  Sculpturing desired shapes in single crystal diamond is ever more crucial in
the realization of complex devices for nanophotonics, quantum computing, and
quantum optics. The crystallographic orientation dependent wet etch of single
crystalline silicon in potassium hydroxide (KOH) allows a range of shapes
formed and has significant impacts on MEMS (microelectromechanical systems),
AFM (atomic force microscopy), and microfluidics. Here, a crystal direction
dependent dry etching principle in an inductively-coupled plasma reactive ion
etcher is presented, which allows to selectively reveal desired crystal planes
in monocrystalline diamond by controlling the etching conditions. The principle
is demonstrated when the kinetic energy of incident ions on diamond surfaces is
reduced below a certain threshold leading to anisotropic etching and faceting
along specific crystal planes. Using the principle, monolithic diamond
nanopillars for magnetometry using nitrogen vacancy centers are fabricated. In
these nanopillars, a half-tapering angle up to 21{\deg} is achieved, the
highest angle reported, which leads to a high photon efficiency and high
mechanical strength of the nanopillar. These results represent the first
demonstration of crystallographic orientation dependent reactive ion etch
principle, which opens a new window for shaping specific nanostructures which
is at the heart of nanotechnology. It is believed that this principle will
prove to be valuable for structuring and patterning of other single crystal
materials as well.
