In this review we present new concepts and recent progress in the application of semiconductor quantum dots (QD) as labels in two important areas of biology, bioimaging and biosensing.
A new class of chiral nanoparticles is of great interest not only for nanotechnology, but also for many other fields of scientific endeavor. Normally the chirality in semiconductor nanocrystals is induced by the initial presence of chiral ligands/stabilizer molecules. Here we report intrinsic chirality of ZnS coated CdSe quantum dots (QDs) and quantum rods (QRs) stabilized by achiral ligands. As-prepared ensembles of these nanocrystals have been found to be a racemic mixture of d- and l-nanocrystals which also includes a portion of nonchiral nanocrystals and so in total the solution does not show a circular dichroism (CD) signal. We have developed a new enantioselective phase transfer technique to separate chiral nanocrystals using an appropriate chiral ligand and obtain optically active ensembles of CdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystals isolated in organic phase, still capped with achiral ligands, now display circular dichroism (CD). We propose that the intrinsic chirality of CdSe/ZnS nanocrystals is caused by the presence of naturally occurring chiral defects.
This review is focused on new concepts and recent progress in the development of three major quantum dot (QD) based optoelectronic devices: photovoltaic cells, photodetectors and LEDs.
We report on an anomalous size dependence of the room-temperature photoluminescence decay time from the lowest-energy state of PbS quantum dots in colloidal solution, which was found using the transient luminescence spectroscopy. The observed 10-fold reduction in the decay time (from ~2.5 to 0.25 μs) with the increase in the quantum dots' diameter is explained by the existence of phonon-induced transitions between the in-gap state-whose energy drastically depends on the diameter-and the fundamental state of the quantum dots.
Carbon
dots (CDots) are a promising biocompatible nanoscale source
of light, yet the origin of their emission remains under debate. Here,
we show that all the distinctive optical properties of CDots, including
the giant Stokes shift of photoluminescence and the strong dependence
of emission color on excitation wavelength, can be explained by the
linear optical response of the partially sp2-hybridized
carbon domains located on the surface of the CDots’ sp3-hybridized amorphous cores. Using a simple quantum chemical
approach, we show that the domain hybridization factor determines
the localization of electrons and the electronic bandgap inside the
domains and analyze how the distribution of this factor affects the
emission properties of CDots. Our calculation data fully agree with
the experimental optical properties of CDots, confirming the overall
theoretical picture underlying the model. It is also demonstrated
that fabrication of CDots with large hybridization factors of carbon
domains shifts their emission to the red side of the visible spectrum,
without a need to modify the size or shape of the CDots. Our theoretical
model provides a useful tool for experimentalists and may lead to
extending the applications of CDots in biophysics, optoelectronics,
and photovoltaics.
Two-dimensional (2D)
nanomaterials have been intensively investigated
due to their interesting properties and range of potential applications.
Although most research has focused on graphene, atomic layered transition
metal dichalcogenides (TMDs) and particularly MoS2 have
gathered much deserved attention recently. Here, we report the induction
of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation
of MoS2 in the presence of chiral ligands (cysteine and
penicillamine) in water. This processing resulted in exfoliated chiral
2D MoS2 nanosheets showing strong circular dichroism signals,
which were far past the onset of the original chiral ligand signals.
Using theoretical modeling, we demonstrated that the chiral nature
of MoS2 nanosheets is related to the presence of chiral
ligands causing preferential folding of the MoS2 sheets.
There was an excellent match between the theoretically calculated
and experimental spectra. We believe that, due to their high aspect
ratio planar morphology, chiral 2D nanomaterials could offer great
opportunities for the development of chiroptical sensors, materials,
and devices for valleytronics and other potential applications. In
addition, chirality plays a key role in many chemical and biological
systems, with chiral molecules and materials critical for the further
development of biopharmaceuticals and fine chemicals, and this research
therefore should have a strong impact on relevant areas of science
and technology such as nanobiotechnology, nanomedicine, and nanotoxicology.
This work presents a comprehensive study of electroabsorption in CdSe colloidal quantum dots, nanorods, and nanoplatelets. We experimentally demonstrate that the exposure of the nanoplatelets to a dc electric field leads to strong broadening of their lowest-energy heavy-hole absorption band and drastically reduces the absorption efficiency within the band. These are results of the quantum-confined Stark and Franz–Keldysh effects. The field-induced change in the nanoplatelets’ absorption is found to be more than 10 times the change in the absorption by the quantum dots. We also demonstrate that the electroabsorption by the nanorods is weaker than that by the quantum dots and nanoplatelets and reveal an unusual dependence of the differential absorption changes on the nanoplatelet thickness: the thicker the nanoplatelet, the smaller the change.
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