Structure and Band Edge Energy of Highly Luminescent CdSei1-xTex Alloyed Quantum Dots

CdSe1-xTex 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 CdSe1-xTex as functions of x. 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 x 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 x> 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 x > 0.5.