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Graphene quantum dots (GQDs)-doped PEDOT:PSS composite layers were utilized to align liquid crystals (LCs) via an ion-beam (IB)-spurting pre-treatment process. LCs were homogeneously aligned between sandwiched GQDs/PEDOT:PSS composite thin layers, and the alignment of LCs was found to be affected by both the quantity of doped GQDs and IB-spurting intensity. Competitive electro-optical switching properties and non-residual DC performance of the cell equipped with GQDs/PEDOT:PSS composite alignment layers were obtained because of the enhanced field effect and charge transport induced by doped GQDs. Notably, using IB-spurted GQDs/PEDOT:PSS layers as alignment layers for next generation high-performance liquid crystal display (LCD) is promising.
© 2015 Optical Society of America
1. Introduction
The alignment of liquid crystals (LCs) has a strong influence on the performance of LC-based devices, especially LC displays (LCD). Historically, rubbing has been the conventional and widespread approach for LCs alignment. However, until now, this approach has been restricted by shortcomings such as the generation of debris, electrostatic discharge, and streaking, even though the process is conducted in a clean room. Ion-beam (IB)-spurting has been proposed as a facile way to achieve multi-domain and large-scale manufactured products [1]; the popular materials used to align LCs via IB-spurting are semiconductors [2–5 ] and carbon-based materials [6]. Ordered atomic arrangement in the inorganic films produced by IB-spurting is favorable because of the destruction of unfavorable atomic orientations and transformation of topology. The modification of chemical interactions that occur when IB-spurting produces anisotropic surfaces is responsible for LCs alignment [4, 7 ]. Graphene, a single layer of carbon atoms in a hexagonal lattice structure, has been widely utilized for optoelectronic devices such as solar cells [8, 9 ], photovoltaics [10, 11 ], light-emitting diodes [12, 13 ], and liquid crystal displays (LCD) [14, 15 ]. A small number of graphene layers have been shown to work as an electrode as well as an alignment-assisting layer after low intensity IB-spurting; the removal of obstacles during the IB-spurting process is expected to enhance the conductance of graphene layers [16]. It has been confirmed that defects are produced during low intensity IB-spurting [17]; formative traps are desirable for LCs alignment because of the interactions between LCs and graphene layers. Unfortunately, the high cost and the complex pre-fabrication process of graphene layers inhibit their further development; therefore, new alternative materials are needed. Recently, graphene quantum dots (GQDs), which are small graphene fragments, have received much attention because of their electronic and opto-electronic properties in addition to their significant quantum confinement effect [16–22 ]. GQDs can be well dispersed in a variety of solvents, especially when their edge carboxylic acid moieties are equipped with functional groups. Consequently, GQDs are promising candidates for electronic device fabrication.
Inspired by above-mentioned reports on the dispersion and aggregation of graphene-doped materials, herein, we present an IB-spurted GQDs-doped PEDOT:PSS composite layer capable of aligning nematic liquid crystals (NLCs). The transparency of prepared GQDs/PEDOT:PSS composite alignment layers was investigated via an UV–Vis spectrophotometer, and the chemical information of new composite alignment layers were characterized via X-ray photoelectron spectroscopy (XPS). The surface morphology of GQDs/PEDOT:PSS was confirmed via atomic force microscopy (AFM). New twist nematic (TN) cells based on GQDs/PEDOT:PSS composite alignment layers were fabricated, and excellent electro-optic characteristics were obtained because of the enhanced electric field effect generated by GQDs as they are good materials for electron transfer and storage.
2. Experimental
Materials and Sample Preparation: Graphene quantum dots (GQDs, 1 mg/mL, ACS MATERIAL, LLC) were doped dropwise into a PEDOT:PSS solution (1.3 wt%, conductive grade, Sigma Aldrich) at concentrations of 0.2 mg/mL, 0.5 mg/mL, and 1.0 mg/mL. The achieved composite solutions were stirred for 2 h at room temperature to disperse the GQDs in the PEDOT:PSS solution. Then, the composite solutions were spin-coated on indium tin oxide (ITO) glass (Samsung Coring 1737; 32 × 22 × 1.1 mm3, sheet resistance 10 Ω/sq). The substrates were cleaned with distilled (DI) water, acetone, and isopropyl alcohol (IPA). ITO substrates coated with a composite layer were baked at 130 °C for 30 min, followed by IB-spurting (Ar plasma IB with the density maintained at 1014–1015 ions/cm2 and an IB current of approximately 1.0–1.2 mA/cm2) at intensities of 0.6, 1.2, 1.8, and 2.4 keV for 2 min with a 45° incident angle. Anti-parallel cells with cell gaps of 5 μm and 60 μm for alignment ability and residual DC performance measurements and TN cells with a gap of 5 μm for opto-electronic measurements were fabricated by injecting a nematic LCs (ne = 1.5702, n0 = 1.4756, and Δε = 10.7; Merck).
Experimental Setup: The characteristics of the GQDs were determined using TEM (JEM-2010, JEOL Ltd, Japan), and the transmittance spectra of the GQDs/PEDOT:PSS composite layers on glass slides were obtained using a double-beam UV–Vis spectrophotometer (UV-2101, Shimadzu, Japan). The binding energy of the GQDs/PEDOT:PSS composite materials was measured by X-ray photoelectron spectroscopy (XPS, ES-CALAB 220i-XL, VG Scientific), and the surface morphologies of a pristine PEDOT:PSS sheet and GQDs/PEDOT:PSS composite films were examined using atomic force microscopy (AFM; Dimension 3100, Digital Instrument Co.). LCs alignment between sandwiched GQDs/PEDOT:PSS composite alignment layers was characterized by polarized optical microscopy (BXP 51, Olympus) without any applied voltage, and the pretilt angles of the LCs were measured by means of a crystal rotation method (TBA 107, Autronic) in which the fluctuating transmittance was recorded while each anti-parallel cell was rotated latitudinally over the range of ± 70°. The voltage–transmittance characteristic of TN cells with crossed polarizers were evaluated using an LCD evaluation system (LCD-700, Otsuka Electronics), with a maximum driving voltage of 5 V, a voltage step of 0.2 V, and a step voltage frequency of 60 HZ. and the characteristic response time of TN cells with crossed polarizers was evaluated using an LCD evaluation system (LCD-700, Otsuka Electronics), with a maximum driving voltage of 5 V, a frequency of 60 HZ, and an integration time of 200 ms. The residual DC behavior was measured using the capacitance–voltage C–V hysteresis method (LCR meter, Agilent 4284A) with a maximum bias voltage of 10 V and a step bias voltage of 0.1 V for 2 circles.
All of the above procedures and measurements were conducted at room temperature.
3. Results and discussion
Initially obtained commercialized GQDs were characterized using TEM as shown in Fig. 1 . GQDs were observed to be arranged in stacks, and one single stacking GQDs cluster was oval shaped (Fig. 1(b)). When GQDs were doped into PEDOT:PSS at a concentration of 0.5 mg/mL, GQDs stacking increased and the size of stacked GQDs clusters was larger (Fig. 1(c)). The transparency of spin-coated PEDOT:PSS layers and GQDs/PEDOT:PSS composite layers on general glass substrates were compared. PEDOT:PSS layers exhibited a transmittance of 83.95% (Fig. 1(a)); however, a much higher transmittance was surprisingly achieved after GQDs doping, and the transmittance of the composite alignment layers gradually increased correspondingly with the increase in GQDs concentration. GQDs-doped PEDOT:PSS composite layers with a GQDs concentration of 1.0 mg/mL exhibited an 86.45% transmittance in the visible light region (780–380 cm−1), which was 2.98% higher than that of the initial pure PEDOT:PSS. The increased transmittance may be because of the original high transparency performance of the graphene [23], which is much higher than that of PEDOT:PSS. In addition, graphene has been reported to promote the re-orientation of PEDOT:PSS molecules into a fine structure with uniaxial anisotropic optical properties, which also induces the corresponding transparency increase.
Fig. 1 (a) Transmittance comparison of a PEDOT:PSS layer and various amount of GQDs doped PEDOT:PSS composite layers. TEM images of GQDs (b) dispersed in water (as received without any modification); (c) doped in PEDOT:PSS (0.5 mg/ml).
The surface morphology of a spin-coated PEDOT:PSS layer (Fig. 2(a) ) and the GQDs/PEDOT:PSS composite layers in Fig. 2(b)–(d) was evaluated using AFM, and the roughness comparison of these thin layers is presented in Fig. 2(e). Initially, the spin-coated PEDOT:PSS layer showed a grain-like structure with a non-homogeneous distribution of the PEDOT and PSS species, and the IB-spurted GQDs-doped PEDDOT:PSS layers presented a morphology similar to that of the initial non-IB-spurted spin-coated PEDOT:PSS layer. GQDs aggregation and/or single GQD were only observed at a GQDs concentration of 1.0 mg/mL, indicating that the GQDs in the composites has little effect on composite thin layer surface morphology even if aggregation occurred. The similarity in morphology between the PEDOT:PSS layer and the IB-spurted GQDs/PEDOT:PSS layers can also be drawn from their similar roughness (Ra and Rq). We contributed the slightly increased roughness in IB-spurted the GQDs (0.5 mg/mL)/PEDOT:PSS layer to the aggregation of GQDs, which indicates that the IB-spurting did not have significant influence on the morphology of the composite layers.
Fig. 2 AFM images of (a) a PEDOT:PSS layer; (b) a 1.2 keV IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS layer; (c) a 0.6 keV IB-spurted GQDs (0.5 mg/mL)/PEDOT:PSS layer; and (d) a 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS layer; and (e) the corresponding roughness comparison.
X-ray photoelectron spectroscopy (XPS) was used to characterize the composition of the PEDOT:PSS and GQDs/PEDOT:PSS layers and to investigate the LCs alignment in the composite layers. The S (2p) core-level spectra of the PEDOT:PSS before and after IB-spurting are displayed in Fig. 3(a) . Two lower binding energy peaks appeared at approximately 165.1 and 163.9 eV, corresponding to the spin-split components of the sulfur atoms in the PEDOT. A higher binding energy peak observed at approximately 168.5 eV corresponded to the sulfur atoms of the PSS, which contained the S (2p3/2) peak of PSS-H and the S (2p3/2) peak of PSS-Na+ salt [24, 25 ]. After IB-spurting, the sulfur–carbon bond was destroyed and new bonds were formed, as inferred from the decreased intensity of the 168.5 eV peak and the increased intensity of the 165.1 and 163.9 eV peaks. Interestingly, we observed that cleavage of the sulfur–carbon bond in the PEDOT was partially inhibited with GQDs doping. The inhibited cleavage of the sulfur–carbon bonds was confirmed with the results shown in Fig. 3(b), which displays the analysis of the C (1s) core-level spectra. The C (1s) core-level spectra of the PEDOT:PSS layer featured two main peaks, a strong peak at 285 eV and a shoulder peak at 287.8 eV. The peak located at 285 eV corresponded to saturated and conjugated carbon atoms from the PEDOT and PSS; and the peak located at 287.8 eV corresponded to –C–O–C– bonds from the PEDOT [24–27 ]. After IB-spurting, no obvious change in the C (1s) core-level spectra was detected, except that the peak corresponding to –C–O–C– decreased in intensity and became broader. Upon GQDs doping, no other significant differences in the C (1s) core-level spectra were observed between the IB-spurted PEDOT:PSS layer and the IB-spurted GQDs/PEDOT:PSS layer, except for a slightly enhanced peak for the conjugated carbon atoms located at 285 eV after GQDs doping. An explanation of GQDs cleavage considering molecule orientation is presented here. Graphene oxide in PEDOT:PSS has been reported to increase the spacing between molecules [28], and because IB-spurting can destroy these unfavorable atomic orientations during the IB-spurting process, we can infer that PEDOT and PSS molecules have been re-orientated. The inhibited cleavage of the sulfur–carbon bonds caused by GQDs doping is because of the favorable orientation of PEDOT and PSS molecules and the unfavorable orientation of GQDs in the composite layer.
Fig. 3 XPS results for (a) S 2p and (b) C 1 s core levels of PEDOT:PSS layer, 1.2 keV IB-spurted PEDOT:PSS layer, 0.6 keV IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS layer, and 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS layer.
LCs alignment on the PEDOT:PSS layers and the GQDs/PEDOT:PSS composite layers was evaluated based on POM images (Fig. 4 ). The LCs alignment direction coincided with the direction of undamaged bonding after ion beam spurting, and inclined ion beam irradiation treatment on composite layers generates a surface anisotropy and it determines the LC alignment direction and property [29, 30 ]. Initially, a random LCs alignment was observed in both the PEDOT:PSS layers and the GQDs/PEDOT:PSS composite layers, and after IB spurting at varying intensities, the LCs began to align homogeneously. The LCs started to be homogeneously aligned for the 1.2 keV IB-spurted PEDOT:PSS layer; however, when 0.2 mg/mL GQDs were doped into the PEDOT:PSS, LCs alignment was observed only after 0.6 keV IB spurting (but with a small amount of light leakage). When the GQDs concentration was increased to 0.5 mg/mL, the LCs in the GQDs/PEDOT:PSS composite layer were found to be perfectly aligned after 0.6 keV IB-spurting (without any light leakage). The blue and red lines in Fig. 4 indicate the simulated data and experimental data of transmittance response to incident angles, respectively; and the consistence of the two lines also indicated the uniform alignment of LCs. The excellent LCs alignment observed in composite layers can be indirectly explained by the XPS analysis described above. In the composite layers, the band of adjacent two carbon atoms in the GQDs and the unfavorable direction of the PEDOT:PSS bonds were destroyed by IB spurting. These broken bonds existed as free radicals generating above mentioned surface anisotropy to align LCs. The GQDs free radicals exist in the same surroundings and have the same interactions with LCs, causing homogeneous alignment of LCs. However, two free radicals from each broken –C–S– bond in PEDOT:PSS form one –S– radical and one –C– radical, and these free radicals experience significantly different surroundings with varying unbalanced interactions with LCs, which induce LCs alignment on PEDOT:PSS layers with small pretilt angles. Thus, it is clear that the chemical interaction (caused by destroyed bonds via IB-spurting) between GQDs/PEDOT:PSS composite alignment layers should be responsible for a more homogeneous alignment of LCs on GQDs/PEDOT:PSS composite layers.
Fig. 4 Polarized optical images and the corresponding pretilt angle measurement result of unequal-IB-spurted PEDOT/PSS alignment layer cells and GQDs-doped PEDOT:PSS alignment layers cells (“A” denotes “analyzer” and “P” denotes “polarizer,” and the red arrow indicates the direction of IB spurting).
The switching of nematic liquid crystals (NLCs) sandwiched between two IB-spurted GQDs/PEDOT:PSS alignment layers in TN mode is shown in Fig. 5 . The voltage-dependent transmittance curves of cells are presented in Fig. 5(a). Initially, the threshold voltage (defined as the voltage to obtain 90% of transmittance and the associated dynamic response) of the IB-spurted PEDOT:PSS alignment layer (1.292 V) was extremely low compared with that of a conventional rubbed PI alignment layer cell (1.413 V) (Table 1 ). When GQDs were doped into the PEDOT:PSS, the threshold voltage of cells fabricated with IB-spurted GQDs/PEDOT:PSS composite alignment layers correspondingly increased with increasing amounts of GQDs. The threshold voltage of cells fabricated with IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS composite alignment layers, IB-spurted GQDs (0.5 mg/mL)/PEDOT:PSS composite alignment layers, and IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS composite alignment layers were 1.506 V, 1.854 V, and 2.071 V, respectively. The rising and decaying response times for IB-spurted GQDs (various amounts)-doped PEDOT:PSS composite alignment layer cells are shown in Fig. 5(b). Because of the rising demand for LCD, IB-spurted PEDOT:PSS composite alignment layers are not a sufficient alignment layer candidate yet; however, when GQDs were doped into the PEDOT:PSS, the switching time of the LCs sandwiched between IB-spurted GQDs/PEDOT:PSS composite alignment layers significantly decreased compared with conventional rubbed PI alignment layer cells. When the amount of GQDs in the GQDs/PEDOT:PSS composite alignment layers were 0.2 mg/mL, 0.5 mg/mL, and 1.0 mg/mL, the corresponding rising/decaying time (defined as transmittance change and the associated dynamic response between 10% and 90%) are 3.414/7.754 ms, 2.411/6.807 ms, and 1.741/5.748 ms, respectively. The decaying time of LCs sandwiched between the IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS composite alignment layers decreased to almost half of that of the cell fabricated with rubbed PI alignment layers, and the total response time decreased to 7.489 ms. The threshold voltage increase and the faster response of LCs sandwiched between IB-spurted GQDs/PEDOT:PSS composite alignment layers may be towing to the enhanced electric field effect generated by GQDs because GQDs are a good material for electron transfer [31, 32 ] and storage [33, 34 ]. When a voltage was applied to the fabricated cell, volume charges accumulated on the GQDs/PEDOT:PSS composite alignment layers [35], and with an increase in the amount of GQDs, the accumulated charges increased and the driving voltage rose as well.
Fig. 5 (a) Voltage-dependent transmittance curves (b) rising and decaying response time of a rubbed PI alignment layer cell; a 1.2 keV IB-spurted PEDOT:PSS alignment layer cell; and a 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS alignment layer cell.
Table 1. Threshold voltage, rising and decaying response time of a rubbed PI alignment layer cell, 1.2 keV IB-spurted PEDOT:PSS alignment layer cell, a 1.2 keV IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS alignment layers cell; a 0.6 keV IB-spurted GQDs (0.5 mg/mL)/PEDOT:PSS alignment layer cells; and a 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS alignment layer cells. Vth presents for threshold voltage, τr presents for rising time, τd presents for the decaying time, and τ presents for the total response time.
Image sticking is a phenomenon wherein a faded previous image is visible on the screen, even though the frame has been refreshed. Image sticking arises from residual charges [36, 37 ] and is an essential issue for the assessment of new-generation displays. Normally, when a voltage is applied, residual charges accumulate on the substrate in localized defect regions, and C–V hysteresis in the LC cells is generated. The residual DC could be calculated from the hysteresis levels RDCPlus and RDCMinus that are defined as the difference in voltages between increasing and decreasing show 50% of maximal capacitance as shown in the following equation:
As shown in Fig. 6 and Table 2 , the rubbed polyimide (PI) alignment layer cell showed a strong C–V hysteresis with a residual DC of 0.6240 V; in comparison, the performance of the IB-spurted PEDOT:PSS alignment layer cell was better, and with doping GQDs into PEDOT:PSS, the hysteresis performance of IB-spurted GQDs/PEDOT:PSS alignment layer cells was further gradually improved. When the amount of doped GQDs was increased to 1.0 mg/mL, an ideal zero C–V hysteresis performance with a residual DC of 0.0140 V was achieved. The prevention of residual charges on the IB-spurted GQDs/PEDOT:PSS composite alignment layer was because of the high conductivity of the GQDs and the advantages of the IB-spurting alignment method. Highly conductive GQDs can be regarded as a charge floating channel to release residual charges on the alignment layer when the applied voltage is removed and thereby effectively decrease the volume of the charges. Furthermore, IB-spurting is a process that never introduces impurities through contact between the alignment layers and equipment, which occurs in the rubbing process. The prevention of impurities also contributed to the remarkably low C–V hysteresis performance without adhesion of charges.Fig. 6 C–V characteristics of the cell with (a) rubbed PI alignment layers; (b) 1.2 keV IB-spurted PEDOT:PSS alignment layers; (c) 1.2 keV IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS alignment layers; (d) 0.6 keV IB-spurted GQDs (0.5 mg/mL)/PEDOT:PSS alignment layers; and (e) 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS alignment layers.
Table 2. Residual DC (RDC) of a rubbed PI alignment layer cell, 1.2 keV IB-spurted PEDOT:PSS alignment layer cell, a 1.2 keV IB-spurted GQDs (0.2 mg/mL)/PEDOT:PSS alignment layers cell; a 0.6 keV IB-spurted GQDs (0.5 mg/mL)/PEDOT:PSS alignment layer cells; and a 0.6 keV IB-spurted GQDs (1.0 mg/mL)/PEDOT:PSS alignment layer cells
4. Conclusions
In conclusion, an IB-spurted, highly transparent, GQDs/PEDOT:PSS-composite, LCs alignment layer has been developed. After low-intensity IB-spurting, PEDOT:PSS composite layers with well-dispersed GQDs presented a random morphology with stably aligning LCs exhibiting perfect electro-optical performance, free of residual DC performance. These excellent properties make the IB-spurted GQDs/PEDOT:PSS composite layer a promising candidate alignment material that can compete with other alignment layer materials and that might pave a way to the next generation of fast-response, large-area, multi-domain LCD.
Acknowledgments
Yang Liu thanks the China Scholarship Council (CSC, No. [2015]3022) for fellowship support.
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