Ultrafast dynamics of excess electrons in molten salts: Part II. Femtosecond investigations of Na–NaBr and Na–NaI melts

Brands, Helge; Chandrasekhar, N.; Hippler, H.; Unterreiner, Andreas‐Neil

doi:10.1039/b511392j

Cited by 6 publications

(14 citation statements)

References 26 publications

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“…They thus broaden the concept of intrinsic polaron trapping to disordered wide gap oxides. Localization of excess electrons in deep states has so far been observed in a small number of systems, such as polar [56,57] and non-polar [58,59] liquids, ammonia and water ice and amorphous films on metal substrates [60][61][62], and alkali halide melts [49,50,63,64]. In the latter case, bipolarons facilitated by fluctuations in the melt have also been observed and calculated [49][50][51], although electron polarons do not form in alkali halide crystals.…”

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confidence: 88%

Deep electron and hole polarons and bipolarons in amorphous oxide

et al. 2016

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Amorphous (a)-HfO2 is a prototype high dielectric constant insulator with wide technological applications. Using ab initio calculations we show that excess electrons and holes can trap in aHfO2 in energetically much deeper polaron states than in the crystalline monoclinic phase. The electrons and holes localize at precursor sites, such as elongated Hf-O bonds or under-coordinated Hf and O atoms and the polaronic relaxation is amplified by the local disorder of amorphous network. Single electron polarons produce states in the gap at ∼2 eV below the bottom of the conduction band with average trapping energies of 1.0 eV. Two electrons can form even deeper bipolaron states on the same site. Holes are typically localized on under-coordinated O ions with average trapping energies of 1.4 eV. These results advance our general understanding of charge trapping in amorphous oxides by demonstrating that deep polaron states are inherent and do not require any bond rupture to form precursor sites.Electron and hole states with energy levels lying deep in the bandgap impair the dielectric quality of insulating layers. In particular, electron transitions facilitated by these states account for multitude of degradation phenomena including enhanced leakage current and charge trapping eventually leading to the dielectric barrier failure and breakdown. Routinely, however, these deep electron states are seen as not inherent to the perfect material but rather associated with the presence of defects and/or impurity centers in the atomic network of an insulator. This belief offers some hope that imperfections can be eliminated by using more clean and optimized synthesis and proper processing of the insulators. On the other hand, self-trapping of excess charges in the form of small electron and hole polarons is well known to occur even in perfect crystalline oxide insulators. However, it is usually shallow, with trapping energies of the order of 0.2 eV (see e.g. [1-3]). As a result, the electron and hole polarons are mobile at room temperature in crystalline reduced TiO 2 and NiO [4], CeO 2 [5, 6], doped ZrO 2 [7,8], and in plethora of other oxides (see e.g. [1,[9][10][11][12]). The intrinsic localization of excess electrons and holes in noncrystalline materials and liquids has also been a subject of extensive experimental and theoretical studies pioneered in [13]. Structural disorder typically induces shallow electron states near the bottom of the conduction band, below the so-called mobility edge (see, e.g. [14] . In these systems, the polaronic relaxation is amplified by the local disorder of amorphous network.Here we turn to amorphous (a)-HfO 2 , which represents a wide class of high dielectric constant oxides recently emerged as the major contenders to replace SiO 2 in a broad spectrum of nano-electronic devices ranging from deep-scaled transistors to DRAM and non-volatile memory cells (see, e.g. [21,22]). Amorphous oxides make the backbone of most electronic devices and charge trapping appears to be the key factor determining devic...

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confidence: 88%

Deep electron and hole polarons and bipolarons in amorphous oxide

et al. 2016

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“…6 Briefly, calculated amounts of the metal and salt were transferred into the sample cell consisting of two sapphire plates with one plate having a small circular groove to serve as a reservoir. Detailed sample preparation procedures were described elsewhere.…”

Section: Methodsmentioning

confidence: 99%

“…Exemplarily, we show an absorption spectrum of Cs-CsI melts in Fig. 6 First, excess electrons in the latter system undergo relaxation on two distinct time scales, namely, subpicosecond and picosecond ͑see Ref. 2.…”

Section: A Transient Response At a Constant Temperature Of 903 Kmentioning

confidence: 99%

“…The formation of bipolaron species in M -MX mixtures was first predicted by molecular dynamics calculations by Parrinello and coworkers 8,11 and confirmed experimentally by Freyland and co-workers. 5,6 The contribution of bipolarons to the total number density of defect states was found not only to depend on the metal concentration but also on the size of the cation. Qualitatively, these observations could be explained by a thermodynamic defect model, 15 which is basically in line with a simple two component model describing highly doped semiconductors.…”

Section: Introductionmentioning

confidence: 99%

“…2 It was shown by von Blanckenhagen et al 2 that the tendency of bipolaron formation decreases progressively with increasing cation size. 5,6 It would be desirable, however, to develop an experimental access to investigate either polaronic or bipolaronic states alone as such studies would further enhance our present knowledge of the microstructural information of these molten solutions. 2 When bipolarons are formed, the increased Coulomb repulsion between the electrons has to be overcome by polarization of the surrounding medium.…”

Section: Introductionmentioning

confidence: 99%

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Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide

Chandrasekhar

Unterreiner

2007

The Journal of Chemical Physics

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Temperature-dependent investigations of excess electrons in molten solutions of cesium-doped cesium iodide (Cs-CsI) (mole fraction of Cs approximately 0.003) were performed applying femtosecond pump-probe absorption spectroscopy. The pulse-limited induced bleach observed at probe wavelengths from 600 to 1240 nm was attributed to the excitation of equilibrated excess electrons which were initially formed by melting a Cs-CsI mixture. The interpretation of the relaxation process is based on strongly localized polarons that constitute the majority of defect states in this melt. As expected, the bipolaron contribution was insignificant. The time constants (tau1) were found to be temperature dependent confirming our earlier findings in Na-NaI melts that ionic diffusion almost exclusively controls the dynamics of excess electrons in high temperature ionic liquids. Apart from this temperature dependence, the relaxation dynamics of excess electrons do not differ irrespective of the excitation regime (blue or red part of the respective stationary spectra).

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Photo- and Radiation-Chemistry of Halide Anions in Ionic Liquids

et al. 2013

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One- and two- photon excitation of halide anions (X(-)) in polar molecular solvents results in electron detachment from the dissociative charge-transfer-to-solvent state; this reaction yields a solvated halide atom and a solvated electron. How do such photoreactions proceed in ionic liquid (IL) solvents? Matrix isolation electron paramagnetic resonance (EPR) spectroscopy has been used to answer this question for photoreactions of bromide in aliphatic (1-butyl-1-methylpyrrolidinium) and aromatic (1-alkyl-3-methyl-imidazolium) ionic liquids. In both classes of ILs, the photoreaction (both 1- and 2-photon) yields bromine atoms that promptly abstract hydrogen from the alkyl chains of the IL cation; only in concentrated bromide solutions (containing >5-10 mol % bromide) does Br2(-•) formation compete with this reaction. In two-photon excitation, the 2-imidazolyl radical generated via the charge transfer promptly eliminates the alkyl arm. These photolytic reactions can be contrasted with radiolysis of the same ILs, in which large yield of BrA(-•) radicals was observed (where A(-) is a matrix anion), suggesting that solvated Br(•) atoms do not occur in the ILs, as such a species would form three-electron σ(2)σ(*1) bonds with anions present in the IL. It is suggested that chlorine and bromine atoms abstract hydrogen faster than they form such radicals, even at cryogenic temperatures, whereas iodine mainly forms such bound radicals. These XA(-•) radicals convert to X2(•-) radicals in a reaction with the parent halide anion. Ramifications of these observations for photodegradation of ionic liquids are discussed.

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Ultrafast dynamics of excess electrons in molten salts: Part II. Femtosecond investigations of Na–NaBr and Na–NaI melts

Cited by 6 publications

References 26 publications

Deep electron and hole polarons and bipolarons in amorphous oxide

Deep electron and hole polarons and bipolarons in amorphous oxide

Time-resolved polaron dynamics in molten solutions of cesium-doped cesium iodide

Photo- and Radiation-Chemistry of Halide Anions in Ionic Liquids

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