Improving and broadening luminescence in Gd_2−xAl_xGaSbO₇:Cr³⁺ phosphors for NIR LED applications

Abstract

Near-infrared (NIR) spectroscopy has garnered substantial attention owing to its diverse merits and widespread applications. Meanwhile developing phosphors with broadband and efficient NIR emission is still an enormous challenge for constructing NIR light sources. Herein, enhanced and broadened NIR luminescence of Gd_2−xAl_xGaSbO₇ (G_2−xA_xGSO):y%Cr³⁺ phosphors can be realized via modifying the crystal field environment of Cr³⁺ induced by Al³⁺ → Gd³⁺ cation substitution. Upon blue light irradiation, G_1.4A_0.6GSO:4%Cr³⁺ exhibits a broadband luminescence ranging from 650 to 1000 nm with a full width at half maximum (FWHM) of 136 nm, excellent thermal stability, and a tiny chromaticity shift. Through incorporating the resulting powders into a blue light-emitting diode (LED) chip, a NIR phosphor-converted (pc) LED with a high output power of 62.99 mW and a photoelectric conversion efficiency of 13.38% was manufactured. The fabricated pc-LED shows application potential in nondestructive food analysis. These findings confirm that the as-synthesized G_2−xA_xGSO:Cr³⁺ phosphor is a promising candidate for application in NIR pc-LEDs.

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1. Introduction

Near-infrared (NIR) spectroscopy, with distinct advantages of real-time monitoring, high penetration ability and minimal damage to biological organs, can be extensively applied in night vision, biological tissue detection, food composition and freshness analysis, plant cultivation, etc.^1–8 But traditional NIR-emitting sources, such as tungsten filaments and halogen lamps, are severely limited in emerging applications on account of their compactness, bulkiness, short lifetime and inefficiency.^8–10 In order to overcome these deficiencies and facilitate various functional applications of NIR light sources, NIR phosphor-converted light-emitting diodes (pc-LEDs) with portability, energy saving, fast-response, long lifetime and efficiency are emerging continuously and are fabricated by coupling broadband NIR phosphors with commercial blue chips.^10–12 As a necessary component of pc-LEDs, a wideband NIR-emitting compound which can be easily irradiated by blue light plays a crucial role in practical applications.

Numerous rare earth ion- (RE³⁺: Nd³⁺, Eu³⁺, Tm³⁺, and Yb³⁺) or transition metal ion (TM: Mn⁴⁺, Cr³⁺, and Ni²⁺)-activated NIR phosphors have been explored.^6–8,12 However, the intrinsically narrow emission and inefficiency of RE³⁺-doped phosphors limit their applicability.^13–15 Although the d–d transition of TM ions typically present broad excitation and emission bands,¹⁰ the poor tunability of Mn⁴⁺ luminescence and the mismatch of the Ni²⁺ excitation band with the blue chip inhibit commercial applications of Ni²⁺- or Mn⁴⁺-incorporated materials.^1,15,16

Alternatively, Cr³⁺ is increasingly viewed as a potential NIR activator.^3,5–7 Owing to the unfilled outer 3d³ electronic configuration, the luminescence of Cr³⁺ is greatly influenced by its crystal field environment.^5,12,17 Generally, in a strong crystal field strength, Cr³⁺ gives out a sharp line emission determined by the spin-forbidden ²E → ⁴A₂ transition.¹ Conversely, the luminescence of Cr³⁺ behaves as a broadband NIR emission resulting from the spin-allowed transition of ⁴T₂ → ⁴A₂ if the crystal field strength is less than 2.3,^18,19 while both wide and narrow emission bands coexist at an intermediate crystal field strength.⁶ Moreover, Cr³⁺ provides strong blue absorption,^16–18 which can be effectively stimulated by commercial blue chips. Consequently, extensive efforts have been devoted to exploring Cr³⁺-doped NIR phosphors, such as, Y₃Al_5−xGa_xO₁₂:Cr³⁺,²⁰ ZnGa₂O₄:Cr³⁺,²¹ BaZrSi₃O₉:Cr³⁺,²² ScBO₃:Cr³⁺,²³ La₂MgZrO₆:Cr³⁺,²⁴ Gd₃Sc₂Ga₃O₁₂:Cr³⁺,²⁵ Na₃Al₂Li₃F₁₂:Cr³⁺²⁶ and Gd₃Zn_xGa_5−2xGe_xO₁₂:Cr³⁺.²⁷ However, developing high-performing Cr³⁺-incorporated materials with highly efficient broadband NIR luminescence and superior thermal stability that meet the requirements of commercial applications is still an immense challenge.

Herein, a sequence of neoteric Gd_2−xAl_xGaSbO₇ (G_2−xA_xGSO):Cr³⁺ phosphors, which derive from the pyrochlore system and have excellent physical and chemical properties, were produced successfully. By designing the proper cation replacement for Gd³⁺ by Al³⁺ and adjusting the concentration of Cr³⁺ in G_2−xA_xGSO, the NIR emission of Cr³⁺ excited by the blue light is significantly improved and widened. The corresponding mechanisms were investigated thoroughly. Temperature-dependent spectra show the remarkable thermal and color stability of the as-prepared phosphors. Furthermore, the representative G_1.4A_0.6GSO:4%Cr³⁺ sample was packaged with a commercial blue chip to generate a NIR pc-LED with satisfactory output power and photoelectric efficiency. The feasibility of the as-obtained pc-LED employed in fruit freshness detection was also studied.

2. Experimental section

2.1 Materials and synthesis

A series of G_2−xA_xGSO:y%Cr³⁺ phosphors (x = 0–0.9, y = 0.25–5) were synthesized via a technique proposed by Wu et al.¹³ The initial ingredients Gd₂O₃, Ga₂O₃ (99.99%, all from Rare Earth Materials Center, China), Al₂O₃ (99.5%, from Sinopharm Chemical Reagent Co., Ltd, China), Sb₂O₃ and Cr₂O₃ (99.5%, all from Shanghai Zhanyun Chemical Co., Ltd, China) were accurately weighed based on their chemical composition, and subsequently ground homogeneously for more than 30 minutes. Meanwhile, 3wt% NH₄Cl (99.5%, from Shanghai Aladdin Biochemical Technology Co., Ltd, China) was added as a flux for producing pure phase samples. Then the mixtures were transferred into aluminum oxide crucibles and sintered at 1400 °C for 4 h twice with an intermediate grinding. The reground samples after cooling to ambient temperature were used for further measurement.

2.2 Characterization

X-ray diffraction (XRD) patterns were acquired by employing a Rigaku MiniFlex/600 XRD apparatus (Tokyo, Japan) with Cu Kα radiation (λ = 0.154056 nm). Structural refinement of the XRD patterns was carried out by the Rietveld method using the General Structure Analysis System (GSAS) package. Diffuse reflectance (DR) spectra were captured with an ultraviolet (UV)-visible (vis)-NIR spectrophotometer (UH-4150, Japan), selecting BaSO₄ compound as the reference standard. Photoluminescence (PL) spectra, PL excitation (PLE) spectra and fluorescence decay curves were measured using an Edinburgh FS5 fluorescence spectrophotometer (Livingston, UK) fitted with a xenon lamp (150 W). The same spectrophotometer equipped with a TCB1402 temperature controller (China) was used to collect the temperature-dependent PL spectra.

The NIR pc-LED was invented by coating the G_1.4A_0.6GSO:4%Cr³⁺ phosphor–silicon mixture onto a commercial blue chip. The mixture was obtained by first combining silicon A and silicon B in a mass ratio of 1 : 2, followed by adding phosphor in a ratio of 1 : 1, and then stirring for 15 minutes before drying in an oven for 1 h at 100 °C. Electroluminescence (EL) spectra were analyzed through a test instrument (LHS-1000, EVERFINE) attached to a high-precision array spectrophotometer (350–1000 nm, HAAS-2000).

3. Results and discussion

3.1 Crystal structure and phase formation

The crystal structure of Gd₂GaSbO₇ (GGSO), which belongs to the pyrochlore structure with a space group Fd3̄m, is depicted in Fig. 1. Gd³⁺ is surrounded by eight O²⁻ ions to form a [GdO₈] dodecahedron, while Ga³⁺ or Sb³⁺ is bonded to six O²⁻ ions to generate a [GaO₆] or [SbO₆] octahedron. In view of the same valence state and comparable ionic radii of Cr³⁺ (r = 0.615 Å) and Ga³⁺ (r = 0.620 Å) in six-fold coordination,^2,3,13 Cr³⁺ is highly likely to occupy the Ga³⁺ site when Cr³⁺ is introduced into GGSO.

Fig. 1 Crystal structure of GGSO.

The XRD patterns of G_2−xA_xGSO:5%Cr³⁺ (0 ≤ x ≤ 0.9) powders are shown in Fig. 2(a). When x is less than 0.6, the diffraction peaks can be well indexed to the standard pattern of GGSO (PDF#38-0822) and almost no significant other phase is identified, proving that the pure phase GGSO:5%Cr³⁺ samples were successfully prepared and the proper introduction of Al³⁺ does not disrupt the phase purity. When x is greater than 0.6, a slightly impure phase mainly from Al₂O₃ appears. It is obvious that the diffraction peaks shift to a larger angle as x increases, which illustrates a decrease in lattice spacing.²⁵ To obtain detailed crystallographic information of the samples, Rietveld refinements of G_2−xA_xGSO:5%Cr³⁺ were performed, and the results are revealed in Fig. S1 and Table S1.† The refined lattice parameters a/b/c and volumes shown in Fig. 2(b) decrease gradually with the increase in x, which emphasizes that the appropriate Al³⁺ in place of Gd³⁺ induces lattice contraction.⁵

Fig. 2 (a) XRD patterns of G_2−xA_xGSO:5%Cr³⁺ (x = 0–0.9) and magnified patterns from 29.5° to 31°. (b) Refined lattice parameters a/b/c and volumes of G_2−xA_xGSO:5%Cr³⁺.

3.2 Luminescence properties of G_2−xA_xGSO:5%Cr³⁺

The PLE spectra of G_2−xA_xGSO:5%Cr³⁺ (x = 0–0.9) samples are shown in Fig. 3(a), which are composed of three excitation bands peaking at about 310 nm, 453 nm and 620 nm, governed by the transitions from the ⁴A₂ ground state to the ⁴T₁(⁴P), ⁴T₁(⁴F) and ⁴T₂(⁴F) excited states of Cr³⁺, respectively.^15,18 It is worth mentioning that the optimal excitation band located at 453 nm matches well with the commercial 450 nm blue chips. With the gradual introduction of Al³⁺, the excitation bands shift slightly towards the shorter wavelength.

Fig. 3 G_2−xA_xGSO:5%Cr³⁺ (x = 0–0.9): (a) PLE, (b) PL, and (c) normalized PL spectra; and (d) integrated intensity and FWHM versus x.

Meanwhile, the PL spectra of G_2−xA_xGSO:5%Cr³⁺ under 453 nm light excitation are dominated by a broadband emission in the NIR range, as shown in Fig. 3(b), stemming from the transition of ⁴T₂ → ⁴A₂ of Cr³⁺.^18,28 With the increase in x, the integrated intensity shown in Fig. 3(d) is progressively enhanced and then declines after reaching a maximum at x = 0.6. The luminescence of G_1.4A_0.6GSO:Cr³⁺ is optimal and will be further discussed later. When the doping concentration x is above 0.2, an evidently narrow band emission at approximately 689 nm is observed, which is attributed to the spin-forbidden ²E → ⁴A₂ transition of Cr³⁺.²⁸ Besides, it can be seen intuitively that the emission peak of G_2−xA_xGSO:5%Cr³⁺ shifts from 795 nm to 779 nm by adding x content, while the FWHM increases consistently from 122 to 146 nm, as delineated in Fig. 3(c) and (d), respectively. Such NIR luminescence performance is mainly induced by the change in the crystal field environment of Cr³⁺ with boosting Al³⁺ concentration.^1,2,25

To comprehend the influence of the proper introduction of Al³⁺ on the crystal field and luminescence of Cr³⁺, the Tanabe–Sugano (T–S) diagram is depicted in Fig. 4. Obviously, the energy-level distribution of Cr³⁺ is strongly associated with the crystal field strength. The values of the crystal field parameter D_q and Racah parameters B and C are determined via the following equations:^29–32

10 × D_q = E(⁴T₂) − ΔS/2 (1)

ΔE = E(⁴T₁) − E(⁴T₂) (3)

where ΔS is the Stokes shift, ΔE is the energy difference between the ⁴T₂ and ⁴T₁ states, and E(⁴T₂), E(⁴T₁) and E(²E) represent the energy levels of ⁴T₂, ⁴T₁ and ²E, respectively. D_q, B and C were calculated respectively and are presented in Table 1. The expected D_q/B value, which is an average value of the crystal field strength that includes a distribution of values reflecting the fluctuation of the cation–anion distance caused by lattice vibrations, defects, and the structural differences in a real system,²⁹ increases from 1.99 to 2.43 as Al³⁺ is introduced, which is close to the intermediate value (∼2.3). Such an enhancement of D_q/B gives rise to the blue shift of the emission peak and the emergence of narrow band emission with increasing Al³⁺ content.^25,33

Fig. 4 The T–S diagram of Cr³⁺ in the G_1.4A_0.6GSO host.

Table 1 Crystal field parameters of G_2−xA_xGSO:5%Cr³⁺

x	D q (cm^−1)	B (cm^−1)	C (cm^−1)	D q/B
0	1403	706	2930	1.99
0.05	1412	680	2996	2.07
0.1	1417	664	3038	2.13
0.2	1424	646	3086	2.23
0.6	1436	623	3144	2.30
0.9	1452	589	3234	2.43

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The DR spectra of G_2−xA_xGSO:5%Cr³⁺ are depicted in Fig. 5(a), which contain four absorption bands peaking at roughly 230 nm, 310 nm, 453 nm, and 620 nm, corresponding to host absorption, ⁴A₂ → ⁴T₁ (⁴P), ⁴A₂ → ⁴T₁ (⁴F) and ⁴A₂ → ⁴T₂ (⁴F) transitions of Cr³⁺, respectively.⁹ This outcome confirms that Cr³⁺ ions were successfully incorporated into this host.¹⁸ The optical band gap (E_g) values of G_2−xA_xGSO:5%Cr³⁺ can be attained from the obtained DR spectra according to the Kubelka–Munk equations:^10,14,27

(αhν)ⁿ = A(hν − E_g) (6)

where R and α represent the reflection and absorption coefficients, respectively, hν is the photon energy, A denotes the proportional constant and n signifies the transition coefficient (for the indirect permitted transition, n = 1/2).³⁴ As presented in Fig. 5(b), the E_g value of G_2−xA_xGSO:5%Cr³⁺ increases with an increase in dopant concentration of Al³⁺ until it reaches the maximum when x = 0.6 and then decreases. In general, the luminescence intensity of phosphors is also related to E_g.^35,36 As x increases, E_g increases, meaning that the energy difference between the conduction band and the lowest excited state widens, which may decrease the likelihood of Cr³⁺ ionization and make it more difficult for electrons in the excited state to transit to the conduction band, thus improving the luminescence intensity.³⁶ But with a further increase in x, E_g decreases and the likelihood of Cr³⁺ ionization increases, leading to the decrease in intensity.³⁶ Of course, the excessive introduction of Al³⁺ causes the generation of an impure phase and may suppress the luminescence. On the other hand, a handful of Al³⁺ incorporation would also modify the lattice distortion (D_dis), influencing the luminescence of Cr³⁺.³³ The variation of the local structural distortion of the [Gd/AlO₈] dodecahedron depending on the substitution of Gd³⁺ by Al³⁺ was studied, which can be described by:^15,35where n refers to the coordination number, d_i is the distance from Gd to the ith coordinating O atom and d_av stands for the average of the Gd–O distance. As displayed in Table S2,† the calculated D_dis value of [Gd/AlO₈] decreases from 5.55% to 4.07% along with the increment of x from 0 to 0.6. This means that the appropriate replacement of Al³⁺ for Gd³⁺ can cause an improvement of the structural symmetry, leading to the increase in luminescence intensity of G_2−xA_xGSO:5%Cr³⁺.¹⁵

Fig. 5 G_2−xA_xGSO:5%Cr³⁺ (x = 0–0.9): (a) DR spectra and (b) optical band gap values.

The broadening of FWHM is usually related to electron–phonon coupling (EPC) effects.⁸ The strength of the EPC can be reflected by the Huang–Rhys parameter S, which is estimated through the following equations:^8,37

C O = (2S − 1)ħω (8)

FWHM = 2.35ħω(S cot h(ħω/kT))^1/2 (9)

in which C O, ħω, k and T denote the Stokes shift, lattice phonon energy, Boltzmann constant and temperature, respectively. As seen from Table 2, the expected S increases gradually with increasing content of Al³⁺ from 0 to 0.6. So, a broader width of Cr³⁺ emission may arise from a stronger EPC.^38,39 Although an intense EPC may cause a strong thermal quenching effect, the values of S are lower than 5, which can ensure the wideband NIR luminescence as well as excellent thermal stability of the obtained samples.^1,14,31

Table 2 Spectral and lattice parameters of G_2−xA_xGSO:5%Cr³⁺

x	C O (cm^−1)	FWHM (cm^−1)	S (cm^−1)	ħω (cm^−1)
0	2877	1941	4.09	400
0.05	2950	1979	4.12	407
0.1	3072	2021	4.25	410
0.2	3164	2056	4.33	413
0.6	3452	2214	4.39	443
0.9	3774	2433	4.31	494

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3.3 Crystal structure of G_1.4A_0.6GSO:y%Cr³⁺

To achieve better luminescence performance of G_1.4A_0.6GSO:Cr³⁺ phosphors, G_1.4A_0.6GSO phosphors doped with different Cr³⁺ contents were prepared successfully. No obvious other impurities are found in the XRD patterns of G_1.4A_0.6GSO:4%Cr³⁺, as shown in Fig. 6(a), implying that the phase purity of the resultant samples was barely altered with adjustments to the Cr³⁺ concentration. The Rietveld refinement of G_1.4A_0.6GSO:4%Cr³⁺ (Fig. 6(b)) shows that the refinement results agree well with the weighted profile R-factor (R_wp) = 10.89% and the profile R-factor (R_p) = 7.90%, indicative of the reliability of the results.

Fig. 6 G_1.4A_0.6GSO:4%Cr³⁺: (a) XRD patterns and (b) Rietveld refinement.

3.4 Luminescence properties of G_1.4A_0.6GSO:y%Cr³⁺

The PLE spectra of G_1.4A_0.6GSO:y%Cr³⁺ (y = 0.25–5) are depicted in Fig. 7(a), in which the excitation peaks show little change with increasing Cr³⁺ content. When excited at 453 nm, the PL spectra for y ranging from 0.25 to 5 exhibit a line emission at 689 nm and a broadband emission covering from 650 to 1000 nm, as shown in Fig. 7(b). Moreover, increasing Cr³⁺ concentration induces the emission peaks of Cr³⁺-doped G_1.4A_0.6GSO to experience a red-shift from 771 to 784 nm (Fig. 7(c)). This behavior has been interpreted as the substitution of a larger Ga³⁺ by a smaller Cr³⁺ reducing the energy difference between the ⁴T₂ excited state and the ⁴A₂ ground state of Cr³⁺, leading to the red-shift in the PL spectra of G_1.4A_0.6GSO:y%Cr³⁺.^8,40

Fig. 7 G_1.4A_0.6GSO:y%Cr³⁺ (y = 0.25–5): (a) PLE, (b) PL and (c) normalized PL spectra; and (d) integrated intensity and FWHM versus y.

Fig. 7(d) depicts the correlation between the integrated intensity, the FWHM and the Cr³⁺ concentration in G_1.4A_0.6GSO. It can be found that the emission intensity of G_1.4A_0.6GSO:y%Cr³⁺ with y = 4% is the strongest. However, further adding Cr³⁺ concentration decreases the emission intensity because of the concentration quenching effect.^39,41 In addition, as y increases, the FWHM value of G_1.4A_0.6GSO:y%Cr³⁺ steadily increases from 132 to 136.5 nm, which is attributed to the stronger EPC effect.²⁵ As listed in Table S3,† although the computed S value of G_1.4A_0.6GSO:y%Cr³⁺ increases from 3.40 to 4.39 cm⁻¹ with the increase in y, all of these values are in the medium electron–phonon coupling (1 < S < 5), which is capable of guaranteeing the broadband NIR emission and good thermal stability of Cr³⁺.⁸

The decay curves of the broadband and narrow emissions of G_1.4A_0.6GSO:y%Cr³⁺ (y = 0.25–5) were measured at the corresponding excitations, respectively, as depicted in Fig. 8(a) and (b). The average lifetimes (τ) are calculated using eqn(10):^13,42–44

where I(t) refers to the luminescence intensity at time t. The τ values of the broadband emissions of G_1.4A_0.6GSO:y%Cr³⁺ phosphors under excitation at 453 nm are respectively estimated to be 131.92, 125.63, 118.09, 110.06, 97.38 and 102.09 μs, while those of the narrow emissions (λ_ex = 437 nm and λ_em = 689 nm) are 426.60, 409.80, 399.58, 373.20, 339.73, and 340.79 μs with y = 0.25, 0.5, 1, 2, 4, and 5, respectively. Although the lifetimes are all on the order of microseconds, it can be noticed clearly that the narrow band emission has a longer lifetime. In contrast to the spin-allowed transition ⁴T₂ → ⁴A₂, the spin-forbidden ²E → ⁴A₂ transition of Cr³⁺ has a weak coupling function with the lattice, resulting in a long-lived sharp line emission.^5,18 As the concentration of Cr³⁺ increases, both the average lifetimes of the narrow and broadband emissions follow similar decay trends, which may be due to the increased non-radiative transition caused by the weakened distance between the activation centers with increasing y.⁵

Fig. 8 Decay curves of G_1.4A_0.6GSO:y%Cr³⁺ (y = 0.25–5): respectively monitored at (a) broadband emissions at 771 nm, 773 nm, 777 nm, 780 nm, 782 nm, and 784 nm (λ_ex = 453 nm) and (b) narrow emission at 689 nm (λ_ex = 437 nm).

3.5 Temperature-dependent luminescence properties

In general, temperature strongly affects the luminescence performance of phosphors. Because the operating temperature of blue LED chips will increase for prolonged work, it is crucial to assess the thermal stability of the as-prepared materials in the temperature range from 303 to 573 K. The temperature-dependent PL spectra of G_1.4A_0.6GSO:4%Cr³⁺ under a 453 nm excitation were recorded and are shown in Fig. 9(a). The intensity decreases with increasing temperature due to the greater non-radiative transition at higher temperature.^5,39,45Fig. 9(b) reflects the emission position as a function of temperature, which shifts slightly to the right as the temperature increases, accompanied by the gradual vanishing of the narrow peak located at 689 nm, ascribed to the decrease in crystal field strength resulting from heat-induced crystal lattice expansion.^26,39 As shown in Fig. 9(c), the intensity at 423 K of G_1.4A_0.6GSO:4%Cr³⁺ is maintained at around 62.65% of the initial value (303 K), displaying an outstanding thermal stability. The activation energy of thermal quenching (ΔE) is generally specified by the following Arrhenius equation:^12,46where I_T and I₀ are the emission intensities at the specified and initial temperatures, respectively, k is the Boltzmann constant and C is a constant. The value of ΔE is determined to be about 0.34 eV. The comparatively high ΔE makes for a lower likelihood of overcoming the energy barrier for the non-radiative relaxation, which is in accordance with the experimental finding that the obtained phosphor possesses favorable thermal stability.^5,14

Fig. 9 G_1.4A_0.6GSO:4%Cr³⁺: (a) temperature-dependent PL spectra (λ_ex = 453 nm) and (b) normalized PL spectra. (c) Integrated intensity, inset: plots of ln (I₀/I_T − 1) versus 1/kT. (d) ΔE versus temperature; inset, CIE chromaticity coordinates at 303, 423 and 573 K temperatures, respectively.

The chromaticity shift (ΔE) ascertained by the following equation can be used to quantify the stability of the emission color with the alteration of temperature:^47–49

where u′ = 4x/(3 – 2x + 12y), v′ = 9y/(3 – 2x + 12y), w′ = 1 − u′ − v′, and x and y are the chromaticity coordinates. The computed results of G_1.4A_0.6GSO:4%Cr³⁺ are illustrated in Fig. 9(d), which exhibit a relatively tiny ΔE value of about 1.471 × 10⁻² even operating at 423 K. The inset in Fig. 9(d) delineates the CIE chromaticity coordinates that are situated near the margin of the NIR.

3.6 NIR pc-LED performance and applications

To investigate the applicability of the resultant sample, a NIR pc-LED was manufactured by incorporating the representative G_1.4A_0.6GSO:4%Cr³⁺ with a commercial blue chip. As shown in Fig. 10(a), the EL spectra include a blue emission band centered at 450 nm from the commercial chip and a NIR luminescence band centered at 650–1000 nm emanating from G_1.4A_0.6GSO:4%Cr³⁺. The emission intensity of the NIR pc-LED increases in step with the working current increasing from 10 to 300 mA, while the spectral distribution remains constant. Images of the pc-LED in natural light and the lighted one with and without a 720 nm optical filter are presented in the insets of Fig. 10(a) from left to right. Fig. 10(b) depicts the output optical power and photoelectric efficiency of the fabricated NIR pc-LED versus operating currents. As the current increases from 10 to 300 mA, the NIR output power increases from 3.48 to 62.99 mW. Meanwhile, the photoelectric efficiency reaches 13.38% at a working current of 10 mA, which is higher than the majority of the reported NIR pc-LEDs, as shown in Table 3. A high thermal stability and excellent photoelectric efficiency suggest that the sample has a great promise in NIR pc-LED applications.

Fig. 10 (a) EL spectra of the as-prepared NIR pc-LED device under various drive currents. Insets from left to right show the photographs of the NIR pc-LED in natural light, and the lighted one with and without a 720 nm optical filter. (b) Output optical power and photoelectric efficiency depending on drive currents, respectively. Measured EL spectra with and without the fresh and dried (c) grape and (d) tomato in the integrating sphere.

Table 3 NIR output power and photoelectric property of NIR pc-LEDs with different Cr³⁺-doped phosphors

Phosphor	NIR output power (mW)	Photoelectric efficiency	Ref.
G1.4A0.6GSO:Cr^3+	62.99@300 mA	13.38%@10 mA	TW
BaZrGe3O9:Cr^3+	6.45@320 mA	0.6%@320 mA	3
Lu2CaMg2Si3O12:Cr^3+	181@320 mA	14.8%@100 mA	6
Gd2GaSbO7:Cr^3+	∼21@100 mA	—	13
LiScGeO4:Cr^3+	4.78@60 mA	4.4%@60 mA	19
Na3Al2Li3F12:Cr^3+	14.3@60 mA	8.05%@60 mA	26
Ca2LaHf2Al3O12:Cr^3+,Yb^3+	33.24@200 mA	10%@200 mA	30
ScF3:Cr^3+	24.15@300 mA	3.19%@20 mA	31
LiInSiO6:Cr^3+	51.6@100 mA	17.8%@100mA	32
Ca2YHf2Al3O12:Cr^3+,Yb^3+	18@100 mA	6%@100 mA	40
NaInP2O7:Cr^3+	61.03@320 mA	12.11%@20 mA	46

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We investigated the application of the as-obtained broadband NIR pc-LEDs in nondestructive food analysis including the measurement of the NIR spectral absorption of various types of fruit to assess their freshness. The measured NIR spectra are presented in Fig. 10(c) and (d). On account of the range of the broadband emission (650–1000 nm) containing the third (730 nm) overtones of the O–H stretching in water, the third (720 nm) overtones of O–H stretching in sugar, and the fourth (750 nm) overtones of C–H stretching in soluble sugar, information on the water and sugar content in fruits can be provided by measuring the NIR spectral absorption.^5,50 For comparison with fresh samples, both tomato and grape were dried in an oven at 100 °C for 1 h to evaporate part of their water (Fig. 10(c) and (d)). The NIR spectra were recorded in the integrating sphere with and without the above fruits, respectively. The intensities of the NIR spectra decrease when the fruits are inside the integrating sphere, since NIR radiation is absorbed by the overtones of the vibrational modes (O–H, C–H and N–H).⁵ During the baking process, a sizable proportion of water evaporates and the sugar content rises. After minimizing the effect of water on the absorption features of the NIR spectra of high-water-content samples, it is evident that the intensities of the dried grape and tomato are weaker than those of the fresh ones, which is attributable to the growth of sugar absorption.⁵⁰ Therefore, this broadband NIR pc-LED presents superior prospects in the food freshness analysis field.

4. Conclusions

A batch of NIR broadband G_2−xA_xGSO:Cr³⁺ phosphors were synthesized successfully via a solid state method. Through incorporating a small amount of Al³⁺ into G_2−xA_xGSO:Cr³⁺, the luminescence under irradiation with 453 nm light is efficiently enhanced and broadened, caused by the variations in the crystal field environment of the Cr³⁺ ions. G_1.4A_0.6GSO:4%Cr³⁺ exhibits a broadband NIR emission ranging from 650 to 1000 nm with a FWHM of 136 nm. The emission intensity at 453 K still remains at 62.65% of the initial value, displaying excellent thermal stability. Additionally, G_1.4A_0.6GSO:4%Cr³⁺ was packaged with the commercial blue chip to generate a NIR pc-LED with an output power of 62.99 mW and a photoelectric efficiency of 13.38%. The application of the as-fabricated NIR pc-LED in nondestructive food freshness analysis is realized. All the remarkable findings demonstrate that the as-synthesized G_2−xA_xGSO:Cr³⁺ NIR luminescent material is a promising candidate for commercial applications in NIR pc-LEDs.

Author contributions

Siyu Guo: investigation, methodology, and writing – original draft. Ligan Ma: investigation, methodology, and review. Muniran Abudureyimu: methodology and review. Fumin Lu: investigation and methodology. Fangfang Hu: methodology. Hai Guo: conceptualization, resources, review, and supervision. RongFei Wei: conceptualization, resources, writing – review & editing, and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant no. 51802285, 11974315 and 11804303), the Natural Science Foundation of Zhejiang Province (Grant no. LZ20E020002), the Zhejiang University Student Science and Technology Innovation Activity Plan (New Seedling Talent Plan) Project (no. 2022R404A036), and the National College Students’ innovation and entrepreneurship training program (202210345017).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi00229b

‡ These authors contributed equally.

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