INTRODUCTION
Background of Luminescence
Many substances that can absorb the energy exerted from the outside, such as light, mechanical energy, chemical energy, etc and use the absorbed energy to supply for their molecules, atoms that are in the ground state moving up to the corresponding excited states From the excited state, electrons in the atom or molecule can return to their ground state by jumping alternately through intermediate states with the generation or destruction of phonons (non-radiative recovery), or emitting light radiation (emission recovery) to shift back to the ground state The wavelength of emission is featured in the materials and the energy transfer processes of the source itself Luminescence is a material that emits light in the visible region, and such the light emitted in the visible area is called luminescent.
If, based on the lifetime of radiation, luminescence can be divided into fluorescence and phosphorescence When is called the time when the material emits radiation after stopping stimulation (or called the lifetime of radiation), it is classified as:
The luminescence process having 10 -8 s is called the phosphorescent process.
So, fluorescence is a process of luminescence that the emission happens almost simultaneously with the absorption of energy by matter and shuts off immediately after stopping the stimulation The phosphorescence is a very long process of luminescence, in which the lifetime of the radiation can be delay hours after finishing the stimulation Furthermore, the lifetime is characterized by the delay between the energy absorption process and time to radiation reaches maximum intensity Phosphors can be excited by either optical or other excitation, such as electrical energy, chemical energy, mechanical energy, etc If semiconductors give light emissions by absorbing incident light, this is call photoluminescence (PL) In the mechanism of PL, electrons in the valence band of a semiconductor absorb photons from light To be absorbed, the photons should have energy either equals or higher than the energy band gap of the semiconductor, but still too low to cause photoionization, leaving an unoccupied state (or hole) in the valence band The process is called the absorption process These excited electrons generated by optical excitation will return to the ground state, accompanied by emitting photons,which is called the emission process Excitation and emission processes are the
There are many reasons for the decrease in the luminous intensity of materials, in which large concentrations of impurities can also cause some from external physical, chemical, and environmental agents such as temperature, humidity, etc or fluorescent depletion.
The luminous intensity of the material is very sensitive to the concentration of luminescent centers (or doped concentrations) in the matrix Typically, when the concentration of luminescent centers is not too large, the concentration of doping increases gradually, the intensity of luminescence increases with the increase of optical centers When the concentration of the emitting centers reaches a certain threshold value (critical value), the further increase in doping concentration will reduce the luminous intensity of the fluorescent powder This phenomenon is called concentration quenching.
The cause of optical quenching due to concentration is the effect of transmitting energy between impurity ions without photon emission when they have high concentrations This phenomenon is created by one of three mechanisms: charge exchange interaction, radiation reabsorption, and multi-electrode interaction. According to Blasse equation [62], the critical RC distance between the optical centers of optical quenching phenomenon due to the concentration is determined:
Where X C is the concentration of the activation center that starts the quenching phenomenon, N is the cation number in the unit cell, and V is the volume of the base cell.
The charge exchange interaction usually takes place when the RC distance reaches a small value, ≈ 5 A ° [62] The reabsorption process may occur if materials contain the luminescent center and sensitive light centers, and there must be an overlap of the excitation spectra with the emission centers of the luminescent centers The first two mechanisms are often difficult to occur in concentration quenching Multi- electrode interactions, according to Dexter's theory [63], often play a significant role in explaining the energy transfer process that reduces fluorescence when the concentration is doping high enough.
Electroluminescence (EL) is a light-emitting phenomenon under electrical excitation It is also considered as an electro-optic conversion process in which a phosphor emits light in response to the injection of electric current In the mechanism of EL, holes, and electrons are separately injected into the valence and conduction bands of the semiconductor The recombination of holes and electrons will release their energy as photons-light The phenomena are often happening in inorganic semiconductors, which have direct band gaps [e.g., the III–V compounds GaAs and indium phosphide (InP)], where holes and electrons in their valence and conduction bands, respectively, are considered as free carriers.
Thermoluminescence (TL) is the phenomenon of light emission of a semiconductor when it is heated at a constant rate from room temperature to some temperature after irradiated at low temperatures (room temperature or liquid nitrogen ) by ionizing radiation such as UV, X-ray, …The TL spectrum is called
“glow-curve,” and the luminescence emitted is a function of temperature Typically, the distinct peaks occurring at different temperatures in the glove - curve relate to the electron traps present in the sample [64].
The TL process can be summarized as follows:
(i) The material must be a semiconductor with a large band gap.
(ii) Before heating, the material absorbs energy during irradiation, and luminescence occurs when heated.
(iii) When the sample irradiated by ionizing radiation is heated from room temperature to some temperature, electrons in traps are excited to conduction band, and some of these reach luminescence centers (L); if so, light (i.e., TL) is emitted as a result of the process of recombination into these centers.
The Zn2SiO4 and Zn2SiO4: Mn phosphors show good thermoluminescence properties [43,65,66].
1.2 Background of Transition Metal (TM) ions in the crystal field
TM ions are formed from atoms in the fourth period of the periodic table; from beyond the calcium atom (element 20 in the periodic table), with electronic configuration (Ar)4s 2 , up to the zinc atom (element 30), with electronic configuration (Ar)3d 10 4s 2 and some atoms in the fifth period from Z = 39 (Ytri) → Z 48 (Cadimi); period 6 from Z = 57 (Lanthan) → Z = 80 (mercury); and the 7 cycle elements starting from Z = 89 (Actini) In general TM shows many different oxidation numbers For example manganese (Mn, Z = 25, configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d 5 4s 2 ) has six oxidation states: +2, +3, +4, +5, +6 and + 7.
Transition metal (TM) ions in the lattice often create magnetic properties for the material but are also used as optically active dopants in host lattices Normally, TM ions have an electronic configuration that is not filled in the outermost 3d subclass.
Thus, they have an electronic configuration 1s 2 2s 2 2p 6 3s 2 3p 6 3d n , where n (15% Mn 2+ ) leads to the extra of Mn 2+ ion and forms the unexpected phase MnO2 in XRD patterns
Table 4.3 Lattice parameters of ZTO undop and dop with Mn 2+ (3-7% Mn) of the samples treated at 1000 C
4.3.2 Phosphor morphology of Zn 2 SnO 4 :Mn2+
Figure 4.13 FESEM images of the as-milled powder (a) and the samples annealed at
Figure 4.13 shows FE-SEM images of 5 wt% Mn2+-doped ZnO/SnO2 milled powders annealed at different temperatures 700 C, 800 C, 900 C, 1000 °C and
1100 C in the air for 2 hours It is clear to see that the particles present uniform size and spherical-like shape The average particle size of the samples without annealing, and that was annealed at 700 C is about 50 nm When sintering temperature increases from 800 °C to 1100 °C, the particle sizes increased significantly At 1000 °C and
1100 C, the average particle size is about 1 m.
However, when annealing temperature increase to 1100 C, there is a re- crystallization or particles cluster together to form either very big particles or very small paricle As a result, when combined with XRD analysis, the 5% Mn 2+ doped
Zn2SnO4 samples calcinated at a temperature of 1000 °C gives both good crystallinity and uniform particle size.
4.3.3 Optical properties of Zn 2 SnO 4 :Mn2+
4.3.3.1 UV-vis spectra of the un-doped and Mn2+ doped Zn 2 SnO 4
The band gap energy Eg of undoped Zn2SnO4 and Mn2+ doped Zn2SnO4 has been estimated by UV – vis spectroscopy (Fig.4.14).
Figure 4.14 UV-Vis spectra of the un-doped and Mn2+doped Zn 2 SnO 4 at 1000 C
As can be seen from the spectra 4.14, the diffuse reflectance reveals a significant rise at 320 nm which can be assigned to intrinsic band gap absorption. The Tauc formula and Kubelka-Mung Function [154,204] has been used to estimated optical absorption band gap of prepared sample as following rule:
Typically, a Tauc plot shows the quantity hν (the energy of the light) on the absorption and the quantity (αhν) 1/r on the ordinate as formular (4.2):
Where α is the absorption coefficient of the material The value of the exponent r denotes the nature of the transition with r = 1/2 for direct allowed transitions and r
= 2 for indirect allowed transitions It is known that both Zn2SnO4 is a direct semiconductor [86,204] According to Kubelka-Mung Function, the relation between absorption coefficient and reflectance is shown in formular (2):
Where (R) is the absolute value of reflectance and F(R) is equivalent to the absorption coefficient Because of α ~ F(R) and combined with (4.2) and (4.3) equation, we have:
(F(R)hv) 2 = 0 as shown in the inset of Fig 4.14 For the undoped and Mn 2+ doped
Zn2SnO4, the estimated band gap Eg value is 3.66 eV and 3.6 eV, respectively The calculated band gap energy of the undoped sample is slightly higher than that of pure Zn 2 SnO4 nanocomposites in the previous study [84], which may be attributed to the effect of particle size It is obvious to see that; there is a light red-shift in the band gap of the Mn-doped Zn2SnO4, which accounts for 3.6 eV compared to 3.66 eV of the undoped sample According to us, this narrowing of the band gap when doping Mn in to host lattice can be explained that Mn 2+ ions substituting for the Zn 2+ ions which leads to the exchange interaction between the ‘d’ electron of Mn atom and ‘s’ and ‘p’ electrons of the host Zn atom (s–d and p–d interactions) [154].
4.3.3.2 Effects of annealing temperature on optical properties of Mn 2+ doped
Figure 4.15 PL spectra of 5 %wt Mn2+ doped Zn 2 SnO 4, powders after milling for 40 hours followed by the annealing at different temperatures
Effects of annealing temperature on the photoluminescence of Zn2SnO4: Mn2+ are also investigated, and results are shown in Fig 4.15 For un-annealing samples,
PL spectra show no emission peak in the visible light region This can be explained by the Zn2SnO4 crystal, which is not observed in the XRD pattern When the sample annealed at 700 C, a broadband emission peak at 523 nm presents in the spectrum.
PL intensity increases when the annealing temperature rises to 1000 C By increasing the annealing temperature up to 1100 C, the PL intensity reduces in comparison with that annealed at 1000 C As observed, the increase and reduction of PL intensity are in accordance with diffraction peak intensity of XRD pattern.
4.3.3.3 Effects of Mn 2+ concentration on optical properties of Mn 2+ doped
To optimize Mn2+ doping concentration into Zn2SnO4, the molar percentages of ion Mn 2+ were modified from 0 to 7 %mol PLE and PL spectra of the phosphors annealed at 1000 o C with the various concentration are shown in Fig 4.16 Both PLE and PL spectra show that intensity increases as a function of Mn 2+ doping concentration in the range of 0 to 5 %mol and decreases when the concentration exceeds 5 %mol due to the concentration quenching, which is a known phenomenon in phosphors [47] Concentration quenching might also be due to the mechanisms of the exchange interaction or the multipole–multipole interaction In order to elucidate which mechanism is responsible for the concentration quenching, an estimation of the critical distance (Rc) is performed according to the following relationship proposed by Blasse:
SnO 4 :5%Mn2+ phosphors
Conclusion
The single phase of the infrared phosphor Zn2SnO4 and single-phase of green phosphor Zn2SnO4: Mn2+ have been successfully synthesized by high energy planetary ball milling combined with annealing at 1000 C in air and reduced gas atmosphere.
The Zn SnO phosphor gives emission in the range of 450-800 nm with a peak mechanisms for this emission: (1) recombination of a deeply trapped electron (VO* state) with a deeply trapped hole (VSn/VZn), or (2) recombination of a shallowly/deeply trapped hole (VO++) with a deeply trapped electron (Zni/Sni).
Zn2SnO4: Mn2+ phosphor was the first synthesis giving green emission spectra with a peak of 523 nm Excitation photoluminescence spectra (PLE) of Zn2SnO4: 5% Mn 2+ show a strong absorption peak characteristic of Mn 2+ ions at 444 nm, so it has a high potential for application in green and white LEDs using the blue LED chip The results of the quenching temperature indicate that green phosphor Zn 2 SnO4: Mn2+ has excellent thermal stability With increasing temperature further to 200 °C, the emission intensity remains at 62 % of the initial value (at 25 °C) so that the prepared phosphor shows excellent promise for LED application Besides, the internal quantum efficiency of the Zn2SnO4: 5%Mn2+ is about 40%, similar value, which has been achieved in previous reports about Mn 2+ doped oxide phosphors
LED device fabricated by coating Zn2SnO4: 5% Mn2+ powder on blue LED chip with color coordinates (x = 0.2419; y = 0.3953) Besides, when using Zn2SnO4:5% Mn2+ powder mixed with Zn2SnO4 red powder: 3% Cr3+, 0.6% Al3+ and covered 450 nm blue LED chip for white LED device with color coordinates x 0.3783 and y = 0.3520, color temperature is 3858 K and color rendering index is 91.
OPTICAL PROPERTIES OF Zn 2 SnO 4 :Cr3+ AND Zn 2 SnO 4 :Cr3+, Al3+ FOR PLANT CULTIVATION LED
Introduction
Recently, the LED technology has been popularly researched as the light source for indoor plant cultivation in the agricultural industry [212,213] The indoor plant cultivating method can control ambient environments for plant growth, such as meaning filtered air, steady temperature, and unique growth media, and consequently, making it inaccessible to damages from bad weather Hence, indoor plant cultivation permits a stable supply of vegetables without the influence from the outside, and it is expected as a solution method to the global food problem
[214] Despite such merits, indoor plant cultivation struggles with natural light scarcity, then, LEDs are functioned to be the major light source for stimulating plant growth.
For the technology of indoor plant cultivation, artificial light is one of the critical factors influencing the growth performances of plant tissues It has been reported that blue light around 450nm (400–500nm), red light around 660nm (620– 690nm), and far-red light around 730nm (700–740nm) play important roles in reactions of photosynthesis, phototropism and photomorphogenesis [215–218]. Among them, far- red light is especially significant for the preparation of higher- value vegetables, because it prevents aggravation of quality and the bitterness caused by the excessive growth of plants [219] However, the proportion of red light in the natural sunlight is higher than in far-red light [220] Meanwhile, with the development of science and technology, the far-red light becomes even less due to the night lighting, which means the far-red light is insufficient for plant cultivation and also influences the entire lifestyle of the plant Thus, finding an alternative light source to meet the requirements of plant growth is urgent, especially in the greenhouse industry In the past, the light emitted from traditional gas-discharge lamps cannot match well with the absorption spectrum of phytochrome, especially the PFR But the light-emitting diodes (LEDs) device with long lifetime and low power consumption can make up for this drawback It means that phosphor- converted far-red-emitting LEDs by combining the blue LED chips and the blue light excitable far-red phosphors are considered as a prospective artificial light source used in indoor plant cultivation Therefore, it is urgent to develop highly- e ciencyffi blue-light-excitable far-red emitting phosphors.
Many research efforts have already been made to develop the far – red ultraviolet
(UV) and blue regions Notably, the red emissions peaked at ~615nm of Eu 3+ ions make them unsuitable for indoor plant growth [224,225] Besides, most of the rare- earth ions are costly and harmful to the environment In recent year, many researchers have intended to find eco-friendly non-rare-earth ions to fabricate red and far-red emitting phosphors Mn 4+ ions activated phosphors show several essential advantages, such as strong excitation peak at 300 to 480 nm and strong emission in the range from 600 to 780 nm, economically cheap and no absorption in the green spectral range [225,232,239,240] Like Mn 4+ ions, Cr 3+ ions activated phosphors show strong absorption in the blue region (400-480 nm) and strong emission in the infrared range (700-740 nm) but, phosphors based on Cr 3+ have not been applied in plant cultivation yet Most of the Cr 3+ activated phosphors were studied their long afterglow properties [29–34].
Zinc stannate (Zn2SnO4) has a typical inverse spinel crystal structure with a band gap of ~ 3.6-3.7 eV Compared with normal spinel, the inverse spinel structure features an alternative cation arrangement In Zn2SnO4, all of the [SnVI] cations and half of the [Zn VI ] cations occupy octahedral sites, whereas the other half of the [Zn IV ] cations occupy tetrahedral sites This configuration, thus, features a path of easy doping ion precipitation into the octahedral [Sn VI ] or [Zn VI ] under the condition of matching geometrical lattice and atomic radius, which occurs with aluminum, chromium, gallium, and others.
We aim to synthesize eco-friendly phosphors based on nonrare-earth elements, which have strong absorption in the blue range (400-450 nm) and strong emission in far-red light around (700-740 nm) so that they are useful in the application of indoor plant cultivation Specifically, a series of efficient near-infrared (NIR) phosphors (Zn2SnO4: 3%Cr3+ (ZTO: Cr3+) and Zn2SnO4: 3%Cr3+,0.6%Al3+ (ZTO: Cr3+, Al3+)) were produced successfully by high energy planetary ball milling technique and post- annealing in air The excitation spectra of Zn2SnO4:3%Cr3+ exhibit strong broad absorption bands around 400 to 550, which are well-matched with the emission of Blue LED Chip Hence, when excited at 460m, Zn2SnO4:3%Cr3+ phosphor presents an intense broad-band emission peaking at 740 nm, which shows a potential application in agriculture The nature of broad band emission is due to spin-allowed 4 T2 to 4A2 transition of Cr3+ ions.
In this work, Al3+ ion co-dopant into Zn2SnO4: 3%Cr3+ to extend the photoemission and photoabsorption The reason is due to the manipulation of the local crystal field around Cr 3+ sites Thus, the emission spectrum (λexci = 460 nm) the excitation spectra of Cr3+, Al3+ co-doped Zn2SnO4 show a blue shift and strong absorption in the blue region compared to the Cr 3+ single doped samples Further research is coating the Cr3+ doped Zn2SnO4 and Cr3+, Al3+ co-doped Zn2SnO4 phosphors on to Blue LED Chips, which both show near-infrared light with the corresponding energy conversion efficiency of 6.6% and 16.61 %, respectively The results demonstrate that we might be able to produce inexpensive and efficient
Zn2SnO4: 3%Cr3+, 0.6%Al3+ for near-infrared broad-band emitting LEDs.
Structural and optical properties of Zn 2 SnO 4 :Cr3+ phosphors
5.2.1 X-ray diffraction of Zn 2 SnO 4 :Cr3+
5.2.1.1 Effect of annealing temperature on structural formation
Figure 5.1 XRD patterns of Zn 2 SnO 4 :3%Cr3+ powder un-annealed and annealed in the range of 900-1200 °C in air
Figure 5.1 shows the XRD patterns of Zn2SnO4:3%Cr3+ powder after milling for 60 hours (un-annealed) and annealed in the range of 500, 900-1200 °C in air As we can see, for the un-annealed sample, the diffraction peaks are observed at26.25°, 33.53°, 37.72°, 51.61°, 54.53°, and 61.68° They respectively correspond to the
31.25° and 35.68°, corresponding to the (100) and (101) plane, respectively (JCPDS card No 00-005-0664] [166] At 500 ˚C, no noticeable change in the XRD peak position can be observed when compared with the as-milled, suggesting no reaction between ZnO and SnO2 However, when the annealing temperature increases up to
900 ˚C, all peaks of Zn2SnO4 crystals are formed, which are corresponding to the cubic structure of the Zn2SnO4 (JCPDS standard No 00-024-1470) When samples are air-annealed in the range of 900-1100 °C, the intensities of diffraction peaks characterized for Zn2SnO4 phase tend to increase gradually and then decrease, reaching the maximum value at 1100 °C In this annealing temperature range, no diffraction peaks from ZnO, SnO2 or other impurities can be observed, and all diffraction peaks can be attributed to the cubic Zn2SnO4 phase (JCPDS card no 00- 024-1470, space group Fd-3m (227) with cell parameters a = b = c = 8.7125 Å, V 648.88 Å 3 ) Further increasing the annealing temperature to 1200 o C gives, beside diffraction peaks of Zn2SnO4, the appearance of other peaks of the ZnO phase beside those of the Zn2SnO4 phase It is also observed that the intensities of diffraction peaks corresponding to Zn2SnO4 are reduced in comparison with the sample annealed at 1100 o C The intensity reduction and the reappearance of ZnO at
1200 o C may be due to the evaporation of SnO2, which leads to a shortage of Sn in the ZTO.
Table 5.1 The crystal sizes of Zn 2 SnO 4 :3%Cr3+ particles calculated by Debye-
It is necessary to notice that ZnO and SnO2 phases disappear at 900 °C, indicating a complete interaction between them However, the ZnO phase is observed to reappear at high temperatures (≥1100 C), suggesting that Sn atoms were probably evaporated at high temperatures Therefore, it is possible to conclude that the Zn2SnO4 crystalline phase began appearing at 900 C, and the best crystal quality was obtained at 1100 C In addition, the crystal size D was estimated from the Full width at half maximum (FWHM) of the peak (311) with Debye-Scherrer’s formula The calculated crystal sizes are shown in table 5.1, indicating that the crystal size increases with the annealing temperature [170].
5.2.1.2 Effect of Cr 3+ concentration on structural formation
Figure 5.2 shows the XRD patterns of Zn2SnO4:x%Cr3+ (x = 0-6%) powder
Figure 5.2 XRD patterns of Zn 2 SnO 4 :x%Cr3+ (x=0-6%) annealed at 1100 °C in air
From Fig 5.2, we can see that all the diffraction peaks are indexed to the cubic phase Zn2SnO4 (JCPDS card no 00-024-1470, space group Fd-3m (227) and cell parameters a = b = c= 8.7125 A 0 ) and the majority peaks positioned at 28.62°, 33.80°, 35.32°, 41.17°, 54.44°, and 59.83° can be attributed to (220), (311),
(511) and (440) planes, respectively As the XRD results show a single Zn2SnO4 phase without any detectable secondary phase, hence, the Cr atoms are deduced to be successfully incorporated into the Zn2SnO4 lattice However, the samples with a higher concentration of Cr 3+ (4% - 6%), intensities of the peaks decrease in comparison with those of the samples with a lower concentration of Cr 3+ (0-3%). This leads to a prediction that the optimal concentration of Cr3+ in Zn 2 SnO4 lattice is 3% mol.
The XRD peaks of Zn2SnO4:x%Cr3+ phosphors shift to higher diffraction angle with increasing the Cr 3+ concentration in the range 0-3% (Figure 5.2b), indicating the decrease of lattice interplanar spacing due to the smaller ionic radius of Cr 3+ This result shows that Cr3+ ions may have substituted the Zn2+/Sn4+ sites in the
Zn2SnO4 matrix as luminescence centers [228] Nevertheless, when Zn2SnO4 is doped with a large Cr 3+ concentration (> 3% Cr 3+ ), the XRD peaks do not shift to the have successful doped Cr3+ ions into Zn2SnO4 lattice, and the optimal concentration of Cr 3+ is about 3% mol.
5.2.2 Phosphor morphology of Zn 2 SnO 4 :Cr3+
Figure 5.3 FESEM of Zn 2 SnO 4 :3%Cr3+ powder un-annealed and annealed in the range of 900-1200 °C in air
To determine phosphor morphology and particle size of phosphors obtained, a typical FESEM was carried out by ultra-high resolution field emission scanning Electron Microscopy (SEM – Jeol JSM-7600F) FE-SEM images of
Zn2SnO4:3%Cr3+ powder after milling for 60 hours and annealing in the range of
500, 900-1200 ˚C in the air are shown in Figure 5.3 For the un-annealed sample, it is clear to see that the particles are about 10-30 nm, granular, nearly spherical (Figure 5.3a), and these small particles agglomerate to form more massive clusters (see inset in Figure 5.3a) The particle size slightly changes to reach 100-200 nm when the annealing temperature is 500 ˚C (Figure 5.3b) However, the particle size increases significantly when the annealing temperature is about ≥ 1000 °C At 1100 °C, the average particle size is in the range of 2.0-3.0 μm, which is significantly larger than the average size of the as- milled powder (see Figure 5.3e).
5.2.3 Optical properties of Zn 2 SnO 4 :Cr3+
5.2.3.1 The effect of annealing temperature on optical properties of
Figure 5.4 PL spectra of the Zn 2 SnO 4 : Cr3+ obtained at different annealing temperature
The PL spectra of samples with different annealing temperatures were measured by using Nanolog - Horiba Jobin Yvon equipment and presented in Figure 5.4 As we can see, PL spectra of all obtained samples at various annealing temperatures show a strong and broad emission band from 650 nm to 1000 nm peaking at 740 nm which is ascribed to the spin-allowed 4T 2 - 4A2 transition of Cr3+ ions [30,31,229]. The PL intensity of this emission band increases with the annealing temperature up to 1100
C By further increasing the annealing temperature to 1200 C, the PL intensity begins to reduce The increase and reduction of PL intensity are consistent with the intensity of Zn2SnO4 diffraction peaks in the XRD patterns This helps us to choose the optimal annealing temperature for the highest PL intensity is about 1100
5.2.3.2 The effect Cr3+ ions concentration on optical properties of
Zn 2 SnO 4 : Cr 3+ shown in Fig 5.5 The PL intensity increases with the Cr 3+ doping concentration in the range from 1 to 3 % and decreases when the doping concentration exceeds 4 %.
Figure 5.5 PL spectra of the Zn 2 SnO 4 : xCr3+ (x=1-6%) obtained at 1100C
The concentration quenching is a well-known phenomenon in phosphors [208]. And it might be related to the two mechanisms of the physics Which are the exchange interaction and the multipole–multipole interaction In order to elucidate which mechanism is responsible for the concentration quenching, an estimation of the critical distance (Rc) is made according to the relationship proposed by Blasse [224].
For Zn2SnO4:Mn2+, N = 8, x c = 0.03 and V = 648.88 Å3, the R c value is calculated to be 14.74 Å It has been well established that for the concentration quenching of a phosphor, the exchange interaction dominates when R c is less than 5 Å and the multipole-multipole interaction becomes predominant when R c is larger than 5 Å. Most likely the multipole-multipole interaction is the main mechanism responsible for the observed concentration quenching.
5.2.3.3 Photoluminescence excitation of Zn 2 SnO 4 : xCr3+(1-6%)
When monitored at 740 nm at room temperature, the PLE spectra of the as- prepared samples were measured by photoluminescence spectroscopy (Nanolog,Horiba Jobin Yvon, 450 W) and presented in Figure 5.6 As we can see, for the un- doped sample (the inset of Fig 5.6), there is a strong absorption in the UV region
Conclusion
The single-phase of Zn2SnO4: Cr3+ and Zn2SnO4: Cr3+, Al3+ phosphors have been successfully synthesized by high-energy planetary ball milling combined with annealing at the low temperature of 1100 C in air.
The Zn2SnO4: Cr3+ material was first investigated for the properties of phosphor- converted LEDs The excitation spectra of Zn2SnO4: Cr3+ phosphors show strong absorption in blue and red areas with the peaks at 460 nm and 630 nm. The emission spectra of this material give emission in the infrared region with a peak at 740 nm.
When Al3+ ions co-doped Zn2SnO4: Cr3+ material, the excitation spectrum and the emission spectrum show blue shift due to the Burstein – Moss effect and the crystal field around the Cr 3+ ion increased from 2.24 to 2.71 Excitation spectra and emission spectra of Zn2SnO4: Cr3+, Al3+ materials give strong absorption in blue and red regions with the peaks at 450 nm and 620 nm and emission in infrared areas with a peak at 730 nm.
LED device was fabricated using blue LED Chip combined with the obtained ZTO: Cr 3+ or ZTO: Cr 3+ , Al 3+ have the corresponding energy conversion efficiency of 6.6% and 16.3%, respectively The infrared phosphors Zn2SnO4: 3%Cr3+ and
Zn2SnO4: 3%Cr3+, 0.6% Al3+ show the potential for the application in manufacturing specialized LED for agricultural lighting.
Through the motivation set up from the beginning, this research focused on three specific orientations: synthesis and study optical properties of (1) Zn2SiO4 and
Zn2SiO4: Mn; (2) Zn2SnO4 and Zn2SnO4: Mn2+ , and (3) Zn2SnO4: Cr3+ and Zn2SnO4:
Cr 3+ , Al 3+ The thesis has achieved some new results, such as:
(1) Zn 2 SiO 4 and Zn 2 SiO 4 : Mn2+
The undoped and Mn2+ doped Zn2SiO4 were first synthesized successfully by high energy ball milling and calcination in air The optimal annealing for the pure phase Zn2SiO4 and maxima PL intensity is about 1250 °C, which is 200 °C -300 °C lower than that of the solid-state reaction method Besides, the PL intensity of
Zn2SiO4: 5%Mn2+ produced by the modified method is nearly 1.8 times higher than that of the sample obtained by the conventional solid-state reaction method.
The Zn2SiO4 gives the PL spectrum with a broadbands centered at 735 nm This broadband was fitted into two Gaussian peaks at 730 nm and 760 nm The origin of the two peaks is ascribed to the NBOHs of the unpaired electron on 2px or 2py orbitals. The Mn2+-doped Zn2SiO4 phosphor emits an intense green band at 525 nm.
The TL glove curve of Zn2SiO4: 5%Mn2+ shows a strong peak at 158 °C and a shoulder at 235 °C, and displays linear dose-response with β-ray exposure time which indicates the phosphor could be useful for the dosimetric application.
A green LED device was fabricated by using a 270 nm UV LED chip combined with 5% Mn2+-doped Zn2SiO4 phosphor, which provides 525 nm green light with CIE chromaticity coordinates of (0.2477; 0.6829) and the color purity of nealy 85%.
(2) Zn 2 SnO 4 and Zn 2 SnO 4 : Mn2+
The undoped and Mn2+ doped Zn2SnO4 were first synthesized successfully by high energy ball milling and calcination in air The optimal annealing for the pure phase Zn2SnO4 and maxima PL intensity is about 1000 C.
The Zn2SnO4 gives the PL spectrum with a new emission peaking at 684. Literally, there are possibly two potential mechanisms for this emission: (1) recombination of a deeply trapped electron (V O* state) with a deeply trapped hole (VSn/VZn), or (2) recombination of a shallowly/deeply trapped hole (VO ++) with a deeply trapped electron (Zni/Sni).
Zn2SnO4: Mn2+ phosphor was the first synthesis giving green emission spectra with a peak of 523 nm Excitation photoluminescence spectra (PLE) of Zn2SnO4:
Mn2+ has excellent thermal stability With increasing temperature further to 200 °C, the emission intensity remains at 62 % of the initial value (at 25 °C) so that the prepared phosphor shows excellent promise for LED application Besides, the internal quantum efficiency of the Zn2SnO4: 5%Mn2+ is about 40%, similar value, which has been achieved in previous reports about Mn 2+ doped oxide phosphors
LED device fabricated by coating Zn2SnO4: 5% Mn2+ powder on blue LED chip with color coordinates (x = 0.2419; y = 0.3953) Besides, when using Zn2SnO4: 5% Mn2+ powder mixed with Zn2SnO4 red powder: 3% Cr3+, 0.6% Al3+ and covered 450 nm blue LED chip for white LED device with color coordinates x 0.3783 and y = 0.3520, color temperature is 3858 K and color rendering index is 91.
(3) Zn 2 SnO 4 : Cr3+ and Zn 2 SnO 4 : Cr3+, Al3+
Efficient far-red emitting ZTO: Cr 3+ and ZTO: Cr 3+ , Al 3+ were successfully synthesized by a combination of high energy planetary ball milling technique and calcination at 1100 C
The excitation spectrum of ZTO: 3%Cr 3+ monitored at 740 nm shows strong absorption in the blue region peaked at 460 nm, which is well-matched with the emission of blue-chip And, its emission spectrum using 460 nm excitation wavelength illustrates a broad band centered at 740 nm because of spin-allowed 4T2- 4A2 transition of Cr3+ ions
The emission spectrum of ZTO: 3%Cr 3+ , 0.6%Al 3+ consists of broadband peaking at 730 nm, and two distinguish narrow features at 691 nm and 725 nm The nature of broadband at 730 nm is due to spin-allowed 4T2- 4A2 transition of Cr3+ ions, but two later peaks are because of a spin forbidden transition 2E1(2G) →4A2.
LED device was fabricated using blue LED Chip combined with the obtained ZTO: 3%Cr 3+ or ZTO: 3%Cr 3+ , 0.6%Al 3+ phosphors and both show near-infrared light output powder of nearly 30 mW, and 35 mW, the corresponding energy conversion efficiency of 6.6% and 16.3%, respectively The results demonstrate that we might be able to produce the inexpensive and efficient Zn2SnO4: Cr3+, Al3+, for broadband near-infrared LEDs.
In the future, we will:
Improve the key parameters for LEDs such as: CCT, CIE, the color rendering index (CRI or Ra), Luminous efficacy, Quantum yield.
Try to apply prepared devices in the indoor plan cultivation.
1 L.T.T Vien, Nguyen Tu, T.T Phuong, N.T Tuan, N.V Quang, H Van Bui,