In a paper recently published in the journal Light: Science & Applications, researchers revealed the quantum-mechanical effects in photoluminescence from thin monocrystalline gold flakes.
a Recorded photoluminescence per incident photon for different flake thicknesses d under 488 nm excitation wavelength. b, c Ratio of the measured signals presented in a to the signal obtained from the 113 nm flake and separated into b thick and c thin flakes, respectively. Dashed lines show calculated profiles based on the model presented in the main text (Eq. 2). Curve colors in b, c are coordinated with a. d Calculated internal luminescence (using an 88 nm thick flake data) for 488 nm and 532 nm light excitation, alongside simulated internal luminescence for 530 nm wavelength excitation. e Schematic of the processes leading to photoluminescence, where εf indicates the Fermi energy and h+/e- represents the excited hole/electron following photoexcitation. The incident laser power on the sample is 0.162 mWμm-2/3.334 mWμm-2 for 488 nm/532 nm excitation wavelength in all panels. Image Credit: https://www.nature.com/articles/s41377-024-01408-2
Understanding Metal Luminescence
Semiconductor luminescence serves as a non-invasive tool for observing various phenomena, including monitoring chemical reactions and dating rocks. This is possible because the processes behind semiconductor luminescence are well understood. In contrast, luminescence from metals is not as well comprehended, limiting its use in exploring nanoscale carrier dynamics. This limitation is compounded by the fact that the luminescence signal from metals is significantly weaker than that from most semiconductors.
Photon emission from metals, particularly within plasmonic nanostructures, is generating significant excitement. This breakthrough has the potential to revolutionize industries such as energy, sensing, and healthcare. The key lies in plasmon-generated hot carriers, which increase local electronic temperatures and enhance solar cell efficiency—opening up exciting new possibilities.
Steady-state luminescence could be key to unlocking the secrets of hot-carrier processes in plasmonic systems. While this type of luminescence from metals has been applied to basic nanoscale research, monitoring charge transfer processes, and investigating interactions between gold molecules, there is still some uncertainty about the origins of the light it emits.
This uncertainty is compounded by factors such as the Purcell enhancement, which intensifies emission at specific light wavelengths that resonate with the plasmonic modes of the metal structure. As a result, we still lack a comprehensive understanding of metal luminescence, particularly steady-state luminescence stemming from interband excitation without resonant effects. This gap in knowledge is limiting its broader use 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.
Researchers studied photon emission from atomically flat, monocrystalline gold flakes ranging from 13 nm to 113 nm in thickness, with exposed (111) surfaces. These samples allowed the team to explore the connection between nanoscale confinement and photon emission, independent of plasmonic enhancement or surface roughness. The findings from this study are broadly applicable, extending to all metals, including those not functioning within 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.
For luminescence measurements, researchers used a Renishaw inVia Raman Microscope RE04 for all 532 nm and 488 nm laser excitation tests. Meanwhile, all other measurements were carried out using a NanoMicroSpec-Transmission™ (NT&C) microscope. This microscope was specially modified to facilitate Raman spectroscopy, enhancing its versatility for various experimental needs.
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 but 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 is characterized by two major components when excited within the interband regime: longer wavelength post-scattered luminescence and pre-scattered luminescence close to the excitation energy. Both components result from the recombination of excited d-band holes with unexcited electrons. This dual feature in the luminescence spectrum highlights the complex interactions within the electronic structure of gold during photoluminescence.
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.
The researchers also proposed that intraband luminescence in gold is not solely due to photoluminescence, based on scaling arguments observed during luminescence experiments within the intraband regime. They successfully reproduced all observations qualitatively using first-principles modeling. This approach has led to a unified description of luminescence in monocrystalline gold flakes, which enhances its potential as a comprehensive probe for studying 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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