Recent Progress On Broadband Near-infrared Phosphors-converted Light .
Optical Materials: X 1 (2019) 100011Contents lists available at ScienceDirectOptical Materials: Xjournal homepage: ed ArticleRecent progress on broadband near-infrared phosphors-converted lightemitting diodes for future miniature spectrometersTVeeramani Rajendrana,b, Ho Changb, , Ru-Shi Liua,b, abDepartment of Chemistry, National Taiwan University, Taipei, 106, TaiwanDepartment of Mechanical Engineering and Graduate Institute of Manufacturing Technology, National Taipei University of Technology, Taipei, 106, TaiwanARTICLE INFOABSTRACTKeywords:Broadband near-infraredpc-NIR LEDCr3 luminescenceMiniature spectrometersNear-infrared spectroscopyPhosphor host systemThe development of phosphor-converted technology-based broadband near-infrared light source for miniaturespectrometers 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 asthe 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 ofCr3 , the association between the number of crystallographic sites and spectral distribution of near-infraredlight is mainly reviewed with the example of known near-infrared phosphors, which may be helpful in exploringfuture 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. IntroductionInfrared 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 namedWilliam 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 thermometerscale different from the red light of the visible region. Herschel [1]assigned that region as infrared light and postulated that infrared lightcan be sensed as heat. Any object with a temperature of 268 C(450 F) can emit infrared radiations. The improper classification ofinfrared light is more common in practice. However, the InternationalCommission 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 20473classified infrared light on the basis of its wavelength, as shown inTable 2 [2]. Tungsten halogen lamps, laser diodes, supercontinuumlasers, and globars are traditional, commercially available light sourcesof near-infrared light. Near-infrared spectrometers are nondestructiveanalytical characterization tools using near-infrared light to performnear-infrared spectroscopy function in the diverse applications, including agriculture, pharmaceutical, food industry, and noninvasivehealth monitoring. The massive size of near-infrared spectrometerslimits the scope of their usage for research activities in the laboratory.The phosphor research community has recently narrowed down themassive 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, acompact light source with competitive performance to traditional lightsources is targeted and highly desirable. Despite the fact that the traditional light sources possess wide spectral distribution of infraredlight, the factors of massive size, unstable spectral stability, high electrical consumption, short lifetimes, and large amount of generated heatmake traditional infrared light sources as an inferior candidate of lightsources for the miniature size or portable hand-held spectrometers.Phosphor-converted near-infrared light-emitting diode (pc-NIR LED) isa 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 GraduateInstitute 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: f10381@ntut.edu.tw (H. Chang), rsliu@ntu.edu.tw (R.S. ived 1 March 2019; Accepted 14 March 2019Available online 05 April 20192590-1478/ 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND ND/4.0/).
Optical Materials: X 1 (2019) 100011V. Rajendran, et al.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 forminiature spectrometers and its design principles. We also include aclear 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 1CIE classification of IR radiations.NameWavelength (μm)/(nm)Photon �1.4 μm (700–1400 nm)01.4–3.0 μm (1400–3000 nm)3.0–1.4 μm (3000–1400 nm)215–430100–2153–100Table 2ISO 20473 standard subdivision of IR.2. Near-infrared spectroscopy history and principlesNameWavelength .003.00–5.005.00–10.00Near-infrared spectroscopy is based on the principle that everymolecule is constructed by several atoms connected with the characteristic bonds between them. When such molecule is excited with aspecific radiation of light, it undergoes short-term vibrations in terms ofreflection, transmission, and absorption within the molecule on thebasis of elemental constituents and bond strength. The nature of suchlight behavior is unique for each organic molecule and acts as a characteristic spectral fingerprint. The multivariate statistical and analyticaltools, such as least squares regression, linear variable technology, andelectromechanical systems, are used to extract requisite informationfrom the spectral fingerprint [6].In 1835, a commercial infrared spectrometer was built by CharlesWheatstone for the first time and utilized for materials study and theirchemical characterizations in academic research and also to studynatural vegetation, planets, and stars. Afterward, the significant advancement was achieved in the infrared spectroscopy through theworks of the famous spectroscopists Hertzberg, Coblenz, and Angstrom[7]. In 1900, infrared spectroscopy has already been the main tool instudying organic compound bonds. From 1964 to 2004, NIR spectroscopy was used in agricultural applications to detect, measure, andmonitor the moistures in the grain, soluble solids, and water content ofapples, onion, and mushrooms [8]. Near-infrared spectroscopy can alsobe utilized for bioimaging and biomedical applications due to the penetrating potential of infrared light into the biological tissues withoutcausing any harm [9]. Near-infrared spectroscopy is a main qualitycontrol tool in the food industry at present and is being extensivelystudied 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 highfor effective and efficient functions. Various organic elements present infoodstuffs and human body possess the absorption and reflectionspectra of light in the blue region and infrared light of the electromagnetic spectrum, respectively, as shown in Fig. 1 [3]. For example, inthe human brain, the absorption and reflectance ranges of hemoglobin,O saturation, and scattering protoporphyrin are 450–600 and700–900 nm wavelengths, respectively. Hence, broadband near-infrared phosphors that are excitable by blue light are highly desirable todevelop miniature spectrometers. Pc-NIR LED also allows the use ofinfrared 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 distributionof near-infrared light with benchmarking performance.The overview of some reported scientific articles shows that theluminescent center for near-infrared light in the phosphor material canbe rare earth elements (i.e., Pr3 , Nd3 , Tm3 , Eu2 ) or transitionmetal elements (i.e., Cr3 , Ni2 , V2 , Mn4 ). Among these elements,the transition element Cr3 is the commonly selected luminescentcenter to obtain broadband near-infrared light because of its highsensitivity of d orbitals to local coordination environments. Cr3 alsooffer considerable tunability of both narrow and broad spectral lightdistributions on the basis of crystal field strength (Dq).Compared with the previous review by Zhuang et al. [4] on thetransition metal-activated red to near-infrared persistent luminescence3. Infrared light sourcesThe sun is the ideal broadband infrared light source because it emitsthe whole range of the electromagnetic spectrum. Infrared light sourcesare classified into four types, as shown in Fig. 2.Fig. 1. Absorption and reflection spectra of various organic elements in the human body.2
Optical Materials: X 1 (2019) 100011V. Rajendran, et al.requires an extremely high temperature for improved spectral stabilityand functioning in the infrared region. Hence, globar is always used inconnection with a nichrome wire heater for the infrared light source inpractical commercial applications [6]. Globar is also widely used inFourier 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 forminiature or handheld portable spectrometers.3.3. Laser diodesSolid-state laser diode is another promising infrared light source forspectroscopy applications. Solid-state laser diodes also exhibit a broadspectral light distribution from visible to infrared region(100–2400 nm) with good spectral stability. The characteristics of smallspot size and low divergence angle of laser diodes make them suitableas light sources for diffuse reflectance diagnosis applications due to theefficient couplings of light emissions to the optical fibers in a sub-mmdiameter [11]. However, the requirements of massive housing arrangements and high electrical consumptions obstruct laser diode aslight 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 and1144 nm [12].Fig. 2. Types of infrared light sources.3.1. Tungsten halogen lamp3.4. pc-NIR LEDTungsten halogen lamps are incandescent incoherent light sourcesoperated by the heating of high-temperature tungsten (W) filamentfilled with the mixtures of noble and N gases at a certain ratio. Theapplication of current heats the W filament and creates vaporized W,which is then redeposited back due to the eradication of halide byhalogen at the operation temperature of 250 C. Tungsten lampsfollow the black-body emission behavior and are also commerciallyused as an infrared light source in spectrometers for agricultural andmedical applications, with the peak at the wavelength of 960 nm withthe temperature of 3000 K. Tungsten lamps also exhibit the distinctivecontinuous spectral distribution of light from visible to infrared light inthe electromagnetic spectrum. Ordinary W lamp without halogen gas isalso available at an inexpensive cost for spectroscopy applications.Meanwhile, the requirement of a stable DC power supply incurs cost ofoperation in addition to high power consumption. W halogen lampsdistributes a large amount of heat during operation, which further requires a large glass envelope volume. Other types of light sources forvisible and UV regions, such as duplex (deuterium and W combinationlamps) and Xenon lamps, are available at the wavelengths in the rangesof 0.2–2.5 and 0.2–2.0 μm, respectively. Xenon lamps do not have asmooth spectral distribution in the infrared region [10]. The factors ofhigh 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 portablehandheld spectrometers.A light source on the basis of pc-NIR LED offers remarkable advantages over the previously mentioned traditional light sources interms of compactness (small size), long lifetime, low electrical energyconsumption, inexpensive manufacturing and fabrication, high spectralstability, and customized tunable broadband spectral distribution[13–15]. The fabrication of a pc-NIR LED device follows the generalprinciples of white LED devices. In brief, the emitted light from the blueLED chip is used to excite the near-infrared phosphor deposited over theblue-chip, which results in near-infrared luminescence.4. Nephelauxetic effect and Dq theoriesThe emission wavelength of inorganic phosphor materials is influenced by the two main effects, namely, nephelauxetic effect and crystalfield splitting. Nephelauxetic effect came from Greeks words that literally mean “cloud expanding.” The incorporation of an activator intothe host lattice results in the formation of chemical bonds with thesurrounding ligands or anions, which affect the delocalization of theouter d orbitals of the activator ions. Hence, the interelectron repulsionwithin the d shell is decreased due to the sharing of some d electronswithin the anions and p- and s- orbitals. The spectroscopic properties ofthe transition metal ions Mn4 , Cr3 , and Ni2 in various hosts arereviewed by Brik et al. [5], who proposed the possible methods to tunethe emission of spin-forbidden transitions (otherwise known as lineemission) and also introduced the new parameter in approximating thenephelauxetic 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 caseof free ion, the energy values of the five degenerated d orbitals of theCr3 activators dxy, dyz, dxz, dx2-y2, and dz2 are the same. However, thebonding of the activator by the anions induces repulsive force in thesystem, which is considerably dependent upon anion orientation.Hence, the five degenerated d orbitals undergo orbital splitting on thebasis of the energy value of each d orbital in the geometric structure. Asshown in Fig. 3a, Cr3 is an activator coordinated by the O ions in theoctahedral 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 energylevel and dxy, dyz, and dxz as triplet state (T2g) with low energy structure,3.2. Nichrome heater and globarThe term “globar” is the combination of the words “glow” and“bar,” which indicates that the material will emit light when the barreaches considerably high temperatures. Globar is a simple ceramic rodmade up of silicon carbide (SiC) assorted with some additives andmanufactured by sintering at high temperatures. Globar is heated viaresistor 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 turnproduces infrared light through the interaction with the downstreaminterference filter. Globar is the continuous thermal radiation source,which is similar to black body radiators, and exhibits a broad spectraldistribution of the wavelength ranging from 1 μm to 50 μm. Globar3
Optical Materials: X 1 (2019) 100011V. Rajendran, et al.Fig. 3. a) Scheme of the nephelauxetic effect and crystal field splitting (CFS) ofCr3 and b) CFS of d orbitals in octahedral and tetrahedral coordination.which is represented as Eg and T2g. The energy value difference betweenEg and T2g levels is called crystal field splitting energy and denotedeither as Δo or 10 Dq. Meanwhile, if Cr3 is surrounded by the O ions intetrahedral structure, then the pattern of orbital splitting is entirelyreversed compared with the octahedral structure, as shown in Fig. 3b.This finding indicates that the Eg state is at a low energy level, while theT2g 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 46R5Fig. 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 emissionspectrum because 2Eg is always below the 4T2g state. In other words, 2Egis the first excited state and is a stable energy storage state. The electronic transitions in the strong crystal field are called spin-forbiddentransition. By contrast, the positions of the 2Eg and 4T2g states areswitched entirely while moving toward the weaker crystal field, therebyindicating that 4T2g is below the 2Eg state and becomes the first excitedstate. The electronic transitions in the weak crystal field always exhibita broad emission spectrum and called as spin-allowed transitions. At theintermediate crystal values ( 2.3), the 4T2g and 2Eg energy states areintersected together and result in orbital mixing. The compounds possessing intermediate crystal field values will experience both strong andweak crystal field characteristics, which result in both narrowband orline (spin-forbidden) and broad lines (spin-allowed) in the emissionspectrum, respectively. Considering the high positive charge, Mn4 always experiences a strong crystal field, but Cr3 can be located at anypoint of the crystal field value. Hence, the emission spectrum of Cr3 can be tuned into narrow lines or broadband and both by modulatingthe Dq.(1)Where Dq is the crystal field strength, z is the anion valence, e is thecharge of an electron, r is the radius of the d wave function, and R is thedistance between the central ion and ligands. As shown in Equation (1),Dq is inversely proportional to the bond length between the activatorand anions. The Dq becomes weaker if the bond length increases andvice 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 thetransition 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 withthe ground state as 4F and 120 possible allowed states, as determined bythe Pauli exclusion principle [17]. The x- and y-axes of the diagramrepresent the Dq/B and E/B values, respectively, where Dq is the crystalfield strength, E is the energy of the transitions, and B is the Racahparameters. Dq/B is 0 on the far-left side of the diagram corresponds tothe free ion behavior. The increment in the Dq/B values leads to theproportional increase in 4T2 energy value state because the 4T2 energystate is directly related to 10 Dq according to crystal field theory.Meanwhile, the variation in the energy value of 2Eg is extremely low oralmost constant, thereby indicating that 2Eg is a highly stable energystate despite any crystal field value. According to the Dq/B value, thestrength of the crystal field is classified into the three following categories, as follows [18,19]:5. Known broadband near-infrared phosphors for spectroscopyapplicationsCr3 -doped materials are investigated well by the scientific community for laser and persistent luminescence applications for the pastseveral decades. However, only few research articles have discussed theimportance and demonstrated the implementation of broadband pc-NIRLED as alternative light sources for spectroscopy applications, as listedin Table 3. All of these research articles have been published from 20171) Strong or high crystal field ( 2.3)4
Optical Materials: X 1 (2019) 100011V. Rajendran, et al.Table 3Known broadband near-infrared phosphors for spectroscopy applications.Host materialIncorporated siteNo. ofsitesCrystal systemSpacegroupEmission range(nm)FWHM (nm)Year of publicationReferenceLu3Al5O12: 0.05 Ce3 , 0.5% Cr3 Ga3 (VI)Ga3 (IV)Ga3 (VI) Ga3 /Ge4 (IV)2CubicIa3̄ [20]–2–CubicIa3̄ rigonalR3̄ cR32700–950650–850 and ��2018La3Ga4.95GeO14:0.05 Cr3 Bi-doped GeO2 glassCa2LuZr2Al3O12:0.08 Cr3 3 ScBO3:0.02 CrYAl3(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 graduallyincrease in the future.To survey the commercial viewpoint regarding the practicability ofbroadband near-infrared light emitting diode for spectroscopy applications, OSRAM, which is a multinational lighting industry headquarters in Munich, Germany, announced that they have successfullydeveloped the world's first infrared broadband emitter (SFH4735) inthe range of 650–1050 nm for near-infrared spectroscopy applicationsin the press meeting on November 2016. SFH4735 was designed basedon phosphor converter technology excitable by blue light [24]. OSRAMalso 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 of2.75 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 tissuesensing for real-time biological tissue characterization by using thetechnique pc-NIR LED through the project InSPECT2020. The vision ofthe InSPECT2020 project is to develop photonic needles for biopsieswith pc-NIR LED light source and utilize in sensing spectral issues fortumor 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 emittingknown Cr3 -activated aluminates and gallate-based chemical systems,as shown in Table 4, in addition to those listed in Table 3. The phosphorhost system for near-infrared light is classified into five types, as shownin 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 andtetrahedral local coordination environments due to similar ionic radiiand 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 coordinationnumber is 4 (tetrahedral), the ionic radii of Al3 , Ga3 , and Cr3 are0.39, 0.47, and 0.41 Å, respectively. Hence, designing near-infraredphosphors containing any one of these chemical elements is the firstpreferred choice for near-infrared luminescence.Table 4Cr3 -activated aluminates and gallate-based near-infrared phosphors.HostIncorporated siteNo. ofSitesEmissionrange (nm)ReferenceZnGa2O4Zn(Ga1-xAlx)2O4Ga3 (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 �950[48][49][50]ZnxGa2O3 xMgGa2O4Zn3Ga2Ge2O10Zn1 5GeO14LiGa5O8β-Ga2O3SrGa12O195.1. Selection of near-infrared inorganic phosphor hostFig. 5. Host classification for broadband near-infrared inorganic phosphors.Ruby crystal is sapphire (Al2O3) with the impurity ions of Cr3 insome Al3 sites. The deep red (near-infrared) luminescence of the famous handsome gemstone named ruby crystal has initiated interestsamong researchers to study Cr3 -doped compounds for laser applications. The emission spectrum of the ruby crystal consists of several5
Optical Materials: X 1 (2019) 100011V. Rajendran, et al.emission lines that are close together. Meanwhile, the Cr3 -dopedGa2O3 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 bothsamples. A high ionic radius of Ga3 ion in comparison with Al3 ionsfor the octahedral coordination induces a low reduction in the Racahparameters (B and C) and changes in the crystal field splitting (Dq)values, which generates new spectroscopic properties. The 2O3: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]. Thisobservation suggests why gallate-based host systems may be a betteroption for broadband near-infrared luminescence than aluminate-basedhost systems.applications [19,56–58].5.1.4. La-gallogermanate systemLa-gallogermanate systems are also investigated for the persistentluminescence capability by doping Cr3 and rare-earth elements ascodopants or sensitizers. The emission spectrum of Cr3 -doped LaGaO3single 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 ofLa3Ga5GeO14:Cr3 , M, where M Li , Pb2 , Zn2 , Eu3 , Tm3 ,Dy3 , and demonstrated its potential for long persistent near-infraredluminescence applications with the broad emission wavelength range of700–1100 nm. The system shows the highest luminescence decay timeof 8 h for the optimized luminescence intensity by doping Dy3 ascoactivators. This result suggests that the La-gallogermanate systemmay be a good potential candidate for fluorescence property and isinferior to phosphorescence. A detailed persistent luminescence mechanism study has been conducted by Yan et al. [47] for the compoundwith the chemical composition of La3Ga5GeO14:Cr3 (1% mole) andZn2 (1% mole) and achieved the persistent luminescence decay timeof 1 h only. The inferior performance of La-gallogermanate has notreceived considerable attraction from the researchers in the phosphorcommunity for the phosphorescence applications. Nevertheless, theinvestigations regarding the applications of the possible new membersof the La-gallogermanate family are also carried out and studied againfor phosphorescence applications; the possible new members includeLa3GaGeO16:Cr3 ,LaGaGe2O7:Mn4 ,La3Ga5SiO14:Cr3 ,andLa3Ga5.5Nb0.5O14: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 thenear-infrared spectroscopy application and proposed that the superbroadband near-infrared luminescence of 650–1200 nm with the fwhmof 330 nm are mainly due to the presence of the two distinct luminescent center.5.1.1. Zn-gallogermanate systemThe spinel crystal structure of MGa2O4 (M Zn, Mg) is focused onpersistent luminescence studies due to the possible generation of antisite defects and O vacancies due to Cr3 transition element doping. Thehigh Dq (Dq/B 3.3) and the observation of the luminescence fromonly the octahedral coordinated Ga site (no luminescence from thetetrahedral 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 crystalstructure of ZnGa2O4 by doping the tetravalent chemical element ofGe4 in the Ga3 site and reported the breakthrough near-infraredpersistent phosphors with the benchmarking persistent luminescencedecay time of 360 h for the first time to the phosphor community,which can be activated by sunlight. However, the authors failed toprove 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
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 hand-held spectrometers because of its remarkable advantages of smaller
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