  The negatively-charged nitrogen-vacancy center (NV) in diamond forms a
versatile system for quantum sensing applications. Combining the advantageous
properties of this atomic-sized defect with scanning probe techniques such as
atomic force microscopy (AFM) enables nanoscale imaging of e.g. magnetic
fields. To form a scanning probe device, we place single NVs shallowly (i.e. <
20 nm) below the top facet of a diamond nanopillar, which is located on a thin
diamond platform of typically below 1 \mu m thickness. This device can be
attached to an AFM head, forming an excellent scanning probe tip. Furthermore,
it simultaneously influences the collectible photoluminescence (PL) rate of the
NV located inside. Especially sensing protocols using continuous
optically-detected magnetic resonance (ODMR) benefit from an enhanced
collectible PL rate, improving the achievable sensitivity. This work presents a
comprehensive set of simulations to quantify the influence of the device
geometry on the collectible PL rate for individual NVs. Besides geometric
parameters (e.g. pillar length, diameter and platform thickness), we also focus
on fabrication uncertainties such as the exact position of the NV or the taper
geometry of the pillar introduced by imperfect etching. As a last step, we use
these individual results to optimize our current device geometry, yielding a
realistic gain in collectible PL rate by a factor of 13 compared to bulk
diamond and 1.8 compared to our unoptimized devices.
