CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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4.3. Results & Discussion:
4.3.1. Steady State Absorption Studies
To investigate charge carrier dynamics of different sized of CdTe QD we have synthesized
particles of different diameters. Figure 4.1 shows optical absorption spectra of CdTe QD
which clearly demonstrate different exciton bands in the spectral region. Particle sizes were
calculated from the sizing curves as demonstrated by Peng et al [4.15] considering spherical
nature of the particles. The particle size of CdTe QD determined to be 3.5 nm which is much
smaller than the Bohr radius of CdTe (r CdTe B ~6.5 nm). We have synthesized different sizes
of QD particles and the optical spectra are shown in supporting information, which clearly
demonstrate that with increasing particle size the exciton band moves to the red region of the
spectra.
The discrete features at 480 nm, 530 nm and 590 nm in the absorption spectrum in Figure 4.1
can be attributed to different exciton states of quantum dot and are labeled according to usual
nomenclature [4.8]. The lowest exciton states corresponds to the transition between two
different states conduction band and valence band, where electronic state (1S e ) in the
conduction band is same, however the valence band states of holes are different (1S 3/2 and
2S 3/2 ). The higher exciton state is the 1P 3/2 -1P e where both the electron and the hole is in state
with l=1. The peak at 590 nm and 530 nm corresponds to 1S 3/2 -1S e and 2S 3/2 -1S e transition,
where as the high energy peak at 480 nm can be attributed to 1P 3/2 -1P e . Particularly striking
is the energy difference between the lowest states which is much higher compared to thermal
energy of ~25meV, this would lead to a much slower relaxation from the higher hole state to