Transient absorption spectra were measured to demonstrate carrier injection in multi-layered stacks of Au nanoparticles sandwiched in between TiO2 atomic layer deposited (ALD) thin films. Similar structures were fabricated with ALD Al2O3 for control samples. Sub-percolation thin films of Au resulted in <20 nm particles with plasmon resonances at ~650 nm (~590 nm) in the TiO2 (Al2O3) samples. Two separate pump-probe experiments were preformed to monitor transient heating of the metal and carrier injection in the TiO2. In the first experiment, the metal nanoparticles were excited at 400 nm, and the metal electron dynamics were probed at wavelengths around the plasmon resonance. We measured a decay time of ~1.7 ps in the TiO2-Au layered samples compared to ~2.2 ps in the Al2O3-Au layered samples. The decay times are attributed to electron-phonon coupling. The faster decay in TiO2 may be the result of charge injection into the TiO2. In the second experiment, carriers were excited in the Au nanoparticles by pumping on the plasmon resonance, and the system was probed in the mid-IR to measure free carrier absorption in the TiO2. The TiO2-Au layered sample exhibited transient signals similar to the free carrier absorption signals following excitation of TiO2 films, however, no signal was observed on the Al2O3-Au layered sample. This provides clear evidence that the signal measured in the TiO2-Au layered sample was not from the Au nanoparticles alone but instead originated from charge injection from the Au into the TiO2.
Daniel Ratchford, Adam D. Dunkelberger, Jeff C. Owrutsky, and Pehr E. Pehrsson, "Probing carrier dynamics in wide-bandgap semiconductor-metal nanoparticle hybrids
(Conference Presentation)," Proc. SPIE 9921, Plasmonics: Design, Materials, Fabrication, Characterization, and Applications XIV, 992112 (Presented at SPIE Nanoscience + Engineering: August 30, 2016; Published: 9 November 2016); https://doi.org/10.1117/12.2237168.5161456669001.
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