4 research outputs found
Photosensitivity Enhancement with TiO<sub>2</sub> in Semitransparent Light-Sensitive Skins of Nanocrystal Monolayers
We propose and demonstrate light-sensitive
nanocrystal skins that
exhibit broadband sensitivity enhancement based on electron transfer
to a thin TiO<sub>2</sub> film grown by atomic layer deposition. In
these photosensors, which operate with no external bias, photogenerated
electrons remain trapped inside the nanocrystals. These electrons
generally recombine with the photogenerated holes that accumulate
at the top interfacing contact, which leads to lower photovoltage
buildup. Because favorable conduction band offset aids in transferring
photoelectrons from CdTe nanocrystals to the TiO<sub>2</sub> layer,
which decreases the exciton recombination probability, TiO<sub>2</sub> has been utilized as the electron-accepting material in these light-sensitive
nanocrystal skins. A controlled interface thickness between the TiO<sub>2</sub> layer and the monolayer of CdTe nanocrystals enables a photovoltage
buildup enhancement in the proposed nanostructure platform. With TiO<sub>2</sub> serving as the electron acceptor, we observed broadband sensitivity
improvement across 350–475 nm, with an approximately 22% enhancement.
Furthermore, time-resolved fluorescence measurements verified the
electron transfer from the CdTe nanocrystals to the TiO<sub>2</sub> layer in light-sensitive skins. These results could pave the way
for engineering nanocrystal-based light-sensing platforms, such as
smart transparent windows, light-sensitive walls, and large-area optical
detection systems
Phonon-Assisted Exciton Transfer into Silicon Using Nanoemitters: The Role of Phonons and Temperature Effects in Förster Resonance Energy Transfer
We study phonon-assisted Förster resonance energy transfer (FRET) into an indirect band-gap semiconductor using nanoemitters. The unusual temperature dependence of this energy transfer, which is measured using the donor nanoemitters of quantum dot (QD) layers integrated on the acceptor monocrystalline bulk silicon as a model system, is predicted by a phonon-assisted exciton transfer model proposed here. The model includes the phonon-mediated optical properties of silicon, while considering the contribution from the multimonolayer-equivalent QD film to the nonradiative energy transfer, which is derived with a <i>d</i><sup>–3</sup> distance dependence. The FRET efficiencies are experimentally observed to decrease at cryogenic temperatures, which are well explained by the model considering the phonon depopulation in the indirect band-gap acceptor together with the changes in the quantum yield of the donor. These understandings will be crucial for designing FRET-enabled sensitization of silicon based high-efficiency excitonic systems using nanoemitters
Understanding the Journey of Dopant Copper Ions in Atomically Flat Colloidal Nanocrystals of CdSe Nanoplatelets Using Partial Cation Exchange Reactions
Unique
electronic and optical properties of doped semiconductor
nanocrystals (NCs) have widely stimulated a great deal of interest
to explore new effective synthesis routes to achieve controlled doping
for highly efficient materials. In this work, we show copper doping
via postsynthesis partial cation exchange (CE) in atomically flat
colloidal semiconductor nanoplatelets (NPLs). Here chemical reactivity
of different dopant precursors, reaction kinetics, and shape of seed
NPLs were extensively elaborated for successful doping and efficient
emission. Dopant-induced Stokes-shifted and tunable photoluminescence
emission (640 to 830 nm) was observed in these Cu-doped CdSe NPLs
using different thicknesses and heterostructures. High quantum yields
(reaching 63%) accompanied by high absorption cross sections (>2.5
times) were obtained in such NPLs compared to those of Cu-doped CdSe
colloidal quantum dots (CQDs). Systematic tuning of the doping level
in these two-dimensional NPLs provides an insightful understanding
of the chemical dopant based orbital hybridization in NCs. The unique
combination of doping via the partial CE method and precise control
of quantum confinement in such atomically flat NPLs originating from
their magic-sized vertical thickness exhibits an excellent model platform
for studying photophysics of doped quantum confined systems
Chiral Ceramic Nanoparticles and Peptide Catalysis
The
chirality of nanoparticles (NPs) and their assemblies has been
investigated predominantly for noble metals and II–VI semiconductors.
However, ceramic NPs represent the majority of nanoscale materials
in nature. The robustness and other innate properties of ceramics
offer technological opportunities in catalysis, biomedical sciences,
and optics. Here we report the preparation of chiral ceramic NPs,
as represented by tungsten oxide hydrate, WO<sub>3–<i>x</i></sub>·H<sub>2</sub>O, dispersed in ethanol. The chirality
of the metal oxide core, with an average size of ca. 1.6 nm, is imparted
by proline (Pro) and aspartic acid (Asp) ligands via bio-to-nano chirality
transfer. The amino acids are attached to the NP surface through C–O–W
linkages formed from dissociated carboxyl groups and through amino
groups weakly coordinated to the NP surface. Surprisingly, the dominant
circular dichroism bands for NPs coated by Pro and Asp are different
despite the similarity in the geometry of the NPs; they are positioned
at 400–700 nm and 500–1100 nm for Pro- and Asp-modified
NPs, respectively. The differences in the spectral positions of the
main chiroptical band for the two types of NPs are associated with
the molecular binding of the two amino acids to the NP surface; Asp
has one additional C–O–W linkage compared to Pro, resulting
in stronger distortion of the inorganic crystal lattice and greater
intensity of CD bands associated with the chirality of the inorganic
core. The chirality of WO<sub>3–<i>x</i></sub>·H<sub>2</sub>O atomic structure is confirmed by atomistic molecular dynamics
simulations. The proximity of the amino acids to the mineral surface
is associated with the catalytic abilities of WO<sub>3–<i>x</i></sub>·H<sub>2</sub>O NPs. We found that NPs facilitate
formation of peptide bonds, leading to Asp-Asp and Asp-Pro dipeptides.
The chiroptical activity, chemical reactivity, and biocompatibility
of tungsten oxide create a unique combination of properties relevant to
chiral optics, chemical technologies, and biomedicine