Structure and Band Edge Energy of Highly Luminescent
CdSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub> Alloyed Quantum Dots
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Abstract
CdSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub> quantum dot (QD) alloys are
characterized by high luminescence
quantum yields and a strong band gap bowing as a function of the Se:Te
ratio, featuring longer emission wavelengths than CdTe or CdSe dots
of identical size. In this contribution, these properties are rationalized
by examining the structure and band edge energy of CdSe<sub>1–<i>x</i></sub>Te<sub><i>x</i></sub> as functions of <i>x</i>. The QDs were synthesized employing the “hot-injection”
method, in the presence of either trioctylphosphine oxide (TOPO) or
octadecene (ODE) as the Cd precursor solvent. Elementary analysis
of the QDs indicated that TOPO plays a crucial role in tuning the
content of Se in the alloys, as only traces of this element were found
when using ODE. Detailed studies based on X-ray diffraction (XRD),
high-resolution transmission electron microscopy (HRTEM) and selected
area electron diffraction (SAED) revealed a high degree of complexity
in the structure of the alloyed dots. The analysis concluded that
the structure of the QDs was essentially wurtzite, although features
associated with zinc blende can be seen due to the presence of stacking
faults and to a small population of nanocrystals with cubic structure.
More importantly, these studies reveal a nonlinear expansion of the
effective lattice constant with increasing Te content. The valence
band edge energy of the alloys in solution was estimated from the
first oxidation potential measured by linear sweep voltammetry at
Au microelectrodes. The results show that the valence band edge exhibits
a very weak dependence on <i>x</i> for values below 0.5,
indicating that the decrease in the optical band gap is mainly linked
to a decrease in the conduction band edge energy. For <i>x</i> > 0.5, the conduction and valence band edges shift to higher
values
with an overall increase in the band gap. The experimental trends
show, for the first time, that the characteristic red shift of the
band gap with low to intermediate Te content is determined by relaxation
of the lattice constant, whereas the contribution arising from the
change in anion electronegativity becomes predominant for <i>x</i> > 0.5