Gate-Tuned Thermoelectric Power in Black Phosphorus

The electric field effect is a useful means of elucidating intrinsic material properties as well as for designing functional devices. The electric-double-layer transistor (EDLT) enables the control of carrier density in a wide range, which is recently proved to be an effective tool for the investigation of thermoelectric properties. Here, we report the gate-tuning of thermoelectric power in a black phosphorus (BP) single crystal flake with the thickness of 40 nm. Using an EDLT configuration, we successfully control the thermoelectric power (S), and find that the S of ion-gated BP reached +510 $\mu$V/K at 210 K in the hole depleted state, which is much higher than the reported bulk single crystal value of +340 $\mu$V/K at 300 K. We compared this experimental data with the first-principles-based calculation and found that this enhancement is qualitatively explained by the effective thinning of the conduction channel of the BP flake and non-uniformity of the channel owing to the gate operation in a depletion mode. Our results provide new opportunities for further engineering BP as a thermoelectric material in nanoscale.