Daejong Yang

Focused Energy Field Method for the Localized Synthesis and Direct Integration of 1D Nanomaterials on Microelectronic Devices

In the focused energy field method, localized heating, and convective mass transfer in a liquid precursor realizes selective synthesis and direct integration of 1D nanomaterials as well as their surface functionalization, all in a low-temperature, liquid environment. This allows facile fabrication of 1D nanomaterial-based nanoelectronic devices.

Multiplexed gas sensor based on heterogeneous metal oxide nanomaterial array enabled by localized liquid-phase reaction.

A novel method for the selective and localized synthesis of nanomaterials and their in situ integration based on serial combination of localized liquid-phase reaction has been developed for the fabrication of heterogeneous nanomaterial array. This method provides simple, fast and cost-effective fabrication process by using well-controlled thermal energy and therefore solves the challenging problems of assembly and integration of heterogeneous nanomaterial array in functional microelectronic devices. We have fabricated a parallel array of TiO2 nanotubes, CuO nanospikes, and ZnO nanowires, which exhibited adequate gas sensing response. Furthermore, we could approximately determine individual gas concentrations in a mixture gas consisting of 0-2 ppm of NO2 and 0-800 ppm of CO gas species by analyzing multiple data from an array of heterogeneous sensing nanomaterials.

Fabrication of heterogeneous nanomaterial array by programmable heating and chemical supply within microfluidic platform towards multiplexed gas sensing application

A facile top-down/bottom-up hybrid nanofabrication process based on programmable temperature control and parallel chemical supply within microfluidic platform has been developed for the all liquid-phase synthesis of heterogeneous nanomaterial arrays. The synthesized materials and locations can be controlled by local heating with integrated microheaters and guided liquid chemical flow within microfluidic platform. As proofs-of-concept, we have demonstrated the synthesis of two types of nanomaterial arrays: (i) parallel array of TiO2 nanotubes, CuO nanospikes and ZnO nanowires, and (ii) parallel array of ZnO nanowire/CuO nanospike hybrid nanostructures, CuO nanospikes and ZnO nanowires. The laminar flow with negligible ionic diffusion between different precursor solutions as well as localized heating was verified by numerical calculation and experimental result of nanomaterial array synthesis. The devices made of heterogeneous nanomaterial array were utilized as a multiplexed sensor for toxic gases such as NO2 and CO. This method would be very useful for the facile fabrication of functional nanodevices based on highly integrated arrays of heterogeneous nanomaterials.

Localized Liquid-Phase Synthesis of Porous SnO2 Nanotubes on MEMS Platform for Low-Power, High Performance Gas Sensors

We have developed highly sensitive, low-power gas sensors through the novel integration method of porous SnO2 nanotubes (NTs) on a micro-electro-mechanical-systems (MEMS) platform. As a template material, ZnO nanowires (NWs) were directly synthesized on beam-shaped, suspended microheaters through an in situ localized hydrothermal reaction induced by local thermal energy around the Joule-heated area. Also, the liquid-phase deposition process enabled the formation of a porous SnO2 thin film on the surface of ZnO NWs and simultaneous etching of the ZnO core, eventually to generate porous SnO2 NTs. Because of the localized synthesis of SnO2 NTs on the suspended microheater, very low power for the gas sensor operation (<6 mW) has been realized. Moreover, the sensing performance (e.g., sensitivity and response time) of synthesized SnO2 NTs was dramatically enhanced compared to that of ZnO NWs. In addition, the sensing performance was further improved by forming SnO2-ZnO hybrid nanostructures due to the heterojunction effect.

Microfabricated and Nanoengineered Chemical Sensors for Air Quality Monitoring System

The importance of air quality monitoring is rapidly increasing. Although state-of-the-art air quality monitoring systems based on sophisticated optical systems or gas chromatography provide high accuracy measurement of air quality parameters, they cannot provide highly portable and/or personalized platform due to high cost, difficult maintenance and poor portability. With the advent of mobile electronic systems such as smartphones or wearable gadgets, people are more interested in obtaining personalized and highly localized environmental information rather than averaged and global information. To meet this need, highly integrated, ultra-compact, and low-power gas sensors that can be put into small electronic systems should be developed. The best approach to enable this is to use microfabricated (i.e., MEMS) sensing platform and nanoengineered sensing materials. In this chapter, we review the design and fabrication MEMS-based gas sensors and their applications to portable air quality monitoring. In addition, the principles, designs and the usage of functional nanomaterials to highly sensitive, highly selective and quickly responding air quality sensors are explained. Finally, we explain our recent development on the controlled synthesis of nanomaterials on microfabricated platform and its application to advanced gas sensing devices.

8.4.4 Enhanced H2S Sensing Properties of Porous SnO2 Nanofibers Modified with CuO

Porous SnO2 nanofibers were fabricated by electrospinning and oxygen plasma treatment. Subsequently, CuO was sputtered on the porous SnO2 fibers as the modifier. H2S sensing properties of the fibers were tested and the effect of CuO amount on the response and recovery characteristics to H2S was investigated. It was found that the response is strongly dependent on the CuO amount and the response time and recovery time become longer with the increase of the CuO amount. The highest response of the fibers to 2 ppm H2S attains 1.9×10 3 at the operating temperature of 100� with the optimum CuO thickness of 24 nm, and the fibers show good selectivity to H2S.