Chang-Ho Choi

Fabrication of high-performance, low-temperature solution processed amorphous indium oxide thin-film transistors using a volatile nitrate precursor

In this study, we fabricate amorphous indium oxide thin film transistors (TFTs) on a display glass substrate at various annealing temperatures from 200 °C to 300 °C. Using a volatile nitrate precursor, we were able to fabricate TFTs with excellent device performance within this annealing temperature range. Amorphous In2O3 films could be obtained by carefully controlling the film thickness and annealing temperature. TFTs based on amorphous In2O3 channel layers with an average mobility as high as 7.5 cm2 V−1 s−1, an Ion/Ioff ratio of 107, and Von = −5 V could be fabricated at 300 °C annealing temperature in air. The devices prepared at 200 °C still exhibit transistor characteristics with an average mobility of 0.04 cm2 V−1 s−1, an Ion/Ioff ratio of 105, and Von = 0 V. The temperature effects on the device performances are elucidated based on X-ray photoelectron spectroscopy and thermal gravimetric analysis characterization results of precursors and the resulting amorphous In2O3 thin films.

Large-scale Generation of Patterned Bubble Arrays on Printed Bi-functional Boiling Surfaces.

Bubble nucleation control, growth and departure dynamics is important in understanding boiling phenomena and enhancing nucleate boiling heat transfer performance. We report a novel bi-functional heterogeneous surface structure that is capable of tuning bubble nucleation, growth and departure dynamics. For the fabrication of the surface, hydrophobic polymer dot arrays are first printed on a substrate, followed by hydrophilic ZnO nanostructure deposition via microreactor-assisted nanomaterial deposition (MAND) processing. Wettability contrast between the hydrophobic polymer dot arrays and aqueous ZnO solution allows for the fabrication of heterogeneous surfaces with distinct wettability regions. Heterogeneous surfaces with various configurations were fabricated and their bubble dynamics were examined at elevated heat flux, revealing various nucleate boiling phenomena. In particular, aligned and patterned bubbles with a tunable departure frequency and diameter were demonstrated in a boiling experiment for the first time. Taking advantage of our fabrication method, a 6 inch wafer size heterogeneous surface was prepared. Pool boiling experiments were also performed to demonstrate a heat flux enhancement up to 3X at the same surface superheat using bi-functional surfaces, compared to a bare stainless steel surface.

Visible-light-sensitive Na-doped p-type flower-like ZnO photocatalysts synthesized via a continuous flow microreactor

A Na-doped p-type flower-like ZnO photocatalyst (Na:ZnO) that is highly visible-light-sensitive in air at room temperature was synthesized by a continuous flow microreactor, where NaOH was used as both the precipitating and doping agent. The results of various characterization techniques (XPS, ICP, ToF-SIMS, XRD, and HRTEM) indicated that the Na ions have been successfully doped into the ZnO lattice. The Na:ZnO demonstrated a much higher photocatalytic degradation rate of methylene blue under simulated sunlight (λmax = 494 nm) than the rates obtained from commercially available TiO2 photocatalysts (P-25) and pure ZnO. This much enhanced rate is likely a result of increased surface defect sites associated with oxygen when Na replaces Zn in the crystal structure. A possible mechanism of the photocatalytic degradation of methylene blue on the Na:ZnO is suggested.

Effects of fluid flow on the growth and assembly of ZnO nanocrystals in a continuous flow microreactor

The assembly of nanocrystals is considered to be one of the most promising approaches to design nano-, microstructures and complex mesoscopic architectures. A variety of strategies to induce nanocrystal assembly have been reported, including directed assembly methods that apply external forces to fabricate assembled structures. In this study, ZnO nanocrystals were synthesized in an aqueous solution using a continuous flow microreactor. The growth mechanism and stability of the ZnO nanocrystals were studied by varying the pH and flow conditions of the aqueous solution. It was found that convective fluid flowing from Dean vortices in a winding microcapillary tube could be used for the assembly of ZnO nanocrystals. The ZnO nanocrystal assemblies formed three-dimensional mesoporous structures of different shapes, including a tactoid and a sphere. The assembly results from a competing interaction between the electrostatic forces caused by the surface charge of the nanocrystals and the collision of the nanocrystals associated with Dean vortices. The dispersion behaviours of the ZnO assembly in some solvents were also studied. MeOH, a strong precipitant, led to the precipitation of the ZnO assembly. This study shows that the external forces from convective fluid flow could be applied to fabricate an assembly of functional metal oxides with complex architectures using a continuous flow microreactor.

Room temperature fabrication and patterning of highly conductive silver features using in situ reactive inks by microreactor-assisted printing

Highly conductive silver was fabricated at room temperature using in situ reactive silver precursor inks by microreactor-assisted printing without any post-processing. Reactive silver nanoinks, synthesized in situ from the microreactor, were directly delivered onto glass and polymeric substrates without any surface treatment to form a highly dense and uniform silver feature. The distribution of the reactive silver nanoinks can be controlled simply by adjusting the flow rate of the continuous flow system. Silver lines were fabricated using the in situ reactive precursors delivered via a micro-channel applicator. The electrical conductivity of the silver film and feature were measured to be around 3.3 × 107 (S m−1), corresponding to about half of the conductivity of bulk silver. The functionality of the silver line was confirmed through the operation of LEDs. This study demonstrates the possibility to fabricate patterned silver features at room temperature from in situ nanoinks without the aid of any post-processing.

Continuous formation of a seed layer and vertical ZnO nanowire arrays enabled by tailored reaction kinetics in a microreactor

ZnO nanowires (NWs) are gaining widespread popularity due to their many unique physical and chemical properties. The hydrothermal process is the most commonly used approach to prepare vertical ZnO NW arrays. Its fundamental issues are the rather slow deposition rate and the variation of reactant concentrations as a function of time. In this study, we employed a continuous flow microreactor and deposition system to prepare both a ZnO seed layer and vertical ZnO NW arrays continuously by simply tuning the flow rate of reactant solutions. The change in flow rate allows reaction kinetics to be selectively tuned, resulting in the generation of molecular zinc species and colloidal ZnO nanocrystals at a high and low flow rate, respectively. The molecular zinc species constitute the seed layer at the high flow rate, and subsequently the colloidal ZnO nanocrystals contribute to the growth of ZnO NWs at the low flow rate. Since the direct delivery of the controlled building blocks on the substrate prevents the formation of homogeneously induced ZnO particles, we were able to obtain highly uniform ZnO NW arrays, with a growth rate as fast as 240 nm min−1, which is significantly higher than typical growth rates from gas-phase or batch hydrothermal processes. Based on the growth mechanism study, the dimensions of the vertical ZnO NW arrays were varied, leading to an aspect ratio of the NWs up to 23. The advantages of our system were highlighted by preparing a high-quality heterogeneous surface having both hydrophilic and hydrophobic nature on one substrate. These results demonstrate the capability of the continuous flow microreactor and deposition system in efficiently preparing functional vertical ZnO NW arrays.

Low-temperature, inkjet printed p-type copper(I) iodide thin film transistors

Low temperature fabrication of printed p-type CuI TFTs was reported for the first time. The printed CuI film was fabricated by printing molecular CuI ink directly onto the device substrate followed by immediate crystallization of CuI nanoparticles as the solvent evaporated. The substrate temperature during inkjet printing was varied in order to obtain continuous CuI films with large grain size for improved device performance. The CuI TFTs printed at 60 °C exhibited an average field-effect mobility of 1.86 ± 1.6 cm2 V−1 s−1, with the maximum value of 4.4 cm2 V−1 s−1 and an average On/Off ratio of 101–102. This study demonstrates potential low temperature, directly printed p-type TFTs for constructing transparent, complementary inorganic TFT circuits.

Microreactor-Assisted Solution Deposition for Compound Semiconductor Thin Films

State-of-the-art techniques for the fabrication of compound semiconductors are mostly vacuum-based physical vapor or chemical vapor deposition processes. These vacuum-based techniques typically operate at high temperatures and normally require higher capital costs. Solution-based techniques offer opportunities to fabricate compound semiconductors at lower temperatures and lower capital costs. Among many solution-based deposition processes, chemical bath deposition is an attractive technique for depositing semiconductor films, owing to its low temperature, low cost and large area deposition capability. Chemical bath deposition processes are mainly performed using batch reactors, where all reactants are fed into the reactor simultaneously and products are removed after the processing is finished. Consequently, reaction selectivity is difficult, which can lead to unwanted secondary reactions. Microreactor-assisted solution deposition processes can overcome this limitation by producing short-life molecular intermediates used for heterogeneous thin film synthesis and quenching the reaction prior to homogeneous reactions. In this paper, we present progress in the synthesis and deposition of semiconductor thin films with a focus on CdS using microreactor-assisted solution deposition and provide an overview of its prospect for scale-up.

The effects of gallium on solution-derived indium oxide-based thin film transistors manufactured on display glass

Metal oxide semiconductor TFTs have been considerably investigated as a promising alternative to hydrogenated amorphous silicon and organic semiconductors. While many multicomponent oxide TFTs have been studied, there are only a few reports of TFTs using amorphous indium gallium oxide channel layers. In this study, the effects of gallium atomic ratio on the performance of solution-derived indium oxide-based TFTs on display glass were investigated for the first time. The morphological, optical, and electrical properties of IGO channel layers with different gallium atomic ratios were characterized. IGO TFTs with various chemical compositions were compared and interpreted based on the analysis of In3d, Ga2p, and O1s XPS data. It was found that gallium dopant suppresses the generation of oxygen vacancies, while promoting the formation of oxygen in the oxide lattice without oxygen vacancies by reducing the density of hydroxides. By adjusting the atomic ratio of gallium, we were able to fabricate IGO TFTs on display glass with an average field-effect mobility as high as 6.1 cm2 V−1 s−1, Von = −2 V, and on–off ratio of 107.

Capillary Rise of Nanostructured Microwicks

Capillarity refers to the driving force to propel liquid through small gaps in the absence of external forces, and hence enhanced capillary force has been pursued for various applications. In this study, flower like ZnO nanostructures are successfully deposited to enhance capillarity of microwick structures that are specially designed to augment boiling heat transfer performance. Microreactor-assisted nanomaterial deposition, MANDTM, is employed with a flow cell to deposit the ZnO nanostructures on a large sized microwick (4.3 cm × 10.7 cm) with dual-channel configuration. A capillary rise experiment based on the mass gain method is first performed using water and ethanol (EtOH) as the working liquids to demonstrate the enhanced capillary force induced by the ZnO nanostructure on the microwick structure. It is found that the coating of ZnO nanostructure effectively propels the working fluids through the nano- or micro pores created from the ZnO nanostructure and consequently improves the capillary force. In order to investigate the wicking mechanism of the ZnO coated microwick structure, the capillary rise result based on height measurement was compared with analytical models. It is found that the gravity effect and viscous force play an important role in wicking rise of the coated wick structure. This study aims at demonstrating the capability of the integrated MAND process with a flow cell for producing a large scaled nanostructured surface, which eventually has a great potential for enhanced boiling heat transfer.