S. Park, D. Nowak, T. Albrecht
Keywords: force microscopy, nanochemical mapping
Summary:Along with the surge in nanodevice and nanomaterial fabrication, the need for analytical probing techniques, which operate at high resolution and with high sensitivity, has grown. In particular, the smaller dimensions of the samples dictate that the spatial resolution of the technique resides in the nanometer range while retaining analytical capabilities. Scan probe techniques have been particularly successful in interrogating materials with nanoscopic precision. Atomic force microscopy (AFM) and related techniques are capable of topographically mapping nanoscale features and probing physical properties of the material. Nonetheless, the AFM technique has only been moderately successful in chemically characterizing nanostructured samples. Whereas chemical bonding forces can be used to dress the AFM with chemical selectivity, such methods are highly specific and not generally applicable for routine probing of materials under ambient conditions. In this regard, the AFM technology would benefit greatly from the excellent chemical selectivity offered through spectroscopic techniques. The use of photons in the tip-sample junction brings about the possibility of studying samples through material-specific electronic or vibrational transitions, which form the basis of spectroscopic analysis. The merger of tip-based techniques with spectroscopic methods is still in its early stage, but the prospects of spectroscopic probing at the nanoscale have tremendous implications for nanotechnology and related fields. Photo-induced force microscopy (PiFM) is a new nano-analytical technique based on AFM, which acquires both topography and spectral response of samples with true nanoscale spatial resolution. When coupled with a broadly tunable infrared (IR) excitation laser, such as a quantum cascade laser (QCL), IR PiFM can generate topographic and chemical maps (based on the unique absorption signature of the material) of sample surfaces with sub-10 nm spatial resolution. This is achieved by detecting the absorption of the IR light by the sample via mechanical force detection. The excitation laser is pulsed at a specific frequency such that the absorption of light by the sample excites a cantilever resonance frequency. Since this optical force exists only when the tip is in extreme close proximity to the sample surface, it leads to several unique traits: (1) excellent spatial resolution; (2) high surface sensitivity, and (3) lack of far-field background signal. The breadth of the capabilities of PiFM will be highlighted by presenting data on various material systems (organics, inorganics, 1D/2D, bio-molecules, and nano-photonic materials). Preliminary results on 2D materials probed via a tunable visible source will be presented as well. By enabling imaging at the nm-scale with chemical specificity, PiFM provides a powerful new analytical method for deepening our understanding of nanomaterials and facilitating technological applications of such materials.