Combination of AFM and Photothermal FT-IR Spectroscopy

The Anasys instrument has been described in several publications [37-41] of which the review paper by Dazzi et al. [41] is the most detailed. The spectra measured with this instrument are essentially identical to those measured on a macrosample of the same material unless, of course, the polymer has microdomains of differing structure either because of immiscibility of two or more copolymers or because of the effect of orientation. The spatial resolution is significantly better than 100 nm, as reported by Dazzi et al. who embedded a 3 pm diameter PS bead in epoxy and demonstrated the very rapid change in spectrum on the edge of the PS bead [41]. 

This type of instrument has been used for a number of applications of polymer characterization, including the spatial differentiation of submicrometer domains in a poly(hydroxyalkanoate) copolymer [42] and multicomponent polymer blends of the type used as organophotovoltaics. An interesting application of AFM-IR was reported by Dazzi etal., in which the chain orientation of the polymers comprising very thin electrospun fibers usually have a diameter (10 nm to 10 pm) 

46] and Lahiri et al [47]. Lahiri et al examined the various factors that determine the sensitivity of this technique [47]. They reported that in order to record an image with the optimum lateral resolution in the AFM scanning direction, the 

The photothermal signal is proportional to the sample s thermal expansion, to the absorbed energy per unit area t/abs for short laser pulses to the duration of the thermal excitation in the sample. The sample s thermal expansion is proportional to its thickness z while is approximately proportional to z. Thus, the photothermal signal S should be proportional to the product of and z. The absorption by the sample will depend on the exponentially decaying evanescent electric field intensity. For thicknesses less than the depth of penetration of the evanescent wave d, the AFM-IR signal increases with an increase in sample thickness z. For nonabsorbing samples, the value of is given by the well-known equation  

The measurement of chemical maps using infrared spectroscopy at a spatial resolution below the diffraction hmit is clearly a rapidly moving field, and now that powerful instruments are commercially available, we look forward to future developments in the future. 

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Combination of AFM and Photothermal FT-IR Spectroscopy 537 When the sample absorbs, is given by the more complex equation [48] ... 

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