Nano-ftir-nanoscale infrared near-field spectroscopy

Resumen

Optical spectroscopy in the infrared (IR) frequency range has tremendous merit in chemical and structural analysis of materials. IR radiation addresses a rich variety of light-matter interactions because photons in this low energy range can excite molecular vibrations and phonons, as well as plasmons and electrons of non-metallic conductors1,2. A widely used analytical tool for chemical identification of inorganic, organic and biomedical materials3, as well as for exploring conduction phenomena4, is Fourier Transform Infrared Spectroscopy2 (FTIR), which will be briefly introduced in chapter 3 of this thesis. In infrared spectroscopy the unique response of materials to broadband infrared illumination is utilized to identify and characterize them. The majority of the characteristic interactions are found in the so-called ¿finger-print¿ region of the infrared spectrum spanning from 5 ¿ 20 ¿m wavelength (corresponding to 400 ¿ 2000 wavenumbers [cm-1]). However, the diffraction-limited spatial resolution of this technique, which is in the range of 10 ¿m, has been preventing applications in nanoscale material and device analysis, industrial failure analysis or quality control.Recently, tip-enhanced near-field optical microscopy5 and scattering-type scanning near-field optical microscopy (s-SNOM)6,7 have become valuable methods for nanoscale material characterization. They enable optical spectroscopies such as fluorescence, Raman, infrared or terahertz spectroscopy to be performed with nanoscale spatial resolution5. At infrared frequencies, s-SNOM enables for example the nanoscale mapping of free carriers in transistors8, semiconductor nanowires9,10 and vanadium oxide11,12, of chemical composition of polymers13-17 and biological objects18,19, of phonons20,21 and strain22 in polar crystals, or of plasmons in graphene23,24. Chapter 3 of this work will introduce the basic principles of this technique. It is typically based on atomic force microscopy (AFM) where the tip is illuminated with a focused laser beam. The tip-scattered light is detected simultaneously to topography providing an excellent optical resolution in the 10 nm range, independent of the wavelength6,7. Using metallic tips, the strong optical near-field interaction between tip and sample modifies the scattered light, allowing for probing the local dielectric properties with nanoscale resolution.Local spectral information is obtained, up to now, by time-consuming imaging at various but limited wavelengths from tunable laser sources13,20,25,26. The set of near-field images yield the wavelength-dependent dielectric sample properties, from which local chemical composition or conductivity can be determined18,27. For chemical identification of unknown nanostructures, however, the acquisition of local near-field spectra in a broad spectral range spanning from near- to far-infrared frequencies is needed. This issue was addressed in this thesis by developing a novel FTIR system that allows for broadband infrared-spectroscopic nano-imaging of dielectric properties, which became established under the acronym nano-FTIR. It combines the analytical power of infrared spectroscopy with the ultra-high spatial resolution of s-SNOM. Utilizing (i) broadband infrared radiation for illuminating the probing tip and (ii) strong signal enhancement employing an asymmetric28 FTIR spectrometer, the spatial resolution of conventional IR spectroscopy could be improved by more than 2 orders of magnitude. The basic working principles of nano-FTIR are presented in chapter 5 of this work.Nano-FTIR has been experimentally realized with different broadband sources during this thesis. In a first step, a thermal source, similar to what is used in conventional infrared spectroscopy has been implemented for broadband infrared illumination (chapter 6). By mapping a semiconductor device, spectroscopic identification of silicon oxides and quantification of the free-carrier concentration in doped Si regions with a spatial resolution better than 100 nm could be demonstrated29.Due to the limited power of the thermal source, however, only strong phonon and plasmon resonances could be mapped, but not the vibrational contrasts of molecular organic substances, which rely on relatively weak resonances30. Therefore the nano-FTIR setup was combined with a novel laser-based infrared continuum source31, which is able to provide broadband illumination of sufficient power for detecting the weak molecular vibrational resonances of organic samples (chapter 7). It could be demonstrated, for the first time, that nano-FTIR can acquire molecular vibrational spectra throughout the mid-infrared fingerprint region at a spatial resolution of 20 nm, by imaging a typical polymer sample (poly(methyl methacrylate), PMMA). Furthermore, first experimental evidence has been provided that the near-field absorption spectra match well with conventional (far-field) FTIR absorption spectra. Theoretical considerations corroborate this observation (chapter 7.4). This relates, in particular, to the spectral line positions, line widths and line shapes, and therefore allows for direct chemical recognition of nanoscale materials by consulting standard FTIR databases. As an application example, the identification of a nanoscale sample contamination was demonstrated.In typical IR s-SNOM studies and also in the first experiments with nano-FTIR, standard (commercial) metalized AFM tips were used. The infrared antenna performance of these tips32, however, is widely unexplored and antenna concepts33-47 have not yet been applied to optimize near-field probes for the infrared spectral range. In chapter 8 of this work a novel concept is proposed, aiming for superior probing tips that utilize geometrical resonances to optimize the optical performance of tips in near-field optical experiments. By focused ion beam (FIB) machining infrared-resonant antenna tips could be successfully fabricated on standard Si cantilevers. Characterization of these tips by electron energy loss spectroscopy (EELS), Fourier transform infrared (FTIR) spectroscopy and Fourier transform infrared near-field spectroscopy (nano-FTIR) clearly revealed geometrical antenna resonances in the tips, which are found to be in good agreement with numerical calculations. Employing these tips for near-field infrared imaging of individual tobacco mosaic viruses (TMV), state-of-the-art AFM performance accompanied by an excellent optical performance could be verified. Due to their well-defined geometry, these antenna tips will furthermore significantly ease the qualitative description of the tip-sample near-field interaction, which will be essential for quantitative measurements of the local sample properties such as dielectric function and molecular absorption.In conclusion, a novel technique for nanoscale infrared spectroscopy (named ¿nano-FTIR¿) has been developed. It enables the analysis of a large variety of materials with nanoscale spatial resolution and has interesting application potential in widely different sciences and technologies, ranging from materials to chemical and biological sciences. Envisioned applications include nanoscale chemical mapping of polymer blends, organic fibers, and biomedical tissue, as well as quantitative and contact-free measurement of the local free-carrier concentration and mobility in doped nanostructures.