Role Of Electronic Excitations Research Articles

Excitation Dependence in Photoiniferter Polymerization.

We report on a fundamental feature of photoiniferter polymerizations mediated with trithiocarbonates and xanthates. The polymerizations were found to be highly dependent on the activated electronic excitation of the iniferter. Enhanced rates of polymerization and greater control over molecular weights were observed for trithiocarbonate- and xanthate-mediated photoiniferter polymerizations when the n → π* transition of the iniferter was targeted compared to the polymerizations activating the π → π* transition. The disparities in rates of polymerization were attributed to the increased rate of C-S photolysis which was confirmed using model trapping studies. This study provides valuable insight into the role of electronic excitations in photoiniferter polymerization and provides guidance when selecting irradiation conditions for applications where light sensitivity is important.

Role of electronic excitation on the anomalous magnetism of elemental copper

The magnetic susceptibility of elemental copper (Cu) shows an anomalous rise at low temperatures superimposed on the expected atypical diamagnetic response. Such temperature-dependent susceptibility, which is also known as the Curie tail, cannot be explained on the basis of the Larmor diamagnetic and Pauli paramagnetic contributions expected in Cu. Using valence band resonant photoemission spectroscopy results and density functional theory calculations, we show the magnetic anomaly appears due to the presence of holes in the Cu $3d$ band, which originates from a thermally excited electronic configuration. Our study therefore highlights that the Curie tail, which is generally overlooked presuming it is either due to paramagnetic impurities or defects, can in fact be intrinsic to a material, and even simple systems such as elemental Cu are susceptible to electronic excitations giving rise to an anomalous magnetic state.

Transmission electron microscopy study of extended defect evolution and amorphization in SiC under Si ion irradiation

ABSTRACTThe damage induced in 3C‐SiC epilayers on a silicon wafer by 2.3‐MeV Si ion irradiation for fluences of 1014, 1015, and 1016 cm−2, was studied by conventional and high‐resolution transmission electron microscopy (TEM/HRTEM). The evolution of extended defects and lattice disorder is followed in both the 3C‐SiC film and Si substrate as a function of ion fluence, with reference to previous FTIR spectroscopy data. The likelihood of athermal unfaulting of native stacking faults by point defect migration to the native stacking faults is discussed in relation to damage recovery. Threshold energy densities and irradiation doses for dislocation loop formation and amorphous phase transformation are deduced from the damage depth profile by nuclear collisions. The role of electronic excitations on the damage recovery at high fluence is also addressed for both SiC and Si.

Formation of Defects in Two-Dimensional MoS2 in the Transmission Electron Microscope at Electron Energies below the Knock-on Threshold: The Role of Electronic Excitations.

Production of defects under electron irradiation in a transmission electron microscope (TEM) due to inelastic effects has been reported for various materials, but the microscopic mechanism of damage development in periodic solids through this channel is not fully understood. We employ non-adiabatic Ehrenfest, along with constrained density functional theory molecular dynamics, and simulate defect production in two-dimensional MoS2 under electron beam. We show that when excitations are present in the electronic system, formation of vacancies through ballistic energy transfer is possible at electron energies which are much lower than the knock-on threshold for the ground state. We further carry out TEM experiments on single layers of MoS2 at electron voltages in the range of 20-80 kV and demonstrate that indeed there is an additional channel for defect production. The mechanism involving a combination of the knock-on damage and electronic excitations we propose is relevant to other bulk and nanostructured semiconducting materials.

Role of electronic excitations and nuclear collisions for color center creation in AlxGa1−xN semiconductors

In this work, AlxGa1−xN (x=0; 0.1; 0.3; 0.5; 0.65; 0.7; 0.8; 1) wurtzite epilayers, grown on c-plane sapphire substrates, have been irradiated with Swift Heavy Ions at GANIL facility. Modifications induced by irradiation are characterized with in-situ optical absorption spectroscopy at 15K. Spectra of these irradiated alloys exhibit optical absorption band formation, related to new energy levels in their bandgaps, whose positions only depend on the composition of the layer. However, these absorption bands are not observed in the AlxGa1−xN with Al molar fraction less than 0.3, likely because the energy level of the corresponding defect is located above the conduction band. Moreover, using different irradiation conditions, a coupled effect between nuclear collisions and electronic excitations for these color center creation have been investigated. A synergy between these two phenomena has been shown and appears to be independent of the composition of the alloy.

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].

Role of Electronic Excitations in Ground-State-Forbidden Inelastic Collisions Between Ultracold Atoms and Ions

The role of electronic excitation in inelastic collisions between ultracold Ca atoms and Ba(+) ions, confined in a hybrid trap, is studied for the first time. Unlike previous investigations, this system is energetically precluded from undergoing inelastic collisions in its ground state, allowing a relatively simple experimental determination and interpretation of the influence of electronic excitation. It is found that while the electronic state of the ion can critically influence the inelastic collision rate, the polarizability mismatch of the neutral atom electronic states suppresses short-range collisions, and thus inelastic processes, involving electronically excited neutral atoms. As a result of these features, it is experimentally demonstrated that it is possible to mitigate inelastic collision loss mechanisms in these systems, marking an important step toward long-lived hybrid atom-ion devices.

Role of electronic excitations in magneto-Raman spectra of graphene

We investigate the signature of the low-energy electronic excitations in the Raman spectrum of monolayer and bilayer graphenes. The dominant contribution to the Raman spectra is due to the interband electron–hole (e–h) pairs, which belong to the irreducible representation A2 of the point group C6v of the graphene lattice, and are characterized by crossed polarization of incoming and outgoing photons. At high magnetic fields, this is manifested by the excitation of e–h inter-Landau-level (LL) transitions with selection rule n− → n+. Weaker Raman-active inter-LL modes also exist. One of those has a selection rule similar to the infrared absorption process, n− → (n ± 1)+, but the created e–h excitation belongs to the irreducible representation E2 (rather than E1) and couples to the optical phonon mode, thus undergoing an anticrossing with the optical phonon G-line in Raman in a strong magnetic field. The fine structure acquired by the G-line due to such anticrossing depends on the carrier density, inhomogeneity of doping and presence of inhomogeneous strain in the sample.

Role of electronic excitation in phase‐change memory materials: A brief review

AbstractPhase‐change memory (PCM) materials, such as chalcogenide alloys, have the ability for fast and reversible transition between their amorphous and crystalline states. Owing to the large optical/electrical contrast of the two states, PCM materials have been developed for data storage. It has been generally accepted that thermal effects, caused by laser irradiation or electrical pulses, control the amorphization by melting the sample and subsequent quenching, while crystallization is realized by thermal annealing. An important element that has not been considered extensively, however, is the role of electronic excitation by optical or electrical pulse. Strictly speaking, until electrons and holes recombine, the system under external stimulus is in a non‐equilibrium environment, especially when the excitation intensity is high. This raises an important question: can the excitation alone induce phase transition for PCM data storage without the usual thermal melting? Here, we will review the recent experimental and theoretical indications and evidence in support of the electronic excitation‐induced phase change in PCM materials and discuss potential ramifications of the athermal phase‐change phenomenon for data storage.

Electron-Ion Coupling in Shocked Energetic Materials

The magnitude and role of electronic excitations in shocked energetic materials are studied theoretically using quantum molecular dynamics simulations. Focusing on the detonating primary explosive ...

Role of Electronic Excitation in the Amorphization of Ge-Sb-Te Alloys

First-principles molecular dynamics simulation reveals the effects of electronic excitation in the amorphization of Ge-Sb-Te. The excitation makes the phase change an element-selective process, lowers the critical amorphization temperature considerably, for example, to below 700K at a 9% excitation, and reduces the atomic diffusion coefficient with respect to that of melt by at least 1 order of magnitude. Noticeably, the resulting structure has fewer wrong bonds and significantly increased phase-change reversibility. Our results point to a new direction in manipulating ultrafast phase-change processes with improved controllability.

Role of electronic excitation on thermodynamic and transport properties of argon and argon-hydrogen plasmas

Thermodynamic and electron transport properties of the argon and argon-hydrogen plasmas have been calculated under the local thermodynamic equilibrium conditions in temperature range of 10 000-40 000 K over the wide range of pressures. Electronic excitation affects strongly these properties especially at high pressures. The inclusion of electronically excited states (EES) in relevant partition function influences the internal contribution to frozen and total specific heat for argon and argon-hydrogen plasma and it has been observed that although the total specific heat of argon plasma is less than that of hydrogen plasma, yet its internal contribution is more. Compensation between different contributions to total specific heat (by including and neglecting EES) occurring in hydrogen plasmas at low pressures has not been observed in argon and argon-hydrogen plasmas. As electron transport properties strongly depend upon the degree of ionization, therefore larger relative errors are found for these properties with and without EES, and in contrast to hydrogen plasma there exist a dominance of electron-atom cross section at low temperatures and EES dominance at intermediate temperatures.

Contribution of Electronic Energy Deposition to the Atomic Cascade Damage in Nanocrystals

AbstractUsing Molecular Dynamics we study the role of electronic excitations in the radiation damage caused by an energetic ion in Ge nanocrystals embedded in amorphous SiO2. The electronic effects are included as heating along the ion path modeled by the thermal spike model. In an ion energy regime where the electronic stopping power is larger than the nuclear, we find that the electronic effects enhance the defect creation significantly. We conclude that the electronic excitations below the track production threshold due to an energetic ion cannot be disregarded as a source of radiation damage.

Modification of atomic solids via electronic subsystem probed by spectroscopic methods

The role of electronic excitations in the modification of surface properties of atomic solids (AS) is considered using solid Kr as an example. The study is focused on the novel phenomenon found recently in AS: anomalous low temperature “post-desorption” (ALTpD) observed after irradiation of AS with low-energy electron beam. The experiments were performed employing cathodoluminescence (CL) method in combination with correlated in real time measurements of thermally stimulated luminescence (TSL), thermally stimulated exoelectron emission (TSEE) and detection of the “post-desorption” yield. Analysis of the yields correlation enabled us to verify a two-stage mechanism of the ALTpD: (i) thermally assisted release of electrons from their traps followed by (ii) intrinsic charge recombination resulting in “excimer dissociation” via radiative transition to a repulsive part of the ground state potential curve.

Modelling swift heavy ion irradiation in iron

Swift heavy ions moving in metals lose most of their energy to inelastic scattering of electrons. The energy deposited in the electronic system is transferred into the atomic system via electron–ion interactions and can lead to melting and creation of new damage and also annealing of pre-existing atomic defects. Using a combination of molecular dynamics and a consistent treatment of electron energy transfer and transport we have modelled experiments performed in Fe to investigate the annealing effect and damage creation under electronic excitations. We observe both annealing and new damage creation at low and high electronic stopping, respectively. Rapid separation of interstitial atoms and vacant lattice sites is seen due to efficient transport via replacement collision sequences. Our results suggest that the role of electronic excitation can be significant in modeling of the behaviour of metals under swift heavy ion irradiation and attempts to modify metals via ion implantation.

Photo-induced surface functionalization of carbon surfaces: The role of photoelectron ejection

Carbon-based materials are attractive for a wide range of applications, from biomaterials to fuel cells; however, their effective use often requires controlling the surface chemistry to incorporate recognition moieties or reactive centers. The high stability of carbon also makes it a challenging material to functionalize; recently, the use of ultraviolet light (254nm) to initiate functionalization of carbon surfaces has emerged as a way to obtain carbon/organic interfaces with tailored properties. The authors have investigated the mechanism of covalent grafting of amorphous carbon surfaces with functional organic molecules using the photochemical reaction of terminal alkenes. Measurements comparing the reactivity of different n-alkenes bearing different terminal groups at the terminus opposite the olefin showed pronounced differences in reactivity. They characterized the rate and final coverage of the resulting organic layers using x-ray photoelectron spectroscopy and infrared reflection-absorption spectroscopy. Ultraviolet photoelectron spectroscopy and photocurrent measurements suggested that the reaction involves photoelectron emission from the carbon surface into the liquid phase. Density functional calculations show a strong correlation between the electron affinity of the alkenes and the observed reactivity. The specific terminal group opposite to the olefin was found to play an important role in the stabilization of excess negative charges on the molecule, thus explaining the strong dependence of reactivity on the particular terminal group. These findings suggest that the reaction involves injection of photoelectrons into the alkene acceptor levels, leading to the formation of radical anions in the liquid phase. Finally, the authors demonstrate that the grafting of marginally reactive alkenes can be enhanced by seeding the surface with a small amount of good electron accepting groups. These results provide fundamental new insights into the role of electronic excitations in controlling rates and mechanisms of olefin reactions at surfaces. While demonstrated here for amorphous carbon, these results may also be significant for the UV initiated grafting of olefins on other semiconductors.

Interatomic potentials for simulating radiation damage effects in metals

We critically review the recent developments in the field of semi-empirical interatomic potentials for molecular dynamics simulations, with particular emphasis on the requirements and criteria associated with the simulations of radiation damage effects in metals. We address a range of issues including the suitability of potentials for large-scale simulations, the role of electronic excitations and electron energy losses, and the part played by the dynamics of internal degrees of freedom of atoms, for example magnetic excitations. To cite this article: K. Nordlund, S.L. Dudarev, C. R. Physique 9 (2008).

Role of Electronic Excitations in Ion Collisions with Carbon Nanostructures

By combining ab initio time-dependent density functional calculations for electrons with molecular dynamics simulations for ions in real time, we investigate the microscopic mechanism of collisions between energetic protons and graphitic carbon nanostructures. We identify not only the amount of energy lost by the projectile, but also the electronic and ionic degrees of freedom of the target that accommodate this energy as a function of the impact parameter and projectile energy. Our results establish validity limits for the Born-Oppenheimer approximation and the threshold energy for defect formation in carbon nanostructures.

Electronic Excitations in Initiation of Chemistry in Molecular Solids

ABSTRACTAn ab initio study is performed for the initiation of chemistry in high explosive crystals from a solid-state physics viewpoint. Specifically, we are looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the Hartree- Fock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the results obtained, an excitonic mechanism of the earliest stages for initiation of high explosive solids is discussed with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental verification of the validity of the proposed model is reported for RDX and heavy metal azides. Thus, the key role of electronic excitations facilitated by edge dislocations in explosive solids is established and analyzed. Practical applications of the suggested mechanisms are discussed.

Collision-induced dissociation dynamics in O2++Ne(Ar) collisions: The role of electronic excitation

Integral collision-induced dissociation (CID) cross sections at center-of-mass (CM) energies ranging from threshold to 22 eV and product ion recoil velocity distributions are presented for O2++Ne(Ar) collisions. The O2+ ions are produced in an electron impact ion source and thus have ∼0.5 eV of vibrational energy. The O2++Ne CID cross sections are observed at energies below the 0 K thermodynamic threshold of 6.66 eV, implying the possibility of significant vibrational effects. A second CID threshold is apparent at 9±1 eV. This is close to the charge-transfer (CT) threshold energy. No CT products are observed. The O2++Ar CID cross section rises rapidly above a relative translational energy onset of 7.5±0.2 eV and reaches a maximum value of ∼0.8 Å2 at 14 eV (CM). A CT onset of 6.0±0.2 eV is observed which is close to the thermodynamic threshold for CID. The energy dependence of the CT cross section is very similar to the CID cross section except for a small step at energies where CID becomes efficient. The CID recoil velocity distributions near threshold are centered at the CM velocity and shift to forward preference at higher energies. The O2++Ar CT recoil velocities are forward scattered near threshold and become centered with respect to the CM velocity at energies where competition with CID is apparent.