Femtosecond laser-induced non-thermal welding for a single Cu nanowire glucose sensor

Abstract

Copper nanowires (CuNWs) are a key building block to facilitate carrier conduction across a broad range of nanodevices. For integration into nanoscale devices, manipulation and welding of these nanowires need to be overcome. Based on high energy density laser processing investigation, we report on innovative welding of single CuNWs to a silver film using a tightly focused laser beam combined with manipulation of CuNWs through the dielectrophoresis (DEP) method. Two types of lasers, femtosecond (FS) and continuous-wave (CW), were employed to analyze, improve, and control Cu-NW melting characteristics under high energy density irradiation. The FS laser welding of CuNWs resulted in a metallic joint with a low contact resistance suitable for functional electronic nanodevices. Computational simulations using the 1-D heat diffusion equation and finite difference method (FDM) were performed to gain an insight into metal-laser interactions for high performance welded contact development. Simulation studies on lasers established contrasting melting behavior of metal under laser irradiation. The device feasibility of CuNW based welded contacts was evaluated in terms of the electrical performance of a glucose sensor. It was possible to sense glucose concentration down to 10^-6 M, demonstrating a path towards integration of CuNWs into wearable, flexible nanoelectronic devices.

Fig. 1. SEM images of a CuNW (a) before and (b) after FS laser irradiation and (c) enlarged image of the FS laser irradiation area. CuNW images (d) before and (e) after CW laser irradiation and (f) enlarged image of the CW laser irradiation area.

Fig. 2. EDS mapping results for the (a) CW laser processed sample in air, (b) FS laser processed sample in air, (c) CW laser processed sample in Ar, and (d) FS laser processed sample in Ar.

Fig. 3. (a) Temperature distribution and (b) heatmap of a modeled CuNW with respect to the distance from the heating area (x, the length direction) at a different heating time (t) when using the CW laser irradiation. (c) Electron and lattice temperatures of the first nodal point (T_e and T_l at x = 0) in a short time period after the FS laser pulse (with an average power of 5 mW) starts (<20 ps). Time evolution of lattice temperature (T_l) from the beginning of a laser pulse (300 fs) at five different locations (i.e., x = 0, 0.75, 1.5, 3.0, and 4.5 μm) of the CuNW with an average FS laser power of (d) 5 mW and (e) 35 mW. (f) Temperature distribution and (g) heatmap of the CuNW in the x-direction at different t when using the FS laser with 5 mW average power. The regions surrounded by green lines in (b) and (f) are above the oxidation temperature and vulnerable to oxidation.

Fig. 4. SEM images of a CuNW (a) before laser welding and (b) after FS laser welding, (c) cross-sectional SEM image of the laser-welded area, (d) enlarged image of the blue welding area, (e) enlarged image of the white welding area and (f) real-time resistance change monitoring results during laser welding.

Fig. 5. (a) Variation of resistance with the concentration of glucose sample 1 and (b) sample 2. (c) Variation of the normalized response current with time at a voltage of 0.2 V with increasing glucose concentration.