Influence of CdSe quantum dot on molecular/ionic relaxation phenomenon and change in physical parameters of ferroelectric liquid crystal

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Influence of CdSe quantum dot on molecular/ionicrelaxation phenomenon and change in physicalparameters of ferroelectric liquid crystalD.P. Singhac, S.K. Guptaa, S. Pandeya, T. Vimala, P. Tripathia, M.C. Variabd, S. Kumarb, S.Manohara & R. Manohara

a Liquid Crystal Research Lab, Department of Physics, University of Lucknow, Lucknow, Indiab Soft Condensed Matter Laboratory, Raman Research Institute, Bangalore, Indiac Unité de Dynamique et Structure des Matériaux Moléculaires, EA 4476, Université duLittoral Côte d’Opale, Dunkerque, Franced Department of Chemistry, B V Shah Science College, C U Shah University, Surendranagar,IndiaPublished online: 28 May 2015.

To cite this article: D.P. Singh, S.K. Gupta, S. Pandey, T. Vimal, P. Tripathi, M.C. Varia, S. Kumar, S. Manohar & R. Manohar(2015) Influence of CdSe quantum dot on molecular/ionic relaxation phenomenon and change in physical parameters offerroelectric liquid crystal, Liquid Crystals, 42:8, 1159-1168, DOI: 10.1080/02678292.2015.1031199

To link to this article: http://dx.doi.org/10.1080/02678292.2015.1031199

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Influence of CdSe quantum dot on molecular/ionic relaxation phenomenon and change in physicalparameters of ferroelectric liquid crystal

D.P. Singha,c, S.K. Guptaa, S. Pandeya, T. Vimala, P. Tripathia, M.C. Variab,d, S. Kumarb, S. Manohara

and R. Manohara*aLiquid Crystal Research Lab, Department of Physics, University of Lucknow, Lucknow, India; bSoft Condensed MatterLaboratory, Raman Research Institute, Bangalore, India; cUnité de Dynamique et Structure des Matériaux Moléculaires, EA4476, Université du Littoral Côte d’Opale, Dunkerque, France; dDepartment of Chemistry, B V Shah Science College, C U ShahUniversity, Surendranagar, India

(Received 16 February 2015; accepted 16 March 2015)

Spherical cadmium selenide (CdSe) quantum dots (QDs), capped with octadecylamine, dispersed in ferroelectricliquid crystal (FLC), can remarkably alter the electro-optical (E-O) parameters (material parameters) of the hostcompound. Here we present an E-O, dielectric, surface anchoring and fluorescence study demonstrating that thephysical properties of host FLC strongly depend on the dopant (QD) concentration. The addition of QDs in FLCchanges the surface anchoring of FLC molecules, which results the change in E-O parameters of pristine FLC as afunction of QDs concentration. The ion–polarisation coupling induces a new temperature-dependent weak ionicrelaxation mode (TDWIRM) in FLC–QDs mixture at a certain concentration of QDs. Dipolar coupling betweenCdSe QDs and FLC molecules readjust the dielectric properties and molecular/ionic relaxation phenomenon inthe FLC–QDs mixtures. The fluorescence of FLC–QDs mixtures is probably due to the coupling between theexciton and photon in LC medium, which leads the radiative process. The behaviour of fluorescence property ofFLC–QDs mixtures reveals that the concentration of uniform-sized QDs only changes the fluorescence intensityof the FLC–QDs mixtures.

Keywords: quantum dot; electro-optical property; relaxation mode; anchoring energy

1. Introduction

During the last few decades, the beauty of numerousmesophases differing in molecular ordering and geo-metry has attracted the research community due totheir vital applications in various fields.[1] Among allof them, ferroelectric liquid crystals (FLCs) have pro-ven their importance in electro-optical (E-O) applica-tion and the symmetry-originated ferroelectric natureof this phase has been explored extensively to produceadvanced E-O devices.[2] A significant research workhas also been carried out on various FLC materials sothat they can be used in display devices due to theirfast response of micro-second order.[3]

Recently, semiconducting quantum dots (QDs)have drawn significant attention because of theirsize-dependent electronic and optical properties.[4]The quantum confinement effects associated withthe size of QDs couple with the organic or inorganicmolecules and present a new platform for the technol-ogy. The QDs have proven their proficient candida-ture to be used in light emitting diodes, photovoltaicdevices and biological devices.[5–7]

In last few years, insertion of small concentrationof QDs into LC matrix came into existence as apromising method for regulating the electrical, optical

and other physical properties of pristine LC materials.[8–10] Cadmium (Cd) based QDs (viz. CdSe, CdTe,CdS, etc.) have attracted immense interest of LCcommunity to use them as dopant in LC matrix inorder to present some interesting changes in the prop-erties of pristine LCs.[8–17] Hegmann et al. [8,9]reported the molecular alignment capability of CdSeQDs for nematic LC whereas Gupta et al. [16] inves-tigated the homeotropic alignment of FLC moleculesin the presence of CdSe QDs. Recently, Dradrachet al. [10] have presented the importance of CdSeQDs based hybrid nematic LC for more efficientholographic data storage devices. Singh et al. [11]reported that the presence of CdSe QDs affects themolecular relaxation dynamics of FLC material andcan induce new relaxation mode in the composites.The phenomenon of memory effect and change inE-O parameters in CdTe dispersed FLC systemobtained by Kumar et al. [12] have also attractedtechnologists for using these systems in optical mem-ory devices. The presence of CdSe QDs also changesthe ordering of mesophases. A focused study on thechange in smectic ordering and SmC*–SmA phasetransition has been reported by Nounesis et al. [13].Strong thermal optical nonlinearity in smectic ionic

*Corresponding author. Email: [email protected]

Liquid Crystals, 2015Vol. 42, No. 8, 1159–1168, http://dx.doi.org/10.1080/02678292.2015.1031199

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liquids, tailoring of switching time and E-O para-meters of FLC mixture and reduction of ionic con-taminations of FLC material in the presence of CdSeQDs are few other examples of such systems thatoblige the research society for more investigationson Cd-based QDs dispersed LC systems for otherapplications.[14,15,17]

In contrast to our previous investigation,[11]describing the effect of CdSe QDs on the FLC mate-rial having low spontaneous polarisation value (stan-dard value: 10 nC/cm2 at 25°C) and induction of aconcentration-dependent new relaxation mode near104 Hz in the QDs dispersed FLC system, we reportthe influence of CdSe QDs on the molecular/ionicrelaxation phenomenon of FLC material of highspontaneous polarisation value (standard value:47 nC/cm2 at 25°C). In the present investigation,ion–polarisation coupling leads the origin of a newlow-frequency temperature-dependent weak ionicrelaxation mode. The presence of spherical QDs (adifferent shape of dopant QDs as compared to the rodshape of FLC molecules) in FLC matrix alters theorder parameter of pure FLC material and alsochanges the molecular alignment. Such changes leadto variations in dielectric and E-O parameters of thepure FLC after the dispersion of QDs. The semicon-ducting nature of QDs influences the variation ofconductivity of the FLC–QDs mixtures. The exci-ton–photon coupling in FLC medium results a fluor-escence emission peak near UV-visible interfacewhich suggests the use of present FLC–QDs mixtureto be used as UV-visible filters. The influence of CdSeQDs on the dielectric, electro-optical, surface anchor-ing, fluorescence and conductivity has been investi-gated as a function of temperature and explainedwithin the scope of this article.

2. Experimental details

In the present study, the investigated FLC material isFelix 17/100 purchased from Clariant Chem. Co.Ltd., Germany. The phase transition scheme of FLCmaterial is as follows:

Cryst:�!�20�CSmC��!72

�CSmA�!84

�CN�

�!90�94�CIso:

The spherical-shaped cadmium selenide (CdSe) QDscapped with octadecylamine was taken as dopant inFLC. The diameter of QDs was 3.5 nm. The detailedinformation about QDs has already been provided inRef. [17] and references therein. CdSe QDs have beendispersed in concentration of 1 and 2 wt/wt% ratio inpure FLC. The preparation of FLC–QDs mixtures has

been done by mixing the QDs and FLC in weight ratio.FLC–QDs mixtures were homogenised with the help ofan ultrasonic mixer for 1 h at isotropic temperature toensure the uniform dispersion of QDs in FLC. Thesample cells were prepared by using Indium tin oxide(ITO) coated glass plates. The planar alignment wasobtained by coating with nylon (6/6). The polymerlayer was dried for 3 h at 120°C and then rubbedunidirectional in anti-parallel way. The thickness ofsample cells was maintained by placing the Mylarspacer (5 µm) in between the plates and then all samplecells were sealed with UV sealant. The assembled cellswere first calibrated with benzene and then filled withpure FLC and FLC–QDs mixtures by capillary action10°C above the isotropic temperature of FLC material.The dielectric behaviour of pure FLC and FLC–QDsmixtures as a function of temperature and frequencyhas been studied by using a computer-controlled impe-dance/gain phase analyser (HP-4194A) in the fre-quency range of 100 Hz–10 MHz. The temperature-dependent measurements have been carried out bykeeping the sample cells on a computer-controlledhot-stage (Instec HCS-302) having an accuracy of±0.01 C. Spontaneous polarisation measurement ofpure FLC and FLC–QDs mixtures has been carriedout by the polarisation reversal method. The triangularwave pulses have been applied by using a functiongenerator (Tektronix AFG-3021B). Value of sponta-neous polarisation was obtained by integrating the areaof current bump visualised on oscilloscope (TektronixTDS-2024C). The optical response of pure FLC andFLC–QDs mixtures has been studied by using a He-Nelaser of 633 nm wavelength as optical source and asquare wave electrical signal (20 VPP, 10 Hz). Theoutput was recorded by a photo detector (InstecPD02-L1). The detailed experimental informationregarding dielectric and electro-optical measurementshas already been reported in Ref. [11,18–20] and refer-ences therein. All the dielectric and E-O measurementshave been conducted on planar anchored sample cells.

3. Results and discussion

The presence of QD affects the interactions amongmesogenic molecules. The QD dispersed FLC systemhas been considered as a homogeneous system, whichmacroscopically behaves as an anisotropic fluid withmodified physical properties. The change in physicalproperties of these dispersed systems strongly dependson the nature of both the QDs and LCs. Active QDs,having a permanent electric and/or magnetic dipole,redistribute the interactions between mesogenic mole-cules causing the increase or decrease in the LC orderparameter. In addition to this, the change in LC orderparameter due to the presence of QDs strongly

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depends on the size, shape and composition of theQDs. Spherical QDs decrease the FLC order para-meter because they do not easily fit in the FLC matrix(i.e. the presence of spherical QD wraps the smecticlayers around QDs and changes smectic layer order-ing) and breach the long-range molecular interactions.

Spontaneous polarisation (PS) is known as thesecondary order parameter for FLC materials. Thechange in PS of pure FLC with the addition of QDs isshown in Figure 1(a). The presence of QDs in FLCmatrix reduces the PS significantly, which is attributedto the decrease in order parameter of pure FLC afterthe dispersion of QDs. As we increase the concentra-tion of QDs in FLC matrix, the change in PS (formixture 1 and mixture 2) becomes prominent athigher temperatures (i.e. 45–65°C) whereas the differ-ence in PS values for mix. 1 and mix. 2 is minusculefor the temperature interval of 30–40°C. In additionto this, the other factors responsible for the reductionof PS values for FLC–QDs mixtures are the loweringof net ferroelectricity and the perturbation in thehelical structure of FLC molecules (or change in

smectic layer ordering). As the spherical CdSe QDs(3.5 nm) have higher dimension with that of the FLCmolecules, therefore, the presence of QDs in FLCmatrix perturbs the helical structure of FLC mole-cules. The QDs might also be altering the smecticlayer ordering due to their size and shape. The changein smectic layer ordering in the presence of sphericalnanoparticles has already been reported by Pratibhaet al. [21] and Singh et al. [22]. The above-mentionedfactors reduce the net ferroelectricity in FLC–QDsmixtures, which corresponds the lowering in PS valuesof FLC–QDs mixtures. In contrast to CdSe QDs,Kumar et al. [12] have reported the dispersion ofCdTe QDs in deformed helix FLC (DHFLC) mate-rial and observed the enhancement in PS. On compar-ing the properties of Se and Te, it is found that Se hashigher electro-negativity and lower atomic radius ascompared to Te (for Se, the atomic radius is 1.16 Åand electro-negativity is 2.6, whereas for Te, theatomic radius is 1.35 Å and electro-negativity is 2.1).Therefore, cadmium-based Se or Te QDs possess dif-ferent electro-negativity and effective atomic radius

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Figure 1. (colour online) The variation of (a) spontaneous polarisation, (b) response time and (c) rotational viscosity of pureFLC and FLC–QDs mixtures with the change in temperature.

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(and thus surface-to-volume ratio) resulting in thenon-similar interactions with mesogenic molecules.

The change in optical response time and rota-tional viscosity with the variation of temperature hasbeen plotted in Figure 1(b) and 1(c), respectively. It isclear from Figure 1(b) that the optical response timeof FLC–QDs mixture is higher as compared to pureFLC. The response time is highest for mixture 2whereas it is least for pure FLC. The higher opticalresponse time of FLC–QDs mixture is attributed tothe change in dispersion anchoring energy of pureFLC due to the addition of QDs. The change inanchoring energy coefficients (dispersion and polari-sation) will be discussed later in the manuscript.

The rotational viscosity of pure FLC material hasalso been enhanced with the dispersion of CdSe QDs.The molecular dynamics of FLC molecules is mainlygoverned by the Zth component of rotational viscos-ity, which strongly depends upon the surface geome-try (i.e. surface anchoring). As rotational viscosity iscalculated by the optical response time and sponta-neous polarisation values (γ ¼ PSτE; where γ, PS, τand E are rotational viscosity, spontaneous polarisa-tion, response time and applied electric field, respec-tively) which has been influenced by the anchoringenergy coefficients, therefore; the increased rotationalviscosity of FLC–QDs mixture is attributed to theperturbation in FLC geometry in the presence ofQDs. The increased rotational viscosity is indirectlyrelated with the dispersion energy coefficient(WD / τ�1 and τ / γ ) WD / γ�1), which indicatesthat the presence of QDs hinders the moleculardynamics of pure FLC molecules.

The variation of dispersion and polarisationanchoring energy coefficients (WD and WP, respec-tively) on the temperature scale has been shown inFigure 2(a) and 2(b), respectively. The description of

both the anchoring energy coefficients and theirdependency on the external bias voltage for the core/shell QDs dispersed FLC has already been reportedby Shivani et al. [20]. QDs are well-known materialfor their ability to modify the surface alignment,[8,9,11,12,16] therefore; they significantly change theFLC molecular ordering if present in FLC matrix.The modification in the molecular ordering and align-ment also depends upon the concentration of QDs inhost material. It is clear from Figure 2(a) and 2(b)that the dispersion of QDs in FLC matrix causes aremarkable reduction in both the dispersion andpolarisation anchoring energy coefficients. Thereduction in anchoring energy coefficients is analo-gous to decrease in the order parameter of FLC–QDsmixtures. The difference in anchoring energy coeffi-cients for mixture 1 and 2 is not so prominent asobserved between pure FLC and FLC-QDs mixtures.The dispersion and polarisation anchoring energycoefficients are related to optical responsetime (WD / τ�1) and spontaneous polarisation(WP / PS

1=2) of the mixtures respectively as discussedin the Ref. [20] and references therein. The WD andWP were calculated with the help of optical responsetime and spontaneous polarisation values. The changein anchoring energy coefficients leads the alteration ofelectro-optical parameters in the FLC–QDs mixtures.

The polarising optical microscopy was performedto analyse the change in surface alignment and mole-cular ordering of FLC molecules after the addition ofQDs into different concentrations. The polarising opti-cal micrographs (POMs) of pure FLC and FLC–QDsmixtures at room temperature associated with bright/dark states are shown in Figure 3. Figure 3(a), 3(c) and3(e) correspond to the bright states of pure FLC,mixture 1 and mixture 2, respectively, whereasFigure 3(b), 3(d) and 3(f) are associated with the dark

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Figure 2. (colour online) The change in (a) dispersion anchoring energy coefficient (WD) and (b) polarisation anchoring energycoefficient (WP) with the variation of temperature.

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states of the same. It is clear from POMs that thepresence of QDs affects the alignment and orderingof FLC molecules remarkably. Some recent reports[8,9,11,12,16] reveal that the presence of QDs caninduce homeotropy in the LC alignment. We havealso found similar result i.e. induction of little home-otropy in the FLC–QDs mixtures. The QDs breach theplanar alignment of pristine FLC molecules due totheir ability to favour homeotropic alignment. It isclear from the POMs that the contrast of FLC–QDsmixtures has been increased as compared to pure FLC.This increase in contrast ratio for FLC–QDs mixturesis attributed to the high white light absorption capabil-ity of QDs in the mixture.

The variation of tangent loss factor (tan δ) on thefrequency scale (for temperature 30–72°C) for pureFLC and FLC–QDs mixtures has been depicted inFigure 4. Figure 4(a) represents the tan δ curve ofpure FLC whereas Figure 4(b) shows the tan δ curveof mixture 1 (1 wt% QDs dispersed in FLC). Thecharacteristic Goldstone relaxation mode, due to azi-muthal fluctuation of the director, was well observed

in both curves. Along with the characteristicGoldstone relaxation mode, temperature-dependentweak ionic relaxation mode (TDWIRM) near300 Hz has also been observed for mixture 1, whichcomes into existence at 55°C and remains at SmC*–SmA phase transition temperature (72°C). As weincrease the temperature from 55°C, the visibility ofTDWIRM becomes clearer [SupplementaryFigure 1]. The existence of TDWIRM is associatedwith the ionic carriers present in the FLC material. InFLC materials, the ionic carriers are more effective atlower frequencies. The high spontaneous polarisationvalue of FLC material couples with the ions at lowfrequencies. This ion–polarisation coupling favoursthe diffusion of ionic carriers (associated with FLCmaterial) that had been adsorbed (or accumulated) onthe surface of QDs after the dispersion of QDs inFLC and results the induction of TDWIRM.[Supplementary material: Model of diffusion of ioniccarriers at certain temperature and frequency]. Theexistence of TDWIRM strongly depends on the con-centration of CdSe QDs and is found to be absent at ahigher dopant concentration (in mixture 2). Thismode is temperature-dependent because both effectivepolarisation and ionic movements are temperature-dependent in the FLC–QDs mixtures. The relaxationfrequency of TDWIRM increases with increasingtemperature. TDWIRM has been suppressed onlyby the application of small bias voltage of 2 volts,which also indicates its origin to be in the ionic carriers[Supplementary Figure 2]. It is believed that theTDWIRM arises due the diffusion of ionic carriers(associated with FLC material) that had been adsorbed(or accumulated) on the surface of quantum dots afterdispersion in FLC [Supplementary material: Model ofdiffusion of ionic carriers at certain temperature andfrequency]. For a different guest–host system, a ther-mally activated sub-hertz frequency (SHF) dielectricrelaxation and the effect of silver nanoparticles(SNPs) on SHF relaxation have also been reportedby Mandal et al. [23], in which they observed theexistence of SHF relaxation due to the space chargeaccumulation of ions in both pure FLC mixture andFLC mixture doped with SNPs. In non-chiral 8CBLC, Kim et al. [24] have reported low-frequency dielec-tric relaxations originating in two different processes,viz. ionisation-recombination and translational diffu-sion. In the present investigation, we have onlyobserved a thermally activated weak ionic relaxationmode at certain concentration of dopant due to theion–polarisation coupling assisted diffusion of accumu-lated charge carriers associated with FLC materialsthat had been adsorbed on the surface of QDs. Inaddition to the existence of TDWIRM, the values oftan δ curve for mixture 1 is also greater than that of

Figure 3. (colour online) The polarising optical micro-graphs (POMs) of pure FLC and FLC–QDs mixtures inbright and dark state at room temperature. The POMs a, cand e correspond to pure FLC, mixture 1 and mixture 2 inbright state whereas the POMs b, d and f are associatedwith dark state for the pure FLC, mixture 1 and mixture 2,respectively.

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pure FLC. The increase in tan δ values for mixture 1 isdue to the increase in the real part of complex permit-tivity. The existence of TDWIRM was further ana-lysed by the Cole–Cole plots, which are shown inFigure 4(c). The Cole–Cole plots reveal that the exis-tence of TDWIRM is not observable at lower tempera-tures (Cole–Cole has been only plotted for 30°C and40°C), whereas at higher temperatures (only shown for70°C and 72°C in Cole–Cole plots), it is well observed.In the inset of Figure 4(c), both the relaxation modeshave been theoretically fitted using the well-knownCole–Cole relation:

ε� ¼ ε0 � ε00

¼ εð1Þ þ ε0 � ε11þ ði2πfτ0Þ1�α � i

σ

2πfεS(1)

where f is the frequency, τo is the relaxation time, α isthe distribution parameter, ε′ and ε′′ are the real(dielectric permittivity) and imaginary (dielectricloss) parts of the ε* (complex dielectric permittivity),respectively, εo and ε∞ are the low- and

high-frequency limits of the electric permittivity, εs isthe electric permittivity of the free space and σ is theconductivity.

Figure 4(d) represents the tan δ curve for themixture 2 (2 wt% QDs in FLC). At low temperatures,the value of tan δ for mixture 2 is almost same as pureFLC material whereas at high temperature it becomesslightly greater than that of pure FLC.

Relative permittivity versus temperature plots forpure FLC and FLC–QDs mixtures (mix. 1 and mix. 2)at 200 and 500 Hz have been made in Figure 5(a)and (b), respectively. The plots show that the beha-viour of relative permittivity for mixture 1 at bothfrequencies is different whereas the relative permittivityfor mixture 2 has been reduced as compared to pureFLC at both frequencies. As observed for the nature ofspontaneous polarisation of the mixtures, the relativepermittivity of the mixtures should also decrease butthe high value of conductivity for mixture 1 interferesin the usual behaviour of relative permittivity.

The conductivity (a.c.) of pure FLC before andafter the dispersion of QDs at 200 and 500 Hz has

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been plotted in Figure 6(a) and (b), respectively. It isobserved that the semiconducting nature of QDsinfluences the behaviour of conductivity. The influ-ence of semiconducting nature of QDs is more effec-tive in mixture 1 whereas it is less effective for mixture2 due to greater QD–QD interaction as compared toQD–FLC interaction. The conductivity of mixture 1is greater as compared to pure FLC whereas it isreduced for mixture 2 at both frequencies. Theincreased value of conductivity evinces the presenceof temperature-dependent ionic motion (or diffusionof ionic carriers) in mixture 1, which is responsible forthe existence of TDWIRM and unusual behaviour ofrelative permittivity for mixture 1 at both frequencies.

The UV-visible and fluorescence measurements ofpure FLC and FLC–QDs mixtures (mix. 1 and mix. 2)have been carried out on the wavelength scale, which isshown in Figure 7(a) and (b), respectively. The inset ofFigure 7(b) shows the photoluminescence of pure FLC

taking two different slit widths but all observationshave been analysed and explained with 4 nm slitwidth to maintain experimental consistency. The PLcurve of pure FLC with 10 nm slit width is not relatedto the material because it does not show repeatability.It is clear from Figure 7(a) that pure FLC and FLC–QD mixtures do not absorb light in visible regionwhereas UV absorbance of pure FLC materialincreases after the addition of QDs. The width of UVabsorption peaks for FLC–QD mixtures also increasesslightly as compared to pure FLC. It is also believedthat QDs behave like UV absorbance promoter forFLC material. Recently, Majumder et al. [25] havereported the enhancement of absorbance and photolu-minescence in CdS nanorods doped AFLC materials.The fluorescence intensity measurement was per-formed at room temperature in solution (mg/ml) tak-ing ethanol as solvent. The excitation wavelength was290 nm for all the samples. It is clear from the inset of

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Figure 5. (colour online) The change in relative permittivity with the variation of temperature for pure FLC and FLC–QDsmixtures at (a) 200 Hz and (b) 500 Hz.

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Figure 6. (colour online) The change in conductivity (a.c.) with the variation of temperature for pure FLC and FLC–QDsmixtures at (a) 200 Hz and (b) 500 Hz.

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Figure 7(b) that pure FLC do not have fluorescence invisible region whereas after the dispersion of octadecy-lamine-capped CdSe QDs into different concentrations(mix. 1 and mix. 2), it shows single emission peak at385 nm. Singh et al. [26] have already reported theabsence of PL property in pure FLC under the sameexperimental conditions. The existence of emissionpeak in FLC–QDs mixtures is probably due to thecoupling between the exciton and photon in FLCmedium, which leads the radiative process. When exci-tation photons are incident on the FLC–QD mixtures,they couple with the excitons (the bound state of theelectron–hole pair) of the QDs in FLC medium. Thestrong light scattering nature of the smectic layers ofFLC medium provides a more favourable backgroundfor photon–exciton coupling. The strong light scatter-ing nature of smectic layers and their role in theenhancement in PL property has already been reportedby Kumar et al. [27] The centre of emission peak isindependent of the concentration of QDs in the mix-tures suggesting that the dispersion of dopantQDs with varying concentration do not produce anyremarkable change in the refractive index of FLC andthus no change in the energy of emission peak (anyblue or red shift in emission peak) due to uniformsize and size-dependent quantum confinement effects([25] and references therein). The change in dopantconcentration only increases the intensity of emissionpeak.

4. Conclusions

In summary, this article has presented the influenceof octadecylamine-capped CdSe QDs on the mole-cular/ionic relaxation phenomenon and changes in

other physical parameters of FLC material with var-iation of concentration. It is observed that CdSeQDs not only alter the E-O and dielectric parametersof FLC but also modify the ionic/molecular relaxa-tion. Spherical-shape QDs reduce the smectic layerordering and thus the order parameter of FLC mate-rial. The change in order parameter leads to thechange in net ferroelectricity in the FLC–QDs mix-tures, which reduces the value of spontaneous polar-isation. The change in anchoring energy coefficientsis also responsible for the change in E-O parameters.The presence of certain concentration of QDsinduces a new temperature-dependent weak ionicrelaxation mode (TDWIRM) in addition to charac-teristic Goldstone mode. This TDWIRM is observedto exist at 55°C and persists till SmC*–SmA phasetransition temperature (i.e. 72°C). The induction ofTDWIRM is attributed to the ion–polarisation cou-pling in the FLC–QDs mixture and the diffusion ofions (charge carriers). The semiconducting nature ofCdSe QDs also influences the nature of relative per-mittivity and the variation of a.c. conductivity as afunction of temperature and frequency. The fluores-cence emission peak has also been observed forFLC–QDs mixtures, which are probably due to thecoupling between the exciton and photon in FLCmedium, which leads to the radiative process.Results suggest possible use of FLC–QDs compo-sites as UV light-storage devices.

Disclosure statementNo potential conflict of interest was reported by theauthors.

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Figure 7. (colour online) (a) UV-visible absorption spectra of pure FLC and mixtures, (b) the fluorescence intensity curve onwavelength scale for the FLC–QDs mixtures. The inset shows the fluorescence intensity curves for pure FLC taking twodifferent slit widths. The fluorescence spectra were recorded at 4 nm slit width. The UV-visible and fluorescence measurementswere performed at room temperature in solution (mg/ml) taking ethanol as solvent. The excitation wavelength was 290 nm forall the samples.

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Funding

The authors are thankful to Department of Science andTechnology, Government of India for the financial assis-tance for present work in the form of project. The authorDPS is thankful to CSIR, India, for the grant of SRFfellowship [no. 09/107(0363)/2012-EMR-I]. DPS is alsothankful to CEFIPRA for Raman-Charpak fellowship inFrance. Authors SP and TV are thankful to UGC, NewDelhi for providing financial assistance in the form ofUGC-BSR Fellowship. RM is thankful to UGC researchaward.

Supplemental dataSupplemental data for this article can be accessed here.

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