Quantum-mechanical effects in photoluminescence from thin crystalline gold films

Luminescence in semiconductors is well-understood and widely used, unlike in metals where it's much weaker and its origin debated. Metal luminescence, particularly from plasmonic nanostructures, is gaining attention due to its potential in sensing and energy applications. This study focuses on steady-state luminescence, a more experimentally accessible process than two-photon photoluminescence. While steady-state luminescence has been used to study nanoscale phenomena, the origin of the emitted light—photoluminescence (PL) from electron-hole recombination or inelastic light scattering—remains unclear. The debate is further complicated by factors such as Purcell enhancement and excitation wavelength relative to the interband transition threshold. This research aims to understand steady-state luminescence from metals without resonant excitations, using monocrystalline gold flakes with varying thicknesses (13 nm to 113 nm) and (111) surface exposed. This approach avoids surface roughness and plasmonic enhancement, making the conclusions applicable to various metals.

Existing literature shows a long-standing debate (spanning over 50 years) on the origin of metal luminescence, with various theoretical and experimental studies proposing different mechanisms. Some studies emphasize the role of Purcell enhancement of emission at wavelengths resonant with plasmonic modes. Others focus on the influence of the excitation wavelength relative to the interband transition threshold and spatial confinement. However, a complete understanding of steady-state luminescence from metals following interband excitation without resonant effects is still lacking, limiting its applications as a probe of material properties.

Monocrystalline gold flakes with thicknesses ranging from 13 nm to 113 nm were synthesized. Measurements were performed using focused laser beams on the central, atomically flat region of the flakes to avoid edge or defect effects and plasmonic enhancement. The photoluminescence (PL) spectra were recorded for different excitation wavelengths (488 nm, 532 nm, and 785 nm) in the interband and intraband regimes. The effects of laser power and sample temperature on the PL signal were also investigated. To determine the spatial origin of the PL, the ratio of PL spectra was measured when exciting from the same and opposite sides of the sample. Angle-resolved luminescence measurements were also performed. A model was developed to describe the luminescence, incorporating dipole emission throughout the material, charge transport, and gold's absorption coefficients. First-principles calculations based on density-functional theory (DFT) were used to support the model.

The study found that when exciting in the interband regime, long-wavelength photon emission is independent of the excitation wavelength, confirming a photoluminescence origin. This indicates that charge carriers undergo relaxation prior to radiative recombination. The short-wavelength emission shows temperature dependence, which enabled the development of a label-free thermometer for gold. A model incorporating local photoluminescence and photon re-absorption accurately reproduces experimental observations, revealing minimal charge diffusion before photon emission. As the flake thickness is reduced below 40 nm, quantum-mechanical confinement near the Fermi level increases long-wavelength pre-scattered luminescence. Intraband luminescence, on the other hand, is not solely due to photoluminescence based on scaling arguments. The external photoluminescence quantum yield (PLQY) was estimated to be ~10⁻¹⁰, consistent with previous studies.

The findings address the long-standing question regarding the origin of gold luminescence, conclusively showing the dominant role of photoluminescence when exciting in the interband regime. The development of a label-free thermometer based on the temperature dependence of the short-wavelength emission provides a valuable tool for nanoscale thermometry. The observation of quantum-mechanical effects in thin gold flakes demonstrates the sensitivity of luminescence to nanoscale confinement. The unified model presented in this study provides a comprehensive framework for understanding and applying luminescence as a probe of carrier dynamics in gold and other materials.

This research provides a comprehensive understanding of gold photoluminescence in monocrystalline flakes, resolving a long-standing debate about its origin. A new label-free nanoscale thermometry technique is demonstrated. The study establishes a unified model that accurately predicts luminescence behavior and highlights the observable quantum effects in thin films. Future work could explore the application of this technique to other materials and investigate the detailed dynamics of hot carriers in these systems.

The angle-resolved luminescence measurements were challenging due to competing luminescence from the objective lens, potentially limiting the accuracy of those specific findings. The study primarily focused on monocrystalline gold flakes with (111) surface orientation, and the generalizability to other crystal orientations or materials needs further investigation. The PLQY measurements are based on external quantum yield, and internal quantum yield would require further investigation to completely understand the overall efficiency of the emission process.