Quantum-mechanical effects in photoluminescence from thin crystalline gold films

The study investigates the microscopic origin and characteristics of steady-state luminescence from metals, focusing on thin, atomically flat monocrystalline gold (Au) flakes. While semiconductor luminescence is well understood and widely used, metal luminescence is much weaker and its origin has been debated for decades, including whether observed emission is true photoluminescence (PL) from electron–hole recombination or other inelastic light-scattering processes. Understanding this distinction is critical for using luminescence as a probe of hot-carrier dynamics in plasmonic and metallic systems for applications in sensing, energy conversion, and nanoscale thermometry. The authors examine photon emission from 13–113 nm thick Au(111) flakes without plasmonic enhancement to determine the emission mechanism under interband excitation and to assess how thickness and quantum confinement affect the PL. They also explore how luminescence can serve as a probe of local temperature and carrier transport, and contrast interband with intraband excitation regimes.

Metal luminescence was first reported in 1969 (Mooradian), but its microscopic origin has remained contentious, with competing interpretations involving PL versus inelastic scattering mechanisms. In plasmonic nanostructures, Purcell enhancement and resonant plasmonic modes can dominate emission spectra, complicating identification of intrinsic mechanisms. Prior work has also emphasized two-photon PL under pulsed excitation, while steady-state luminescence received less attention despite its experimental accessibility. Confounding factors include excitation relative to the interband threshold, surface roughness, plasmonic enhancements, and spatial confinement. Recent summaries and studies highlight these debates and the need for measurements free of plasmonic resonances to isolate intrinsic metal PL behavior.

• Samples: Monocrystalline, atomically flat Au flakes (111) on quartz, thickness 13–113 nm, lateral dimensions >5 μm, synthesized following Kiani et al. Measurements were performed far from edges to avoid plasmonic enhancement and surface plasmon excitation. Signals were spatially uniform across multiple points and stable over months; results matched independently synthesized monocrystalline gold.
• Excitation and detection: Focused continuous-wave lasers at interband wavelengths (488 nm, 532 nm) and intraband (785 nm). Emission spectra recorded and normalized per absorbed photon or power. Long-wavelength overlap tests between 488/532 nm excitations to assess mechanism. Power-dependence studies at 488 nm to examine temperature effects; independent external heating to calibrate luminescence as a thermometer using only Stokes-side (λout > λin) emission.
• Absorption and emission modeling: Reflection/transmission measurements to obtain absorption spectra; modeling parameterized by the refractive index of Au. A dipole-emitter formalism describes PL as a combination of intrinsic dipole emission strength (linked to band structure), charge transport prior to emission, and photon re-absorption/escape. Two transport limits considered: (i) local PL (emission at the excitation location) and (ii) maximally delocalized PL (emission uniformly throughout the film). Derived expressions for external PL include depth-dependent absorption f_abs(z, λin) and escape probability f_emit(z, λout).
• Spatial origin and transport: A two-sided excitation experiment measured the ratio of PL collected from the same side when exciting from the same versus opposite sides of the film. Comparison to model predictions distinguishes localized versus delocalized emission and assesses surface versus bulk origin.
• Angular emission: Back-focal-plane (BFP) measurements of angle-resolved PL at selected wavelengths (550, 600, 700, 790 nm) to validate the escape model; addressed challenges from objective-induced luminescence.
• Thickness dependence: Systematic PL measurements across thicknesses (13–113 nm) to disentangle effects of absorption versus escape probability and infer changes in internal PL with confinement.
• Theory: First-principles (DFT-based) modeling of band-structure-related emission components and quantum confinement effects, including separation of pre-scattered (near-excitation) and post-scattered (longer-wavelength) PL components, and incorporation of photon re-absorption.

• Mechanism under interband excitation: The long-wavelength PL spectra per absorbed photon for 488 nm and 532 nm excitation overlap in absolute magnitude and shape, demonstrating that carriers relax prior to radiative recombination; therefore, long-wavelength emission is due to photoluminescence, not inelastic scattering without relaxation.
• Near-excitation behavior: Emission close to the excitation wavelength does not overlap between different interband excitations, consistent with prior observations and with a pre-scattered PL component sensitive to excitation energy.
• Temperature dependence and nanoscale thermometry: For 488 nm excitation, the short-wavelength PL (λout < 600 nm) per absorbed photon decreases with increasing incident power; the effect is reversible and replicates with external sample heating, enabling non-invasive local temperature readout from the Stokes signal alone. Estimated lattice temperature rises: ~200 K in thin films (<20 nm) and ~70 K in thick films (>50 nm) under the used intensities.
• Linearity and PLQY: External PL quantum yield is ~1e-10 and remains constant with excitation power (apart from temperature effects), indicating linear response and recombination of an excited carrier with an unexcited partner. Reported PL lifetimes after interband excitation are ~50 fs, consistent with PL rather than instantaneous inelastic scattering.
• Minimal charge transport before emission: Two-sided excitation experiments show ratios consistent with the local PL model, indicating negligible carrier redistribution prior to emission and confirming bulk, not surface, origin of PL under these conditions.
• Angular emission validation: Angle-resolved PL matches predictions from the photon escape model, supporting the treatment of re-absorption and emission geometry.
• Thickness effects and quantum confinement: With decreasing thickness (113→13 nm), short-wavelength emission decreases while long-wavelength emission increases. Modeling and first-principles calculations attribute changes in internal PL to quantum-mechanical confinement of states near the Fermi level for films thinner than ~40 nm, enhancing pre-scattered luminescence at longer wavelengths compared to thick flakes. Quantum effects are observable up to ~40 nm thickness.
• Intraband excitation: Under 785 nm excitation, emission is weaker but comparable in order of magnitude to interband-induced PL, indicating that intraband luminescence is not solely due to PL; a peak near 900 nm is an experimental artifact (seen also on a silver mirror).
• Experimental conditions: Measurements conducted free of plasmonic enhancements by avoiding edges/roughness and verifying spatial localization of emission to the illuminated region.

The overlap of long-wavelength spectra under interband excitation directly addresses the longstanding question of whether steady-state metal luminescence originates from PL or inelastic scattering without relaxation. The results show that charge relaxation precedes emission, establishing PL as the dominant mechanism for long-wavelength emission in monocrystalline Au under interband excitation. The validated local-emission model, supported by two-sided excitation and angle-resolved measurements, reveals minimal carrier diffusion before recombination and confirms bulk origin, enabling accurate modeling that includes depth-dependent absorption and photon re-absorption/escape. Thickness-dependent trends, captured qualitatively by first-principles theory, identify quantum confinement of near-Fermi-level states as a key driver of internal PL changes below ~40 nm, thus linking macroscopic PL signatures to microscopic electronic structure modifications. The temperature dependence of short-wavelength PL at 488 nm enables practical, label-free thermometry using only Stokes-side emission, expanding the utility of steady-state luminescence as a non-invasive probe of local heating in metallic nanostructures. Finally, intraband excitation behavior suggests additional non-PL channels contribute, motivating further mechanistic studies beyond interband conditions. Collectively, these findings unify disparate observations of Au luminescence, disentangle roles of excitation regime, transport, and re-absorption, and establish steady-state PL as a robust probe of carrier dynamics in metals.

The work provides a unified, experimentally validated and theoretically supported framework for steady-state luminescence in monocrystalline Au flakes without plasmonic enhancement. It demonstrates that, under interband excitation, long-wavelength emission is true photoluminescence following carrier relaxation, with minimal transport prior to recombination and bulk origin. Incorporating photon re-absorption and escape accurately reproduces angle- and thickness-dependent behavior, while first-principles modeling links observed spectral changes to quantum confinement of states near the Fermi level for films thinner than ~40 nm. Practically, the study introduces a simple Stokes-side luminescence thermometer for Au at 488 nm excitation. The insights and modeling approach generalize to other metals and nanostructures, enabling luminescence as a quantitative probe of hot carriers and light–matter interactions. Future work should: (i) resolve the detailed mechanisms of intraband-induced emission and quantify non-PL contributions; (ii) extend to different crystallographic orientations, substrates, and metals; (iii) explore time-resolved PL to correlate dynamics with confinement; and (iv) integrate with plasmonic structures to separate intrinsic PL from resonant enhancements.

• The detailed affiliations of all co-authors and complete methods are referenced but not fully provided in the excerpt.
• Angle-resolved PL measurements were experimentally challenging due to objective-induced luminescence, potentially limiting signal-to-noise.
• For very thin films (<25 nm), the two-sided excitation ratio approaches unity regardless of transport due to more uniform excitation, reducing sensitivity to discriminate transport effects.
• The internal PL construct is defined for modeling convenience and may not map uniquely to a physical observable.
• Intraband excitation results suggest non-PL contributions, but their mechanisms are not fully resolved here.
• Publication provides qualitative agreement from first-principles modeling; full quantitative validation across all thicknesses and wavelengths may require additional parameters and measurements.