Near-field IR nanoprobe: s-SNOM and AFM-IR modes

The  unique experimental setup at B22 of DLS is based on a customized (Neaspec) IR-neaSCOPE system coupled to a levitation chopper (developed by DLS with Celeroton) and  a N₂ purged optics box for coupling SR to the system.

A module for s-SNOM, including an asymmetric FTIR interferometer, is operated with SR IR, with the scattered light collected by a high sensitivity MCT detector. The IR spectral range is defined by the detector coverage, the beamsplitter material and the tip-cantilever geometry. For routine experiments on reflective samples, nanospectra between ca. 650 and 4000 cm^-1 can generally be achieved. For each imaging measurement, the topography and mechanical response, as well as the IR optical amplitude/phase signal (spectrally unresolved i.e. whitelight) are recorded. Spectra can be recorded in single points or by performing linescans or maps (hyperspectral imaging).

A distinguishing feature of the endstation is the use of SR IR for resonance-enhanced AFM-IR in a separate module, using a symmetric FTIR interferometer. This is possible through a unique feature of our system, a fast vacuum-operated light chopper (up to 200 kHz).The routinely usable IR range for these experiments is from 600 to 5000 cm^-1 based on the current setup. This can be enhanced towards higher/lower spectral regions by Si or ZnSe beamsplitters and/or changing the SR IR alignment. The system allows for both top and bottom illumination.

The endstation also features a QC-Laser 4-chip system (MIRCat, Daylight Solutions) which can cover the IR range from about 900 to 1800 cm^-1 usable for nanospectroscopy and nanoimaging in s-SNOM (pseudo-heterodyne) and AFM IR modes.

A detailed description of the working principles can be found in the dedicated sections.

In general, suitable samples should fit the following parameters of the AFM stage:

• max lateral diagonal of substrate ca. 40 mm; ideally disks with max diameter 30 mm
• overall height less than 8 mm; ideally substrates height at max 3 mm

The optical field of view is circa 0.75mm by a 10x magnification optical microscope for sample area identification, by a resolution better than 0.8 µm and a vertical focusing range of circa 250 µm. Visible Images are recorded via a 5 MPixel digital camera.

The region of interest defined by the AFM x-y-z piezo stage is as follow:

• lateral range 100x100 µm²
• max topography height 7 µm
• sample features in the spatial scale 10 nm to several µm

Samples must be fixed and adherent to the substrate given the tapping or contact AFM mode of operations. Soft matter matrices are suitable depending on non-contact force. Currently, the system supports samples at standard P and T conditions, including purging by N₂ gas for best IR nanospectra.

Suitable substrate and sample matrix depend on the nanoIR method used:

for s-SNOM

For measuring the surface region of reflective samples, the sample can be any thickness up to several mm (cf. above). For low reflectivity samples, the signal can be enhanced with the sample on a reflective substrate - in this case the sample should ideally be less than 100 nm thick. Silicon or Germanium disks and Gold-coated glass substrates work well.

for AFM-IR

In top illumination, the top few microns are generally probed and samples of any thickness up to mm can be measured. The spatial resolution is better for thinner samples, but the signal intensity is then lower, with the ideal compromise being for samples between about 250-1000 nm. Substrates with low thermal conductivity/small thermal expansion are ideal; classic IR transparent materials (CaF₂ etc) work well, and are necessary in transmission mode.

Please consider the following guidelines for a nanoIR project:

• A concise description of the science case, including the research context and any useful information like microscopy/AFM images or spectroscopy data/simulations
• The nanoIR measurement goal, either as spectroscopic bands/changes or spatial IR maps, plus any key information on the sample micro/nano features of interest
• Characteristics of the sample
  • overall size and regions of interest
  • info on the sample topography and indicative matrix composition
  • substrate material or self-standing sample
• The preferred method (s-SNOM or AFM IR)
• Parameters of the measurements
  • number of samples (including reference samples)
  • size of the regions of interest
  • nanoimaging or nanospectroscopy details (single points nanospectra, 1D linescans, 2D maps)
  • spectral resolution and range