(color online) (a) PL spectra with bias voltages sweeping from −0.5 V to +0.5 V of a single QD embedded in an n–i–Schottky device with an excitation power of 2.37 μW. The PL peaks from X³⁻, X²⁻, X⁻, X⁰, and X⁺ are labeled in the figure. The dotted lines mark the beginning of emission lines of X⁰ and X⁺. (b) Band profiles of the n–i–Schottky diode structure under bias voltages of −0.5 V and +0.5 V. The energy bands tilt with the two bias voltages due to the built-in electric field of the structure. A positive bias voltage of 0.75 V calculated by one-dimensional Poisson–Schrödiner solver and measured via the photocurrent signal is required to achieve zero total electric field in this structure. With non-resonant excitation, the photo-generated carriers first emerge at GaAs matrix, then relax to wetting layer, finally are captured by QDs, as marked by blue and purple arrows.^[20]