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High-Resolution Nanochemical Mapping


Figure 5 : Analysis of a polystyrene-polyethylene blend. (a) Topography of a PS-LDPE sample, (b) simultaneously acquired modulus, and (c) infrared refl ection at 1880 cm -1 . Lower modulus in (b) identifi es the softer LDPE domains compared to the PS matrix. The refl ection image in (c) confi rms this assignment where LDPE with a lower refractive index in the fl at region of its absorption-free dispersion exhibits lower near-fi eld refl ection.


carbonyl group of PHBV is used to assign both morphologies in the topographic image to PHBV. Absorption and refl ection behave similarly to the PS-PMMA example of Figure 3 with a resonance around 1720 cm -1 . On resonance, darker, that is, less absorbing, boundaries are observed within the island-like structure. This might indicate incomplete phase separation with PS or could be related to a local diff erence in degree of crystallinity within PHBV. Clearly, s-SNOM uncovers that PHBV exhibits more heterogeneity than the topography image alone provides.


Simultaneous modulus and reflection mapping of a polystyrene-polyethylene blend . Because s-SNOM is an AFM-based technique, it can be supplemented and enhanced by AFM modes other than tapping mode [ 31 ]. Here, we demonstrate the combination of s-SNOM with PeakForce Tapping using a low-density polyethylene (LDPE) sample. LDPE is most commonly known from plastic bags. T e topography of a PS-LDPE blend is displayed in Figure 5a . Modulus mapping in Figure 5b reveals soſt disks within a stiff matrix of higher modulus. T is contrast already identifi es the high-modulus


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material as PS with a nominal 2 GPa modulus with embedded LDPE disks of 0.1 GaP modulus. This assignment and the topography data inform us that the soſt er LDPE disks protrude out of the surface by 10 nm. Both materials are not absorptive at the infrared frequency of 1880 cm -1 , but Figure 5c reveals a clear contrast in the simultaneously acquired refl ection channel. LDPE with a lower refractive index exhibits a lower near-fi eld refl ection signal compared to PS. T is is an example where the nanoscale refl ection information alone is suffi cient to distinguish materials, a fact oſt en used, for example, when studying metallic domains in insulator-to-metal transition materials [ 11 ]. In general, metals can be distinguished in this way from insulators, whereas phonon or plasmon resonances can result in resonant enhancement with signals exceeding that of metals [ 32 ]. Simultaneous modulus and absorption mapping of a contamination on a PS-PMMA film . In contrast to the previous example ( Figure 5 ), we will turn to a system where topography, modulus, and reflection information are not suffi cient to characterize the sample, but where IR absorption properties need to be measured as well. T e combination of IR and mechanical data was demonstrated on the previous PS-PMMA blend, examined in Figure 3 . The topography ( Figure 6a ) reveals taller PMMA domains in a PS matrix, similar to what was observed in Figure 3 . T e highest feature in the image however, located close to the image center, exhibits less internal structure than the PMMA domains that also protrude from the surface. From topography information alone, it is unclear whether this feature is PMMA or a contami- nation. Its corresponding modulus mapped in Figure 6b appears to lie between the one measured for PMMA and PS. Adding IR refl ection data ( Figure 6c ) does not solve the puzzle: the refl ection acquired at 1728 cm -1 resonant with the PMMA absorption (compare with Figure 3d ) shows a PMMA-like signal for the feature. So far the topography, modulus, and refl ection signals are ambiguous. T is underlines the need for additional IR absorption data ( Figure 6d ), which reveals a vanishing absorption similar to that of PS for the tall feature observed in topography. Combining refl ection and absorption data, we learn that the dielectric properties of the contamination must be diff erent from PS and PMMA, and hence a contamination of some sort must be present.


Discussion


In this paper we have introduced the underlying working principles of Bruker’s Inspire system and its application to polymers for imaging chemical phases. T e presented s-SNOM technique based on short-range near-fi eld interaction shows much higher spatial resolution in the infrared fingerprint region than diff raction-limited, far-fi eld infrared spectroscopy. A PS-b-PMMA block copolymer system ( Figure 1 ) served to demonstrate a spatial resolution down to 10–20 nm in agreement with previous publications [ 1 , 2 , 17 ]. In this model system PMMA domain sizes of 30–50 nm were measured, a similar size as recently published [ 30 ]. AFM topography and chemical material distribution deduced from s-SNOM were shown not to correlate in this sample, a fact that underlines the necessity for nanoscale chemical identifi cation. T e chemical specificity was presented using a vibrational resonance in a different, phase-segregated PS-PMMA film ( Figure 3 ). Resonantly driving the carbonyl mode that is present in PMMA


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