Scanning Near‐Field Optical Microscopy ( SNOM )

Abstract

The successful artifact‐free scanning near‐field optical microscopy (SNOM) for rough surfaces with the versatile apertureless internal reflection in the shear‐force gap technique is reviewed and compared with the difficulties associated with apertureless scattering and aperture SNOM techniques. The most economical technique requires sharp, economically self‐pulled, tapered waveguide tips and is enabled by a unique reflection enhancement in the shear‐force gap that depends linearly on the damping settings. Owing to numerous factors that determine the enhancement, strong chemical contrasts are obtained even between similar materials on composite materials. Unlike aperture SNOM, the contrast in an internal‐reflection SNOM is precisely at the site of the chemical interface and the optical resolution is below 10 nm, which is equal to or better than the simultaneous AFM resolution at tip radii of about 10 nm. This enables efficient local spectroscopy (fluorescence, Raman, SERS), industrial (dental alloys, textile dyeing, nanoparticle dyes, sustainable chemical production, etc.), and medicinal applications (organelles, cancer prediagnosis, etc.), and helps in the elucidation of supramolecular mechanisms, all at the nanometric level. SNOM analyses of waste‐free supramolecular gas–solid and solid–solid reactions secure far‐reaching anisotropic molecular migrations within crystals, and also in solid‐state photoreactions. Such mechanistic foundation was not possible with previous “topochemistry” hypotheses, which impeded the maturing of the unparalleled sustainable supramolecular productions, by forbidding these migrations.

The Enhancement Effect at Local Reflectance and Emission Back to Apertureless SNOM Tips in the Shear-Force Gap

“…The nominal diameters were 100-200 nm. The depth of the resin cover on the correctly measured nanoparticles could therefore be judged to be 5 nm and the optical resolution is better than the AFM resolution [33]. In another experiment a skew (slope of 10°-15°) anthracene surface with anthraquinone features on it exhibited several 18-20 nm peak-to-peak distances in the optical image [23].…”

“…Similarly, local Raman SNOM spectra of silicon (519.7 cm -1 ) and its freshly grown (5 nm) silica layer (500 cm -1 ) (non-resonant) as well as of gallium nitride on alumina [E 1 (TO) and E 2 Raman modes at 560.8 and 570.4 cm -1 ] at total collection times of 10 min are reported in [33]. These enhanced Raman emissions on flat and rough surfaces are unsurpassed.…”

Organic Solid-State Reactions

“…For example, anthracene reacts slowly in air or oxygen atmosphere to give anthraquinone. This reaction has been studied by AFM and SNOM [8,10]. It turns out that the prominent (0 0 1) and also the (1 1 0) faces are unreactive due to the crystal packing, but there is reaction on random crater slopes that start at (1 1 0), where geometrically both the 9-and 10-H positions of anthracene are available for hydrogen transfer to O 2 as the first reaction step.…”

“…[8]. Independent support of the three-stage mechanism was provided by grazing incidence diffraction (GID) of reacting crystals [36] and by the chemical contrast of SNOM [7,9,10]. We are now in a situation to proceed with a systematic discussion of intracrystalline and intercrystalline and gas-solid thermal or photochemical molecular (including salts) solid-state reactions of all types on a sound mechanistic basis.…”

“…Further images can be found in the cited original publications. In addition, scanning near-field optical microscopy (SNOM) on rough surfaces was developed to independently secure these facts [8][9][10]. If the crystal packing does not offer a means for migration, there will be no chemical reaction independent of short distances of the centers of a putative reaction and that can be shown by the lack of slip planes, channels, and appropriate voids for accommodation.…”