Calculated Absorption Spectra Research Articles

Optical absorption coefficient calculations of GaSb/GaAs quantum dots for intermediate band solar cell applications

Absorption coefficients of GaSb/GaAs quantum dots (QDs) are calculated by the 8-band strain-dependent k· p method and Fermi’s golden rule. A more realistic but simple approach to model the QD ensemble with wetting layer is described. Effects of the QD size and density, and the GaAs spacer thickness for multi-stacked QDs on absorption characteristics are studied. Absorption spectra of the single QD, single layer of QDs, and multi-stacked QDs are presented and discussed. Interband absorption is found to be more intense than intraband absorption. The calculated absorption spectra are brought into the drift-diffusion model coupled with rate equations to determine the current density-voltage curves of the GaSb/GaAs QD solar cells, which are compared with measured data in literature for validation. The models proposed in this work are capable of predicting the short-circuit current density and open-circuit voltage of real devices, and would have the potential to investigate the impact of doping and position of the QD layers, which is necessary for intermediate band solar cell analysis and design.

Theoretical study on the detailed excited state triple proton transfer mechanism of cyclic 6-Azaindole trimer

The proton transfer mechanism of 6AI trimer in cyclohexane has been theoretical study by time-dependent density functional theoretical methods. After three 6AI monomers are connected to form trimer by hydrogen bonds, the calculated absorption spectra confirm that the experimental absorption peaks of 6AI monomer (about 280 nm) and 6AI trimer (310 nm) are attributed to the 6AI trimer and TPT tautomer respectively. Through the potential energy curves and the potential energy surface, it can be concluded that the 6AI trimer reaches the TPT tautomer in the S1 state through the concerted mechanism rather than the stepwise mechanism.

Study of super absorption properties of 1D graphene and dielectric photonic crystal for novel applications

In this article, we have theoretically analyzed the variation of real and imaginary components of electric permittivity and also the variation of refractive index of the graphene material in the Terahertz frequency range. The optical characteristics of One-Dimensional Periodic Structure (1-DPS) of graphene and dielectric materials have analyzed using Transfer Matrix Method. The transfer matrix method is very eminent method to analyze the optical characteristics of 1-DPS like dispersion relation, phase velocity, group velocity, reflection, transmission and absorption characteristics, etc. The absorption characteristics of considered graphene-based multilayered periodic structure have been analyzed with the variation of chemical potential ( $$\mu_{c}$$ ) of the graphene layer. The calculated absorption spectra of considered 1-DPS at the certain threshold frequency have found high and threshold value for the high chemical potential (gate voltage) of the graphene having the large thickness of the dielectric layer. Such the high and threshold absorption property of graphene-based 1-DPS, which acts as super absorber at terahertz region of the electromagnetic spectrum, may be used for the novel super absorber applications.

Towards understanding physical origin of 2175 Å extinction bump in interstellar medium

ABSTRACT The 2175 Å ultraviolet (UV) extinction bump in interstellar medium (ISM) of the Milky Way was discovered in 1965. After intensive exploration of more than a half century, however, its exact origin still remains a big conundrum that is being debated. Here, we propose a mixture model by which the extinction bump in ISM is argued possibly relevant to the clusters of hydrogenated T-carbon (HTC) molecules (C40H16) that have intrinsically a sharp absorption peak at the wavelength 2175 Å. By linearly combining the calculated absorption spectra of HTC mixtures, graphite, MgSiO3, and Fe2SiO4, we show that the UV extinction curves of optional six stars can be nicely fitted. This work poses an alternative explanation towards understanding the physical origin of the 2175 Å extinction bump in ISM of the Milky Way.

Design of silver nanoparticles with graphene coatings layers used for LSPR biosensor applications

A new hybrid graphene nanoplasmonic structure, using silver nanoparticles (AgNPs) coated with a graphene film and deposited on a substrate is proposed in this paper. The aim is to obtain a plasmonic response with high sensitivity for plasmonic biosensors. We show, through the calculation of absorption spectra, that the localized surface plasmon resonance (LSPR) obtained in the SiOx/AgNPs/Graphene nanostructure is very sensitive to variations in the thickness of the graphene and those of the refractive index of the detection medium. The study reveals a shift in plasmon resonance peaks resulting from the coupling between the AgNPs networks and the covering graphene layer. We obtained the red-shift of LSPR modes from 412 nm to 548 nm when the thickness of the graphene layer increases from 0.34 nm to 9 nm. We have shown that the increase in graphene thickness significantly affects the sensitivity of the device under study. An optimal sensitivity value is obtained for a graphene thickness of 9 nm, with a 304.60% gain in sensitivity value compared to the structure without graphene. These properties should enhance these nano-device and make them a preferred choice for biosensors applications, over other current biosensors.

Ab-initio Calculations of the Structural, Electronic and Optical Properties of (Cdse)2 Clusters

The distinctive properties of cadmium selenide (CdSe) semiconductor situated it in a multitudinous number of applications. Although (CdSe)2 cluster has more than one isomer, the previous studies concentrated merely on one isomer. The goal of this study was to determine the various stable geometric structure isomers of (CdSe)2 clusters; also, structural, electronic, and optical properties of the stable isomers are investigated using density functional theory (DFT). First, geometry optimization calculations of the possible geometric isomers were carried out using the BroydenFletcher-Goldfarb-Shanno minimization (BFGS) algorithm. Total ground-state energy calculations showed that all the converged isomers have a high probability of existing in any experiment, relying on the implemented experimental technique. Twenty initial possible geometric structures were investigated, in which eleven isomers were converged. However, all of them are relaxed in the 2D planar geometry. The results showed that eleven possible stable isomers were disclosed, where the final structures of the converged isomers produced six different structures; three of them were not detected before. The rhombus structure was ascertained to be the most stable isomer followed by the trapezoidal structure of (CdSe)2. The isomers’ Cd-Se bond length are 2.50-2.74 A, and the average Cd-Se-Cd, Se-Cd-Se angles were 64.5o-123o and 56.3o-114.2o, respectively. Furthermore, the bond angles show that the selenium atom lone-pairs electrons are responsible for shifting the isomers’ structure from the linearity. The total ground-state energy differences were 0.00-1.82 eV. The calculated highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) gap of the isomers implied that the gap depends on the symmetrical geometry of the isomer. Furthermore, it was evident that the most stable isomers are accompanied with larger gaps. The HOMO-LUMO graphs demonstrated that HOMO orbitals were localized around the selenium atom, while LUMO orbitals were distributed around both cadmium and selenium atoms. The calculated absorption spectrum was unique for each isomer. The absorption edges for the isomers are ranging from 2.53 to 3.73 eV. The results show that the obtained absorption spectra peaks’ values (nm) are smaller compared to CdSe experimental results. (CdSe)2 clusters are very active that they straightforwardly react to produce larger clusters. Finally, the results of this study corroborate with previous computational studies.

Fast optical absorption spectra calculations for periodic solid state systems

We present a method to construct an efficient approximation to the bare exchange and screened direct interaction kernels of the Bethe-Salpeter Hamiltonian for periodic solid state systems via the interpolative separable density fitting technique. We show that the cost of constructing the approximate Bethe-Salpeter Hamiltonian scales nearly optimally as $\mathcal{O}(N_k)$ with respect to the number of samples in the Brillouin zone $N_k$. In addition, we show that the cost for applying the Bethe-Salpeter Hamiltonian to a vector scales as $\mathcal{O}(N_k \log N_k)$. Therefore the optical absorption spectrum, as well as selected excitation energies can be efficiently computed via iterative methods such as the Lanczos method. This is a significant reduction from the $\mathcal{O}(N_k^2)$ and $\mathcal{O}(N_k^3)$ scaling associated with a brute force approach for constructing the Hamiltonian and diagonalizing the Hamiltonian respectively. We demonstrate the efficiency and accuracy of this approach with both one-dimensional model problems and three-dimensional real materials (graphene and diamond). For the diamond system with $N_k=2197$, it takes $6$ hours to assemble the Bethe-Salpeter Hamiltonian and $4$ hours to fully diagonalize the Hamiltonian using $169$ cores when the brute force approach is used. The new method takes less than $3$ minutes to set up the Hamiltonian and $24$ minutes to compute the absorption spectrum on a single core.

First-principles study on the structure and optical spectroscopy of the redox-active center of blue copper proteins

We present first-principles calculations on the geometrical structures and optical absorption properties of redox-active centers in two types of blue copper proteins, plastocyanin (Pc) and pseudoazurin (PAz). Two four-coordination (4C) cluster models consisting 111-atom were constructed based on the crystal structures of the two proteins. The optimized configurations of the proposed models took on highly identical geometry as the crystal structures and the calculated absorption spectra of both models acceptably reproduced the corresponding experimental spectra. Based on spectral simulations, we were able to confirm the indispensable role of axial ligand methionine (Met) in both proteins. The difference in the absorption spectroscopy of the two types of proteins was caused by the changes to the ground state wave functions which originate mainly from the different coordination structures of the redox-active center, especially the different Cu-SMet bond lengths. The effects of density functionals and the size of basis sets were also tested.

Magneto-plasmonic Co@M (M = Au/Ag/Au-Ag) core-shell nanoparticles for biological imaging and therapeutics

In this paper, the optical properties of magnetic-plasmonic core-shell nanoparticles are studied using the Mie theory and DDA method. DDA results are compared with Mie theory results for spherical core-shell NPs as it gives the exact solution. It is observed that the position of LSPR peak in core-shell nanostructures is tuned by changing the cores and shell thickness in the core-shell spherical nanostructures and aspect ratio as well as shell thickness in spheroidal core-shell nanostructures. In addition, the yolk-shell nanostructures exhibit enhancement of LSPR peaks, bandwidth, and their corresponding intensities, as compared to spherical core-shell nanostructures. The calculated absorption spectra at λmax are obtained in the wavelength range of 336-698 nm. This study opens up the new potential of appropriate core-shell nanostructures in bio-imaging, drug delivery, biomedicine, and therapeutic uses.

Designing Donor-Acceptor Conjugated Macrocycles with Polyradical Character and Global (Anti)Aromaticity

Polyradical character and global (anti)aromaticity are fundamental concepts that govern rational design of cyclic conjugated macromolecules for optoelectronic applications. Here, we have designed donor-acceptor (D-A) macromolecules with ring topology along with their pi-spacer derivatives, and analyzed their unique electronic properties with density functional theory (DFT). The macromolecules with n = 8 and 16 repeat D-A units display similar singlet-triplet energy gap and have closed-shell electronic configuration with no diradical character (y0 = 0). These macromolecules also display global nonaromatic character, which is supported by NICS(0), AICD, and 2D-ICSS analysis. However, the derivatives with pi-spacer (n = 8-pi) develop near pure open-shell character (y0=1) with increased singlet-triplet gap. Furthermore, these derivatives display global antiaromaticity in the singlet ground-state according to Huckel's rule, and global aromaticity in the lowest triplet state following Baird's rule, and have a very high polyradical character, which is not reported for D-A type macromolecules. The calculated absorption spectra with time-dependent DFT (TDDFT) indicates intensive light absorption in the near-infrared (NIR) region broadening to 2500 nm. These macrocycles also display a size-dependent absorption maxima when the number of repeat units increased from n = 8 to 16. Furthermore, due to the development of open-shell character in the pi-spacer derivatives, a significant red-shift in the absorbance wavelength is observed.

Ring currents modulate optoelectronic properties of aromatic chromophores at 25 T

The properties of organic molecules can be influenced by magnetic fields, and these magnetic field effects are diverse. They range from inducing nuclear Zeeman splitting for structural determination in NMR spectroscopy to polaron Zeeman splitting organic spintronics and organic magnetoresistance. A pervasive magnetic field effect on an aromatic molecule is the aromatic ring current, which can be thought of as an induction of a circular current of π-electrons upon the application of a magnetic field perpendicular to the π-system of the molecule. While in NMR spectroscopy the effects of ring currents on the chemical shifts of nearby protons are relatively well understood, and even predictable, the consequences of these modified electronic states on the spectroscopy of molecules has remained unknown. In this work, we find that photophysical properties of model phthalocyanine compounds and their aggregates display clear magnetic field dependences up to 25 T, with the aggregates showing more drastic magnetic field sensitivities depending on the intermolecular interactions with the amplification of ring currents in stacked aggregates. These observations are consistent with ring currents measured in NMR spectroscopy and simulated in time-dependent density functional theory calculations of magnetic field-dependent phthalocyanine monomer and dimer absorption spectra. We propose that ring currents in organic semiconductors, which commonly comprise aromatic moieties, may present new opportunities for the understanding and exploitation of combined optical, electronic, and magnetic properties.

Atomistic analysis of radiative recombination rate, Stokes shift, and density of states in c-plane InGaN/GaN quantum wells

Recent experimental studies have revealed an unusual temperature dependence of the radiative recombination rate in polar InGaN/GaN quantum wells. We show, by direct atomistic evaluation of the radiative recombination rate, that the experimentally observed trend of an increasing rate with increasing temperature results from the population of energetically higher lying electron and hole states with dipole matrix elements larger than those of the band edge states relevant to low temperature studies. Given that the overall evolution of this recombination rate is tightly linked to the energetic distribution of localized states, we investigate the hole density of states and absorption spectra. Based on the calculated absorption spectra, Stokes shift energies are extracted for InGaN quantum wells with In contents ranging from 5% to 25%. Here, good agreement with experimental literature results is found. We provide also hole tail state characteristic energies as a function of the In content, a quantity that indicates the localization character of the ensemble of hole states and which serves often as a key component in modified continuum-based models to capture carrier localization effects in transport or optical gain calculations.

Synthesis of metal-free tetraazaisobacteriochlorin and via template condensation of phthalogenes

The indium(III) complexes of (4-(tert-butyl)phenyl)-substituted tetraazaisobacteriochlorin (TAiBC) and tetraazachlorin (TAC) were synthesized by direct template condensation of bis(4-(tert-butyl)phenyl)fumaronitrile and tetramethylsuccinonitrile using indium(III) ion as a matrix. The corresponding metal-free tetraazaisobacteriochlorin and tetraazachlorin were obtained by demetallation of their indium(III) complexes. These metal-free complexes were characterized using elemental analysis, mass-spectrometry, 1H and [Formula: see text]C{1H}NMR spectroscopy, UV-vis and MCD spectroscopy as well as DFT and TD-DFT calculations. Due to the low symmetry of the molecules, the NMR data were complex, but could be assigned by collecting 1D- and 2D NMR data and comparing with the results of quantum chemical calculations. From the position of the pyrrole proton signal (6.78 ppm), it was found that the diatropic current of TAiBC is much weaker than that of TAC, and plausibly the weakest among porphyrinoids so far reported. Absorption and MCD spectra were reasonably interpreted using the calculated absorption spectra.

TD-DFT+TB: An Efficient and Fast Approach for Quantum Plasmonic Excitations

We study the quantum plasmonic features of gold and silver nanoparticles using TD-DFT+TB, a new density functional theory approach to the calculation of excited states, which combines a full DFT ground state with tight-binding approximations in the linear response calculation. In this framework, the optical properties of closed-shell Ag, Au and bimetallic Ag–Au nanoparticles with tetrahedral symmetry (with 20, 56, 120, and 165 atoms) and icosahedral structure (with 13, 55, and 147 atoms) were obtained and compared to full linear response time-dependent density functional theory (TD-DFT) as a reference and also to time-dependent density functional based tight binding (TD-DFTB) as a low-cost alternative approach. We find an excellent agreement of TD-DFT+TB calculated absorption spectra with the TD-DFT reference with errors less than 0.15 eV in peak positions, while TD-DFTB shows larger errors of about 1 eV. The computational cost for the ground state calculation is identical for TD-DFT and TD-DFT+TB, but the excited state calculation becomes about a hundred times faster when applying the TB approximation and is then almost negligible for the overall timing of the calculation. In contrast to TD-DFTB, which can only be applied to element combinations for which a suitable DFTB parametrization is available, TD-DFT+TB can be applied to any combination of elements. To assess the accuracy of TD-DFT+TB for different combinations of atoms, the plasmonic properties of bimetallic clusters with different ratios of Ag and Au atoms were obtained and the trend of energy and intensity reproduced in good agreement with TD-DFT, which is not possible using TD-DFTB with standard parameter sets.

Giant proximity exchange and valley splitting in transition metal dichalcogenide/ hBN /(Co, Ni) heterostructures

We investigate the proximity-induced exchange coupling in transition-metal dichalcogenides (TMDCs), originating from spin injector geometries composed of hexagonal boron-nitride (hBN) and ferromagnetic (FM) cobalt (Co) or nickel (Ni), from first-principles. We employ a minimal tight-binding Hamiltonian that captures the low energy bands of the TMDCs around K and K' valleys, to extract orbital, spin-orbit, and exchange parameters. The TMDC/hBN/FM heterostructure calculations show that due to the hBN buffer layer, the band structure of the TMDC is preserved, with an additional proximity-induced exchange splitting in the bands. We extract proximity exchange parameters in the 1--10 meV range, depending on the FM. The combination of proximity-induced exchange and intrinsic spin-orbit coupling (SOC) of the TMDCs, leads to a valley polarization, translating into magnetic exchange fields of tens of Tesla. The extracted parameters are useful for subsequent exciton calculations of TMDCs in the presence of a hBN/FM spin injector. Our calculated absorption spectra show large splittings for the exciton peaks; in the case of MoS$_2$/hBN/Co we find a value of about 8 meV, corresponding to about 50 Tesla external magnetic field in bare TMDCs. The reason lies in the band structure, where a hybridization with Co $d$ orbitals causes a giant valence band exchange splitting of more than 10 meV. Structures with Ni do not show any $d$ level hybridization features, but still sizeable proximity exchange and exciton peak splittings of around 2 meV are present in the TMDCs.

Effects of halogen substitutions on the properties of CH3NH3Sn0.5Pb0.5I3 perovskites

First-principles density functional theory calculations are employed to provide insight into the structures and properties of metal- and halide-mixed perovskites, CH3NH3Sn0.5Pb0.5I3−yXy (X = Br, Cl; y = 0.083, 0.167, 0.250, 0.333). Our calculations show that the substitution of I− with Cl− or Br− enlarges the band gap of the material, decreases the mobility of carriers and reduces the performance of the perovskites. The calculated absorption spectra show that a blue-shift occurs for the Cl−/Br− substituted perovskite, and the absorption intensity decreases with the increase of y value. Sn atom in the (0 0 1) surface of CH3NH3Sn0.5Pb0.5I3−yXy (X = Cl, Br) is more attractive to the oxygen than the Pb atom, providing a reasonable explanation for the rapid oxidation of Sn2+ to Sn4+ observed experimentally. Unlike the O2 molecule, H2O molecule prefers to adsorb on the Pb atom of the surface. The adsorption of water is much stronger than that of oxygen. Water is a critical factor of causing the degradation of the perovskites solar cell.

Efficient Treatment of Large Active Spaces through Multi-GPU Parallel Implementation of Direct Configuration Interaction.

We have extended our graphical processing unit (GPU)-accelerated direct configuration interaction program to multiple devices, reducing iteration times for configuration spaces of 165 million determinants to only 3 s using NVIDIA P100 GPUs. Similar improvements in the one- and two-particle reduced density matrix formation allow for fast analytical energy gradients and electronic properties. Our parallel algorithm enables the calculation of arbitrarily large configuration spaces (limited only by available system memory), with iteration times of 13 min for an active space of 18 electrons in 18 orbitals (2.4 billion determinants) using six consumer grade NVIDIA 1080Ti GPUs. These advances enable routine molecular dynamics simulations, geometry optimizations, and absorption spectrum calculations for molecules with large configuration spaces, a task that has heretofore required massive computational effort. In this work, we demonstrate the utility of our program by generating the absorption spectrum for diphenyl acetylene at the floating occupation molecular orbital complete active space configuration interaction level of theory. Several active spaces were investigated to assess the dependence of spectral features on orbital space dimension.

Excitons in two-dimensional atomic layer materials from time-dependent density functional theory: mono-layer and bi-layer hexagonal boron nitride and transition-metal dichalcogenides.

Time-dependent density functional theory (TDDFT) has been applied to the calculation of absorption spectra for two-dimensional atomic layer materials: mono-layer and bi-layer hexagonal boron nitride (h-BN) and mono-layer transition metal dichalcogenides, MoS2 and MoSe2. We reveal that the character of the first bright exciton state of bi-layer h-BN is dependent on the layer stacking type through the use of many-body perturbation theory (MBPT) calculations, i.e., the electron and hole in the AA' stacking are present in the same layer (an intralayer exciton) while the A'B stacking exhibits an interlayer exciton. We demonstrate that the TDDFT approach with the meta-generalized gradient approximation to the exchange-correlation (XC) potential and the Bootstrap XC kernel can capture the characteristics of the absorption peaks that correspond to these excitons without computationally heavy GW and Bethe-Salpeter equation calculations. We also show that the TDDFT method can capture the qualitative features of the absorption spectra for mono-layer transition metal dichalcogenides, MoS2 and MoSe2, although the exciton binding energies are underestimated. This study elucidates the usefulness of the TDDFT approach for the qualitative investigation of the optical properties of two-dimensional atomic layer materials.

Electronic and photocatalytic properties of two-dimensional boron phosphide/SiC van der Waals heterostructure with direct type-II band alignment: a first principles study.

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as opening opportunities for applications in solar energy conversion and nanoelectronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of a boron phosphide–SiC (BP–SiC) vdW heterostructure using first-principles calculations. The relaxed configuration is obtained from the binding energies, inter-layer distance, and thermal stability. We show that the BP–SiC vdW heterostructure has a direct band gap with type-II band alignment, which separates the free electrons and holes at the interface. Furthermore, the calculated absorption spectra demonstrate that the optical properties of the BP–SiC heterostructure are enhanced compared with those of the constituent monolayers. The intensity of optical absorption can reach up to about 105 cm−1. The band edges of the BP–SiC heterostructure are located at energetically favourable positions, indicating that the BP–SiC heterostructure is able to split water under working conditions of pH = 0–3. Our theoretical results provide not only a fascinating insight into the essential properties of the BP–SiC vdW heterostructure, but also helpful information for the experimental design of new vdW heterostructures.

Calculation of vibrationally resolved absorption and fluorescence spectra of the rylenes.

A generating function method was used to simulate the vibrationally resolved absorption and emission spectra of perylene, terrylene and quaterrylene. This method operates on the basis of adiabatic excitation energies and electronic ground and excited state vibrational frequencies. These parameters were calculated using density functional theory with the PBE0 functional for perylene and terrylene and with the BH-LYP functional for quaterrylene. The vertical excitation energies of the lower excited states were calculated using functionals with differing amounts of Hartree-Fock exchange. The optimal functional for each molecule was chosen by comparing these energies to literature excitation energies. Using this technique the calculated absorption spectra and the calculated emission spectrum of perylene were found to be in excellent agreement with the literature experimental spectra after introducing a shift and a scaling factor. The most prominent bands of the absorption spectra were assigned to their respective vibronic transitions.