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. 2024 Aug 2;146(32):22807–22817. doi: 10.1021/jacs.4c08011

Photoelectric Studies as the Key to Understanding the Nonradiative Processes in Chromium Activated NIR Materials

Natalia Majewska Ψ, Mu-Huai Fang †, Sebastian Mahlik Ψ,*
PMCID: PMC11328124  PMID: 39091220

Abstract

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In this study, we synthesized a series of Ga1.98–xInxO3:0.02Cr3+ materials with varying x values from 0.0 to 1.0, focusing on their broadband near-infrared emission and photoelectric properties. Interestingly, photocurrent excitation spectra exhibited behavior consistent with the absorption spectra, indicating the promotion of carriers into the band structure by the 4T1, and 4T2 states of Cr3+ ions. This association suggests that photocurrent in this material is influenced not only by valence to conduction band transitions but also by transitions involving Cr3+ dopants. Our investigation of luminescence quenching mechanisms revealed that nonradiative processes were not directly linked to thermally induced relaxation from the excited state 4T2 to the ground state 4A2, as usually suggested in the literature for this type of material. Instead, we linked it to the thermal ionization of Cr3+ ions. Unexpectedly, this process is unrelated to the transfer of electrons from Cr3+ impurities to the conduction band but is associated with the formation of holes in the valence band. This study provided novel evidence of luminescence quenching via the hole-type thermal quenching process in Cr3+-doped oxides, suggesting potential applicability to other transition metal ions and host materials. Finally, we demonstrated the dual-purpose nature of Ga1.98–xInxO3:0.02Cr3+ as a practical emitter for NIR-pc-LEDs and effective photocurrent for UV detectors. This versatility underscores these materials’ practicality and broad application potential in optoelectronic devices designed for near-infrared and ultraviolet applications.

Introduction

The utilization of Cr3+-activated optical materials, which exhibit efficient emission within the visible and near-infrared (NIR) spectrum, has garnered significant interest due to emerging demands in fields such as biological imaging, freshness analysis, luminescence thermometry and luminescence manometry exploiting their NIR luminescence capabilities.1−7 Particularly, these materials have found practical application in NIR phosphor-converted light-emitting diodes (pc-LEDs), representing a new frontier in NIR light sources owing to their cost-effectiveness, superior efficiency, and compact form factor poised to replace conventional tungsten halogen lamps.8 The pc-LEDs leverage high-power blue LED chips combined with inorganic phosphors to achieve targeted NIR emissions.

Despite the extensive research on luminescent materials activated by Cr3+ ions documented in the literature, certain fundamental luminescence properties remain inadequately elucidated. Among these is thermal luminescence quenching, a phenomenon observed in all luminescent materials at sufficiently high temperatures, suppressing luminescence. While the radiative processes are well understood, the nonradiative processes and their temperature dependencies, not only for Cr3+ ions but also other transition metal ions, are not comprehensively explained. Addressing these nonradiative relaxation processes is pivotal in engineering materials with tailored properties for potential phosphor applications.

According to the models proposed by Struck and Fonger, the activation energy (Enr) represents the energy required to transition between excited and ground electronic states by the intersystem relaxation.9 Despite the widespread acceptance of this approach, these authors have demonstrated that it does not offer an accurate quantitative description of nonradiative quenching processes in materials like Cr3+ in ruby and emerald.10 The activation energy values observed experimentally for luminescence quenching of transition metal ions such as Cr3+ and Mn4+ are notably lower by an order of magnitude than predictions from the intersystem relaxation model. Grinberg and Lesniewski have systematically shown that the activation energy for nonradiative transitions (Enr) for Mn4+ ions (which are isoelectronic with Cr3+) is significantly smaller than theoretically predicted.11 They attribute this discrepancy to the interaction with phonons in a multidimensional configurational space, leading to increased initial state degeneracy. In this context, the final state could be any electronic manifold facilitating nonradiative relaxation, such as an electron trap, impurity-trapped exciton, or conduction band edge. A similar mechanism can be found also in materials activated by Cr3+ ions.12,13 In certain studies, the activation energy is interpreted as crossing between the 2E and 4A2 levels, despite both belonging to the same electronic configuration and not undergoing such a crossing at all.14−17 This highlights the complexity and nuances involved in accurately describing the nonradiative processes governing the luminescent properties of transition metal-activated materials.

Another phenomenon contributing to the reduction of luminescence intensities is autoionization, where an electron in an excited state, influenced by phonon interactions, transitions thermally to the conduction band (CB). In this context, the opposite situation can also be considered, namely the transfer of an electron from the valence band (VB) to the Cr state, causing the formation of intermediate states of the charge transfer (CT) state, known as intervalence charge transfer (IVCT)18 or impurity trapped exciton (ITE) state.19,20 Instead of electron ionization to the CB, quenching can occur by hole ionization to the VB. These states can significantly affect radiative processes, repeatedly shown in materials doped with, for example, Eu3+ ions.21,22

The resulting electron in the CB or holes in VB contributes to current carriers, which can be studied through photocurrent measurements. By examining the temperature dependence of photocurrent intensity, we can determine the activation energy of the ionization process. Comparing this with temperature-dependent photoluminescence intensity allows us to establish a link between ionization and luminescence quenching.

Studies on temperature-dependent photoconductivity in dielectric materials activated by transition metal ions, providing insights into luminescence quenching phenomena, remain largely absent in the literature. Some photoconductivity investigations have been conducted on lanthanide ions, particularly Pr3+23−25 and Ce3+ ions,26−29 in various matrices. For instance, Tanabe and Ueda observed photoconductivity excitation in Ce3+-doped garnets like Y3Al2Ga3O12 and Y3Ga5O12, revealing luminescence quenching attributed to thermally activated ionization from the 5d excited level, as evidenced by photocurrent excitation spectra.26,27,30 They noted that the photocurrent excitation spectrum matches the photoluminescence excitation spectrum at room temperature, indicating that the 5d1 (lowest excited state) and 5d2 (higher excited state) states promote carriers into the CB. Concurrently, they observed a decrease in photoluminescence intensity alongside a monotonically increasing photocurrent with rising temperature, demonstrating the role of thermal ionization in the quenching process of Ce3+. Similar behavior was observed in other series of Ce3+-doped garnets, like Y3Sc2Al3–xGaxO12,29 as well as Gd3Al2Ga3O12 and Gd3Ga5O12.28 While some studies have reported photoconductivity excitation spectra for Cr3+-doped LiNbO3 materials, which demonstrate photoconductivity associated with Cr3+ impurities, these investigations have not yet explored the implications of these findings in relation to luminescence quenching.31−33

The Ga2O3 matrix was chosen as a model sample for investigation for several compelling reasons. First, this material, when codoped with Cr3+, exhibits efficient near-infrared luminescence, making it a subject of widespread study as a near-infrared emitter for NIR phosphor-converted light-emitting diodes (pc-LEDs), persistent luminescence phosphors, and optical thermometers. Additionally, Ga2O3 demonstrates higher current conductivity and photocurrent in the UV region, making it a key candidate for UV detectors, which show numerous applications in space communication, missile and flame warning, or ozone hole detection.34,35

Modifying this material with In3+ ions allows us to tune the luminescence properties of Cr3+ for more suitable performance in NIR-pc-LEDs applications.36 Furthermore, this modification can help narrow the band gap, potentially leading to a more significant enhancement of carriers related to the Cr3+ ions.36,37 This approach holds promise for optimizing the material’s performance and expanding its applicability in optoelectronic devices.

Our research highlights the interplay between photoconductivity and luminescence properties in Cr3+-doped materials. Understanding the luminescence quenching mechanisms of materials doped with transition metal ions is crucial for optimizing their luminescence properties for practical applications. This study investigates the luminescence properties of the Ga1.98–xInxO3:0.02Cr3+ (GIOC) solid solution, focusing on its optical and photoelectric characteristics, including the influence of temperature on luminescence and photocurrent. We also demonstrate the potential of Ga1.78In0.2O3:0.02Cr3+ as a dual-purpose material, as a potential emitter for NIR-pc-LEDs and UV detectors. This dual functionality showcases the versatility and applicability of this material in optoelectronic devices for both near-infrared and ultraviolet applications. Through this investigation, we aim to pave the way for further advancements in developing efficient and multifunctional materials.

Results

Structural Analysis

To confirm that our experiment successfully synthesized Ga1.98–xInxO3:0.02Cr3+ (GIOC) with x = 0.0–1.0, the in-house X-ray diffraction is examined, as shown in Figure S1. The position of the diffraction peak of GIOC fits well with the Ga2O3 standard pattern converted through the crystallographic information framework (CIF) in inorganic crystal structure database (ICSD) with the number of ICSD-34243.38 Besides, the diffraction peaks shift toward the lower angles due to the larger ionic radii of In3+ (0.80 Å; CN = 6) (CN represents the coordinated number) than that of Ga3+ (0.62 Å; CN = 6).39 However, due to the influence of Kα2 radiation and the relatively broader peak width of the in-house XRD instrument, the XRD peaks with closed position cannot be well resolved, as denoted by the pound sign in Figure S1. Furthermore, some peaks are connected, hindering the judgment of the contribution from different phases, especially for the x = 1.0 sample, as denoted with the asterisk in Figure S1. To address the aforementioned problem, the high-resolution synchrotron XRD patterns of GIOC (x = 0.0–1.0) are characterized, as shown in Figure 1a. One can observe that the two diffraction peaks with close positions at 12.6321° and 12.6338° gradually split into two independent peaks. Furthermore, the peaks of x = 1.0 at around 12.3° indicate the existence of the two phases, Ga2O3-phase and In2O3-phase, proving the benefit of high-resolution synchrotron X-ray diffraction in determining the structural properties. The Ga2O3 in this study belongs to β-Ga2O3, possessing a monoclinic structure with a space group of C2/m, as depicted in Figure 1b. There are two Ga sites, Ga1 and Ga2, in the structure of Ga2O3. Ga1 is coordinated by four O2– ions, forming a [GaO4] tetrahedron, and Ga2 is coordinated by six O2– ions, consisting of a [GaO6] octahedron. To further ascertain the structural evolution after incorporating In3+ ions, the Rietveld refinement of GIOC (x = 0.0–1.0) is conducted, as shown in Figure S2. The atomic positions, occupancies, atomic displacement parameters, and refined parameters are listed in Tables S1–S7. The results indicate that the In3+ ions did not distribute evenly at the Ga1 and Ga2 sites. Instead, In3+ ions show highly preferred occupation at the Ga2 sites. Only when Ga2 is close to fully occupied by In3+ ions (x = 1.0), In3+ ions will start to incorporate into the Ga1 sites. Besides, the lattice parameters of a, b, c and the volume of the unit cell gradually increase with the increase of In3+ ions. In contrast, the β angles of the unit cell are reduced by incorporating In3+ ions. Moreover, the XRD pattern of x = 1.0 sample can be attributed to the combination of Ga2O3 phase doped with In and In2O3 phase doped with Ga, namely Ga2–yInyO3 (monoclinic, C2/m) and In2–zGazO3 (cubic, Ia3̅), respectively.40 The weight ratio between the Ga2O3 and In2O3 for x = 1.0 sample is around 75%:25%. These results indicate we can only obtain the GIOC in a single phase for x = 0.0–0.8. Accordingly, we will use the GIOC (x = 0.0–0.8) for the following analysis.

Figure 1.

Figure 1

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(a) Synchrotron XRD patterns of GIOC (x = 0.0–1.0). (b) Crystal structure of Ga2O3 depicted based on the ICSD-34243. (c) Preferred occupation of In3+ at Ga1 and Ga2 sites. (d) Lattice parameters of GIOC (x = 0.0–1.0).

Room Temperature Photoluminescence Analysis and Energy Gap Determination

To examine the fundamental optical and photoelectrical properties, room temperature (RT) absorption (ABS), photoluminescence excitation (PLE) and photoluminescence (PL) spectra of GIOC for x = 0.0–0.8 are shown in Figure 2. The ABS spectra reveal one overlapping band in the ultraviolet (UV) and two distinct bands in the visible (VIS) region. The absorption spectra of the undoped samples Ga2–xInxO3 (GIO, crystal matrix) are depicted in Figure 2a with dashed lines, showing a single band in the UV region. This indicates that the UV bands observed in GIO and GIOC are associated with band-to-band (VB → CB) absorption with the energy related to the band gap (Eg). In contrast, the lower energy bands observed solely in GIOC are attributed to Cr3+ dopants. Specifically, the 300 nm band for x = 0.0 could be attributed to the charge transfer transition (CTT).41 The ∼450 nm and ∼620 nm bands correspond to the 4A2 → 4T1 and 4A2 → 4T2 transitions of Cr3+ ions in a 6-fold octahedral coordination. All bands exhibit a wavelength red shift (shift toward lower energies) with increasing x. This red shift is attributed to the decreased crystal field strength (Dq) around Cr3+ ions due to substituting larger In3+ ions for Ga3+ ions, increasing in lattice volume, as evidenced by XRD studies in Figure 1. The decrease in Dq value lowers the energy levels of the 4T1 and 4T2 states relative to the 4A2 ground state.

Figure 2.

Figure 2

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RT (a) ABS spectra of GIOC samples and GIO matrix, (b) PLE spectra upon observation at maximum of luminescence, (c) PL spectra upon excitation at 442 nm. (d) Optical band gap determination for direct band gap. (e) Schematic diagram of energy levels.

Upon observation at the maximum luminescence of GIOC (740–820 nm), the PLE spectra reveal three excitation bands of Cr3+ ions, consistent with the ABS spectra (Figure 2b). Furthermore, the lower energy excited band associated with the 4A2 → 4T2 transition exhibits significant broadening with increasing x. This broadening can be attributed to the increased disorder or variability in the local crystal field environment surrounding the Cr3+ ions as the concentration of x increases.

For x = 0.0, both narrow-line and broadband emissions of Cr3+ are observed simultaneously (Figure 1c). The narrow-line emission around 690 nm corresponds to the 2E → 4A2 transition (R1 and R2 lines), accompanied by a weak phonon structure. The presence of the line emission suggests that the x = 0.0 sample exhibits a strong crystal field environment. The line emission is visible only for samples with low In3+ concentration, specifically x = 0.0 and 0.2. The broadband emission from 650 to 950 nm corresponds to the transition from the 4T2 excited to the 4A2 ground state. Co-doping with In3+ induces a red shift in the emission spectra related to the 4T2 → 4A2 transition due to the decreasing Dq value.

At RT, the full width at half-maximum (fwhm) for x = 0.0 is 2064 cm–1. Upon incorporating In3+, the fwhm increases significantly to 2351 cm–1 for x = 0.2 and then slightly increases up to 2385 cm–1 for x = 0.8. Similarly, Sℏω values, which describe the energy of electron lattice relaxation, significantly increase from 1415 cm–1 (x = 0.0) to 1639 cm–1 (x = 0.2) and then slightly increase up to 1654 cm–1 for x = 0.8. As expected, the crystal field strength (Dq) decreases with increasing x value, from 1647 to 1490 cm–1. The increasing fwhm and Sℏω values are attributed to the enhanced disorder in the environment surrounding the Cr3+ centers, induced by the incorporation of In3+ into the crystal lattice. Detailed analysis of these parameters is provided in the Supporting Information (SI) in Table S8.

Figure S3a displays the RT concentration-dependent decay profiles upon excitation at 440 nm. The decay profile is single-exponential for the sample without In3+ dopants (x = 0.0), whereas In3+-doped samples exhibit increasingly multiexponential decay behavior, due to aforementioned variations of the local surroundings. The average decay time (τav) for all studied materials was calculated using the following eq 1:

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where I(t) is the luminescence decay profile. The calculated average decay times of Cr3+ ions are illustrated in Figure S3b. The incorporation of In3+ ions into the crystal structure of Ga2O3:Cr3+ results in an extended crystal lattice, which reduces the crystal field strength around the Cr3+ ion. Consequently, as the x increases, there is an increase in the thermal occupation of the 4T2 state, resulting in two observable effects: increasing prominence of the 4T2 → 4A2 broadband emission and shortening of the emission decay time. This latter effect is demonstrated in the RT luminescence decay time study shown in Figure S3a and S3b.

The optical band gap energy (Eg) of the studied samples can be estimated for each sample from the diffuse reflectance spectra (Figure 2d) using the following eq 2:42

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where hv, R, Eg, and A refer to the photon energy, reflectance, band gap, and proportionality constant, respectively. The value of the exponent n in the equation used to estimate the nature of the electronic band and transition type determines the optical band gap. For materials with direct band gaps, n typically has a value of 1/2, while for those with indirect band gaps, n is 2.

In β-Ga2O3, there is an indirect band gap, with the direct band gap only slightly larger by approximately 0.04 eV. The direct transitions have a higher probability, so we assume that these transitions are observed in the absorption spectra. Therefore, we used n = 1/2 for the direct band gap calculations,43 resulting in the Eg = 4.60 eV for x = 0.0, which are slightly smaller than those reported in refs (36 and 44−46), Eg = 4.69–4.8 eV. The band gap gradually narrows with increasing x, with determined values of 4.36, 3.15, 3.98, and 3.82 eV for GIO (x = 0.2–0.8), respectively (Figure 2d). As demonstrated by He et al., the band gap of β-In2zGa2(1–z)O3 decreases linearly with increasing In3+ concentration, attributed to the rising valence-band maximum and the lowering conduction-band minimum.37 The band gap values estimated for the indirect transitions are shown in Figure S3c. These values are significantly lower than those reported in the literature.36,44−46Figure 2e shows the schematic diagram of energy levels showing the decrease of the Eg, and the Cr3+ dopant states. The energy levels of the Cr3+ dopants were taken from the ABS/PLE spectra, and energy equal to 0 was fixed at the edge of the VB.

Room Temperature Photocurrent Analysis

Figure 3a illustrates the schematic diagram of the photocurrent excitation measurement system used in the study. The setup begins with a xenon lamp emitting a broad-spectrum light beam that passes through a grating monochromator, which selects a specific 10 nm fwhm section of the lamp spectrum. This monochromatic beam is then directed through a focusing lens onto the sample, where two gold electrodes, separated by approximately 200 μm, are positioned (Figure 3b). The size of the light spot from the monochromator determines the separation distance between the gold electrodes. The gold electrodes, shaped like semicircles and deposited through a shadow mask using magnetron sputtering, have a thickness of approximately 100 nm. Due to the persistent photocurrent observed in the studied material and the associated charging and discharging processes,47−49 a lock-in amplifier combined with chopper modulation is employed to isolate the photocurrent signal from background noise. This setup allows for accurate measurement of the photocurrent response.

Figure 3.

Figure 3

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(a) Schematic photocurrent excitation measurement system. (b) Schematic view of the sample preparation. RT PCE spectra of (c) GIOC samples and (d) GIOC samples compared with GIO matrix as a reference. (e) Comparison of PCE and ABS of GIOC x = 0.2 and 0.8.

Figure 3c displays the normalized photocurrent excitation (PCE) spectra of GIOC for x = 0.0–0.8. The applied voltage was 500, 300, 200, 20, and 10 V, for x = 0.0–0.8, respectively. These applied voltages are adjusted to optimize measurement conditions and ensure the dark current in the pA range across the sample series. For x = 0.0, a prominent band is observed in the UV region at 300 nm in the PCE spectra. Additionally, less intense bands in the visible (VIS) range are observed at 440 and 610 nm. As x increases, the UV band red shifts. Similarly, the VIS bands also red shift with increasing x. However, for x ≥ 0.4, the spectra exhibit changes where an additional band at 500 nm appears. Further temperature dependence studies reveal that this additional band, appearing for samples x ≥ 0.4, covers the observed two VIS bands, still clearly resolved at lower temperatures. Comparatively, the PCE spectra for the GIO (undoped matrix) are shown in Figure 3d (dashed line) alongside the PCE of GIOC. Notably, the band at ∼300–400 nm observed in GIOC is absent in the PCE spectra of the GIO matrix. This indicates that the PCE in the GIOC samples is associated with band-to-band transitions within the matrix together with transitions involving Cr3+ dopants. The UV band in the PCE spectra consists mainly of the band-to-band transition (Eg) at 260 nm and the CTT from the VB to Cr3+ state (as further analyses prove) at 300 nm for x = 0.0 in GIOC. The Eg and CCT bands red shift with increasing x. For x = 0.8, the CCT band decreases significantly, and the Eg band dominates the PCE spectra. Comparison of the ABS and PCE spectra of GIOC for x = 0.2 and 0.6 (Figure 3e) indicates that the PCE bands in the visible range align with the ABS bands. Both ABS and PLE (photoluminescence excitation) spectra of x = 0.6 show red shifts compared to x = 0.2. Furthermore, the lower energy band around 650 nm is broadened for both PCE and ABS spectra. Notably, these bands are not observed in samples without Cr3+ (GIO matrix), highlighting their association with the 4A2 → 4T1 and 4A2 → 4T2 transitions of Cr3+. It should be noted this study represents the first observation of photocurrent related to the Cr3+ doped dielectric lattice.

Temperature-Dependent Photoluminescence and Photocurrent Analysis

PL and PCE measurements were conducted while varying the temperature to enhance understanding of radiative and nonradiative processes, focusing specifically on samples x = 0.0, 0.2, and 0.6. Figure 4a shows the temperature-dependent PL spectra in the temperature range of 77–550 K for x = 0.2 and 0.6 upon excitation at 455 nm. The temperature-dependent PL spectra of x = 0.0 are shown in Figure S4. Given the widely reported temperature behavior of emission spectra in Ga2O3:Cr3+ systems, we will not focus on the temperature-induced changes in emission spectra for the x = 0.0 sample.50−52 However, it is essential to note that the broadband 4T2 → 4A2 emission, extending from 650 to 1000 nm, is observed for all samples in the tested temperature range, with the emission broadening observed with increasing temperature. In the case of the x = 0.2 sample, additional emission lines around 700 nm related to the 2E → 4A2 transition are also observed. This can be attributed to the distribution of Cr3+ local environments and the crystal field strength, which, in the case of the x = 0.2 sample, spans across the crossing point of the 2E and 4T2 states for luminescent centers. This phenomenon has been described in similar materials in refs (53 and 54).

Figure 4.

Figure 4

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Temperature-dependent (a) emission spectra of GIOC, x = 0.2, and 0.6, and (b) integrated PL intensity of GIOC, x = 0.0, 0.2, and 0.6 sample. (c) Configuration diagram of GIOC x = 0.0, x = 0.2, and x = 0.6. The temperature dependence of PCE spectra of GIOC (d) x = 0.0, (e) x = 0.2, and (f) x = 0.6. Comparison of integrated PL and PCE intensities of (g) x = 0.0, (h) x = 0.2, and (i) x = 0.6.

Figure 4b illustrates the temperature-dependent emission intensity of Cr3+ for samples x = 0.0, 0.2, and 0.6. In the case of the x = 0.0 sample, the material exhibits excellent thermal stability, with only a gentle decrease in emission intensity of about 10% observed up to 400 K. However, a significant reduction in emission intensity occurs at higher temperatures. For samples doped with In3+, the considerable decrease in emission intensity starts at lower temperatures. Specifically, for x = 0.2, the emission intensity decreases significantly around 250 K, while for x = 0.6, the decrease starts at 150 K. The experimental data on the temperature dependence of Cr3+ emission intensity can be adequately fitted using a formula (3) for two deexcitation processes, reflecting the complex thermal behavior observed in these materials.

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where I0 is the PL intensity at 77 K, EA,1, EA,2 are the activation energy, and A1 and A2 are the relative probability of the nonradiative deexcitation processes. The fitting curves represented by solid lines in Figure 4b correspond to the obtained parameters listed in Table 1.

Table 1. Parameters Obtained from Fitting the Temperature-Dependent Integrated Emission Intensity.

  x = 0.0 x = 0.2 x = 0.6
EA,1(cm–1) 450 ± 10 930 ± 50 760 ± 9
A1 0.5 ± 0.2 9 ± 2 43 ± 3
EA,2(cm–1) 5460 ± 30 3270 ± 80 2020 ± 20
A2 (7.3 ± 5.4) × 106 (3.3 ± 0.7) × 104 (1.2 ± 0.1) × 104
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The activation energy EA,1 is relatively low; moreover, the probability of this process is negligible, as evidenced by the A1 parameter, which is several orders of magnitude lower than the A2 parameter describing the following quenching mechanism. This process may be related to the tunneling effect to the ground state 4A2 from the excited 4T2 after excitation to the excited vibronic states in Cr3+ centers.

The activation energy EA,2 corresponds to the nonradiative thermal quenching of Cr3+ luminescence, which can be attributed to three primary mechanisms:

  • (i)

    Photoionization: This mechanism involves the transfer of the excited electron from the 4T2 state of Cr3+ to the CB due to thermal energy.

  • (ii)

    Crossover mechanism: In this process, thermal activation energy associated with the intersection of the excited and ground states allows electrons to relax directly from the 4T2 to the 4A2 ground state.

  • (iii)

    Electron transfer from the 4T2 state of Cr3+ to the CT(ITE) state (Cr3++e).

The Stokes shift between excitation and emission spectra allowed us to construct one-dimensional configurational coordinate diagrams, which consist of the ground state (represented by a black parabola) and excited states (represented by colored parabolas) of Cr3+ for x = 0.0, 0.2, and 0.6 (Figure 4c). By analyzing these configuration diagrams, we can conclude that the nonradiative processes are not solely attributed to the thermally induced nonradiative relaxation process directly from the excited state to the ground state (4T2 → 4A2 transition). The intersections of the ground and excited state parabolas occur at much higher energies for Cr3+ (∼20 000 cm–1), suggesting that additional processes or energy levels are involved in the nonradiative relaxation pathways of the Cr3+ ions. For further insights into the temperature dependence of luminescence kinetics, refer to the discussion in the SI, specifically Figure S5.

To assess whether the ionization process may cause luminescence quenching, we analyzed the temperature dependence of PCE spectra for GIOC samples x = 0.0, 0.2, and 0.6, Figure 4d, e, and f, respectively, covering a temperature range of 50–300 K. Insets in the figures show a smaller PCE range to highlight less intense bands in the visible range. Notably, for x = 0.6, the inset is shown in the log scale due to the stronger temperature dependence observed in this case. As depicted in Figure 4d, e, and f, the overall PCE increases with rising temperature. Visible range bands related to the 4A2 → 4T1 and 4A2 → 4T2 transitions of Cr3+ are also observed at low temperatures. Integrated PCE intensities are presented in Figure 4f on a log scale, revealing a significant increase for the x = 0.6 sample compared to x = 0.0 and 0.2.

Comparing the integrated PCE intensity with PL intensity for samples x = 0.0, 0.2, and 0.6 (Figures 4g, h, i, respectively), we note a slight decrease in PL intensity up to 300 K for x = 0.0 and 0.2, accompanied by a slight increase in PCE intensity. Conversely, for x = 0.6, a significant decrease in PL intensity aligns with a growth of photocurrent. This suggests that the decreasing PL intensity is related to the promotion of carriers to the bands. These results prove that luminescence quenching in these materials is linked to ionization. The observed photocurrent in which the Cr3+ excitation bands are visible even at low temperatures would suggest that the 4T2 state lies very close to the CB edge, allowing for the thermal activation of electrons from 4T2 to the CB. In such a case, the higher excited state 4T1 of Cr3+ ions would degenerate with the CB. This means that excitation to higher states would give a higher photocurrent, and other temperature dependences of the photocurrent would be expected when excited to the 4T2 and 4T1. No such effect was observed; moreover, considering previous papers showed significant distance between the 4T2 state and CB edge, this mechanism of thermal excitation of electrons from Cr3+ into the CB should be excluded.36 The observed photocurrent should therefore be considered as the transport of electric charges in VB namely a hole current.

Model

The kinetics of the processes occurring in the tested systems are presented in Figure 5a. Energy zero was set at the basic level of the Cr3+ ion. The Eg defines the width of the energy band gap that changes for the sample series as presented in Figure 2. Material x = 0.2 was chosen as an example; however, the process described applies to all materials tested. Selected levels of the Cr3+ ion and the CT state (in which the additional electron is trapped on the Cr3+ ion) are marked in the energy gap. The position of the Cr3+ states in relation to the band edges was taken from ref (36). When the Cr ion captures electrons (Cr3+ + e), strong lattice relaxation (LR) occurs, which is illustrated by lowering the energy of the CT state in the diagrams. This situation is better illustrated by the configuration diagram in Figure 5b, in which a red parabola describes the CT state. The position of the CT state (Cr3+ + e) was determined considering the CTT transition and the EA,2 activation energy, which allowed us to determine the lattice relaxation energy LR.

Figure 5.

Figure 5

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Energy diagram describing the kinetics of the observed processes. (b) Configuration diagram for the Cr3+ ion, including the CT state (Cr3++e).

The ground state of the Cr3+ ion is close to the VB edge, which may favor the transfer of charges from the VB to the Cr3+ ion states. At the same time, the excited 4T2 state is away from the CB edge, which prevents electrons thermal ionization into the CB. Upon excitation of the Cr3+ ion (1), a fast nonradiative relaxation to the 4T2 excited state occurs. From the 4T2 state, we can observe broadband emission (bright pink arrow), which is extinguished as the temperature increases. This quenching occurs due to electron transfer to the CT (2) state, which is distant from the 4T2 state by the activation energy EA,2. The proximity of the ground state of the Cr3+ ion to the VB edge allows for electron capture by the Cr2+ ion (Cr3+ – e), which gave up an electron to the CT state, leaving a hole in the VB (3). We can also observe the creation of holes by directly exciting electrons from VB to the CT state (4). Such CTT is clearly visible in the photocurrent excitation spectra (see Figure 3b). The electron from the CT state can be transferred back to the Cr ions. Still, this process can only occur if the Cr3+ ion has not captured the electron from the VB. All this causes the CT to become a metastable state, and persistent luminescence55 and photocurrent are observed in the tested systems. It should be noted that the electron current observed in undoped materials is strongly quenched by the CT states (which are deep electron traps).

One Sample-Two Applications

Finally, we present a dual-purpose material, GIOC, highlighting its suitability as both an emitter for NIR-pc-LEDs and a UV detector. This dual functionality underscores the versatility and practicality of this material in optoelectronic devices designed for applications in both the near-infrared and ultraviolet regions.

The x = 0.2 sample was selected for NIR-pc-LED preparation due to its highest intensity upon a blue 450 nm LED excitation (see Figure S7a). Figure 6a illustrates the schematic of the NIR-pc-LED package. The fabricated LED device was driven by currents ranging from 2 to 20 mA, resulting in an emission peak centered at 770 nm with fwhm of 115 nm (Figure 6b). As depicted in Figure 6b, increasing the current from 2 to 20 mA led to a gradual increase in NIR intensity. It is important to note that we used an available blue LED package, allowing only small currents up to 20 mA to be applied. However, a higher-power excitation light source could achieve higher NIR output power. Photographs obtained under VIS light and pc-NIR-LED light are presented in Figure 6c and d, respectively. Figure 6c shows that the fabricated pc-LED emits NIR light, as demonstrated by the 800 nm long-pass filter. When the pc-LED is turned off, the light does not pass through the filter, hindering visibility behind the setup. However, upon switching on the pc-LED, NIR light passes through the filter, revealing what is behind the setup. Figure 6d shows a conventional color photo illuminated by VIS light, while Figure 6e depicts a black-and-white photo captured under the same illumination. Turning off the VIS light and turning on the NIR-pc-LED, as well as using a NIR camera, enables the capture of black-and-white images (Figure 6f).

Figure 6.

Figure 6

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(a) Schematic illustration of NIR-pc-LED. (b) Photoluminescence spectrum of the fabricated NIR-pc-LED device that combines a 450 nm InGaN blue LED chip with Ga1.78In0.2O3:0.02Cr3+ NIR phosphor under a forward bias of 2–20 mA, and the insets show the photographs of the LED device. Photographs obtained under VIS light and pc-NIR-LED captured by the corresponding VIS camera and a NIR camera, showing (c) that the produced NIR light passes through the 800 nm long pass filter and (d–f) the obtained images. (g) Schematic illustration of fabricated UV detector. (h) Photocurrent measured at 20 V and under 290 nm UV-LED excitation. (g) Dependence of photocurrent on applied excitation source UV-LED current.

The potential application as a UV detector was also investigated for the same sample. Figure 6g illustrates the schematic of the fabricated UV photodetector, where a voltage of 20 V is applied to the sample. A photocurrent is observed when UV light from a 290 nm diode (20 mA) is switched on, as demonstrated in Figure 6h. The photocurrent builds up relatively quickly, followed by a slight increase before stabilizing (bottom panel). A persistent photocurrent is observed after switching off the UV light, reproducible with each consecutive pulse. To mitigate the charging and discharging process associated with this behavior, modulation was implemented using a lock-in amplifier, as described previously. As shown in Figure 6h (upper panel), this approach minimizes the effects related to the persistent photocurrent, and the process becomes reproducible with each pulse, highlighting the utility of this method. Furthermore, Figure 6i depicts the photocurrent dependence on the UV LED current. The UV pulse was gradually increased from 2 to 20 mA, resulting in a linear increase in photocurrent with the increasing UV light intensity.

These findings underscore the promising potential of the Ga2–xInxO3:Cr3+ material in both NIR-pc-LED and UV photodetector applications, demonstrating its versatility and effectiveness across different optoelectronic devices.

Conclusions

A series of Ga1.98–xInxO3:0.02Cr3+ materials with varying x values, from 0.0 to 1.0, were synthesized, exhibiting broadband near-infrared emission. Under 442 nm excitation, Ga1.98–xInxO3:0.02Cr3+ demonstrated a tunable ultrabroadband NIR emission spanning 650 to 1100 nm, primarily from Cr3+ ions. The absorption spectra revealed a UV band corresponding to the band-to-band absorption at 260 nm, the charge transfer transition at 300 nm, and two distinct bands in the visible region at 440 and 610 nm. This visible bands are attributed to the 4A2 → 4T1 and 4A2 → 4T2 transitions of Cr3+ ions, respectively. Based on configuration diagrams and estimated activation energies of luminescence quenching, it was determined that nonradiative processes did not result directly from thermally induced relaxation from the excited state 4T2 to the ground state 4A2. Photocurrent excitation spectra showed behavior consistent with the absorption spectra, promoting holes into the VB, indicating that photocurrent in these materials involves Cr3+ dopants. Notably, a decrease in photoluminescence intensity was observed alongside a monotonically increasing photocurrent with rising temperature, demonstrating the role of thermal ionization in the quenching process of Cr3+ in these materials. This study provided the first evidence of luminescence quenching via the hole-creating process in Cr3+-doped oxides, suggesting potential applicability to other transition metal ions in dielectric materials.

Finally, the potential of Ga1.78In0.2O3:0.02Cr3+ as a dual-purpose material was demonstrated, highlighting its suitability as an emitter for NIR-pc-LEDs and UV detectors. This dual functionality underscores the versatility and practicality of this material in optoelectronic devices for both near-infrared and ultraviolet applications.

Experimental Section

Gallium oxide (Ga2O3, 99.99%), indium oxide (In2O3, 99.99%), and chromium oxide (Cr2O3, 99.99%) were purchased from Gredmann. To synthesize the Ga1.98–xInxO3:0.02Cr3+, all the precursors were weighed in the stoichiometric ratio and mixed using the agate mortar for 30 min. The mixing powder was transferred into alumina crucibles and put in a muffle furnace. All the samples were heated to 1400 °C for 5 h in an air atmosphere with a heating and cooling rate of 5 °C/min. After the furnace was cooled to room temperature, the samples were again grounded with the agate mortar, and the final products could be obtained.

Acknowledgments

N.M. acknowledges the National Science Center Poland Preludium No. 2022/45/N/ST3/00576. S.M. acknowledges the National Science Center Poland GRANT OPUS17 NO. 2019/33/B/ST3/00406. M.-H.F. acknowledges the National Science and Technology Council of Taiwan (Contract No. 112-2113-M-001-039-MY3). We thank the synchrotron X-ray characterization support by the National Synchrotron Radiation Research Center (NSRRC, Taiwan) with the beamlines of TPS BL19A1.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c08011.

  • Characterization methods, supplementary tables and figures (PDF)

The authors declare no competing financial interest.

Supplementary Material

ja4c08011_si_001.pdf (1,012.6KB, pdf)

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