Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 20
Filtrar
Mais filtros










Base de dados
Intervalo de ano de publicação
1.
Sensors (Basel) ; 19(20)2019 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-31640259

RESUMO

We address wideband direct coherent localization of a radio transmitter by a distributed antenna array in a multipath scenario with spatially-coherent line-of-sight (LoS) signal components. Such a signal scenario is realistic in small cells, especially indoors in the mmWave range. The system model considers collocated time and phase synchronized receiving front-ends with antennas distributed in 3D space at known locations connected to the front-ends via calibrated coaxial cables or analog radio frequency over fiber links. The signal model assumes spherical wavefronts. We propose two ML-type algorithms (for known and unknown transmitter waveforms) and a subspace-based SCM-MUSIC algorithm for wideband direct coherent position estimation. We demonstrate the performance of the methods by Monte Carlo simulations. The results show that even in multipath environments, it is possible to achieve localization accuracy that is much better (by two to three orders of magnitude) than the carrier wavelength. They also suggest that the methods that do not exploit knowledge of the waveform have mean-squared errors approaching the Cramér-Rao bound.

2.
J Biophotonics ; 12(12): e201900218, 2019 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-31452335

RESUMO

Modern document protection relies on the simultaneous combination of many optical features with micron and submicron structures, whose complexity is the main obstacle for unauthorized copying. In that sense, documents are best protected by the diffractive optical elements generated lithographically and mass-produced by embossing. The problem is that the resulting security elements are identical, facilitating mass-production of both original and counterfeited documents. Here, we prove that each butterfly wing-scale is structurally and optically unique and can be used as an inimitable optical memory tag and applied for document security. Wing-scales, exhibiting angular variability of their color, were laser-cut and bleached to imprint cryptographic information of an authorized issuer. The resulting optical memory tag is extremely durable, as verified by several century-old insect specimens still retaining their coloration. The described technique is simple, amenable to mass-production, low cost and easy to integrate within the existing security infrastructure.

3.
Sci Rep ; 8(1): 15557, 2018 Oct 22.
Artigo em Inglês | MEDLINE | ID: mdl-30348982

RESUMO

Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (-1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations.

4.
Phys Chem Chem Phys ; 20(40): 25693-25700, 2018 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-30280172

RESUMO

The effects of thermal disorder on the electronic properties of organic/inorganic halide perovskites were investigated using ab initio molecular dynamics simulations. It was generally found that band gap variations due to effects of thermal disorder are the largest in materials with the smallest lattice constant. The factors that may lead to departure from this trend include the degree of rotational and translational motion of the organic cation and the strength of its dipole. It was found that the contribution of the flexible organic part to the band gap variations is considerably smaller than the contribution of the inorganic part of the material. The results of our simulations indicate that band gap variations in halide perovskites fall within the range exhibited in inorganic semiconductors.

5.
EURASIP J Adv Signal Process ; 2018(1): 33, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-29904392

RESUMO

In this paper, we propose a massive MIMO (multiple-input-multiple-output) architecture with distributed steerable phased antenna subarrays for position estimation in the mmWave range. We also propose localization algorithms and a multistage/multiresolution search strategy that resolve the problem of high side lobes, which is inherent in spatially coherent localization. The proposed system is intended for use in line-of-sight indoor environments. Time synchronization between the transmitter and the receiving system is not required, and the algorithms can also be applied to a multiuser scenario. The simulation results for the line-of-sight-only and specular multipath scenarios show that the localization error is only a small fraction of the carrier wavelength and that it can be achieved under reasonable system parameters including signal-to-noise ratios, antenna number/placement, and subarray apertures. The proposed concept has the potential of significantly improving the capacity and spectral/energy efficiency of future mmWave massive MIMO systems.

6.
J Phys Chem Lett ; 9(13): 3604-3611, 2018 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-29902010

RESUMO

We report an investigation of lead halide perovskite CH3NH3PbBr3 nanocrystals and associated ligand molecules by combining several different state-of-the-art experimental techniques, including synchrotron radiation-based XPS and VUV PES of free-standing nanocrystals isolated in vacuum. By using this novel approach for perovskite materials, we could directly obtain complete band alignment to vacuum of both CH3NH3PbBr3 nanocrystals and the ligands widely used in their preparation. We discuss the possible influence of the ligand molecules to apparent perovskite properties, and we compare the electronic properties of nanocrystals to those of bulk material. The experimental results were supported by DFT calculations.

7.
Phys Chem Chem Phys ; 16(47): 25950-8, 2014 Dec 21.
Artigo em Inglês | MEDLINE | ID: mdl-25358087

RESUMO

The effects of thermal disorder on the electronic properties of crystalline polymers were investigated. Atomic configurations of the material were obtained using classical Monte Carlo simulations at room temperature, while electronic structure calculations were performed using the density functional theory based charge patching method and the overlapping fragment method. We investigated two different stable configurations of crystalline poly(3-hexylthiophene) (P3HT) and calculated the density of electronic states and the wave function localisation. We found that the effect of disorder in side chains is more pronounced in the more stable configuration of P3HT than in the other one due to the larger conformational freedom of side chains. The results show that disorder in main chains has a strong effect on the electronic structure and leads to the localisation of the wave functions of the highest states in the valence band, similar to localisation that occurs in amorphous polymers. The presence of such states is one possible origin of thermally activated electrical transport in ordered polymers at room temperature.

8.
J Phys Chem Lett ; 5(8): 1335-40, 2014 Apr 17.
Artigo em Inglês | MEDLINE | ID: mdl-26269977

RESUMO

We developed an accurate model accounting for electron-phonon interaction in colloidal quantum dot supercrystals that allowed us to identify the nature of charge carriers and the electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids, the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping.

9.
J Am Chem Soc ; 135(32): 12048-56, 2013 Aug 14.
Artigo em Inglês | MEDLINE | ID: mdl-23855781

RESUMO

The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems. Here, we developed a binder polymer with multifunctionality to maintain high electronic conductivity, mechanical adhesion, ductility, and electrolyte uptake. These critical properties are achieved by designing polymers with proper functional groups. Through synthesis, spectroscopy, and simulation, electronic conductivity is optimized by tailoring the key electronic state, which is not disturbed by further modifications of side chains. This fundamental allows separated optimization of the mechanical and swelling properties without detrimental effect on electronic property. Remaining electronically conductive, the enhanced polarity of the polymer greatly improves the adhesion, ductility, and more importantly, the electrolyte uptake to the levels of those available only in nonconductive binders before. We also demonstrate directly the performance of the developed conductive binder by achieving full-capacity cycling of silicon particles without using any conductive additive.

10.
Phys Chem Chem Phys ; 15(10): 3543-51, 2013 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-23376937

RESUMO

The comparison of hole density of states (DOS) and hole mobilities of several organic polymer based systems was performed to gain insight into the main factors that determine the electrical properties of conjugated polymers. The DOS and the mobility of the systems under investigation were evaluated using an atomistic multiscale procedure. The results suggest that the irregularities in the shape of the polymer chains increase the diagonal disorder, while alkyl side chains act as spacers that reduce the diagonal disorder which originates from long range electrostatic interactions. Intrachain electronic coupling in relatively ordered polymers narrows the tail of the DOS, while in less ordered polymers it represents the additional component of disorder and widens the tail of the DOS. The width of the DOS tail was confirmed to be an important factor that determines the activation energy for charge carrier transport. However, it is not the only factor since the system with a smaller width of the DOS tail can have a larger activation energy due to, for example, smaller wave function overlap between transport states.

11.
Phys Rev Lett ; 109(12): 126407, 2012 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-23005969

RESUMO

We consider electron-phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a first-principles approach, we compute the changes in the selfconsistent Kohn-Sham potential corresponding to different phonon modes and go on to obtain the carrier-phonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowest unoccupied (LUMO), and holes at the top of the highest occupied (HOMO), band, obtaining Z(e) ≈ 0.74 and Z(h) ≈ 0.78, respectively. Along with the widely accepted notion that the carrier-phonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide strong microscopic evidence for the previously conjectured nonpolaronic nature of bandlike carriers in these systems.

12.
J Phys Chem Lett ; 3(17): 2442-6, 2012 Sep 06.
Artigo em Inglês | MEDLINE | ID: mdl-26292130

RESUMO

Time-resolved terahertz spectroscopy was employed for the investigation of charge-transport dynamics in benzothiadiazolo-dithiophene polyfluorene ([2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)]) (APFO-3) polymers with various chain lengths and in its monomer form, all blended with an electron acceptor ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM). Upon photoexcitation, charged polaron pairs are created, negative charges are transferred to fullerenes, while positive polarons remain on polymers/monomers. Vastly different hole mobility in polymer and monomer blends allows us to distinguish the hole and electron contributions to the carrier mobility.

14.
Nano Lett ; 11(10): 4107-12, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21848283

RESUMO

Using conducting probe atomic force microscopy (CAFM) we have investigated the electrical conduction properties of monolayer films of a pentathiophene derivative on a SiO(2)/Si-p+ substrate. By a combination of current-voltage spectroscopy and current imaging we show that lateral charge transport takes place in the plane of the monolayer via hole injection into the highest occupied molecular orbitals of the pentathiophene unit. Our CAFM data suggest that the conductivity is anisotropic relative to the crystalline directions of the molecular lattice.

15.
Phys Chem Chem Phys ; 13(32): 14500-9, 2011 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-21738941

RESUMO

Insight into the electronic structure of disordered poly-2,5-bis(phenylethynyl)-1,3,4-thiadiazole in an amorphous region, in comparison to an ideal two-planar cofacial oligomer system, is pursued. The atomic structure of the amorphous polymer was obtained from classical molecular dynamics. It was subsequently used to calculate the electronic states and inter- and intrachain electronic coupling integrals using the density functional theory based charge patching method. The interchain electronic coupling integrals in the amorphous system were found to be an order of magnitude smaller than in the ordered system with similar distances between the chains. The results also suggest that the electronic structure of the whole system cannot be understood as a collection of the electronic structures of individual chains. The band gap of the whole system is significantly smaller than the band gaps of individual chains. This decrease originates from the disordered long range electrostatic potential created by the dipole moments of polymer repeat units, which should be minimized if one seeks good transport properties.


Assuntos
Elétrons , Polímeros/química , Tiadiazóis/química , Modelos Moleculares , Estrutura Molecular , Teoria Quântica
16.
J Chem Phys ; 134(9): 094119, 2011 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-21384962

RESUMO

We present a method for the calculation of the electronic structure of systems that contain tens of thousands of atoms. The method is based on the division of the system into mutually overlapping fragments and the representation of the single-particle Hamiltonian in the basis of eigenstates of these fragments. In practice, for the range of the system size that we studied (up to tens of thousands of atoms), the dominant part of the calculation scales linearly with the size of the system when all the states within a fixed energy interval are required. The method is highly suitable for making good use of parallel computing architectures. We illustrate the method by applying it to diagonalize the single-particle Hamiltonian obtained using the density functional theory based charge patching method in the case of amorphous alkane and polythiophene polymers.


Assuntos
Alcanos/química , Elétrons , Polímeros/química , Teoria Quântica , Tiofenos/química , Estrutura Molecular
17.
J Phys Chem B ; 115(8): 1792-7, 2011 Mar 03.
Artigo em Inglês | MEDLINE | ID: mdl-21291182

RESUMO

We present large-scale calculations of electronic structure of strongly disordered conjugated polymers. The calculations have been performed using the density functional theory based charge patching method for the construction of single-particle Hamiltonian and the overlapping fragments method for the efficient diagonalization of that Hamiltonian. We find that the hole states are localized due to the fluctuations of the electrostatic potential and not by the breaks in the conjugation of the polymer chain. The tail of the density of hole states exhibits an exponentially decaying behavior. The main features of the electronic structure of the system can be described by an one-dimensional nearest neighbor tight-binding model with a correlated Gaussian distribution of on-site energies and constant off-site coupling elements.

18.
Nano Lett ; 9(12): 3996-4000, 2009 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-19908900

RESUMO

We developed an ab initio multiscale method for simulation of carrier transport in large disordered systems, based on direct calculation of electronic states and electron-phonon coupling constants. It enabled us to obtain the never seen before rich microscopic details of carrier motion in conjugated polymers, which led us to question several assumptions of phenomenological models, widely used in such systems. The macroscopic mobility of disordered poly(3-hexylthiophene) (P3HT) polymer, extracted from our simulation, is in agreement with experimental results from the literature.


Assuntos
Modelos Químicos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Polímeros/química , Simulação por Computador , Condutividade Elétrica , Eletricidade Estática
19.
J Phys Chem B ; 113(2): 409-15, 2009 Jan 15.
Artigo em Inglês | MEDLINE | ID: mdl-19099442

RESUMO

Electronic structure of disordered semiconducting conjugated polymers was studied. Atomic structure was found from a classical molecular dynamics simulation, and the charge patching method was used to calculate the electronic structure with the accuracy similar to the one of density functional theory in local density approximation. The total density of states, the local density of states at different points in the system, and the wave functions of several states around the gap were calculated in the case of poly(3-hexylthiophene) (P3HT) and polythiophene (PT) systems to gain insight into the origin of disorder in the system, the degree of carrier localization, and the role of chain interactions. The results indicated that disorder in the electronic structure of alkyl-substituted polythiophenes comes from disorder in the conformation of individual chains, while in the case of polythiophene there is an additional contribution due to disorder in the electronic coupling between the chains. Each of the first several wave functions in the conduction and valence band of P3HT is localized over several rings of a single chain. It was shown that the localization can be caused in principle both by ring torsions and chain bending; however, the effect of ring torsions is much stronger. PT wave functions are more complicated due to larger interchain electronic coupling and are not necessarily localized on a single chain.

20.
J Chem Phys ; 128(12): 121102, 2008 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-18376901

RESUMO

The development of the charge patching method for the calculation of the electronic structure of organic systems containing a large number of atoms was presented. The method was tested on a range of systems including alkane and alkene chains, polyacenes, polythiophenes, polypyrroles, polyfuranes, polyphenylene vinylene, and poly(amidoamine) dendrimers. The results obtained by the method are in very good agreement with direct calculations based on density functional theory, since the eigenstate errors are typically of the order of a few tens of meV.

SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA