High Electronic Excitation Research Articles

Molecular dynamics simulation of surface phenomena due to high electronic excitation ion irradiation in amorphous silica

We studied by means of an atomistic model based on molecular dynamics the thermal evolution of surface atoms in amorphous silica under high electronic excitation produced by irradiation with swift heavy ions. The model was validated with the total and differential yields measured in sputtering experiments with different ions and ion energies showing a very good quantitative prediction capability. Three mechanisms are behind the evolution of the surface region: (1) an ejection mechanism of atoms and clusters with kinetic energy exceeding their binding energy to the sample surface, which explains the experimentally observed angular distributions of emitted atoms, and the correlation of the total sputtering yield with the electronic stopping power and the incidence angle. (2) A collective mechanism of the atoms in the ion track originated by the initial atom motion outwards the track region subsequently followed by the return to the resulting low-density region in the track center. The collective mechanism describes the energy dissipation of bulk atoms and the changes in density, residual stress, defect formation and optical properties. (3) A flow mechanism resulting from the accumulation and subsequent evolution of surface atoms unable to escape. This mechanism is responsible for the crater rim formation.Graphic Evolution of the molecular dynamics simulation of amorphous silica for 12 keV/nm electronic stopping power with a hot cylinder of radius 3.0 nm and an incidence angle of 20º

Nanofiber Semiconductor Experiment with Distance Variation Electrospinning Method to Improve DSSC Performance

Dye-Sensitized Solar Cell (DSSC) is a type of solar cell that uses dyes to transfer sunlight to electrical energy. DSSC construction uses a layered system (sandwich) that consisting of a working electrode and an opposing electrode, both of which are placed on conducting glass and electrolytes to allow electron cycling. This research aims to determine the effect of the distance between the tip and the rotating collector in an effort to increase the efficiency of Dye-Sensitized Solar Cell (DSSC) and examine its impact on the morphology of the ZnO nanofiber. This experiment is carried out by varying the distance between the tip to the rotating collector, which are 4 cm, 6 cm, and 8 cm. The results of this research indicates that at a distance of 8 cm it produces a small, uniform and regular ZnO nanofiber structure with Voc, Jsc, FF, and DSSC efficiency values of 0.559 V, 9.809 mA / cm2, 43.3% and 2.3%. In addition, at a distance of 8 cm it also produces the highest DSSC electrical efficiency from the other distances due to the absorbance of the dye and high electron excitation.

Ultrafast Laser Processing of Diamond Materials: A Review

Diamond laser engineering is of great importance for designing devices, which find applications in radiation sensing and quantum technologies. A review of the present state of the art of experimental and theoretical studies on ultrashort laser irradiation of diamond is presented. For a wide range of laser parameters, the optimization of laser-induced electronic, optical and structural modifications of diamond requires quantitative understanding of the microscopic processes underlying the high electronic excitation in the material.

Electronic Spectra of C60 Films Using Screened Range Separated Hybrid Functionals.

We study computationally the electronic spectra of C60 thin films using the recently developed density functional theory (DFT) framework combining a screened range separated hybrid (SRSH) functional with a polarizable continuum model (PCM). The SRSH-PCM approach achieves excellent correspondence between the frontier orbital's energy levels and the ionization potential and electron affinity of the molecular system at the condensed phase and consequently leads to high quality electronic excitation energies when used in time-dependent DFT calculations. Our calculated excited states reproduce the experimentally main reported spectral peaks at the 3.6-4.6 eV energy range and when addressing excitonic effects also reproduce the red-shifted spectral feature. Notably, we analyze the low-lying peak at 2.7 eV and associate it to an excitonic state.

Experimentally validated modeling of the optical energy deposition in highly ionized ambient air by strong femtosecond laser pulses

The deposition of femtosecond laser optical energy in gases leads to the emission of secondary electromagnetic and acoustic radiation. These optoacoustic components have a complex nonlinear dependency on the laser beam characteristics, such as the pulse energy, duration, wavelength and the focusing conditions, as well as on the optical and elastic characteristics of the gaseous medium. The initial interaction times are governed by the high electronic excitation and ionization. These phenomena result in a self-modulation of the laser pulse, significantly affecting the optical energy deposition on the medium. Such complex nonlinear phenomena are very difficult to be studied via analytical equations. To address this, a multiphysics Particle-In-Cell model is applied for the evaluation of the optical energy deposition and plasma generation from tightly focused femtosecond pulses in ambient air. The computational domain of the model is built to describe optical energy deposition in its full spatiotemporal scale. The model is validated by experimental results of the absorbed energy. The agreement between the computational and experimental results provides the basis for the future development of an advanced microstructural Finite Element Method model, which, combined with the Particle-In-Cell model, will have the ability of delivering detailed insights for all the sub-domains and timescales varying from nano- to femto-seconds of the laser-induced breakdown phenomenon.Graphic

Anisotropic nanostructure formation by vapor etching of ion tracks in α-quartz

In this study, latent and etched ion tracks generated by high electronic excitation in alpha quartz (α-SiO2) were characterized. Single crystals of Y- and Z-cut α-SiO2 were irradiated at room temperature with 20 MeV Ni6+ ions and 40 MeV I7+ ions. The track morphology depends on the energy of the incident ion and the stopping power on the target material. Subsequent chemical vapor-etching with hydrofluoric acid solutions was conducted with varying etching times and acid concentrations. The vapor etching process produced nanostructures whose dimensions increased with etching time and etchant concentrations. Y-cut samples etched more slowly than Z-cut samples and exhibited anisotropic track etching behavior. Production of nanowells with different aspect ratios was accomplished by altering the etching time and etchant concentration. The nanowells were characterized by Atomic Force Microscopy. The etched nanostructure templates could be used in the fabrication of novel nanodevices with unique optical, thermal, and electronic properties.

The Effects Induced by Microwave Field upon Tungsten Wires of Different Diameters.

The effects induced by microwave field upon tungsten wires of different diameters were investigated. Tungsten wires with 0.5 and 1.0 mm diameters were placed in the focal point of a single-mode cylindrical cavity linked to a microwave generator and exposed to microwave field in ambient air. The experimental results showed that the 0.5 mm diameter wire was completely vaporized due to microwaves strong absorption, while the wire with 1 mm diameter was not ignited. During the interaction between microwaves and tungsten wire with 0.5 mm diameter, a plasma with a high electronic excitation temperature was obtained. The theoretical analysis of the experiment showed that the voltage generated by metallic wires in interaction with microwaves depended on their electric resistance in AC and the power of the microwave field. The physical parameters and dimension of the metallic wire play a crucial role in the ignition process of the plasma by the microwave field. This new and simple method to generate a high-temperature plasma from a metallic wire could have many applications, especially in metal oxides synthesis, metal coatings, or thin film deposition.

Nonthermal phase transitions in metals

It is well known that sufficiently thick metals irradiated with ultrafast laser pulses exhibit phonon hardening, in contrast to ultrafast nonthermal melting in covalently bonded materials. It is still an open question how finite size metals react to irradiation. We show theoretically that generally metals, under high electronic excitation, undergo nonthermal phase transitions if material expansion is allowed (e.g. in finite samples). The nonthermal phase transitions are induced via an increase of the electronic pressure which leads to metal expansion. This, in turn, destabilizes the lattice triggering a phase transition without a thermal electron-ion coupling mechanism involved. We find that hexagonal close-packed metals exhibit a diffusionless transition into a cubic phase, whereas metals with a cubic lattice melt. In contrast to covalent solids, nonthermal phase transitions in metals are not ultrafast, predicative on the lattice expansion.

Nonthermal phase transitions in irradiated oxides

It is predicted theoretically that various oxides (Al2O3, MgO, SiO2 and TiO2) under ultrafast excitation of the electronic system exhibit nonthermal phase transitions. In the bulk, Al2O3 transiently forms a superionic phase via nonthermal phase transition, MgO and SiO2 disorder, TiO2 experiences solid–solid phase transition while thermal effects lead to melting. In the finite-size samples and near-surface regions, MgO undergoes solid–solid phase transition at lower doses than those required for atomic disorder. All studied oxides but TiO2, if allowed to expand, exhibit a lower damage threshold, whereas in TiO2 expansion releases the stress and prevents solid–solid phase transition thereby increasing the damage threshold up to the melting one. The results suggest that a nonthermal phase transition is a general response of oxides to sufficiently high ultrafast electronic excitation. A comparison with nonadiabatic simulations demonstrates that Born–Oppenheimer approximation systematically overestimates damage thresholds, and in some cases misses a phase transition entirely.

Relaxation of electronic excitations in ions in cold krypton plasma

Reaction rate constants have been calculated for electronic transitions in ions and for their decay as induced by collisions with krypton atoms and/or spontaneous radiation processes. The rate constants have then been used in a series of modelings of electronic relaxation in the ions in cold krypton plasmas with the main focus on relaxation times and final states. It has been shown that the collision-induced (non-radiative) relaxation is much faster than the radiative one and completely dominates with typical relaxation times ranging between nanoseconds and microseconds. The relaxation always ends up in the electronic ground state, high electronic excitations survive for longer times than lower excitations due to spin–orbit coupling effects.

“Thermal Spike” model applied to thin targets irradiated with swift heavy ion beams at few MeV/u

High electronic excitations in radiation of metallic targets with swift heavy ion beams at the coulomb barrier play a dominant role in the damaging processes of some metals. The inelastic thermal spike model was developed to describe tracks in materials and is applied in this paper to some systems beams/targets employed recently in some nuclear physics experiments. Taking into account the experimental conditions and the approved electron-phonon coupling factors, the results of the calculation enable to interpret the observation of the fast deformation of some targets.

Applying Tersoff-potential and bond-softening models in a molecular dynamics study of femtosecond laser processing

In the molecular dynamics study of short-pulsed laser processing of semiconductors, potential models capable of describing the atomistic behavior during high electronic excitations is the most critical issue at the current stage. This study of the molecular dynamics adopts the Tersoff-potential model to analyze the ultrafast laser processing of silicon. The model was modified to include electronic excitation effects by reducing the attraction of the antibonding state by half. It offers an excellent description of the experimental behavior during nonthermal melting. Subpicosecond melting is achieved above certain threshold levels of superheating and carrier density as required in experiments. Energy conservation is demonstrated with a bandgap energy of the order obtained in experiments. The modification of the potential mimics an absorption of bandgap energy and a subsequent lattice heating on a time scale within 0.3 ps. The melting kinetics establishes a correlation between nonthermal melting and thermal bulk melting. For superheating of less than two, the electronic melting of bond softening proceeds via homogeneous nucleation. The associated thermal theory, corrected with a limit on the nucleus radius to bond length, is still valid for the higher superheating regime. The original Tersoff model shows that this superheating by a factor of two is isothermal for spallation—the lowest-energy ablative mechanism. Its proximity to the evaporating point suggests the role of thermal boiling during spallation.

Promoting ozone synthesis from oxygen by a high performance volume-surface hybrid discharge

A high performance hybrid discharge reactor with a flat-plate configuration, in which volume and surface discharges can occur simultaneously, is designed to achieve a high ozone energy yield at high concentrations. Compared with the single volume/surface dielectric barrier discharge, this hybrid discharge acquires much higher discharge voltage, number density of current pulses, and thus discharge power at the same input power. The optical emission spectroscopy diagnosis indicates that the designed reactor possesses a relatively high electron density, excitation temperature, and low rotational temperature, which facilitates the production of ozone. The excellent performance of the hybrid discharge in ozone production from oxygen demonstrates its high energy efficiency and reliability for the practical application of the ozone synthesis.

Real-Time Observation of a Coherent Lattice Transformation into a High-Symmetry Phase

Excursions far from their equilibrium structures can bring crystalline solids through collective transformations including transitions into new phases that may be transient or long-lived. The direct spectroscopic observation of far-from-equilibrium rearrangements provides fundamental mechanistic insight into chemical and structural transformations and a potential route to practical applications, including ultrafast optical control over material structure and properties. However, in many cases, photoinduced transitions are irreversible or only slowly reversible, or the light fluence required exceeds material damage thresholds. This requirement precludes conventional ultrafast spectroscopy, in which optical excitation and probe pulses irradiate the sample many times, each measurement providing information about the sample response at just one probe delay time following excitation, with each measurement at a high repetition rate and with the sample fully recovering its initial state in between measurements. Using a single-shot, real-time measurement method, we are able to observe the photoinduced phase transition from the semimetallic, low-symmetry phase of crystalline bismuth into a high-symmetry phase whose existence at high electronic excitation densities is predicted based on earlier measurements at moderate excitation densities below the damage threshold. Our observations indicate that coherent lattice vibrational motion launched upon photoexcitation with an incident fluence above 10 mJ/cm2 in bulk bismuth brings the lattice structure directly into the high-symmetry configuration for several picoseconds, after which carrier relaxation and diffusion restore the equilibrium lattice configuration. Published by the American Physical Society 2018

Excitonic gain and laser emission from mixed-cation halide perovskite thin films

Organic–inorganic halide perovskites have recently developed into a potential semiconductor coherent light emitter candidate beyond their promise in solar cell applications. However, despite the ample demonstrations of perovskite lasers, experimental results on the origin of optical gain in perovskites are still elusive. Here, we analyze the excitonic gain in the green from mixed-cation halide perovskites Cs0.17[CH(NH2)2]0.83PbBr3 (Cs0.17FA0.83PbBr3) by both low temperature absorption/emission spectroscopies and ultrafast pump–probe transient absorption experiments. The perovskite thin films show a robust excitonic feature up to room temperature, with estimated exciton binding energy E b =43.8 meV, which can be maintained under high electronic excitations that are required for lasers. By using a high-quality (Q=1350) vertical cavity consisting of sputtered dielectric HfO2/SiO2 distributed Bragg reflectors with perovskite optical gain medium embedded inside, we have demonstrated excitonic-gain-enabled optically pumped lasing, with improved threshold of 13.5±1.4 μJ/cm2 and device longevity lifetime >35 h (108 laser shots) at ambient environment under sustained pulsed optical excitations (3.493 eV, τpulse=0.34 ns, 1 kHz). Understanding and exploiting excitonic gain from perovskite thin film materials may help to further boost the performance of perovskite-based lasers.

Permanent modifications in silica produced by ion-induced high electronic excitation: experiments and atomistic simulations

The irradiation of silica with ions of specific energy larger than ~0.1 MeV/u produces very high electronic excitations that induce permanent changes in the physical, chemical and structural properties and give rise to defects (colour centres), responsible for the loss of sample transparency at specific bands. This type of irradiation leads to the generation of nanometer-sized tracks around the ion trajectory. In situ optical reflection measurements during systematic irradiation of silica samples allowed us to monitor the irradiation-induced compaction, whereas ex situ optical absorption measurements provide information on colour centre generation. In order to analyse the results, we have developed and validated an atomistic model able to quantitatively explain the experimental results. Thus, we are able to provide a consistent explanation for the size of the nanotracks, the velocity and thresholding effects for track formation, as well as, the colour centre yield per ion and the colour centre saturation density. In this work we will discuss the different processes involved in the permanent modification of silica: collective atomic motion, bond breaking, pressure-driven atom rearrangement and ultra-fast cooling. Despite the sudden lattice energy rise is the triggering and dominant step, all these processes are important for the final atomic configuration.

Virtual gap states induced modifications in charge neutrality level in cadmium oxide thin films

Present manuscript reports on the creation of virtual gap states (ViGS) in sol–gel spin coated cadmium oxide thin films by tin doping and irradiation at different density of electronic excitations (EEs) induced by energetic ions. Effects of ViGS on charge neutrality level (CNL), which affects the modifications in band gap (BG) of such oxide semiconductors are investigated. Studies reveal that there were insignificant changes in the structural and morphological properties of the films. Interestingly, the transmittance spectrum shows insignificant changes in the films irradiated by low density EEs but an overall decrease in transmittance for high density EEs. However, Raman spectroscopy reveals a significant change in longitudinal optical (LO) modes with tin doping which get intensified and they exhibit an insignificant influence upon irradiation. The BG with respect to bulk is observed to be increased by 0.57 eV and 0.83 eV upon tin doping and irradiation, respectively. These variations are attributed to the combined effect of Burstein–Moss shift (BMS) and ViGS, which were created by tin doping and EEs. It is observed that density of ViGS strongly depends on the density of EEs. A schematic band diagram is also developed for demonstrating the observed influence of ViGS on band gap modifications. Thus the reported investigations are very interesting for the in-depth understanding of fundamental interactions and besides their possible potential applications in the development of optoelectronic devices.

Structure of ion tracks in ceria irradiated with high energy xenon ions

Structure and accumulation behavior of ion tracks in CeO2 irradiated with 200 MeV Xe ions were examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to obtain fundamental knowledge on the microstructure evolution induced by fission fragments in nuclear fuels and transmutation targets, which is of importance for the development of advanced fuel/target materials at high burn-up conditions. Bright-field (BF) TEM images of ion tracks from an inclined direction showed Fresnel contrast along penetrating path of incident ions. The signal intensity of high-angle annular dark-field (HAADF) STEM images was decreased at the core damage region of ion tracks along the path of ions, revealing the reduction of atomic density inside the ion track. Preferential formation of smaller and larger ion tracks was observed at a high ion fluence of 1 × 1014 cm−2 compared to a low ion fluence of 1 × 1011 cm−2. Results were discussed due to the coalescences and incomplete recovery of the core damage regions during the overlap of high density electronic excitation damage, which is induced during the repetition of the formation and recovery of ion tracks within an influence region.

Structure of Defects and Microstructure Evolution in Oxide Ceramics – Role of Electronic Excitation and Selective Displacement Damage

Fluoriteand spinel-type oxide ceramics, such as yttria-stabilized zirconia, ceria, magnesium aliminate spinel, are known to be exceptionally resistant to irradiation damage with energetic particles. Those oxide compounds, thereofore, have potential applications to advanced nuclear fuels and innert matrices of transmutaion targets. Fundamental understanding of radiation damage is apparently essential to assess the microstructure stability of fuel/target materials under the hostile environment. In this presentation, structure and stability of radiation-induced defects in the oxide ceramics is reported. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is utilized to examine the following topics focucing on role of electronic excitation and selective displacement damage: (1) effects of selective displacement damage of oxygen sublattice in fluorite-type oxides, such as stabilided cubic ZrO2, CeO2 [1-3], (2) atomic structure of ion tracks in CeO2 and MgAl2O4 induced by swift heavy ions, and microstructure evolution at high fluences under overlapping irradiation of high density electronic excitation damage [4-6], and (3) production and stability of defects in MgAl2O4 and alumina (-Al2O3) under synergistic irradiation with dispacement damage and electronic excitation [7].

Modification of photosensing property of CdS–Bi2S3 bi-layer by thermal annealing and swift heavy ion irradiation

The CdS–Bi2S3 bi-layer thin films have been deposited on Indium Tin Oxide (ITO) glass substrates at room temperature by Chemical Bath Deposition Technique (CBD) and bi-layer thin films were annealed in air atmosphere for 1 h at 250 °C. The air annealed sample was irradiated using Au9+ ions at the fluence 5 × 1011 ion/cm2 with 120 MeV energy. Effects of Swift Heavy Ion (SHI) irradiation on CdS–Bi2S3 bi-layer thin films were studied. The results are explained on the basis annealing and high electronic excitation, using X-ray diffraction (XRD), Selective Electron Area Diffraction (SEAD), Atomic Force Microscopy (AFM), Raman Spectroscopy, UV spectroscopy and I–V characteristics. The photosensing property after illumination of visible light over the samples is studied. These as-deposited, annealed and irradiated bi-layer thin films are used to sense visible light at room temperature.