Enhanced Photoemission Efficiency of hot electrons in Au Nanodisk-Cluster Complexes on TiO2

Localized surface plasmon resonances are collective oscillations of free electrons in metal nanoparticles when excited by light and can generate hot electron-hole pairs through plasmon decay with promising applications in photovoltaics, photodetection, and catalysis. However, the practical utilization of these nonequilibrium charge carriers is currently limited because of the low photoemission efficiencies in harvesting photons with energies lower than the semiconductor band gap.  To this end, new study published in ACS Nano and conducted by Professor Yurui Fang, Dr. Nan Gao from the Dalian University of Technology alongside Professor Lei Shao from the Sun Yat-sen University investigated the photoemission efficiency of hot electrons at the Au nanodisk-cluster complex/TiO₂ interface and developed an innovative optical nanoantenna-sensitizer design that enhanced the light absorption and hot electron injection efficiency. First the team fabricated samples with small Au clusters, Au nanodisks, and nanodisk-cluster complexes on a TiO₂ substrate. The Au clusters were less than 3 nm in size, while the nanodisks had a diameter of 100 nm and a height of 35 nm. They created these structures using a hole-mask colloidal lithography method, followed by sputtering and annealing processes to ensure proper formation and adhesion. The coverage percentages of the clusters, nanodisks, and complexes on the TiO₂ were carefully controlled through this fabrication process. The team tested the hypothesis that smaller Au clusters, when coupled with larger nanodisk antennas can significantly enhance the absorption and subsequent emission of hot electrons. Moreover, the authors validated using scanning electron microscopy the successful fabrication of the structures with distinct differences in the morphology and distribution of the Au clusters and nanodisks. Afterward, they performed optical transmission measurements on the fabricated samples using a spectrophotometry setup and found that the small Au clusters on TiO₂ to have a low extinction peak around 550 nm due to their small absorption cross-section. However, when combined with large Au nanodisks, the extinction and IPCE spectra showed enhanced peaks. Moreover, the nanodisk-cluster complexes showed photoemission efficiencies approximately three times higher than the sum of the individual clusters and nanodisks, despite having a smaller total covered area. This enhancement was attributed to the strong near-field optical confinement provided by the large nanodisks, which effectively focused light onto the small clusters. The researchers performed electron energy loss spectroscopy (EELS) on the samples to further understand the near-field enhancement. EELS technique can map the plasmonic resonance and field enhancement at the nanodisk-cluster interfaces. The EELS spectra revealed that the plasmon resonance from the large nanodisks dominated the overall feature due to its high optical density of states. They observed that the small clusters benefited significantly from the plasmon excitation of the large nanodisks. The enhancement factor of the electric field intensity decayed with distance from the nanodisk, following an exponential decay model. The authors’ data showed that the average field intensity enhancement near the nanodisk was approximately 8.7 times which confirms the strong near-field effect predicted by the simulations. Moreover, the researchers studied the quantum dot-like behavior of the small Au clusters using high-resolution transmission electron microscopy and density of states calculations and found that the clusters had discrete energy levels similar to quantum dots and that these discrete levels combined with charge doping effects resulted in shifts in the resonant peaks of the clusters. Furthermore, the team evaluated the internal quantum efficiency of the hot electrons injected from Au to TiO₂ and found the IPCE measurements to be normalized to the surface area and metal volume and that the efficiency was highly dependent on the amount of gold and the enhancement factors from the nanodisk antennas.

In conclusion, Professor Yurui Fang, Dr. Nan Gao, and Professor Lei Shao successfully developed a new optical nanoantenna-sensitizer capable of enhancing light absorption and hot electron generation and this innovation can directly impact the efficiency of solar energy harvesting systems and the Au nanodisk-cluster complexes integration with semiconductor materials such as TiO₂, solar cells could achieve higher conversion efficiencies and ultimately makes solar energy a more viable and cost-effective alternative to traditional energy sources. Additionally, the principles and mechanisms the authors proposed in their study can be applied to the development of advanced photodetectors with improved hot electron generation and transfer which will lead to more accurate and efficient detection systems for medical diagnostics, environmental monitoring, and communications.