Realizing efficient broadband near-infrared emission under blue light excitation in Sb³⁺-doped zero-dimensional organic tin(IV)-based metal halides via coordination structure modulation

Abstract

Realizing Sb³⁺-activated efficient broadband near-infrared (NIR) emission under blue light excitation remains a significant challenge in lead-free metal halides. To overcome the above difficulties, a coordination structure modulation strategy was adopted, and the broadband NIR emission under blue light excitation was achieved in Sb³⁺-doped zero-dimensional (0D) organic tin(IV) bromide. Compared to the weak visible light emission with a photoluminescence quantum yield (PLQY) of 2.4% for pure (TBP)₂SbBr₅ (TBP = tetrabutylphosphonium), Sb³⁺-doped (TBP)₂SnBr₆ exhibits an efficient broadband NIR emission band at 705 nm with a PLQY of 33.2% upon 452 nm excitation, which stems from self-trapped exciton emission. Combined with experiments and theoretical calculations, we find that the large excited-state lattice distortion degree compared to the ground state and the narrow bandgap are dominant reasons for Sb³⁺-doped (TBP)₂SnBr₆ showing efficient NIR emission under blue light excitation. Specifically, Sb³⁺-doped (TBP)₂SnBr₆ also has excellent anti-water stability, existing stably in water for more than 4 hours while maintaining a high luminous efficiency. Based on the excellent stability and unique optical properties of Sb³⁺-doped (TBP)₂SnBr₆, a high-performance NIR light-emitting diode (LED) was fabricated by combining Sb³⁺-doped (TBP)₂SnBr₆ with a commercial blue LED chip, and its application in night vision was demonstrated.

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1. Introduction

Near-infrared (NIR) light sources are highly desired due to their low thermal effect, non-destructive nature, and deep tissue penetration, and are an ongoing pursuit for a variety of high-end applications such as night vision, food quality analysis, and biomedical imaging.^1,2 In particular, broadband NIR sources in the 700–1100 nm range have received much attention because the interaction between infrared light and infrared-active molecular bonds in food and human tissues produces wide-response characteristic absorption signals in this spectral region.³ However, traditional NIR light sources, including halogen and tungsten lamps, are unsuitable for portable applications due to their large volumes and high working temperatures. Recently, NIR phosphor conversion LEDs (pc-LEDs), which combine broadband NIR phosphors with commercial blue LED chips, have sparked widespread attention because of their compact size, tunable broadband emission, and excellent efficiency. The NIR emitter, as an important component of NIR pc-LEDs, and its photophysical properties directly determine the performance of the device.⁴ Therefore, it is crucial to develop efficient broadband NIR luminescent materials with excellent stability under blue light excitation.

Currently, many broadband NIR emitters have been reported, mainly focusing on the study of metal-ion doped oxides, such as Cr³⁺-doped Ga_2−xSc_xO₃,⁵ Eu²⁺-doped K₃LuSi₂O₇,⁶ and Bi³⁺-doped BaAl₁₂O₁₉.⁷ However, these oxide phosphors have a high lattice energy and require high-temperature (>1000 °C) calcination to obtain the target products. Obviously, high-temperature synthesis not only results in serious energy consumption but also poses a series of safety risks in the preparation process. The above shortcomings drive us to develop efficient broadband NIR luminescent materials that can be synthesized at low temperatures. Low-dimensional lead-free metal halides have drawn much attention because of their simple preparation method, low toxicity, and adjustable optical properties. Parallelly, they generally exhibit broadband self-trapped exciton (STE) emission due to lattice distortion and strong electron–phonon coupling. Thus, this provides an opportunity for the development of a new generation of NIR luminescent materials.

Recently, some lead-free metal halides with broadband NIR emission have been developed. For instance, Wang et al. synthesized Sn²⁺-doped all-inorganic 0D Cs₂ZnBr₄, which exhibits broadband NIR emission at 700 nm with a photoluminescence quantum yield (PLQY) of 40%.⁸ Moreover, Yang et al. reported Cu(I)-based metal halides with highly efficient broadband NIR emission.⁹ However, the poor stability seriously limits their application in high-performance NIR light sources. In contrast, Sb³⁺-activated lead-free metal halides with NIR emission have attracted wide attention due to their high efficiency and remarkable stability. For example, Sb³⁺-doped Cs₂ZnCl₄ emits a broadband NIR emission band at 745 nm under 316 nm excitation,¹⁰ and Sb³⁺-doped (NH₄)₂SnCl₆ exhibits an NIR emission band at 736 nm under 360 nm excitation.¹¹ Although the aforementioned Sb³⁺-activated NIR emitters exhibit excellent optical properties, their excitation bands are limited to the ultraviolet (UV) region, which is difficult to match with commercial blue LED chips.

To address the above concerns, we report herein a new NIR emitter of Sb³⁺-doped tin(IV)-based metal halides via coordination structure modulation, which can exhibit bright broadband NIR emission under blue light excitation. Compared to the Sb(III)-based metal halide of (TBP)₂SbBr₅ with weak red emission (PLQY ∼ 2.4%), Sb³⁺-doped (TBP)₂SnBr₆ exhibits a broadband NIR emission band at 705 nm with a PLQY of 33.2% under blue light excitation. Combined with experiments and theoretical calculations, we find that the large excited-state lattice distortion degree compared to the ground state and the narrow bandgap are the dominant reasons for Sb³⁺-doped (TBP)₂SnBr₆ showing efficient NIR emission under blue light excitation. Finally, given the fascinating optical properties as well as the remarkable air and anti-water stability of Sb³⁺-doped (TBP)₂SnBr₆, a high-performance NIR pc-LED was fabricated by combining a commercial 460 nm blue LED chip with Sb³⁺-doped (TBP)₂SnBr₆ phosphors, and we further demonstrated its application in night vision. Therefore, this work provides a feasible approach for designing efficient broadband NIR light sources based on lead-free metal halides.

2. Experimental section

Materials

Tetrabutylphosphonium bromide (TBPBr, 98%), tin(IV) bromide (SnBr₄, 99.0%), antimony(III) bromide (SbBr₃, 99%), acetonitrile (99.5%), N,N-dimethylformamide (DMF, 99.9%), and ethyl ether (Et₂O, 99.5%) were purchased from Aladdin Reagent and all materials were used without any purification.

Growth of (TBP)₂SbBr₅ single crystals (SCs)

(TBP)₂SbBr₅ SCs were synthesized by the slow evaporation method. Typically, 0.3615 g of SbBr₃ and 0.6786 g of TBPBr were dissolved in 5 mL of acetonitrile to form a transparent solution. Afterwards, the solution was evaporated in ambient air at 40 °C for about two days, and the bulk (TBP)₂SbBr₅ SCs can be obtained.

Synthesis of undoped and Sb³⁺-doped (TBP)₂SnBr₆ single crystals

(TBP)₂SnBr₆ SCs were synthesized using the antisolvent recrystallisation method. Typically, 0.4383 g of SnBr₄ and 0.6786 g of TBPBr were dissolved in DMF to form a transparent solution. The (TBP)₂SnBr₆ SCs can then be harvested by diffusing Et₂O into the precursor solution at room temperature. In order to synthesize the Sb³⁺-doped (TBP)₂SnBr₆ SCs, only x mmol SbBr₃ was used under otherwise identical conditions.

Characterization

The crystal structures of the two compounds were determined through single-crystal X-ray diffraction (SCXRD, IPDS II-STOE). The crystallographic data of (TBP)₂SbBr₅ and (TBP)₂SnBr₆ can be obtained for free from the Cambridge Crystallographic Data Centre (CCDC) with accession numbers 2382097 and 2382098.† Powder XRD (PXRD) measurements were acquired via a Bruker D8 Discover X-ray diffractometer. The PL and PLE spectra, temperature-dependent PL spectra, time-resolved photoluminescence (TRPL) spectra and PLQY were recorded using a Horiba Jobin Yvon Fluorolog-4 spectrometer. The Raman and power-dependent PL spectra were obtained using a WITec alpha300R instrument with 532 and 405 nm lasers, respectively. UV-Vis absorption spectra were recorded using a Shimadzu UV3600 Plus instrument. The morphology was observed via a Hitachi SU8020 scanning electron microscope (SEM), and an energy-dispersive spectrometer (EDS, Oxford X-Max Aztec) was employed to determine the composition and distribution of elements. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo Fisher ESCALAB 250Xi. Thermal stability was measured using a Shimadzu DTG-60H instrument in a nitrogen environment with a heating rate of 15 °C min⁻¹.

Calculation methods

The Vienna ab initio simulation package (VASP) was used for all calculations.¹² For the exchange and correlation functionals, the Perdew–Burke–Ernzerhof (PBE) parameterization was approximated using a generalized gradient method through the use of the projector-augmented wave method.^13,14 For the wavefunction basis set, a 4 × 4 × 3 Monkhorst–Pack k-mesh for (TBP)₂SbBr₅ and 4 × 3 × 3 for (TBP)₂SnBr₆, and a 500 eV kinetic energy cutoff were used. For each element, ultrasoft pseudopotentials were employed. For structural relaxations, the energy convergence threshold was established at 1.0 × 10⁻⁵ eV. To calculate the lattice distortion of the excited state structure, the crystal structure of the excited state must first be obtained. The detailed process is as follows: first, the ground state structure is obtained through a standard VASP optimization and relaxation process. Then, the EIGENVAL file from the static calculation is read to examine the occupation of relevant electrons. By adding FERWE and FERDO to control the spin occupation of electrons, the excited state structure is relaxed. Finally, using eqn (1), the bond lengths of the excited state structure are analyzed to determine the degree of distortion in the excited state.where d_n is the distances of Sb–Br bonds, d_ave denotes the average Sb–Br bond length, and n is the number of Sb–Br bonds.

NIR pc-LED fabrication

The as-synthesized Sb³⁺-doped (TBP)₂SnBr₆ SCs were ground into powders, then mixed in AB glue to form a uniform mixture, and finally coated onto a 460 nm LED chip. Subsequently, the performance parameters of the as-fabricated NIR pc-LED were measured using an Everfine HAAS-2000 instrument.

3. Results and discussion

Two bulk assemblies of 0D organic metal halides of (TBP)₂SbBr₅ and (TBP)₂SnBr₆ with various coordination structures were synthesized via a simple solution method at room temperature (RT), and their crystal structures were determined by single-crystal X-ray diffraction (SCXRD). As shown in Fig. 1a and b, the crystallographic units of the two as-synthesized compounds both contain two TBP⁺ cations and a different inorganic polyhedron, that is, square-pyramidal [SbBr₅]²⁻ clusters for (TBP)₂SbBr₅ and octahedron-shaped [SnBr₆]²⁻ clusters for (TBP)₂SnBr₆. The isolated polyhedra are separated from each other and periodically embedded in the framework of organic cations in both compounds, thus constituting a typical 0D structure. Moreover, (TBP)₂SbBr₅ crystallizes in the triclinic space group P1̄, while (TBP)₂SnBr₆ adopts the monoclinic space group C2/c. As shown in Fig. 1c, the distance of Sb–Br drops in the range of 2.52–2.78 Å in the [SbBr₅]²⁻ unit of (TBP)₂SbBr₅, while that of Sn–Br ranges from 2.59 to 2.63 Å in the [SnBr₆]²⁻ octahedron of (TBP)₂SnBr₆. More detailed crystallographic data of (TBP)₂SbBr₅ and (TBP)₂SnBr₆ are summarized in Tables S1–S3,† and their different crystal structure will bring different optical properties. In particular, there are large M–M (M = Sn and Sb) distances >9.6 Å in both compounds, which indicates that there is a negligible interaction between adjacent inorganic polyhedra.¹⁵Fig. 1d shows the PXRD measurements of (TBP)₂SbBr₅ and (TBP)₂SnBr₆, and the diffraction patterns were consistent with the simulated ones, which demonstrates the dependability of SCXRD results.

Fig. 1 Crystalline structures of (a) (TBP)₂SbBr₅ and (b) Sb³⁺-doped (TBP)₂SnBr₆. (c) A ball-and-stick model of [SbBr₅]²⁻ and [SnBr₆]²⁻ clusters. (d) Experimental and calculated PXRD patterns of (TBP)₂SbBr₅ and (TBP)₂SnBr₆. (e) PXRD patterns of Sb³⁺-doped (TBP)₂SnBr₆. (f) SEM and EDS elemental mapping of 0.15Sb³⁺-doped (TBP)₂SnBr₆.

Then, a series of Sb³⁺-doped (TBP)₂SnBr₆ with various doping concentrations were synthesized. The structures of Sb³⁺-doped (TBP)₂SnBr₆ are given in Fig. 1b. Clearly, Sb³⁺ ions can be substituted for Sn⁴⁺ sites due to their similar ion radii (CN = 6, r_Sb³⁺ = 76 pm and r_Sn⁴⁺ = 69 pm) and exist in the form of the [SbBr₆]³⁻ octahedron in the Sb³⁺-doped (TBP)₂SnBr₆ compound. The PXRD patterns of Sb³⁺-doped (TBP)₂SnBr₆ with various doping concentrations are similar to that of the pristine (TBP)₂SnBr₆, indicating that the doping of Sb³⁺ does not change the crystal structure of the host matrix (Fig. 1e). Table S4† shows the EDS results of Sb³⁺-doped (TBP)₂SnBr₆, and it can be seen that the doping concentration of Sb³⁺ is much lower than the feed content. Fig. S1† shows the high-resolution X-ray photoelectron spectroscopy (HRXPS) spectra of Sb 3d. Compared with (TBP)₂SbBr₅, the characteristic peaks of Sb³⁺ 3d_5/2 and Sb³⁺ 3d_3/2 in Sb³⁺-doped (TBP)₂SnBr₆ shift towards a lower binding energy (0.3 eV), which should be due to Sb³⁺ ions having different coordination chemical environments in the above two compounds. The weak signal-to-noise ratio of Sb³⁺ in HRXPS indicates the low Sb³⁺ content, which echoes the EDS results well. Moreover, the SEM images of these two compounds are given in Fig. 1f and S2,† and the elemental mapping shows the uniform distribution of Sn, Sb, and Br in (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆. The morphology and size of single crystals are shown in Fig. S3;† both types of single crystals exhibit a yellow color under natural light, with sizes ranging from 2 to 10 mm. They display irregular block-like structures and have good transparency.

The optical properties of the as-synthesized compounds were measured at RT. For (TBP)₂SnBr₆, the UV-vis absorption spectrum showed an optical absorption edge at 468 nm, and the bandgap value was determined to be 2.78 eV through the Kubelka–Munk function (Fig. S4†). Moreover, (TBP)₂SnBr₆ is a yellow crystal in natural light and shows very weak PL emission under photoexcitation (Fig. S3a†). The PL and PLE spectra of (TBP)₂SnBr₆ are given in Fig. S5.† Clearly, this compound exhibits a broadband emission band at 480 nm with a full width at half-maximum (FWHM) of 78 nm. Normally, there are three mechanisms for the single PL emission in metal halides apart from the influence of organic molecules, namely, permanent defects, free excitons, and STEs.¹¹ We analyzed the possibility of a respective mechanism that governs the emission in (TBP)₂SnBr₆ as follows: (i) the optical properties of TBPBr were also measured (Fig. S6†), which are greatly different from those of (TBP)₂SnBr₆, thus excluding the influence of the organic ligand on the optical properties of (TBP)₂SnBr₆; (ii) we measured the variation of emission with respect to the excitation power and found that the PL intensity exhibits linear dependence on the excitation power (Fig. S5a†). This rules out the permanent defect mechanism, otherwise the emission intensity should be saturated with an increase in the power density;¹⁶ (iii) the broad FWHM and large Stokes shift in (TBP)₂SnBr₆ are completely inconsistent with the free exciton mechanism since free excitons usually have smaller Stokes shifts and narrower FWHMs (Fig. S5b†);¹⁷ and (iv) the excitation wavelength-dependent PL spectra of (TBP)₂SnBr₆ show an identical profile, which illustrates that the observed PL stems from the same excitation state (Fig. S5c†). The average lifetime of the emission is 17.6 ns (Fig. S5d†), which is comparable to those of recently reported organic Sn(IV) halides with STE emission.^16,18 Combining the unique photophysical properties discussed above, we can reasonably attribute the broadband emission observed in (TBP)₂SnBr₆ to STE emission.¹⁹

However, (TBP)₂SnBr₆ shows weak emission (PLQY < 1%), which severely restricts its further applications in photoelectric devices. Recently, Sb³⁺-activated low-dimensional metal halides have attracted extensive attention due to their fascinating optical properties.^20–22 In order to improve the optical properties of (TBP)₂SnBr₆ and further understand the reasons for their efficient emission, we synthesized Sb³⁺-doped (TBP)₂SnBr₆ and the pure Sb(III)-based metal halide of (TBP)₂SbBr₅, respectively. Then, the PL and PLE spectra of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ were recorded at RT, and the corresponding optical images are given in Fig. S3b and S3c.† As shown in Fig. 2a and b, both compounds have an intense excitation band within the blue light region (470 nm for (TBP)₂SbBr₅ and 452 nm for Sb³⁺-doped (TBP)₂SnBr₆). Upon blue light excitation, (TBP)₂SbBr₅ shows a broad visible emission band at 660 nm, while Sb³⁺-doped (TBP)₂SnBr₆ exhibits a NIR emission band at 705 nm. As a consequence, the two compounds have a large Stokes shift of 0.76 eV for (TBP)₂SbBr₅ and 0.98 eV for Sb³⁺-doped (TBP)₂SnBr₆. Moreover, the above two compounds also exhibit long PL decay lifetimes of 0.73 and 1.22 μs, respectively (Fig. S7†). Parallelly, the UV-vis absorption spectra of (TBP)₂SbBr₅ and 0.15Sb³⁺-doped (TBP)₂SnBr₆ are depicted in Fig. S8 and S9.† Clearly, both compounds show strong absorption in the visible light region, which corresponds to the yellow crystal in natural light (Fig. S3b and S3c†), but they have different optical absorption edges and bandgaps: 498 nm and 2.60 eV for (TBP)₂SbBr₅ and 475 nm and 2.76 eV for 0.15Sb³⁺-doped (TBP)₂SnBr₆. The greatly different optical properties of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ should be attributed to their different crystal structures, which we will discuss in detail later.

Fig. 2 (a) PL and PLE spectra of (TBP)₂SbBr₅ at RT. (b) PL and PLE spectra of Sb³⁺-doped (TBP)₂SnBr₆ SCs with various Sb/(Sn + Sb) ratios at RT. Three-dimensional excitation–emission matrix fluorescence spectra of (TBP)₂SbBr₅ (c) and 0.15Sb³⁺-doped (TBP)₂SnBr₆ (d).

Subsequently, the PL spectra of Sb³⁺-doped (TBP)₂SnBr₆ with various Sb³⁺ doping concentrations were recorded. As shown in Fig. 2b, the PL intensity increases first with the increase of Sb³⁺ doping concentration because more luminescence centers are generated. When the doping concentration of Sb³⁺ further increases, the emission intensity begins to decrease, which is caused by concentration quenching.²² The strongest NIR emission in Sb³⁺-doped (TBP)₂SnBr₆ can be witnessed when the feed content of Sb³⁺ is 15%, accompanied by an elevated PLQY of 33.2% (Fig. S10†). Furthermore, our Sb³⁺-doped (TBP)₂SnBr₆ also has the blue light excitation characteristic, which represents the most advanced NIR emitter in lead-free metal halides (Table S5†). In contrast, the PLQY of (TBP)₂SbBr₅ is only 2.4%, which is difficult to meet the practical application. To further confirm the nature of broadband emission, the varying PLE and PL spectra of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ were recorded. As shown in Fig. 2c and d, both compounds show a broadband emission with a change in the excitation wavelength at RT, indicating that there are no other impurities or additional energy levels in the two samples. The power-dependent PL spectrum of Sb³⁺-doped (TBP)₂SnBr₆ is shown in Fig. S11,† and it can be seen that the PL intensity exhibits linear dependence on the excitation power, which rules out the assumption that the emission in Sb³⁺-doped (TBP)₂SnBr₆ stems from the permanent defect mechanism.²³ For (TBP)₂SbBr₅, besides the broadband emission band observed in Fig. 2a, there is an additional weak emission band at 510 nm (band A) with a nanosecond lifetime (3.2 ns, Fig. S12 and S13†) under 309 nm excitation at 80 K, which is different from the broadband emission band at 660 nm (band B). Although Sb³⁺-doped (TBP)₂SnBr₆ exhibits a negligible change in broadband PL spectra (band B) at RT (Fig. 2d), an additional emission band at 528 nm (band A) (Fig. S14a†) with a decay lifetime of 3.18 ns can be observed under 368 nm excitation in the low-temperature PL spectra (80 K, Fig. S14b†). Moreover, the PLE spectra monitored at band A and band B exhibit different PLE spectra, which should correspond to ¹S₀ → ¹P₁ (360 nm) and ¹S₀ → ³P₁ (410 nm) transition of Sb³⁺.²¹ Therefore, (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ exhibit different optical properties, and the dual-emission bands in both compounds should stem from different excited states. Generally, for Sb³⁺ ions with a 5s² electron configuration, the ground state is ¹S₀, while the excited state is ¹P₁, ³P₀, ³P₁, and ³P₂.²² In particular, ¹S₀–³P₀ and ¹S₀–³P₂ are forbidden transitions, while ¹S₀–³P₁ and ¹S₀–¹P₁ are permitted transitions because of spin–orbit coupling.^24,25 Under photoexcitation, the electrons are excited to ³P₁ and ¹P₁ levels, and they will quickly be self-trapped to form two different STEs due to the large lattice distortion and strong electron–phonon coupling.²³ In our studies, the band A emission shows a short lifetime (nanosecond level) and the band B emission exhibits a long lifetime (microsecond level), which should be attributed to the two emission bands that stem from different excited states. Generally, the decay lifetime of a singlet STE is significantly shorter than that of a triplet STE, and hence the observed band A and band B emissions come from singlet and triplet STEs, respectively.²⁶

Recently, numerous lead-free metal halides with efficient broadband emission have been reported, making the relationship between the crystal structure and luminescence properties a key area of research. As we know, the crystal structure of Sb³⁺ has an important influence on its optical properties.²⁷ In our findings, the Sb³⁺ ions have different coordination structures in (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆, which enables them to exhibit different photophysical properties. To be more specific, (TBP)₂SbBr₅ exhibits visible light emission, while Sb³⁺-doped (TBP)₂SnBr₆ has a larger Stokes shift, resulting in efficient broadband NIR emission. Recent studies have shown that increasing the lattice distortion degree (η) of the excited state compared to the ground state is the key to yielding large Stokes shifts.^28,29 Thus, the lattice distortion parameters (Δd) of [SbBr₅]²⁻ in (TBP)₂SbBr₅ and [SbBr₆]³⁻ in Sb³⁺-doped (TBP)₂SnBr₆ in the ground state (GS) and excited state (ES) were calculated using eqn (1). The calculated Δd values of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ are given in Tables S6 and S7.† Although the Δd value of (TBP)₂SbBr₅ in GS (Δd_GS) and ES (Δd_ES) is greater than that of Sb³⁺-doped (TBP)₂SnBr₆, the lattice distortion degree (η = (Δd_ES − Δd_GS)/Δd_GS) of [SbBr₆]³⁻ is larger than that of [SbBr₅]²⁻, which enables Sb³⁺-doped (TBP)₂SnBr₆ to exhibit a large Stokes shift and yield a broadband NIR emission. Parallelly, a large lattice distortion can increase nonradiative losses of excitation energy, thereby quenching the PL of low-dimensional metal halides with STE emission and vice versa.^30,31 Therefore, Sb³⁺-doped (TBP)₂SnBr₆ with appropriate lattice distortion can emit efficient emission under photoexcitation.

To further understand the broadband emission mechanism of the as-synthesized samples, the band structures and projected density of states (PDOS) of (TBP)₂SbBr₅, (TBP)₂SnBr₆, and Sb³⁺-doped (TBP)₂SnBr₆ were calculated via density functional theory (DFT). All compounds show a direct bandgap with values of 1.15 eV for (TBP)₂SbBr₅ (Fig. 3a), 2.19 eV for (TBP)₂SnBr₆ (Fig. 3b), and 1.36 eV for Sb³⁺-doped (TBP)₂SnBr₆ (Fig. 3c), which are smaller than the experimental values (Fig. S3, S8, and S9†), and this should be attributed to the fact that DFT calculations typically underestimate the bandgap value. Compared with pure (TBP)₂SnBr₆, when Sb³⁺ is introduced into the lattice, the bandgap decreases significantly, which corresponds to the redshift of the absorption band after Sb³⁺ doping. In addition, all compounds exhibit a negligible dispersion in the valence band maximum (VBM) and conduction band minimum (CBM), which illustrates a strong quantum confinement effect and is further consistent with its long M–M (M = Sn⁴⁺, Sb³⁺) distance in our 0D structures.³² In particular, the calculated bandgaps of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ were much smaller than those of other 0D lead-free metal halides,^33–35 which makes them have strong absorption in the visible light region and yield bright emission under blue light excitation. Fig. 3d and g show the PDOS and partial charge density distribution of (TBP)₂SbBr₅. The VBM is composed of organic ligands, while Br p, Sb s, and Sb p states mainly dominate the CBM. Thus, a charge is transferred between the organic counterparts and inorganic clusters in (TBP)₂SbBr₅, which will weaken the PL intensity. In contrast to (TBP)₂SbBr₅, the VBM and CBM of (TBP)₂SnBr₆ are mostly contributed by an inorganic unit (Fig. 3e and h), that is, the VBM is mainly composed of the Br p state, while the CBM mainly consists of Br p and Sn s orbitals. After Sb³⁺ doping, there is little effect on the CBM, but a new impurity band appears in the VBM, which is contributed by Sb s and Br p characteristics (Fig. 3f and i). Moreover, the doping of Sb³⁺ will cause the downshift of Br p and Sn s states, which leads to the bandgap contraction. Although heterovalent doping introduces elements with different valence states, this does not necessarily lead to an increase in defects or a decrease in PLQY. In contrast, in some cases, appropriate heterovalent doping can improve the influence of defects by regulating the lattice structure and electronic environment of the material. For example, the incorporation of Sb³⁺ can form a stable local structure that passivates inherent defects in the material, thereby reducing non-radiative recombination and improving luminescence efficiency.¹⁸ This has been demonstrated in practical studies, where the lone pair electrons of Sb³⁺ can significantly enhance the optical performance of Sn⁴⁺ systems by modulating the electronic structure.^22,36 In low-dimensional halide materials, Sb³⁺ doping is often accompanied by lattice distortion, which promotes the formation of STEs. STEs refer to electrons and holes that are confined to locally deformed regions of the material and release energy in the form of radiative recombination.³⁷ This mechanism not only prevents a decrease in luminescence efficiency but also helps to generate broadband emission and enhance the PLQY of the material. Therefore, Sb³⁺ ion doping can effectively regulate the electronic properties of (TBP)₂SnBr₆, which makes Sb³⁺-doped (TBP)₂SnBr₆ exhibit fascinating optical properties compared with pure (TBP)₂SnBr₆.

Fig. 3 Band structures and PDOS of (a and d) (TBP)₂SbBr₅, (b and e) (TBP)₂SnBr₆, and (c and f) Sb³⁺-doped (TBP)₂SnBr₆. Partial charge density distribution for the CBM and VBM of (g) (TBP)₂SbBr₅, (h) (TBP)₂SnBr₆ and (i) Sb³⁺-doped (TBP)₂SnBr₆.

To better understand the photophysical properties of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆, temperature-dependent PL spectra were investigated at 80 to 300 K. As shown in Fig. 4a and b, the PL intensities of both compounds gradually increase with a decrease in temperature, which should be attributed to the suppression of nonradiative transitions at low temperatures. Then, the thermal activation energy (E_b) values for the two compounds were calculated using the Arrhenius equation (eqn (2)):

where k_B is the Boltzmann constant and I₀ is the PL intensity at 0 K. The E_b values of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ are 295.6 and 83.2 meV (Fig. 4c), both of which are larger than the RT thermal ionization energy (≈26 meV). Consequently, the STEs observed in Sb(III)-based and Sb³⁺-doped compounds can exist in a stable state and overcome thermal quenching.²²

Fig. 4 Temperature-dependent PL spectra of (a) (TBP)₂SbBr₅ and (b) Sb³⁺-doped (TBP)₂SnBr₆ within 80–300 K. (c) Integrated PL intensity of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆ as a function of the reciprocal temperature. (d) Huang–Rhys factor of (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆. (e) PLE spectra of Sb³⁺-doped (TBP)₂SnBr₆ with various temperatures. (f) Raman spectra of undoped and Sb³⁺-doped (TBP)₂SnBr₆ SCs.

The Huang–Rhys factor (S) is another important physical parameter that can be used to evaluate the strength of electron–phonon coupling, which can be obtained using eqn (3):

where ℏω_phonon represents the phonon energy. Here, the S values are 20 and 30 for (TBP)₂SbBr₅ and Sb³⁺-doped (TBP)₂SnBr₆, respectively. Such large values of S indicate that both compounds have strong electron–phonon coupling, which guarantees the formation of STEs in these two compounds.³⁸ Furthermore, the difference in values of S in the above two compounds should be attributed to their different coordination configurations. Fig. 4e exhibits the temperature-dependent PLE spectra of Sb³⁺-doped (TBP)₂SnBr₆. With a decrease in temperature, the PLE band exhibits a distinct blueshift, which should be attributed to the suppression of exciton–phonon coupling as well as the reduction in thermally populated vibrations in the inorganic moiety.¹⁸ In other words, low temperature promotes electron excitation to higher excited energy levels, resulting in the shift of the PLE band from 452 nm at RT to 410 nm at 80 K.

To gain further insight into the electron–phonon interaction, Raman spectra were obtained for pure and Sb³⁺-doped (TBP)₂SnBr₆ under 532 nm laser excitation (Fig. 4f). Clearly, a series of Raman peaks are observed, which should stem from inorganic units. Among them, the Raman bands at 254, 186, 147, and 93 cm⁻¹ perfectly meet the following formula in the octahedron: , where A_1g, F_2g, and E_g are the symmetric stretching, bending, and asymmetric stretching modes in [SnBr₆]²⁻ units, respectively.³⁹ After Sb³⁺ doping, all the Raman modes shift to a lower wavenumber, which is caused by lattice expansion due to the larger ion radius of Sb³⁺ than that of Sn⁴⁺.⁴⁰ In particular, Sb³⁺ doping can significantly narrow the FWHM of the Raman mode of the E_g mode and weaken the Raman intensity of the A_1g mode, which indicates that Sb³⁺ doping can effectively regulate the symmetry and electron–phonon coupling of the host lattice, thus bringing rich photophysical properties. Furthermore, the Raman mode at the lowest energy should be the strongest in strong-confined metal halides in the small polaron (STE) system.⁴¹ However, the strongest Raman mode in this doped system is 186 cm⁻¹, which is the overtone of the 93 cm⁻¹. Their simultaneous presence means that their strong electron–phonon coupling Raman mode can be found in the just one octahedron with strong anharmonicity, resulting in an STE state within a cluster.

Based on the abovementioned results, Sb³⁺-doped (TBP)₂SnBr₆ exhibits strong NIR emission under blue light excitation. Moreover, the optical properties of Sb³⁺-doped (TBP)₂SnBr₆ are greatly different from those of (TBP)₂SnBr₆ and (TBP)₂SbBr₅. Combined with the crystal structure and DFT calculations, the NIR emission in Sb³⁺-doped (TBP)₂SnBr₆ should stem from [SbBr₆]³⁻ clusters. To identify the source of the luminescence, the Huang–Rhys factor of (TBP)₂SbBr₅ was calculated to be 30, which illustrates a strong electron–phonon interaction in this compound. Then, we recorded the power-dependent PL spectra of Sb³⁺-doped (TBP)₂SnBr₆, and the PL intensity exhibits a linear dependence on the excitation power, which rules out the assumption that the emission in Sb³⁺-doped (TBP)₂SnBr₆ stems from the permanent defect mechanism. Moreover, Sb³⁺-doped (TBP)₂SnBr₆ also exhibits a large lattice distortion, broad FWHM (152 nm), large Stokes shift (0.98 eV), and long decay lifetime (1.22 μs), and thus we can confirm that the broadband NIR emission in Sb³⁺-doped (TBP)₂SnBr₆ stems from STEs, and the photophysical mechanism is given in Fig. S16.† Under photoexcitation (e.g., 452 nm), the electrons in [SbBr₆]³⁻ clusters are excited from the ground state to the excited state. Subsequently, the excited electrons undergo quick intersystem crossing from the excited state to the triplet STE state, thus yielding a broad NIR emission peaking at 705 nm. In particular, compared with (TBP)₂SbBr₅ with visible light emission, Sb³⁺-doped (TBP)₂SnBr₆ exhibits a larger excited state lattice distortion degree compared to the ground state, and thus this compound exhibits NIR emission under blue light excitation.

Given the excellent NIR emission characteristics of Sb³⁺-doped (TBP)₂SnBr₆, we then investigated the stability. When Sb³⁺-doped (TBP)₂SnBr₆ was stored in an atmospheric environment for 3 months, the PXRD patterns (Fig. S17a†) and the emission spectra show a similar outline to the pristine one. In the meantime, the PL intensity remains basically unchanged (Fig. S17b†). The thermogravimetric analysis revealed that the decomposition temperature of Sb³⁺-doped (TBP)₂SnBr₆ is 333 °C (Fig. S17c†). Surprisingly, Sb³⁺-doped (TBP)₂SnBr₆ also exhibits remarkable anti-water stability. As shown in Fig. 5a, Sb³⁺-doped (TBP)₂SnBr₆ SCs can still maintain their original shape and have a high NIR emission intensity when soaked in water for 4 h. Moreover, the PL spectrum of the sample after water treatment shows a similar profile to the fresh sample (Fig. 5b), and the PLQY only dropped from 33.2 to 28.9% after soaking in water for 4 h (Fig. S18†). The PXRD patterns of Sb³⁺-doped (TBP)₂SnBr₆ after water treatment show an identical profile to that of the fresh sample, indicating that this compound did not undergo structural degradation after soaking in water (Fig. 5c). Hence, the above results illustrate that Sb³⁺-doped (TBP)₂SnBr₆ has excellent anti-water stability and further show that this compound emits bright NIR emission even when immersed in water. To explore the intrinsic mechanism of Sb³⁺-doped (TBP)₂SnBr₆ with excellent water stability, the HRXPS spectra were recorded (Fig. 5d). Compared with the fresh sample, the sample after water treatment has an additional satellite peak at 531.9 eV, which can be assigned to the O 1s. Therefore, we believe that after this compound has been treated with water, an amorphous tin layer will form on the surface, further protecting it from degradation.⁴² In the meantime, the absorption spectra exhibit a blue shift compared with the pristine sample, indicating that the formation of the oxide layer makes the optical bandgap larger (Fig. S19†). Therefore, when Sb³⁺-doped (TBP)₂SnBr₆ is treated with water, an amorphous tin oxide layer can be formed on its surface to protect it from further degradation, which is the dominant reason for its excellent anti-water stability.

Fig. 5 (a) Digital photos of Sb³⁺-doped (TBP)₂SnBr₆ under daylight (left) and 450 nm blue light (right) in water. PL stability (b), PXRD (c), and Sb 3d HRXPS results (d) of Sb³⁺-doped (TBP)₂SnBr₆ after immersion in water for 4 h.

Finally, a high-performance NIR pc-LED was prepared by combining an Sb³⁺-doped (TBP)₂SnBr₆ NIR phosphor with a blue LED chip (460 nm). Fig. 6a shows the electroluminescence (EL) spectra of the as-fabricated device, which shows a similar profile to the PL spectra. Moreover, the emission intensity of the NIR pc-LED increases gradually with an increase of driving currents (Fig. 6b), and the output power and photoelectric conversion efficiency of the device at different driving currents are given in Fig. 6c. The NIR output power increases gradually as the driving current increases, while the photoelectric conversion efficiency shows the opposite trend. The output power reaches a maximum of 9 mW at a current of 60 mA, at which point the photoelectric conversion efficiency drops to 5.4% due to the reduced efficiency of commercial LED chips.¹⁰ In particular, the device performance parameters of the Sb³⁺-doped (TBP)₂SnBr₆ based NIR pc-LED are much higher than those of the recently reported Sb³⁺-doped Cs₂ZnCl₄ based NIR pc-LED under the same driving current,¹⁰ which should be attributed to the fact that Sb³⁺-doped (TBP)₂SnBr₆ can be excited by a blue LED chip. In further experiments, we demonstrated the application of the Sb³⁺-doped (TBP)₂SnBr₆ based NIR pc-LED in night vision. In Fig. 6d, the image of a sunflower can be captured using a visible camera under sunlight, but nothing can be observed when the sunlight is turned off. In particular, the black-and-white image of a sunflower is captured using an NIR camera when the NIR pc-LED is turned on (Fig. 6f). Moreover, the operational stability of the NIR pc-LED device was measured, and the emission intensity remained high (Fig. S20†). These results demonstrate the great potential application of Sb³⁺-doped (TBP)₂SnBr₆ in night vision due to the efficient NIR emission and blue-light excitation characteristics.

Fig. 6 (a) The EL spectra of the NIR pc-LED. (b) NIR EL spectra of the NIR pc-LED at various driving currents. (c) NIR output power and the photoelectric conversion efficiency of the device as a function of the driving current. (d–f) Physical drawing captured using visible and NIR cameras under natural and NIR light.

4. Conclusions

In summary, we report a new NIR emitter of Sb³⁺-doped 0D (TBP)₂SnBr₆via coordination structure modulation. Compared with pure (TBP)₂SbBr₅ with [SbBr₅]²⁻ clusters, the coordination structure of Sb³⁺ ions in Sb³⁺-doped (TBP)₂SnBr₆ can be effectively regulated to the [SbBr₆]³⁻ octahedron. Combined with steady state/transient PL spectra and DFT calculations, it was found that [SbBr₆]³⁻ has a larger excited state lattice distortion compared to the ground state, which will cause a large Stokes shift in Sb³⁺-doped (TBP)₂SnBr₆. Moreover, Sb³⁺-doped (TBP)₂SnBr₆ exhibits a narrow bandgap and multi-level non-radiative relaxation. Thus, we can obtain the efficient broadband NIR emission under blue light excitation with a PLQY of 33.2%. The study of photophysical properties illustrates that the observed broadband NIR emission in Sb³⁺-doped (TBP)₂SnBr₆ stems from STE emission due to the strong electron–phonon coupling. Specifically, Sb³⁺-doped (TBP)₂SnBr₆ also exhibits remarkable air and anti-water stability, and we also demonstrated the application of Sb³⁺-doped (TBP)₂SnBr₆ in a high-performance NIR pc-LED. Consequently, this work not only presents a realistic proposal for designing low-dimensional lead-free metal halides with efficient broadband NIR emission under blue light excitation but also helps in understanding the relationship between the crystal structure and photophysical properties.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the Guangxi NSF project (no. 2020GXNSFDA238004), the “Guangxi Bagui Scholars” fund, the Guangxi Science and Technology Major Project (AA23073018), and the Scientific and Technological Bases and Talents of Guangxi (Guike AD21238027 and AD23026119) for financial support. The calculation was supported by the high-performance computing platform of Guangxi University.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2382097 and 2382098. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01904k

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