Noble metal nanoparticles and nanofilms with strong localized surface plasmon resonances (LSPRs) have attracted great interests in fields such as nanoscale photonics, biological sensing 1 2 , surface-enhanced Raman scattering (SERS) 3 4 , photocatalytic and photoelectrochemical processes 5 , and plasmonic absorption enhancement in solar cells 6 7 8 9 10 11 12 13 14 15 16 17 18 19 . The LSPRs arise from the excitation of a collective electron oscillation within the metallic nanostructure induced by the incident light, leading to enormous optical local-field enhancement and a dramatic wavelength-selective photon scattering at the nanoscale 20 21 22 23 . The exceptional optical properties introduced by LSPRs have spurred tremendous efforts to design and fabricate highly SERS-active substrates for molecular sensing. The most studied and best established systems are substrates sprayed with Ag or Au colloids that give high SERS signals at some local ‘hot junctions’ 24 . In order to fabricate noble nanoparticle arrays with high SERS activity and improve the uniformity, lithographic techniques have been employed.

We have recently reported a relatively simple approach in fabricating uniform gold nanocrystal-embedded nanofilms via a conventional magnetron sputtering method. In this method, one can more conveniently assemble noble metals with precise gap control in the sub-10-nm regime 25 than any other method. As a continual effort in supporting the above claim, here we report further evidence such as visible absorption spectra of the Au film on indium tin oxide (ITO) glass substrates, the blend films of poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) and poly(3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) on ITO glass substrates, and the SERS measurements of molecules adsorbed on gold nanocrystals deposited on ITO glass substrates. Our results suggest that the continuous ultrathin nanofilm can obviously enhance visible-range absorption in the active layer of solar cells and obtain an ultrasensitive SERS-active coating.

To compare the impact of continuous ultrathin gold nanofilms on the absorption of visible light, plasmonic enhancement of the P3HT:PCBM bulk heterojunction system is demonstrated in a spin-cast device with an incorporated continuous ultrathin gold nanofilm thicknesses of 2 nm or so which are chosen to be sufficiently thin to limit the amount of light absorbed before reaching the active layer. The nanofilm incorporated with gold in the active P3HT:PCBM layer is shown to have significantly greater absorbance enhancement than the nanofilm without gold in the entire excitation spectral range in Figure 3. As shown in Figure 2, the optical absorption spectrum of the continuous ultrathin gold nanofilm has high light transmittance and broad surface plasmon resonance band in the wavelength range of 300 to 1,000 nm. Therefore, the results demonstrate that the enhancement of absorption in the wavelength range of 350 to 1,000 nm is due to the surface plasmon resonance absorption. The much higher plasma frequency of Au ensures a better overlap between plasmon resonance and absorption band of organic semiconductors. The light energy is trapped mainly in the P3HT:PCBM layer, leading to enhanced absorption in the active layer.

For the ITO/Au film/PEDOT:PSS/Au film/P3HT:PCBM and ITO/PEDOT:PSS/Au film/PEDOT:PSS/Au film/P3HT:PCBM structures, the plasmon resonance is located at a wavelength range of 350 to 1,000 nm. The plasmonic peak better overlaps the P3HT:PCBM absorption band. These enhancements concerning light absorption in the visible region can be explained by the surface plasmon polariton resonance of metallic nanoparticles in the gold nanofilm. When metallic nanoparticles are in close proximity, their plasmon resonances couple with each other and generate a light-scattering spectrum that depends strongly on the interparticle distance. The two-dimensional distinctive ultrathin continuous gold nanofilms can be used as subwavelength antennas in which the plasmonic near-field is coupled to the organic semiconductor, increasing its effective absorption cross section. A corrugated metallic film on the back surface of the P3HT:PCBM photosensitive layer can couple light into surface plasmon polariton (SPP) modes supported at the metal/P3HT:PCBM organic semiconductor interface as well as guided modes in the organic semiconductor slab. Both the shape and size of metal nanoparticles are key factors in determining the coupling efficiency. The two-layer ultrathin nanofilm increases the nanoparticle density; according to the Mie theory, the extinction coefficient is proportional to the nanoparticle density. Consequently, optical local-field enhancement of the two-layer continuous ultrathin gold nanofilm is stronger than that of the one-layer ultrathin continuous gold nanofilm. Figure 3 embodies the absorbance of the two-layer ultrathin continuous gold nanofilm which far outweighs that of ITO/PEDOT:PSS/Au film/P3HT:PCBM and ITO/Au film/PEDOT:PSS/P3HT:PCBM. In brief, the enhanced efficiency is shown to stem from field enhancement originating both from localized plasmonic resonances and periodic similar nanopatch antenna configuration and SPP modes in the peculiar gold nanofilm.

To investigate the performance for electromagnetic enhancement, SERS spectroscopic measurements were carried out using Rhodamine 6G, a well-characterized test molecule. Spectra obtained from Rhodamine 6G molecules at a concentration of 10⁻³ to 10⁻⁶ M are shown in Figure 4 which exhibit repeatable high SERS sensitivity. The distances between the centers of two adjacent particles and the particle diameter are important parameters affecting SERS activity. This ultrathin continuous gold nanofilm produces a high Raman signal due to its periodic arrangements, high nanoisland density, and control of the gap between the nanostructures in the sub-10-nm regime. The observed SERS efficiency can be explained in terms of interparticle coupling-induced Raman enhancement. Thus, the distinctive continuous gold nanofilm is very effective in providing abundant hot spots for SERS enhancement.

In conclusion, we have produced continuous ultrathin gold nanofilms with high local-field enhancement effect and a high SERS activity. Spectral analysis suggests that the prominent light absorption in organic photosensitive materials and the high SERS activity arise from the near-field effect of localized surface plasmons of nanoparticles. Owing to their distinctive morphology and high light transmittance, continuous ultrathin gold nanofilms can be used in multilayer organic solar cells to trap light without affecting the physical thickness of solar photovoltaic absorber layers and yielding new options for solar cell design. Further work is needed to research two-dimensional distinctive continuous gold nanofilms that are utilized to trap light in solar cells which may be suitable for application to the high photoelectric conversion efficiency of organic solar cells.

The authors declare that they have no competing interests.

L-DW carried out the design and the characterization of ultrathin gold films, performed the ultrathin gold nanofilm surface plasmon resonance analysis, and drafted the manuscript. R-ZL participated in the fabrication of gold films. S-QZ participated in the SERS measurement. TZ, X-YZ, Q-LW, and XL read the manuscript and contributed to its improvement. All authors read and approved the final manuscript.