1. Introduction

NIR spectroscopy is uniquely suited for applications in non-destructive testing [1], plant lighting [2], food analyzed [3], and night vision [4,5]. Especially for light sources in the 700–1100 nm spectral range, the organic composition is detected by the absorption intensity of C-H, O-H, and N-H groups in the 700–1100 nm NIR band [5]. The NIR radiation can effectively penetrate biological tissues and can also be used for nondestructive imaging of subcutaneous blood vessels, thus providing a harmless and non-invasive diagnosis of the human physiological state [6]. For multi-disciplinary applications, a wider detection range and faster response time, as well as portable NIR light sources, need to be developed. Compared with traditional NIR light sources, NIR pc-LED technology has the advantages of long operating life, small size, low power consumption, etc. [7,8], which can be achieved by integrating NIR pc-LED into portable electronic devices to realize micro NIR systems. And phosphor as the core component of pc-LED needs to provide both the NIR output of the whole system and match the corresponding LED chip.

In recent years, NIR phosphors with transition metal ions (Cr³⁺, Ni²⁺, Mn²⁺, etc.) [9–11] and rare-earth ions (Eu²⁺, Yb³⁺, etc.) [12,13] as activating ions have been well reported, e.g., (Sr,Ba)Y₂O₄:Eu²⁺ [14], Y₃Al_5-xGa_xO₁₂:Ni²⁺ [9], Li₂ZnGe₃O₈:Mn²⁺ [11], AlP₃O₉:Cr³⁺ [15], LiScP₂O₇:Cr³⁺, Yb³⁺ [16], etc. Among the many activators, Cr³⁺ exhibits broadband absorption in the near-UV-Vis range with its unique 3d³ configuration, which can achieve tunable narrowband or broadband spectra from 600–1600 nm [17]. Among the many Cr³⁺-doped NIR phosphors, the garnet structure has been widely studied for its unique luminescence properties in the form of A₃B₂C₃O₁₂ [18]. However, in general, most of the phosphors with garnet structures as hosts exhibit narrow fluorescence emission ranges and full-width at half maximum (FWHM), such as CaLu₂Mg₂Si₃O₁₂:Cr³⁺ [19], Ca₃Sc₂Si₃O₁₂:Cr³⁺ [20], Ca₄ZrGe₃O₁₂:Cr³⁺ [21], Ca₂LuScGa₂Ge₂O₁₂:Cr³⁺ [22], etc., and their emission ranges most of them do not exceed 650–1000 nm, and the FWHM does not exceed 180 nm. Garnet usually has a large band gap, which can inhibit the electron-phonon coupling (EPC) effect of Cr³⁺ to the host lattice, hence its emission spectrum shows a relatively narrower spectrum [23]. However, a wider spectral range can be obtained by screening garnet with a smaller band gap as a host. The luminescent properties of phosphors are determined by the electronic configuration of the activator ion, and by the coordination environment of the activator ion in the matrix [24]. The effect of the coordination environment on the luminescence properties of activator ions is mainly reflected in the nephelauxetic effect (NE) and crystal field splitting effect. The strength of the NE depends on the bonding properties of the activator ion and the ligand ion (the ratio between the ionic bonding content and the covalent bonding content) [25]. The smaller the electronegativity difference (ED) between the atoms of two elements, the stronger the covalent bonding and the stronger the NE, resulting in the redshift phenomenon of the spectrum.

Herein, through Cr³⁺ doping, the host band gap is indirectly reduced, so as to achieve the effect of broadband emission, a series of NIR broadband GZGG:Cr³⁺/Yb³⁺ phosphors were finally made. When x = 2%, y = 0.2%, the broadband emission of 650 nm–1200 nm was exhibited under 459 nm excitation with a FWHM of 260 nm. By co-doped Cr³⁺/Yb³⁺, the IQY increased from 47.15% to 54.9%, and at 150 °C the emission intensity remains at room temperature (RT) of 60%. Eventually, the obtained phosphor was fabricated into pc-LED for its application in night vision illumination and vascular visualization.

2. Materials and methods

Series of GZGG:xCr³⁺, yYb³⁺ phosphors were successfully synthesized by the high-temperature solid state reaction method. Gd₂O₃ (99.9%), ZnO (99%), Ga₂O₃ (99.999%), GeO₂ (99.9999%), Cr₂O₃ (99%) and Yb₂O₃ (99.99%) were applied as raw material, weighed according to stoichiometric composition and no substrates in excess, then mixed and ground in an agate mortar. The mixture was then fired at 1350 °C for 5 h in air. After cooling, the mixture was ground to a powder using an agate mortar and then tested.

X-ray diffraction (XRD) patterns were recorded by a Bruker D8 Advance X-ray diffractometer at 40 KV and 40 mA with Cu-K-α (λ = 1.54056 Å) irradiation. Morphological characteristics were recorded by field emission scanning electron microscopy (Nova Nano SEM 450). Elemental composition and distribution were determined using an energy dispersive X-ray spectroscope attached to SEM. Rietveld refinement of the measured XRD data was performed using General Structure Analysis System (GSAS) software. Photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra were recorded by a HORIBA FLuorolog-3 fluorescence spectrometer with a 450 W Xe lamp as the excitation source, the temperature spectra were tested with an external device, and the PL QYs were tested with Quanta-φ (HORIBA Scientific), using BaSO₄ as a standard reference. The diffuse reflectance spectra (DRS) were measured by a Hitachi U-4100, using BaSO₄ as a standard reference. Raman spectra were recorded with a HORIBA Raman spectrometer (LabRAM HR Evolution, Jobin Yvon) using a 532 nm Nd-YAG laser as the excitation source.

Pc-LED is a combination of 460 nm blue LED and GZGG:2%Cr³⁺. First, a certain amount of phosphor and silicone are fully mixed and stirred in a cup for 40 min to get the phosphors-silicone mixture coated on the blue diode chip. Then, the structure was baked in an oven at 150 °C for 1 h and then fixed on the substrate.

3. Results and discussion

GZGG belongs to a typical garnet structure with cubic crystal system $\mathrm{Ia\bar{3}d}$ space group, where Gd combines with O^2- to form [GdO₈] dodecahedra, Zn combines with O^2- to form [ZnO₆] hexahedron, Ga/Ge combines with O^2- to form [Ga/GeO₄] tetrahedron [26]. Figure 1(a) gives the XRD of GZGG:2%Cr³⁺, yYb³⁺ (y = 0–2%). By comparing the standard card PDF#04-007-8694, it is found that the prepared samples are pure phase. Figure 1(b) depicts the Rietveld refinement XRD pattern of GZGG and GZGG:2%Cr³⁺, 0.2%Yb³⁺ obtained by GASA software. The parameters R_p = 4.4%, R_wp = 5.9%, χ² = 1.85% and R_p = 4.3%, R_wp = 5.6%, χ² = 1.62% are within the confidence range, respectively. The Raman spectra of GZGG:xCr³⁺ revealed in Fig. 1(c), the intensity of Raman diffraction peaks decreases gradually with the doping of Cr³⁺, Yb³⁺. Figure 1(d) depicts the structure of GZGG, Cr³⁺ and Yb³⁺ generally occupy [ZnO₆] six-coordination octahedron structure. Figure 1(e) portrays the SEM of GZGG:2%Cr³⁺, 0.2%Yb³⁺. The synthesized sample has a uniform particle distribution, smooth and flat surface, and a particle size of about 2 µm, which proves that the sample was synthesized very successfully.

 

Fig. 1. (a) X-ray diffraction pattern of GZGG:2%Cr³⁺, yYb³⁺ (y = 0-2%). (b) Rietveld refinement of GZGG and GZGG:2%Cr³⁺, 0.2%Yb³⁺. (c) Raman spectra of GZGG:xCr³⁺, yYb³⁺. (d) Structure of GZGG. (e) SEM image of GZGG:2%Cr³⁺, 0.2%Yb³⁺.

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Figure 2(a) shows the PLE spectra of GZGG:2%Cr³⁺ monitored at 855 nm and PL spectra monitored at 459 nm of GZGG:2%Cr³⁺, 0.2%Yb³⁺. The excitation spectra consisted of ⁴A₂→⁴T₁(⁴P) transition at 240 nm-380 nm, ⁴A₂→⁴T₁(⁴F) transition at 380 nm-570 nm and ⁴A₂→⁴T₂(⁴F) transition at 570 nm-750 nm [27]. Under the 459 nm excitation, the phosphor demonstrates an ultra-broadband NIR emission of 650-1200 nm with a FWHM of 218 nm, which is generated by the spin-allowed transition of ⁴T₂(⁴F)→⁴A₂ [28]. When co-doped Yb³⁺, the spectrum is widened to 260 nm. The PL spectra of GZGG:xCr³⁺ are gradually redshifted with increasing Cr³⁺ concentration and redshifted by 50 nm (Fig. 2(b)). The emission intensities reached a maximum at x = 2%, followed by concentration quenching and a slight broadening of the spectrum was obtained (see Supplement 1, Fig. S1).

 

Fig. 2. PLE and PL spectra of GZGG:2%Cr³⁺ and GZGG:2%Cr³⁺, 0.2%Yb³⁺. (b) PL spectra of GZGG:xCr³⁺. (c) Schematic diagram of the process of reducing electronegativity difference due to Cr³⁺ replacement of Zn²⁺. (d) Diffuse reflection spectrum of GZGG:2%Cr³⁺, 2%Yb³⁺ and PL spectrum of GZGG:2%Cr³⁺. (e) PL spectra of GZGG:2%Cr³⁺, yYb³⁺.

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Coordination chemistry theory is used to explain this redshift phenomenon. In Fig. 2(c), a schematic diagram of the process of Cr³⁺ substitution into the [ZnO₆] six-coordination octahedron were displayed, where Cr³⁺ has an electronegativity of 1.66, which is bigger than that of Zn²⁺. And Cr³⁺ enters [ZnO₆], replacing Zn²⁺ to reconstruct the [Zn/CrO₆] polyhedron, and the previous Zn-O bond gradually turns into Cr-O bond, and the ED of Zn-O bond is 1.79, while the ED of Cr-O bond is 1.78, which is less than Zn-O bond, and both are bigger than 1.7, hence the ionic bond is formed, but the ionicity gradually decreases. With the doping of Cr³⁺, the ionicity of [ZnO₆] gradually decreases, while the covalency gradually increases, leading to the bond length of Zn/Cr-O bond gradually becoming shorter and the NE getting enhanced, which eventually leads to the redshift phenomenon of the spectrum, and the shortening of the bond length also matches with the results obtained from the refinement (see Supplement 1, Fig. S2). At the same time, we also did a comparative experiment and chose Mg²⁺ as its central ion (R = 0.72 Å), which is close to the radius of Zn²⁺, and then doped with equal amount of Cr³⁺. It was found that the spectrum of Gd₃Mg₂GaGe₂O₁₂:2%Cr³⁺ (GMGG) was blue shifted toward the short band by 30 nm compared with GZGG:2%Cr³⁺, while the electronegativity of Mg was 1.31, and the ED of the Mg-O bond (2.13) was much larger than that of the ED of the Cr-O bond, hence this is the reason for the blueshift of the spectrum (see Supplement 1, Fig. S3a, b).

Figure S4a presents the DRS of GZGG:xCr³⁺ under the RT. The absorption gradually increases with the increase of concentration. When x = 2%, the calculated band gap is 3.63 eV, which is smaller than the band gap of GZGG (E_g = 4.15 eV). In Supplement 1, Fig. S4b, with the increase of concentration, the band gap gradually decreases. Smaller band gaps exhibit weaker ionic properties, which will lead to increased covalency in the octahedral coordination environment in which Cr³⁺ resides, resulting in increased the NE and ultimately causing redshift [29,30].

Figure 2(d) tested the DRS of GZGG:2%Cr³⁺, 2%Yb³⁺, and the absorption of Yb³⁺ appeared around 950 nm, which partially overlapped with the emission of GZGG:2%Cr³⁺, indicating that ET became possible. Figure 2(e) demonstrates the PL spectral of GZGG:2%Cr³⁺, yYb³⁺, which broadens to 260 nm under the excitation of 459 nm at y = 0.2%. With the increase of Yb³⁺ concentration, the emission intensity of Yb³⁺ gradually increases while the emission intensity of Cr³⁺ gradually decreases, indicating the ET from Cr³⁺ to Yb³⁺.

The thermal stability of the phosphors is extremely important to the pc-LED. Figure 3(a) depict the temperature-dependent PL spectra of GZGG:2%Cr³⁺ from 25 °C to 200 °C. With the increase of temperature, the emission intensity decreases gradually, while the spectral shape hardly changes, indicating that the crystal structure does not change with the change of temperature. At 100 °C, the emission intensity is 66.7% of 25 °C, and 41.2% can be maintained even at 150 °C (Fig. 3(b)). The Huang-Rhys factor (S) can reflect how strongly electrons couple to phonons and can be obtained by fitting the temperature- dependent full width at FWHM of photoluminescence peaks using the following equation [31]

Based on the temperature-dependent PL spectra, Fig. 3(c) demonstrates the fitting results of square of FWHM (FWHM²) as a function of 1/2kT, the result explains that the values of ħω and S are 0.0514 eV and 4.14, respectively. The relatively strong S indicates the strong EPC effect in GZGG:2%Cr³⁺. Stronger EPC effect results in a wider Cr³⁺ emission width, which in turn generally results in a more severe thermal quenching effect. In order to further improve the stability of GZGG:2%Cr³⁺ as well as to broaden its spectral range, Yb³⁺ was chosen to be co-doped with Cr³⁺. The temperature-dependent PL spectra of GZGG:2%Cr³⁺, 0.2%Yb³⁺ were next tested (Fig. 3(d)), and the intensity of the spectra gradually decreased with increasing temperature, and at 100 °C, the emission intensity was calculated by the integrated area to be about 77.9% of that at RT, while it was also maintained at 60% at 150 °C, and the stability was greatly improved (Fig. 3(e)). To better understand thermal quenching process, the activation energy (Ea) could be obtained by Arrhenius equation [32]

 

Fig. 3. (a) Temperature-dependent PL spectra of GZGG:2%Cr³⁺. (b) PL intensity variation. (c) Fitting results of square of FWHM² as a function of 1/2kT. (d) Temperature-dependent PL spectra of GZGG:2%Cr³⁺, 0.2%Yb³⁺. (e) The total integral intensity, peak intensity of Cr³⁺ and peak intensity of Yb³⁺ at different temperatures of GZGG:2%Cr³⁺, 0.2%Yb³⁺. (f) Plotting of ln[(I₀/I)-1] versus 1/(kT) of optimal GZGG:2%Cr³⁺, 0.2%Yb³⁺.

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The QY of the phosphor was improved by Cr³⁺/Yb³⁺ co-doping. In Supplement 1, Fig. S5, the fluorescence QYs as well as the absorption efficiency of GZGG:2%Cr³⁺ and GZGG:2%Cr³⁺, 0.2%Yb³⁺ were tested, respectively. The IQY increased from 47.15% to 54.9% and the EQY also increased from 22.77% to 24.65%. Through the way of ET, the QY of phosphors is improved, which will provide a theoretical basis for the application of phosphors.

To verify the application value of the phosphor, it was packaged with a blue light chip as pc-LED. Figure 4(a) portrays the PL spectrum of the pc-LED at RT, the spectrum covers the emission range of 750-1200 nm, and in the inset pictures of pc-LED without and with the current. The temperature of the pc-LED at different operating currents is also recorded in Fig. 4(b). With the increase of the operating current, the temperature of pc-LED gradually increases and reaches 89.3 °C at 300 mA of current. Due to the unique spectral characteristics of the NIR light source, including invisibility to the naked eye, excellent penetration capability, and low biological tissue-related absorption and scattering, the manufactured pc-LED can be used for night vision surveillance and biological tissue penetration applications. Figure 4(c) displays a photograph taken by a NIR camera using a pc-LED as a light source penetrating a finger in darkness, where the blood vessels of the finger can be clearly seen. The encapsulated pc-LED was simulated for nighttime illumination, and the glass bottle was photographed at 0.5 m using the NIR pc-LED as the light source, and a clearly visible photograph was obtained using the NIR camera, indicating the commercial value of the manufactured phosphor for night vision.

 

Fig. 4. (a) The output spectrum of fabricated NIR pc-LED packaged by combining a 460 nm chip with GZGG:Cr³⁺/Yb³⁺. The inset shows the fabricated LED devices. (b) Thermal images of the pc-LED at different operating currents over 0-300 mA. (c) Photograph of human fingers, glass stopper, and glass bottle after irradiating by the NIR pc-LED.

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4. Conclusions

A series of efficient and stable NIR broadband garnet GZGG:Cr³⁺/Yb³⁺ phosphors were synthesized by actively screening of host with smaller band gaps and selection of suitable electronegativity of the central coordination ions in combination with NE. Through the Cr³⁺/Yb³⁺ co-doping, GZGG: Cr³⁺/Yb³⁺ exhibits broadband emission of 650-1200 nm with an FWHM of 260 nm, while the IQY and EQY of the phosphors are also well enhanced, and reach 54.9% and 24.65%, respectively. The luminescence stability was also greatly enhanced by NE, and at 150 °C, the emission intensity was 60% of that at RT. The produced phosphors were combined with a blue light chip to fabricate pc-LED and tested for their operating temperature at different operating currents. The phosphors were used for vascular visualization and night vision, and the experimental results revealed their application prospects.