Fabrication and application of novel highly ordered nano-heterostructure arrays in optoelectronics

• Main Author: Yadian, Boluo
• Other Authors: Huang Yizhong
• Format: Theses and Dissertations
• Language: English
• Published: 2016
• Subjects: DRNTU::Engineering::Materials::Photonics and optoelectronics materials
• Online Access: http://hdl.handle.net/10356/66325
• Institution: Nanyang Technological University

Nano-heterostructures are material system formed by the joint of at least two dissimilar semiconductor material components at nanoscale. For nano-hetero-semiconductors, the coupling between them could provide more functional and effective properties than individual components. This is due to the formation of the interface between these semiconductors with unequal band gaps when they are in contact. To date, a great deal of effort has been devoted to the design, fabrication and characterization of various nano-heterostructures in terms of its morphology, dimension, defects, strain, and lattice mismatch. However, the fabrication and manipulation of highly ordered nano-heterostructure arrays are still challenging. This thesis presents the fabrication and characterization of the novel highly ordered ZnO/Si nano-heterostructure arrays, and the subsequent modification to optimize its optoelectronic properties. Focused ion beam system is employed to fabricate the highly ordered Si nanocone arrays, and then the ZnO nanorods are grown on top of each Si nanocone through hydrothermal method. The obtained ZnO/Si nano-heterostructure array is then subject to further modification, including heat treatment, gallium ion irradiation, and metal sulfide decoration to enhance the optoelectronic properties. As an example, attempt has been made to use this highly ordered nano-heterostructure array as the photoelectrode in the photoelectrochemical water splitting system, to demonstrate its application in optoelectronic fields. To fabricate the highly ordered ZnO/Si nano-heterostructure array, ZnO seed layer was first deposited on the surface of Si (p{111}) substrate. The nucleation sites were then defined by patterning the surface with gallium ion beam. After that the hydrothermal process was used to grow ZnO nanorods from the nucleation sites. The whole fabrication process is simple, facile and offers a direct control of the space, length and aspect ratio of the array. The photoluminescence (PL) measurement of the nano-heterostructure arrays was performed. It is found that ZnO/Si nano-heterostructures show an improved interface after heat treatment. The enhanced photoresponse to white light after the heating process is contributed to the recrystallization of ZnO and the developed tensile lattice strain of Si. Photoluminescence measurement of ZnO/Si nano-heterostructure arrays subject to gallium ion irradiation at different ion energies (0.5 keV~16 keV) was performed. The result shows that the intensity of UV emission increases rapidly with the increase of gallium ion energy and reaches the maximum at around 2 keV, approximately 50 times higher than the UV intensity produced from as-grown nanorods. The gentle bombardment of low energy gallium ions removes the defects from ZnO nanorod surfaces. The gallium ions, on the other hand, are implanted into the nanorods, resulting in compressive strain. It is believed that the perfect arrangement of crystal lattice after the removal of surface defects and the introduced compressive strain are two factors that contribute to the significant enhancement of UV emission in the PL spectra. In the present thesis, attempt has been successfully made to decorate ZnO/Si nano-heterostructure arrays with ZnS and Ag2S. ZnS is known to be very stable in aqueous solutions. Meanwhile, Ag2S with a narrow band gap enables more efficient absorption of incident light and faster separation of photogenerated electron-hole pairs. As a result, the Ag2S/ZnS/ZnO/Si nano-heterostructure array has been demonstrated to be a good potential candidate as the photoelectrode in photoelectrochemical water splitting system.