Structural investigations of hydrogenated epitaxial graphene grown on SiC(0001) are presented. It is shown that hydrogen plays a dual role. In addition to contributing to the well-known removal of the buffer layer, it goes between the graphene planes, resulting in an increase of the interlayer spacing to 3.6 Å–3.8 Å. It is explained by the intercalation of molecular hydrogen between carbon planes, which is followed by H2 dissociation, resulting in negatively charged hydrogen atoms trapped between the graphene layers, with some addition of covalent bonding to carbon atoms. Negatively charged hydrogen may be responsible for p-doping observed in hydrogenated multilayer graphene.

Van der Waals heterostructures based on hexagonal boron nitride (h-BN) and other 2D materials may pave the way for future electronic applications. Wafer-scale uniform h-BN substrates are a must in this respect. In this work, we demonstrate a new growth regime which allows for scalable, uniform synthesis of high quality h-BN layers on 2’ sapphire substrates. We propose a new approach to metal organic vapour phase epitaxy of h-BN layers on sapphire substrates. The growth scheme involves an intermediary BN buffer layer grown under self-limiting conditions (continuous flow) followed by the final growth of h-BN with flow modulated epitaxy in one growth run. This scheme can be regarded as homoepitaxial growth of h-BN on a self-limiting buffer. Our studies show that the buffer layer allows to control the nucleation at the crucial early stages of BN layer growth, suppressing unwanted out-of-plane growth. It can also be used to control the density of point-like defects responsible for unwanted luminescence from the h-BN layer. Moreover, our results show that the buffer effectively suppresses the creation of amorphous BN at the sapphire/h-BN interface.

This work describes an oxidation process of iron-iron oxide core-shell nanowires at temperatures between 100 °C and 800 °C. The studied nanomaterial was synthesized through a simple chemical reduction of iron trichloride in an external magnetic field under a constant flow of argon. The electron microscopy investigations allowed determining that the as-prepared nanowires were composed of self-assembled iron nanoparticles which were covered by a 3 nm thick oxide shell and separated from each other by a thin interface layer. Both these layers exhibited an amorphous or highly-disordered character which was traced by means of transmission electron microscopy and Mössbauer spectroscopy. The thermal oxidation was carried out under a constant flow of argon which contained the traces of oxygen. The first stage of process was related to slow transformations of amorphous Fe and amorphous iron oxides into crystalline phases and disappearance of interfaces between iron nanoparticles forming the studied nanomaterial (range: 25-300 °C). After that, the crystalline iron core and iron oxide shell became oxidized and signals for different compositions of iron oxide sheath were observed (range: 300-800 °C) using X-ray diffraction, Raman spectroscopy and Mössbauer spectroscopy. According to the thermal gravimetric analysis, the nanowires heated up to 800 °C under argon atmosphere gained 37% of mass with respect to their initial weight. The structure of the studied nanomaterial oxidized at 800 °C was mainly composed of α-Fe2O3 (∼ 93%). Moreover, iron nanowires treated above 600 °C lost their wire-like shape due to their shrinkage and collapse caused by the void coalescence.

No abstract