1. Introduction

Indium tin oxide (ITO) films have been widely used as transparent conducting electrodes of optoelectronic devices such as displays and solar cells due to high optical transmission and good electrical conductivity. However, the ITO film is expensive because of limited resources, brittle [1], and deposited by sputtering at high temperature onto glass substrate. Recent researches are focused on replacing ITO electrodes with alternative materials that can be solution-processed and can reproduce optical and electrical characteristics of sputtered ITO on various substrates including plastics [2–6]. These alternatives such as single-walled carbon nanotubes (SWNTs) [2], graphenes [3], conductive polymers [4], silver nanowires [5], and ITO nanoparticles [6] do not currently match the performance of the ITO film.

A nanoimprint lithography (NIL) used in this study is a promising technique for quick, simple, and low-cost surface patterning with submicron resolution [7]. Especially, periodic or aperiodic nanostructures introduced by the NIL process are attractive for high efficiency or functional photonic devices such as solar cells [8], organic light-emitting diodes (OLEDs) [9,10], and distributed feedback (DFB) lasers [11,12], and liquid crystal (LC) devices [13]. Most reports were limited to organic polymeric systems with thermoplastics and UV-curable resists [7–13]. Therefore, additional polymeric layers that can negatively affect device performance or additional fabrication process are required to introduce specific patterns into metal or metal oxide layer of the devices.

In this paper, we directly fabricate ITO films with one-dimensional (1D) periodic nanostructures by all-solution processing of ITO nanoparticles and the NIL method. The resulting nanostructured ITO films on glass substrates show low sheet resistance of ~200 Ω/sq at high optical transparency of ~80%. This sheet resistance at same transmittance, being higher than that of the conventional ITO and similar to that of the SWNT electrode [2,5] is suitable for LC devices and polymer LEDs. In addition, the experimental transmission spectra from the nanostructured ITO films with various periodicities have specific transmission dips due to light diffractions and are well simulated by calculations. By using this ITO film with 1D periodic structure as transparent electrodes and alignment layers, we present a successful alignment of nematic LC (NLC) molecules and examine electro-optic properties of the fabricated twisted nematic (TN) LC device.

2. Fabrication and characterization of nanostructured ITO films

To fabricate nanostructured ITO films by all-solution processing of Fig. 1(a) , we used ITO nanoparticles with diameter below ~20 nm dispersed in a cyclododecene solvent (Ulvac Materials). Figure 1(b) shows a transmission electron microscope (TEM; JEM-2100F, JEOL) image of the ITO nanoparticles prepared by annealing at 230 °C for 30 min to evaporate the solvent after drop-casting the ITO nanoparticle solution. ITO films were fabricated on glass substrates by spin-coating the nanoparticle solution, annealing in air at 330 °C for 30 min, and repeating this procedure for applications as a transparent electrode with low sheet resistance. By reaction with oxygen for thermal treatment, the ITO film was changed from opaque dark brown to transparent color, as shown in Fig. 1(c).

 

Fig. 1 (a) Schematic illustration of fabrication process of the nanostructured ITO film. (b) TEM image of the ITO nanoparticles. (c) Photographs of the ITO film before (upper) and after (lower) annealing in air at 330 °C for 30 min. (d) Cross-sectional TEM image of the nanostructured ITO films with p = 379 nm, d_grating = 100 nm, and d_ITO = 508 ± 56 nm.

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Then, we introduced a 1D periodic nanostructure on the ITO film by using NIL process of Fig. 1(a). As a master mold, we used holographic gratings with pitches p = 546 nm (grating depth d_grating = 84 nm), 405 nm (d_grating = 132 nm), and 273 nm (d_grating = 71 nm), and a Si mold with p = 379 nm (d_grating = 151 nm) fabricated by e-beam lithography method, where each of the pitches and depths was confirmed by an atomic force microscopy (AFM; XE-100, Park Systems) measurement. First, the master molds were converted to poly(dimethylsiloxane) (PDMS; RT-601, Wacker) molds by drop-casting the PDMS materials and curing at 70°C for 30 min. Next, the ITO nanoparticle solution was spin-coated on the ITO film, and the patterned PDMS mold was pressed onto it at 230 °C for 30 min. Figure 1(d) shows a cross-sectional TEM image of the nanostructured ITO film with p = 379 nm and d_grating = 100 nm on substrate, prepared by a focused ion beam (FIB; Quanta 3D FEG, FEI) after Pt sputtering as a protection layer. This TEM image clearly indicates that the periodic nanostructure of the master mold was replicated onto the ITO film with a thickness d_ITO = 508 ± 56 nm.

Before the NIL process mentioned above, we investigated optical and electrical properties of the ITO films made from the ITO nanoparticle solution. Figure 2(a) shows AFM images of two kinds of ITO films on glass substrates with annealing temperatures T_a = 230 °C and 330 °C, respectively. The root-mean-square roughness of the ITO surfaces in 10 μm × 10 μm area of Fig. 2(a) were 1.2 nm for T_a = 230 °C and 2.6 nm for T_a = 330 °C, while the sheet resistance were 7.46 MΩ/sq for the ITO film with d_ITO = 87 ± 15 nm and T_a = 230 °C, and 10.4 kΩ/sq for the film with d_ITO = 116 ± 14 nm and T_a = 330 °C. The thickness and the sheet resistance of each film were measured by using a surface profiler (P-10, KLS-Tencor) and a four-point probe meter (Model280, Four Dimensions), respectively. In addition, the structures of the ITO films were characterized by using an X-ray diffraction (XRD) system (D/max-2500V/PC, Rigaku). In Fig. 2(b), both XRD patterns match well with those of a cubic bixbyite In₂O₃ [14], and the pattern for the ITO film with T_a = 330 °C reveals clear and strong peaks indicating high crystalline nature. Hence, as the annealing temperature increased, the ITO films have higher surface roughness and lower sheet resistance due to enhanced crystallinity of Fig. 2(b).

 

Fig. 2 (a) AFM images (10 μm × 10 μm) and (b) XRD patterns of ITO films annealed at T_a = 230 °C and 330 °C. Dependences of (c) transmission spectra and (d) sheet resistances on a number of ITO layers. The dashed line refers to a transmission spectrum of the nanoimprinted ITO film of Fig. 1(d).

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In order to obtain transparent electrodes with low sheet resistance of ~200 Ω/sq, we stacked a single ITO layer nanoimprinted at 230 °C onto 5-layered ITO films annealed at 330 °C. Figure 2(c) and (d) exhibits the dependences of transmission spectra and sheet resistances on a number of ITO layers. Transmission spectra were measured by using a spectrometer (Lambda 14P, Perkin Elmer). The transmittance in the visible region are fixed at ~90% except for the nanostructured ITO film on glass substrate (dashed line of Fig. 2(c)), but the sheet resistance decreases from 11.3 kΩ/sq to 213 Ω/sq with increasing the number of ITO layers. Here, the decrease in transmittance of the nanostructured ITO film can be understood from the fact that the PDMS mold prevents the reaction with oxygen during NIL process. This behavior is similar to that of the ITO film during thermal treatment, shown in Fig. 1(c). If we can increase the oxygen reaction and T_a of the ITO film during NIL process, an improved performance in the transparency and the sheet resistance can be obtained.

Next, we prepared four kinds of nanostructured ITO films on glass substrates with different p values, and measured AFM images, transmission spectra, and sheet resistances of them. From AFM images of Fig. 3 , we can clearly observe the formation of patterned ITO structures with p and d_grating values intentionally attempted. In Fig. 3, all experimental transmission spectra for normal incidence of unpolarized light (solid lines) show more than ~80% transmittance in the visible regions, and sheet resistances were (a) 182 Ω/sq, (b) 215 Ω/sq, (c) 221 Ω/sq, and (d) 202 Ω/sq, respectively.

 

Fig. 3 Measured transmission spectra (solid lines) and AFM images (insets) of nanostructured ITO films with (a) p = 544 nm (d_grating = 86 nm), (b) 397 nm (d_grating = 113 nm), (c) 379 nm (d_grating = 100 nm), and (d) 273 nm (d_grating = 71 nm). Photographs and schematic illustration represent diffracted reflection light from the sample with p = 544 nm in (a). Dashed lines indicate simulated transmission spectra of nanostructured ITO films with various p values for normal incidence of TE and TM polarized light.

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Black arrows of Fig. 3 correspond to characteristic dips in transmission spectra by light diffractions through 1D periodic nanostructures of the ITO films. Figure 3(a) displays photographs of diffracted reflection lights from the nanostructured ITO film with p = 544 nm and d_grating = 86 nm for various diffraction angles. The diffraction condition for the light with a wavelength λ and an incident angle α is given by mλ = p (sin α + sin β), where m is the diffraction order and β is the diffracted angles of light [9]. Therefore, as the diffraction angle increased, the colors (or spectral positions) of the reflected diffraction lights changed from blue to red because of the increase in path difference p sin β, as shown in Fig. 3(a).

To examine diffraction characteristics for the case of periodic ITO films in detail, we performed simulations based on rigorous coupled wave analysis (RCWA) [15] by using commercial software DiffractMOD [16]. Here, we assumed normal incidence of transverse electric (TE) or transverse magnetic (TM) polarized light shown in Fig. 3(b) (lower inset). Dashed lines of Fig. 3(b), (c) and (d) exhibit transmission spectra calculated from the periodic nanostructure consisting of an ITO layer with a refractive index n_ITO = 1.8 as a typical value of the ITO used [17] and a thickness d_ITO = 500 nm, and an ITO nanostructure with a refractive index n_grating = 1.8, d_grating = 100 nm, a grating fill factor f = 0.3, and different p values. All these spectra clearly indicate the presence of transmission dips from nanostructured ITO films due to light diffractions, in which the spectral positions of simulated transmission dips accord with those of experimental results. One may notice the disagreement between experimental and simulated depths in transmission dips. This is because we assumed perfectly the same values of d_grating, d_ITO, and f for all simulations and used different incident polarizations. From theoretical fittings with experimental results of Fig. 3(b), (c), and (d), the determined values are p = 397 nm for (b), 370 nm for (c), and 268 nm for (d) that are similar to pitches 397, 379, and 273 nm, experimentally measured from AFM images of Fig. 3. This diffraction from the periodic nanostructure is attractive for high efficiency photonic applications based on light enhancement [9–12,17], whereas it can be a demerit for display devices.

3. Nanostructured ITO films as transparent electrodes and alignment layers of NLCs

As transparent electrodes and alignment layers of NLCs, we employed the nanostructured ITO films with p = 273 nm of Fig. 3(d), chosen to avoid light diffractions in the visible region. Without any further treatments, a pair of nanostructured ITO films on glass substrates was stacked face-to-face with a spacer in between, and then sealed to fabricate an empty cell. The cell gap was ~13.5 μm, and the grating directions of the two facing films were set parallel to each other, as shown in Fig. 4(a) . Then, a NLC (ZLI2293, Merck) was introduced into the cell by capillary action at 88 °C in the isotropic phase. The textures of NLCs in the cells were observed with a polarizing microscope (BX41-P, Olympus) under crossed polarizers. Figure 4(b) shows textures measured at rotation angles of ϕ = 0°, 45°, 90°, and 135°, where the rotation angle is the angle between the polarization direction of one polarizer and the grating direction of the nanostructured ITO cell. Here, it is evident that a homogeneous alignment of NLC molecules is achieved, and that the nanostructured ITO film works as an alignment layer for NLCs.

 

Fig. 4 Schematic illustrations of (a) the NLC cell and (c) the TN-LC cell fabricated with the nanostructured ITO films with p = 273 nm. (b) Crossed-polarizer micrographs of the NLC cells for various rotation angles ϕ = 0°, 45°, 90°, and 135°. (d) The transmitted intensity-applied voltage (T-V) characteristic of the TN-LC cell.

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To investigate electro-optic properties of the NLC device with the nanostructured ITO films, a TN-LC cell was also fabricated. In the TN-LC cell, the grating directions or alignment directions of the ITO films were rotated 90° relative to each other, as shown in Fig. 4(c). Cross-polarized transmission from a He-Ne laser (632.8 nm, away from the spectral position of diffractions of Fig. 3(d)) was examined at a normal incidence while an external voltage (1 kHz square wave) was applied across the two nanostructured ITO electrodes. As shown in Fig. 4(d), the transmittance begins to decrease at ~1.0 V and vanishes beyond ~5.0 V in the normally white mode. The contrast ratio is ~100:1 at 5.0 V. This electro-optic performance is similar to that of conventional TN-LC cell fabricated with a rubbed polyimide alignment layer, and indicates that an anchoring of NLCs on nanostructured ITO is sufficiently strong to enable the filed-induced reorientation [13]. The experiments and their quantitative analysis about the anchoring energies at nanostructured ITO are undertaken in our lab and will be reported in the future.

4. Conclusion

In summary, we investigated the nanostructured ITO films fabricated by solution processing of ITO nanoparticles and the NIL technique. From electro-optical measurements for NLC devices made of these ITO films, it was proven that the nanostructured ITO film is not only a transparent electrode contributing the device performance but also an alignment layer of NLCs. Our functional transparent electrode can provide new opportunities in photonic applications such as solar cells, OLEDs, organic DFB lasers, and LC devices.