Two-photon absorption induced nanowelding for assembling ZnO nanowires with enhanced photoelectrical properties

The joining of semiconductor nanowires (NWs) is fundamental for the construction and assembly of high performance nanoelectronic devices, but the development of reliable methods of nanojoining and nanowelding of these components has been elusive to date. In this work, we report a methodology for laser welding of wide bandgap NWs based on two-photon absorption. Two photon excitation during femtosecond laser irradiation leads to the generation of excitons forming an electron-hole plasma. As an application of this technique, we show that two-photon excitation is effective in the nanowelding of two ZnO NWs. A nanoweld, resulting in the formation of an interconnected structure, occurs when the energy in the solid state plasma is deposited in the contact area between the two ZnO NWs. During excitation with ultrashort laser pulses, rapid melting and solidification result in the generation and freezing out of oxygen vacancies in the irradiated area and the region near the contact between the two components. This enhances exciton trapping and energy deposition at the contact, facilitating the formation of a bond between the two NWs. It is also found that the absorption of visible light is significantly increased in ZnO NW structures assembled via two-photon femtosecond laser processing. In addition, the junction between two ZnO NWs created in this way exhibits a photoresponse that is not present prior to nanojoining. These results indicate that two-photon excitation is a promising technique for the selective deposition of thermal energy in semiconductor NWs in the absence of plasmonic interactions.