Ultrafast dynamics of excess electrons in a molten salt: Femtosecond investigation of K–KCl melts

Dogel, Stanislav A.; Freyland, W.; Hippler, H.; Nattland, D.; Nese, Chandrasekhar; Unterreiner, Andreas‐Neil

doi:10.1039/b303378c

Cited by 10 publications

(17 citation statements)

References 15 publications

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“…The time constant t 1 ¼ (200 AE 50) fs for Na-NaBr is found to be equal to the value reported earlier for K-KCl melt 20 at the same temperature and has been attributed to polaron dynamics. Furthermore, this time constant is in fairly good agreement with predictions from theoretical calculations.…”

Section: Polaron Dynamicssupporting

confidence: 72%

“…Ultrafast investigations of the dynamics of excess electron in molten alkali halides are scarce, but may allow for a direct observation of these processes. In our previous studies 20,21 on K-KCl and Na-NaBr melts we identified a fast (B200 fs) bleach recovery attributable to the relaxation to strongly localized electrons (polarons). Furthermore, we observed a signature of bipolaron dynamics for Na-NaBr melts with a time constant of about 3 ps.…”

Section: Introductionmentioning

confidence: 84%

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

Brands

Chandrasekhar

Hippler

et al. 2005

Phys. Chem. Chem. Phys.

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Ultrafast dynamics of excess electrons in Na-NaBr and Na-NaI molten solutions at elevated temperatures (T = 953-1128 K) were investigated over an extended wavelength range. Modelling the time profiles resulted in two time constants tau 1 = (200 +/- 40) fs and tau 2 = (2.8 +/- 0.4) ps for both systems at 1073 K. All transients can be understood in terms of dynamical equilibria between polaron and Drude-type electrons as well as polaron and Drude-type electron forming bipolarons. In agreement with our earlier results for K-KCl melts the fast component is assigned to the relaxation of Drude-type electrons into polarons while the longer component, tau 2, represents the time during which Drude-type electrons recombine with polarons leading to bipolarons. In addition, the temperature dependence was studied in Na-Nal: Decreasing the temperature to 953 K resulted in an increase of the time constants to tau 1 = (360 +/- 50) fs and tau 2 = (4.3 +/- 0.7) ps, respectively. At temperatures, where the ionic diffusion in Na-NaI melts becomes comparable to Na-NaBr melts, the time constants for the relaxation processes also coincide. The temperature-dependent investigations resulted in an Arrhenius activation energy of (25 +/- 5) kJ mol(-1) for Na-NaI melts in good agreement with literature data.

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Section: Polaron Dynamicssupporting

confidence: 72%

Section: Introductionmentioning

confidence: 84%

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

Brands

Chandrasekhar

Hippler

et al. 2005

Phys. Chem. Chem. Phys.

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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.…”

mentioning

confidence: 86%

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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“…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: 98%

“…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%

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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Ultrafast dynamics of excess electrons in a molten salt: Femtosecond investigation of K–KCl melts

Cited by 10 publications

References 15 publications

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

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

Deep electron and hole polarons and bipolarons in amorphous oxide

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

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