Metal nanoparticles (NPs) respond to electromagnetic waves by creating surface plasmons (SPs), which are localized, collective oscillations of conduction electrons on the NP surface. The coupled particles can then act as optical antennae capturing and refocusing light. Furthermore, a molecule linking such NPs can be affected by these interactions.
The device platform consists of a disordered array of Au NPs fabricated on insulating glass substrates, as shown in Fig a. The average separation of Au NPs has the length scale of dithiolated (porphinato)zinc(II) oligomers (Fig b, c), so that NPs can be interconnected by the dithiol-PZn3 molecule.
Illumination of laser of different wavelengths brought remarkable differences in the photocurrent responses. With red (655nm), green (532nm) and blue (405nm) laser illumination, there were increases of conductivity over the dark experiment (Fig. d, e). Enhancement factors as high as 12.1x for red light over blue light were observed, in spite of a lower photon absorption cross section of the molecule in the red region. This enhancement has been attributed to the optical focusing and increased photon flux resulting from coupled SPs in AuNP arrays.
As schematically shown in Fig. f, in dark, an applied voltage results in hole tunneling through the small barrier; in the presence of blue light, an initially prepared exciton state relaxes to a lower energy state, producing excess current; in the presence of red light, coupled resonating SPs focus excitation between particles leading to multiple exciton states and higher current.