Chapter 4 Results
4.4 Luminescence Property
Luminescence properties with different factors were studied, such as annealing temperatures, Mn concentrations, applied voltages and so on.
For the PL measurements, we use an UV lamp as a light source with a wave length of 254 nm. The spectrum is shown in Fig. 4- 16. Although there were noise existing in the spectrum, it did not make sense to our experiments due to the maximum emission peak for Zn2SiO4:Mn locate at about 520-530 nm as a visible green light. Fig. 4- 17 shows the images of coated samples after heat treatment and exposed under visible and UV light.
The doping amount of Mn in this study was concerned. Fig. 4- 18 shows the PL intensity varies with Mn doping. The PL property increased slightly with amount of Mn which performed a best PL intensity at 5 mol% doping. According to the concentration quenching effect [70], it is believed that there’s a critical doping amount existing in phosphor, i.e. if the doping amount of activator is over than a specific value, the distance between activators inside the system decreased and thus the energy transferred from excited state to ground state became in heat loss in the matrix phase (silicate) instead of visible light and the PL intensity decreased. Therefore, when Mn doping in this case reach up to 10 mol%, then the PL intensity low down due to the concentration
In section 4.2, it has been mentioned that the higher annealing temperature led to the growth of cristobalite phase. When the temperature reached up to 1200oC, the silica particles sintered and became a nearly homogeneous surface in Fig. 4- 7 (case of ZSpII).
This phenomenon resulted in the decreasing of PL (photoluminescence) intensity as Fig.
4- 19. In the figure, the PL intensity increased with temperature, and the 1000oC and 1100oC heat treatment (for 2 hr) performed a better PL property. However, the sample annealed at 1200oC showed a lower emission intensity which was believed that (1) the existing of the overaged cristobalite phase induced a strong lattice distortion and also the different reflection index (or even adsorption index) from the original amorphous SiO2 matrix phase and (2) the great change of surface area, i.e. the amount of phosphor coating on the silica was different between un-sintered (more phosphor located at the surface) and sintered (less phosphor) one in a unit surface area. These reasons resulted to the lower PL intensity. This outcome is agreeable to the case in ZSpVI which shown in Fig. 4- 12 (d) and Fig. 4- 20 which represented that PL intensity increased with annealing temperature and performed a best PL property at 1000oC but the intensity low down as the temperature rose up to 1200oC.
In section 4.3, adding alcohol into Zn/Mn precursors (for the case in ZSpVI) did improve the uniformity to the coating process. Intuitively, the precursor without modifying contains the most precipitates on silica template and thus gave rise to the
highest PL intensity shown in Fig. 4- 21, and then decreased with diluted ratio. The samples diluted to 1/2, 1/3, 1/5, 1/10 and 1/20 represented a lower intensity. However, samples those with diluted precursors did not show a great change. The possible reason is that adding of alcohol resulting to a uniform distribution of Zn and Mn during coating process, and the retained ions were discarded instead of precipitated as segregations.
Therefore, the coated ions remain saturated and just slightly affected by alcohol.
ZSpII sample was used as phosphor layer in a CL device composed of CNT cathode and ITO anode, which installed in a chamber as shown in Fig. 4- 22. ZSpII sample performed a green-blue light under accelerated voltage as the applied voltage rose up. The I-V curve (Fig. 4- 23) illustrates the electrical behavior of ZSpII sample that contained three regions:
(1) sub-threshold region, which represented that the whole system acted as an open circuit without any current passing through.
(2) Linear region (175-310 V), which shows that the measured current increased with applied voltage.
(3) Cut of region (over 310 V), the current maintained as constant which was set to protect the testing machine against current flow over than a specific value.
The CIE analysis for ZSpII sample consisted of x = 0.2899 and y = 0.5048 where
Considering of the application for this material, a low threshold voltage is a beneficial advantage for a flat panel display because of a higher threshold voltage consumes more power (more energy was wasted), and also releases heat in device.
Therefore, this material is potential used for displays.
200 300 400 500 600 0
1000 2000 3000
Intensity, a.u.
Wavelength, nm
UV source
Fig. 4- 16 Optical spectrum of the UV source which was used in this study.
Fig. 4- 17 Photos of samples (a) and (b) prepared via ZSpII, and (c) and (d) via ZSpVI processes. (a) and (c) were exposed under visible light while (b) and (d) were exposed under 254 nm UV light.
1cm
a b
c d
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500 1000 1500 2000 2500
Intensity, a.u.
Wavelength, nm
zZSpII01Mn
ZSpII03Mn
®ZSpII05Mn SZSpII10Mn
Fig. 4- 18 PL spectra of ZSpII intensity with different amounts of Mn ion doping. The doping ratio of Mn was defined as Mn/Zn.
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500 1000 1500 2000 2500 3000
Intensity, a.u.
Wavelength, nm
»ZSpII-900 SZSpII-1000
¦ZSpII-1100
ZSpII-1200
Fig. 4- 19 Spectra illustrating the PL intensity of ZSpII sample annealed at different annealing temperatures. The samples of 1000oC and 1100oC perform a better emission property, while the intensity decreased if the temperature rose up to 1200oC.
450 500 550 600 650
Intensity, a.u.
Wavelength, nm
ZSpVI-600oC SZSpVI-800oC zZSpVI-1000oC {ZSpVI-1200oC
Fig. 4- 20 PL spectra of ZSpVI samples annealed at different annealing temperatures.
The one annealed at 1000oC performed a better PL property.
1000 1500 2000 2500
450 500 550 600 650
Fig. 4- 21 PL spectra of ZSpVI samples diluted by ethyl alcohol to specific concentration, then annealed at 1000oC for 2 hr.
Fig. 4- 22 Photos showing CL device in a vacuum chamber.
focused
1 cm electrode
ITO sample holder
0 100 200 300 400 500 0.000
0.002 0.004 0.006 0.008 0.010
Current, mA
Applied Voltage, V
ZSpII
Fig. 4- 23 I-V curve illustrating the electrical property of ZSpII sample assembled in TECO/CL device existed by a threshold voltage at 175 V and saturated at 310V.
Fig. 4- 24 CIE analysis result of ZSpII sample in TECO CL device. The sample of studied is located at x = 0.2899 and y = 0.5048, which corresponds to the circle in this diagram [71].