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Atomic Force Microscopy Laboratory #12 MET 231 Laboratory In this lab the basics of atomic force microscopy are presented and techniques developed that are used to solve problems in materials science and engineering. Report Requirements Research Atomic Force Microscopy and prepare a memorandum report. Scanning Probe Microscopy-Background Prior to 1990 almost all high-resolution images were performed with either a scanning electron or transmission electron microscope (SEM/TEM). Both SEM and TEM are considered two-dimensional imaging techniques. Invented in 1981, the scanning tunneling microscope (STM) was able to scan surfaces at atomic scale resolution in all three orientations (x-y-z). STM represents a field of measurements called scanning probe techniques. STM is limited to conducting samples, and must be probed with a conducting tip (more on tips later) The second generation of scanning probe microscopy was ushered in by the advent of the atomic force microscope (AFM) in 1986. The AFM allows high quality three-dimensional images like the STM, but does not require that the sample be conducting-thus analysis of samples such as polymer and biomaterials became possible. Basic AFM Components The basic AFM components are relatively simple. The heart of the AFM is the piezoelectric cylindrical scanner, which moves the system at sub-angstrom motion increments. A very sharp-tipped cantilever is used to either tap or continually touch the sample surface, while scanning. A probe motion unit senses the force between the probe and the sample and provides a correction signal to the piezoelectric scanner to keep the force constant. The most common design uses a laser shining onto and reflecting off the back of the cantilever and onto a segmented photodiode to measure the probe motion. Controller electronics provide interfacing between a computer, the scanning system, and the probe motion sensor. The controller also supplies the voltages that control the piezoelectric scanner, accepts the signal from the probe motion sensor, and contains the feedback control system for keeping the force between sample and tip constant. AFM Uses AFM uses are many and varied. Examples of AFM uses include: Measure magnetic force gradient and distribution above the sample surface using amplitude, phase or frequency shifts (magnetic force microscopy) Measure frictional forces between the probe tip and the sample surface (lateral force microscopy) Measure relative stiffness of surface features (force modulation) Measure surface topography (contact or tapping probe onto surface) Measures variations in surface properties (stiffness, adhesion, etc.) as the phase lag of the cantilever oscillation relative to the piezo drive (phase imaging) Measure hardness by indenting with a diamond tipped cantilever Perform nano-lithography with a high aspect ratio tip Demonstration During the laboratory period basic AFM usage will be demonstrated on a variety of material samples. Two examples of images generated by the AFM method are provided below.
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