Changdeuck Bae

High-performance low-temperature solution-processable ZnO thin film transistors by microwave-assisted annealing

Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm2 V−1s−1 mobility and a ∼107 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.

Bias-Stress-Stable Solution-Processed Oxide Thin Film Transistors

We generated a novel amorphous oxide semiconductor thin film transistor (AOS-TFT) that has exellent bias-stress stability using solution-processed gallium tin zinc oxide (GSZO) layers as the channel. The cause of the resulting stable operation against the gate bias-stress was studied by comparing the TFT characteristics of the GSZO layer with a tin-doped ZnO (ZTO) layer that lacks gallium. By photoluminescence, X-ray photoelectron, and electron paramagnetic resonance spectroscopy, we found that the GSZO layer had a significantly lower oxygen vacancy, which act as trap sites, than did the ZTO film. The successful fabrication of a solution-processable GSZO layer reported here is the first step in realizing all-solution-processed transparent flexible transistors with air-stable, reproducible device characteristics.

Origin of surface potential change during ferroelectric switching in epitaxial PbTiO3 thin films studied by scanning force microscopy

We investigated the surface potential of the ferroelectric domains of the epitaxial PbTiO3 (PTO) films using both Kelvin probe and piezoresponse force microscopy. The surface potential changes as a function of applied biases suggested that the amount and sign of surface potentials depend on the correlation between polarization and screen charges. It also suggested that the trapped negative charges exist on the as-deposited PTO surfaces. Injected charges and their resultant surface potentials are investigated by grounded tip scans. The results unveiled the origin of surface potential changes during ferroelectric switching in the epitaxial PTO films.

Inkjet-printed Cu source/drain electrodes for solution-deposited thin film transistors

We report on the first utility of Cu nanoparticle inks as low-cost, printable electrodes in the fabrication of solution-deposited amorphous oxide semiconductor thin film transistors. The performance of printed Cu electrodes was studied in terms of involvements of surface states in the devices. The surface chemical structures of Cu nanoparticulate electrodes were observed to be modified, dependent on the molecular weight of the polyvinylpyrrolidone capping molecules used in their synthesis. The surface dipoles became weak, and the work function of the printed electrodes decreased with increasing the molecular weight. The work function tailored by introducing the larger capping agents allowed for a better energetic leveling with the metal oxide semiconductor layer, resulting in the improved device performance.

Hierarchical Titania Nanotubes with Self-Branched Crystalline Nanorods

Surface decoration strategy for one-dimensional nanostructures will improve their electrical, optical, mechanical, and electrochemical performances dramatically. Heterogeneous growth/deposition on surfaces, however, may create undesired junction interfaces in the system. Here we report a procedure during which amorphous titania nanotubes are readily self-branched with crystalline titanate nanorods at room temperature. The starting amorphous titania nanotubes were prepared by low-temperature atomic layer deposition combined with the template-directed approach. We routinely observed the self-branching phenomenon of crystalline titanate nanorods with a few nanometers in diameter onto the surfaces of the amorphous titania nanotubes in mild alkali solutions. The resulting structures were analyzed by field-emission scanning electron microscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. The reactivity of the hierarchical titania nanotube arrays was observed to be improved as a Li secondary battery electrode. Upon complete consumption of the amorphous body of titania nanotubes, in addition, titanate nanosheets/layers consisting of single TiO(2) layers with unit-cell thickness were obtained, elucidating the formation mechanism of layered titanate materials by alkali treatment.

Thermopower engineering of Bi2Te3 without alloying: the interplay between nanostructuring and defect activation

We report on the interplay between nanostructuring and defect activation in dense polycrystalline Bi2Te3 thin films in terms of the thermopower engineering. The Bi2Te3 thin films were prepared at relatively low temperatures (100–160 °C) by atomic layer deposition and their grains showed different sizes in the range of 50–200 nm according to the deposition temperatures. We monitored the conductivity, Seebeck coefficient, and power factor of all samples from the temperature of 50–400 K. By increasing the growth temperature, remarkably, we observed the gradual defect activation from the nominal p-type to n-type in our binary end compound, Bi2Te3 without any alloying. The present results give us an insight on the optimization of thermoelectric materials not only by nanostructuring (i.e., phonon engineering) but also by controlled defect activation (i.e., electron engineering).

Aging Dynamics of Solution-Processed Amorphous Oxide Semiconductor Field Effect Transistors

This study determined the aging characteristics of solution-processed amorphous In−Ga−Zn oxide (a-IGZO) layers as amorphous oxide semiconductors (AOSs) for transparent, flexible thin-film transistor (TFT) applications. The work function of a-IGZOs upon exposure to air immediately after vacuum annealing was monitored using Kelvin probe force microscopy (KPFM). An increase was observed in the work function with time. Additionally, X-ray photoelectron spectroscopy combined with the KPFM results revealed two competing factors responsible for the aging characteristics: adsorption of oxygen molecules on the nanopore surfaces within the films, and the creation of defects, including oxygen vacancies and zinc interstitials. Although the former was reversible by alteration of ambient conditions, the latter was irreversible. On the basis of modified band theory, we proposed an operative mechanism for solution-processed AOS TFTs based on both the nature of porous amorphous structures and the aging dynamics. The persp...

Nanotubular Heterostructure of Tin Dioxide/Titanium Dioxide as a Binder‐Free Anode in Lithium‐Ion Batteries

Titanium dioxide (TiO2 ), tin dioxide (SnO2 ), and heterostructured TiO2 /SnO2 nanotube (NT) arrays have been fabricated by template-assisted atomic-layer deposition (ALD) for use as anodes in a lithium-ion battery (LIB). TiO2 NT arrays with 8 nm thick walls showed higher capacity (≈250 mA h g(-1) after the 50th cycle at a rate of C/10) than the typical theoretical capacity of bulk TiO2 and a radically improved capacity retention property upon cycling. SnO2 NT arrays with different wall thicknesses (8, 10, 13, and 20 nm) were also fabricated and their electrochemical performances were measured. All of the SnO2 NT arrays showed substantially higher initial irreversible capacity and higher reversible capacity than those of bulk TiO2 . Thinner walls of the SnO2 NTs result in better capacity retention. Heterotubular structures of TiO2 (5 nm)/SnO2 (10 nm)/TiO2 (5 nm) were successfully fabricated, and displayed a sufficiently high capacity (≈300 mA h g(-1) after 50 cycles) with exceptionally improved cycling performance up to the 50th cycle.

Confined crystallization of anatase TiO2 nanotubes and their implications on transport properties

Nanotubes of TiO2 (anatase) and their ordered arrays are emerging, promising candidates as efficient host materials in many applications such as photovoltaic cells, batteries, sensors and catalysts/catalytic supports, but the interplay between these structures and their transport properties has been reported only rarely. Monodisperse, stoichiometric TiO2 nanotubes with smooth morphology and controlled wall thickness were fabricated by template-directed low-temperature atomic layer deposition (ALD), followed by annealing at elevated temperatures. We present a study on the wall thickness-dependent crystallization behaviors due to physical and/or self-confinement, as well as on the corresponding electrical properties. Over certain wall thicknesses, unexpectedly, our TiO2 nanotubes were found to be a new type of mesoporous wide gap semiconductor in which they possess similar porosity, but in terms of conductivity differ from previously known mesoporous photoanodes (i.e., anodized surfaces of Ti films and sintered films consisting of TiO2 nanoparticles). These results were ascribed to the large, elongated anatase domains (by a factor of up to 15–40 wall thicknesses) that developed via boosted crystal growth on porous alumina templates (physical confinement) as well as to the highly curved tubular shape (self-confinement). Indeed, nanotube arrays with walls thicker than 10 nm exhibited an enhancement in conductivity, by more than three orders of magnitude, compared to sintered, mesoporous TiO2 (anatase) particles, approaching the bulk value. The nearly single-crystalline TiO2 nanotubes presented here should allow for a good model system to study TiO2-based surface chemistry and have potential for many applications in photovoltaic and/or catalytic systems.

Screening effect on photovoltaic performance in ferroelectric CH3NH3PbI3 perovskite thin films

Organic and inorganic hybrid materials of CH3NH3PbX3 with a perovskite crystal structure have been conceived as emerging light absorbing materials for high efficiency photovoltaic devices. Here, we demonstrate the screening effect of polarization states on charge redistribution related to the photovoltaic performance of ferroelectric CH3NH3PbI3 thin films using atomic force microscopy. We show the interplay between polarization and injected charges to have significant effects on charge transfer which potentially influences photovoltaic performance. The obtained results reveal that the direction and the amount of charge transfer can be influenced by the screening of polarization states at the interface. These results could deliver fundamental information regarding the influence of ferroelectricity on CH3NH3PbX3 solar cells.