It is a generally
accepted perspective that type-II nanocrystal
quantum dots (QDs) have low quantum yield due to the separation of
the electron and hole wavefunctions. Recently, high quantum yield
levels were reported for cadmium-based type-II QDs. Hence, the quest
for finding non-toxic and efficient type-II QDs is continuing. Herein,
we demonstrate environmentally benign type-II InP/ZnO/ZnS core/shell/shell
QDs that reach a high quantum yield of ∼91%. For this, ZnO
layer was grown on core InP QDs by thermal decomposition, which was
followed by a ZnS layer via successive ionic layer adsorption. The
small-angle X-ray scattering shows that spherical InP core and InP/ZnO
core/shell QDs turn into elliptical particles with the growth of the
ZnS shell. To conserve the quantum efficiency of QDs in device architectures,
InP/ZnO/ZnS QDs were integrated in the liquid state on blue light-emitting
diodes (LEDs) as down-converters that led to an external quantum efficiency
of 9.4% and a power conversion efficiency of 6.8%, respectively, which
is the most efficient QD-LED using type-II QDs. This study pointed
out that cadmium-free type-II QDs can reach high efficiency levels,
which can stimulate novel forms of devices and nanomaterials for bioimaging,
display, and lighting.
Capacitive charge transfer at the electrode/electrolyte interface is a biocompatible mechanism for the stimulation of neurons. Although quantum dots showed their potential for photostimulation device architectures, dominant photoelectrochemical charge transfer combined with heavy-metal content in such architectures hinders their safe use. In this study, we demonstrate heavy-metal-free quantum dot-based nano-heterojunction devices that generate capacitive photoresponse. For that, we formed a novel form of nano-heterojunctions using type-II InP/ZnO/ZnS core/shell/shell quantum dot as the donor and a fullerene derivative of PCBM as the electron acceptor. The reduced electron–hole wavefunction overlap of 0.52 due to type-II band alignment of the quantum dot and the passivation of the trap states indicated by the high photoluminescence quantum yield of 70% led to the domination of photoinduced capacitive charge transfer at an optimum donor–acceptor ratio. This study paves the way toward safe and efficient nanoengineered quantum dot-based next-generation photostimulation devices.
In the next decade,
we will witness the replacement of a majority
of conventional light sources with light-emitting diodes (LEDs). Efficient
LEDs other than phosphors can enhance their functionality and meet
different lighting needs. Quantum dots (QDs) have high potential for
future LED technology due to their sensitive band-gap tuning via the
quantum confinement effect and compositional control, high photoluminescence
quantum yield (PLQY), and mass-production capacity. Herein, we demonstrate
white LEDs using QDs that reach over 150 lumens per electrical Watt.
For that we synthesized green- and red-emitting ZnCdSe/ZnSe core/shell
QDs by low-temperature nucleation, high-temperature shell formation,
and postsynthetic trap-state removal. Their cadmium concentration
is lower than 100 ppm, satisfying the current EU RoHS regulations,
and their PLQY reaches a high level of 94%. The PLQY of QDs is maintained
within the device on blue LED via liquid injection, and their integration
at optimized optical densities leads to 129.6 and 170.4 lm/W for red-green-blue
(RGB)- and green-blue (GB)-based white LEDs, respectively. Our simulations
further showed that an efficiency level of over 230 lm/W is achievable
using ultraefficient blue LED pumps. The simple fabrication and high
performance of white LEDs using QD liquids show high promise for next-generation
lighting devices.
This review summarizes optical nanomaterials, devices, and systems for neuromodulation. We describe their structures, working principles and bioelectronic applications with challenges and prospects.
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