Objective With manufacturing being downsized to the nanoscale, the welding and joining of nanoscale materials (“nanowelding” for short) has become key in the integration of advanced nanodevice fabrication and packaging. As a bottom-up process, nanowelding can flexibly combine nanostructures with different chemical compositions and selectively modify the interfaces of semiconductor heterojunctions in order to obtain complex electrical and optical properties. In recent years, although the research on nanowelding has made significant progress, the basic theory of low-damage joining of heterojunctions at the nanoscale has not been well-established. Femtosecond (fs) laser is now widely used in the precision machining of materials with high melting point or high damage threshold due to its unique “cold” processing characteristics. In particular, optical irradiation-induced surface plasmonic effect on the metal-dielectric interface can affect the energy redistribution in the nanostructures, which is beneficial for heterogeneous interface modification of semiconductors at the nanoscale. In this study, the nanowelding process of Pt electrodes and TiO2 nanowires under localized fs laser irradiation has been reported. The creation of this welded structure shows good performance of the synaptic response, which indicates a new method to realize the modification of a metal-oxide heterointerface.
Methods Pt/Ti (the thick is 200 nm/5 nm) electrodes with special finger spacing were fabricated by optical lithography and lift-off processes on oxidized Si substrate. TiO2 nanowires with pure rutile phase were synthesized by a hydrothermal process, dispersed and diluted in a high-purity acetone solution, which was then drop-cast on the chip with Pt electrodes. The fs laser beam, with 50 fs pulse duration, 800 nm wavelength, and 1 kHz frequency, was generated from a Ti:sapphire laser system and focused by an objective lens with a numerical aperture of 0.5 at a nanoscale spot overlapping a small portion of the nanowire at the junction. COMSOL Multiphysics v5.2 with a RF module was used to simulate the electric field distribution around the Pt-TiO2 nanoscale structure under polarized Gaussian beam excitation. The morphology of the welded structure was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrical characteristic was measured by a precision source and s measuring unit (Angilent B2901A) in a voltage sweeping mode at room temperature.
Results and Discussions Under focused fs laser irradiation, the TiO2 nanowire deposited on a Pt substrate exhibits a limited damaging effect. By adjusting the incident laser fluence to 5.02 mJ/cm 2, the nanowelding of Pt-TiO2 was obtained (Fig. 3). The mechanical strength of the welded Pt-TiO2 bonding can be evaluated by “contact AFM”. The coincidence of the loading/unloading force curves, corresponding to the extend/retract process of the probe, indicates a reliable welded bonding at the irradiated location. In contrast, the separation of the loading/unloading force curves indicates that the nanowire does not remain bound to the Pt electrode without laser irradiation and moves away during the extend/retract process (Fig. 4). The bonding between the TiO2 nanowire and the Pt electrode is mainly facilitated by the formation of “hotspots”, which is known to result from localized plasmonic field enhancement (Fig. 6). During the welding process, the Magnéli phase (oxygen deficient TiO2-x) is formed at the Pt-TiO2 contact by redox reactions under high intensity excitation. The layer contains a high defect concentration, which is beneficial for the wettability of the Pt-TiO2 interface as well as the reduction of the heterointerface barrier height. The electrical characteristics show that after fs laser welding on one side of the TiO2 nanowire contacted with the two Pt electrodes, there is an obvious self-rectifying current response at a bias of -10/+10 V (Fig. 7). Besides, the introduced TiO2-x layer during nanowelding is helpful for the synaptic plasticity of the TiO2nanowire as an artificial synapse. For the initial TiO2 synapses, multilevel excitatory postsynaptic current amplification is observed under voltage cycling. Unlike the unwelded units, the maximum amplified current in the welded structure is stabilized during the first ten cycles (Fig. 10). The current accumulation and decay properties of the artificial synapse to simulate the learning/forgetting response of human memory are also investigated. The repeated application of input pulses induces an enhancement in the current response stability, which suggests the transition from short-term potentiation to long-term potentiation in the TiO2 synapse by repeated stimulation.
Conclusions We have demonstrated a method for welding TiO2 nanowires on Pt electrodes that utilizes plasmonic effect induced by fs laser irradiation. The welded bonding of Pt-TiO2 can be obtained at a fluence of 5.02 mJ/cm 2, which has been confirmed by the “contact AFM”. Strong plasmonic-enhanced electric fields induced by tightly-focused fs laser irradiation exist at the Pt-TiO2 interface and contribute to the formation of localized oxygen deficiencies (Magnéli phases). This introduced component improves the wettability of the TiO2 nanowire on the Pt electrodes and reduces the interfacial barrier height, which results in the stability of the TiO2 synapse. Based on the welded Pt-TiO2 structure, the synaptic plasticity of a single TiO2 nanowire is presented, which shows potential in replicating the complicated learning/forgetting process.
