Structure and Band Edge Energy of Highly Luminescent CdSe_1–<i>x</i>Te_<i>x</i> Alloyed Quantum Dots

CdSe_1–<i>x</i>Te_<i>x</i> 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_1–<i>x</i>Te_<i>x</i> 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

Density of Deep Trap States in Oriented TiO₂ Nanotube Arrays

Correlations between the population of deep trap states in an array of TiO₂ nanotubes (NT) and the dynamic photocurrent responses under supra-band-gap illumination are investigated. Ordered arrays of TiO₂ NT of 10 μm length, 125 nm inner diameter, and 12 nm wall thickness featuring strong anatase character were obtained by anodization of Ti in ethylene glycol solution containing NH₄F. Cyclic voltammograms at pH 10 show the characteristic responses for nanostructured TiO₂ electrodes, in particular a sharp cathodic peak as the electron density in the film increases. These responses are associated with the population of deep trap states with an average value of 5 × 10⁴ electrons per NT. Dynamic photocurrent measurements clearly show that the characteristic rise time of the photocurrent increases as the potential is increased above the onset region for charging deep trap states. At potentials in which deep trap states are fully depopulated in the dark, photocurrent rise time approaches values just below 1 s, which is more than 3 orders of magnitude slower than the estimated <i>RC</i> time constant. The occupancy of the deep trap states under photostationary conditions is a fraction of the density of states estimated from voltammetric responses. These findings are discussed in the context of current views about trap states in high surface area TiO₂ electrodes