Solvation dynamics of surface-trapped electrons at NH3 and D2O crystallites adsorbed on metals: from femtosecond to minute timescales

Stähler, Julia; Meyer, Michael; Bovensiepen, Uwe; Wolf, Martin

doi:10.1039/c0sc00644k

Cited by 16 publications

(27 citation statements)

References 49 publications

(93 reference statements)

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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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“…Previous ultrafast surface science investigations have shown that the dynamics of an electronic state in a molecular film can differ based on the molecular properties of the film, the electronic state localization, influence of the metal substrate, and molecular film thickness. 8,10,[27][28][29][30][31][32][33][34][35] The contrasting dynamics of electron localization in nonpolar n-heptane versus polar water and ammonia highlight the influence of the molecular properties of the film. In ultra-thin non-polar n-heptane layers on a Ag(111) surface, electron injection leads to the formation of small polarons that are self-trapped via the in-phase methylene rocking of n-heptane on an approximately 360 fs timescale.…”

Section: Introductionmentioning

confidence: 99%

“…4,36,37 In frozen water and ammonia, electrons form solvated electrons in pre-existing electron traps on ultrafast timescales. 28,31,34 Solvated electrons are electrons cooperatively supported by the dipole moments of the solvent and are typically described in a cavity or pseudo-cavity model. 38,39 For many electronic states near metal interfaces, recombination with the metal substrate is the primary decay mechanism that determines the electronic state lifetime.…”

Section: Introductionmentioning

confidence: 99%

“…In both frozen ammonia and ionic liquids, for instance, the lifetime of the localized solvated electron increases as a function of coverage as the electrons are separated from the metal by more and more dielectric material, which reduces the electron transfer rate. 31,40,41 In molecular films with strong screening this can lead to states with lifetimes on the order of seconds to minutes, as observed in crystalline ice, crystalline ammonia, and amorphous solid water. 31,[42][43][44][45] The extremely long lifetimes of these normally unoccupied electronic states is sufficient for chemical reactivity both within molecular layers as well as with molecules exposed from the gas-phase.…”

Section: Introductionmentioning

confidence: 99%

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Multistep and multiscale electron transfer and localization dynamics at a model electrolyte/metal interface

King

Broch

Demling

et al. 2018

The Journal of Chemical Physics

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The lifetime, coupling, and localization dynamics of electronic states in molecular films near metal electrodes fundamentally determine their propensity to act as precursors or reactants in chemical reactions, crucial for a detailed understanding of charge transport and degradation mechanisms in batteries. In the current study, we investigate the formation dynamics of small polarons and their role as intermediate electronic states in thin films of dimethyl sulfoxide (DMSO) on Cu(111) using time-and angle-resolved two-photon photoemission spectroscopy. Upon photoexcitation, a delocalized DMSO electronic state two monolayers from the Cu surface is initially populated, becoming a small polaron on a 200 fs timescale, consistent with localization due to vibrational dynamics of the DMSO film. The small polaron is a precursor state for an extremely long-lived and weakly-coupled multilayer electronic state, with a lifetime of several seconds, thirteen orders of magnitude longer than the small polaron. Although the small polaron in DMSO has a lifetime of 140 fs, its role as a precursor state for long-lived electronic states could make it an important intermediate in multistep battery reactivity.

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“…50,270,271,272 The processes that are anticipated to play a fundamental role in the electron dynamics at the interfaces are electron injection, localization, solvation and simultaneous back transfer to the metal. The simulation of this complex scheme is beyond the capabilities of the computational techniques that have been implemented so far.…”

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confidence: 99%