  In this work, we reported uniform layer-by-layer sublimation of black
phosphorus under heating below 600 K. The uniformity and crystallinity of BP
samples after thermal thinning were confirmed by Raman spectra and 2D Raman
imaging. A uniform and crystalline bilayer black phosphorus flake with an area
of 180 um^2 was prepared with this method. No micron scale defects were
observed. The sublimation rate of BP was around 0.18 nm / min at 500 K and 1.5
nm / min at 550 K. Both room and high temperature Raman peak intensity ratio
Si/A2g vs. BP thickness relations were established for in-situ thickness
determination. The sublimation thinning method was shown to be a controllable
and scalable approach to prepare few-layer black phosphorus.
