Energy Absorption Spectra Research Articles

Computational study of the absorption spectrum of defected ZnS nanoparticles

Energy levels and absorption spectra of defected ZnS nanoparticles (NPs) were calculated with Density Functional Theory (DFT) and Time Dependent DFT. Several types of defects were examined such as vacancies and substitutions. NPs with S vacancies were found to have their absorption spectra moved to lower energies well inside the visible spectrum with significantly high oscillator strength. Also, NPs with substitution of S atoms with Cl, Br, or I showed significant absorption. In general, this type of defect moves the absorption spectra in lower energies, thus bringing the absorption edge into the visible spectrum, while the unperturbed NPs have absorption edges in the UV region. In addition, ZnS NPs are made from more abundant and less toxic elements than the more commonly used CdSe NPs. For that reason, they may find significant applications in solar cells and other photonic applications, as well as in biosensing applications as biomarkers.

Effect of interdiffusion and external magnetic field on electronic states and light absorption in Gaussian-shaped double quantum ring

The effect of interdiffusion and magnetic field on confined states of electron and heavy hole as well as on interband absorption spectrum in a Ga1−xAlxAs/GaAs Gaussian-shaped double quantum ring are investigated. It is shown that both interdiffusion and magnetic field lead to the change of the charge carriers' quantum states arrangement by their energies. The oscillating behavior of the electron ground state energy as a function of magnetic field induction gradually disappears with the increase of diffusion parameter due to the enhanced tunneling of electron to the central region of the ring. For the heavy hole the ground state energy oscillations are not observable in the region of the values of magnetic field induction B=0−10T. For considered transitions both the magnetic field and the interdiffusion lead to a blue-shift of the absorption spectrum and to decreasing of the absorption intensity. The obtained results indicate on the opportunity of purposeful manipulation of energy states and absorption spectrum of a Gaussian-shaped double quantum ring by means of the post growth annealing and the external magnetic field.

Study of the Structural, Electronic, Magnetic, and Optical Properties of Mn2ZrGa Full-Heusler Alloy: First-Principles Calculations

In this work, the structural, electronic, magnetic, and optical properties of Mn2ZrGa full-Heusler alloy were investigated by using density functional theory (DFT) calculations. It is found that the spin-up states have a metallic character, but the spin-down bands have a pseudo-gap at the Fermi level. The total spin magnetic moment of Mn2ZrGa (per formula unit) is 3.00 µ B at an equilibrium lattice parameter of 6.15 A. The calculations show that Mn2ZrGa is a ferrimagnetic with 81% spin polarization at equilibrium lattice parameter. The effect of lattice parameter distortion on the magnetic properties and spin polarization is also studied. It is found that the total magnetic moment preserves its value for a lattice parameter range of 5.96–6.30 A. The decreasing of the lattice parameter leads to improvement of spin polarization. The real and imaginary parts of dielectric function and hence the optical properties including energy absorption spectrum, reflectivity, and optical conductivity are also calculated. The value of plasma frequency for spin-up and down electrons is located at 1.78 and 0.74 eV, respectively.

Coupled-square-well model and Fano-phase correspondence

This paper investigates the Fano-Feshbach resonance with a two-channel coupled-square-well model in both the frequency and time domains. This systems is shown to exhibit Fano lineshape profiles in the energy absorption spectrum. The associated time-dependent dipole response has a phase shift that has recently been understood to be related to the Fano lineshape asymmetric $q$ parameter by $\phi=2arg(q-i)$. The present study demonstrates that the phase-$q$ correspondence is general for any Fano resonance in the weak coupling regime, independent of the transition mechanism.

Theoretical study of geometric structures and electronic absorption spectra of Iridium(III) complexes based on 2-phenyl-5-nitropyridyl with different ancillary ligands

In this paper, the influence of different ancillary ligands on geometric structures, frontier molecular orbitals character, gap energy and electronic absorption spectra of [Ir(5-NO2-ppy)2(PPh3)(L)] (L=Cl−, NCO−, NCS− and N−3) has been theoretically studied by density functional theory and time-dependent density functional theory calculations with B3LYP and PBE0 hybrid functionals. Calculated geometric parameters of studied complexes are in good agreement with the available experimental values. The theoretical absorption spectra reproduce the main properties of the experimental spectra. Both energy gaps and absorption spectra depend on the composition of HOMO Orbital, π-donor orbitals reduce the energy gaps and red shift the absorption spectra while π-acceptor orbitals increase the energy gaps and blue shift the absorption spectra. All the spectra are characterized by mixed MLCT/LLCT states in the visible energy domain.

Reference line correcting method and accuracy analysis of fingernail electron paramagnetic resonance

Objective The spectrum has a greater baseline drift when wind speed, temperature and humidity change in the experiment of fingernail electron paramagnetic resonance dose reconstruction, and thus the read-out value does not reflect the true radical concentration, which ultimately reduces the credibility of the test results. This study aims to propose a reference line correcting method to solve the problem. Methods The drifted baseline was found out first, and then spectral correction range and the corresponding correction function were selected or the original spectrum was reduced to energy absorption spectrum. Finally, the point correction method was applied to modify the spectrum. Results The probability of the value obtained without reference line correcting for the spectrum of baseline drift equal to the value of spectrum without baseline drift was only 31%, while the probability was increased to 91%, improving nearly 3 times after using the reference line correcting method in the range of acceptable error. Conclusions This reference line correcting method for spectrum should be applied before reading the spectrum in order to increase the accuracy of dose estimation. Key words: Electron paramagnetic resonance; Spectrum; Reference line correcting; Fingernail; Baseline drift

Plasmon excitations in the dimers formed by atom chains

Based on the linear response theory in the random-phase approximation and the free-electron gas model, we study the plasmon excitations in the dimers formed by atom chains. With the help of energy absorption spectrum and charge distribution, the evolutions of longitudinal and transverse plasmon, and the effect of the system parameters such as size, atomic separation and electron filling on plasmon are obtained. In addition, the dipole, quadrupole, end and central plasmon are observed.

Theory of dielectric loss in Graphene-on-substrate: A tight- binding model study

Graphene-on-substrate exhibits interesting dielectric behaviour due to screening of coulomb interaction induced by many body effects. In this communication we attempt to study the dielectric loss property of graphene within tight-binding model approach. The Hamiltonian consisting of electron hopping upto third-nearest-neighbour's with impurities in two in equivalent sub-lattices. The graphene-on-substrate raises the energy +Δ at one sub lattice and reduces energy -Δ at other sub lattice. Further we introduced coulomb interaction between π - electrons at the two sub lattices separately with the same effective coulomb interaction. We calculate polarization function Π(q, ω) which is a two particle Green's function arising due to charge-charge correlation by using Zubarev's Green's function technique. Finally we calculate dielectric function of graphene i.e. ε(q, ω) =1+Π(q,ω) at arbitrary wave vector q and frequency ra. The dielectric loss in graphene calculated from the imaginary part of dielectric function which is a measure of absorption spectrum. Only a few Fragmentary theoretical attempts have been made to utilize the full frequency and wave vector dependent dielectric function. We compute numerically the frequency dependent dielectric loss function for 100x100 momentum grid points. We observe a low energy Plasmon resonance peak and a high energy flat peak arising due to absorption of optical energy at substrate induced gap. With increase of small Plasmon wave vector, both low and high energy peaks approach each other. The dielectric loss at low energies exhibits a parabolic curve, but it exhibit a clear peak on introduction of higher order electron hopping's. The Coulomb interaction suppresses induced gap in graphene and decreases the optical energy absorption spectra. The increase of substrate induced gap shifts the high energy flat peak to higher energies and enhances the dielectric loss throughout the frequency range. Finally the effect of doping on dielectric loss is investigated and compared with the experimental results.

Energy and optical absorption spectra of endohedral metallofullerenes with Gd or Ho as strongly correlated π-electron systems

Isomerically pure endohedral metallofullerenes Gd@C82(C2v), Ho@C82(C2v), and their monoanions have been synthesized and separated. The optical absorption spectra of solutions of obtained compounds in o-dichlorobenzene have been studied. Within the Hubbard model, the energy spectrum of isomer of C2v symmetry (no. 9) of fullerene С82 has been calculated. Based on the obtained spectrum, optical absorption spectra of endohedral metallofullerenes Gd@C82 and Ho@C82 and their monoanions have been simulated. The calculated optical absorption spectra have been compared with experimental ones; it has been found that qualitative agreement between them is observed.

A Bromo-Functionalized Conjugated Polymer as a Cross-Linkable Anode Interlayer of Polymer Solar Cells.

A cross-linkable conjugated polymer with bromo groups on the side chain has been developed and used as an anode interlayer to improve the active layer morphology and consequently enhance the device performance of polymer solar cells (PSCs). The polymer, PCDTBT-Br, has cross-linkable bromo groups attached to the side chain of a widely-used donor polymer, poly[N-9-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2,1,3-benzothiadiazole)] (PCDTBT). The pendant bromo groups do not significantly change the LUMO/HOMO energy levels and absorption spectrum of the PCDTBT polymer backbone. PDCTBT-Br can crosslink under UV irradiation to give a robust film, which enables multilayer PSC device fabrication. Moreover, the much lower surface energy of PCDTBT-Br (20.4 mJ m(-2) ) compared to PSS (91.6 mJ m(-2) ) is beneficial for achieving optimal active layer morphology. As a result, with PCDTBT:[6,6]-phenyl-C71 butyric acid methyl ester (PC71 BM) as the active layer, the PSC device with PCDTBT-Br as the underlying layer shows a power conversion efficiency (PCE) of 6.59 %, in comparison to a PCE of 5.86 % of the control device. The device performance enhancement is ascribed to the much improved phase separation with a fibrillar nanostructure in the active layer.

Theoretical Study of Graphene Nanoflakes with Nitrogen and Vacancy Defects: Effects of Flake Sizes and Defect Types on Electronic Excitation Properties

We have established finite-sized model molecules of hexagonal graphene nanoflakes which consisted of up to about 400 carbon atoms and certain defects and calculated their geometric structures, molecular orbitals, and valence electron excitation properties by DFT and TD-DFT approaches. Several types of defects such as single nitrogen atom dopants, vacancies, combinations of nitrogen atoms and vacancies, etc. have been investigated. The vertical excitation energies and the corresponding absorption spectra of all low-lying excited states of all these defects are found strongly depending on nanoflake sizes with few exceptions. The reason is revealed by examining molecular orbital configuration changes upon excitation, where in most cases the electron density does not localize in the defect center but disperses to the edges of the nanoflake instead. As a result, graphene nanoflakes with these defects do not present definite spectral features like a zero-phonon line in the nanodiamond. The energy gaps and absorption spectra could be tuned by varying nanoflake sizes and defect types, where the size determines the positions of the first major absorption peaks and the defect type affects spectral shapes of the following bands.

Endohedral fullerene Gd2C2@C82 as a strongly correlated electron system

Isomerically pure endohedral fullerene with a gadolinium carbide cluster Gd2C2@C82(C s ) has been synthesized by the electric-arc method, extracted from soot by o-dichlorobenzene, separated chromatographically, and characterized by mass spectroscopic and spectrophotometric methods. The energy and optical absorption spectra of the synthesized compound have been calculated for the first time within the Hubbard model. Good agreement of the experimental and calculated optical absorption curves has been observed.

Design and characteration of planar star-shaped oligomer electron donors for organic solar cells: a DFT study

To seek high-performance oligomer donor materials used in organic solar cells, four star-shaped molecules with a planar donor core derived from the recent reported molecule 3T-P-DPP (phenyl-1,3,5-trithienyl-diketopyrrolopyrrole) were designed. The molecular properties affecting the cell performance, such as structural characteristics, frontier molecular orbital energy level, absorption spectra, exciton character, and charge transfer/transport, were investigated by means of the density functional theory and time-dependent density functional theory methods. Comparative analysis showed that the new designed molecule 3 with a TTT (2,4,6-tri(thiophen-2-yl)-1,3,5-triazine) core has better planarity, a lower HOMO energy level, and a higher absorption efficiency, as well as more favorable exciton dissociation and charge transfer than the others, potentially improving the open-circuit voltage and short-circuit current density. Consequently, 3 maybe superior to 3T-P-DPP and may act as a promising donor material candidate for organic solar cells.

Förster resonance energy transfer, absorption and emission spectra in multichromophoric systems. I. Full cumulant expansions and system-bath entanglement.

We study the Förster resonant energy transfer rate, absorption and emission spectra in multichromophoric systems. The multichromophoric Förster theory (MCFT) is determined from an overlap integral of generalized matrices related to the donor's emission and acceptor's absorption spectra, which are obtained via a full 2nd-order cumulant expansion technique developed in this work. We calculate the spectra and MCFT rate for both localized and delocalized systems, and calibrate the analytical 2nd-order cumulant expansion with the exact stochastic path integral method. We present three essential findings: (i) The role of the initial entanglement between the donor and its bath is found to be crucial in both the emission spectrum and the MCFT rate. (ii) The absorption spectra obtained by the cumulant expansion method are nearly identical to the exact spectra for both localized and delocalized systems, even when the system-bath coupling is far from the perturbative regime. (iii) For the emission spectra, the cumulant expansion can give reliable results for localized systems, but fail to provide reliable spectra of the high-lying excited states of a delocalized system, when the system-bath coupling is large and the thermal energy is small. This paper also provides a simple golden-rule derivation of the MCFT, reviews existing methods, and motivates further developments in the subsequent papers.

Förster resonance energy transfer, absorption and emission spectra in multichromophoric systems. III. Exact stochastic path integral evaluation.

A numerically exact path integral treatment of the absorption and emission spectra of open quantum systems is presented that requires only the straightforward solution of a stochastic differential equation. The approach converges rapidly enabling the calculation of spectra of large excitonic systems across the complete range of system parameters and for arbitrary bath spectral densities. With the numerically exact absorption and emission operators, one can also immediately compute energy transfer rates using the multi-chromophoric Förster resonant energy transfer formalism. Benchmark calculations on the emission spectra of two level systems are presented demonstrating the efficacy of the stochastic approach. This is followed by calculations of the energy transfer rates between two weakly coupled dimer systems as a function of temperature and system-bath coupling strength. It is shown that the recently developed hybrid cumulant expansion (see Paper II) is the only perturbative method capable of generating uniformly reliable energy transfer rates and emission spectra across a broad range of system parameters.

Förster resonance energy transfer, absorption and emission spectra in multichromophoric systems. II. Hybrid cumulant expansion.

We develop a hybrid cumulant expansion method to account for the system-bath entanglement in the emission spectrum in the multi-chromophoric Förster transfer rate. In traditional perturbative treatments, the emission spectrum is usually expanded with respect to the system-bath coupling term in both real and imaginary time. This perturbative treatment gives a reliable absorption spectrum, where the bath is Gaussian and only the real-time expansion is involved. For the emission spectrum, the initial state is an entangled state of the system plus bath. Traditional perturbative methods are problematic when the excitations are delocalized and the energy gap is larger than the thermal energy, since the second-order expansion cannot predict the displacement of the bath. In the present method, the real-time dynamics is carried out by using the 2nd-order cumulant expansion method, while the displacement of the bath is treated more accurately by utilizing the exact reduced density matrix of the system. In a sense, the hybrid cumulant expansion is based on a generalized version of linear response theory with entangled initial states.

Synthesis and thermal annealing treatment of octylphosphonic acid-capped CdSe-tetrapod nanocrystals for bulk hetero-junction solar cell applications

CdSe-tetrapod nanocrystals (NCs) were synthesized by using octylphosphonic acid (OPA) as a capping ligand and cadmium oxide (CdO) as a cadmium precursor. The effects of thermal annealing in nitrogen (N2) environment on the chemical composition, morphology, crystal structure and optoelectronic properties of the CdSe-tetrapods have been investigated. Remarkable difference in the morphological and optoelectronic properties between as-synthesized and N2-annealed CdSe NCs was observed. The photoluminescence (PL) peak of N2-annealed CdSe NCs shifted to lower energy and UV-vis absorption spectra shifted to longer wavelength, indicating the size increase and improvement of the crystallinity of the CdSe tetrapods. The power conversion efficiency of bulk hetero-junction solar cells made with the annealed CdSe NCs showed higher value compared with the efficiency of cells made with as-synthesized CdSe NCs.

Synthesis of two-dimensional π-conjugated polymers pendent with benzothiadiazole and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole moieties for polymer solar cells

Four new 2D donor-acceptor conjugated polymers were designed and synthesized. These new polymers comprised fluorene- alt -triphenylamine or carbazole- alt -triphenylamine as the backbones, and pendants with 2,1,3-benzothiadiazole (BT) or naphtho[1,2- c :5,6- c ]bis[1,2,5]thiadiazole (NT) in a triphenylamine unit as the side groups. By changing the acceptor BT for a stronger electron-withdrawing unit of NT moiety in the side chain, the energy levels, absorption spectra, band gaps, and charge-transport abilities of the resultant polymers could be effectively tuned. Bulk heterojunction solar cells with these polymers as the electron donors and (6,6)-phenyl-C71-butyric acid methyl ester as the electron acceptor exhibited high open-circuit voltage (more than 0.8 eV). The power conversion efficiency can be improved from 1.37% to 3.52% by replacing the BT with an NT moiety, which indicates that introducing NT as the side-chain building block can be an effective strategy to construct efficient 2D conjugated polymers for PSCs.

Spectra and Charge Transport of Polar Molecular Photoactive Layers Used for Solar Cells

The ground state structures, HOMO and LUMO energy levels, band gapsΔH-L, ionization potentials (IP), and electron affinities (EA) of three types of copolymer P1 and its derivatives P2, P3, and PBDT-BTA were investigated by using density functional theory (DFT) with B3LYP and 6-31G (d) basis set. On the base of optimized structures of ground states, their absorption spectra were obtained by using TD-DFT//Cam-B3LYP/6-31 G (d). Research shows that with the increasing conjugated units, HOMO energy levels increased, LUMO energy levels decreased, and band gaps decreased gradually. Moreover, their ionization potentials decreased and electron affinities increased along with the increase of conjugated chains, and absorption spectra red-shifted. In addition, the side chain has a significant effect on the properties of ground and excited states. In order to investigate the influence of conjugated units and side chain on the charge transport, their hole and electron reorganization energies were calculated, and the results indicated that Pb have a good hole transport capability. Considering the practical application, the HOMO and LUMO energy levels, band gaps, and absorption spectra under external electric field were studied, and the results proved that the external electric field has an effect on the optical and electronic properties.

Energy structure and absorption spectra of colloidal CdS nanocrystals in gelatin matrix

We investigate the energy structure of colloidal CdS nanocrystals by measuring the UV–vis absorption spectra. Nanocrystals were synthesized by sol–gel method in a gelatin matrix in the size range from 2.5 to 3.9 (±0.2)nm. In order to interpret the UV–vis absorption spectra we calculate the energy spectrum of electron quasi-stationary states using the model of open nanocrystal as well as the hole stationary spectrum in a two-band approach. It is shown that the main contribution to the absorption spectrum is made by interband transitions 1S3/2→1Se and 1P3/2→1Pe, and its shape is determined by the size distribution of nanoparticles. For this system the estimated values of the effective masses of the heavy hole and light hole are 1.44m0 and 0.28m0, respectively.