Plastic optoelectronic materials and thin film devices are very attractive in future optical sensor and solar energy applications due to their lightweight, flexible shape, high photon absorption coefficients, low cost, and environmental benefits. In this study, optoelectronic properties of D, D/fA blend, DfA, and a series of DBfA type of conjugated block copolymers has been investigated, where D is a donor type PPV conjugated block, B is a non-conjugated and flexible aliphatic hydrocarbon bridge chain containing different number of aliphatic methylene units, and fA is a fluorinated acceptor type PPV conjugated block. The optical absorptions of the D/fA blend, DfA, and DBfAs are typical overlaps of individual absorptions of D and fA blocks, while the solution steady state photoluminescence (PL) emission of D were quenched to different levels in blends and block copolymers, with DBfAs containing one methylene unit bridge (DB1fA) quenched most. This could be attributed to an intra-molecular photo induced electron transfer or charge separation in DBfA systems. Theoretical first principles study of the equilibrium atomic configuration of DfA reveals the existence of twisting angles between the D and fA blocks in DfA stable states which may account for a less PL quenching of DfA as compared to DB1fA. These results are important for designing and developing high efficiency polymer based optoelectronic systems.
Efficient engineering of metamaterials involves modeling of electric field profiles around these structures. Realistic modeling of the electric field in metamaterials requires accurate knowledge of optical constants of the compo- nents for which traditionally the bulk values are taken. Further progress in the developing of metamaterials is characterized by a reduction of the pattern size, dimensions of single layers in multilayered structures etc. It has been understood that optical functions in low-dimensional and nano-sized materials substantially differ from their bulk values increasingly affecting by quantum processes. In this work we develop a complex method for analytical modeling of electric field profiles in metamaterials including quantum processes in nano-sized multi-layered structures. In particular based on first principles density functional theory we obtained simple analytical functions allowing predictions the optical functions variations with the size reduction of single metamaterial components over a wide spectral region. It is shown that optical functions of nano-sized films substantially (by 50 percent and more) differ from those in bulk. The new calculated optical functions of the components are used for electric field profile modeling of nano-sized multilayered structures by nonlocal Green function technique including effects of spatial dispersion. Silicon, silicon dioxide, and water layers are used as an example. The method effectively incorporates real atomic structure reconstruction on surfaces and inner interfaces thus providing with a more realistic picture for modeling. By comparison with experiment it is demonstrated that our method predicts image potential of the nanostructures in better agreement with experiment than if using traditional classic electrodynamics approach neglecting the quantum effects. The results are discussed in comparison with literature.
The pulsed laser deposited nanocrystalline ZnO films doped by Ga up to six weight percent are studied by X-ray
difraction and generalized spectro-ellipsometry. We report substantial atomic structure modification of heavy
Ga-doped ZnO resulted and a concentration dependent increase of inter-planar distance. Measured dielectric
function spectra show strong blue-shift of the samples studied. Equilibrium atomic configurations and electron
energy structure of ZnO containing defects (voids and Ga impurities) are studied by the density functional theory
(DFT) and generalized gradient approximation (GGA). Atomic geometries are obtained from the total energy
minimization method. Optical functions are calculated within the random phase approximation including the
quasi-particles corrections and plasma excitation effects. We report energetically favorable paths of the voids
growth and aggregation in ZnO. Comparative analysis of experimental and theoretical results indicate that
measured blue-shift in ZnO:Ga substantially exceeds the Burstein-Moss shift as used in many previous work to
interpret concentration dependence of optical functions in heavy doped ZnO. We demonstrate that additional
mechanisms, such as structural and alloying effect, should be involved for quantitative interpretation of optics
of the nano-crystalline heavy-doped ZnO films.
Chemical engineering of metamaterials to reduce optical losses is studied by first principle density functional
theory. Contribution of the surface states to optical losses is studied by calculations of the imaginary part of the
dielectric function for several organic molecules (water, methanol, and ethanol) adsorbed on the (111) surface of
Ag nano-slabs. Substantial modifications of optical functions of metallic nano-slabs in near infrared and visible
spectral regions, caused by surface states and molecular adsorption, are predicted, discussed, and compared to
experimental data.
During the past several decades enormous effort has been dedicated to experimental and theoretical studies of metal-radical
organic complexes. Equilibrium geometries and electron energy structure of divalent lead Pb(II) complexes with
ortho-semiquinone radicals have been calculated by generalized gradient approximation method (GGA) within density
functional theory (DFT). Optical absorption spectra were calculated within random phase approximation (independent
particles picture). Predicted substantial modification of the absorption spectra in visible infrared regions is attributed to
the atomic configuration changes and to modifications of electronic energy due to the metal-radical coupling. The results
of the calculations are discussed in comparison with available experimental data on electron spin resonance spectra of
Pb(II) with paramagnetic ortho-semiquinone ligands.
Optical absorption spectra of poly(thienylenevinylene) (PTV) conjugated polymers are measured at room temperature
in spectral range 400 to 800 nm. A dominant peak located at 575 nm and a prominent shoulder at
614 nm are observed. Equilibrium atomic geometries of PTV conjugated polymers are studied by first principles
density functional theory (DFT). Electron energy structure is obtained through self-consistent solution of eigen
energy problem using ab initio ultrasoft pseudopotentials and generalized gradient approximation method. This
is a non traditional approach for complex organic systems which is shown to be very promising especially for
optical simulations. Linear optical absorption is calculated within Random Phase Approximation (RPA) picture.
By comparative analysis of experimental and theoretical data it is demonstrated that dominant contribution to
the optical excitations of PTV in visible spectral range are related to the delocalized electrons within the polymer
chains. Obtained optical data together with equilibrium geometry analysis indicate that interchain interactions
substantially effect electronic structure and optical absorption of PTV conjugated polymers.
We have demonstrated that an addition of highly concentrated rhodamine 6G chloride dye to the PMMA film adjacent to
a silver film causes three-fold reduction of the imaginary part of the dielectric constant of Ag (absorption loss in metal)
and 30% elongation of the propagation length of surface plasmon polaritons (SPP). The possibility to elongate the SPP
propagation length without optical gain opens a new technological dimension to low-loss nanoplasmonics.
Sum Frequency Generation (SFG) spectra of nanocrystalline porous silicon (por-Si) exposed to different chemical treatments are studied. We report the first SFG studies of por-Si in direct contact with a liquid. SFG is excited by a regeneratively amplified Ti:sapphire system (787 nm, 120 fs, 1 kHz). The sum frequency is generated by combining this light with infrared that is generated with an optical parametric amplifier (OPA) that delivers 100-200 μJ pulses at 1370-1770 nm. Por-Si is made from a 10-20 Ω cm p-type Si(001) wafer. Comparisons are made to planar Si(001) as well as GaAs(001). First principle electronic structure theory based on density functional theory (DFT) is used to study the adsorption and optical response functions from the system of ethanol molecule adsorbed on Si(001) and Si(111) surfaces. Equilibrium atomic geometries are obtained through molecular dynamics and total energy minimization methods. Electron energy structure and optical properties are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted differential optical absorption spectra for chemisorbed Si(001) and Si(111) surfaces are analyzed in comparison with SFG data measured on differently treated porous silicon. Substantial modifications of the surface atomic and electron energy structures of silicon surfaces due to chemisorption are shown to provide the dominant contributions to the SFG response.
The semiconducting conjugated polymers, poly-phenylene-vinylene (PPV), are studied using linear and non-linear optical spectroscopy methods: ultra-fast optical spectroscopy, optical absorption, and luminescence. Study of polarization resolved absorption and excitation spectra at room temperature revealed strong anisotropy of the optical responses. Microscopic interpretation of the results is based of the first principal analysis of the electronic structure of PPV using the density functional theory (DFT). The dominant contribution of the optical excitations in visible spectral range is related to the delocalized pi-electronics of the PPV chains. Time-resolved luminescence measurements indicate the separate contributions from electronic transitions associated to the localized and delocalized electrons in PPV.
Electron energy structure and optical response functions of semiconducting conjugated polymers, Poly-Phenylene-
Vinylenes (PPV) are studied by first principle modeling based on the density functional theory (DFT). Equilibrium
atomic geometries are obtained through total energy minimization. Electron energy structure and optical properties
are calculated using generalized gradient approximation method with ab initio pseudopotentials. Predicted data are
compared with measured optical absorption spectra of PPV. Dominant contribution of the optical excitations related
to the delocalized π-electrons within the PPV chains is demonstrated by comparative analysis of experimental and
theoretical data. Optical absorption spectra of acceptor type PPV chains show substantial blue shift with respect to
the donor type of the PPV polymer. Comparison between calculated and measured optical spectra demonstrates that
the shift could be attributed to the structural deformations of the benzene rings of the host PPV unit due to the
interaction with the atoms of the derivative groups.
Silicon carbide is an interesting high-temperature large band gap
semiconquctor. it ispromising as a basical material for optoelectronic
devices . The optical properties of SiC have been studied by several authors.
The absrption coefficient of SiC 6H3 has been measured by Choyke and
Patrick up to 4.9 eV and by Makarov to 5.8 eV. Reflection spectra of 6H,
15R, and 3C SiC in the range 3.0 to 13 eV have been stidied in . The optical
constants of SiC 6H have been measured by reflectivity in the range 4 to 25
eV,
The energies of direct optical transitions between subbands in the
conduction band, resulting from confinement in a one dimensional superI,at,,tice,
have been measured in8sveral polytypes of SiC by absorption ' and
electroreflection (ER) '
The electron energy band structure (85) of SiC of1he1 halerite
structure (3C SiC) has been calculated by several 1tu4r2 ' . BS of
wurtzite modification of SiC have been calculated 1 in ' ' ' for 2H iC.
BS of 4H and 6H SiC has been calculated by the semiempirical
pseudopotential method at high-symmetry points of the Brillouin zone (BZ).
Tight binding calculations of 2H SiC show valence bands which agree with
experiment, but unrealistic conductive bands due to the restriction to nearest
neighbours in the Hamiltonian matrix
In this work we report the electroreflectance (ER) spectra of hexagonal
(4H and 6H) and cubic SiC measured in the range 1.0 to 5.6 eV. Values of
direct optical gps1ave been obtained from the ER spectra using a multiple
oscillator model ' . BS of SiC has been calculated by the first-principles
self-consistent linear muffin-tin orbital (LMTO-ASA) method (2H, 4H, and 6H
SiC) and by the semiempirical pseudopotential method (3C SiC). Calculated BS
parameters have been compared with experimental data measured in this work and
those available in the literature.
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