Electronic Transitions Research Articles

Functional Nano-Metallic Coatings for Solar Cells: Their Theoretical Background and Modeling

We have collected theoretical arguments supporting the functional role of nano-metallic coatings of solar cells, which enhance solar cell efficiency via by plasmon-strengthening the absorption of sun-light photons and reducing the binding energy of photoexcitons. The quantum character of the plasmonic effect related to the absorption of photons (called the optical plasmonic effect) is described in terms of the Fermi golden rule for the quantum transitions of semiconductor-band electrons induced by plasmons from a nano-metallic coating. The plasmonic effect related to the lowering of the exciton binding energy (called the electrical plasmonic effect) is of particular significance for metalized perovskite solar cells and is also characterized in quantum mechanics terms. The coupling between plasmons in nanoparticles from a coating with band electrons in a semiconductor substrate significantly modifies material properties (dielectric functions) both of the particles and the semiconductor, beyond the ability of the classical electrodynamics to describe.

Unique Applications of para-Hydrogen Matrix Isolation to Spectroscopy and Astrochemistry.

Cryogenic solid para-hydrogen (p-H2) exhibits pronounced quantum effects, enabling unique experiments that are typically not possible in noble-gas matrices. The diminished cage effect facilitates the production of free radicals via in situ photolysis or photoinduced reactions. Electron bombardment during deposition readily produces protonated and hydrogenated species, such as polycyclic aromatic hydrocarbons, that are important in astrochemistry. In addition, quantum diffusion delocalizes hydrogen atoms in solid p-H2, allowing efficient H atom reactions with astrochemical species and introducing new concepts in astrochemical models. Some H atom reactions display anomalous temperature behaviors, highlighting the rich chemistry in p-H2. The investigation on quantum diffusion of heavier atoms and molecules is also important for our understanding of the chemistry in interstellar ice. Additionally, matrix shifts of electronic transitions of polycyclic aromatic hydrocarbons in p-H2 are less divergent than those in solid Ne such that systematic measurements in p-H2 might help in the assignment of diffuse interstellar bands.

Optical and surface properties of Schiff base ligands and Cu(II) and Co(II) complexes

Objectives. To study the transition of electrons in 1,2-phenyl(4’-carboxy)benzylidene Schiff base ligand and transition metal ions, optical properties, as well as the surface chemistry of supported transition metals using diffuse reflectance spectroscopy (DRS); to study the roughness and morphology of the Schiff base ligand and its complexes using atomic force microscopy (AFM).Methods. DRS, AFM, and Fourier-transform infrared spectroscopy instruments were used to identify electron transitions, optical properties, and surface morphology in Schiff base ligands and their complexes.Results. The DRS revealed the d–d transitions and charge transfer shifts of all compounds, and helped identify the structure of the ligand. One of the optical properties studied was the energy gap calculation of the ligand and its complexes. The copper complex exhibited more semiconducting behavior with surface morphology properties such as surface roughness parameters lower than those of the ligand and the cobalt complex. This can be attributed to the smaller size of the copper atom, as well as lower electron transitions compared to the cobalt complex and the square planar bonding shape.Conclusions. In Schiff base ligands, the reflectance spectrum bands reveal three electron transitions: n→π*, π→π*, and σ→σ* transitions. In cobalt complexes, four transitions are indicated: 4A2(F)→4T1(F), 4A2(F)→4T1(P), charge transfer bands, and tetrahedral geometry. Copper complexes exhibit three transitions: 2B1g→2A1g, 2B1g→2Eg, and charge transfer bands, with a square planar geometry for their structure. The energy gap calculations were 2.42, 2.29, and 2.30 eV, respectively. In the case of the SH ligands, copper complexes, and cobalt complexes, all compounds exhibited semiconductor properties. However, the complexes displayed increased conductivity due to the influence of the metal and coordination structure.

Observing vibronic coupling in a strongly hydrogen bonded system with coherent multidimensional vibrational-electronic spectroscopy.

The coupled structural and electronic parameters of intramolecular hydrogen bonding play an important role in ultrafast chemical reactions, such as proton transfer processes. We perform one- and two-dimensional vibrational-electronic (1D and 2D VE) spectroscopy experiments to understand the couplings between vibrational and electronic coordinates in 10-Hydroxybenzo[h]quinoline, an ultrafast proton transfer system. The experiments reveal that the OH stretch (νOH) is strongly coupled to the electronic excitation, and Fourier analysis of the 1D data shows coherent oscillations from the low frequency backbone vibrational modes coupled to the νOH mode, resulting in an electronically detected vibronic signal. In-plane low-frequency vibrations at 242 and 386 cm-1 change the hydrogen bond distance and modulate the observed electronic signal in the polarization-selective 1D VE experiment through orientation-dependent coupling with the νOH mode. Resolution of the excitation frequency axis with 2D VE experiments reveals that excitation frequency, detection frequency, and experimental delay affect the frequency and strength of the vibronic transitions observed. Our results demonstrate evidence of direct coupling of the high frequency νOH mode with the S1 ← S0 electronic transition in 10-Hydroxybenzo[h]quinoline (HBQ), and orientation-dependent couplings of the low-frequency 242 and 386 cm-1 modes to the νOH mode and the electronic transition. This demonstration of multidimensional VE spectroscopy on HBQ reveals the potential of using 1D and 2D VE spectroscopy to develop a quantitative understanding of the role of vibronic coupling in hydrogen bonding and ultrafast proton transfer for complex systems.

Excitation of Helium by Proton and Antiproton Impact

The electron transitions in atoms caused by charged particle impact are benchmarks for the study of electron dynamics. In the present paper we focus on the excitation of helium by proton and antiproton impact. We perform both ab initio and perturbational calculations, revealing the importance of electron correlations and higher-order effects. The influence of the projectile charge sign on the excitation cross section is also studied.

The A2Π–X2Σ+ Band System of ScO for M-type Star Spectral Analysis

M-type stars are the most abundant stars in our Galaxy. Their visible spectra are characterized by strong TiO and VO absorption bands with occasional ScO bands near 6000–6100 Å. This region corresponds to the A2Π–X2Σ+ band system and can be used to measure the abundance of ScO. In this work, a new Doppler-limited resolution spectrum of ScO obtained in a hollow-cathode experiment is presented. The A2Π–X2Σ+ electronic transition is analyzed with special care dedicated to fitting the A′2Δ(v+2) ∼ A2Π(v) perturbation. Spectroscopic constants of the A2Π(v) states are reported for v ≤ 9, and a partial fit of spectroscopic constants is made for the A′2Δ(v) states 2 ≤ v ≤ 6. The modeled interaction of the two states reproduces the experimental data remarkably well. New equilibrium constants are extracted from the fit. Band strengths are calculated with the help of a scaled transition dipole moment function, previously calculated with ab initio methods. Finally, a line list of the A2Π–X2Σ+ band system is provided, including Einstein A coefficients and oscillator strengths. Using this line list, the spectrum of ScO can be simulated for different temperature and pressure conditions in stellar atmospheres.

Structural consideration on Eu2+ distribution inside glaserite-type Na(Ba, Sr)PO4: Eu2+ phosphor: high-intensity and broadband emission consisting of three overlapping Eu2+ emission bands

Glaserite-type NaBaPO4: Eu2+ phosphor has attracted considerable attention because of the intense blue emission. Although this emission was discussed under the assumption that Eu2+ ions were distributed only in A- and B-sites of the crystal structure, this assumption has proved inconsistent with previous results and hindered the quantitative evaluation of the emission. This study aims to quantitatively interpret the emission characteristics in Eu2+-activated Na(Ba, Sr)PO4 solid solutions by tracking spectroscopic changes resulting from the modifications in the crystal structure when Ba2+ in NaBaPO4 is replaced by Sr2+. The broadband emission observed in Na(Ba, Sr)PO4: Eu2+ consists of three partially overlapping bands. Electron spin resonance (ESR) spectroscopy reveals that the three bands correspond to 4f65d1-4f7 interconfigurational electron transitions in crystallographically distinct three Eu2+. The distribution in three sites has not been observed in M3MgSi2O8 or A2MMgP2O8 (A: Li, Na, M: Ba, Ca, Sr), and specific to NaBaPO4 host. Because the emission wavelengths of the three Eu2+ ions are close, NaBaPO4: Eu2+ exhibits a high resultant emission intensity. Based on the Eu2+’s preference for sites with high Sr2+ occupancy, Eu2+ distribution and the associated Na(Ba, Sr)PO4: Eu2+ emission color can be tailored through Ba2+/Sr2+ replacement. The structure-emission relationship revealed in this study provides valuable insights for designing high-luminous-efficiency in glaserite-type phosphors.

Dramatic enhancement in lithium-ion battery capacity through synergistic effects of electronic transitions in light-assisted organic coordination cathode material Co(bpy)(dhbq)2

Dramatic enhancement in lithium-ion battery capacity through synergistic effects of electronic transitions in light-assisted organic coordination cathode material Co(bpy)(dhbq)2

Synthesis of CuOQDs/g-C3N4/C Composites Via Stem Template Induction and their Photocatalytic Properties

Introduction: The synthesis of bio-templated porous CuOQDs/g-C3N4/C composites with controllable morphology and suitable energy band structure was successfully carried out via a simple thermal condensation and hydrothermal method. Method: Dicyandiamide and cadmium chloride were selected as starting materials, while Hollyhock stem was chosen as the biological template. The results indicated that, in comparison to pure g-C3N4, g-C3N4/C had a rich porous structure and better-photogenerated carrier separation efficiency. The CuOQDs were anchored evenly on the surface of the g-C3N4, thus resulting in the formation of a greater number of reactive sites. Results: The type-Z heterojunction formed between the CuOQDs and g-C3N4/C reduced the energy required for electron transition, thereby facilitating the separation of photo-generated electronhole pairs. The highest photocatalytic degradation efficiency of CuOQDs/g-C3N4/C for tetracycline (TC) was 65.1%, which was 3.3 times that of pure g-C3N4. Conclusion: In the photocatalytic process, the main reactive species is O2−. The CuOQDs/g- C3N4/C synthesized by stem induction in multi-phase heterojunction form has a stable microstructure to improve the charge separation efficiency. Further, it represents practical photocatalytic environmental protection.

Electronic, thermodynamic, NLO, and spectroscopic properties of OAFLC compounds via DFT: A focus on 1F6B, 1F6Bi, 1F6BiN, and 1F6BN

This study presents a comprehensive quantum mechanical investigation of four novel liquid crystalline compounds: 1F6B, 1F6Bi, 1F6BiN, and 1F6BN. Using Density Functional Theory (DFT) with the B3LYP functional and 6-311G(d,p) basis set, we optimized the molecular geometries and analyzed their electronic and vibrational properties. Atomic Polar Tensor (APT) charge analysis showed significant charge distributions. Electrostatic potential (ESP) mapping identified potential reactive sites, while Electron Localization Function (ELF) and Localized Orbital Locator (LOL) analyses depicted the electron density distributions. Thermodynamic parameters indicated the compounds’ vibrational stability and responsiveness to thermal changes. Non-linear optical (NLO) properties were characterized by significant dipole moments and polarizabilities, suggesting potential in optoelectronic applications. Frontier molecular orbital analysis indicated stability with HOMO-LUMO gaps ranging from 4.15 to 4.64 eV. UV–Vis spectra revealed key electronic transitions, while Raman spectroscopy detailed various vibrational modes. The study offers valuable insights into the molecular behavior of these compounds, laying the groundwork for their future applications in material science and advanced technologies.

Vibrational assignments and energy analyses of 3,4-difluorophenylacetylene using resonance-enhanced multiphoton ionization and slow electron velocity-map imaging techniques

Here, we report spectroscopic investigations of 3,4-difluorophenylacetylene applying two-color resonant two-photon ionization (2C-R2PI) and slow electron velocity-map imaging (SEVI) techniques. With the assistance of density functional theory (DFT) calculations, vibrational modes of the S0 neutral ground state, S1 first electronic excited state, and D0 cationic ground state were assigned. The adiabatic excitation energy of the S1 ← S0 electronic transition was determined to be 35639 ± 5 cm−1 from the resonance-enhanced multiphoton ionization (REMPI) spectroscopy. Additionally, the adiabatic ionization potential was found to be 72398 ± 5 cm−1 based on the SEVI spectra.

Ultrafast Time-Domain Spectroscopy Reveals Coherent Vibronic Couplings upon Electronic Excitation in Crystalline Organic Thin Films.

The coherent coupling between electronic excitations and vibrational modes of molecules largely affects the optical and charge transport properties of organic semiconductors and molecular solids. To analyze these couplings by means of ultrafast spectroscopy, highly ordered crystalline films with large domains are particularly suitable because the domains can be addressed individually, hence allowing azimuthal polarization-resolved measurements. Impressive examples of this are highly ordered crystalline thin films of perfluoropentacene (PFP) molecules, which adopt different molecular orientations on different alkali halide substrates. Here, we report polarization-resolved time-domain vibrational spectroscopy with 10 fs time resolution and Raman spectroscopy of crystalline PFP thin films grown on NaF(100) and KCl(100) substrates. Coherent oscillations in the time-resolved spectra reveal vibronic coupling to a high-frequency, 25 fs, in-plane deformation mode that is insensitive to the optical polarization, while the coupling to a lower-frequency, 85 fs, out-of-plane ring bending mode depends significantly on the crystalline and molecular orientation. Comparison with calculated Raman spectra of isolated PFP molecules in vacuo supports this interpretation and indicates a dominant role of solid-state effects in the vibronic properties of these materials. Our results represent a first step toward uncovering the role of anisotropic vibronic couplings for singlet fission processes in crystalline molecular thin films.

Mott resistive switching initiated by topological defects

Avalanche resistive switching is the fundamental process that triggers the sudden change of the electrical properties in solid-state devices under the action of intense electric fields. Despite its relevance for information processing, ultrafast electronics, neuromorphic devices, resistive memories and brain-inspired computation, the nature of the local stochastic fluctuations that drive the formation of metallic regions within the insulating state has remained hidden. Here, using operando X-ray nano-imaging, we have captured the origin of resistive switching in a V2O3-based device under working conditions. V2O3 is a paradigmatic Mott material, which undergoes a first-order metal-to-insulator phase transition together with a lattice transformation that breaks the threefold rotational symmetry of the rhombohedral metallic phase. We reveal a new class of volatile electronic switching triggered by nanoscale topological defects appearing in the shear-strain based order parameter that describes the insulating phase. Our results pave the way to the use of strain engineering approaches to manipulate such topological defects and achieve the full dynamical control of the electronic Mott switching. Topology-driven, reversible electronic transitions are relevant across a broad range of quantum materials, comprising transition metal oxides, chalcogenides and kagome metals.

Synthesis, spectroscopic, computational, topology, and molecular docking studies on N,N'-(4-methyl-1,3-phenylene)bis(1-(2,4-dichlorophenyl)methanimine)

The study extensively examined N,N'-(4-methyl-1,3-phenylene)bis(1-(2,4-dichlorophenyl)methanimine) (6D). Advanced spectroscopic techniques, including IR, Raman, NMR, and UV-VIS spectroscopies, were employed to analyse the molecule. The Schiff base was ultimately confirmed using NMR spectrum analysis. The UV-VIS study revealed a notable bathochromic change in the compound, indicating their electronic transitions. By using the HOMO–LUMO bond gap the titled compound reactivity sites were identified. The compound 6D HOMO–LUMO band gap is 3.70 eV. Various wave function investigations like MESP, HOMO–LUMO, RDG, ELF, LOL, and ALIE were conducted to elucidate the distribution of electronic charge, providing valuable insights into the behaviour of molecules. The biological probability of the synthesised compound was extensively assessed using a docking study. Using MEP and HOMO–LUMO investigations, we confirm nucleophilic and electrophilic attacking sites. In the docking study, the protein Bovine cytochrome bc1 complex stigma Tellin bound (PDB ID–1PP9) was downloaded. The docking study identified the most stable minimum binding energy is –6.26 kcal/mol.

Optimization of Ultrafast Singlet Fission in One-Dimensional Rings Towards Unit Efficiency

Singlet fission (SF) is an electronic transition that in the last decade has been under the spotlight for its applications in optoelectronics, from photovoltaics to spintronics. Despite considerable experimental and theoretical advancements, optimizing SF in materials like multichromophoric systems and molecular crystals remains a challenge, due to the complexity of its analysis beyond perturbative methods. Here, we tackle the case of one-dimensional rings, aiming to promote singlet fission and prevent its backreaction. We study ultrafast SF nonperturbatively, by numerically solving a spin-boson model, via exact propagation and tensor network methods. By optimizing over a parameter space relevant to organic molecular materials, we identify two classes of solutions that can take SF efficiency beyond 85% in the nondissipative (coherent) regime, and to 99% when exciton-phonon interactions can be tuned. After discussing the experimental feasibility of the optimized solutions, we conclude by proposing that this approach can be extended to a wider class of optoelectronic optimization problems. Published by the American Physical Society 2024

High-pressure phase transitions of series of catenated nitrogen energetic crystals Nx (x = 4, 8, 10): A comparative DFT-D study.

High-pressure chemistry has advantages in exploring novel energetic materials and is the key to the development of new high-energy materials. The complexity and danger of experimental processes require a deeper understanding by advanced simulation techniques. Therefore, a high-precision comparative DFT-D study was performed to investigate the effect of pressure on series of catenated nitrogen energetic crystals. The results show that there exist phase transitions for N4, N8, and N10 at 4 GPa, 3 GPa, and 2 GPa respectively, which are embodied in various properties of these crystals. Studies on band gap and DOS indicate pressure-induced improvement on the ability for electrons transition from occupied orbitals to empty ones. Hirshfeld surface analysis qualitatively suggests that hydrogen bonding interactions are becoming dominant inter-molecular interactions. The topological analysis quantitatively reveals that pressure is beneficial to enhancing the inter-molecular hydrogen bonding energy, thereby playing an important role in the stability of high-pressure phases. The discussions on mechanical properties imply that pressure can improve the rigidity of these energetic systems and enhance their mechanical properties. Our findings evidence the high-pressure phase transitions for catenated nitrogen energetic crystals, which lay the theoretical foundation for the development of novel energetic materials. Series of catenated nitrogen energetic crystals N4, N8 and N10 were obtained from experiments. Optimizations were performed by GGA/PBE functional and G06 dispersion correction within the framework of CASTEP code, and the cutoff energies of the plane waves were set to 700 eV. The particular moiety in the crystals was extracted by Multiwfn 3.6 and subsequent analysis was conducted by Gaussian 09W package.

Demonstration of efficient Thomson cooler by electronic phase transition.

In the 1850s, Lord Kelvin predicted the existence of a thermoelectric cooling effect inside a whole material (the Thomson effect) according to thermodynamics1, in addition to the Peltier effect that enables cooling at the junction between dissimilar materials. However, the Thomson effect is usually negligible (ΔT/T < 2%) in conventional thermoelectric materials because the entropy change in charge carriers is fairly small2, leading to the guiding principles for advancing thermoelectric cooling to be based on the framework of the Peltier effect and that the figure of merit ZT should be maximized to optimize performance. Here, we demonstrate a Thomson-effect-enhanced thermoelectric cooler using a large Thomson coefficient (τ) induced by the direct manipulation of charge entropy through an electronic phase transition in YbInCu4. The devices achieve a steady temperature span (ΔT) of >5 K from T = 38 K. Our findings suggest not only another approach to advance thermoelectric coolers in addition to improving ZT but also technologically opens opportunities for solid-state cryogenic cooling applications.

Superorders and terahertz acoustic modes in multiferroic BiFeO3/LaFeO3 superlattices

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone, which gives rise to additional electronic bands and phonon modes. This phenomenon has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices. Here, we show that multiferroic BiFeO3/LaFeO3 superlattices exhibit several structural orders parallel and perpendicular to the growth direction, not existing in individual bulk materials. Using transmission electron microscopy, x-ray diffraction, and first-principles calculations, we reveal in particular a new long-range order of tilted FeO6 octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e., a superorder. The effect of this new structural order on the phonon dynamics is studied with ultrafast optical pump-probe experiments. While a folded-mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural out-of-plane superorder. Our work shows that multiferroic BiFeO3/LaFeO3 superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

Pressure induced tunable physical properties of cubic KHgF3 fluoro-perovskite: A first principle study

The structural, electronic, optical, mechanical, and thermodynamic properties of KHgF3 perovskite under various hydrostatic pressures are examined in this study in the frameworkof Density Functional Theory (DFT) with the Generalized Gradient Approximation (GGA) Perdew-Burke-Ernzerhof (PBE) exchange–correlation function. Tunable physical properties have been observed in KHgF3 as pressure is incrementally applied. The investigated compound displays mechanical instability beyond the pressure range of 0 to –10 GPa, as confirmed by the elastic constant analysis. Under ambient pressure, the lattice constants of KHgF3 closely align with theoretically calculated values, affirming the accuracy of this investigation. In addition, the bandgap of KHgF3 narrows with increasing pressure, facilitating easier electron transition from the valence band to the conduction band. The study reveals that when subjected to different hydrostatic pressures, fluctuations are observed in the peaks of the conductivity spectrum, loss spectrum, and absorption spectrum, particularly shifting towards higher photon energies, as analyzed through the optical properties. Moreover, KHgF3 exhibits ductile behavior at 0 GPa pressure, and this ductility rises with pressure. Thermodynamic analysis reveals a decrease in Debye temperature with increasing pressure, while the melting temperature rises correspondingly. The findings of this investigation also imply that KHgF3 can be a good candidate for optoelectronic device application under different hydrostatic pressures.

Investigating the Potential of Cr3+-Doped Pyroxene for Highly Sensitive Optical Pressure Sensing.

Luminescent manometry has gained significant popularity in recent years due to its capability to provide in situ pressure measurements in a remote manner. Therefore, there is a growing need to identify phosphors with pressure-dependent spectroscopic properties that can be utilized to develop highly sensitive pressure sensors operating over a wide pressure range. Hence, we present a novel temperature-invariant luminescent manometer based on Cr3+ ion emission in pyroxene Ca0.8Sr0.2MgSi2O6:Cr3+. We utilized two readout modes, including an innovative luminescent pressure sensing ratiometric approach based on the broad emission band associated with the 4T2g → 4A2g electronic transition of Cr3+ ions. This approach provided an exceptionally high sensitivity of SR = 50.7 ± 0.5% GPa-1 and ensured temperature-independent pressure measurements, thus offering highly reliable readouts. Furthermore, the proposed readout mode, which leverages changes in luminescence kinetics, demonstrated high sensitivity at high pressure at around 5 GPa (SR ∼ 8 ± 0.2% GPa-1) surpassing the performance of luminescence kinetics-based manometers reported to date. Consequently, Ca0.8Sr0.2MgSi2O6:Cr3+ emerges as a highly promising phosphor with significant application potential for pressure sensing across a broad pressure range.