6 research outputs found
Graphene-Based Fluorescence-Quenching-Related Fermi Level Elevation and Electron-Concentration Surge
Intermolecular p-orbital
overlaps in unsaturated Ļ-conjugated systems, such as graphene
and fluorescent molecules with aromatic structure, serve as the electron-exchanged
path. Using Raman-mapping measurements, we observe that the fluorescence
intensity of fluorescein isothiocyanate (FITC) is quenched by graphene,
whereas it persists in graphene-absent substrates (SiO<sub>2</sub>). After identifying a mechanism related to photon-induced electron
transfer (PET) that contributes to this fluorescence quenching phenomenon,
we validate this mechanism by conducting analyses on Dirac point shifts
of FITC-coated graphene. From these shifts, Fermi level elevation
and the electron-concentration surge in graphene upon visible-light
impingements are acquired. Finally, according to this mechanism, graphene-based
biosensors are fabricated to show the sensing capability of measuring
fluorescently labeled-biomolecule concentrations
FoĢrster Energy Transport in MetalāOrganic Frameworks Is Beyond Step-by-Step Hopping
Metalāorganic
frameworks (MOFs) with light-harvesting building
blocks designed to mimic photosynthetic chromophore arrays in green
plants provide an excellent platform to study exciton transport in
networks with well-defined structures. A step-by-step exciton random
hopping model made of the elementary steps of energy transfer between
only the nearest neighbors is usually used to describe the transport
dynamics. Although such a nearest neighbor approximation is valid
in describing the energy transfer of triplet states via the Dexter
mechanism, we found it inadequate in evaluating singlet exciton migration
that occurs through the FoĢrster mechanism, which involves one-step
jumping over longer distance. We measured migration rates of singlet
excitons on two MOFs constructed from truxene-derived ligands and
zinc nodes, by monitoring energy transfer from the MOF skeleton to
a coumarin probe in the MOF cavity. The diffusivities of the excitons
on the frameworks were determined to be 1.8 Ć 10<sup>ā2</sup> cm<sup>2</sup>/s and 2.3 Ć 10<sup>ā2</sup> cm<sup>2</sup>/s, corresponding to migration distances of 43 and 48 nm within their
lifetimes, respectively. āThrough spaceā energy-jumping
beyond nearest neighbor accounts for up to 67% of the energy transfer
rates. This finding presents a new perspective in the design and understanding
of highly efficient energy transport networks for singlet excited
states
Exciton Migration and Amplified Quenching on Two-Dimensional MetalāOrganic Layers
The dimensionality dependency of
resonance energy transfer is of great interest due to its importance
in understanding energy transfer on cell membranes and in low-dimension
nanostructures. Light harvesting two-dimensional metalāorganic
layers (2D-MOLs) and three-dimensional metalāorganic frameworks
(3D-MOFs) provide comparative models to study such dimensionality
dependence with molecular accuracy. Here we report the construction
of 2D-MOLs and 3D-MOFs from a donor ligand 4,4ā²,4ā³-(benzene-1,3,5-triyl-trisĀ(ethyne-2,1-diyl))Ātribenzoate
(BTE) and a doped acceptor ligand 3,3ā²,3ā³-nitro-4,4ā²,4ā³-(benzene-1,3,5-triyl-trisĀ(ethyne-2,1-diyl))Ātribenzoate
(BTE-NO<sub>2</sub>). These 2D-MOLs and 3D-MOFs are connected by similar
hafnium clusters, with key differences in the topology and dimensionality
of the metalāligand connection. Energy transfer from donors
to acceptors through the 2D-MOL or 3D-MOF skeletons is revealed by
measuring and modeling the fluorescence quenching of the donors. We
found that energy transfer in 3D-MOFs is more efficient than that
in 2D-MOLs, but excitons on 2D-MOLs are more accessible to external
quenchers as compared with those in 3D-MOFs. These results not only
provide support to theoretical analysis of energy transfer in low
dimensions, but also present opportunities to use efficient exciton
migration in 2D materials for light-harvesting and fluorescence sensing
FoĢrster Energy Transport in MetalāOrganic Frameworks Is Beyond Step-by-Step Hopping
Metalāorganic
frameworks (MOFs) with light-harvesting building
blocks designed to mimic photosynthetic chromophore arrays in green
plants provide an excellent platform to study exciton transport in
networks with well-defined structures. A step-by-step exciton random
hopping model made of the elementary steps of energy transfer between
only the nearest neighbors is usually used to describe the transport
dynamics. Although such a nearest neighbor approximation is valid
in describing the energy transfer of triplet states via the Dexter
mechanism, we found it inadequate in evaluating singlet exciton migration
that occurs through the FoĢrster mechanism, which involves one-step
jumping over longer distance. We measured migration rates of singlet
excitons on two MOFs constructed from truxene-derived ligands and
zinc nodes, by monitoring energy transfer from the MOF skeleton to
a coumarin probe in the MOF cavity. The diffusivities of the excitons
on the frameworks were determined to be 1.8 Ć 10<sup>ā2</sup> cm<sup>2</sup>/s and 2.3 Ć 10<sup>ā2</sup> cm<sup>2</sup>/s, corresponding to migration distances of 43 and 48 nm within their
lifetimes, respectively. āThrough spaceā energy-jumping
beyond nearest neighbor accounts for up to 67% of the energy transfer
rates. This finding presents a new perspective in the design and understanding
of highly efficient energy transport networks for singlet excited
states
Warm-White-Light-Emitting Diode Based on a Dye-Loaded MetalāOrganic Framework for Fast White-Light Communication
A dye@metalāorganic
framework (MOF) hybrid was used as a
fluorophore in a white-light-emitting diode (WLED) for fast visible-light
communication (VLC). The white light was generated from a combination
of blue emission of the 9,10-dibenzoate anthracene (DBA) linkers and
yellow emission of the encapsulated Rhodamine B molecules. The MOF
structure not only prevents dye molecules from aggregation-induced
quenching but also efficiently transfers energy to the dye for dual
emission. This light-emitting material shows emission lifetimes of
1.8 and 5.3 ns for the blue and yellow components, respectively, which
are significantly shorter than the 200 ns lifetime of Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:Ce<sup>3+</sup> in commercial WLEDs.
The MOF-WLED device exhibited a modulating frequency of 3.6 MHz for
VLC, six times that of commercial WLEDs
Modeling Fe/N/C Catalysts in Monolayer Graphene
Pyrolyzed Fe/N/C
is one of the most promising non-precious-metal
catalysts for the oxygen reduction reaction (ORR), which is supposed
to boost the commercialization of proton exchange membrane fuel cells
(PEMFC). However, the nature of the active sites of Fe/N/C is not
clear and has long been debated. The challenges mainly come from highly
heterogeneous structures formed during the pyrolysis process as well
as no suitable surface probes. To elucidate the active sites, the
most effective approach is building well-defined model catalysts as
single-crystal planes in surface sciences. Herein, we designed a single-atomic-layer
Fe/N/C model catalyst based on monolayer graphene (FeN-MLG) to explore
the active sites. The model catalyst was prepared by 950 Ā°C heat
treatment of graphene with controlled defects under an FeCl<sub>3</sub>(g)/NH<sub>3</sub> atmosphere. The as-prepared model catalyst exhibits
ORR activity and SCN<sup>ā</sup> suppressive effect comparable
to those of normal nanoparticle-like Fe/N/C catalysts, indicating
that active sites are successfully created in the model catalyst.
The effect of defect density, the layer number of graphene, and nitrogen
species on the ORR activity has been investigated. The main content
of nitrogen species on FeN-MLG is N<sub><i>x</i></sub>-Fe,
and quantitative correlation between N<sub><i>x</i></sub>-Fe and ORR activity demonstrates that N<sub><i>x</i></sub>-Fe species are the active site of Fe/N/C catalysts. The proposed
model catalyst serves to simplify the catalyst structures and to simulate
the topmost atomic layer of normal Fe/N/C, where ORR is catalyzed.
This model system opens an opportunity to further understand the highly
heterogeneous Fe/N/C catalysts