the PVA concentration in the dip coating solution of SiOx was explained [14 15] by assuming that the variation of the alignment angle is due to the alteration of the initial columnar topography of SiOx surface PVA molecules would be deposited preferentially on the hollow of SiOx micro-columnar surface thus reducing the height of these columns and modifying the aspect ratio of the evaporation topography

The results presented in this paper clearly show that the different alignment of E7 and MBBA on rubbed PVI PVK PVCi and PVA as function of the terminal (cyan or methoxy) group is also manifested onto the same polymer layers covering SiOx82 substrates

MBBA molecules are tilted in the evaporation plane onto PVK or PVI layers deposited onto SiOx82 like onto bare SiOx82 substrate showing that the changes in the SiOx topography by polymer covering process do not lead to the change of the alignment plane In addition doping of PVK with C60 leads to the alignment of E7 molecules tilted in the evaporation plane as in the case of bare SiOx82 additionally supporting the idea that the topography changes by PVK covering of SiOx82 are not responsible for the alignment perpendicular on the evaporation plane observed in the cells with PVKSiOx82 layers Therefore we may suppose the alignment of E7 molecules perpendicular on the evaporation plane onto PVK or PVI covered SiOx82 substrate cannot be related to changes in the SiOx topography by polymers deposition It rather might reflect an ordering of the polymer lateral


group planes and implicitly of the polymer main chains in the presence of the SiOx anisotropy known to consist in a tilted columnar structure [41] Such an ordering could be favored by bulkiness of carbazole group which compels the polymer chain of PVK to form stiff rods [42] and hindered by the presence of C60 groups on the SiOx surface In fact the polymers and C60 interact weakly [43]

4 CONCLUSIONS

Alignment layers of polyvinylcarbazole were obtained by withdrawing glass plates with SiOx nanostructured layers (obliquely evaporated in vacuum) from PVK solutions in toluene of different concentrations Other polymer layers (PVI PVCi) were investigated for comparison A new alignment effect of nematic liquid crystals was revealed that consists in the change of alignment direction of liquid crystal molecules imposed by SiOx layers evaporated at incidence angles of 82deg or more when they are coated with PVK (or PVI) This effect is inhibited by doping the PVK with C60 (01) when the liquid crystal orientation is that specific for SiOx obliquely evaporated substrates The observed alignment is quite stable It was supposed that the alignment of E7 molecules perpendicular on the evaporation plane onto PVK (or PVI) covered SiOx82 substrate reflects an ordering of the polymer lateral group planes and implicitly of the polymer main chains in the presence of the SiOx MBBA molecules do not show the same behavior as the E7 ones when comparing the orienting properties of SiOx82 substrate and those of PVK (or PVI) covered SiOx82 substrates

Acknowledgements The authors gratfully acknowledge the financial support of Romanian

Authority ANCS - UEFISCDI by the Project Partnerships 12-11108 and the XPS measurements by Drs F Frumosu and C Negrila (NIMP)

REFERENCES

1 J Cognard Mol Cryst Liq Cryst Suppl1 1ndash78 (1982) 2 JL Janning Appl Phys Lett 21 173ndash174 (1972) 3 W Urbach M Boix and E Guyon ApplPhysLett 25 479ndash482 (1974) 4 EP Raynes DK Rowell and IA Shanks Mol Cryst Liq Cryst 34 105ndash110 (1976) 5 LA Goodman JT Mc Ginn CH Anderson and F Digeronimo IEEE Trans Electron Devices

24 795ndash804 (1977) 6 MR Johnson and PA Penz IEEE Trans Electron Devices 24 805ndash807 (1977) 7 P Jagemalm and L Komitov Liq Cryst 23 1ndash8 (1997) 8 G Barbero P Jagemalm and A K Zvezdin Phys Rev E 64 021703 (2001)


9 Yu Kolomzarov P Oleksenko V Sorokin P Tytarenko and R Zelinskyy Semicond Phys Quant Electron Optoelectron 9 58ndash62 (2006)

10 M Lu Jpn J Appl Phys 43 8156ndash8160 (2004) 11 C Chen PJ Bos J Kim and Q Li J Appl Phys 99 123523 (2006) 12 L Z Ruan F Yang and J R Sambles Appl Phys Lett 93 031909 (2008) 13 B Jerome P Pieranski and M Boix Europhys Lett 5 693ndash697 (1988) 14 S Frunza R Moldovan T Beica D Stoenescu and M Tintaru Liq Cryst 14 293ndash296 (1993) 15 R Moldovan G Barbero S Frunza and T Beica Mol Cryst Liq Cryst 257 125ndash130 (1994) 16 M Lu K H Yang T Nakasogi and S J Chey SID Int Symp Digest Tech Papers 29 446ndash449

(2000) 17 JM Geary J W Goodby AR Kmetz and J Patel J Appl Phys 62 4100ndash4108 (1987) 18 K Nakajima H Wakemoto S Sato F Yokotani S Ishihara and Y Matsuo Mol Cryst Liq

Cryst 180 B 223ndash232 (1990) 19 S Frunza G Grigoriu A Grigoriu C Luca L Frunza R Moldovan and DN Stoenescu Cryst

Res Technol 32 989ndash997 (1997) 20 S Frunza T Beica R Moldovan I Zgura and L Frunza J Optoelectron Adv Mater 7 2149ndash2157

(2005) 21 R Karapinar M OrsquoNeill S M Kelly A W Hall and G J Owen ARI ndash An Intern J Phys

Engn Sci 51 61ndash65 (1998) 22 M Kaczmarek and A Dyadyusha J Nonlinear Opt Phys Mater 12 547ndash555 (2003) 23 Y Wang Nature 356 585ndash587 (1992) 24 T Beica S Frunza M Giurgea and R Moldovan Cryst Res Technol 20 875ndash878 (1985) 25 S Frunza R Moldovan T Beica M Giurgea and D Stoenescu Europhys Lett 20 407ndash412 (1992) 26 R Moldovan S Frunza T Beica M Tintaru and V Ghiordanescu Cryst Res Technol 32

669ndash675 (1997) R Moldovan MTintaru T Beica S Frunza and D N Stoenescu Rom J Phys 42 353ndash358 (1997)

27 I Hasegawa and S Nonomura J Sol-Gel Sci Technol 19 297ndash300 (2000) 28 Y Itoh B Kim RI Gearba NJ Tremblay R Pindak Y Matsuo E Nakamura and C

Nuckolls Chem Mater 23 970ndash975 (2011) 29 S-J Rho B-K Jeon K-O Park H-K Lee H-K Baik K-C Kim J-B Kim B-H Hwang

D-C Hyun and H-J Ahn US Pattent 0279562 (2007) 30 T E Madey C D Wagner and A Joshi J Electron Spectrosc Relat Phenom 10 359ndash388 (1977) 31 V A Terekhov SYu Turishchev KN Pankov I E Zanin EP Domashevskaya DI Tetelbaum

A N Mikhailov A I Belov D E Nikolichev and S Yu Zubkov Surf Interface Anal 42 891ndash896 (2010)

32 F Verpoort P Persoon L Fiermans G Dedoncker and L Verdonck J Chem Soc Faraday Trans 93 3555ndash3562 (1997)

33 C M Herzinger B Johns WA McGahan and J A Woollam J Appl Phys 83 3323ndash3336 (1998) 34 H Yokoyama S Kobayashi and H Kamei J Appl Phys 61 4501ndash4518 (1987) 35 K Antonova K Slyusarenko O Buluy Ch Blanc S Joly Yu Reznikov and M Nobili Phys

Rev E 83 050701(R) (2011) 36 M Monkade M Boix and G Durand Europhys Lett 5 697ndash702 (1988) 37 J Papanek and Ph Martinot-Lagarde J Phys II (France) 6 205ndash210 (1996) 38 MG Samant J Stohr H R Braun TP Russell JM Sand and SK Kumar Macromolecules

29 8334ndash8342 (1996) 39 PPagliusi CY Chen and YR Shen J Chem Phys 125 201104 (2006) 40 R Yamaguchi and S Sato Mol Cryst Liq Cryst 301 67ndash72 (1997) 41 D Reznikov Effect of surface alignment layer on electro-optical properties of ferroelectric liquid

crystal displays Thesis Kent 1227562895 (2008) 42 A Kimura S Yoshimoto Y Akana H Hirata S Kusabayashi H Mikawa and N Kasai

J Polymer Sci A2-Polymer Phys 8 643ndash648 (1970) 43 W Osikowicz MthinspP dethinspJong and WthinspR Salaneck Adv Mater 19 4213ndash4217 (2007)


on the sign of LC dielectric anisotropy [10ndash12] At incidence angles greater than 60deg and less than 72deg a bistable orientation between planar and oblique orientation is induced for SiOx films of the thickness comparable with the molecular length [13]

A technique was developed which uses polyvinyl alcohol (PVA) to cover SiOx layers evaporated under incident angles higher than 80deg (eg [14ndash16]) PVA film deposited by dip coating from aqueous solution allows obtaining alignment angles for liquid crystal molecules from 27deg (for PVA concentration of 025) down to 7deg (for solution concentration of 165) For concentrations of 17 or higher the PVA film ceases to present alignment properties

On the other hand unidirectionally rubbed polymer layers have aligning properties [17] parallel to the rubbing direction rubbing the polymer films was thus imposed as the most widely used technique in the display devices There were also found polymer films which induce an alignment perpendicular to the rubbing direction for nematic liquid crystals with a cyan-end group for example polystyrene (PS) [18] polyvinylimidazole (PVI) [1920] or polyvinylcinnamate (PVCi) [19 21] Rubbing a layer of polyvinylcarbazole (PVK) (which is in addition an excellent film-forming material) induces homogeneous alignment of LCs with their director axes perpendicular to the rubbing direction [22]

In order to obtain new knowledge on the alignment mechanisms of liquid crystal molecules we combined the orienting properties of thin polymer layers and those of SiOx layers obliquely evaporated in vacuum Polymers with bulky or polar side chains as PVK PVI or PVCi were thus chosen and deposited over SiOx by dip or spin coating The withdrawing direction was chosen either parallel with the evaporation plane of SiOx or making an angle with this plane For the cells made of plates with SiOx evaporated at 82deg and coated with PVK from toluene solution with concentration between 025 and 150 we found that the final alignment of liquid crystal was perpendicular to the evaporation plane whereas uncoated SiOx evaporated at 82deg induced alignment in the evaporation plane The nematic LC used was a mixture of cyan-end group components (named E7) its behavior was compared with that of another nematic which has a methoxy-end group and imine structure Over layers of fullerene (C60) doped PVK [23] does not show this found perpendicular alignment

2 EXPERIMENTAL

21 MATERIALS

The liquid crystal (LC) used in this study was E7 (from Merck) a mixture of four cyanobiphenyl components This LC was selected among the nematics for






22 ALIGNMENT LAYERS














G

G



SiO x

S iOx


p o lym e r

a)

b)










96 98 100 102 104 106 1080

2000

4000

6000

8000

Binding Energy (eV)

Inte

nsity

(cou

nts)

a)

528 530 532 534 536 5380

2000

4000

6000

8000

10000

Binding Energy (eV)

Inte

nsity

(cou

nts)

b)


sample SiOx82




4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)



Table 1




Observations











a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES



























22 ALIGNMENT LAYERS














G

G



SiO x

S iOx


p o lym e r

a)

b)










96 98 100 102 104 106 1080

2000

4000

6000

8000

Binding Energy (eV)

Inte

nsity

(cou

nts)

a)

528 530 532 534 536 5380

2000

4000

6000

8000

10000

Binding Energy (eV)

Inte

nsity

(cou

nts)

b)


sample SiOx82




4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)



Table 1




Observations











a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES






























G

G



SiO x

S iOx


p o lym e r

a)

b)










96 98 100 102 104 106 1080

2000

4000

6000

8000

Binding Energy (eV)

Inte

nsity

(cou

nts)

a)

528 530 532 534 536 5380

2000

4000

6000

8000

10000

Binding Energy (eV)

Inte

nsity

(cou

nts)

b)


sample SiOx82




4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)



Table 1




Observations











a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES
























96 98 100 102 104 106 1080

2000

4000

6000

8000

Binding Energy (eV)

Inte

nsity

(cou

nts)

a)

528 530 532 534 536 5380

2000

4000

6000

8000

10000

Binding Energy (eV)

Inte

nsity

(cou

nts)

b)


sample SiOx82




4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)



Table 1




Observations











a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES























4

8

12

400 500 600 700 800 900

400 500 600 700 800 90040

80

120

160

Ψ

(O)

∆ (o )

λ (nm)



Table 1




Observations











a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES


























a) b) c) d)

e) f) g) h)







a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES
























a)

b)

c)



Table 2



33 DISCUSSION











4 CONCLUSIONS




REFERENCES































4 CONCLUSIONS




REFERENCES






















condensed matters. frunza 734 et al. 2 on the sign of lc dielectric anisotropy [10–12]. at...

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