Fig. 1. Thinning of BP by rinsing with DI water. (a) Scheme illustrating the used concept for the layer-by-layer thinning of micromechanically exfoliated BP flakes on Si/SiO2-substrates. During oxidation PxOy-species form (25% relative humidity, 21 ± 1 °C), which coalesce over time before they can be rinsed with DI water (second step) leading again to a flat BP surface. This process can be repeated several times in order to get the desired thickness. (b) Sequence of optical images of a BP flake showing its pristine (1), oxidized after 5 days (2) and the washed (3) form with DI water. Further oxidation for 2 days (4) and subsequent rinsing with DI water (5) complete the series. The color change from yellow to blueish of the flake already indicates a thinning of the BP flake. (c) AFM images corresponding to the pristine BP flake (1) as well as after each washing procedure (3 & 5). (d) Statistical AFM evaluation recorded in the white square of the corresponding AFM image above visualizing the difference in height between the underlying substrate and the BP flake clearly confirming the thinning effect. Note that the first peak in the topography AFM statistics histograms accounts for the substrate, and the second peak to the flake.
