Influences of chemical modification and environment on friction and electrical properties of nanostructures investigated by atomic force microscopy

Abstract

Atomic force microscopy (AFM) is a versatile tool which can observe various properties (e.g., friction, adhesion, surface potential, and conductance) at nanoscale using a short-range interaction between atoms at a sharp AFM tip and the surface of sample. Unlike other microscopes, non-optical AFMs are also capable of precise calibration to have atomic-level resolution. In recent years, many studies have shown remarkable results using AFM. The existence of wrinkles on exfoliated graphene was revealed by AFM measuring periodic friction forces. Furthermore, it is possible to produce electricity rubbing the sample surface with an AFM tip. In this dissertation, Chapter 1 introduces the research background of AFM in surface chemistry. In Chapter 2, vanadium dioxide nanowires fabricated by chemical vapor deposition were hydrogenated prohibiting formation of oxygen vacancies, and the changes in structural and electrical properties were observed. In particular, the Kelvin probe force microscopy (KPFM), one of the modes of AFM, was used to measure the work function and infer the energy band structure. In Chapter 3 and 4 of this thesis, it was shown that UV irradiation and hydrogenation on ultrananocrystalline diamond (UNCD) films causes chemical modification on the surface, and the resulting an enhancement of triboelectric properties. The graphitization and oxidation of the surface by UV/ozone treatment and the increase in work function were observed by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), and the fact that radicals were formed on the surface could be speculated from the contact angle measurements. Furthermore, contact electrification induced hydrogenation produces more hydrocarbons on UNCD resulting in increment of work function and decrease in electrical conductivity. The contact mode of AFM was used to generate triboelectricity, and it was possible to quantitatively analyze the produced tribocharges using a KPFM that the characteristics of triboelectricity were improved by the oxidized surface and radicals. In Chapter 5, the friction characteristics of the surface of UNCD were studied by controlling the gas environment of oxygen, nitrogen, and water from ultra-high vacuum (UHV) to atmospheric pressure through a gas manifold on an UHV-AFM. It was confirmed by AP-XPS that the surface of the UNCD was oxidized in an oxygen condition, and that the frictional force increased by 6 times was confirmed with an ambient pressure AFM (AP-AFM). Nitrogen did not significantly change the surface properties because of its high dissociation energy. In the presence of water, oxidation occurred at initial dosing state, and the friction force increased slightly, whereas when water was injected close to ambient pressure, the friction decreased significantly. When water molecules are present on the UNCD surface, the surface is passivated by dissociated water molecules due to normal pressure at the diamond tip-UNCD interface, thereby reducing friction forces. Based on these studies, AFM has potential for various applications (e.g. nanodevices, energy materials).