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

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.

The photocurrent excitation (PCE) spectral measurements were conducted using a custommade setup comprising a 150 W xenon lamp (LOT Quantum Design) coupled to a grating monochromator (Omni-λ 1509), which operated in the spectral range of 250-1000 nm as the excitation source.The photocurrent was measured using a digital electrometer (Keysight B2987A).The excitation light was modulated at 5 Hz using an optical chopper to enhance the signal-to-noise ratio, and the photocurrent signal was extracted using a lock-in amplifier (Signal Recovery 7270, Ametek Scientific Instruments).
The decay profiles were measured using a setup designed for time-resolved spectroscopy, featuring a PG 401/SH optical parametric generator pumped by a PL2251A pulsed YAG:Nd laser (EKSPLA) as the excitation source.For detection, a 2501S grating spectrometer (Bruker Optics) was used in conjunction with a C4334-01 streak camera (Hamamatsu).The data were recorded as streak images on a 640 by 480 pixel CCD array.The analysis involved software employing a photon counting algorithm, which transformed the recorded data into a 2D matrix representing photon counts plotted against both wavelength and time (streak image).This comprehensive setup facilitated detailed investigation and analysis of the decay dynamics in the spectroscopic measurements. 1r temperature-dependent photoluminescence (PL) experiments, temperature control was achieved using the THMSG600 temperature controller integrated with the Linkam stage and the LNP95 liquid nitrogen cooling pump system, enabling precise temperature regulation within the range of 77-600 K.During decay profile measurements, samples were cooled using an APD Cryogenics closed-cycle DE-202 optical cryostat, which provided temperature control ranging from 10 to 450 K. Similarly, for photocurrent excitation (PCE) measurements, sample cooling was facilitated by a custom-made cryogenics closed-cycle optical cryostat, allowing temperature adjustment within the range of 10 to 300 K.  Table S1.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 0. Table S2.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 0.2.Table S3.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 0.4.Table S4.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 0.6.Table S5.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 0.8.Table S6.Atomic positions, occupancies, and atomic displacement parameters of GIOC with x = 1.0.Table S7.Refined parameters of GIOC with x = 0.0-1.0.

Room Temperature Photoluminescence Analysis
Table S8 displays the energies of the maxima for the 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions determined from excitation spectra, along with the 4 T 2 → 4 A 2 transition from emission spectra.This information allows us to calculate the crystal field parameter Dq, which describes the interactions between 3d electrons and ligand ions, and the Racah parameter B, representing the interaction between 3d electrons in Cr 3+ .The energy of the excitation band maximum for the 4 A 2 → 4 T 2 transition is equivalent to the 10Dq.The Racah parameters B can be calculated using the following equation: where ΔE is the difference between the energy of the 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions.The calculated Dq and B parameters are gathered in Table S8.The Sℏω value was estimated using following equation: where E( 4 A 2 → 4 T 2 ) is the energy of 4 A 2 → 4 T 2 transition, taken from the band maksimum in PLE spectra, and E( 4 T 2 → 4 A 2 ) is the energy of 4 T 2 → 4 A 2 transition, taken from the band maximum in PL sepctra.Figures S5a and b illustrate the temperature dependence of luminescence decays for GIOC samples with x = 0.2 and 0.6.The decays exhibit a multi-exponential nature due to the distribution of crystal field strengths around Cr 3+ in mixed ion samples.Figure S5c presents the calculated decay times as a function of temperature for x = 0.2 and 0.6.Due to the multiexponential decay, the average decay time was computed, eq. ( 1).
Typically, for Cr 3+ ions in a strong crystal field at 10 K, the radiative lifetime values remain unaffected by the thermal occupation of the 4 T 2 state, thus representing the radiative lifetime of the 2 E→ 4 A 2 transition.However, the transition probability of 2 E→ 4 A 2 is influenced by the presence of the 4 T 2 state due to spin-orbit interaction. 3It is noteworthy that the lifetime at 10 K is not solely the radiative lifetime of the 2 E→ 4 A 2 transition but is affected by a faster 4 T 2 → 4 A 2 transition, owing to the distributions of the Cr 3+ local environment and crystal field, resulting in broadband emission with faster decay even at 10 K.With increasing x, the contribution of the 4 T 2 → 4 A 2 transition increases.Consequently, the lifetime at 10 K varies significantly, from 88 µs for x = 0.2 to 29 µs for x = 0.6.In the case of x = 0.2 (where line emission is also observed at 100 K), two temperature regions exhibit decreasing decay times.In the low-temperature region (10-200 K), the decrease in decay times is accompanied by an increase in the intensity of the 4 T 2 → 4 A 2 broadband emission with a shorter decay time compared to the 2 E→ 4 A 2 line emission.In the high-temperature region (>300 K), the decrease in decay times is attributed to luminescence quenching.For x = 0.6, the emission consists solely of broadband related to the 4 T 2 → 4 A 2 transition, and the lifetime remains stable up to 200 K, equaling the decay time of the 4 T 2 → 4 A 2 broadband emission.The decrease in decay times at higher temperatures is again related to luminescence quenching.

Figure
Figure S1.In-house XRD of GIOC (x = 0.0-1.0)with Cu Kα radiation source.The asterisk and pound sign indicate the undistinguishable diffraction peaks.

Figure S3 .
Figure S3.RT x dependence of (a) decay profiles and (b) the calculated average decay times of GIOC of x = 0.0 -0.8.The decay times taken from the single-exponential fitting are given for x = 0.0 (grey circle).(c) Optical band gap determination for indirect bandgap.

Figure
Figure S7.(a) The x-dependent emission intensity of GIOC x = 0.0-0.8upon excitation at 450 nm.PCE dependence of excitation source UV-LED current.

Table S8 .
Energy of 4 A 2 → 4 T 1 (4F) and 4 A 2 → 4 T 2 (4F) excitation bands and 4 T 2 → 4 A 2 emission band maximum, value of estimated Sℏω and crystal filed and Racah parameters Dq and B