UC Berkeley

Nanostructures have unique properties that will be leveraged for next generation sensor and energy applications: large surface-to-volume ratios, increased sensitivity at lower input power levels, and higher current densities all contribute to increased functionality of nanostructure devices. The synthesis and integration of carbon-, metal-, and metal oxide-based nanostructures for sensor and energy applications are studied with the goal of nano-to-micro devices utilizing well-established micro- and nano-fabrication tools and techniques: (1) self-assembled, integrated carbon nanotube gas/pressure sensors; (2) titanium dioxide (TiO₂) nanostructures for energy applications; and (3) silver nanowires and graphene for biological and optical sensing platforms. Suspended carbon nanotubes are grown by in situ heating of microstructures, self-assembled and integrated directly for electrothermal gas/pressure sensing. Titanium dioxide nanoswords, synthesized by induction heating, exhibit large areas of crystalline faces demonstrating enhanced photocatalytic activity. TiO₂ nanosword synthesis is scaled for bulk growth and combined with microfabrication techniques to study solar absorption under visible light irradiation for photoelectrochemical cell applications. Silver nanowires are synthesized using oblique angle deposition (OAD), allowing for control over nanowire porosity and morphology based on substrate deposition angle. Graphene devices are fabricated using nanoimprint lithography (NIL) techniques.

Suspended carbon nanotubes (CNTs) are grown by in situ heating of microstructures, allowing for the placement of CNTs at specific locations without conventional global furnace heating during the synthesis process. These CNT devices are self-assembled and the integration process monitored via electrical feedback. Electrothermal pressure sensing with selective gas and pressure sensing applications has been demonstrated. Furthermore, control and improvement of the self-assembled contact resistances of nano-to-micro interfaces have been investigated by means of local annealing via Joule heating.

Titanium dioxide nanoswords have been synthesized using a rapid induction heating technique. These rutile-phase, single-crystalline nanostructures, typically 40-90 nanometers in thickness, 200-1000 nanometers in width, and up to tens of micrometers in length, exhibit large areas of exposed crystalline faces that show an enhanced photocatalytic reaction. Scaling up the synthesis method for bulk growth and ease of integration with existing microfabrication techniques has been studied for enhanced solar absorption of the titania nanostructures for possible large-scale water splitting applications under direct sunlight.

In the third area of this dissertation, silver nanowires arrays were synthesized using oblique-angle deposition (OAD). Based on the angle of metal deposition, the thin-film porosity and nanowire morphology can be controlled. These nanostructure substrates have been used as biological test platforms for nanotoxicology studies, with trends indicating cell viability decreased as the thin film porosity increased. Finally, experimental results of graphene nanopatterning with nanoimprint lithography (NIL) for electrical and optical devices are discussed. Chemical vapor deposition (CVD) graphene is transferred onto silicon oxide substrates and patterned using different NIL mold configurations.