In a paper recently published in the journal Light: Science & Applications, researchers revealed the quantum-mechanical effects in photoluminescence from thin monocrystalline gold flakes.
Understanding Metal Luminescence
Semiconductor luminescence is commonly used as a non-invasive probe of different phenomena, such as chemical reaction monitoring and dating of rocks, as the processes causing luminescence from semiconductors are sufficiently understood. However, luminescence from metals remains poorly understood, and its potential for elucidating nanoscale carrier dynamics has not been sufficiently exploited as the signal is substantially weaker compared to most semiconductors.
Recently, photon emission from metals has gained significant attention while considering plasmonic nanostructures, which could revolutionize industries like energy, sensing, and healthcare owing to the plasmon-generated hot carriers’ ability to substantially increase local electronic temperatures and enhance solar cell absorption.
Steady-state luminescence can potentially unravel hot-carrier processes within plasmonic systems. Although steady-state luminescence from metals was used for fundamental nanoscale studies, charge transfer process monitoring, and gold-molecule interaction probing, uncertainty remains regarding the emitted light’s origin.
This uncertainty has been further complicated due to additional effects like the Purcell enhancement of emission at particular light wavelengths that resonate with the metal structure’s plasmonic modes. Thus, a comprehensive understanding of metal luminescence/steady-state luminescence from metals caused by interband excitation without such resonant excitations is lacking, which is hampering its extensive application as a probe.
The Study
In this study, researchers aimed to reveal the quantum-mechanical effects in the luminescence originating from thin monocrystalline gold flakes. They presented experimental evidence supported by first-principles simulations to demonstrate the photoluminescence origin when exciting in the interband regime.
The photon emission from 13 nm to 113 nm thick, atomically flat, monocrystalline gold flakes with exposed (111) surfaces was studied. These samples enabled the researchers to investigate the relationship between nanoscale confinement and photon emission without plasmonic enhancement or surface roughness. Thus, the conclusions drawn from this investigation can be applied to all metals, even those metals that are not operating in the plasmonic regime.
Researchers fabricated monocrystalline gold flakes using the bottom surface instead of the inter-substrate surface. The samples were fabricated on quartz substrates as glass photoluminescence outcompetes the photoluminescence from thin gold structures. Substrates were cleaned in ultrasonic ethanol baths, followed by de-ionized water, before sample fabrication.
In luminescence measurements, all 532 nm laser excitation measurements and 488 nm laser excitation measurements were performed on a Renishaw inVia Raman Microscope RE04, while all other measurements were conducted using a NanoMicroSpec-Transmission™ (NT&C) microscope, which was further adapted to enable Raman spectroscopy.
A Thorlabs S170C or S130C power meter was employed to record laser power. Two lenses were placed between the image plane of the spectrometer and the microscope for back-focal-plane measurements. Photoluminescence quantum yield (PLQY) was estimated using a calibrated light source that was coupled to an integrating sphere with a known spectral response. Additionally, the NT&C system was used to record absorption measurements, while sample thicknesses were measured using atomic force microscopy.
Significance of the Study
Results showed that the long-wavelength photon emission was not affected by the excitation wavelength when illuminating in the interband regime, which conclusively proved that this signal was only due to photoluminescence and not due to other inelastic scattering forms.
Researchers also demonstrated that gold luminescence could be utilized as a local temperature probe only using the Stokes signal when excited at 488 nm. Stokes signal refers to a signal at longer wavelengths compared to the excitation wavelength.
The use of photon re-absorption to further comprehend the emission revealed that the charged diffusion was minimal after photoexcitation before photon emission, which enabled the development of a luminescence model. This luminescence model included density-functional theory (DFT)-based first-principles calculations and photon re-absorption and produced results that were in good agreement with photoluminescence experiments.
Gold photoluminescence contained two major components, including longer wavelength post-scattered luminescence and pre-scattered luminescence close to the excitation energy, when exciting within the interband regime. Both of them arose from the recombination of excited d-band holes with unexcited electrons.
The quantum-mechanical effects were observable in the luminescence signal from flakes up to 40 nm in thickness. Using the bulk luminescence model, it was identified that the quantum-mechanical confinement of states closer to the Fermi level increased the pre-scattered luminescence at longer wavelengths compared to thick flakes as the flake thickness was reduced below 40 nm.
Researchers also proposed that intraband luminescence was not only due to photoluminescence by invoking scaling arguments while investigating luminescence signals when exciting within the intraband regime. All observations were qualitatively reproduced using first-principles modeling, establishing a unified description of gold monocrystalline flake luminescence and enabling its extensive application as a probe of light-matter interactions and carrier dynamics in this material.
To summarize, this study provided a comprehensive gold photoluminescence theory in monocrystalline flakes that can be readily applied to other nanoparticles and metals, with the key finding that quantum-mechanical effects could emerge in the luminescence of metallic flakes with less than 40 nm thickness.
Journal Reference
Bowman, A. R., Rodríguez Echarri, A., Kiani, F., Iyikanat, F., Tsoulos, T. V., Cox, J. D., Sundararaman, R., Javier, F., Tagliabue, G. (2024). Quantum-mechanical effects in photoluminescence from thin crystalline gold films. Light: Science & Applications, 13(1), 1-12. https://doi.org/10.1038/s41377-024-01408-2, https://www.nature.com/articles/s41377-024-01408-2
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