Optical and electrical control of circularly polarised fluorescence in CdSe quantum dots
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Liquid Crystals
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Optical and electrical control of circularly
polarised fluorescence in CdSe quantum dots dispersed polymer stabilised cholesteric liquid crystal shutter
Rishi Kumar & K.K. Raina
To cite this article: Rishi Kumar & K.K. Raina (2016) Optical and electrical control of circularly polarised fluorescence in CdSe quantum dots dispersed polymer stabilised cholesteric liquid crystal shutter, Liquid Crystals, 43:7, 994-1001, DOI: 10.1080/02678292.2016.1155771To link to this article:
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Optical and electrical control of circularly polarised fluorescence in CdSe quantum dots dispersed polymer stabilised cholesteric liquid crystal shutter
Rishi Kumar a,b and K.K.Raina a
a
Material Research Lab,School of Physics and Materials Science,Thapar University,Patiala,India;b Department of Applied Physics,Giani Zail Singh Campus College of Engineering &Technology,Bathinda,India
ABSTRACT
In the present communication,we report on the synthesis and electro-optic investigation of photosensitive CdSe quantum dots (QDs)dispersed polymer stabilised liquid crystal (PSCLC)luminescent gel.The assembly of the fluorescence properties of CdSe QDs facilitate the aniso-tropy of PSCLC gel and hence manipulates the optical and electro-optic switching properties,which was further investigated using polarised fluorescence spectrophotometer.The circularly polarised fluorescence intensity was tuned electrically so as to affect the orientations of liquid crystal in the helix.It was found that the electro-optic switching behaviour of QDs-doped sample predicts the improvement in threshold voltage and hence makes them applicable for the switch-able liquid crystal contrivances with low power
consumption.
ARTICLE HISTORY
Received 31December 2015Accepted 14February 2016KEYWORDS
Quantum dots (QDs);polarised fluorescence spectroscopy;electro-optic switching;morphology
Introduction
In the past decades,semiconductor quantum dots (QDs)as an emerging nanomaterials has received sub-stantial attention of researchers around the world due to their unique chemical and physical properties.[1–3]So it becomes a subject of excruciating research activity targeting a broad reach of possible applications such as
light-emitting diodes (LEDs),photovoltaic,transistors and ?uorescent tags for biological imaging.[4–6]QDs have a few advantages over the conventionally used organic dye for making high-contrast liquid crystal displays because of their unique size-dependent optical properties such as high optical absorption,quantum yield,photo stability etc.[7–9]CdSe QDs also have
CONTACT Rishi Kumar
rishikumar.phd@https://www.sodocs.net/doc/8f6753722.html,
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relatively minute energy band gaps and thus are cap-able of harvesting photons in the UV-Vis and IR region.Capping of organic functional group around the CdSe nanocrystals provides isolation from the environment and additionally helps to facilitate charge carrier,which as a result provides narrow photo-lumi-nescence spectra.[6,10]
Aside from these major characteristic properties of semiconductor QDs,researchers around the globe have made an endeavour to utilise them in liquid crystal contrivances to amend their photoluminescence.[11–14]However,over the preceding years it has been visually perceived that the doping of QDs in LC mate-rials introduces novel effects related to pattern and defect formation.[15–17]Individual merits such as optical stability and flexible tuning of nematic and cholesteric liquid crystals were also achieved with the dispersion of colloidal CdSe QDs.Bobrovsky et al.[18] reported that the?uorescence emission of CdSe QDs embedded in chiral LCs is circularly polarised and that the dissymmetry factor of this polarisation could be optically or electrically controlled via conformational changes in the helical structure of the LC matrix.The lasing wavelength of the QD–CLC contrivances can be reversibly tuned by successive UV irradiation in a semiconductor laser.Semiconducting QDs should have an ability to build up a charge much more rarely than the LC molecules,therefore enabling them to decrement threshold voltage(V th).It gives the clue that CdSe QDs can tailor the other electro-optic prop-erties of liquid crystals such as an alteration of LC alignment,more expeditious switching time and opti-cal contrast.
In this article,we have made an effort to report here synthesis of CdSe QDs doped polymer stabilised cho-lesteric liquid crystal gels and their characterisation for their morphological,optical and electro-optic perfor-mance,which can improve the optical performance of liquid crystal shutters.Experiment
Synthesis of CdSe quantum dots
For the synthesis of CdSe QDs,the solvo-chemical co-precipitation route was followed via aqueous phase approach.[19]The analytical grade chemical reagents cadmium chloride,selenium and thioglycolic acid (TGA)were purchased from SD Fine Chemi.Ltd., Mumbai.These chemicals were used as received with-out any further puri?cation.
CdSe QDs were synthesised by direct reaction of Cd2+source solution and an Se2–source solution containing sodium hydrogen selenide(NaHSe). Colourless selenium source(Se2–)solution(Figure1
(a))was obtained by dissolving0.015g Se powder and
0.015g NaBH4into100ml deionised water followed by continuous magnetic stirring for1hours under nitro-gen.On the other side,cadmium(Cd2+)source (Figure1(b))was obtained by dissolving0.07g CdCl2·2.5H2O and0.07ml TGA into150ml deionised water.Its pH value was adjusted to11by adding1.0M NaOH solution.After separately deoxygenating by bubbling nitrogen for at least30min,the Cd and Se source solutions were directly mixed in a three-neck ?ask under nitrogen atmosphere.The chemical reac-tion that occurs is
Cd2ttSe2àtTGA!CdSeeTGAT(1) The molar ratio of Cd2+/Se2?/TGA in the round-bottom three-neck?ask was kept to be2/1/6.The pH of the solution was maintained at11by adding appro-priate amount of NaOH solution.As the nucleation and growth kinetics phenomena played a very impor-tant role in the synthesis of CdSe QDs nanocrystals,the refluxing temperature of the reaction during the synth-esis was kept at70°C with4hours of refluxing time. The appearance of yellow colour in the solution (Figure1(c))indicates the growth of fluorescent
CdSe Figure1.(colour online)Image of the prepared(a)selenium source,(b)cadmium source,(c)yellow colour emission of CdSe QDs
during the synthesis process.
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QDs.The solution was centrifuged at 12,000rpm for 15minutes to remove the aqueous solvent.The impu-rities within CdSe QDs were washed with ethanol solvents and filtered out,dried at 40°C in vacuum oven for half an hour.
For characterising the optical properties of CdSe QDs,UV-Vis absorption and FTIR spectra were recorded in Perkin Elmer spectrophotometer.The yel-low emission of freshly prepared CdSe QDs was char-acterised in Agilent fluorescence spectrophotometer (model:Cary Eclipse).
CdSe QDs dispersed PSCLC cell fabrication
The freshly synthesised CdSe QDs was dispersed into PSCLC gel in very minute concentrations of 0.02,0.04and 0.06weight%in polymer stabilised cholesteric liquid crystal (PSCLC)composite material,[20]which act as the base matrix of an undoped sample.It is composed of nematic liquid crystal BL036(purchased from E.Merck),UV curable NOA65optical adhesive and CB15chiral dopant in the ratio 90:5:5(wt%),respectively.These suspended mixtures of CdSe QDs doped PSCLC were allowed to dispense into chloro-form solution,ultrasonicated at 40°C for 30minute to ensure the homogeneity and appropriate dispersion of QDs in PSCLC gel.After that,these materials were allowed to fill into planar aligned LC cells at isotropic temperature via capillary action.These cells were com-posed of two glass substrates with a diameter of 1cm ×1cm covered by transparent conducting layers of indium tin oxide (ITO)followed by antiparallel alignment layers of polyamide.These sample cells were cured into UV radiations (intensity ~2mW/cm 2)for an hour to induce phase separation via poly-merisation induced phase separation (PIPS)process.[21]Electrical contact with conducting ITO substrate was made by using indium solder to perform the elec-tro-optical measurements.
Results and discussion
Characterisation of CdSe QDs
UV-Vis and FTIR spectroscopy make a valuable imple-ment to determine the physical properties of CdSe QDs as these techniques are optically sensitive to the QDs ’shape,size and the capping agent used.Figure 2shows typical absorption and PL scan of dispersed CdSe QDs in ethanol solvent.UV-Vis absorption spectra clearly demonstrate that the absorption onset appeared at around 520nm get blue-shifted by an order of ~196nm in comparison with bulk CdSe (716nm).
[22]This more vigorous blue-shifting denotes the mag-nification of CdSe nanocrystals (QDs)with quantum confinement effect.In addition,the CdSe QDs exhib-ited very narrow PL bands with FWHM ~55.95nm,which may be attributed to defect-free emission of CdSe nanocrystals having prodigiously high surface-to-volume ratio.It suggests excellent size distribution as well as the best colour purity in the fluorescent CdSe sample.
The size (diameter)and structure of CdSe QDs were determined using transmission electron microscopy (TEM)with diameter turning out to be 6–7nm.The morphology (Figure 3)suggests that the CdSe nano-crystals were well dispersed in ethanol solution and no aggregation was observed.The vicissitude in image contrast was additionally observed in the nanostructure around the boundary of CdSe particles,which suggests the capping of TGA around the CdSe
nanocrystals.
Figure 2.(colour online)UV-Vis absorption and photolumines-cence spectra of synthesised CdSe QDs.The red line shows the onset of absorption at 520
nm.
Figure 3.TEM micrograph of CdSe QDs dispersed in ethanol solution.
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The capping of TGA on CdSe nanocrystals was additionally corroborated using FTIR spectroscopy (Figure 4).The absorption peaks at 1380and 1672cm ?1substantiated the shifting of asymmetrical vibration and C=O stretching of the carboxylic group in TGA,respectively.Peaks around 2969and 3225cm ?1are due to sp3stretching of C-H and due to vibration of O-H group present in TGA.[23]Shifting of S-H group peak from 2560to 2342cm ?1was observed.This may be attributed to S-Cd bonds for-mation between thioglycolic acid (TGA)and CdSe.
Hence FTIR spectroscopy attested the capping of TGA on the CdSe QDs.
Characterisation of CdSe QDs dispersed polymer stabilised cholesteric films
Morphology analysis
To enable the evaluation of CdSe QDs in polymer-stabilised cholesteric liquid crystal films,a comparative morphological analysis of undoped and doped PSCLC sample cells has been performed (Figure 5).The mor-phology of sample cells was investigated in the switch off state (Figure 5(a –c))at 100X magnification through crossed polarisers in Olympus polarising microscope (Model BX-51P)interfaced with a charge coupled device (CCD).In undoped PSCLC sample cell,a poly-domain texture (Figure 5(a))composed of oily streaks was observed.It provides information about the perpendicular orientation of supramolecular helix to the conducting ITO glass substrates under planar anchoring.The authentic visualisation of oily streaks network of declination lines is dependent on the elas-ticity and surface anchoring to the liquid crystalline helix.At 0.02wt%CdSe QDs in PSCLC gel,the oily streaks network structure vanishes with the appearance of more minute yellow domains in the optical
textures
Figure 4.FTIR spectra of TGA capped CdSe
QDs.
Figure 5.(colour online)Morphology analysis of CdSe QDs dispersed polymer stabilised cholesteric liquid crystal films at CdSe QDs concentration 0,0.02and 0.06wt%,respectively.(a –c)Optical textures,(d –f)SEM microstructure analysis of polymer network after extracting liquid crystals,(g –i)hypothetical models of light scattering through QDs doped sample cells.
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(Figure5(b)).It suggests the helical superstructure deforms due to effective change in elastic interactions with the dispersion of CdSe QDs as the surface anchor-ing was kept constant in both undoped and doped sample cell.The appearance of yellow domains in the optical textures demonstrates the deportment of bend-ing of transmitted light intensity from the deformed helical supra-structure when the sample was viewed under crossed polarisers.The corresponding SEM microstructure(Figure5(d-f))was obtained by dipping the sample cell into acetone and abstracted the liquid crystal for obtaining fibre morphology.Figure5(e) clearly demonstrate that0.02wt%of CdSe QDs don’t perturb the fibrous morphology as that was observed in undoped sample(Figure5(d)).Even the growth of fibrous network in this sample is looking much denser and homogeneously dispersed than undoped sample.It designates that CdSe QDs availed in the fibre growth mechanism in PSCLC films by absorbing UV radiation during the polymerisation process.But above the cri-tical concentration0.06wt%of CdSe QDs,Figure5(f) depicts that the fibre growth rate is very slow during polymerisation process because of agglomeration of quantum dots with polymer matrix.This agglomerated QDs reduced the optical contrast of its corresponding polarised optical micro texture(Figure5(c))as well as induced poor phase separation.Hence the morpholo-gical investigations clearly reveal that CdSe QDs affect the fibre magnification mechanism.The light scattering through the QDs doped sample cell can be well under-stood with the help of hypothetical model(Figure5(g–i)).This reflects the optical and electro-optic switching behaviour.
Optical properties analysis
When exhilarated light passed through the supramo-lecular helical axis of CdSe QDs dispersed PSCLC samples,it gets circularly polarised.[24]The two plane-polarised beams,oscillating along parallel I ll and perpendicular I components,were scanned using polarised fluorescence spectrophotometer.Here one of the beams was retarded by90o,which was out of phase.Integrating both components together induced circularly polarised dichroism.Therefore,the polari-sation of circularly polarised light in PSCLC samples doped with fluorescent species followed the rela-tion[25]
P?
I IIàI
I IItI(2) where I II and I represent parallel and perpendicular component of emitted PL intensity,obtained(Figure6) with the help of polarised fluorescence spectroscopy. Figure7(a)represents the spectral shift of32nm (towards red shift from548to580nm)of fluorescence polarisation in visible yellow region with the dispersion of CdSe QDs in PSCLC matrix,whereas the fluores-cence polarisation in undoped PSCLC matrix was observed in deep blue(410nm)region(Figure7(b)). At0.02wt%of CdSe in PSCLC,the value of circularly polarisation increases approximately four times
than Figure6.(colour online)Polarised fluorescence spectra of CdSe QDs dispersed PSCLC sample cells with QDs concentrations:(a)0,
(b)0.02,(c)0.04and(d)0.06wt%.
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undoped (Table 1).This signi ?cant difference in the polarisation values (shown in Table 1)showed that CdSe QDs availed in aligning the helical liquid crystal and hence contribute to the inductively authorising.These calculated spectroscopic parameters favour the increment in authoritatively mandating at 0.02wt%CdSe QDs as suggested by the optical micrographs (Figure 5).The lowest order parameter was observed at 0.06wt%CdSe QDs in PSCLC matrix due to aggre-gation.The authoritatively mandating caused by CdSe QDs tailors the electro-optic switching properties of circularly polarised light.
Electro-optic switching analysis
Electrically controlled circularly polarised fluorescence replication was examined [26–28]by fluorescence spec-trophotometer (Agilent Technologies-Model Cary Eclipse).Square wave pulse was implemented to sample cells and recorded electrically tuned photoluminescence spectra (Figure 8)in Cary eclipse scan application soft-ware.The slit size of excitation and emission filters was maintained at 5nm during scanning of fluorescence.Figure 8shows that the relative fluorescence intensity %was gradually decremented with incrementing the abruptly applied electric field [24,26,29,30]until it reaches the saturation where it remained unchanged significantly.The applied electric field at 10%of the maximum value of relative fluorescence intensity was contemplated to be the threshold field (E 10)whereas 90%were considered as the driving field (E 90)of each sample cell.The slope between E 10and E 90values denotes the driving efficiency at a given applied electric field.As a result of electrically tuned scan (Figure 8),we observed that the driving field (E 90)get reduced to 2.6V/μm in 0.02wt%concentration of CdSe QDs dispersed PSCLC sample whereas it was 4.17V/μm for
the
Figure 7.Fluorescence polarisation measurements (a)Undoped PSCLC sample cells;(b)0.02,0.04and 0.06wt%CdSe QDs dispersed PSCLC sample cells.
Table 1.Calculated optical parameters from the polarised fluorescence spectra of CdSe QDs dispersed PSCLC sample cells using dichroism measurements.
Polarised intensity
CdSe (wt.%)Emission wavelength (nm)
I II I ⊥Fluorescence polarisation Fluorescence anisotropy Order parameter 0410694515+0.28970.21380.10380.025********+0.84790.84790.45310.0456*******+0.44240.44280.16860.06
580
664
606
+0.0915
0.0912
0.0309
Figure 8.(colour online)Electrically controlled fluorescence scan of CdSe QDs dispersed PSCLC cells.
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undoped.This clearly denotes that liquid crystal mole-cules oriented much more expeditious in homeotropic state when CdSe QDs were dispersed 0.02wt%in PSCLC sample cell.Whereas the orientations of liquid crystal molecules get constrained in 0.06wt%CdSe QDs dis-persed sample.This hindrance [25,31]in electro-optic switching behaviour of this sample supports the reduc-tion in optical contrast of texture (Figure 5(c))where the aggregation of QDs was found.Also,the analysis of Figure 8depicts that the aggregation of QDs obstruct the electrical reorientations and hence additionally reduces the decrementing amplitude of fluorescence intensity during emission.This mechanism of electrically tuned fluorescence suggests CdSe QDs tailor the electro-optic switching performance of polymer stabilised cho-lesteric liquid crystal shutter.
Conclusion
In summary,we have successfully investigated the effects of CdSe QDs on PSCLC films.We conclude that a combination of fluorescent properties of CdSe QDs with PSCLC matrix manipulates circularly polarised fluorescence intensity and simultaneously improves the electro-optic switching behaviour.Hence CdSe QDs can become potential candidates for their utility in high-contrast and less power con-suming liquid crystal displays.Further research has to be carried out to explore their more practical aspects,which can prove a most promising avenue for devel-oping photonic band gap structures.
Acknowledgement
The valuable comments from the reviewer are gratefully acknowledged by the authors.
Disclosure statement
No potential conflict of interest was reported by the authors.
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