Semiconductor quantum dots possess attractive optical properties useful in the photonic field. Thanks to the evolution of core-shell colloidal synthesis, quantum dots exhibit almost ideal emitting properties. Their employment in optoelectronic devices is encouraged by their size-tunable emission as well as by their high absorption cross-section and high photoluminescence quantum yield. In this work, starting from the archetypal CdSe based system, the influence of shell thickness and composition on exciton dynamics in CdSe/CdxZn1-xS core-shell QDs is investigated. By means of steadystate and transient optical spectroscopy, single- and multi-exciton recombination processes are characterized and correlated with different shell materials. The present study clarifies why graded CdSe-CdS-Cd0.5Zn0.5-ZnS core-shell represents the best performing heterostructure for optical gain applications, since such quantum dots combine the high radiative rates of CdS with the superior confinement potential of ZnS in a single dot.
In the last years inorganic semiconductor (particularly CdSe and CdS) quantum dots (QDs) have received great attention
for their important optical properties. The possibility to tune the emission wavelength, together with their high
fluorescence quantum efficiency and photostability, can be exploited in photonic and optoelectronic technological
applications. The design of DFB devices, based on QDs as active optical material, leads to the realization of compact
laser systems. In this work we explore the use of an inorganic/organic hybrid material composed of CdSe-ZnS
semiconductor quantum dots doped into a zirconia sol-gel matrix for optical gain applications. Through the use of soft
lithography on a sol-gel germania-silica hybrid, large scale distributed feedback gratings can be created. Used in
conjunction with the CdSe-ZnS/ZrO2 hybrids, these gratings can act as microcavities and allow for the realization of true
lasing action. The lasing properties within these devices are characterized in the femtosecond regime by both one- and
two-photon excitation. From experimental data the value of the optical gain of the core-shell quantum dot samples has
been estimated. Moreover, one- and two-photon lasing threshold and stability are reported.
We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect
transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the
field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact
We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold
voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane
(OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases.
The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the
semiconductor polymer can be related to the density of the oxydryl groups (-OH ), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA),
polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of
the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced
enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS
and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.
We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a
better device to device reproducibility.
We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The
electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.