Electro-optic and dielectric properties of a ferroelectric liquid crystal doped with chemically and thermally stable emissive carbon dots

Abstract

We investigated the effect of carbon dot doping at concentrations ranging from 0.05 and 0.10 wt% on the electro-optic and dielectric dynamics of a ferroelectric liquid crystal mixture. The phase transition temperature remained unchanged in the presence of the carbon dots, while the change of the tilt angle is on the borderline of experimental accuracy. A remarkable enhancement of 31% in the switching response, 15% in spontaneous polarization and 20% in dielectric constant has been noticed for the nanocolloids comprising 0.10 wt% carbon dots. The increase in response time is attributed to the increase in the conductivity and rotational viscosity. The modification of the spontaneous polarization and dielectric constant are attributed to the parallel coupling between carbon dots and dipoles of Ferroelectric Liquid Crystals (FLCs) and may also be due to the enhancement of the ordering of the FLC upon carbon dot doping. The Goldstone mode relaxation frequency decreased with carbon dot doping, while the dc conductivity increased by one order of magnitude for the 0.1 wt% mixture. No ion capturing has been seen in these nanocolloids and the localized electric field also remained unaffected by carbon dot doping.

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1. Introduction

Nanocarbons like carbon nanotubes, fullerenes, nanodiamond, graphite, and graphene remain at the centre of interest owing to their fascinating properties and device applications. In the recent past, a new form of nanocarbon, namely carbon dots, has been reported and explored for a variety of applications. Carbon dots (C-dots) are small carbon nanoparticles (<10 nm) with various unique properties.¹ C-dots have great sp² character, which is symbolic of nanocrystalline graphite and hold high amounts of oxygen with lower amounts of carbon.^2–4

Carbon dots possess exceptional biocompatibility,^1,5 unique optical properties,⁶ and high aqueous solubility compared to the semiconductor quantum dots like CdTe and CdSe.^7,8 In addition, C-dots shows strong, excitation wavelength-dependent luminescence, which is why these nanoparticles are sometimes termed as carbon nanolights⁴ and are used for photothermal therapy of tumors.^9,10 C-dots are also of great interest in photovoltaic conversion, photochemical transformation and energy storage as well as electroluminescence.⁴

With continuous growth of nanoscience a great deal of attention has been paid to engineer functional smart materials for display, memory storage and optoelectronic devices by combining the anisotropy and ordering of liquid crystals (LCs) with smart functional nanomaterials. In this context ferroelectric liquid crystals were considered an ideal candidate for device application prospectives owing to their fast switching response, lower operational voltage, large optical contrast, and memory effect etc.¹¹ Liquid crystals that show a spontaneous electric polarization and whose direction of the spontaneous polarization can be reversed by an applied electric field are termed as ferroelectric liquid crystals. Various nanomaterials/FLC nanocolloids were reported in the literature and tested for device application. The details and application prospectives of such nanocolloids are well documented in a recent review by Glushchenko et al.¹¹ In the existing premise, carbon nanotubes (single walled or multi walled) were studied most extensively among the nanocarbons in LCs.¹¹ It was found that the insertion of the carbon nanotubes significantly alters the properties of FLCs leading to many fascinating effects like reduction of switching time,^11–14 enhanced spontaneous polarization,^12–14 and altered dielectric strength.^11–14

Recently, our group reported on the effect of fullerene doping on FLC parameters. A significant reduction in the switching response and polarization has been noticed for fullerene/FLC nanocolloids.¹⁵ Strong modulation of the localized electric field has been seen in these nanocolloids (about 76% at 0.5 wt% concentration). In another report we demonstrated the effect of detonation nanodiamond (DND) on the FLC parameters.¹⁶

Very recently Basu et al. report on graphene/FLC nanocolloids.¹⁷ The authors described a faster ferroelectric switching response, an increase in the spontaneous polarization, and a reduction in the rotational viscosity with graphene doping. Such modification of the properties was attributed to the π–π stacking among FLC molecules and the graphene honeycomb structure, which may enhance the ordering of the nanocolloids.

A literature review suggests that C-dots are used as sensitizer for solar cell,¹⁸ their composites with polymers are used in the organic solar cells,¹⁹ C-dot based hybrids are used as excellent electrode materials for supercapacitors,²⁰ and C-dots also emerged as light emitting diodes LEDs materials.²¹ Considering the fascinating optical properties of C-dots and in the light of significant results of earlier reports of nanocarbon (CNTs, MWCNTs, DND, C₆₀, graphene)/FLC nanocolloids, the present study is devoted to investigate the effects of chemically inert, hydrophobic C-dot doping on the FLC's properties. Here, we particularly focus on the effects of such C-dots on the electrooptic and dielectric parameters of the FLC host.

2. Material and methods

There are numerous methods for the synthesis of C-dots, including chemical or thermal oxidation of carbonaceous compounds.²² However, most of these methods provide hydrophilic C-dots that are not suitable for LC applications where there is a need for compatibility of the C-dots with weakly polar LC molecules. As shown in Scheme 1, the synthesis of hydrophobic C-dots has been accomplished by modifying previously reported methods.^23,24

Scheme 1 Synthesis of hydrophobic C-dots.

A phenylpyrimidine-based FLC mixture exhibiting a phase sequence (heating cycle) 25.0 °C – SmC* – 65.4 °C – SmA* – 69.8 °C – iso (onset of the SmC* phase may be at <25.0 °C) was used as a host material for the nanocolloids. The component of the FLC is shown below

with weight concentrations for m = 8, n = 10 of 43.3%, m = 9, n = 8 of 33.3% and m = 8, n = 8 of 23.4%. The chiral dopant terphenyl used in the prepared FLC mixtures, with two chiral C₅H₁₁CH*(CH₃)–OCO groups in the p, p′ positions, the chemical structure of chiral dopant was specified in ref. 25 The FLC mixture is comprised of a 75 : 25 wt% ratio of mesogenic compound and chiral dopant.

A small amount of the C-dots was first dispersed in n-hexane and sonicated for about 5 minutes to reduce the aggregation tendency of the C-dots, which results in a dispersed solution of the C-dots and n-hexane. Two FLC nanocolloids were prepared by adjusting the C-dot concentration to 0.05 and 0.10 wt% in the FLC host. The colloidal mixtures were heated until the complete evaporation of organic solvent. No significant change observed in transition temperatures was observed in heating and cooling cycle. We carried out all the characterizations in the heating cycle. The details of cell preparation are given elsewhere ((ref. 15, 25 and 26) and references therein). The cell thickness ≈3.5 μm was maintained for all the experiments by using ball spacers. Electrooptic parameters were examined via a setup featuring a He–Ne laser at a wavelength of λ = 632.8 nm, a rotating table, a Linkam LTS 350 hot stage attached with the temperature controller Linkam CI 94 (with an accuracy ±0.1 °C), function generator HP 33120A, and the digital oscilloscope HP Infinium. The frequency and the strength of the bipolar rectangular electric field used for all the electrooptic measurements were 3 Hz and 10 V μm⁻¹, respectively, while, 15 V μm⁻¹ for polarization measurements. Field saturation calculated for nanocolloids was well below that field strength used for the measurements. The switching time was measured with accuracy ±1 μs. The optical tilt measured with the accuracy ±1°. The spontaneous polarization was measured by using well known reverse current technique with triangular wave electric field.²⁷ The spontaneous polarization was measured with accuracy of ±1 nC cm⁻². Dielectric measurements were carried out by an impedance analyzer 4192A (frequency range: 100 Hz to 1 MHz, probing voltage: 100 mV).

Visible absorption spectra of the C-dots in n-hexane were recorded using a dual-cell OLIS14 clarity spectrophotometer. Photoluminescence (fluorescence) spectra were collected using a Varian Cary Eclipse with variable excitation wavelengths. TEM analysis was performed with either a FEI Tecnai TF20 TEM or a Hitachi H 7000 instrument at an accelerating voltage of 200 kV. Samples were prepared by evaporating a drop of dilute solution of C-dots onto carbon-coated copper TEM grids (400 mesh) and dried overnight.

3. Results and discussion

The C-dots were first characterized by photoluminescence and transmission electron microscopy (TEM). As can be seen in Fig. 1, the as-synthesized C-dots show a bright, excitation wavelength-dependent photoluminescence (Fig. 1a–c), and have an average size of 10 ± 2 nm in diameter (Fig. 1d).

Fig. 1 Colloidal dispersion of the C-dots in n-hexane (a) under ambient light and (b) under 366 nm illumination indicating the bright photoluminescence of the C-dots; (c) PL spectra of purified C-dots at different excitation wavelengths from 360 nm to 540 nm increasing at 20 nm intervals; (d) TEM image of the C-dots.

The non-doped FLC and C-dot/FLC nanocolloids were first examined by polarized light optical microscopy to examine the phase sequence and potential textural changes that may indicate C-dot aggregation or segregation. Optical measurements showed characteristic SmC* textures at 0.05 and 0.10 wt% doping concentrations of the C-dots without unusual defects indicating C-dot aggregation and phase separation (texture not shown here). This implies that the C-dots were uniformly distributed in the FLC matrix without perturbing the phase sequence and director field. Variation of tilt angle as a function of temperature is shown in the Fig. 2a. A minor decrease of the tilt angle seems to be possible but being on the borderline of experimental accuracy (±1°). These results were found in good agreement with earlier reports on nanocarbon/FLC nanocolloids (CNTs/FLC,^11–14 C₆₀/FLC,¹⁵ and graphene/FLC¹⁷ nanocolloids).

Fig. 2 Temperature dependence of (a) tilt angle, (b) switching time (τ) and (c) spontaneous polarization (P_s) for non doped FLC and of nanocolloids.

An increase of about 15.5% and 31% in the switching response τ was observed for the 0.05 and 0.10 wt% C-dot doped samples, respectively, as compared to the non-doped FLC at 30 °C as evident from the Fig. 2b. It is interesting to note that τ doubled as we doubled the doping concentration of the C-dots in the FLC matrix. Variation of spontaneous polarisation (P_s) as a function of temperature is presented in the Fig. 2c. P_s was found to increase by 10.5% and 15.5% respectively for 0.05 and 0.10 wt% C-dots doping concentration comparing to the non-doped FLC at 30 °C. However, no further pronounced change in P_s is observed when doubling the C-dot concentration from 0.05 to 0.10 wt%.

These findings were inconsistent with the results of GNRs/FLC,²⁸ CNTs/FLC,^11–14 C₆₀/FLC,¹⁵ CdSe QD/FLC,²⁶ harvested BaTiO₃,²⁹ LiNbO₃/FLC²⁵ nanocolloids, where the capturing of the impurity ions strongly modulated the localized electric field leading to shorter response times. To examine this effect in the C-dot/FLC nanocolloids, the number of ions were calculated using the relation n_ions = d_LC/zeμR_LCS (ref. 15 and 26), where d_LC stands for cell thickness, z is the ionic charge, e symbolizes the charge of the electron, μ represents the ion mobility, R_LC and S stand for the resistance and area of the sample. n_ions calculated for the non-doped FLC and the 0.05 and 0.10 wt% C-dot/FLC nanocolloids were 1.5 × 10²¹, 1.45 × 10²¹ and 1.62 × 10²¹ m⁻³, respectively. These data confer that there was no ion capturing for the C-dot/FLC nanocolloids, as the variation in the magnitude of n_ions was insignificant compared to the non-doped FLC. Complementary to this, we also calculated the local electric field E_loc for non-doped FLC and the C-dot/FLC nanocolloids by using the relation γ_φ = τP_sE_loc.³⁰ The obtained magnitude of E_loc for the non-doped FLC as well as the 0.05 and 0.10 wt% CDs nanocolloids were 50.0, 51.4, 52.7 V μm⁻¹, respectively. From these results it can be concluded that C-dot doping does not modulate the ion concentration and localized electric field as is reported for the other FLC/nanocolloids.^15,25,26 While the increase in the τ may be due to an increase in the rotational viscosity as these quantity follow a direct proportionality.

The increase in the P_s of the C-dot/FLC can be understood via parallel coupling among the C-dot and FLC dipole. The application of an electric field in the FLC systems gives rise to the charge separation resulting in the formation of dipoles in the system. If these dipoles are arranged parallel to the dopant species, the net dipole moment is modulated, and hence, the coupled response enhances the net polarization of the nanocolloids. These results for the C-dot/FLC mixtures were found in good agreement with the graphene/FLC nanocolloids, where P_s increased due to the enhancement in smectic-C ordering via π–π electron stacking between the LC's benzene rings and the graphene-honeycomb structure.¹⁷ Probably the parallel coupling amid C-dots and FLC dipoles and the enhancement of the ordering of the nanocolloids could be responsible for the enhancement of P_s in C-dot/FLC nanocolloids.

Dielectric dynamics of the non-doped FLC and C-dot/FLC nanocolloids was investigated in the frequency range of 100 Hz to 1 MHz. Experimental and fitted data (with standard Cole–Cole process) of complex permittivity is presented in the Fig. 3a and b. The fitted parameters at 30 °C for non-doped FLC were ε₀ = 22.0 ± 0.05, ε_∞ = 3.0 ± 0.05, α = 0.72 ± 0.01 and τ = 11.4 ± 1 μs, for the nanocolloid with 0.05 wt% C-dot concentration ε₀ = 24.3 ± 0.05, ε_∞ = 2.6 ± 0.04, α = 0.74 ± 0.01, τ = 18.4 ± 1 μs, and for the 0.10 wt% concentration ε₀ = 26.4 ± 0.05, ε_∞ = 3.0 ± 0.04, α = 0.76 ± 0.01 and τ = 15 ± 1 μs. An 11.5% and 20% enhancement in the dielectric permittivity has been noticed for the 0.05 and 0.10 wt% dispersion of the C-dots in the FLC host. Similarly, the absorption spectra also reflect an enhancement of the loss of Goldstone mode by increasing the amount of dopant as evident from Fig. 3b ε′′ was found to be increased by 12% and 23% (ε′′ maxima for 0.05 wt% and 0.10 wt% C-dots) compared to the non-doped FLC at 30 °C. Conversely, the Goldstone mode relaxation frequency (f_G) was reduced upon doping the FLC with the C-dots compared to the non-doped FLC, while there was no change in the f_G with increase in the dopant concentration from 0.05 to 0.10 wt% as evident from the Fig. 4a. Goldstone mode process is connected to the phase fluctuations in the azimuthal orientation of the director. It is worthy to mention here that for FLCs there is one other collective process, defined as soft mode. The elastic forces controlling the tilt fluctuation turned weak as the system come close to the SmA* phase, resulting the amplitude of the tilt fluctuation to increase drastically. Soft mode occur at much higher frequency 100 kHz to 1 MHz comparing to the Goldstone mode.^30,31 For the non-doped FLC, f_G showed the linear increase as a function of temperature up to 45 °C, remained constant up to 55 °C, and then declined with further increase of the temperature. However, in nanocolloids the variation of f_G was not significant with the variation of temperature. A linear increase in the dielectric strength Δε was observed as a function of temperature up to 65 °C for the non-doped FLC and the 0.05 wt% and 0.1 wt% C-dot/FLC nanocolloids as shown in Fig. 4b.

Fig. 3 Frequency dependence of (a) dielectric permittivity, (b) absorption strength for non doped FLC and nanocolloids at temperature 30 °C.

Fig. 4 Temperature dependence of (a) Goldstone mode frequency f_G (b) Goldstone mode strength Δε and (c) rotational viscosity γ_G (calculated using dielectric parameter) of non doped FLC and of nanocolloids.

The Goldstone mode rotational viscosity for the non-doped FLC and the C-dot/FLC nanocolloids can be estimated using the relation³¹

The variation of γ_G as a function of temperature for the non-doped FLC and the C-dot/FLC nanocolloids is plotted in Fig. 4c. Quantitatively, an about 65% and 74% increase in γ_G was noticed for the 0.05 wt% and 0.1 wt% C-dot/FLC nanocolloids, respectively, compared to the non-doped FLC.

The increase observed for dielectric parameters for the C-dot/FLC nanocolloids was attributed to the parallel correlation among C-dots and FLC dipoles. Such enhancement in the dielectric constant can also be understood by considering the increase in the dc conductivity (σ_dc) (details of the calculation are given in the ref. 25) of the nanocolloids 5.1 × 10⁻⁷ S m⁻¹, 6.7 × 10⁻⁷ S m⁻¹, 1.7 × 10⁻⁶ S m⁻¹ for the non-doped FLC, the 0.05 and the 0.10 wt% C-dot concentrations, respectively. It was interesting to note that σ_dc increased by one order of magnitude for the 0.10 wt% C-dot sample, while, the change was less pronounced at lower doping concentration 0.05 wt%. A reduction in f_G with C-dot doping can be attributed to the strong modification of the Goldstone mode rotational viscosity as γ_G ∝ 1/f_G. The presence of the C-dots could perhaps present some hindrance to the flipping of FLC dipoles and the increase in the dc conductivity may also be responsible for the reduction in f_G.

The variation of ac conductivity (σ_ac) for non-doped FLC and both nanocolloids is presented in Fig. 5. No significant variation in the σ_ac was observed in the lower frequency window, however, σ_ac was found higher for the nanocolloids in the higher frequency range. Probably, such behaviour of σ_ac may be due to the higher conductivity of the C-dots and contributions from the ITO.¹¹

Fig. 5 Variation of ac conductivity as a function o frequency at 30 °C for non doped FLC and nanocolloids.

4. Conclusions

We demonstrated that the dispersion of the chemically inert hydrophobic CDs did not perturb the phase sequence and director field of FLC. The electrooptic and dielectric results were found contrary to the other nanocarbon/FLC nanocolloids. The increase in the τ for the C-dot/FLC nanocolloids can be understood by considering the invariance of n_ions and E_loc as well as an enhancement in the conductivity and rotational viscosity upon C-dot doping. However, the increase in the P_s, ε′, ε′′ and Δε may be attributed to the parallel coupling amid C-dots and FLC dipoles and an enhancement of the FLC ordering through C-dot doping. A significant finding to highlight is the one order of magnitude enhancement in the σ_dc for the 0.10 wt% C-dot/FLC nanocolloid compared to the non-doped FLC. The hindrance exerted by the C-dot dopant, increasing σ_dc and γ_G, may responsible for the decrease in the relaxation frequency for the nanocolloids.

Acknowledgements

W.H. acknowledges financial support from German Science Foundation (Ha 782/98-1). T.H. and D.H. would like to thank the National Science Foundation for funding the NSF-REU Program (CHE-1263087). T.H. gratefully acknowledges financial support from the Government of Ohio's Third Frontier Program for Ohio Research Scholars, which also supports the TEM facility at the Liquid Crystal Institute.

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Footnote

† Present address: Department of Physics, Dehradun Institute of Technology University, Dehradun, Uttarakhand-248009, India.

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