Effect of ZnO nanoparticles on the morphology

Journal of Molecular Liquids 250 (2018) 381–387

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Effect of ZnO nanoparticles on the morphology, dielectric, electro-optic and photo luminescence properties of a confined ferroelectric liquid crystal material Divya Jayoti a, Praveen Malik a,⁎, S. Krishna Prasad b a b

Liquid Crystal Lab, Dr B R Ambedkar National Institute of Technology, Jalandhar 144011, India Centre for Nano and Soft Matter Research, Jalahalli, Bangalore 560013, India

a r t i c l e

i n f o

Article history: Received 25 September 2017 Received in revised form 27 November 2017 Accepted 8 December 2017 Available online 10 December 2017 Keywords: Polymer dispersed ferroelectric liquid crystal ZnO-NPs Morphology Dielectric relaxation Spontaneous polarization Photoluminescence spectroscopy

a b s t r a c t The influence of spherical zinc oxide nanoparticles (ZnO-NPs) incorporated in low concentrations (0.45, 0.7, 1.0 wt/wt%) into a polymer confined ferroelectric liquid crystal (FLC) has been investigated. Varying the concentration of ZnO-NPs is found to have a profound impact on the morphology of the polymer dispersed ferroelectric liquid crystal (PDFLC) composites. With increasing ZnO-NP content, the real and imaginary parts of the permittivity and the dielectric strength of a relevant relaxation mode depict an increase. However, the associated relaxation frequency shifts to lower values; a concomitant increase in spontaneous polarization is also observed. The response time of the composites slightly improved on doping with the ZnO-NPs. The changes in electro-optic and dielectric parameters are explained in terms of change in elastic energy as well as surface morphology of the composites. Interestingly, the polymer/liquid crystal environment is also seen to enhance the photoluminescence response of confined FLC. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Confined liquid crystal (LC) composites have been a subject of keen interest from both fundamental as well as promising applications point of view. Among these, polymer dispersed liquid crystals (PDLCs) based on nematic LCs have been studied extensively due to their interesting electro-optical effects, LC droplet configurations, droplet orientation in electric field and dielectric behavior [1–8]. The control of LC molecular alignment inside the droplets using external electric fields is the basis of working and applications of PDLCs. Employing low to moderate electric fields, interesting electro-optic behavior can be realized in the PDLC films. In the absence of external electric field, the LC molecules are randomly oriented inside the droplets and hence there is strong scattering of incident light between the neighbouring droplets and inside the droplets which makes the film appear translucent. However, in the presence of electric field, a decrease in the scattering occurs and the PDLC film becomes transparent. The transparency of PDLC film is caused by the unidirectional alignment of the LC inside the droplets and the matching of ordinary refractive index of the LC (no) with the polymer refractive index (np) i.e. no = np. The light scattering behavior of PDLCs in the presence of external electric field give rise to potential ⁎ Corresponding author. E-mail address: [email protected] (P. Malik).

https://doi.org/10.1016/j.molliq.2017.12.035 0167-7322/© 2017 Elsevier B.V. All rights reserved.

applications in holographic films, large flexible displays, privacy windows, shutters and sensors [9]. Despite of various advantages, in addition to being polarizer-free and non-requirement for surface pretreatment, PDLCs suffer from a major shortcoming i.e. high threshold voltage, low contrast. In order to overcome these issues, the PDLC composites are doped with nanoparticles/microparticles which affect the reorientation processes around the LC droplets leading to a decreased threshold voltage [10,11]. The introduction of nanoparticles (NPs) into the PDLCs changed the transmission [12], threshold voltage and the reorientation processes. LC droplet reorientations by incorporating alumina NPs was proposed by a two mode model i.e. the surface interaction with the polymer and due to the bulk LC [10]. In another work, gold NPs were found to enhance the transmission by almost 20% of a conventional PDLC [13]. Yaroshchuk et al. investigated metal oxide NPs (SiO2, TiO2, Sb2O5) basic PDLC and polymer stabilized liquid crystals and it was concluded that the NPs are mainly engaged with the polymer phase during PDLC formation by phase separation and alter the absorption coefficient, refractive index, and optical homogeneity of the polymer matrix [14]. Confining the FLC into an isotropic polymer matrix makes polymer dispersed ferroelectric liquid crystals (PDFLCs). Unlike the nematic PDLCs wherein the LC orients inside the droplets on the application of electric field, the PDFLC composites involve a different switching mechanism. The principle of switching of a PDFLC involves the unwinding of the helix in the SmC* phase [15]. The structure of the FLC is expected to

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change near the FLC polymer walls depending upon the anchoring conditions. Owing to the conditions under which the cavities are created during the polymerization process, their dimensions are not uniform, and if smaller than the pitch of the FLC, the helix may get distorted. All these features are known to influence the static and dynamic response of the FLC confined in the polymer matrix. It has been show that the switching time of the FLC in cavities is faster due to the enhanced rotational mobility as well as the soft anchoring behavior at the cavity walls [16]. In order to understand the role of various types of NPs in FLCs, a lot of research has been performed and it is inferred that NPs play a key role in modifying the electro-optic and physical properties of the host FLCs [17–21]. Very limited attention has been given to the influence of NPs on the physical properties of PDFLC composites although there are studies mentioning the use of nanoparticles with nematic LC based PDLCs [22–27]. In this study, ZnO-NPs are chosen to be the dopant because they possess a wide band gap of 3.37 eV which make them an attractive material for short wavelength opto-electronic applications and LC based laser applications. ZnO-NPs also show piezoelectric property which find potential applications in sensors and actuators. In addition to this, it was seen that ZnO-NP doping into pristine FLC resulted in the improved electro-optic behavior due to the enhanced anchoring of FLC molecules surrounding the ZnO-NPs [28]. This motivates us to further to extend the study to the PDFLC composites. The present work explores the influence of the incorporated ZnO-NPs on the morphology of the FLCpolymer dispersions, dielectric relaxation spectroscopy, spontaneous polarization (Ps) and switching response and photoluminescence response. These investigations are also among the first in which PDFLC is doped with a metal oxide NP. 2. Experimental 2.1. Preparation of the ZnO-PDFLC composites The FLC used in this study was W206E (Awat. Co. Poland) with a 87:6 °C

92:5 °C

97:6 °C

phase sequence SmC ↔ SmA ↔ N ↔ I, and having a saturated (or well in the SmC* phase) spontaneous polarization of 15 nC/cm2, tilt angle ~25.2° and rotational viscosity ~172 kg/ms. The monomer NOA65 (Norland, NJ) was used with the FLC to make the required PDFLC film. The physical properties of NOA65 have been given in previous works [29,30]. As the nano component, spherical ZnO-NPs of size b 50 nm (Sigma-Aldrich) was used. The refractive index of the cured NOA65 and W206E is 1.524 and 1.514 respectively. Composites with four different ZnO-NP concentrations were prepared, the details of which are given in Table 1. The optimized quantities of the three components i.e. LC, ZnO NPs and polymer as shown in Table 1 were mixed together in order to get the homogeneous mixture of three components and homogenous composites were obtained by mechanically mixing the components on a hot stage (Mettler, FP82 HT), maintained at a temperature of 100 °C. The prepared composites were then filled, by capillary action, into planer aligned indium tin oxide (ITO) coated cells (thickness-5 μm and active area-25 mm2 from Instec, USA). The polymerization of the monomer was carried out by irradiation for 2 h with UV light (peak wavelength ~365 nm and intensity 10 mW/cm2).

Table 1 The composition of the PDFLC composites studied. Sample

FLC (wt%) W-206E

Polymer (wt%) NOA65

ZnO NPs (wt%) ZnO

1 2 3 4

60 59.55 59.3 59

40 40 40 40

0 0.45 0.7 1

2.2. Physical measurements The filled LC cells were heated to the isotropic phase and cooled at a uniform rate of 0.5 °C/min to encourage the uniform dispersion of FLC inside the polymer matrix. The optical micrographs were obtained using Linksys software connected to a polarizing optical microscope (Nikon LV100POL, Japan) fitted with a CCD camera (Q28378). The dielectric studies were performed using an impedance analyzer (HP4194A) over the frequency range of 100 Hz to 100 kHz. The spontaneous polarization (Ps) was determined using the standard polarization current reversal technique [31]. A triangular wave (20 Hz) from a function generator (AFG3021B, Tektronix, USA) in conjunction with an broadband linear amplifier (A400, FLCE, Sweden) was applied to the LC cell samples and output recorded on a digital storage oscilloscope (TDS2024B,Tekronix, USA). The detailed procedure is given elsewhere [32]. For switching time measurement, an amplified square wave was applied to the PDFLC samples and output was recorded in digital storage oscilloscope. The photophysical studies were performed using a spectroflourometer (Flurolog-3, Horiba JobinYvon, Model – FL-1039/ 40). 3. Results and discussion 3.1. Morphology The droplet morphology of the PDLC composites is an important parameter which decides the resulting optical and dielectric properties of the composites. The droplet morphology is dependent on a number of factors such as rate of polymer curing, relative amount of polymer and LC, the UV curing intensity in case of photo polymerization, curing time, curing temperature and polymer viscosity [33–35]. A typical evolution of NP-PDFLC morphology is shown in Fig. 1. The micro textures of the PDFLC (undoped) and ZnO NP-PDFLC samples at 30 °C are shown in Fig. 2(a–d). The polarizing optical microscopy studies unveil that in comparison to the undoped PDFLC composites the droplet morphology is quite distinct in the ZnO NP-PDFLC. In case of the undoped PDFLC, the FLC droplets are ellipsoidal and closely packed inside the polymer matrix (see Fig. 2a). In contrast, the FLC cavities are elongated and nebulous in the ZnO NPs-PDFLC as can be seen in Fig. 2(b–d). Such morphological features were also observed in the ZnO quantum dots dispersed in PDLC [36]. Here, inside the droplets, the majority of FLC molecules are oriented non uniformly while some are oriented along the alignment direction. This was confirmed by applying electric field to the composites and it was found that complete extinction was not obtained inside the droplets which suggest that FLC molecules inside the droplets were not completely oriented along the rubbing direction. As the ZnO-NP content is increased further in the composites (Fig. 2c, d) the FLC attain random structure and the smaller droplets coalesce to form large random domains (Fig. 2d). It is reported that the droplet morphology is dependent on the size and quality of dispersion of NPs inside the PDLC composite. Optimum doping amounts of NPs are found to give better morphology while large amount of nanoparticle loadings cause aggregation and prevention of the formation of a homogeneous film morphology [23]. Presence of NPs inside the polymer matrix may modify the refractive index of the polymer and hence change the scattering behavior of the PDLC film [37]. Here, the LC content and the polymerization conditions were kept constant which indicates that the changes observed in the morphology are due to the presence of ZnO-NPs in the PDFLC. ZnONPs can scatter the incident UV light, thereby changing the effective light reaching the monomer. The nano dopants also provide steric hindrances to the growing polymer chains which can slow down the phase separation process resulting in large droplet domains (Fig. 2d) [14,26]. The phase transition temperatures as observed under the polarization optical microscopy show that the presence of ZnO NP dopants decrease the N*-Iso transition temperature up to 2 °C, attributable to


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Fig. 1. Illustrative diagram to show the evolution of NP doped PDFLC morphology.

the presence of some amount of FLC inside the polymer matrix and viceversa [37]. 3.2. Dielectric relaxation Dielectric spectroscopy is an important tool which gives measure of different physical properties of a polar material like permittivity, conductivity, polarization as a function of temperature and frequency. The dielectric measurements were carried out over the frequency range, 100 Hz–100 kHz in the SmC* phase of the confined FLC, and the real and imaginary parts of the permittivity were fitted to the Havriliak– Negami (HN) relation given as, ε ¼ ε∞ þ h

1þ

Δε α iβ

compared to the broad peak obtained for the undoped PDFLC. Fig. 4(a, b) represent the temperature dependence of the dielectric strength (ΔεG) and the relaxation frequency (fG) of GM respectively. The magnitude of ΔεG (Fig. 4a) increases with an increasing ZnO NP doping concentration of PDFLC. The maximum value of Δ εG was achieved for 1% ZnO NP content in the present work. As per the generalized Landau model, the parameters ΔεG and fG can be expressed as ΔεG ¼

fG ¼

2 1 Ps 2ϵ0 K eff q20 θs

K eff q20 2 πγeff

ð2Þ

ð3Þ

ð1Þ

if fR

where, ε∗ is the complex permittivity at a frequency f. The first term on the right hand side of Eq. (1) is the high frequency permittivity, the second term is the Havriliak–Negami (HN) equation, fR is the relaxation frequency and Δε is the dielectric strength, α and β are the symmetric and asymmetric distribution parameters of the relaxation peak for the mode. Fig. 3(a) and (b) depict, respectively, the frequency dependence of ε′ and ε″ obtained in the SmC* phase at 30 °C. The low frequency (f b 104 Hz) dielectric response of the composites is strongly ZnO-NP concentration dependent while at higher frequencies (f N 104 Hz), the dielectric response is observed to be nearly constant in all composites. As shown in Fig. 3(a), at low frequency (100 Hz), ε′ increases from 8 to 11. A prevalent feature of all composites in the ε″ spectra is the presence of a characteristic relaxation peak of Goldstone mode (GM) which is due to the azimuthal angle fluctuations in the SmC* phase of the FLC. It is also evident from the ε″ spectra that the absorption peak increases from 2 to 4 as the ZnO NPs content is increased from 0% to 1% in small steps. The GM relaxation peak shifts towards the lower frequency as the ZnO NP content is increased in the composites and is sharper

Here, we assume that the above expressions, formulated to the nonPDLC systems, remain valid for the currently studied PDFLC systems with NP dopants. Keff and γeff denote the effective twist elastic constant and GM rotational viscosity of the PDFLC films respectively. θs is the tilt angle. The quantity q0 is expressed as q0 = 2π p , where p is the helical pitch of the FLC which is assumed to be nearly constant in the bulk SmC* phase in the present study. The temperature dependence of dielectric strength of the all the composites (Fig. 4a), can be attributed to the collective contribution of Pθss and Keff. As we shall see in the next section, the Ps of the composites increases as the amount of ZnO NP content is increased. It is worth mentioning here that ΔεG do not change significantly in the PDFLC composites having 0.45 and 0.7 wt% of ZnO-NPs with respect to the undoped PDFLC (see Fig. 4a). However, ΔεG almost doubles in 1% ZnO NP-PDFLC in comparison to the undoped PDFLC (Fig. 4a) and the increase becomes significant as the 1% ZnO NP-PDFLC composite approaches the SmA-SmC* transition. In the systems containing spatially confined FLC molecules like polymer stabilized liquid crystal composited where the polymer content is relatively very low, it has been generally observed that the dielectric strength of the GM decreases substantially as the polymer content/network is increased due

384


Fig. 2. The optical micrographs of (a) 0% (b) 0.45% (c) 0.7% (d) 1% ZnO NP doped PDFLC (10× magnification) samples at 30 °C.

to the increase in the effective elastic energy. In the present study, the increased dielectric strength of the GM (almost doubled) in the 1% ZnO-NP doped PDFLC could be possibly due to the increase in the spontaneous polarization due to the effective coupling of dipole moment of ZnO NPs and FLC and decrease in the effective elastic constant [16]. The temperature dependent behavior of fG for the different composites studied is shown in Fig. 4b. fG increases slightly and then attains a maxima before dropping precipitously near the SmC*-SmA

transition. It is observed that the relaxation frequency decreases in the ZnO NP-PDFLC composites. Plotting the concentration dependence of ΔεG and fG (see Fig. 5a) obtained at a fixed relative temperature of (T = TSmC⁎-SmA − 35 °C)., it is seen that ΔεG and fG change on varying the concentration of ZnO NPs. If we assume that the behavior is entirely controlled by NP concentration, then the magnitude of variation of ΔεG

Fig. 3. Frequency dependence of (a) ε′ and (b) ε″ in the SmC* phase of PDFLC and ZnO NPPDFLC composites.

Fig. 4. Thermal behavior of (a) dielectric strength (b) relaxation frequency in the ZnO NP dispersed PDFLC composites.


385

Fig. 5. Influence of ZnO NP concentration (X) on the dielectric strength (Δε) and relaxation frequency (fG) of the PDFLC composites at a fixed relative temperature (T = TSmC⁎-SmA − 35 °C). The dots are guides to the eyes.

and fG should be same, but of opposite sign. This can also be corroborated by plotting 1 / (ΔεG fG) vs. X (Fig. 5b). The Eqs. (2) and (3) suggest that this product should be independent of Keff which can be realized in Fig. 5b for all the composites except 1.0% ZnO NP-PDFLC composite. All the composites exhibit a linear dependence of relaxation frequency on inverse temperature (1/T) except in the region approaching SmC*-SmA transition (see Fig. 6), the behavior accounted for by the Arrhenius equation. f G ¼ f o exp:

−Ea kB T

ð4Þ

where, Ea is the activation energy, kB is the Boltzman constant, fo being a constant [38]. Ea is evaluated by performing a least square fit of the data in the SmC* phase. The concentration dependence of Ea is depicted in Fig. 6 (inset). The ZnO-NPs environment clearly seems to influencing Ea and it is seen to vary non-monotonically with ZnO-NP concentration with a maximum for 0.45 wt% ZnO-NPs. The composites containing polymers usually have high values of Ea because the relaxation processes involve a larger region. The Arrhenius plots depicted in Fig. 6 shifts to the lower left part of the plot with an increase in the ZnO-NP concentration due to the slowing down of the molecular processes contributing to the Ea. The decrease in Ea (Fig. 6 (inset)) might be due to increased number of dipoles in case of doped (FLC-Polymer-ZnO NP) composites.

Fig. 6. Arrhenius plots for the (a) 0% (b) 0.45% (c) 0.7% (d) 1.0% ZnO NPs doped PDFLC composites. The inset shows dependence of activation energy on the ZnO NP concentration (X %) of the composites. Lines are mere guide to the eyes.

Fig. 7. The spontaneous polarization as function of applied voltage at 25 °C.

3.3. Electro-optical Parameters Fig. 7 shows the Ps as a function of applied voltage for all the composites. The thermal variation of Ps follows an expected behavior in the SmC* phase and vanishes near the SmC*-SmA transition in all the composites. Interestingly, the Ps increases with ZnO NP concentration throughout the thermal range of the SmC* phase. It is possible that the dopant ZnO NPs increases the dipole moment per unit volume inside the smectic domains. An increase in the number of dipoles per unit volume with the increasing ZnO NP content might result in a corresponding enhancement in Ps. The response time on logarithmic scale is shown as a function of applied voltage in Fig. 8. The response time (τ) of a PDFLC is given by γ = τPsE, where γ is the rotational viscosity and E is the applied electric field. Addition of ZnO-NPs accelerates the electro-optic switching by a factor of ~ 5% as compared to the PDFLC.

Fig. 8. The response time as function of applied voltage at 25 °C.

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which results in a non-uniform and nebulous domain like morphology. An increase in the dielectric strength of GM as well as decrease in relaxation frequency was observed which can be attributed to the change in elastic interactions between the FLC and polymer in the presence of ZnO-NPs. The spontaneous polarization and response time were found to be improved in the doped composites. An increase in the PL intensity was also observed after doping with ZnO-NPs but the emission wavelength did not show any noticeable shift. Acknowledgements One of the authors (DJ) would like to thank Director, Center of Nano and Soft Matter Research, Bangalore for giving permission to carry out the work as a visiting student and Marlin Baral (Research Scholar, CENS) for helping with the experimental set ups. DJ would also like to acknowledge the CSIR (9/1127(0002)/2017-EMR-I), New Delhi. The financial support by DST, New Delhi under the research project no. EMR/2016/003540 is acknowledged. References

Fig. 9. Photoluminescence response of (a) PDFLC (b) 1% ZnO doped PDFLC in the SmC* phase. The black lines in the curve show the Gauss fit of PL data. (c) The emission spectra at 30 °C for all the composites. (d) The maximum PL intensity for all the samples in the SmC* phase.

This is due to the faster switching dynamics of the bulk FLC due to reduced elastic forces. 3.4. Photoluminescence spectroscopy In order to tune the PL response of the FLC, dispersion of NPs is suggested by various workers [39,40]. The PL spectra of all the samples were taken in the temperature ranging from 30 °C to 90 °C. The samples were excited at 303 nm and the slit width was kept to be 2 nm. Fig. 9(a & b) shows the temperature dependent PL spectra of PDFLC and 1% ZnONPs doped PDFLC, respectively in the temperature range 30 °C–90 °C. One clearly notices a single prominent PL feature in pure as well as doped composites. The pristine FLC was also found to emit in the similar energy region around 425 nm due to the presence of terphenyl derivatives. Fig. 9(c) depicts the PL spectra for all composites at 40 °C. The PL emission intensities as a function of temperature were determined by fitting the spectra with normalized Gaussians in all of the cases (Fig. 9(d)). In the SmC* phase, the PL intensity reduces as the FLC approaches the SmC*-SmA transition. It is evident from Fig. 9(c & d) that PL intensity is strongly dependent on the amount of ZnO-NP in the PDFLC composite. This twofold increase in the PL intensity can be attributed to large ordered domains in the ZnO NPs doped PDFLC composites [41,42]. The increased ordering of the FLC molecules inside the domains which cause large amount of FLC molecules to absorb light and hence a corresponding enhancement in the PL intensity. For the ZnO-NP doped PDFLC composite, a slight blue shift (Fig. 9(c)) was observed might be due to the quasi confinement of the charge carriers within the irregular FLC domains. 4. Conclusions In the present work, effect of ZnO-NPs on the morphology, dielectric, electro-optic and PL response of PDLC composites based on a FLC has been studied. The polarizing optical microscopy studies reveal rather significant changes in the morphology of the composites after the dispersion of ZnO-NPs. These changes are attributed to the steric hindrances provided by the ZnO-NPs during the phase separation process

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