Vibrational analysis with scanning probe microscopy
[5] The combination of Raman scattering and NSOM techniques was first realized in 1995, when it was used for imaging a Rb-doped KTP crystal at a spatial resolution of 250 nm.
[8] In 2000, T. Kalkbrenner et al. used a single gold particle as a probe for apertureless scanning and presented images of an aluminium film with 3 μm holes on a glass substrate.
[9] The resolution of this apertureless method was 100 nm, that is comparable to that of fiber-based systems[9] Recently, a carbon nanotube (CNT) having a conical end, tagged with gold nanoparticles, was applied as a nanometer-resolution optical probe tip for NSOM.
In 2000, Stȍckle et al.[13] first designed a setup combining apertureless NSOM, Raman and AFM techniques, in which the tip had a 20 nm thick granular silver film on it.
They reported a large gain in the Raman scattering intensity of a dye film (brilliant cresyl blue) deposited on a glass substrate if a metal-coated AFM tip was brought very close to the sample.
[14][15] High-resolution spatial maps of Raman signals were obtained with this technique from molecular films of such compounds as brilliant cresyl blue, malachite green isothiocyanate and rhodamine 6G,[16] as well as individual carbon nanotubes.
Previously, IR-NSOM was realized by applying a solid immersion lens with a refractive index of n, which shortens wavelength (λ) to (λ/n), compared to FTIR-based IR microscopy.
[19] IR-NSOM uses an AFM to detect the absorption response of a material to the modulated infrared radiation from an FTIR spectrometer and therefore is also referred to as AFM/FTIR spectroscopy.
In 2007, AFM was combined with infrared attenuated total reflection (IR-ATR) spectroscopy to study the dissolution process of urea in a cyclohexane/butanol solution with a high spatial resolution.
To obtain a high resolution Raman micrograph/spectrum, the following conditions should be met: (1) the size of the aperture must be on the order of the wavelength of the excitation light.
An important AFM feature is the ability to accurately control the distance between the sample and probe tip, which is the reason why the AFM-Raman combination is preferred for realizing Raman-NSOM.
If the focusing objective lens is also used for collecting the scattered photons (backscattering geometry), the optimum angle is around 55° with respect to the surface normal.
Whereas SEM and TEM usually require vacuum and an extensive sample preparation, SPM measurements can be performed in atmospheric or liquid conditions.
AFM and NSOM can be used to detect the response when a modulated infrared radiation generated by an FTIR spectrometer is absorbed by a material.
In the AFM-IR technique the absorption of the radiation by sample will cause a rapid thermal expansion wave which will be transferred to the vibrational modes of the AFM cantilever.
This approach can map chemical composition beyond the diffraction-limit resolution and can also provide three-dimensional topographic, thermal and mechanical information at the nanoscale.
FEL is an excellent IR source, with 2–20 μm spectral range,[50][51] short pulses (picosecond) and high average power (0.1-1 W).
[44][52] The essence of NSOM/FTIR is that it allows the detection of non-propagating evanescent waves in the near-field (less than one wavelength from the sample), thus yielding high spatial resolution.
One main challenge in apertureless NSOM/FTIR is a strong background signal because the scattering is obtained from both near-field and remote area of the probe.
Lateral (x-y plane) oscillation of the fiber tip is induced by applying an AC voltage to a dither piezo-scanner.
[3] In the first developed setup of AFM-IR, a sample is mounted onto an infrared-transparent zinc selenide prism for excitation purposes (Figure 12), then an optical parametric oscillator (OPO)-based tunable IR lased is radiated on the molecules to be probed by the instrument.
Similar to conventional ATR spectroscopy, IR beam illuminates the sample through total internal reflection mechanism (Figure 12).
The amplitudes of the cantilever provide information of local absorption spectra, whereas the oscillation frequencies depend on the mechanical stiffness of the sample (Figure 12).
It usually takes several minutes to accumulate a conventional Raman spectrum, and this time could be much longer in Raman-NSOM; for example, 9 hours for a 32×32-pixel image.
[6][19] As to near-field IR/AFM, high optical losses in aqueous environments (water is strongly absorbing in the IR range) reduces the signal-to-noise ratio.
[18][63] Improving the resolution and enhancing the instrumentation with user-friendly hardware and software will make AFM/NSOM coupled with IR/Raman a useful characterization tool in many areas including biomedical, materials and life sciences.
[64] For example, this technique was used in detecting the spin-cast thin film of poly(dimethylsiloxane) with polystyrene on it by scanning the tip over the sample.
[66] These techniques can be also used in numerous biological related applications including the analysis of plant materials, bone, and single cells.
Biological application was demonstrated by detecting details of conformation changes of cholesteryl-oleate caused by FEL irradiation with a spatial resolution below the diffraction limit.
[70] Because of their high spatial resolution, NSOM/AFM-Raman/IR techniques can be used for measuring the width of multilayer films, including layers which are too small (in the x and y directions) to be probed with conventional IR or Raman spectroscopy.
