Heterogeneous nanostructural design. In our experiment, we designed a heterogeneous core-multishell structure to suppress surface quenching and achieve tunable emissions. In a conventional design29,30, under 808-nm excitation, Nd3+ sensitizers harvest excitation photons and subsequently pass them to Yb3+ ions with an excited state at ~ 10,000 cm-1. Energy migration through a network of high concentration Yb3+ ions promotes back-energy transfer of the NIR excitation to Tm3+ emitters with ladder-like metastable intermediate states, facilitating sequential upconversion processes from NIR to visible/UV. By doping of Gd3+, upconverted UV emission from high-lying states of Tm3+ can be further transferred to Gd3+ ions as deep-UV energy reservoirs.
The key to our design is the optical inert NaYF4 layer locating in the first shell layer of NaGdF4:Yb,Tm@ NaGdF4:Yb@NaGdF4:Yb,Nd@NaGdF4 (Gd-CSGdS2S3) nanoparticle (Scheme 1). This inert layer can lock-in the upconverted excitation energy of Gd3+ ions. The Gd3+ network can reuse the upconverted excitation energy and prevent depopulation by deleterious energy traps within the nanoparticles. The NaYF4 layer plays a key role in interdicting detrimental energy transfer between Gd3+ and interior traps, enhancing five- and six-photon-upconverted UV emissions.
Upconverted excitation lock-in (UCEL) mode. The UCEL mode requires both an optical inert NaYF4 interlayer and a network of Gd3+ ions to recycle upconversion energy for deep-UV emission amplification. Fig. 1 illustrates a typical upconversion process in heterogeneous, core-multishell nanoparticles upon 808-nm excitation. In brief, 808 nm photons are first sensitized by Nd3+ sensitizer ions, being populated at the 4F5/2 energy state and quickly relaxed to the 4F3/2 energy state of Nd3+. The excited Yb3+ ions serve as an energy migrator to populate the 3P2 state of Tm3+ through a five-photon upconversion process. The 6DJ state of Gd3+ is further populated by the appropriate energy matching of the following transitions of Tm3+: 3P0,1→1D2 (~7600 cm-1, ~8200 cm-1), 3F2,3→3H5 (~6700 cm-1, ~6200 cm-1), and 1G4→3H4 (~8600 cm-1), via an energy transfer process31. However, the probability of nonradiative relaxation of 6IJ→6PJ is larger than that of the radiative transition of 6IJ→8S7/2, resulting in an efficient population of the 6P7/2 state, commonly observed in Gd-based homogeneous nanostructures18. In our design, the NaYF4-based first shell layer selectively blocks the energy transfer from Gd3+ to interior energy traps (e.g., lattice defects and impurities). It preserves and recycles the excitation energy within the core region, leading to increased populations in the 6DJ, 6IJ, and 6PJ states of Gd3+ and intense UV emission of Gd3+.
Controlled synthesis. We used a layer-by-layer epitaxial growth method20 to synthesize a batch of Gd-CSYS2S3 nanoparticles with optimized concentrations of co-dopants30 following the design of NaGdF4:49%Yb,1%Tm@NaYF4:20%Yb@NaGdF4:10%Yb,50%Nd@NaGdF4 (Fig. 2a). Transmission electron microscopy (TEM) images of obtained Gd-CSYS2S3 nanoparticles show the average size of ~29 nm with each layer ~2.5 nm in thickness (Supplementary Fig. S1). High-resolution TEM shows the single-crystalline structure of the as-synthesized core-multishell nanoparticles (Fig. 2b inset), and X-ray powder diffraction result (XRD, JCPDS file number 27-0699, Supplementary Fig. S2) confirms the hexagonal phase of the as-prepared nanoparticles. High-angle annular darkfield scanning TEM identified the formation of the heterogeneous core-multishell structures (Fig. 1b), in which the brighter regions correspond to heavier elements (Gd, Yb, and Nd) and the darker parts correspond to lighter ones (Y). Energy-dispersive X-ray mapping analysis further confirmed the heterogeneous core-multishell structures (Fig. 1c, Supplementary Fig. S3).
Remarkable deep-UV enhancement. To investigate the unusual deep-UV upconversion emission from Gd3+, we recorded the photoluminescence spectra of the as-synthesized nanoparticles at room temperature. Usually, in favor of the lower 6P7/2 (311 nm) energy level, the Gd3+ emission in the deep-UV range is quenched, and optical transitions of (6DJ, 6IJ, 6P5/2→8S7/2) could hardly be spectroscopically detected (Supplementary Figs. S4-7)12. In contrast, as shown in Fig. 1c and Supplementary Fig. S8, intense upconversion emissions from 6DJ and 6IJ of Gd3+ peaked at 253 nm (6D9/2→8S7/2), 273 nm (6IJ→8S7/2), 276 nm (6IJ→8S7/2), 279 nm (6IJ→8S7/2), 306 nm (6P5/2→8S7/2) and 311 nm (6P7/2→8S7/2) in the UV region were observed either under 808 nm or 980 nm excitation. Moreover, we observed more than 50-fold and 30-fold enhancements in Gd3+ emission (311 nm) by our Gd-CSYS2S3 heterogeneous core-multishell design compared with the conventional Gd-CSGdS2S3 nanoparticles under 808 nm and 980 nm excitation, respectively (Supplementary Figs. S9 and S10), although the absorption profile of Gd-CSYS2S3 is not changed compared with that of Gd-CSGdS2S3 nanoparticles (Supplementary Fig. S11). As verified by the emission spectra of several different batches of as-prepared nanoparticles (Supplementary Fig. S12), our protocol to enhance the deep-UV upconversion emissions is reproducible.
We further studied the excitation power dependence of luminescence intensity from higher-lying 6DJ, 6IJ and 6PJ excited states of Gd3+ (Fig. 2f). The number of photons (n) required to populate the upper emitting state can be calculated by the luminescence intensity If, and the pump power of laser P following relation of If∝Pn32. The output slope for 253 nm emission band was calculated as 5.92, indicating that six 808 nm photons needed to populate the 6DJ level, following a six photon upconversion process (Fig. 2g), while n values obtained for 276 and 311 nm emissions were 5.07 and 5.09, indicating five-photon processes (Fig. S13).
Gd3+ energy recycling above 6PJ. To probe the role of NaYF4 layer in locking-in and recycling Gd3+ excitation energy, we have compared the excited state lifetime of Gd3+. As shown in Fig. 3 and Supplementary Fig. S14, a significant prolonged (~4 times) lifetime of Gd3+ emission from the 6P7/2 level was achieved when the NaYF4 first layer was applied. In contrast, there were negligible changes in the Gd3+ lifetimes for emissions from 6DJ and 6IJ energy levels, indicating the energy loss from Gd3+ to interior energy traps was mainly through 6P7/2 energy level of Gd3+ due to small energy gap between 6DJ, 6IJ and 6PJ (Supplementary Fig. S15). In addition, the emission intensities of Nd3+ at 893 nm (4F3/2→4I9/2), 1057 nm (4F3/2→4I11/2), and 1330 nm (4F3/2→4I13/2) and Tm3+ at ~1460 nm (3H4→3F4) in the near-infrared range were essentially unaltered (Supplementary Fig. S16). These results indicate that the NaYF4-assisted UCEL mechanism favors the upconversion emissions from high-lying energy levels.
The role of the first layer of NaYF4 shell. To further verify the role of NaYF4 layer in enhancing the deep-UV emissions, we synthesized a group of Gd-CSYS2S3, Gd-CS1SYS3 and Gd-CS1S2SY heterogeneous nanoparticles, in which NaGdF4 was selectively replaced by NaYF4 host lattice in the first, second and third layer, respectively (Fig. 4a). The intense deep UV emission was only observed in Gd-CSYS2S3 nanoparticles. The emission profiles of Gd-CS1SYS3 and Gd-CS1S2SY were quite similar to Gd-CSGdS2S3 nanoparticles. Moreover, when the optically inert Y3+ ions in the first layer were replaced by half of the Gd3+ ions, a drastic reduction of the Gd3+ emission was observed, indicating that the optical inert NaYF4 layer can effectively prevent the back energy transfer from Gd3+ (Supplementary Fig. S17).
We further prepared a group of Gd-CSYS2S3 nanoparticles doped with Tb3+ or Eu3+ ions in the first layer (Gd-CSY-15%TbS2S3 or Gd-CSY-15%EuS2S3), which can extract the excitation energy from Gd3+ to emit green and red upconversion emissions through the scheme of energy migration upconversion (EMU)18. Upon excitation at 808 nm, the characteristic emissions of Tb3+ and Eu3+ (highlighted in color) were observed (Fig. 4b-c and Supplementary Fig. S18), but no enhancement in deep-UV emission. Doping with 15% Tb3+ or Eu3+ in the outmost layer only led to weak emission of Tb3+ or Eu3+ (Supplementary Fig. S19). The weak Tb3+ and Eu3+ emissions were attributed to the interior energy trapping of the excitation energy in the Gd3+ sublattice. Together, these results indicate that an efficient energy transfer pathway (Nd3+→Yb3+→Tm3+→Gd3+) occurs33, and the excitation energy of Gd3+ can be easily dissipated through the emission of Tb3+, Eu3+, or interior traps without the NaYF4 first-shell layer.
Determination of the interior traps. An efficient energy transfer can occur between Gd3+ and Nd3+ ions34. However, in our design, the energy transfer between these two ions did not happen. To preclude the possibility of the interior Nd3+ energy trapping, we prepared a series of Gd-CSGdS2S3 nanoparticles with and without Nd3+ dopant (Gd-CSGdS50%NdS3 and Gd-CSGdS0%NdS3). The lifetimes of Gd3+ (6DJ, 6IJ, 6PJ) and Tm3+ (1I6, 1D2) were virtually unchanged after removing Nd3+ dopants in nanoparticles (Fig. 4d and Supplementary Fig. 20). These results confirm that intense deep-UV emission from Gd3+ is enabled by obstructing the energy transfer from Gd3+ to interior lattice defects or impurities.
Furthermore, we compared the amount of light-to-heat conversion in Gd-CSYS2S3 and Gd-CSGdS2S3 nanoparticles by using an infrared thermal imaging camera. As a higher concentration of Nd3+ in nanoparticles would generate more heat under single-beam infrared laser excitation35, we measured the concentrations of Nd3+ in these two types of nanoparticles (Supplementary Table 1). The measured temperature rises of the solution of Gd-CSYS2S3 and Gd-CSGdS2S3 are 5.0 oC and 7.2 oC under irradiation at 808 nm light, 1.9 oC and 3.2 oC under irradiation at 980 nm light, respectively (Fig. 5). These results suggest that less excitation energy is converted to lattice heating in heterogeneous core-multishell structures than in conventional nanoparticles.
Enhancement in highly doped single nanoparticles. To further evaluate UCEL mode in enhancing the high-order upconversion emissions in the heterogenous core-multishell structures, we implemented the similar design in the highly doped UCNP core, e.g. Gd-C8%TmSYS2S3 and Gd-C8%TmSGdS2S3, and quantify the brightness of single UCNPs using a purpose-built confocal microscopy system (see Supplementary Fig. S21). Due to the significant UV absorption by the optical components, including the objective lens and mirrors, instead of a direct quantification of the deep UV emissions at a single nanoparticle level, we monitored the amount of the blue band emissions from a single nanoparticle. Under the same excitation power from both 808 nm and 976 nm lasers, the emission intensities of Gd-C1%TmSYS2S3 and Gd-C1%TmSGdS2S3 nanoparticles under the 808 nm excitation were ~4 times and ~5 times higher than those under the 976 nm excitation, respectively (Supplementary Fig. S22). In contrast, much higher enhancement factors of the highly doped Gd-C8%TmSYS2S3 (~25 times) and Gd-C8%TmSGdS2S3 (~15 times) nanoparticles were achieved under the 808 nm v.s. 976 nm excitations. These results suggest UCEL mode could be broadly applied to a variety of UCNP core concentrations36 and under a large dynamic range of excitation power densities37, suitable for both ensemble and single nanoparticle applications38.