Manufacturing Bits: July 28

Nanoscale IR imaging; synchrotron IR; pulsed force microscopy.


Nanoscale IR imaging
The Nanooptics Group at CIC nanoGUNE has made some major advances in the emerging field of nanoscale infrared microscopy.

The group’s technology, called nano-FTIR spectroscopy, is an infrared characterization technique. Infrared (IR) isn’t new. Invisible to the human eye, infrared wavelengths range between 760nm to 1,000nm. For years, infrared inspection/metrology have been used for thin-film measurements in chips. IR can penetrate the thin films and provide measurements. The drawback is that infrared inspection is slow and expensive.

Nano IR is the next big thing. Meanwhile, using nano-FTIR, researchers from CIC nanoGUNE have found a way to identify materials that are located up to 100nm below the surface of a sample.

Nano-FTIR is related to a characterization technique called scattering-type scanning near-field optical microscopy (s-SNOM). Basically, s-SNOM is a scanning probe microscopy technique. It provides images for various samples.

“In s-SNOM, monochromatic electromagnetic radiation of the visible, infrared, or terahertz spectral range is focused onto the tip of a standard, metallized atomic force microscope (AFM) probe,” explained Lars Mester from the CIC nanoGUNE in Nature Communications, a technology journal. Others contributed to the work.

AFM uses a tiny probe to enable measurements in structures. “At infrared frequencies, s-SNOM offers the possibility for highly sensitive compositional mapping based on probing vibrational excitations such as the one of molecules or phonons, analogously to infrared microscopy,” Mester said. “The technique—named nano-FTIR spectroscopy—yields near-field phase spectra that match well the absorptive properties of organic samples, and thus allows for nanoscale chemical identification based on standard FTIR references.”

As stated, infrared spectroscopy isn’t new. Optical spectroscopy with infrared light, such as Fourier transform infrared (FTIR) spectroscopy, allows for chemical identification of materials at the micrometer-scale. In comparison, nano-FTIR can resolve objects at the nanoscale.

In nano-FTIR, infrared light is scattered using a sharp metallized tip of a scanning-probe microscope. “The tip is scanned across the surface of a sample of interest and the spectra of scattered light are recorded using Fourier transform detection principles,” according to CIC nanoGUNE.

It probes the nanometric volume below the tip. The technology has demonstrated the ability to detect spectral signatures of materials below the surface of a sample. In this case, the materials can be detected and chemically identified up to a depth of 100nm. It shows that the thin surface layers differ from the sub-surface layers.

“Our experimental findings are confirmed and explained by a semi-analytical model for calculating nano-FTIR spectra of multilayered organic samples. Our results are critically important for the interpretation of nano-FTIR spectra of multilayer samples, particularly to avoid that geometry-induced spectral peak shifts are explained by chemical effects,” Mester said in Nature Communications.

Synchrotron IR
Kings College London, the University of Vienna, and the Diamond Light Source (DLS) have conducted the world’s first measurements using synchrotron infrared nanospectroscopy.

More specifically, researchers used synchrotron resonance-enhanced infrared atomic force microscopy (RE-AFM-IR). This is a near-field photothermal vibrational nanoprobe using the synchrotron at DLS. RE-AFM-IR is capable of measuring mid-infrared absorption spectra with spatial resolution around 100nm, according to researchers.

DLS is the UK’s national synchrotron. Using a giant storage ring, the machine accelerates electrons to near light speeds so that they give off light 10 billion times brighter than the sun. These bright beams are then directed off into laboratories within the storage ring known as beamlines.

Using these capabilities, researchers measured drug-induced molecular changes within a cell at sub-wavelength scale. Specifically, researchers measured “biomolecular changes induced by a drug (amiodarone) within human cells (macrophages) and localized at 100nm.”

In this case, a cellular model of drug-induced phospholipidosis (DIPL) was developed. Traditionally, DIPL is evaluated using visual confirmation by electron microscopy or the use of fluorescence labelling techniques.

Instead, researchers used the IR broadband illumination via synchrotron together with AFM. “Our experiment is – to my knowledge – a world first by synchrotron photothermal IR nanospectroscopy in life sciences, and proved that photothermal IR nanospectroscopy can successfully scan across mammalian cells and reveal the inner molecular fingerprint via the full IR spectrum, thanks to synchrotron IR broadband coverage,” said Gianfelice Cinque from the DLS.

Pulsed force microscopy
Lehigh University has devised a new characterization technique called Pulsed Force Kelvin Probe Force Microscopy (PF-KPFM).

The technology allows for less than 10nm measurements of work function and surface potential in a single-pass AFM scan.

The technology works like AFM. “In Pulsed Force Kelvin Probe Force Microscopy, we removed the need for the AC voltage by implementing a custom circuit of a field effect transistor between the tip and the sample, which acts as a binary switch,” said Xiaoji Xu, assistant professor in Lehigh’s Department of Chemistry. “When the switch is on, the circuit acts as a simple wire, allowing charges to pass between tip and sample. A small amount of charges spontaneously migrates between tip and sample based on the relative difference in their intrinsic Fermi levels. When the switch is off, the circuit does not allow for charges to pass, and acts as a capacitor to re-absorb the charges from the tip and sample region.

“The next logical step was to combine PF-KPFM with Peak Force Infrared (PFIR) microscopy, an infrared imaging technique invented in our lab, since both techniques use the pulsed force mode,” said Xu. “The resulting technique, named PFIR-KPFM, provides topographical, mechanical, chemical, and electrical information at <10nm spatial resolution.”

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