CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

Intensity (arb.units)
25000
20000
15000
10000
5000
0
Asprepared
300 °C
500 °C
700 °C
900 °C
λ exc
=253 nm
350 400 450 500 550 600 650
Wavelength (nm)
Normalised intensity
1
0.1
0.01
Asprepared
300 °C
500 °C
700 °C
900 °C
0 10 20 30 40 50
Time (μs)
(a)
(b)
Fig.111. Emission spectra (a) and excited state decay curves (b) of CaWO 4 nanoparticles as a
function of heat treatment temperatures (particle size). (λ exc =253 nm and λ em = 425
nm)
To confirm above inferences, decay curves corresponding to the excited state for the
emission (425 nm) is recorded for different samples and are shown in Fig.111 (b). The
corresponding lifetime values and their relative percentages are given in the Table 15. Decay
curves corresponding to as prepared, 300 and 500°C heated CaWO 4 nanoparticles are found
to be bi-exponential in nature, whereas that of 700 and 900°C heated samples are found to be
single exponential. In the case of samples showing bi-exponential decay curves, the shorter
lifetime values are assigned to surface WO 2- 2-
4 and longer lifetime values due to bulk WO 4
structural units.
Table 15. Excited state lifetime values of CaWO 4 nanoparticles as a function of heat
treatment temperature and particle size.
Sample Avg. crystallite Lifetime values
size (in nm) τ 1 (µs) τ 2 (µs)
As prepared 7 0.5 (25%) 2.602 (75 %)
300°C 8 0.398 (13%) 2.798 (87 %)
500°C 26 1.661 (9%) 7.004 (91 %)
700°C 90 8.214 (100%) -
900°C 99 7.829 (100%) -
The lifetime value increases and relative percentage of shorter component decreases with
increase in heat treatment temperatures. Hence from the steady state and lifetime
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