By wiring molecular triplets into lanthanide-doped nanoparticles, researchers create the first electrically driven NIR-II LEDs from an insulating host.
Study: Triplets electrically turn on insulating lanthanide-doped nanoparticles. Image Credit: Vershinin89/Shutterstock.com
In a recent Nature article, researchers have reported the first proof-of-concept electrically driven light-emitting diodes (LEDs) based on lanthanide-doped nanoparticles (LnNPs), a novel route to narrowband emission in the second near-infrared window (NIR-II).
Lanthanide-doped nanoparticles are prized for their narrow linewidths, high photostability, and non-blinking, non-bleaching emission in the NIR-II range. These properties make them attractive for bioimaging, sensing, and optical communication.
However, their insulating fluoride or oxide hosts have large band gaps (~8 eV), which prevent efficient charge injection and have limited their use in electrically driven devices.
Most existing LnNP applications rely on optical excitation. The new work addresses this long-standing problem by using organic molecules as an electrical “bridge” between injected charges and the lanthanide ions, enabling electrically driven NIR-II emission from materials that are not semiconductors.
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Device Concept And Design
The team created a nanohybrid design by coupling an organic dye, 9-anthracenecarboxylic acid (9-ACA), to NaGdF4 nanoparticles doped with Nd3+, Yb3+, or Er3+. 9-ACA was chosen because its triplet energy (~1.8 eV) aligns with the ladder-like energy levels of these lanthanide ions.
These LnNP@9-ACA nanohybrid particles become the emissive layer in a multilayer LED stack on ITO/glass.
Electrons and holes are injected from the contacts, transported through TmPyPB (ETL) and poly-TPD (HTL), and recombine primarily on the 9-ACA ligands. This generates singlet and triplet excitons on the organic molecules.
The key energetic step is the triplet energy transfer (TET) from the T1 state of 9-ACA to the lanthanide ions through a Dexter-type process that requires close spatial proximity and orbital overlap.
The lanthanides then emit photons in the NIR-II, effectively turning electrical energy into narrowband infrared light using an otherwise insulating host.
Probing The Hybrid System
To confirm the formation of the uniform LnNPs (~6 nm) and clean multilayer device cross-sections, the researchers used transmission electron microscopy (TEM) and HAADF-STEM.
X-ray diffraction (XRD) identifies the hexagonal phase of the NaGdF4 host, while Fourier-transform infrared (FTIR) spectroscopy, supported by DFT calculations, shows that 9-ACA preferentially binds to surface Ln³? sites and partially replaces the native oleic acid ligands.
FTIR-based analysis estimated surface coverage of 9-ACA at 6.8 % (NdNPs), 1.0 % (YbNPs), and 3.6 % (ErNPs), indicating that most sites remain capped with oleic acid, but that sufficient 9-ACA is present to mediate efficient energy transfer.
Steady-state photoluminescence (PL) measurements revealed that coupling 9-ACA to LnNPs dramatically boosted NIR-II emission under UV excitation, with enhancements of 6.6×, 34.1×, and 23.6× for Nd, Yb, and Er systems, respectively.
Time-correlated single-photon counting (TCSPC) and transient absorption spectroscopy revealed that:
- The singlet lifetime of 9-ACA shortens markedly when bound to LnNPs, indicating accelerated intersystem crossing.
- The triplet rise and decay dynamics show very efficient TET from 9-ACA to the lanthanides, with transfer efficiencies above 98 % depending on the specific ion.
- Oxygen strongly quenches the NIR PL, consistent with triplet-mediated excitation.
Together, these measurements confirm that triplet excitons on the ligands, rather than direct singlet transfer, dominate the excitation pathway into the lanthanide ions.
LnNP LED Performance
The LnNP-based LEDs (LnLEDs) exhibit narrow NIR-II electroluminescence with peak wavelengths at approximately 1,058 nm for NdLEDs, 976 nm for YbLEDs, and 1,533 nm for ErLEDs.
The full-width at half maximum (FWHM) values are as low as 20 nm (Nd), 43 nm (Yb), and 55 nm (Er), far narrower than typical NIR-II emission from quantum dots or organic emitters, which often exceed 150 nm.
Turn-on voltages, defined at a radiance of 0.01 mW sr-1m-2, were found to be around 5 V, and the devices operated at up to 15 V without catastrophic failure in tests.
Peak radiances reach ~1.2 mW sr-1 m-2 for Nd and Yb LEDs and ~0.4 mW sr-1 m-2 for Er LEDs. However, the initial external quantum efficiencies (EQEs) are modest:
- ~0.01 % for NdLEDs
- ~0.04 % for YbLEDs
- ~0.004 % for ErLEDs
Optical simulations of the full stack revealed a decrease in light-extraction efficiency in the NIR-II region, which contributed to these low EQE values.
Loss Channels And Optimization
Several factors limited the performance of the first-generation devices: modest PLQEs of ultrasmall, highly doped core-only LnNPs, where surface-related nonradiative pathways are significant. Charge leakage and unwanted visible emission from poly-TPD also hindered performance, resulting from recombination outside the nanohybrid layer.
Low 9-ACA surface coverage (of less than 10 %) constrained the number of effective energy-transfer sites and reduced light extraction at NIR-II wavelengths due to the optical stack design.
To address some of these issues, the study's authors introduced core–shell Yb@Nd nanoparticles (NaGd0.8F4:Yb0.2@NaGd0.4F4:Nd0.6), which substantially improve PLQE (to approximately 3 % under 375 nm excitation) and enable more efficient harvesting of energy transferred from 9-ACA.
The nanoparticles also work to optimize the hole-transport layer and add a half-ball outcoupling lens on the substrate.
With these changes, the Yb@Nd-based devices achieve peak NIR EQEs above 0.6%, representing an order-of-magnitude improvement over the initial structures and surpassing most organic LEDs operating beyond 1,000 nm.
The work also shows that emission can be tuned across the NIR-II range by varying the lanthanide ion type and concentration, highlighting the spectral flexibility of the platform.
The study suggests that further gains may be possible by increasing lanthanide PLQEs through tailored doping strategies, improving surface passivation, and more advanced nanostructures, alongside further refinement of device architectures for better charge balance and light extraction.
The Future of Lanthanide LEDs
This study introduced a practical method for electrically exciting insulating lanthanide-doped nanoparticles by harvesting long-lived molecular triplet excitons at low voltages. The resulting LnLEDs combine narrow NIR-II emission with a clear roadmap for efficiency improvements.
As materials chemistry and device engineering advance, drawing on insights from the OLED and quantum dot LED communities, lanthanide-based hybrid LEDs could become valuable light sources for deep-tissue imaging, optogenetics, optical communication, and other NIR-II technologies.
Journal Reference
Yu Z. et al. (2025). Triplets electrically turn on insulating lanthanide-doped nanoparticles. Nature 647, 625–631. DOI: 10.1038/s41586-025-09601-y