Enabling Efficient Mid‐Infrared Luminescence of Tm3+ in a Single Core–Shell Nanocrystal through Erbium Sublattice

Mid‐infrared luminescence at around 1.8 μm has shown great potential in many frontier photonic fields. However, how to realize the 1.8 μm emission of Tm3+ with multiple pump wavelengths and in particular in nanosized hosts has remained a challenge so far. Herein, an erbium sublattice–based core–shell nanostructure is proposed to achieve the multiwavelength excitable mid‐infrared emission of Tm3+ at around 1.8 μm from its 3F4 → 3H6 transition. The core–shell engineering and cross‐relaxation help to improve the population of Er3+ at its 4I13/2 energy level and subsequent energy transfer to Tm3+ (3F4) for its efficient 1.8 μm emission upon 808, 980, and 1530 nm excitations. The modulation of energy‐transfer channels by codoping other rare‐earth ions shows that introducing a small amount of Ce3+ into the erbium sublattice can enhance the 1.8 μm emission of Tm3+ through favorable cross‐relaxation processes. Moreover, the 1.8 μm emission is further significantly enhanced by designing a core–shell–shell nanostructure with a NaYF4:Yb‐sensitizing interlayer, which is able to maximize the absorption of 980 nm excitation energy. These results provide a new conceptual nanosized model for mid‐infrared luminescent materials toward infrared biophotonics and microlasers.


Introduction
[3][4][5][6][7] Among rare-earth ions, Tm 3þ is an important luminescent emitter that is able to generate infrared emission at around 1.8 μm through its 3 F 4 ! 3 H 6 transition, [8] and the emission band can cover a broad spectral range of 1700-2000 nm. [9]Tm 3þ has absorption at around 808 nm due to its 3 H 6 ! 3 H 4 absorption transition, and consequently is excitable by commercial %800 nm semiconductor laser in early works. [10]However, due to the concentration quenching effect, the doping concentration of Tm 3þ is usually very low, which heavily limits its luminescence brightness. [11,12]To enhance the 1.8 μm emission, Yb 3þ is usually co-doped as a sensitizer.15] However, this pump scheme is only available for the 980 nm excitation wavelength and is useless for other infrared-excitation wavelengths.Recent work showed that Er 3þ is also a good sensitizer apart from Yb 3þ .More importantly, it can expand the excitation source to a series of infrared wavelengths such as 808 and 1530 nm. [16,17]It has been reported that doping of Er 3þ in glasses can effectively enhance the 1.8 μm luminescence of Tm 3þ ions, [18][19][20][21][22][23] and the energy-transfer efficiency of Er 3þ ( 4 I 13/2 ) ! Tm 3þ ( 3 F 4 ) can reach 85.9%. [24]owever, the host materials were mainly focused on glass materials, which usually have low rare-earth ion-doping concentration and inherent color center defects, restricting their luminescence efficiency. [7,25]These bulk host materials also limit their applications in nanophotonics. [4][28][29][30][31][32][33][34] It would be an ideal nanoscale candidate for mid-infrared luminescence.Recently, doping an appropriate amount of Er 3þ into LiTmF 4 @LiYF 4 core-shell nanocrystals leads to the 1.8 μm luminescence of Tm 3þ . [35]However, to date, there is a lack of systematic research on the mid-infrared luminescence properties for sodium-based lanthanide fluoride nanocrystals, which is a class of typical nanoparticles for upconversion.The 1.8 μm emission of Tm 3þ has to be further improved, the multiple wavelength excitations and the energy-transfer mechanism also needs an in-depth investigation.
Here, the Er sublattice is used as the sensitized matrix to activate the 1.8 μm luminescence of Tm 3þ .Due to the multiwavelength absorption property of Er 3þ , the NaErF 4 :Tm@NaYF 4 Mid-infrared luminescence at around 1.8 μm has shown great potential in many frontier photonic fields.However, how to realize the 1.8 μm emission of Tm 3þ with multiple pump wavelengths and in particular in nanosized hosts has remained a challenge so far.Herein, an erbium sublattice-based core-shell nanostructure is proposed to achieve the multiwavelength excitable mid-infrared emission of Tm 3þ at around 1.8 μm from its 3 F 4 ! 3 H 6 transition.The core-shell engineering and cross-relaxation help to improve the population of Er 3þ at its 4 I 13/2 energy level and subsequent energy transfer to Tm 3þ ( 3 F 4 ) for its efficient 1.8 μm emission upon 808, 980, and 1530 nm excitations.The modulation of energy-transfer channels by codoping other rare-earth ions shows that introducing a small amount of Ce 3þ into the erbium sublattice can enhance the 1.8 μm emission of Tm 3þ through favorable cross-relaxation processes.Moreover, the 1.8 μm emission is further significantly enhanced by designing a core-shell-shell nanostructure with a NaYF 4 :Yb-sensitizing interlayer, which is able to maximize the absorption of 980 nm excitation energy.These results provide a new conceptual nanosized model for mid-infrared luminescent materials toward infrared biophotonics and microlasers.core-shell nanostructure is able to generate efficient midinfrared luminescence of Tm 3þ under excitation wavelengths of 808, 980, and 1530 nm (Figure 1a).The optimal doping concentrations of Er 3þ and Tm 3þ are optimized and possible energy-transfer mechanisms (Figure 1b) are discussed in detail.The effect of surface quenching on infrared luminescence in this system was studied by altering the thickness of inert NaYF 4 shell.Moreover, the modulations on both luminescence and energytransfer channels through codoping Ce 3þ , Yb 3þ , Ho 3þ , Eu 3þ , Tb 3þ , Dy 3þ , and Sm 3þ were also explored.The mid-infrared luminescence of Tm 3þ with 980 nm excitation can be greatly enhanced by introducing a NaYF 4 :Yb interlayer into the coreshell nanostructure.These results provide an effective conceptual model for intense 1.8 μm mid-infrared emission of Tm 3þ in rare-earth-doped nanocrystals, which is of great significance for both fundamental research of the luminescence properties of rare-earth ions and the development of new kinds of gain medium for mid-infrared fluorescence and laser.

Results and Discussion
We first synthesized a series of NaErF 4 :Tm@NaYF 4 core-shell nanocrystals by coprecipitation method.As shown in Figure 2a,b, the synthesized core and core-shell nanocrystals are uniform in size and well dispersed.The particle size distribution shows that the average size of the core-shell nanocrystals is 27.2 nm, and the thickness of NaYF 4 shell is 5.4 nm.The X-ray diffraction patterns show that both core and core-shell nanocrystals are in hexagonal phase (Figure S1, Supporting Information).Interestingly, intense 1.8 μm mid-infrared luminescence Tm 3þ ( 3 F 4 ! 3 H 6 transition) was observed under 808, 980, and 1530 nm excitations (Figure 2c).It is found that the 1530 nm excitation that contributes to the highest luminescence intensity under identical pumping power density might be due to the strongest absorption of Er 3þ at 1530 nm and efficient Er 3þ ( 4 I 13/2 )-to-Tm 3þ ( 3 F 4 ) energy transfer (Figure 1b) by comparison to other two-excitation wavelengths.Also, more nonradiative relaxation processes may occur for the sample with other excitation wavelengths, resulting in additional energy loss and lower energy-transfer efficiency.Here, note that the absorption of Tm 3þ ions at 808 nm can be negligible because of its much lower doping concentration (5 mol%) than that of the NaErF 4 host.
To optimize the 1.8 μm luminescence, a series of NaErF 4 : Tm(0.5-9 mol%)@NaYF 4 core-shell samples with different Tm 3þ -doping concentrations were synthesized.As shown in Figure 2d, the 1.8 μm luminescence exhibits a gradual increase with increasing Tm 3þ -dopant concentration before reaching a decline when it is 5 mol%.Other excitation wavelengths show similar emission spectral results (Figure S2, Supporting Information).It is also found that an appropriate amount of Tm 3þ doping can enhance the red light emission of Er 3þ (Figure S3, Supporting Information).This is because that Tm 3þ can act as an energy-trapping center to suppress the energy migration within Er sublattice.The concentration of Er 3þ was also investigated, and the control samples of NaYF 4 :Er (20-95 mol%)/Tm(5 mol%)@NaYF 4 core-shell nanocrystals were prepared by using inert Y 3þ to replace Er 3þ in the lattice.As shown in Figure 2e and S4, Supporting Information, the 1.8 μm luminescence shows a decrease with reducing the concentration of Er 3þ .This further confirms that Er sublattice is an ideal host to facilitate efficient infrared emission of Tm 3þ .The smaller average Er-Tm ionic separation contributes to more efficient energy transfer between them.Note that in this sample, Er 3þ itself also contributes to infrared and upconverted visible emissions (Figure S5, Supporting Information).More importantly, the 1.8 μm emission intensity of the optimal NaErF 4 : Tm(5 mol%)@NaYF 4 core-shell nanocrystals is much higher than that of conventional NaYF 4 :Yb/Tm(30/1 mol%)@NaYF 4 sample under 980 nm excitation (Figure 2f and S6, Supporting Information).This further demonstrates the advantage of our design in achieving intense mid-infrared luminescence.
[38] To investigate the effect of surface quenching on 1.8 μm luminescence of Tm 3þ , we synthesized a series of NaErF 4 :Tm(5 mol%) @NaYF 4 core-shell nanocrystals with variable NaYF 4 shell thicknesses (Figure 3a).Their mid-infrared luminescence intensity shows a gradual increase when the shell thickness is less than 6.9 nm (Figure 3b and S7, Supporting Information).The lifetime of Tm 3þ at its 3 F 4 level was also prolonged during this process (Figure 3c).These results indicate that the coating of inert NaYF 4 shell can indeed minimize the surface quenching on luminescent centers and enhance their mid-infrared luminescence.To further understand this issue, we measured both nearinfrared and visible emission spectra of the samples.As shown in Figure 3d and S8, Supporting Information, the near-infrared emission of the sample exhibits a similar result as that of the mid-infrared emission.In contrast, the visible emissions are different, and thicker shell layer leads to stronger visible emissions without obvious trend of saturation in luminescence intensity (Figure S9, Supporting Information).These results indicate that the downshift infrared transition of nanocrystals competes with the up-transition for the lower intermediate and emitting states.When the inert shell is sufficiently thick, Er 3þ ions are more likely to transition up to higher energy levels to produce its visible luminescence rather than relax to lower energy levels, such as 4 I 13/2 level of Er 3þ , which consequently leads to a decrease in the population of Tm 3þ at its 3 F 4 level.This might be a leading reason for the decline of infrared luminescence intensity when the NaYF 4 shell layer thickness exceeds 6.9 nm.With regard to the ion diffusion, it could not lead to a decline of the infrared emission because it helps to enhance the emission by reducing the concentration quenching effect.And, the effect of hydroxyl group can also be removed because the total synthesis process is hydrophobic.
Among the three-excitation wavelengths, 808 and 980 nm excitations are less efficient because the energy-transfer processes from Er 3þ to Tm 3þ become more complex and more nonradiative relaxation processes would occur.[41] Next, we attempted to introduce rare-earth ions as an energy mediator to further tune and enhance the 1.8 μm emission under short wavelength excitations.[44][45] Here, we first introduced Ce 3þ into the NaErF 4 :Tm(5 mol%) core to tune energytransfer processes.As shown in Figure 4a, the 1.8 μm emission intensity exhibits an initial improvement followed by a decline with the increase of Ce 3þ content under 980 nm excitation, and 0.5 mol% Ce 3þ has the best luminescence performance.The population of Er 3þ at its 4 I 13/2 level can be promoted by the cross-relaxation process of [Er 3þ ( 4 I 11/2 ); Ce 3þ ( 2 F 5/2 )] ![Er 3þ ( 4 I 13/2 ); Ce 3þ ( 2 F 7/2 )] (CR1; see Figure 4b).This was also confirmed by the increased lifetime values of Er 3þ ( 4 I 13/2 state) together with enhanced near-infrared luminescence (Figure 4c,d).In addition, it is also worth noting that the cross-relaxation process of [Tm 3þ ( 3 H 5 ); Ce 3þ ( 2 F 5/2 )] ![Tm 3þ ( 3 F 4 ); Ce 3þ ( 2 F 7/2 )] (CR2) between Ce 3þ and Tm 3þ may also occur, which is helpful to promote the population of Tm 3þ at its 3 F 4 -emitting level and contribute to an enhancement of mid-infrared luminescence.The relevant energy-transfer processes were illustrated in Figure 4b.A similar luminescence enhancement tendency was also recorded for the excitations of 808 and 1530 nm lasers (Figure S10, Supporting Information).Another interesting observation is that the 980 nm excitation produces the largest luminescence enhancement by comparison to 808 and 1530 nm excitations.This may be due to that Er 3þ has the best population of its 4 I 11/2 energy level under 980 nm excitation and a stronger crossrelaxation process (CR1) with Ce 3þ can further contribute to the population of its 4 I 13/2 level.
[48] The role of codoping Yb 3þ on the mid-infrared luminescence of nanocrystals was investigated.As shown in Figure 5a, the mid-infrared luminescence of the core-shell nanocrystals becomes weaker when compared to the sample without doping Yb 3þ in the core under 980 nm excitation.The near-infrared emission intensity of Er 3þ also shows a monotonic decrease (Figure 5b).However, the upconversion luminescence of the samples is slightly enhanced (Figure 5c).This can be attributed to the energy-transfer upconversion process from Yb 3þ to Er 3þ , namely [Yb 3þ ( 2 F 5/2 ); Er 3þ ( 4 I 13/2 )] ![Yb 3þ ( 2 F 7/2 ); Er 3þ ( 4 F 9/2 )] (see Figure 5d).This leads to enhanced upconversion luminescence of Er 3þ and subsequently a decrease in the energy transfer to Tm 3þ .In addition, the back energy transfer from Er 3þ to Yb 3þ (ET5) may become severe under 808 and 1530 nm excitations, resulting in an energy loss.This was further proved by the decrement of both infrared and visible emissions under 808 and 1530 nm excitations (Figure S11 and S12, Supporting Information).Furthermore, the codoping of other lanthanide ions including Ho 3þ , Eu 3þ , Tb 3þ , Dy 3þ , and Sm 3þ only leads to greatly suppressed 1.8 μm emission (Figure S13 and S14, Supporting Information).This might be due to the significant nonradiative depletion of the 3 F 4 -emitting energy level of Tm 3þ through energy transfer to the dopants as illustrated in Figure S13d and S15, Supporting Information.Therefore, doping the lanthanide ions with appropriate energy levels and proper content is crucial for enhancing 1.8 μm emission of Tm 3þ .
It is found that direct introducing of Yb 3þ in NaErF 4 :Tm would reduce the 1.8 μm luminescence intensity.To shed more light on the role of Yb 3þ , we further employ a NaYF 4 :Yb interlayer through the core-shell design to enhance the absorption of   980 nm excitation photons, which could inhibit back energytransfer processes.Figure 6a shows the schematic of coreshell nanostructure design of NaErF 4 :Tm(5 mol%)@NaYF 4 : Yb@NaYF 4 and its internal energy-transfer processes.Under 980 nm excitation, the 1.8 μm emission of the sample is substantially enhanced after the addition of the NaYF 4 :Yb interlayer (Figure 6b,c).The luminescence enhancement is more significant with increasing concentration of Yb 3þ in the intermediate layer and the optimal enhancement effect was achieved when it reaches 80 mol%.In contrast, this sample shows no obvious luminescence enhancement under 808 or 1530 nm excitation (Figure S16, Supporting Information), because Yb 3þ only has response to the 980 nm excitation.The near-infrared and visible luminescence of the samples was also enhanced significantly (Figure 6d,e), being similar to the mid-infrared luminescence.This indicates that our core-shell-shell nanostructure design is an effective strategy to improve the harvest of the excitation energy and enhance the luminescence intensity of the sample in both visible and infrared spectral regions.

Conclusion
In conclusion, efficient 1.8 μm mid-infrared luminescence of Tm 3þ has been realized by designing the Er-sublattice-sensitized core-shell nanostructure.Multiple-excitation wavelengths of 808, 980, and 1530 nm are able to effectively activate such mid-infrared emission by taking advantage of the unique absorption property of Er 3þ at these wavelengths.Introducing suitable lanthanide ions is a facile way to tune energy-transfer processes and Ce 3þ works well in enhancement of the 1.8 μm mid-infrared emission because it can facilitate the population of Er 3þ from upper energy levels to 4 I 13/2 via cross relaxations, while codoping other lanthanide ions (e.g., Yb 3þ , Ho 3þ , Eu 3þ , Tb 3þ , Dy 3þ and Sm 3þ ) only results in a decline or even quenching of the emission.Moreover, the 1.8 μm mid-infrared luminescence can be further significantly enhanced by introducing a NaYF 4 :Yb interlayer in a core-shell-shell nanostructure which enhances the absorption of 980 nm excitation greatly in addition to Er sublattice.These results not only demonstrated a new strategy for mid-infrared luminescent nanomaterials, but also help the development of new classes of nanosized gain media and devices for future cutting-edge nanophotonic applications.