Harmonic force microscope: A new tool for biomolecular identification and material characterization based on nanomechanical measurements.

摘要

At the molecular level, physical and chemical properties of materials are tightly coupled to the mechanical properties. The potential of mechanics for interacting with matter at the nanoscale has been largely unexplored due to lack of instruments capable of performing mechanical measurements at nanometer length scales.; This thesis describes nanomechanical sensing techniques and applications based on time-resolved tip-sample force measurements in tapping-mode atomic force microscopy. Tapping mode is the most successful operation mode of atomic force microscopes. Theoretical calculations presented in the first part of this thesis show that time variations of the tip-sample forces in the tapping-mode depend on the physical and chemical properties of the sample and therefore, have the potential to be used for nanomechanical measurements. Unfortunately, the force-sensing probe of the tapping-mode atomic force microscope, the vibrating cantilever, is limited in its response to the variations of forces in time within a period of oscillations. We are describing two types of special micromachined cantilevers that enable measurements of time variations of tip-sample forces: the harmonic cantilever and the coupled torsional cantilever. These special cantilevers allow sensitive mechanical measurements at the nanoscale and single molecular level. The operation of these cantilevers does not require any modifications to the existing atomic force microscopy systems.; With the nanomechanical sensing techniques we have developed, we investigated phase transformations of sub-micron domains of composite polymers and observed their glass transitions for the first time. Conventional measurements on bulk properties of these samples do not provide information on the physical changes at the nanoscale. Studies on nucleic acids attached to a surface, a configuration commonly used in DNA microarray technology, showed that the hybridized DNA molecules can be detected at the single molecule level with unprecedented sensitivity. Conventional techniques mainly use electrical or optical techniques and they are limited by their spatial resolution and sensitivity.; By providing data on chemistry and mechanics with high sensitivity and nanometer spatial resolution, time-resolved tip-sample force measurements in tapping-mode atomic force microscope has the potential to enable a new class of biological and chemical sensors and instruments for materials design and characterization.