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Abstract
All-inorganic perovskite quantum dots (PQDs) have been effectively incorporated in the three-dimensional ordered structure of blue phase liquid crystals (BPLCs) to stabilize the BPLCs. Uniform dispersion, reduced phase transition temperature, widened BP temperature range, dynamic and fast electro-optical response and static optical display of selective reflection mode and photoluminescence mode have been confirmed with a given concentration of PQDs. Such a novel strategy of assembling all-inorganic PQDs in BPLCs shows favorable prospects for wide-range and near room temperature BPLCs, responsive BPLCs, multifunctional display materials and tunable bandgap lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Blue phase liquid crystals (BPLCs) are a distinct class of chiral LCs in which the director axes self-assemble into double twisted cylinders and form three-dimensional (3D) ordered lattices. The dielectric constant inside the BPLCs is tailor-made to vary periodically in three dimensions, giving rise to a 3D photonic bandgap. These extraordinary physical and optical properties make BPLCs attractive materials for versatile manipulation of light, such as display devices, optical sensors, smart gratings, tunable lasers and so on [1–5].
However, one disadvantage of BPLCs as predicted theoretically and proved experimentally, is that they have limited thermal stability, they tend to exist over a narrow temperature range (~1 °C) between the isotropic (I) phase and the chiral nematic (CN) phase, which restricts their practical applicability. In the meantime, for some BPLCs, their BP temperature ranges are inevitably located at high temperatures (70 °C or above) due to their high melting points [6, 7], which limits their optical application at room temperature. Therefore, the widening of BP temperature range and the shift of BP temperature range from high temperature zone to low temperature zone are of great significance. Various alternatives, including polymer networks, nanoparticles and the synthesis of new mesogens, have been widely applied to widen the BP temperature range [8–11]. And the most common method of shifting BP temperature range to low temperature zone is to use nematic liquid crystals or nematic mixtures (5CB, E7, or other commercial liquid crystals) with low melting points as main component of BPLCs [12, 13].
Owing to the tremendous progress in nanoscience and quantum science, nanomaterials have been widely applied in the field of BPLCs to fabricate functional photonic devices with enhanced electrical, optical, thermal and magnetic properties [14–19]. All-inorganic perovskite quantum dots (PQDs) with a size of less than 10 nm possess the miscibility with BPLC matrix and the potential ability to stabilize the microscopic structure of BPLCs. Compared with polymer network in polymer-stabilized BPLCs, PQDs cannot only act as a stabilization element, but also can act as a functional element to fabricate multifunctional BPLC-PQD composites owing to the superior optical properties of all-inorganic PQDs in the aspects of photoluminescence, light-emitting diodes, lasers and optoelectronic devices [20–22]. In the present study, all-inorganic PQDs CsPbBr3 are assembled in the 3D structure of BPLCs, the incorporation of PQD guest in the BPLC host in the micro level are characterized, the effect of the incorporated PQDs on the phase transition temperatures, BP temperature range and electro-optical response are analyzed. As the CsPbBr3 PQDs possess photochromism characteristic, the resulted BPLC/PQD composites can be considered as a kind of fluorescent BPLC, which possesses potential application in the field of liquid crystal display, optical sensors, laser host and fluorescent displays.
2. Experimental section
2.1. Materials
The CsPbBr3 PQDs are synthesized based on a phase separation method, in details, firstly, PbBr2 (0.1 mmol) and CsBr (0.1 mmol) are dissolved in DMF (3 mL). Oleic acid (OA) (0.3 mL) and oleylamine (OAM) (0.2 mL) are added to stabilize the precursor solution. Then, 1 mL of the precursor solution is quickly added into toluene (10 mL) under vigorous stirring. Due to the poor solubility of the precursor in toluene, PbBr2 and CsBr occurr phase separation in toluene and form CsPbBr3 PQD colloid. The PQD colloid is centrifuged for 10 min at 2000 rpm to obtain the precipitation, and followed by washing with acetone and toluene to get purified PQDs. The BPLCs are prepared by mixing 32 wt% 4-(1-methylheptyloxycarbonyl)phenyl-4-hexyloxy benzoate (S-811) with nematic liquid crystals (BHR59001) at 100 °C. And the BPLCs doped with different concentration (1 wt%, 2 wt% and 3 wt%) of PQDs are prepared by dispersing the PQDs into BPLCs at their clearing point and followed by ultrasound for 10 mins.
2.2. Instruments and Characterization
The X-ray powder diffraction spectrum to determine the crystal structure is performed for the diffraction angle 2θ from 5° to 70° using Shimadzu XRD-6000 powder diffractometer, the transmission electron microscopy (TEM) (JEOL, JEM-2100F) image of CsPbBr3 PQDs is taken to confirm their presence, the fluorescence spectrum of CsPbBr3 PQDs is recorded by a fluorescence spectrometer (HORIBA Scientific, Jobin Yvon), the observation of polarized optical textures of BPLCs and BPLC/PQD composites at different temperatures is conducted by using polarized optical microscope (POM) (Olympus, BX51), the temperatures in the whole study are controlled by a Linkam heating and cooling stage at a rate of 0.1 °C min−1. The electro-optical response measurements of BPLCs and BPLC/PQD composites are conducted based on the previously reported procedure [8]. The sample cell was placed between two polarizers, a He-Ne laser was incident to the sample cell at the angle of 45°. The reflected light is detected by an optical fiber, and recorded with LCD tester (North LCD Engineering Center).
3. Results and discussion
Results in Fig. 1(a) show that the as-prepared CsPbBr3 PQDs possess characteristic XRD signals at 11.6°, 15.3°, 21.6°, 31.0°, 38.1° and 44.1°, which is in accordance with the previously reported work [20], TEM image in Fig. 1(b) shows that the size of PQDs is basically around 5 nm with a polydispersity of 9.17%, strong green emission in Fig. 1(c) can be observed after the PQD colloid in toluene is under the irradiation of UV light of 365 nm, and the central emission wavelength in Fig. 1d is located at 519 nm [23]. As the surface of the all-inorganic PQDs is modified and stabilized by OA and OAM with weak polar, the miscibility and dispersivity of CsPbBr3 PQDs in nonpolar BPLCs can be achieved theoretically. The emitted green fluorescence of PQDs can be used to figure out the miscibility between PQDs and BPLCs, and the effect of the incorporated PQDs on the microstructure of BPLCs. The fluorescence images of BPLC + 1% CsPbBr3 PQDs, BPLC + 2% CsPbBr3 PQDs and BPLC + 3% CsPbBr3 PQDs in I state, BP state and CN state are listed in Fig. 2(a)-2(c), the first and the last image in Fig. 2(a)-2(c) correspond to the fluorescence images in I state and CN state responsively, the middle four images in Fig. 2(a)-2(c) correspond to the fluorescence images in BP state. Results illustrate that PQDs have been effectively embedded into the BPLC matrix as the fluorescence intensities increase with the increase of the PQD addition. Theoretical simulation has demonstrated that the nanoparticles dispersing into BPLCs would most likely be trapped in the disclination lines as a result of elastic interactions [24, 25]. Since the PQDs being doped into the BPLC have small sizes of 5 nm and the diameter of the disclination line core in BPLC is reported to be about 10 nm [26], it can be deduced that the freely moving PQDs get trapped when meeting a disclination line, and the free energy around the disclination in the BPLC would be significantly reduced. The distribution area of PQDs in Fig. 2(a)-2(c) basically remains unchanged at different temperatures for a specific BPLC/PQD composites, the fluorescence peaks are all located at 519 nm, and the fluorescence intensity of PQDs in BPLC/PQD composites also remains unchanged at different temperatures (see Fig. 2(d)-2(f)), indicating the stable incorporation of PQDs. As for the incorporation of the PQDs in CN phase, we assume that the PQDs are embedded around the defects in the interlayers of LCs. As for the arrangement of the PQDs in I phase, we assume the PQDs are randomly embedded in the LCs.
Fig. 1 The structure, morphology and optical properties of CsPbBr3 PQDs. XRD pattern (a), TEM image (b), fluorescent image (c) and fluorescence spectrum (d) of CsPbBr3 PQDs. The insert shows the size distribution of PQDs.
Fig. 2 The fluorescent microscopy and fluorescent spectroscopy of BPLC/PQD composites at different temperatures, the concentrations of PQDs in BPLCs are 1wt% (a, d), 2wt% (b, e) and 3wt% (c, f). The first and the last image in Fig. 2(a)-2(c) correspond to the fluorescence images in I state and CN state responsively, the middle four images in Fig. 2(a)-2(c) correspond to the fluorescence images in BP state.
POM is used to analyze the effect of CsPbBr3 PQDs on the phase transition process of BPLCs. The BPLCs consisting of nematic liquid crystals and chiral dopants reach an I phase at 51 °C, while keeping cooling at a rate of 0.1 °C min−1, the liquid crystal mixture starts to form a BP with typical platelet textures from red to cyan, when the temperature is further down to 33.0 °C, the focal conic textures appear, indicating a secondary phase transition from BP to CN phase (see Fig. 3(a)). As shown in Fig. 3(b-c), the CsPbBr3 PQD embedded BPLCs sequentially present I phase, BP and CN phase with the decrease of the temperature, and the Bragg reflection exhibits the change from red to green, which is the same with neat BPLCs, indicating that the dispersed CsPbBr3 PQD do not damage the original 3D structure of BPLCs. We note that the BPLCs and BPLC/PQD composites in Fig. 3 share a common feature of blue shift during the decrease of the temperature, which is a typical feature of BPII [27–31]. It is noteworthy that the phase transition temperatures (TI-BP and TBP-CN) of these BPLC/PQD composites are quite different from neat BPLCs (see Fig. 4). The TI-BP dramatically decreases from 51 °C to 44.6 °C when the content of PQDs increases from 0% to 3%, the TBP-CN also obviously decreases from 33.0 °C to 23.8 °C. The reason for the decreased phase transition temperatures (TI-BP and TBP-CN) can be ascribed to the following reasons, the interaction between CsPbBr3 PQDs and BPLCs makes the PQDs in situ incorporated in the BPLC matrix, the flexible long chains of OA and OAM twine round the rod-like BPLC mesogens, meanwhile, the PQDs localize with the disclination lines of BPLCs and stabilize the microcosmic defect of BPLCs, as the OA and OAM modified CsPbBr3 PQDs cannot form ordered micro structure in BPLC matrix, and exist in the form of amorphous dopant, which is similar with the amorphous polymer chains in the polymer-stabilized BPLCs [8]. Therefore, the physical twine and the defect stabilization of PQDs are supposed to make the mesogens in the BPLC matrix tend to keep their orientation in the process of decreasing the temperature, which causes the TI-BP and TBP-CN a little delay towards the low temperature (see Fig. 4(a)). It is worth noting that the decrease magnitude of TBP-CN is larger than that of TI-BP, leading to a widened BP temperature range (TI-BP-TBP-CN) in Fig. 4b, the reason for this phenomenon is that as BPLCs are tightly stacked into 3D periodic structure in the form of highly viscous fluids, it is more difficult for BPLCs to occur the phase transition than for isotropic fluids tooccur the phase transition, therefore, the physical twine and the defect stabilization have greater impact on the BP-CN transition than on the I-BP transition. In order to confirm the thermal stability of BPLCs and BPLC/PQD composites, these samples are kept for 36 h within their BP temperature range (Fig. 5), the POM images are maintained without any texture change. Thus, it is confirmed that the formation of BPLC is not due to super-cooling but the obtained BP is a thermally stable phase. Due to the limited dispersity of PQDs in BPLCs, when the concentration of CsPbBr3 reaches 3.5%, the PQDs cannot be effectively dispersed in the BPLC matrix. The BPLC/PQD composite exists phase separation in the cooling process. In this case, the BP range of BPLC/PQD composite cannot be accurately quantified. The dispersed CsPbBr3 PQDs also have effect on the domain structure of BPLCs, the size of single BPLC domain gradually decreases with the increase of PQD content. In order to quantify the size change of mature BPLC domain, the area distributions of single BP domain of BPLCs (42 °C), BPLC + 1% CsPbBr3 PQDs (37 °C), BPLC + 2% CsPbBr3 PQDs (37 °C) and BPLC + 3% CsPbBr3 PQDs (40 °C) are figured out as Fig. 6, the average domain areas of BPLCs (42 °C), BPLC + 1% CsPbBr3 PQDs (37 °C), BPLC + 2% CsPbBr3 PQDs (37 °C) and BPLC + 3% CsPbBr3 PQDs (40 °C) are 130.4 um2, 74.6 um2, 49.5 um2 and 34.0 um2, which also proves the effective dispersion of CsPbBr3 PQDs in BPLCs. As for the decreasing domain size at increasing PQD concentrations, this phenomenon can be explained by the following mechanism, the PQDs modified with long alkyl chain ligands cannot self-assemble into liquid crystals and cannot be induced to form LCs by BPLCs, therefore, the PQDs act as heterogeneous building block to physically divide the BPLCs into lots of domains, the more PQDs means the more physical boundary and the smaller domain sizes of the BPLCs. Similar phenomenon of reducing domain sizes by increasing the concentration of heterogeneous polymer network can be found in the LC-polymer system [32–34].
Fig. 3 Typical textures of the BPLCs (a) and BPLC/PQD composites, the concentrations of PQDs in BPLCs are 1wt% (b), 2wt% (c) 3wt% (d) and 3.5wt% (e). The phase separation areas are marked by white frame.
Fig. 4 The phase diagram (a) and the BP temperature range (b) determined by POM observation for the BPLC/PQD composites.
Fig. 5 The stability of BPLCs and BPLC/PQD composites. Images a-d correspond to the original POM textures of BPLCs (at 40 °C), BPLC + 1% CsPbBr3 PQDs (at 35 °C), BPLC + 2% CsPbBr3 PQDs (at 35 °C) and BPLC + 3% CsPbBr3 PQDs (at 40 °C). Images e-h correspond to the POM textures of BPLCs (at 40 °C), BPLC + 1% CsPbBr3 PQDs (at 35 °C), BPLC + 2% CsPbBr3 PQDs (at 35 °C) and BPLC + 3% CsPbBr3 PQDs (at 40 °C) after preserved for 36 h.
Fig. 6 The area distributions of single BP domain of BPLCs (42 °C) (a), BPLC + 1% CsPbBr3 PQDs (37 °C) (b), BPLC + 2% CsPbBr3 PQDs (37 °C) (c) and BPLC + 3% CsPbBr3 PQDs (40 °C) (d).
One of outstanding advantages of BPLCs is the ultra-fast electro-optical response (sub-millisecond level), Fig. 7(a) shows the optical switch-like response curves of the reflected light intensity through the in-plane switching (IPS) cell (40 µm thick) on the application of a 20 kHz AC. electric voltage of 45 V at 30 °C. The change in the reflected light intensity results from the rotational change in the polarizing direction of the incident light. The response times (the rise time and the decay time) of BPLCs and BPLC/PQD composites are all tested to be sub-millisecond at 30 °C (see Fig. 7(b)), this result indicates that the liquid crystal mesogens in the BPLC/PQD composites are able to response quickly to an external field in spite of the presence of the heterogeneous CsPbBr3 PQDs. It is important to figure out whether the electro-optical response of the CsPbBr3 PQDs stabilized BPLCs is resulted from the change in the mesogen orientation or the change in the lattice constant. As the latter effect is connected with the evolution of domain structure, its response to an electric field would be much slower, usually in second level. It seems that the fast electro-optical response reported here is caused by a local director orientation within the unit lattice. We also note that the response times of BPLC/PQD composites are slightly higher than those of neat BPLCs, the higher content of CsPbBr3 PQDs in composites, the larger the response time is. The rise times of BPLCs, BPLC + 1% CsPbBr3 PQDs, BPLC + 2% CsPbBr3 PQDs and BPLC + 3% CsPbBr3 PQDs are 0.36 ms, 0.4 ms, 0.46 ms and 0.5ms, the corresponding decay times are 0.28 ms, 0.32 ms, 0.4 ms and 0.46 ms. The increased response times of BPLC/PQD composites can be explained by the following equation [35],
Where, γ1 is the rotational viscosity, EC is the critical field and VC critical voltage for unwinding the double twisted cylinders, ε is the vacuum dielectric constant and Δε is the dielectric anisotropy of the BPLCs. As discussion on Fig. 3, the incorporated CsPbBr3 PQDs do not damage the basic structure of BPLC host, therefore, the primary physical properties (EC, VC and Δε) possess no obvious variation. On the contrary, the dispersed viscous CsPbBr3 PQDs inevitably increase the bulk viscosity of BPLC/PQD composites, thus leading to the increased response time. The above mentioned ultra-fast electro-optical response makes the as prepared BPLC/PQD composites possess potential application in the field of advanced dynamic display materials, besides that, the flow characteristics of BPLCs endows the CsPbBr3 PQDs with additional fluidity and processability, based on this advantage, the BPLC/PQD composites can be easily used to fabricate optical pattern by screen printing, the selective reflection under white light irradiation and the photoluminescence under UV light irradiation make the BPLC/PQD composite a static dual-mode optical material (see Fig. 8).Fig. 7 The electro-optical response time curves of reflectance versus time (a), the rise response time and decay response time for BPLCs, BPLC + 1% CsPbBr3 PQDs, BPLC + 2% CsPbBr3 PQDs and BPLC + 3% CsPbBr3 PQDs (b) at 30 °C.
Fig. 8 The optical image of BPLC/PQD composites (3%) under white light irradiation at 30 °C (a) and the photoluminescence image under UV light (365 nm) irradiation at 30 °C (b), the pattern printed by screen printing is the emblem of soft matter center of Guangdong University of Technology.
4. Conclusions
In conclusion, all-inorganic CsPbBr3 PQDs are incorporated into the 3D ordered structure of BPLCs for the first time, to the best of our knowledge, using the melting blend method and synchronous ultrasound. The physical winding between the flexible carbon chains and the BPLC domains and the stabilization of the disclination lines of BPLCs by CsPbBr3 PQDs are conducive to the enhancement of the thermal stability of BPLCs when decreasing the temperatures, thus leading to the decreased phase transition temperatures (TI-BP and TBP-CN), as the above stabilization mechanism has more significant effect on the TBP-CN than on TI-BP, which results in the widened BP temperature range. The BPLC/PQD composites exhibit fast electro-optical response (submillisecond level) due to the local director orientation within the unit lattice. The flow characteristic of BPLCs endows the CsPbBr3 PQDs with additional fluidity and processability, therefore, an advanced optical pattern possessing selective reflection and photoluminescence can be easily fabricated by screen printing. Such a win-win strategy of assembling all-inorganic PQDs in 3D ordered BPLC matrix combines the superior optical properties of all-inorganic PQDs and the fluidity, 3D order and stimul-response of BPLCs, showing favorable prospects for wide-range and room-temperature BPLCs, responsive BPLC devices, multifunctional display materials and bandgap lasers.
Funding
Guangzhou Science Technology and Innovation Commission (No. 201807010108); National Natural Science Foundation of China (Grant No.51736005); Foshan Municipal Science and Technology Bureau project (2015IT100162); Innovative project of College Students of Guangdong University of Technology (201711845027, 201711845154,xj201711845085).
Acknowledgment
The authors gratefully acknowledge Qi Yan, Zhan Wei and Ying Chen for helpful discussions and thank Xuezhen Wang and Zhengdong Cheng for his contribution to the construction of the early-stage experimental setup.
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