Abstract
The electronic and the optoelectronic properties of graphene-based nanocomposites are controllable, making them promising for applications in diverse electronic devices. In this work, tetrapod-shaped zinc oxide (T-ZnO)/reduced graphene oxide (rGO) core/coating nanocomposites were synthesized by using a hydrothermal-assisted self-assemble method, and their optical, photoelectric, and field-emission properties were investigated. The ZnO, an ideal ultraviolet-light-sensitive semiconductor, was observed to have high sensitivity to visible light due to the rGO coating, and the mechanism of that sensitivity was investigated. We demonstrated for the first time that the field-emission properties of the T-ZnO/rGO core/coating nanocomposites could be dramatically enhanced under visible light by decreasing the turn-on field from 1.54 to 1.41 V/μm and by increasing the current density from 5 to 12 mA/cm2 at an electric field of 3.5 V/μm. The visible-light excitation induces an electron jump from oxygen vacancies on the surface of ZnO to the rGO layer, resulting in a decrease in the work function of the rGO and an increase in the emission current. Furthermore, a field-emission light-emitting diode with a self-enhanced effect was fabricated making full use of the photo-assisted field-emission process.
Highlights
Field emission has been extensively studied for its importance in both fundamental research and high-power device applications, such as microwave power tubes, terahertz generators, and X-ray generators[1,2,3]
We report that the zinc oxide (ZnO) material, an ideal ultraviolet-light-sensitive semiconductor[33], can exhibit properties that are highly sensitive to visible light when coated with reduced graphene oxide; such properties have never before been observed
The peculiar tetrapod structure of the tetrapod-shaped zinc oxide (T-ZnO) may cause one of the crystal whiskers to protrude from the T-ZnO layer at a large oblique angle regardless of the random orientation of the T-ZnO, as shown the inset of Fig. 1(c) and Supplementary Fig. S4, which is beneficial for potential applications in field-emission devices
Summary
Field emission has been extensively studied for its importance in both fundamental research and high-power device applications, such as microwave power tubes, terahertz generators, and X-ray generators[1,2,3]. We have demonstrated that ZnO nanostructures, including nanorods, tetrapod-shaped ZnO (T-ZnO), nanoparticles, and ZnO film[4,5,6,7], exhibit excellent field-emission properties Their relatively low conductivity and low aspect ratio have limited their emission current density and have hindered their practical applications in efficient field-emission devices. The reduced graphene film can be used as a bottom electrode, as well as a substrate, in flexible field-emission devices due to its large flexibility and high conductivity, and to the low contact barrier between the reduced graphene film and the ZnO nanowires. We have demonstrated that the field-emission properties of T-ZnO can be improved by coating it with graphene-oxide (GO) sheets, which enhance the mechanical connection between the T-ZnO and the bottom electrode[31]. The fabrication of T-ZnO/graphene core/coating nanocomposites, the modulation of their photoelectric properties, and their possible visible-light-assisted field-emission performances have not yet been demonstrated
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