Noncontact atomic force microscopy II

In order to visualize the atomic structure of materials in real space, a microscope with sub-nanometer resolution is needed. As such, breaking the resolution limit associated with the wavelength of visible light employed in traditional optical microscopy has been a long-standing dream of scientists around the world. This goal was finally reached in the early 1980s with the invention of the scanning tunneling microscope (STM). While it is possible to obtain atomic-resolution images of material surfaces by using STM with relative ease, its basic operational principle depends on the phenomenon of quantum tunneling, rendering the technique applicable only to conductive and semi-conductive samples. The atomic force microscope (AFM), which was invented only a few years after the introduction of the STM, overcame this fundamental limitation and was used with great success to image a number of sample surfaces with nanometer resolution without limitations associated with electrical conductivity. However, unlike the STM, the operation of the AFM in its traditional form requires the establishment of a permanent – albeit light – contact between the probe tip and the sample surface, leading to a finite contact area, which prevents true atomic-resolution imaging. True atomic resolution imaging through AFM was finally achieved in 1994 with the invention of noncontact atomic force microscopy (NC-AFM). The basic idea behind NC-AFM is based on the detection of minor changes in the resonance frequency of a micro-machined cantilever carrying a sharp probe tip due to attractive force interactions while it is oscillated above the sample surface to be investigated. Since actual contact with the sample is avoided, the probe tip retains its sharpness and atomic-resolution images may be obtained. Since its introduction two decades ago, NC-AFM has indeed been used to image a large number of conducting, semi-conducting, and insulating material surfaces of technological and scientific importance with atomic resolution, thus contributing to nano-scale science in a major way with each passing year. The capabilities of NC-AFM are not only limited to atomic-resolution imaging: Force spectroscopy allows characterization of inter-atomic forces with unprecedented resolution in three spatial dimensions, while manipulation experiments at both low temperatures and room temperature have demonstrated the capability of the technique to controllably construct atomic-scale structures on surfaces.