Fig. 1. SEM images of Ag nanowire-CNF heterojunctions formed under fs laser irradiation at different fluences and irradiation time: (a) 7.0 mJ/cm2 for 30 s, (b) 8.5 mJ/cm2 for 30 s, (c) 8.5 mJ/cm2 for 60 s and (d) 10.0 mJ/cm2 for 30 s.

Fig. 2. TEM morphology and composition analysis of T-type Ag NW tip-CNF heterojunction structures treated by fs laser irradiation. The fluence was 8.5 mJ/cm2, the pulse repetition frequency was 1000 Hz, and the irradiation time was 30 s: (a) Overall morphology of two Ag tip-CNF heterojunctions formed on the same CNF; (b) HRTEM image of one of these Ag-CNF heterojunctions; (c) Magnified image of (b), indicating the area of FFT processing; (d, e) Diffraction patterns of Ag, and CNF adjacent to the Ag-CNF interface; (f) Diffraction pattern within the Ag-CNF interface; (g)?(i) EDX mappings of the Ag, C and O elemental abundances in the region shown in (b).

Fig. 3. TEM images of morphology together with compositional analysis of Ag nanowire-CNF cross structures treated by fs laser irradiation. The fluence was 8.5 mJ/cm2, the pulse repetition frequency was 1000 Hz, and the irradiation time was 30 s: (a) Overall morphology of an Ag NW-CNF cross-structured heterojunction; (b) HRTEM image of the Ag-CNF heterojunction; (c) Magnified image of (b), indicating the area of FFT processing; (d)?(f) The diffraction patterns of Ag, CNF and the Ag-CNF interface, respectively; (g)?(i) EDX mappings of (b), indicating the elemental distribution of Ag, C, and O, respectively.

Fig. 4. FDTD simulations of the distribution of electric field intensity for four different structures under fs laser irradiation with two orthogonal polarization directions as indicated by arrows in the figure: (a, b) T-type Ag NW tip-CNF; (c, d) T-type CNF tip-Ag NW; X-type cross-structures (e, f) Ag NW on top of CNF and (g, h) CNF on top of Ag NW.

Fig. 5. (a) Schematic of the resistance measurement configuration used for a single Ag nanowire-CNF heterojunction; (b) SEM image of the Ag nanowire-CNF heterojunction before laser irradiation; (c) Conductivity change with increasing laser irradiation time; (d?f) I?V responses of the heterojunction after laser irradiation under 8.5 mJ/cm2 for 0 s, 15 s, and 30 s, respectively. Sequential voltage sweeps are indicated as 1 to 5.

Fig. 6. Performance of fabricated strain sensors with and without fs laser processing: (a) SEM image showing the structure of a CNF-Ag NW hybrid nanowire strain sensor after fs laser joining. (b) Schematic of the experimental configuration for the determination of resistance vs strain in a hybrid sensor consisting of a mixture of CNFs and Ag NWs. The density of CNFs and Ag NWs is much higher than that shown in this figure; (c) Relative resistance change with strain for sensors coated with CNF & Ag hybrid nanowires. (d) Relative resistance change versus applied strain for sensors coated only with CNF nanowires. (e) Time-dependent resistance in a CNF-Ag NW sensor over several cycles to strains of 15.8% and 31.7%. (f) SEM image of the surface of a typical sensor after multiple strain cycles where the maximum strain exceeded 70%. (g, h) Schematic of the breakage and recovery of CNF-Ag NW junctions with strain cycles, which explains how strain influences the resistance of strain sensor.