Atomic force microscopy
Atomic force microscopy (AFM) is a technique which maps the surface of a material using a microfabricated cantilever as a scanning probe. A piezoelectric scanner is used to create precision nanoscale displacements of the sample and cantilever. A laser beam, typically a red or infrared laser, is used to monitor the cantilever movement as the cantilever and sample are scanned relative to each other. Laser light is reflected off the flexible, free end of the cantilever, onto a position-sensitive photodetector. The feedback system is used to maintain the cantilever’s vertical motion at a set point value, permitting the acquisition of surface topography during scanning. The sensitivity afforded by this technique provides imaging resolution at the nanometer scale. As well as the resolution this technique allows, the samples may be analyzed in both dried and hydrated conditions, and therefore offers a significant advantage over techniques such as histology and EM. Sample preparation for AFM is mostly associated with reducing the possibility for mechanical motion. For biomineralization, this technology can potentially capture the different phases of mineralization over time (Habraken et al., 2013; Hamm et al., 2014). A variation in this technique is dynamic force microscopy, whereby the cantilever tip can be functionalized to interact with the sample surface and provide force and chemical bond information. Collectively these data may provide information on chemical moieties and functional groups present on the sample and how they change over time.
Atom probe tomography
Atom probe tomography (APT) requires samples to be milled to a sharp point before using a pulsed laser to induce atom evaporation, and therefore sample preparation is extensive. However, the imaging resolution is less than 1 nm and the technique offers atomic level information to be obtained (Miller et al., 2012). Other than sample preparation, the main limitations of this technique are (1) the small size of sample that can be analyzed and (2) that the technique is destructive. APT utilizes a position-sensitive detector to reconstruct the location of atoms, affording 3-D structural information. Apatites (Gordon et al., 2012) and the organic/inorganic interface (Gordon and Joester, 2011) are common topics for study using APT.