Time-Dependent Density Functional Theory Investigation of the Electronic Structure and Chiroptical Properties of Curved and Helical Silver Nanowires.

Time-dependent density functional theory methods are employed to examine the evolution of the absorption and circular dichroism (CD) spectra of neutral bare silver helical nanostructures as a function of their geometrical parameters. Calculations of excited states to determine optical absorption and CD spectra were performed using the SAOP/TZP level of theory. In our model, the geometry of the helical silver chain is dependent on the Ag-Ag-Ag bond angle and the Ag-Ag-Ag-Ag dihedral angle. The influence of different geometrical structures on the optical absorption and CD spectra were studied for helical and planar Ag8. Silver nanowires Agn (n = 4, 6, 8, 10, 12) were examined to determine the effect of the helical chain length on the electronic properties. The results show that when the metal atomic chain loses planarity, strong CD signals arise; the intensities of the CD peaks for these structures are strongly affected by the shape and length of the silver nanowires. The theoretically predicted CD spectra of the nonplanar Ag4 and Ag6 model systems show good agreement in spectral shapes and reasonable agreement in peak locations compared to experimental data for silver-DNA clusters. However, the theoretical and experimental results for the longer Ag12 wire show larger differences in the peak locations, which could potentially be caused by effects such as the presence of DNA and cationic silver atoms in the experimental system.