Abstract
The mechanism for the growth of 3D ZnO nanowall networks in nest-like structure by a pulsed laser deposition (PLD) technique have been investigated for dye sensitized solar cells (DSSCs) and biosensor applications. As complex oxides shift towards large volume productions and practical applications, PLD offers a good advantage over other techniques due to the scalability of the produced film/structures, good adhesion between nanostructures and substrates, and preservation of stoichiometric ratio of the target material. In this study, a two-step temperature–pressure approach has been used to grow ZnO nanostructures on Si(100) using PLD. Optimizing this technique, we have structured a 3D ZnO nanowall networks with nest-like feature without any catalyst. Investigating the effect of substrate temperature from 300°C to 750°C and O2 background from 10mTorr to 10Torr, we have grown the nanowall structures by first depositing a thin ZnO seed layer on Si(100) at a substrate temperature of ~300°C and a background oxygen pressure of 10mTorr, subsequently growing the nanowalls at ~550°C and 500mTorr background. The relationship of the PLD parameters and the morphology of the grown nanowalls were examined by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), X-ray Diffraction (XRD), Atomic Force Microscope (AFM), Photoluminescence (PL) and X-ray Photoemission Spectroscopy (XPS).
The grown ZnO nanowall networks are single-crystals and highly-oriented on the c-axis. The nest-like structures initially grow on the thermodynamically active sites along the grain boundaries of randomly oriented Zn-rich ZnO (011̅0) plane, which undergoes self-nucleation on the amorphous layer of the Si (100) substrates. The growth mechanisms for the synthesis of the ZnO nanowalls network can be explained by the vapor–solid (VS) model.
The PLD-grown 3D ZnO nanowall network has a very high surface-to-volume ratio and appears to be denser than the previously reported ZnO nanowall structures grown by other techniques. This makes the PLD-grown ZnO nanowall a very promising material as photoanodes in DSSCs and an excellent matrix for biomolecule binding for potential applications in biosensors.
•Highly c-axis oriented nest-like ZnO nanowalls were grown on Si using the PLD technique.•Two-step low temperature–pressure templating/seeding approach was used.•The nanowalls were grown at temperature (550°C) and low oxygen background pressure (500mTorr).•The nest-like ZnO nanowalls are thinner with high surface-to-volume ratio.