Electrical Features of Liquid Crystal Composition Doped with Cobalt Ferrite: Possible Sensing Applications

Abstract

The effects of CoFe₂O₄ nanoparticles on the properties of an electro-optical liquid crystal cell based on the nematic composition of 4-Cyano-4′-pentylbiphenyl (5CB) under the influence of different forms of bias voltage were studied. Detailed results were established for the application of sinusoidal voltages with various frequencies and amplitudes. At the input signal, with a frequency of 500 kHz, a resonant current increase was obtained in the electrical circuit, followed by a decrease in the current with an increase in the frequency. This indicates the formation of a consistent oscillatory circuit. The quality factor of the nanoparticle system does not depend on the amplitude of the controlled voltage. Liquid crystal cells with constant quality can be used in a number of devices and technologies, including extended sensing devices, where stable electrical properties are required.

1. Introduction

Liquid crystal (LC) mesophase, especially nematic liquid crystals (NLCs) and the compositions based on them, is widely used in materials science and in different techniques. This is due to the rather easy process of LC mesophase control using thermal, mechanical, electrical and optical signals [1,2,3,4,5]. LC-based compositions are applied in the laser technique, in the set-ups for holographic recording and for information reading [6,7,8,9,10]. LC cells and modulators based on them can be used for the switching, modulation, conversion and limitation of radiation [11,12,13,14,15], in various photovoltaic and sensor devices [16,17,18,19,20] and in displays [21,22,23,24,25]. At the same time, the control of the electrical external action can be varied by modification of the shape of the electric pulse, its amplitude, its duration and the frequency of its repetition [26,27,28,29,30,31,32,33,34,35,36,37,38,39,40]. Thus, a wide range of parameters of the controlled electrical signal allows one to obtain unique optimized LC device properties. On the other hand, the recently used nanostructuring process for LC and polymers with different nanoparticles (NPs) makes it possible to change the polarization of the new organic composite and its order parameter, the refractive index, which also affects both the basic physical–chemical properties of conjugated organic composites, including LC ones, and, in particular, their performance [41,42,43,44,45,46].

In the present study, the effect of CoFe₂O₄ NPs on the properties of an LC cell based on the 5CB group under the influence of different forms of bias voltage were studied. Detailed results were established for the application of sinusoidal voltages with various frequencies and amplitudes. At the input signal, with a frequency of 500 kHz, a resonant current increase was obtained in the electrical circuit, followed by a decrease in the current with an increase in the frequency. Thus, if the resonant current is increased, it causes a decrease in LC cell impedance. This indicates the formation of a consistent oscillatory circuit. Under these conditions, the quality factor of the nanostructured LC system does not depend on the amplitude of the controlled voltage.

2. Results

The purpose of this study is to analyze the electrical response of an LC cell based on 5CB injected with CoFe₂O₄ nanoparticles when exposed to sinusoidal voltages of various amplitudes and frequencies. Special attention was paid to identifying resonant effects, changing the current properties of the system, and establishing dependencies that can serve as the basis for new types of the devices developed with controlled frequency properties.

In this study, two LC cells (pure and structured with CoFe₂O₄ NPs) based on the nematic composition 4-Cyano-4′-pentylbiphenyl (5CB) were manufactured. The structure of the manufactured cells is shown in Figure 1. The sandwich construction of an LC cell is presented, which is connected with two glass substrates (Crown glass K8), two conducting and orientation layers (where conducting ITO coating works as both the conducting and orienting layers) and LC layers inside this construction. The ITO layers are brought to the outer edges of the cell, where the contacts of the control voltage generator are connected to them. It should be noted once again that the ITO conducting layers also act as an orienting (alignment layer) for the LC molecules. The laser orienting deposition (LOD) technique has been used to achieve this aim, which is described in papers [47,48,49]. One cell was constructed by using pure LC, and the second cell was injected with CoFe₂O₄ nanoparticles.

During the experiment, sinusoidal voltages of various amplitudes, including 1 V, 5 V and 10 V, were applied to both LC cells; the changes in the amplitude of the current in the circuit were recorded, with the quality factor measured based on the resonant properties. Figure 2 shows the resulting relationship. One can see an increase in the current with increasing frequency obtained at different applied voltage amplitudes for the pure and sensitized LC cells. It should be noted that the form of the applied voltage is shown in the middle part of Figure 2 as a sine wave.

For a more detailed display of the changes, the quality factors of the systems were calculated. The calculated Q-factor values for a pure LC cell and a cell with the addition of magnetic nanoparticles at various amplitudes of the control voltage are shown in

Table 1. The calculated Q-factor values of the systems at different values of control voltage amplitude.

Systems	The Amplitude of the Control Voltage, V
	1	5	10
5CB	0.245	0.390	1.17
5CB + CoFe2O4	0.58	0.59	0.56

.

The Q-factor was calculated according to the above graph using the formula

$$Q={\displaystyle \frac{f_{res}}{∆f}},$$

where $f_{res}$ is the resonant frequency, Hz, and $∆f$ is the frequency dependence at a current amplitude level of $\sqrt{2}$ times less than the resonant frequency, Hz.

Based on the obtained results the following can be concluded:

Resonance occurs for both pure 5CB and its sensitized version, while the resonant frequency of the system, measured when exposed to a sinusoidal voltage, remains unchanged at 500 kHz, as in the case of a pure LC cell and with the addition of CoFe₂O₄ nanoparticles. Although ferritic nanoparticles have magnetic properties, their concentration and effect on 5CB molecules are limited within local regions. Nanoparticles do not significantly affect the macroscopic inductance of the system, which could shift the resonant frequency.

Moreover, in a pure 5CB cell, the Q-factor increases with an increase in control voltage, since it lacks nanoparticles that could create local fields and additional magnetic losses. Cobalt ferrite nanoparticles injected into the cell create local fields and can interact with liquid crystal molecules, thus limiting their orientation in one direction. This leads to additional losses and a decrease in Q-factor, especially at high voltages, when the external electric field begins to compete with the local fields of the nanoparticles.

Furthermore, the quality factor of the nanoparticle system does not depend on the amplitude of the controlled voltage. The CoFe₂O₄ nanoparticles can partially shield an external electric field, creating local areas with a high field around themselves. These local fields stabilize the LC molecules and make the system less susceptible to changes in external voltage. Thus, regardless of the increase in the control voltage (in the studied range), the electrical response of the cell with nanoparticles remains at the same level, maintaining a constant quality factor.

We should note that the introduction of CoFe₂O₄ nanoparticles into the 5CB LC system potentially makes it possible to significantly change its electrical and optical properties, which opens up opportunities for creating more sensitive sensors and stable displays with improved properties [49]. In addition, the presence of ferrite nanoparticles can cause the effect of local electric and magnetic fields, which can affect the orientation and dynamics of the liquid crystal molecules, modifying both the electrical and optical responses of the LC cell. Indeed, this warrants the future study of these features.

3. Materials and Methods

We sensitized the composite materials based on the NLC structure with the addition of CoFe₂O₄ nanoparticles. LC cells were made based on the nematic composition 4-Cyano-4′-pentylbiphenyl (5CB), purchased from Alfa Aesar Co., Lancashire, England, LOT 10166530, No. B21856. To construct the LC cells, we used additional alignment layers. The conductive ITO coatings were treated with a CO₂ laser in order to make the gratings. These gratings permitted us to align the LC molecules with good accuracy.

CoFe₂O₄ nanoparticles were synthesized following the approach proposed in [50], providing non-agglomerated nanoparticles of a uniform shape, size and surface chemistry; CoFe₂O₄ nanoparticles were synthesized solvothermally. Using a typical synthesis method, NaOH (10 mmol) was dissolved in 2 mL of distilled water. After that, 10 mL of 1-pentanol was added, followed by the addition of oleic acid, which is used as a capping agent. Amounts of 2 mmol of iron nitrate and 1 mmol of cobalt nitrate were dissolved in 18 mL of distilled water, and aqueous solution was poured into the above solution and underwent vigorous stirring for 2 h. After that, the autoclave was placed into an oven at 180 °C for 8 h. Then, the obtained particles were separated from the liquid phase and washed three times by redispersing them in n-hexane and precipitating them with ethanol. Finally, the nanoparticles were redispersed in n-hexane, transferred to a watch glass and left to dry in air overnight. The prepared CoFe₂O₄ nanoparticles were non-agglomerated, uniform in size (5 ± 1 nm) and sphere-like in shape, covered with a monolayer of oleic acid [50].

Based on our previous experiments with different nanoparticles (the fullerenes C₆₀ and C₇₀, carbon nanotubes, shungites, graphene oxides, WS₂ and MoS₂ nanotubes, quantum dots, etc.), we used approximately 0,1 wt.% of ferrite NPs in the LC matrix. This concentration permitted us to make a homogenous composition with increased polarizability. Thus, we predicted that the LC cells would have a high speed. Moreover, our previous study of this system showed that the introduction of cobalt ferrite NPs into the nematic LC has significant effects [46]. Based on the data presented in [46], it is possible to detect a tendency towards an increase in the refractive index of the LC medium when CoFe₂O₄ nanoparticles are introduced. This is of additional practical interest, since the possibility of the refractive index varying is an additional tool for the optical matching of LC media with orientation coatings and other functional layers in LC cells. Moreover, this material has a number of valuable properties, including a high level of the magnetic anisotropy, mechanical and chemical stability, and pronounced electrical properties.

In the present study, the 5CB and CoFe₂O₄ composite was examined under the conditions of different shapes, amplitudes, durations and repetition rates of the electric pulse.

It should be noted that the composition of LC 4′-Pentyl-4-biphenyl-carbonitrile was used as a classical LC matrix. CoFe₂O₄ nanoparticles were synthesized following the approach in Ref. [50], providing non-agglomerated nanoparticles of a uniform shape, size and surface chemistry. The concentration of nanoparticles injected into the LC composition was at the level of 0.1 wt.%. The thickness of the LC cells was 10 µm.

An experimental scheme for the current experiments is shown in Figure 3.

The light source in the experimental set-up was a red semiconductor laser with a wavelength of λ = 633 nm and an output power of 200 mW (Model: LSM-SRA650-200; country of manufacture: China). The laser provided a stable and narrow beam, essential for passing through the liquid crystal cell. The laser was powered by a 12V constant voltage source, ensuring its stable operation. Neutral density filters were used to vary the laser intensity. The laser beam was linearly polarized using a polarizer placed before the liquid crystal cell. An analyzer was positioned after the LC cell at a perpendicular orientation relative to the polarizer. This configuration allowed for the detection of light intensity changes caused by the reorientation of the liquid crystal molecules under the influence of an electric field. It should be mentioned that both the thin-film polarizer (element 3 shown in the set-up) and analyzer (element 5 shown in the set-up) were developed in Prof. N. Kamanina’s lab (Vavilov State Optical Institute, Saint-Petersburg, Russia) using their own patented technology [51].

The light transmitted through the analyzer was detected by a photodiode (model: FD-26; country of manufacture: Saint-Petersburg-Moscow, Russia, Key Specifications of Xibo XBI3005). The photodiode converted the optical signal into an electrical current proportional to the light intensity. A 12 V bias voltage, supplied by a Xibo XBI3005 power source, was applied through a load resistor. The resistor acted as a current-to-voltage converter, enabling the registration of the electrical signals corresponding to changes in the light intensity. These voltage signals were then analyzed using an oscilloscope. The control voltage was generated using a Tektronix AFG3011C signal generator (country of manufacture: Beaverton, OR, USA), which allowed for the selection of sinusoidal, triangular or rectangular waveforms. The generated signal was amplified using a non-inverting amplifier based on an operational amplifier. The amplifier ensured stable output signal amplitude, which was critical for ensuring the accuracy of the experiment. The system’s response to the applied control voltage was recorded using a Rigol DS1102D oscilloscope (country of manufacture: Rigol Technologies, Inc. Headquarters, Suzhou, China) connected to the photodiode’s load resistor. The oscilloscope captured changes in the electrical signal, reflecting variations in the light intensity at the cell’s output. This set-up enabled real-time visualization and analysis of the system’s dynamic response to the control voltage, including its temporal properties.

The developed set-up facilitates the determination of response times, including the device turn-on time and the relaxation time of the electro-optical response (device turn-off time).

Other instruments used: To study the spectral properties, a SF-26 spectrophotometer (USSR production) was used, operating in the wavelength range of 200–1200 nm. The calibrated filters were used to control the spectral measurements in the visible spectral range. The error in the measurements of the spectra was about 0.2%. The spectra in the IR range were diagnosed on an FSM-1211 device (country of manufacture: Infraspec Co., Saint-Petersburg, Russia).

4. Conclusions

It should be noted that constant-quality liquid crystal cells, including those with the addition of cobalt ferrite nanoparticles, can be used in a number of devices and technologies where stable electrical properties are critically important. For example, in radio frequency devices, such as filters, frequency stabilizers and modulators, the use of the constant-quality cells can ensure stable operation at different signal power levels. This is especially important in telecommunications, where the frequency properties must remain stable regardless of changes in the input signal power.

It is worth noting that these LC cells worked according to Frederick’s scheme, taking into account the installation of the thin-film polarizers and analyzers based on a polyvinyl–alcohol matrix (PVA). It should be mentioned that Frederick’s scheme has always been used by scientists working with LC materials. This scheme permits us to measure the time-on and time-off parameters, as well as the contact.

So, in terms of identifying the novelty of this work, it is worth saying that both the geometric factors and the physical processes of the functioning of the sensitized LC cell were taken into account to optimize its operation, but a different pulse shape of the control signal was also used. At the same time, this case was shown to yield the best result precisely when controlling the waveform as a sine wave.