Donor-acceptor Transitions Research Articles

CALCULATION of HOMA AND BIRD AROMATICITY INDICES, NLO and NBO PROPERTIES OF BENIDIPINE

Benidipine hydrochloride, being the derivate of 1,4-dihydropyridine is a calcium channel blocker antihypertensive drug. The aromaticity, natural bond orbital (NBO) and Nonlinear Optical (NLO) parameters properties of Benidipine (BEN) and BENHCl compounds were theoretically investigated using density functional theory (DFT) electronic structure method. Among the values of common aromaticity indices, the Harmonic Oscillator Aromaticity Model (HOMA) and BIRD's (Aromaticity Index) for electronic delocalization show that the aromaticity of the nitrophenyl group is higher in BEN and BENHCl compounds, and also the aromaticity in the gas phase is higher than that in water, octanol and DMF phase. In compound BEN, it was observed that as the dielectric constant of the medium increased, the aromaticity difference increased slightly. In the compound BEN, the correlation coefficient between HOMA values and aromatic fluctuation index (FLU), para delocalization index (PDI) and para-linear response (PLR) for the nitrophenyl group is higher than that the phenyl group. Donor-acceptor transitions, stabilization energies, intramolecular charge transfer were determined by natural population analyses. NLO parameters such as dipole moment, polarisibility and first order hyperpolarizability values of the BEN and BENHCl compound were also studied.

Synthesis, Theoretical studies, and antibacterial activity of Novel Zn(II) metal complex with salicylaldehyde semicarbazone, and ortho phenylene diamine ligands.

Computational and experimental investigation of synthesized Zn(II) complex. Salicylaldehyde semicarbazone (SSC) and ortho phenylene diamine (OPDA) were the ligands for the preparation of Zn(II) metal complex. Using quantum chemistry simulations at DFT/B3LYP/LANL2DZ level of theory. FT-IR, UV, and Mass spectrometry methods were characterized the complex. The computed small gap energy demonstrated that the Zn(II) molecule had good biological activity. Measured dipole moment and first-order hyperpolarizability values were useful to investigate the complex NLO activity. Intra-molecular charge transfer, donor-acceptor transitions, and stabilization energies were determined by analyzing natural bond orbital (NBO) and Mulliken charges. Electrophilic and nucleophilic interactions were performed with the MEP map. QTAIM framework explained the nature of different intermolecular interactions within the molecule. Moreover, the ADMET studies of Zn(II) compound were also analyzed. The title molecule showed good antibacterial activity.

Comprehensive structural characterization and Quantum mechanical exploration of o-phenylenediamine Dihydrogenphosphate: An integrated theoretical and experimental approach

Inorganic-organic hybrid materials merge the best of both inorganic and organic properties, enabling enhanced performance in diverse applications. The present study elucidates the crystallographic attributes of O-Phenylenediamine Dihydrogenphosphate (OPDP; C6H11N2O4P), crystallized in an ethanol medium. Characterized by a triclinic lattice, the asymmetric unit contains a single molecule (Z'=0), while the unit cell is comprised of eight molecules (Z = 2). The overall architecture is connected by intermolecular NH···O hydrogen bonds, which give rise to a bi-dimensional supramolecular framework. To substantiate the experimental findings and to probe the molecule's particular attributes, Fourier Transform Infrared Spectroscopy was deployed, owing to its sensitivity to inter/intra-molecular interactions within crystal systems. Density Functional Theory (DFT) calculations were performed using B3LYB functional, yielding molecular structure optimization, vibrational frequencies, and molecular properties such as Mulliken atomic charge distribution and frontier molecular orbital. Functional groups were discerned using FTIR spectroscopy and compared with theoretical predictions. The molecule features fourteen HH and three OH bonds. Natural Bond Analysis (NBO) was conducted to evaluate stabilization energy in donor–acceptor transitions. Surface morphology was examined using Scanning Electron Microscopy (SEM) in conjunction with Energy-Dispersive X-ray Spectroscopy (EDX). This comprehensive study advances our understanding of the unique structural and molecular properties of OPDP, demonstrating its potential for multifaceted applications.

Structure, chemical reactivity, NBO, MEP analysis and thermodynamic parameters of pentamethyl benzene using DFT study

This study explores the structural, molecular and electronic properties of the pentamethyl benzene (PMB) using quantum chemical calculations employing DFT/ B3LYP/6–311++G(d,p) level of theory. Structural parameters, HOMO-LUMO energies, chemical reactivity descriptors and molecular electrostatic potential (MEP) were evaluated from the optimized molecular geometry. The computed small band gap energy, correlated with the UV–visible spectrum, demonstrates that the test molecule has good biological activity. NLO activity of the molecule was also studied by evaluating dipole moment and first order hyperpolarizability values. Intra-molecular charge transfer, donor-acceptor transitions and stabilizations energies were determined from the analyses of natural bond orbital (NBO) and natural population. 1H and 13C chemical shifts were computed from the simulated NMR spectrum following GIAO approach in DMSO‑d6 solvent at the same level of DFT. Furthermore, thermodynamic parameters and rotational constants were also calculated for this molecule.

Photoluminescence emission in the red band at low temperatures in type-island layers of the Al0.32Ga0.68 As/Al0.29Ga0.71As/GaAs structure obtained via liquid phase epitaxy, and its description by the mechanism of Stranski-Krastanov growth

Type-island layers for the Al0.32Ga0.68 As/Al0.29Ga0.71As/GaAs structure were grown on GaAs substrates at 600 °C, via liquid phase epitaxy with a growth rate of 0.5 °C/min for 4 min. Air leaks, as well as severe temperature gradients across the graphite sliding boat, were related to the discontinuous growth (type-island). On the other hand, the mechanism of Stranski-Krastanov growth was used to describe the growth of the type-island layers. High-resolution x-ray diffraction showed a minimal lattice mismatch between layers and substrate with a value of 0.0371%. Scanning electron microscopy showed a type-island rectangular shape with a size of 2.059 μm in width and 3.586 μm in large, with an average thickness of 291 nm. Photoluminescence studies at low temperature and room temperature demonstrated energy emission located in the red band at 1.76 eV (703 nm), which was related to free-to-bound and donor-acceptor transitions. I–V measurements for the Al0.32Ga0.68As/Al0.29Ga0.71As/GaAs structure showed the typical curve for light emitting diode, with a threshold voltage of 0.429 V.

Low-temperature deposition of Cd1-xZnxTe layers by laser sputtering and their physical properties

CdZnTe films were grown by the method of modulated infrared laser deposition at a substrate temperature Tsub ≤ 1200C from appropriate sources on oriented single-crystal substrates Si, GaAs, InSb in the same technological conditions in one technological cycle. Surface morphology and spectra of low-temperature photoluminescence (T = 4.2K) in the energy range from 1.30 to 1.70 eV were studied. Luminescence spectra were analyzed and presented from three different energy regions: from 1.70 eV to 1.60 eV with exciton emission, from 1.60 eV to 1.55 eV by donor-acceptor transitions (DAP) and region A-centers from 1, 55 to 1.40 eV. The presence in the low-temperature photoluminescence spectra of free exciton bands, excitons on the neutral acceptor and neutral donor, and their phonon replicas on CdZnTe/InSb films testifies to the high structural perfection inherent in materials of detector quality with composition corresponding of the CdZnTe-target.

Direct observation of site-specific dopant substitution in Si doped (Al x Ga1− x )2O3 via atom probe tomography

In this work, the interaction of n-type dopants in Si doped (Al x Ga1−x )2O3 films with varying Al content over the entire composition range (x = 0%–100%) was analyzed using atom probe tomography. An almost uniform dopant distribution with dopant density in the range of 1018 cm−3 was obtained in (Al x Ga1−x )2O3 layers with Al contents, x < 0.60. At x , small Si segregated zones were observed which was attributed to the increased donor-acceptor transition centers or bond length differences in Al–O, Ga–O and Si–O at high Al content. We have demonstrated that for the single phase β-(Al x Ga1−x )2O3 films with Al content of x < 0.30, dopants prefer to occupy on Ga sites while Al site is preferred for high Al content (x > 0.50) (Al x Ga1−x )2O3 layers. It was also observed for Al content, x = 0.30–0.50, no specific cationic site occupancy was observed, Si occupies either Al or Ga sites. This can be attributed to highly inhomogeneous layers within this Al composition range due to which dopant Si atoms are either in the Al-rich or Al-depleted regions.

Property manipulation through pulsed laser annealing in high dose Mg-implanted GaN

The generation of p-type GaN through ion implantation is an attractive proposition in the massive production of GaN-based bipolar devices, whereas the removal of implantation induced lattice disturbances and defects is a difficult exercise and hampers the conversion of conductivity in GaN. Pulsed laser annealing is an effective annealing technique to recover lattice crystallinity and activate dopants with the preserved implanted profile. In this work, the effect of pulsed laser annealing on structural and optical recovery in high-dose magnesium (Mg) ion-implanted GaN has been investigated. The structural evolution and vibrational dynamics indicate an obvious structural recovery and partial strain release of Mg-implanted GaN during the pulsed laser annealing process, with a threshold laser fluence of 400 mJ/cm2, while rough surface structures are a result of the regrowth mechanism similar to liquid phase epitaxy. The enhanced donor–acceptor transition at 3.35 eV after pulsed laser irradiation is a sign of the effective activation of Mg from interstitial sites into the substitution of Ga ions. These results suggest that further optimization of the laser annealing technique has promising potential to manipulate the p-type conductivity of Mg-implanted GaN and to be implemented in GaN bipolar devices for practical applications.

Donor-acceptor nature of orange photoluminescence in AlN

Recombination dynamics, photoluminescence and photoluminescence excitation spectra have been investigated for 1.9 eV photoluminescence band in AlN in the temperature range of 5–650 K. The recombination dynamics for the 1.9 eV photoluminescence band has been described by a model of donor-acceptor recombination with taking into account a broadening due to electron coupling with local lattice vibrations of a deep level defect. The experimental results have been compared with density functional theory calculations of luminescence peak energies and line shapes of band to defect and donor-acceptor transitions, and possible origin of the orange photoluminescence band in AlN has been discussed.

Luminescence line shapes of band to deep centre and donor–acceptor transitions in AlN

Energy structure and electron coupling with local lattice vibrations have been investigated for deep centres in AlN using hybrid functional density functional theory. Local phonon energies and Huang–Rhys parameters have been calculated for defects and defect complexes containing most common unintentional impurities of carbon, oxygen and silicon, and for intrinsic vacancies, nitrogen split-interstitial defect, and complexes of Al and N vacancies in AlN. Luminescence line shapes of band to deep centre transitions in AlN have been calculated in dependence on temperature for most abundant defects in AlN. Donor–acceptor luminescence line shapes for shallow donor to deep acceptor and deep donor to deep acceptor transitions have been considered theoretically. Configuration diagrams of oxygen and silicon DX centres have been calculated by density functional theory with hybrid functional, and peak energies of optical transitions of an electron from the DX-centres to deep acceptors have been estimated. Possible assignments of the experimental luminescence bands in AlN based on the calculations have been discussed.

Phonon coupling and shallow defects in CuInS2

$\mathrm{Cu}(\mathrm{In},\mathrm{Ga}){\mathrm{S}}_{2}$ is an emerging material for solar cells, reaching already efficiencies above 15%. For any solar cell material, it is essential to understand the recombination processes and doping behavior of its defects. Therefore, in this work, we present a detailed photoluminescence and admittance investigation of polycrystalline $\mathrm{CuIn}{\mathrm{S}}_{2}$ thin films to study the shallow intrinsic defects. We find evidence for two acceptors, 105 and 145 meV from the valence band, their predominance depending on the Cu excess, plus one donor, 30 meV from the conduction band. The high crystalline and electronic quality of our samples allows the detection of low intensity luminescence peaks. Based on these emissions we can analyze the phonon coupling of the observed defects and show that an emission previously attributed to a second donor is in fact a phonon replica of the first donor-acceptor transition.

Modulating donor-acceptor transition energies in phosphorus-boron co-doped silicon nanocrystals via X- and L-type ligands.

In this work, we explore the effect of ligand binding groups on the visible and NIR photoluminescent properties within phosphorus-boron co-doped silicon nanocrystals (PB:Si NCs) by exploiting both the X-type (covalent) and L-type (Lewis donor molecule) bonding interactions. We find that the cooperative nature of both X- and L-type bonding from alkoxide/alcohol, alkylamide/alkylamine, and alkylthiolate/alkylthiol on PB:Si NCs results in photoluminescence (PL) energy blue shifts from the as-synthesized, hydride-terminated NCs (PB:Si-H) in excess of 0.4 eV, depending on the surface termination. These PL blue shifts appear greatest in the most strongly confined samples with diameters <4 nm where the surface-to-volume ratio is high and, therefore, the ligand effects are most pronounced. A correlation between the donor group strength (either X-type or L-type) and the degree of D-A state modulation is found, and the proportion of the PL blue shift from the X- and L-type interactions is quantified. Raman spectroscopy is used to provide additional evidence of the strength of the L-type donor groups. Additionally, we probe how the nature of the ligand chemistry affects the radiative lifetime and PL efficiency and find that the ligands do not significantly change the D-A emission dynamics, and all samples retain the long 50-130 μs lifetimes characteristic of these transitions. Finally, we describe three mechanisms that operate to affect the D-A recombination energies: (1) X-type ligands that modulate the PB:Si-X NC wavefunction; (2) L-type ligands that perturb the donor and acceptor states via a molecular orbital theory picture; and (3) X- and L-type ligands that cause a dielectric increase around the PB:Si NC core, which provides Coulomb screening and modulates the donor and acceptor states even further.

Donor–acceptor pair emission via defects with strong electron–phonon coupling in heavily doped AlxGa1−xN:Si layers with Al content x > 0.5

We report the results of time-resolved and temperature-dependent stationary photoluminescence investigations of the defects responsible for emission in the visible spectral range in heavily silicon-doped AlxGa1–xN layers grown by molecular beam epitaxy on sapphire substrates. The emission band was attributed to donor–acceptor transitions. The transitions were described using the one-dimensional configuration coordinate model taking into account the high-doping regime. An increase in Al content from 0.56 to 1 leads to an increase in the acceptor ionization energy from 1.4 to 1.87 eV. The value of the Franck–Condon shift is about 1 eV at x = 0.56–0.74 and decreases to 0.8 eV at x > 0.74. The changes in the donor–acceptor transition energy parameters with increasing silicon concentration are discussed.

Determination of the energy structure of recombination centers in heavily doped AlxGa1-xN:Si epitaxial layers with x > 0.5

The photoluminescence properties of the intensive defect-related emission in heavily doped AlxGal-xN:Si layers with x > 0.5 have been investigated by photoluminescence (PL) spectroscopy. The PL band in AlN was attributed to donor-acceptor (DA) transitions. At the lowest Al content, the impurity band merges with the conduction band and DA transitions are replaced by electron-acceptor transitions involving the same acceptor. The energy structure of recombination centers was obtained using the model of configuration coordinates for Al0.67Ga0.33N.

Fabrication of cerium-doped β-Ga2O3 epitaxial thin films and deep ultraviolet photodetectors.

High-quality cerium-doped β-Ga2O3 (Ga2O3:Ce) thin films could be achieved on (0001)α-Al2O3 substrates using a pulsed-laser deposition method. The impact of dopant contents concentration on crystal structure, optical absorption, photoluminescence, and photoelectric properties has been intensively studied. X-ray diffraction analysis results have shown that Ga2O3:Ce films are highly (2¯01) oriented, and the lattice spacing of the (4¯02) planes is sensitive to the Ce doping level. The prepared Ga2O3:Ce films show a sharp absorption edge at about 250nm, meaning a high transparency to deep ultraviolet (DUV) light. The photoluminescence results revealed that the emissions were in the violet-blue-green region, which are associated with the donor-acceptor transitions with the Ce3+ and oxygen vacancies related defects. A simple DUV photodetector device with a metal-semiconductor-metal structure has also been fabricated based on Ga2O3:Ce thin film. A distinct DUV photoresponse was obtained, suggesting a potential application in DUV photodetector devices.

Statistical approach to tunneling time in attosecond experiments

Tunneling, transport of particles through classically forbidden regions, is a pure quantum phenomenon. It governs numerous phenomena ranging from single-molecule electronics to donor–acceptor transition reactions. The main problem is the absence of a universal method to compute tunneling time. This problem has been attacked in various ways in the literature. Here, in the present work, we show that a statistical approach to the problem, motivated by the imaginary nature of time in the forbidden regions, lead to a novel tunneling time formula which is real and subluminal (in contrast to various known time definitions implying superluminal tunneling). In addition to this, we show explicitly that the entropic time formula is in good agreement with the tunneling time measurements in laser-driven He ionization. Moreover, it sets an accurate range for long-range electron transfer reactions. The entropic time formula is general enough to extend to the photon and phonon tunneling phenomena.

Properties of intensive defect-related band in photoluminescence spectra of heavily doped AlxGa1-xN:Si layers

We report photoluminescence investigations of heavily doped AlxGa1-xN:Si films grown by molecular beam epitaxy on sapphire substrates. The wide intensive defect-related band dominates in the photoluminescence spectra of AlxGa1-xN:Si films with the Al content higher than 0.38 covering the whole visible spectral range. This band is attributed to donor-acceptor and free electron-acceptor transitions involving the same acceptor. The acceptor ionization energy of about 1.87 eV for heavily doped AlN:Si was obtained, decrease of Al content leads to decrease of the acceptor ionization energy. The donor was assigned to the Si atom on the Ga/Al site; the acceptor might be (0/-) transition level of the CN or (2-/3-) transition level of the VAl.

Characterization of Co-existing In2O3-ZnO Nanostructures

In this study, we report the simultaneous growth of both In2O3 and ZnO nanostructures on the same substrate when attempting to achieve heavily Zn-doped In2O3 nanowires with Zn. These oxide structures were synthesized by vapor–liquid–solid growth. Scanning electron microscope imaging and transmission electron microscopy shows the presence of nanostructures with different morphologies while energy-dispersive x-ray and x-ray photoelectron spectroscopy study confirms the elemental structure. Room-temperature photoluminescence (PL) study reveals the presence of oxygen vacancies and surface defects in the structures. Emission related to free and bound excitons as well as the donor–acceptor transitions were observed using temperature-dependent PL. Raman spectroscopy measurement using 442-nm non-resonant excitation sources shows the presence of four phonon modes associated with a cubic In2O3 lattice structure along with the observation of the dominant E 2 high and quasi-longitudinal optical phonon modes associated with wurtzite ZnO. We find that the introduction of high zinc content results in the formation of ZnO nanowires in addition to the In2O3 nanowires, and not the formation of a highly doped In2O3 nanowires.

Nature of intensive defect-related broadband luminescence of heavily doped AlxGa1-xN:Si layers

We report photoluminescence investigations of heavily doped AlxGa1-xN:Si films grown by molecular beam epitaxy on sapphire substrates. The wide intensive defect-related band dominates in the photoluminescence spectra of AlxGa1-xN:Si films with the Al content higher than 0.46 covering the whole visible spectral range. This band is attributed to donor-acceptor transitions. The acceptor ionization energy of about 1.87 eV for heavily doped AlN:Si was obtained, decrease of Al content leads to decrease of the acceptor ionization energy. The donor was assigned to the Si atom on the Ga/Al site; the acceptor might be the (2-/3-) transition level of the VAl.

Investigating State Restriction in Fluorescent Protein FRET Using Time-Resolved Fluorescence and Anisotropy.

Most fluorescent proteins exhibit multiexponential fluorescence decays, indicating a heterogeneous excited state population. FRET between fluorescent proteins should therefore involve multiple energy transfer pathways. We recently demonstrated the FRET pathways between EGFP and mCherry (mC), upon the dimerization of 3-phosphoinositide dependent protein kinase 1 (PDK1), to be highly restricted. A mechanism for FRET restriction based on a highly unfavorable κ2 orientation factor arising from differences in donor–acceptor transition dipole moment angles in a far from coplanar and near static interaction geometry was proposed. Here this is tested via FRET to mC arising from the association of glutathione (GSH) and glutathione S-transferase (GST) with an intrinsically homogeneous and more mobile donor Oregon Green 488 (OG). A new analysis of the acceptor window intensity, based on the turnover point of the sensitized fluorescence, is combined with donor window intensity and anisotropy measurements which show that unrestricted FRET to mC takes place. However, a long-lived anisotropy decay component in the donor window reveals a GST-GSH population in which FRET does not occur, explaining previous discrepancies between quantitative FRET measurements of GST-GSH association and their accepted values. This reinforces the importance of the local donor–acceptor environment in mediating energy transfer and the need to perform spectrally resolved intensity and anisotropy decay measurements in the accurate quantification of fluorescent protein FRET.