Nonradiative Electron Transitions Research Articles

Femtosecond exciton relaxation in chlorosomes of the photosynthetic green bacterium <i>Chloroflexus aurantiacus</i>

In the green bacteria Chloroflexus (Cfx.) aurantiacus, the process of photosynthesis begins with the absorption of light by chlorosomes, peripheral antennas consisting of thousands of bacteriochlorophyll c (BChl c) molecules combined into oligomeric structures. In this case, excited states are formed in BChl c, the energy of which migrates along the chlorosome towards the baseplate and further to the reaction center, where the primary charge separation occurs. Energy migration is accompanied by nonradiative electronic transitions between numerous exciton states, that is exciton relaxation. In this work, we studied the dynamics of exciton relaxation in Cfx. aurantiacus chlorosomes using difference femtosecond spectroscopy at cryogenic temperature (80 K). Chlorosomes were excited by 20 fs light pulses at wavelengths from 660 to 750 nm, and the difference (light-dark) kinetics were measured at a wavelength of 755 nm. Mathematical analysis of the data obtained revealed the kinetic components with characteristic time constants of 140, 220, and 320 fs responsible for the exciton relaxation. As the excitation wavelength decreased, the number of components and their relative contribution increased. Theoretical modeling of the results obtained was carried out on the base of the cylindrical model of BChl c aggregates. Nonradiative transitions between groups of exciton bands were ascribed by the system of kinetic equations. The model in which the energetic and structural disorder was taken into account turned out to be the most adequate.

Targeting cooling for quantum dots by 57.3°C with air-bubbles-assembled three-dimensional hexagonal boron nitride heat dissipation networks

The non-radiative electron transitions of Quantum dots (QDs) in White light-emitting diodes (WLEDs) not only generates thermal phonons with temperature rise but also degrades their photonic properties. These nanoscale heat sources are usually embedded in low-thermal-conductivity polymer matrix, which is difficult for building efficient heat dissipation pathways without sacrificing the optical performance simultaneously. Herein, we reported an air-bubbles-assembly strategy to construct three-dimensional (3D) thermal dissipation network inside the QDs-WLEDs. Owing to the lateral expulsion of massive microscale air-bubbles, an interconnected hexagonal boron nitride (hBN) network was established under an extremely low hBN loading of 2.74 wt%. With this efficient 3D/hBN network, the highest working temperature of QDs was dramatically decreased by 57.3°C at 700 mA. More than that, the 3D/hBN-WLEDs also show high luminous efficacy of 84.6 lm/W as well as superior color rendering index of Ra = 94.1 and R9 = 93.8, which are comparable to those of traditional WLEDs. The proposed strategy is expected to pave a new door for targeting cooling QDs and other photoluminescent nanoparticles at high-power applications.

Reduction of non-radiative recombination by inserting a GaAs strain-relaxation interlayer in InGaAs/GaAsP superlattice solar cells investigated by photo-thermal spectroscopy

The role of a GaAs strain-relaxation interlayer inserted into InGaAs/GaAsP superlattice solar cells was evaluated by measuring the piezoelectric photothermal (PPT) signals in the temperature range from 100 K to a device operation temperature of around 340 K. The PPT signals caused by the non-radiative recombination of electrons photo-excited to the first quantized level were observed. The temperature-dependent PPT signal intensities were assessed using an electron carrier relaxation model comprising four processes: radiative recombination, non-radiative recombination, thermionic emission, and tunneling of carriers through the e2-miniband after thermal excitation from the e1-level. The contribution of holes in the hh1 state was also included in this model, in which e1 and e2 are the first and second electron levels in the conduction band, respectively, and hh1 is the first heavy hole level in the valence band of the quantum wells. A similar analysis was conducted using photoluminescence (PL) spectra to elucidate the carrier transition dynamics in greater detail, because PPT and PL measurements are complementary to each other in terms of non-radiative and radiative electron transitions. Consequently, although the non-radiative recombination remained dominant around room temperature, the quantum yield of the carrier tunneling process increased and became comparable to that of non-radiative recombination. This implies that the recombination loss of the photo-excited carriers is suppressed by the insertion of the GaAs interlayer. By clarifying the role of the inserted interlayer with respect to the non-radiative recombination process, the usefulness of the PPT method is demonstrated.

Temperature dependence of dislocation-related photoluminescence (D1) of self-implanted silicon subjected to additional boron implantation

The goal of this work is the qualitative and quantitative interpretation of experimental data obtained for the effect of additional boron ion implantation on the temperature dependence of dislocation-related luminescence (D1 line) of self-implanted silicon. This goal is motivated by the need in development of effective light emitter for silicon-based optoelectronics. The model explaining the previously established improvement of the D1 line temperature dependence is suggested and supported by computer calculations based on a set of kinetic equations taking into account the radiative and nonradiative electron transitions between energy levels.

Silicon-Vacancy Centers in Ultra-Thin Nanocrystalline Diamond Films.

Color centers in diamond have shown excellent potential for applications in quantum information processing, photonics, and biology. Here we report the optoelectronic investigation of shallow silicon vacancy (SiV) color centers in ultra-thin (7–40 nm) nanocrystalline diamond (NCD) films with variable surface chemistry. We show that hydrogenated ultra-thin NCD films exhibit no or lowered SiV photoluminescence (PL) and relatively high negative surface photovoltage (SPV) which is ascribed to non-radiative electron transitions from SiV to surface-related traps. Higher SiV PL and low positive SPV of oxidized ultra-thin NCD films indicate an efficient excitation—emission PL process without significant electron escape, yet with some hole trapping in diamond surface states. Decreasing SPV magnitude and increasing SiV PL intensity with thickness, in both cases, is attributed to resonant energy transfer between shallow and bulk SiV. We also demonstrate that thermal treatments (annealing in air or in hydrogen gas), commonly applied to modify the surface chemistry of nanodiamonds, are also applicable to ultra-thin NCD films in terms of tuning their SiV PL and surface chemistry.

Additional Specific Channel of Photoactivation of Solid Semiconductors. A Revisit of the Thermo-/Photo-Stimulated Bleaching of Photoinduced Ti3+ Color Centers in Visible-Light-Active Photochromic Rutile Titania

The vis–NIR-stimulated bleaching and thermostimulated bleaching of UV–vis-induced Ti3+ color centers in visible-light-active rutile titania were revisited to gain a fuller understanding of the photoinduced formation and separation of charge carriers. The prime photophysical process of photostimulated bleaching of color centers is the absorption of the light quanta by Ti3+ centers. However, as expected from the photoexcitation and ionization of a certain type of Ti3+ centers, no selectivity of photostimulated bleaching could be ascertained. Moreover, thermoprogrammed annealing of color centers showed that photogenerated holes captured at a set of traps are also participants in the photostimulated bleaching of color centers. The extent of involvement of these trapped holes exhibited a rather puzzling dependence on the concentration and depth of the traps and on trap filling. Based on our current findings and earlier results, it is hypothesized that the heat released during nonradiative electron transitions,...

Observation of Reverse Saturable Absorption of an X-ray Laser.

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^{16} -10^{17}  W/cm^{2}. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray free-electron laser pulses.

Bismuth and vanadate activators in BiMVO5 (M=Ca, Mg, Cd) phases: Structural, electronic and optical specificities

In the BiMVO5 series, only three representatives were found to show photoluminescence (PL) at room temperature, namely the compounds with M = Mg, Ca and Cd. Their optical properties are complex, but understood on the basis of the calculated electronic structures. Indeed, two antagonist behaviors are distinguishable by the existence (Mg, Ca) or not (Cd) of a broad area (∼0.9 eV above the bottom of the conduction band) allowing for non-radiative electronic transitions at room temperature. In spite of the existence of the two structural polytypes of a cis- (Ca, Cd) and trans-like (Mg) connectivity to the oxygen bridges, the structural topology is not directly related to the two contrasted PL processes. It seems rather correlated to the degree of the distortion of the VO43− tetrahedra and the splitting of the 3d levels that favor a violation of forbidden transitions.

Dynamics of ground state absorption spectra in donor-acceptor pairs with ultrafast charge recombination.

A theoretical approach to simulation of the transient spectra in molecular systems with ultrafast photoinduced nonradiative electronic transitions is developed. The evolution of the excited and ground state populations as well as the nonradiative transitions between them are calculated in the framework of the stochastic multichannel point-transition model involving the reorganization of the medium and the intramolecular high frequency vibrational modes. Simulations of transient spectra of donor-acceptor pairs excited in the charge-transfer band that are accompanied by ultrafast charge recombination into the ground state demonstrate a possibility of positive band appearance in the transient absorption spectrum caused by those systems in the ground state, which returned there from the excited state. The region of the parameters of the donor-acceptor systems where a positive ground state absorption signal can be observed is discussed. A qualitative comparison of the simulated transient spectra with the experimental data on betaine-30 is presented.

Multiscale approach combining nonadiabatic dynamics with long-time radiative and non-radiative decay: Dissociative ionization of heavy rare-gas tetramers revisited

A multiscale approach is proposed to address short-time nonadiabatic dynamics and long-time decay. We show the role of both radiative and non-radiative processes in cluster decay mechanisms on examples of rare-gas cluster fragmentation after electron impact ionization. Nonadiabatic molecular dynamics is used as an efficient tool for theoretical study on femto- and picosecond scales and a multiscale approach based on kinetic rates of radiative as well as non-radiative transitions, both considered as parallel reaction channels, is used for the analysis of the long-time system relaxation spanning times over microseconds to infinity. While the radiative processes are typically slow, the system relaxation through non-radiative electronic transitions connected with electron-nuclear interchange of energy may, on the other hand, significantly vary in kinetic rates according to kinetic couplings between relevant adiabatic states. While the predictions of picosecond molecular dynamics themselves fail, the results of the multiscale model for the electron-impact post-ionization fragmentation of krypton and xenon tetramers are in agreement with experiment, namely, in leading to the conclusion that charged monomers prevail. More specifically, on microsecond and longer scales, mainly slow radiative processes are substantial for krypton cluster decay, while for xenon the radiative and slow non-radiative processes compete. In general, the role of slow decay processes through non-radiative transitions is comparable with the role of radiative decay mechanism. The novel multiscale model substantially improves theoretical predictions for the xenon tetramer decay and also further improves the good agreement between theory and experiment we reached previously for krypton.

On the Interaction of Dislocations with Impurities in Silicon

Dislocations and impurities in silicon have been widely investigated since many years, nevertheless many questions on this subject remain still unsolved. As an example, theory, models and experimental phenomena provide evidence of the existence of shallow bands in silicon induced by the dislocation strain field. Nevertheless, only deep bands, likely associated with contamination at dislocations, have been detected up to now by junction spectroscopy. The present contribution reviews several results, obtained by the authors, on dislocation impurity interactions and their effects on the electronic properties of defect states in silicon. Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical methods. Different materials (oxygen precipitated and deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation and metallic contamination on non-radiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated to dislocation-related impurity centers, while additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms. Moreover, experimental results obtained by junction spectroscopy assessed the existence of dislocation related shallow states. These were found to be located at 70 and 60 meV from the valence and conduction band edge, respectively.

Nonradiative transition to the ground state and superelastic scattering of exited atoms upon impact on the surface of wide-bandgap dielectrics

The impact of excited cesium atoms on sapphire and glass surfaces have been experimentally studied. It is established that the probability of electron excitation quenching upon impact of an atom on the dielectric surface is close to unity. The velocity distribution of unexcited atoms upon scattering from the surface has been determined using the time-of-flight technique. The kinetic energies of most of these atoms are several tens of times greater than the energy of thermal motion of the excited atoms impinging on the surface. Conversion of the internal energy of atoms into their kinetic energy is explained in terms of nonradiative electron transitions with simultaneous excitation of lattice vibrations in the dielectric crystal. This mechanism of atomic excitation quenching near the dielectric surface explains the significant difference between the energies of atoms upon superelastic scattering and upon photodesorption from an adsorbed state.

Electronic transitions at defect states in Cz p-type silicon

Point and extended defects introduced in p-type Cz Si by oxygen precipitation and plastic deformation have been investigated with electrical and optical methods. Different materials (oxygen precipitated and/or deformed Cz Si and Fz Si) were examined in order to separate the role of oxygen precipitation, plastic deformation, and metallic contamination on the radiative and nonradiative electronic transitions at defect centers. A deep hole trap, named T1, has been associated with dislocation-related impurity centers; additional deep traps have been related to contamination by grown-in transition metals and to clusters involving oxygen atoms.

Optical properties of GaAsNSe/GaAs superlattice investigated by means of piezoelectric photothermal spectroscopy for nonradiative electron transitions

Optical properties of the GaAsNSe/GaAs superlattice (SL) were investigated using piezoelectric photothermal (PPT) and photoluminescence (PL) spectroscopies. The sample was grown by metalorganic molecular beam epitaxy on a semi-insulating (SI-)GaAs substrate. The SL consists of five periods of GaAsNSe and GaAs layers of thickness 11 nm. The temperature variation of the PPT spectrum of this sample was measured down to liquid nitrogen temperature and three peaks were observed at 0.7, 0.90 and 1.25 eV at 80 K. The 1.25 eV peak seems to be due to the bandgap transition of GaAsNSe. Comparing the PPT spectra with the PL results, it was considered that the localised levels caused the PPT signal peaks at 0.7 and 0.90 eV. The intensity of the 0.90 eV peak drastically changed with secondary light illumination. The effect of the electron nonradiative recombination at the interface states should be considered.

Proton-beam-induced defect levels in CuInSe2 thin-film absorbers: An investigation on nonradiative electron transitions

Electron nonradiative relaxation through the proton-irradiation-induced defects in CuInSe2 solar cell material were investigated by using a piezoelectric photothermal spectroscopy (PPTS). Among the observed three peaks at 1.01, 0.93, and 0.84eV, it was concluded that the peak at 0.84eV was due to the proton-irradiation-induced defect. This is because this peak appeared after irradiation with the proton energy of 0.38MeV and the fluence of 1×1014cm−2. The peaks at 1.01 and 0.93eV were attributed to free band-edge exciton and intrinsic defect level, respectively. The intensities for the latter two peaks were not affected by the irradiation. Since the irradiation defect was clearly observed at room temperature, we concluded that the PPTS technique was a very sensitive tool to study the defect level in the irradiated semiconductor thin-film solar cell structures.

Another important coordinate in the photoisomerization of cis-stilbene

Detailed simulations are reported for the coupled dynamics of electrons and nuclei in the isomerization reaction of cis-stilbene stimulated by laser excitation. The results demonstrate that, in addition to the traditional vinyl and vinyl–phenyl torsions, the HCCH torsional coordinate of the vinyl group also makes a significant contribution to the HOMO and LUMO couplings that yield nonradiative electronic transitions to the ground state. Moreover, variations in the CCH angles of the vinyl group in the vicinity of the HOMO–LUMO couplings indicate that pyramidalization of the vinyl-group carbon atoms is strongly involved in these molecular orbital couplings.

Electron and hole trap levels in semi-insulating GaAs investigated by a temperature variation of piezoelectric photo-thermal spectra

Temperature variation of the piezoelectric photo-thermal (PPT) spectra of semi-insulating (SI) GaAs from 15 K to room temperature was measured. Curve fitting procedure was carried out for the experimental results in terms of a theoretical analysis based on rate equations for electrons in conduction band (CB) having regard to deep levels. We identified that the observed peaks were due to the nonradiative electron transitions into the deep electron trap levels. By changing the wavelength of an excitation light, new peaks were observed. Considering a photo-ionization cross section spectra for electrons and holes in GaAs, these peaks are considered due to hole traps.

Population dynamics in vibrational modes during non-Born-Oppenheimer processes: CARS spectroscopy used as a mode-selective filter.

The coupling of specific nuclear and electronic degrees of freedom of a molecular system during non-radiative electronic transitions plays a central role in photochemistry and photobiology. This breakdown of the Born-Oppenheimer approximation during processes such as internal conversion determines the mechanism and product distribution of photochemical reactions and is responsible for the high efficiency of photobiological processes. In order to explore this phenomena in beta-carotene, a molecule that plays a primary role as an auxiliary light-harvesting pigment in photosynthesis, a spectroscopic method was employed that allows for the individual vibrational modes to be monitored selectively within the dynamics of an internal conversion process. This spectroscopic technique employs an initial pump laser to excite the molecule into an excited electronic state and resolves the subsequent relaxation process by interrogating the system with a time-delayed, coherent anti-Stokes Raman process (CARS), which acts as a mode-selective filter for observing the population flow within specific vibrational modes with a time resolution in the femtosecond regime.

Investigation of EL6 Deep Level in Semi-Insulating GaAs by Means of a Temperature Dependence of Piezoelectric Photothermal Signal

The temperature dependence of the piezoelectric photothermal (PPT) signal intensity of semi-insulating (SI) GaAs from 160 to 297 K was measured. One peak at 205 K was observed in the temperature dependence of the PPT signal. Curve fitting to experimental results by a theoretical analysis based on the rate equations of electrons in the conduction band and a corresponding deep level was carried out. The temperature dependence of the quasi-Fermi level in the band gap of SI-GaAs was also taken into account in the present theoretical model for the first time. We then identified that the observed PPT peak was due to the nonradiative electron transitions through the deep level EL6 in GaAs. It is demonstrated that the deep level was clearly characterized in SI-GaAs by using the PPT measurement.

Calculation of the probability of nonradiative electronic transitions from deep centers in GaAs in the one-coordinate approximation

A study was made of the experimental optical bands of radiative VGaSAs and VGaSnGa complexes in GaAs in a wide temperature range. Parameters of the one-coordinate model of the centers were determined. The configuration-coordinate diagram of the VGaSAs complex was constructed. Using parameters of the one-coordinate model of the VGaSAs complex as the base, the field dependences of emission rates were calculated. The calculation results were compared with the experimental data. A conclusion was made that the one-coordinate model can be used for the description of the field dependence of the rate of hole emission from the VGaSAs center.