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"Hexagonal Boron Nitride Nanowalls: Physical Vapour Deposition, Structural and Optical Properties."

Since the discovery of graphene1, the two-dimensional (2D) honeycomb crystal receives a renewed attention with a particular focus on nanostructures due to its exceptional properties and potential application in materials science. Besides single layer graphene, a nanowall is described as a 2D nanostructure which typically has a platelet-like structure that rises upwards from the surface in a vertically uniform direction.

Like carbon nanomaterials, boron nitride (BN) has attracted overwhelming attention owing to its structural similarity. Such as carbon, BN exists in sp2-bonded form, i.e. hexagonal BN (h-BN) or in sp3-bonded type, namely cubic BN c-BN). A layered h-BN structure has excellent physical properties analogous to graphite, being in-plane mechanically strong and showing high chemical and thermal stability2-4. Recently, h-BN has shown to be an alternative substrate for graphene deposition.

The objectives of this thesis are strengthening the efforts that are going on to investigate high quality h-BN thin film deposition for various applications and in particular to the growth of 2D h-BN nanowalls. In this work, the use of physical vapor deposition techniques is proposed. Although there are few reports on the synthesis of h-BN films by the CVD technique, this work is based on the easy to handle, scalable and cost effective reactive radio-frequency magnetron sputtering.

The first chapter provides primarily background information on the BN material. This chapter describes the synthesis of different BN allotropes with a focus on h-BN as a thin film and the mechanism responsible for its formation. Among several growth techniques for h-BN thin films, I will introduce the unbalanced magnetron sputtering as a physical vapor deposition technique used during these four years of investigations at the Institute for Materials Research (IMO), Hasselt University. I will discuss the influence of the deposition parameters.

In chapter 2, a homemade RF unbalanced magnetron sputtering system will be completely described (MS-PVD). Different characterization techniques will be presented in two distinct sections. The spectroscopy techniques and the microscopic imaging that enable to identify the h-BN features, will be further detailed throughout this chapter.

In chapter 3, optimized parameters to control the MS-PVD setup will be briefly introduced. I will focus on technical enhancements and deposition conditions for the growth of good quality, reproducible and homogenous h-BN films. In this chapter I will introduce as well the deposition parameters responsible for a specific structure identified as 2D h-BN "nanowalls"(h-BN NWs).

Chapter 4 deals with the optical properties of h-BN (NWs) films. I will present different methods based on infrared techniques and micro-Raman spectroscopy to assess the structural properties of these nanowalls films. The measurements using the UV-Vis-NIR spectrophotometer will allow me to evaluate the quality of my h-BN layers. The cathodoluminescence technique will be used as well in order to highlight the nature of the h-BN (NWs) excitons. Two different optical techniques will be used for measurements to confirm the porosity of h-BN (NWs) films.

Chapter 5 will describe some possible future applications of the h-BN (NWs) films. The super-repellent behavior of the h-BN (NWs) will be investigated using a contact angle measuring device. I propose a method of fabrication and characterization of h-BN (NWs) flakes in order to use them as a template for graphene and DNA-attachment. An experimental approach assessing the magnetic properties of the h-BN (NWs) will be briefly discussed. Finally, I will end with a short overview of the main conclusions and achievements that are presented in this thesis. Some guidelines are given for the further improvements and suggestions are made for future research in this promising field.