Development of self-assembled ZnO nanostructures in diblock copolymers on large area Si wafers and gas sensor applications

Abstract

ZnO nanoparticles with improved optical properties and increased surface areas have the potential for advanced optoelectronic, gas sensor and biosensor applications. In order to exploit these unique properties of ZnO nanoparticles for the realization of nanoscale devices, we developed novel techniques for the self-assembly and functionalization of ZnO nanoparticles through diblock copolymers on large area (100)Si surfaces. These novel techniques allowed us to subsequently develop the first ZnO nanoparticle based device. Thus, a novel ZnO-nanocomposite/Si n-p heterojunction diode and a high performance hydrogen gas nanosensor have been developed, for the first time.

The thesis presents the novel technique developed for the self-assembly of ZnO nanostructures with spherical morphology through diblock copolymers on large area Si substrates. Correlation between the physical parameters of the nanoparticles and the copolymers was evaluated from AFM studies. Control of the nanoparticle size and density was achieved by varying copolymer block lengths. The largest nanoparticles had average sizes of 250 nm and densities of 1x10⁷cm^-2 while the smallest nanoparticles had average sizes of 20nm and densities of 1x10¹⁰10cm^-2. XRD studies showed that the wurtzite crystal structured nanoparticles assumed the same orientation (100) as the Si substrate, indicating a pseudo-epitaxial nanostructure. Room temperature photoluminescence studies showed quantum confinement effects with a blue shift from 372 nm (large particles) to 363 nm (small particles). A broad defect related green-yellow luminescence centered at 555 nm indicative of n-type conductivity of the nanoparticles was also observed.

The n-type nanoparticles on p-type Si resulted in the development of a ZnO-nanocomposite/pSi n-p heterojunction diode for the first time. The nanodiode showed good rectification and low leakage currents. LogI-V characteristics gave built-in voltages of 0.69 and 0.7 eV, saturation currents of 2 and 2.34 x10^-8A, and ideality factors of 5.9 and 5.7 for the small and large particles, respectively. The transport mechanisms of the nano-diodes were studied. C-V characteristics showed abrupt p-n junctions, suggesting an intimate junction interface consistent with the pseudo-epitaxial nature of the structure. A novel hydrogen gas nanosensor based on the ZnO-nanocomposite/Si heterojunction diode was developed for high sensitivity, rapid, room temperature sensing. Response and recovery times were reduced by a factor of 100 and smaller and denser nanoparticles were found to be faster and more sensitive.