18 research outputs found
Level Anticrossing of Impurity States in Semiconductor Nanocrystals
The size dependence of the quantized energies of elementary excitations is an essential feature of quantum nanostructures, underlying most of their applications in science and technology. Here we report on a fundamental property of impurity states in semiconductor nanocrystals that appears to have been overlooked—the anticrossing of energy levels exhibiting different size dependencies. We show that this property is inherent to the energy spectra of charge carriers whose spatial motion is simultaneously affected by the Coulomb potential of the impurity ion and the confining potential of the nanocrystal. The coupling of impurity states, which leads to the anticrossing, can be induced by interactions with elementary excitations residing inside the nanocrystal or an external electromagnetic field. We formulate physical conditions that allow a straightforward interpretation of level anticrossings in the nanocrystal energy spectrum and an accurate estimation of the states\u27 coupling strength
Band Structure and Intersubband Transitions of Three-Layer Semiconductor Nanoplatelets
This paper presents the first general theory of electronic band structure and intersubband transitions in three-layer semiconductor nanoplatelets. We find a dispersion relation and wave functions of the confined electrons and use them to analyze the band structure of core/shell nanoplatelets with equal thicknesses of the shell layers. It is shown that the energies of electrons localized inside the shell layers can be degenerate for certain electron wave vectors and certain core and shell thicknesses. We also show that the energies of intersubband transitions can be nonmonotonic functions of the core and shell thicknesses, exhibiting pronounced local minima and maxima which can be observed in the infrared absorption spectra. Our results will prove useful for the design of photonic devices based on multilayered semiconductor nanoplatelets operating at infrared frequencies
Age of deposits of Caspian sea early khvalyn transgressive stage: New data
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Effect of Extinction on Separation of Nanoparticle Enantiomers with Chiral Optical Forces
Toward Bright Red-Emissive Carbon Dots through Controlling Interaction among Surface Emission Centers
Relatively weak red photoluminescence of carbon dots (CDots) is a major challenge on the way to their successful implementation in biological and optoelectronic devices. We present a theoretical analysis of the interaction among the surface emission centers of CDots, showing that it may determine efficiency of the red photoluminescence of CDots. Based on the previous experimental studies, it is assumed that the optical response of the CDots is determined by the molecule-like subunits of polycyclic aromatic hydrocarbons (PAHs) attached to the CDots surface. Three characteristic types of coupling of these PAH subunits are considered: non-interacting monomers, noncovalently bound dimers, and covalently bound dimers with two, three, or four carbon linkers. We demonstrate that the CDots photoluminescence broadens, redshifts, and weakens by 2 orders of magnitude when the free monomers are substituted by the covalently bridged centers. These and other results of our study show that the realization of CDots with many weakly interacting surface emission centers may constitute an efficient way to achieve their efficient red photoluminescence, which is highly desirable for biological and optoelectronic applications
Level Anticrossing of Impurity States in Semiconductor Nanocrystals
The size dependence of the quantized energies of elementary excitations is an essential feature of quantum nanostructures, underlying most of their applications in science and technology. Here we report on a fundamental property of impurity states in semiconductor nanocrystals that appears to have been overlooked—the anticrossing of energy levels exhibiting different size dependencies. We show that this property is inherent to the energy spectra of charge carriers whose spatial motion is simultaneously affected by the Coulomb potential of the impurity ion and the confining potential of the nanocrystal. The coupling of impurity states, which leads to the anticrossing, can be induced by interactions with elementary excitations residing inside the nanocrystal or an external electromagnetic field. We formulate physical conditions that allow a straightforward interpretation of level anticrossings in the nanocrystal energy spectrum and an accurate estimation of the states\u27 coupling strength
Toward Bright Red-Emissive Carbon Dots through Controlling Interaction among Surface Emission Centers
Optical Anisotropy of Topologically Distorted Semiconductor Nanocrystals
Engineering
nanostructured optical materials via the purposeful distortion of
their constituent nanocrystals requires the knowledge of how various
distortions affect the nanocrystals’ electronic subsystem and
its interaction with light. We use the geometric theory of defects
in solids to calculate the linear permittivity tensor of semiconductor
nanocrystals whose crystal lattice is arbitrarily distorted by imperfections
or strains. The result is then employed to systematically analyze
the optical properties of nanocrystals with spatial dispersion caused
by screw dislocations and Eshelby twists. We demonstrate that Eshelby
twists create gyrotropy in nanocrystals made of isotropic semiconductors
whereas screw dislocations can produce it only if the nanocrystal
material itself is inherently anisotropic. We also show that the dependence
of circular dichroism spectrum on the aspect ratio of dislocation-distorted
semiconductor nanorods allows resonant enhancing their optical activity
(at least by a factor of 2) and creating highly optically active nanomaterials
Optically Active Semiconductor Nanosprings for Tunable Chiral Nanophotonics
The
search for the optimal geometry of optically active semiconductor
nanostructures is making steady progress and has far-reaching benefits.
Yet the helical springlike shape, which is very likely to provide
a highly dissymmetric optical response, remains somewhat understudied
theoretically. Here we comprehensively analyze the optical activity
of semiconductor nanosprings using a fully quantum-mechanical model
of their electronic subsystem and taking into account the anisotropy
of their interaction with light. We show that the circular dichroism
of semiconductor nanosprings can exceed that of ordinary semiconductor
nanocrystals by a factor of 100 and be comparable to the circular
dichroism of metallic nanosprings. It is also demonstrated that nanosprings
can feature a total dissymmetry of optical response for certain ratios
between their length and coil height. The magnitude and sign of the
circular dichroism signal can be controlled by stretching or compressing
the nanosprings, which makes them a promising material base for optomechanical
sensors, polarization controllers, and other types of optically active
nanophotonic devices