Improved Morphologies and Enhanced Field Emissions of CuO Nanoneedle Arrays by Heating ZnO Coated Copper Foils

Large-area and crack-free films of CuO nanoneedle arrays were synthesized by heating ZnO coated copper foils at a temperature of 400 °C. Scanning electron microscopy shows that the cracking and exfoliation of CuO nanoneedle films are effectively eliminated by predeposition of ZnO layers by a solution route before the heating processes. In addition, the diameters, lengths, and area densities of CuO nanoneedles are tunable by varying the thickness of the ZnO layers. High-resolution transmission electron microscopy demonstrates that the CuO nanoneedles are single-crystalline structures growing along the <31̅0> direction. X-ray diffraction and energy dispersive X-ray spectra reveal that the predeposited ZnO prevents quick oxidation of the copper substrate and prohibits the formation of Cu 2 O. Field-emission measurements show enhanced emission of the crack-free CuO nanoneedle film with a low turn-on field of 0.85 V/μm. The enhanced emission is attributed to the improved morphologies of CuO nanoneedles, as well as elimination of cracks of nanoneedle films. The work provides an efficient route to synthesize well-controlled CuO nanoneedle arrays for nanodevice applications.