A thermo-enhanced field emission phenomenon was observed from ZnO nanowires. The field emission current increased by almost two orders of magnitude under a constant applied electric field, and the turn on field decreased from 6.04 MV/m to 5.0 MV/m when the temperature increased from 323 to 723 K. The Poole-Frenkel electron excitation from the defect-induced trapping centers to the conduction band under high electric fields is believed to be the primary cause of the observed phenomenon. The experimental results fit well with the proposed physical model. The field emission from ZnO nanowires with different defect concentrations further confirmed the role of defects. Using the thermo-enhanced field emission phenomenon, a diode flat panel X-ray source was demonstrated, for which the energy and dose can be separately tuned. The thermo-enhanced field emission phenomenon observed from ZnO nanoWires could be an effective way to realize a large area flat panel multi-energy X-ray source. (C) 2016 Elsevier B.V. All rights reserved.