Principles of AFM operation
Since its conception by Binnig, Quate, and Gerber in 1986, the Atomic Force Microscope (AFM) has become a powerful tool for surface imaging, force spectroscopy, and biochemical sensing. Although individual applications may vary, the basis for all AFM experiments lies in the instrument's ability to detect minute forces on a thin, microscopic cantilever. This is typically accomplished using the laser beam deflection method. In this method a laser beam is focused onto a cantilever mounted within the AFM head. Because the cantilever has reflective surface, the laser is then reflected off of the cantilever and onto a position sensitive photodetector. As the tip deflects and twists, the motion is detected by the photodetector and the interaction forces and displacements are recorded.
The research conducted by the Clark Group focuses on AFM instrumentation development. Specifically, we focus on developing appropriate control methodologies and signal processing techniques for advanced AFM research. By studying the AFM from a dynamic systems, measurements, and controls approach, we are able to customize the instrument and controller architecture for specific applications thereby increasing the capabilities and sensitivity of the instrument. As part of this research, the Clark group has constructed two custom multi-axis AFMs and numerous software packages to operate the equipment.
AFM used for anodization based nano- fabrication and templating.
AFM used for single molecule force spectroscopy.
AFM Centering Algorithm
In atomic force microscopy based single molecule force spectroscopy (AFM-SMFS); it is assumed that the pulling angle is negligible and that the force applied to the molecule is equivalent to the force measured by the instrument. Recent studies, however, have indicated that the pulling geometry errors can drastically alter the measured force-extension relationship of molecules.
AFM-based force spectroscopy pulling geometries.
In an effort to overcome these pulling geometry errors, we developed a software-based alignment technique that repositions the cantilever such that the molecule is aligned with the measurement axis of the instrument. Alignment is accomplished by subjecting a partially stretched molecule to small, continuous circular movements. By measuring the phase lag between the circling input and the force output, the angular location of the substrate attachment site can be ascertained. The program then moves along this, continuously updating angular path until a stall position is reached, indicating the molecule attachment sites are vertically aligned.
Forces measured with AFM during circling.
Computational and experimental results verify the ability of the program to minimize pulling geometry errors in force-extension measurements. Combined, these results are a promising step towards improving the accuracy of AFM –SMFS studies.
Alignment simulation results. The contour plot was generated via l-phage DNA pulling geometry studies. Each black line represents an alignment trajectory. Even with travel distances of over 1 µm, the alignment program was capable of aligning the cantilever and substrate attachment sites to within 1.5 nm in under 30 seconds.
Experimental alignment results. 58 nm separate the original (blue) and ‘aligned’ (green) cantilever positions. The average force increase due to alignment was ~129 pN during the conformational transition of the dextran conjugate.
Of primary interest is local anodic oxidation (LAO), which is a technique that utilizes the cantilever tip to locally oxidize metallic and semiconducting substrates. All that is required is a bias voltage between the tip and the substrate. Experiments in our lab are underway to quantify and control the current that passes from tip to samples as the process unfolds.
Schematic of the anodization process.
In order to determine the result of nanolithographic experiments, the AFM is raster scanned over an area and the resulting cantilever deflection/twisting motion is recorded. Shown here is an example of a substrate that has been modified using our system and then imaged. A series of arcs and circles have been exactly duplicated from a CAD environment and reproduced at the nanoscale.
(left) The resulting image map of the structure resulting from scanning within the anodization AFM. (right) Image of the same pattern performed on a seperate AFM, the solid scale bar is 1 micron and the height colormap range is 5 nm dark to light.