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High-Resolution Nanochemical Mapping


Another aspect is that force curves can be acquired and evaluated online during imaging to obtain mechanical properties of the sample in a quantitative way, including, for example, the modulus, adhesion, or the deformation [ 16 , 26 , 27 ]. In addition, PeakForce Tapping can be combined with potential or current measurements in PeakForce Kelvin probe force microscopy (PF-KPFM) [ 28 ] and PeakForce tunneling AFM (PF-TUNA) [ 29 ], respectively. T is versatility is now further enhanced in Bruker’s Inspire by the addition of s-SNOM, allowing correlated, even simultaneous nanochemical, nanoelectrical, and nanome- chanical data acquisition.


Results


Infrared detection of PMMA phase . As an example of the high chemical sensitivity of the s-SNOM technique, its spectral accuracy, and its ability to deliver easily interpretable data that corresponds to far-fi eld spectra, we investigated a spin-casted PS-PMMA blend (70:30 ratio) on a Si substrate. Mapping of the PMMA distribution is shown in Figure 3 a and 3 b where nanoscale s-SNOM reflection and absorption data were acquired at diff erent infrared frequencies. Figure 3c presents the topography with taller PMMA domains embedded in the PS matrix. T e laser was tuned around the carbonyl resonance of PMMA. As outlined above, a non-absorbing reference is needed to set the reference mirror phase and obtain near-fi eld absorption data. Conveniently, PS serves this purpose because it has a flat infrared response in the displayed frequency range. T e absorption and refl ection profi les ( Figure 3d ) were extracted on the PMMA domain from multiple images acquired at different, discrete frequencies. Both quantities exhibit a shape characteristic of a Lorentzian absorption line, as verifi ed


by fi tting with the common Lorentz oscillator model (solid lines). We note that only recently the same repetitive imaging at discrete laser frequencies has been employed to sweep over the PMMA carbonyl resonance in a PS-b-PMMA block copolymer system [ 30 ], similar to the one imaged in Figure 1 . Both, absorption line center frequencies and line widths, were measured with 0.2 cm -1 spectral precision to resolve intermo- lecular Stark shiſt s of the carbonyl resonance occurring between the center and interface of the PMMA domains due to locally varying electric fi elds [ 30 ]. T at study and our data underline that near-fi eld absorption on nanoscale systems is routinely obtainable, highly reproducible, and matches far-fi eld data that allows material identifi cation without the need for modeling of the exact tip-sample near-fi eld interaction. Furthermore, it highlights the high chemical sensitivity and spectral accuracy consistently achievable with s-SNOM that makes it possible to analyze on a quantitative level the physics and chemistry of such minuscule eff ects as the Stark shiſt between polymer chains. IR reflection and absorption in a biodegradable


polymer . To emphasize the wide applicability of s-SNOM and its capability to provide additional information about sample properties, we turn to PHBV or poly(3-hydroxybutyrate-co- 3-hydroxyvalerate), a biodegradable polymer. T e topography of the PS-PHBV sample (ratio 70:30) obtained from spin-casting is presented in Figure 4a , and the corresponding nanoscale refl ection and absorption in Figures 4 b and 4 c, respectively. T e topography reveals two diff erent morphologies: disk-like structures deeply embedded in a background matrix and an extended, island-like structure at the bottom right that is only slightly lower in height than the matrix. In panels 4b and 4c, the


Figure 4 : Analysis of a biodegradable polymer. (a) Topography of a PS-PHBV blend showing two morphologies: disk-like ones and an extended, island-like one at the bottom right, both embedded into the background matrix. (b) Nanoscale refl ection and (c) absorption for different frequencies around the carbonyl vibrational mode. The PHBV phase is identifi ed from resonant absorption mapping at 1720 cm -1 in (c). It occurs in the two different morphologies and exhibits some internal structure within the island-like region.


48 www.microscopy-today.com • 2016 May


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