domain: chim phys

text:
Lingling Mao

Weijun Ke

Laurent Pedesseau

Yilei Wu

Claudine Katan

Jacky Even

Michael R Wasielewski

Constantinos C Stoumpos

Mercouri G Kanatzidis

à la diffusion de documents scientifiques de niveau

recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Hybrid organic-inorganic halide perovskites materials with three-dimensional (3D) AMX3 structures enable solar cells with power conversion efficiency (PCE) over 22%. [1][2][3][4][5][6][7] With impressive structural diversity and great potential in optoelectronic applications, two-dimensional (2D) hybrid organic-inorganic halide perovskites are evolving into an important class of high-performance semiconductors. [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] 2D halide perovskites, A'2An-1MnX3n+1 or A'An-1MnX3n+1, (A' = 1+ or 2+, A = 1+ cation, M = Pb 2+ , Sn 2+ , Ge 2+ , Cu 2+ , Cd 2+ etc., X = Cl -, Br -and I -), are classified depending on the stacking orientation of the inorganic layers ((100), (110) or (111) with respect to the ideal cubic perovskite), but also on the number of the inorganic layers (n = 1, 2, 3 etc, in the chemical formula). 10,23 The single-layered 2D perovskites (n = 1) which have a general formula of A2MX4 or AMX4 (have been extensively explored and there is a large number of structural types reported to date, differing in the nature of the organic spacers and the configuration of the inorganic layers. 10,[24][25] For the higher number of layers (n ≥ 2), however, there are only few crystallographically characterized examples: (PEA)2(MA)n-1PbnI3n+1 (n = 2, 3) 14,26 (PEA = phenylethylammonium, MA = methylammonium), (BA)2(MA)n-1PbnI3n+1 (n = 2 -5) (BA = butylammonium), [27][28] (GA)(MA)nPbnI3n+1 (GA = guanidinium, n = 2 -3), 29 (BA)2(MA)n-1SnnI3n+1 (n = 2 -3), 23,30 (CH3C6H4CH2NH3)2 (MA)Pb2I7, 31 (HO2C(CH2)3NH3)2(MA)Pb2I7, 32 (C4H3SCH2NH3)2 (MA)Pb2I7, 33 (EA)4Pb3X10 34 (X = Cl, Br) (EA = ethylammonium), (BA)2(MA)2Pb3Br10 35 and Cs2[C(NH2)3]Pb2Br7. 36 The layered structures, historically, can be divided into several categories, based on the nomenclature of oxide perovskites: 37 a) Ruddlesden-Popper (RP) phases, 27,38 b) Dion-Jacobson (DJ) [39][40] (Figure 1a), the oxide perovskite specific c) Aurivilius (AV) phases 41 and the halide perovskite specific d) alternating cation in the interlayer space (ACI) type. 29 The differences between these categories are shown in the relative stacking of the layers. The halide perovskites are dominated by the RP archetypes which are characterized by two offset layers per unit cell (Figure 1a). having pairs of interdigitated interlayer spacers (1+). The DJ perovskites feature divalent (2+) interlayer spacers, requiring only one cation per formula unit, [42][43][44][45] and tend to adopt the RbAlF4 structure-type. 46 Because of this, DJ perovskites have a rich configurational stereochemistry with the layers being able to stack in a perfect (0,0 displacement, as reported here) or imperfect (0, ½ or ½, ½ displacements) arrangement according to the steric demands of the interlayer cations, as derived from oxide chemistry. [START_REF] Tilley | Perovskites: Structure-Property Relationships[END_REF] The oxide DJ perovskites have been studied extensively due to their interesting ion-exchange [START_REF] Jacobson | [END_REF][49] and intercalation 50 properties. In halide perovskites, beyond the single-layer perovskites (n = 1) very little is known regarding the higher n-members in the perovskite hierarchy.

Here we report the first examples of hybrid DJ hybrid 2D lead iodide perovskites which consist of thick perovskite slabs (n>1) with layer number (n) ranging from 1 to 4. We describe two new DJ perovskite series based on bivalent (+2) organic cations deriving from a piperidinium (C5NH12) organic backbone (Figure 1b). The new DJ perovskites are built from 3AMP (3AMP = 3-(aminomethyl)piperidinium) and 4AMP (4AMP = 4-(aminomethyl)piperidinium) cations between the layers ("spacers") and methylammonium (MA) cations inside the 2D layers ("perovskitizers") to form (A') (MA)n-1PbnI3n+1 (A' = 3AMP or 4AMP, n = 1-4) homologous series. 51 With the exception of the n = 1 members, the 3AMP and 4AMP series with a representative crystal structure sequence shown in Figure 1b. We find that the difference in the position of the-CH2NH3 + group on the piperidine chair (3-and 4-position with respect to the piperidine nitrogen) influence the crystal structure through different hydrogen bonding modes, which is further reflected on the distortion of the inorganic layers. This difference has a major impact on the optical and electronic properties, which see a narrowing of the bandgap and an enhanced charge transport performance for the least distorted structure (3AMP). Density functional theory calculations (DFT) calculations are in good agreement with the observed trends.

We further demonstrate the superior optoelectronic properties of these materials on photovoltaic (PV) devices. Owing to the less distorted crystal structure the 3AMP series shows a superior performance (~7% champion efficiency for n = 4) to the 4AMP series (~5% champion efficiency for n = 4). These two series of examples showcase the power of utilizing different templating organic cations to influence the semiconducting properties of the inorganic part of the perovskites, which broaden the horizons of 2D perovskites for achieving new solar cells and other optoelectronic devices with better characteristics. The structural differences between RP and DJ halide perovskites are mainly caused by the inter-layer cations (spacers), where RP phases have two sheets of interdigitating cations (1+) while the DJ phases only have one sheet of inter-layer cations (2+) between the inorganic slabs. The influence of the spacers on inorganic slabs is exerted in many levels, depending on the cation size and shape (steric effect), charge (electrostatic attraction) and the position of the functional groups (H-bonding and dispersion forces). This difference between RP and DJ perovskites is also reflected on the general formula, where RP phase has a general formula of A'2An-1MnX3n+1 and DJ phase has a general formula of A'An-1MnX3n+1 (A' = inter-layer cation). In hybrid DJ phases, the inter-layer organic cations are 2+, having less degrees of freedom, making the layers closer to each other.

Results and discussion

In RP phases, the organic cations are 1+, which results in more flexible layer stacking.

The Dion-Jacobson series of layered perovskites, (A')(MA)n-1PbnI3n+1 (A' = 3AMP or 4AMP, n = 1-4), produce uniform, square plate-like crystals, except 3-AMPPbI4, which is an elongated plate as seen in Figure 1c. For the 3AMP series, the color of the crystal gets progressively darker from n = 1 (red) to n = 4 (black). The 4AMP has a similar trend, but it starts from lighter colors than the 3AMP for the n = 1 (orange) and n = 2 (red) members. The bulk crystals exhibit good stability in ambient environment and can be handled without any protection during characterizations.

Both (A')(MA)n-1PbnI3n+1 series form isostructural analogues for n>1. Detailed crystallographic data and structural refinements for all eight compounds reported here are listed in Table 1. They consist of n layers (~6.3 Å is the thickness of one octahedron) of corner-sharing [PbI6] 4-octahedra with xAMP 2+ (x = 3, 4) separating the perovskite slabs and MA + filling in the perovskite voids (Figure 1b). The difference between the two DJ perovskite families is highlighted in Figure 2a and2b, where specific crystallographic characteristics are stressed. The n = 1 and n = 2 members of each AMP series are selected as representative examples. For n = 1, the layers stack almost exactly on top of one another from the top-down viewing direction. (3AMP)PbI4 is somewhat mismatched due to an out-of-plane tilting. (4AMP)PbI4 matches perfectly as it displays exclusively large in-plane tilting. The 3AMP and 4AMP behave alike when it comes to n = 2, where the difference only lies in the Pb-I-Pb angle. The trend continues for the higher numbers (n = 3 and 4). Viewing along the inorganic layers (Figure 2b), the hydrogen bonding networks for 3AMP and 4AMP are drastically different. In (3AMP)(MA)Pb2I7, the 3AMP cation forms weak Hbonds (highlighted in red) with the terminal I -. Bonding with the terminal I -has a small effect on the in-plane Pb-I-Pb angles as the terminal I -does not contribute to the in-plane distortion directly. On the contrary, in (4AMP)(MA)Pb2I7, the H-bonds are formed with the bridging I -anions deeper inside the layers as seen in Figure 2b, which amplify the in-plane distortion.

The differences in hydrogen bonding have an impact on the Pb-I-Pb angles, which are directly related to the optical and electrical these materials (see below). To illustrate this point, we classify the Pb-I-Pb angles into two categories, the axial (along the longest crystallographic axis) and the equatorial (along the inorganic plane) as shown in Figure 2e. In these systems, the axial Pb-I-Pb angles are very close to 180°, as they are much less affected by the interaction (e.g. hydrogen bonding) with the spacing cations. On the other hand, the equatorial Pb-I-Pb angles are much more distorted since they are directly exposed to the spacing cations, especially for the case of n = 1 and n = 2. The evolution of the Pb-I-Pb angles is summarized in Figure 2c and2d, where Figure 2c shows only the averaged equatorial angles and Figure 2d shows the averaged (both axial and equatorial) Pb-I-Pb angles. From Figure 2c, it is clear the gap between the average of the equatorial angles of the 3AMP and 4AMP gradually closes as the layer thickness increases from n = 1 to 4. For 3AMP, the averaged equatorial Pb-I-Pb decreases from 165.1 ̊ to 162.2 ̊ while for 4AMP it increases from 155.1 ̊ to 159.7 . This indicates that the effect of organic cation on the inorganic slabs is gradually diminished as they get thicker (increasing n number). As the axial Pb-I-Pb angles in both series are close to 180 , when they are averaged with the equatorial angles as shown in Figure 2d, the average is increased for both series up to n = 4.

Another interesting structural feature is that the I···I distance between the inorganic layers is very short. Since the layers lay exactly on top of each other (eclipsed configuration), the I···I distance essentially defines the closest interlayer distance. The 3AMP series has generally smaller I···I distance than 4AMP (Figure 2d), while for both series the I···I distance gradually decreases slightly as the layer gets thicker. This is possibly a result of increased stacking fault formation in the perovskite layers as n increases, expressed indirectly in the determined average crystallographic structure.

The close I···I interlayer distance (~ 4.0 Å) is one of the shortest among reported 2D lead iodide perovskites and plays a crucial role in affecting the electronic band structure of these materials which will be discussed below.

The optical band gaps of both 3AMP and 4AMP series follow a general trend that has the energy gap (Eg) decreasing as the layer number (n) increases (Table 2). From n = 1 to n = ∞ (MAPbI3), the band gap decreases from 2.23 eV to 1.52 eV for the 3AMP series, while for the 4AMP the range is much wider (2.38 eV to 1.52 eV) (Figures 3a,3b). The spectra of both series show clear excitonic features similarly to other 2D perovskites, 13, 18, 28, 51-52 which become less prominent as the n number increases and finally disappear for n = ∞. The steady-state photoluminescence (PL) spectra of these materials, shown in Figures 3c,d, exhibit an analogous trend with the band gaps. The 3AMP series demonstrates constantly lower PL emission energy than the 4AMP, until n = 5 when they become equal. Though the evolution of the band gap of both series matches the PL trend, from n = 3 and above, the difference between the two series is negligible (Figures 3e,f). The lifetimes of both AMP series (Figure S6) are comparable to the previously reported 2D layered perovskite PEA (PEA = phenylethylamine) series, for which the lifetimes lie in the 0.1 -0.2 ns range. 53 Overall the 3AMP series has longer lifetime than the 4AMP (except n = 1), which indicates slower carrier combination, more ideal for the PV devices.

The optical properties of the DJ iodide perovskites are quite different from those observed in RP perovskites. 51 Relative to the corresponding RP perovskites (Figure 4), the emission energy observed in PL is characteristically red shifted by ~ 0.1 eV in the case of 3AMP (1.90 eV) and 0.03 eV in the case of 4AMP (1.97 eV) with respect to the BA analogue (2.00 eV), taking n = 3 as the reference example. 51 The absorption edges of the compounds containing AMPs (1.70 eV) are also 0.1 eV lower than the BA analogue. The recently reported structural type ACI perovskite GAMA3Pb3I10 falls in between the RP and DJ perovskites, with a Eg of 1.73 eV and PL emission peak at 1.96 eV. 29 The optical properties of these materials correlate very well with their structural characteristics. As discussed above, the Pb-I-Pb angles for 3AMP are systematically larger than the 4AMP. The larger Pb-I-Pb bond angles (closer to 180 ), the more the Pb s and I p orbitals overlap. [54][55][56] The strong anti-bonding interaction pushes up the valence band maximum (VBM), resulting in a reduced band gap. Thus, the systematically narrower band gap observed for 3AMP vs 4AMP can be attributed to the more linear Pb-I-Pb angles (i.e. smaller octahedral tilting (Figure S7))for the former. The results of Density Functional Theory (DFT) electronic structure calculations are shown in Figure 5a-g. The calculated band gap for (3AMP)PbI4 is determined at the Ґ point (1.13 eV) (Figure 5a), whereas the band gap of (4AMP)PbI4 is determined at the BZ edge (1.14 eV) (Figure 5b). The calculated band gaps for the higher numbers between the 3AMP and 4AMP series have larger differences, where Eg is 0.48 eV for n = 2, 3AMP, and 0.70 eV for n = 2, 4AMP (Figure 5c-d). For n = 3, the band gaps at Z point for 3AMP and 4AMP are 0.47 eV and 0.74 eV, respectively (Figure 5e -f). The calculated gap for 3AMP (n = 4) is very small (0.07 eV), much lower than 0.54 eV for n = 4, 4AMP as seen in Figure 5g -h. The DFT computed band gaps do not include many-body interactions needed to properly assess optical response, which is why calculated values are systematic underestimated without GW corrections. 57 The band gaps of 3AMP series DJ are systematically smaller than those computed for 4AMP series as shown in Figure 5h, in agreement with experimental findings. The stacking of perovskite sheets in the DJ structure type, which aligns the perovskite layers, allows for a better interlayer electronic coupling through van der Waals I … I interactions. These I … I contacts participate in anti-bonding interactions that further destabilize the VBM (Figure S9), contributing to the reduction of the band gap as compared to RP phases with respect to the same n-value, as discussed above.

Based on the attractive properties of the new 2D DJ perovskites, in a preliminary study we investigated the higher layer numbers (n = 3 and 4) as light absorbers for solar cells. A planar solar cell structure was adopted for device fabrication (Figure 6a), consisting of a fluorine doped tin oxide (FTO) substrate, a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) hole transport layer (HTL), a 2D perovskite light absorber, a C60/BCP electron transport layer (ETL) and a Ag electrode. The devices were fabricated using a modified solvent engineering method (see Methods). The photocurrent density-voltage (J-V) curves of the solar cells using the 2D DJ perovskites are in Figure 6b, (measured using a reverse voltage scan). The thickness of the perovskite films was ~250 nm. Among the n = 3 and 4 for 3AMP and 4AMP, (3AMP)(MA)3Pb4I13 achieved the highest power conversion efficiency (PCE) of 7.32% with a short-circuit current density (Jsc) of 10.17 mA•cm -2 , an open-circuit voltage (Voc) of 1.06 V, and a fill factor (FF) of 67.60%. This value is significantly higher that the corresponding n = 3 and n = 4 RP perovskites prepared using a regular mesoporous TiO2 device structure.

The remarkable performance of (3AMP)(MA)3Pb4I13 can be mainly attributed to the reduced band gap and the improved mobility originating from the increased band dispersion (see above). While the device based on (3AMP)(MA)2Pb3I10 has the lowest PCE of 2.02% with a Jsc of 3.05 mA•cm -2 , a Voc of 0.99 V, and a FF of 66.54%, attributed to the largest band gap and the intense presence of a secondary phase, identified as the n = 2 member (2θ = ~11° and ~16°). (4AMP)(MA)2Pb3I10 and (4AMP)(MA)3Pb4I13 have PCE below 5%, mainly due to the much lower Jsc relative to the 3AMP. The average photovoltaic parameters of the devices using the various absorbers are summarized in Table S6. The Jsc integrated from the external quantum efficiency (EQE) curves (Figure 6c) of the devices based for (3AMP)(MA)2Pb3I10, (3AMP)(MA)3Pb4I13, (4AMP)(MA)2Pb3I10 and (4AMP)(MA)3Pb4I13 are 2.92, 10.16, 5.83, and 7.08 mA•cm -2 , respectively, which are in good agreement with the trend of the Jsc obtained from the J-V curves. In Figure 6d, powder x-ray diffraction (PXRD) of the (3AMP)(MA)3Pb4I13, (4AMP)(MA)2Pb3I10 and (4AMP)(MA)3Pb4I13 films show preferred orientation in the "perpendicular" direction judging from the strongest hkl (110) and (220) at ~14° and ~28°, which facilitates the carriers to travel through the layers. 8,13 Further results of the fabrication of higher quality films and higher efficiency solar cells will be reported in the future. PL emission properties of the films (Figure 6e) are quite different (red shifted) from the bulk materials, which can be attributed to the so-called "edge effect". 52 The edge effect is observed in single crystals of both 3AMPPbI4 and (3AMP)(MA)2Pb3I10 as seen in Fig- ure S11. The second PL emission at lower energy were observed when excited the sample through "the edge", which is parallel to the layers. The results show larger separation of the higher (2.21 eV from bulk) and lower energy emission (1.64 eV edge) of the n = 1 than for n = 3 (1.91 eV and 1.68 eV) for 3AMP. The lower energy emission (1.68 eV) is very close to the emission of the thin film (1.66 eV). This result is similar to the previously reported (BA)2(MA)2Pb3I10 example, where the higher energy emission is 2.00 eV and lower energy emission is 1.70 eV. 52 Attempts for measuring edge states of the other layered number crystals such as n = 2 and 4 were not successful owing to the thin crystal morphology which has caused handing difficulty.

The absorption edges of the films have the same trend as in the EQE spectra, though multiple slopes appear, indicating the formation of some lower layer numbers (Figure 6f). Even though these 2D DJ perovskite devices are not completely optimized, the initial results show great promise as they compare well with the corresponding 2D RP perovskites which lead the 2D perovskite solar cells field. 8,15,58

Conclusions

We have shown that a new crystal motif based on the DJ class of perovskites forms 2D hybrid lead iodide perovskites. The Figure 6. Solar cell architecture for the higher layer numbers (n = 3 and 4) of 3AMP and 4AMP. a, Scheme of the adopted inverted device structure. b, J-V curves of the 2D perovskite solar cell devices. c, External quantum efficiency (EQE) spectra. d, PXRD of the thin-films. e, steady-state PL spectra, where the emission peaks: 746 nm, 1.66 eV(3AMPPb3I10), 764 nm, 1.62 eV (3AMPPb4I13), 752 nm, 1.65 eV (4AMPPb3I10 and 4AMPPb4I13). f, Absorption spectra for the thin-films. special spacer cations used 3AMP and 4AMP have strong influence on the overall properties. Detailed crystallographic investigations on all eight compounds (n = 1 to 4, for 3AMP and 4AMP), have provided the structural insights for understanding the structure-property relationships. In particular, by understanding the angular distortion (Pb-I-Pb angle) within the system, we manage to show the subtle difference in the cations causes large differences in the optical properties by affecting the Pb-I-Pb angles, where the 3AMP series has systematically larger angles and smaller band gaps than the 4AMP series. Compared to the most common 2D RP perovskites, namely the BA series, the 3AMP and 4AMP series possess lower band gaps because of a less distorted inorganic framework and closer I···I interlayer distances. Our analysis suggests more superior optoelectronic properties of the 3AMP over the 4AMP series, which as demonstrated in the actual device fabrication, where the preliminary result shows the champion device has PCE over 7%.

The strong correlation between the materials and their applications' performance validates the importance of understanding structure-property relationships and discovering new materials in the halide perovskite systems. ). The collected data was integrated and applied with numerical absorption corrections using the STOE X-AREA programs. The rest of the compounds were collected using a Bruker Molly instrument with MoKα IμS microfocus source (λ = 0.71073 Å) with MX Optics at 250 K. The collected data was integrated and applied with numerical absorption corrections using the APEX3 software. Crystal structures were solved by charge flipping and refined by full-matrix least squares on F 2 with the Jana2006 package.

Methods

Materials

Optical Absorption Spectroscopy. Optical diffuse reflectance measurements were performed using a Shimadzu UV-3600 UV-VIS-NIR spectrometer operating in the 200 -1000 nm region using BaSO4 as the reference of 100% reflectance. The band gap of the material was estimated by converting reflectance to absorption according to the Kubelka-Munk equation: α/S = (1-R) 2 (2R) -1 , where R is the reflectance and α and S are the absorption and scattering coefficients, respectively.

Steady State and Time-resolved Photoluminescence. Steady-state PL spectra were collected using HORIBA LabRAM HR Evolution Confocal RAMAN microscope. 473 nm laser (0.1% power) was used to excite all samples at 50× magnification. Time-resolved photoluminescence (TRPL) spectra were acquired using HORIBA Fluorolog-3 equipped with a 450-W xenon lamp and a TCSPC module (diode laser excitation at λ = 375 nm) and an integrating sphere (Horiba Quanta-φ) for absolute photoluminescence quantum yield determination. The spectra were corrected for the monochromator wavelength dependence and photomultiplier response functions provided by the manufacturer.

Electronic Structure Calculations. First-principles calculations are based on density functional theory (DFT) as implemented in the VASP package. [59][60][61] All calculations are carried out on the experimentally determined crystal structures. We used the GGA functional in the PBE form, the projector augmented wave (PAW) method [62][63] with the PAW data set supplied in the VASP package with the following valence orbitals: Pb [5d 10 6s 2 6p 2 ], I [5s 2 5p 5 ], N [2s 2 2p 3 ], H [1s 1 ] and C [2s 2 2p 2 ]. In addition, the wavefunctions are expanded using a plane-wave basis set with an energy cut-off of 500 eV. Spin-orbit coupling is systematically taken into account.

For band structures, the reciprocal space integration is performed over a 4×4×1 Monkhorst-Pack grid for compounds with n=1 and n=3, and over a 4x4x4 grid for compounds with n=2 and n=4 in their primitive cells. [64][65] Device fabrications. FTO glass substrates were coated with PEDOT:PSS by spin-coating at 4000 rpm for 30s, and then annealed at 150 °C for 30 min in air. The 2D perovskite precursors with a molar concentration of 0.6 M were prepared by dissolving the 2D perovskite crystal powders in a mixed solvent of DMF and DMSO with a volume ratio of 4:1. After the crystal powders dissolved, adding 0.8 vol% HI into the perovskite precursors. Then the precursors were coated on the substrates with a spin rate of 4000 rpm for 60s in a N2-filled glove box. During the spin-coating, 0.7 mL diethyl ether was dropped on the rotating substrates at 20 s.

Figure 1 .

 1 Figure 1. a, Comparison between Dion-Jacobson phases and Ruddlesden-Popper phases for both oxide and halide perovskite. Crystal structure of CsBa2Ta3O10, Ca4Mn3O10 and (BA)2(MA)2Pb3I10 are adopted from reference 41, 38 and 27. b, General crystal structure of the two series of DJ perovskite reported here, from n = 1 to 4. Structures of the cation 3AMP and 4AMP are listed in the lower left corner. c, Optical images of the 3AMP and 4AMP crystals. Scale bar on the bottom right applies to all.

Figure 2 .

 2 Figure 2. a, Top-view of (3AMP)PbI4, (4AM)PPbI4, (3AMP)(MA)Pb2I7 and (4AMP)(MA)Pb2I7. b, Side-view of (3AMP)(MA)Pb2I7 and (4AMP)(MA)Pb2I7, hydrogen bonding is marked in red. c, Average equatorial Pb-I-Pb angles for 3AMP and 4AMP series from n = 1 to 4. d, Average axial and equatorial angles for 3AMP and 4AMP. e, Definition of axial and equatorial Pb-I-Pb angles. f, I•••I distance trend in 3AMP and 4AMP, where the 3AMP series has closer distance.

Figure 3 .

 3 Figure 3. Optical properties of the 3AMP and 4AMP series. a,b, Optical absorption spectra of 3AMP and 4AMP series. c,d, Steady-state photoluminescence (PL) spectra of 3AMP and 4AMP series. e,f, Summary of absorption and PL in energy from n = 1 to 5.

Figure 4 .

 4 Figure 4. Comparison of the a. optical absorption spectra and b. PL spectra between (3AMP)(MA)2Pb3I10, (4AMP)(MA)2Pb3I10, (GA)(MA)3Pb3I10 and (BA)2(MA)2Pb3I10.

Figure 5 .

 5 Figure 5. a-g, DFT calculations of band structures for the 3AMP and 4AMP series with SOC. The calculated gaps are 1.13 eV for (3AMP)PbI4(at Ґ), 1.14 eV for (4AMP)PbI4 (at B), 0.48 eV for (3AMP)(MA)Pb2I7 (at Yo), 0.70 eV for (4AMP)(MA)Pb2I7 (at Yo), 0.47 eV for (3AMP)(MA)2Pb3I10 (at Z), 0.74 eV for (4AMP)(MA)2Pb3I10 (at Z), 0.07eV for (3AMP)(MA)3Pb4I13 (at Yo), 0.54 eV for (4AMP)(MA)3Pb4I13 (at Yo)

Table . 1

 . Crystal data and structure refinement for A′MAn-1PbnI3n+1 (A' = 3AMP or 4AMP). N 2 H 16 )(CH 3 NH 3 )Pb 2 I 7 (C 6 N 2 H 16 )(CH 3 NH 3 ) 2 Pb 3 I 10 (C 6 N 2 H 16 )(CH 3 NH 3 ) 3 Pb 4 I 13

	Compound	(3AMP)PbI 4			(3AMP) (MA)Pb 2 I 7	(3AMP) (MA) 2 Pb 3 I 10	(3AMP) (MA) 3 Pb 4 I 13
	Empirical formula (C 6 Crystal system C 6 N 2 H 16 PbI 4	Monoclinic
	Space group	P2 1 /c			Ia		Pa		Ia
		a = 8.6732(6) Å,			a = 8.8581(11) Å,	a = 8.8616(3) Å,		a = 8.8627(18) Å,
	Unit cell	b = 18.4268(9) Å,	b = 8.8607(4) Å,		b = 8.8624(3) Å,		b = 8.8689(18) Å,
	dimensions	c = 20.4522(14) Å,	c = 33.4749(5) Å,	c = 23.0316(7) Å,	c = 58.842(12) Å,
		β = 99.306(6) °			β = 90°		β = 90°		β = 90°
	Volume (Å 3 )	3225.67(35)			2627.4(3)		1808.79(10)		4625.1(16)
	Z	8			4		2		4
	Density (g/cm 3 )	3.4224			3.6681		3.8024		3.8645
	Indepd. refl.	5033 [R int = 0.1102]			4422 [R int = 0.0202]		8035 [Rint = 0.0361]		5893 [R int = 0.0405]
	Data /
	restraints /	5033 / 32 / 145			4422 / 17 / 115		8035 / 28 / 163		5893 / 19 / 201
	param.
	Final R indi-	R obs = 0.0869,			R obs = 0.0323,		Robs = 0.0395,		R obs = 0.0901,
	ces [I>2σ(I)]	wR obs = 0.1622			wR obs = 0.0922		wRobs = 0.1063		wR obs = 0.2062
	R indices [all	R all = 0.1471,			R all = 0.0365,		Rall = 0.0612,		R all = 0.1136,
	data]	wR all = 0.1686			wR all = 0.0941		wRall = 0.1296		wR all = 0.2138
	Largest diff. peak and hole 3.97 and -4.88 e•Å	-3	1.44 and -1.23 e•Å	-3	2.014 and -1.382 e•Å -3	11.07 and -5.34 e•Å	-3
	Compound	(4AMP)PbI 4			(4AMP) (MA)Pb 2 I 7	(4AMP) (MA) 2 Pb 3 I 10	(4AMP) (MA) 3 Pb 4 I 13
	Empirical for-mula	C 6 N 2 H 16 PbI 4			(C 6 N 2 H 16 )(CH 3 NH 3 )Pb 2 I 7	(C 6 N 2 H 16 )(CH 3 NH 3 ) 2 Pb 3 I 10	(C 6 N 2 H 16 )(CH 3 NH 3 ) 3 Pb 4 I 13
	Crystal system					Monoclinic
	Space group	Pc			Ia		Pc		Ia
	Unit cell di-	a = 10.4999(13) Å,	a = 8.8412(11) Å,	a = 23.1333(7) Å,	a = 8.8587(18) Å,
	mensions	b = 12.5429(9) Å,	b = 8.8436(4) Å,		b = 8.8365(3) Å,		b = 8.8571(18) Å,
		c = 12.5289(13) Å,	c = 33.6045(5) Å,	c = 8.8354(3) Å,		c = 58.915(12) Å,
		β = 89.984(9)°			β = 90°		β = 90°		β = 90°
	Volume (Å 3 )	1650.05(43)			2627.5(4)		1806.11(10)		4622.6(16)
	Z	4			4		2		4
	Density	3.3441			3.6681		3.8081		3.8666
	(g/cm 3 )
	Indepd. refl.	4646			4558		7954		8002
		[R int = 0.1198]			[R int = 0.028]		[R int = 0.0291]		[R int = 0.1339]
	Data /
	restraints /	4646 / 36 / 141			4558 / 17 / 116		7954 / 18 / 158		8002 / 19 / 202
	param.
	Final R indi-ces [I>2σ(I)]	R obs = 0.0797, wR obs = 0.1093			R obs = 0.0330, wR obs = 0.0915		R obs = 0.0351, wR obs = 0.0889		R obs = 0.0585, wR obs = 0.0839
	R indices [all data]	R all = 0.1618, wR all = 0.1275			R all = 0.0410, wR all = 0.0949		R all = 0.0559, wR all = 0.0970		R all = 0.1700, wR all = 0.1000
	Largest diff. peak and hole	4.48 and -4.31 e•Å	3	1.69 and -1.20 e•Å	-3	1.80 and -1.39 e•Å	-3	4.23 and -2.94 e•Å	-3
	R = Σ||Fo|-|Fc|| / Σ|Fo|, wR = {Σ[w(|Fo| 2 -|Fc| 2 ) 2 ] / Σ[w(|Fo| 4 )]} 1/2 and w=1/(σ 2 (I)+0.0004I 2 )

Table 2 .

 2 Optical properties and color of the (A')(MA)n-1PbnI3n+1 (A' = 3AMP or 4AMP, n = 1-4) DJ perovskites.

		Compound	Eg (eV)	PL (eV)	Color	Compound	Eg (eV)	PL (eV)	Color
	n = 1	(3AMP)PbI4	2.23	2.22	red	(4AMP)PbI4	2.38	2.33	orange
	n = 2	(3AMP)(MA)Pb 2 I 7	2.02	2.00	dark red	(4AMP)(MA)Pb 2 I 7	2.17	2.13	red
	n = 3	(3AMP)(MA) 2 Pb 3 I 10	1.92	1.90	black	(4AMP)(MA) 2 Pb 3 I 10	1.99	1.97	black
	n = 4	(3AMP)(MA) 3 Pb 4 I 13	1.87	1.84	black	(4AMP)(MA) 3 Pb 4 I 13	1.89	1.88	black

  Powder X-ray Diffraction. PXRD analysis was performed using a Rigaku Miniflex600 powder X-ray diffractometer (Cu Kα graphite, λ = 1.5406 Å) operating at 40 kV/15 mA with a Kβ foil filter.

	(31.5%), 531 mg (25.6%) and 477 mg (17.7% based on total Pb
	content), respectively.
	Single Crystal X-ray Diffraction. Full sphere data were collected
	after screening for a few frames using either a STOE IPDS 2 or
	IPDS 2T diffractometer with graphite-monochromatized Mo Kα
	radiation (λ = 0.71073 Å) (50 kV/40 mA) under N2 at 293 K
	(3AMPPbI4 , 4AMPPbI4 and (3AMP)(MA)3Pb4I13
	. PbO (99.9%), 3-(aminomethyl)piperidine (Aldrich CPR ),
	4-(aminomethyl)piperidine (96%), hydroiodic acid (57 wt. % in
	H2O, distilled, stabilized, 99.95%) and hypophosphorous acid so-
	lution (50 wt. % in H2O) were purchased from Sigma-Aldrich and
	used as received. Methylammonium iodide (>99.5%) was pur-
	chased from Luminescence Technology Corp. and used as re-
	ceived.
	Synthesis of (3AMP)(MA)n-1PbnI3n+1. For n = 1, an amount of 669
	mg (3 mmol) 99.9% PbO powder was dissolved in 6 ml of hy-
	droiodic acid and 1 ml hypophosphorous acid solution by heating
	under stirring for 5-10 min at ~130°C until the solution turned to
	clear bright yellow. 0.5 ml hydroiodic acid was added to 342 mg (3
	mmol) 3-(aminomethyl)piperidine (3AMP) in a separate vial under
	stirring. The protonated 3AMP solution was added into the previ-
	ous solution under heating and stirring for 5 min. Red plate-like
	crystals precipitate during slow cooling to room temperature. Yield
	963 mg (38.6% based on total Pb content). For n = 2, an amount
	of 669 mg (3 mmol) 99.9% PbO powder was dissolved in 6 ml of
	hydroiodic acid and 1 ml hypophosphorous acid solution by heat-
	ing under stirring for 5-10 min at 130°C until the solution turned to
	clear bright yellow. 318 mg (2 mmol) of methylammonium iodide
	(MAI) was added directly to the above solution under heating. 0.5
	ml hydroiodic acid was added to 57 mg (0.5 mmol) 3AMP in a
	separate vial under stirring. The protonated 3AMP solution was
	added into the previous solution under heating and stirring for 5
	min. Dark red plate-like crystals precipitated (Figure 1c) during
	slow cooling to room temperature. Yield 487 mg (22.4% based on
	total Pb content). For the synthesis of higher numbers, they follow
	the same route except the ratio were change to 37.6 mg (0.33 mmol)
	3AMP, 477 mg (3 mmol) MAI, 669 mg (3 mmol) PbO for n = 3,
	yield 252 mg (12.2% based on total Pb content) and 34.2 mg (0.3
	mmol) 3AMP, 636 mg (4 mmol) MAI, 892 mg (4 mmol) PbO for
	n = 4, yield 301 mg (11.2% based on total Pb content).
	Synthesis of (4AMP)(MA)n-1PbnI3n+1. Similar synthetic proce-
	dures were used to synthesize the 4AMP series. However, the
	amount of 4AMP was reduced as the 4AMP series precipitate faster
	than the 3AMP. The experimental ratios (4AMP: MAI: PbO) (in
	mmol) of the 4AMP are 3:0:3 for n = 1, 0.5:2:3 for n = 2, 0.33:3:3
	for n = 3 and 0.27:4:4 for n = 4. Yield 1155 mg (46.3%), 684 mg

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After spin-coating, the films were annealed at 100 °C for 10 min in the glove box. To complete the devices, C60 (20 nm)/BCP (5 nm)/Ag (100 nm) were sequentially thermally evaporated on top of the perovskites. The active area of the solar cells was 0.09 cm 2 . Characterizations. J-V curves were measured by a Keithley model 2400 instrument under AM1.5G simulated irradiation with a standard solar simulator (Abet Technologies). The light intensity of the solar simulator was calibrated by a National Renewable Energy Laboratory-certified monocrystalline silicon solar cell. EQE curves were measured by an Oriel model QE-PV-SI instrument equipped with a National Institute of Standards and Technology-certified Si diode.

ASSOCIATED CONTENT

Additional crystallographic details (CIF), thermal analysis (DSC, TGA and DTA), calculation and device data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. *Email: m-kanatzidis@northwestern.edu *konstantinos.stoumpos@northwestern.edu


Wenqian Cao

Feifei Huang
email: author.huangfeifei@cjlu.edu.cn

Renguang Ye

Muzhi Cai

Ruoshan Lei

Junjie Zhang

Shiqing Xu
email: shiqingxu@cjlu.edu.cn.

Xianghua Zhang

Structural and fluorescence properties of Ho 3+ /Yb 3+ doped germanosilicate glasses tailored by

Keywords: Germanosilicate glasses, Lanthanide additive Lu 2 O 3, NBO and BO, Energy transfer

Structural and fluorescence properties of Ho 3+ /Yb 3+ co-doped germanosilicate glasses have been modified by tailoring the composition using lanthanide additive Lu 2 O 3 . Raman spectra and X-ray Photoelectron spectra reveal that the addition of Lu 2 O 3 can change the structure by increasing non-bridging oxygens (NBO) in this glasses. Meanwhile, an improved thermal stability (∆T: from 110 to 187 ˚C) has also been obtained via Lu 3+ 'lanthanide contraction'. Futhermore, the positive effect of changed glass structure gives excellent fluorescence properties by the decreasing cross relaxation process which are proved by the experimental upconversion, near-infrared and mid-infrared fluorescence spectra. Additionally, a double enhancement of a 2.0 µm emission has been achieved successfully in this silica-germanate glass with 7 mol% Lu 3+ addition, which possesses a larger emission cross section (4.41 × 10 -21 cm 2 ) at 2022 nm. These results indicate that the optimized emission of Ho 3+ for optical fiber laser can be achieved by tuning the glass structure using lanthanide additive Lu 2 O 3 .

Introduction

Rare earth ions doped glass fibers that exhibit laser emission around ~2.0 µm have been paid

considerable attention due to their potential applications in eye safe laser radar, atmosphere pollution monitoring, measurement of water vapor and biomedicine, etc [START_REF] Geng | Kilowatt-peak-power, single-frequency, pulsed fiber laser near 2 µm[END_REF][START_REF] Zheng | Scaling all-fiber mid-infrared supercontinuum up to 10W-level based on thermal-spliced silica fiber and ZBLAN fiber[END_REF][START_REF] Qi | Long lifetime of Er 3+ : 4 I 11/2 in low phonon-energy fluoro-chloride glasses for mid-infrared optical applications[END_REF][START_REF] Sun | Preparation and mid-infrared 2.7µm luminescence property of high content Er 3+ -doped (Y 0.9 La 0.1 ) 2 O 3 transparent ceramics pumped at 980nm[END_REF][START_REF] Yang | Mid-infrared luminescence of Dy 3+ ions in modified Ga-Sb-S chalcogenide glasses and fibers[END_REF]. A number of experiments have proved that Tm 3+ and Ho 3+ perform well in achieving 2.0 µm laser. Compared with Tm 3+ , the emission band of Ho 3+ is broader and the stimulated emission cross section of Ho 3+ is practically five times larger than that of Tm 3+ , which suggests that more efficient laser output can be achieved by Ho 3+ . Meanwhile, the longer fluorescence lifetime of Ho 3+ is conducive to Q-switched lasers [START_REF] Xu | 2.0µm emission properties and energy transfer processes of Yb 3+ /Ho 3+ codoped germanate glass[END_REF][START_REF] Li | Watt-level output rectangular-core neodymiun-doped silicate glass fiber laser[END_REF]. However, Ho 3+ cannot directly be pumped by 808 nm or 980 nm commercial laser diode (LD) source for lacking of a suitable ground absorption band. Traditionally, in order to solve this problem, other rare earth ions such as Er 3+ , Yb 3+ , Tm 3+ and Nd 3+ with obvious ground absorption under commercial LD excitations are usually introduced to sensitize the Ho 3+ [START_REF] Bai | Light emission at 2 µm from Ho-Tm-Yb doped silicate glasses[END_REF][START_REF] Gao | Investigation of 2.0 µm emission in Tm 3+ and Ho 3+ co-doped TeO 2 -ZnO-Bi 2 O 3 glasses[END_REF][START_REF] Li | ∼2 µm Luminescence and energy transfer characteristics in Tm 3+ /Ho 3+ co-doped silicate glass[END_REF]. Among all the sensitizers, Yb 3+ ion with the simple energy level structure (ground state 2 F 7/2 and excited state 2 F 5/2 ) and a broad absorption band in the wavelength region of 860-1060 nm can be a useful sensitizer [START_REF] Zhou | Near-and mid-infrared photoluminescence in Ho 3+ doped and Ho 3+ -Yb 3+ codoped low-phonon-energy germanotellurite glasses[END_REF][START_REF] Ye | Reduced photon quenching in Ce-doped NaYF 4 :Yb/Ho upconversion nanoparticles with core/shell structure[END_REF].

In order to develop more efficient fiber lasers for 2.0 µm emission, besides the rare earth ions, the host material must be considered as well. Thus, searching for suitable host materials for mid-infrared lasers operating in this wavelength is quite necessary. Recent years have witnessed the great development of various glasses, such as silicate, heavy-metal oxide and fluoride glasses. Although the phonon energy of fluoride-based glass is low, its low mechanical strength limits its applications [START_REF] He | Research and development of new neodymium laser glass[END_REF]. In parallel, the low loss and high physical strength of silicate glasses have made them remarkably useful hosts for rare-earth cations emitting in the 2.0 µm region. However, compared with other glasses, the larger multiphonon relaxation rate induced by higher phonon energy in silicate glass (~1000 cm -1 )

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

3 lowers the quantum efficiency and causes thermal damage of the fiber laser [START_REF] Li | Investigation on Tm 3+ -doped silicate glass for 1.8µm emission[END_REF]. Thus, the heavy-metal oxide glasses have become a new investigative hotspot of mid-infrared laser to achieve lower phonon energy. Germanate glass as the typical example of heavy-metal oxide glasses, is one of the most promising materials benefited from its high rare-earth solubility, superior infrared transparency, comparative low phonon energy (~900 cm -1 ), strong mechanical and high damage threshold.

Unfortunately, the high cost of GeO 2 causes its slow development in optical amplifiers [START_REF] Bai | Enhanced light emission near 2.7µm from Er-Nd co-doped germanate glass[END_REF][START_REF] Bayya | Germanate glass as a window for high energy laser systems[END_REF][START_REF] Zhang | Enhanced NIR emission from nanocrystalline LaF 3 :Ho 3+ germanate glass ceramics for E-band optical amplification[END_REF]. So we try to research mixed germanosilicate (SG) glasses, which can compensate for the deficiencies of silicate and germanate glasses. The phonon energy has been decreased by introducing appropriate amount of GeO 2 into silicate glass, which can reduce the multiphonon relaxation rate and enhance the quantum efficiency of 2.0 µm. Moreover, the addition of GeO 2 in silicate host can enhance the refractive index of the glass host and which is beneficial for improving the 2.0 µm emission cross section [START_REF] Bayya | VIS-IR transmitting windows[END_REF][START_REF] Wei | Broadband 1.53µm emission property in Er 3+ doped germa-silicate glass for potential optical amplifier[END_REF]. Therefore, germanosilicate glass which combines the merits of germanate and silicate glasses can be a suitable optical material for mid-infrared fiber lasers [START_REF] Wei | Broadband 2µm fluorescence and energy transfer evaluation in Ho 3+ /Er 3+ codoped germanosilicate glass[END_REF].

Additionally, the structure properties of host glasses usually can be modified by tailoring the composition using lanthanide additives. Cations with higher coordination numbers and the higher field strength can enhance the structure of glass and increase glass transition temperature. Lanthanide cations with the high field strength are often used as network modifiers in the glass, which are incorporated in the space between Si-(O, N)x tetrahedral and strengthen the structure of glass [START_REF] Lofaj | Thermal expansion and glass transition temperature of the rare-earth doped oxynitride glasses[END_REF].

Therefore, in 1994, Jewell J. et al. reported that the addition of La 2 O 3 is able to improve glass forming ability and enhance thermal stability due to the larger molecular ratio and coordination number of La 3+ ion, while Y 2 O 3 component is able to improve thermal stability [START_REF] Jewell | Properties of BaO-R 2 O 3 -Ga 2 O 3 -GeO 2 (R = Y, Al, La, and Gd) Glasses[END_REF]. Since then, many authors have been using lanthanide as the modifier to tailor the optical properties of glasses. For example, in 2017, [START_REF] Hao | Influence of Y 2 O 3 on the structure and luminescence of Eu 2+ doped borosilicate glasses[END_REF]. At the same time, Wang X. et al also reported the influence of La 2 O 3 on the structure and spectroscopy of Tm 3+ -doped Lanthanum aluminosilicate glasses [START_REF] Wang | Influence of La/Al ratio on the structure and spectroscopy of Tm 3+ doped Al 2 O 3 -La 2 O 3 -SiO 2 glasses[END_REF]. Especially, the glass network with Lu 3+ would have tighter structure and better properties than the glasses containing other lanthanides owing to its network with smaller atoms. [START_REF] Lofaj | Thermal expansion and glass transition temperature of the rare-earth doped oxynitride glasses[END_REF]. Additionally, Lu 3+ has a stronger ability for bonding with O 2-than Si 4+ due to the oxyphilic character of Lu 3+ [START_REF] Fouquet-Parry | Composition and local bonding in RE-Si-M-O-N (M=Mg, Al ; RE=La, Lu) glasses[END_REF]. However, there is no detailed report on mid-infrared fluorescence of Lu 2 O 3 modified germanosilicate glasses from the aspect of regulating its structure. Thus, in this paper we investigated primarily different concentration of Lu 2 O 3 in the Ho 3+ /Yb 3+ co-doped SiO 2 -GeO 2 -Ga 2 O 3 -BaO-Li 2 O (SG) glasses to analysis its structure and mid-infrared spectroscopic properties. The structure of glasses has been studied by X-ray Photoelectron Spectroscopy (XPS) and Raman spectra. The introduction of Lu 2 O 3 improved the thermal stability and mid-infrared fluorescence in present system. Additionally, we researched the absorption spectra, and then calculated the J-O intensity parameters Ω λ (t = 2, 4, 6), radiative parameters (such as spontaneous radiation transition probability A rad , radiation lifetime τ rad ) of Ho 3+ in present glass. Meanwhile, the absorption (σ abs ) and emission cross sections (σ em ) of Ho 3+ at 2.0 µm have been calculated and discussed.

Experimental

Germanosilicate glasses with molar compositions of (99-x)(SiO 2 -GeO were well-mixed and melted in a corundum crucible in a Si-Mo resistance electric furnace at 1520 °C for 40 min. Then, they were quenched on preheated stainless steel plate and annealed near the temperature of glass transition for 2 h before they were cooled to room temperature. The annealed samples were finally cut and optically polished to the size of 10 mm × 10 mm × 1 mm for the optical property measurements.

The characteristic temperatures (temperature of glass transition T g and temperature of onset crystallization peak T x ) of samples were determined using a NetzschSTA449/C differential scanning calorimetry at a heating rate of 10 K/min with error limit of ±0.2%. The densities of samples were tested via the Archimedes principle using distilled water as the immersion medium with error of ±0.001 g/cm 3 . Refractive indexes were measured by the prism minimum deviation method at the wavelength of 1053 nm with error limit of ±0.05%. The visible UC (500~800 nm) fluorescence spectra were obtained with a TRIAX550 spectrofluorimeter upon the excitation of 980 nm LD. Absorption spectra were obtained using a PerkinElmer Lambda 900UV-VIS-NIR spectrophotometer in the range of 300~2200 nm a resolution of 1 nm. The emission spectra (500~700 nm, 1100~1300 nm, 1800~2300 nm and 2700~300 nm) were measured via a Triax 320 type spectrometer upon excitation of 980 nm.

The Raman spectra of the glasses were measured with a Renishaw invia Raman microscope in 200-1200 cm -1 spectrum range using the 532 nm excitation line. The XPS measurements were conducted using a high-performance K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific, United States) in ultra high vacuum. Monochromatic Al Kα radiation (hυ=1846.6 eV) was used with 15 kV accelerating voltage. The C-1s peak arising from pump oil impurities was used as an internal reference, with binding energy of 284.6 eV. The same experimental conditions for different

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 6
samples were maintained so as to get comparable results. All the measurements were carried out at room temperature. Additionally, the maximal measured error in binding energy is ± 0.5%.

Results and discussion

The Raman spectra of prepared glasses are measured at room temperature as shown in Fig. 1. The spectrum of SG glass is deconvoluted into six peaks using Gaussian distribution. These six bands are denoted as 260, 465, 480, 670, 840 and 1020 cm -1 bands. The obvious band C in Raman spectra is related to bending and stretching vibrations of the T-O-T (Ge or Ga) bond. The partial vibrations of Ge-O and Ga-O tetrahedron units the peak E is obtained [START_REF] Wang | Spectroscopic properties and energy transfer process in Tm 3+ -doped Silica-germanate glasses[END_REF]. Hence, in the Raman spectra the contribution of germanate bands is dominate, this is perhaps because of the higher polarizability of Ge 4+ than Si 4+ and therefore only the primary bands of germanate structures were observed [START_REF] Verweij | Raman study of the structure of alkali germanosilicate glasses (I): Sodium and potassium metagermanosilicate glasses[END_REF][START_REF] Chen | Thermal and luminescent properties of 2 µm emission in thulium-sensitized holmium-doped silicate-germanate glass[END_REF].

Especially, from the inset of Fig. 1, two weak bands (Peak D and F in the SG glass) at 670 and 1020 cm -1 may be assigned to the Si-O-Ge and Si-O-Si modes [START_REF] Sharma | Raman study of the structure of glasses along the join SiO 2 -GeO 2[END_REF], and then, the two weak bands exhibit a sharp reduction in the relative intensity with the introduction of the Lu 2 O 3 . A possible mechanism for this reduction is related to the oxyphilic character of Lu 3+ , and the Lu 3+ is preference for bonding with O exceeds Si, which means that the Lu 3+ will compete with Si for O 2- [START_REF] Fouquet-Parry | Composition and local bonding in RE-Si-M-O-N (M=Mg, Al ; RE=La, Lu) glasses[END_REF]. Hence, in this paper, the introduction of Lu 2 O 3 instead of Ga 2 O 3 might break the primary structure of [SiO 4 ]/[GeO 4 ] and lead a change between the relative amount of NBO and BO. It will be detailedly researched to further understand the change of glass structure in the next. A detailed analysis of the XPS binding energies was performed in order to get insight into the bonding in these glasses as a function of the rare earth. Fig. 2 shows the O1S spectra of present glasses with a Shirley background subtraction. Each spectrum can be deconvoluted into two peaks according to the asymmetry of the shoulders and peaks. The low and high binding energy peaks of O1S spectra of prepared glasses are attributed to non-bridging (NBO) and bridging oxygens (BO), respectively [START_REF] Chowdari | The role of Bi 2 O 3 as a network modifier and a network former in xBi 2 O 3 • (1 -x)LiBO 2 glass system[END_REF][START_REF] Honma | Electronic polarizability, optical basicity and XPS spectra of Sb 2 O 3 -B 2 O 3 glasses[END_REF].

M A N U S C R I P T A C C E P T E D

From Table 1, the area fraction F of NBO and BO are 0.33, 0.54, 0.70 and 0.71 in SG, SGLu3, SGLu7 and SGLu9 glasses, which suggests that the number of non-bridging oxygens increases in the glass network structures with the introduction of Lu 2 O 3 . A larger non-bridging oxygens number means the structure of primary glass changed and exhibits a lower extent of local Ho 3+ clustering in the SGLU-X glasses [START_REF] Wang | Efficient Enhancement of Bismuth NIR Luminescence by Aluminum and Its Mechanism in Bismuth-Doped Germanate Laser Glass[END_REF]. Therefore, we can know that the addition of modifier Lu 2 O 3 in this germanosilicate glass can change the structure of glasses to decrease the probability of rare-earth clustering, conducing to better fluorescence characteristic of germanosilicate glass. an important factor for laser glass, which gives glass good thermal stability to resist thermal damage at high pumping intensities [START_REF] Chan | Nonstoichiometry in Acceptor-Doped BaTiO 3[END_REF]. Meanwhile, ∆T has been frequently used for evaluating the thermal stability and forming ability of glass, a larger ∆T parameter means the better thermal stability against crystallization and nucleation process. Usually, ∆T should be more than 120 °C to minimize probability of crystallization during the fiber drawing process [START_REF] Yang | Single-mode low-loss optical fibers for long-wave infrared transmission[END_REF].

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 10
730°C which indicates that the introduction of lanthanide cations Lu 3+ can improve the thermal stability of this germanosilicate glass system because of so called 'lanthanide contraction' [START_REF] Lofaj | Thermal expansion and glass transition temperature of the rare-earth doped oxynitride glasses[END_REF]. In addition, the ∆T of SGLu5, SGLu7 and SGLu9 are 172, 180 and 187°C, respectively, which are much larger than that of SG glass. These results suggest that the introduction of Lanthanide cations Lu 3+ with the high field strength in this germanosilicate glass can be a good way to improve the thermal stability, defined as the resistance to the permanent change of properties, particularly, the resistance against crystallization during heating [START_REF] Burtan-Gwizdala | The influence of Pr 3+ content on luminescence and optical behavior of TeO 2 -WO 3 -PbO-Lu 2 O 3 glass[END_REF]. Compared with the characteristic temperatures in Table 2 and Fig. 3, a conclusion can be drawn that the prepared germanosilicate glasses with the addition of Lu 2 O 3 in this study possess a better thermal stability, which is good for crystal-free fiber drawing and fabrication.

Fig. 4 indicates the room temperature absorption spectra of Ho 3+ singly and Yb 3+ /Ho 3+ co-doped samples in the wavelength region of 300-2200 nm. The inset is the photo of prepared glasses. Referring to Ho 3+ singly doped SG glass, the absorption spectrum consists of seven absorption bands centered at 1949, 1151, 644, 540, 487, 474 and 450 nm, corresponding to the transitions starting from the 5 I 8 ground state to excited states 5 I 7 , 5 I 6 , 5 F 5 , ( 5 F 4 , 5 S 2 ), ( 5 G 6 , 5 F 1 ) and 5 G 5 , respectively. In addition, the broadband absorption with a strong peak at 980 nm can be observed in Ho 3+ /Yb 3+ co-doped SG and SGLUX glasses due to the Yb 3+ : 2 F 5/2 transition, otherwise absorption spectra are similar. This suggests that there is no obvious change in the absorb sites of Ho 3+ after the introduction of sensitizer Yb 3+ . It is implied that Yb 3+ /Ho 3+ co-doped germanosilicate glass can be excited quite efficiently by a conventional 980 nm LD.

M A N U S C R I P T A C C E P T E D

Fig. 4 Absorption spectra of the prepared glasses. The inset is the photo of prepared glasses.

The Judd-Ofelt theory is commonly applied to evaluate the important spectroscopic and laser properties of rare-earth ions doped glasses [START_REF] Judd | Optical Absorption Intensities of Rare-Earth Ions[END_REF]. According to the J-O theory, absorption spectrum and other parameters, some spectroscopic parameters of rare earths in solids, such as intensity parameters Ω λ radiative transition probability A rad , and radiative lifetime τ rad can be calculated. Table 3 shows the J-O intensity parameters (Ω 2 , Ω 4 , and Ω 6 ) of Ho 3+ ions in various glass hosts. Previous studies proved that the parameter Ω 2 closely depends on the ligand symmetry of local environments of Ho 3+ ion sites, and is strongly related to the amount of covalent bonds [START_REF] Wang | emission performance in Ho 3+ doped fluorophosphate glasses sensitized with Er 3+ and Tm 3+ under 800 nm excitation[END_REF]. As is showed in Table 3, it can be obtained that Ω 2 in SGLuX is lower than that of SG glass, the results illustrated that the presence of Lu 3+ in the glass structure reduces the covalent of rare earth ions. So, the germanosilicate glass with the addition of Lu 2 O 3 has lower asymmetry and covalent environment around Ho 3+ ions. On the other hand, the Ω 6 parameter is a vibronic dependent parameter and the radio of Ω 4 /Ω 6 determines the spectroscopy quality of the materials [START_REF] Watekar | Optical properties of Ho-doped alumino-germano-silica glass optical fiber[END_REF]. The Ω 4 /Ω 6 values of prepared glasses are showed in Table 3. The larger Ω 4 /Ω 6 value means the higher lasing efficiency for the laser transition of Ho 3+ ions in a given matrix.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 12

From results as shown in To research the effect of addition of Lu 2 O 3 on the mid-infrared emission properties in Ho 3+ /Yb 3+ co-doped germanosilicate glass, the 2.0 µm fluorescence spectra of Ho 3+ /Yb 3+ co-doped SG and SGLU glasses were measured upon 980nm LD and revealed in Fig. 5 (a), it can be seen that an obvious emission peak centered near 2.0 µm in all samples, which is corresponding to the Ho 3+ : 5 I 7 → 5 I 8 transition. Meanwhile, the concentrations of Ho 3+ /Yb 3+ remain unchanged, and it can be found that the relative emission intensity of 2.0 µm firstly increases and then decreases with the increasing Lu Therefore, the energy level diagram and energy transfer mechanism of Ho 3+ /Yb 3+ are showed in Fig. 6 so as to further comprehend the relation of mid-infrared (2.0 and 2.8 µm), near-infrared and up-conversion emissions. The electrons of Yb 3+ ions at 2 F 7/2 are excited to Yb 3+ : 2 F 5/2 by ground state absorption (GSA) when the sample is pumped by 980 nm LD. Then the energy transfers from the ions on the excited Yb 3+ to the 5 I 6 state of Ho 3+ by ET: Ho 3+ : 5 I 8 +Yb 3+ : 2 F 5/2 →Ho 3+ : 5 I 6 +Yb 3+ : 2 F 7/2 . On one hand, some of the electrons at Ho 3+ : 5 I 6 level are excited to the 5 F 4 , 5 S 2 levels by an energy transfer upconversion (ETU1) and excited state absorption (ESA1: 5 I 6 +a photon→ 5 F 4 , 5 S 2 ), the decay of the ions from these states ( 5 F 4 , 5 S 2 ) to the ground state Ho 3+ : 5 I 8 result in the green emission ( 5 F 4 , 5 S 2 → 5 I 8 +662 nm). Due to the small energy gap between 5 F 4 , 5 S 2 and 5 F 5 level, ions of 5 F 4 , 5 S 2 level relax non-radiative to the next 5 F 5 level, the 552 nm radiations take place according to the 5 F 5 → 5 I 8 transition.

On the other hand, a part of ions on the Ho 3+ : 5 I 6 level decay to the next lower Ho 3+ : 5 I 7 and 5 I 8 states, adding concentration is around 7% mol in this germanosilicate glass.

Fig. 6 Energy level diagrams and energy transfer mechanism between Yb 3+ and Ho 3+ ions.

To research the number of photons involved in the up-conversion luminescence process of this glass, the up-conversion of SGLu7 excited at 980 nm with different pump power has been measured and shown in Fig. 7 (a). The result clearly illustrates that the up-conversion increases with pump power increasing. It is known that for the up-conversion process, the number of photons required to populate the upper emitting state can be described by the following relation [START_REF] Dong | Influence of erbium substitution on structural, electrical, and up-conversion photoluminescence properties of unfilled tungsten bronze oxides Ba 3.75 La 0.833-x Er x Nb 10 O 30[END_REF][START_REF] Zhao | Li + ion doping: an approach for improving the crystallinity and upconversion emissions of NaYF 4 :Yb 3+ , Tm 3+ nanoparticles[END_REF] Fluorescence lifetime is an important parameter to estimate the ions behavior of the excited state.

I ∝ P ୬ (1)
According to the measured and fitted decay curves, the fluorescence lifetime can be determined by the time of the intensity of 1/e. R-square of the single exponential fittings in prepared glass are all above 0.998. The coincidence between the measured and fitted decay curves suggests that energy level 5 I 7 in these glasses single-exponentially. Hence, from Fig. 8 and Table .4, it can be found the fluorescence lifetime of Ho 3+ : 5 I 7 level becomes longer with increasing Lu 2 O 3 concentrations. Higher lifetime suggests that the upper level of 2.0 µm emission for SGLuX is more favorable for ions accumulation, and SGLuX is more helpful for population inversion between upper level and lower level than SG glass.

The lifetime results further proved that the cross relaxation among rare-earth (Ho 3+ ) ions has been

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 16
decreased with the addition of Lu 3+ , and hence, fluorescence lifetime of Ho 3+ : 5 I 7 level increased owing to the reduction of multistep energy transfer among rare-earth ions. Moreover, the longer radiation lifetime is beneficial for reducing the laser oscillation threshold [START_REF] Wei | Mid-infrared fluorescence, energy transfer process and rate equation analysis in Er 3+ doped germanate glass[END_REF]. Therefore, this Ho 3+ /Yb 3+ co-doped germanosilicate glass with the addition of Lu 2 O 3 can be considered as an appropriate medium to achieve 2.0 µm laser. The fluorescence decay curves of Yb 3+ ions at the wavelength of 1064 nm in Yb 3+ singly, and Yb 3+ /Ho 3+ co-doped glasses were measured to research the energy transfer process . From Fig. 9, it can be found the fluorescence lifetime of Yb 3+ becomes longer with the Lu 2 O 3 addition, which further proved that the cross relaxation among rare-earth ions (Yb 3+ ) has been decreased with the addition of where τ and τ 0 are the measured lifetimes of Yb 3+ with and without the co-doping of Ho 3+ ions, respectively. The calculated energy transfer efficiencies of SG and SGLu7 are as high as 73.5% and 72.6 %, respectively, which indicates that the energy transfer from Yb 3+ to Ho 3+ is efficient. The decrease of energy transfer efficiency might be due to the increase of average distances for Ho 3+ -Yb 3+ . The average distance of rare earth ion pairs is related to the cross relaxation and energy transfer among rare earth ions. Hence, the average distance for Ho 3+ -Ho 3+ and Yb 3+ -Ho 3+ are counted using the following equation [START_REF] Sun | Eu 3+ -activated B 2 O 3 -GeO 2 -RE 2 O 3 (RE=Y 3+ , La 3+ and Gd 3+ ) borogermanate scintillating glasses[END_REF]

R ୧ = ቀ ఽ ρଡ଼ ቁ ଵ ଷ ⁄ (3)
where N A is the Avogadro number, ρ is the density of prepared glass, M is the mean molecular weight of glass, and x is the total concentration of the Yb 3+ -Ho 3+ or Ho 3+ -Ho 3+ ions. The calculated values are listed in Table 5. The change of average distances is in line with the above discussion. The increasing average distance of Yb 3+ -Ho 3+ slightly decreases the energy transfer efficiency. However, the

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 18
increasing average distance of Ho 3+ -Ho 3+ also results in the decrease of cross relaxation, which can enhance the fluorescence emission of Ho 3+ ions.

Based on the above results, the ideal net structure diagram of the prepared glasses in short range has been proposed as shown in Fig. 10. From the discussions above, we can conclude that SGLu7 possesses optimal fluorescence characteristic. So we calculated the absorption, emission and gain cross sections of SGLu7 sample in order to further research its possibility as mid-infrared laser material. The emission cross section (σ em ) of 2.0 µm fluorescence spectra can be calculated according to Fuchtbauer-Ladenburg (FL) equation [START_REF] Payne | Infrared cross-section measurements of the Er 3+ , Tm 3+ , and Ho 3+ ions in crystals[END_REF] ( )

∫ × = λ λ λ λ λ π λ λ σ d I I cn A rad em ) ( ) ( 8 2 4 (4)
where A rad is the spontaneous radiative transition probability of Ho 3+ : 5 I 7 → 5 I 8 transition, λ is the emission wavelength, n is the refractive index of glass host, c is the velocity of light in vacuum, ∫I(λ)dλ is the integrated fluorescence intensity, and I(λ) is the 2.0 µm fluorescence spectra intensity.

Based on emission cross section, absorption cross section can be obtained by using the

M A N U S C R I P T A C C E P T E D ( ) ( )               - × × × = λ λ λ σ λ σ 1 1 exp ZL u l abs em kt hc Z Z (5)
where Z l and Z u are the respective partition functions of the lower and the upper levels involved in the considered optical transition is equal to 17/15. Parameter K and T are the Boltzmann constant (1.38 ×10 -23 J/K) and the temperature (here it is the room temperature), respectively. λ ZL =2014 nm is the zero-phonon line which means the wavelength for the transition between the lower Stark sublevels of the emitting multiplets and the lower Stark sublevels of the receiving multiplets.

The absorption and emission cross sections of Ho 3+ in SGLu7 sample are shown in Fig. 11 (a). It can be seen that the maximum emission cross section σ em for Ho 3+ : 5 I 7 → 5 I 8 is 4.41×10 -21 cm 2 at 2022 nm, which is higher than those of germanate-tellurite (4.36 ×10 -21 cm 2 ) [START_REF] Peng | Ho 3+ /Yb 3+ -codoped germanate-tellurite glasses for 2.0µm emission performance[END_REF] and silicate (3.07 ×10 -21 cm 2 ) [START_REF] Li | ∼2µm Luminescence and energy transfer characteristics in Tm 3+ /Ho 3+ co-doped silicate glass[END_REF] glasses. Thereby, the prepared SGLu7 glass possessing higher emission cross section might be a superior laser material for achieving intense 2.0 µm laser.

In addition, gain coefficient is another characteristic parameter to evaluate 2.0 µm emission properties quantitatively. Therefore, the gain spectra G (λ) can be calculated by where P is the population inversion given by the ratio between the population of Ho 3+ : 5 I 7 level and the total Ho 3+ concentration and N is the total concentration of Ho 3+ Fig. 11 (b) indicates the gain spectra of 2.0 µm of SGLu7 glasses. Obviously, when the population inversion P>0.4, the gain cross sections in the range of 1700-2300 nm become positive. It is suggested that the prepared Ho 3+ /Yb 3+ co-doped SGLu7 glass would be a suitable optical material to obtain 2.0 µm infrared emission thanks to the excellent gain properties.

  et al. demonstrated the proper introduction of Y 2 O 3 can enhance the luminescence properties of borosilicate glasses by decreasing the bridge oxygens of glass host

  2 -20Ga 2 O 3 -BaO-Li 2 O)-xYb 2 O 3 -1Ho 2 O 3 (x=0, 1, namely SG-Ho 3+ , and SG-Ho 3+ /Yb 3+ , respectively) and 98(SiO 2 -GeO 2 -(20-y)Ga 2 O 3 -yLu 2 O 3 -BaO-Li 2 O)-1Yb 2 O 3 -1Ho 2 O 3 (y=0, 5, 7 and 9, named SG, SGLu5, SGLu7 and SGLu9, respectively) were fabricated by using reagent-grade SiO 2 , GeO 2 , Ga 2 O 3 , BaO, M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

Fig. 1

 1 Fig. 1 Deconvoluted Raman spectra of SG glass. The inset is Raman spectra of SG, SGLu5, SGLu7 and SGLu9 glasses.

Fig. 2

 2 Fig. 2 O1S spectrum of prepared glasses (a) SG; (b) SGLu5; (c) SGLu7; (d) SGLu9.

Fig. 3

 3 Fig. 3 shows the measured DSC curves for the prepared glasses.Table 2 lists the characteristic

Fig. 3

 3 Fig. 3 DSC curves of prepared glasses.

2 O 3 concentration

 23 . Additionally, the maximum fluorescence peak intensity for prepared glasses is observed around 7 mol % Lu 2 O 3 . Although the 2.0 µm emission intensity of SGLu9 decreases, it is still higher M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 13 than original SG glass.

Fig. 5

 5 Fig. 5 Fluorescence spectra around (a)2.0 µm (b)2.8 µm (c)up-conversion (d) near-infrared spectra of prepared glasses.

  2.8 and 1.2 µm emissions, respectively. Finally, the ions in 5 I 7 level decay radiatively to the ground state 5 I 8 and 2.0 µm emission occurs. According to Fig.5, it can be obtained that the intensity of 2.8 µm, near-infrared and up-conversion emissions are in good agreement with the behavior of 2.0 µm fluorescence spectra. It might be attributed to the difference of local environment around Ho 3+ /Yb 3+ ions and, hence, leading to different fluorescence behaviors between SGLUX and SG glasses. It means that a moderate quantity of Lu 3+ can decrease clustering of rare-earth (Ho 3+ ) ions in this glass, therefore decrease the cross relaxation among rare-earth (Ho 3+ ) ions and enhance the emission intensity of Ho 3+ . Additionally, the maximum fluorescence peak intensity for all fluorescence spectra is observed in SGLu7, which indicates that for fluorescence characteristic the optimum Lu 2 O 3

  the up conversion luminescence intensity, P is the pump laser intensity, and n is the number of pump photons required. Fig.7(b) shows the logarithm-logarithm plots for the up conversion green (552 nm) and red emission (662 nm) intensities of SGLu7 as function of excitation power. The plots are well fitted linearly(R 2 = 0.998), giving out the slope for 5 F 5 → 5 I 8 and 5 F 4 → 5 I 8 transitions are 1.41 ± 0.02 and 1.47 ± 0.02, respectively. This indicates that a two-photon absorption process is responsible for the up conversion emission.

Fig. 7

 7 Fig. 7 (a) Up-conversion of SGLu7 excited at 980 nm with different pump power (mW) in the region of

Fig. 8

 8 Fig. 8 Decay curves of 2.0 µm emission in prepared glasses excited by 980 nm LD. The solid lines represent the

  Lu 3+ . Additionally, the fluorescence lifetime of the SG and SGLu7 samples both show obvious M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 17 decrease with the addition of Ho 3+ . According to the following equation,

Fig. 9

 9 Fig. 9 Decay curves of 1064 nm emission in prepared glasses excited by 980 nm LD (a) SG sample,

Fig. 10

 10 Fig. 10 The ideal net structure diagram of the prepared glasses in short range (a) without Lu 2 O 3 (b) with Lu 2 O 3 .

20 Fig. 11 (

 2011 Fig. 11 (a) The calculated absorption and emission cross sections (b) gain coefficient for prepared glass (SGLu7).

  This study investigated the structure and fluorescence characteristic in the Ho 3+ /Yb 3+ co-doped germanosilicate glasses by using lanthanide additives Lu 2 O 3 . The introduction of Lu 2 O 3 improves the thermal stability of glass system owing to the oxyphilic character of Lanthanide cations Lu 3+ . And Lu 2 O 3 changes the structure of glass to decrease the probability of rare-earth clustering (Ho 3+ ), conducing to the fluorescence characteristic of germanosilicate glass. Meanwhile, the J-O intense parameters and radiative lifetimes of Ho 3+ /Yb 3+ co-doped germanosilicate glass with different concentration Lu 2 O 3 are calculated and discussed. The results of test and discussion indicate that SGLu7 possesses optimal fluorescence characteristic. The emission cross section at 2.0 µm reaches as high as 4.41 ×10 -21 cm 2 in SGLu7 glass. According to the experimental results, the germanosilicate glass with the modification of Lu 2 O 3 might be a promising candidate for 2.0 µm laser.

Table 1

 1 Binding energy (eV), FWHM (eV) and the area ratios F of NBO and BO in XPS spectra.

	Glass samples	Binding energy(eV)	FWHM(eV)	F
		532.40	2.34
	SG			0.33
		531.00	2.17
		532.21	1.98
	SGLu5			0.54
		531.42	2.11
		531.73	2.13
	SGLu7			0.70
		531.05	2.21
		531.71	2.07
	SGLu9			0.71
		530.79	2.73

Table 2

 2 The glass transition temperature (T

	Sample	T g (°C)	T x (°C)	T p (°C)	∆T(°C)
	SG	645	755	795	110
	SGLu5	682	854	911	172
	SGLu7	716	896	943	180
	SGLu9	730	917	950	187

g ), onset crystallization temperature (T x ), top crystallization peak (T p ), thermal stability parameters ∆T in prepared glasses.

As shown in Table

2

, it can be found the glass transition temperature (T g ) increases from 645°C to

Table 3 ,

 3 it can be found that the Ω 4 /Ω 6 value increases when the Lu 2 O 3 is introduced in primal germanosilicate glass host. It turns out that the Lu 2 O 3 contents in germanosilicate glass can improve the efficiency of laser and be a useful modifier for mid-infrared emission. Besides,

	the radiative transition probabilities for Ho 3+ : 5 I 7 → 5 I 8 level can be evaluated via J-O parameters. The
	radiative transition probability A rad of prepared sample which possesses highest Ω 4 /Ω 6 value (SGLu7) is
	calculated (103.42 s -1 ), which is larger than those of silicate glass (78.71s -1 )[39] and germanate glass
	(70.15 s -1 ) [40]. High spontaneous transition probability is beneficial for high gain and more
	opportunity to achieve laser action.

Table 3

 3 The J-O intensity parameters Ω λ (λ=2, 4, 6) (×10 -20 cm 2 ) of Ho 3+ in prepared glasses.

	Sample	Ω 2	Ω 4	Ω 6	Ω 4 /Ω 6
	SG	5.28±0.2	3.93±0.1	1.25±0.05	3.14
	SGLu5	5.08±0.2	3.84±0.1	1.19±0.05	3.23
	SGLu7	4.76±0.2	3.62±0.1	1.08±0.05	3.35
	SGLu9	4.52±0.2	3.44±0.1	1.03±0.05	3.33

Table 4

 4 Measured fluorescence lifetimes of the Ho 3+ : 5 I 7 level of prepared glasses.

	Sample	SG	SGLu5	SGLu7	SGLu9
	Lifetime	485 µs	616 µs	707 µs	691 µs

Table 5

 5 The average distances of Yb 3+ -Ho 3+ or Ho 3+ -Ho 3+ ion pairs

	Average distance (Å)	SG	SGLu5	SGLu7	SGLu9
	Yb 3+ -Ho 3+	10.48	10.60	10.69	10.77
	Ho 3+ -Ho 3+	13.20	13.35	13.47	13.57

Acknowledgements

This research was financially supported by the Chinese National Natural Science Foundation (Nos. 61605192, and 51472225), the Natural Science Foundation of Zhejiang Province (LY18E020008), and the open Fund of the Guangdong Engineering Technology Research Development Center of Special Optical Fiber Materials and Devices (South China University of Technology).

3. The increase of NBO can decrease the cross relaxation effect among RE-RE ions. 4. Fluorescence properties of Ho 3+ have been improved due to the increase of NBO.