Advances in Fingerprint Technology.pdf

Absorbance
4
3
2
1
0
300 350 400 450 500
Wavelength (nm)
Figure 6.10 Absorption spectra of CdS nanocrystals in hexane (solid line) and
heptane (dashed line).
spectra, are shown in Figure 6.10. They serve to determine the sizes of the
nanocrystals, 23 which were deduced to have radii of 3 to 4 nm. The nanocrystal
luminescence (band width about 100 nm) peaked at about 580 nm,
with a second, weaker luminescence band in the 400 to 450-nm range. In
terms of intensities as a function of excitation power, excitation wavelength,
solvent system, etc., the luminescence spectroscopy of nanocrystals tends to
be more complex than that of typical fluorescent molecules because in nanocrystals
one likely has a number of different, and quite variable, trap states
that contribute to the luminescence, including surface traps, as well as the
intrinsic semiconductor recombination process. Thus, the spectroscopic features
generally need to be examined for the samples at hand, with literature
results serving as approximate guides only. Lifetime measurements for heptane
solution used phase-shift/demodulation techniques, as described earlier,
and employed a HeCd laser (operating at 325 nm). The lifetime measurement
corresponded to the total luminescence (transmitted to the detector through
a long-wavelength-pass filter). Laser modulation frequencies ranged from 0.1
to 100 MHz. The fit to the obtained phase shift and demodulation curves,
analogs to the curves shown in Figure 6.5, yielded three lifetime components.
Approximately 70% of the luminescence intensity corresponded to emission
with lifetime of about 1000 ns, about 25% to emission with lifetime of about
70 ns, and about 5% to emission with lifetime of 0.54 ns. This latter is excitonic
emission and the obtained lifetime is in good agreement with the literature. 25
Total luminescence lifetime measurement was made, rather than lifetime measurement
at a particular wavelength or a set of particular wavelengths, because
fingerprint luminescence imaging usually involves the total luminescence.