Lanthanide-doped upconversion nanophosphors (UCNPs) convert near-infrared light to visible emission through sequential photon absorption and energy transfer, but their quantum yield is intrinsically low, and placing them near metal nanoparticles can amplify their emission by factors of several to tens. Saboktakin et al. (ACS Nano 2012) demonstrated spacer-thickness-tunable enhancement of NaYF4:Yb,Er and NaYF4:Yb,Tm monolayers near Au and Ag nanoparticle films, but could not determine how the metal modifies the radiative and nonradiative decay rates separately: their quantitative estimates substituted a literature absolute upconversion quantum yield of 10^-3 for the sample-specific intrinsic quantum yield, their simulations used bulk literature dielectric functions that may misdescribe 5–6 nm metal particles, and they left open how the enhancement/quenching balance transfers to brighter emitters. The problem is to establish this partition experimentally and theoretically for metal–spacer–UCNP architectures and thereby place the metal/spacer design rules on a quantitative footing across intrinsic quantum-yield regimes.

Background

Photon upconversion (UC) is an anti-Stokes process in which a lanthanide-doped material absorbs two or more low-energy photons and emits one higher-energy photon. In the standard UCNP system, hexagonal-phase sodium yttrium fluoride (β-NaYF4) doped with ytterbium (Yb3+, the sensitizer absorbing the 980 nm pump) and erbium or thulium (Er3+/Tm3+, the activators emitting visible light), upconversion proceeds by sequential Yb3+ absorption and energy transfer to the activator. The relevant f–f electronic transitions of lanthanides are parity-forbidden, so absorption cross sections and radiative decay rates are small; absolute quantum yields (the fraction of absorbed pump photons converted to emitted photons, abbreviated UCQY) of small UCNPs are typically on the order of 10^-4 to 10^-2, limiting applications in bioimaging, sensing, and solar spectral conversion.

One strategy to raise UCNP emission is to place the nanoparticles near metal nanoparticles (NPs) supporting localized surface plasmon resonances (collective electron oscillations that concentrate optical fields at sub-wavelength scale). Two beneficial channels are usually invoked: nonresonant near-field enhancement that increases the local pump intensity at Yb3+ (accelerating the sequential excitation/energy-transfer steps), and a Purcell-like resonant modification of the local photonic density of states that increases the radiative decay rate of the emitting transitions. Competing against both is quenching: energy transferred to the metal is dissipated as heat (an increase of the nonradiative decay rate). If Γr and Γnr denote the intrinsic radiative and nonradiative decay rates of an emitting transition, and Γr' and Γnr' their metal-modified values, then the measured decay time gives only the sum 1/τ = Γr + Γnr (or Γr' + Γnr' near metal), and separating the beneficial radiative enhancement from the quenching loss additionally requires the intrinsic quantum yield, QY = Γr/(Γr + Γnr). This intrinsic quantum yield is the emitting transition's radiative fraction; it is distinct from the system-level absolute upconversion quantum yield UCQY introduced above, and the two are related by UCQY = η_UC·QY, where η_UC is the multi-photon excitation/energy-transfer efficiency, so the two must not be used interchangeably.

Saboktakin, Ye, Oh, Hong, Fafarman, Chettiar, Engheta, Murray and Kagan (ACS Nano 2012, DOI 10.1021/nn302466r) built a model architecture for this problem: a close-packed monolayer of β-NaYF4:Yb,Er or β-NaYF4:Yb,Tm UCNPs separated from a dense spin-coated film of ~5–6 nm Au or Ag NPs by an atomically layer-deposited (ALD, a gas-phase monolayer-by-monolayer growth technique) Al2O3 spacer whose thickness was varied between 2 and 15 nm. They measured steady-state upconversion photoluminescence enhancements of up to 5.2-fold near Au (optimal spacer 5 nm) and up to 45-fold near Ag (optimal spacer 10 nm), with concurrent shortening of the luminescence decay and rise times, and interpreted the effect as combined nonresonant pump-field enhancement and resonant plasmonic modification of the emission. However, converting their measured lifetimes (for example τ = 298.2 µs for the 540 nm Er3+ emission of a UCNP monolayer without metal and τ = 144.8 µs for the Ag/10 nm spacer sample) into radiative and nonradiative rates required the intrinsic quantum yield of the emitting transition; because that of the actual samples was not measured, the absolute upconversion quantum yield (UCQY) of 10^-3 reported for similar but not identical colloids (Boyer and van Veggel, Nanoscale 2010) was used in its place; only the sum Γr' + Γnr' was experimentally determined. Their electromagnetic simulations (finite-element, single 6 nm metal sphere with Johnson–Christy data, i.e. the standard tabulated bulk optical constants for gold and silver) further left open how well literature dielectric functions describe very small metal NPs, whose plasmon resonance is broadened and weakened by surface- and size-limited electron scattering — an effect expected to be strongest for Ag — and how strongly the dense multilayer NP film deviates from the isolated-sphere model.

Later theory has substantially clarified what is at stake. Rate-equation modeling combined with exact electrodynamic calculations (Fischer et al., Optics Express 2012 and 2016) shows that near a spherical Au nanoparticle the upconversion luminescence, the absorption, and the upconversion quantum yield are maximized at different emitter–metal distances, that strong quenching of the quantum yield occurs at small separations, and that for an upconverter whose absolute upconversion quantum yield (UCQY) is already around 12% (β-NaYF4:Er) the achievable absolute quantum-yield gain from a nearby Au sphere is modest and shrinks with increasing pump irradiance. Comprehensive Mie-theory-based analyses (Mie theory being the exact classical solution for electromagnetic scattering by a sphere; Rasskazov et al., Optical Materials Express 2018) map the competition of enhancement and quenching for Ag and Au spheres and provide design guidelines. On the experimental side, single-particle spectroscopy has directly imaged both outcomes on the same type of emitter: Piątkowski et al. (Materials 2017) found individual NaYF4:Er,Yb nanocrystals near Au spheres whose emission is enhanced with shortened decay (Purcell-type radiative enhancement) and others whose resonant emission band is almost completely quenched, and Ćwierzona et al. (Phys. Chem. Chem. Phys. 2026) used controlled polymer spacers and fluorescence lifetime imaging to resolve three distance-controlled regimes near metallic nanowires — quenching, plasmonic enhancement, and effective decoupling. Quantum-yield metrology for UCNPs has matured (Kaiser et al., Nanoscale 2017, on the power-dependent absolute UCQY; Jones, Gakamsky and Marques-Hueso, 2021, on standardizing upconversion quantum-yield measurement), but the absolute upconversion quantum yield is itself a function of pump irradiance because of the nonlinear kinetics, which complicates any single-number rate partition.

Problem Statement

For upconversion nanophosphors (β-NaYF4 doped with Yb3+ and Er3+ or Tm3+) placed at nanometer-scale, spacer-controlled separation from metal nanoparticle structures, determine quantitatively and unambiguously how the metal modifies the decay rates of the upconversion emission: measure, for the same samples, the modified radiative rate Γr' and the modified nonradiative rate Γnr' separately — via sample-specific determinations of the intrinsic quantum yield QY = Γr/(Γr + Γnr) (measured directly, or inferred from absolute upconversion-quantum-yield measurements together with the excitation efficiency η_UC, since UCQY = η_UC·QY) combined with measured decay times, rather than an assumed literature value — and thereby establish the partition between beneficial channels (nonresonant near-field pump enhancement and resonant Purcell-type radiative-rate increase) and detrimental channels (metal-induced quenching and absorption loss), including its dependence on spacer thickness, metal type (Au versus Ag), metal nanoparticle size, and pump irradiance. These dependences — together with the same-sample Γr' and Γnr' partition for one spacer series in each of the Au and Ag systems at a stated pump irradiance, extended across both intrinsic-quantum-yield regimes (the low regime of the original demonstration and the several-percent-brightness regime of modern bright upconversion nanophosphors) — are the scientific target. Establish, on this basis and with adequate modeling of the size- and surface-corrected dielectric response of very small metal nanoparticles, how the optimal metal–spacer design rules change as the intrinsic quantum yield QY of the upconverter is raised from the low regime assumed in the original demonstration to the higher regime of modern bright upconversion nanophosphors (these brightness regimes are quoted in the literature as absolute upconversion quantum yields UCQY of ≈10^-3 and several percent) — that is, determine for which emitter brightness regimes metal proximity remains a net enhancement and for which it quenches.

References

  1. M. Saboktakin, X. Ye, S. J. Oh, S.-H. Hong, A. T. Fafarman, U. K. Chettiar, N. Engheta, C. B. Murray, C. R. Kagan, 'Metal-Enhanced Upconversion Luminescence Tunable through Metal Nanoparticle–Nanophosphor Separation', ACS Nano 2012, 6(10), 8758–8766, DOI: 10.1021/nn302466r, https://doi.org/10.1021/nn302466r
  2. J.-C. Boyer, F. C. J. M. van Veggel, 'Absolute quantum yield measurements of colloidal NaYF4: Er3+, Yb3+ upconverting nanoparticles', Nanoscale 2010, 2(8), 1417–1419, DOI: 10.1039/c0nr00253d, https://doi.org/10.1039/c0nr00253d
  3. D. Lu, S. K. Cho, S. Ahn, L. Brun, C. J. Summers, W. Park, 'Plasmon Enhancement Mechanism for the Upconversion Processes in NaYF4:Yb3+,Er3+ Nanoparticles: Maxwell versus Förster', ACS Nano 2014, 8(8), 7780–7792, DOI: 10.1021/nn5011254, https://doi.org/10.1021/nn5011254
  4. Q. C. Sun, J. Casamada-Ribot, V. Singh, H. Mundoor, I. I. Smalyukh, P. Nagpal, 'Effect of plasmon-enhancement on photophysics in upconverting nanoparticles', Optics Express 2014, 22(10), 11516–11527, DOI: 10.1364/OE.22.011516, https://doi.org/10.1364/OE.22.011516
  5. Y.-L. Wang, N. E. Mohammadi Estakhri, A. Johnson, H.-Y. Li, L.-X. Xu, Z. Zhang, A. Alù, Q.-Q. Wang, C.-K. Shih, 'Tailoring Plasmonic Enhanced Upconversion in Single NaYF4:Yb3+/Er3+ Nanocrystals', Scientific Reports 2015, 5, 10196, DOI: 10.1038/srep10196, https://doi.org/10.1038/srep10196
  6. D. Piątkowski, M. K. Schmidt, M. Twardowska, M. Nyk, J. Aizpurua, S. Maćkowski, 'Spectral Selectivity of Plasmonic Interactions between Individual Up-Converting Nanocrystals and Spherical Gold Nanoparticles', Materials 2017, 10(8), 905, DOI: 10.3390/ma10080905, https://doi.org/10.3390/ma10080905
  7. S. Fischer, D. Kumar, F. Hallermann, G. von Plessen, J. C. Goldschmidt, 'Enhanced upconversion quantum yield near spherical gold nanoparticles – a comprehensive simulation based analysis', Optics Express 2016, 24(6), A460–A475, DOI: 10.1364/OE.24.00A460, https://doi.org/10.1364/OE.24.00A460
  8. S. Fischer, F. Hallermann, T. Eichelkraut, G. von Plessen, K. W. Krämer, D. Biner, H. Steinkemper, M. Hermle, J. C. Goldschmidt, 'Plasmon enhanced upconversion luminescence near gold nanoparticles–simulation and analysis of the interactions', Optics Express 2012, 20(1), 271–282, DOI: 10.1364/OE.20.000271, https://doi.org/10.1364/OE.20.000271
  9. I. L. Rasskazov, L. Wang, C. J. Murphy, R. Bhargava, P. S. Carney, 'Plasmon-enhanced upconversion: engineering enhancement and quenching at nano and macro scales', Optical Materials Express 2018, 8(12), 3787–3801, DOI: 10.1364/OME.8.003787, https://doi.org/10.1364/OME.8.003787
  10. M. Kaiser, C. Würth, M. Kraft, I. Hyppänen, T. Soukka, U. Resch-Genger, 'Power-dependent upconversion quantum yield of NaYF4:Yb3+,Er3+ nano- and micrometer-sized particles – measurements and simulations', Nanoscale 2017, 9(28), 10051–10058, DOI: 10.1039/c7nr02449e, https://doi.org/10.1039/c7nr02449e
  11. H. Lakhotiya, A. Nazir, S. Roesgaard, E. Eriksen, J. Christiansen, M. Bondesgaard, F. C. J. M. van Veggel, B. B. Iversen, P. Balling, B. Julsgaard, 'Resonant Plasmon-Enhanced Upconversion in Monolayers of Core–Shell Nanocrystals: Role of Shell Thickness', ACS Applied Materials & Interfaces 2019, 11(1), 1209–1218, DOI: 10.1021/acsami.8b15564, https://doi.org/10.1021/acsami.8b15564
  12. C. M. S. Jones, A. Gakamsky, J. Marques-Hueso, 'The upconversion quantum yield (UCQY): a review to standardize the measurement methodology, improve comparability, and define efficiency standards', Science and Technology of Advanced Materials 2021, 22(1), 810–848, DOI: 10.1080/14686996.2021.1967698, https://doi.org/10.1080/14686996.2021.1967698
  13. F. Zhang, P. R. A. Oiticica, J. Abad-Arredondo, M. S. Arai, O. N. Oliveira, D. Jaque, A. I. Fernández Domínguez, A. S. S. de Camargo, P. Haro-González, 'Brownian Motion Governs the Plasmonic Enhancement of Colloidal Upconverting Nanoparticles', Nano Letters 2024, 24(12), 3785–3792, DOI: 10.1021/acs.nanolett.4c00379, https://doi.org/10.1021/acs.nanolett.4c00379
  14. M. Ćwierzona, M. Żebrowski, P. Dziarhai, J. Niedziółka-Jönsson, M. Jönsson-Niedziółka, M. Nyk, S. Maćkowski, D. Piątkowski, 'Quench, glow, or stay silent: distance-controlled up-conversion emission near metallic nanowires', Physical Chemistry Chemical Physics 2026, 28(30), 18470–18477, DOI: 10.1039/d6cp01454b, https://doi.org/10.1039/d6cp01454b
  15. X. Liu, C. Jiang, G. Chen, 'Brightening Upconversion Nanoparticles', Accounts of Chemical Research 2026, 59(8), 1354–1365, DOI: 10.1021/acs.accounts.5c00910, https://doi.org/10.1021/acs.accounts.5c00910