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The influence of the surface nanostructured relief on the near IR-spectral shift and on the structural properties of the nematic liquid crystal (NLC) materials doped with nanoparticles from lanthanide group has been considered. The relief mentioned above has been made on the interface between glass substrate with ITO-coatings and the LC mesophase. The specific feature of this relief is based on the applying of the contactless laser deposition technique and on the its modification by using surface electromagnetic waves (SEW) or carbon nanotubes (CNTs) treated with SEW. The modified relief permits to orient LC molecules without direct polymer orienting layers and can be used as conducting layer too. The features mentioned above lead to decrease drastically the resistivity and the bias voltage as well as that permits to increase the transparency and reveal the near IR-shift in the spectral characteristics of the lanthanide-doped LC mesophase.
© 2015 Optical Society of America
Corrections
10 February 2016: A correction was made to the author listing.
1. Introduction
It is well known that the liquid crystal (LC) materials can be considered as the best model to confirm the classical effects and to study the novel electro-optical and elastic effects too. LCs, being a unique mesomorphic phase of matter, combine the properties of both solids (long-range orientation order, manifestation of Bragg diffraction) and liquids (fluidity, viscosity). Important features of the LCs are the weak dispersion forces between the molecules and the strong orienting fields. Based on this point of view it should be remarked that at present time one can indicate two important directions of the LC-mesophase parameters modification and improving. The first one is coincided with the successfully sensitization of the LC with the different types of the nanoparticles; the second one is connected with the structuration of the interface between the LC and the solid substrate. Both of these methods permit to correct and to improve the LC-cells and the LC-modulators technical characteristics which can be useful to be applyied in the information processing, basically in the LC-display and in the biomedicine.
So many technical and scientific teams have studied the improvement of the LC-mesophase features [1–10 ]. Our own research occupies the special place in this area too. We have demonstrated the dramatic change of the laser-induced refractive index, the IR-shift of the spectra, the increase of the order parameters, the increase of the charge carrier mobility, etc. of the nano- and bio-structured LC-mesophase [11–20 ]. Moreover, the unique contactless laser technique has been revealed, which has permited to make with good advantage the vertically oriented carbon nanotubes (CNTs) deposition on the ITO-conducting layers [19,21 ]. That leads to decrease of the resistivity of the conducting layers, to increase of the transparency, to change of the wetting angle and to extend the area of the application of the LC devices in the complex optoelectronic schemes.
In the current paper the types of the nanostructures that can be successfully used in order to modify the LC-mesophase properties have been extended via applying for this purpose the perspective lanthanide nanoparticles (NPs), such are cerium, praseodymium, and samarium ones. Furthermore, the interface between the solid substrate and the LC system has been structured by CNTs and treated with surface electromagnetic waves (SEWs).
2. Experimental conditions
The results of the spectral measurements are shown in Fig. 1 . The spectrophotometer SF-26 and “Infralum” with calibrated light filters have been used. The thickness of the all LC-cells have been closed to 10 micrometers; the LC-cells have been constructed in the S-configuration. The nematic LC-mesophase has been chosen from cyanobiphenyl group. The relation of the LC mixture and lanthanide nanoparticles (NPs) emulsion have been made of 5:2 that indicated in the Fig. 1 as, LC + CeNP(2), LC + PrNP(2), and LC + Sm(2). It should be mentioned that the ratio of the lanthanide nanoparticles to liquid was volume ratio. Moreover, the size of the lanthanide nanoparticles affects the transmission spectrum.
Fig. 1 Transmittance spectra of the LC-cells: pure LC and LC doped with the lanthanide NPs. 1 – spectral of the pure LC-cell; 2,3,4 - spectral characteristics of the LC-cells with the lanthanide NPs when ITO coatings have been treated with SEW; 2*,3*,4*- spectral characteristics of the LC-cells with the lanthanide NPs when CNTs have been deposited on the ITO coatings and then have been treated with SEW
In order to observe the self-arrangement process in the LC-cells sensitized with the lanthanide NPs the scheme presented in Fig. 2 has been applied.
Fig. 2 1 - He-Ne laser; 2 – microscope objective; 3 – lens or objective to make the parallel laser beam; 4 – diaphragm to correct the laser beam; 5 – rectangular diaphragm with the relation of the sides as 16:9 in order to find the coinciding conditions between diameter of the laser bean and the dimension of the camera; 6 – lens to collect the beam on the entrance of the camera; 7 – studied LC-cells; 8 – camera.
To analyze the relief modification under the conditions, when the CNTs have been deposited and SEW has been applied, the AFM Solver Next has been used. The initial CNTs with the diameter from 0.7 to 1.1 nm have been purchased from Alfa Aesar Co. (Karlsruhe, Germany). The laser deposition of CNTs and their subsequent SEW treatment have been obtained using a quasi-continuous CO2 laser with p-polarized radiation at a wavelength of 10.6 μm and with a power of 30 W. CNTs have been deposited on a substrate with ITO conducting layers in the presence of an electric field with the strength of 100 – 250 V × cm−1.
3. Results and discussion
Let us to consider the results received in the framework of the current study. It shoud be drawn the attention that the unique spectral results with indicated near IR-shift are shown in Fig. 1. The spectra are corresponded, for the first, to the doping of the body of the LC-mesophase with lanthanide’s nanoparticles and, for the second, they are regarded to the modification of the interface between the solid substrate and the LC-mesophase.
It should be remarked that the bending of the curve for the pure LC structure (see Fig. 1, curve 1) has been obtained at the wavelength that is close to 450 nm. But, one can testify that the bathochromic spectral shift up to 500-550 nm (see Fig. 1, curves 2,3) has been observed when LC-molecules have been doped with the lanthanoids nanoparticles, such as Ce and Pr. In this case the LC-dipoles have been oriented using general ITO-coating treated with the surface electromagnetic waves. It should be mentioned that the LC mixture doped with Sm nanoparticles (see Fig. 1, curve 4) has been prepared directly before the complex experiments. Thus, the time to organize the link between LC molecules and the Sm nanoparticles is not enough. As the results, the essential spectral shift has not now been observed for SmNPs under these conditions, but our experience permits to testify that when the self-arrangement process will be complited the near IR-shift will be always observed via LC intermolecular doping procedure. In addition to a bathochromic shift we can observe the increase in transparency of the LC when this material has been doped with Ce, Pr and SmNPs in series. It should be noticed if the bathochromic shift is connected with the intermolecular charge transfer complex (CTC) formation process, but the increase in transparency is regarded to the increase of the order parameter when the core of the lanthanide nanoparticles can be increased too. That is the evidence of the influence of the classical electronic mechanism correlated with the strong energetic levels and light-induced complex formation mechanism connected with the weak bonding between the host matrix molecular and nano-sensitizers.
Furthermore, drastic near IR-shift has been found up to 650-750 nm (see Fig. 1,curves 2*,3*,4*) when LC-molecules have been aligned using the ITO conducting layers with the laser-deposited oriented carbon nanotubes treated in consequence by the SEW.
To discuss the effect of the observed spectral shift, the LC-cells with the forming of the self-arrangement structures have been studied in details. Figure 3 presents the results of the structures based on the LC doped with the praseodymium nanoparticles, for example. One can see that the difference in the nematic mesophase can be found and the novel network can be established. Really, the classical ordering in one direction is corresponded to the pure nematic LC, but the two dimension structure can be found with forming of the clear domain systems via nanoparticles doping. Let us to remind that the self-arrangement organization has been provoked via including in the LC matrix the lanthanide’s nanoparticles and the creation of the binder (link) between them due to, for example, an intermolecular CTC formation procedure. It should be remarked that the analogous character for LC-mesophase doped with the different nanoparticles, such as the fullerenes, shungites, quantum dots, etc. has been established in the previous studies and connected with the bathochromic spectral shift and the mass-spectrometry data [22]. It has been supported for the LC with the fullerenes C70 by the observation of the transition from the nematic phase to the quasi-smectic one via changing of the order parameter [14]. The change of the doped LC system polarizability has been predicted in the previously published papers [13,18,22 ]. It follows from the results that the physical mechanism of the accelerated rotation of the LC dipoles is likely to be caused by a the creation of a new polarization field due to intermolecular CTC formation between the nanoparticles with the large electron affinity energy and a donor part of the initial organic molecules. This process provokes the bathochromic spectral shift, an order parameter change, etc. during the quasi-transition from the nematic phase to the smectic one.
Fig. 3 Observation of the self-arrangement 2D dimentional LC-mesophase by the doping process via using the lanthanoids nanoparticles, such as praseodymium one.
Moreover, the same organization of the smectic phase and spectral shift have been established in the LC doped by the bio-particles based on DNA [20]. Thus, it can be postulated that an incorporation of the different types of the nano- and bio-particles in the classical nematic LC-mesophase always provokes the bathochromic spectral shift in the near IR- range and the self-arrangement process change too. One of the reason of this modification of the LC-mesophase under the condition of the structurisation namely the body of the LC can be connected with the direct intermolecular CTC formation which regarded to the increase of the electron pathway and higher dipole moment of the doped LC-mesophase.
Anather vision can be done to explain the dramatic near IR-shift of the structured LC when the interface between liquid crystal and the solid substrate has been improved via nanostructuration (see Fig. 1,curves 2*,3*,4*). It can be assumed that the CNTs due to the large number of the electrons from the core of the CNTs can change the energy system levels of the LC-mesophase. It can provoke the formation of the quasi-graphene layer due to the reason that CNTs can reveal not only donor, but the acceptor properties too. Really, the change of the resistivity of the ITO-coatings with the CNTs has been found and the 3-5 times decrease of this parameter has been revealed that can support the prognosis mentioned above.
Moreover, the AFM-images support the relief modification by deposition of the CNTs with good advantage too. Figure 4 shows the compared pictures corresponded to the ITO-coating treated with SEW and ITO with oriented CNTs deposited on the ITO and then treated with SEW. The dimension of the considered pictures is corresponded to the part of the surfaces with 30 × 30 micrometers. It should be remarked that the relief grating is connected with the wavelength of CO2-laser at 10,6 micrometers.
Fig. 4 AFM-images of the relief regarded to ITO treated with SEW (a) and one connected with the ITO with oriented CNTs and treated by SEW.
4. Conclusion
Analyzing the obtained results one can postulate the following summary.
- 1). Structuration of the body of the nematic LC-mesophase by lanthanides nanoparticles provokes the bathochromic spectral shift and forms of the 2D network inside the doped LC that connected with the previously obtained data for LC doped with fullerenes, quantum dots, DNA, other nano- and bio-particles.
- 2). Structuration of the interface between LC-mesophase and solid substrate directly influences on the dramatic near IR-spectral shift and can be connected with the formation of the quasi-graphene intermediate layer due to the donor-acceptor properties of the CNTs and the large number of the electrons involved in the charge transfer from the CNTs core.
- 3). Both of the mentioned above structuration processes of the LC body and of the interface lead to change of the energy levels of the LC system and can be considered as the novel key point and method to form the organic photonic liquid crystals.
- 4). Both of the mentioned above structuration processes of the LC body and of the interface can extend drastically the area of the application of the electro-optical devices operated in the nea-IR spectral range in the real time.
- 5). The modified interface between LC and solid substrate can decrease drastically the number of the layers in the sandwich LC-cells and electro- and optically-addressed spatial light modulators based on the LC materials due to the reason that to orient LC-molecules in the planar or in the homeotropic direction it is not necessary to use high resistivity polyimide orienting coatings (with the specific resistance of close to 1013 Ohm × cm in comparison with the analogous value of the LC structure which is close to 1010 Ohm × cm) solved in the toxic tetrachloretane solution.
- 6). Control of the near IR-spectral shift in the LC matrixes doped with the lanthanide nanoparticles can be made via modification of the surface relief between the liquid crystal matrixes doped with the lanthanide nanoparticles and the solid substrate.
Acknowledgments
The authors would like to acknowledge their colleagues Prof. D.P. Uskokovic (Institute of Technical Sciences of the Serbian Academy of Sciences and Arts, Belgrade, Serbia), Prof. F. Kajzar and Prof. Chantal Andraud (Laboratoire de Chimie, UMR CNRS 5182, Ecole Normale Supґerieure de Lyon, France) for the useful discussion at the international conferences.
The presented results are partially supported by Russian Foundation for Basic Research Fond, Grant no. 13-03-00044 (2013–2015) as well as by the FP7 Program, Marie Curie International Researchers Exchange Proposal “BIOMOLEC” (2011–2015). As an additional, the work of Cosmina Lazar has been funded by the Sectorial Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/159/1.5/S/132397.
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