A sandwiched luminescent heterostructure based on lanthanide‐doped Gd2O2S@NaYF4 core/shell nanocrystals

Lanthanide (Ln3+) oxysulfide nanocrystals (NCs) have great prospect in many advanced technologies; however, they suffer from a low photoluminescence efficiency due to the volatility of sulfur and deleterious surface quenching effect. Herein, we report a novel sandwiched luminescent heterostructure based on Ln3+‐doped Gd2O2S@NaYF4 core/shell NCs with tunable sulfur content in the sandwich layer. By means of Eu3+ as the sensitive structural probes, we unravel the ligand‐mediated structure control of the NCs from Gd2O3: Ln3+@NaYF4 to Gd2O2S: Ln3+@NaYF4 with tailored S2– deficiency. Such a sandwich‐type core/shell heterostructure enables us to achieve efficient and multicolor downshifting and upconversion luminescence (UCL), with up to 208.8 folds of enhancement in UCL intensity as compared to that of their core‐only counterparts. These findings provide a general approach for the controlled synthesis of lanthanide oxysulfide@fluoride heterostructure, which offers a new way for the materials design towards diverse emerging applications.


INTRODUCTION
However, on nanoscale, lanthanide oxysulfides normally suffer from a low PL efficiency due to the volatility of sulfur and the deleterious surface quenching effect, [23][24][25][26][27] especially for upconversion Ln 3+ emitters with smaller energy gaps between the emitting states and their next low-lying states.[44][45][46][47] However, because of the drastically different physicochemical properties and growth kinetics between oxysulfide and fluoride, it remains notoriously difficult for the controlled synthesis of oxysulfide@fluoride heterostructure with desirable properties.
Herein, we report a unique strategy via epitaxial growth of α-NaYF 4 on Ln 3+ -doped Gd 2 O 2 S NCs for the controlled synthesis of heterogeneous Gd 2 O 2 S: Ln 3+ @NaYF 4 core/shell NCs with a sandwiched structure and bright luminescence.We demonstrate that the ligand of oleylamine (OAm) plays a critical role in stabilizing the sulfide layer of the Gd 2 O 2 S core, which enables us to finetune the heterostructure with tailored S 2-deficiency in the sandwich layer by varying the ligand ratio of OAm and oleic acid (OA).Specifically, such ligand-mediated structure change can be monitored by utilizing Eu 3+ as the sensitive structual probe.Furthermore, based on the novel oxysulfide@fluoride heterostructured design, we achieve multicolor UCL (Tm 3+ , Er 3+ , and Ho 3+ ) and DSL (Eu 3+ , Tb 3+ , Dy 3+ , and Sm 3+ ) under single-wavelength excitation at 980 and 254 nm, respectively, with remarkably improved intensities as compared to those of Gd 2 O 2 S: Ln 3+ core.

RESULTS AND DISCUSSION
Gd 2 O 2 S has a hexagonal structure (space group of P3__m1) with one layer of edge-sharing [Gd 2 O 2 ] 2+ units and one sulfur layer arranged alternatively along the c-axis (Figure 1A).Gd 3+ ion is coordinated with three S 2-ions and four O 2- ions and has a single site symmetry of C 3V . [21][50] transmission electron microscope (TEM) images showed that the NCs were lamellar with a lateral size of 5.4 ± 0.7 nm and a thickness of ∼3.4 nm (Figure 1B and Figure S1, Supporting Information).High-resolution TEM (HRTEM) image exhibited clear lattice fringes with observed d-spacings of 0.33 and 0.25 nm (Figure 1C), corresponding to the (100) and (102) planes of hexagonal Gd 2 O 2 S, respectively.Selected-area electron diffraction (SAED) pattern of the NCs displayed intense diffraction rings that can be well indexed into hexagonal Gd 2 O 2 S (Figure 1D).Powder X-ray diffraction (XRD) pat-tern showed that all diffraction peaks of the NCs matched well with the standard pattern (JCPDS No. 26-1422) of hexagonal Gd 2 O 2 S (Figure 1E), confirming high crystallinity and phase purity of the resulting NCs.Compositional analyses through energy dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) identified the elements of Gd 3+ , S  1E).TEM images showed that the heterostructured NCs were roughly spherical with a mean size of 10.6 ± 0.8 nm (Figure 1F and Figure S4, Supporting Information), indicating a shell thickness of ∼3.3 nm.Specifically, the HRTEM images of the heterostructured NCs exhibited distinct lattice spacings of 0.33 nm and 0.31 nm in different areas of a single nanoparticle (Figure 1G and Figure S5, Supporting Information).The lattice spacing of 0.33 nm in the central area can be assigned to the (100) plane of hexagonal Gd 2 O 2 S, and that of 0.31 nm in the peripheral area belongs to the (111) plane of cubic NaYF 4 .The clear lattice fringes combined with the strong diffraction rings observed in the SAED pattern (Figure 1H) revealed the high crystallinity of both Gd 2 O 2 S core and α-NaYF 4 shell.Moreover, it was found that the Gd 2 O 2 S: Ln 3+ core was sandwiched by the α-NaYF 4 shell on the hexagonal plane, as visualized by the contrasted HAADF-STEM image of the heterostructured NCs (Figure 1I), where the brighter regions correspond to the heavier Gd 3+ ions in the sandwich layer (core) and the darker regions correspond to the lighter Y 3+ ions in the cover layer (shell).Such a sandwiched heterostructure was further confirmed by the electron energy loss spectroscopy (EELS) and EDX elemental mapping of the NCs (Figures 1J-Q), where Gd 3+ , S 2-, and the doped Ln 3+ ions were detected mainly in the central region of the NCs, while Na + , Y 3+ , and F -ions were observed primarily in the periphery.The sandwiched structure of Gd 2 O 2 S: Ln 3+ @NaYF 4 suggests that α-NaYF 4 grows along the c-axis (namely, the (001) plane) of Gd 2 O 2 S (Figure 2A).This can be ascribed to the fact that the (001) plane of Gd 2 O 2 S has a higher density of Gd 3+ and is terminated by [Gd 2 O 2 ] 2+ layer, [51] which has high affinity with F -ions to facilitate the epitaxial growth of α-NaYF 4 .
It should be noticed that, Gd 2 O 2 S: Ln 3+ NCs are susceptible to degradation in an acidic environment owing to the volatility of sulfur. [39]Therefore, the synthesis of Gd 2 O 2 S: Ln 3+ core and Gd 2 O 2 S: Ln 3+ @NaYF 4 core/shell heterostructure should be carried out in an alkaline environ- ment.It implies that the claim by Zou and some of us et al. in a recent report about the synthesis of oxysulfide@fluoride heterostructure in the acidic solvents of OA/ODE is a misinterpretation.Indeed, in Zou's work, [45] the characteristics of oxysulfide phase including the XRD peaks of hexagonal Gd 2 O 2 S and the element of sulfur were not detected in the claimed Gd 2 O 2 S: Ln 3+ @β-NaYF 4 core/shell NCs, due to the degradation of Gd 2 O 2 S during the shell growth.We have also synthesized the core/shell NCs according to Zou's protocol and obtained similar XRD patterns and TEM images, which showed merely single phase of β-NaYF 4 (Figures S6 and S7, Supporting Information).By utilizing Eu 3+ as the structural probe, we have identified that Zou's synthesis yielded the core/shell NCs of β-NaGdF 4 : Ln 3+ @β-NaYF 4 instead of the nominal Gd 2 O 2 S: Ln 3+ @β-NaYF 4 (Figure S8, Supporting Information).These inspections combined with our findings reveal that the ligand of OAm played a critical role in the synthesis of oxysulfide@fluoride heterostructure.
It is known that the oleate (OA) ligand has a high binding affinity to metal ions and can stabilize both Gd 2 O 2 S and α-NaYF 4 to maintain the integrity and monodispersity of the heterostructured NCs. [52]Nonetheless, excess OA may lead to the degradation of Gd 2 O 2 S due to the sulfur volatilization.By contrast, OAm prefers to combine with S 2-ion and can promote the OA protonation, [51] and thus OAm should be the key factor that stabilizes the sulfide layer of Gd 2 O 2 S. As such, it is expected that, the sulfur content as well as the size of Gd 2 O 2 S: Ln 3+ @NaYF 4 heterostructured NCs can be controlled by tuning the ligand ratio of OAm/OA in the synthesis (Figure 2A).To this regard, we performed a group of controlled experiments for the synthesis of Gd 2 O 2 S: Eu 3+ @NaYF 4 heterostructured core/shell (0.4/0.5 mM) NCs with a fixed amount of OA (1 mL) and different amounts of OAm (0-5 mL).TEM images showed that all the NCs were monodispersed with the mean size increase from 7.2 ± 0.5 nm to 10.6 ± 0.8 nm as the OAm/OA ratio increased from 0 to 5 (Figures 2B-F and Figures S9-S12, Supporting Information), due mainly to the suppressed sulfur loss in the sandwich layer during the shell growth at higher OAm/OA ratios.Accordingly, the phase and composition of the heterostructured NCs changed with varying the OAm/OA ratio.At low OAm/OA ratios (≤1), single phase of α-NaYF 4 was observed (Figures 2G,H and Figures S9 and  S10, Supporting Information).At high OAm/OA ratios (>1), the sandwiched heterostructure with dual phases of hexagonal Gd 2 O 2 S and α-NaYF 4 was identified (Figures 2I-K and Figures S11 and S12, Supporting Information).This indicates that Gd 2 O 2 S: Ln 3+ @NaYF 4 heterostructure can only be achieved at a relatively high OAm/OA ratio (>1).Besides, it was observed that the XRD peaks of hexagonal Gd 2 O 2 S (e.g., 26.7 • , 29.9 • , and 38.1 • ) were intensified upon increasing the OAm/OA ratio, while those of α-NaYF 4 remained nearly unchanged (Figure 2L), indicative of an increased amount of Gd 2 O 2 S phase in the sandwich layer of the heterostruc- tured NCs.Such ligand-mediated composition evolution of the heterostructure was further confirmed by EDX, XPS, and ICP-AES measurements (Table S2).The results showed that the molar ratio of S/(Gd + Eu) increased gradually from 0 to ∼0.54 upon elevating the OAm/OA ratio from 0 to 5 (Figure 2M), affirming the critical role of OAm in stabilizing the sulfide layer of Gd 2 O 2 S. By contrast, the molar ratio of Y/(Gd + Eu) showed merely a slight fluctuation around ∼1.27 with varying the OAm/OA ratio (Figure 2N), suggesting that the (Gd + Eu) elements in the core were well preserved during the shell growth.Note that the exfoliation of the sulfide layers from Gd 2 O 2 S may yield Gd 2 O 3 .There-fore, the surplus (Gd + Eu) in the S 2--deficient Gd 2 O 2 S: Eu 3+ @NaYF 4 heterostructure was deduced to derive from Gd 2 O 3 : Eu 3+ in the sandwich layer, which cannot be distinguished by XRD patterns and HRTEM images because of its low contents and close lattice parameters to those of α-NaYF 4 .
To gain deep insights into the fine structure of the assynthesized Gd 2 O 2 S: Eu 3+ @NaYF 4 heterostructured NCs, we carried out optical spectroscopic analyses by using Eu 3+ as the sensitive structural probe.Figure 3 shows the PL excitation and emission spectra and PL decay curves of the nominal Gd 2 O 2 S: Eu 3+ @NaYF 4 heterostructured NCs synthesized with OAm/OA ratios of 0, 1, 2, 3, and 5 (denoted as CS-0, CS-1, CS-2, CS-3, and CS-5, respectively).The PL spectra of Gd 2 O 2 S: Eu 3+ and Gd 2 O 3 : Eu 3+ NCs were used as the spectroscopic fingerprints for reference (Figure S13, Supporting Information).As shown in Figure 3A, upon excitation to the S 2--Eu 3+ charge transfer band (CTB) at 325 nm, CS-2, CS-3, and CS-5 exhibited nearly identical emission pattern to that of Gd 2 O 2 S: Eu 3+ NCs, with characteristic and sharp emission peaks arising from the 5 D 0 → 7 F J transitions of Eu 3+ at 582.0 ( 7 F 0 ), 594.6 ( 7 F 1 ), 616.0 ( 7 F 2 ), 625.4 ( 7 F 2 ), 655.0 ( 7 F 3 ), and 705.6 nm ( 7 F 4 ), which resembled those of bulk Gd 2 O 2 S: Eu 3+ previously reported. [53,54]PL excitation spectra of the NCs by monitoring the Eu 3+ emission at 625.4 nm showed also similar excitation pattern to those of bulk Gd 2 O 2 S: Eu 3+ with a broad excitation band covering from 220 to 450 nm, which can be deconvoluted into three bands (Figure S14, Supporting Information): the host absorption band of Gd 2 O 2 S centered at 234 nm, the O 2--Eu 3+ CTB at 250 nm, and the S 2--Eu 3+ CTB at 325 nm. [53,54]The O 2--Eu 3+ CTB was found to be enhanced in the heterostructured NCs with respect to that in Gd 2 O 2 S: Eu 3+ NCs, as a result of the O 2-surplus and S 2-deficiency in the sandwich layer of the heterostructure.It was also noticed that the number of the crystal-field (CF) transition lines of Eu 3+ in the heterostructured NCs deviated from that of the theoretical prediction for Gd 2 O 2 S: Eu 3+ with a single site symmetry of C 3V (Figure S15, Supporting Information), [55,56] indicating multiple site symmetries of Eu 3+ in the nominal Gd 2 O 2 S: Eu 3+ @NaYF 4 heterostructure.
In comparison with that of CS-2, CS-3, and CS-5, the Eu 3+ emission was not detected in CS-0 and CS-1 under 325 nm excitation, due to the lack of S 2-ions and consequently the absence of the S 2--Eu 3+ CTB.By constrast, they displayed strong and typical 5 D 0 → 7 F J emissions of Eu 3+ at 576.8 ( 7 F 0 ), 593.0 ( 7 F 1 ), 610.0 ( 7 F 2 ), 627.2 ( 7 F 2 ), 653.2 ( 7 F 3 ), and 707.0 nm ( 7 F 4 ), upon excitation to the O 2--Eu 3+ CTB at 250 nm, in parallel with nearly identical excitation and emission patterns to those of cubic Gd 2 O 3 : Eu 3+ NCs (Figure 3B), thus identifying the single phase of Gd 2 O 3 : Eu 3+ instead of Gd 2 O 2 S: Eu 3+ in CS-0 and CS-1.Owing to the S 2-deficiency in the core, the fingerprint emission of Gd 2 O 3 : Eu 3+ was also observed in CS-2, CS-3 and CS-5 under 250 nm excitation, in addition to that of Gd 2 O 2 S: Eu 3+ .The component of Gd 2 O 2 S: Eu 3+ emission increased gradually at the expense of that of Gd 2 O 3 : Eu 3+ , as the OAm/OA ratio increased, since the S 2-deficiency was suppressed at higher OAm/OA ratios.Specifically, the fingerprint emission of Gd 2 O 3 : Eu 3+ was negligible in CS-5, verifying high efficiency of OAm in stabilizing the sulfide layer of Gd 2 O 2 S in the heterostructure.Such dual-phase emissions from Gd 2 O 2 S: Eu 3+ and Gd 2 O 3 : Eu 3+ in the heterostructured NCs can be distinguished by measuring their PL decay curves at 625.4 and 610.0 nm, respectively.As shown in Figures 3C and D, the PL lifetimes (610.0 nm: 820-2203 μs) of Gd 2 O 3 : Eu 3+ in the heterostructure were typically longer than those (625.4nm: 433-572 μs) of Gd 2 O 2 S: Eu 3+ (Table S3, Supporting Information), due mainly to the larger oscillator strength of Eu 3+ in Gd 2 O 2 S than that in Gd O 3 . [54]Moreover, owing to the surface passivation by α-NaYF 4 , both the PL lifetimes of Gd 2 O 2 S: Eu 3+ and Gd 2 O 3 : Eu 3+ in the heterostructured NCs were lengthened in comparison with those of their core-only counterparts.These results demonstrate unambiguously that, the sandwiched heterostructure changed from Gd 2 O 3 : Eu 3+ @NaYF 4 (CS-0 and CS-1) to Gd 2 O 2 S: Eu 3+ @NaYF 4 (CS-2, CS-3 and CS-5) with increasing S 2-content in the sandwich layer by elevating the OAm/OA ratio in the synthesis.

CONCLUSIONS
In summary, we have developed a novel sandwiched core/shell heterostructure with tunable size and composition based on the epitaxial growth of α-NaYF 4 along the c-axis of Gd 2 O 2 S: Ln 3+ NCs.By means of Eu 3+ as the sensitive structural probes, we have unraveled the important role of the ligand of OAm in stabilizing the sulfide layer of the Gd 2 O 2 S core and realized the precise control of the heterostructure from Gd 2 O 3 : Ln 3+ @NaYF 4 to Gd 2 O 2 S: Ln 3+ @NaYF 4 with tailored S 2-deficiency.Furthermore, by doping different Ln 3+ ions in these heterostructured NCs, we have achieved efficient and multicolor UCL and DSL under single-wavelength excitation at 980 and 254 nm, respectively, with markedly improved intensities in comparison with their core-only counterparts.These findings provide a general approach for boosting the luminescence efficiency of lanthanide oxysulfide NCs via heterostructured core/shell engineering, thereby opening up new opportunities for the advanced materials design based on lanthanide oxysulfides towards versatile applications such as multimodal bioimaging, nanoscintilators, and photocatalysis.

Synthesis of Gd 2 O 2 S: Ln 3+ @NaYF 4 heterostructured NCs
Gd 2 O 2 S: Ln 3+ @NaYF 4 heterostructured NCs were synthesized through a high-temperature coprecipitation method by using Gd 2 O 2 S: Ln 3+ NCs as the seed to mediate the shell growth of α-NaYF 4 .In a typical synthesis of Gd 2 O 2 S: 10%Eu 3+ @NaYF 4 heterostructured NCs, 0.5 mmol of Y(CH 3 COO) 3 ⋅4H 2 O was mixed with 1 mL of OA, 5 mL of OAm, and 4 mL of ODE in a 100 mL three-necked roundbottom flask.The resulting mixture was heated to 160 • C under a N 2 flow with constant stirring for 30 min to form a clear solution.After cooling down to RT, 10 mL of methanol solution containing 1.25 mmol of NaOH and 2 mmol of NH 4 F was added and the solution was stirred at 70 • C for 30 min to remove methanol.After methanol was evaporated, 10 mL cyclohexane solution containing 0.4 mmol of Gd 2 O 2 S: 10%Eu 3+ NCs was added and the solution was stirred at 80 • C for 30 min to remove cyclohexane.Thereafter, the solution was heated to 280 • C under a N 2 flow with vigorous stirring for 90 min, and then cooled down to RT.The obtained NCs were precipitated by addition of 20 mL of ethanol, collected by centrifugation, purified with ethanol for twice, and finally re-dispersed in cyclohexane.Gd 2 O 2 S: Ln 3+ @NaYF 4 heterostructure with different S 2-contents was synthesized by varying the ligand ratio of OAm/OA (0, 1, 2, 3, and 5) under otherwise identical conditions, and the total amount of (OA + OAm + ODE) was fixed at 10 mL.

Synthesis Gd 2 O 3 : Eu 3+ NCs
Gd 2 O 3 : Eu 3+ NCs were synthesized via a modified thermal decomposition method. [2]In a typical synthesis, 0.9 mmol of Gd(CH 3 COO) 3 ⋅4H 2 O, 0.1 mmol of Eu(CH 3 COO) 3 ⋅4H 2 O, and 1 mmol of LiOH were mixed with 3 mL of OA, 7 mL of OAm, and 10 mL of TOA in a 100 mL three-necked roundbottom flask.The resulting mixture was heated to 120 • C under a N 2 flow with constant stirring for 30 min to remove the residual water and oxygen, and then heated to 160 • C and stirred for another 30 min to form a clear solution.Thereafter, the solution was heated to 320 • C under a N 2 flow with vigorous stirring for 60 min, and then cooled down to RT.The obtained NCs were precipitated by addition of 20 mL of ethanol, collected by centrifugation, purified with ethanol for twice, and finally re-dispersed in cyclohexane.

Characterization
Powder XRD patterns were collected with an X-ray diffractometer (MiniFlex2, Rigaku) using Cu Kα1 radiation (λ = 0.154187 nm).TEM measurements, including the lowand high-resolution TEM, HAADF-STEM and EDX ele-ment mapping, were performed on a TECNAI G 2 F20 TEM.ICP-AES analysis was conducted on an ICP-AES spectrometer (Ultima2, Jobin Yvon).XPS measurements were carried out on a Thermo Fisher ESCALAB 250Xi using Al Kα (1486.6 eV) and He Iα (21.2 eV) as the sources of radiation.PL excitation and emission spectra and PL decay curves were measured on FLS980 spectrometer (Edinburgh) equipped with both continuous (450 W) and pulsed xenon lamps.UCL spectra were recorded on FLS980 spectrometer upon excitation with a 980-nm continuous-wave (CW) diode laser (2 W).UCL lifetimes were measured on FLS980 spectrometer equipped with a tunable midband Optical Parametric Oscillator (OPO) pulse laser as the excitation source (410-2400 nm, 10 Hz, pulse width ≤5 ns,Vibrant 355II, OPOTEK).
The UCL and DSL photographs were taken by using a Huawei Nova5Pro cell phone without using any filter.The absolute PL QYs of the NCs were measured by employing a standard barium sulfate coated integrating sphere (150 mm in diameter, Edinburgh) as the sample chamber that was mounted on FLS980 spectrometer with the entry and output port of the sphere located in 90 • geometry from each other in the plane of the spectrometer.A standard tungsten lamp was used to correct the optical response of the instrument.All the spectral data were recorded at RT by using the powder samples unless otherwise noted, and corrected for the spectral response of both the spectrometer and the integrating sphere.

A C K N O W L E D G M E N T S
This work was supported by the National Key R&D Program of China (grant number: 2022YFB3503700), the NSFC (grant numbers: 12074379, 12174391, U22A20398, and 22135008), the Youth Innovation Promotion Association of CAS (grant number: 2020305), and the Natural Science Foundation of Fujian Province (grant numbers: 2020I0037, 2021L3024, and 2022H0040).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

3+ -doped Gd 2 O 2 S NCs
to 320 • C under a N 2 flow with vigorous stirring for 60 min, and then cooled down to RT.The obtained NCs were precipitated by addition of 20 mL of ethanol, collected by centrifugation, purified with ethanol for twice, and finally re-dispersed in cyclohexane.
• C under a N 2 flow with constant stirring for 30 min to remove the residual water and oxygen, and then heated to 160 • C and stirred for another 30 min to form a clear solution.After cooling down to RT, 10 mL of ethanol solution containing 3 mmol of DPTU was added and the solution was stirred at 80 • C for 30 min to remove ethanol.After ethanol was evaporated, the resulting solution was