Open Access
Add to BibSonomy
Abstract
Hybrid fluorescent cholesteric liquid crystalline (CLC) materials are representatives of “smart” soft matter, and are characterized by light emission that can be flexibly controlled by various external stimuli. This fact is due to the many possibilities for potential applications in the fields of photonics and optics stimulating design, and study of this type of hybrid materials. Here, we report on the optical and fluorescence properties of the hybrid CLC material based on a low-molecular-weight CLC matrix and CdSe/ZnS quantum dots (QDs) stabilized by LC diblock copolymers. The hybrid CLC material is characterized by the cholesteric phase in a wide temperature range, the high loading of QDs, and no QD aggregation. We demonstrate that the cholesteric stop band alters characteristics of the QD emission due to the resonance effect. This makes the polarization state and wavelength of the QD emission thermo- and angle-dependent. This work provides a way for the design of a wide range of field-controllable photonic devices for various applications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fluorescent liquid crystalline (LC) materials exhibit a fluorescence emission that can easily be tuned under various external stimuli making these soft materials promising for diverse photonic applications. Due to optical anisotropy of liquid crystals, which are easily tunable by external fields, the fluorescent LC materials allow controlling intensity [1], polarization state [2], wavelength [3] and even direction [4] of their fluorescence emission. Another peculiarity of liquid crystals is their capability of self-assembly, which makes photonic devices based on the fluorescent LC materials low-cost and easily accessible.
Although various types of LC phases are used for designing LC based photonic devices, the cholesteric LC (CLC) phase (or chiral nematic phase) stands out among other because of its intrinsic helix structure. The formation of the helix structure causes the 1D photonic stop band for the circularly polarized light with the same handedness as the chiral molecular order [5]. For normal incidence, the photonic stop band of a planar-aligned CLC is centered at the wavelength of λc = P (no+ne) / 2, where P is the pitch of the helix structure, no and ne are ordinary and extraordinary refractive indices. The pitch of the helix structure is sensitive to various external fields (light [6], electrical [7,8], thermal [9] and so on [10,11]) allowing flexible tuning of the cholesteric stop band. These features of CLC have allowed various photonic devices such as tunable color filters [12] and LED displays with wide color gamut [13] to be designed.
A vast diversity of fluorescent materials and photonic devices have been engineered through embedding of organic fluorescent dyes in the CLC phase [14–16]. However, most organic florescent dyes suffer from photodegradation, which hinders practical application of these materials. To overcome this disadvantage, organic dyes can be replaced by inorganic fluorescent semiconductor nanoparticles or quantum dots (QDs) that have high photostability [17]. Besides that, QDs are characterized by size-tunable properties, high fluorescence quantum yield and emission brightness [18,19]. Following the above consideration, many hybrid CLC materials and devices based on them have been designed and studied including light sources with controllable polarization [20–22], photo-, thermo- and electrocontrollable lasers [23,24], and single-photon sources of circularly polarized light [25,26].
Despite the significant progress in designing hybrid LC-QD materials, including the ones based on CLCs, achieving phase compatibility between inorganic nanoparticles and an organic LC media remains a sophisticated task. If compatibility between QDs and the LC media is poor, nanoparticles may be induced to form aggregates resulting in macrophase separation in the material. Over time, these effects can worsen optical properties of the material in uncontrollable (or even unpredictable) manner. One of the ways to improve the phase compatibility is in using QD ligands containing mesogenic-like groups. This approach has allowed hybrid CLC materials with the QD loading up to 0.1 wt.% to be prepared [27]. Such materials exhibited circularly polarized fluorescence emission and didn’t suffer from QD aggregation. In addition, resonance between the QD emission and the cholesteric stop band was observed resulting to the alteration of the polarization state and emission wavelength of the QDs [22]. However, the QD loadings in hybrid materials available by this approach are quite low and insufficient for some applications such as a lasing. On the other hand, lasing was demonstrated in the hybrid CLC materials with higher QD loading as reported in [24] but the distribution of QDs within organic matrix as well as the material stability over time were not revealed. Thus, the further development of approaches to the preparation of hybrid fluorescent LC-QD materials and study of their properties are desirable.
Recently, we have reported a new approach to stabilizing dispersions of CdSe/ZnS QDs in LC media [28,29]. Our approach is based on capping QDs with LC block copolymers containing polyvinylpyridine (pVP) block and a LC block consisting of nematogenic phenyl benzoate (PhM) units (Fig. 1(a)). As the pVP block can form coordinate bonds with the QD surface through nitrogen atoms and the LC block is compatible with LC media, these LC block copolymers can be used as polymer ligands for QDs that assure high compatibility between QDs and LC phase. Using this approach, we have managed to raise the QD loading to more than 1.0 wt. % in CLCs with no QD aggregation being observed.
Fig. 1. (a) Chemical structures of the constituent compounds and (b) schematic representation of the inner structure of the hybrid CLC material.
Here, we aim to study optical and fluorescence features of the hybrid CLC materials that were prepared according to our previous approach with the focus on thermal and angular dependence of their fluorescence emission. Before discussing these properties, we consider the phase behavior and the QD distribution in the hybrid materials that are fundamental for subsequent consideration of their optical properties.
2. Experimental
Chloroform, pyridine, 4′-pentyl-4-biphenylcarbonitrile (5CB), and 2,6-di-tert-butyl-4-methylphenol (BHT) were purchased from Aldrich. Chloroform was passed through aluminum oxide and distilled. Pyridine was dried over KOH and distilled over CaH2. Nematogenic diacrylate RM257 and photoinitiator Irgacure 651 (Fig. 1(a)) were purchased from Synthon and Ciba company, respectively. The synthesis of LC diblock copolymers was performed by reversible addition-fragmentation transfer polymerization as described previously [30]. CdSe/ZnS QDs covered with oleylamine and encapsulated in TOPO storage matrix were synthesized according to a previous report [31]. To prepare QDs coated with LC block copolymers a ligand exchange procedure was applied [28]. In the course of this procedure, oleylamine on the surface of CdSe/ZnS QDs was first replaced by pyridine which was later exchanged to a LC block copolymer containing poly(vinylpyridine) block. Chiral additives HexSorb and ButSorb (Fig. 1(a)) were synthesized as described previously [32].
Polarizing optical microscopic (POM) observations were conducted on a LOMO P-112 polarizing microscope equipped with a CCD camera and a Mettler FP 80 heating stage.
TEM images were taken with a LEO 912 AB Omega transmission electron microscope (Carl Zeiss) operating at an accelerating voltage of 100 kV. TEM specimen were prepared according to the following procedure. The dispersion of composite in chloroform were drop-casted onto a PET substrate, and the obtained films were annealed at 140 °C for 3 h. Then, the annealed samples were embedded in an epoxy resin and cured overnight. The sample was subsequently microtomed to a thickness of about 50 nm using a Reichert-Gung ultramicrotome with a diamond knife (Diatome) at room temperature. The microtomed sections floating in water were placed on copper TEM grids and stained with iodine for 1 h.
The phase transition temperatures of the samples were studied by differential scanning calorimetry (DSC) using a PerkinElmer DSC-7 thermal analyzer at the scanning rate of 10 K/min. The samples were prepared as 10–20 mg pellets.
To measure transmittance, reflectance and fluorescence spectra the hybrid CLC material was placed into the 20 µm-thickness plane-parallel glass cell, both inner surfaces of which were coated with rubbed polyimide layer (ZLI 2650). Samples were annealed at room temperature for 48 h. Transmittance spectra of the samples were recorded by Unicam UV-500 UV-Vis spectrophotometer. Fluorescence and reflectance spectra were recorded using an M266 automated monochromator/spectrograph (SOLAR Laser Systems, Belarus) equipped with a CCD detector U2C-16H7317 (Ormins, Belarus) in a homemade light-collecting inverted system using a 100X/0.80 MPLAPON lens (Olympus, Japan) and a homemade confocal unit with two 100-mm objective lenses. Exciting light was cut off by Semrock 488-nm RazorEdge ultrasteep longpass edge filters (Semrock, USA). Fluorescence of QDs was excited by an KLM-473/h-150 laser (Plazma, Russia) operating at 473 nm. An incident light intensity was equal to 100 mW/cm2 as measured with a LaserMate-Q (Coherent) intensity meter. To heat samples a Mettler FP 80 heating stage was used. To achieve the equilibrium state, samples were annealed at each temperature for 30 min before spectral measurements.
3. Results and discussion
The chemical structures of the constituents used for the preparation of the hybrid CLC material and their weight fractions are shown in Fig. 1(a). The main part of the material consists of nematogenic compounds, 5CB and RM257, the ratio of which specifies the isotropization temperature of the LC phase. Chiral additives HexSorb and BuSorb were added to the compound to induce the formation of a cholesteric phase. Their weight fractions were adjusted to provide an overlap of the cholesteric stop band with the fluorescence spectrum of QDs. The inhibitor BHT was added to prevent undesirable polymerization of RM257, which is a diacrylic compound. To stabilize the dispersion of QDs and prevent their aggregation, the LC diblock copolymer pVP60-b-PhM40 was used as the QD ligand as described earlier [28].
As determined by DSC, the glass transition temperature of the hybrid material is −51 °C and the isotropization temperature of the CLC phase is equal to 50 °C (Fig. 2(a)). Both temperatures are close to those of the initial CLC matrix, what indicates indirectly a good compatibility between inorganic QDs and organic CLC matrix. The temperature range of the CLC phase covers an interval of about 100 °C and includes the ambient temperatures that are common for numerous practical applications. Below the isotropization temperature the hybrid material forms a cholesteric phase as confirmed by POM imaging (Fig. 2(b)), where the so-called “oily streaks” are observed, being the typical defects of a cholesteric phase [33].
Fig. 2. (a) DSC curves of the CLC matrix and the hybrid CLC material. (b) POM and (c) TEM images of the hybrid CLC material.
The essential characteristic of hybrid LC materials containing inorganic nanoparticles is the distribution of the latter in the bulk and the possible presence of their aggregates. We used TEM to study the distribution of QDs in our hybrid material because this method enables direct visualization of inorganic nanoparticles in an organic matrix. Since common TEM technique is intended for a study of solid samples, radical polymerization of the diacrylic compound RM257 was induced to prepare TEM specimen resulting in the formation of a polymer network. For this purpose, the photoinitiator Irgacure 651 was added to the sample of the hybrid material and the photopolymerization was initiated by exposure to UV light (λ = 365 nm, I = 1 mW/cm2, t = 10 min).
As can be seen from the TEM image of the cross-linked sample (Fig. 2(c)), QDs appear as black spots that are randomly distributed in the bulk, and no QD aggregates are present. Note that the same QD distribution patterns were observed in different parts of the TEM specimen, what indicates that QD distribution is quite homogeneous in the whole sample. This finding corroborates the good compatibility between QDs decorated with the LC diblock copolymer and the CLC matrix.
A cholesteric phase is known to be capable of selective reflection of circularly polarized light. The handedness of the reflected circularly polarized light and its wavelength are defined by the handedness of the cholesteric helix and its pitch, respectively. As seen from Fig. 3(a), the reflectance spectrum of the hybrid material is characterized by the pronounced peak located within the range of 500-650 nm. Since the pitch of the cholesteric helical structure is temperature-dependent, the cholesteric stop band can be controlled by varying the temperature. As can be seen from Fig. 3(a), mild heating of the hybrid material resulted in red-shifting of the cholesteric stop band, with spectral position of the band edges being linearly dependent on temperature (Fig. 3(b)). The same thermal dependence was observed for transmittance spectra of the hybrid CLC material (Fig. S1). These properties are typical of nematic LCs doped with chiral additives [34] in contrast to cholesterol derivatives where non-linear blue-shifting with increase in temperature is more typical. Altering temperature from 25 °C to 44 °C, the cholesteric stop band can be shifted on about 60 nm that is enough to tune the band edges through the whole QD emission spectrum, as it will be shown below. At higher temperature the shift of selective light reflection peak becomes more pronounced and the reflection peak eventually disappears at 48 °C. More pronounced shift observed above 46 °C can be associated with the thermal dependence of refractive indices, which can demonstrate strong non-linear behavior near isotropization temperature of a LC phase [35]. Note that on heating the spectral positions of band edges become closer and closer to each other (Fig. 3(b)). It can be explained by difference in thermal dependence of liquid crystal refractive indices, namely, ne decreases and no increases when temperature is increased [35].
Fig. 3. (a) Changes in reflectance spectra and (b) the spectral position of the band edges of the hybrid CLC material at different temperatures.
Introducing light emitters such as fluorescent dyes or QDs into the cholesteric phase can modify the polarization and wavelength of their emission if the fluorescence spectra and the cholesteric stop band overlap [10,27]. Since the cholesteric phase reflects circularly polarized light with the same handedness, the light emission of the fluorescent material should be circularly polarized with the opposite handedness. Here, we use isosorbide derivatives as chiral dopants that induce the formation of a right-handed cholesteric helical structure [32], thus a left-handed circularly polarized emission should be expected for our hybrid material.
As seen from Fig. 4(a), the circularly polarized components of the fluorescence emission of the hybrid CLC material demonstrate quite different spectra. The spectrum of the left-handed component is virtually similar to that of the QDs coated with the block copolymer pVP60-b-pPhM40 (Fig. S2). On the contrary, the spectrum of the right-handed component exhibits two peaks the spectral position and shape of which differ from that of initial QDs. These effects can be explained with optical features of the cholesteric phase, which can play a role of microcavity for circularly polarized light. When QDs are embedded in the cholesteric phase their emission can be altered due to coupling with cholesteric microcavity. This microcavity resonance results in intensity enhancement of the right circularly polarized component at the band edges in comparison with opposite one. As can be seen from Fig. 4(a), the resonance emission peaks are located at short- and long-wavelength band edges of the cholesteric stop band, but inside the stop band light emission is low. The spectral position of the resonance peaks can be described by the photonic density of states, which is high at the band edges [36].
Fig. 4. (a) Reflectance spectrum, left-handed and right-handed circularly polarized fluorescence spectra, and (b) fluorescence dissymmetry factor of the hybrid CLC material. QDs were excited with a blue laser (λ = 473 nm, I = 100 mW/cm2).
The degree of circular polarization of the QD emission at the specific wavelength can be characterized by dissymmetry factor ge calculated using the equation:
where IL and IR are the intensities of left and right-handed circularly polarized light, respectively. For our hybrid material, the dissymmetry factor reaches 1.5 within the cholesteric stop band indicating that the QD emission has left-handed circular polarization and the polarization degree is relatively high (Fig. 4(b)). Near the edges of the cholesteric stop band the dissymmetry factor becomes negative due to the predominant right-handed circularly polarized component. This domination is related to the resonance in this region as it has been discussed above.Thermal dependence of the pitch of the helical structure implies that the resonance between the QD emission and the cholesteric cavity may also be temperature-dependent. Temperature-dependent spectra of the right-handed circularly polarized fluorescence component represented in Fig. 5(a) shows that as temperature is increased the intensity and spectral position of the long-wavelength and the short-wavelength peaks are changed. The resonance peaks are red-shifted in a linear manner (Fig. 5(c)) that agrees well with thermal dependence of the band edges (Fig. 3(b)). These resonance effects become inefficient above 44 °C and the fluorescence spectrum of the right-handed component becomes similar to that of left-handed component because the cholesteric stop band disappears at this temperature. Note that the cholesteric stop band disappears at slightly lower temperature than the isotropization temperature that is likely to be aroused by distortion of the helical structure and the reduction of order parameter near the isotropization temperature. As the result of the temperature-induced shift in the right-handed component of fluorescence spectra, corresponding red-shifting of dissymmetry factor spectra is observed (Fig. 5(b)). The increase in temperature provokes some decrease in dissymmetry factor but its magnitude remains relatively high. This decrease appears to be associated with the reduction of order parameter at higher temperatures.
Fig. 5. (a) Right-handed circularly polarized component of fluorescence spectra and (b) dissymmetry factor spectra of the hybrid material for temperature in the 27-48 °C range. (c) Thermal dependence of the short- and long-wavelength peaks of right-handed circularly polarized component of fluorescence spectra. The dash line marks the spectral position of the peak of left-handed circularly polarized component.
Besides the thermal effects discussed above, the cholesteric stop band becomes shifted towards shorter wavelengths if the sample is observed at oblique angles [37]. The angular dependence of the photonic stop band means that resonance peaks in the florescence spectrum should be also angle-dependent. Indeed, in the case of dye-doped cholesteric liquid crystals, fluorescence spectra measured at oblique angles has demonstrated strong variations of both intensity and polarization [38,39].
We limited ourselves to the angles of emergence from 0° to 30° because the spectral characteristics of cholesteric samples at larger angles are not trivial and beyond the scope of our report. As can be seen from Fig. 6(a), reflectance spectra of the hybrid material became blue-shifted when the angle of incidence was increased. The transmittance spectra show the same behavior (Fig. 3(S)). These findings indicate that the spectral position of the photonic stop band is angle-dependent. As expected, the intensity and spectral position of the right-handed circularly polarized component of the fluorescence emission are also strongly angle-dependent (Fig. 6(c)). The increase in the angle of emergence results in blue-shifting of the resonance peaks that finally disappear at 25°. On the contrary, in the case of the left-handed circularly polarized component, the lineshape of emission peak is invariable (not shown here). The observed blue-shifting of the resonance peaks is related to the corresponding shift of the cholesteric stop band which becomes off-resonance at angles of more than 25°. The fluorescence emission collected at oblique angles is characterized by relatively high degree of circular polarization for angles up to 15° as it can be seen from dissymmetry factor spectra (Fig. 6(b)). Thus, the light emission of the hybrid CLC material exhibits blue shifting at oblique angle, what allows controlling of the resonance condition. In the combination with the thermal dependence, this fact allows shifting the resonance emission peaks in both directions of the visible spectrum.
Fig. 6. (a) Reflectance spectra, (b) dissymmetry factor spectra and (c) right-handed circularly polarized fluorescence of the hybrid material measured for angle of incidence in the 0° – 30° range.
Note that the observed angular dependence of the fluorescence emission should be taken into account for the case of designing photonic devices to optimize their performance. In addition, this feature can be used to develop specialized devices, for instance, multiwave out-of-normal band-edge lasers [40], where emission wavelength can be controlled by an angle of emergence.
4. Conclusion
We have investigated phase behavior, morphology and optical properties of the hybrid material based on low-molecular-weight cholesteric liquid crystals and CdSe/ZnS quantum dots stabilized with LC diblock copolymer. We found that the hybrid CLC material forms the cholesteric phase in wide temperature range, from −51°C to 50°C, that is similar to that of the initial CLC matrix. In addition, no aggregation of QDs was observed indicating good compatibility between nanoparticles and the CLC matrix. It was shown that the hybrid material preserved the fluorescent properties intrinsic to QDs and the fluorescence emission of the material is left-handed circularly polarized within the wavelength range of cholesteric stop band. At the both band edges the resonance between the QD emission and the cholesteric cavity was observed. The emission wavelength and polarization are temperature-dependent, with resonance peaks being red-shifted at higher temperature. On the contrary, the increase in emergence angle of fluorescence emission leads to blue-shifting of resonance peaks. The obtained results may be helpful for further designing of fluorescent hybrid CLC materials with field-controllable light emission, which can be exploited in various photonic devices.
Funding
Russian Science Foundation (19-13-00029, 19-73-00058, 20-13-00358).
Acknowledgments
This study was financially supported by the Russian Science Foundation (grant no. 19-73-00058) – hybrid material preparation, optical, thermal and TEM measurements (M. Bugakov). The authors (N. Boiko and V. Shibaev) acknowledge the support of the Russian Science Foundation, grant no. 19-13-00029 (synthesis of LC block copolymers). The part of this study dealing with the synthesis of CdSe/ZnS quantum dots was supported by the Russian Science Foundation, grant no. 20-13-00358 (P. Samokhvalov).
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
Supplemental document
See Supplement 1 for supporting content.
References
1. B. Liu, Z. Zheng, X. Chen, and D. Shen, “Low-voltage-modulated laser based on dye-doped polymer stabilized cholesteric liquid crystal,” Opt. Mater. Express 3(4), 519 (2013). [CrossRef]
2. B. A. San Jose, J. Yan, and K. Akagi, “Dynamic switching of the circularly polarized luminescence of disubstituted polyacetylene by selective transmission through a thermotropic chiral nematic liquid crystal,” Angew. Chem. Int. Ed. 53(40), 10641–10644 (2014). [CrossRef]
3. I. P. Ilchishin, L. N. Lisetski, and T. V. Mykytiuk, “Reversible phototuning of lasing frequency in dye doped cholesteric liquid crystal and ways to improve it [Invited],” Opt. Mater. Express 1(8), 1484 (2011). [CrossRef]
4. S. Cho, H. Yoshida, and M. Ozaki, “Emission direction-tunable liquid crystal laser,” Adv. Opt. Mater. 8(16), 2000375 (2020). [CrossRef]
5. Z. He, K. Yin, and S.-T. Wu, “Peculiar polarization response in chiral liquid crystal stacks for multispectral camouflage,” Opt. Express 29(2), 2931 (2021). [CrossRef]
6. H. Huang, T. Orlova, B. Matt, and N. Katsonis, “Long-lived supramolecular helices promoted by fluorinated photoswitches,” Macromol. Rapid Commun. 39(1), 1700387 (2018). [CrossRef]
7. A. Bobrovsky and V. Shibaev, “Novel type of combined photopatternable and electro-switchable polymer-stabilized cholesteric materials,” J. Mater. Chem. 19(3), 366–372 (2009). [CrossRef]
8. Y.-C. Hsiao, I. V. Timofeev, V. Y. Zyryanov, and W. Lee, “Hybrid anchoring for a color-reflective dual-frequency cholesteric liquid crystal device switched by low voltages,” Opt. Mater. Express 5(11), 2715 (2015). [CrossRef]
9. W. Zhang, A. P. H. J. Schenning, A. J. J. Kragt, G. Zhou, and L. T. de Haan, “Reversible thermochromic photonic coatings with a protective topcoat,” ACS Appl. Mater. Interfaces 13(2), 3153–3160 (2021). [CrossRef]
10. H. Finkelmann, S. T. Kim, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Tunable mirrorless lasing in cholesteric liquid crystalline elastomers,” Adv. Mater. 13(14), 1069–1072 (2001). [CrossRef]
11. K. Yin, Y.-H. Lee, Z. He, and S.-T. Wu, “Stretchable, flexible, rollable, and adherable polarization volume grating film,” Opt. Express 27(4), 5814 (2019). [CrossRef]
12. Z. He, H. Chen, Y.-H. Lee, and S.-T. Wu, “Tuning the correlated color temperature of white light-emitting diodes resembling Planckian locus,” Opt. Express 26(2), A136 (2018). [CrossRef]
13. E.-L. Hsiang, Y. Li, Z. He, T. Zhan, C. Zhang, Y.-F. Lan, Y. Dong, and S.-T. Wu, “Enhancing the efficiency of color conversion Micro-LED display with a patterned cholesteric liquid crystal polymer film,” Nanomaterials 10(12), 2430 (2020). [CrossRef]
14. M. H. Song, B. Park, K.-C. Shin, T. Ohta, Y. Tsunoda, H. Hoshi, Y. Takanishi, K. Ishikawa, J. Watanabe, S. Nishimura, T. Toyooka, Z. Zhu, T. M. Swager, and H. Takezoe, “Effect of phase retardation on defect-mode lasing in polymeric cholesteric liquid crystals,” Adv. Mater. 16(910), 779–783 (2004). [CrossRef]
15. M. Humar and I. Muševič, “3D microlasers from self-assembled cholesteric liquid-crystal microdroplets,” Opt. Express 18(26), 26995 (2010). [CrossRef]
16. H. Coles and S. Morris, “Liquid-crystal lasers,” Nat. Photonics 4(10), 676–685 (2010). [CrossRef]
17. U. Resch-Genger, M. Grabolle, S. Cavaliere-Jaricot, R. Nitschke, and T. Nann, “Quantum dots versus organic dyes as fluorescent labels,” Nat. Methods 5(9), 763–775 (2008). [CrossRef]
18. J. M. Pietryga, Y.-S. Park, J. Lim, A. F. Fidler, W. K. Bae, S. Brovelli, and V. I. Klimov, “Spectroscopic and device aspects of nanocrystal quantum dots,” Chem. Rev. 116(18), 10513–10622 (2016). [CrossRef]
19. P. Reiss, M. Carrière, C. Lincheneau, L. Vaure, and S. Tamang, “Synthesis of semiconductor nanocrystals, focusing on nontoxic and Earth-abundant materials,” Chem. Rev. 116(18), 10731–10819 (2016). [CrossRef]
20. A. Bobrovsky, K. Mochalov, V. Oleinikov, A. Sukhanova, A. Prudnikau, M. Artemyev, V. Shibaev, and I. Nabiev, “Optically and electrically controlled circularly polarized emission from cholesteric liquid crystal materials doped with semiconductor quantum dots,” Adv. Mater. 24(46), 6216–6222 (2012). [CrossRef]
21. A. Bobrovsky, K. Mochalov, V. Oleinikov, and V. Shibaev, “Glass-forming photoactive cholesteric oligomers doped with quantum dots: novel materials with phototunable circularly polarised emission,” Liq. Cryst. 38(6), 737–742 (2011). [CrossRef]
22. A. L. Rodarte, C. Gray, L. S. Hirst, and S. Ghosh, “Spectral and polarization modulation of quantum dot emission in a one-dimensional liquid crystal photonic cavity,” Phys. Rev. B 85(3), 035430 (2012). [CrossRef]
23. L.-J. Chen, J.-D. Lin, and C.-R. Lee, “An optically stable and tunable quantum dot nanocrystal-embedded cholesteric liquid crystal composite laser,” J. Mater. Chem. C 2(22), 4388–4394 (2014). [CrossRef]
24. L.-J. Chen, J.-D. Lin, S.-Y. Huang, T.-S. Mo, and C.-R. Lee, “Thermally and electrically tunable lasing emission and amplified spontaneous emission in a composite of inorganic quantum dot nanocrystals and organic cholesteric liquid crystals,” Adv. Opt. Mater. 1(9), 637–643 (2013). [CrossRef]
25. S. G. Lukishova, L. J. Bissell, J. Winkler, and C. R. Stroud, “Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host,” Opt. Lett. 37(7), 1259 (2012). [CrossRef]
26. S. G. Lukishova, L. J. Bissell, C. R. Stroud, and R. W. Boyd, “Room-temperature single photon sources with definite circular and linear polarizations,” Opt. Spectrosc. 108(3), 417–424 (2010). [CrossRef]
27. A. L. Rodarte, Z. S. Nuno, B. H. Cao, R. J. Pandolfi, M. T. Quint, S. Ghosh, J. E. Hein, and L. S. Hirst, “Tuning quantum-dot organization in liquid crystals for robust photonic applications,” ChemPhysChem 15(7), 1413–1421 (2014). [CrossRef]
28. M. Bugakov, N. Boiko, P. Samokhvalov, X. Zhu, M. Möller, and V. Shibaev, “Liquid crystalline block copolymers as adaptive agents for compatibility between CdSe/ZnS quantum dots and low-molecular-weight liquid crystals,” J. Mater. Chem. C 7(15), 4326–4331 (2019). [CrossRef]
29. M. Bugakov, S. Abdullaeva, P. Samokhvalov, S. Abramchuk, V. Shibaev, and N. Boiko, “Hybrid fluorescent liquid crystalline composites: directed assembly of quantum dots in liquid crystalline block copolymer matrices,” RSC Adv. 10(26), 15264–15273 (2020). [CrossRef]
30. M. A. Bugakov, N. I. Boiko, E. V. Chernikova, S. S. Abramchuk, and V. P. Shibaev, “New comb-shaped triblock copolymers containing a liquid-crystalline block and polyvinylpyridine amorphous blocks: synthesis and properties,” Polym. Sci. Ser. C 60(1), 3–13 (2018). [CrossRef]
31. V. Krivenkov, P. Samokhvalov, M. Zvaigzne, I. Martynov, A. Chistyakov, and I. Nabiev, “Ligand-mediated photobrightening and photodarkening of CdSe/ZnS quantum dot ensembles,” J. Phys. Chem. C 122(27), 15761–15771 (2018). [CrossRef]
32. A. Y. Bobrovsky, N. I. Boiko, and V. P. Shibaev, “New chiral-photochromic dopant with variable helical twisting power and its use in photosensitive cholesteric materials,” Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 363(1), 35–50 (2001). [CrossRef]
33. O. D. Lavrentovich and D.-K. Yang, “Cholesteric cellular patterns with electric-field-controlled line tension,” Phys. Rev. E 57(6), R6269–R6272 (1998). [CrossRef]
34. H. F. Gleeson and H. J. Coles, “Optical properties of chiral nematic liquid crystals,” Mol. Cryst. Liq. Cryst. Inc. Nonlinear Opt. 170(1), 9–34 (1989). [CrossRef]
35. J. Li, S. Gauza, and S.-T. Wu, “Temperature effect on liquid crystal refractive indices,” J. Appl. Phys. 96(1), 19–24 (2004). [CrossRef]
36. J. Schmidtke and W. Stille, “Fluorescence of a dye-doped cholesteric liquid crystal film in the region of the stop band: theory and experiment,” Eur. Phys. J. B 31(2), 179–194 (2003). [CrossRef]
37. C. Oldano, E. Miraldi, and P. T. Valabrega, “Comparison between measured and calculated reflectance spectra from a monodomain cholesteric liquid crystal,” Jpn. J. Appl. Phys. 23(Part 1, No. 7), 802–809 (1984). [CrossRef]
38. A. M. Risse and J. Schmidtke, “Angular-dependent spontaneous emission in cholesteric liquid-crystal films,” J. Phys. Chem. C 123(4), 2428–2440 (2019). [CrossRef]
39. L. Penninck, J. Beeckman, P. De Visschere, and K. Neyts, “Light emission from dye-doped cholesteric liquid crystals at oblique angles: simulation and experiment,” Phys. Rev. E 85(4), 041702 (2012). [CrossRef]
40. S. P. Palto, N. M. Shtykov, B. A. Umanskii, and M. I. Barnik, “Multiwave out-of-normal band-edge lasing in cholesteric liquid crystals,” J. Appl. Phys. 112(1), 013105 (2012). [CrossRef]
