Luận án Synthesis and properties of undoped and transition metal (Mn₂⁺, Cr³⁺) doped Zn₂SiO⁴ and Zn₂SnO⁴ phosphors

results, as has been described above. It is noticed 
that the vibration modes of Zn-O and Si-O-Si bonds are disappeared completely, 
while the stretching vibrations of the ZnO4, and SiO4 groups become sharp and well-
defined peaks in the FTIR spectrum of the samples annealed above 1150 °C, as shown 
in Fig. 3.10(d-g). 
3.3.3.2. Raman spectra 
Figure 3.11. Raman spectra of 5 wt% Mn2+-doped ZnO2SiO4
 powders after milling for 40 
hours (a), the samples milled for 40 hours followed by annealing at different temperatures 
of 500 °C (b), 900 °C (c), 1150 °C (d), 1200 °C (e), 1250 °C (f) , 1300 °C (g), for 2 h in air 
 Fig. 3.11 displays Raman spectra of Zn2SiO4:5%Mn2+ powders after ball-
milling for 40 hours and annealed at different temperatures for 2 hours in the air. The 
results show that the Raman spectra of the as-milled sample and the sample annealed 
at 500 C consist of several vibration modes at 320, 428, 460, 543, and 642 cm-1 
which originated from ZnO and SiO2 materials (Fig. 3.11a-b)[123,132]. After 
annealing at 900 °C, a new and weak peak emerges in the Raman spectrum at 865 
cm-1, besides the characteristic peaks of the ZnO and SiO2 (Fig. 3.11c). This peak is 
contributed to the surface vibrational mode of the siloxane group (the Si–O–Si 
linkage) in -Zn2SiO4 structure [120,131]. This result demonstrates that the -
 61 
Zn2SiO4 phase was formed at the annealing temperature as low as 900 °C. With 
further increasing annealing temperature, three peaks related to the surface siloxance 
group at 870, 907, 947 cm-1 appear and dominate in the Raman spectra 
[16,75,120,131]. The intensity of these peaks first increases with increasing annealing 
temperature up to 1250 C and then reduce at high temperature. This result is in good 
agreement with both XRD and FTIR results, as shown in Fig. 3.7, and Fig. 3.10 and 
revealing the optimum heat-treatment condition for the -Zn2SiO4 phase is 1250 C. 
3.3.4. Optical properties of Zn2SiO4:Mn2+ 
3.3.4.1. PLE and PL spectra of the un-doped and Mn2+ doped Zn2SiO4 
PLE (a) and PL (b) spectra of Zn2SiO4 powders after ball-milling (without and 
with Mn-doped) followed by the annealing at 1250 °C for 2 hours in air are shown in 
Fig. 3.12. The Figure 3.12a candidates that the samples absorb the UV region strongly 
around 270 nm, which related to the electronic transitions from 6A1g (6S) state in Mn2+ 
ions to the conduction band of Zn2SiO4 host lattice. Four weak absorption peaks at 
357, 379, 421 and 433 nm are also observed in the PLE spectrum of Mn-doped sample 
(see inset of Fig. 3.12a) and can be attributed to the electronic transitions 6A1→4E 
(4D), 6A1→4T2 (4D), 6A1 →4A1(4G)/4E(4G) and 6A1g→4T2 (4G) in Mn2+ ions, 
respectively [16]. This is an indication that the Mn2+ ions are diffused into the 
Zn2SiO4 lattice. The PL spectrum of the un-doped sample consists of a strong, broad 
and intense near-infrared emission band centered at around 735 nm, and a weak green 
emission band at 520 nm (see Fig. 3.9b). The NIR peak at 735 nm is attributed to the 
energy transition from non-bridging oxygen hole centers (NBOHs) of SiO2 to the zinc 
interstitial (Zni) and oxygen vacancy (Vo) states of Zn2SiO4 [26,71,75,125]. The green 
peak at 520 nm is explained as due to defect-related emission in ZnO, SiO2, or/and 
Zn2SiO4 [133-135]. For the sample doped with five wt% Mn2+, the PL spectrum 
reveals an intensive green emission band peaking at around 525 nm. This emission 
band is the well-known green emission band of α-Zn2SiO4:Mn2+ and is originated 
from the electronic transitions 4T1-6A1 in Mn2+ ions, [71,75,118,125,135]. 
Here, the disappearance of both NIR and 520 nm emission bands upon Mn2+ 
doping can be explained as due to the extinction of all defects because of charge 
compensation or oxygen vacancies compensation. When Mn2+ ions are introduced 
into the host lattice, they could either substitute for Zn2+ sites in Zn2SiO4 or stay at 
interstitial sites that lead to the reduction of oxygen vacancies and compensate surface 
charge defects. Alternatively, the more efficient energy transfer process from host 
defects (donor) to the Mn2+ luminescence center (acceptor) that results in strong 525 
nm green emission would also be the cause of the disappearance of defect-related 
emissions [73,79]. 
 62 
Figure 3.12. (a) PLE spectra measured at maxima of the emission at 740 nm (curve 1) and 
525 nm (curve 2); and (b) photoluminescence spectrum of Zn2SiO4 sample after milling 
without doping and doped with 5% Mn 
3.3.4.2. Effects of dopant concentration and annealing temperature on 
photoluminescence of Mn2+ doped Zn2SiO4 
Figure 3.13. PL spectra of 5 %wt Mn2+ doped Zn2SiO4 
 powder after milling for 40 hours 
followed by the annealing at different temperatures in the range of 500-1350 C 
 Effects of dopant concentration and annealing temperature on the 
photoluminescence of Zn2SiO4: Mn2+ are also investigated, and results are shown in 
735 
 63 
Fig. 3.13, and Fig. 3.14, respectively. It is clearly seen that the PL intensity first 
increased with increasing annealing temperature and reached the maximum at 1250 
C, and then reduced at higher annealing temperatures (Fig. 3.13). Since the 
luminescence intensity is dependent on the crystalline quality of the host lattice, from 
the XRD, FTIR, Raman as described above, we can conclude that the enhancement 
of the PL intensity is due to the improvement of the crystalline quality of the phosphor 
[45,136]. 
Figure 3.14 shows PL emission spectra of -Zn2SiO4 samples doped with Mn2+ 
ions at different concentrations. The -Zn2SiO4 sample with a Mn doping 
concentration of 5 wt% presented the maximum PL intensity. The reduction of the 
PL intensity at higher doping concentration is due to concentration quenching effect 
that reduces the radiative recombination process of Mn2+ ions, and therefore reduce 
the PL intensity [16,131]. 
Figure 3.14. PL spectra of Zn2SiO4:x%Mn
2+ (x=0-8) samples after milling followed by 
the annealing in air at 1250 C. The inset displays the dependence of the PL intensity on 
Mn2+ doping concentrations 
Besides, to demonstrate the advantage of our synthesis method, Zn2SiO4 powder 
doped with 5 wt% Mn2+ was also prepared by using conventional solid-state reaction 
method under similar experimental conditions. The PL emission spectra measured 
under the same conditions of the two samples prepared by the two methods are plotted 
in Fig. 3.15. It is shown the PL intensity of the sample produced by the modified 
method is nearly 1.8 times higher than that of the sample obtained by conventional 
solid-state reaction method. Here, the high-energy planetary ball-mill step may help 
 64 
to improve mixing of Mn2+-doped source into the host powder before high 
temperature diffusion process. 
Figure 3.15. Comparison of the PL emission spectrum of the -Zn2SiO4:5%Mn2+ sample 
prepared by HEBM-SSR method with that of the sample prepared by conventional solid- 
state reaction method 
3.3.5. Thermoluminescence (TL) properties and Decay time of Mn2+ 
doped Zn2SiO4 
 Thermoluminescence (TL) spectroscopy has emerged as an important technique 
that can be used to understand the dynamics of electron trapping centers. In this study, 
TL properties of the phosphor Zn2SiO4:5%Mn2+ phosphor was investigated by β-ray 
beam (Sr-90) source irradiation. Fig. 3.16a shows the TL glow curve for five wt% 
Mn2+-doped Zn2SiO4 with different β-ray exposure time at a constant heating rate i.e., 
2 C.s-1. The sample reveals two peaks at 158 C and 235 C (see Fig. 3.16b) and 
linear response with a dose up to 25 minutes β-ray exposure time. The low 
temperature and high-intensity peak (at 158 C) can be attributed to traps of Mn2+ 
[64,137] while the weak and high-temperature peak (at 235 C) may be due to non-
bridging oxygen (NBOHs) on which holes are trapped when the sample is β-ray 
irradiated. Here, similar to the PL results, no defect-related TL spectra were observed 
in the Mn2+-doped Zn2SiO4 sample. This is generally due to the competition between 
the radiative and nonradiative center or between different kinds of trapping centers 
[137]. As the main basis in the thermoluminescence dosimetry (TLD) is that TL 
output is directly proportional to the radiation dose received by the phosphor and 
 65 
hence provides the means of estimating the dose from unknown irradiations [64]. 
Thus, the TL results obtained in our study show a high potential application of the 
Mn2+-doped Zn2SiO4 in TLD. 
Figure 3.16. TL spectra of Zn2SiO4:5%Mn
2+under various β-ray exposure time(a) and 
decomposition of glow curve into individual peaks (b) 
 The estimated kinetic parameters (E, s, b, µg) for Zn2SiO4:5%Mn2+ phosphor is 
examined by using curve fitting of experimental data (see Fig.3.16b), and the results 
are shown in Table 3.3. As can be seen from table 3.2, the activation energy of the 
two peaks is 0.78 eV and 1.19 eV, and the frequency factor is in the range 1010 (s-1). 
These parameters give valuable information about the mechanism responsible for the 
emission in the present phosphor. Further, we have measured the 
thermoluminescence emission spectra at 158 °C and 235 °C and the result, as 
depicted in Fig. 3.17a confirmed that the green emission at 525 nm of the 
Zn2SiO4:5%Mn2+ phosphor could be observed at both temperatures. This is in 
good agreement with previously reported experimental results [38]. It concludes 
that both the traps at 0.78 eV and 1.19 eV belong to energy states in the band gap, 
which act as electron traps for energy transfer to Mn2+ and cause green emission. 
In addition, the lifetime that electrons were trapped in the traps was also 
measured. As shown in Fig. 3.17b, the decay time of the Mn2+ doped sample is 10.5 
ms, while the un-doped Zn2SiO4 sample shows a very fast decay, which accounts for 
nearly 1000 ns (inset Fig.3.17b). In our opinion, the reduction of the distance of 
neighbouring ions because of defects in the host lattice combined with Mn2+ states in 
the Mn2+-doped sample may be responsible for the longer decay time. 
 66 
Figure 3.17. Thermoluminescence emission spectra measured at 158 and 235 oC (a) and 
the decay curve of the Zn2SiO4:5%wt Mn
2+ phosphor (b). The decay curve of Zn2SiO4 
sample is shown in the inset of Fig. 3.14b 
Table 3.2. Kinetic parameters of Zn2SiO4: Mn
2+(5% wt) phosphor evaluated using R. Chen 
method. 
Peaks T1(°C) Tmax(°C) T2(°C) µg b E (eV) s (s-1) 
Peak 1 126 158 192 0.515 general 0.78 672.109 
Peak 2 212 235 283 0.693 general 1.19 1129.105 
3.3.6. Application of Mn2+ doped Zn2SiO4 on UV LED 
Figure 3.18. Electroluminescence spectrum (a) and the CIE coordinate plot of the 
prototype green-emitting LED under drive current of 60 mA (b). The inset of Fig. 12b is 
the digital image of the actual green-emitting LED 
 67 
 In order to explore the potential application of Mn2+-doped Zn2SiO4 phosphor 
prepared by HEBM-SSR method, green emission LED is assembled by coating 5 
wt%Mn2+-doped Zn2SiO4 phosphor on 270 nm UV-LED. The image of the green 
LED device underdrive current of 60 mA, and its CIE chromaticity coordination is 
displayed in Fig. 18. Its CIE coordinates of (0.2477; 0.6829) were located in the green 
wavelength range, and especially, it exhibited strong green emission under 270 nm 
UV excitation, which can be clearly seen with the naked eyes. 
3.4. Conclusion 
Near-infrared Zn2SiO4 and green Zn2SiO4: Mn2+ phosphor have been produced 
successfully by the high-energy ball milling technique followed by annealing 
temperature of 1250 C in air. 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 
conventional solid-state reaction method. 
Zn2SiO4 phosphor emits a broad spectrum in the infrared (maximum at 735 nm) 
due to the overlap of two emission bands with the maximum of 730 and 760 nm, 
respectively. The origin of these two emission peaks is explained by the non-bridging 
oxygen defects in the Zn2SiO4 lattice. 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 nearly 85%. 
 68 
Chapter 4. STRUCTURE AND OPTICAL PROPERTIES OF 
Zn2SnO4 AND Zn2SnO4:Mn2+ PHOSPHORS 
4.1. Introduction 
Light-emitting devices such as LEDs, WLED, and other optoelectronic devices 
based on wide band gap (WBG) semiconductor oxide materials are showing high 
efficiency and usefulness in recent years because of their stable physical thermal and 
chemical properties [110,138]. Zinc stannate (Zn2SnO4) is an essential n-type 
semiconductor with a band gap of about 3.67 eV [139]. In recent years, Zn2SnO4 
(ZTO) has attracted great attention from many research groups because of their 
excellent sensing performance [2,3], superior optical, optoelectronic, and 
photoelectric properties [141]. Therefore, ZTO has been used in a wide range of 
applications such as sensors [142], batteries [143], transparent thin films [144], solar 
cells [145], and photocatalyst [60]. There are several methods for synthesizing ZTO, 
such as sol-gel [146], hydrothermal [147], sputtering [148], and solid-state interaction 
[149]. Among them, the solid-state interaction method shows a lot of advantages, 
such as mass production, less toxic, and especially high optical efficiency. In 
addition, there are various structural morphologies of Zn2SnO4, such as wires [150], 
belts [151], particles, and hybrid structures [152]. These structures are strongly 
dependent on experimental conditions or synthesis methods, and they are all directed 
towards specific applications [1,4,17]. Interestingly, different structures also have 
their unique optical properties and can be tunable by controlling their internal-defect 
states. 
Thanks to a large band gap (3.67 eV), a near-ultraviolet emission peak around 
340 nm can be emitted by Zn2SnO4 [139]. In semiconductor materials, there are two 
defect types as either intrinsic or extrinsic defects due to impurities (doped elements). 
By optimizing the intrinsic defects of Zn2SnO4, emissions in the visible region from 
blue [154], green [155], yellow-orange [139], red [156] and even far-red [85] have 
been obtained and reported. These emissions originated from various causes such as 
oxygen vacancies (VO) [15,19], zinc vacancies (VZn) [1,22], tin vacancy defects (VSn) 
[16,18], Zn-interstitial (Zni) [20,23], Sn-interstitial (Sni) [159], lattice distortion or 
oxygen vacancy interactions (VO) and zinc interstitial (Zni), interstitial oxygen (Oi) 
in the lattice [1,20]. Red or far-red emission of Zn2SnO4, however, has been rarely 
observed in works of literature. Besides, the origin of these emissions has not been 
clearly explained yet, and they strongly depend on fabrication methods. By the 
Gaussian fitting method, Hsiu-Fen Lin et al. reported an emission peak at 671.1 nm 
from Zn2SnO4 powder synthesized by thermal plasma method, and it could be related 
to the Zn-interstitial (Zni) [139]. Annamalai et al. [153] and Jun Zhang et al. [160] 
 69 
proposed that the emission peak at 680 nm from Zn2SnO4 nanoparticles fabricated by 
hydrothermal method and chemical vapor deposition (CVD) method,respectively are 
caused by zinc or tin vacancies (VZn, VSn). However, the red or far-red emission of 
Zn2SnO4 has not been clearly observed, and its origin is still controversial. It is 
interesting to notice that the high-energy planetary ball milling technique could be 
used to create new defects-related states [161], which could lead to new emission 
bands in ZTO structures. 
In the past years, Zn2SnO4 doped with various ions as Eu3+, Cr3+, Dy3+, Ti4+, 
Co2+, Ni2+, and Cu2+ have been investigated for phosphor and photocatalytic activity 
application [19,82,162]. Recently, Mn ions are common non-rare earth luminescence 
centers that have been widely used in phosphor material. Beside Mn4+ ion-doped 
phosphors, Mn2+ ion-doped materials have been investigated the luminescence 
behaviours intensively [52,163]. According to reports, the low energy levels of Mn2+ 
ions strongly depend on the co-valency interaction with the host crystal or the crystal 
field. In a suitable host lattice, Mn2+ may tuneable emission color from blue to red 
region due to its 4T1→6A1 transition. Thus, Mn2+ ions are an ideal non-rare earth 
luminescent centers for phosphor material. Until now, Mn2+ ions doped CaAlSiN3, 
ZnAl2O4, Zn2SiO4, Zn2GeO4, MgGa2O4, CaCl2, CdSiO3, NaCaPO4, CaZnOS had 
been studied as phosphors used in fluorescent lamps and WLED [164]. Although 
there were researches about the synthesis, investigation of magnetic and electrical 
properties of Zn2SnO4: Mn2+, most of the researches have not yet been reported 
optical properties so far. Our point, however, that the aspect is essential and deserves 
an optical properties investigation to extend the application of this material. 
In this work, we report the undoped and Mn2+ doped Zn2SnO4 synthesized by a 
high energy planetary ball milling technique followed by calcination in air. The 
crystal structure, morphologies elemental analysis, and luminescence properties of 
Zn2SnO4: Mn2+ phosphors were investigated in detail. 
4.2. Structural and optical properties of Zn2SnO4 phosphors 
4.2.1. X-ray diffraction of Zn2SnO4 
4.2.1.1. Effect of annealing temperature on structural formation 
Figure 4.1 shows the XRD patterns of ZnO/SnO2 powder after milling for 60 
hours and annealing in the range of 600-1200 °C in air. On the one hand, some 
featured diffraction peaks are observed at 26.57°, 33.87°, 37.94°, 51.75°, 54.75°, 
61.88°, and 64.74°, as shown in Figure 4.1a. They respectively correspond to the 
crystal planes (110), (101), (200), (211), (220), (310), and (112), which belong to the 
face-centered structure of SnO2 material (JCPDS card No. 00-021-1250) [165]. On 
the other hand, the diffraction peaks of the wurtzite structure of ZnO are also observed 
 70 
at 31.71° and 36.25°, corresponding to the (100) and (101) plane, respectively 
(JCPDS card No. 00-005-0664] [166]. At 600 ˚C, no noticeable change in the XRD 
peak position can be observed when compared with the as-milled sample (see Figure 
4.1b), suggesting no reaction between ZnO and SnO2 occurs at this temperature. 
However, when the annealing temperature is increased up to 700 ˚C, three new peaks 
are observed in the XRD pattern (Figure 4.1c) at 34.29°, 41.68° and 60.40°, 
corresponding to the (311), (400) and (440) planes of the inverse spinel structure of 
Zn2SnO4 (JCPDS standard No. 00-024-1470). 
Table 4.1. The phase composition of the obtained samples annealed at different 
temperatures in air 
Sample Phase composition 
As-milled SnO2 ZnO - 
600 °C SnO2 ZnO - 
700 °C SnO2 ZnO Zn2SnO4 
800 °C SnO2 ZnO Zn2SnO4 
900 °C - - Zn2SnO4 
1000 °C - - Zn2SnO4 
1100 °C - ZnO Zn2SnO4 
1200 °C - ZnO Zn2SnO4 
 When the samples are annealed in the range of 800-1200 °C, the intensity of 
diffraction peaks first increases to reach the maximum value at 1000 °C (Figure 4.1d-
f) and then decreases with higher calcination temperatures. It is worthy to notice that 
ZnO and SnO2 phases disappear at 900 °C (Figure 4.1e), indicating a complete 
interaction between them. However, the ZnO phase is observed to reappear at high