Developing alternating current scanning tunneling microscopy and atomic force microscopy to measure thin film properties on the nanoscale

An alternating current scanning tunneling microscope (AC-STM) has been designed and built for the study of insulating thin films on the nanometer scale. In order to achieve the detailed resolution of STM on insulating surfaces, the AC-STM uses a high frequency alternating current bias voltage across the tip-sample junction to produce an AC signal. The inherent nonlinearity of an STM junction generates higher harmonics. The AC-STM uses the third harmonic of the applied bias frequency as the measured quantity for imaging and control. By using this AC signal as the measured quantity to control the tip-sample distance, STM quality images have been produced without regard to the conductivity of the surface.;Several experiments have been completed using the AC-STM with both DC and AC feedback control signals. By comparing three sulfides (CuS, MoS 2, and PbS) and H-Si(111), the third harmonic signal generation mechanism and ultimate resolution of the technique have been explored. A more in depth analysis covers the application of the AC-STM to investigate the tip-induced surface oxidation of natural PbS. With the AC-STM, the oxidation of PbS has been imaged in real time as the surface was covered by an insulating overlayer.;In addition, atomic force microscopy (AFM) techniques have been used to probe interfacial properties. AFM cantilevers have been functionalized using different silanes to produce hydrophobic tips. These tips have been used to study the effect of humidity on capillary forces at different surfaces. The measurements indicate that pull-off force discontinuities are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in the relative humidity. The origin of the adhesive interactions as a function of relative humidity is critically discussed.;Logarithmic relationships have been observed between the friction and the scanning velocity both in dry contacts and in lubricated environments. Solid-like interfacially confined boundary layers are found to be responsible for the unexpected logarithmic rate behavior of Newtonian-like bulk liquids. Non-Newtonian properties are induced in an interfacial boundary layer of more than 2 nm thickness in n-hexadecane and OMCTS as a result of a solid-surface induced entropic cooling effect.