Tailoring the physical parameters of ferroelectric liquid crystal mixture with cadmium selenide quantum dots

ABSTRACT Present studies demonstrate the dispersion effect of cadmium selenide quantum dots on ferroelectric liquid crystal mixture properties. Varying concentrations of cadmium selenide quantum dots (0.05 wt.%, 0.10 wt.% and 0.50 wt.%) have been taken to investigate its influence on the physical parameters of ferroelectric liquid crystal mixture KCFLC 10R. Phase transition, textural, electro-optical and photoluminescence studies have been performed in planar aligned cells. The presence of QDs disturbs the geometrical symmetry and molecular tilt of ferroelectric liquid crystal molecules which results in the modification of physical parameters of ferroelectric liquid crystal. It has been observed that doped samples have shown decrement in spontaneous polarisation and rotational viscosity. However, there is an improvement in the response time of the doped samples. The reduction in the spontaneous polarisation with voltage signifies that doped samples can operate at low voltage (~6 V) as compared to ferroelectric liquid crystal mixture (~12 V). Photoluminescence studies reveal that all the samples absorb UV light ~340 nm and produce visible emission at ~ 400 nm which can behave as UV light storage and high contrast display. This would be beneficial for luminescent displays as well as UV to visible light conversion applications. Highlights Dispersion of cadmium selenide quantum dots with varying concentration in ferroelectric liquid crystal mixture has been investigated. Temperature- as well as voltage-dependent electro-optical parameters and photoluminescence studies have been done and analysed. Fast response time has been observed in the doped samples as compared to FLC mixture. Doped samples can be operated at low voltage (~6 V) as compared to FLC mixture (~12 V) These prepared samples can be used for UV to visible light conversion applications. GRAPHICAL ABSTRACT


Introduction
Ferroelectric liquid crystals (FLCs) are a special class of liquid crystalline materials possessing tilted LC molecules with reduced symmetry [1].The existence of permanent dipole moment even in the absence of external electric field makes them suitable for various applications like in displays with memory, spatial light modulators, optical imaging etc [2,3].Fast response time in microsecond, biaxiality, good contrast, inherent ferroelectric order and memory effect are also the key aspects that make them a vibrant topic of research in the scientific community [4,5].Despite these outstanding FLC characteristics, attaining consistent/uniform molecular alignment and improved properties of mesogens over a broad surface area is still a challenge, preventing their use in commercial displays and other applications.
In this regard, LC research encompasses two perspectives for its accomplishment: reconfigurable LC approach and dispersion of guest entity (nanomaterials) into LCs (host).We have chosen the second perspective which is quite approachable to tailor the physical properties of LCs [6][7][8].The emergence of nanoscience and nanotechnology has created an intriguing route for tailoring the distinctive properties of LCs by dispersion of nanomaterials (NMs).It is noteworthy that such a technology could not only expedite the process of creating materials with improved qualities but also envision new perspectives to investigate cutting-edge uses for nanocomposites.The major concern in today's era is to fabricate high-resolution, lightweight, compact and energy-efficient devices.Metallic, semiconducting, ferroelectric, metal-oxide NMs have been dispersed in LCs.These NMs are anticipated to show improved contrast, dielectric permittivity, rotational viscosity, wide viewing angle, ion capturing phenomenon etc [9][10][11].
Tunable bandgap, the greatest asset of quantum dots (QDs), makes them a special class of 0D nanostructure materials.This owes to the quantum confinement effect which deals with the motion of valence band holes and conduction band electrons resulting in astonishing properties.For instance, narrow PL bands, broad absorption bands, size-dependent optical and electronic properties, high luminescence and large surface to volume ratio just name to a few [12].
The superiority of optical efficiency, narrow emission line width and stable luminescence of QDs over other nonmesogens (dyes, polymer, nanoparticles, graphene) have broadened the LC horizon and amalgamation of FLC-QDs has opened up new avenues in various multidisciplinary fields [13][14][15].A comprehensive study of the interaction between FLC and QDs to understand the structure-property relationship is a prerequisite condition, which depends upon the concentration of QDs, size, shape, capping agent, etc. [16].When QDs are dispersed into FLC, there are mainly FLC-QDs and QDs-QDs interaction takes place.If the concentration of QDs is small, FLC-QDs interaction occurs either by electro-static or dipolar interaction while strong van der waals interaction exists between neighbouring QDs in the latter case.It generally occurs at high dopant concentration which leads to the aggregation of QDs in FLC [17][18][19].The basic idea of dispersing QDs into FLC is motivated by (i) monodisperse QDs of varying sizes can be achieved due to size-dependent PL property (ii) Versatile synthesis of QDs (iii) Optical defects, orientation and spatial distribution of QDs can be traced out [15].This would help to study a detailed investigation of FLC-QDs coupling.Despite such prospective and promising applications, still it has some issues (i) to get uniform alignment, highly stable and defect free system (ii) tailoring various properties of LC (iii) understanding plausible FLC-QDs and QDs-QDs mechanism (iv) transferring self-organisation of FLC onto QDs.So, there is a requirement to explore FLC-QDs nanocomposites in more depth.
Cadmium selenide (CdSe) QDs, belonging to group II-IV elements, are popular amongst the QDs due to high luminescence and quantum yield.CdSe QDs provoked as fluorescent tags for light emitting diodes, lasing and solar cell applications [20][21][22].Interfacial charge transfer in FLC-QDs nanocomposites is one of the important characteristics which modulates the local electric field [23][24][25].This will further help in elucidating the underlying interaction mechanism responsible for the modification of FLC's properties.
An ample amount of literature is available on the studies of different QDs doped FLC nanocomposites and presented in Table 1.Few studies are available for CdSe doping in single compound FLC [34,35].However, the impact of CdSe QDs on the FLC mixture has not been explored in depth.In light of this, it would be worthwhile to conduct an extensive study on FLC mixture by doping different concentrations of CdSe QDs.
In this work, we have chosen FLC mixture (KCFLC 10R) and CdSe QDs of varying concentration from 0.05 wt.% to 0.50 wt.% to analyse the phase transition temperature, electro-optical, photoluminescence and dielectric properties.An insight view of how different concentrations of QDs behaves in LC scaffold is needed.The novelty of the work lies in (i) how these QDs affect the physical parameters of FLC, (ii) to understand the role/interaction mechanism of QDs inside FLC mixture (iii) finding potential applications in UV to visible light conversion applications.

FLC mixture used
A commercial available FLC mixture KCFLC 10R (Kingston Chemicals, UK) was used in the present study as received.The phase sequence of used FLC is as follows [36,37]: Where, K, SmC*, SmA, N*and Iso are crystal, chiral smectic C, smectic A, chiral nematic and isotropic phases respectively.FLC mixture consists of dialkyl ortho difluoroterphenyl as a host, and cyanohydrin ester (10%) as a chiral dopant.The ester moiety acts as linking group that can aid lamellar packing and generate molecular tilt in SmC* phase.The fluoro-substituted compound is highly electronegative, small in size and polar group which is responsible for high spontaneous polarisation.

Method used
For the synthesis of CdSe QDs, 1mmol CdO and 4mmol OA were added to 15mL ODE in a 50mL four-necked round bottom flask and the mixture was degassed first at room temperature for 5min.Then temperature is increased to 120°C under vacuum and degassed for 30min.Then switched to inert atmosphere and heated to 240°C until a clear solution is obtained under constant stirring.The reaction mixture was brought to 80°C.Further 1.5 g ODA and 1 g TOPO were added to the solution and heated again to 280°C with continuous stirring under inert atmosphere.At this temperature, TOP-Se solution (0.4 M, 2.5 mL) was injected swiftly into the Cd-oleate solution.The temperature fell down to 265°C and the reaction was allowed to proceed at 270°C until the desired size of the nanocrystals were achieved.The nanocrystals were purified by phase separation with a 1:1 mixture of hexane and methanol to remove unreacted precursors.The nanocrystals were precipitated by adding acetone and finally redispersed in toluene [38].

Characterization of CdSe QDs
The UV-vis absorption and PL spectra of CdSe QDs are shown in Figure 1(a,b), respectively.The first excitonic peak of CdSe QDs appears at 580 nm in the absorption spectrum and they exhibit PL peak at 588 nm after excitation at 377 nm. Figure 1(c) shows powder X-ray diffraction (PXRD) patterns of CdSe QDs along with the diffraction patterns of the bulk CdSe in zinc blende (ZB) and wurtzite (WZ) phase obtained from the database.The diffraction peaks of QDs are matching well with the wurtzite CdSe which confirms that these QDs exhibit wurtzite crystal structure.These QDs are monodispersed and of spherical shape as can be seen in Figure 1.The average size of these QDs comes out to be around 3.8 nm as calculated from transmission electron microscope (TEM) image and the corresponding histogram is also shown in inset 1 (d).The Fourier transform infrared (FTIR) spectra of CdSe QDs and FLC with different wt.% QDs are recorded and shown in Figure 2. The peak at ∼1463 cm −1 is due to P=O stretching vibrations of bound TOPO molecules on QDs as ligands.The three peaks at 805, 2858 and 2924 cm −1 are associated with the bending, symmetric and asymmetric stretching modes of C-H respectively that are present in the alkyl chains of QDs and FLC molecules.The peak around 1095 cm −1 may be due to C-F stretching while peak at 1310 cm −1 is associated with C≡N stretching.The peaks at 1180 and 1262 cm −1 correspond to the symmetric and asymmetric C-O-C stretching vibration.The combination mode of C=C-C aromatic ring and C-H asymmetric bending are obtained at 1457 cm −1 .The peaks at 1609 and 1733 cm −1 are assigned to the C-C stretching and C=O stretching of ester molecules in the FLC core.

Liquid crystal cell fabrication
Commercially available planar LC cells (purchased from Instec, USA) of dimensions 17 mm × 15.25 mm × 1.5 mm were used.Two indium tin oxide (ITO) glass substrates of resistance 100Ω were coated with polyimide layer (KPI-300B) of thickness 0.06 µm and coating speed of 2000-3000 rpm.After being cured at 240°C for 1-1.5 h, ITO substrates were placed in such a way that both conducting sides of substrates are towards each other.The effective area was kept 25 mm 2 .To maintain a gap between substrates, a spacer of thickness (5 ± 0.5) µm was placed between glass substrates.

Preparation of FLC-QDs nanocomposites
For the preparation of QDs doped FLC nanocomposites, firstly 1 mg of CdSe was measured in a vial using digital weighing balance (Sartorius, CPA225D) and then dispersed in a non-polar solvent (n-hexane).The dispersion was ultrasonicated for an hour at 33 kHz frequency.
Thereafter, different concentrations (0.05 wt.%, 0.10 wt.%, 0.50 wt.%) of CdSe QDs were mixed with FLC followed by ultrasonication for another one hour for homogeneous mixing.The prepared mixtures were then placed in an oven at 70°C to remove the excess solvent.Further, the prepared mixtures of FLC-QDs nanocomposite were kept for another 24 h to remove the solvent left over.The pure FLC and FLC-QDs samples were then filled into empty LC cells at a temperature above the isotropic temperature of FLC via capillary action.The filled LC cells were then allowed to cool slowly at room temperature.The prepared samples are termed as FLC, FLC + 0.05 wt.% QDs, FLC + 0.10 wt.% QDs, and FLC + 0.50 wt.% QDs in the manuscript.

Textural, phases and phase transition temperatures
To determine the phases, textures and phase transition temperatures, polarising optical microscope (POM) (Nikon, LV100POL, Japan) in crossed polarised position was used.POM was interfaced with charge coupled device (CCD) camera (Q28378)  through Linksys software.The LC cell was put in the hot stage (THMS600E) which is also connected to the temperature controller (Linkam TP94, UK).To achieve stability, filled LC cells were heated and cooled @ 1.0°C/min.

Electro-optical measurement
Spontaneous polarisation (P s ) is the secondary order parameter of FLC device.The measurement of P s was carried out by a well-known polarisation reversal technique.A triangular wave of 20 V pp and 100 Hz frequency was fed to LC cell through a function generator (Tektronix AFG-3021B).The detailed procedure to measure P s is given in our earlier work [39,40].The optical tilt angle was measured by rotating the optical microscope stage.The two extinction points in +E, -E states give rise to cone angle (2θ).The half of the measured cone angle gives an optical tilt angle of FLC material.

Photoluminescence (PL) measurement
PL spectra was performed using fluorescence spectrophotometer (Cary Eclipse, Agilent) equipped with Xenon discharge lamp as an excitation source at room temperature.An excitation wavelength of 340 nm and slit width of 5 nm were used for all the samples.

Phases, phase transition temperature and analysis of QD dispersion effect on FLC
In order to study the QDs dispersion effect in FLC, we have recorded the microphotographs of studied samples using POM in heating as well as in cooling cycles.Figure 3 shows the microphotographs of various phases present in the FLC and FLC-QDs samples recorded during cooling cycle @1°C/min.It has been observed that doped samples possess the same phases as FLC and the corresponding   phase transition temperatures are also listed in Table 2.
There is a considerable shift in phase transition temperature for FLC-QDs samples is seen.Similar type of results were also reported by other groups [34,35].Oily streak textures were observed in pure FLC as shown in Figure 3 (a) upon cooling from the isotropic liquid which is believed to be due to the edge dislocations in the sample [41].It is worth to mention here that due to the planar anchoring of the cell, long molecular axis of LC is aligned parallel to the glass substrate implying that the helix axis of chiral nematic (N*) phase is oriented parallel to the substrates.Upon further cooling, elongated germs called bâtonnets [42] were observed and started to grow anisotropically (Figure 3 In FLC + 0.50 wt.% QDs, the simultaneous emergence of fan-shaped and fingerprint textures (Figure 5(a,b)) were noticed upon cooling from isotropic liquid.However, a fingerprint followed by oily streaks textures were observed in FLC + 0.05 wt.% QDs (Figure 6(a,b)) which confirms the partial planar as well as homeotropic alignment of FLC molecules.On further cooling, paramorphotic focal conic textures were observed in SmA phase in both type of samples (Figures 5(d In terseness, optical microphotographs suggest that presence of QDs affects the FLC molecular orientation and induce perturbation in helical structure and hence properties of FLC.

Optical tilt angle measurement
Being a primary order parameter and intrinsic property of FLC, the study of temperature effect on optical tilt angle is necessary.The measured tilt angle with temperature is shown in Figure 7.It is observed that FLC has highest tilt angle among all the samples.The   measured values for FLC, FLC + 0.05 wt.% QDs, FLC + 0.10 wt.% QDs and FLC + 0.50 wt.% QDs were found ~19.5°,10.5°,16° and 7.5° respectively.The change in the orientation of FLC molecules and perturbation in molecular tilt cause a variation in the tilt angle.It is observed that optical tilt angle of FLC system is greatly affected by the dispersion of QDs.In case of FLC, the interaction exists only between the FLC-FLC molecules.However, when QDs are considered with FLC as a dopant, FLC-QDs and QDs-QDs interactions also come into the picture and play an important role in easy tilting of the LC molecules as well.More precisely, there may be two possibilities (i) QDs may be surrounded by FLC molecules (ii) FLC molecules can be surrounded by QDs as presented in Figure 8 (The animated video is given in the supplementary sheet for more clarification).QDs being spherical in shape shows their effect on FLC from all sides and nullifies the overall effect.Thus, FLC-QDs interaction does not contribute much.In QDs-QDs interaction, FLC molecule is surrounded by QDs on all sides and finds difficulty in tilt as desired.Therefore, it suppresses the conical geometry of the FLC [43].Here, in our case, FLC + 0.05 wt.% QDs and FLC + 0.50 wt.% QDs samples easily perturb the SmC* phase and hence the tilt angle changes substantially with temperature.However, there is less variation of optical tilt angle with temperature in FLC + 0.10 wt.% QDs nanocomposite is obtained.The low values of P s in the nanocomposite system can be understood by taking into account the coupling of dipole moment of QDs with FLC.It is anticipated that orientation of QDs inside FLC medium takes place in such a manner that net dipole moment per unit volume of the sample decreases and hence a corresponding decrease in P s .Besides this, a significant decrease in SmC*-SmA transition temperature (~22°C, 15°C and 21°C in FLC + 0.05 wt.% QDs, FLC + 0.10 wt.% QDs and FLC + 0.50 wt.% QDs respectively) in FLC-QDs samples could be seen.This decrease in phase transition temperatures may be attributed to low ordering in FLC molecules or decrease in average strength of intermolecular interaction between FLC and QDs. Figure 10 represents the response time (τ) with the change in temperature in the SmC* phase.It has been observed

Electro-optic studies
that response time improve with the doping of QDs and τ was found 400 μs, 260 μs, 280 μs and 270 μs for FLC, FLC + 0.05 wt.% QDs, FLC + 0.10 wt.% QDs and FLC + 0.50 wt.% QDs respectively.Variation of rotational viscosity (η) with increasing temperature in SmC* phase is also calculated by the relationship η = τ P s E loc [44,45] and shown in Figure 11.Where E loc is the net electric field.It has been observed that η decreases with the doping of QDs into FLC material.This can be understood by considering that E loc has contribution from two terms i.e.E loc = E ext -E dep .Here, E ext and E dep are the external applied and depolarisation electric fields respectively.The direction of E dep is just opposite to the E ext and arises at the interface between FLC medium and electrodes.E loc arises due to the addition of foreign entity (NMs, QDs) into the FLC medium.It is somewhat different from the E ext thereby giving the net electric field in the particular system [46].It is clearly seen from Figure 11 that there is decrease in η in doped samples.As a result, the electric field inside the doped samples is greater than electric field in pure FLC sample.This causes the system for faster switching time whilst the decrease in P s values.Figure 12 shows the variation of spontaneous polarisation with change in applied voltage at 30°C for pure and FLC-QDs samples.It is clearly seen that after dispersion of QDs, P s decreases than the P s observed for pure FLC sample.The contributing factor for such behaviour is change in dipole moment and optical tilt angle.As mentioned in earlier section, there is decrease in optical tilt angle with the doping of QDs.Changing in the tilt angle causes variation in P s as a function of QDs concentration.Also, the overall decrement in the dipole moment of FLC-QDs leads to decrease in P s .Figure 12 infers that with increasing the applied  voltage, the dipoles of FLC and QDs aligned towards the electric field direction and tend to reach a saturation condition at high voltage (~20 V or above in our case).
One can clearly see that in doped sample, the molecules get aligned more easily in electric field direction at low voltages (~6 V) than voltage required for pure FLC (~12 V).In this situation, FLC molecules attain a hometropic orientation due to applied e-field.An usual decrease in P s value for FLC + 0.05 wt.% QDs is found.We expect that this decrease may be due to overall orientation of FLC molecules i.e. planar orientation dominant over homeotropic alignment.Due to this type of configuration, the composite may have lower value of P s than found in alignment configuration.However, when the concentration of QDs is increased beyond 0.05 wt.%, a slight increment in P s values was observed due to favourable planar alignment acquired by FLC molecules as discussed earlier.The decrement in the P s values was also reported earlier in the literature [34].We can conclude that composite system can operate at low voltages as compared to pure FLC. Figure 13 shows the response time behaviour with voltage at a fixed temperature 30°C.It has been observed that there is substantial improvement in the response time after doping of QDs in FLC samples as compared to pure FLC.However, there is no remarkable change noticed with respect to doping of QDs concentration.The improvement in response time can be attributed to loosening in FLC molecular packing, increase in anchoring energy and decrease in rotational viscosity.
As FLC molecules are arranged in particular geometry and their molecules are tightly packed.But doping of QDs helps in loosening the molecular packing so that they respond freely with electric field as compared to pure FLC.The distortion/perturbation in optical tilt angle and P s as discussed earlier is responsible for the decrement in rotational viscosity of FLC samples.In addition, the composite system experiences a torque due to the dielectric anisotropic behaviour of FLC on the application of electric field and hence a decrease in rotational viscosity [47].The effect of anchoring energy on the response time also plays an important role in FLC-QDs samples.According to the relation [48,49], The response time (τ) depends upon the anchoring energy (W) as well as cell gap (d).Here in our case, d = 5 μm is constant throughout the investigation, so there is no effect of d on the response time.
But anchoring energy (strong as well as weak) varies inversely with the τ.. τ should be lowered only if W is high i.e. strong anchoring between QDs and FLC molecules increases the elastic energy of FLC samples.However, in our case, lower anchoring energy of FLC-QDs was noticed with faster response time (Supplementary sheet Figure S1, S2).This can be explained more precisely by taking QDs' contribution from two factors (i) decrease in rotational viscosity (ii) lesser torque required to switch the molecules due to reduced P s in the composites system.Thus, the combined effect of P s and η subdue the opposite behaviour of anchoring energy on τ.

Photoluminescence (PL) study
In order to understand the quenching phenomena, enhancement and peak position of the system, PL spectra of pure FLC and FLC-QDs samples were recorded at an excitation wavelength of 340 nm (UV region, 3.6 eV) in the range 360-800 nm at a slit width of 5 nm.A peak around 400 nm was found in all the samples as depicted in Figure 14.This emission peak may be ascribed to the radiative relaxation of electrons from lowest energy unoccupied molecular orbital (LUMO) to the highest energy occupied molecular orbital (HOMO) in the system [50,51].It has been observed that QDs affect the PL emission of FLC.There is an enhancement of PL signal with 0.05 wt.% doping whereas further increase in QDs concentration leads to decrease in PL signal i.e. quenching effect arises.In addition to this, no shifting of peak with the doping of QDs was observed.The basic mechanism that works for such a behaviour is the formation of radiative and non-radiative levels which arises due to the intermolecular interaction between FLC and QDs.With the addition of QDs, the local field is generated between the excitons of FLC and QDs which provides extra radiative recombination levels.These levels further felicitate the easy transfer of energy from QDs to FLC molecules leading to the enhancement of PL signal.On the other hand, with further increase in concentration of QDs (> 0.05 wt.%), PL signal get quenched.The dispersion of QDs among FLC are relatively close in higher doping concentration which ultimately increases the chances of energy transfer among QDs itself rather than in between FLC and QDs.This would further lead to the formation of non-radiative paths in the system and hence the quenching phenomenon occurs.However, pure FLC as well as FLC-QDs samples absorb UV light ~340 nm and produce visible emission at ~400 nm which can behave as UV light storage and high contrast display.This would be beneficial for luminescent displays as well as UV to visible light conversion applications.

Conclusion
In conciseness, different concentrations of CdSe QDs (0.05,0.10 and 0.50 wt.%) were dispersed into FLC mixture.The objective of current work is to analyse how these QDs influence the physical parameters of FLC.The electrooptical, photoluminescence and textural studies were performed in pure FLC and FLC-QDs samples to accomplish the task.There is considerable difference in phase transition temperatures for FLC-QDs samples was seen.FLC + 0.10 wt.% QDs show the planar alignment while FLC + 0.05 wt.% QDs and FLC + 0.50 wt.% QDs show the partial alignment (planar as well as homeotropic).This would lead to change in the spontaneous polarisation and rotational viscosity for doped samples.However, fast response time was observed with the doping of QDs.Optical tilt angle was found to be decreased with the addition of QDs.Voltagedependent electro-optical studies were also compiled and found that doped samples would operate at low voltage as compared to pure FLC.PL spectra show the enhancement for FLC + 0.05 wt% QDs whereas quenching phenomenon is observed for higher doping concentration.The absorption in UV regime and emission in visible regime make them suitable for UV to visible light conversion applications.Thus, the outcome of the present work provides an efficient way of tailoring the physical parameters of FLC mixture and a deep understanding of the underlying interaction mechanism involved in FLC-QDs samples.

Figure 3 .
Figure 3. (Colour online) Optical microphotographs of pure FLC comprises of (a) N*phase; (b) transition from N* to SmA; (c) SmA phase; (d) SmC* phase under cross polarisers at a magnification of 10x.The scale bar ( ) corresponds to 100μm.
Sample T SmC*-SmA (°C) T SmA-N* (°C) T N*-Iso (°C) FLC (Pure) 60.0 101.7 109.1 FLC + 0.05 wt.% QDs 37.0 110.1 117.5 FLC + 0.10 wt.% QDs 45.0 102.0 109.5 FLC + 0.50 wt.% QDs 38.4 103.4 109.8 (b)) which gradually transformed into planar and uniform texture (Figure3(c)).Bâtonnets generally arise due to screw dislocations in the material.Besides, the optic axis is lying along the rubbing direction of LC cell i.e. in the plane of substrate whereas the smectic layer planes are lying perpendicular to the substrate plane.A slight change i.e. occurrence of striated textures in Figures3(d) and 4(f)) was observed which confirms that the smectic layer configuration does not change during a phase transition from SmA to SmC*.In FLC + 0.10 wt.% QDs, fingerprint textures (Figure4(a,b)) were observed during cooling cycle which finally get coalesced to form textures as shown in Figure4(c) at 102.5°C.Planar textures with some defects as shown in Figure4(e) were also observed in SmA and SmC* phase with some optical contrast variation.
) and 6(d)).The transition from SmA to SmC* was noticed by the observation of broken/patchy focal domains in SmC* phase (Figures5(e) and 6(e)).The patchy textures appeared to anticipate the different orientation of molecules in SmC* phase which is further related to the changes in optical tilt angle.

Figure 4 .
Figure 4. (Colour online) Optical microphotographs for FLC + 0.10 wt.% QDs sample showing (a) transition from isotropic to N* phase; (b) zoomed out window for the clear view of fingerprint textures; (c) N* phase; (d) transition from N* to Sm A phase; (e) SmA phase (f) SmC* phase under cross polarisers at a magnification of 10x.The scale bar ( ) corresponds to 100μm.

Figure 6 .
Figure 6.(Colour online) Optical microphotographs of FLC + 0.05 wt.% QDs sample showing bar (a) transition from isotropic to N* phase; bar (b) Oily streaks textures in N* phase; bar (c) transition from N* phase to SmA phase; bar (d) paramorphotic focal conic textures in SmA phase; bar (e) patchy focal textures in SmC* phase under cross polarisers at a magnification of 10x.The scale bar ( ) corresponds to 100μm.

Figure 5 .
Figure 5. (Colour online) Optical microphotographs of FLC + 0.50 wt.% QDs sample showing (a) transition from isotropic to N* phase; (b) simultaneous occurrence of fan shaped and fingerprint textures; (c) transition from N* phase to SmA phase; (d) paramorphotic focal conic textures in SmA phase; (e) patchy focal textures in SmC* phase under cross polarisers at a magnification of 10x.The scale bar ( ) corresponds to 100μm.

Figure 9
Figure9depicts the temperature dependence on spontaneous polarisation (P s ) for FLC and FLC-QDs nanocomposite samples at 20 V (100 Hz) in SmC* phase.It is found that P s decreases with the doping of QDs as well as an increase in temperature.In ferroelectrics, P s depends upon net ferroelectricity in the given sample.The calculated values of P s were found ~21, 10.5, 15.6 and 13.2 nC/cm 2 for FLC, FLC + 0.05 wt.% QDs, FLC + 0.10 wt.% QDs and FLC + 0.50 wt.% QDs respectively.

Figure 7 .
Figure 7. (Colour online) Optical tilt angle variation with reduced temperature for FLC and FLC-QDs samples at 20V.T C * is the SmC*-SmA transition temperature.

Figure 10 .Figure 11 .
Figure 10.(Colour online) Variation of response time with reduced temperature in SmC* region for FLC and FLC-QDs samples.

Figure 12 .
Figure 12. (Colour online) Behaviour of spontaneous polarisation with the variation in applied voltage for FLC and FLC-QDs samples.

Figure 13 .
Figure 13.(Colour online) Behaviour of response time with the variation in voltage for FLC and FLC-QDs samples.para.

Figure 14 .
Figure 14.(Colour online) Photoluminescence spectra of FLC and FLC-QDs samples at excitation wavelength of 340 nm.

Table 1 .
A literature review of doping different QDs in different FLC media.

Table 2 .
Phase transition temperatures for FLC and FLC-QDs samples during cooling cycle.