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Plasmonic welding of hybrid Au-ZnO nanostructure

We report that 532 nm CW laser can be used to obtain non-wetting metal-semiconductor (Au-ZnO) Schottky heterojunctions by plasmonic welding. Single crystal Au and n-type ZnO nanowires are placed on gold (Au) and titanium (Ti) electrodes, respectively, and the junction welding is realized. The current-voltage (I-V) characteristic curve of the single Schottky rectifier is also measured.


Introduction
One-dimensional metal/semiconductor nano-materials have received widespread attention for their potential applications in micro/nano-electronics and -photonics. Metal/semiconductor Schottky junction with nonlinear electrical response can be applied in integrated circuits, including high-speed switches [1] and rectifier units [2] . The condition of Schottky junction formation is a large bias between the work function of the metal and the fermi level of the semiconductor. Different sizes and doping concentration can lead to different barrier heights.

Experimental setup
The length of n-type ZnO nanowire ranges from 30 μm to 100 μm, and its diameter is about 150 nm. The length of single-crystal Au nanowire ranges from 20 μm to 30 μm, and its diameter is about 200 nm. Figure 1 is the schematic of the experimental set-up for photothermal nano-welding. A Nd: YAG laser is used as the light source. The operating wavelength is 532 nm. An attenuator is placed next to the laser to control the laser intensity. To regulate the exposure time, a shutter is used. Here, the exposure time is set as 4 ms. For uniform heating, circularly polarized light is used. A CCD is used to monitor the nanomanipulation by two fiber probes.

Results and analysis
A single crustal Au nanowire is placed on top of a n-type ZnO nanowire using a nano-manipulator. The circularly polarized laser beam of wavelength 532 nm is focused on the top of the nanowire junction using a microscope objective (100 X). The scanning electron microscope (SEM) images are used to observe the quality of the welded junction. Figure 2 shows the welded junction of the single crystal Au and n-type ZnO nanowires. We studied electrical property of this structure towards further probing the quality of the welded junction. In order to obtain the characteristic current-voltage (I-V) curve, the single crystal Au and ntype ZnO nanowires were placed on Au and Ti electrodes, respectively. We welded three junctions: (1) single crystal Au nanowire on Au electrode, (2) single crystal Au and n-type ZnO nanowires, and (3) ntype ZnO nanowire on Ti electrode (see figure 3. a). Junction 1 and 3 are ohmic contacts because the barrier height, which is the difference between work function of Ti (4.1 eV) and Fermi level of ZnO (4.3 eV), is less than 1.0 eV. On the other hand, junction 2 is Schottky contact due to the large barrier height between single-crystal Au (work function is 5.2 eV) and n-type ZnO nanowires. The I-V characteristic curve (see figure 3.b) shows that the current remains about zero when the applied bias voltage is below +0.7 V, and it starts increasing beyond this voltage.
bias voltage 3 V. This is typical behavior of single Schottky rectifier. It can be concluded that good quality junction is realized by plasmonic welding.

Conclusion
Our experimental results demonstrate the feasibility of the nano-scale device fabrication of metalsemiconductor (Au-ZnO) Schottky heterojunctions by photothermal welding technique. It has great significance in large-scale integration applications such as high-speed switches and rectifier units.