Gradient area-selective deposition for seamless gap-filling in 3D nanostructures through surface chemical reactivity control.

The integration of bottom-up fabrication techniques and top-down methods can overcome current limits in nanofabrication. For such integration, we propose a gradient area-selective deposition using atomic layer deposition to overcome the inherent limitation of 3D nanofabrication and demonstrate the applicability of the proposed method toward large-scale production of materials. Cp(CH3)5Ti(OMe)3 is used as a molecular surface inhibitor to prevent the growth of TiO2 film in the next atomic layer deposition process. Cp(CH3)5Ti(OMe)3 adsorption was controlled gradually in a 3D nanoscale hole to achieve gradient TiO2 growth. This resulted in the formation of perfectly seamless TiO2 films with a high-aspect-ratio hole structure. The experimental results were consistent with theoretical calculations based on density functional theory, Monte Carlo simulation, and the Johnson-Mehl-Avrami-Kolmogorov model. Since the gradient area-selective deposition TiO2 film formation is based on the fundamentals of molecular chemical and physical behaviours, this approach can be applied to other material systems in atomic layer deposition.

On-screen fingerprint sensor with optically and electrically tailored transparent electrode patterns for use on high-resolution mobile displays.

In this study, a mutual capacitive-type on-screen fingerprint sensor, which can recognize fingerprints on a display screen to provide smartphones with full-screen displays with a minimal bezel area, is fabricated. On-screen fingerprint sensors are fabricated using an indium tin oxide transparent conductor with a sheet resistance of ~10 Ω/sq. and a transmittance of ~94% (~86% with the substrate effect) in the visible wavelength range, and assembled onto a display panel. Even at this high transmittance, the electrodes can degrade the display quality when they are placed on the display. The interference between periodic display pixel arrays and sensor patterns can lead to the Moiré phenomenon. It is necessary to find an appropriate sensor pattern that minimizes the Moiré pattern, while maintaining the signal sensitivity. To search for appropriate patterns, a numerical calculation is carried out over wide ranges of pitches and rotation angles. The range is narrowed for an experimental evaluation, which is used to finally determine the sensor design. As the selected sensor pitches are too small to detect capacitance variations, three unit patterns are electrically connected to obtain a unit block generating a larger signal. By applying the selected sensor pattern and circuit driving by block, fingerprint sensing on a display is demonstrated with a prototype built on a commercial smartphone.

Low diffuse reflection of silver nanowire/ruthenium oxide nanosheet hybrid films for high-performance transparent flexible electrodes.

Transparent conducting electrodes (TCEs) based on silver nanowire (AgNW) networks possess high conductance, transmittance, and mechanical flexibility. However, due to the relatively high diffuse reflection of incident light on AgNWs, they cannot be practically implemented in displays requiring low pattern visibility. One promising strategy for solving this problem is to place an optical stack with high refractive index underneath the AgNW layer. In this work, AgNW-RuO2 nanosheet hybrid TCEs with low diffuse reflections are fabricated using metallic RuO2 nanosheets as undercoats. As predicted by theoretical simulations, RuO2 nanosheets with high refractive indices reduce the diffuse reflections of AgNWs by almost 8%. Moreover, after the partial etching of AgNWs, the difference in the diffuse reflections of their etched and non-etched regions becomes equal to about 0.003, leading to the formation of an invisible pattern. The film consisting of micro-sized RuO2 nanosheets is not damaged during etching, but instead forms a current path between different AgNWs broken by cyclic bending, resulting in a tenfold decrease in the resistance of the AgNW TCE after 170 000 cycles. Further, RuO2 nanosheets suppress the diffusion of humid air from the outside, thus improving the environmental stability of the AgNW-RuO2 nanosheet hybrid TCEs.

Moiré-free fingerprint sensors based on multilayer oxide-metal-oxide electrodes.

The display quality of touchscreen devices with on-screen fingerprint sensors is reduced by moiré patterns, interference phenomena caused by an overlap between the pixel pattern of the display, and the electrode pattern of the fingerprint sensor. A promising strategy for resolving this issue is to reduce the visibility of the moiré pattern, by including a filling layer with a transmittance similar to that of the electrodes, between the different patterns. We propose a moiré-free fingerprint sensor that uses an oxide-metal-oxide (IZO/Ag/IZO) multilayer as a highly transparent electrode. To verify the moiré reduction effect, we conducted a two-dimensional spectral analysis to calculate the spatial frequencies of the superimposed image of the display and the sensor patterns, and demonstrated experimentally that the proposed electrode greatly reduces the undesirable moiré phenomenon.

Low-visibility patterning of transparent conductive silver-nanowire films.

A partial etching mechanism is proposed to meet the requirement for low-visibility patterning of silver nanowire (AgNW)-based transparent conductive electrodes (TCEs) by reducing the difference in optical properties between conductive and nonconductive regions of the pattern. Using the finite difference time domain (FDTD) method, etched geometries that provide the smallest difference in transmittance after etching are theoretically determined. A sodium hypochlorite-based etchant capable that allows the etched geometry to be varied by controlling the pH is used to create a low-visibility pattern with a transmittance and haze difference of 0.07 and 0.04%, respectively. To the best of our knowledge, this is the first time that a partial etching mechanism such as this has been studied in relation to AgNW-based TCEs.

Single crystal-like Si patterns for photonic crystal color filters.

A novel fabrication method for a two-dimensional photonic crystal color filter based on guided mode resonance is proposed. An amorphous silicon layer deposited through the low-temperature plasma enhanced chemical vapor deposition (PECVD) process is patterned into two-dimensional structures using low-cost nanoimprint lithography. It is then effectively crystallized using multi-shot excimer laser annealing at low energy. We have demonstrated analytically and experimentally that single crystal-like silicon patterns on a glass substrate can offer high-efficiency photonic crystal color filters for reflective display applications. The highly crystallized silicon patterning scheme presented here may be very attractive for a variety of devices requiring high carrier mobility and high optical efficiency.

Nanoimprinted photonic crystal color filters for solar-powered reflective displays.

A novel concept for reflective displays that uses two-dimensional photonic crystals with subwavelength gratings is introduced. A solar-powered reflective display with photonic crystal color filters was analyzed by a theoretical approach. We fabricated the photonic crystal color filters on a glass substrate by using low-cost nanoimprint lithography and multi-scan excimer laser annealing to produce RGB color filters through a single patterning process. The theoretical and experimental results show that the color filters have high reflectance and angular tolerance, which was qualitatively confirmed by chromaticity coordination analysis.

Two-dimensional photonic crystal color filter development.

Reflective color filters using two-dimensional photonic crystals based on sub-wavelength gratings were proposed and constructed. Using low-cost nanoimprint lithography, an amorphous silicon layer was deposited through the low-temperature PECVD process and patterned into two-dimensional structures. The isolated amorphous silicon patterns were readily crystallized using a multi-shot excimer laser annealing at low energy. A study of the close relationship between color filter reflectance and silicon pattern crystallinity is introduced. Theoretical and experimental results show that the proposed color filters have high reflectance and, moreover, decrease the dependence on incident angle compared to one-dimensional photonic crystal color filters.