NMR Observation of Mobile Protons in Proton-Implanted ZnO Nanorods

Electronic and diffusion properties of hydrogen in ZnO have been extensively studied, since efficient defect engineering is essential to fabricate electronic, ferroelectric, and optical devices1,2,3,4. Furthermore, diffusion properties of H donors may help in understanding the mechanism responsible for ferromagnetic order triggered by proton implantation5. The electronic characteristics on ZnO can be modified by proton irradiation, as the physical origin is attributed to the hydrogen shallow donors6,7,8,9. Recently, it has also been reported that radiation-induced defects cause the threshold voltage shift and the electrical conductance modulation, making them potentially applicable in nanoelectronic devices7,10. Although previous works addressed effects of proton implantation on electrical properties, it is still far from being well understood from a microscopic point of view. Here, we observe atomic-scale features on protons and their diffusion properties in proton-implanted ZnO nanorods by using proton NMR spectroscopy. Extensive works have been made to investigate the site and stability of interstitial H (Hi) in ZnO by employing infrared and secondary ion mass spectroscopy11,12. The activation barrier for the diffusion of Hi was determined to be 0.4–0.5 eV, corroborating the occurrence of thermally unstable species as suggested by theoretical calculations2,4,11,12. The Hi and hydrogen trapped within the O vacancy site (HO) were believed to be the cause of n-type conductivity. The Hydroxyl group on the ZnO surface also exhibits pronounced effects on the chemical activity and electronic properties of oxide surfaces13,14. The hydroxyl groups on ZnO have been recently identified by observing their vibrational modes by infrared spectroscopy13,14. 1H NMR spectroscopy can be one of the best method to identify hydrogen species and to observe their diffusion properties in ZnO. The dynamical properties of mobile protons in the lattice site of ZnO were previously investigated by 1H NMR spectroscopy15,16,17. However, the assignment of interstitial H and surface hydroxyl group on the NMR spectra is a matter of controversy15,16. Previously, Wang et al. attributed the NMR line at 4.8 ppm to the mobile proton in the lattice of ZnO15. In contrast, it has been recently reported that the resonance line is due to the hydroxyl group on the surface of ZnO16. Thus, the identification of Hi and surface hydroxyl group on ZnO and their diffusion properties are still not clear. NMR relaxometry is a powerful technique of atomic-scale access to probe ion hopping motion in solids18,19,20. The laboratory-frame relaxation rate is effective for probing nuclear spin precessing in radio frequency range, i.e., fast diffusing spins. The rotating-frame relaxation rate, on the other hand, effectively probes motions occurring at ultralow-frequencies21. Mobile and rigid hydrogen species in ZnO have distinct values for the spectral density in the ultralow-frequency region, thus giving an opportunity to simultaneously investigate their diffusion properties17,21. Here we identify comprehensive hydrogen species in proton-implanted ZnO nanorods and investigate their dynamical properties by employing the rotating-frame spin-lattice relaxation technique. Our work gives manifest evidence for the first time from a microscopic point of view that implanted protons become mobile in the lattice.