Self-growth of silver tree-like fractal structures with different geometries

Silver tree-like fractal structures are self-grown, where needle growths and branching are iterated, via rapid reduction of silver ions in a solution. Many self-similar frameworks exist at different scales in the whole structure, and geometry is characterized by a fractal dimension. Here, we perform a control of the fractal geometry by facile growth engineering related to the viscosity of the solvent, with acetone and ethanol, on the silver tree-like structure. The geometry changed from needle leaf-like to broadleaf-like structure as the viscosity increased, which resulted in the difference in the fractal dimensions ranging from 1.743 to 1.812, at 303 K. The silver fractal structures exhibited optical responses over a wide wavelength range from 400 to 900 nm, which was also visualized by the dark-field observations with an optical microscope. We demonstrated surface-enhanced Raman scattering (SERS) measurements of para-aminothiophenol at 532, 633, and 785 nm on the silver tree-like fractal structures with three types of different fractal geometries. Compared with the common SERS substrates, the merit of using dendrite structures is the availability of multiple laser wavelengths for SERS excitation and detection on the same sample, with keeping significant enhancement effect. This feature also allows us direct observation of plasmon-mediated chemical transformations of molecules through multi-color excitation SERS measurements.