Three-color plasmon-mediated reduction of diazonium salts over metasurfaces

Abstract

Surface plasmon-mediated chemical reactions are of great interest for a variety of applications ranging from micro- and nanoscale device fabrication to chemical reactions of societal interest for hydrogen production or carbon reduction. In this work, a crosshair-like nanostructure is investigated due to its ability to induce local enhancement of the local electromagnetic field at three distinct wavelengths corresponding to three plasmon resonances. The structures are irradiated in the presence of a solution containing diazonium salts at wavelengths that match the resonance positions at 532 nm, 632.8 nm, and 800 nm. The resulting grafting shows polarization and wavelength-dependent growth patterns at the nanoscale. The plasmon-mediated reactions over arrays of the crosshair structures are further investigated using scanning electron microscopy and supported by finite domain time domain modelling revealing wavelength and polarization specific reactions. Such an approach enables nanoscale molecular printing using light source opening multiplexing applications where different analytes can be grafted under distinct opto-geometric conditions.

Fig. 1. Distribution of the near-field at 530 nm (a), 630 nm (b) and 800 nm (c) for a horizontal polarization. The corresponding isowavelength map is shown in (d) and represents the spatial distribution of the field for wavelengths varying from 500 nm to 800 nm with a crosshair structure outlined in black.

Fig. 2. (a) Depiction of a grafting experiment. (b) Produced individual structures.

Fig. 3. (a) Representative SEM image of an array of dendrimers used for grafting with a single structure shown in the inset. (b) FDTD far-field scattering for ideal (defect-free) and real structures (as shown in Fig. 1b) and experimental absorption measurements for a linearly polarized light oriented horizontally or vertically with respect to the structure axes.

Fig. 4. SEM images of the dendrimers after irradiation with an 800 nm fs-pulsed laser, for both (a) horizontally and (b) vertically polarized light. (c) FDTD modelling of the near-field distribution upon irradiation at 800 nm for the real structure with a horizontal polarization.

Fig. 5. SEM images of the structures after irradiation using a 632.8 nm continuous-wave laser, for both (a) horizontally and (b) vertically polarized light. (c and d) The same images but colorized in red to highlight the areas on which grafting mainly occurred. (e) FDTD modelling of the near-field distribution upon irradiation at 630 nm for the real structure with a horizontal polarization.

Fig. 6. SEM images of the crosshairs after irradiation using a 532 nm laser both (a) horizontally and (b) vertically polarized. (c) and (d) the colorized images of (a) and (b), respectively, with the outline of the cross in black as a guide for the eye. (e) FDTD modelling of the near-field distribution upon irradiation at 530 nm for the real structure with a horizontal polarization. The reference background image used to generate (c) and (d) is provided in Fig. S4.