Chemical contrast in scanning near-field optical microscopy

Abstract

Repeatable and stable scanning near-field optical microscopy (SNOM) images are obtained under shear-force control if uncoated, i.e. cold and sharp, tapered fiber tips are used. True chemical contrast is seen for the first time on organic crystals due to the different near-field reflectivities of different chemical species on partly oxidized anthracene. The topography spans a Z-range of 500 nm. The surface roughness is tolerable and the sites of reaction can be related to crystal structure data. A submicroscopic local resolution of 18 nm has been obtained for chemical contrast, discriminating different chemical species even with poorly reflecting organics on rough crystal surfaces. Thus, reflectionback-to-the-fiber SNOM will find wide applications for both transparent and opaque samples. All drawbacks with blunt and hot metal-coated tips at a < 10 nm distance from the surface in previous SNOM setups are overcome. ©

Organic Solid-State Reactions

“…Thus, the autoxidations can continue from there. SNOM localizes also the position of the intermediate charge-transfer complex between anthracene and benzoquinone molecules [84]. An extreme sensitivity toward traces of oxygen exhibits the conjugated blue-green benzo-bis-cyclobutadiene derivative in Scheme 10.…”

Waste‐free synthesis and production all across chemistry with the benefit of self‐assembled crystal packings

“…[3,25] Even better and more sensitive proof is provided if AFM is combined with SNOM, which provides identification of products by chemical contrast. Air autoxidation at slopes on unreactive (110) of anthracene or around a nucleation site on the unreactive (001) face of 2-mercaptobenzothiazole have been imaged in Reference [26] . Another example there is the diazotization of sulfanilic acid with NO 2 gas only at a slope on the unreactive (010) face.…”

Organic Solid-State Reactions with 100% Yield

“…Mechanistic investigations of gas-solid and solid-solid reactions as well as their proper engineering require identifiable crystal surfaces for atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) [1,3,[13][14][15] in combination with X-ray diffraction data, which are the basis of crystal packing analyses [1,3,[16][17][18].…”

“…On the other hand, the combination anthracene and anthraquinone did not form the chargetransfer complex upon milling or from solution but only from the rapidly cooled melt [15]. This brown complex separates slowly (several months) into anthracene and anthraquinone upon standing at room temperature in a complete solid-state reaction.…”

“…2-Mercaptobenzothiazole (32) was oxidized by nitrosonium nitrates such as 8 to give 453 and 454 upon milling. This reaction has also been mechanistically studied by AFM [1] and SNOM [15]. While solid 2-mercaptobenzothiazole (32) is not efficiently oxidized by solid iodine, its methylamine salt 33 is quantitatively oxidized by milling with iodine and gives 453 with 100% yield after washing with water and drying [22].…”