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High-Resolution Nanochemical Mapping of Soft Materials


Martin Wagner* and Thomas Mueller Bruker Nano Surfaces Division , 112 Robin Hill Road , Santa Barbara , CA 93117


* martin.wagner@bruker.com


Abstract : Identifying chemical phases in inhomogeneous materials via their infrared fi ngerprints is routinely performed, but the employed infrared spectroscopy is limited in spatial resolution to ~10 μm. Scattering scanning near-fi eld optical microscopy (s-SNOM) circumvents this limitation, accessing the 10–20 nm scale. Here, we introduce Bruker’s Inspire™ tool that is based on s-SNOM and that provides non-destructive, modeling-free absorption mapping. We discuss application examples in polymer research, highlighting the high spatial resolution and chemical sensitivity. In combination with other atomic force microscopy (AFM) modes, in particular PeakForce Tapping ® , the full breadth of nanooptical, nanomechanical, and nanoelectrical properties becomes accessible for comprehensive material characterization.


Introduction


Identification and characterization of materials with nanoscale resolution has always been the ultimate goal in atomic force microscopy (AFM). This knowledge is essential for correlating the macroscopic properties and performance of materials such as biomembranes or organic photovoltaics with their nanoscale inhomogeneity. Infrared spectroscopy is a standard technique for identi- fi cation of materials, but its spatial resolution is limited by diff raction. Scattering scanning near-fi eld optical microscopy (s-SNOM) in the infrared spectral region is an AFM-based technique that overcomes the diff raction limit and provides non-destructive, surface-sensitive chemical identifi cation with a spatial resolution down to 10 nm [ 1 , 2 ]. Standard AFM tapping mode operation is inherent to s-SNOM, which renders this technique well suited for soft materials where contact mode could cause tip and sample damage. Even more advantageous, as implemented in Bruker’s Inspire system for the fi rst time, s-SNOM can be combined with PeakForce Tapping. In this article, we discuss new opportunities this aff ords for correlated and simultaneous nanochemical, nanome- chanical, and nanoelectrical measurements on soſt polymeric samples. Additional applica- tions of the Inspire system are not covered here, for example, imaging of local strain [ 3 , 4 ], nano-antenna modes [ 5 , 6 ], graphene plasmons [ 7 , 8 ], boron nitride phonon polaritons [ 9 , 10 ], phase-transitions [ 11 ] and biomaterials [ 12 – 14 ].


Materials and Methods Model polymer example . Designing novel material properties of polymer blends used, for instance, in organic photovoltaics or liquid


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crystals requires the ability to characterize the system at the nanoscale in order to correlate phase distribution with macroscopic performance. Figure 1 presents an example of an inhomogeneous, nanoscale polymer system, the model system poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Panel (a) shows the topography of the 50 nm spin-cast thin fi lm, an image unable to answer the question of whether the two components PS and PMMA coexist as a homogeneous mixture or phase-separated. We note that conventional tapping mode routinely provides topography information while preserving soſt samples such as polymers. PeakForce Tapping, discussed below, can additionally inform us on a quantitative level about certain material properties, such as the modulus, and is preferred for imaging of the most delicate samples, such as DNA [ 15 ] or


Figure 1 : Example polymer analysis. (a) Topography of a spin-cast, 50 nm thin PS-b-PMMA block copolymer fi lm on Si substrate. (b) Corresponding nanoscale absorption at the 3 rd harmonic obtained with Inspire at 1725 cm -1 . At this frequency the infrared light resonantly excites the carbonyl C=O stretch that is only present in PMMA. Consequently, only the PMMA domains appear bright while PS does not absorb in this frequency range. A trench outside the displayed image and accessing the Si substrate was used as a reference material. The yellow dotted line marks the position of the line profi les for topography and IR absorption presented in (c) and (d), respectively. No correlation between the height in (c) and the absorption in (d) is found, indicating that the absorption image maps the nanoscale chemistry rather than topography artifacts. The step in absorption at a position of 260 nm has a width of 15 nm and represents a measure for the obtained spatial resolution that is limited by the tip radius of < 25 nm.


doi: 10.1017/S1551929516000298 www.microscopy-today.com • 2016 May


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