Electronic Relaxation Research Articles

Zero-point averaged dynamics of Arn+He1000: Dopant-size influence on potential-energy surfaces, mass spectra and fragmentation patterns

• Effective He-He potentials are little sensitive to the dopant embedded in HNDs. • No Ar + He q fragments detected for Ar n + He 1000 clusters ( n = 3 - 5 ) . • Fast ion ejections, ion trappings, and geminate recombinations for all Ar n + He 1000 . • Inefficient relaxation of Ar 3 + initially excited in its uppermost electronic state. The fragmentation of Ar n + He 1000 ( n = 3 , 5 ) is investigated by zero-point averaged dynamics (ZPAD) and compared with recent results on Ar 4 + He 1000 to investigate the influence of the dopant size. At the core of ZPAD, effective He-He potentials converged for He 1000 and Ar n He 1000 are found alike which suggests that ZPAD simulations on argon-doped helium nanodroplets (HNDs) could restrict to effective potentials obtained for pure HNDs. The fragmentation dynamics of Ar n + He 1000 clusters share a number of similarities, like the absence of Ar + -containing fragments, the occurrence of ion ejections and trappings, Ar n + electronic relaxation typically extending on a picosecond time scale, or primary fragmentation events which are not necessarily representative of the final distribution of fragments. However, Ar 3 + He 1000 clusters also exhibit unexpected behaviors like an inefficient electronic relaxation when the dynamics starts on the uppermost Ar 3 + electronic state which makes the dopant trapped within the droplet.

Расчёт толщины висмутовых нанослоёв, согласованных с характеристическим импедансом поля двух встречных электромагнитных волн.

Расчёт толщины висмутовых нанослоёв, согласованных с характеристическим импедансом поля двух встречных электромагнитных волн.

Intramolecular Charge Transfer in the Azathioprine Prodrug Quenches Intersystem Crossing to the Reactive Triplet State in 6-Mercaptopurine.

The thiopurine prodrugs 6-mercaptopurine and azathioprine are among the world's essential medications for acute lymphoblastic leukemia, immunosuppression and several autoimmune conditions. Thiopurine prodrugs are efficient UVA absorbers and singlet oxygen generators and the long-term treatment with these prodrugs correlates with a high incidence of sunlight-induced skin cancer in patients. In this contribution, we show that the electronic relaxation mechanisms and photochemical properties of azathioprine are remarkably different from those of 6-mercaptopurine upon absorption of UVA radiation. UVA excitation of 6-mercaptopurine results in nearly 100% triplet yield and up to 30% singlet oxygen generation, whereas excitation of azathioprine with UVA leads to triplet yields of 15-3% depending on pH of the aqueous solution and <1% singlet oxygen generation. While photoexcitation of 6-mercaptopurine and other thiopurine prodrugs can facilitate oxidatively generated cell damage, azathioprine's poor photosensitization ability reveals the use of interchromophoric charge-transfer interactions for the molecular design of photostable prodrugs exhibiting a remarkable reduction in photocytotoxic side effects before drug metabolization.

Vacancy-induced enhancement of electron–phonon coupling in cubic silicon carbide and its relationship to the two-temperature model

The electron–phonon coupling factor was calculated for both pristine and vacancy-rich 3C-SiC. Ab initio calculations were performed within the framework of the density functional perturbation theory. Wannier functions were used to interpolate eigenvalues into denser grids through the electron–phonon using Wannier code. The coupling factor was determined through calculations of the electron self-energy, electron–phonon relaxation time, and electronic specific heat. These parameters were extrapolated to high temperatures using a hybrid model which mixes band calculations for electrons below an energy cutoff with the free electron gas model for electrons above the energy cutoff. The electron relaxation times, specific heats, electron drift mobilities, and electron–phonon coupling factors were calculated as a function of electron temperature. Si and C vacancies were found to have a profound effect on electron–phonon coupling for all temperatures, while electronic specific heat capacity was found to be most affected at cryogenic temperatures. The electron drift mobility was calculated at different temperatures using the scattering time. Calculated mobilities were validated with Hall mobility measurements reported in the literature. The importance of structural defects on the electron–phonon coupling is discussed in the context of the two-temperature model, a model that has been widely used to understand aspects of the interaction of solids with pulsed laser irradiation and swift heavy ion irradiation.

Ultrafast relaxation dynamics of Au38(SC6H13)24 monolayer-protected clusters resolved by two-dimensional electronic spectroscopy.

Electronic relaxation dynamics of neutral Au38(SC6H13)24 monolayer-protected clusters (MPCs), following excitation of the mixed 15875cm-1 charge transfer resonance, were studied using femtosecond transient absorption (fsTA) and two-dimensional electronic spectroscopy (2DES). The excited carriers relax by three different mechanisms, including an ∼100fs HOMO-12/-13 to HOMO-4/-6 hole transfer, picosecond HOMO-4/-6 to HOMO hole transfer, and subsequent electron-hole recombination that persisted beyond the hundreds of picoseconds measurement range. The fsTA data revealed two transient bleach components at 15820 and 15625cm-1, where the lower frequency component exhibited a delayed first-order buildup of 80 ± 25fs that matched the decay of the high-energy bleach component (110 ± 45fs). These results suggested that the excited charge carriers internally relax within the exited-state manifold in ≈100fs. 2DES resolved multiple electronic fine-structure transient peaks that spanned excitation frequencies ranging from 15500 to 16100cm-1. State-to-state dynamics were understood by the analysis of time-dependent 2DES transient signal amplitudes at numerous excitation-detection frequency combinations. An off-diagonal cross peak at 15825-15620cm-1 excitation-detection signified the HOMO-12/-13 to HOMO-4/-6 hole transfer process. The lowest-frequency (15620cm-1) 2DES diagonal fine-structure peak exhibited instantaneous amplitude but intensified following a 75 ± 10fs buildup when compared to diagonal peaks at higher frequencies. This observation indicated that the charge transfer resonance in Au38(SC6H13)24 MPCs is comprised of several electronic transitions of unique spectral weights, which may result from different orbital contributions associated with specific cluster domains. The use of 2DES in combination with structurally precise MPCs can provide a platform for understanding structure-dependent electronic dynamics in metal nanoclusters and technologically important metal-chalcogenide interfaces.

The dynamic variation of upconversion luminescence dependent on shell Yb3+ contents in NaYF4: Yb3+,Tm3+@NaYF4: Yb3+,Er3+ nanoparticles

In this study, a series of NaYF4:Yb3+, Tm3+@NaYF4:Yb3+, Er3+ core@shell nanocrystals with different sensitizer contents in the shell were synthesized by a facile solvothermal method to investigate the impact of sensitizer Yb3+ on the upconversion luminescence. Transmission electron microscopy (TEM) results show that all prepared nanocrystals are well-crystallized with a hexagonal phase and have a desirable size distribution. The particle size increases after shell coating as shown in TEM images; however, it changes a little for samples with different sensitizer contents in the shell. Therefore, the effect of different particle sizes on luminescence properties, which is caused by different Yb3+ contents, could be minimized. Subsequently, it was assumed that the luminescent center content (Er3+) is same in all samples and does not influence the emission spectrum differences. Therefore, we investigated the underlying upconversion luminescence mechanism by determining the sample’s fluorescence spectrum and assuming that the energy transfer mechanism occurred in all the samples. The results show that the green upconversion luminescence decay was faster than its red counterpart when the Yb3+ content was increased and samples were excited using a 980 nm pulse laser. The transient spectra measured at different times after the excitation pulse reveal the intensity ratio associated with the green to red upconversion luminescence. It was concluded that the back energy transfer (BET) process from Er3+ to Yb3+ led to a conversion of the green emissions into the red emissions, which was beneficial to the red upconversion and the electron relaxation from 4I11/2 to 4I13/2. Therefore, the red to green upconversion luminescence ratio increases significantly in samples highly doped with Yb3+.

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D'yakonov-Perel' spin relaxation mechanism in wurtzite GaN.

Synthesis of red photoluminescent nickel doped self-assembled copper nanoclusters and their application in biothiol sensing

Copper nanoribbons (Cu NRs) and Ni-doped copper nanowires (Cu NWs) with red photoluminescent (PL) were prepared and employed for detection of biothiol. Using 4-(trifluoromethyl) thiophenol as both reducing agent and protecting ligand, the copper nanoclusters (Cu NCs) were synthesized and assembled to nanoribbons. And the Cu NCs were assembled to nanowires by adding nickel ion precursor in synthetic process. The Cu NWs with 3% Ni doping showed a 3-fold emission enhancement than Cu NRs, and the absolute quantum yield was obviously increased from 4.53% to 19.76%. The doped Ni induced Ni-Cu metallophilic interaction, which facilitated the radiative relaxation of excited electrons and furtherly resulted in stronger PL emitting. The morphologies, composition, electronic states, geometrical configuration and arrangement of Cu NCs in assembled architecture were studied by experimental and theoretical investigation. Moreover, the PL of Cu NCs assembly was sensitive to cysteine, homocysteine, and glutathione attributed to surface ligands exchange of Cu NCs through interaction between metal and sulfydryl group. Therefore, a rapid, sensitive, and selective sensing strategy for detection of biothiol was proposed. And the PL test strips based on Cu NCs assembly were fabricated for visual detection of biothiol. Finally, the practical application of sensing system and PL test strips were verified in fetal bovine serum samples spiked with cysteine and homocysteine.

Hole injection from P1 dye hot-excited states in p-type dye-sensitized films: a fluorescence study.

We present a study of the excited state relaxation dynamics of the photosensitizer P1 used in p-type dye-sensitized solar cells. Comparative femtosecond fluorescence upconversion measurements in solution and in films show that the dye undergoes a picosecond electronic relaxation from the bright Franck-Condon (FC) state to a low-emitting charge-transfer (CT) state in polar environment. The fluorescence is moderately quenched in solution and on the mesoporous Al2O3 isolator but dramatically more on NiO semiconductor. We assign this sub-picosecond process to the hole injection thus confirming that the electron transfer is from the FC state directly into the NiO valence band.

Oxidation-enhanced thermoelectric efficiency in a two-dimensional phosphorene oxide

We performed density functional theory calculations to investigate the thermoelectric properties of phosphorene oxide (PO) expected to form by spontaneous oxidation of phosphorene. Since thermoelectric features by nature arise from the consequences of the electron-phonon interaction, we computed the phonon-mediated electron relaxation time, which was fed into the semiclassical Boltzmann transport equation to be solved for various thermoelectric-related quantities. It was found that PO exhibits superior thermoelectric performance compared with its pristine counterpart, which has been proposed to be a candidate for the use of future thermoelectric applications. We revealed that spontaneous oxidation of phosphorene leads to a significant enhancement in the thermoelectric properties of n-doped phosphorene oxide, which is attributed to the considerable reduction of lattice thermal conductivity albeit a small decrease in electrical conductivity. Our results suggest that controlling oxidation may be utilized to improve thermoelectric performance in nanostructures, and PO can be a promising candidate for low-dimensional thermoelectric devices.

Enhancement of electron-impact ionization induced by warm dense environments.

Studies have shown significant discrepancies between the recent experiment [Berg etal., Phys. Rev. Lett. 120, 055002 (2018)PRLTAO10.1103/PhysRevLett.120.055002] and current theoretical calculations on the electron-impact ionization cross section of ions in warm dense magnesium. Here, we present a systematic study the effects of the ionic correlations and free-electron screening on the electron-impact ionization of ions in warm dense matter. The ionic correlation and the free-electron screening effects yield additional Hermitian terms to the calculation of the ionic central-force-field potential, which significantly change the electronic structure compared with that of the isolated ion. In calculating the electron-impact ionization, we describe the impact and ionized electrons using a damped-distorted wave function, which considers the momentum relaxation of free electrons due to collisions with other free electrons and ions. We reproduce the electron-impact ionization process for Mg^{7+} in the solid-density plasma and increase the ionization cross section by one order of magnitude compared with that of the isolated ion, which excellently agrees with the experimental result of Berg etal.

Emission characteristics and dynamics of neutral, ionic and molecular species in a laser produced CaF2 plasma

The emission characteristics of a laser-ablated calcium fluoride CaF2 plasma have been investigated using time and space optical emission spectroscopy. The plasma-plume was produced by focusing 266 nm, 15 ns pulses from a Nd: YAG laser on a calcium fluoride crystal substrate placed in a vacuum chamber. The emission observed in the plasma region is due to electronic relaxation of excited neutral Ca and F, ionic fragments Ca2+, Ca+ and F+ and molecular features of CaF (A2Π- X2Σ+, B2Σ+- X2Σ+ and C2Π- X2Σ+) band systems. Plasma species were analyzed both as a function of the delay from the laser pulse incidence and location from the target surface. Spectroscopic diagnostics were employed to estimate time-resolved electron densities, electron temperatures and velocity expansion of different species. The results show a faster decay of the continuum emission, ionic F+, Ca2+, Ca+, F species than in the case of neutral Ca atoms and molecular species of CaF. Different excitation temperatures were measured by comparing Ca and F atomic/ionic Saha-local thermodynamic equilibrium (LTE) spectra with experimental ones at various delay times after the ablation event. Vibrational temperatures of CaF were assessed from the temporal analysis of the A2Π - X2Σ+ Δv = 0 band sequence molecular emission. We observed a non-equilibrium plasma although the plasma is found to maintain partial LTE.

A single nucleobase tunes nonradiative decay in a DNA-bound silver cluster.

DNA strands are polymeric ligands that both protect and tune molecular-sized silver cluster chromophores. We studied single-stranded DNA C4AC4TC3XT4 with X = guanosine and inosine that form a green fluorescent Ag10 6+ cluster, but these two hosts are distinguished by their binding sites and the brightness of their Ag10 6+ adducts. The nucleobase subunits in these oligomers collectively coordinate this cluster, and fs time-resolved infrared spectra previously identified one pointof contact between the C2-NH2 of the X = guanosine, an interaction that is precluded for inosine. Furthermore, this single nucleobase controls the cluster fluorescence as the X = guanosine complex is ∼2.5× dimmer. We discuss the electronic relaxation in these two complexes using transient absorption spectroscopy in the time window 200 fs-400 µs. Three prominent features emerged: a ground state bleach, an excited state absorption, and a stimulated emission. Stimulated emission at the earliest delay time (200fs) suggests that the emissive state is populated promptly following photoexcitation. Concurrently, the excited state decays and the ground state recovers, and these changes are ∼2× faster for the X = guanosine compared to the X = inosine cluster, paralleling their brightness difference. In contrast to similar radiative decay rates, the nonradiative decay rate is 7× higher with the X = guanosine vs inosine strand. A minor decay channel via a dark state is discussed. The possible correlation between the nonradiative decay and selective coordination with the X = guanosine/inosine suggests that specific nucleobase subunits within a DNA strand can modulate cluster-ligand interactions and, in turn, cluster brightness.

Photoexcited carrier dynamics of thin film Cd3As2 grown on a GaAs(111)B substrate by molecular beam epitaxy

The topological Dirac semimetal ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ has drawn great attention for its novel physics and technical applications in optoelectronic devices operating in the infrared and THz regimes. Exploring the photoexcited carrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ is helpful to understand the transient carrier occupation and cooling processes that are closely associated with its unconventional band characteristic. Here, the photoexcited carrier dynamics in the ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ thin film epitaxially grown on the $\mathrm{GaAs}(111)B$ substrate was investigated by employing the time-resolved midinfrared transient reflectance measurements, along with a theoretical modeling concerning the electron-polar optical ($e$-PO) phonon interaction. The measured photoexcited electron relaxation time of ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ increases with the probe wavelength, and increases with temperature at temperatures higher than 30 K. Theoretical modeling under the framework of the two-temperature model can give qualitatively good description on both the temperature and probe wavelength dependence of the carrier relaxation time. Additionally, theoretical modeling shows that charge screening of the interaction of electrons with polar optical phonons can greatly reduce the hot carrier relaxation rate. The measured transient reflectance analyses indicate that the hot phonon effect plays a negligible role on the carrier relaxation in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$. Our findings provide further insight into the photoexcited carrier dynamics in ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$ and valuable reference for developing its high-performance infrared optoelectronic devices.

Efficient third-harmonic generation in asymmetric InxGa1-xN/InN double quantum wells

We report on enhanced third-harmonic generation based on intersubband transitions in an asymmetric InxGa1-xN/InN double quantum well. We give a comprehensive account of the influences of different structural parameters such as doping concentration, thickness of layers, and indium mole composition of barriers on the intersubband transition quantities and then their nonlinear response. The conduction band parameters are computed by the numerical solution to Schrödinger and Poisson equations within nonparabolic band approximation. More importantly, the participation of each electron scattering process, e.g., LO phonon scattering in the electron relaxation time and linewidth values, is calculated, so that it can be considered a reference to provide a heterostructure with optimum functionality. Finally, two optimized double quantum wells are designed at the fundamental photon energies of 117 and 144 meV, which exhibit remarkable third-order susceptibility up to 1.2×10−3 and 10−3μm2/V2, respectively. This study opens a new path to design a suitable InxGa1-xN/InN heterostructure for a third-harmonic generation process from the far- to near-infrared band, which promises various applications in optoelectronic devices.

Role of Solvent in Electron-Phonon Relaxation Dynamics in Core-Shell Au-SiO2 Nanoparticles.

Relaxation dynamics of plasmons in Au-SiO2 core-shell nanoparticles have been followed by femtosecond pump-probe technique. The effect of excitation pump energy and surrounding medium on the time constants associated with the hot electron relaxation has been elucidated. A gradual increase in the electron-phonon relaxation time with pump energy is observed and can be attributed to the higher perturbation of the electron distribution in AuNPs at higher pump energy. Variation in time constants for the electron-phonon relaxation in different solvents is rationalized on the basis of their thermal conductivities, which govern the rate of dissipation of heat of photoexcited electrons in the nanoparticles. On the other hand, phonon-phonon relaxation is found to be much less effective than electron-phonon relaxation for the dissipation of energy of the excited electron and the time constants associated with it remain unaffected by thermal conductivity of the solvent.

X-ray transient absorption reveals the 1Au (n\u03c0*) state of pyrazine in electronic relaxation

Electronic relaxation in organic chromophores often proceeds via states not directly accessible by photoexcitation. We report on the photoinduced dynamics of pyrazine that involves such states, excited by a 267 nm laser and probed with X-ray transient absorption spectroscopy in a table-top setup. In addition to the previously characterized 1B2u (ππ*) (S2) and 1B3u (nπ*) (S1) states, the participation of the optically dark 1Au (nπ*) state is assigned by a combination of experimental X-ray core-to-valence spectroscopy, electronic structure calculations, nonadiabatic dynamics simulations, and X-ray spectral computations. Despite 1Au (nπ*) and 1B3u (nπ*) states having similar energies at relaxed geometry, their X-ray absorption spectra differ largely in transition energy and oscillator strength. The 1Au (nπ*) state is populated in 200 ± 50 femtoseconds after electronic excitation and plays a key role in the relaxation of pyrazine to the ground state.

Energy relaxation of quantum dot hot electrons in hybrid quantum dot—Bose–Einstein condensate system

The theory of electron energy relaxation in a hybrid structure consisting of quantum dot interacting with a two dimensional exciton gas in Bose–Einstein condensate (BEC) regime is developed. A new type of the relaxation mechanism in the presence of BEC is introduced and theoretically analyzed. It is shown that, in the first order of electron-exciton interaction, two microscopic processes of energy relaxation appear. The first one is related to the emission of a single Bogoliubov excitation (bogolon) by an electron, whereas the second process is associated with the emission of two bogolons. It is shown that the second type processes dominate in the QD electron energy relaxation.

Tailoring electronic properties and polarization relaxation behavior of MoS2 monolayers for electromagnetic energy dissipation and wireless pressure micro-sensor

Electromagnetic radiation has become a severe problem due to the widespread utilization of wireless communications and smart electronic devices. Hence, the development of high-performance electromagnetic wave absorbers to overcome the electromagnetic pollution is of utmost significance. Herein, density functional theory (DFT) calculations are adopted to guide the design of high-performance electromagnetic wave absorbers based on layered MoS2. The results indicate that the electronic properties, the dipole moment and the electric polarization of vertically-aligned MoS2 monolayers on N-doped graphene are significantly tuned compared to horizontally-aligned MoS2 monolayers on N-doped graphene and MoS2 nanosheets, favoring the absorption of electromagnetic waves. Based on theoretical predictions, we have combined vertically-aligned MoS2 monolayers and N-doped graphene nanomesh by the spatial confinement effect of the nanomeshs. The experimental results demonstrate that the MoS2 monolayers on N-doped graphene exhibit excellent electromagnetic wave absorption performance with a minimal reflection loss of –72.83 dB and an effective absorption bandwidth of 4.81 GHz even at a matching thickness below 2.0 mm, remarkably outperforming MoS2 nanosheets. The excellent consistency between theoretical and experimental results highlights that the DFT calculations can be employed as a design tool for high-performance electromagnetic wave absorber. Based on the excellent electromagnetic absorption performance of the MoS2 monolayers, a highly sensitive wireless pressure micro-sensor is designed, which has potential apllication in internet of things.

Signature of magnon polarons in electron relaxation on terbium revealed by comparison with gadolinium

Terbium and gadolinium have eight and seven 4$f$ electrons, respectively. This one-electron difference sparks surprisingly different behaviors. Here, the authors use spin-, angle-, and energy-dependent photoemission to investigate the relaxations of the photo-hole in the surface states of terbium and gadolinium metal. They find distinct, spin-dependent linewidths for gadolinium, in contrast to spin-independent relaxation for terbium. This peculiar difference is explained by strong magnon-phonon interaction in terbium leading to magnon polarons that couple to both spin-decay channels, in contrast to gadolinium.