Abstract
Room temperature atmospheric plasma treatments are widely used to activate and control chemical functionalities at surfaces. Here, we investigated the effect of atmospheric pressure plasma jet (APPJ) treatments in reducing atmosphere (Ar/1‰ H2 mixture) on the photoluminescence (PL) properties of single crystal ZnO nanorods (NRs) grown through hydrothermal synthesis on fluorine-doped tin oxide glass substrates. The results were compared with a standard annealing process in air at 300 °C. Steady-state photoluminescence showed strong suppression of the defect emission in ZnO NRs for both plasma and thermal treatments. On the other side, the APPJ process induced an increase in PL quantum efficiency (QE), while the annealing does not show any improvement. The QE in the plasma treated samples was mainly determined by the near band-edge emission, which increased 5–6 fold compared to the as-prepared samples. This behavior suggests that the quenching of the defect emission is related to the substitution of hydrogen probably in zinc vacancies (VZn), while the enhancement of UV emission is due to doping originated by interstitial hydrogen (Hi), which diffuses out during annealing. Our results demonstrate that atmospheric pressure plasma can induce a similar hydrogen doping as ordinarily used vacuum processes and highlight that the APPJ treatments are not limited to the surfaces but can lead to subsurface modifications. APPJ processes at room temperature and under ambient air conditions are stable, convenient, and efficient methods, compared to thermal treatments to improve the optical and surface properties of ZnO NRs, and remarkably increase the efficiency of UV emission.
Highlights
ZnO is long known as an environmentally friendly wide bandgap semiconductor with an exciton binding energy of 60 meV.1,2 The high exciton binding energy ensures that excitonic emission is significant at room temperature, and ZnO is a promising candidate for stable room temperature luminescent and lasing devices.3,4 ZnO with various morphologies, such as bulk ZnO, nanoparticles, films, nanowires, and tetrapods, has been extensively investigated for different applications.5–8 Among all morphologies, the one-dimensional (1D) ZnO nanorods (NRs) and nanowires (NWs) are the most common and studied structures
We report a simple room temperature atmospheric pressure plasma jet (APPJ) treatment, using a reducing process gas, on hydrothermally synthesized ZnO NRs vertically grown on fluorine-doped tin oxide (FTO) conductive glass substrates
We demonstrated the effect of atmospheric H2plasma treatment on the optical properties of hydrothermally synthesized ZnO NRs
Summary
ZnO is long known as an environmentally friendly wide bandgap semiconductor with an exciton binding energy of 60 meV. The high exciton binding energy ensures that excitonic emission is significant at room temperature, and ZnO is a promising candidate for stable room temperature luminescent and lasing devices. ZnO with various morphologies, such as bulk ZnO, nanoparticles, films, nanowires, and tetrapods, has been extensively investigated for different applications. Among all morphologies, the one-dimensional (1D) ZnO nanorods (NRs) and nanowires (NWs) are the most common and studied structures. An alternative post-treatment able to reduce the green emissions due to ZnO NW defects is the exposure to vacuum plasma using a hydrogen-containing process gas. In this case, the vacancies are filled by hydrogen, which diffuses through the ZnO crystal. The use of process gases containing oxygen or nitrogen does not allow us to achieve results similar to hydrogen in the visible band emission reduction and NBE increase In this case, the plasma effect is limited to the surface and no gas species diffusion in the ZnO crystals can be achieved.. The optical performances of APPJ-treated ZnO NRs were improved, and the possible mechanism was studied and compared with the as-prepared and thermally treated ones
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