An Effective Method for the Preparation of Stable LC Composites with High Concentration of Quantum...

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modifi cation of QDs with complex mesogen containing ligands, which resulted in the preparation of thermodynamically stable QDs solution in LC matrix, yet with still low concentration of the additive (0.25%). In the most of other papers devoted to QDs solutions in LC media the concentration of QDs does not exceed 0.1%.

There are several recent works concerning the LC–QDs mixtures with higher concentration of the QDs. [ 21,22 ] Yet, these experimental results are in strong contradiction with other publications devoted to the similar mixtures and lack strong experimental proofs of the thermodynamic stability of these composites.

In most cases, the methods of preparation of QD–LC com-posites are based on simple mixing of components in common solvent followed by spin coating or drying. [ 23,24 ] In some cases, bi- or multilayer fi lms containing QDs-rich layers were pre-pared by sequential spin-coating method. [ 25,26 ]

As a resume, from the literature data, we may conclude that the development of new methods for embedding of high amount of QDs in LC media without the loss in the optical properties of any of the component is still challenging and highly relevant. In the present paper, we propose a new method of preparation of stable QD–LC composites containing very high (up to 10 wt%) amount of QDs.

The idea of the elaborated procedure is schematically depicted in Figure 1 . On the fi rst stage, the polymer-stabilized LC fi lm (with any type of LC phase) is prepared (Figure 1 a). For this purpose, LC mixture consisting of ≈ 10% of meso-genic diacrylate, ≈ 15% of LC monoacrylate, ≈ 75% of low-molar-mass LCs and 1%–2% of photoinitiator (see Table 1 ) is placed into asymmetrical cell with two glass substrates coated by different polymer layers — polyvinyl alcohol (PVA) and polyvinyl-cinnamate (PVCin). Rubbing of the substrates with polymer layers allows one to achieve uniaxial orientation of LCs molecules. UV irradiation of these cells leads to photopo-lymerization of acrylates. The presence of reactive C C double bonds in the cinnamate fragment of PVCin warrants a good adhesion of polymer network to PVCin-coated glass substrate, which makes mechanical removal of the PVA-coated glass plate (Figure 1 b) on the next stage more easy. The LC fi lm obtained after photo polymerization has microheterogeneous structure consisting of LC polymer network and low-molar-mass LC microphase. Nevertheless, under appropriate irradia-tion conditions (light intensity, monomers, and photoinitiator structure and their concentration), characteristic dimensions of these microphases are comparable or even smaller than the wavelength of visible light, which minimizes light scattering of LC-composite fi lms. [ 20,21 ] LC-polymer network stabilizes the alignment of the LCs molecules, and has the possibility

An Effective Method for the Preparation of Stable LC Composites with High Concentration of Quantum Dots

Alexey Bobrovsky ,* Pavel Samokhvalov , and Valery Shibaev

Dr. A. Bobrovsky, Prof. V. Shibaev Faculty of Chemistry Moscow State University Leninskie gory , Moscow 119991 , Russia E-mail: [email protected] Dr. P. Samokhvalov Laboratory of Nano-Bioengineering National Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Moscow 115409 , Russian Federation

DOI: 10.1002/adom.201400215

Organic–inorganic hybrid materials, in particular, liquid crys-tals doped with nanoparticles, quantum dots (QDs) and other nanometer-scale colloid particles attract a great attention of large number of research groups due to their unique optical properties, switchability, and the variety of possible applications in optics, optoelectronics, and photonics. [ 1–19 ]

Cholesteric liquid crystals doped with QDs present a special interest because the photonic bandgap structure of cholesteric media when combined with emission properties of QDs pro-vides the opportunity to realize a variety of specifi c photooptical properties. Cholesteric mesophase has a 1D photonic bandgap structure, which stands for its unique optical properties such as Bragg selective light refl ection, huge optical rotation, and so on. [ 20 ] In several recent papers, the optical and photophys-ical properties of the cholesteric low-molar-mass and polymer liquid crystalline (LC) materials doped with QDs were dem-onstrated. [ 16–18 ] For example, in recent publication, [ 16 ] the modulation of recombination lifetimes of CdSe/ZnS QDs (0.01–0.02 wt%) dispersed in cholesteric low-molar-mass liquid crystal and time-resolved emission from QD ensembles in LC matrices with different alignment were investigated. The differ-ence in decay time characteristics of QD’s emission depending on planar or homeotropic alignment was revealed in the case of overlapping of emission and photonic bandgap peaks. It was shown that coupling between the excitonic and the photonic cavity modes leads to the enhancement and modulation of spontaneous emission.

In the papers [ 18,19 ] we have described the novel types of cho-lesteric QDs-doped materials with photo- and electro-tunable fl uorescence. The possibility of manipulation of fl uorescence intensity and degree of circular polarization (dissymmetry factor) was demonstrated for the fi rst time.

Despite a great progress in the fi eld of the QD–LC compos-ites, the introduction of large concentration of QDs into liquid crystal media is still a challenging task. The most successful achievement in the area was published recently. [ 15 ] The authors of this work have offered an approach based on the surface

Adv. Optical Mater. 2014, DOI: 10.1002/adom.201400215

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to “memorize” supramolecular organization of low-molar-mass LC molecules after polymerization. In other words, any external action or fi eld can easily change the orientation or ordering of LCs, but after turning fi eld off LC network pro-motes fast relaxation of low-molar-mass LCs back to their initial state. [ 27 ]

During the second stage of composite preparation (Figure 1 b) the PVA-coated glass substrate is removed mechanically and low-molar-mass components are washed off from the fi lm by dissolving them in ethanol (Figure 1 c). After drying under ambient conditions, the highly cross-linked LC-polymer fi lm is formed. This fi lm is placed into concentrated solution of QDs in toluene (Figure 1 d). During this step QDs penetrate into the cross-linked LC-polymer sample, and after its drying (Figure 1 e) and immersion in low-molar-mass nematic liquid crystal mix-ture (Figure 1 f), LC polymer-stabilized fi lm with high content of QDs is obtained.

It is noteworthy that the developed method is simple and versatile because it does not imply any side chemical reac-tions or modifi cation of QDs with new surface ligands. The nanoparticles of different nature with any types of ligands could be used for LC-composite preparation. Furthermore, the sug-gested approach is quite universal and could be applied to any LC system forming nematic, cholesteric, smectic, or blue meso-phases. In the present work, we have applied this approach for the preparation of QDs-doped cholesteric composites due to their promising optical properties. In the previous section we have defi ned the main principles of introduction of high con-centration of QDs into the LC-polymer composites, and in the following part we would like to discuss the application of the abovementioned principle to the particular system and describe the results we obtained.

The photopolymerizable mixture containing nematic mixture E48 , chiral dopant HexSorb , diacrylate RM257 , monoacrylate A4CB, and photoinitiator Irgacure 651 was prepared. Chemical structures and contents of all components are listed in Table 1 . The concentration of the chiral dopant was chosen in order to warrant spectral overlapping of the selective light refl ec-tion band with QDs emission peak. The contents of the polymerizable compounds, mono- and di-acrylates, were experimentally optimized to obtain the fi lms which would be mechanically stable after treatment them with ethanol (Figure 1 c).

The prepared mixture does not show phase separation and forms cholesteric phase at room temperature. UV irradiation of glass cells fi lled with the mixture leads to photopolymeriza-tion (Figure 1 a) of the acrylates and formation of cholesteric cross-linked fi lm with selective light refl ection peak centered at 580 nm. Polarizing optical microscopy (POM) reveals the typical planar texture with oily streaks (Figure S2, Supporting Information).

On the second and the third stages of composite prepara-tion the PVA-coated glass substrate (Figure 1 b) is removed and the fi lm is immersed in ethanol (Figure 1 c). Ethanol dissolves all low-molar-mass components of cross-linked LC fi lm. As a result, a shift of selective light refl ection into the UV-spectral range takes place. During the immersion, the color of selective light refl ection is consequently changed from orange to green and blue within several minutes. In order to ensure complete extraction of the low-molar-mass components from the fi lm, it was usually immersed in ethanol for ca. 12 h.

The next stage implies introduction of toluene solution of nanoparticles into the LC-polymer cross-linked fi lm. CdSe/



Figure 1. Scheme of method of LC QD-containing composites preparation proposed in this work.

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ZnS QDs coated with trioctylphosphine oxide (TOPO) ligands with emission peak at 575 nm were used (see Experimental Section for synthesis details) in this work. The dry LC-polymer cross-linked fi lm, obtained after removal of all low-molar-mass components with ethanol, was immersed in concentrated tol-uene solution of QDs (Figure 1 d) and kept there for 2–3 h, followed by gentle removing of excess QDs solution with fi lter paper and drying at 65 °C (Figure 1 e). Since toluene is a good solvent for QDs as well as for the LC-polymer network, during the immersion of the fi lm into toluene solution of QDs, the polymer network partially swells and becomes saturated by QDs, which are absorbed by polymer matrix.

Figure 2 a shows POM and fl uorescent microphotos of the same area of polymer-stabilized cholesteric fi lm after its immersion in toluene solution of QDs and drying it at 65 °C (Figure 1 e). POM image demonstrates a number of oily streaks which are typical for cholesteric mesophase, [ 22 ] whereas fl uorescent microscopy reveals very bright and homo-geneous fl uorescence, and absence of visible large aggregates of QDs. It is interesting to note that the LC-polymer network does not contain any chiral molecules at this stage of com-posite preparation, because low-molar-mass chiral dopants were completely removed from the fi lm together with other nonpolymerizable compounds. Nevertheless, as mentioned above, one can observe that the cross-linked LC-polymer net-work structure “memorizes” the cholesteric order of initial sample.

The selective light refl ection band of such fi lm is likely to be in the UV region and could not be detected by available spectral methods ( Figure 3 ).

The last stage of LC-composite preparation is the refi lling of QD-doped LC- polymer network with nematic mixture E48 (Figure 1 f). During this process, selective refl ection band of the fi lm reappears and complete restoration of orange color refl ection takes place. At this stage, POM and fl uorescent images demonstrate the colored cholesteric planar texture with oily streaks and, again, absence of large QDs aggregates (Figure 2 b). The fi nally obtained QDs-doped cholesteric fi lms possess clearly visible selective light refl ection with orange



Table 1. Chemical structure of components and composition of photopolymerizable cholesteric mixture.

Substance Content [wt%]

Nematic mixture E48

72.3

Chiral dopant HexSorb

2.1

Diacrylate RM257

9.8

Monoacrylate A4CB

15.0

Photoinitiator Irgacure 651

0.8

Figure 2. a) Polarizing optical (POM, crossed polarizers) and fl uores-cent optical microscopy (FOM) images of polymer-stabilized cholesteric fi lm washed by ethanol and drying followed by immersion in QD toluene solution (100 mg mL −1 ) and drying at 65 °C; b) the same fi lm after refi lling with E48 . Excitation wavelength is 546 nm. Scale bars correspond to 20 µm. Room temperature.

Figure 3. Absorbance spectra of fi lm after photopolymerization of cho-lesteric mixture, removing of low-molar-mass component (washing with ethanol), fi lling with QDs, and refi lling with E48 . Blue-dashed line shows the absorbance of QDs in chloroform solution.


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tilt of the sample transforms its visible color to green, which is the typical feature of Bragg refl ection of cholesteric systems. [ 28 ]

The changes in absorbance spectra of the fi lm during the aforementioned steps of composite preparation are presented in Figure 3 . It is noteworthy, that the excitonic absorbance peak of QDs in LC composite is clearly seen in the spectra, and its position completely coincides with the absorbance maximum of the same QDs in chlorophorm solution (dashed blue line in Figure 3 ). Refi lling of the fi lm with nematic mixture E48 results in reappearance of selective light refl ec-tion peak, which is centered at ca. 580 nm and is partially overlapped with fl uorescence band of QDs. The obtained QDs-doped LC-composite fi lms are quite stable and the position of selective light refl ection does not change at room temperature, even consid-ering the low vapor pressure of some of E48 components.

The rough content of QDs in the obtained LC-composite fi lms was calculated from the absorbance spectra of LC composite and equations derived in ref. [29 ] . Approximate concentration of QDs was found equal to ≈ 6 wt%, and this value is one or almost two orders of magnitude higher than the ones reported in previous works. [ 10,14–19 ]

It is important to note that TOPO-coated QDs used in our work are completely insol-uble in nematic mixture E48 , as well as in mono-, and di-acrylates. In this way, the obtained QD–LC composite should be con-sidered as microheterogeneous system. But, nevertheless, due to the absence of a signifi -cant light scattering, the characteristic dimen-sions of these heterogeneities are, probably, less or comparable with the wavelength of visible light.

QD–LC composite fi lms possess very bright and homogeneous fl uorescence that is clearly seen under excitation with light of the different wavelengths: 365 nm (Figure S4, Supporting Informa-tion), 436 nm (Figure S3, Supporting Information), and 546 nm (Figure 2 ). In order to confi rm the homogeneous distribution of the QDs within the polymer matrix, we have performed confocal fl uorescence microscopy measurements of thin slices of the cholesteric network fi lled with QDs by using technique described recently. [ 19 ] Fluorescence intensity map obtained by this method shows the absence of large aggregates and homo-geneous QDs distribution (Figure S5, Supporting Information).

Figures 4 a, b shows nonpolarized and circularly polarized emission spectra of the obtained QD–LC composites, respec-tively. Fluorescence maximum of QD–LC composite fi lm is redshifted by ca. 10 nm in comparison with chlorophorm solution of QDs (Figure S4, Supporting Information). This shift

is probably caused by the higher optical density of the QD–LC fi lm (Figures 3 and 4 a) in comparison with the solution, which results in reabsorption of short-wavelength part of the emission of QDs in LC composite and, therefore, visible redshifting of the emission is maximum.

Analysis of circularly polarized components of QDs fl uo-rescence reveals a signifi cant degree of polarization of emitted light (Figure 4 b). As clearly seen from Figure 4 b, the intensity of right-handed circularly polarized component of the emission is signifi cantly lower. This difference in intensity of polarized components is associated with the presence of right-handed cholesteric helical structure in QD–LC composite, which has selective light refl ection (photonic bandgap) that completely covers QDs emission spectrum, because spectral width of



Figure 4. a) Absorbance and fl uorescence spectra of polymer-stabilized cholesteric fi lm con-taining high concentration of QDs. Excitation wavelength is 365 nm. b) Circularly, polarized fl uorescence spectra of cholesteric fi lm. Excitation wavelength is 532 nm. Insert shows wave-length dependence of dissymmetry factor.

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selective light refl ection band is noticeably larger. Using spectral data and Equation ( 1) , dissymmetry factor g e was calculated.

= 2( )/( + )e L R L Rg I I I I− (1)

where I L and I R are the intensities of left- and right-handed circularly polarized light, respectively. At the wavelengths cor-responding to the maximum of fl uorescence, the value of dis-symmetry factor is approximately equal to one (see insert in Figure 4 b). Thus, the obtained QD–LC composites possess noticeable degree of polarization that could be further improved by better alignment of a cholesteric planar texture and optimi-zation of the proposed method of composite preparation. It should be pointed out that high values of dissymmetry factor of polarized emission are quite important for creation of devices for different applications in optics and optoelectronics. [ 30 ]

In summary, the new effective method of preparation of hybrid organic–inorganic multifunctional LC composites with high content of QDs was elaborated. The obtained QD–LC composites possess very bright fl uorescence, clearly visible by naked eye under the excitation by UV or visible light of dif-ferent wavelengths. The absence of large QDs aggregates and high degree of circular polarization of emission make these composites promising for different applications in photonics. The proposed technique is quite reproducible; we have success-fully repeated preparation of the QD–LC composites several times, as well as varied the composition of photopolymerizable mixtures with different components ratio, and were able to impregnate the LC fi lm with QDs of different sizes (3.5–8 nm). It should be pointed out that this method is universal and could be applied for any type of nanoparticles or substances intrin-sically insoluble in the LC matrix. In addition, any type of mesophase could be used as the LC media. The future efforts will be concentrated on detailed investigation of the possible phenomena of QDs aggregation and nanoparticles localiza-tion in such LC composites by high-resolution microscopy methods.

Experimental Section Materials : As nematic liquid crystal, the commercial mixture of

cyanobiphenyl derivatives ( E48 , Merck, Δ n = 0.231 at 589 nm) was used. Chiral dopant 2,5-bis(4-hexyloxybenzoyl)-1,4;3,6-dianhydro-D-sorbitol, HexSorb , was synthesized by us according to our previous paper. [ 31 ] Bifunctional monomer RM257 (Merck) was used as received. LC monomer A4CB was synthesized according to the procedure described elsewhere. [ 32 ] Photoinitiator 2,2-dimethoxy-1,2-diphenylethan-1-one, Irgacure 651 , was purchased from Chiba company.

Synthesis of CdSe/ZnS Quantium Dots : The procedure for the synthesis of CdSe/ZnS core/shell QDs was adopted from ref. [ 33 ] and is given in detail in the Supporting Information. Briefl y, wurtzite CdSe cores with a diameter of ≈ 2.9 nm were synthesized using n -hexadecylphosphonic acid as capping agent. After isolation and purifi cation the CdSe cores were coated with a three monolayer-thick ZnS shell in octadecene–oleylamine solution, and the native surface capping ligands were further replaced by TOPO. The fl uorescence quantum yield of the core–shell QDs was determined to be 68% using rhodamine 6G as a reference standard. The QD photoluminescence was found to be stable during photopolymerization and fl uorescence excitation, with the variation of photoluminescence emission during photopolymerization not exceeding 10%.

Mixture and Cells Preparation : Photopolymerizable mixture was prepared by dissolving the components in chloroform, followed by slow evaporation of the solvent and drying in vacuum. Phase separation did not take place during preparation of the mixture and fi nally it was completely homogeneous.

Two substrates were prepared by spin-coating of PVA (solution in ethanol, ≈ 0.1 mg mL −1 ) and PVCin (solution in chloroform, 2 mg mL −1 ) onto fl at glass plates. 30-µm-thick cells were constructed from these substrates; the thickness of the test samples was controlled by Tefl on spacers. The planar texture of both polymer coatings was achieved by rubbing them with a cloth.

After fi lling the cell with photopolymerizable mixture by capillary forces at room temperature, the prepared cells were irradiated by UV light (10 min, 365 nm, ≈ 1 mW cm −2 ) in order to induce photopolymerization of acrylates and formation of LC network.

Optical Microscopy : Polarizing optical microscopy investigations were performed using LOMO P-112 polarizing microscope. Fluorescence microscopy observations were performed using Micromed 3 LUM microscope (LOMO).

Spectral Investigations : Absorbance spectra of fi lms were recorded by Unicam UV-500 UV–Vis spectrophotometer.

Nonpolarized fl uorescence spectra of the fi lms and QDs solution were obtained using AvaSpec-2048-USB2 spectrometer and 365 nm LED as an excitation source.

Circularly polarized fl uorescence spectra were recorded using Automated Monochromator/Spectrograph M266 (SOLAR Laser Systems) with CCD detector U2C-16H7317 (Ormins) and homemade light-collection system. Diode laser with wavelength of 532 nm was used for fl uorescence excitation. Detection of the emitted light was realized in direction normal to plane of the fi lm, whereas the excitation beam was positioned at the certain angle from the back side of the fi lms. Circularly polarized components of fl uorescence were measured by using a combination of a linear polarizer with a broadband quarter-wave plate.

Acknowledgements This research was supported by the Russian Foundation of Fundamental Research (13–03–00648, 13–03–12071, and 13–03–12456). The part of research concerning the synthesis of QDs was supported by the Russian Science Foundation, grant no. 14–13–01160. The authors are very thankful to Dr. K. Mochalov for fl uorescence measurements and Dr. A. Efi mov for sliced samples preparation using ultramicrotome.

Received: May 8, 2014 Revised: September 24, 2014

Published online:

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