  Nitrogen-vacancy (NV) center in diamond is a promising quantum sensor with
remarkably versatile sensing capabilities. While scanning NV magnetometry is
well-established, NV electrometry has been so far limited to bulk diamonds.
Here we demonstrate imaging external alternating (AC) and direct (DC) electric
fields with a single NV at the apex of a diamond scanning tip under ambient
conditions. A strong electric field screening effect is observed at low
frequencies due to charge noise on the surface. We quantitatively measure its
frequency dependence, and overcome this screening by mechanically oscillating
the tip for imaging DC fields. Our scanning NV electrometry achieved an AC
E-field sensitivity of 26 mV um^(-1) Hz^(-1/2), a DC E-field gradient
sensitivity of 2 V um^(-2) Hz^(-1/2), and sub-100 nm resolution limited by the
NV-sample distance. Our work represents an important step toward building a
scanning-probe-based multimodal quantum sensing platform.
