Microstructure Observations and EDS Measurements

Fig. 4- 9 (a) to Fig. 4- 9 (c) are the SEM images of three types coated SiO2

particles. The initial silica particles in Fig. 4- 9 (a) had an average particle size of about 325 nm which was synthesized by the precursors in a ratio of TEOS: NH4OH:C2H5OH

= 24:25:450 aging for 24 hr. After the coating process, the particle size became 375 nm as shown in Fig. 4- 9 (b), which tells us that the thickness of the coating layer is about 25 nm. The Si/Zn ratio in this case was measured. 7 points were chosen in Fig. 4- 9 (c) randomly and calculated by t-distribution method. The average value of Si/Zn ratio is 6.088 with standard deviation of 0.612. The samples would have a 95% confidence ranged from 5.522 to 6.654.

The silica particle were assembled to photonic band-gap (PBG) structure, as shown in Fig. 4- 10 which started with TEOS:NH4OH:C2H5OH = 10:20:200 aging for 2 hr. In order to provide strength, the assembled SiO2 PBG was annealed at 1000^oC for 1 hr first.

The average particle size in this case is 240 nm with a polydispersity of 5.5%. Then immersed the template in the designed molar ratio (as the previous case with 5 mol% of Mn doping) of Zn and Mn solution for 2 hr. Fig. 4- 11(a) represents non-uniform coating of precipitates. The reason to the non-uniform coating resulted from a strong surface tension that make the Zn and Mn solution did not “penetrate into” the pores of

useful chemical, was added to reduce the surface tension of the solution coating and improve the wettability of the solution. In Fig. 4- 11(c), the Zn and Mn solution was diluted by alcohol to 3 times in volume. The improvement was obvious in the image, and so were the cases in Fig. 4- 11 (d) to Fig. 4- 11 (f), which were diluted to 5, 10 and 20 times, respectively. The segregation of non-uniform Zn/Mn coverage on the SiO2

PBG surface was no longer existed. Therefore, reducing the surface tension of the precursors by adding ethyl alcohol was a good step to offer a better coating condition.

The morphologies of coated SiO2 PBG crystal with different annealing temperature are shown in Fig. 4- 12. Different annealing temperatures seemed not to change the morphology, except the case by 1200^oC (Fig. 4- 12 (d)). Zn2SiO4:Mn coated silica particles remained its original spherical shape below 1100^oC. The particles could still be distinguished clearly at 1000^oC. While the PBG structure disappeared and densified after sintering at 1200^oC for 2 hr which was similar to the trend in Fig. 4- 7. The SiO2

template experienced an extensive sintering behavior. Therefore, proper conditions for fabricating Zn2SiO4:Mn coated SiO2 PBG crystals is sintering at <1200^oC 2 hr in air.

The EDS measurement of ZSpVI series was shown in Fig. 4- 13. The sample in this case was diluted to 5 times. The results from top site to bottom were 1.05, 0.32, 0.33, and 1.24 (At% base on Si), respectively. The reason is that during the drying process, the un-coated (or excess) ions were discard instead of forming precipitates, and

also, the solution diffusions and evaporates from sample to the surface and thus resulting in a higher concentration at the surface and that of a lower one inside the sample.

Fig. 4- 14 was the microstructure of pure silica particles assembled as PBG crystal.

The particles were arranged in nearly ordered closed-pack structure with mono- and two-layer stacking in (a) and (b), respectively. The cross-sectional view of the coated particles was shown in Fig. 4- 15. The coated particle with original concentration shows a phosphor layer in a thickness of about 50 nm. In the other hand, particles coated a phosphor layer became extremely thin if the Zn(NO3)2 and Mn(NO3)2 precursors were diluted to 5 times by alcohol as shown in Fig. 4- 15. The thickness of the coating layer for diluted sample is about 20 nm. Some black spots in image are copper contamination after ion-milling. The diffraction pattern in this case is too weak to identify the Zn2SiO4

crystalline phase because of the G1 glue covering on observation area. However, in section 4.2 and 4.4, the XRD pattern and PL spectra can be evidence to prove that the Zn2SiO4 phase does exist on the surface of silica particles as annealed at temperature higher than 800^oC. The thickness of the coating layers determine the PL intensity, i.e.

the thinner the phosphor layer, the weaker the PL intensity (which will be shown in chapter 4.4).

Fig. 4- 9 SEM images showing the morphologies of (a) silica particles prepared by Stöber method, (b) silica particles coated with Zn²⁺ and Mn²⁺ before calcinations, and (c) dried and after 1100^oC calcination for 2hr.

(a)

(b)

(c)

Fig. 4- 10 SEM image showing silica particles with PBG structure at 1000^oC heat treatment for 2 hr. The precursors consists of TEOS:NH4OH:C2H5OH = 10:20:200.

Fig. 4- 11 SEM images of SiO2-template coated with Zn and Mn solution (a) original concentration, or diluted by ethanol in volume to (b) 2, (c) 3, (d) 5, (e) 10 and (f) 20 times, then dried and annealed at 1000^oC for 2 hr. The white circles illustrate that there are still a little amount of non-uniform coating, however, this non-uniform coating did not exist at all as the precursors were diluted to 3 times.

(a) (b)

(c) (d)

1 μm (f) (e)

Fig. 4- 12 SEM images of coated SiO2-template by the Zn/Mn solution diluted with alcohol to 5 times. The samples were dried and annealed at (a) 600^oC, (b) 800 ^oC, (c) 1000 ^oC, and (d) 1200 ^oC, respectively.

(a) (b)

(c) (d)

Fig. 4- 13 SEM image showing the positions of EDS measurement on ZSpVI sample.

x x

x x

Fig. 4- 14 TEM images showing the detail morphologies of assembled SiO2-template with (a) one layer and (b) two layers packing, respectively.

(b) (a)

200 nm 200 nm

\

Fig. 4- 15 TEM cross-sectional view of SiO2 particles coated with precursors (a) directly and (b) diluted by alcohol to 5 times, respectively.

(a)

100 nm (b)

core ~20 nm shell layer

Cu spot G1 glue