Plasmon-Enhanced Upconversion

Abstract

Upconversion, the conversion of photons from lower to higher energies, is a process that promises applications ranging from high-efficiency photovoltaic and photocatalytic cells to background-free bioimaging and therapeutic probes. Existing upconverting materials, however, remain too inefficient for viable implementation. In this Perspective, we describe the significant improvements in upconversion efficiency that can be achieved using plasmon resonances. As collective oscillations of free electrons, plasmon resonances can be used to enhance both the incident electromagnetic field intensity and the radiative emission rates. To date, this approach has shown upconversion enhancements up to 450×. We discuss both theoretical underpinnings and experimental demonstrations of plasmon-enhanced upconversion, examining the roles of upconverter quantum yield, plasmonic geometry, and plasmon spectral overlap. We also discuss nonoptical consequences of including metal nanostructures near upconverting emitters. The rapidly expanding field of plasmon-enhanced upconversion provides novel fundamental insight into nanoscale light-matter interactions while improving prospects for technological relevance.

Near‐Infrared‐Plasmonic Energy Upconversion in a Nonmetallic Heterostructure for Efficient H2 Evolution from Ammonia Borane

“…The time‐resolved luminescence spectroscopy indicated that the lifetimes of 2 I 11/2 → 4 I 15/2 (521 nm), 4 S 3/2 → 4 I 15/2 (545 nm), and 4 F 9/2 – 4 I 15/2 (660 nm) decays for the NaYF 4 :Yb‐Er/W 18 O 49 film were shorter than the corresponding lifetimes obtained from the individual NaYF 4 :Yb‐Er film (Figure S7, Supporting Information). Combining with the results of selective luminescence enhancement, we confirmed the existence of plasmon‐mediated competition between the radiative and nonradiative processes in the NaYF 4 :Yb‐Er/W 18 O 49 film after 980 nm excitation 4, 7. The radiative rate enhancement is originated from the plasmon‐enhanced emission field, while the nonradiative process in the NaYF 4 :Yb‐Er/W 18 O 49 film is mainly attributed to the mentioned energy transfer.…”

“…This is mainly due to their unique capability to concentrate and amplify the incident light intensity near the surface of plasmonic nanostructures 1, 2, 3. As a classic plasmonic “optical antenna,” noble metal nanostructures with tunable LSPR energy are frequently introduced into nanomaterials to promote their performance in light absorption and/or emission via plasmonic energy transfer from noble metal to neighboring optical nanomaterials 4, 5, 6. For example, coupling appropriate nanostructures of plasmonic Au or Ag with upconversion nanomaterials, in particular trivalent lanthanide ions (Ln 3+ )‐doped NaYF 4 nanoparticles (NPs), can achieve enhanced upconversion luminescence in the visible light region by converting lower frequency incident photons at 980 nm with high effectivity 7, 8, 9.…”

“…Upon 980 nm excitation, the W 18 O 49 NWs serve as the “plasmonic antenna” to locally concentrate the NIR energy near the NWs, and then resonantly transfer this energy to adjoining NaYF 4 :Yb‐Er NPs ( Figure 2 a) 28, 29. Therefore, the NaYF 4 :Yb‐Er NPs near the NaYF 4 :Yb‐Er/W 18 O 49 interface would experience a far more intense excitation electric field based on the surface enhancement effect, which could promote electron population on the excited‐state energy levels of Er 3+ ions, thus resulting in an enhanced upconversion luminescence 4, 7, 30. Meanwhile, the LSPR‐enhanced localized electric field can interact with the emission electric field of NaYF 4 :Yb‐Er NPs to boost the radiative decay rate of upconversion process 4, 7.…”

“…Therefore, the NaYF 4 :Yb‐Er NPs near the NaYF 4 :Yb‐Er/W 18 O 49 interface would experience a far more intense excitation electric field based on the surface enhancement effect, which could promote electron population on the excited‐state energy levels of Er 3+ ions, thus resulting in an enhanced upconversion luminescence 4, 7, 30. Meanwhile, the LSPR‐enhanced localized electric field can interact with the emission electric field of NaYF 4 :Yb‐Er NPs to boost the radiative decay rate of upconversion process 4, 7. However, the direct contact of W 18 O 49 NWs and NaYF 4 :Yb‐Er NPs quenches the upconversion emission to a certain degree due to the nonradiative energy transfer from the NaYF 4 :Yb‐Er NPs (donor) and the adherent W 18 O 49 NWs (acceptor) 31, 32, 33, 34.…”

Enhanced UV Upconversion Emission Using Plasmonic Nanocavities

“…Thus far, photon UC has been utilized in various applications such as IR quantum counters, solid-state lasers, in lighting, displays, and solar cells [4][5][6]. In recent years, the successful synthesis of colloidally stable solutions of monodisperse upconverting nanoparticles (UCNPs) with diameters on the order and less than 50 nm has opened up new opportunities in this general area [7][8][9]. It is envisioned that such UCNPs may replace dye molecules as diagnostic and therapeutic tools for biological imaging, or can be used in targeted drug delivery [10][11][12].…”