Auger-Excited Photoluminescence from Gold Nanoflowers

Abstract

Photoluminescence from metal nanostructures offers a promising means of studying excited charge processes in metal nanostructures. Moreover, they have many potential applications in sensing, imaging, and nanothermometry. However, a general understanding of the emission from metal nanoparticles has not yet been achieved. In particular, the possible presence of sequential emission mechanisms involving the excitation of conduction band electrons via interband Auger scattering remains unclear. In this article, we provide spectroscopic evidence of Auger-excited intraband emission from gold nanoflowers. We employ a combination of photoluminescence and photoluminescence excitation spectroscopy to investigate the excitation pathways in films of gold nanoflowers. While, on the one hand, the excitation spectrum clearly demonstrates absorption by interband transitions, the emission spectra can be unequivocally assigned to intraband recombination. The combination of these two observations can be conclusively explained only by Auger-excited intraband emission. These results suggest Auger excitation to be a promising route to generate energetic nonthermal electrons with energies substantially above the Fermi level. Exploiting this effect could strongly benefit applications for nanoluminescent probes and the progress of plasmon catalysis.

Keywords: Auger processes; hot electrons; intraband emission; metal emission; photoluminescence; photoluminescence excitation; plasmons.

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Emission processes in gold nanostructures discussed in the literature (excitation process are colored blue, emission processes yellow): (a) Interband excitation and recombination of a d-band hole, ,,,, (b) intraband excitation and recombination from and to the Fermi level within the conduction band, ,,− (c) electronic Raman scattering at the Fermi level combining excitation and emission into a single event, ,, and (d) Auger excitation by nonradiative recombination of a d-band hole , followed by intraband recombination. ,,

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(a) Experimental setup: different excitation wavelengths can be selected from the output of supercontinuum laser using a monochromator. The laser is focused onto the sample through a 60× objective and the emission is collected from the same side. The reflected laser is subsequently blocked by a dichroic mirror and a long-pass filter. The remaining emission is measured using a spectrometer. (b) Optical micrograph and (c) scanning electron micrograph of the sample. (d) Transmission electron micrograph of AuNFs with the same structure than the ones used to prepare the film in panels (b, c).

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(a) Absorption of the AuNF film (obtained with an integrating sphere, orange line) shows a very broad absorption feature. (b) Extinction of the AuNFs in aqueous solution exhibits a pronounced plasmon resonance. (c) Emission of the AuNF films excited at λ_ex = 355 nm (blue line) closely resembles the plasmon resonance in solution (dashed line).

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(a) Excitation wavelength-dependent emission spectra with long-pass filters at 550 nm (window I, blue lines) and 650 nm (window II, red lines). Darker colors represent longer excitation wavelengths. The intensities of the spectra in window II were adjusted to the spectra in window I. (b) Photoluminescence excitation (PLE) spectrum of the AuNF film calculated from panel (a) (blue points). The excitation measurements closely follow the simulated interband absorption spectrum (black line), which consists of contributions from the L- and X-points (orange and red dashed lines, respectively).

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Band structure of gold at optically relevant regions close to the X- and L-point (a, b). The excited charge distributions used in the simulations of interband (c) and intraband (d) luminescence, as proposed by Boyd and Sivan and Dubi.

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Simulated I/I ₄₂₅ emission spectrum for interband emission (a) differs strongly from the measured spectrum (b), while the emission simulated for intraband emission reproduces the experimental spectrum rather well (c). The spectra were determined for excitation wavelength from λ_ex = 425–530 nm.