Position-controlled growth of ZnO nanostructure array on large-area CVD-graphene for flexible piezoelectric pressure sensors

Abstract

Semiconductor nanostructures fabricated by bottom-up approach have been considered as ideal building blocks for electronic and optoelectronic device applications due to their own easy miniaturization of nanodevices and high crystallinity of the materials. Catalyst-free metal-organic vapor phase epitaxy (MOVPE) growth of ZnO nanostructures demonstrated their high purity and crystallinity with a few crystal defects, thus the method provides the desirable way for nano- and micrometer scale devices. In particular, individually position-controlled and vertically-aligned one-dimensional (1D) nanostructures are desirable for the practical manipulation of these nanodevices. While this can be achieved with a number of specific materials on single-crystalline substrates such as sapphire and silicon wafers, expanding this to more general material/substrate combinations remains challenging due to limitations in growth compatibility. <br/> To overcome the narrow combination of material/substrate, recently, the growth of 1D nanostructures on two-dimensional (2D) semiconductor layers such as graphene and h-BN has been studied as a method to allow the preparation of vertically aligned 1D nanostructures on traditionally incompatible substrates. There, the 2D layers act as a growth buffer layer that can be easily attached to, and also mechanically/chemically detached from, arbitrary substrates. However, while position-controlled and vertically aligned growth has been performed on mechanically exfoliated 2D layers, this approach cannot allow scalability to a wafer area. This thesis presents the position-controlled selective growth of ZnO nanostructures on CVD-graphene films and demonstrated piezoelectric pressure sensor as a representative device application. <br/> This thesis consists of 7 parts. Chapter 1 decribes general introduction and chapter 2 reviews previous research activities for growing 1D nanostructures on graphene and spatial controlling trials. In chapter 3, the experimental set-ups and procedures for growing nanomaterials and device fabrication using them are described. Chapter 4 describes two different growths of wide bandgap semiconductor nanostructures on graphene films and their characteristics. Chapter 5 propose a way to controlling spatial position and shape of individual ZnO nanostructures on CVD-graphene films. Using the nanostructures grown in a controlled manner, Chapter 6 describes a preparation method for flexible nanomaterials and demonstration of piezoelectric pressure sensors as a representative application. Finally, chapter 7 summarizes this thesis with a suggestion for future works.<br/>