Faster and low-power-operated highly luminescent CdSe nanoplatelets incorporated ferroelectric liquid crystals

ABSTRACT The ferroelectric liquid crystals (FLC) composites with improved electro-optic properties, obtained by incorporating CdSe (CS) nanoplatelets (NPLs), are reported. Noticeable improvementin optical contrast, saturation voltage, response time, and brightness of FLC have been found. The improvements of electro-optical properties are the results of enhanced molecular alignment of FLC due to the presence of CS NPLs. Time resolved photoluminescence (TRPL) measurements reveal NPLs surface energy transfer in FLC. Two types of Fӧrster resonant energy transfer (FRET) is observed in CS incorporated FLC. First is homo FRET which is the transfer of exciton within stacked CS NPLs and second is FRET between FLC and CS NPLs. FRET are supporting the CS incorporated FLC to get superior electro-optic properties. These findings may create opportunity of future CS NPLs incorporated FLC based display devices having better electro-optical properties. Graphical abstract


Introduction
Liquid crystals (LCs) have shown their potential in modern display devices such as laptop screen, mobile phones, smart-watches, and television screen etc [1][2][3][4].Ferroelectric liquid crystals (FLCs), which are thermotropic LC, show properties useful in display devices viz.inclusive viewing angle, fast switching time, less operating power, and high contrast [5,6].But FLCs have drawbacks such as meagre energy efficiency and brightness due to their lower molecular ordering.The nanomaterial having various intrinsic characteristics like shape, type, doping, alloying can be incorporated in FLC to manipulate their elastic, physical, electrooptic, and dielectric properties [7][8][9].CdSe nanoplatelets (CS NPLs) show unique anisotropic optoelectronic properties because of quantum confinement effect along thickness axis such as narrow photoluminescence (PL) line width [10][11][12].The incorporation of CS NPLs in LC reduce director field distortion crafting unique anchoring condition [13].The main challenge of incorporation of 2D nanomaterial in LC is uniform dispersion since strong interparticle interaction between NPL-LC molecules (Van der Waals, hydrophobic, and depletion attraction) resulted in aggregation of NPLs and creating problem of concentration optimisation [14].Incorporation of CS NPLs into LC can control their molecular alignment and photoluminescence properties with external magnetic field [12].The incorporation of gold NPLs reduces polar surface anchoring energy of LCs which can be useful for switchable plasmonic polarisers, electro-optic devices, colour filters and smart windows etc [15,16].Anisotropic optical and electronic properties of MoS 2 NPLs utilised to affect optical properties of host LC material [17,18].Incorporation of NPLs of different material (CdSe, Au, and MoS 2 ) affects the optical properties of host LC material by controlling their molecular alignment and ordering.Henceforth, present work demonstrate improved electro-optic properties of host FLC material by presence of CS NPLs which is significant for future NPL based display devices.Moreover, time resolved photoluminescence (TRPL) technique can be used to give experimental evidence about Fӧrster resonance energy transfer (FRET) in FLC-quantum dot (QD) composite system (CdSe QD/FLC) [19].The incorporation of carbon QDs in LC resulted in PL quenching and, conservation of lifetime (Absence of FRET), and increase in zeta potential [20].In present work TRPL technique used to confirm FRET between NPLs and FLC material.
In summary, presence of CS NPLs in FLC improves optical, electro-optic, and dielectric properties of host FLC material.The improved electro-optic properties were verified by measuring tilt angle, spontaneous polarisation, response time, contrast and PL intensity of FLC-CS NPL composites.TRPL measurements reveal noticeable lifetime reduction (4.5 nsec to 3.9 nsec) by presence of CS NPLs of host FLC.The lifetime reduction is explained as nanoplatelets surface energy transfer in FLC composites.These findings are discussed in the light of localisation of electron and hole wave functions in the stacked CS NPLs as a result of homo FRET.

Experimental details
CS NPLs were synthesised by the reported high temperature colloidal synthesis route [21].Crystallite phase of CS NPLs confirmed by Bruker D8 advance powder X-ray diffraction (XRD) instrument with Cu K α (λ = 1.54 Å).The XRD data matched with standard data of JCPDS card No. 19-0191 of CdSe material.To study size, shape and stacking of CS NPLs, transmission electron microscopy (TEM, FEI, Netherland, Tecnai G 2 20 200 kV ultra-twin microscope) was used.To confirm purity of CS NPLs optical absorption measurements (Lambda from Perkin Elmer spectrophotometer-950) were performed in range of 250 nm to 600 nm.The PL measurements were performed using Horiba FluoroLog spectrophotometer (FL-1057).
The CS NPL-FLC composite cells are formed using conducting (~20 Ω/sq.) indium tin oxide (ITO) coated glass plates 25 mm 2 electrode pattern obtained by photolithography and using rubbed polyamide method [22].The uniform thickness of FLC cells (~7 ± 0.5 μm) was achieved using mylar spacer between electrodes.The CS NPL incorporated FLC composites were prepared by homogeneously mixing (0.002, 0.004, 0.006, 0.008, 0.010 wt%) of CS NPLs and 4 mg host FLC mixture.The CS NPL-FLC mixture was homogenised by 3-4 cycles of ultrasonication and heating.The homogenised CS NPL-FLC mixture was then filled into assembled cells by capillary action method and copper wires are soldered with indium metal to electrodes.The optical texture, and tilt angle for CS NPL-FLC composite cells were observed by polarising optical microscope (POM) of Eclipse 200 polarising microscope, Nikon Corporation, Japan interfaced by CCD camera.The p-E ferroelectric loop of pure FLC and CS NPL incorporated FLC composites were recorded using Radiant technologies p-E loop tester PMF 20,516-326 at 10 Hz frequency and 10 V.The response time of FLC and CS NPL incorporated FLC composites were measured by applying 10 Hz square wave by function generator (Aplab, MSG3M) [23,24].The sample cell was kept at 45° between diode laser (Besto, power 5 mW, plane polarised, and wavelength of 532 nm) and polariser [25].The photodiode was used to detect signal and connected to digital storage oscilloscope (Nvis, NB102C).The dielectric measurements of composites were performed by LCR metre (Hioki IM3523) in the frequency range of 100 Hz to 2 KHz for 1 V oscillating voltage.Time resolved photoluminescence spectra was recorded using timecorrelated single photon counting (TCSPC) method by Horiba spectrophotometer (FL-1057) using 300 nm LED source with 50 picosecond pulse at repetition rate of 100 MHz.

Results and discussion
The XRD pattern of CS NPL is shown in supporting information show NPLs having four monolayers (ML) with zinc-blende crystal structure.Figure 1 depictTEM images of CS NPL and show CS NPL have nearly square shape with sharp boundaries.TEM images also reveal that CS NPLs are of lateral dimension of ~13.4 ± 0.7 nm and ~13.5 ± 0.8 nm with thickness of~ 2.4 ± 0.2 nm.In CS NPL quantum confinement on flat surface (13.4 × 13.5 nm 2 ) is negligible since these dimensions are higher than Bohr exciton radius (5.4 nm) [10,26].The presence of polar solvent (Viz.ethanol) into solution of apolar solvent (Viz.hexane) favours stacking of CS NPLs [26,27].In present work, for size selective precipitation of NPLs ethanol was added to hexane dispersed NPLs and QDs colloidal solution of CS.Ethanol reduces surface energy of NPLs since it is an anti-solvent for ligand (Oleic acid) which was bonded on surface of NPLs during synthesis [26].The detailed analysis of oleic acid ligand substitution by ethanol from CS NPLs surface can be found in other report [28].Hence, presence of ethanol triggers formation of column like 1D stacking of NPLs due to stout Van der Waals forces and clearly observed in Figure 1 [26].Noteworthy, once NPLs stacked together, Van der Waals forces hold them together (even in FLC matrix) [26].Figure 1 also showcentre-to-centre distance between stacked NPLs ~4.6 ± 0.4 nm which is less than first exciton.However, there are appreciable numbers of NPLs which are not stacked (See Figure 1).Stacking in NPLs substantially increases exciton trapping and transfer corroborated by homo ultra-efficient Fӧrster resonance energy transfer (FRET) [26].Such exciton trapping may show largest wave function overlap and reduce possibility of charge leakage.
The effect on molecular alignment of FLC by incorporating CS NPLs (C-0.002/FLC,C-0.004/FLC, C-0.006/ FLC, C-0.008/FLC, and C-0.010/FLC) is studied by polarising optical microscope (POM) analysis.POM images of bright and dark states for CS NPLs incorporated FLC shown by Figure 2. Figure 2(b) clearly show leakage of light in dark state, which is due to lowered molecular alignment (Intrinsic defects) of host FLC material [29,30].Presence of intrinsic defects could hamper molecular alignment and cost optical contrast of FLC material [30].Incorporation of CS NPLs in FLC has improved molecular alignment of FLC evidenced by reduced light leakage centres in dark state (Figure 2(d,f,g)).Importantly, CS NPLs align their surface normal parallel to LC director which reduces director field distortion and resulted to enhanced order parameter of FLC [12,13,[31][32][33][34].However, due to observed agglomeration for C-0.010/ FLC (Figure 2(k,l)), concentrations of 0.01 wt% and beyond have not been considered for further measurements.
Figure 3 illustrates the photoluminescence (PL and PLE) measurements of composites in presence of CS NP.The CS NPL incorporated composite show unique broad emission peak centred ~396 nm, due to light scattering SmC* phase of FLC [22].Improvement in PL intensity along with slightly broadening of emission peak (Compared to host FLC) for C-0.002/FLC, C-0.004/FLC and C-0.006/FLC composites is noted.Ratio of area under curve of emission spectra for composite to pure FLC is calculated, the maximum value of ratio is 1.4, which is highest for C-0.002/FLC composite (Inset of Figure 3).The maximum PL intensity and area ratio is observed for lower concentration (0.002 wt%) due to higher improved ordering of host FLC material [35].The increased ordering of FLC composite may be due to reducing charge leakage by NPLs stacking effect.The stacking of CS NPLs (Figure 1(b,c)) in host FLC substantially increases exciton trapping and transfer by homo (amongst same CS NPLs) ultra-efficient Fӧrster resonance energy transfer (FRET) in the range of ~100 nm [26].The mechanism is illustrated schematically in Figure 4 show homo-FRET is ultra-efficient due to stacking, huge extinction coefficient and minor Stokes shift (~18.9meV) of CS NPLs, resulted into a higher Fӧrster radius ~135 Å.Such FRET trapped exciton of NPLs in host FLC matrix may cause largest wave function overlap along with enhanced oscillator strength and assures charge transfer is minimal in case of C-0.002/ FLC, C-0.004/FLC and C-0.006/FLC composites.As ordering of FLC enhanced by presence of CS NPLs electro-optic properties of FLC also altered [35].The reduced director field distortions explained earlier is also responsible for improving order parameter.As a result, the absorption coefficient is improved which resulted in enhancement of PL and PLE intensity for CS NPLs incorporated FLC [35].The improved PL intensity and broadening would be beneficial for emissive display devices.
An optical tilt angle (θ, primary order parameter) reveal information about FLC molecular ordering.The saturation voltage which is measured by applying external electric field is a significant property of FLC material.Figure 5(a) show plot for tilt angle versus voltage in presence of CS NPLs FLC composites.The nature of plot Figure 5(a) show saturation voltages are 7 V for host FLC, 4 V for C-0.002/FLC, 5 V for C-0.004 and C-0.006/FLC composites.The C-0.002/FLC show highest tilt angle (~28.9 ± 0.8°) and minimum saturation (Operating) voltage compared to pure FLC (~22.2 ± 0.5°) and other composites.Incorporation of CS NPLs triggers far-off FLC molecular dipole interaction as a result of minimised director field distortions.CS NPL align with their thickness axis along FLC director, and improves anchoring of FLC material evidenced as improved tilt angle [12].The dielectric constant is also reduced by presence of CS NPLs may be due to   minimised director field distortions (See supporting information).The alignment of NPLs along FLC director improves magnitude of moment of force experienced by FLC-NPL composite by external electric field due to higher dipole moment moments of NPLs.As a result FLC-NPL composite may be switched with lower value of applied external electric field as compared to host FLC [12,22].However, homo FRET trapped exciton in stacked NPLs (as discussed earlier) result into effectively localised charges in stacked NPLs inside host FLC matrix with aid of increased dipolar interaction gives best result for C-0.002/FLC composite.The decreased saturation voltage is applicable for sinking operating power consumption of FLC-NPL-based display device.
Spontaneous polarisation (P s , secondary order parameter) is important parameter of molecular ordering which can be measured by recording ferroelectric P-E loop. Figure 5(b) show P-E ferroelectric loop of CS NPL incorporated FLC composites.The Figure 5(b) shows by presence of CS NPL does not affect the ferroelectric nature of host FLC material.Noteworthy, the significant improvement of spontaneous polarisation (see Table 1) is observed for C-0.002/FLC, C-0.004/FLC, C-0.006/FLC and C-0.008/ FLC composites.The maximum value for P s (~23.7 nC/ cm 2 ) is for C-0.002/FLC composite.The improved spontaneous polarisation validate enhancement of anchoring energy [3].The improved P s is attributed to improved localised charges, a result of FRET as discussed earlier in this report.The response time (τ R ) is important parameter of LC for display applications [29].The response time of pure FLC (~622 μs at 10 V) is observed to reduce appreciably by presence of CS NPLs (~211 μs at 10 V for C-0.002/FLC).The measured values of response time of FLC composites are given in Table 1 and supplementary material.The reduction of response time may be attributed to improved value of spontaneous polarisation of FLC by presence of CS NPLs [36].The improved order parameters (θ, P s ), and reduced response time (τ R ) enhances the electrooptic properties of host FLC material.
Time resolved photoluminescence (TRPL) is very accurate technique to confirm the FRET between donor (Pure FLC) and acceptor (CS NPLs) [19].Inset of Figure 6 show spectral overlap of emission bands of host FLC and absorption bands of CS NPLs [37,38].Figure 6 depict the TRPL spectra recorded for host FLC and CS NPLs incorporated FLC for C-0.002/FLC composite.The TRPL data fitted to tri-exponential decay model by equation [39].where, τ 1 is attributed to emission from radiative relaxation of excited electron to ground state; τ 2 is attributed to radiative decay from exciton recombination on the surface with surface localised states (defect states) and τ 3 is attributed to carrier recombination process [40].The TRPL data fitting parameters are given in Table 1 (See supporting information).The significant lowering of average lifetime is observed (~4.5 ± 0.2 ns to ~ 3.9 ± 0.2 ns) of FLC by presence of CS NPLs and attributed to non-radiative energy transfer from FLC to the conduction bands delocalised electron of CS NPLs [37,41].
The energy transfer efficiency is 13.5% for LC-NPL composite calculated from equation [37].
The multiple lifetime component (α i τ i ) shows, photon numbers following decay dynamics of related τ i process [42].The α i τ i for decay process in pure FLC is dominated by τ 2 i.e. by radiative decay exciton recombination from surface states (intrinsic defect states) concluded by value of α 2 τ 2 (55.3%, Table 1).Importantly, the presence of CS NPLs in FLC enhances the radiative relaxation of exciton to ground state (τ 1 ) evidenced by improved value of α 1 τ 1 (49.3%,Table 1).The improved α 1 τ 1 is due to FRET from FLC to CS NPLs in excited state.The exciton which are dominant in pure FLC is due to intrinsic defect states are transferring its energy in excited state to CS NPLs since CS NPLs are close to surface of FLC molecules by FRET.The exciton energy received by CS NPLs is then transferred within CS NPLs by homo FRET followed by relaxation as radiative recombination.The radiative recombination in presence of CS NPL is reflected by improved FLC material properties i.e. contrast, PL intensity and tilt angle.The energy transfer via FRET between FLC-NPL may play a key role for memory in FLC material.

Conclusions
In summary, presence of CS NPL in FLC at 0.002 wt% concentration has shown notable improvement of dielectric and electro-optic properties as a result of improved molecular ordering.The tilt angle measurement show lowered saturation voltage as consequence of homo FRET trapped exciton in stacked NPLs.The TRPL measurements validate FRET between FLC and CS NPL.The presence of homo FRET (Among stacked NPLs) and FRET (Among FLC-NPL surface) are responsible for improved FLC material properties.The present work may guide for future FLC-NPL based display device which have quick response, less power consumption, improved brightness, optics contrast.

Figure 3 .
Figure 3. Graph of Photoluminescence (emission and excitation) spectra for CdSe nanoplatelets incorporated FLC; Inset show broadening of emission peak by calculating area under curve ratio.

Figure 4 .
Figure 4. Schematic illustration for CdSe NPLs stacking alignment in FLC matrix along with exciton trapping and transport in stacked NPLs assemblies (homo FRET).The ñ denotes the director of liquid crystals.

Figure 5 .
Figure 5. (a) Plot of tilt angle versus external applied voltage for CdSe nanoplatelets incorporated FLC composites.(b) p-E ferroelectric loop of CdSe nanoplatelets incorporated FLC composites.

Figure 6 .
Figure 6.Time resolved photoluminescence spectra of pure FLC and CdSe NPLs incorporated FLC (C-0.002/FLC) at RT. Inset show normalised absorption spectra of CdSe NPLs and photoluminescence spectra of pure FLC.