  The capability to finely tailor material thickness with simultaneous atomic
precision and non-invasivity would be useful for constructing quantum platforms
and post-Moore microelectronics. However, it remains challenging to attain
synchronized controls over tailoring selectivity and precision. Here we report
a protocol that allows for non-invasive and atomically digital etching of van
der Waals transition-metal dichalcogenides through selective alloying via
low-temperature thermal diffusion and subsequent wet etching. The mechanism of
selective alloying between sacrifice metal atoms and defective or pristine
dichalcogenides is analyzed with high-resolution scanning transmission electron
microscopy. Also, the non-invasive nature and atomic level precision of our
etching technique are corroborated by consistent spectral, crystallographic and
electrical characterization measurements. The low-temperature charge mobility
of as-etched MoS$_2$ reaches up to $1200\,$cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$,
comparable to that of exfoliated pristine counterparts. The entire protocol
represents a highly precise and non-invasive tailoring route for material
manipulation.
