Chemical Nano-Analytics with Nano-FTIR

Interview conducted by Will SoutterMar 4 2015

In this interview, Rainer Hillenbrand, Nanooptics Group leader at CIC nanoGUNE and co-founder of neaspec, talks to AZoNano about the incredible possibilities of nano-FTIR, which neaspec implements in their neaSNOM near-field microscope.

Can you give us a brief introduction to nano-FTIR?

An ultimate goal in modern chemistry and materials sciences is the non-invasive chemical mapping of materials with nanometer scale resolution.

Although a variety of high-resolution imaging techniques exist (e.g. electron microscopy or scanning probe microscopy), their chemical sensitivity often cannot meet the demands of modern chemical nano-analytics. On the other hand, infrared spectroscopy offers high chemical sensitivity, but its resolution is limited by diffraction to about half the wavelength, thus preventing nanoscale resolved chemical mapping. This is precisely what now becomes possible with nano-FTIR.

nano-FTIR is a near-field optical scanning-probe technique that combines scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. In short, the metalized tip of an atomic force microscope (AFM) is illuminated with a broadband infrared laser beam. The tip acts as an infrared antenna and concentrates the laser beam at the tip apex to an infrared spot of about 20 nm diameter.

The near fields of this “nanofocus” are reflected at the sample surface and modify the scattering of the tip, dependent on the sample´s local infrared properties. By collecting and analyzing the backscattered light with a specially designed and patented Fourier Transform spectrometer, we can thus perform local infrared spectroscopy with a spatial resolution of about 20 nm. This enables reflectivity and absorption measurements of diverse materials at nanoscale resolution.

nano-FTIR allows for fast and reliable chemical identification of virtually any infrared-active material on the nanometer scale. Moreover, nano-FTIR spectra of organic materials match well with conventional FTIR spectra, while the spatial resolution is improved by a factor of about 500 compared to conventional infrared spectroscopy. The high sensitivity to chemical composition combined with ultra-high resolution makes nano-FTIR a unique tool for research, development and quality control in polymer chemistry, biomedicine and the pharmaceutical industry.

Infrared nanoimaging of a single protein complex (ferritin, 5000 C=O bonds, 1 attogram mass) with 100 to 1000 times improved resolution compared to conventional infrared microscopy. Left: Illustration of the infrared illuminated tip of a near-field microscope on top of a ferritin complex. Right: Topography image (height map) of three nanoparticles. The infrared absorption image of the three nanoparticles reveals that only two of them are protein complexes. The nano-FTIR spectrum (red) was taken on the complex marked by the red dot in the topography image. Remarkable, the nano-FTIR spectrum of the single ferritin complex matches well the far-field FTIR spectrum of a large ensemble of ferritin complexes. Image credit: I. Amenabar, et al., Nat. Commun. 4:2890 doi: 10.1038/ncomms3890 (2013)

What is unique about neaspec's technology in this area?

The patented technology from neaspec allows for reliable suppression of background signals that are unavoidable when detecting light scattered from a tips, which can disturb the measurement. The interferometric technique furthermore yields both the amplitude and phase of the tip-scattered light, which enables a complete characterization of the dielectric sample properties, including the permittivity and refractive index.

nano-FTIR possesses the unique capability to combine this interferometric detection technique with Fourier transform spectroscopy. With this capability, we can measure background-free infrared amplitude and phase spectra with 20 nm resolution, which allows for direct measurement of local refractive index or absorption.

The neaSNOM near-field microscope is also capable of performing nano-imaging and nano-spectroscopy throughout the wide spectral range between visible, infrared and terahertz frequencies. Owing to its patented two-path mirror objective, it even allows for simultaneous operation in different spectral ranges, or for easy combination with terahertz time domain spectroscopy.

neaspec´s technology also allows for mapping both amplitude and phase of the near fields at the surface of photonic and plasmonic nanostructures. It can therefore be applied for comprehensive studies of the electromagnetic near-field distribution of photonic structures, including field enhancement, field confinement, mode profiles or the dispersion of propagating surface polaritons.

As many photonic applications require sample illumination from below, we developed an interferometric transmission module. It enables retardation-free illumination of the samples, which is often a prerequisite for studying specific photonic features, for example Fano resonances in plasmonic metamolecules.

An important aspect of nano-FTIR is the infrared light source. It needs to cover the broad spectral infrared fingerprint region, while offering enough power to achieve the highest possible signal-to-noise ratio.

Standard FTIR sources such as glow bars can be used in principle, and we have also successfully obtained nano-FTIR spectra using thermal radiation. For most practical applications, however, such sources are too weak and measurement times are too long.

To address this issue, we developed a novel and unique broadband infrared laser source. The laser source is fully automated, and offers a spectral coverage from 600 to 2200 wavenumbers, and with a remarkable spectral power density, higher than that of typical infrared synchrotrons.

With this new source, we obtain nano-FTIR spectra of single viruses on a time scale of minutes.

What were the drivers behind the development of nano-FTIR technology, and why did you decide to spin it out as a company?

In 1998 I started to work on my PhD thesis with Fritz Keilmann at the Max Planck Institute for Biochemistry in Martinsried (near Munich). We wanted to develop an interferometric near-field microscope based on light scattering at an AFM tip. During that time we developed the basic concepts that enable amplitude and phase resolved near-field imaging with efficient background suppression.

Later, I received a Nanofutur grant from the German Federal Ministry for Education and Research (BMBF), which anticipated, amongst other things, a spin-off company commercializing our near-field microscope.

Within this project, my PhD student Nenad Ocelic developed a pseudoheterodyne detection technique for operating at visible, infrared and terahertz frequencies with one and the same interferometer. We patented this technique, which also set the basis for nano-FTIR.

We were able to obtain nanoscale chemical maps of polymers, proteins, crystals and mobile carriers in semiconductors with infrared light in a straightforward manner, which had been impossible before.

Because infrared spectroscopy is so broadly used in science, technology and industry, our capability to perform it with hundred-fold improved resolution allowed us to start neaspec GmbH in 2007.

Importantly, s-SNOM systems were not available at that time commercially, but we were convinced about the huge potential of this technique. In the following years, the nano-FTIR technology was developed within the PhD project of Florian Huth, and in collaboration between neaspec GmbH and the Basque research center CIC nanoGUNE, where my research group has been located since 2008.

The neaSNOM nano-FTIR system from neaspec

Can you share an example of some recent research carried out using nano-FTIR?

We are currently applying nano-FTIR to study the material distribution in organic nanocomposites, e.g. polymer blends. We have also managed to map the secondary structures of insulin fibrils, showing that with our technique we are able to map the distribution of alpha-helical and beta-sheet structures with nanoscale resolution.

This could have application potential in studying conformational changes and misfolding processes on the nanometer scale, which might be interesting for neurodegenerative disease research.

We are also using nano-FTIR to study the confinement and propagation of infrared light in 2D materials such as graphene and boron nitride. nano-FTIR allows us to measure the dispersion and losses, for example.

What is the full range of information that nano-FTIR able to provide, and how is this obtained?

nano-FTIR employs standard metal AFM tips to measure and map the local complex-valued permittivity as a function of wavelength. From this information, a vast variety of information can be obtained.

Converting the measurements into absorption spectra, we can perform local chemical identification, analogous to conventional FTIR spectroscopy, just with up to 1000 times better resolution.

Additionally, it can provide structural and conformational information, e.g. in proteins, on the nanometer scale. We can also measure the local carrier concentration in highly doped semiconductors or map nanoscale strain fields in infrared active crystals, ceramics or minerals.

With dielectric tips, we can map the near fields at the surface of photonic structure, such as metallic antennas, or the nanoscale concentrated electromagnetic fields (plasmons) propagating along graphene layers.

Why is nano-FTIR such a useful technology for analyzing graphene in particular?

Graphene samples are typically doped, and therefore can support electromagnetic surface waves (called plasmons) at mid-infrared frequencies. Graphene plasmons strongly depend on the carrier concentration, carrier mobility and defects, and thus can be used for characterization of the local electronic properties of graphene.

However, plasmons in unstructured graphene samples cannot be analyzed by far-field infrared spectroscopy, as their wavelength is more than an order of magnitude smaller than the wavelength of the illuminating light.

This wavelength difference, and the respective momentum mismatch, between graphene plasmons and photons can be overcome by nano-FTIR. By concentrating the light at the nano-FTIR tip, we can excite graphene plasmons. Near-field interaction between the graphene plasmons and the tip modifies the tip-scattered light, which subsequently contains information about the graphene plasmons, and thus about the local electronic properties of graphene.

What practical implications does nano-FTIR have for the commercialization of graphene?

nano-FTIR could be applied for nanoscale, noninvasive and current-free analysis of graphene, which could be useful for graphene quality control. nano-FTIR might be also applied for characterizing graphene during the development of growth processes that aim to optimize the graphene quality.

What are the next steps for nano-FTIR?

We recently developed a novel technique for faster amplitude- and phase-resolved near-field imaging. It combines nano-FTIR technology with digital holography methods, so we named it synthetic optical holography (SOH).

As a first application, we demonstrated record-fast non-invasive infrared nanoimaging of grain boundaries in CVD-grown graphene. We were able to record 65 kilopixel near-field images in 26 seconds, and 2.3 megapixel images in 13 min.

In the future, we want to improve the resolution and sensitivity by optimizing the infrared performance of the probing tips, with the ultimate goal of obtaining infrared spectra of single (macro-) molecules. We believe that this ambitious goal can be reached, as already with standard AFM tips we currently obtain infrared spectra of single ferritin particles, which are protein complexes of only 5000 C=O bonds and 1 attogram mass.

What other areas can nano-FTIR be applied to?

nano-FTIR is applicable in many in other fields, such as semiconductor technology, mineralogy, 2D materials and photonics. nano-FTIR can be used to study local carrier concentration in semiconductors and 2D material, composition and structure of crystals and (bio)minerals or localized and propagating electromagnetic waves in nanophotonic structures, devices and 2D materials.

Also, polymer nanostructures and heterostructures can be studied, as well as biomolecules and membranes. We assume that there are many more possibilities than we can think of at the moment! Many customers come and ask us for specific applications, which drives us to think outside the box to constantly improve and develop our neaSNOM near-field microscope.

About Rainer Hillenbrand

Rainer Hillenbrand is Ikerbasque Research Professor and Nanooptics Group Leader, CIC nanoGUNE in San Sebastian (Basque Country, Spain), and is also co-founder and scientific advisor of the company neaspec GmbH (Martinsried, Germany), which develops and manufactures near-field optical microscopes.

From 1998 to 2007 he worked at the Max-Planck-Institut fuer Biochemie (Martinsried, Germany), where he led the Nano-Photonics Research Group from 2003 to 2007. He obtained his PhD degree in physics from the Technical University of Munich in 2001.

Hillenbrand’s research activities include the development of optical near-field nanoscopy and infrared nanospectroscopy, and its applications in nanophotonics, graphene plasmonics, materials sciences and biology.

In 2014 he received the Ludwig-Genzel-Price “for the design and development of infrared near-field spectroscopy and the application of the novel spectroscopy method in different fields of natural sciences”.

Hillenbrand published more than 85 peer-reviewed articles (among others in Science and Nature) on pioneering developments and applications in the field of scattering-type near-field optical microscopy and infrared nanospectroscopy, is coinventor of several key patents of neaspec´s technologies, and presented more than 90 invited talks.