Because of the diffraction limit in the resolution of conventional lens-based microscopes, namely D = 0.61λ/nsinθ, the maximum resolution obtainable with an optical microscope is ~200 nm. A new type of lens using multiple scattering of light allowed to improve the resolution to about 100 nm. Several new microscopy techniques with a sub-nanometer resolution have been developed in the last several decades, such as electron microscopy (SEM and TEM) and scanning probe microscopy (NSOM, STM and AFM). SPM differs from other techniques in that the excitation and signal collection are very close (less than diffraction limit distance) to the sample. Instead of using a conventional lens to obtain magnified images of samples, an SPM scans across the sample with a very sharp probe. Whereas SEM and TEM usually require vacuum and an extensive sample preparation, SPM measurements can be performed in atmospheric or liquid conditions.
Despite the achievable resolution of atomic scale for AFM and NSOM techniques, it does not provide chemical information of the sample. The infrared part of the electromagnetic spectrum covers molecular vibrations which can characterize chemical bonding within the sample.
By combining SPM and vibrational spectroscopy, AFM/IR-NSOM and AFM-IR have emerged as useful characterization tools that integrate the high spatial resolution abilities of AFM with IR spectroscopy. This new technique can be referred to as AFM-FTIR, AFM-IR and NSOM/FTIR. AFM and NSOM can be used to detect the response when a modulated infrared radiation generated by an FTIR spectrometer is absorbed by a material. In the AFM-IR technique the absorption of the radiation by sample will cause a rapid thermal expansion wave which will be transferred to the vibrational modes of the AFM cantilever. Specifically, thermal expansion wave induces a vertical displacement of the ATM tip (Figure 6). A local IR absorption spectrum then can be obtained through the measurement of the amplitude of the cantilever, which is a function of the IR source wavelength. For example, when the radiation laser wavelength is tuned at the resonance frequency with the vibrational absorption frequency of the sample, the displacement intensity of the cantilever will increase until the laser wavelength reaches the maximum of sample absorption. The displacement of the cantilever will then be reduced as the laser wavelength is tuned past the absorption maximum. This approach can map chemical composition beyond the diffraction-limit resolution and can also provide three-dimensional topographic, thermal and mechanical information at the nanoscale. Overall, it overcomes the resolution limit of traditional IR spectroscopy and adds chemical and mechanical mapping to the AFM and NSOM.