Electron-vibrational Interaction Research Articles

Electron transport through degenerate electron level in single-molecular junction in the presence of electron-vibrational coupling and attractive electron-electron correlations

Electron transport through a molecular junction consisted of a single molecule coupled to macroscopic leads is studied in the nonequilibrium regime. The molecule is modeled as a degenerate energy level with an electron-vibrational interaction and attractive electron-electron correlation which lifts the degeneracy. The level occupancy and differential conductance are calculated in the antiadiabatic regime with an account of molecule-lead coupling at low temperature and finite voltage. The method of transport spectroscopy made it possible to determine through which of the split levels tunneling takes place depending on the applied voltages and to estimate the current through these levels. The cases of a negative differential conductance are discussed. The influence of the electron-vibrational coupling of different strength on electron tunneling through 2- and 4-fold degenerate level on the molecule in the presence of the attractive electron-electron interaction is analyzed.

Photoluminescence features of nickel-nitrogen complexes in Ib HPHT diamond matrix

The multiple color centers based on nickel-nitrogen complexes in synthetic Ib HPHT diamond matrices were optically characterized with an accent on a comparative analysis of temperature dependencies of photoluminescence spectra of two different Ni-N complexes with the phononless electronic transitions at 1.56 eV and 1.66 eV. The comparison shows that the intensities, widths, and energies of the bands of electronic and vibronic optical transitions, as well as temperature dependences of these parameters, measured in temperature range of 77–300 K, are essentially differ due to distinction in electron-vibrational interaction in the complexes.

Luminescence and energy transfer of Ce3+, Eu2+ co-doped Ba2Mg(BO3)2 for potential anti-counterfeiting applications

The phosphors Ba2Mg(BO3)2 activated with Ce3+ and Eu2+ ions are prepared by a solid-state reaction method at high-temperature and the Rietveld structure refinement of the host compound is performed. The synchrotron radiation VUV - UV excitation, UV – vis emission and luminescence decay spectra, as well as the dependencies of luminescence intensity and lifetime on doping content and temperature are systematically investigated. In addition, the electron-vibrational interaction (EVI) of Eu2+ ions with coordination surroundings and the Ce3+→Eu2+ energy transfer (ET) dynamics are studied in detail. Based on above discussion, the potential applications of co-doped phosphors in excitation wavelength and temperature dual-response optical anti-counterfeiting are demonstrated.

EPR, optical and thermometric studies of Cr3+ ions in the α-Al2O3 synthetic single crystal

The optical spectra of a single crystal of α-Al2O3Cr3+ were studied in a wide temperature range. The crystal was demonstrated to have a potential for optical thermometric applications. Three ways of measuring temperature were tested and analyzed: i) luminescence intensity ratio of the R1 and R2 lines; ii) change of the full width of half maxima of the R1 and R2 lines, and iii) thermal shifts of the R1 and R2 lines maxima. The highest absolute and relative sensitivities were obtained at cryogenic temperatures. In addition, the thermal shifts of the R lines were analyzed using the McCumber-Sturge equation to estimate the Debye temperature and the electron-vibrational interaction parameter.

Photoluminescence and electron-vibrational interaction in 5d state of Eu2+ ion in Ca3Al2O6 down-conversion phosphor

In the present work, a series of Eu2+ activated Ca3Al2O6 down-conversion phosphors have been successfully prepared via solid-state reaction method at 800 °C. Under 334 nm excitation, PL emission spectra shows broad band emission with the peak centred at 445 nm. This broad band emission is assigned to the allowed 4f65d1 → 4f7 electronic transition of Eu2+ ions. The highest emission intensity was observed for Ca3Al2O6: 0.5 mol%Eu2+ phosphor. The CIE chromaticity diagram shows high colour purity and excellent chromaticity coordinates. This CIE diagram reveals (0.2127, 0.1102) chromaticity coordinates in the blue region with 84.21% colour purity. Further, the electron-vibrational interaction parameters such as Stokes shift, Huang-Rhys factor, effective phonon energy, zero phonon line position, and red shift have been calculated using a spectrum fitting method. These results indicate that the Eu2+ activated Ca3Al2O6 down-conversion phosphor has great potential in the field of white LEDs.

Blue-emitting Ca3Mg3(PO4)4:Eu2+ phosphor: Study of electron-vibrational interaction in the 5d states of Eu2+ ions

A blue-emitting Eu2+-doped Ca3Mg3(PO4)4 phosphor was prepared by the polyethylene glycol assisted sol-gel synthesis method. The prepared phosphor was analyzed by using X-ray diffraction and scanning electron microscopy for the investigation of its structural and morphological features. From the photoluminescence (PL) excitation and emission spectra, it was found that the prepared phosphor emits blue light under ultraviolet and near-ultraviolet excitation. Temperature-dependent PL spectra and PL lifetime decay measurements were also executed for determining the detailed features of the Eu2+ emission in this phosphor. Under 325 nm excitation, the phosphor demonstrated excellent thermal stability with the activation energy for thermal quenching of 0.291 eV. The electron-vibrational interaction in the 5d states of the optimum Eu2+-concentration present in the blue-emitting Ca3Mg3(PO4)4:Eu2+ phosphor was evaluated and discussed by using the spectrum-fitting method.

Site Occupancies, VUV-UV-vis Photoluminescence, and X-ray Radioluminescence of Eu2+-Doped RbBaPO4.

RbBaPO4:Eu2+ phosphors have been prepared by a high-temperature solid-state reaction method, and the structure was determined by Rietveld refinement based on powder X-ray diffraction (P-XRD) data. Their VUV-UV-vis photoluminescence properties are systematically investigated with three objectives: (1) based on low-temperature spectra, we clarify the site occupancies of Eu2+, and demonstrate that the doublet emission bands at ∼406 and ∼431 nm originate from Eu2+ in Ba2+ [Eu2+(I)] and Rb+ [Eu2+(II)] sites, respectively; (2) an electron-vibrational interaction (EVI) analysis is conducted to estimate the Huang-Rhys factors, the zero-phonon lines (ZPLs) and the Stokes shifts of Eu2+ in Rb+ and Ba2+ sites; (3) the studies on luminescence decay of Eu2+(I) reveal that dipole-dipole interaction is mainly responsible for the energy transfer from Eu2+(I) to Eu2+(II), and the energy migration between Eu2+(I) is weak. Finally, the X-ray excited luminescence (XEL) spectrum indicates that the light yield of the sample RbBa0.995Eu0.005PO4 is ∼17 700 ph/MeV, showing its potential application in X-ray detecting.

Theoretical analysis of electron vibrational interaction (EVI) parameters in 5d states of Eu2+ activated BaSiF6 downconversion phosphor

The excitation and emission spectra of Eu2+ ions are dependent on the host environment in which it is doped. This is because the spectra arise from the inter-band transitions between the 4f and 5d orbitals and the electrons transiting between these two orbitals interact with the phonons in the host lattice. In the present work, low temperature solution combustion method was employed for the preparation of Eu2+ activated BaSiF6 phosphor by using urea as a fuel. The structure, vibrations and morphology of the samples were analyzed by X-ray Diffraction (XRD), and Scanning Electron Microscope (SEM) which showed that the samples were crystallized in a well known structure. Elemental composition of the given phosphor was confirmed by EDX analysis. The photoluminescence (PL) excitation and emission spectra were recorded for all the samples and the optimum photoluminescence (PL) intensity was obtained for the 0.5 mol% sample. Electron-vibrational interaction parameters were evaluated using a spectrum-fitting method. The expected parameters are found to be satisfactory which is evident from the good agreement between the experimental and simulated emission bands. The International Commission on Illumination chromaticity coordinates was located in the blue region, indicating the possibility of employing this phosphor as a blue-emitting component in Ultraviolet-pumped light emitting diode.

An old system revisited: Al2O3:Ti3+ - Microscopic crystal field effects explored by the crystal field and first-principles calculations

The exchange charge model of crystal field has been used to analyze the microscopic crystal field in the Al2O3:Ti3+ system. The effects of lattice relaxation upon doping were treated in the framework of density functional based calculations of the optimized geometry of the doped host. The optimized Al2O3:Ti3+ structure was used as an input for the calculations of distance dependence of the crystal field parameters and crystal field splittings of the Ti3+ 3d states. From these calculations, the constants of the electron-vibrational interaction and the Stokes shift between the absorption and emission spectra of Ti3+ ions in Al2O3:Ti3+ were estimated to yield good agreement with the experimental results. In addition, four parameters of the superposition model of the crystal field (which in general are considered as the fitting parameters) were extracted from the results of the exchange charge model calculations. The proposed method of linking together two independent models of the crystal field is a reliable way of calculating the superposition model parameters (rather than allowing them to vary freely) and can be readily applied to other systems.

The Electron-Vibrational Interaction in a Thiophene—Phenylene Cooligomer and Its Relationship to the Raman Spectrum

Electron-vibrational interactions play a key role in limiting charge mobility in organic semi-conductors. This paper reports a theoretical study of the electron—phonon interaction in the 5,5′-diphenyl-2,2′-bitiophene (PTTP) molecule, which belongs to the class of thiophene-phenylene cooligomers, which are of great interest for organic optoelectronics due to their electron-transport and luminescent properties; these results are compared with anthracene, which is a model organic semiconductor. The contributions of various vibrational modes to the reorganization energy of PTTP and the anthracene molecules are revealed and it is shown that these contributions correlate with the intensities of the corresponding bands in the Raman spectrum. In particular, it is found that for the PTTP molecule the so-called I-mode with a frequency of ∼1460 cm−1, which corresponds to collective vibration of atoms of all oligomer units, has the highest intensity in both spectra. These results indicate the promise of Raman spectroscopy for studying electron-vibrational interactions in organic semiconductors. Finally, the mobility of holes in PTTP and anthracene is estimated in the framework of the jump model and the reasons for their difference are analyzed. Based on these results, we propose some ways to reduce the electron-vibrational interaction in thiophene—phenylene cooligomers, which is important for the directional molecular design of organic semiconductors.

Analysis of the electron-vibrational interaction in the 5d states of Eu2+ ions in LiSrPO4 host matrix

Different concentrations of Eu2+ doped LiSrPO4 phosphors were synthesized by a sol-gel/Pechini method. The photoluminescence (PL) excitation and emission spectra were recorded for all the samples and the optimum photoluminescence (PL) intensity was obtained for the 5 mol% sample. The Rietveld refinement method was performed on the optimum sample structure. Electron-vibrational interaction parameters were evaluated using a spectrum-fitting method. The International Commission on Illumination chromaticity coordinates were located in the blue region, indicating the possibility of employing this phosphor as a blue-emitting component in Ultraviolet-pumped light emitting diode.

Study of Electron-Vibrational Interaction in Molecular Aggregates Using Mean-Field Theory: From Exciton Absorption and Luminescence to Exciton-Polariton Dispersion in Nanofibers

We have developed a model in order to account for electron-vibrational effects on absorption, luminescence of molecular aggregates and exciton-polaritons in nanofibers. The model generalizes the mean-field electron-vibrational theory developed by us earlier to the systems with spatial symmetry, exciton luminescence and the exciton-polaritons with spatial dispersion. The correspondence between manifestation of electron-vibrational interaction in monomers, molecular aggregates and exciton-polariton dispersion in nanofibers is obtained by introducing the aggregate line-shape functions in terms of the monomer line-shape functions. With the same description of material parameters we have calculated both the absorption and luminescence of molecular aggregates and the exciton-polariton dispersion in nanofibers. We apply the theory to experiment on fraction of a millimeter propagation of Frenkel exciton polaritons in photoexcited organic nanofibers made of thiacyanine dye.

Theory of Vibrational Spectra of Excited Quadrupolar Molecules with Broken Symmetry

Quadrupolar molecules of the structure AL–D–AR, where D is an electron donor and AL and AR are identical acceptors, are investigated. A general theory of vibrational spectra of such molecules undergoing symmetry breaking after photoexcitation is developed. The approach is based on the expansion of the electron–vibrational interaction in a series in powers of the displacements of the vibrational modes and the dissymmetry parameter. The theory provides a possibility to calculate the intensities of the IR bands and the splitting of the frequencies of local vibrations associated with vibrations of small groups of atoms symmetrically located in the left and right arms of the molecule. The theory explains the regularities of influence of symmetry breaking on the IR band intensities and the splitting of the frequencies observed recently using time-resolved infrared spectroscopy.

Electron-vibrational interaction in the 3d states of Cu+ ions activated in Na5Ca4(PO4)4F

Cu+-doped Na5Ca4(PO4)4F fluorophosphate was synthesized by wet-chemical method. This blue-emitting phosphor was characterized by XRD, FTIR and photoluminescence measurements. Rietveld refinement method was employed to analyse the phase formation of the as-prepared phosphor. The electron-vibrational interaction (EVI) parameters were calculated by the spectrum-fitting method. Na2Ca4(PO4)3F:Cu+ phosphor demonstrated blue emission under near-UV excitation and its CIE coordinates were found at (0.151, 0.037).

Vacuum Referred Binding Energy Scheme, Electron–Vibrational Interaction, and Energy Transfer Dynamics in BaMg2Si2O7:Ln (Ce3+, Eu2+) Phosphors

The host structure and the synchrotron radiation VUV–UV luminescence properties of samples BaMg2Si2O7 (BMSO):Ln (Ce3+, Eu2+) at different doping levels and different temperatures were investigated in detail. Three important aspects are studied to elucidate the luminescence properties of samples: (1) the vacuum referred binding energy (VRBE) scheme is constructed with the electron binding in the BMSO host bands and in the Ce3+ and Eu2+ impurity levels with the aim to explain the different thermal stabilities of Ce3+ and Eu2+ emissions; (2) the electron–vibrational interaction analysis on the narrow Eu2+ emission indicates a weak electron–phonon interaction in the current case; (3) by using three models (Inokuti–Hirayama, Yokota–Tanimoto, and Burshteĭn models) at different conditions, the energy transfer dynamics between Ce3+ and Eu2+ was analyzed. It reveals that the energy transfer from Ce3+ to Eu2+ via electric dipole–dipole (EDD) interaction is dominant while energy migration between Ce3+ is negligible....

Energy transfer controlled by dynamical Stark shift in two-level dissipative systems

A strong electromagnetic field interacting with an electron system generates both the Rabi oscillations and the Stark splitting of the electron density. Changing of the electron density gives rise to nonadiabatic effects due to existence of the electron-vibrational interaction in a dissipative system. In this Letter, the mechanism of energy transfer between the electron system and the phonon reservoir is presented. This mechanism is based on establishment of the coupling between the electron states dressed by the electromagnetic field and the forced vibrations of reservoir oscillators under the action of rapid changing of the electron density with the Rabi frequency. The photoinduced vibronic coupling results in appearance of the states that are doubly dressed by interaction, first time due to the electron-photon interaction, and second time due to the electron-vibrational interaction. Moreover, this coupling opens the way to control energy which can be transferred to (heating) or removed from (cooling) the phonon reservoir depending on the parameters of the electromagnetic pulse.

Resonance tunneling electron-vibrational spectroscopy of polyoxometalates.

The tunneling spectra of the ordered monolayer films of decamolybdodicobaltate (DMDC) compounds deposited from aqueous solutions on HOPG were measured by scanning tunnel microscopy in air. The DMDC spectra, as well as the tunneling spectra of other polyoxometalates (POMs), exhibit well-defined negative differential resistances (NDRs). The mechanism of formation of these spectral features was established from the collection of revealed NDR dependences on the external varying parameters and found to be common to all systems exhibiting Wannier-Stark localization. A model of biresonance tunneling was developed to provide an explanation for the totality of experimental data, both the literature and original, on the tunneling POM probing. A variant of the tunneling electron-vibrational POM spectroscopy was proposed allowing the determination of the three basic energy parameters-energy gaps between the occupied and unoccupied states, frequencies of the vibrational transitions accompanying biresonance electron-tunneling processes, and electron-vibrational interaction constants on the monomolecular level.

Nonlinear dynamics of electron localization in four-center linear and cyclic type clusters in an external electric field

This paper presents a quantum-mechanical model to describe the electron localization dynamics in a tetrameric cluster under an external low-frequency electric field, taking into account the electron-vibrational interaction. A system of differential equations concerning the time-dependent amplitudes of the electron localization probability at each of the tetramer centers has been developed. The proposed model is used to describe the tetramers of the linear and cyclic (square-planar) configurations. For both types of tetramer configuration, the numerical calculations are performed at various values of the model parameters. At the same time, this study is focused on the tetramer in which the centers are weakly tunnel-connected and the electron-vibrational interaction with the ligand environment at each of the centers is significant. The control role of the electric field has been found, the variation of the amplitude and frequency of which allows us to implement different electron localization regimes in a cluster by adjusting the duration of the full localization of an electron on the cluster centers and switching a cluster between states with a localized and delocalized electron.

Synthesis and photoluminescence properties of Li2SO4: RE (RE = Eu3+, Tb3+, Gd3+ and Ce3+) phosphors

Li2SO4 activated with trivalent rare earth ions Eu3+, Tb3+, Gd3+ and Ce3+ were prepared by the wet chemical method which is suitable for large scale production and is an eco-friendly method. Prepared phosphors were characterized by photoluminescence technique. Photoluminescence emission spectra of Eu3+ and Tb3+ doped Li2SO4 compound shows red and green emission, respectively, under efficient n-UV (350–400nm) excitation. This indicates that these phosphors are good and promising candidates for eco-friendly solid state lighting application. Photoluminescence of Li2SO4:Gd3+ compound shows narrow band (NB) UV-B (311nm) emission which is in good agreement with TL-01 lamp used for treatment of psoriasis. Li2SO4 doped with Ce3+ ions possess broad emission from 300nm to 400nm. Electron vibrational interaction (EVI) parameters were determined for Ce3+ emission which show intermediate coupling regime. Emission spectrum of Li2SO4: Ce3+ compound matches well with emission spectrum of Sylvania broad band (UV6) lamp which is also used for phototherapy treatment.

Investigation of thermoluminescence and electron-vibrational interaction parameters in SrAl2O4:Eu2+, Dy3+ phosphors

Combustion synthesis method was employed for the synthesis of green-emitting monoclinic SrAl2O4:Eu2+, Dy3+ phosphors. The phase-purity of the prepared phosphors were examined using X-ray diffraction (XRD). The prepared phosphors exhibited green light emission with the peak centred at 510 nm, under 350 nm UV excitation. The excitation and emission spectra were analysed and the parameters of electron-vibrational interaction (EVI), such as the Huang–Rhys factor, effective phonon energy and zero-phonon line position were estimated using the spectrum fitting method. Thermoluminescence (TL) behaviour of the as-prepared phosphors were analysed for UV and 137Cs γ-ray source irradiation. TL glow curves for UV-irradiated SrAl2O4:Eu2+, Dy3+ phosphors were analysed.