Preparation of graphene and graphene-analogue two-dimensional nanomaterials and their applications in electronics

Abstract

Two-dimensional (2D) nanomaterials have regained worldwide attention over the past decade since the discovery of graphene in 2004. Tremendous efforts have been devoted to the synthesis and applications of 2D nanomaterials due to their extraordinary and diverse properties in electronics, photonics, catalysis, etc. upon the exfoliation from their bulk counterparts. Regarding the production of 2D nanomaterials, one of the great challenges we have to deal with is how to produce high-quality 2D nanomaterials in a reliable and scale-up way. Although various synthetic strategies have been developed including mechanical exfoliation, chemical vapor deposition (CVD), chemical exfoliation, liquid-phase exfoliation and so on, they all exhibit some sort of disadvantages. Moreover, numerous promising applications, such as energy storage and conversion, electronic devices, catalysis and the like, have been demonstrated due to their diverse properties. Among these applications, we're quite concerned about the photovoltaic (PV) devices, which can convert the solar energy for human beings to use. Currently there are some issues that gradually restrict the rapid development of PV technology. For example, the widely used ITO electrode is becoming a problem due to the scarce reserve, rigid nature and vulnerability to bending test. In this context, this thesis introduces two approaches to produce 2D nanomaterials and investigate their applications in electronics. One is the liquid-based exfoliation method named salt-assisted direct exfoliation, in which the presence of inorganic salt can facilitate the exfoliation of bulk-layered materials in liquid media. This method can produce high-quality, aqueous-dispersible, single-and few-layered 2D nanomaterials without using any oxidants or long time sonication. The as-produced 2D nanosheets including graphene and TMDs can be easily fabricated into thin films via solution processable manners and be applied into photovoltaic devices either as the transparent electrodes or the buffer layers. Another method is CVD that can produce electronic grade graphene sheets with high carrier mobility, which are applied to field-effect transistors. Firstly, this thesis details the background and challenges we are confronted with and then puts forward the research objectives and significance. Secondly, a comprehensive literature review is presented including the preparation approaches developed till now to produce 2D nanomaterials, 2D nanomaterialsbased composites and their applications in PV aspects. Then the general methodologies involved are introduced. In chapter 4, a salt-assisted direct exfoliation method is first developed and as-produced graphene nanosheets are characterized. Conductive and transparent graphene thin films with various thicknesses are fabricated by vacuum filtration method. Chapter 5 introduces the graphene hybrid thin films with enhanced conductivity by the combination with 1D silver nanowires. This hybrid films based on as-produced graphene nanosheets can be employed as the electrode in perovskite solar cells. Chapter 6 extends the salt-assisted exfoliation method to prepare 2D graphene-analogue transition metal dichalcogenide nanomaterials. Single-and few-layered 2D nanosheets are obtained and can readily disperse in aqueous solutions. And MoS₂ nanosheets are solution processed into thin films and integrated into organic solar cells as a hole transport layer. Chapter 7 develops a CVD method to synthesize high-quality monolayer graphene and investigates the electrical properties of graphene FET devices with functionalization of polymer brushes and subsequent immobilization of biomolecules. Finally, chapter 8 concludes this thesis and provides future perspectives.