Anomalous Acoustoelectric Currents in Few-Layer Black Phosphorus Nanocrystals

We report. on the anomalous acoustoelectric (AE) currents in few-layer black phosphorus (BP) nanocrystals as the result of strong Bragg reflections. The device consists of a surface acoustic wave (SAW) delay line and a BP field-effect transistor, which uses a graphene film as its gate electrode with an atomically thin boron nitride gate dielectric layer. It was found that the Bragg reflection could give rise to large anomalous AE currents when the hole concentration was sufficiently large. Our experimental results showed that the magnitude and the direction of the anomalous AE currents were related to the applied bias and the position of the device with respect to the nodes/antinodes of the standing waves. The observations have been explained based on some assumption. Our work provides more insights into the interaction between SAW and 2D materials and highlights the particularity of anomalous AE effect in such systems.