LnNPs consist of an inorganic insulating host, typically fluorides or oxides such as NaGd/Y/LuF 4 with a large energy gap of approximately 8 eV (ref. 6), with lanthanide ions embedded in the host lattice. LnNPs have high photo and chemical stability in various environments and have narrow and tunable emission in the NIR-II range (1,000–1,700 nm). This is in contrast to semiconductor-based systems, such as NIR-II emissive organic dyes or semiconducting colloidal quantum dots (QDs), which show broad emission spectra in this region owing to homogeneous broadening. This has motivated research into the application of LnNPs in stimulated-emission depletion microscopy7, deep-tissue theranostics4,8,9,10, sensing11 and optical communication12. However, as these systems are not semiconductors, they cannot be used to construct electrically driven devices, as can be done for colloidal QDs13,14, metal halide perovskites15,16,17 or organic semiconductors18,19.
It has previously been shown that triplet excitons on organic molecules can couple to the f-f transitions in lanthanide ions and that this enables TET between organic molecules and LnNPs20. Organic dye sensitization has proved effective to enhance the emission of LnNPs21,22,23,24,25. Here we use molecular triplet excitons to mediate the function of electrically driven LnNP-based optoelectronic devices, using triplets to efficiently turn on these insulating materials. The first step in this process is to engineer the coupling between organic molecules and LnNPs. The inset of Fig. 1a shows a schematic of the LnNP. The as-prepared LnNPs have oleic acid (OA) on the surface. However, OA is an insulating ligand, which cannot mediate electrical excitation. We therefore partially replace OA with 9-anthracenecarboxylic acid (9-ACA), a widely studied organic dye with a singlet energy of 3.2 eV and triplet energy around 1.8 eV (ref. 26). As shown in Fig. 1b, the triplet energy level of 9-ACA (ref. 27) can, in principle, allow for TET to the ladder-like energy levels of Ln3+ ions (Ln = Nd, Yb, Er). These hybrid materials allow us to construct the first LnLEDs.
Fig. 1: Fabrication of LnNP-based NIR-II LEDs. a, Schematic illustration of the device architecture of LnLEDs with a close-up schematic of LnNP@9-ACA nanohybrids. b, Simplified schematic showing electron and hole injection through organic molecules to turn on lanthanide ions in an insulating host lattice. CB, conduction band; VB, valence band. c, Normalized EL spectra of LnLEDs. a.u., arbitrary units. d, Reported FWHMs of the EL at different wavelengths from different types of LED, including LnLEDs and QD LEDs. Full size image
Figure 1a shows the device architecture of LnLEDs, consisting of glass/indium tin oxide (ITO)/poly(ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/poly(4-butylphenyl-diphenylamine) (poly-TPD)/LnNP@9-ACA/1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB)/lithium fluoride (LiF)/aluminium (Al). ITO and LiF/Al function as electrodes. PEDOT:PSS acts as hole injection layer. TmPyPB and poly-TPD serve as electron and hole transport layers (ETL and HTL), respectively. The LnNP@9-ACA nanohybrids serve as the light-emitting layer. Electrons and holes injected from the contacts travel through the charge transport layers and recombine on the 9-ACA ligands. This will lead to the formation of singlet and triplet excitons on 9-ACA in a 1:3 ratio as governed by the spin–statistics theorem. We note that triplet excitons can undergo efficient energy transfer to the Ln3+ ions as shown in Fig. 1b and as experimentally demonstrated in Fig. 3. The Ln3+ ions can then emit photons, leading to EL from the device. We keep the device architecture constant but vary the type of Ln3+ ions doped into the LnNPs to achieve a range of EL emission from 1,000 to 1,533 nm.
Figure 1c shows the EL spectra obtained from the LnLEDs. The spectra are narrow and consistent with the main peaks of NIR-II photoluminescence (PL) spectra of LnNP@9-ACA nanohybrids under 350-nm photoexcitation. The full widths at half maximum (FWHMs) of LnLEDs EL spectra are calculated to be 20, 43 and 55 nm for Nd/Yb/ErLEDs, which are much lower than the FWHMs found in semiconducting QDs/bulk materials-based systems (FWHM normally above 150 nm)5 (Fig. 1d). The large FWHM of QD LEDs, which is limited by homogeneous line broadening, creates complications for their use in optical communication and chemical/biomedical imaging/sensing applications. The narrow linewidths we achieve here, combined with the inherent ease of processing, flexibility, wide-area compatibility and potential low cost of organic–LnNP hybrids offers exciting possibilities for a new generation of light sources across the NIR-II range. A quantitative comparison of our LnLEDs and other NIR-II LEDs and laser diodes is included in Supplementary Tables 1 and 2.
To obtain high-quality NIR-II light-emitting layers, we synthesized uniform and ultrasmall (<10 nm) LnNPs. Transmission electron microscopy (TEM) images show that all of the LnNPs had good size monodispersity, with an average size of around 6 nm (Supplementary Fig. 1). The dopant ratio of fluorescent Ln3+ (Ln = Nd, Yb, Er) ions has been fixed to 20 mol% in the form of NaGd 0.8 F 4 :Ln 0.2 , which will be subsequently referred to as NdNPs, YbNPs and ErNPs, respectively. This dopant ratio guarantees that enough fluorescent Ln3+ ions receive energy transferred from organic molecules and a fair comparison of energy transfer efficiencies among different Ln3+ ions, while avoiding severe cross-relaxation to maintain a relatively high NIR-II fluorescence28. The high-resolution TEM images and X-ray diffraction (XRD) patterns show that these LnNPs are hexagonal phase (Supplementary Figs. 2 and 3).
As shown in Fig. 2a, the LnNPs show weak and narrow absorption peaks, which is one of the key limitations of LnNPs for various applications. Coupling 9-ACA onto the surface of LnNPs endows LnNP@9-ACA nanohybrids with strong absorption in the ultraviolet range (Fig. 2b). The absorption of these nanohybrids is hence dominated by organic molecules and overcomes the aforementioned limitation of LnNPs. LnNP@9-ACA nanohybrids also show a 5-nm redshift of absorption compared with pure 9-ACA owing to the coupling between organic molecules and LnNPs (Supplementary Fig. 4). Investigations of the ligand exchange process using Fourier-transform infrared (FTIR) spectroscopy and corresponding density functional theory (DFT) simulations (see Fig. 2c,d and Supplementary Figs. 5–7 for details) indicate that the 9-ACA preferentially binds to the Ln3+ ion site on the surface of the LnNPs, in contrast to the OA, which also binds to the Na+ sites. DFT-predicted FTIR spectra of 9-ACA bonded to Gd3+ reproduces the experimentally observed spectrum, whereas 9-ACA bonded to Na+ does not, and introduces peaks at 1,600 cm−1, which are not observed (vertical lines in Fig. 2c). DFT-predicted FTIR spectra for OA show peaks shared at 1,450 and 1,590 cm−1 for OA bonded to Na+ or Gd3+ (vertical lines in Fig. 2d). On the basis of the FTIR data, we estimate the replacement ratios of 9-ACA on different LnNPs to be 6.8%, 1.0% and 3.6% for NdNPs, YbNPs and ErNPs, respectively (Extended Data Fig. 1 and Supplementary Table 3). The important point here is the preferential binding of 9-ACA to the Ln3+ ion sites, which will promote efficient energy transfer.
Fig. 2: Characterization of the LnNP@9-ACA nanohybrid system. a,b, Absorption spectra of LnNPs (a) and LnNP@9-ACA nanohybrids (b). c, DFT-simulated FTIR spectra of free 9-ACA molecules, bound 9-ACA molecules to Gd3+ and Na+ ions and experimental data of YbNP@9-ACA nanohybrids. d, DFT-simulated FTIR spectra of free OA molecules, bound OA molecules to Gd3+ and Na+ ions and experimental data of OA-capped YbNPs. e, Comparison of NIR-II emission between LnNPs and LnNP@9-ACA nanohybrids under the excitation of a 350-nm lamp (concentration 20 mg ml−1). Full size image
Ligand exchange is a dynamic process and can be influenced by numerous factors. We find that the ligand exchange rate is first increased by prolonging the reaction time and then reaches a plateau, by monitoring the absorbance change of YbNP@9-ACA nanohybrids and PL excitation spectra (Supplementary Fig. 8). We observe that the energy transfer efficiency is not greatly influenced by the ligand exchange rate when the ligand exchange process reaches an equilibrium (Supplementary Figs. 8d–f and 9). We also note that the short distance between 9-ACA and the surface of the LnNPs, linked by the carboxyl group, should allow for efficient TET, as this process is considered to be a Dexter-type energy transfer process. As well as the TET, energy transfer from the singlet state of the 9-ACA to the Ln3+ ions is also possible by means of Förster resonance energy transfer (FRET), although the low absorption cross-section of the Ln3+ ions and poor spectral overlap with the 9-ACA blue emission make this process inefficient29.
As shown in Fig. 2e, the coupling of organic molecules leads to a notable enhancement of the NIR-II emission under ultraviolet excitation, achieving a large Stokes shift. The LnNP@9-ACA nanohybrids show 6.6-fold, 34.1-fold and 23.6-fold enhancement in NIR-II PL compared with NdNPs, YbNPs and ErNPs, respectively. The NIR photoluminescence quantum efficiencies (PLQEs) of LnNPs and LnNP@9-ACA nanohybrids are measured in Supplementary Table 4. Tuning the doping ratio of Ln3+ is a straightforward and effective approach to enhance the fluorescent performance of LnNPs. Increasing the doping ratio of Yb3+ would substantially enhance the downconversion intensities for both YbNPs and YbNP@9-ACA nanohybrids (Supplementary Fig. 10), for which cross-relaxation between Yb3+ ions is not a notable loss, unlike in Er3+ and Nd3+. The ratios of several peaks in the NIR-II EL have changed compared with the PL spectra, indicating distinct energy transfer mechanisms under photoexcitation and electroexcitation for LnNP@9-ACA nanohybrids. To study the energy transfer mechanisms, we further perform steady-state PL, PL decay and transient absorption measurements. Owing to the different amounts of attached 9-ACA in the nanohybrids, we cannot directly compare the intensity of the visible PL to determine the efficiency of the energy transfer (Fig. 3a). PLQE measurements show that bound 9-ACA molecules on LnNPs have markedly decreased PLQE compared with pristine 9-ACA (Supplementary Fig. 11).
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