AFM-IR

AFM-IR (atomic force microscope infrared-spectroscopy) is one of a family of techniques that are derived from a combination of two parent instrumental techniques; infrared spectroscopy and scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope (a type of SPM) equipped with a sharp probe that measured the local absorption of infrared light by a sample; it required that the sample be coupled to an infrared-transparent prism and be less than 1μm thick. It improved the spatial resolution of photothermal AFM-based techniques from microns to circa 100 nm.It is worth noting that the first infrared spectrum obtained by measuring thermal expansion using an AFM was obtained by Hammiche and co-workers without inducing resonant motions in the probe cantilever. In this early example the modulation frequency was too low to achieve high spatial resolution but there is nothing, in principle, preventing the measurement of thermal expansion at higher frequencies without analysing or inducing resonant behaviour. Possible options for measuring the displacement of the tip rather than the subsequent propagation of waves along the cantilever include; interferometry focused at the end of the cantilever where the tip is located, a torsional motion resulting from an offset probe (it would only be influenced by the motions of the cantilever as a second order effect) and exploiting the fact that the signal from a heated thermal probe is strongly influenced by the position of the tip relative to the surface thus this could provide a measurement of thermal expansion that wasn’t strongly influenced by or dependent upon resonance. The advantages of a non-resonant method of detection is that any frequency of light modulation could be used thus depth information could be obtained in a controlled way (see below) whereas methods that rely on resonance are limited to harmonics. The thermal-probe based method of Hammiche et al. has found a significant number of applications.AFM-IR nanospectroscopy of a laser printer toner particle, showing spatially resolved chemical analysis. Toner particles are typically complex composites of various binders and transfer agents; these can be revealed by AFM-IRAFM-IR compositional mapping of Streptomyces bacteria. Left: AFM topographic image of bacterial cells. Middle: AFM-IR absorption at 1650 cm−1, corresponding to the amide I band associated with protein. Right: AFM-IR absorption at the carbonyl band 1740 cm−1, indicating the distribution of triglyceride vesicles within bacterial cells. AFM-IR (atomic force microscope infrared-spectroscopy) is one of a family of techniques that are derived from a combination of two parent instrumental techniques; infrared spectroscopy and scanning probe microscopy (SPM). The term was first used to denote a method that combined a tuneable free electron laser with an atomic force microscope (a type of SPM) equipped with a sharp probe that measured the local absorption of infrared light by a sample; it required that the sample be coupled to an infrared-transparent prism and be less than 1μm thick. It improved the spatial resolution of photothermal AFM-based techniques from microns to circa 100 nm. Recording the amount of infrared absorption as a function of wavelength or wavenumber creates an infrared absorption spectra that can be used to chemically characterize and even identify unknown materials. Recording the infrared absorption as a function of position can be used to create chemical composition maps that show the spatial distribution of different chemical components. Novel extensions of the original AFM-IR technique and earlier techniques have enabled the development of bench-top devices capable of nanometer spatial resolution, that do not require a prism and can work with thicker samples, and thereby greatly improving ease of use and expanding the range of samples that can be analysed. One of these techniques has achieved spatial resolutions down to around 20 nm, with a sensitivity down to the scale of molecular monolayer AFM-IR is related to techniques such as tip-enhanced Raman spectroscopy (TERS), scanning near-field optical microscopy (SNOM), nano-FTIR and other methods of vibrational analysis with scanning probe microscopy. The earliest measurements combining AFM with infrared spectroscopy were performed in 1999 by Hammiche et al. at the University of Lancaster in the United Kingdom, in an EPSRC-funded project led by M Reading and H M Pollock. Separately, Anderson at the Jet Propulsion Laboratory in the United States made a related measurement in 2000. Both groups used a conventional Fourier transform infrared spectrometer (FTIR) equipped with a broadband thermal source, the radiation was focused near the tip of a probe that was in contact with a sample. The Lancaster group obtained spectra by detecting the absorption of infrared radiation using a temperature sensitive thermal probe. Anderson took the different approach of using a conventional AFM probe to detect the thermal expansion. He reported an interferogram but not a spectrum; the first infrared spectrum obtained in this way was reported by Hammiche et al. in 2004: this represented the first proof that spectral information about a sample could be obtained using this approach. Both of these early experiments used a broadband source in conjunction with an interferometer; these techniques could, therefore, be referred to as AFM-FTIR although Hammiche et al. coined the more general term photothermal microspectroscopy or PTMS in their first paper. PTMS has various subgroups; including techniques that measure temperature measure thermal expansion use broadband sources. use lasers excite the sample using evanescent waves, illuminate the sample directly from above etc. and different combinations of these. Fundamentally, they all exploit the photothermal effect. Different combinations of sources, methods, methods of detection and methods of illumination have benefits for different applications. Care should be taken to ensure that it is clear which form of PTMS is being used in each case. Currently there is no universally accepted nomenclature. The original technique dubbed AFM-IR that induced resonant motion in the probe using a Free Electron Laser has developed by exploiting the foregoing permutations so that it has evolved into various forms. The pioneering experiments of Hammiche et al and Anderson had limited spatial resolution due to thermal diffusion - the spreading of heat away from the region where the infrared light was absorbed. The thermal diffusion length (the distance the heat spreads) is inversely proportional to the root of the modulation frequency. Consequently, the spatial resolution achieved by the early AFM-IR approaches was around one micron or more, due to the low modulation frequencies of the incident radiation created by the movement of the mirror in the interferometer. Also, the first thermal probes were Wollaston wire devices that were developed originally for Microthermal analysis (in fact PTMS was originally considered to be one of a family of microthermal techniques). The comparatively large size of these probes also limited spatial resolution. Bozec et al. and Reading et al. used thermal probes with nanoscale dimensions and demonstrated higher spatial resolution. Ye et al described a MEM-type thermal probe giving sub-100 nm spatial resolution, which they used for nanothermal analysis. The process of exploring laser sources began in 2001 by Hammiche et al when they acquired the first spectrum using a tuneable laser (see Resolution improvement with pulsed laser source). A significant development was the creation by Reading et al. in 2001 of a custom interface that allowed measurements to be made while illuminating the sample from above; this interface focused the infrared beam to a spot of circa 500μm diameter, close to the theoretical maximum. The use of top-down or top-side illumination has the important benefit that samples of arbitrary thickness can be studied on arbitrary substrates. In many cases this can be done without any sample preparation. All subsequent experiments by Hammiche, Pollock, Reading and their co-workers were made using this type of interface including the instrument constructed by Hill et al. for nanoscale imaging using a pulsed laser. The work of the University of Lancaster group in collaboration with workers from the University of East Anglia led to the formation of a company, Anasys Instruments, to exploit this and related technologies (see Commercialization). In the first paper on AFM-based infrared by Hammiche et al., the relevant well-established theoretical considerations were outlined that predict that high spatial resolution can be achieved using rapid modulation frequencies because of the consequent reduction in the thermal diffusion length. They estimated that spatial resolutions in the range of 20 nm-30 nm should be achievable. The most readily available sources that can achieve high modulation frequencies are pulsed lasers: even when the rapidity of the pulses is not high, the square wave form of a pulse contains very high modulation frequencies in Fourier space. In 2001, Hammiche et al. used a type of bench-top tuneable, pulsed infrared laser known as an optical parametric oscillator or OPO and obtained the first probe-based infrared spectrum with a pulsed laser, however, they did not report any images Nanoscale spatial resolution AFM-IR imaging using a pulsed laser was first demonstrated by Dazzi et al at the University of Paris-Sud, France. Dazzi and his colleagues used a wavelength-tuneable, free electron laser at the CLIO facility in Orsay, France to provide an infrared source with short pulses. Like earlier workers, they used a conventional AFM probe to measure thermal expansion but introduced a novel optical configuration: the sample was mounted on an IR-transparent prism so that it could be excited by an evanescent wave. Absorption of short infrared laser pulses by the sample caused rapid thermal expansion that created a force impulse at the tip of the AFM cantilever. The thermal expansion pulse induced transient resonant oscillations of the AFM cantilever probe. This has led to the technique being dubbed Photo-Thermal Induced Resonance (PTIR), by some workers in the field. Some prefer the terms PTIR or PTMS to AFM-IR as the technique is not necessarily restricted to infrared wavelengths. The amplitude of the cantilever oscillation is directly related to the amount of infrared radiation absorbed by the sample. By measuring the cantilever oscillation amplitude as a function of wavenumber, Dazzi's group was able to obtain absorption spectra from nanoscale regions of the sample. Compared to earlier work, this approach improved spatial resolution because the use of short laser pulses reduced the duration of the thermal expansion pulse to the point that the thermal diffusion lengths can be on the scale of nanometres rather than microns.