Scanning Probe Microscopy (SPM)

In modern microscopy, spatial and spectral resolutions are of great importance in tackling questions related to material properties. For example, the study of the physical and compositional properties of materials at the nanoscale is increasingly demanding due to the level of complexity of systems such as biological cells and fabricated composite nanostructures. The emergence of methods based on mechanical interactions such as the atomic force microscopy (AFM), which by far surpasses what can be achieved optically due to the inherent diffraction limit, has opened numerous research opportunities for investigating surfaces across the full spectrum of scientific disciplines. However, a contemporary challenge in nanoscience is the non-destructive nanoscale characterization of soft materials and their subsurfaces. The ability to noninvasively explore subsurface as well as surface domains for the presence of material inhomogeneities or internal structures is of tremendous importance. In addition, non-destructive, nanoscale characterization techniques that provide both physical and chemical information are needed in order to reach a comprehensive understanding of the composition and behavior of complex systems such as the plant cell walls in bioenergy research, or the response of cells to nanoparticles exposure, which is an important problem in nanotoxicology.

In order to explore the subsurface physical and spectral imaging, a new concept of Mode Synthesizing Atomic Force Microscopy (MSAFM) has been developed. MSAFM is a technique which capitalizes on the nonlinear interaction forces between the atoms of the apex of an AFM probe tip and those of a given sample surface immediately below the apex. The engendered nanomechanical coupling between the probe and the sample offers tremendous potential for obtaining a host of material characteristics. By means of applying a multi-harmonic mechanical forcing to the AFM probe and another multi-harmonic forcing to the sample under investigation, we obtain, via frequency mixing, a series of new operational modes, which are sensitive to the variation of the composition at the subsurface level. By varying the nature of the excitation, using elastic coupling or photonic coupling, it is then possible to obtain physical and chemical signatures of a heterogeneous medium with high spatial resolution.

Combining the MSAFM with photothermal deflection spectroscopy when carried out in the mid infrared red region could provide selectivity to AFM imaging. Efforts are presently underway in developing these images tools.