Near-infrared luminescence from Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho)

Abstract

In this work, ceramic compounds Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho), belonging to the family of germanate olivines, were studied. The structural properties were examined using X-ray diffraction, scanning electron microscopy, IR and Raman spectroscopy. Near-infrared luminescence spectra showed that the bands located in the spectral range of 1400–2100 nm were due to the electronic transitions of lanthanide ions: ⁴I_13/2 → ⁴I_15/2 (Er³⁺), ³H₄ → ³F₄ and ³F₄ → ³H₆ (Tm³⁺), and ⁵I₇ → ⁵I₈ (Ho³⁺). Decay curve analysis indicated that the near-infrared luminescence from the excited states of lanthanide ions in Li₂ZnGeO₄ was relatively long-lived. The results suggest that germanate olivines Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho) are promising inorganic phosphors and can be successfully used as ceramic sources emitting near-infrared radiation.

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Introduction

In recent years, near-infrared (NIR) luminescent materials,^1–5 with particular focus on NIR inorganic persistent phosphors,^6,7 have gained significant attention due to their numerous multifunctional applications. Recently published papers report interesting findings regarding the potential applications of NIR persistent phosphors in the third bio-imaging window (NIR-III). Xu and co-workers^8–10 suggested that NIR long persistent luminescence of erbium ions in inorganic phosphors such as garnet Y₃Al₂Ga₃O₁₂ or perovskite LaAlO₃ can be realized by utilizing efficient energy transfer from Ce³⁺ to Er³⁺ or Cr³⁺ to Er³⁺, respectively. These inorganic phosphors exhibit long (>10 h) NIR persistent luminescence centered at approximately 1550 nm due to the ⁴I_13/2 → ⁴I_15/2 transition of erbium ions, operating in the NIR-III biological window. These aspects are particularly interesting for low-phonon inorganic matrices, especially germanate ceramics, due to their promising optical and dielectric properties.

In general, germanate-based ceramics are very attractive materials for modern visible and near-infrared optoelectronics. Doping with lanthanide ions (Ln³⁺) enables important multifunctional applications of germanate ceramics, including their use as inorganic phosphors in red-green-blue (RGB) technology. Phosphors of ABGe₂O₇ (A = La or Y; B = Al or In) doped with Er³⁺, Tb³⁺, Pr³⁺ or Tm³⁺, emitting red, green or blue light with high color purity, are of interest for displays and lighting devices.^11–14 Eu³⁺-doped germanate ceramics, such as Zn₂GeO₄:Eu³⁺, KNaGaGeO₄:Eu³⁺, and Ba₂AGe₂O₇:Eu³⁺ (A = Mg or Zn), are thermally stable, efficient red-emitting phosphors, which can be successfully applied to high-resolution and high-sensitivity accurate latent fingerprint detection, multi-mode anti-counterfeiting and white-LED applications.^15–17 Phosphors with the general formula A₂B₂GeO₇ (where A = Ca or Sr; B = Al or Ga) doped with Pr³⁺ ions^18,19 and co-doped with Pr³⁺/Yb³⁺ ions²⁰ may be applied for c-Si solar cells, biomedical imaging and dynamic multicolor anti-counterfeiting under UV irradiation. The energy transfer processes in germanate ceramics co-doped with Dy³⁺/Tm³⁺ and Tb³⁺/Eu³⁺ ions^21,22 have been studied in relation to their practical applications as white-light-emitting diodes. Phosphors, such as Zn₂GeO₄,²³ Ca₂Al₂Ge₃O₁₂,²⁴ and La₄GeO₈,²⁵ co-doped with Er³⁺/Yb³⁺ ions have been designed for fingerprint verification, anti-counterfeiting and luminescent thermometry. The influence of Yb³⁺ content on the up-conversion luminescence of Er³⁺ ions in SrGe₄O₉ phosphors has also been examined in detail.²⁶ Further studies revealed that germanate ceramic materials co-doped with transition metal and lanthanide ions are also useful for a wide range of photonic applications, including c-Si solar cells, infrared detectors and long-wavelength NIR LED chips.^27,28

Among the germanate ceramics, special attention has been paid to compounds with an olivine structure. The AYGeO₄ (A = Na or Li) germanate olivines doped with Eu³⁺, Sm³⁺ or Dy³⁺ have been developed for potential applications in solid-state lighting.^29–31 The variation in the up-conversion luminescence of LiYGeO₄:Er³⁺/Yb³⁺ phosphors with temperature has been also investigated for optical thermometers and anti-counterfeiting.³² The results indicate that the modulation of emission color can be realized by changing the laser excitation power, temperature and pumping wavelength. Ceramics with the chemical formula Li₂ZnGeO₄ belong to the same olivine-type germanate family. The energy band gap for Li₂ZnGeO₄ calculated from the Kubelka–Munk equation and Tauc plot method is equal to 5.78 eV.³³ However, the data in the literature indicate that the optical properties of Li₂ZnGeO₄ have been rarely studied. Recently, the published papers mainly focused on the luminescence of Li₂ZnGeO₄ germanate ceramics doped with transition metal ions such as Mn²⁺ and Cr⁴⁺. The experimental results suggest that Mn²⁺-doped Li₂ZnGeO₄ ceramics^34,35 are promising green-emitting phosphors in field-emission displays with high color purity, whereas Cr⁴⁺-doped Li₂ZnGeO₄ ceramics are recommended as efficient broadband NIR-II phosphors.³⁶ Factually, recent work on Ln³⁺-doped germanate olivines was limited to Li₂ZnGeO₄ with Pr³⁺, Nd³⁺ or Gd³⁺, which exhibited enhanced persistent blue luminescence.³⁷ However, to date, the infrared emission properties of Li₂ZnGeO₄:Ln³⁺ have not been studied. Lastly, the emission properties of NaYGeO₄:Tm³⁺ powders in the range of 1300–1600 nm and 1600–2200 nm due to the ³H₄ → ³F₄ and ³F₄ → ³H₆ transitions of Tm³⁺ have been analyzed under 808 nm laser diode excitation.³⁸ At this moment, it should be also pointed out that Nd³⁺/Ho³⁺ co-doped NaLa₉(GeO₄)₆O₂ phosphors with an apatite structure have been proposed for infrared luminescence applications.³⁹

The aim of this work is related to germanate olivines Li₂ZnGeO₄:Ln³⁺ emitting near-infrared radiation in the spectral range of 1400–2100 nm. Lanthanide ions playing an important role as optical dopants were limited to trivalent erbium, thulium and holmium due to their near-infrared luminescence bands located at about 1500 nm (Er³⁺), 1450/1800 nm (Tm³⁺) and 2000 nm (Ho³⁺), respectively. Luminescence bands corresponding to the ⁴I_13/2 → ⁴I_15/2 (Er³⁺), ³H₄ → ³F₄ and ³F₄ → ³H₆ (Tm³⁺), and ⁵I₇ → ⁵I₈ (Ho³⁺) transitions of Ln³⁺ were analyzed. Our studies showed that the Li₂ZnGeO₄:Ln³⁺ germanate olivines are promising inorganic phosphors and attractive candidate for ceramic sources useful in near-infrared optoelectronics. Our intention was to study the infrared emission properties of Ln³⁺-doped germanate olivines for the first time. The unique advantages of this study are the characterization of the near-infrared luminescence of Er³⁺, Tm³⁺ and Ho³⁺ ions in germanate ceramic systems in the range of 1400–2100 nm, which has not been examined and reported before, to the best of our knowledge.

Experimental

Li₂Zn_(1−x)Ln_xGeO₄:xLn³⁺ (Ln³⁺ = Er³⁺, Tm³⁺ or Ho³⁺ with a concentration equal to 0.5 mol%) samples in the form of pellets were prepared using the high-purity initial reagents of ZnO (99.99%), GeO₂ (99.99%), and Li₂CO₃ (99.997%), and the appropriate lanthanide oxides of Er₂O₃ (99.999%), Tm₂O₃ (99.999%) or Ho₂O₃ (99.999%) from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA) via the conventional high-temperature solid-state reaction method. The appropriate amounts of raw materials were milled and homogenized in an agate mortar for 1 h with C₂H₅OH medium (POCH basic 96% pure). The ground samples were calcined in a non-covered Pt crucible at 1100 °C/6 h in an air atmosphere. The calcination process was divided into two steps, i.e., reaching the temperature of 800 °C/0.5 h, and then 1100 °C/10 min. The calcinated samples were ground again and divided into smaller batches. Pellets with a diameter of 10 mm were formed using a binder (PVA) and cold pressed at 375 MPa. Next, the pellets were subjected to heat treatment to remove the binder at 550 °C/2 h and cooled to room temperature. The samples were sintered at 1200 °C/5 h and cooled to room temperature. The sintering process included several steps of heating to 800 °C/1 h, then sintering for 15 min, heating to 1200 °C and sintering for 5 h.

To characterize the undoped and Ln³⁺-doped ceramic samples, several measurements were performed, including XRD, SEM, transmittance, Raman and near-infrared emission spectroscopy. The nature of the studied samples was identified using a diffractometer (X'Pert-Pro, PANalytical, Eindhoven, The Netherlands). X-ray diffraction measurements were performed with CuK_{α1 and 2} radiation. The microstructure of the samples was observed using a JSM6480 scanning electron microscope (SEM) along with JSM-7100F TTL LV (Jeol Ltd., Tokyo, Japan). Transmittance spectra were recorded on a Nicolet™ iS™ 50 IR spectrometer. Raman spectra were measured using a Thermo Scientific™ DXR™2xi Raman imaging microscope. Data were recorded with a 455 nm laser (the power 4 mW on samples). Near-infrared luminescence measurements were carried out using a Photon Technology International (PTI) Quanta-Master 40 (QM40) UV/VIS steady-state spectrofluorometer including a double 200 mm monochromator, an Xe lamp (75 W) as the light source, and Hamamatsu H10330B-75 and InGaAs detectors. The spectra were measured with a resolution of ±0.5 nm. For the decay curve measurements, a pulsed tunable optical parametric oscillator (OPO) pumped by the third harmonic of an Nd:YAG laser (Opotek Opolette 355 LD, Carlsband, CA, USA) was used. The decay curves with an accuracy of ±0.5 μs were recorded using a PTI ASOC-10 USB-2500 oscilloscope (Horiba Instruments).

Absolute photoluminescence quantum yield was measured using a Petite integrating sphere (PTI Horiba Instruments), which is the standard accessory for QM40. It replaces the sample holder, and thus no modification of excitation or emission optics was required. The integration sphere had a powder or pellet holder inside it. The method applied used two-step luminescence measurement, with the excitation set to λ_exc. Firstly, the entire luminescence spectrum, including the scatter of excitation line around λ_exc, was recorded with the “active sample” in the pellet mount. In the second step, the same measurement was executed with the “reference sample”, which was prepared in the same way as the active sample, but without Ln³⁺ ions. This procedure guaranteed that all the conditions for sample and reference runs were as similar as possible. Both spectra were actively recorded with real-time emission correction. A QM40 EXCORR real-time excitation source intensity monitor was used to verify the stability of the Xe lamp source. Dark background subtraction before each run was also applied. All measurement conditions were kept identical in both runs with the exception of a neutral density filter to reduce the intensity of the strong scattered band at λ_exc to keep the intensity of this band in the same order as the luminescence band of interest around λ_em. Then, the attenuation coefficient was carefully measured. The integral intensities of the scatter I(λ_exc) band and luminescence with any possible trace of the scatter at I(λ_em) for the active and reference samples were calculated and subtracted, respectively, yielding a good representation of the number of photons absorbed and emitted by the sample. The ratio of the latter to the former, corrected by the attenuation factor of the ND filter used resulted in the absolute quantum yield of the I(λ_em) emission band.

Results and discussion

The X-ray diffraction patterns of the undoped Li₂ZnGeO₄ and lanthanide-doped Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho) germanate ceramics are presented in Fig. 1. All the diffraction peaks can be assigned to the Li₂ZnGeO₄ ceramic compounds, which crystallize in a monoclinic lattice with the P2₁/n space group (ICDD PDF-4 database – card no. 00-038-1082). It is also well evident that Ln³⁺ doping had no effect on the crystalline structure and no additional phases related to impurities existed compared to the pure Li₂ZnGeO₄ monoclinic phase. This suggests that Ln³⁺ ions well entered Li₂ZnGeO₄ with an olivine structure.

Fig. 1 X-ray diffraction patterns of Li₂ZnGeO₄ and Li₂ZnGeO₄:Ln³⁺.

Further studies using the SEM-BS images (Fig. 2) sensitive to the contrast of the chemical composition confirmed that no other phases, precipitation, or inclusions existed in the undoped and Ln³⁺-doped ceramic samples. Similar to previously published results for Li₂MgGeO₄ doped with Pr³⁺ and Tm³⁺ ions,⁴⁰ the objects shown in the SEM-BS images have an even shade of gray, suggesting that all the ceramic components reacted and only one crystalline phase of Li₂ZnGeO₄ was formed.

Fig. 2 Ceramic particles observed with SEM-BE images: (a) undoped sample and samples doped with (b) Er³⁺, (c) Tm³⁺ and (d) Ho³⁺ ions.

Previous investigations on Li₂ZnGeO₄ ceramics using Raman spectroscopy revealed that bands observed in the low frequency range below 400 cm⁻¹ correspond to the wagging and twisting vibrations of the GeO₄ tetrahedra.^41,42 Fig. 3 shows the Raman spectrum measured in the higher frequency region, which consists of two groups of bands. These two groups are located in the frequency range of 440–550 cm⁻¹ and 650–850 cm⁻¹, respectively. The first group of bands near 442 cm⁻¹, 494 cm⁻¹ and 529 cm⁻¹ is related to the bending vibrations of the GeO₄ groups.³³ In the second group, the three bands located at about 725 cm⁻¹, 752 cm⁻¹ and 820 cm⁻¹ are due to the stretching vibrations of the GeO₄ groups.⁴³ Based on previous results obtained for zinc germanate nanomaterials, Zn₂GeO₄, the Raman bands located at 750 cm⁻¹ and 820 cm⁻¹ were assigned to the symmetric and asymmetric stretching vibration of the Ge–O–Zn bond, respectively.⁴⁴

Fig. 3 Raman spectrum of germanate ceramics Li₂ZnGeO₄.

Numerous near-infrared luminescent materials containing lanthanide and/or transition metal ions have been reported.¹ Among them, low-phonon germanate ceramics seem to be perspective emerging infrared luminescent materials for next-generation optoelectronic devices.⁴⁵ In particular, there has been a significant increase in the search for germanate-based materials emitting infrared radiation in the spectral range of 1400–2100 nm.^46,47 Near-infrared laser sources are an attractive scientific topic due to their various fields of applications such as laser remote chemical sensing, medical surgery and atmospheric monitoring. They can be also applied as the main components in eye-safe laser radars and optical amplifiers. In this spectral region, materials doped with Er³⁺, Tm³⁺ and Ho³⁺ can emit infrared radiation due to their characteristic intraconfigurational 4f–4f electronic transitions. Based on absorption and luminescence spectra measurements, the energy level diagram including main laser transitions at 1450 nm, 1550 nm 1800 nm and 2000 nm was constructed for numerous germanate-based compounds doped with Er³⁺, Tm³⁺ and Ho³⁺, which have been studied systematically by us in detail.^48–54 The near-infrared luminescence bands of the Ln³⁺ ions correspond to the ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transition located at 1550 nm, the ³H₄ → ³F₄ and ³F₄ → ³H₆ (Tm³⁺) transitions near 1450 nm and 1800 nm, and the ⁵I₇ → ⁵I₈ (Ho³⁺) transition at 2000 nm. They are schematically illustrated in the energy level diagram of Ln³⁺ ions shown in Fig. 4.

Fig. 4 Energy level diagram of Ln³⁺ ions, with near-infrared luminescence transitions indicated.

According to transmittance spectrum measurements, it is well known that the infrared band located near 3400 cm⁻¹ is due to the stretching vibrations of hydroxyl groups. The relatively high content of hydroxyl groups effectively quenches the emission from the excited states of Ln³⁺ ions in different compounds. This phenomenon is especially important for Er³⁺,^55–57 Tm³⁺,⁵⁸ and Ho³⁺ ions⁵⁹ in inorganic glasses emitting near-infrared radiation at 1550 nm (6450 cm⁻¹), 1800 nm (5550 cm⁻¹) and 2000 nm (5000 cm⁻¹) because less than two vibrations of OH⁻ groups can effectively bridge the energy gaps between ⁴I_13/2 and ⁴I_15/2 (Er³⁺), ³F₄ and ³H₆ (Tm³⁺), and ⁵I₇ and ⁵I₈ (Ho³⁺), and quench the near-infrared emission. Structural transformation from glasses (completely amorphous) to glass–ceramic materials (partially crystallized) progressively results in a decrease in the number of hydroxyl groups with an increase in heat-treatment temperature.⁶⁰ Further systematic studies suggest that the increase in the photoluminescence intensity of the lanthanide ions with the calcination temperature is attributed to the removal of the hydroxyl groups.⁶¹ Thus, the high-temperature sintering process (1200 °C/5 h) used for the preparation of the Li₂ZnGeO₄:Ln³⁺ germanate ceramics, where Ln denotes Er, Tm or Ho (see Experimental section), can efficiently reduce the OH⁻ content. The results from the transmittance spectrum measurements given in Fig. 5 show that the IR band intensity near 3400 cm⁻¹ is extremely low and nearly invisible compared to our previous results for oxide and oxyfluoride germanate glass systems based on BaO–Ga₂O₃–GeO₂ (BGG), where the IR absorption coefficients, α_OH, are close to 0.25 cm⁻¹ (0% BaF₂) and 0.020 cm⁻¹ (10% BaF₂), respectively.⁵⁰ This suggests that the presence of OH⁻ groups cannot make a substantial contribution to the luminescence quenching from the excited states of Ln³⁺ ions (Ln = Er, Tm, Ho) in the Li₂ZnGeO₄ germanate olivines.

Fig. 5 Transmittance spectrum of germanate ceramics Li₂ZnGeO₄. Inset shows the transmittance bands due to the stretching vibration of OH⁻ groups. For comparison, the results for germanate glass (BGG) are also given.

Initially, the near-infrared emission spectra and decays of Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho) were tested under different excitation wavelengths. The spectra measured for the Ln³⁺-doped samples show near-infrared luminescent bands corresponding to the characteristic electronic transitions of Ln³⁺ ions at 1550 nm (Ln = Er), 1450 nm and 1800 nm (Ln = Tm) and 2000 nm (Ln = Ho), as schematized in the energy level diagram (Fig. 4). These optical effects were not observed for the undoped Li₂ZnGeO₄ sample, which is experimental proof for the presence of lanthanide ions (Er³⁺, Tm³⁺ or Ho³⁺) in the final products. Generally, it is also accepted for inorganic phosphors that each excited state of Ln³⁺ ions further splits under the influence of the crystal field produced by the chemical environment.⁶² Thus, the broad emission bands consist of several Stark components induced by the crystal field and their spectral profiles are usually unsymmetrical. The comparative studies indicated that the infrared emission peaks originating from sub-levels of erbium ions are quite well-resolved and the Stark level structures of multiplets depending on the crystalline field of the inorganic matrices are completely different for the Y₂O₃:Er³⁺, Sc₂O₃:Er³⁺ and YF₃:Er³⁺ systems.⁶³ In the case of Li₂ZnGeO₄:Ln³⁺ (where Ln denotes Er, Tm or Ho), its Stark level structure of multiplets is rather less evidenced and the experimental results are comparable to the NaLaMgWO₆ inorganic phosphors emitting near-infrared radiation from sub-levels of Nd³⁺ and Er³⁺ ions.⁶⁴

The preliminary spectroscopic results for the Er³⁺, Tm³⁺ and Ho³⁺ ions in the Li₂ZnGeO₄ olivines indicate that the profiles of the emission bands and decays from the excited states of Ln³⁺ are nearly independent of the excitation wavelength. This suggests that the Ln³⁺ ions occupy only one site in Li₂ZnGeO₄, in contrast to Y₂SiO₅ (ref. 65) or CaSc₂O₄,^66,67 where two Er³⁺ centers were identified using emission spectroscopy. At this moment, it should be noticed that the atomic positions of Zn²⁺ may be occupied by Ln³⁺. This was observed earlier for similar germanate olivines, Li₂SrGeO₄:RE³⁺, where the positions of the divalent Sr²⁺ were occupied by trivalent lanthanide (Ce³⁺, Tb³⁺, or Dy³⁺) ions.⁶⁸

Fig. 6 presents the near-infrared luminescence spectrum of Li₂ZnGeO₄:Er³⁺ excited at 980 nm line. The characteristic emission band at 1542 nm corresponds to the ⁴I_13/2 → ⁴I_15/2 (Er³⁺) transition. The excitation spectrum for Li₂ZnGeO₄:Er³⁺ measured by monitoring the emission wavelength at 1542 nm is presented in the inset of Fig. 6. The band located at 980 nm corresponds to the transition originating from the ⁴I_15/2 ground state to the ⁴I_11/2 excited state of Er³⁺.

Fig. 6 Near-infrared emission spectrum of Li₂ZnGeO₄:Er³⁺. The inset shows the excitation spectrum under monitoring the emission wavelength at 1542 nm.

Fig. 7 shows the emission decay from the ⁴I_13/2 state of Er³⁺. Previous studies demonstrated that the emission decays from the ⁴I_13/2 state of Er³⁺ in some ceramic compounds can vary greatly with experimental values ranging from microseconds to milliseconds. In fact, the ⁴I_13/2 lifetime of Er³⁺ ions changed from 370 μs for Ca₂SiO₄:Er³⁺ phosphors obtained from agro-food waste materials⁶⁹ to 9.5 ms for Ba(Zr, Mg, Ta)O₃:Er³⁺ perovskite ceramics.⁷⁰ In our case, the decay curve was well fitted using a mono-exponential function given by I(t) = I₀ exp(t/τ_m) and the calculated luminescence lifetime ⁴I_13/2 (Er³⁺) in the Li₂ZnGeO₄ germanate ceramics possessing an olivine structure is nearly 8.27 ms. The Li₂ZnGeO₄:Er³⁺ germanate ceramics exhibit a relatively large ⁴I_13/2 lifetime compared to the values obtained for similar oxide phosphors documented in the literature. For example, the luminescence decay from the ⁴I_13/2 state is considerably longer compared with the values for Er³⁺ ions in ZnO semiconductor quantum dots⁷¹ and germanate compounds.^72–74

Fig. 7 Luminescence decay from the ⁴I_13/2 excited state of Er³⁺ ions.

Fig. 8 presents the near-infrared luminescence spectrum of the Li₂ZnGeO₄:Tm³⁺ germanate olivines under 797 nm excitation. The inset shows the excitation spectrum measured on monitoring the emission wavelength at 1802 nm, where the two characteristic bands are related to the ³F₂,³F₃ → ³H₆ (650–700 nm) and ³H₄ → ³H₆ (797 nm) transitions of the Tm³⁺ ions. The latter transition near 800 nm, i.e. ³H₄ → ³H₆ transition, is often used as the excitation line to examine the up-conversion processes and near-infrared luminescence properties of thulium ions in numerous inorganic compounds.^75,76

Fig. 8 Near-infrared emission spectrum of Li₂ZnGeO₄:Tm³⁺. The inset shows the excitation spectrum under monitoring the emission wavelength at 1802 nm.

Two emission bands were observed for Tm³⁺ in the studied spectral range of 1400–2100 nm under 797 nm excitation. The near-infrared emission bands correspond to the ³H₄ → ³F₄ and ³F₄ → ³H₆ transitions of Tm³⁺. The near-infrared luminescence band near 1450 nm corresponding to the ³H₄ → ³F₄ transition of the Tm³⁺ ions was successfully quenched due to the presence of a cross-relaxation (CR) process between the ³H₄ excited state and ³H₆ ground state. The ³H₄ state is quickly depopulated because the cross-relaxation process between pairs of Tm³⁺: ³H₄ + ³H₆ → ³F₄ + ³F₄ is very efficient.^77–81 Consequently, the intensity of the near-infrared emission band centered at 1802 nm due to the ³F₄ → ³H₆ transition is significantly strong. Moreover, the emission linewidth for the ³F₄ → ³H₆ transition of thulium in Li₂ZnGeO₄:Tm³⁺ defined as the full width at half maximum (FWHM) is relatively broad and equals 145 nm.

Fig. 9 shows the emission decays from the ³H₄ and ³F₄ states of thulium ions in the olivine Li₂ZnGeO₄:Tm³⁺. In both cases, the emission decay curves become nearly mono-exponential. Based on the decays, the luminescence lifetimes for both the ³H₄ and ³F₄ excited states of Tm³⁺ ions were determined. The values of the emission lifetimes are close to 0.2 ms (³H₄) and 2.14 ms (³F₄), respectively. This indicates that the lifetime of Tm³⁺ (0.5 mol%) is nearly 10-fold larger for the ³F₄ state than the ³H₄ state. Interestingly, similar effects were also observed for Tm³⁺ ions in apatite-type germanate phosphors with the chemical formula NaLa₉(GeO₄)₆O₂, where the activator concentration-dependent decay times were larger for the ³F₄ state (133–1313 μs) than the ³H₄ state (19–432 μs).⁸²

Fig. 9 Luminescence decays from the ³H₄ and ³F₄ excited states of Tm³⁺.

Finally, the germanate ceramics with an olivine structure doped with Ho³⁺ ions were investigated. In general, Ho³⁺-doped germanates belonging to the family of low-phonon systems are known as efficient compounds emitting near-infrared radiation at 2000 nm.⁵³ However, the infrared emission properties of Ho³⁺ ions in germanate ceramics have not been studied to date, to the best of our knowledge. Recently, Ho³⁺:Y₂O₃–MgO nanocomposite ceramics have been proposed as new promising IR emitting materials for use in high-power eye-safe laser systems operating in the 2 μm wavelength range.⁸³ Previous works published by Singh et al.^84–86 focused on green light emitting phosphors containing Ho³⁺. Here, the results for Li₂ZnGeO₄:Ho³⁺ emitting IR radiation near 2000 nm are presented and discussed. The near-infrared emission spectrum corresponding to the ⁵I₇ → ⁵I₈ transition of Ho³⁺ ions is presented in Fig. 10.

Fig. 10 Near-infrared emission spectrum of Li₂ZnGeO₄:Ho³⁺. The inset shows the excitation spectrum under monitoring the emission wavelength at 1998 nm.

The emission spectrum centered at 1998 nm was measured under 450 nm excitation. The value of FWHM for the ⁵I₇ → ⁵I₈ transition of Ho³⁺ is equal to 112 nm. The inset presents the excitation spectrum for Li₂ZnGeO₄:Ho³⁺ measured by monitoring the emission wavelength at 1998 nm. The spectrum consists of bands assigned to transitions of Ho³⁺ ions, which originate from the ⁵I₈ ground state to the higher-lying ⁵G_4,5, ⁵G₆, ⁵F_2,3 and ⁵S₂, ⁵F₄ excites states. The intensity of band centered at 450 nm due to the ⁵I₈ → ⁵G₆ transition is considerably higher compared to other bands. Therefore, the 450 nm excitation line is often chosen to measure the near-infrared emission properties of holmium ions.⁸⁷ Fig. 11 shows the nearly mono-exponential luminescence decay from the ⁵I₇ state of holmium ions in the Li₂ZnGeO₄:Ho³⁺ germanate olivine.

Fig. 11 Luminescence decay from the ⁵I₇ excited state of Ho³⁺ ions.

Based on the decay measured from the ⁵I₇ state of holmium in Li₂ZnGeO₄:Ho³⁺, the emission lifetime was calculated and its experimental value is close to 6.98 ms. The lifetime ⁵I₇ (Ho³⁺) in the Li₂ZnGeO₄:Ho³⁺ germanate olivine is comparable with the values obtained for Y₃Al₅O₁₂:Ho³⁺ ceramics (τ = 7.04 ms) produced from laser-ablated nanoparticles⁸⁸ and Yb₃Ga₅O₁₂ nanocrystals with 0.5% Ho³⁺ (τ = 7.77 ms) synthesized via the sol–gel combustion technique using citric acid.⁸⁹

Finally, the photoluminescence quantum yield (PLQY), which is defined as the ratio of the number of emitted photons to the number of absorbed photons, was determined by the absolute method using an integrating sphere.⁹⁰ Among the infrared phosphors, the determination of the absolute photoluminescence quantum yield is mainly limited to erbium-doped compounds emitting radiation near 1500 nm under 980 excitation. For example, literature data indicates that the PLQY values changed significantly from 3.6% (in water) for poly(acrylic acid) (PAA)-modified NaLnF₄:40Gd/20Yb/2Er nanorods⁹¹ up to even 18.7% for NaErF₄@NaYbF₄@NaYF₄ nanoparticles,⁹² 32.8% for NaCeF₄:Er/Yb nanocrystals,⁹³ and 35.74% for Ce³⁺/Er³⁺:LiYbF₄ nanocrystals.⁹⁴ Further luminescent studies on up-converted PbF₂:Er³⁺/Yb³⁺ crystals fabricated using the Bridgman method well demonstrated that the PLQY values varied from 2.7% to 4.5%, depending on the activator (Er³⁺/Yb³⁺) content.⁹⁵ In our case, the absolute quantum yield of NIR luminescence near 1550 nm (excited at 988 nm) for the Li₂ZnGeO₄:Er³⁺ sample in the powder pellet form is close to 5.9% and its value is comparable to the experimental results (PLQY = 6%) obtained for core–shell nanostructures with homogeneous core (α-NaYF₄:Yb³⁺/Er³⁺) and shell (α-NaYF₄) domains.⁹⁶ However, the latter PLQY value increased to 30% in the case of α-NaYF₄:Yb³⁺/Er³⁺ core–shell structures with heterogeneous (CaF₂) shell domains. A similar situation was observed for the same α-NaYF₄:Yb³⁺/Er³⁺ core–shell systems with Ce³⁺ co-doping. The structural transformation from homogeneous (α-NaYF₄) to heterogeneous (CaF₂) shell domains strongly influenced the PLQY values determined for α-NaYF₄:Yb³⁺/Er³⁺/Ce³⁺ core nanocrystals. Thus, the absolute quantum yield increased drastically from 24% up to 50% at 60 mW cm⁻²; one of the highest reported PLQY values to date.⁹⁶

In summary, we can conclude that the near-infrared luminescence of lanthanides (Er³⁺, Tm³⁺, Ho³⁺) in the studied Li₂ZnGeO₄ olivines is quite long-lived. Our experimental studies based on near-infrared luminescence spectra and their decays indicate that the Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho) germanate olivines are promising inorganic phosphors and can be successfully used as ceramic sources emitting near-infrared radiation.

Conclusion

The near-infrared luminescence properties of Li₂ZnGeO₄:Ln³⁺ (Ln = Er, Tm, Ho) ceramic phosphors belonging to the family of germanate olivines were investigated for the first time. Their spectra showed near-infrared emission bands centered at 1550 nm, 1450 nm, 1800 nm and 2000 nm, corresponding to ⁴I_13/2 → ⁴I_15/2 (Er³⁺), ³H₄ → ³F₄ and ³F₄ → ³H₆ (Tm³⁺), as well as ⁵I₇ → ⁵I₈ (Ho³⁺) transitions, respectively. The decay curve analysis demonstrated that the lifetimes for the excited states of Ln³⁺ (Ln = Er, Tm, Ho) are relatively long. The experimental results showed that the Li₂ZnGeO₄:Ln³⁺ germanate olivines are attractive inorganic phosphors and can be applied as ceramic sources emitting near-infrared radiation.

Data availability

The data are available upon request from the authors.

Author contributions

Conceptualization, project administration, resources and funding acquisition: Joanna Pisarska. Investigation and validation: Nikola Bednarska-Adam prepared the ceramic samples. Nikola Bednarska-Adam and Marta Kuwik performed spectroscopic and optical experiments. Tomasz Goryczka performed the XRD and SEM experiments. Vitalii Ivanov and Józef Dresner performed the absolute photoluminescence quantum yield test. Methodology: Joanna Pisarska and Wojciech A. Pisarski. Formal analysis and supervision: Wojciech A. Pisarski. Writing – original draft: Joanna Pisarska. Writing – review & editing: Joanna Pisarska.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The National Science Centre (Poland) supported this work under project no. 2019/35/B/ST5/01924. The publication was co-financed by the funds granted under the Research Excellence Initiative of the University of Silesia in Katowice.

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