Effects of hydrogen plasma treatment on the electronic, optical, mechanical and chemical properties of Mo, Rh, Au, HOPG and Graphene

Interaction of ionic and atomic species of hydrogen isotopes (protium, deuterium and tritium) with transition metals and carbon materials is an important subject of research in the condensed matter physics and material science fields. Other than the fundamental aspects of physics, the topic also has a special importance in magnetic confinement fusion and nanoelectronics applications. With the aim of providing some insight to this topic, this thesis reports experimental investigations on the electronic, optical, mechanical and chemical properties of several material systems including molybdenum/deuterium, rhodium/deuterium, gold/protium and hydrogenated layered carbon. Introduction of hydrogen into initially pure metals and onto layered carbon materials is achieved by plasma treatment.
The next generation fusion reactor (ITER) will include plasma facing, electromagnetic radiation reflecting components which will be made of either molybdenum or rhodium. Hydrogen in transition metals may act as an electron donor or an acceptor, changing the electronic band structure of its host. It can also result in increased number of lattice distortions, defect sites and inelastic scattering events reducing the optical transitions. Both directly imply that optical properties of these metals are changed. In this thesis, it is shown that deuterium acts as an electron acceptor in molybdenum, but an electron donor in rhodium. Both cases are investigated with various experimental techniques including photoelectron spectroscopy, spectroscopic ellipsometry, spectroscopic reflectometry, spectrophotometry, specific resistivity and direct surface morphology imaging techniques. Rhodium/deuterium system is not stable in air due to a catalytic reaction between deuterium and oxygen, whereas molybdenum/deuterium system is stable because deuterium is strongly bound to defect sites. In the case of molybdenum, the research is extended to investigations on the partial delamination of the coated films and kinetic roughening of the surface caused by high ion flux. Further research on thin film buckling was performed using gold coatings, where intense partial delamination from the substrate was observed even after exposure to protium plasma with a low ion flux. This mechanical deformation is attributed to high compressive stress exerted on the gold films as a result dynamic protium inventory exceeding the protium solubility of the film.
The wall materials of some of the fusion reactors currently in operation consist of graphite. Therefore, interactions of graphite with hydrogen plasma have been investigated extensively in the fusion community. A recent interest is utilizing hydrogen plasma in milder conditions for chemical modifications of graphite and graphene without chemical or physical sputtering. Hydrogenated graphite and graphene were also investigated in this thesis. It is shown with scanning probe techniques, photoelectron spectroscopy and Raman spectroscopy that hydrogenation of graphite changes its surface corrugation, valence band structure, surface electron density and vibrational modes. Moreover, it is shown that hydrogenation can be achieved locally and work function changes of the graphene surface can be mapped with Kelvin probe force microscopy.
The outcomes of the thesis are aimed to aid the fusion community in terms of material choice for the light reflecting components considered to be used in the new generation reactors, as well as the carbon community in terms of helping the comprehension of properties of hydrogenated graphene.