NICKEL-GRAPHENE-BASED SYSTEMS AS FIELD EMITTERS

Abstract

The aim of this doctoral research is to synthesize and apply graphene-based hybrid structures for field emission or cold cathode emission. Graphene is one atomic layer thick carbon that forms an sp2 hybridized hexagonal structure. Graphene has attracted immense attention in the last two decades due to its unique properties. It possesses excellent mechanical, electrical, optical, thermal, and electronic mobility properties. As a 2D material, it has been used in many applications and has been projected for many advanced applications. 2D materials deposited or grown on flat surfaces are considered inefficient field emitters. The flat surface hinders the electron tunnelling through the surface. In order to achieve attractive field emission properties, either graphene has to be placed on a nanostructure surface or surface treatment of the graphene is required, or a composite with other nanostructure materials has to be prepared. These processes break the continuous structure of graphene and make sharp protrusions at the surface of the graphene. These sharp protrusions or graphene placed on a nanostructure surface modify its surface topography and act as field emission sites. Several methods can produce graphene; among these methods, some processes give a bulk quantity of graphene that compromise the quality, such as chemical exfoliation and liquid-phase exfoliation. Some graphene synthesis methods, such as mechanical exfoliation, produce good quality of graphene, but it is hard to scale them up to the industrial production level. Electrical conductivity is not an important parameter for applications such as supercapacitors, structural composites, etc., but the high surface area is. These applications can use graphene produced through chemical exfoliation or liquid-phase exfoliation. In electronic device applications, high-quality graphene is required. The challenge is the production of large-area, defect-free graphene on a large scale. Chemical synthesis of graphene was generally by oxidation of graphite to form graphite oxide, followed by sonication to produce graphene oxide (GO). It is a method for the mass production of graphene-based material. Well-known methods for producing GO by liquid-phase exfoliation are Staudenmaier's', Modified Hummers,' and Tour's Methods. Oxidation of graphite introduces a significant number of oxygen-containing functional groups attached to the basal planes and edge. The graphite becomes an insulator during oxidation as part of the planar sp2-hybridized geometry transfers to disordered sp3-hybridized geometry, which loses many excellent properties, especially those related to charge movement. The reduction and doping are performed on GO to achieve good electrical conductivity. The main goal of this study was to synthesize graphene by the Hummers method and CVD technique on textured Nickel (Ni)-Copper (Cu) alloys and use these graphenes as graphene-nickel-based hybrid systems in field emission. NiO was synthesized directly on the Ni-foil or Ni-foam by wet chemical or hydrothermal method to achieve this goal. This NiO-containing Ni-foil or Ni-foam surface was used for making NiO-GO/rGO/CNT/graphene hybrid. Nickel oxide (NiO) is a p-type wide bandgap semiconductor. Many synthesis methods are available to produce NiO nanostructures. Here, NiO nanostructures were grown on Ni foil using a simple wet chemical method. The NiO nanostructure does not show field emission responses or has low field emission responses. So graphene was placed on 1D/2D nanostructure surfaces to make a hybrid to enhance field emission properties. NiO nanoparticles were coated with graphene oxide (GO) by dip-coating technique, graphene was transferred on nanostructure substrates, and carbon nanotubes (CNTs) were grown by the chemical vapor deposition (CVD) method. The combination of 1D/2D and 2D/2D structures was used for enhanced field emission application. In this doctoral research work, attempts were also made to synthesize and optimize the graphene growth on the Ni-Cu alloy substrate. In the CVD method, the carbonaceous precursor material is dehydrogenated or dissociated on top of the catalytic surface and forms graphene during cooling. During the process, graphene is deposited on the catalytic substrate surface. So, the nature of the substrate, i.e., carbon solubility, crystal structure, roughness, grain size, and texture, also plays a crucial role in graphene synthesis. Graphene is hexagonal crystalline in nature. In order to synthesize on top of FCC metal and alloy, FCC (111) is preferred due to the close crystallographic relationship between (111) and (0001). That forms a perfect epitaxy between the substrate and graphene. Single-crystalline FCC metal surfaces are the most preferable, but single-crystalline substrates are costly. So, the easier way is to control the texture of the substrate surface, i.e., most of the grain will be in the particular direction that can help to get better quality and better graphene domain size. An easier way to develop texture in metal and alloys is deformation or deformation followed by annealing. The graphene was synthesized on the Ni-Cu substrates, and the texture was studied before and after the graphene synthesis. The number of layers of graphene and quality were correlated with the substrate texture. Details of this doctoral thesis have been described in 7 different chapters. The first chapter is "Introduction," which delivers the background and theories of the current research work. It briefs about the advantages of graphene and its synthesis methods. Notably, this chapter establishes the motivations for selecting graphene for field emission application. It also provides the aim of the research and the objectives of the thesis. The 2nd chapter of the thesis, "Literature Review," depicts the currently available literature on graphene-based field emitters and NiO-based field emitters, texture study of the Ni and Ni-Cu alloys, and graphene synthesis by CVD method on Ni-Cu substrate, orientation and compositional effect on the graphene quality. The 3rd chapter, "Materials and Methods," details procured materials used for synthesis. The detailed synthesis methods and schemes adopted for synthesis methods include the synthesis of GO, rGO, NiO nanostructures, synthesis of CNT, and graphene. The characterization technique used for studying samples, such as XRD, FESEM, Raman spectroscopy, XPS, UPS, pole figure, EBSD, field emission, nano-scratch, etc., are also discussed in this chapter. The 4th chapter, entitled "NiO-GO/rGO nanostructural hybrid," presents the field emission of these hybrid structures. The 5th chapter is about the texture study of "Ni-Cu alloys at different degrees of reduction and different processes of reduction, annealing, and graphene growth." The graphene synthesis by the CVD method on textured Ni-Cu alloys and the study of the graphene quality in relation to substrate texture are included in this chapter. The 6th chapter compares optical, electrical, and field emission responses of GO, rGO, and CVD-grown graphene. Chapter 7th summarizes the main conclusions of the results of this research. The 8th chapter overviews the future scope of the research work presented. The findings of this thesis indicate the potential of Ni-graphene-based hybrids for field emission applications