Breaking the barriers – chemical analysis on the nanoscale

Craig Prater and Kevin Kjoller discuss how atomic force microscopy and infrared spectroscopy combine to characterise chemicals on a nanoscale.

One of the long term goals of probe microscopy has been to perform bulk type analytical measurements on the nanoscale. While Atomic Force Microscopy (AFM) can measure mechanical, electrical, magnetic and thermal properties of materials, the technique has lacked the robust ability to chemically characterise unknown materials. Infrared spectroscopy is the accepted technique for chemical identification and is used in a broad range of sciences and industry to characterise and identify materials via their vibrational resonances of chemical bonds. AFM has been integrated with IR spectroscopy to allow measurement of high quality IR spectra at arbitrary points in an AFM image to give nanoscale chemical characterisation at a resolution not previously available from a commercial instrument. 

AFM has been enormously successful in addressing problems in basic nanoscale research as well as applied problems in both the materials and life sciences.  However, a gap in the AFM’s capabilities has been the ability to chemically characterise regions of the sample. This is especially important in the study of heterogeneous materials like polymer blends, multilayer films and nanocomposites. Several AFM probe-based techniques have been used to beat the diffraction limit of conventional IR measurements. Traditional IR microscopy has a resolution limit roughly three times its wavelength, on the scale of many to tens of microns. Various optical scattering methods attempt to relate spectral optical properties of materials to their chemical composition. In general, near field approaches are single or narrow band and do not produce rich spectra that can be used to characterise a broad range of vibrational resonances associated with different chemical species. Other IR techniques are based on measuring the local temperature rise from spectral absorption through the use of AFM cantilevers integrated with conventional Fourier Transform IR (FTIR) spectrometers. These approaches allow broader spectrum measurement than near-field approaches, but the spatial resolution is typically limited due to thermal diffusion to the scale of many microns. 

Anasys has released a novel lab-based instrument, nanoIR, which combines

atomic force microscopy (AFM) and infrared spectroscopy to enable chemical characterisation of polymers and other samples at scales below the diffraction limit. The instrument employs a photothermal measurement that uses an AFM probe to measure local thermal expansion from IR light incident upon a sample. 

The technique was originally developed by Dazzi using a tunable infrared source based on an institutional free electron laser source1. This capability has been developed into an instrument using a lab scale IR source.  As shown in figure 1a, the sample is illuminated via the pulsed tunable IR laser light source. When IR radiation is absorbed by the sample, it creates a rapid thermal expansion wave that excites resonant oscillations of the AFM cantilever. By measuring the amplitude of the cantilever vibrations as a function of the IR source wavelength, a local IR absorption spectrum can be created.

The nanoIR uses a new IR source, a nanosecond optical parametric oscillator that is continuously tunable from 2.5 μm to 10 μm (4000 to 1000 cm-1). This covers a major portion of the mid-IR including important CH, NH and CO bands, as well as carbonyl and amide I/II bands. The IR source is based on an Nd-YAG laser that pumps nonlinear crystals to generate higher wavelengths. The spectral width is less than 16 cm-1 over the range between 1200 and 3600 cm-1. Samples are mounted on a zinc selenide (ZnSe) prism and the IR beam is arranged to illuminate the sample by total internal reflection similar to the attenuated total reflection (ATR) technique employed in IR spectroscopy. The cantilever deflection is recorded with a traditional AFM optical lever system with a detection bandwidth of roughly 2 MHz. A high speed data acquisition system records each cantilever ring-down event. The IR source is pulsed at ~1 kHz and we typically synchronously average multiple ring-downs (e.g. 256) to improve the measurement sensitivity. With measurement and tuning time, each spectrum currently takes about one minute. To provide an alternative output, the IR source may be tuned to a single wavelength and the absorption at that wavelength can be spatially mapped over the sample.

The co-averaged ring-down events are analysed to extract both IR absorption and mechanical properties. The ring-down events typically include one or more "contact resonances" meaning that the modes of oscillation that couple the cantilever's resonant properties with the mechanical stiffness and damping of the sample area in contact with the AFM tip. The peak amplitude of the ring-down and/or the amplitude of one or more contact resonance modes is extracted for each wavelength to construct an absorption spectrum. The contact resonant frequencies can be used to extract relative stiffness and sample viscosity, figure 1b.

Samples are prepared on ZnSe prisms in one of two ways. Ultramicrotomy has

been used to cut sections with thicknesses between 100 nm and 1000 nm. These may then be transferred to a prism surface. Other sample preparations include casting thin films from solvent directly on the prism. A variety of cantilevers may be used including standard contact mode cantilevers, nanothermal analysis cantilevers and novel enhanced resonance probes.

Infrared nanospectroscopy has been used to measure and map a variety of polymer samples. Figure 2 shows measurements performed on degradable polymers. Biodegradable polymers are important materials in a variety of applications ranging from tissue engineering, drug delivery, food packaging and textiles. Such materials are increasingly complex blends of base materials and performance enhancing additives. The nanoIR system has been used to map, characterise and even identify specific polymer additives.

The spatial mapping of polymer matrix and additives shows the variations in chemical components. In the line spectral map (figure 3), the spatially varying concentration of the C=O carbonyl band (1740 cm-1) and the single bond C-O peak at around 1100 cm-1 are clearly seen.

Infrared nanospectroscopy has also been used to make multifunctional measurements on composite materials. In this example, figure 3, polyethylene terephthalate fibres in a polyamide (nylon) matrix is imaged. The point spectra clearly identify the materials through the carbonyl and amide peaks.

Figure 4 illustrates the combination of chemical and thermal maps of the composite.

Such a combination of chemical and physical measurements with resolution on the nanoscale has not been illustrated before the advent of nanoIR. Nanothermal analysis is useful as often it will give the confirmation of a chemical identification by reporting the softening temperature of individual component materials.

The final example illustrates another composite system, a nylon and ethylene acrylic acid copolymer multilayer sample. Figure 5 shows measurements of topographic, chemical, mechanical and thermal properties. The mechanical and spectroscopic data was obtained simultaneously, thus allowing direct correlation of mechanical stiffness information with chemical composition data. Note that the transitions in contact stiffness correlates extremely well with the strength of the CH absorption.  Nanothermal analysis was also performed on the same sample clearly identifying softening at different temperatures for the nylon and EAA layers.

The correlation of structure and function is of critical importance to materials

science and engineering at the nanoscale. With infrared nanospectroscopy, spatially resolved topographic, chemical, mechanical and thermal properties have been shown. The nanoscale resolution of these measurements operates well below the diffraction limits of conventional IR spectroscopy. Furthermore, these measurements can be performed either simultaneously and/or with the same probes on the same samples.  As a result, it is possible to measure and study materials with greater richness and spatial resolution than previously available. Because of the high correlation to FTIR spectra, infrared nanospectroscopy can leverage the enormous existing materials databases of IR absorption reference spectra. It is possible to export infrared nanospectroscopy data into Bio-Rad's KnowItAll spectral analysis and successfully identify unknown materials via correlation with IR absorption databases. 

Applications of infrared nanospectroscopy that combine infrared spectroscopy and atomic force microscopy to provide high resolution topographic, chemical, mechanical and thermal mapping have been demonstrated.  This combination provides spatial resolution at length scales well below the diffraction limit of conventional IR spectroscopy and adds chemical spectroscopy to the field of atomic force microscopy.