An Investigation of Spin and Charge Transport in Doped and Defected Graphene
Graphene has proven to be an interesting and exciting material for experimental and theoretical researcher. The unique properties have motivated proposed applications including high frequency transistors, flexible touch screens, sensors, and spin logic gates, all of which will require integration of graphene with other materials. Additionally, exotic theoretical predictions involving magnetic phenomena in doped and defected graphene provide a new area of physics to explore. A central issue for both fundamental and applied physics involves understanding how charge and spin transport properties are affected by the presence of foreign materials or defects, a topic investigated here by intentionally introducing the graphene surface to transition metals, insulators, water, atomic hydrogen and lattice vacancies.
The first portion of this thesis provides the reader with a general introduction to graphene, the field of spintronics, device fabrication, measurement techniques, and sample characterization methods. Included are basic theoretical concepts of charge and spin transport in graphene. Details of the unique ultra high vacuum system (UHV) that combines in situ variable temperature electrical measurement, molecular beam epitaxy, hydrogen doping, and sputtering capabilities are provided.
The second portion discusses experimental results. Firstly, all transition metals (Ti, Fe, Pt, Au) investigated result in ¬n-type doping for sub monolayer coverage. This behavior provides evidence for the presence of a strong interfacial dipole. Secondly, metallic and insulating dopants are directly compared by in situ oxidation of Ti. Monitoring the charge transport properties throughout oxidation provides evidence of short range scattering due to insulating titanium dioxide impurities. Thirdly, the spatial distribution of impurities is found to strongly affect charge transport. Using a fixed amount of gold impurities, the formation of clusters from point like charged impurities reduces the electronic doping and scattering. Fourthly, in regards to spin transport in graphene, exposure to water is found to result in a substantial enhancement of spin signal, providing a simple method by which researchers can improve graphene spin valve device performance. Finally, spin scattering experiments provide direct evidence for the formation of paramagnetic moments in graphene exposed to atomic hydrogen or lattice vacancies, resolving a long standing controversy in the field.