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While PiFM provides excellent nanoscale chemical mapping and spectral analysis capabilities when coupled with broadly tunable IR sources, it can also provide tremendously useful information about samples even with a few diode laser of fixed wavelengths in the visible spectrum. In this note, we highlight some examples.
As with IR sources, PiFM will map out the absorption of visible light by the sample. Figure 1 shows the results obtained by PiFM on minute level of Alexa Fluor 750 (AF750) dye molecules that are deposited onto a glass slide. As shown in the figure, it has an absorption maximum at 749 nm (emission maximum at 775 nm) and virtually no absorption at 835 nm. PiFM image at 764 nm (with < 50 mW focused by 100X, 1.4NA objective lens from below) clearly highlights dye molecules while at 835 nm, no molecules are highlighted; note that the topography and phase images acquired together with PiFM look identical at both excitation wavelengths. Looking at the cross-sectional profile of the single particle in the yellow ellipse, we see that it is only ~0.5 nm thick and ~34 nm in extent. Given that a metal coated tip has a typical radius of about 30 nm, the particle may be ~5 nm in size and most likely a single layer, which should not consist of many molecules. PiFM cross-section shows that it is less convoluted than topography (due to tigher E-field profile compared to physical tip shape) and the 10% – 90% transition is 9 nm, again demonstrating excellent spatial resolution of PiFM. Recently, visible PiFM was used to track the changing absorption profile of a perovskite photovoltaic device due to ionic migration caused by different bias voltages .
Figure 2 shows exfoliated MoS2 film that is examined by PiFM. Looking at the absorbance date obtained from a far-field measurement , PiFM should be able to generate progressively higher signal with more layers at an excitation wavelength of 500 nm but show no such differentiation at 1000 nm, which is nicely demonstrated by the accompanying PiFM images.
Since PiFM measures the complex polarizability of the sample, PiFM can create high resolution maps of materials based on their index of refraction. Figure 3 shows an array of TiO2 in a matrix of E-beam resist (EBR). As seen in the SEM image, TiO2 contains a shallow hemispherical troughs due to slight over-etching. This results in circular features in the topography. The two materials have transmission higher than 87% and no intrinsic absorption in this sample at 532 nm. However, PiFM can create a clear material map based on the different index of refraction . The authors report that a gap between two TiO2 structures as small as 35 nm is clearly distinguishable via PiFM.
Figure 4 shows a modelled map of longitudinal electric field associated with a tightly focused light with high NA objective lens from below. Even though the light is polarized in the sample plane, the high NA creates an appreciable amount of longitudinal component, which is typically utilized in tip-enhanced measurements such as tip-enhanced Raman spectroscopy (TERS). The modelling of Ez2 shows two lobes that exhibit the strongest longitudinal field. When the tip is over such a lobe, the field will drive the electrons along the tip axis and induce a dipole, which creates an image dipole in the glass substrate. The two dipoles will attract each other, which is measured by PiFM. The measured PiFM image agrees well with the model. In a similar manner, plasmonic field associated with nano-particles can be imaged with PiFM .
A visible far-field detector can be added onto VistaScope to perform s-SNOM measurements. Figure 5 shows gold discs prepared on a glass slide that are excited by a laser at 650 nm (near plasmon resonance of the discs) at two different polarizations. The dipolar plasmonic fields are clearly observed in the s-SNOM images, which behave consistently with the polarization directions.
Figure 6 shows the near-field associated with a plasmonic structure at the end of a waveguide in a heat-assisted magnetic recording head, observed by both s-SNOM at multiple harmonics of the tapping mode frequency and PiFM; due to PiFM’s sensitivity, it can also detect fields that are leaking from the waveguide structure. Figure 7 shows concurrent measurements of topography, PiFM at 488nm, and photoluminescence (PL) at 650nm on an exfoliated MoS2 sample, allowing measurements of nanoscale absorption along with the far-field PL at the same time. Figure 8 shows a TERS spectrum acquired on a MoS2 sample by using the PiFM field mapping to select the optimal alignment of the tip .
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