Overlap Of Electron Wave Functions Research Articles

π-π Stacking Modulation via Polymer Adsorption for Elongated Exciton Diffusion in High-Efficiency Thick-Film Organic Solar Cells.

Developing efficient organic solar cells (OSCs) with thick active layers is crucial for roll-to-roll printing. However, thicker layers often result in lower efficiency. This study tackles this challenge using a polymer adsorption strategy combined with a layer-by-layer approach. Incorporating insulator polystyrene (PS) into the PM6:L8-BO system creates PM6+PS:L8-BO blends, effectively suppressing trap states and extending exciton diffusion length in the mixed donor domain. Adding insulating polymers with benzene rings to the donor enhances π-π stacking of donors, boosting intermolecular interactions and electron wave function overlap. This results in more orderly molecular stacking, longer exciton lifetimes, and higher diffusion lengths. The promoted long-range exciton diffusion leads to high power conversion efficiencies of 19.05% and 18.15% for PM6+PS:L8-BO blend films with 100 and 300nm thickness, respectively, as well as a respectable 16.00% for 500nm. These insights guide material selection for better exciton diffusion, and offer a method for thick-film OSC fabrication, promoting a prosperous future for practical OSC mass production.

Unravelling the Interfacial Dynamics of Bandgap Funneling in Bismuth-Based Halide Perovskites.

An environmentally friendly mixed-halide perovskite MA3 Bi2 Cl9- x Ix with a bandgap funnel structure has been developed. However, the dynamic interfacial interactions of bandgap funneling in MA3 Bi2 Cl9- x Ix perovskites in the photoelectrochemical (PEC) system remain ambiguous. In light of this, single- and mixed-halide lead-free bismuth-based hybrid perovskites-MA3 Bi2 Cl9- y Iy and MA3 Bi2 I9 (named MBCl-I and MBI)-in the presence and absence of the bandgap funnel structure, respectively, are prepared. Using temperature-dependent transient photoluminescence and electrochemical voltammetric techniques, the photophysical and (photo)electrochemical phenomena of solid-solid and solid-liquid interfaces for MBCl-I and MBI halide perovskites are therefore confirmed. Concerning the mixed-halide hybrid perovskites MBCl-I with a bandgap funnel structure, stronger electronic coupling arising from an enhanced overlap of electronic wavefunctions results in more efficient exciton transport. Besides, MBCl-I's effective diffusion coefficient and electron-transfer rate demonstrate efficient heterogeneous charge transfer at the solid-liquid interface, generating improved photoelectrochemical hydrogen production. Consequently, this combination of photophysical and electrochemical techniques opens up an avenue to explore the intrinsic and interfacial properties of semiconductor materials for elucidating the correlation between material characterization and device performance.

Lieb Lattices Formed by Real Atoms on Ag(111) and Their Lattice Constant-Dependent Electronic Properties

Scanning tunneling microscopy is a powerful tool to build artificial atomic structures that do not exist in nature but possess exotic properties. In this study, we constructed Lieb lattices with different lattice constants by real atoms, i.e., Fe atoms on Ag(111), and probed their electronic properties. We obtain a surprising long-range effective electron wavefunction overlap between Fe adatoms as it exhibits a dependence with the interatomic distance r instead of the theoretically predicted exponential one. Combining control experiments, tight-binding modeling, and Green’s function calculations, we attribute the observed long-range overlap to being enabled by the surface state. Our findings enrich the understanding of the electron wavefunction overlap and provide a convenient platform to design and explore artificial structures and future devices with real atoms.

First-principles calculations of phonon transport across a vacuum gap

Phonon transport across a vacuum gap separating intrinsic silicon crystals is predicted via the atomistic Green's function method combined with first-principles calculations of all interatomic force constants. The overlap of electron wave functions in the vacuum gap generates weak covalent interaction between the silicon surfaces, thus creating a pathway for phonons. Phonon transport, dominated by acoustic modes, exceeds near-field radiation for vacuum gaps smaller than ~ 1 nm. The first-principles-based approach proposed in this work is critical to accurately quantify the contribution of phonon transport to heat transfer in the extreme near field.

Graphene Field-Effect Transistor on a Calcium Fluoride Substrate

We report the electrical-transport properties of a monolayer graphene device put on a 500 µm-thick calcium fluoride (CaF2) single-crystal substrate. From the gate-voltage dependence, clear n-type behavior was observed in a graphene/CaF2 field-effect transistor. This can be understood on the basis of the strong interaction between graphene and the CaF2 substrate, where the substantial electron wavefunction overlap between graphene and fluorine atoms of CaF2 occurs. This observation indicates that the strong n-type doping to graphene is possible only through the contact of graphene with the fluorinated substrate due to the strong electron affinity of a fluorine atom. Various electrical and optical characterizations for the graphene on CaF2 substrates are provided.

Angle-Resolved Photoemission Study on the Band Structure of Organic Single Crystals

Angle-resolved photoemission spectroscopy (ARPES) is a vital technique, collecting data from both the energy and momentum of photoemitted electrons, and is indispensable for investigating the electronic band structure of solids. This article provides a review on ARPES studies of the electronic band structure of organic single crystals, including organic charge transfer conductors; organic semiconductors; and organo-metallic perovskites. In organic conductors and semiconductors, band dispersions are observed that are highly anisotropic. The Van der Waals crystal nature, the weak electron wavefunction overlap, as well as the strong electron-phonon coupling result in many organic crystals having indiscernible dispersion. In comparison, organo-metallic perovskite halides are characterized by strong s-p orbitals from the metal and halide at the top of the valence bands, with dispersions similar to those in inorganic materials.

Layer-resolved magnetic proximity effect in van der Waals heterostructures.

Magnetic proximity effects are integral to manipulating spintronic1,2, superconducting3,4, excitonic5 and topological phenomena6-8 in heterostructures. These effects are highly sensitive to the interfacial electronic properties, such as electron wavefunction overlap and band alignment. The recent emergence of magnetic two-dimensional materials opens new possibilities for exploring proximity effects in van der Waals heterostructures9-12. In particular, atomically thin CrI3 exhibits layered antiferromagnetism, in which adjacent ferromagnetic monolayers are antiferromagnetically coupled9. Here we report a layer-resolved magnetic proximity effect in heterostructures formed by monolayer WSe2 and bi/trilayer CrI3. By controlling the individual layer magnetization in CrI3 with a magnetic field, we show that the spin-dependent charge transfer between WSe2 and CrI3 is dominated by the interfacial CrI3 layer, while the proximity exchange field is highly sensitive to the layered magnetic structure as a whole. In combination with reflective magnetic circular dichroism measurements, these properties allow the use of monolayer WSe2 as a spatially sensitive magnetic sensor to map out layered antiferromagnetic domain structures at zero magnetic field as well as antiferromagnetic/ferromagnetic domains at finite magnetic fields. Our work reveals a way to control proximity effects and probe interfacial magnetic order via van der Waals engineering13.

A coupled pseudoforce model for quantum plasmon excitations

A coupled nonlinear pseudoforce system is derived from hydrodynamic equations for the nonlinear electron plasma oscillations in an electron-ion plasma with arbitrarily degenerate electron fluid in a background fixed ion potential. The dynamics of the fully nonlinear electron plasma excitations are studied using this generalized coupled pseudoforce model including the damping and driving pseudoforce elements. The fully nonlinear plasma oscillations are shown to be of quite different nature in the extreme classical and quantum limits. In particular, high frequency quantum oscillations due to electron tunneling effect modulates the ordinary large amplitude low frequency periodic Langmuir excitations in the quantum plasma regime. It is shown that such characteristic quantum oscillations appear due to the electron wavefunction overlap which also appear on the quantum shock wave structures. It is found that the stimulation of dense plasmas with the driving frequency matching that of characteristic quantum oscillations leads to the chaotic behavior of the electron fluid. The presented coupled pseudoforce theory can be used to investigate a large number of interesting nonlinear phenomena such as the quantum electron fluid chaos and turbulence, autoresonance phenomenon, quantum harmonic generation, nonlinear resonances, and bistability of quantum electron plasma oscillations.

Influence of interparticle interaction on the structural, optical and magnetic properties of NiO nanoparticles

In this work, we have synthesized nickel oxide (NiO) nanoparticles using a simple wet chemical method. The synthesized nanoparticles were annealed at different temperatures ranging from 573 to 973 K. The structural, magnetic, and optical properties were characterized using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM), Superconducting Quantum Interference Device (SQUID), UV–VIS and Photoluminescence (PL) spectrophotometer. The optical properties depict that as NiO particles grow bigger, the intensity of the UV emission increases. The broad emission band can be attributed to the direct recombination of photogenerated charge carriers from the conduction band with the holes in the valence band. This kind of emission due to the transition of 3d8 electrons of the Ni2+ ions was also observed in the PL spectra. NiO nanoparticles exhibit antiferromagnetic behaviour with varied magnetic moment values of 3.08–5.72 μB. The ZFC curves of all samples show cusp and sharp decrease in magnetization at temperatures in the region of 10–21 K. This is due to the surface cluster spins whose thermal fluctuations freeze in a cluster-glass-like state. The bifurcation temperature between ZFC-FC curves increases with respect to the particle size due to the exchange field which is related to the overlap of electron wave functions of two magnetic ions (Ni2+). This field acts indirectly through the neighbouring anions O2−, as Ni2+ ions are too far apart for direct exchange forces mechanism called super exchange and originates from Pauli exclusion principle and electrostatic force.

The impact of quadrupole moment of 4f shell on the hyperfine interactions anisotropy in RAl2 (R=Sm, Tb) intermetallic compounds

Abstract The magnetic hyperfine interactions for 119 Sn impurity atoms in SmAl 2 and TbAl 2 ferromagnetic compounds have been investigated by Mossbauer spectroscopy technique. These compounds have the same structure but differ in the sign of the quadrupole moment of the R 3+ ion. In both cases, the spectrum contains two magnetic subspectra with the ratio of the intensities 1:3, which correspond to a and b Al sites with significantly different hyperfine parameters. The phenomenon change the order the component of inversion component of the Mossbauer spectra was found. This phenomenon is explained by the influence of the quadrupole moment 4f-shell of R 3+ ions on the electron density distribution in the valence band. The degree of overlap of electron wave functions being on hybrid orbitals greatly depends on the sign of the 4f-shell quadrupole moment, which gives rise to huge anisotropy in the hyperfine magnetic field and the electric field gradient. Quadrupole deformation induced by the 4f quadrupole moment and the electric field gradient, greatly affects the d-like and p-like components of the electron wave functions, but little effect on the its s-components.

Evidence in Support of Exciton to Ligand Vibrational Coupling in Colloidal Quantum Dots

The Perspective focuses on the investigation of an unresolved conflict in semiconductor colloidal quantum dots (CQDs) research, concerning the influence of the immediate surrounding on the optical properties of the materials. Today's advanced synthetic colloidal procedures offer formation of a high-quality inorganic crystallite, capped with various organic/inorganic molecular ligands. The Perspective aims to clarify whether exciton recombination processes in CQDs are influenced by the type of crystallite-ligand bonding and, moreover, whether these excitonic processes experience direct coupling to the ligands' vibrational modes. Most ligands used have redox characteristics whose functional groups are added on to the CQDs' surface via coordination, covalent or ionic bonding. The surface-ligand bonding introduces electronic states either above or below the intraband/interband energy gap, resulting in electronic passivation or in creation of trapping states that affect intraband and interband relaxation processes. Furthermore, crystalline electronic states may have a direct coupling to molecular vibrational states via direct overlap of electronic wave functions or through a long-range energy-transfer process. Also, photoejected carriers resulting from an Auger process or ionization processes may diffuse temporarily onto a ligand site. These scenarios are discussed in the current publication with supporting theoretical and experimental observations.

Green functions of graphene: An analytic approach

In this article we derive the lattice Green Functions (GFs) of graphene using a Tight Binding Hamiltonian incorporating both first and second nearest neighbour hoppings and allowing for a non-orthogonal electron wavefunction overlap. It is shown how the resulting GFs can be simplified from a double to a single integral form to aid computation, and that when considering off-diagonal GFs in the high symmetry directions of the lattice this single integral can be approximated very accurately by an algebraic expression. By comparing our results to the conventional first nearest neighbour model commonly found in the literature, it is apparent that the extended model leads to a sizeable change in the electronic structure away from the linear regime. As such, this article serves as a blueprint for researchers who wish to examine quantities where these considerations are important.

Integration of the Ferromagnetic Insulator EuO onto Graphene

We have demonstrated the deposition of EuO films on graphene by reactive molecular beam epitaxy in a special adsorption-controlled and oxygen-limited regime, which is a critical advance toward the realization of the exchange proximity interaction (EPI). It has been predicted that when the ferromagnetic insulator (FMI) EuO is brought into contact with graphene, an overlap of electronic wave functions at the FMI/graphene interface can induce a large spin splitting inside the graphene. Experimental realization of this effect could lead to new routes for spin manipulation, which is a necessary requirement for a functional spin transistor. Furthermore, EPI could lead to novel spintronic behavior such as controllable magnetoresistance, gate tunable exchange bias, and quantized anomalous Hall effect. However, experimentally, EuO has not yet been integrated onto graphene. Here we report the successful growth of high-quality crystalline EuO on highly oriented pyrolytic graphite and single-layer graphene. The epitaxial EuO layers have (001) orientation and do not induce an observable D peak (defect) in the Raman spectra. Magneto-optic measurements indicate ferromagnetism with a Curie temperature of 69 K, which is the value for bulk EuO. Transport measurements on exfoliated graphene before and after EuO deposition indicate only a slight decrease in mobility.

Deciphering the Influence of Short-Range Electronic Couplings on Optical Properties of Molecular Dimers: Application to “Special Pairs” in Photosynthesis

The excited states of chromophore dimers are, in general, delocalized, and the transition energies and transition dipoles are different from those of the monomers. The intermolecular interaction that is responsible for these effects has two contributions: Forster-type Coulomb coupling and a short-range coupling, which depends on the intermolecular overlap of electronic wave functions. The latter contains the Dexter-type exchange coupling and the coupling of excited states to intermolecular charge-transfer (CT) states. Recently, we developed a method (TrEsp) for an accurate and numerically efficient calculation of the Forster-type Coulomb part (Madjet et al. J. Phys. Chem. B 2006, 110, 17268). Here, we combine the latter with quantum chemical calculations to evaluate the short-range contribution, extending a method developed earlier by Scholes et al. (J. Phys. Chem. B 1999, 103, 2543). An effective two-state model is used, which relates the transition energies and transition dipole moments obtained by quantum chemical calculations of the monomers to those calculated for the dimer. From this relation, the short-range excitonic coupling and effective shifts of the local transition energies due to the coupling to intermolecular CT states can be inferred including a consistency check to evaluate quantum chemical methods that differ in the treatment of electron correlation. The method is applied to the special pairs of the reaction centers of purple bacteria (bRC) and photosystem I (PSI). We find that the short-range coupling represents the dominant contribution to the total excitonic coupling in both special pairs (80% in PSI and 70% in the bRC) and exhibits a monoexponential dependence on the distance between the pi-planes of the pigments with an attenuation factor of 2.8 A(-1). We obtain significant red-shifts of the local transition energies, which show a biexponential distance dependence with one attenuation factor being 2.8 A(-1) and another factor being in the range 0.3-0.7 A(-1) for PSI and 0.8-0.9 A(-1) for bRC. Both effects of the short-range coupling determine the excitation energy sink in the reaction centers at the special pairs.

Methods of Analysis for Imaging and Detecting Ions and Molecules

Abstract For imaging molecular processes in living cells, we developed novel intracellular fluorescent and bioluminescent indicators for second messengers, protein phosphorylation, protein/protein interactions, and protein localizations that work in single living cells. Key molecules and steps of cellular signaling pathways were visualized in target live cells under fluorescent microscopy using developed fluorescent indicators. A second new approach to molecular imaging is also described. Chemically modified STM tips (molecular tips) were developed for molecular imaging at the interfaces. The molecule-specific imaging is based on an increase in a tunneling current due to the overlap of electronic wave functions induced by the chemical interactions between tip and sample molecules. Chemically selective imaging using STM molecular tips can be described as intermolecular tunneling microscopy.

Theoretical Study of Oxygen Isotope Exchange and Quenching in the O(1D) + CO2 Reaction

Ab initio multireference configuration interaction calculations have been carried out for the CO3 system in singlet and triplet electronic states to investigate the mechanism of the O(1D) + CO2 reaction. The reaction has been shown to occur through the formation of an O−CO2 complex s0, which then isomerizes to a cyclic O(CO2) structure s1 over a barrier at s-TS0 located ∼0.3 kcal/mol below the reactants. The cyclic isomer s1, 48.8 kcal/mol lower in energy than O(1D) + CO2, can in turn rearrange to a D3h structure s2, only slightly higher in energy. The isomers s1 and s2 formed in the reaction possess high internal energy and can decompose into the initial reactants or undergo singlet−triplet intersystem crossing to form the triplet isomer t1, which can dissociate to O(3P) + CO2. If the attacking oxygen atom is isotope-labeled, isotope exchange can occur due to symmetry properties of s1 and s2. The ab initio energies, molecular parameters, and spin−orbit coupling constants were employed in statistical calculations of various isomerization and dissociation reaction rates. For intersystem crossing rates, we used the theory of radiationless transitions, in which the rates are determined as a product of the overlap of electronic wave functions (spin−orbit coupling) and the overlap of vibrational wave functions (Franck−Condon factor). The calculated rate constants were then used to compute the product branching ratios both for the case of the 16O(1D) + 44CO2 reaction and for the isotope-labeled 18O(1D) + 44CO2 reaction. For the latter, the calculated relative branching ratios of the 16O(1D) + 46CO2 and 16O(3P) + 46CO2 products at collision energies of 4.2 and 7.7 kcal/mol, 17/83 and 42/58, agree reasonably well with the experimental values of 16/84 and 33/67 (ref 4), respectively, and reproduce the qualitative trend with Ecol. The overall relative yields computed for 44CO2 and 46CO2 show that the attacking O(1D) atom can be incorporated into the product CO2 molecule with a near-statistical probability of 2/3.

Multiexcitons confined within a subexcitonic volume: Spectroscopic and dynamical signatures of neutral and charged biexcitons in ultrasmall semiconductor nanocrystals

The use of ultrafast gating techniques allows us to resolve both spectrally and temporally the emission from short-lived neutral and negatively charged biexcitons in ultrasmall (sub-10 nm) CdSe nanocrystals (nanocrystal quantum dots). Because of forced overlap of electronic wave functions and reduced dielectric screening, these states are characterized by giant interaction energies of tens (neutral biexcitons) to hundreds (charged biexcitons) of meV. Both types of biexcitons show extremely short lifetimes (from sub-100 picoseconds to sub-picosecond time scales) that rapidly shorten with decreasing nanocrystal size. These ultrafast relaxation dynamics are explained in terms of highly efficient nonradiative Auger recombination.

Seeing what was unseen: new analytical methods for molecular imaging.

For nondestructive analysis of chemical processes in living cells, we developed novel intracellular fluorescent indicators for second messengers, protein phosphorylation, and protein/protein interactions that work in single living cells. Key molecules and steps of cellular signaling pathways were visualized under a confocal laser microscope in target live cells using developed fluorescent indicators. A second new approach to molecular imaging is also described. When chemically modified tips were used for STM measurements, contrast enhancements at specific regions in the STM images occurred on the basis of hydrogen bond and metal-coordination interactions. This enabled us to detect not only the distribution of specific chemical species and functional groups but also the orientation of functional groups. The contrast enhancements reflect the increase in a tunneling current due to the overlap of electronic wave functions induced by the chemical interactions between tip and sample.

Uranium (and Cerium) Compounds At High Pressures and Magnetic Fields

ABSTRACTCorrelated-electron systems are so named due to strong interactions between electrons unlike traditional metals (e.g. copper) that have “free electrons” that interact very weakly. Knowledge of the Fermi surface, density of electron states and band structure are the starting points for a first-principles understanding of the electronic and electronically related macroscopic properties, e.g. equation of state. The use of high pressure and high magnetic fields to alter the electron-electron (hybridization) and electron-lattice interactions give us powerful tools to understand complicated rare earth and actinide correlated-electron systems and allows precise testing of experiment to theory. Correlated-electron systems yield a wide variety of ground states that are a direct result of the hybridization strength including: short and long range magnetic order, spin fluctuating, enhanced Pauli paramagnetism, heavy fermion behavior and superconductivity. We will review some results on U compounds in high magnetic fields and high pressures. By comparing the results to Ce compounds that have significantly more localized f electrons, the effect of direct 5f electron wavefunction overlap in U compounds can be discerned. Consequences on the search for U based heavy fermion superconductors will be discussed.

Electron–phonon coupling of Cr 3+ in YAG and YGG

The electron–phonon (EP) coupling of the 2 E→ 4 A 2 -emission from Cr 3+ has been measured for the aluminum garnet Y 3Al 5O 12 (YAG) and the gallium garnet Y 3Ga 5O 12 (YGG) using three different methods; determination of the vibronic transition probability A vib from the decay time τ and the ratio between the vibronic- and zero-phonon line intensity (1), temperature-dependent line-broadening resulting in a strength parameter α ̄ (2) and temperature-dependent line shift measurements, resulting in an electron–phonon coupling strength parameter α (3). All three methods show a stronger EP coupling for the gallium garnet YGG compared to the aluminium garnet YAG. This increase in EP coupling is explained by an increase in covalency in the gallium garnet compared to the aluminium garnet, which results in larger overlap of electron wave functions of the transition metal ion Cr 3+ with the ligands. Also lowering of the 4 T 2 state going from Cr 3+-doped YAG to YGG could give a contribution to the increase of EP coupling for YGG : Cr 3+.