4 research outputs found
Tunable Azacrown-Embedded Graphene Nanomeshes for Ion Sensing and Separation
Remarkable
selectivity with which crown ethers served as macrocyclic hosts for
various guest species has led to numerous investigations on structure-specific
interactions. Successful fabrication of graphene nanomeshes has opened
up a plethora of avenues for sensing and separation applications.
Embedding crown ether backbones in graphene frameworks can therefore
be an interesting strategy for exploring the advantages offered by
crown ether backbones, yet having the properties of graphene-based
materials. Motivated by the recent success in fabrication of crown
ether-based graphene nanopores, herein we investigate their performance
toward ion sensing and separation using electronic structure methods.
The effect of topology and electronic properties of the nanopore are
probed by considering a series of oxygen-based and nitrogen-based
graphene crown ethers (crown-<i>n</i>; <i>n</i> = 1–6). Our computations have revealed the excellent alkali
ion binding properties of azacrown-based graphene nanomeshes over
conventional oxygen crown-based graphene nanomeshes and normal crown
ethers. Selectivity in ion transmission through the nanomeshes is
demonstrated by employing graphene crown ethers [crown-<i>n</i> (<i>n</i> = 4–6)]. To the best of our knowledge,
this article is the first report on azacrown-based graphene nanomeshes
and their possible applications in ion sensing and separation, an
aspect that we hope will be demonstrated in experiments soon
Overwhelming Analogies between Plasmon Hybridization Theory and Molecular Orbital Theory Revealed: The Story of Plasmonic Heterodimers
Plasmon
hybridization theory (PHT), an analogue of molecular orbital
theory (MOT) for plasmonic molecules, has enjoyed tremendous success
over the last decade in discerning the optical features of hybrid
nanostructures in terms of their constituent monomeric nanostructures.
Dimers of metal nanoparticles served as prototypes in elucidating
many of the key aspects of plasmon hybridization. Employing quantum
two-state model, in conjunction with the quasi-static approximation
and the finite-difference time-domain simulations, we demonstrate
that the analogy between PHT and MOT can be further propelled by a
theoretical estimation of the plasmon-coupling strengths and the relative
contributions of the unhybridized monomeric states toward the hybrid
dimeric states in plasmonic Ag–Au nanorod heterodimers. The
aspect ratio of the constituent nanorods and the gap size between
the monomeric nanorods can further be used as handles to tune the
relative contributions of (i) the bonding and the antibonding modes
to the total extinction and (ii) the monomeric states toward the dimeric
states, with meaningful implications for surface-enhanced spectroscopy.
The tunability in light absorption properties of heterodimers in the
400–800 nm region arising as a result of broken symmetry is
also suggestive of their potential role as plasmonic rulers for measuring
distances
Adsorption of Monocyclic Carbon Rings on Graphene: Energetics Revealed via Continuum Modeling
Gas-phase spectroscopic
detection of tiny carbon clusters is a
recent success story in the area of carbon cluster research. However,
experimental production and isolation of these clusters are extremely
difficult because of their high reactivity. One possibility to isolate
the generated clusters would be to deposit them on graphene and to
desorb them for subsequent use. One of the pertinent questions toward
realizing this would be the energetics of the adsorption process.
Therefore, in this work, the energetics for the adsorption of the
monocyclic carbon rings (C<sub><i>n</i></sub> with <i>n</i> = 10, 12, 14, 16, 18, 20, and 22) on a graphene sheet
are investigated using the analytical approaches, developed earlier
by Hill and co-workers. The adsorption process here is driven by the
noncovalent interactions between the carbon rings and the graphene
sheet. The analyses of the interaction energies as a function of both
the vertical distance <i>Z</i> and the rotational angle
Ï• are performed in order to determine the preferred orientations,
equilibrium positions, and binding energies for the adsorption of
various carbon rings on graphene. We find that the preferred orientation
of the rings with respect to the graphene sheet is the parallel orientation.
The results from continuum, discrete–continuum, and discrete
models are in good agreement. Further, computations using density
functional theory and quantum mechanics/molecular mechanics approaches
are performed, and comparisons of the computed energetics with the
data from the models are reported. Finally, we highlight the scope
and the limitations of the analytical models
Plexcitons: The Role of Oscillator Strengths and Spectral Widths in Determining Strong Coupling
Strong
coupling interactions between plasmon and exciton-based
excitations have been proposed to be useful in the design of optoelectronic
systems. However, the role of various optical parameters dictating
the plasmon-exciton (plexciton) interactions is less understood. Herein,
we propose an inequality for achieving strong coupling between plasmons
and excitons through appropriate variation of their oscillator strengths
and spectral widths. These aspects are found to be consistent with
experiments on two sets of free-standing plexcitonic systems obtained
by (i) linking fluorescein isothiocyanate on Ag nanoparticles of varying
sizes through silane coupling and (ii) electrostatic binding of cyanine
dyes on polystyrenesulfonate-coated Au nanorods of varying aspect
ratios. Being covalently linked on Ag nanoparticles, fluorescein isothiocyanate
remains in monomeric state, and its high oscillator strength and narrow
spectral width enable us to approach the strong coupling limit. In
contrast, in the presence of polystyrenesulfonate, monomeric forms
of cyanine dyes exist in equilibrium with their aggregates: Coupling
is not observed for monomers and H-aggregates whose optical parameters
are unfavorable. The large aggregation number, narrow spectral width,
and extremely high oscillator strength of J-aggregates of cyanines
permit effective delocalization of excitons along the linear assembly
of chromophores, which in turn leads to efficient coupling with the
plasmons. Further, the results obtained from experiments and theoretical
models are jointly employed to describe the plexcitonic states, estimate
the coupling strengths, and rationalize the dispersion curves. The
experimental results and the theoretical analysis presented here portray
a way forward to the rational design of plexcitonic systems attaining
the strong coupling limits