High-Resolution Nanochemical Mapping


cells [ 16 ]. Identifying chemistries and phases by means other than their sometimes-unknown nanomechanical properties is oſt en desired. Infrared spectroscopy is able to provide valuable information on the chemical composition because many materials show a unique fi ngerprint in the infrared spectral range. However, conventional far-field techniques such as Fourier-transform infrared (FTIR) spectroscopy are limited in spatial resolution to approximately half of the employed wavelength, that is, typically at least 10 μ m in the fi ngerprint region and much larger than the 30–50 nm diameter of grains in Figure 1a . On the other hand, s-SNOM is a well-established near-fi eld infrared technique that can overcome the far-fi eld diff raction limit, allowing an improvement in spatial resolution by three orders of magnitude down to 10 nm. Applying s-SNOM imaging to the block copolymer sample reveals the quasi-lamellar PMMA domains in Figure 1b . T ese domains are detected by probing the carbonyl resonance at 1725 cm -1 that is present in PMMA but not in the PS chains. T e height distribution does not correlate with the PMMA distribution, as can be seen from the line profi les taken along the yellow line in Figure 1b and extracted in Figure 1c for height and Figure 1d for IR absorption. Consequently, the absorption channel only probes the chemical composition and not topography artifacts. For the present example, the Pt-Ir metallized AFM probe had a nominal tip radius less than 25 nm. A spatial resolution of <15 nm can be estimated for the chemical mapping from the step in the line profi le of Figure 1d at the position of 260 nm, in an area of minimal topography changes to rule out crosstalk with topography. We note that a spatial resolution in the IR of < 15 nm can be achieved without special tip/sample preparations, and the avoidance of tip/sample damage in tapping mode (vs. contact mode) results in highly reproducible results during multiple scans. T is resolution is comparable to the tip radius due to strong confi nement of the interaction volume by the exponential decay of the near-fi elds with distance and the fi eld enhancement at the tip apex as discussed below. It has been shown before that tips with smaller radius of 10–15 nm allow for an even higher IR spatial resolution down to 8 nm [ 17 ]. T e s-SNOM mode . In s-SNOM, as implemented in the Inspire system, scattered light from an AFM tip is detected. Infrared light from a laser source, for example, from a continuous wave quantum cascade laser, is focused onto the end of a metallized AFM tip. Figure 2a shows the schematic of the AFM tip in an asymmetric Michelson interferometer that is used for near-fi eld amplitude and phase image extractions. T e infrared light incident on the tip is polarized along the tip in order to effi ciently couple the electric fi eld to a “nanoscale antenna” that gets polarized. T e metal coating of the tip results in a lightning- rod eff ect for the incident light, that is, fi eld concentration at the apex of the tip [ 18 ]. Brought into contact with a sample, the exponentially decaying evanescent fi elds at the tip apex result in a local sample polarization according to the optical constants of the material. T e sample polarization in turn infl uences the tip polarization and eventually the tip, as a nano-antenna, couples the polarization as radiation into the far-fi eld. In other words, the tip elastically scatters the light, and this scattered radiation


46


Figure 2 : Infrared signal acquisition. (a) Schematic diagram of the s-SNOM implementation in Inspire. Light from an infrared laser source is split into a reference beam that is refl ected from a motorized mirror and a sample beam that is focused on an AFM tip. Backscattered light from the tip is detected with a MCT detector where it interferes with the reference light. In two-phase homodyne detection the reference mirror is set to two positions that allow phase-sensitive measurements to extract absorption and refl ection data with nanoscale spatial resolution. (b) Schematic of the AFM tip illustrating the confi nement of near-fi eld signals E nf to small tip-sample distances compared to background signals E b . (c) Highly nonlinear decay of the near-fi eld signal with height above the sample in contrast to the linear background signal allows extraction of the near-fi eld contri- bution via tapping mode operation. The harmonic background signal in time can be distinguished from the anharmonic near-fi eld signal by Fourier transformation. Detection at higher harmonics (2, 3, 4…) of the tip oscillation frequency allows suppression of the background signal.


now contains information on the local dielectric properties of the sample under the tip with a spatial resolution given in fi rst order by the radius of the tip, typically ~10–25 nm. T e radiation is then collected by the focusing element, for example, a lens or parabolic mirror, followed by analysis in a Michelson-type interferometer. Signal and background . T e near-fi eld scattering signal of the s-SNOM is usually buried in a much larger background scattering signal that does not contain the desired, sample- specifi c localized information. T e background signal originates from scattering off the tip, the cantilever, and the sample surface because the focus of the incident light is still diff raction-limited to a few tens of micrometers. Despite its larger magnitude compared to the near-fi eld signal, suppression of the background and subsequent near-fi eld extraction turns out to be straight- forward. T e most common scheme is based on modulating the tip-sample distance, which conveniently corresponds to standard tapping mode AFM operation. T e near-fi eld signal increases strongly nonlinearly close to the sample surface, while far-fi eld scattering shows only a linear signal versus distance dependence, as shown in Figures 2 b and 2 c. Consequently, the harmonic motion of the AFM tip in tapping mode at the cantilever resonance frequency Ω results in a background signal that varies only slightly with Ω , while the near-fi eld signal shows higher harmonics nΩ (with integer n ≥2). Typically, near-fi eld signals are acquired at the second or third harmonic


www.microscopy-today.com • 2016 May


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