Ionics 5 (1999) 275

Polarity Sensitive Electro-Optical Response in Liquid Crystal Devices Due to Ionic Diffusion in WO3 Thin Films

G. Strangi a),*, N. Scaramuzza a), C. Versace a), E. Cazzanelli a), F. Simone b), A. Pennisi c) and R. Bartolino a)

a)Dipartimento di Fisica, Universit~ della Calabria and Istituto Nazionale per la Fisica della Materia (INFM),

Unit~ di Cosenza; 1-87036 Rende (Cosenza), Italy.

b)Dipartimento di Fisica, Universith di Catania and INFM, Unith di Catania 1-95129 Catania, Italy.

c)CNR and INFM, Unit?a di Catania, Corso Italia 57,I-95129 Catania, Italy.

Abstract. The present work is aimed to a deeper understanding of the polarity sensitive electro-

optical response observed in a liquid crystal cell with asymmetric insertion of a thin film of

tungsten trioxide, deposited by r.f. sputtering on a transparent ITO substrate. After preliminary

structural characterization of the films, the electro-optical response of the liquid crystal cells con-

taining a WO 3 layer has been analyzed. The voltage thresholds of the Fr6dericksz transition are

increased or decreased for the anodic or the cathodic charge of the tungsten trioxide film,

respectively. In such a way a polarity-sensitive electro-optical response is generated, with the same

frequency of the applied field. Impedance and cyclic voltammetry measurements have been

performed on liquid crystal cells having WO3 films inserted as electrode, for as-deposited films as

well as for annealed films at increasing temperatures. A strong decrease of the ionic current is ob-

served in the cell where the film underwent the highest temperature treatment, above 300 ~ All

the measurements suggest that a charge redistribution occurs for the hydrogen ions, always present

in the films, during the intercalation-deintercalation processes induced by the applied voltage, and it

is responsible for the changes of the effective electric field acting on the liquid crystal layer.

1. In troduc t ion Tungsten trioxide (WO3) is among the most studied

materials for active electrochromic electrodes, because of

its properties of good ionic conductivity for small cations

(H +, Li § Na +) coupled with a reasonable electronic con-

ductivity [1-7] and a remarkable change in the electronic

band structure and consequently in the optical trans-

mission spectrum, when cations are intercalated. Many

applications have been studied, like hydrogen sensors [8],

energy and light control system in buildings and vehicles,

information display panels and others [7, 9]. In principle,

the reversible intercalation-deintercalation processes of

small ions occur in WO3 bulk crystals. It shows a very

complex polymorphism [ 10-14], strongly dependent also

*) Author for correspondence: E-mail: Strangi@fis.unical.it

on electronic impurities, grain size distribution, thermal

and mechanical history [15], but all the various phases

can be considered as successive deformations, associated to

phonon condensation [16], of a simple cubic structure of

ReO 3 type, based on corner-sharing octahedral units WO6;

the guest cations can enter in available interstitial sites

between such octahedra.

On the other hand, the high efficiency of intercalation

for small ions, needed for practical applications, has been

achieved in thin films [17, 18] deposited by using several

different techniques [19-23] like r.f. magnetron sputtering,

evaporation, sol-gel synthesis. In these films the basic

units are still the WO6 octahedra, but they can be arranged

in disordered configurations of three-, four- and six-mem-

bered rings, leaving many empty spaces [24, 25], appre-

276 Ionics 5 (1999)

ciably larger than the interstitial sites in the bulk crystal.

A refined study [26], aimed to model the radial distri-

bution function (RDF) given by X-ray measurements,

suggests a dominant six-membered rings configuration,

making these "amorphous" films quite similar to the

crystals with hexagonal symmetry. Obviously the real

symmetry is expected to depend on the deposition tech-

nique and on the specific parameters used during the film

preparation, but a preferred hexagonal distribution seems

to be frequent in the tungsten trioxide films. Another

ubiquitous character of these films is the presence of water

in remarkable amount. In fact, the open disordered struc-

ture allows for the intercalation of water molecules during

the process of film deposition, and this fact seems to be

correlated to the high values of proton conductivity [25,

27-29] and even'to Li + ion conductivity [30].

Moreover, the study of the stoichiometry of the de-

posited tungsten trioxide films reveals an uptake of the

water molecules during storage after film production

[31]:

WO3.0H0.3 + aH20 (1)

Ion beam analysis was used to investigate the presence of

hydrogen contained in the tungsten oxide films at different

times after production, obtaining significantly different

profiles [31 ]. This increase indicates that the film takes up

hydrogen-containing species during storage with a ratio

H/W = 0.3 three hours after production.

Dehydration reactions and structural transformations of

the films, strongly affecting the ionic conductivity, can be

obtained by thermal annealing [32, 33]. In general, high

temperature annealing of the WO 3 samples lead to a cry-

stalline structure belonging to the standard sequence [16]

derived from cubic ReO3 structure. The temperature value

necessary to this transition depends in some amount on

the bulk or thin film character of the sample and on the

deposition technique; its variations are reported between

300 ~ for evaporated films [32] and 500 ~ for films

prepared by the sol-gel process [22]. For lower tempe-

ratures intermediate steps are observed on the way toward

crystallization and structural collapse, in particular the

elimination of water molecules trapped in the film struc-

ture [32] . This process occurs over a wide range of

temperature, reflecting the different trapping potential for

such molecutes; in principle, the deposition methods and

the thermal history are relevant. For this reason, specific

structural, spectroscopic (IR and Raman) and electro-

chemical measurements are quite useful for any set of film

samples.

Beside the amorphous or crystalline structure, the sur-

face characters on a larger scale are very important to

determine the interactions of WO 3 thin films with mole-

cular compounds outside. The micromorphologic study of

the tungsten trioxide films shows the presence of a

columnar structure in which the inclination of the co-

lumns strongly depends on the deposition geometry [34].

This fact suggests that interesting effects would derive

from the combination of such kinds of oxide film surfaces

with a liquid crystal (LC) layer, following the specu-

lations proposed in previous works dealing with similar

oxides having columnar surface topography [34,35].

However, to our knowledge, no study has been made on

the static anchoring of LC molecules on tungsten oxide

films, and no device has been made to explore the

correlation between the properties of mixed ionic-

electronic conduction of the WO3 and the electro-optic

response with LC, before the experiments described in our

previous works [36, 37].

The aim of the present work is to explore the static

anchoring interaction of the nematic liquid crystal on the

sputtered thin film of WO3, as well as the interplay

between the electrochemistry of the ionic-electronic solid

conductor and the reorientational dynamics of the LC

layer. Therefore, the electrooptical response of several

cells has been studied in different ways. Beside the

standard measurements of the white light transmission

between crossed polarizers under the action of a square

wave applied voltage, more detailed investigations have

been made by using a specific experimental set-up for the

study of the Frtdericksz transition [38].

In addition, the impedances and the currents of the LC

cells containing WO3 have been investigated (complex

impedance plots and cyclic voltammetry). To highlight

the role of the WO3 film structure and the water content

on the electrochemical properties of the entire cell, a com-

parative investigation was carried out, including also cells

in which the WO3-coated electrodes were purposedly

annealed via heat-treatments at different temperatures.

2. Experimental Description 2.1. Sample Preparation. Thin WO 3 films were deposited

by r.f. sputtering in a reactive atmosphere of Ar and

02 gas. The flow rates were 1.2 scem for oxygen and 12

Ionics 5 (1999) 277

sccm for argon; the ratio oxygen/argon was maintained at

a value of 10% during deposition. Sputtering parameters

varied for the r.f. power between 100 and 250 W and for

the deposition times between 10 and 30 minutes, in order

to obtain a film thickness of some hundred nm. A cathode

target of 100 mm diameter was used; a distance of 80 mm

separates the target and the substrate, which consisted of

glass covered by an ITO (In203:Sn) conducting layer

(provided by Donnelly Corp.). The sheet resistance of ITO

layer was 20 D)Q. The samples were made using different

deposition times for tungsten trioxide to investigate the

influence of the film thickness on the electro-optical res-

ponse of the liquid crystal.

The heating treatment on the film used for LC cells

has been carried out in air, by using a standard laboratory

oven (Carbolite' Furnaces, Mod. RMF14/3). The sample

labeled A has been heated up to 260 ~ starting from

room temperature (RT), with a rate of 10 ~ up to

180 ~ and a slower rate, 5 ~ between 180 and 260

~ After 10 minutes at the maximum temperature, the

sample was cooled down with a rate of about 10 ~

to RT. The sample B was heated with a rate of 6-7

~ up to 250 ~ remained at such temperature for 15

minutes and underwent a further heating step up to 303 ~

with a rate of about 3 ~ After staying for 5 minutes

at the maximum temperature, the sample was cooled

down with the same rate of the former sample, 10

~

Both the electric and electro-optical characterizations of

the purposedly heat-treated glass plates coated by WO 3

have been carried out some days after the thermal anneal-

ing. In any case, they were kept in a sealed envelope

within a container filled with desiccant silica gel to avoid

water absorption.

The ITO transparent electrodes, without any surface

treatment to induce preliminary alignment of the liquid

crystal, were enclosed in the standard sandwich con-

figuration (see Fig. 1). The thickness of the cells was

ensured by stripes of Mylar (8-36 lxm). The introduction

of the liquid crystal in the space enclosed between the

asymmetric glass plates was made slowly to prevent any

orientational alignment induced by the flow. The nematic

liquid crystal inserted in the cell is known commercially

as BL001 by Merck (former E7); it is an eutectic mixture

of four different cyanobiphenyls compounds. The tem-

perature range of existence for the nematic mesophase is

extendeds from 20 ~ up to 61 ~ and the dielectric

anisotropy Ae = eli - e.L = + 13.8 (at 20 ~ being ell

and E.L the dielectric permittivities parallel and perpen-

dicular to the molecular director, respectively.

2.2. Experimental Measurements. The XRD characte-

rization of the sputtered film has been made by an x-ray

diffractometer Siemens D5000, using the Cu K-ct line,

and the electron diffraction pattern has been obtained by a

JEOL microscope. The Raman spectra have been collected"

by a Lab-Ram spectrometer with a confocal microscope

and a CCD detector. The exciting light was the 633 nm

line of a He-Ne laser, kept at low power (15 mW).

Observations of the electro-optical response of the

cells, between crossed polarizers, were made by a pola-

rizing microscope Axioskop Pol (Zeiss). Videomicro-

scopy was performed by a 3CCD color camera TCM 112

(GDS Elettronica) connected to a PC equipped to visualize

and to store the images of the samples.

The investigation of the transmitted light intensities

was carried out by a large area silicon photodiode (Hama-

matsu) mounted on the polarizing microscope. The photo-

current was collected by a digital oscilloscope (Tektronics,

Mod. TDS 784).

Moreover, to better characterize the polarity sensitive

effect, investigations were performed to examine the vol-

tage thresholds of the Frtdericksz transition. A special

equipment was arranged for this purpose (see Fig. 2), a

Fig. 1. Schematic drawing of the liquid crystal cell.

Fig. 2. Experimental set-up to study the Frtdericksz transition. L: He-Ne laser, P: polarizer, M: Mirror, O: Oven, S: Sample, A: Analyzer, PD: Photodiode, OA: Amplifier, WG: Waveform generator, DM: Digital multimeter, PC: computer.

278 Ionics 5 (1999)

polarized He-Ne laser beam was directed normal to the

boundary surfaces of the sample. The light transmitted

between crossed polarizers was detected by a silicon

photodiode and the signal was sent to a digital 3478A

multimeter (Hewlett Packard). An electric field, coming

from a waveform generator 8904A (Hewlett Packard) was

applied to the cell, perpendicular to the boundary glass

plates. Both waveform generator and digital multimeter

were connected to a PC.

For the comparative electrical measurements on liquid

crystal cells containing as-deposited and annealed elec-

trodes, glass plates coated by WO3 were chosen in such a

way to have sputtering parameters quite similar to each

other, so that chemical and structural characters are quite

comparable.

A preliminary electrochemical characterization of both

the as-deposited and the annealed samples was carried out

by a Potentiostat/Galvanostat/Impedance-meter EG&G

Mod. 273A.

The impedance of the samples was investigated in the

frequency range 1 mHz to 100 kHz. The amplitude of

the applied voltage was chosen to be 100 mV so to

avoid undesired electrically induced reorientation of the

liquid crystal.

Cyclic voltammetry measurements were also carried

out to complete the comparative analysis of the electro-

chemical behavior. The interval of the applied voltage was

-2 to 2 V and the scan rate was set to 50 mV/sec.

3. Resul ts and Discuss ion

3.1. WO 3 Films Characterization. The tungsten trioxide

films used in this work inside a LC cell have been

characterized with the usual techniques for structural de.

termination. Standard X-ray diffraction does not reveal any

crystalline symmetry in the as-deposited films, while

typical crystal patterns appear in XRD control measure-

ments made on the same films after high temperature

treatments (2 h at 400 ~ A different picture is given by

electron diffraction on the as-deposited films; as it can be

seen in Fig. 3, different regions of the same sample

generate hexagonal crystal-like diffraction patterns (Fig.

3a) or typical halos of amorphous structures (Fig. 3c). In

some region a coexistence of the two patterns is observed

(Fig. 3b). To get an additional information on the struc-

tural order in the films, some Raman measurements have

been carried out by using a confocal microscope. A com-

parison is possible between the as-deposited sample

Fig. 3. Electron diffraction patterns in different regions of a WO 3 film, as deposited, a) hexagonal pattern, b) mixed amorphous halos and cristalline diffraction spot, c) amor- phous halos

Ionics 5 (1999) 279

,=

_ . s ( a )

200 u I I I I I i j

400 600 800 1 00

Raman shift (cm -1)

(b) WO 3 annealed film

I I i I , i I

201 400 600 800 1000

Raman shift (era -1) Fig. 4. Raman spectra at room temperature of tungsten trioxide film on ITO substrate, deposited following the same relevant sputtering parameters, a) as deposited- sample, b) sample thermally annealed up to 305 ~ with a time duration of all the annealing process of about 3 hours.

(see Fig. 4a) and the film annealed up to 305 ~ using

heating rates and treatment periods similar to those of the

film samples inserted in the LC cells (see Fig. 4b). The

main character of the spectra is not changed after the

annealing: all these samples reveal the typical spectrum of

"amorphous" r.f. sputtered films [39], and no appreciable

indication of a transition toward the "cubic" crystal phase

appears for the annealed samples; the only remarkable and

interesting change concern the highest frequency band,

shifted from about 950 cm -1 up to 965 cm -1 in the ther-

mally treated film, and appreciably broadened. It is typical

of the terminal W-O bond [25] and is also observed in the

hydrated crystals [40] and films [17]. The observed trans-

formation after the above said annealing treatment suggest

a more inhomogeneous environment for the "inner sur-

faces", due to the removal of water molecules, but, on the

other side, it reveals that no destruction of the terminal

Fig. 5. Typical AFM images of the WO 3 layer surface. A square of 5 p.m on an edge is shown, a) Top view. b) Iso- metric view.

bonds and structural compaction occurs, as it would be

expected after crystallization of the film into a crystal

phase belonging to the "standard" series derived from a

cubic ReO3 structure. In conclusion, the WO3 films used

as electrode in LC cells have similar structural character as

the ones commonly reported in the existing literature used

for electrochromic devices.

A columnar character of the surface has been observed

by AFM scanning, on a submicron scale, as it is shown

in Fig. 5. Moreover, the sputtered films of tungsten

trioxide seem to be not homogeneous on mesoscopic

scale: domains are observed on the surface of the WO3

280 Ionics 5 (1999)

films by using polarimetric techniques. An optical aniso-

tropy is present in these films, related to the sputtering

geometry, and fluctuation of transmittance occurs on the

scale of hundred micrometers (in fact, a change of 90 of

the orientation of the ITO coated glass in the sputtering

chamber induced a rotation of the preferred direction of

optical transmissivity modulation).

3.2. Anchoring of the Liquid Crystals. The video-

microscopic observation of the cells, prepared as above

described, shows that the nematic liquid crystal molecules

are organized in macro-regions (some hundred of microns

wide) where it is possible to define a common orientation

parallel to the plates (planar alignment) and these macro-

regions are related to the surface domains of the under-

lying oxide film. A reason for this correlation between

liquid crystal alignment and surface topography of the

electrode can be the columnar character of the tungsten

trioxide surface; it is noteworthy that the columns and

voids revealed by the AFM measurement shown in Fig. 5

occur on a scale much smaller than the observed trans-

missivity modulation. In any case, the molecular orien-

tations in these macro-regions have a maximum deviation

of about 10 degree from the mean direction determined by

the sputtering geometry. With regard to the alignment of

the LC on the ITO electrode, it can be supposed also

planar, on the basis of many observations of similar be-

haviour of the same LC mixtures on ITO untreated

surfaces.

3.3. Electro-optical Response of the LC Cells. Figure 6

presents the images acquired during the application of a

low frequency square-wave electric signal to the cell con-

taining the as-deposited WO3 film. The bright state (Fig.

6a) corresponds to the anodic charge of the WO3 film

whereas the dark state (Fig. 6b) corresponds to the catho-

dic charge (the electrode not coated by WO 3 film was al-

ways connected to the ground).

A more detailed characterization of the electro-optical

response is shown in Fig. 7. In such figures the polarity

sensitive response of the as-deposited sample (Fig. 7a) are

presented compared with the usual quadratic response of a

customary BL001 LC cell (Fig. 7b) confined by electrodes

of purely electronic conductors (ITO). The optical

response of the as-deposited sample is plotted along with

the exciting field; two distinct states are observed with an

applied voltage of amplitude 1.2 V, in a wide frequency

range. In the range of low frequency square waves applied

to the cell, the electro-optical response appears roughly as

a rectified square wave, while at increasing applied field

3 - m ( a )

2

| _ w

!i

.~ ]fical response

-2.

-3 �9 ~

-4 -2

E : I

0 2

Time (sec)

4 , ,

2--~"f : f ; ~ . . . . " 7 7 " ~ i r ' ,

l : ! j :t L,'~j,' t d . . . . . . . . . . . . . . . . . . . . t_. ...........

- 2 - i '

I

-4 t ; -2 -1 0 I

T i m e ( sec )

. . ~ 20

1.5

1.0

0.5

070 / "

068

0.66 ~

064

062

Fig. 6. Video-microscope images between crossed pola- rizers relative to the optical switching induced by a low frequency voltage, a) Anodic polarization of the WO 3 film. b) Cathodic polarization of the WO 3 film.

Fig. 7. The polarity sensitive electro-optical response of the presented cell (a) is compared with the usual quadratic response of a customary LC cell (b) confined with electrodes of purely electronic conductors like ITO.

Ionics 5 (1999) 281

frequency (more than 100 Hz) the wave shape of the

photodiode signal approaches a triangular pattern [37].

However, it is important to note that in all the explored

frequency range the electro-optical response occurs with

the same periodicity of the applied field, while in the

usual cells with similar nematic liquid crystal, confined

by purely electronic conductors the response is doubled in

frequency with respect to the stimulus.

A better understanding of the electro-optical response

of the LC cells investigated in this work requires some

brief discussion within the framework of the theory of

Fr&lericksz transition in planarly aligned nematic liquid

crystals.

Following that theory it is possible to observe the

Electrically Controlled Birefringence (EBC) effect in the

case of planar alignment, depending on the value of di-

electric anisotropy of the LC molecules (when Ae > 0)

[411.

Let us suppose that the polarization vector of the in-

cident beam is oriented at an angle (~o with respect to the

director of the nematic liquid crystal initially parallel to

the glass plates of the cell. The analyzer is crossed with a

polarizer. In its initial position the nematic liquid crystal

layers manifest birefringence An = n E - no, ne and no

being the refractive indices for the extraordinary ray and

ordinary ray, respectively.

If the external field exceeds a certain threshold value

[38, 41]:

Eth = rc (4~Kl l / IAEI) 1/2 (1)

a change in the phase difference A~) between the extra-

ordinary and ordinary ray for monochromatic light of

wavelength ~, appears. Here Kll represents the splay

elastic constant of the material.

The intensity of light, passing through the cell, de-

pends on the angle ~o between the polarization vector of

the incident beam and the initial orientation of the director

of the nematic liquid crystal, i.e.:

I = I o sin 2 2r sin 2 (AO/2) (2)

where I o is the intensity of the planar polarized light

incident on the cell.

A change in the phase difference A~ results in an

oscillatory dependence of the optical signal at the exit of

the analyzer.

1 0 - I

~ 7 -

6. "0

v :~ 5.

0 1

r "-x -I "--- 2SY ) ~,\il

f b ) S ,~ a~c charge

l ii �9 1

; I

2 3 4

Applied voltage (Volts)

Fig. 8. Transmitted light vs. sweep voltage show a different Frrdericksz transition threshold for the anodic and for the cathodic polarization.

The maximum amplitude of these oscillations corres-

ponds to an angle ~o = 45~ and the maximum possible

number of oscillations (i.e., the number of maximum

Nmax during a complete reorientation of director) is

approximately:

Nmax = (ne - no )d / ~ (3)

Figure 8 shows the Frrdericksz transition measurements

for both the anodic and the cathodic charges of the

tungsten trioxide film. The transmitted light is reported

versus the external applied voltage. As can be seen, the

Fredericksz transition threshold appears to be different in

the two cases. In the cathodic charge case the observed

threshold value corresponds to about 1 V, whereas in the

anodic charge one its value is about 1.8 V. Moreover, the

oscillatory dependence of the optical signal exhibits a

different behavior in the two cases. In the cathodic charge

case it is quite similar to that observed in usual liquid

crystal cells [38] but rather different in the anodic charge

case especially for high applied electric fields.

These last results seem to confirm the hypothesis

made in previous works [36, 37] about the establishment

of an internal electric field during the anodic charge of the

inserted electrochromic film. Moreover, the internal elec-

tric field should be dependent from the external applied

electric field. The explanation of such peculiar behavior

must be searched in the interaction between the charge

distribution in the cell, determined by the ionic diffusion

of the WO3 film, and the orientational configuration of

the nematic liquid crystal.

282 Ionics 5 (1999)

3.4. Electrical Measurements on the LC Cells. The 20M

established internal electric field can be associated to a

diffusion process of protons, coming from the amorphous

film and migrating toward the ITO electrode during the 1 ~M

anodic phase. To gain a deeper comprehension of the role

played by the adsorbed water molecules in the ionic ~ ~OM

conductivity of the protons, which are responsible of the •

polarity sensitive optical response, a comparative investi- 5O 0 Ok - - - -

gation has been carried out on several cells, in which one

of the glass plates was coated by a sputtered WO 3 thin

film having different structures or water content, as a con-

sequence of different heat treatments. Different experi-

mental techniques (complex impedance measurements and

cyclic voltammetry) have been used for such LC cells.

The thermal annealing of the arbitrarily chosen glasses

was carded out'to expel the adsorbed water molecules and

to study the electrochemical process at the boundary

surface of the LC cells, related to the established internal

electric field, depend on the hydrogen content of the films.

The analysis of the Nyquist diagrams (see Fig. 9) has

been performed both on the cells containing as-deposited

WO3 films (Fig. 9a) and in those with thermally annealed

tungsten oxide films (Figs. 9b and 9c); as it is pointed

out in the figures, the ionic diffusion process becomes

dominant in the plot for frequencies becoming more and

more lower, when passing from the cell with as-deposited 1400M-

film to the one containing annealed films. 120.OM . . . . .

The almost linear dependence between X = Im(Z) and

R = Re(Z) (Z being the impedance of the cell) indicates lOOOM-

that a diffusion process appears well defined in the former ~" 8OOM-

case (starting frequency is approximately 0.5 Hz), while ;K

in the latter cases, a more complex relationship between

X and R can be approximated to a straight line for a

lower frequency value, i.e. about 20 mHz. It is also

important to note that the diameters of the semicircle in

the Nyquist plots are quite different. In fact, the diameter

of the semicircle is related to the resistance of the cell and

it is remarkably increased in the cell labeled B (Fig. 9c).

The water molecules, desorbed after the heating of the

surface, do not contribute anymore to the diffusion

process of protons toward the liquid crystal layer; therefore

the observed behavior is quite reasonable.

But the picture becomes clearest analyzing the cyclic

voltammetry measurements performed on the same sam-

pies. Taking into account that the sweeping rate is main-

tained constant during all the measurements, the total

O0 O0

J iiiiii _i I

i I (a)

freq.= 0.5 H z "

500 Ok 10M

R(f2) 1 5M

IO.OM �9 - - - - - - ~ - -

0.0

0 .0 5 0 M IO.OM 15.0M 2 0 . 0 ~

RCf2)

60.OM -

40.OM -

20.OM -

O 0 A

0 .0

1

(c)

I

l r

F•e-l= 6 m H z

1. w v

; I i 20.OM ,40.OM 60.OM

T

W

I ,

80.0M 100.OM 120.OM

R(~)

2 0 M

25 .0M

140.0M

Fig. 9. Nyquist diagrams relative to the impedance mea- surement of the as deposited sample (a); sample A : maxi- mum annealing temperature T = 260 ~ (b); sample B: max. annealing temperature T = 305 ~ (c).

transferred charge is represented by the area enclosed in the

cycle.

Ionics 5 (1999) 283

150.0n

100 0

5O 0

r j -50.On

-100 On i

-150.0n

-2

(a)

J / /

Applied voltage(Volts)

transferred charge is considerably reduced. Therefore it is

evident that more relevant structural and chemical trans-

formations for the protonic conductivity occur between

260 ~ and 300 ~

This current analysis on the cells containing WO3

films undergoing different treatments support the hypo-

thesis that an important diffusive process in the as-

deposited film plays a specific role for the creation of an

interfacial double layer by charge separation. This charge

separation is responsible for the internal electric field

establishment which modifies unipolarly the effective

electric field acting on the LC layer.

150 On

100 On

50 On

o.o

-50 On

-100.On

-150.0n

-2 -1

/ Applied voltage (Volts)

lO0.On

50 On

0o

-50 On

(c)

f

-100 On -2 -1

Z

I

I

II Applied voltage (Volts)

Fig. 10. Cyclic voltammetry of the as-deposited sample (a); 260 ~ annealed sample A (b); 305 ~ annealed sample B (c).

By comparing the plot of the measurement relative to the

sample without heat-treatment (Fig. 10a) with that rela-

tive to the annealed sample (Fig. 10c), it appears well

evident that in the case of annealed film above 300 ~ the

4. Conclusions

The main result of the present work is the unusual po-

larity sensitive electro-optical response for a nematic

liquid crystal device. The innovation consists in the

asymmetric confinement of the liquid-crystalline material

by insertion of an amorphous tungsten trioxide film that

is covering one of the boundary surfaces. The inserted

film is a material with mixed conduction properties

(electronic and ionic). In fact a large series of studies

conducted on these materials reveals that some amount of

hydrogen-containing species enters these films during the

sputtering deposition or after deposition, because of the

exposition to wet air.

The analysis of the measurements reveals an important

electrochemical process associated to the presence of OH

and O1-/2 groups in WO 3 films exposed to wet air. More

precisely, during the anodic charge of the WO3 thin film a

H § diffusion process occurs [42]. The consequence of this

displacement of charge is a reverse internal electric field

which counteracts unipolarly the external perturbation; in

such a way a greater modulus of the external applied field

is needed to reach the Fredericksz transition threshold. On

the contrary for the cathodic charge the same applied

voltage is sufficient to trigger the optical switching.

The ionic conductivity properties of the inserted films

play an important role in the rectifying behavior of the

optical response of the LC cells, because of a diffusion-

controlled charge redistributionprocess, which is mainly

responsible of the asymmetric effective electric field.

Experiments on asymmetric cells containing thermally

annealed WO 3 films demonstrate that the important

process involved in the considered device is due to the

protons, whose conductivity is strongly enhanced by the

water content of the films.

284 Ionics 5 (1999)

Further studies are planned with regard to different

cationic species, having also different initial concentra-

tions with the aim to develop an original electrooptical

device.

5. Acknowledgements The authors would like to thank the firm ISA Italia for

kindly providing the LABRam spectrometer used for the

Raman measurements. Another special acknowledgement

is reserved to Cristina Armellini, of CEFSA-CNR at

Trento, for her assistance in the thermal treatment of the

films.

Finally, the authors are indebted to Prof. L. M. Blinov

for the fruitful discussion and the helpful suggestions.

The work has been carried out in the framework of the

ORCHIS TMR RESEARCH NETWORK (Contract

Number: ERBFMRXCT97-0119).

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Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12-19, 1999.

Manuscript rec. Sept. 14, 1999; acc. Oct. 20, 1999.