3 research outputs found
Structure and Band Edge Energy of Highly Luminescent CdSe<sub>1ā<i>x</i></sub>Te<sub><i>x</i></sub> Alloyed Quantum Dots
CdSe<sub>1ā<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>1ā<i>x</i></sub>Te<sub><i>x</i></sub> as functions of <i>x</i>. The QDs were synthesized employing the āhot-injectionā
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 TiO<sub>2</sub> Nanotube Arrays
Correlations between the population
of deep trap states in an array
of TiO<sub>2</sub> nanotubes (NT) and the dynamic photocurrent responses
under supra-band-gap illumination are investigated. Ordered arrays
of TiO<sub>2</sub> NT of 10 Ī¼m 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 NH<sub>4</sub>F. Cyclic voltammograms at pH 10 show the characteristic responses
for nanostructured TiO<sub>2</sub> 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 Ć 10<sup>4</sup> 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 TiO<sub>2</sub> electrodes
Fast One-Pot Synthesis of MoS<sub>2</sub>/Crumpled Graphene pān Nanonjunctions for Enhanced Photoelectrochemical Hydrogen Production
Aerosol processing enables the preparation
of hierarchical graphene nanocomposites with special crumpled morphology
in high yield and in a short time. Using modular insertion of suitable
precursors in the starting solution, it is possible to synthesize
different types of graphene-based materials ranging from heteroatom-doped
graphene nanoballs to hierarchical nanohybrids made up by nitrogen-doped
crumpled graphene nanosacks that wrap finely dispersed MoS<sub>2</sub> nanoparticles. These materials are carefully investigated by microscopic
(SEM, standard and HR TEM), diffraction (grazing incidence X-ray diffraction
(GIXRD)) and spectroscopic (high resolution photoemission, Raman and
UVāvisible spectroscopy) techniques, evidencing that nitrogen
dopants provide anchoring sites for MoS<sub>2</sub> nanoparticles,
whereas crumpling of graphene sheets drastically limits aggregation.
The activity of these materials is tested toward the photoelectrochemical
production of hydrogen, obtaining that N-doped graphene/MoS<sub>2</sub> nanohybrids are seven times more efficient with respect to single
MoS<sub>2</sub> because of the formation of local pān MoS<sub>2</sub>/N-doped graphene nanojunctions, which allow an efficient
charge carrier separation