Research Article | Open Access
Nanoparticle-Enabled Ion Trapping and Ion Generation in Liquid Crystals
Nowadays, nanomaterials in liquid crystals and their possible applications in the design of tunable, responsive, and wearable devices are among the most promising research directions. In the majority of cases, all liquid crystal based devices have one thing in common; namely, they are driven by electric fields. This type of device driving can be altered by minor amounts of ions typically present in liquid crystal materials. Therefore, it is very important to understand how nanodopants can affect ions in liquid crystals. In this paper, a recently developed model of contaminated nanoparticles is applied to existing experimental data. The presented analysis unambiguously indicates that, in general, nanomaterials in liquid crystals can behave as a source of ions or as ion traps. Physical factors determining the type of the nanoparticle behaviour and their effects on the concentration of ions in liquid crystals are discussed.
Thermotropic liquid crystals are widely used in the design of tunable electro-optical devices. They include liquid crystal displays (LCD) , tunable lenses , filters , wave plates , retarders , diffractive optical elements , optical shutters [5, 6], and smart windows , to name a few. In the majority of cases, all of them are driven by electric fields . Ions, normally present in liquid crystals in small quantities, can alter the performance of liquid crystals [9–12]. Typically, liquid crystal devices such as LCD and tunable optical elements (filters, retarders, etc.) utilize the electric field effect when the applied electric field reorients liquid crystal molecules. This type of liquid crystal based applications considers ions a nuisance because of many negative side effects caused by ions in liquid crystal devices (image sticking, image flickering, reduced voltage holding ratio, overall slow response) [9–12]. There are also electro-optical devices relying on ions in liquid crystals (optical shutters and smart windows) [5–7]. That is why an understanding of possible sources of ion generation in liquid crystals is very important .
Ions in molecular liquid crystals can be generated in different ways: (i) the dissociation of neutral molecules in the bulk of liquid crystals (these dissociating species can be inherently present or added intentionally) [10–12, 14, 15]; (ii) ionic impurities as chemicals left over from the chemical synthesis [11, 16–18]; (iii) chemical degradation of liquid crystals ; (iv) ionic contaminants originated from the glue  and from the alignment layers [21–25] of the liquid crystal cell; (v)-(vii) ions generated by means of ionizing radiation [26, 27], through electrochemical reactions [28–30], and by relatively high electric fields [31–33].
Recently, the dispersion of nanomaterials in liquid crystals has emerged as a promising way to modify their properties and design novel materials suitable for many applications [[37, 38] and references therein]. From perspectives of ion generation in liquid crystals, a very important question is how can nanodopants affect the behaviour of ions in liquid crystals. Ion-related effects of nano-objects in liquid crystals were reported in many publications reviewed in a recent paper [ and references therein]. Different research groups reported that various types of nanomaterials (metal [39–43], semiconductor [35, 44], dielectric [34, 36, 45], ferroelectric [46–51] magnetic , and carbon-based [53–56]) changed the concentration of ions in liquid crystals in different ways. Despite the variety of existing experimental results on ions and nano-objects in liquid crystals, they can be broadly categorized into the following groups: (i) papers reporting the decrease in the concentration of ions in liquid crystals (the ion trapping regime); (ii) publications presenting the increase in the concentration of ions (the ion generation regime); (iii) the combination of both ion trapping and ion generation regimes (depending on the concentration of nanodopants in liquid crystals) . A very important finding is that the same type of nanomaterials dispersed in different liquid crystals can result in different regimes (ion trapping or ion generation) [34, 39, 53]. An elementary model of these regimes was recently proposed and developed in a series of papers [57–61]. This model introduced the ionic contamination of nanomaterials as a key factor enabling the possibility of different regimes (ion trapping (or ion capturing regime), ion generation (or ion releasing regime), and no change regime) in liquid crystals doped with nanomaterials [57–61]. However, the origin of the ionic contamination of nanomaterials remains practically unexplored.
In this paper, the aforementioned model of contaminated nanoparticles in liquid crystals is applied to existing experimental results with the aim of shedding some light on the nature of ionic contamination of nanodopants and ion generation/ion trapping in liquid crystals doped with such nanomaterials.
2. Elementary Model
To simplify the discussion, consider liquid crystals containing some mobile ions and nanoparticles contaminated with the same type of fully ionized ionic species. These ionic species are characterized by their volume concentration . The discussion of contaminated nanomaterials is needed to account for the possibility of experimentally observed ion trapping and ion generation regimes [57–61]. If contaminated nanoparticles are dispersed in liquid crystals, interactions between ions and nanoparticles will result in the change of the total concentration of mobile ions in liquid crystal/nanoparticle colloids. In short, a fraction of ionic contaminants can leave the surface of nanoparticles thus enriching the liquid crystal host with ions. This process can be considered as nanoparticle-enabled ion generation in liquid crystals. The reverse process, namely, the trapping of mobile ions by the surface of nanoparticles, also takes place. The competition between these two processes will result in the steady state characterized by a constant concentration of mobile ions in liquid crystals doped with nanoparticles (). In the simplest case, the ion releasing (or ion generation) process can be associated with the desorption of ions from the surface of nanoparticles, while the ion trapping process can be described by the adsorption of ions onto the surface of nanoparticles. As a result, the concentration of mobile ions in this system is governed by the following rate equation :The first term of (1) describes the adsorption of ions onto the surface of nanoparticles (ion trapping process), and the second term accounts for the ion desorption from the surface of nanoparticles (ion generation process). In (1), is the concentration of mobile ions; denotes time; is the surface area of a single nanoparticle (for simplicity, consider spherical nanoparticles characterized by their radius ; in this case and this parameter determines the dependence of the concentration of mobile ions in liquid crystal nanocolloids on the size of nanoparticles); is the volume concentration of nanoparticles (in many practical cases the weight concentration of nanoparticles , which is related to the volume concentration as ( is the volume of a single nanoparticle, and () is the density of liquid crystals (nanoparticles)), is preferred); is the surface density of all adsorption sites on the surface of a single nanoparticle; is the fractional surface coverage of nanoparticles defined as ( is the surface density of adsorption sites on the surface of nanoparticles occupied by ions); is the adsorption rate constant; and is the desorption rate constant. The aforementioned physical parameters (, , and ) are material-dependent and, for a given system under study (liquid crystals-nanoparticles), are considered as constants.
The applicability and limitations of (1) to compute the concentration of mobile ions in liquid crystals doped with nanoparticles were recently discussed in papers [63–65]. It should be stressed that (1) is an approximation which is reasonably applicable to describe ions in molecular liquid crystals. In a general case, a more rigorous approach utilizing Poisson-Boltzmann equation should be considered [66–69].
In the steady-state regime (), typically achieved in experiments, (1) should be solved along with the conservation law of the total number of ions expressed bywhere is the initial concentration of ions in liquid crystals and is the contamination factor of nanoparticles. The contamination factor of nanoparticles accounts for their ionic contamination and equals the fraction of the adsorption sites on the surface of nanoparticles occupied by ionic contaminants prior to dispersing nanodopants in liquid crystals [70, 71].
The ionic contamination of nanoparticles is a key factor enabling the possibility of different regimes, namely, the ion trapping regime (Figure 1, solid and dashed-dotted curves), the ion generation regime (Figure 1, dashed curve), and no change regime (Figure 1, dotted curve).
The switching between these regimes is controlled by the aforementioned contamination factor of nanoparticles . The ion trapping regime is observed if (), the ion generation regime is reached if (), and no change regime is achieved if (), where is the critical contamination factor of nanoparticles defined as and is the critical concentration of ions written as and (Figure 1) . Both ion trapping and ion generation regimes are more pronounced if higher concentrations of nanoparticles are used (Figure 1). It should be noted that the dispersion of 100% pure nanoparticles in liquid crystals results in the ion trapping regime only (Figure 1, dashed-dotted curve). Another very important feature of the behaviour of contaminated nanoparticles in liquid crystals is an existence of the saturation effect. Upon increasing the concentration of nanoparticles in liquid crystals, the concentration of mobile ions increases or decreases towards its saturation level given by the critical concentration (Figure 1, solid and dashed curves). On the contrary, the dispersion of 100% pure nanoparticles () in liquid crystals does not exhibit the saturation effect (Figure 1, dashed-doted curve).
3. Case Study: TiO2 Nanoparticles in Liquid Crystals 
The effects of titanium dioxide nanoparticles on the concentration of mobile ions in nematic liquid crystals were recently reported by Shcherbinin and Konshina . By dispersing TiO2 nanoparticles (Plasmotherm, Moscow) in commercially available nematic liquid crystals (ZhK1282, NIOPIK, Moscow) the ion releasing regime was observed (Figure 2, squares) . These results indicate that, in the considered case, nanoparticles in liquid crystals act as a source of ions. In other words, nanoparticles, upon their dispersion in liquid crystals, increase the total concentration of mobile ions (Figure 2). According to the model described in the previous section, this increase in the concentration of ions can be caused by the ionic contamination of nanoparticles. By applying (1)-(2) of the presented model, a very good agreement between theoretical predictions and experimental results can be achieved (Figure 2). Interestingly, a minor level of the ionic contamination of nanoparticles () is enough to increase the concentration of ions in liquid crystals (Figure 2). Given its small values, this uncontrolled ionic contamination of nanoparticles can easily happen during the preparation and handling of these nanomaterials.
By adding the ionic surfactant (cetyltrimethylammonium bromide, abbreviated as CTABr) to the same nematic liquid crystals (ZhK1282), the total concentration of ions was increased by nearly one order of magnitude . Dispersing the same TiO2 nanoparticles in these, enriched with ions, liquid crystals, the ion trapping regime was achieved . To analyse these experimental results, we can consider two dominant types of ionic species in ZhK1282 liquid crystals. Pristine liquid crystals (prior to doping them with CTABr surfactants and nanoparticles) are characterized by dominant ionic species of the concentration . Liquid crystals doped with surfactants are characterized by two dominant types of ionic species, namely, and . In the regime of low surface coverage of nanoparticles ( and ), these two dominant types of ionic species can be treated independently. As a result, the dispersion of nanoparticles in liquid crystals increases the concentration of ions (Figure 2) and decreases the concentration of ionic species (Figure 3) (Table 1).
The ion trapping regime, corresponding to the effects of nanoparticles on the concentration of ions , is shown in Figure 3. As can be seen from Figure 3, there is a very good agreement between theoretical predictions based on the model of contaminated nanoparticles ((1)-(2)) and reported experimental data. As expected, TiO2 nanoparticles were not contaminated with ionic species prior to dispersing them in liquid crystals (). As a result, the only possibility is an experimentally observed ion trapping regime (Figure 3).
4. Case Study: CdSe/ZnS Core/Shell Nanoparticles in Liquid Crystals 
The behaviour of ionic impurities in nematic liquid crystals (ZhK1289, NIOPIK, Moscow) doped with CdSe/ZnS core/shell quantum dots was studied by Shcherbinin and Konshina . Prior to dispersing them in liquid crystals, quantum dots were functionalized with trioctylphosphine oxide (TOPO) ligands. Shcherbinin and Konshina reported that functionalized quantum dots dispersed in liquid crystals enriched the liquid crystal host with a new type of ions. These ions (“slow” ions as called in paper ) were not present in liquid crystals prior to mixing them with nanodopants. Thus, functionalized quantum dots were considered a primary source of these ions. It was found that the concentration of “slow” ions was a linear function of the weight concentration of nanodopants in liquid crystals . This linear dependence can also be obtained by means of (1)-(2) (steady-state regime) and assuming the following conditions: and . In this case, the concentration of mobile ions in liquid crystals doped with nanoparticles can be written as
An inset (Figure 4) also shows the effect of the nanoparticle size on the concentration of ions in liquid crystals. In (1)–(3) this size effect is caused by the surface area of nanoparticles, . In the case of spherical nanodopants, . According to Figure 4 (inset), at the same concentration of nanoparticles, the ion releasing effect is greater if smaller nanoparticles are used. Additional details can also be found in recently published papers [63, 65].
5. Case Study: Cu7PS6 Nanoparticles in Liquid Crystals 
An increase in the electrical conductivity of 6CB nematic liquid crystals through doping them with Cu7PS6 nanoparticles was recently reported by Kovalchuk and coworkers . An interesting feature of the observed effect was the saturation of the increase in the electrical conductivity at higher concentrations of nanoparticles .
The concentration of mobile ions can be estimated throughwhere is the measured electrical conductivity, is the charge of a single ion, and is its mobility [10–12]. As a result, the obtained values of the concentration of mobile ions in 6CB liquid crystals doped with Cu7PS6 are in a very good agreement with theoretical predictions based on (1)-(2) of the model of contaminated nanoparticles (Figure 5).
In the case shown in Figure 5, nanoparticles act as a source of ions in liquid crystals, thus increasing the electrical conductivity of the system under study. The concentration of ions in 6CB liquid crystals doped with Cu7PS6 nanoparticles as a function of the nanoparticle concentration exhibits a monotonous increase towards the saturation level. In the framework of the proposed model, this saturation level is given by the critical concentration of ions, . In the considered example, , , and thus . Relatively high values of the contamination factor can indicate that, in the case of Cu7PS6 nanoparticles, ionic contaminants are inherently present in these nanoparticles. In other words, the origin of these ions can be associated with the chemical structure of Cu7PS6 nanoparticles and the possibility of their self-dissociation rather than with ionic contaminants originated during their preparation and handling. The dissociation of Cu7PS6 nanoparticles resulting in the generation of copper ions can be reasonably expected for this type of materials .
According to existing experimental reports, nanoparticles in liquid crystals can behave either as ion traps or as ion generating objects. If nanodopants are 100% pure, the ion trapping regime is the only possible outcome of their dispersion in liquid crystals. On the contrary, liquid crystals doped with contaminated nanoparticles can exhibit three different regimes, namely, the ion trapping regime, ion generation regime, and no change regime. Both ion trapping and ion generation regimes are characterized by the saturation effect (Figure 1, solid and dashed curves, and Figures 2, 5). This effect, absent in the case of 100% pure nanoparticles in liquid crystals (Figure 1, dashed-dotted curve), is quantitatively described by the critical concentration of ions, . Thus, the ionic contamination of nanodopants is an important physical quantity, enabling the type of the achieved regime (ion trapping or ion generation) and leading to the saturation effect. In the framework of the model of contaminated nanomaterials, this ionic contamination is quantified by means of the dimensionless contamination factor (). The predictions of this model are in very good agreement with reported experimental results (Figures 2–5). In addition, an analysis of experimental results using the model of contaminated nanoparticles can shed some light on the origin of ionic contaminants.
The ionic contamination of nanomaterials can originate from different sources during their production and handling. For example, it can be a minor fraction of ionic contaminants left over from the chemical synthesis. In the considered case study (Figure 4), trioctylphosphine oxide (TOPO) ligands are prone to uncontrolled contamination (including ionic contaminants) during the chemical synthesis . As a result, once quantum dots functionalized with TOPO are dispersed in liquid crystals, ionic contaminants can be released in the bulk of the liquid crystal host. This assumption is also consistent with recent findings reported by Urbanski and Lagerwall . In the case of uncontrolled ionic contamination of nanodopants, the values of the contamination factor are typically relatively low. Interestingly, even such small values of the contamination factor (, on the order of 10−3 -10−4 or less, see Figures 1–4) are enough to alter the concentration of mobile ions in liquid crystals.
Ionic contaminants can also be inherently present in nanoparticles. In this case, the value of the contamination factor is relatively high (). An example of this, inherent ionic “contamination,” includes self-dissociating nanomaterials. Such nanomaterials can dissociate and generate ions in liquid crystals (Figure 5).
In general, contaminated nanoparticles should be considered as an important source of ion generation in liquid crystals. In addition, under certain conditions, they can act as ion traps. Further studies are needed to understand physical-chemical mechanisms governing these processes and identities of ionic contaminants. In the long run, this understanding will enable numerous applications of liquid crystals doped with nanomaterials.
Conflicts of Interest
The author declares that there are no conflicts of interest.
The author would like to acknowledge the support provided by the UCCS BioFrontiers Center at the University of Colorado.
- K. Naoyuki, Ed., The Liquid Crystal Display Story. 50 Years of Liquid Crystal R&D that lead The Way to the Future, Springer, Tokyo, Japan, 2014.
- Y. Lin, Y. Wang, and V. Reshetnyak, “Liquid crystal lenses with tunable focal length,” Liquid Crystals Reviews, vol. 5, no. 2, pp. 111–143, 2017.
- I. Abdulhalim, “Non-display bio-optic applications of liquid crystals,” Liquid Crystals Today, vol. 20, no. 2, pp. 44–60, 2011.
- L. De Sio, D. E. Roberts, Z. Liao et al., “Beam shaping diffractive wave plates [Invited],” Applied Optics, vol. 57, no. 1, pp. A118–A121, 2018.
- M. W. Geis, P. J. Bos, V. Liberman, and M. Rothschild, “Broadband optical switch based on liquid crystal dynamic scattering,” Optics Express, vol. 24, no. 13, pp. 13812–13823, 2016.
- E. A. Konshina and D. P. Shcherbinin, “Study of dynamic light scattering in nematic liquid crystal and its optical, electrical and switching characteristics,” Liquid Crystals, vol. 45, no. 2, pp. 292–302, 2017.
- R. Dabrowski, J. Dziaduszek, J. Bozetka et al., “Fluorinated smectics - New liquid crystalline medium for smart windows and memory displays,” Journal of Molecular Liquids, 2017.
- D. K. Yang and S. T. Wu, Liquid Crystal Devices, John Wiley & Sons, Hoboken, Hoboken, NJ, USA, 2006.
- S. Naemura, “Electrical properties of liquid-crystal materials for display applications,” Materials Research Society - Proceedings, vol. 559, pp. 263–274, 1999.
- V. G. Chigrinov, Liquid Crystal Devices: Physics and Applications, Artech House, Boston, MA, USA, 1999.
- L. M. Blinov, Structure and Properties of Liquid Crystals, Springer Netherlands, New York, NY, USA, 2010.
- K. Neyts and F. Beunis, Handbook of Liquid Crystals: Physical Properties and Phase Behavior of Liquid Crystals, vol. 2, Chapter 11, Ion transport in liquid crystals, Wiley-VCH, Germany, 2014.
- P. P. Korniychuk, A. M. Gabovich, K. Singer, A. I. Voitenko, and Y. A. Reznikov, “Transient and steady electric currents through a liquid crystal cell,” Liquid Crystals, vol. 37, no. 9, pp. 1171–1181, 2010.
- R. Chang and J. M. Richardson, “The Anisotropic Electrical Conductivity of M.B.B.A. Containing Tetrabutyl-Ammonium Tetraphenyl-Boride,” Molecular Crystals and Liquid Crystals, vol. 28, no. 1-2, pp. 189–200, 1974.
- M. I. Barnik, L. M. Blinov, M. F. Grebenkin, S. A. Pikin, and V. G. Chigrinov, “Electrohydrodynamic instability in nematic liquid crystals,” Sov Phys JETP, vol. 42, no. 3, pp. 550–553, 1976.
- G. Briere, F. Gaspard, and R. Herino, “Ionic residual conduction in the isotropic phase of a nematic liquid crystal,” Chemical Physics Letters, vol. 9, no. 4, pp. 285–288, 1971.
- S. Naemura and A. Saivada, “Ionic conduction in nematic and smectic a liquid crystals,” Molecular Crystals and Liquid Crystals, vol. 400, pp. 79–96, 2003.
- H.-Y. Hung, C.-W. Lu, C.-Y. Lee, C.-S. Hsu, and Y.-Z. Hsieh, “Analysis of metal ion impurities in liquid crystals using high resolution inductively coupled plasma mass spectrometry,” Analytical Methods, vol. 4, no. 11, pp. 3631–3637, 2012.
- M. Sierakowski, “Ionic interface-effects in electro-optical LC-cells,” Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals, vol. 375, pp. 659–677, 2002.
- S. Murakami and H. Naito, “Electrode and interface polarizations in nematic liquid crystal cells,” Japanese Journal of Applied Physics, vol. 36, no. 4 A, pp. 2222–2225, 1997.
- H. Naito, Y. Yasuda, and A. Sugimura, “Desorption processes of adsorbed impurity ions on alignment layers in nematic liquid crystal cells,” Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals, vol. 301, pp. 85–90, 1997.
- M. Mizusaki, S. Enomoto, and Y. Hara, “Generation mechanism of residual direct current voltage for liquid crystal cells with polymer layers produced from monomers,” Liquid Crystals, vol. 44, no. 4, pp. 609–617, 2017.
- R. Kravchuk, O. Koval'Chuk, and O. Yaroshchuk, “Filling initiated ion transport processes in liquid crystal cell,” Molecular Crystals and Liquid Crystals Science and Technology Section A: Molecular Crystals and Liquid Crystals, vol. 384, no. I, pp. 111–119, 2002.
- E. A. Konshina and D. P. Shcherbinin, “Effect of granular silver films morphology on the molecules orientation and ion contamination of nematic liquid crystal,” Bulletin of the Moscow State Regional University (Physics and Mathematics), no. 4, pp. 103–113, 2017.
- Y. Garbovskiy, “Time-dependent electrical properties of liquid crystal cells: unravelling the origin of ion generation,” Liquid Crystals, pp. 1–9, 2018.
- A. V. Kovalchuk, O. D. Lavrentovich, and V. A. Linev, “Electrical conductivity of γ-irradiated cholesteric liquid crystals,” Sov. Tech. Phys. Lett, vol. 14, no. 5, pp. 381-382, 1988.
- H. Naito, K. Yoshida, M. Okuda, and A. Sugimura, “Transient current study of ultraviolet-light-soaked states in n-pentyl-p-n-cyanobiphenyl,” Japanese Journal of Applied Physics, vol. 33, no. 10, pp. 5890-5891, 1994.
- S. Barret, F. Gaspard, R. Herino, and F. Mondon, “Dynamic scattering in nematic liquid crystals under dc conditions. I. Basic electrochemical analysis,” Journal of Applied Physics, vol. 47, no. 6, pp. 2375–2377, 1976.
- S. Barret, F. Gaspard, R. Herino, and F. Mondon, “Dynamic scattering in nematic liquid crystals under dc conditions. II. Monitoring of electrode processes and lifetime investigation,” Journal of Applied Physics, vol. 47, no. 6, pp. 2378–2381, 1976.
- H. S. Lim, J. D. Margerum, and A. Graube, “Electrochemical properties of dopants and the D-C dynamic scattering of a nematic liquid crystal,” J. Electrochem. Soc.: Solid State Science and technology, vol. 124, no. 9, pp. 1389–1394, 1977.
- T. C. Chieu and K. H. Yang, “Transport properties of ions in ferroelectric liquid crystal cells,” Japanese Journal of Applied Physics, vol. 28, no. 11 R, pp. 2240–2246, 1989.
- S. Murakami and H. Naito, “Charge injection and generation in nematic liquid crystal cells,” Japanese Journal of Applied Physics, vol. 36, no. 2, pp. 773–776, 1997.
- H. De Vleeschouwer, A. Verschueren, F. Bougrioua et al., “Long-term ion transport in nematic liquid crystal displays,” Japanese Journal of Applied Physics, vol. 40, no. 5 A, pp. 3272–3276, 2001.
- D. P. Shcherbinin and E. A. Konshina, “Impact of titanium dioxide nanoparticles on purification and contamination of nematic liquid crystals,” Beilstein Journal of Nanotechnology, vol. 8, pp. 2766–2770, 2017.
- D. P. Shcherbinin and E. A. Konshina, “Ionic impurities in nematic liquid crystal doped with quantum dots CdSe/ZnS,” Liquid Crystals, vol. 44, no. 4, pp. 648–655, 2017.
- O. V. Kovalchuk, I. P. Studenyak, V. Yu. Izai et al., “Saturation effect for dependence of the electrical conductivity of planar oriented liquid crystal 6CB on the concentration of Cu7PS6 nanoparticles, Semiconductor Physics,” Quantum Electronics & Optoelectronics, vol. 20, no. 4, pp. 437–441, 2017.
- Y. A. Garbovskiy and A. V. Glushchenko, “Liquid crystalline colloids of nanoparticles: Preparation, properties, and applications,” Solid State Physics - Advances in Research and Applications, vol. 62, pp. 1–74, 2010.
- J. P. F. Lagerwall and G. Scalia, Liquid Crystals with Nano and Microparticles (Series in Soft Condensed Matter: Volume 7), vol. 7, World Scientific Publishing Co., Singapore, Singapore, 2016.
- Y. Garbovskiy and I. Glushchenko, “Nano-objects and ions in liquid crystals: Ion trapping effect and related phenomena,” Crystals, vol. 5, no. 4, pp. 501–533, 2015.
- R. K. Shukla, X. Feng, S. Umadevi, T. Hegmann, and W. Haase, “Influence of different amount of functionalized bulky gold nanorods dopant on the electrooptical, dielectric and optical properties of the FLC host,” Chemical Physics Letters, vol. 599, pp. 80–85, 2014.
- F. V. Podgornov, R. Wipf, B. Stühn, A. V. Ryzhkova, and W. Haase, “Low-frequency relaxation modes in ferroelectric liquid crystal/gold nanoparticle dispersion: impact of nanoparticle shape,” Liquid Crystals, vol. 43, no. 11, pp. 1536–1547, 2016.
- M. Urbanski and J. P. F. Lagerwall, “Why organically functionalized nanoparticles increase the electrical conductivity of nematic liquid crystal dispersions,” Journal of Materials Chemistry C, vol. 5, no. 34, pp. 8802–8809, 2017.
- F. V. Podgornov, M. Gavrilyak, A. Karaawi, V. Boronin, and W. Haase, “Mechanism of electrooptic switching time enhancement in ferroelectric liquid crystal/gold nanoparticles dispersion,” Liquid Crystals, pp. 1–9, 2018.
- E. Konshina, D. Shcherbinin, and M. Kurochkina, “Comparison of the properties of nematic liquid crystals doped with TiO 2 and CdSe/ZnS nanoparticles,” Journal of Molecular Liquids, 2017.
- A. Chandran, J. Prakash, J. Gangwar et al., “Low-voltage electro-optical memory device based on NiO nanorods dispersed in a ferroelectric liquid crystal,” RSC Advances, vol. 6, no. 59, pp. 53873–53881, 2016.
- R. K. Shukla, C. M. Liebig, D. R. Evans, and W. Haase, “Electro-optical behaviour and dielectric dynamics of harvested ferroelectric LiNbO3 nanoparticle-doped ferroelectric liquid crystal nanocolloids,” RSC Advances, vol. 4, no. 36, pp. 18529–18536, 2014.
- Y. Garbovskiy and I. Glushchenko, “Ion trapping by means of ferroelectric nanoparticles, and the quantification of this process in liquid crystals,” Applied Physics Letters, vol. 107, no. 4, 2015.
- R. Basu and A. Garvey, “Effects of ferroelectric nanoparticles on ion transport in a liquid crystal,” Applied Physics Letters, vol. 105, no. 15, p. 151905, 2014.
- Y.-C. Hsiao, S.-M. Huang, E.-R. Yeh, and W. Lee, “Temperature-dependent electrical and dielectric properties of nematic liquid crystals doped with ferroelectric particles,” Displays, vol. 44, pp. 61–65, 2016.
- P. Kumar, S. Debnath, N. V. Rao, and A. Sinha, “Nanodoping: a route for enhancing electro-optic performance of bent core nematic system,” Journal of Physics: Condensed Matter, vol. 30, no. 9, p. 095101, 2018.
- S. Al-Zangana, M. Turner, and I. Dierking, “A comparison between size dependent paraelectric and ferroelectric BaTiO3 nanoparticle doped nematic and ferroelectric liquid crystals,” Journal of Applied Physics, vol. 121, no. 8, 2017.
- Khushboo, P. Sharma, P. Malik, and K. K. Raina, “Electro-optic, dielectric and optical studies of NiFe2O4-ferroelectric liquid crystal: a soft magnetoelectric material,” Liquid Crystals, vol. 43, no. 11, pp. 1671–1681, 2016.
- S. Tomylko, O. Yaroshchuk, O. Kovalchuk, U. Maschke, and R. Yamaguchi, “Dielectric properties of nematic liquid crystal modified with diamond nanoparticles,” Ukrainian Journal of Physics, vol. 57, no. 2, pp. 239–243, 2012.
- B.-R. Jian, C.-Y. Tang, and W. Lee, “Temperature-dependent electrical properties of dilute suspensions of carbon nanotubes in nematic liquid crystals,” Carbon, vol. 49, no. 3, pp. 910–914, 2011.
- M.-J. Cho, H.-G. Park, H.-C. Jeong et al., “Superior fast switching of liquid crystal devices using graphene quantum dots,” Liquid Crystals, vol. 41, no. 6, pp. 761–767, 2014.
- P.-C. Wu, L. N. Lisetski, and W. Lee, “Suppressed ionic effect and low-frequency texture transitions in a cholesteric liquid crystal doped with graphene nanoplatelets,” Optics Express, vol. 23, no. 9, pp. 11195–11204, 2015.
- Y. Garbovskiy, “Switching between purification and contamination regimes governed by the ionic purity of nanoparticles dispersed in liquid crystals,” Applied Physics Letters, vol. 108, no. 12, 2016.
- Y. Garbovskiy, “Impact of contaminated nanoparticles on the non-monotonous change in the concentration of mobile ions in liquid crystals,” Liquid Crystals, vol. 43, no. 5, pp. 664–670, 2016.
- Y. Garbovskiy, “Electrical properties of liquid crystal nano-colloids analysed from perspectives of the ionic purity of nano-dopants,” Liquid Crystals, vol. 43, no. 5, pp. 648–653, 2016.
- Y. Garbovskiy, “Adsorption of ions onto nanosolids dispersed in liquid crystals: Towards understanding the ion trapping effect in nanocolloids,” Chemical Physics Letters, vol. 651, pp. 144–147, 2016.
- Y. Garbovskiy, “Ion capturing/ion releasing films and nanoparticles in liquid crystal devices,” Applied Physics Letters, vol. 110, no. 4, 2017.
- Y. Garbovskiy, “Kinetics of Ion-Capturing/Ion-Releasing Processes in Liquid Crystal Devices Utilizing Contaminated Nanoparticles and Alignment Films,” Nanomaterials, vol. 8, no. 2, p. 59, 2018.
- Y. Garbovskiy, “Adsorption/desorption of ions in liquid crystal nanocolloids: the applicability of the Langmuir isotherm, impact of high electric fields and effects of the nanoparticle’s size,” Liquid Crystals, vol. 43, no. 6, pp. 853–860, 2016.
- Y. Garbovskiy, “The purification and contamination of liquid crystals by means of nanoparticles. The case of weakly ionized species,” Chemical Physics Letters, vol. 658, pp. 331–335, 2016.
- Y. Garbovskiy, “Ions and size effects in nanoparticle/liquid crystal colloids sandwiched between two substrates. The case of two types of fully ionized species,” Chemical Physics Letters, vol. 679, pp. 77–85, 2017.
- G. Barbero and L. R. Evangelista, Adsorption Phenomena and Anchoring Energy in Nematic Liquid Crystals, Taylor & Francis, Boca Raton, FL, USA, 2006.
- V. Steffen, L. Cardozo-Filho, E. A. Silva, L. R. Evangelista, R. Guirardello, and M. R. Mafra, “Equilibrium modeling of ion adsorption based on Poisson-Boltzmann equation,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 468, pp. 159–166, 2015.
- F. Batalioto, A. M. Figueiredo Neto, and G. Barbero, “Ion trapping on silica nanoparticles: Effect on the ζ-potential,” Journal of Applied Physics, vol. 122, no. 16, 2017.
- V. Steffen, E. Silva, L. Evangelista, and L. Cardozo-Filho, “Debye–Hückel approximation for simplification of ions adsorption equilibrium model based on Poisson–Boltzmann equation,” Surfaces and Interfaces, vol. 10, pp. 144–148, 2018.
- Y. Garbovskiy, “Nanoparticle enabled thermal control of ions in liquid crystals,” Liquid Crystals, vol. 44, no. 6, pp. 948–955, 2017.
- Y. Garbovskiy, “Ions in liquid crystals doped with nanoparticles: conventional and counterintuitive temperature effects,” Liquid Crystals, vol. 44, no. 9, pp. 1402–1408, 2017.
- F. Wang, R. Tang, J. L.-F. Kao, S. D. Dingman, and W. E. Buhro, “Spectroscopic identification of tri-n-octylphosphine oxide (TOPO) impurities and elucidation of their roles in cadmium selenide quantum-wire growth,” Journal of the American Chemical Society, vol. 131, no. 13, pp. 4983–4994, 2009.
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