The Role of Diamonds Dispersed in Ferronematic Liquid Crystals on Structural Properties

Abstract

A study of the role of diamond nanoparticles on 5CB liquid crystal composites with Fe₃O₄ nanoparticles is presented. Composite ferronematic systems based on the nematic liquid crystal 5CB doped with Fe₃O₄ magnetic nanoparticles and additionally bound to diamond nanoparticles (DNPs), of a volume concentration of 3.2 mg/mL, 1.6 mg/mL and 0.32 mg/mL, were investigated using both magneto-optical effect and surface acoustic waves (SAWs) to study the role of diamond nanoparticles on the structural properties of ferronematic liquid crystals. The responses of light transmission and SAW attenuation to an external magnetic field were investigated experimentally under a linearly increasing and decreasing magnetic field, respectively. Investigations of the phase transition temperature shift of individual composites were also performed. The experimental results highlighted a decrease in the threshold field in the ferronematic LC composites compared to the pure 5CB as well as its further decrease after mixing Fe₃O₄ with diamond powder. Concerning the transition temperature, its increase with an increase in the volume fraction of both kinds of nanoparticles was registered. The role of diamond nanoparticles in the structural changes and the large residual light transition and/or attenuation (memory effect) were also observed. The presented results confirmed the potential of diamond nanoparticles in nematic composites to modify their properties which could lead to final applications.

1. Introduction

Composite systems based on liquid crystals (LCs) and nanomaterials [1] are of particular interest because of the possibility of extending the original LC properties, inducing essential changes in the viscoelastic, thermal, dielectric, electro- and magneto-optical properties [2,3,4,5]. One of the scientific interests in recent years is modifying the LC properties by doping with nanoparticles to improve the basic original properties of host LCs. The doping process can prepare new original LC composites that can have interesting properties that are absent in pure LCs, and thus they can offer many practical applications in the LC display industry, optical processing, photonics and electro- and magneto-optics [6]. Following the primary idea [7] supposing that LCs doped by magnetic nanoparticles may have enhanced sensitivity to magnetic fields, many works have focused in particular on these LC composites (ferronematics) [8,9,10]. The basic feature of ferronematics is a strong coupling between magnetic moments of nanoparticles and LC molecules, which can result in the LC molecules’ reorientation [8,11]. The applied magnetic field due to the coupling between the magnetic moments of the nanoparticles and the director, n, characterizing the magnetic moments of LC molecules, can change the magnetic nanoparticles’ orientation. Ferronematics are composites of ferromagnetic particles in nematic LCs and their response to an external magnetic field markedly exceeds that of pure nematic LCs.

Due to remarkable thermal, electrical and mechanical properties, carbon-based nanomaterials such as carbon nanotubes (CNTs), fullerenes or carbon nanocrystals are another group of dopants for LCs [12,13]. CNTs bring a number of improvements to LC layers, such as a reduction in the driving voltage and response time, and are used primarily in electro-optical devices [14,15]. Among the other candidates for the numerous applications of carbon-based nanomaterials are diamond nanoparticles (DNPs). Due to the optical transparency, chemical inertness, high specific area, spherical surface and hardness of nanosized diamond particles, they are considered for biomedical applications and nanotechnology [16,17,18]. It was found that the insertion of DNPs results in the significant ionic rectification of LCs, and a small concentration of DNPs can reduce the threshold voltage of LCs [19].

In the present study, we have presented for the first time the results of an investigation of composites of nematic 5CB with magnetic Fe₃O₄ nanoparticles bound to DNPs using light transmission and SAW measurements. The prepared LC (5CB) composites are investigated with the main intention being to determine the effect of DNPs on structural changes under an external magnetic field, including their effect on the threshold field shift and transition temperatures for individual LC composites. The connection of the utilized experimental techniques, the results of which we report here, provided interesting results in the last decade [20,21,22].

2. Experimental

The LC sample was based on the nematic LC 4-cyano-4′-pentylbiphenyl (5CB) that has a nematic to isotropic transition temperature of ~35 °C and was purchased from the Military University of Technology (MUT, Warsaw, Poland). The synthesis of magnetic nanoparticles was based on the coprecipitation of Fe²⁺ and Fe³⁺ ions from FeSO₄·7H₂O and FeCl₃·6H₂O dissolved in deionized water, followed by the addition of NH₄OH and subsequent heating to 60 °C to obtain Fe₃O₄ precipitates. The size of the Fe₃O₄ nanoparticles was indicated as 11–14 nm in diameter. The nanodiamond powders analyzed in this study were obtained from Federal State Unitary Enterprise “Technolog” (St. Petersburg, Russia) [22]. The production process involved detonating a mixture of trinitrotoluene and hexogen at a ratio of 60/40 and subsequent purification. The initial powders underwent an X-ray diffraction (XRD) study, and according to Scherrer analysis of the coherent scattering region, the average crystallite size of the diamond was found to be approximately 3.1 nm.

To prepare magnetically modified diamond nanoparticles, 100 mg of nanodiamond powder was thoroughly mixed with 500 μL of magnetic fluid stabilized with perchloric acid (concentration 30.4 mg/mL) in a 10 mL centrifuge tube, and the mixture was dried at room temperature for several days. Then, the magnetically modified diamond nanoparticles were repeatedly washed with methanol. Figure 1 presents bright-field TEM images of a studied composite, Fe₃O₄ nanoparticles bound to diamond nanoparticles (DNPs), performed using a transmission electron microscope Tecnai G2 20 TWIN (FEI, Hillsboro, OR, USA) (LaB₆ cathode, 200 kV). Magnetite nanoparticles and magnetically modified nanodiamond particles in powder form were added to 5CB liquid crystal to prepare the composites with desired mass concentrations. The 5CB liquid crystal with the particles added in powder form was sonicated for one hour. The composites with lower particle concentrations were prepared by gradual dilution of the concentrated sample while the samples were sonicated before each dilution. In this manner, samples with concentrations of nanoparticles of 3.2 mg/mL, 1.6 mg/mL and 0.32 mg/mL were prepared. In the case of magnetic nanoparticles, the mass represents the mass of pure Fe₃O₄ particles, and in the case of magnetically modified nanodiamond particles, the mass represents the mass of both components: magnetic and bound diamond nanoparticles.

The role of diamond nanoparticles in ferronematic 5CB and its structural change development in a magnetic field was studied using two experimental techniques: measurements of light transmission and SAW attenuation responses. The experimental investigation of light transmission was performed in LC cells (D = 50 μm) commercially prepared in MWAT (Warsaw, Poland). The cell surfaces were coated with ITO transparent conductive layers and the alignment layers were rubbed to ensure parallel alignment. The cell was filled in the isotropic phase of the LC. A green laser beam (532 nm, 5 mW) was used to illuminate the cells with a linearly polarized incident light beam. The initial LC position was provided in the case of parallel polarizer maximal transmittance. The intensity of the transmitted light was recorded using a photodetector and subsequently registered by a computer monitoring the light transmission as a function of the magnetic field. A more detailed configuration of the experimental arrangement has been previously described [20,22].

The other experimental technique used represented the SAW attenuation response measurement. The LC cells of the investigated composites (D ≈ 100 μm) were prepared right on the center of the LiNbO₃ piezoelectric line furnished with two interdigital transducers. Cells with LC composites were installed in the sample holder that was a component of the thermostatic measuring chamber connected to a temperature stabilizer Julabo ED. The first transducer generated SAW pulses at a frequency of 10 MHz and a width of ~1 µs and another transducer received the SAW signal after passing an LC cell. The SAW attenuation response as a function of the magnetic field or temperature was monitored and recorded by a computer. The initial intrinsic arrangement of LC molecules is supposed to have a predominate alignment in the LC cell plane, and the magnetic field was then oriented vertically. A SAW amplitude at contact with an LC layer is attenuated because of the additional generation of a longitudinal wave into the LCs by SAW that gives rise to the propagation losses [23]. The absorption of longitudinal waves by the LCs is then the reason that the SAW attenuation can respond to any LC orientational changes initiated by external terms and conditions, including a magnetic field or temperature so that the SAW can be used for the experimental investigation. The instability for attenuation measurement was better than ±0.02 dB and the temperature accuracy in the range of 5–80 °C was in the interval ±0.2 °C. A more detailed measurement procedure and experimental arrangement were previously described [20,22].

3. Results and Discussion

The light transmission was investigated in the set of the 5CB composites doped with Fe₃O₄ nanoparticles as well as Fe₃O₄ nanoparticles bound to diamond nanoparticles (DNPs) using the procedure described in the previous section. The results of the light transmission measurements are presented here for the case of parallel polarizers. The light transmission is expressed after the ratio I/I₀, where I₀ and I are the intensity of incident light passing through the LC cell without the magnetic field and under the field, respectively. Figure 2 illustrates the results of the light transmission dependences on the magnetic field for the 5CB composites with Fe₃O₄ nanoparticles of concentrations 0.32, 1.60 and 3.20 mg/mL (Figure 2a) and composites with Fe₃O₄ nanoparticles bound to DNPs of the same concentrations (Figure 2b), measured in the increasing regime both including pure 5CB. The characteristics obtained for the composites doped only with Fe₃O₄ nanoparticles showed similar development. In the beginning, there was almost constant light transmission up to 100–150 mT, depending on the Fe₃O₄ concentration, followed by a rapid decrease characteristic for the threshold field. After achieving their minimum, only a small increase in the light transmission, however, with superimposed oscillations was registered for the increasing magnetic field up to its maximal measured value (400 mT).

The situation in the case of the composites with Fe₃O₄ nanoparticles bound to DNPs, especially for two higher concentrations, was quite different. While the characteristics of the light transmission development in the composite with the lowest concentration (0.32 mg/mL) were like the previous case, the light transmission in the case of higher concentrations (1.62 and 3.20 mg/mL) decreased just after the application of the magnetic field. Such development was attributed to the coexistence of several regimes of the threshold orientational behavior relevant to the compensated ferronematics [24,25]. The reason can be related to the successive magnetization of the ferronematic, the director rotation in the magnetic field, and, finally, the synchronous rotation of the director and magnetization of the compensated ferronematics along the applied field. However, after reaching the minimum, a gradual increase in the light transmission following saturation, again with superimposed oscillations, was registered for the increasing magnetic field. A similar development of light transmission characteristics was registered in the case of 6CHBT doped with CNTs [20] and/or rod-like magnetic nanoparticles [26]. The amplitudes of the superimposed oscillations were comparable with previous ones (Figure 2a). Regarding the oscillations occurring when the magnetic field passes through a threshold field, it is supposed that a planar-aligned molecular director underwent a reorientation between the initial planar orientation and the final perpendicular. Therefore, when the LC is subjected also to a laser beam, the maxima and minima of the light transmitted through the LC cell appear [8]. When the magnetic field decreases, the LC returns to the initial planar orientation and the same situation occurs.

Moreover, the obtained results also proved the role of the concentration on the magnetic threshold field shift and on the process of further light transmission development. While the threshold field for the 5CB composites doped only with Fe₃O₄ nanoparticles decreased with increasing concentration and was only negligible, except for the slightly higher decrease registered in the case of the concentration 1.6 mg/mL, the position of the threshold field in the case of the composites with Fe₃O₄ nanoparticles bound to DNPs decreased quite rapidly. These results unambiguously pointed to the marked lowering of the threshold magnetic field with increasing concentration for the DNP mixed composites. The decrease in the threshold field values was the most significant, especially for the composites with the highest nanoparticle volume concentrations (1.62 and 3.20 mg/mL). The position of the threshold field, however, was influenced in particular by the presence of DNPs. However, generally, this is attributed to the combination of both the anchoring and ferromagnetic energies, i.e., it depends on the size and shape of the nanoparticles, their concentration or nanoparticle formation.

Significant differences in the electro-optical response dependences of polymer/LC (E7) systems were observed for cases with and without DNPs. The electro-optical response dependences of the pure polymer/LC system without DNPs revealed quite different development than the optical response when the DNPs were added to the polymer/LC system. The latter system gave rise to an electro-optical response [27] characterizing a much slower increase, and the alteration from a switched-off to a switched-on state occurred on a much larger voltage scale than in the DNP-free case. As the transmission values in the decreasing regime remained nearly the same before and after applying an electrical field, no or only weak optical memory effects were registered.

A comparison of the light transmission dependences on the magnetic field for the 5CB composites with Fe₃O₄ nanoparticles and the composites with Fe₃O₄ nanoparticles bound to DNPs, but measured also in the decreasing regime, are illustrated in Figure 3 for all the investigated concentrations. It is evident that the light transmission dependences are evidently different for the composites containing extra DNPs (Figure 3b) from the composites with only Fe₃O₄ nanoparticles added (Figure 3a) also for a decreasing field. The composites containing DNPs show quite clear hysteretic light transmission dependences, mainly after passing the threshold field. The observed hysteresis indicates that some structural forms created at higher fields can remain also after the removal of the field. The memory effect registered at a zero field evidently depends on the nanoparticle concentration and it increases from practically zero for the lowest concentration (0.32 mg/mL) up to values representing 20% for the concentration 1.60 mg/mL and 70% for the highest concentration of 3.20 mg/mL. While the Fe₃O₄ nanoparticles of all the used concentrations cause only a negligible memory effect (Figure 3a), the powder of the DNPs bound to Fe₃O₄ nanoparticles (Figure 3b) is able to create partly freezing structural changes induced by the external magnetic field, which are responsible for the creation of a less transparent LC layer.

A similar behavior was registered also in the case of the 5CB composites with SiO₂ nanoparticles containing extra Fe₃O₄ nanoparticles [28]. While the SiO₂ nanoparticles in that case practically caused the freezing of the structural changes induced by the external field, their mixing with the Fe₃O₄ nanoparticles disrupted this property, but the memory effect remained considerably large—comparable with the present one. The reason for such behavior was attributed to the SiO₂ network structure creation as a consequence of both hydrogen bonds among the aerosil surface and between the aerosil and LC molecules.

The micrograph of the DNPs dissolved in nematic LC (E7) pointed out the presence of DNP aggregates with diameters from 2 up to 10 mm, which appeared in nontransparent forms [27]. Aggregates were formed also using a lower DNP concentration (0.5 wt.%). In addition, some small aggregates grouped together to form bigger ones, and these aggregates were randomly scattered within the sample. It was also found that due to the trapping of ionic impurities by the DNPs in the LC, the dispersion of the DNPs resulted in a significant LC ionic correction. If we take into account that DNPs dispersed in nematic LC can significantly influence its properties [27,28,29], and we suppose that during the process of the binding of Fe₃O₄ nanoparticles to DNPs, not all DNPs create the bond with ferroparticles, some free DNPs can be present in our LC composites and the creation of some aggregation cannot be excluded. This fact suggests that diamond aggregates in our case perform a similar role to that of SiO₂ networks.

Figure 4 presents a comparison of the light transmission dependences on the magnetic field for the 5CB composites with Fe₃O₄ nanoparticles of concentrations of 3.20 mg/mL, composites with Fe₃O₄ nanoparticles bound to DNPs of the same concentrations and pure 5CB, all measured in both an increasing and decreasing regime. In the case of the composite with only Fe₃O₄ nanoparticles, despite some hysteresis being registered, no significant memory was obtained. The presence of DNPs could cause that the marked memory effect was recorded.

The effects of the external magnetic field on the SAW attenuation response in the 5CB composites with Fe₃O₄ nanoparticles of concentrations 0.32, 1.60 and 3.20 mg/mL and the composites with Fe₃O₄ nanoparticles bound to DNPs of the same concentrations, both including pure 5CB and measured in an increasing regime, are presented in Figure 5. The characteristics of the SAW results correspond to, concerning the threshold field shift, previous results obtained using light transmission measurements. It is evident that the Fe₃O₄ nanoparticles shift the threshold field of the 5CB to lower values, however, with interesting progression. The 5CB composites doped with only Fe₃O₄ nanoparticles (Figure 5a) showed a decrease in the threshold field with regard to pure 5CB but also its gradual increase with an increasing nanoparticle volume fraction. This means an increase in the threshold field from the composites with 0.32 mg/mL up to the composites with 3.20 mg/mL. Concerning the composites with Fe₃O₄ nanoparticles bound to DNPs (Figure 5b), the threshold field decreases even more, and in the case of a higher concentration (1.60 and 3.20 mg/mL), the beginning of the SAW attenuation increase is already at the zero field like in the light transmission measurements. The reason for such a threshold field shift was already discussed in a description of the light transmission results.

Figure 6 presents a comparison of the SAW attenuation responses in both the increasing and decreasing magnetic field for the 5CB composites with only Fe₃O₄ nanoparticles and the composites with Fe₃O₄ nanoparticles bound to DNPs. While in the case of the composites doped only with Fe₃O₄ nanoparticles (Figure 6a) the memory effect is very similar for all the concentrations and it presents no more than 20% of the total SAW attenuation changes, in the case of the composites with Fe₃O₄ nanoparticles bound to DNPs (Figure 6b), the memory effect is quite large and in two higher concentrations it represents almost 80% of the total SAW changes. This effect is evidently connected to the presence of the extra DNPs in the LC composites. The observed hysteresis also indicates that a process of structural changes continues at decreasing fields up to magnetic fields of about 150 mT. The memory effect registered at the zero field depends on the nanoparticle concentration and it increases from 6% for the lowest concentration up to values representing 75% or 80% for the two highest concentrations, respectively. Such behavior, observed in the SAW investigation, coincides quite well with the light transmission results. It should be noted that the fact that DNPs dispersed in LC can significantly influence its properties [27,29,30] suggests that the memory effect can be connected with the creation of some diamond aggregations similar to that of SiO₂ networks in nematic LC [28]. The memory in pure 5CB can be attributed to structural deformation.

The behavior of the investigated composites in the magnetic field by means of the representative dependences of the 5CB liquid crystal doped with Fe₃O₄ nanoparticles and Fe₃O₄ nanoparticles bonded with DNPs of a concentration of 3.20 mg/mL, measured in increasing and decreasing regimes three times one after another in 25 min. intervals, is illustrated in Figure 7. The total structural changes for the increasing magnetic field are similar for the first and second runs; however, in the decreasing regime, the decrease in the memory effect is already registered. The SAW attenuation response in the case of the third run is weaker compared to the previous ones including the memory effect. It is evident that the presence of the DNPs bound to the Fe₃O₄ nanoparticles is responsible in the nematic 5CB for some structural changes, which occur in addition to the composites doped only with ferroparticles or pure nematic LC. Regarding the memory effect, it is evident that structural changes are solidified for a certain time (during the measurement process), but after a longer time (see measurement after 25 min), some demerging of the structure is registered so that the following driving of the magnetic field leads again to the network changes resulting in the reorganized structure. A similar development, but with a lower memory effect, was obtained for the composite with a concentration of 1.60 mg/mL. The characteristic feature of the dependences of the composites with higher diamond concentration is the continued increase in the SAW attenuation response in the increasing regime from the zero field that continues even in a decreasing field. This characteristic feature remains unchanged also after repeated measurements. The repeated measurements, i.e., the measurements after some time (~25 min), confirm some level of residual SAW attenuation and, thus, structural changes. The behavior of the memory effect could correspond to structural changes due to the creation of some aggregate structure by the DNPs that can form some network in the LC matrix.

The SAW attenuation was dependent on both an increasing and decreasing magnetic field measured for the 5CB composites only with Fe₃O₄ nanoparticles bound to DNPs. However, the magnetic field B parallel to the cell plane and parallel to the k vector for all concentrations (0.32, 1.60 and 3.20 mg/mL) is shown in Figure 8. While the changes in the SAW attenuation measured for the same LC composites for the magnetic field B perpendicular to the cell plane (Figure 6b) showed a distinctive memory effect compared with the 5CB doped only with Fe₃O₄ nanoparticles and increasing with an increasing concentration, these dependences show a memory effect of less than 10% of the total SAW attenuation changes. Concerning repeated measurements, the dependences were only slightly different. In summary, the residual attenuations registered for this magnetic field orientation were negligible compared to the previous case. The reason for such behavior can be related to the stronger anchoring effect of the LC molecules for the direction parallel to the cell plane.

The temperature dependences of the SAW attenuation measured for the 5CB composites consisting of both Fe₃O₄ nanoparticles and Fe₃O₄ nanoparticles bonded to DNPs for all concentrations (0.32, 1.60 and 3.20 mg/mL), including pure 5CB, focused on the structural transitions from the nematic to isotropic phase, are illustrated in Figure 9. The presented dependences show very similar progress for all concentrations compared to the pure LC. A rapid decrease in the SAW attenuation was registered at temperatures close to ~34–36 °C, corresponding to the nematic–isotropic transition (T_NI) when the nematic structure changes to the isotropic phase and there is considerable shift. The characteristic feature of temperature characteristics is namely the increase in the transition temperature, T_NI, in the composites caused by the Fe₃O₄ nanoparticles, followed by an additional increase in the transition temperature, T_NI, when bound to the DNPs. Nevertheless, the spherical nanoparticles in the LC should reduce the T_NI [30]. However, the shift of the transition point towards a higher temperature could be attributed to the diamond aggregates’ structure in an LC matrix, which can induce local magnetic moments with a sufficient number of neighboring Fe₃O₄ and/or Fe₃O₄/DNP nanoparticles and LC molecules. This additional interaction can be the reason for an increasing transition point. The anomalous shift of the transition temperature in the case of the concentration 1.60 mg/mL could be, thus, influenced by the different levels of nanoparticle aggregation.

It is apparent that measurements using light transmission measurements provide results that coincide significantly with results obtained using SAW attenuation response measurements. Some dissimilarities, however, can originate from the particular principles of individual techniques. While SAW measurement detects cumulative structural changes in the LC layer located just over the LiNbO₃ substrate, light transmission measurement can provide a response to structural changes in any LC layer, even in its center. It is apparent that both methods of investigation used are able to contribute individually to the extension of knowledge about the studied LC composites’ behavior, but their current utilization provides a more complex view of the studied problem.

4. Conclusions

The role of diamond nanoparticles bound to Fe₃O₄ nanoparticles and dispersed in nematic liquid crystal 5CB on LC structural properties was studied using light transmission measurements and the SAW technique The study of the 5CB composites with only Fe₃O₄ nanoparticles and the composites with Fe₃O₄ nanoparticles mixed with diamond nanoparticles using both techniques confirmed the particular role of the diamond in the total structural changes, including its influence on the threshold field, transition temperature and the increasing memory effect in comparison not only with pure LC but also with composites doped with Fe₃O₄ nanoparticles. The prepared nematic composites exhibited a memory effect represented by hysteresis in both the light transmission and SAW attenuation response measurements, occurring in the nematic phase, increasing with the diamond nanoparticles concentration. However, the observed memory effect relative to the diamond nanoparticle presence was not persistent for a long time. Moreover, it was found that the threshold field decreased compared with the pure LC in the case of Fe₃O₄ nanoparticles dispersed in 5CB composites. However, the mixing with diamond nanoparticles shifted the threshold field even more and for higher concentrations, to the zero magnetic field. The other characteristic feature of the investigated composites is the increase in the transition temperature, T_NI, caused by adding Fe₃O₄ nanoparticles. Moreover, the additional presence of the diamond leads to a further increase in the transition temperature, T_NI. The presented results confirm that diamond nanoparticles substantially modify the properties of ferronematic LCs, which can lead, after additional and extended studies, to the potential development of magneto-optical memory devices.