(INVITED) Recent progress on broadband near-infrared phosphors-converted light emitting diodes for future miniature spectrometers

Optical Materials: X 1 (2019) 100011

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Invited Article

(INVITED) Recent progress on broadband near-infrared phosphorsconverted light emitting diodes for future miniature spectrometers

T

Veeramani Rajendrana,b, Ho Changb,∗∗, Ru-Shi Liua,b,∗ a b

Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei, 106, Taiwan

ARTICLE INFO

ABSTRACT

Keywords: Broadband near-infrared pc-NIR LED Cr3+ luminescence Miniature spectrometers Near-infrared spectroscopy Phosphor host system

The development of phosphor-converted technology-based broadband near-infrared light source for miniature spectrometers to perform spectroscopy applications has recently attracted remarkable attention among researchers in the academe and industry. The transition metal element Cr3+-activated luminescent materials act as the potential candidates to meet the demands for increased near-infrared light spectral distribution. In this minireview, the most recently developed broadband near-infrared phosphors activated by Cr3+ are listed and classified according to their major chemical element constituents. In addition to the luminescence mechanism of Cr3+, the association between the number of crystallographic sites and spectral distribution of near-infrared light is mainly reviewed with the example of known near-infrared phosphors, which may be helpful in exploring future broadband near-infrared phosphors. Finally, the performance-evaluating parameters of phosphor-converted near-infrared light-emitting diode are discussed and compared with those of known broadband nearinfrared phosphors for spectroscopy applications.

1. Introduction Infrared light is a portion between the visible and microwave regions of the electromagnetic spectrum ranging from 700 nm to 1 mm. Infrared light was discovered by a German-British astronomer named William Herschel in 1800 while investigating the temperature difference among the colors in the visible spectrum by using thermometers. He perceived the value of increased temperature in the thermometer scale different from the red light of the visible region. Herschel [1] assigned that region as infrared light and postulated that infrared light can be sensed as heat. Any object with a temperature of > 268 °C (450 °F) can emit infrared radiations. The improper classification of infrared light is more common in practice. However, the International Commission on Illumination classified infrared light into three categories on the basis of photon energy, as presented in Table 1 [2]. Alternatively, the International Organization for Standardization 20473 classified infrared light on the basis of its wavelength, as shown in Table 2 [2]. Tungsten halogen lamps, laser diodes, supercontinuum lasers, and globars are traditional, commercially available light sources of near-infrared light. Near-infrared spectrometers are nondestructive

analytical characterization tools using near-infrared light to perform near-infrared spectroscopy function in the diverse applications, including agriculture, pharmaceutical, food industry, and noninvasive health monitoring. The massive size of near-infrared spectrometers limits the scope of their usage for research activities in the laboratory. The phosphor research community has recently narrowed down the massive size of standard desktop laboratory spectrometers into miniature size or portable hand-held spectrometers, thereby allowing realtime nondestructive investigations and use by the nonscientific community, especially in food analysis and health monitoring. Therefore, a compact light source with competitive performance to traditional light sources is targeted and highly desirable. Despite the fact that the traditional light sources possess wide spectral distribution of infrared light, the factors of massive size, unstable spectral stability, high electrical consumption, short lifetimes, and large amount of generated heat make traditional infrared light sources as an inferior candidate of light sources for the miniature size or portable hand-held spectrometers. Phosphor-converted near-infrared light-emitting diode (pc-NIR LED) is a promising alternative light source for miniature NIR or portable handheld spectrometers because of its remarkable advantages of smaller

∗ Corresponding author. Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan and Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei, 106, Taiwan. ∗∗ Corresponding author. Department of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei, 106, Taiwan. E-mail addresses: [email protected] (H. Chang), [email protected] (R.S. Liu).

https://doi.org/10.1016/j.omx.2019.100011 Received 1 March 2019; Accepted 14 March 2019 Available online 05 April 2019 2590-1478/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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phosphors and the special focused review by Brik et al. [5] on the nephelauxetic effect relationship of the transition metal ions of Mn4+, Cr3+, and Ni2+ in various host crystals, this brief mini-review highlights the state-of-the-art development of near-infrared phosphors for miniature spectrometers and its design principles. We also include a clear background of near-infrared spectroscopy and its light sources, proper phosphor host system selection, and well-defined tuning approaches to understand the recent progress on this research field.

Table 1 CIE classification of IR radiations. Name

Wavelength (μm)/(nm)

Photon energies

Near-infrared Mid-infrared Far-infrared

0.7–1.4 μm (700–1400 nm) 01.4–3.0 μm (1400–3000 nm) 3.0–1.4 μm (3000–1400 nm)

215–430 100–215 3–100

Table 2 ISO 20473 standard subdivision of IR.

2. Near-infrared spectroscopy history and principles

Name

Wavelength (μm)

Near-infrared Mid-infrared Far-infrared

0.78–3.00 3.00–5.00 5.00–10.00

Near-infrared spectroscopy is based on the principle that every molecule is constructed by several atoms connected with the characteristic bonds between them. When such molecule is excited with a specific radiation of light, it undergoes short-term vibrations in terms of reflection, transmission, and absorption within the molecule on the basis of elemental constituents and bond strength. The nature of such light behavior is unique for each organic molecule and acts as a characteristic spectral fingerprint. The multivariate statistical and analytical tools, such as least squares regression, linear variable technology, and electromechanical systems, are used to extract requisite information from the spectral fingerprint [6]. In 1835, a commercial infrared spectrometer was built by Charles Wheatstone for the first time and utilized for materials study and their chemical characterizations in academic research and also to study natural vegetation, planets, and stars. Afterward, the significant advancement was achieved in the infrared spectroscopy through the works of the famous spectroscopists Hertzberg, Coblenz, and Angstrom [7]. In 1900, infrared spectroscopy has already been the main tool in studying organic compound bonds. From 1964 to 2004, NIR spectroscopy was used in agricultural applications to detect, measure, and monitor the moistures in the grain, soluble solids, and water content of apples, onion, and mushrooms [8]. Near-infrared spectroscopy can also be utilized for bioimaging and biomedical applications due to the penetrating potential of infrared light into the biological tissues without causing any harm [9]. Near-infrared spectroscopy is a main quality control tool in the food industry at present and is being extensively studied in both the academe and industry sector to improve its accuracy.

size, longer lifetime, stable spectral stability, and lower cost than traditional light sources. The resulting spectral light distribution and radiant power from pc-NIR LED light source should be considerably high for effective and efficient functions. Various organic elements present in foodstuffs and human body possess the absorption and reflection spectra of light in the blue region and infrared light of the electromagnetic spectrum, respectively, as shown in Fig. 1 [3]. For example, in the human brain, the absorption and reflectance ranges of hemoglobin, O saturation, and scattering protoporphyrin are 450–600 and 700–900 nm wavelengths, respectively. Hence, broadband near-infrared phosphors that are excitable by blue light are highly desirable to develop miniature spectrometers. Pc-NIR LED also allows the use of infrared light in other application fields, such as in iris/facial recognition, surveillance camera (850–940 nm), gaming notebook, car sensors, virtual reality, and light detection and ranging technology. Thus, understanding the design and development concepts is important to develop a phosphor system that can generate a broad spectral distribution of near-infrared light with benchmarking performance. The overview of some reported scientific articles shows that the luminescent center for near-infrared light in the phosphor material can be rare earth elements (i.e., Pr3+, Nd3+, Tm3+, Eu2+) or transition metal elements (i.e., Cr3+, Ni2+, V2+, Mn4+). Among these elements, the transition element Cr3+ is the commonly selected luminescent center to obtain broadband near-infrared light because of its high sensitivity of d orbitals to local coordination environments. Cr3+ also offer considerable tunability of both narrow and broad spectral light distributions on the basis of crystal field strength (Dq). Compared with the previous review by Zhuang et al. [4] on the transition metal-activated red to near-infrared persistent luminescence

3. Infrared light sources The sun is the ideal broadband infrared light source because it emits the whole range of the electromagnetic spectrum. Infrared light sources are classified into four types, as shown in Fig. 2.

Fig. 1. Absorption and reflection spectra of various organic elements in the human body. 2

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requires an extremely high temperature for improved spectral stability and functioning in the infrared region. Hence, globar is always used in connection with a nichrome wire heater for the infrared light source in practical commercial applications [6]. Globar is also widely used in Fourier transform infrared spectrometer, light sources for infrared microscopy, and heating elements. The factors of high operating temperature and low radiant flux make globar an unsuitable candidate for miniature or handheld portable spectrometers. 3.3. Laser diodes Solid-state laser diode is another promising infrared light source for spectroscopy applications. Solid-state laser diodes also exhibit a broad spectral light distribution from visible to infrared region (100–2400 nm) with good spectral stability. The characteristics of small spot size and low divergence angle of laser diodes make them suitable as light sources for diffuse reflectance diagnosis applications due to the efficient couplings of light emissions to the optical fibers in a sub-mm diameter [11]. However, the requirements of massive housing arrangements and high electrical consumptions obstruct laser diode as light sources for miniature spectrometers or handheld portable spectrometers. For example, the YAG:Nd laser shows 10 emission lines between 1052 and 1123 nm and 9 lines in the region between 1319 and 1144 nm [12].

Fig. 2. Types of infrared light sources.

3.1. Tungsten halogen lamp

3.4. pc-NIR LED

Tungsten halogen lamps are incandescent incoherent light sources operated by the heating of high-temperature tungsten (W) filament filled with the mixtures of noble and N gases at a certain ratio. The application of current heats the W filament and creates vaporized W, which is then redeposited back due to the eradication of halide by halogen at the operation temperature of > 250 °C. Tungsten lamps follow the black-body emission behavior and are also commercially used as an infrared light source in spectrometers for agricultural and medical applications, with the peak at the wavelength of 960 nm with the temperature of 3000 K. Tungsten lamps also exhibit the distinctive continuous spectral distribution of light from visible to infrared light in the electromagnetic spectrum. Ordinary W lamp without halogen gas is also available at an inexpensive cost for spectroscopy applications. Meanwhile, the requirement of a stable DC power supply incurs cost of operation in addition to high power consumption. W halogen lamps distributes a large amount of heat during operation, which further requires a large glass envelope volume. Other types of light sources for visible and UV regions, such as duplex (deuterium and W combination lamps) and Xenon lamps, are available at the wavelengths in the ranges of 0.2–2.5 and 0.2–2.0 μm, respectively. Xenon lamps do not have a smooth spectral distribution in the infrared region [10]. The factors of high bulb temperature, unstable spectral stability with respect to temperature, noncompact size, and high power consumption make Tungsten halogen lamps an inferior candidate for miniature or portable handheld spectrometers.

A light source on the basis of pc-NIR LED offers remarkable advantages over the previously mentioned traditional light sources in terms of compactness (small size), long lifetime, low electrical energy consumption, inexpensive manufacturing and fabrication, high spectral stability, and customized tunable broadband spectral distribution [13–15]. The fabrication of a pc-NIR LED device follows the general principles of white LED devices. In brief, the emitted light from the blue LED chip is used to excite the near-infrared phosphor deposited over the blue-chip, which results in near-infrared luminescence. 4. Nephelauxetic effect and Dq theories The emission wavelength of inorganic phosphor materials is influenced by the two main effects, namely, nephelauxetic effect and crystal field splitting. Nephelauxetic effect came from Greeks words that literally mean “cloud expanding.” The incorporation of an activator into the host lattice results in the formation of chemical bonds with the surrounding ligands or anions, which affect the delocalization of the outer d orbitals of the activator ions. Hence, the interelectron repulsion within the d shell is decreased due to the sharing of some d electrons within the anions and p- and s- orbitals. The spectroscopic properties of the transition metal ions Mn4+, Cr3+, and Ni2+ in various hosts are reviewed by Brik et al. [5], who proposed the possible methods to tune the emission of spin-forbidden transitions (otherwise known as line emission) and also introduced the new parameter in approximating the nephelauxetic effect. By contrast, spin-allowed transition, that is, broadband emission, is the focus of interest in this review for spectroscopy applications. Crystal field theory assumes that ions are point charges. In the case of free ion, the energy values of the five degenerated d orbitals of the Cr3+ activators dxy, dyz, dxz, dx2-y2, and dz2 are the same. However, the bonding of the activator by the anions induces repulsive force in the system, which is considerably dependent upon anion orientation. Hence, the five degenerated d orbitals undergo orbital splitting on the basis of the energy value of each d orbital in the geometric structure. As shown in Fig. 3a, Cr3+ is an activator coordinated by the O ions in the octahedral structure. The five degenerated d orbitals dxy, dyz, dxz, dx2-y2, and dz2 are split into dx2-y2 and dz2 as doublet state (Eg) with high energy level and dxy, dyz, and dxz as triplet state (T2g) with low energy structure,

3.2. Nichrome heater and globar The term “globar” is the combination of the words “glow” and “bar,” which indicates that the material will emit light when the bar reaches considerably high temperatures. Globar is a simple ceramic rod made up of silicon carbide (SiC) assorted with some additives and manufactured by sintering at high temperatures. Globar is heated via resistor heating by applying an electric current ranging from 5 A to 10 A (50 W–100 W) to create exothermic heat of up to 1400 K, which in turn produces infrared light through the interaction with the downstream interference filter. Globar is the continuous thermal radiation source, which is similar to black body radiators, and exhibits a broad spectral distribution of the wavelength ranging from 1 μm to 50 μm. Globar 3

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Fig. 3. a) Scheme of the nephelauxetic effect and crystal field splitting (CFS) of Cr3+ and b) CFS of d orbitals in octahedral and tetrahedral coordination.

which is represented as Eg and T2g. The energy value difference between Eg and T2g levels is called crystal field splitting energy and denoted either as Δo or 10 Dq. Meanwhile, if Cr3+ is surrounded by the O ions in tetrahedral structure, then the pattern of orbital splitting is entirely reversed compared with the octahedral structure, as shown in Fig. 3b. This finding indicates that the Eg state is at a low energy level, while the T2g state is at a high energy level due to the lack of inversion center. Thus, the degree of crystal field splitting is calculated using the following equation (Equation (1)) [16]:

Dq =

ze 2r 4 6R5

Fig. 4. Tanabe–Sugano diagram of d3 electronic configuration.

2) Intermediate crystal field (∼2.3) 3) Weak or Low crystal field (< 2.3) A strong crystal field always exhibits a narrow or line emission spectrum because 2Eg is always below the 4T2g state. In other words, 2Eg is the first excited state and is a stable energy storage state. The electronic transitions in the strong crystal field are called spin-forbidden transition. By contrast, the positions of the 2Eg and 4T2g states are switched entirely while moving toward the weaker crystal field, thereby indicating that 4T2g is below the 2Eg state and becomes the first excited state. The electronic transitions in the weak crystal field always exhibit a broad emission spectrum and called as spin-allowed transitions. At the intermediate crystal values (∼2.3), the 4T2g and 2Eg energy states are intersected together and result in orbital mixing. The compounds possessing intermediate crystal field values will experience both strong and weak crystal field characteristics, which result in both narrowband or line (spin-forbidden) and broad lines (spin-allowed) in the emission spectrum, respectively. Considering the high positive charge, Mn4+ always experiences a strong crystal field, but Cr3+ can be located at any point of the crystal field value. Hence, the emission spectrum of Cr3+ can be tuned into narrow lines or broadband and both by modulating the Dq.

(1)

Where Dq is the crystal field strength, z is the anion valence, e is the charge of an electron, r is the radius of the d wave function, and R is the distance between the central ion and ligands. As shown in Equation (1), Dq is inversely proportional to the bond length between the activator and anions. The Dq becomes weaker if the bond length increases and vice versa. The weak crystal field leads to a red shift in the photoluminescence spectrum. The Tanabe–Sugano diagram shown in Fig. 4 is applicable for the transition metals in octahedral coordination complexes with d3 electronic configuration. V2+, Cr3+, Mn4+, and Fe5+ are transition elements that fall under the category of d3 electronic configuration with the ground state as 4F and 120 possible allowed states, as determined by the Pauli exclusion principle [17]. The x- and y-axes of the diagram represent the Dq/B and E/B values, respectively, where Dq is the crystal field strength, E is the energy of the transitions, and B is the Racah parameters. Dq/B is 0 on the far-left side of the diagram corresponds to the free ion behavior. The increment in the Dq/B values leads to the proportional increase in 4T2 energy value state because the 4T2 energy state is directly related to 10 Dq according to crystal field theory. Meanwhile, the variation in the energy value of 2Eg is extremely low or almost constant, thereby indicating that 2Eg is a highly stable energy state despite any crystal field value. According to the Dq/B value, the strength of the crystal field is classified into the three following categories, as follows [18,19]:

5. Known broadband near-infrared phosphors for spectroscopy applications Cr3+-doped materials are investigated well by the scientific community for laser and persistent luminescence applications for the past several decades. However, only few research articles have discussed the importance and demonstrated the implementation of broadband pc-NIR LED as alternative light sources for spectroscopy applications, as listed in Table 3. All of these research articles have been published from 2017

1) Strong or high crystal field (> 2.3) 4

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Table 3 Known broadband near-infrared phosphors for spectroscopy applications. Host material

Incorporated site

No. of sites

Crystal system

Space group

Emission range (nm)

FWHM (nm)

Year of publication

Reference

Lu3Al5O12: 0.05 Ce3+, 0.5% Cr3+

Ga3+ (VI) Ga3+ (IV) Ga3+ (VI) Ga3+/Ge4+ (IV)

2

Cubic

Ia3¯ d

500–850

–

2017

[3]

3

Trigonal

P321

600–1200

330

2018

[20]

– 2

– Cubic

Ia3¯ d

1000–1600 650–850

– 117

2018

[21]

1 1

Rhombohedral Trigonal

R3¯ c R32

700–950 650–850 and 980

120 –

2018 2018

[22] [23]

1

Monoclinic

C2/c

750–950

–

2018

La3Ga4.95GeO14:0.05 Cr3+ Bi-doped GeO2 glass Ca2LuZr2Al3O12:0.08 Cr3+ 3+

ScBO3:0.02 Cr YAl3(BO3)4:0.04 Cr3+, 0.01 Yb3+ NaScSi2O6: 0.06 Cr3+

Ga3+ (VI) Ga3+ (IV) Sc3+ (VI) Al3+ (VI) Al3+ (IV) Sc3+ (VI)

to 2018, which reveals the freshness and importance of this mini-review. The publication of the related research articles may gradually increase in the future. To survey the commercial viewpoint regarding the practicability of broadband near-infrared light emitting diode for spectroscopy applications, OSRAM, which is a multinational lighting industry headquarters in Munich, Germany, announced that they have successfully developed the world's first infrared broadband emitter (SFH4735) in the range of 650–1050 nm for near-infrared spectroscopy applications in the press meeting on November 2016. SFH4735 was designed based on phosphor converter technology excitable by blue light [24]. OSRAM also introduced another infrared broadband light emitter diode (SFH4776) as a shrunken version of SFH4735 for spectroscopy applications with only 0.6 mm high and space-saving imprint of 2.75 mm × 2.0 mm as an ideal one for smartphones in September 2018 [25]. OSRAM also discussed the potential of seven broadband nearinfrared light-emitting phosphors for spectroscopy and endoscopy applications [13]. Another multinational lighting industry, that is, Philips, is also engaged in developing an integrated spectrometer for spectral tissue sensing for real-time biological tissue characterization by using the technique pc-NIR LED through the project InSPECT2020. The vision of the InSPECT2020 project is to develop photonic needles for biopsies with pc-NIR LED light source and utilize in sensing spectral issues for tumor screening by oncology physicians with ease [3,26]. Similarly, several small- and medium-scale lighting industries are focusing actively in developing novel broadband near-infrared phosphors. We also attempted to list the possible near-infrared light emitting known Cr3+-activated aluminates and gallate-based chemical systems, as shown in Table 4, in addition to those listed in Table 3. The phosphor host system for near-infrared light is classified into five types, as shown in Fig. 5 and discussed in section 5.1. As presented in Tables 3 and 4, the incorporation of Cr3+ ions will be substituted at the crystallographic site positions of Al3+, Ga3+, and Sc3+ in both octahedral and tetrahedral local coordination environments due to similar ionic radii and valence state (+3). With the coordination number of 6 (octahedral), the ionic radii of Al3+, Ga3+, Sc3+, and Cr3+ are 0.535, 0.62, 0.745, and 0.615 Å, respectively. Meanwhile, when the coordination number is 4 (tetrahedral), the ionic radii of Al3+, Ga3+, and Cr3+ are 0.39, 0.47, and 0.41 Å, respectively. Hence, designing near-infrared phosphors containing any one of these chemical elements is the first preferred choice for near-infrared luminescence.

Table 4 Cr3+-activated aluminates and gallate-based near-infrared phosphors. Host

Incorporated site

No. of Sites

Emission range (nm)

Reference

ZnGa2O4 Zn(Ga1-xAlx)2O4

Ga3+ (VI) Ga3+ (VI) Al3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ca2+ (VIII) Ge4+ (IV) Al3+ (VI) Al3+ (IV) Ga3+ (VI) Ga3+ (VI) Al3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (IV) Ga3+ (VI) Ga3+ (VI) Ga3+ (VI) Ga3+ (IV) Ga3+ (V)

1 1

650–750 675–800

[27–29] [30]

1 1 1 1 1 1 3

650–750 650–770 650–1000 650–800 600–800 600–800 670–1100

[31] [32] [33] [34] [35,36] [37] [38–40]

2

650–750

[41]

1 1 1 3

500–800 650–800 500–850 600–1200

[42] [43,44] [45] [20,46,47]

1 1 3

650–850 650–850 650–950

[48] [49] [50]

ZnxGa2O3+x MgGa2O4 Zn3Ga2Ge2O10 Zn1+xGa2-2x(Ge,Sn)xO4 Zn3Ga2Ge2O10 Zn3Ga2Sn1O8 Ca3Ga2Ge3O12 Ca14Zn6Al10O35 Y3Al2Ga3O10 Gd3Ga5O10 Lu3Al5O12 La3Ga5GeO14 LiGa5O8 β-Ga2O3 SrGa12O19

5.1. Selection of near-infrared inorganic phosphor host Fig. 5. Host classification for broadband near-infrared inorganic phosphors.

Ruby crystal is sapphire (Al2O3) with the impurity ions of Cr3+ in some Al3+ sites. The deep red (near-infrared) luminescence of the famous handsome gemstone named ruby crystal has initiated interests among researchers to study Cr3+-doped compounds for laser applications. The emission spectrum of the ruby crystal consists of several 5

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emission lines that are close together. Meanwhile, the Cr3+-doped Ga2O3 crystal samples show a narrowband emission spectrum (600–800 nm) centered at 690 nm under the excitation of 310 nm. However, spin-forbidden transition predominately occurs in both samples. A high ionic radius of Ga3+ ion in comparison with Al3+ ions for the octahedral coordination induces a low reduction in the Racah parameters (B and C) and changes in the crystal field splitting (Dq) values, which generates new spectroscopic properties. The quantitative spectroscopic parameters are as follows: Ga2O3:Cr3+ Dq = 1667 cm−1, B = 529 cm−1, C = 3413 cm−1 and Al2O3:Cr3+ Dq = 1664 cm−1, B = 640 cm−1, and C = 3300 cm−1 [51,52]. This observation suggests why gallate-based host systems may be a better option for broadband near-infrared luminescence than aluminate-based host systems.

applications [19,56–58]. 5.1.4. La-gallogermanate system La-gallogermanate systems are also investigated for the persistent luminescence capability by doping Cr3+ and rare-earth elements as codopants or sensitizers. The emission spectrum of Cr3+-doped LaGaO3 single crystal reveals several narrow lines with the two maximum positions at 739 and 729 nm [59]. Jia et al. [46] proposed the La-gallogermanate system for the first time with the chemical composition of La3Ga5GeO14:Cr3+, M, where M = Li+, Pb2+, Zn2+, Eu3+, Tm3+, Dy3+, and demonstrated its potential for long persistent near-infrared luminescence applications with the broad emission wavelength range of 700–1100 nm. The system shows the highest luminescence decay time of > 8 h for the optimized luminescence intensity by doping Dy3+ as coactivators. This result suggests that the La-gallogermanate system may be a good potential candidate for fluorescence property and is inferior to phosphorescence. A detailed persistent luminescence mechanism study has been conducted by Yan et al. [47] for the compound with the chemical composition of La3Ga5GeO14:Cr3+ (1% mole) and Zn2+ (1% mole) and achieved the persistent luminescence decay time of > 1 h only. The inferior performance of La-gallogermanate has not received considerable attraction from the researchers in the phosphor community for the phosphorescence applications. Nevertheless, the investigations regarding the applications of the possible new members of the La-gallogermanate family are also carried out and studied again for phosphorescence applications; the possible new members include La3GaGeO16:Cr3+, LaGaGe2O7:Mn4+, La3Ga5SiO14:Cr3+, and La3Ga5.5Nb0.5O14:Cr3+ [60–62]. The inferior phosphorescence behavior and high bandwidth in the near-infrared region suggest that the Lagallogermanate system can be the proper host for the broadband nearinfrared spectroscopy applications. In 2018, Rajendran et al. [20] investigated the La-gallogermanate system of La3Ga5GeO14:Cr3+ for the near-infrared spectroscopy application and proposed that the super broadband near-infrared luminescence of 650–1200 nm with the fwhm of 330 nm are mainly due to the presence of the two distinct luminescent center.

5.1.1. Zn-gallogermanate system The spinel crystal structure of MGa2O4 (M = Zn, Mg) is focused on persistent luminescence studies due to the possible generation of antisite defects and O vacancies due to Cr3+ transition element doping. The high Dq (Dq/B = 3.3) and the observation of the luminescence from only the octahedral coordinated Ga site (no luminescence from the tetrahedral coordination Ga site) result in the narrow band emission (650–770 nm) spectrum with a decreased full width at half maximum (fwhm) [27,33,53]. Pan et al. [34] modified the inverse spinel crystal structure of ZnGa2O4 by doping the tetravalent chemical element of Ge4+ in the Ga3+ site and reported the breakthrough near-infrared persistent phosphors with the benchmarking persistent luminescence decay time of > 360 h for the first time to the phosphor community, which can be activated by sunlight. However, the authors failed to prove the structural modifications of Ge substitution and the mechanism of broadband near-infrared persistent luminescence (650–1000 nm) with several emission lines when excitable by ultraviolet light [54]. Thus, many persistent luminescence studies are conducted in the Zn-gallogermanate system by codoping the sensitizers and chemical substitution [35–37]. Although the emission bandwidth is high, the presence of antisite defects may deteriorate the fluorescence performance and increase the decay time due to spin-forbidden transition alone, thereby making the Zn-gallates and Zn-gallogermanate system as inferior candidates for broadband near-infrared spectroscopy applications.

5.1.5. Borate system The borate-based host system is also focused on broadband nearinfrared spectroscopy applications in recent times. The borate-based host system possesses low thermal quenching behavior, which is one of the main important parameters required for LED semiconductor. Meanwhile, the use of the borate host system eliminates the high-cost starting precursors, such as Ge and rare earths, in the host, which is an additional advantage considering its practical implementations. Shao et al. [22] reported ScBO3:Cr3+ for near-infrared spectroscopy applications with the broad emission spectrum in the range of 700–950 nm centered at 800 nm and a fwhm of 120 nm. The crystal field value (Dq/B value) is 2.15, which perfectly lies in the weak crystal field sites of the Tanabe–Sugano diagram. Hence, only spin-allowed transitions offer a considerable range of tuning possibility through crystal field engineering because the 4T2 state is highly sensitive to the changes in the crystal field. Shao et al. [23] also reported another promising borate host system for near-infrared luminescence with the chemical composition of Y0.99Al2.96(BO3)4:0.04Cr3+/0.01 Yb3+, which exhibits broad emission spectrum in the range of 670–800 nm in the weak crystal field side (Dq/B = 2.23) of the Tanabe–Sugano diagram centered at 710 and 980 nm.

5.1.2. Ca-gallogermanate system The Ca-gallogermanate system is another category of a host system for the broadband near-infrared spectroscopy applications. The selection design principle and chemical composition of this system are almost similar to those of the Zn-gallogermanate system, but Ca is present in the host system instead of Zn. Most Ca-gallogermanate compounds are located in the weak crystal field region of the d3 Tanabe–Sugano diagram due to the presence of large divalent cation in the host. Hence, Cr3+ in Ca-gallogermanate compounds follows the spin-allowed transitions and results in tunable broadband emission spectrum. For example, Cr3+-doped Ca3Ga2Ge4O14 shows the Dq/B value of 2.07 [55]. However, Cr3+-doped Ca3Ga2Ge3O12 with three crystallographic sites shows a broadband emission spectrum in the range of 600–1000 nm centered at 702.8, 723, and 748 nm, with the Dq/B value of approximately 2.4 [39,40]. 5.1.3. Garnet-type system The garnet-type crystal structure with the chemical formula of A3B5O12 (A = Y, Gd, La, Lu and B = Al, Ga) is also investigated for Cr3+ doping in the B3+ site and focused mainly on persistent luminescence studies. Garnet-type persistent luminescent phosphors show uniform single broadband near-infrared luminescence in the range of 650–800 nm centered at 700 nm, which is in contrast to those of Zngallate and Zn-gallogermanate systems, and the emission bandwidth of which is extremely narrow for broadband near-infrared spectroscopy

6. Roles of crystallographic sites In this section, the three kinds of gallate-based near-infrared phosphors, namely ZnGa2O4, Ca2LuZr2Al3O12:Cr3+, and La3Ga4.95GeO14:Cr3+, are considered as examples to discuss the effects of a number of crystallographic sites on broadband near-infrared luminescence. 6

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Fig. 6. a) Cubic spinel structure of ZnGa2O4 and b) normalized photoluminescence emission and persistent luminescence spectra of ZnGa2O4:Cr3+. [Picture adapted with permission from Ref. [28] © 2013 American Chemical Society].

6.1. Type I: single crystallographic site The spinel crystal structure with the chemical composition of AB2O4 (A = Zn, Mg, Bi; B = Al, Ga) is an attractive near-infrared light emitting phosphor with two crystallographic sites for Cr3+ ion luminescence, as shown in Fig. 6 [28]. The crystal structure can be depicted as tetrahedral sites for A2+ and octahedral sites for B3+. The phosphor host system with the spinel crystal structure exhibits a near-infrared luminescence of 650–750 nm with the presence of R line, phonon side bands, and antiphonon side bands. The near-infrared host system with the single crystallographic sites always results in decreased fwhm, which may be an unsuitable parameter for broadband spectroscopy applications. Spinel structure compounds are mainly studied for persistent luminescence applications. For example, ZnGa2O4:Cr3+ and MgGa2O4:Cr3+ emit the near-infrared emissions in the ranges of 650–750 and 650–770 nm, respectively [28,33]. The borate-based near-infrared host system also possesses a single crystallographic site for Cr3+ ion luminescence. ScBO3:Cr3+ belongs to the rhombohedral crystal structure (space group = R3¯c ) with the layered calcite-like structure (Fig. 7) [22] and shows the near-infrared luminescence in the range of 700–950 nm with the fwhm of 120 nm. Meanwhile, another borate system YAl3(BO3)4:Cr3+ codoped with Yb3+ shows the near-infrared luminescence range of 650–850 nm centered at 710 nm [23]. 6.2. Type II: two crystallographic site systems The chemical systems with the garnet crystal and inverse spinel crystal structures are the most famous near-infrared host systems with the presence of two crystallographic sites for the incorporation of Cr3+ ions into the crystal lattice. The garnet structure exhibits the chemical formula of A3B5O12:Cr3+, where A = Lu, Gd, Y, La, and B = Al, Ga. The crystal structure was evaluated as the presence of B3+ crystallographic site with the Wyckoff position of 24d and 16a in the local environments of octahedral and tetrahedral coordinations, respectively. The local surrounding environments of A3+ site will be dodecahedral. Generally, the substitution of Cr3+ ions at the crystallographic site of B3+ in the crystal lattice will result in the near-infrared luminescence in the range of 660–850 nm. The position of luminescence center can be easily controlled by the chemical substitution of cations at the A3+ and B3+ sites on the basis of the Dq. In all cases, Cr3+ions in tetrahedral local coordination possess the valence of 3. Hence, the fwhm of the nearinfrared luminescence is always decreases when choosing Cr3+ as the activator. Reports have yet to explain the tetrahedral luminescence of Cr3+ ions in garnet structure. Meanwhile, the absorption of Cr3+ ions is extremely weak due to the occurrence of a transition between the d-d orbitals only. Hence,

Fig. 7. a) Crystal structure representation of ScBO3, b) coordination environment of [ScO6] octahedron, and c) excitation and emission spectra of ScBO3:0.02Cr3+ phosphors. [Picture adapted with permission from Ref. [22] © 2018 Royal Society of Chemistry].

introducing a sensitizer, such as Ce3+, Yb3+, and Sm3+, is essential and preferable in designing the garnet structure for near-infrared luminescence. The garnet structure can also be tailored by the nonequivalent substitution of the divalent (i.e., Ca2+ and Sr2+) and tetravalent ions (i.e., Zr4+ and Ge4+) in the crystallographic positions of A3+ and B3+ sites with the maintenance of the charge balance to induce the disturbance in electron orbits, which results in new spectroscopic properties and improved near-infrared luminescence. For example, Zhang et al. [21] synthesized the garnet structure with the chemical composition of Ca2LuZr2Al3O12:Cr3+ (Ca2+ in the A3+ site and Zr4+ in the Al3+ site) and achieved the near-infrared luminescence range of 7

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infrared luminescence with the wavelength range of 600–1200 nm at the excitation wavelength of 473 nm. The photoluminescence emission spectra of La3Ga5GeO14:0.05 Cr3+ consist of sharp R lines at 690 nm and the two distinct emission peaks centered at 750 and 920 nm. The sharp line and emission peak at 750 nm are believed to be the Cr3+ ion luminescence in the octahedral coordination structure, and the transitions are labeled as per the Tanabe–Sugano diagram of the 3d3 electron configuration. By contrast, the emission peak centered at 920 nm was investigated as the Cr3+ ion luminescence in the tetrahedral local coordination structure. As shown above, the observation of Cr3+ luminescence in the tetrahedral coordination environment is unusual and impossible. However, in the La3Ga5GeO14:0.05 Cr3+ system, the valence of Cr3+ ions in the tetrahedral coordinating environment is +4 on the basis of the X-ray absorption near-edge structure, electron paramagnetic resonance, and time-resolved spectroscopy experiment results. The fwhm of 330 nm is obtained as the combined luminescence of Cr3+ and Cr4+ in the La-gallogermanate chemical system of La3Ga5GeO14. Li et al. [63] also achieved tunable broadband near-infrared luminescence in the range of 650–1500 nm with the spectral gap of 950–1000 nm in an amorphous solid system, such as aluminate glass, by doping Cr ions. The emission range of 650–950 and 1000–1500 nm is categorized as the luminescence of Cr3+ and Cr4+, respectively, as shown in Fig. 10. The transformation of Cr ions into Cr4+ state through self-redox reaction and crystal field modulation by doping Sr2+ ions are anticipated as the possible reasons for the emission intensity and broadness. Other chemical systems with three crystallographic sites, such as SrGa12O19:Cr3+ (Ga on octahedron, tetrahedron, and hexahedron), also exhibit broadband near-infrared luminescence in the range of 650–950 nm at the excitation wavelength of 425 nm.

Fig. 8. a) Crystal structure of Ca2LuZr2Al3O12 and b) excitation and emission spectra of Ca2LuZr2Al3O12:Cr3+ phosphors. [Picture adapted with permission from Ref. [21] © 2018 Royal Society of Chemistry].

550–750 nm with the two emission bands at 754 and 813 nm, as shown in Fig. 8. Lin et al. [40] achieved the broadband near-infrared luminescence with the emission range of 650–1100 nm in the chemical system of Ca3Ga2Ge3O12:Cr3+. The emission spectra of Ca3Ga2Ge3O12:Cr3+ contain three attractive luminescence peaks at 749, 803, and 907 nm, which are ascribed to the occupancy of Cr ions at Ca2+, Ga3+, and Ge4+, respectively.

7. Infrared light-emitting diode structure LED is a semiconductor device constructed by two semiconductor materials, which include both p-type and n-type semiconductors, as shown in Fig. 11a. On the application of voltage in the forward bias condition, the electrons from n-side and holes from p-side are recombined in the depletion zone and photons are released. The wavelength of the emitted photons is substantially determined by the use of phosphor conversion materials, and the elements are included in the semiconductor device. The structure of pc-NIR LED optoelectronic component shown in Fig. 11b consists of a blue semiconductor chip generally made up of aluminum indium gallium phosphide or indium gallium nitride mounted on the epitaxial layer sequence in emitting blue light. The semiconductor chips are connected to the cavity in the base housing through a bonding wire. The base housing is embedded in a recess. The weighted amount of inorganic phosphor conversion materials is mixed with the matrix materials (i.e., silicone) and filled the cavity in the base

6.3. Type III: three crystallographic site systems La3Ga5GeO14 belongs to the crystal system of trigonal with the space of P321 (150). The crystal structure of La3GaGe5O16 is characterized by the presence of three Ga3+ crystallographic sites for the substitution of Cr3+ ions in the crystal lattice, as shown in Fig. 9 [20]. Out of the three sites, the Ga1 and Ga3 crystallographic sites are in the local coordination of octahedral structure with the Wyckoff positions of 1a and 3f, respectively. Meanwhile, the crystallographic site of Ga2 is in the local coordination structure of tetrahedral with the partial sharing of the 2 d Wyckoff position with the Ge4+ ion. When Cr3+ is incorporated into the crystal lattice at the crystallographic site position of Ga3+, it exhibits the super broadband near-

Fig. 9. a) Crystal structure of La3Ga5GeO14 and b) excitation and emission spectra of La3Ga4.95GeO14:0.05 Cr3+ phosphors. [Picture adapted with permission from Ref. [20] © 2018 American Chemical Society]. 8

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Thus, increasing the driving current leads to the subsequent increase in near-infrared power but decrease in photoelectric energy conversion efficiency. Table 5 shows a list of reported near-infrared phosphors with the photoelectric properties of the fabricated pc-NIR LED. Hayashi et al. [3] achieved a high radiant flux of 47 mW as the combination of phosphor conversion elements, namely, crystalline Lu3Al5O12:Ce3+, Cr3+, and Bi2+-doped GeO2 amorphous glass, to cover the spectral distributions of 470–1600 nm with the spectral gap ranging from 850 to 1000 nm [26]. The photoelectric energy conversion efficiency value is missing in Table 5. Hence, the value is not considered for comparison with the other phosphor host systems shown in Table 5. As listed in Table 5, the La-gallogermanate host system of La3Ga4.95GeO14:Cr3+ with the characteristic features of three crystallographic sites for Cr3+ ions exhibits the highest photoelectric energy conversion efficiency of 76.59% but lowest radiant power (18.2 mW) compared with the radiant power of borate host system with a single crystallographic site of Cr3+ ions. The photoelectric energy conversion efficiency of garnet and borate host systems is < 10%. 8. Conclusions

Fig. 10. Emission spectra of Cr-doped aluminate glass upon excitation at 310 and 670 nm [Picture adapted with permission from Ref. [63] © 2019 Royal Society of Chemistry].

Although the new near-infrared phosphors are also being continuously developed by the phosphor community, chemical systems that are qualified for the practical applications, especially for broadband-related spectroscopy applications, are extremely few. In this minireview, a short survey is performed for the possible phosphor host systems activated by Cr3+ for the broadband near-infrared light-emitting miniature spectrometers for spectroscopy applications. The main criteria for the effective and efficient functions of the miniature spectrometers are the high fwhm and radiant flux with optimum photoelectric energy conversion efficiency. The comparison of the crystallographic availability for Cr3+ ions suggests that the phosphor host system with an increased number of crystallographic sites (La3Ga5GeO14) can promote high spectral distribution of near-infrared light and photoelectric energy conversion efficiency but end with decreased radiant flux. Meanwhile, the phosphor host system with single crystallographic sites (ScBO3) for Cr3+ can exhibit high radiant flux but restricted spectral distribution of near-infrared light and photoelectric energy conversion efficiency. Future perspectives on broadband nearinfrared light for spectroscopy application can include the exploration of fluoride and oxynitride-based phosphor host systems other than the oxides and the enhancement of the pc-NIR LED performance of the existing systems.

housing frame. The reflector is mounted on the base housing unit with suitable materials coated on the inner walls of the recess to attain the maximum reflection of primary radiations from the blue semiconductor chip and infrared luminescence from the conversion materials embedded in the matrix materials. During operation, the blue light emitted from the semiconductor device will act as an excitation source of light for the phosphor conversion materials and results in infrared luminescence. The near-infrared LED device appears bluish white in addition to invisible infrared LED during the operation due to the possible combinations of visible red and blue lights [13]. 7.1. Evaluation parameters of pc-NIR LED Given that infrared light is invisible to human eyes, the performance of the pc-NIR LED is evaluated in the terms of radiant flux, which generally quantifies the radiant energy emerging from the source per unit time. Radiant flux is denoted in the SI units W, mW, or μW. Another performance evaluating parameter of the pc-NIR LED is the photoelectric energy conversion value. The term photoelectric energy conversion describes the emissions of photons from the pc-NIR LED device surface due to the interaction between incident light and matter. Hence, photoelectric energy conversion is the ratio of the output powers to the input power. Given that the photoelectric energy conversion efficiency value is indirectly proportional to the input power, thereby indicating that obtaining high photoelectric energy conversion by operating the pc-NIR LED device at low input power is beneficial.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 11. a) PN junction design and its working function and b) LED design structure. 9

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Table 5 Photoelectric properties of reported broadband near-infrared phosphors. Host material

Excitation wavelength

Operating current

NIR power Photoconversion efficiency Year of publication Reference

Lu3Al5O12:0.05 Ce3+, 0.5% Cr3+ + Bi-doped GeO2 glass La3Ga4.95GeO14:0.05 Cr3+ Ca2LuZr2Al3O12:0.08 Cr3+ ScBO3:0.02 Cr3+ YAl3(BO3)4:0.04 Cr3+, 0.01 Yb3+ + NaScSi2O6:0.06 Cr3+

470 450 460 460 460

100 350 20 120 100

47 18.2 2.448 26 26

Acknowledgments

– 76.59 4.1 7 8.6

2017 2018 2018 2018 2018

[3] [20] [21] [22] [23]

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