sensors
Communication Research of Fluorescent Properties of a New Type of Phosphor 2+ with Mn -Doped Ca2SiO4
Xiaozhou Fan 1,2, Wenqi Zhang 1,2,*, Fangcheng Lü 1,2, Yueyi Sui 1,2, Jiaxue Wang 1,2 and Ziqiang Xu 3
1 Department of Electrical Engineering, College of Electrical and Electronic Engineering, North China Electric Power University, Baoding 071003, China; [email protected] (X.F.); [email protected] (F.L.); [email protected] (Y.S.); [email protected] (J.W.) 2 Hebei Provincial Key Laboratory of Power Transmission Equipment Security Defense, North China Electric Power University, Baoding 071003, China 3 State Grid Nanjing Power Supply Company, Nanjing 210000, China; [email protected] * Correspondence: [email protected]
Abstract: Fluorescent optical fiber temperature sensors have attracted extensive attention due to their strong anti-electromagnetic interference ability, good high-voltage insulation performance, and fast response speed. The fluorescent material of the sensor probe directly determines the temperature measurement effect. In this paper, a new type of fluorescent material with a Mn2+-doped 2+ ◦ Ca2SiO4 phosphor (CSO:Mn ) is synthesized via the solid-state reaction method at 1450 C. The X-ray diffraction spectrum shows that the sintered sample has a pure phase structure, although the diffraction peaks show a slight shift when dopants are added. The temperature dependence of the fluorescence intensity and lifetime in the range from 290 to 450 K is explored with the help of a fluorescence spectrometer. Green emission bands peaking at 475 and 550 nm from Mn2+ are observed in the fluorescence spectra, and the intensity of emitted light decreases as the temperature rises. The average lifetime of CSO:Mn2+ is 17 ms, which is much higher than the commonly used
Citation: Fan, X.; Zhang, W.; Lü, F.; fluorescent materials on the market. The fluorescence lifetime decreases with increasing temperature Sui, Y.; Wang, J.; Xu, Z. Research of and shows a good linear relationship within a certain temperature range. The research results are of Fluorescent Properties of a New Type great significance to the development of a new generation of fluorescence sensors. of Phosphor with Mn2+-Doped
Ca2SiO4. Sensors 2021, 21, 2788. Keywords: fluorescent optical fiber; fluorescence intensity; lifetime; temperature sensor https://doi.org/10.3390/s21082788
Academic Editor: Hiroshi Ueda 1. Introduction Received: 1 March 2021 As one of the fundamental physical parameters, temperature is of great significance in Accepted: 9 April 2021 Published: 15 April 2021 scientific research, industrial and agricultural production, national defense modernization, and other fields [1]. According to different temperature measurement principles, there
Publisher’s Note: MDPI stays neutral are three main types of thermometers: liquid thermometers based on thermal expansion with regard to jurisdictional claims in of materials [2], thermocouple thermometers based on the Seebeck effect [3], and optical published maps and institutional affil- thermometers, including fluorescent temperature measurement systems [4]. With the iations. continuous development of industry and science, many temperature measurement systems face increasingly harsh environments, and higher requirements are placed on these systems. In recent years, optical fiber temperature sensors have received widespread attention due to their strong anti-electromagnetic interference ability, high-voltage insulation, fast response speed, and non-contact measurement [5]. According to different optical principles, optical Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. fiber temperature sensors can be divided into the following types: (1) based on fluorescence This article is an open access article signals and fluorescence temperature-measuring devices with temperature information distributed under the terms and correlation [6]; (2) based on optical interference principles such as F-P interference, thin- conditions of the Creative Commons film interference, and white light interference [7,8]; (3) based on the light absorption Attribution (CC BY) license (https:// characteristics of semiconductors such as gallium arsenide [9]; (4) based on the thermal light creativecommons.org/licenses/by/ radiation of black-body cavity, quartz, and light guide rods [10]; and (5) based on the light- 4.0/). carrying temperature information forming Raman scattering and Rayleigh scattering in the
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light [11]. Among them, fluorescent optical fibers have become a research hotspot in the field of optical fiber temperature measurement. Generally, all the temperature-dependent fluorescent properties, such as excitation spectra, emission spectra, and decay lifetime, can be exploited to detect temperature due to their variations with temperature [12–15]. The generated fluorescence can penetrate media so that real-time and remote operations can be realized, which makes the luminescence-based measurement more competitive. According to the different temperature measurement principles, sensors are mainly divided into fluorescence-intensity-type, fluorescence-proportional-type, and fluorescence lifetime sensors [16]. Compared with the first two types, the fluorescence lifetime sensor is not affected by the fluctuation of the excitation light source, and the temperature measurement is realized only according to the length of the fluorescence afterglow lifetime [17]. The duration of the fluorescence afterglow depends on the optical temperature charac- teristics of the fluorescent substance located on the temperature probe [18]. At present, most of the temperature measurement research is concentrated on the fluorescent material doped with rare earth ions, Dy3+, Tm3+, Nd3+, and Eu3+, and other rare earth ions have been successfully applied in fluorescent probe materials [19–23]. However, rare earth elements are expensive due to their small abundance, and not all of their fluorescence characteristics are feasible or sufficiently sensitive. In recent years, researchers have discovered that some transition metals can also become luminescence-activating ions or sensitizing ions, which exchange energy with the luminescence center in the matrix to increase luminescence intensity [24–26]. For example, Wu [27] prepared Cr3+-doped spinel fluorescent materials and explored the relationship between the fluorescence lifetime and temperature. At room temperature, the fluorescence lifetime of the most commonly used ruby is 4.2 ms, that 3+ 3+ of YAG:Cr is 2 ms, and that of MgAl2O4:Cr is 10 ms. Chi [28] developed a new type 2+ of fluorescent material with Mn -doped ZnGeO4 and tested the fluorescence intensity and lifetime in the temperature range of 250 to 420 K; a maximum relative sensitivity of 12.2% K−1 was achieved, and the best temperature resolution was about 0.68 K. Wang et al. [29] added Zn ions to ZnO, which improved the ability of the trap to capture electrons and enhanced the luminescence intensity of the material. Researchers added Fe3+ ions to LiAlO2 or Co and Ni ions to ZnGa2O4 to generate near-infrared light [30,31]. The transi- tion metal Mn has rich chemical valence states and can be used alone in either organic luminescent materials or inorganic luminescent materials [32]. In addition, the d-d orbital transition of Mn2+ belongs is a spin-forbidden transition, so when Mn2+ is used as an active ion, its fluorescence decays slowly and the fluorescence lifetime is usually on the order of milliseconds, which is convenient for measurement [33]. In the choice of a phosphor matrix, silicate has excellent physical, chemical, and thermal stability; moreover, the cost is low, which is suitable for preparing phosphors with excellent performance, so rare-earth-doped silicate phosphors are subject to extensive re- 2+ search [34]. Sato et al. [35] developed a Ca2SiO4:Eu phosphor in 2014, whose fluorescence lifetime was 2.5 ms. This phosphor realizes a broad spectrum of yellow light emission under 365 nm excitation, and with an increase of Eu2+ doping, the emission peak of the 3+ spectrum gradually shifts to red. Kalaji et al. [36] reported a Ca2SiO4:Ce phosphor, and this phosphor is excited at 450 nm and has an emission peak at 565 nm, which is a new type of yellow phosphor. In this paper, Ca2SiO4 (CSO) is used as the phosphor matrix, and a new type of fluorescent material (CSO:Mn2+) is synthesized by doping with the transition metal Mn2+. The dependence between the optical properties of the material and the temperature change is explored, and a good linear relationship between the fluorescence lifetime and temperature change of the material is found. The findings are of great significance to the development of a new generation of fluorescent optical fiber temperature sensors.
2. Materials and Methods 2.1. Synthesis Pure and 2% Mn2+-doped CSO phosphors were synthesized via the high-temperature solid-phase method. High-purity SiO2 (AR), CaO (AR), and MnCO3 (AR) raw materials Sensors 2021, 21, 2788 3 of 11
were accurately weighed according to a certain stoichiometric ratio. Stoichiometric mixtures of raw powders were placed in a corundum mortar and fully ground for 2 h, and then the obtained powder was put into a mold and pressed by a tablet press with 20 MPa pressure for 1 min to form a disc with a diameter of 15 mm. Subsequently, the obtained sample was sintered in air at 1450 ◦C for 4 h, the final product was cooled to room temperature, and a fluorescent material sample was obtained.
2.2. Experimental Setup The crystal structure of the sample was tested using a Rigaku UItimate IV X-ray diffractometer (Cu/Kα radiation) with a scanning step of 0.02◦ in the 2θ range from 20◦ to 70◦. The excitation spectrum, emission spectrum, and fluorescence lifetime of the sample were tested using an Edinburgh FLS1000 fluorescence spectrometer with a 450 W ozone- free xenon arc lamp that covers a range of 230 to 1000 nm for steady-state measurements. The time resolution of the fluorescence spectrometer is 1e-9s (nanosecond level), which can meet the measurement range of fluorescence attenuation. To explore the dependence between the optical properties and temperature changes, temperature control was realized using a temperature controller (OMRON E5CC-800) with a type-K thermocouple and a heating tube.
3. Results and Discussion 3.1. Structure Properties The synthesized fluorescent material was characterized with an X-ray diffraction (XRD) diffractometer, and the obtained diffraction peaks were compared with the standard Powder Diffraction File (PDF) #49-1672 of Ca2SiO4. As shown in Figure1, the diffraction peaks of the material, especially the three main peaks distributed at 29.7◦, 33.2◦, and 47.4◦, were well matched, which proves that Ca2SiO4 is generated when the reactants react fully in the high-temperature solid-phase process, and doping with Mn2+ does not change the ◦ matrix phase of Ca2SiO4, indicating that all the samples synthesized at 1450 C are in the pure phase. A small part of the weak impurity peaks in the picture may be derived from impurities such as calcium oxide with insufficient reaction. The crystallographic planes corresponding to the main diffraction peaks are shown in the figure, and the crystal planes corresponding to the three main peaks are (112), (130), and (222). By comparing the existing unit cell parameters in the crystallography open database (COD), we found that the crystal lattice parameters of the sample and γ-Ca2SiO4 (COD: 1546025) are consistent; hence, the crystal form of Ca2SiO4 synthesized is γ-type. As shown in Figure2, it is worth noting that the main peak of CSO:Mn 2+ distributed at 29.7◦ is slightly shifted back relative to the standard peak position, and the shift of the diffraction peak implies that the crystal lattice of the doped sample has been shrunk. According to the Bragg equation, 2dsin θ = nλ, where d is the interplanar spacing, n is the reflection order, and λ is the incident wavelength. Since the radius of the manganese ion (0.66Å) is smaller than the radius of the calcium ion (0.1Å), when Mn2+ replaces Ca2+ sites in the dicalcium silicate lattice, the lattice shrinks and the interplanar spacing d decreases, so the diffraction angle θ increases. The unit cell parameters of pure and Mn2+-doped CSO samples were calculated, and the results are given in Table1.
2+ Table 1. The calculated lattice parameters of pure and 2% Mn -doped Ca2SiO4 samples.
Sample a (Å) b (Å) c (Å) V (Å3) CSO 5.076 11.214 6.758 384.7 CSO:Mn2+ 5.032 11.173 6.701 376.4 Sensors 2020, 20, x FOR PEER REVIEW 4 of 11
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Figure 1. The XRD pattern of Ca2SiO4 (CSO):Mn2+ and the standard PDF of CSO.
As shown in Figure 2, it is worth noting that the main peak of CSO:Mn2+ distributed at 29.7° is slightly shifted back relative to the standard peak position, and the shift of the diffraction peak implies that the crystal lattice of the doped sample has been shrunk. Ac- cording to the Bragg equation, 2dsin θ = nλ, where d is the interplanar spacing, n is the reflection order, and λ is the incident wavelength. Since the radius of the manganese ion (0.66Å ) is smaller than the radius of the calcium ion (0.1Å ), when Mn2+ replaces Ca2+ sites in the dicalcium silicate lattice, the lattice shrinks and the interplanar spacing d decreases, so the diffraction angle θ increases. The unit cell parameters of pure and Mn2+-doped CSO 2+2+ FigureFiguresamples 1.1. The were XRD patterncalculated ofof CaCa22,SiO and4 ((CSO):MnCSO the):Mn results andand theare the standard standardgiven PDFin PDF Table of of CSO. CSO. 1 .
As shown in Figure 2, it is worth noting that the main peak of CSO:Mn2+ distributed at 29.7° is slightly shifted back relative to the standard peak position, and the shift of the diffraction peak implies that the crystal lattice of the doped sample has been shrunk. Ac- cording to the Bragg equation, 2dsin θ = nλ, where d is the interplanar spacing, n is the reflection order, and λ is the incident wavelength. Since the radius of the manganese ion (0.66Å ) is smaller than the radius of the calcium ion (0.1Å ), when Mn2+ replaces Ca2+ sites in the dicalcium silicate lattice, the lattice shrinks and the interplanar spacing d decreases, so the diffraction angle θ increases. The unit cell parameters of pure and Mn2+-doped CSO samples were calculated, and the results are given in Table 1.
FigureFigure 2. 2.Peak Peak shift shift of CSO:Mn of CSO:Mn2+ in the2+ rangein the of range 29◦ to of 30 ◦29. ° to 30°. 3.2. Fluorescence Spectrum and Lifetime Table 1. The calculated lattice parameters of pure and 2% Mn2+-doped Ca2SiO4 samples. Under 365 nm excitation, the excitation spectrum of CSO:Mn2+ is as shown in Figure3a . The materialSample has two obvious excitation peaksa (Å) at 475 and 550b (Å nm,) and the peakc (Å intensity) at V (Å3) 475 nm is obviously smaller than that at 550 nm. CSO 5.076 11.214 6.758 384.7 Figure 2. Peak shift of CSO:Mn2+ in the range of 29° to 30°. CSO:Mn2+ 5.032 11.173 6.701 376.4
Table 1. The calculated lattice parameters of pure and 2% Mn2+-doped Ca2SiO4 samples.
Sample a (Å) b (Å) c (Å) V (Å3) CSO 5.076 11.214 6.758 384.7 CSO:Mn2+ 5.032 11.173 6.701 376.4
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3.2. Fluorescence Spectrum and Lifetime Under 365 nm excitation, the excitation spectrum of CSO:Mn2+ is as shown in Figure 3a. The material has two obvious excitation peaks at 475 and 550 nm, and the peak inten- sity at 475 nm is obviously smaller than that at 550 nm. Figure 3b describes the energy-level state diagram of Mn2+-doped Ca2SiO4 [37]. Under laser excitation at 365 nm, the extranuclear electrons transition from the ground state 6A1 (6S) to the excited state 4E (4D), and then the electrons fall back from the lowest excited state 4T1 (4G) to the ground state 6A1(6S) due to the band-gap transition between the d-d energy levels and emit 550 nm yellow-green light. A small part of the electrons transition Sensors 2021, 21, 2788 from the 4T2 (4G) energy level to the ground state 6A1 (6S) to emit blue-green light. 5The of 11 number of electrons limited by the transition is small, so the light intensity is relatively weak.
(a) (b)
2+ FigureFigure 3. 3. (a()a The) The excitation excitation spectrum spectrum of of CSO:Mn CSO:Mn2+ underunder light light excitation excitation at at 365 365 nm nm.. (b (b) )Energy Energy-level- level transition diagram of CSO:Mn2+. transition diagram of CSO:Mn2+.
2+ AsFigure shown3b describesin Figure the4a,energy-level when the excitation state diagram light ofsource Mn stops-doped exciting, Ca2SiO the4 [37 fluores-]. Under 2+ 6 cencelaser intensity excitation of at CSO:Mn 365 nm, the drops extranuclear suddenly electrons. However transition, the fluorescence from the ground decay time state re-A1 mains(6S) to long, the excited the decay state curve4E(4D), can and be thenwell thefitted electrons via an fall exponential back from Equation the lowest (1), excited and statethe 4 4 6 6 averageT1 ( G) fluorescence to the ground lifetime state ofA1 the( S) material due to the is calculated band-gap transitionby Equation between (2). the d-d energy levels and emit 550 nm yellow-green light. A small part of the electrons transition from the 4 4 tt126 6 t3 T2 ( G) energy levelI(t) to the A1 ground exp( state ) AA 21 exp(( S) to emit ) blue-green A 3 exp( light. ) The number(1) of electrons limited by the transition is small,1 so the light 2 intensity is relatively 3 weak. As shown in Figure4a, when the excitation light source stops exciting, the fluorescence intensity of CSO:Mn2+ drops suddenly.2 However, 2 the fluorescence 2 decay time remains AAA1 1 2 2 3 3 long, the decay curve can be well = fitted via an exponential Equation (1), and the average(2) a AAA fluorescence lifetime of the material is1 calculated 1 2 by 2 Equation 3 3 (2).
where I is the fluorescence intensityt; A1, A2, and A3 tare constants; t tis time; τ1, τ2, and τ3 ( ) = (− 1 ) + (− 2 ) + (− 3 ) represent the fluorescenceI t A lifetime1 exp s; and τAa 2representsexp theA average3 exp fluorescence lifetime.(1) τ1 τ2 τ3 After calculation, the average fluorescence lifetime of CSO:Mn2+ at 290 K is 17 ms, which 2 2 2 is far beyond the fluorescence lifetimeA1 of· τ similar1 + A2 ·materialsτ2 + A3 · [τ273 ,35]. τa = (2) At 290 K, the values of τ1, τ2, and τ3 are 0.047, 1.495, and 18.48 ms, respectively. A1, A1 · τ1 + A2 · τ2 + A3 · τ3 A2, and A3 are 691.2637, 62.5746, and 85.9381, respectively, which represent the signal where I is the fluorescence intensity; A1,A2, and A3 are constants; t is time; τ1, τ2, and τ3 strength of each part, and it can be seen that the signal strength of τ1 is much greater than represent the fluorescence lifetimes; and τa represents the average fluorescence lifetime. that of τ2 and τ3, which is the main process of transition. According to the transition prin- After calculation, the average fluorescence lifetime of CSO:Mn2+ at 290 K is 17 ms, which is ciple, the τ1 process corresponds to the radiation transition, and the electrons return to the far beyond the fluorescence lifetime of similar materials [27,35]. ground state directly from the excited state; hence the fluorescence lifetime is shorter. The At 290 K, the values of τ1, τ2, and τ3 are 0.047, 1.495, and 18.48 ms, respectively. A1,A2, processes of τ2 and τ3 correspond to the trap transitions, and when electrons in the excited and A are 691.2637, 62.5746, and 85.9381, respectively, which represent the signal strength state are3 captured by deep and shallow traps on the crystal surface, instant transitions of each part, and it can be seen that the signal strength of τ1 is much greater than that of τ2 often cannot occur. After a period of time, the electrons in the shallow traps are released and τ3, which is the main process of transition. According to the transition principle, the τ1 process corresponds to the radiation transition, and the electrons return to the ground state directly from the excited state; hence the fluorescence lifetime is shorter. The processes of τ2 and τ3 correspond to the trap transitions, and when electrons in the excited state are captured by deep and shallow traps on the crystal surface, instant transitions often cannot occur. After a period of time, the electrons in the shallow traps are released by thermal energy or other disturbances, returning to the ground state and producing fluorescence, while electrons in deep traps often take longer. Sensors 2020, 20, x FOR PEER REVIEW 6 of 11
by thermal energy or other disturbances, returning to the ground state and producing fluorescence, while electrons in deep traps often take longer. To show the fluorescence decay process more intuitively, the lnI(t) function was used to express the relationship between the fluorescence intensity I and the time t, as shown Sensors 2021, 21, 2788 in Figure 4a. It can be roughly seen that the curve contains three linear relationships,6 of 11 which correspond to the third-order fitting process in Equation (1).
(a) (b)
2+ FigureFigure 4. 4.(a)( aThe) The fluorescence fluorescence intensity intensity decay decay curve curve of of CSO:Mn CSO:Mn.2+ (b.() bEnergy) Energy-level-level diagram diagram and and the the long persistent fluorescence mechanism of CSO:Mn2+. long persistent fluorescence mechanism of CSO:Mn2+.
AsTo shown show thein Figure fluorescence 4b, under decay light process excitation more at intuitively, 365 nm, themost lnI(t) excitation function energy was used is transferredto express to the the relationship fluorescent between centers the, followed fluorescence by the intensity transition I and of theMn time2+: 4T t,1( as4G) shown to the in groundFigure state4a. It 6A can1(6S). be When roughly electrons seen that in the conduction curve contains band three (CB) linear are captured relationships, by electron which trapscorrespond caused by to interstitial the third-order Ca defects fitting (V processCa1) and in oxygen Equation vacanc (1). ies (VO), which are intrinsic defectsAs of shownCa2SiO4 in, and Figure holes4b, in under the valence light excitation band (VB) at 365are captured nm, most by excitation cation vacancies energy is 2+ 4 4 VCa2transferred and traps to V theSi, part fluorescent of the energy centers, is followedstored. Since by the the transition traps for ofstoring Mn :energyT1( G) in tothe the 6 6 latticeground are statefar apartA1(, S).the When electrons electrons and holes in the do conduction not recombine band (CB)in a areshort captured time. However, by electron undertraps the caused disturbance by interstitial of thermal Ca defects energy, (VCa1 the) andelectrons oxygen escape vacancies from (VtrapsO), whichand transfer are intrinsic via thedefects CB to ofthe Ca fluorescent2SiO4, and centers holes in or the directly valence tunnel band in (VB) an inefficient are captured and by slow cation way vacancies to the 2+ excitedVCa2 andstates traps of Mn VSi,. partAt the of same the energy time, the is stored. holes move Since from the traps the traps for storing to the ground energy state in the oflattice Mn2+ via are the far VB. apart, The the gradual electrons recombination and holes do of not electrons recombine and holes in a short produces time. the However, long persistentunder the fluorescence disturbance. of thermal energy, the electrons escape from traps and transfer via the CB to the fluorescent centers or directly tunnel in an inefficient and slow way to the 3.3.excited The Relationship states of Mn between2+. At theSpectrum same time,and L theifetime holes with move Temperature from the traps to the ground state 2+ of MnTo furthervia the explore VB. The the gradual dependence recombination of the optical of electrons properties and of holessynthetic produces materials the longon temperature,persistent fluorescence. we tested the excitation spectrum and fluorescence lifetime of materials in the range of 290 to 450 K. Figure 5a shows the temperature-dependent emission spectra 3.3. The Relationship between Spectrum and Lifetime with Temperature of CSO:Mn2+ under laser excitation at 365 nm with the temperature increasing from 290 to 450 K. ThereTo further are two explore broad the emission dependence bands of ranging the optical from properties 450 to 600 of nm synthetic, with a materialsmaximum on centeredtemperature, at 475 and we tested550 nm, the and excitation the intensity spectrum of the and two fluorescencepeaks decreases lifetime signif oficantly materials with in the range of 290 to 450 K. Figure5a shows the temperature-dependent emission spectra of increasing 2+temperature. The peak intensity of the emission spectrum is highest at 290 K andCSO:Mn lowest atunder 450 K laser, only excitation 1/4 of the atmaximum. 365 nm with Figure the temperature5b shows the increasingcorresponding from tem- 290 to 450 K. There are two broad emission bands ranging from 450 to 600 nm, with a maximum perature-dependent mapping of CSO:Mn2+ under laser excitation at 365 nm. centered at 475 and 550 nm, and the intensity of the two peaks decreases significantly with increasing temperature. The peak intensity of the emission spectrum is highest at
290 K and lowest at 450 K, only 1/4 of the maximum. Figure5b shows the corresponding temperature-dependent mapping of CSO:Mn2+ under laser excitation at 365 nm. As shown in Figure6a, the integrated fluorescence intensity of the material decays with increasing temperature, indicating that CSO:Mn2+ has inferior thermal stability. The phenomenon can be explained by thermal quenching theory. As shown in Figure6b, under 6 6 light excitation at 365 nm, electrons transition from the ground A1 ( S) state to the excited state 4E(4D). When the temperature rises, the excited-state electrons absorb thermal energy and jump to the conduction band, instead of light emission. The free electrons in the conduction band can be captured by traps on the surface of the material, and only a small part of the electrons can move back to the ground state, so the intensity of the emitted light is greatly reduced, and even fluorescence quenching occurs. Sensors 2020, 20, x FOR PEER REVIEW 7 of 11
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(a) (b) Figure 5. (a) The temperature-dependent emission spectra of CSO:Mn2+ under laser excitation at 365 nm, with the temperature increasing from 290 to 450 K. (b) The corresponding temperature- dependent mapping of CSO:Mn2+ under laser excitation at 365 nm.
As shown in Figure 6a, the integrated fluorescence intensity of the material decays with increasing temperature, indicating that CSO:Mn2+ has inferior thermal stability. The phe- nomenon can be explained by thermal quenching theory. As shown in Figure 6b, under light
excitation at 365 nm, electrons transition from the ground 6A1 (6S) state to the excited state 4E (4D). When the(a )temperature rises, the excited-state electrons (absorbb) thermal energy and
jump to the conduction band, instead of light emission. 2+The free electrons in the conduc- Figure 5.Figure (a) The 5. temperature(a) The temperature-dependent-dependent emission emission spectra of spectra CSO:Mn of CSO:Mn under laser2+ under excitation laser excitationat at tion band can be captured by traps on the surface of the material, and only a small part of 365 nm,365 with nm, the with temperature the temperature increasing increasing from 290 fromto 450 290 K. to(b) 450 The K. corresponding (b) The corresponding temperature temperature-- dependentthe mappingelectrons of can CSO:Mn move2+ underback tolaser the excitation ground atstate, 365 nm. so the intensity of the emitted light is dependent mapping of CSO:Mn2+ under laser excitation at 365 nm. greatly reduced, and even fluorescence quenching occurs. As shown in Figure 6a, the integrated fluorescence intensity of the material decays with increasing temperature, indicating that CSO:Mn2+ has inferior thermal stability. The phe- nomenon can be explained by thermal quenching theory. As shown in Figure 6b, under light excitation at 365 nm, electrons transition from the ground 6A1 (6S) state to the excited state 4E (4D). When the temperature rises, the excited-state electrons absorb thermal energy and jump to the conduction band, instead of light emission. The free electrons in the conduc- tion band can be captured by traps on the surface of the material, and only a small part of the electrons can move back to the ground state, so the intensity of the emitted light is greatly reduced, and even fluorescence quenching occurs.
(a) (b)
2+ Figure 6. ((aa)) Temperature Temperature dependence dependence of of the the integrated integrated intensity intensity of of CSO:Mn CSO:Mn.2+ (b.() Energyb) Energy-level-level 2+ diagram and the fluorescencefluorescence processprocess ofof CSO:MnCSO:Mn2+..
Relative sensitivity SR isis often often used used to measure temperature and measurement accu- racy. ItIt cancan bebe defineddefined byby thethe relationshiprelationship betweenbetween emissionemission intensityintensity andand temperaturetemperature asas Equation (3).
11 dI dI SSRI = = 100% × 100% (3) (3) RI I dTI dT (a) (b) 1 dτ S τ = × 100%2+ (4) Figure 6. (a) Temperature dependence of the integratedR 1dτ dT intensity of CSO:Mn . (b) Energy-level diagram and the fluorescence process of CSO:MnSR =2+. 100% (4) As shown in Figure7a, the relationship dT between 1/I (I represents the integrated emis- sion intensity from 475 to 800 nm) and 1/T (T is the temperature from 290 to 450 K) is RelativeAs sensitivity shown in SFigureR is often 7a, theused relationship to measure between temperature 1/I (I andrepresents measurement the integrated accu- emis- gained, which can be fitted with the Arrhenius-type equation. According to Equation (3), racy. Itsion can beintensity defined from by the 47 5relationship to 800 nm) between and 1/T emission (T is the intensitytemperature and temperaturefrom 290 to 45as 0 K) is the relative sensitivity SRI of the fluorescence intensity with temperature can be calculated, Equationgained (3). , which can be fitted with the Arrhenius-type equation. According to Equation (3), and the variation of relative sensitivity1 dI with temperature is shown in Figure7b. The rela- τ τ tionship between 1/ ( representsSRI = the fluorescence 100% lifetime) and 1/T (T is the temperature(3) from 290 to 450 K) is shown in FigureI dT7c, the relative sensitivity S τ is calculated according R to Equation (4), and the variation is shown in Figure7d. It can be seen that at the tempera- 1d ture range of 330 to 410 K, all relative sensitivities are higher than 3%, the maximum SRI SR = 100% (4) reaches 4.18% at 370 K, and the maximumdT SRτ reaches 4.25% at 375 K, which are much better than previously reported rare-earth-ion-doped temperature sensors [38–41]. As shown in Figure 7a, the relationship between 1/I (I represents the integrated emis- sion intensity from 475 to 800 nm) and 1/T (T is the temperature from 290 to 450 K) is gained, which can be fitted with the Arrhenius-type equation. According to Equation (3),
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the relative sensitivity SRI of the fluorescence intensity with temperature can be calculated, and the variation of relative sensitivity with temperature is shown in Figure 7b. The rela- tionship between 1/ ( represents the fluorescence lifetime) and 1/T (T is the temperature from 290 to 450 K) is shown in Figure 7c, the relative sensitivity SR is calculated according to Equation (4), and the variation is shown in Figure 7d. It can be seen that at the temper- Sensors 2021, 21, 2788 ature range of 330 to 410 K, all relative sensitivities are higher than 3%, the maximum8 S ofRI 11 reaches 4.18% at 370 K, and the maximum SR reaches 4.25% at 375 K, which are much better than previously reported rare-earth-ion-doped temperature sensors [38–41].
(a) (b)
(c) (d)
2+ FigureFigure 7. 7.(a)( aThe) The relationship relationship between between 1/I and 1/I and1/T of 1/T CSO:Mn of CSO:Mn at 292+0–at45 290–4500 K. (b) The K. ( brelationship) The relation- between relative sensitivity of fluorescence intensity and temperature. (c) The relationship be- ship between relative sensitivity of fluorescence intensity and temperature. (c) The relationship tween 1/ and 1/T of CSO:Mn2+ at 290–450 K. (d) The relationship between relative sensitivity of between 1/τ and 1/T of CSO:Mn2+ at 290–450 K. (d) The relationship between relative sensitivity of fluorescence lifetime and temperature. fluorescence lifetime and temperature.
2+ TheThe fluorescence fluorescence lifetime lifetime of ofCSO:Mn CSO:Mn from2+ from 290 to 290 45 to0 K 450 was K further was further measured measured.. Fig- ureFigure 8a shows8a shows that when that when the excitation the excitation light stops, light stops, the decay the decaycurve curveconsists consists of rapid of at rapidten- uationattenuation and a long and apersistent long persistent decay decayprocess. process. To more To moreintuitively intuitively show showthe relationship the relationship be- tweenbetween the thefluorescence fluorescence lifetime lifetime and and the the temperature, temperature, the the coordinate coordinate ofof the the diagram diagram was was representedrepresented by by the the semi semi-logarithmic-logarithmic lnI(t),lnI(t), asas shownshown in in Figure Figure8b. 8b. It canIt can be seenbe seen clearly clearly that thatas theas the temperature temperature rises, rises, the the decay decay rate rate of the of the curve curve continues continues to increase. to increase. The The mechanism mech- anismof fluorescence of fluorescence afterglow afterglow decay hasdecay been has explained been explained above: asabove the temperature: as the temperature increases, in- the creases,recombination the recombination of electrons of andelectrons holes and intensifies holes intensifies under the under disturbance the disturbance of thermal of energy, ther- malwhich energy, promotes which the promotes increase the of increase the fluorescence of the fluorescence afterglow decay afterglow rate. decay rate. The average fluorescence lifetime of the material at different temperatures was cal- culated and can be fitted well by the Boltzmann function, as Figure9 shows. At the temperature range of 310 to 410 K, the curve of the fluorescence lifetime and temperature is almost linear (R = 0.9892), which provides a good foundation for subsequent fluorescence temperature measurement work.
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Sensors 2021, 21, 2788 9 of 11 Sensors 2020, 20, x FOR PEER REVIEW 9 of 11
(a) (b) Figure 8. (a) The fluorescence afterglow decay curve of CSO:Mn2+ at different temperatures. (b) The relationship between the semi-logarithmic of I and t at different temperatures.
The average fluorescence lifetime of the material at different temperatures was cal- culated and can be fitted(a) well by the Boltzmann function, as Figure(b) 9 shows. At the tem- perature range of 310 to 410 K, the curve of the fluorescence lifetime and temperature is 2+ FigurealmostFigure 8. 8.(lineara)( aThe) The (fluorescenceR fluorescence= 0.9892), afterglowwhich afterglow provides decay decay curve curvea good of of CSO:Mn foundation CSO:Mn2+ atat different for different subsequent temperatures temperatures. fluorescence. (b (b) ) The The relationship between the semi-logarithmic of I and t at different temperatures. temperaturerelationship between measurement the semi-logarithmic work. of I and t at different temperatures. The average fluorescence lifetime of the material at different temperatures was cal- culated and can be fitted well by the Boltzmann function, as Figure 9 shows. At the tem- perature range of 310 to 410 K, the curve of the fluorescence lifetime and temperature is almost linear (R = 0.9892), which provides a good foundation for subsequent fluorescence temperature measurement work.
(a) (b)
FigureFigure 9. 9. (a(a) )The The relationship relationship between between fluorescence fluorescence lifetime lifetime and and temperature temperature in the in the range range of 29 of0 290to 450 K. (b) The relationship between fluorescence lifetime and temperature in the range of 310 to to 450 K. (b) The relationship between fluorescence lifetime and temperature in the range of 310 to 410 K. 410 K.
4. Conclusions 4. Conclusions (a) (b) InIn this this paper, paper, 2% 2% Mn Mn2+-2+doped-doped Ca Ca2SiOSiO4 waswas successfully successfully synthesized synthesized via the via high the- high-tem- Figure 9. (a) The relationship between fluorescence2 4 lifetime and temperature in the range of 290 to perature solid-state method. X-ray powder diffraction was performed to prove the syn- 45temperature0 K. (b) The relationship solid-state between method. fluorescence X-ray powder lifetime diffraction and temperature was performed in the range toof prove310 to the 41theticsynthetic0 K. sample sample has a has pure a purephase. phase. The excitation The excitation spectrum, spectrum, emission emission spectrum spectrum,, and fluores- and cencefluorescence lifetime lifetimeof the sample of the samplewere tested were to tested explore to exploreits fluorescent its fluorescent properties. properties. Under light Un- 4.excitation derConclusion light excitationat 365s nm, atthe 365 sample nm, has the two sample emission has two peaks emission at 475 and peaks 550 atnm, 475 corresponding and 550 nm, 2+ 4 6 42+ 4 6 6 4 tocorresponding the parity-forbidden to the parity-forbidden d-d band transition d-d of band Mn transition: T2 to A of1 and Mn T1: toT 2Ato1. TheA1 averageand T1 In this paper, 2% Mn2+-doped Ca2SiO4 was successfully synthesized via the high-tem- fluorescenceto 6A . The lifetime average of fluorescence the sample lifetimeat room oftemperature the sample is at 17 room ms, which temperature far exceeds is 17 the ms, peratur1e solid-state method. X-ray powder diffraction was performed to prove the syn- commonlywhich far exceeds fluorescent thecommonly materials. To fluorescent further observe materials. the To relationship further observe between the relationshipthe fluores- thetic sample has a pure phase. The excitation spectrum, emission spectrum, and fluores- cencebetween characteristics the fluorescence and the characteristics temperature, andthe fluorescence the temperature, intensity the fluorescenceand fluorescence intensity life- cence lifetime of the sample were tested to explore its fluorescent properties. Under light timeand fluorescenceof the sample lifetime in the oftemperature the sample range in the temperatureof 290 to 450 rangeK were of tested. 290 to 450 As Kthe were tempera- tested. excitation at 365 nm, the sample has two emission peaks at 475 and 550 nm, corresponding tureAs the increases, temperature the fluorescence increases, the intensity fluorescence of theintensity sample continues of the sample to decrease continues, and to the decrease, max- to the parity-forbidden d-d band transition of Mn2+: 4T2 to− 6A1 and 4T1 to 6A1. The average imumand the relative maximum sensitivity relative reaches sensitivity 4.18% reaches K−1 at 37 4.18%0 K. KThis1 atphenomenon 370 K. This phenomenoncan be explained can fluorescence lifetime of the sample at room temperature is 17 ms, which far exceeds the asbe electrons explained in as the electrons excited instate the absorbing excited state thermal absorbing ener thermalgy and jumping energy and to the jumping conduction to the commonlyconduction fluorescent band instead materials. of the To emission further band.observe The the fluorescence relationship lifetimebetween of the the fluores- sample cencedecreases characteristics as the temperature and the temperature, rises, and the the maximum fluorescence relative intensity sensitivity and fluorescence reaches 4.25% life- K−1 time of the sample in the temperature range of 290 to 450 K were tested. As the tempera- at 375 K, which is caused by the increase in the recombination rate of electrons and holes ture increases, the fluorescence intensity of the sample continues to decrease, and the max- in the defect levels (VCa1, VO and VCa2, VSi) under the disturbance of thermal energy. −1 imumIt is worthrelative mentioning sensitivity thatreaches the 4. curve18% K of theat 37 fluorescence0 K. This phenomenon lifetime at thecan temperature be explained of as electrons in the excited state absorbing thermal energy and jumping to the conduction
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290–450 K can be fitted well with the Boltzmann function, and in the temperature range of 310–410 K, the change trend shows an excellent linear relationship, which may have some value for the development of new fluorescent temperature measurement materials.
Author Contributions: Conceptualization, X.F.; methodology, W.Z.; software, Y.S. and; formal analy- sis, W.Z.; investigation, F.L.; resources, X.F.; data curation, Z.X.; writing—original draft preparation, W.Z.; writing—review and editing, J.W. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Research on Fire Mechanism of Transformer Bushing and Technology of Fire Prevention and Explosion (grant no. 5442GY190008). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.
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