Rb2InCl5·H2O doped with Te4+ for fluorescence thermometry and coordinated water-related structural phase transformation
  • photonics1
  • Oct. 26, 2024

Abstract

 

Here, we synthesized Rb2InCl5·H2O with x Te4+ doping by the coprecipitation method, which converts the original nonluminance for pure Rb2InCl5·H2O into orange luminescence ascribed to Te4+ incorporation with an emission peak at 648?nm and a full width at half maximum of 142?nm (420.3?meV) at room temperature. The photoluminescence (PL) lifetime of Te4+-doped Rb2InCl5·H2O exhibits a strong temperature dependence, with a maximum absolute sensitivity (SA) of 12 × 10−3?K−1 at 260?K and a relative sensitivity (SR) of 3.5%K−1 at 300?K, demonstrating its capability for non-contact remote temperature measurement. Additionally, we capture the dynamic changes of coordinated water with temperature in Rb2InCl5·H2O through temperature-dependent Raman spectroscopy, analyze the resulting structural phase transformation associated with coordinated water, and investigate the reversibility of this phase transformation.

 

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Introduction. Metal halide perovskites have been widely studied for their excellent photoluminescent properties [1,2], which have demonstrated strong application potential in the areas of solar cells [3], light-emitting diode (LED) [4], photoelectric detection [5], fluorescence thermometry [6,7], and fluorescent anti-counterfeiting [8,9]. Among these, the photoluminescence (PL) intensity and lifetime of metal halide materials doped with ns2-metal ion (e.g., Bi3+, Sb3+, and Te4+) generally exhibit strong temperature dependence, which facilitates its applications in optical temperature measurement [10]. In recent years, In/Sc-based metal halides (e.g., Cs2InCl5·H2O, Rb2InBr5·H2O, and Cs2ScCl5·H2O) containing coordinated water molecules have been reported [11]. Doped ns2-metal ion and water-induced phase transformation endow these materials with diverse optical properties, enabling their use in fluorescent anti-counterfeiting, LEDs, and fluorescence thermometry.

However, limited studies exist on temperature-induced structural phase transformation due to coordinated water release in Rb2InCl5·H2O, with no reports on the dynamic changes of coordinated water with temperature as well as the luminescent properties of Te4+-doped Rb2InCl5·H2O. In this work, Te4+-doped Rb2InCl5·H2O powders are fabricated using a simple coprecipitation method. The addition of Te4+ doping induces orange emission in the matrix of non-luminous Rb2InCl5·H2O. In addition, the experimental studies determined the strong temperature dependence of its PL intensity and lifetime, which can be utilized for non-contact remote fluorescence thermometry. Moreover, for the first time, the dynamic process of coordinated water and the temperature-dependent phase transformation of Te4+-doped Rb2InCl5·H2O are being investigated using temperature-dependent Raman spectroscopy. At the same time, the reversibility of this phase transformation is also being analyzed through cooling experiments under vacuum/air conditions.

Results and discussion. Similar to the previously reported Cs2InCl5·H2O single crystal, pure Rb2InCl5·H2O (hereinafter referred to as RICH) belongs to the orthorhombic crystal structure of the Pnma space group. The [InCl5·H2O]2- octahedral unit, formed by coordinating one water molecule, five Cl atoms, and one In atom, is divided by Rb atoms to create the 0D structure [12,13]. As shown in Fig. 1(a), the Te4+ (0.97?Å) and In3+ (0.81?Å) ions, due to their similar ionic radii, are more likely to substitute the In3+ ions with Te4+ ions, forming a new octahedral structure [14]. To determine the purity of the synthesized RICH as well as different Te4+ doping levels, the findings of the powder X-ray diffraction (PXRD) are displayed in Fig. 1(b). It can be seen that the synthesized RICH is in good agreement with the standard diffraction data (PDF#78-1821), and the diffraction peaks are shifted to a lower angle with the increase of the Te4+ feeding ratio, which implicitly suggests the gradual replacement of In3+ by Te4+ [10]. When the Te4+ feeding ratio is 5%, new diffraction peaks begin to appear around 35 and 50°, and the phase is more pronounced at 30% feeding ratio, and the appearance of the Rb2TeCl6 phase can be confirmed by comparing the diffraction data with that of Rb2TeCl6. The Raman spectrum is displayed in Fig. 1(c), where the vibrations of the In-Cl and In-O bonds in pure RICH contribute to the peak at 340?cm−1, while symmetric vibrational stretching in the [InCl5·H2O]2- octahedron accounts for the remaining peaks at 155, 190, and 278?cm−1 [15]. As Te4+ doping increases, Raman spectra show no significant changes at trace doping, but Raman peaks belonging to Rb2TeCl6 appear when the feeding ratio is 5% [16]. In the absorption spectrum of Fig. 1(d), pure RICH has a strong absorption from 200 to 300?nm, after which it belongs to the weak absorption up to 370?nm. Two prominent absorption peaks at around 350?nm are observed upon the introduction of Te4+, and these can be ascribed to the 1S01P1 and 1S03P1 transitions, respectively [17]. When the concentration of Te4+ rises, the absorption band widens and the band edge redshifts, which is similar to that of the Cs2SnCl6 material, and it is noteworthy that the absorption band gradually evolves toward Rb2TeCl6 when the Te4+ feeding ratio is 5% [18]. As shown in Fig. 1(e), the steady-state PL spectra excited at 405?nm laser show that the doping of Te4+ gives the non-luminous RICH powder an orange emission. At trace doping, with a full width at half maximum (FWHM) of roughly 142?nm, the Te4+-doped RICH powder displayed a broad PL emission band at 648?nm, whereas a shift in the spectrum is observed at 5% doping and above, which is attributed to the generation of excessive [TeCl6]2-. Figure 1(f) shows the PLQY values at the feeding ratio from 0.1 to 5.0%, with the highest point occurring at 1% Te4+ doping, with a PLQY value of 12.26% (Fig. S2), after which the main object of study is RICH doped with 1% Te4+ (hereafter referred to as RICHT). The PLQY decrease at higher doping concentrations is due to concentration quenching from strong self-absorption [13].

 figure: Fig. 1.

Fig. 1. (a) Schematic crystal structure of Te4+-doped RICH. (b) PXRD patterns and (c) Raman spectra of RICH: x Te4+. (d) Normalized UV–vis absorption spectra of RICH: x Te4+. (e) Normalized PL spectra and (f) PLQY values (inset: images of RICHT powders under daylight and UV lamp at room temperature) of RICH: x Te4+ under a 405?nm excitation.

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The excitation power-dependent PL spectra of RICHT are experimentally measured as shown in Fig. 2(a). The relationship between emission intensity and excitation power density is revealed in the inset of Fig. 2(a), with a linear fitting slope of 0.91 after logarithmization, confirming single-photon excitation and excluding permanent defect emissions [19]. It is imperative to underscore that the permanent defect emission is commonly associated with broadband emission, and the PL intensity tends to saturate with an increase in the excitation power density. To gain insight into the exciton–phonon interactions of RICHT and its luminescence mechanism, relevant temperature-dependent PL spectroscopy is carried out using a 405?nm laser. In Figs. 2(b) and S5, show that the PL intensity decreases with temperature, accompanied by a blueshift in the peak position. The attenuation of the PL intensity is more significant at temperatures closer to room temperature. This phenomenon is because more phonons are involved in the non-radiative recombination when warming up, leading to a weakening of radiative recombination [20].

 figure: Fig. 2.

Fig. 2. (a) RICHT excitation power-dependent PL spectra (inset: integrated PL intensity as a function of excitation power). (b) Pseudo-color plots of the temperature-dependent PL spectra of RICHT from 80 to 360?K. (c) Normalized integral PL intensity and (d) FWHM of RICHT as a function of reciprocal temperature.

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The calculated Ea value of 406.5?meV, as shown in Fig. 2(c), is attributed to the 0D structure of RICHT, where excitons (electron–hole pairs) are more strongly localized within a small region, which is responsible for this high activation energy and also indicates the production of self-trapped exciton states (STEs) with highly confined behavior [21]. The lattice’s thermal expansion is the cause of the blueshift in the PL peak position [17]. The Huang–Rhys factor (S) and phonon frequency (phonon) indicate the degree of electron–phonon coupling, while the temperature-related changes in FWHM can be represented using this equation [22]:

FWHM(T)=2.36S?ωphononcoth?(?ωphonon2kBT).
The FWHM tends to widen as the temperature rises, typically due to robust interactions between electrons and phonons. The calculated S and phonon values, as displayed in Fig. 2(d), are 16.61 and 35.92?meV, respectively, which are similar to those observed in other metal halides doped with Te4+ [23,24], demonstrating the significant electron–phonon coupling existed in the RICHT. Larger S values indicate strong electron–phonon coupling, which induces small polarons that trap charge carriers and lead to STE formation [25].

 

To confirm the thermometric potential of RICHT and to have a better understanding of its PL emission process, measurements are conducted on the time-resolved PL (TRPL) spectra. The concentration quenching effect causes the PL lifetime to decrease as the Te4+ doping level increases from 0.1 to 5.0% [26], as seen in Figs. 3(a) and S7 (full fitted data are presented in Table S1). The TRPL spectra are shown in Figs. 3(b) and 3(c) for temperatures between 80 and 360?K. The PL lifetime of RICHT reflects a strong temperature dependence, and the PL lifetime reflects a gradual decreasing trend at 80–360?K, which decays from 6.07?μs to 27.74?ns (detailed fitted data are shown in Table S2). While the temperature range of 80–360?K can be divided into two parts, the PL lifetime is maintained in the order of microseconds at 80–240?K, which results from radiative recombination’s supremacy at low temperatures. When the temperature rises from 260 to 360?K, the PL lifetime rapidly decays from the microsecond scale to the nanosecond scale due to the gradual dominance of non-radiative recombination. Figure 3(d) provides a distinct relationship between the PL lifetime and temperature. The PL lifetime of RICHT shows a significant quenching with increasing temperature (especially after room temperature), which provides a theoretical basis for its use in lifetime thermometry of fluorescence. It is important to evaluate the performance of a thermometric material based on absolute sensitivity (SA) and relative sensitivity (SR). The results of the calculations are presented in Fig. 3(e). SA and SR have maximum values of 12 × 10−3?K−1 at 260?K and 3.5%K−1 at 300?K, respectively. The value of SR is higher compared to some other temperature measurement materials (detailed data presented in Table S3). This indicates that RICHT has temperature measurement capabilities and promising applications in non-contact temperature measurement. Additionally, it is worth noting that this is the first comprehensive report on the temperature measurement of RICHT. Based on the spectral data analysis presented above, we propose the physical luminescence mechanism of RICHT as illustrated in Fig. 3(f). Like other ns2 ions, Te4+ possesses a singlet state of 1P1 and a triplet state of 3P1, 2, 3. The two primary absorption peaks of RICHT can be observed through the absorption spectra matching the Te4+ transitions 1S01P1 and 1S03P1. The transition from 1P1 to 1S0 is not allowed, with the electrons at 1P1 first transitioning to the 3P1 state, followed by a transition back down to the 1S0 state by the electrons at 3P1. Under a 405?nm laser irradiation, electrons in the ground state are stimulated to the Te4+ triplet state, and subsequently, they become trapped in the more stable STEs due to the strong electron–phonon coupling [27]. Subsequently, a broad orange emission occurs during the transition from the STEs to the ground state, which exhibits a significant Stokes shift.

 figure: Fig. 3.

Fig. 3. (a) TRPL decay curves of RICH at different Te4+ doping levels. (b) and (c) Temperature-dependent TRPL decay curves of RICHT from 80 to 360?K. (d) Dot plot of RICHT lifetime from 80 to 360?K. (e) Calculated values of SA and SR for RICHT from 80 to 360?K. (f) Schematic diagram of the PL emission mechanism of RICHT.

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In Fig. S11, FTIR spectra confirmed the presence of coordinated water in RICH and RICHT. The H–O stretching vibration (νH–O) is responsible for the broad absorption peaks between 3000 and 3600?cm−1, whereas the H–O bending vibration (δH–O) is responsible for the narrow absorption peaks between 1500 and 1600?cm−1 [28]. In addition, as shown in Figs. 4(a) and 4(b), Raman spectra also capture vibrational peaks belonging to coordinated water at the same wavenumber, reflecting the changes in RICHT’s vibrational peaks with temperature. Raman vibrational peaks (55, 190, 278, and 340?cm−1) in the first section shift toward lower wavenumbers as temperature rises, attributable to anharmonic phonon–phonon interactions, electron–phonon coupling, or temperature-induced lattice expansion [29]. The second part comprises the Raman vibrational peaks of coordinated water, displaying a bimodal peak at low temperatures with intensity decreasing as the peak position shifts to higher wavenumbers with increasing temperature. This phenomenon is attributed to the elongation of the van der Waals bond in coordinated water under warming conditions, and the Coulomb force between two neighboring oxygen atoms compresses the intramolecular polar covalent link (O–H bond), leading to the shift of the O–H Raman vibration to higher wavenumbers [30,31]. Following 420–440?K, Raman peaks belonging to coordinated water show a gradual disappearance, consistent with the TG curve, with a new Raman peak emerging at 304?cm−1, distinct from that observed at room temperature. Evidently, the loss of the coordinated water alters the structure of the original RICHT, leading to the emergence of a new phase. To explore whether this phase is reversible, we heated the RICHT to 480?K and then cooled it in separate environments. In Fig. 4(c), the Raman peak pattern remains unchanged after cooling to room temperature in a vacuum environment, whereas it reverts to the state observed before heating after cooling to room temperature in the air environment. The difference can be attributed to the presence of water in the air environment, which means that the absence of water in the vacuum environment prevented the material from returning to its original state, whereas the opposite is true for the air environment. The RICH also shows the same features in Fig. S12, indicating that trace doping does not affect this phase transformation. In Fig. S13, RICHT generates new phases with rising temperature, mainly consisting of Rb3In2Cl9 as identified through standard diffraction comparison [32]. Heating RICHT to 480?K and cooling it to room temperature in different environments reveals changes in the PXRD consistent with the Raman spectra. This indicates that the Rb3In2Cl9 is observed only upon cooling in the vacuum environment, while the phase transformation of RICHT is reversible exclusively upon cooling in the air environment. Figure 4(d) illustrates the changes in the PL intensity of RICHT after heating to 480?K and cooling in different environments. At room temperature, the PL disappears upon heating to 480?K, yet cooling to room temperature in a vacuum environment does not induce PL, while cooling in an air environment leads to the reappearance of PL. This indirectly indicates that the phase transformation of RICHT with temperature is reversible only in an air environment. Figure 4(e) shows the whole phase transformation of RICHT, which emits orange PL at room temperature and has an SR of 3.5%K−1 at 300?K, qualities that make it suitable for use as a non-contact thermometry material. The PL disappears when heated to 360?K, which results from the Te4+ thermal quenching action. Continuing heating to about 420–440?K, the coordinated water in RICHT disappears to produce an impurity phase transformation with Rb3In2Cl9 as the main body. This temperature-dependent phase transformation is only reversible in an air environment, because only in air is there a replenishment of coordinated water and thus a return of RICHT to its original state. As far as we know, this is the first work detailing the reversible phase transformation belonging to RICHT using temperature-dependent Raman spectroscopy.

 figure: Fig. 4.

Fig. 4. (a) Temperature-dependent Raman spectrum of RICHT and (b) its pseudo-color maps. (c) Raman spectra of RICHT cooled down from 480?K to room temperature in vacuum/air environments. (d) PL spectra from room temperature up to 480?K and cooled to room temperature in different environments. (e) Flow chart of RICHT phase transformation with temperature.

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Conclusions. Te4+-doped Rb2InCl5·H2O shows broadband orange emission at 648?nm under a 405?nm laser excitation. The Te4+-induced STEs responsible for this emission are confirmed through temperature-dependent PL measurements and theoretical calculations. RICHT also demonstrates good relative sensitivity (SR = 3.5%K−1) at 300?K, representing the first detailed report on its thermometric properties. Additionally, the loss of coordinated water and the associated phase transformation are identified via temperature-dependent Raman spectroscopy, revealing a reversible phase transformation process in an air environment. This investigation enhances the understanding of Rb2InCl5·H2O photophysical properties and its environmentally reversible phase transformations, along with similar metal halides.