27 research outputs found
Kinetic Density Functional Theory for Plasmonic Nanostructures: Breaking of the Plasmon Peak in the Quantum Regime and Generation of Hot Electrons
We
develop a quantum kinetic theory of the dynamic response of
typical noble metals. Our approach is based on the density functional
theory (DFT) and incorporates new important elements as compared to
the conventional time-dependent DFT formalism. The kinetic DFT is
derived starting from the master equation of motion for the density
matrix, which involves both momentum and energy relaxation processes.
Therefore, the quantum system is described by two relaxation parameters,
unlike the conventional time-dependent DFT incorporating only one
relaxation parameter. This allows us to describe both the absorption
of light and the generation of hot plasmonic electrons. Using our
kinetic DFT theory, we also observe the transition from the multiple
peaks in small size-quantized systems to the intensive plasmonic resonance
in large classical systems. Unlike the standard picture of collisional
broadening of the plasmon peak in small systems, we observe a very
different scenario: the formation of multiple plasmonic-like peaks
in small quantized systems. These peaks are the result of a hybridization
of the collective plasmon mode and the single-particle transitions
in a quantized electron gas. There are a few experimental observations
that seem to correlate with such a scenario of the plasmonic broadening
in small systems. Our approach also incorporates the interband transitions,
which are important for a qualitative description of gold and silver.
Although this paper gives an application of our kinetic DFT only to
the slab geometry, our theory can be applied to nanocrystals of arbitrary
shape. The kinetic DFT formalism developed here can be employed to
model and predict a variety of metal and hybrid nanostructures for
applications in photocatalysis, sensors, photodetectors, metamaterials,
etc
Optical Generation of Hot Plasmonic Carriers in Metal Nanocrystals: The Effects of Shape and Field Enhancement
We investigate theoretically photogeneration
of excited carriers in plasmonic nanocrystals. The theory is based
on the solution of the quantum equation of motion for the density
matrix. Efficient photogeneration of plasmonic electrons and holes
in small nanocrystals becomes possible due to the nonconservation
of the electron momentum. The confinement and reflection of electrons
in small nanocrystals allowed photon-assisted electron transitions
with high excitation energies and therefore lead to a large number
of energetic carriers. This process is a surface-scattering effect
and efficient only for nanostructures with small sizes. Other important
factors for the photogeneration effect are the field enhancement and
the inhomogeneity of electromagnetic fields inside a plasmonic nanostructure.
The plasmonic field effects strongly depend on the shape of the nanocrystal.
For example, a plasmonic nanocube is more efficient for the electron
photogeneration than a nanosphere and a nanosphere generates more
energetic carriers compared to a plasmonic slab. The results obtained
here can be used for designing plasmonic nanostructures for solar
and photocatalytic applications
Chiral Nanocrystals: Plasmonic Spectra and Circular Dichroism
The life is inherently chiral. Consequently, chirality
plays a
pivotal role in biochemistry and the evolution of life itself. Optical
manifestation of chirality of biomolecules, so-called circular dichroism,
is a remarkable but relatively weak effect appearing typically in
the UV. In contrast to the biomolecules, plasmonic nanocrystals offer
an interesting opportunity to create strong circular dichroism (CD)
in the visible spectral range. Here we describe plasmonic properties
of single chiral nanocrystals and focus on a new mechanism of optical
chirality originating from a chiral shape of a nanocrystal. After
careful examination, we found that this CD mechanism is induced by
the mixing between different plasmon harmonics and is qualitatively
different from the previously described dipolar CD effect in chiral
assemblies of spherical nanoparticles. Chiral plasmonic nanocrystals
studied here offer a new approach for the creation of nanomaterials
with strong chiroptical responses in the visible spectral interval
Amplified Generation of Hot Electrons and Quantum Surface Effects in Nanoparticle Dimers with Plasmonic Hot Spots
Plasmonic
excitations in optically driven nanocrystals are composed
of excited single-particle electron–hole pairs in the Fermi
sea. In large nanostructures, most of the excited plasmonic electrons
have relatively small excitation energies due to the conservation
of linear momentum. However, small optically driven nanocrystals may
have large numbers of hot electrons with large energies. In this study,
we develop the concept of hot electron generation further by considering
the effect of a plasmonic hot spot. Plasmonic hot spots are areas
in a nanostructure with highly inhomogeneous and enhanced electric
fields. In our model of a nanoparticle dimer, the hot spot region
appears near the gap between the nanoparticles. We then apply the
quantum formalism based on the density matrix to describe this system.
We show that the electromagnetic enhancement and the nonconservation
of linear momentum in the hot spot of the nanoparticle dimer lead
to strongly increased rates of generation of energetic (hot) electrons.
The rates of hot electron generation grow faster than the absorption
cross section and the electromagnetic enhancement factor with the
decrease of the gap between the nanoparticles. This happens due to
the breaking of the linear momentum conservation of electrons in the
hot spot regions. We also show that hot electron generation effect
leads to the quantum mechanism of surface-induced absorption in nanocrystals
that is an intrinsic property of any confined plasmonic system. The
results obtained in this study can be useful for understanding and
designing plasmonic photodetectors and hybrid materials for efficient
photocatalysis
Theory of Photoinjection of Hot Plasmonic Carriers from Metal Nanostructures into Semiconductors and Surface Molecules
We
investigate theoretically the effects of generation and injection
of plasmonic carriers from an optically excited metal nanocrystal
to a semiconductor contact or to surface molecules. The energy distributions
of optically excited hot carriers are dramatically different in metal
nanocrystals with large and small sizes. In large nanocrystals, the
majority of hot carriers has very small excitation energies, and the
excited-carrier distribution resembles the case of a plasmon wave
in bulk. For nanocrystal sizes smaller than 20 nm, the carrier distribution
extends to larger energies and occupies the whole region <i>E</i><sub>F</sub> < ε < <i>E</i><sub>F</sub> + ℏ<i>ω</i>. The physical reason for the above behaviors is
nonconservation of momentum in a nanocrystal. Because of the above
properties, nanocrystals of small sizes are most suitable for designing
opto-electronic and photosynthetic devices based on injection of plasmonic
electrons and holes. For gold nanocrystals, the optimal sizes for
efficient generation of hot carriers with overbarrier energies are
in the range of 10–20 nm. Another important factor is the polarization
of the exciting light. For efficient excitation of carriers with high
energies, the electric-field polarization vector should be perpendicular
to a prism-like nanoantenna (slab or platelet). We also show the relation
between our theory for injection from plasmonic nanocrystals and the
Fowler theory of injection from a bulk metal. Along with a prism geometry
(or platelet geometry), we consider cubes. The results can be applied
to design both purely solid-state opto-electronic devices and systems
for photocatalysis and solar-energy conversion
Orientation-Sensitive Peptide-Induced Plasmonic Circular Dichroism in Silver Nanocubes
Polyproline
II-based helical peptides were adsorbed to silver nanocubes
through a cysteine linker and induced circular dichroism (CD) in two
of their plasmon resonance modes. Inversion of the peptide’s
orientation with respect to the surface led to inversion of the plasmonic
CD signal. This phenomenon could not be explained by the simple molecule–plasmon
dipolar interaction model and could be due to the multipolar nature
of the plasmon modes. Elongation of the polyproline peptide led to
a significant increase of the induced CD signal. Surprisingly, the
effect did not change with sharpening of the edges of the silver nanocubes.
The change to a polyproline I helix caused nulling of the induced
plasmonic CD
Optical Properties of Chiral Plasmonic Tetramers: Circular Dichroism and Multipole Effects
Chiral metal nanoparticle assemblies
exhibit plasmonic circular
dichroism (CD) in the visible spectral interval. It was found previously
that the circular dichroism signals can be induced by dipolar interactions
between nanoparticles in a chiral assembly. In order to enhance plasmonic
circular dichroism response, one can take advantage of multipole effects
and anisotropy of nanostructures. We calculate the plasmonic circular
dichroism of several nanoparticle (NP) assemblies using the interacting
point-dipole approach and the purely numerical method based on the
discrete dipole approximation (DDA). We found that the multipole effects
revealed by the DDA calculations are crucial to describe and understand
CD responses of tightly packed assemblies. The chiral equilateral
tetrahedral 4-NP complexes are especially interesting because they
do not have the dipolar contribution to the CD signal. Therefore,
CD signals of equilateral tetrahedral 4-NP complexes originate solely
from the multipole interactions. The strength of CD signals rapidly
decreases with the particle–particle distance as 1/<i>R</i><sup>9.7</sup> for the helices and as 1/<i>R</i><sup>18.1</sup> for the equilateral tetrahedral 4-NP complexes, where <i>R</i> is a particle–particle distance. We show that the
CD spectra are much more sensitive to the geometry of a plasmonic
complex compared to the extinction spectra. Small variations in geometry
can result in large changes in CD responses. This study can be used
to design nanostructures with strong CD for optical and sensor applications
Plasmonic Metamaterials and Nanocomposites with the Narrow Transparency Window Effect in Broad Extinction Spectra
We
propose and describe plasmonic nanomaterials with unique optical
properties. These nanostructured materials strongly attenuate light
across a broad wavelength interval ranged from 400 nm to 5 ÎĽm
but exhibit a narrow transparency window centered at a given wavelength.
The main elements used in our systems are nanorods and nanocrosses
of variable sizes. The nanomaterial can be designed as a solution,
nanocomposite film or metastructure. The principle of the formation
of the transparency window in the broad extinction spectrum is based
on the narrow lines of longitudinal plasmons of single nanorods and
nanorod complexes. To realize the spectrum with a transmission window,
we design a nanocomposite material as a mixture of nanorods of different
sizes. Simultaneously, we exclude nanorods of certain lengths from
the nanorod ensemble. The width of the plasmonic transparency window
is determined by the intrinsic and radiative broadenings of the nanocrystal
plasmons. Nanocrystals can be randomly dispersed in a solution or
arranged in metastructures. We show that interactions between nanocrystals
in a dense ensemble can destroy the window effect and, simultaneously,
we design the metastructure geometries with weak destructive interactions.
We also describe the effect of narrowing of the transparency window
with increasing the concentration of nanocrystals. Two well-established
technologies can be used to fabricate such nano- and metamaterials,
the colloidal synthesis, and lithography. Nanocomposites proposed
here can be used as optical materials and smart coatings for shielding
of electromagnetic radiation in a wide spectral interval with a simultaneous
possibility of communication using a narrow transparency window
Fractal Nanoparticle Plasmonics: The Cayley Tree
There has been strong, ongoing interest over the past decade in developing strategies to design and engineer materials with tailored optical properties. Fractal-like nanoparticles and films have long been known to possess a remarkably broad-band optical response and are potential nanoscale components for realizing spectrum-spanning optical effects. Here we examine the role of self-similarity in a fractal geometry for the design of plasmon line shapes. By computing and fabricating simple Cayley tree nanostructures of increasing fractal order <i>N</i>, we are able to identify the principle behind how the multimodal plasmon spectrum of this system develops as the fractal order is increased. With increasing <i>N</i>, the fractal structure acquires an increasing number of modes with certain degeneracies: these modes correspond to plasmon oscillations on the different length scales inside a fractal. As a result, fractals with large <i>N</i> exhibit broad, multipeaked spectra from plasmons with large degeneracy numbers. The Cayley tree serves as an example of a more general, fractal-based route for the design of structures and media with highly complex optical line shapes
Mie Sensing with Neural Networks: Recognition of Nano-Object Parameters, the Invisibility Point, and Restricted Models
In this work, we use artificial neural networks (ANNs) to recognize the material composition, sizes of nanoparticles and their concentrations in different media with high accuracy, solely from the absorbance spectrum of a macroscopic sample. We construct ANNs operating in the following two schemes. The first scheme is designed to recognize the dimensions and refractive indices of dielectric scatterers in mixed ensembles. The second ANN model simultaneously recognizes the dimensions of gold nanospheres in a mixture and the refractive index of a matrix. A challenge in the first scheme arises at and near the invisibility point, i.e., when the refractive index of nanoparticles is close to that of the medium. Of course, particle recognition in this regime faces fundamental physical limitations. However, such recognition near the invisibility point is possible, and our study reveals its unique properties. Interestingly, the recognition process for the refractive index in the vicinity of the invisibility point shows very small errors. In contrast, the errors for the recognition of the radius grow strongly near this point. Another regime with limited recognition occurs when the extinction spectra are not unique and can correspond to different realizations of nanoparticle mixtures. Regarding multi-particle or polydisperse solutions, the ML-based models should in such cases be rationally restricted to maintain the feasibility of the recognition process. Overall, the recognition schemes proposed and investigated by us can find their applications in the field of sensing