Dai-Hong Kim

SnO2 nanotubes fabricated using electrospinning and atomic layer deposition and their gas sensing performance

A novel method is developed to fabricate a SnO(2) nanotube network by utilizing electrospinning and atomic layer deposition (ALD), and the network sensor is proven to exhibit excellent sensitivity to ethanol owing to its hollow, nanostructured character. The electrospun polyacrylonitrile (PAN) nanofibers of 100-200 nm diameter are used as a template after stabilization at 250 degrees C. An uniform and conformal SnO(2) coating on the nanofiber template is achieved by ALD using dibutyltindiacetate (DBTDA) as the Sn source at 100 degrees C and the wall thickness is precisely controlled by adjusting the number of ALD cycles. The calcination at 700 degrees C transforms the amorphous nanofibers into SnO(2) nanotubes composed of several nanometer-sized crystallites. The SnO(2) nanotube network sensor responds to ethanol, H(2), CO, NH(3) and NO(2) gases, but it exhibited an extremely high gas response to ethanol with a short response time (<5 s). The results demonstrate that the combination of electrospinning and ALD is a very effective and promising technique to fabricate long and uniform metal oxide nanotubes with the precise control of wall thickness, which can be applied to various applications such as gas sensors and lithium ion batteries.

Electromechanical properties of CNT-coated cotton yarn for electronic textile applications

Smart fabrics have attracted considerable attention due to their potential applications. The essential features of smart fabrics include wearability, weaveability, and stretchability, as well as their sensing/response capability, which is frequently based on electrical measurement. Thus, the electromechanical behavior of these fabrics is considered the most important material property. Here, we report the negative piezoresistance of single-walled carbon nanotube coated cotton yarn (SWNT-CY). The gauge factor (the ratio of the normalized change in piezoresistance to the change in strain) of SWNT-CY is measured to be − 24. It is noteworthy that the factor is negative and an order of magnitude higher than that for conventional metal strain gauges. The negative piezoresistance is due to mechanical contact between fabric fibers, which leads to better electrical paths of SWNT networks. The conduction behavior can be modeled as fluctuation-induced tunneling (FIT) through the contact barriers between conducting regions. The effective barrier strength of strained SWNT-CY is measured to be ~ 30% lower than that of unstrained SWNT-CY. This characteristic may offer new design opportunities for wearable electronics and has significant implications for sensor applications.

Direct printing synthesis of self-organized copper oxide hollow spheres on a substrate using copper(II) complex ink: gas sensing and photoelectrochemical properties.

The direct printing synthesis of metal oxide hollow spheres in the form of film on a substrate is reported for the first time. This method offers facile, scalable, high-throughput production and device fabrication processes. The printing was carried out via a doctor-blade method using Cu(II) complex ink with controllable high viscosity based on formate-amine coupling. Following only thermal heating in air, well-defined polycrystalline copper oxide hollow spheres with a submicrometer diameter (≤1 μm) were formed spontaneously while being assembled in the form of a film with good adhesion on the substrate. This spontaneous hollowing mechanism was found to result from the Kirkendall effect during oxidation at elevated temperature. The CuO films with hollow spheres, prepared via direct printing synthesis at 500 °C, led to the creation of a superior p-type gas sensor and photocathode for photoelectrochemical water splitting with completely hollow cores, a rough/porous shell structure, a single phase, high crystallinity, and no organic/polymer residue. As a result, the CuO hollow-sphere films showed high gas responses and permissible response speeds to reducing gases and high photocurrent density compared to conventional CuO powder films and the values previously reported. These results exemplify the successful realization of a high-throughput printing fabrication method for the creation of superior nanostructured devices.

Fabrication of SnO2 nanotube microyarn and its gas sensing behavior

Continuously aligned and assembled forms of nanomaterials (e.g., carbon nanotube yarns) have been recognized as an effective means of realizing the marvelous properties of individual nanomaterial in the macro/microscale. Many efforts have been made to develop metal oxide nanotubes, however, few researches have been dedicated to fabricating their microyarns. In this study, we report a fabrication method for SnO2 nanotube microyarns and their potential applications. The aligned polyacrylonitrile nanofibers were first prepared using electrospinning and they were twisted into a microyarn. SnO2 was then coated onto the nanofibers in the yarn by atomic layer deposition (ALD). Finally the microyarn consisting of the coated nanofibers was calcined, resulting in a SnO2 nanotube microyarn. The average diameter of the obtained SnO2 nanotubes in the microyarn was around 500?nm, and their wall thickness was approximately 70?nm when 1000 cycles of the ALD process were applied. The lattice fringes in the high resolution transmission electron microscope image and selected area electron diffraction pattern revealed that the nanotubes were polycrystalline SnO2 with a rutile structure. The SnO2 nanotube microyarn fabricated in this study has the potential to be applied for the development of a multiple-celled gas sensor, which has been confirmed by carrying out H2 gas sensing at 400??C using a one-celled sensor. The results showed the stable and reversible gas sensing of the nanotube yarn, demonstrating that the nanotube yarns can be incorporated into a multiple-celled sensor due to their handling convenience, stable structure, and gas sensing performance.

Tunable conductivity at LaAlO3/SrxCa1−xTiO3 (0 ≤ x ≤ 1) heterointerfaces

The two-dimensional electron gas formed at LaAlO3/SrTiO3 heterointerfaces exhibits a variety of interesting physical properties. Herein, we report on tunable conductivity at LaAlO3/SrxCa1−xTiO3 heterostructures. By changing the Sr content in the SrxCa1−xTiO3 (0 ≤ x ≤ 1) layers, the orthorhombicity of the films, which inevitably accompanies TiO6 octahedral distortions in the unit cells, could be varied. As a result, the interfacial conductivity can be tuned over 6 orders of magnitude. We suggest that the use of pseudosubstrates with chemical substitution or alloying is a promising route to finely tune conductivity at oxide heterointerfaces.

Brookite TiO2 Thin Film Epitaxially Grown on (110) YSZ Substrate by Atomic Layer Deposition

Epitaxial brookite TiO2 (B-TiO2) film was deposited on (110) yttria-stabilized zirconia (YSZ) substrate using plasma-enhanced atomic layer deposition, and its structural, optical, and gas sensing properties were investigated. As-deposited TiO2 film was a pure brookite and (120) oriented. The determined in-plane orientation relationships were [21̅0]B-TiO2//[1̅10]YSZ and [001]B-TiO2 //[001]YSZ. The B-TiO2 film showed ∼70% transmittance and the optical band gap energy was 3.29 eV. The B-TiO2 film-based gas sensor responded to H2 gas even at room temperature and the highest magnitude of the gas response was determined to be ∼150 toward 1000 ppm of H2/air at 150 °C. In addition, B-TiO2 sensor showed a high selectivity for H2 against CO, EtOH, and NH3.

Integrating Metal-Oxide-Decorated CNT Networks with a CMOS Readout in a Gas Sensor

We have implemented a tin-oxide-decorated carbon nanotube (CNT) network gas sensor system on a single die. We have also demonstrated the deposition of metallic tin on the CNT network, its subsequent oxidation in air, and the improvement of the lifetime of the sensors. The fabricated array of CNT sensors contains 128 sensor cells for added redundancy and increased accuracy. The read-out integrated circuit (ROIC) was combined with coarse and fine time-to-digital converters to extend its resolution in a power-efficient way. The ROIC is fabricated using a 0.35 μm CMOS process, and the whole sensor system consumes 30 mA at 5 V. The sensor system was successfully tested in the detection of ammonia gas at elevated temperatures.

Synthesis of well-aligned SnO2 nanowires with branches on r-cut sapphire substrate

Well-aligned single crystal SnO2 nanowires were epitaxially grown on r-cut sapphire substrate viaAu-catalyzed vapor–liquid–solid (VLS) growth, and their hetero-epitaxial relationships were determined by pole figure and high resolution transmission electron microscopy (HRTEM). In addition, the growth of branched nanowires by homo-epitaxy was demonstrated in one step without a multiple catalyst deposition.

Lateral epitaxial growth of faceted SnO2 nanowires with self-alignment

Self-aligned, lateral SnO2 nanowires (NWs) were grown on r-cut (012) sapphire substrate by thermal evaporation via a Au-catalyzed vapor–liquid–solid (VSL) mechanism. The lateral SnO2 NWs were hetero-epitaxially grown on r-cut sapphire and the growth direction was [01] of SnO2, which was confirmed by an X-ray pole figure and high-resolution transmission electron microscopy (HRTEM). The SnO2 NWs exhibited sawtooth-like morphology in the length direction, and rectangular shape with round corners in the perpendicular direction. The top surface of the SnO2 NW was (101) and the side surfaces were (020). Consequently, the lateral SnO2 NWs grew in the direction of a large lattice mismatch with the formation of surface facets and coherent interface in the perpendicular direction. In addition, the effects of gas supply direction and the size of Au catalysts on the growth direction and growth mode of the SnO2 NWs were examined.

Hetero-epitaxial growth of vertically-aligned TiO2 nanorods on an m-cut sapphire substrate with an (001) SnO2 buffer layer

Vertically well-aligned rutile TiO2 nanorods were epitaxially grown on an m-cut sapphire substrate by a hydrothermal method with an SnO2 buffer layer deposited by PEALD. Hetero-epitaxial relationships between the TiO2 nanorods, the SnO2 buffer layer and the sapphire substrate were demonstrated by X-ray pole figures and high resolution transmission electron microscopy (HRTEM).