AFM FOR THE IMAGING OF LARGE AND STEEP SUBMICROSCOPIC FEATURES, ARTIFACTS AND SCRAPING WITH ASYMMETRIC CANTILEVER TIPS

Very high and steep features of a microporous membrane from regenerated cellulose and of chemically generated solids, which are produced by photodimerization of crystalline 9-chloroanthracene (1) or by surface hydration of crystalline hydroxylamine hydrochloride with moist air, are probed with contact atomic force microscopy (AFM) using pyramidal Si3N4 tips. Wide scans under high and rapid feedback control produce stable images if the cantilever tips do not scrape and if all scan parameters are properly set. Both tip/sample convolutes of closely packed cones (about 1 mu m high and steeper than 55 degrees) and virtually undisturbed floes (with slope angles of 42 degrees at both sides) are reliably imaged. Validity tests are provided by establishing image stability during more than ten consecutive scans, by imaging with high magnification in the Z direction, and by comparing perspective views at 0 degrees and 90 degrees. The stability proves an absence of surface alterations and an absence of nanoliquids, and the images show an absence of scan tracks and of scan-generated asymmetries in all 3D features. The artifact of a tip not hitting the ground while the cantilever slides over remote obstacles is investigated on a microporous membrane. Odd planes and very steep wedge-like indentations amongst the regular features are artificially imaged. Materials of low hardness are probed and scraping of the surfaces must be strictly avoided. Thus, the reasons for erratic surface alterations in contact AFM are elucidated. Asymmetric cantilever tips scrape at low forces already, while symmetric ones do not at low or higher forces. Conversely, asymmetric cantilever tips are used for writing nanostructures of considerable deepness into organic surfaces. No piling up of material occurs at the edges when deep square holes are scraped out of crystalline trans-3-benzylidenebutyrolactone (3). Depths of 50 nm or 120 nm are achieved within five or ten AFM scans with an asymmetric cantilever tip at a 10(-8) N force. The application of AFM for elucidation of further types of organic solid-state reactions is discussed.