Charge transport across metal/molecular (alkyl) monolayer-Si junctions is dominated by the LUMO level

We compare the charge transport characteristics of heavy doped p- and n-Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p- and n-type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current

Phonon-phonon interactions in the polarizarion dependence of Raman scattering

We have found that the polarization dependence of the Raman signal in organic crystals can only be described by a fourth-rank formalism. The generalization from the traditional second-rank Raman tensor $\mathcal{R}$ is physically motivated by consideration of the light scattering mechanism of anharmonic crystals at finite temperatures, and explained in terms of off-diagonal components of the crystal self-energy. We thus establish a novel manifestation of anharmonicity in inelastic light scattering, markedly separate from the better known phonon lifetime.Comment: 31 pages, 17 figure

Engineering plastic phase transitions via solid solutions: the case of “reordering frustration” in ionic plastic crystals of hydroxyquinuclidinium salts

A family of salts of R-(+)-(3)-hydroxyquinuclidinium [QH]+, with SO42−, BPh4−, BF4− and PF6− counter-anions, have been prepared by the metathesis of [QH]Cl and metal salts of the corresponding anions. Solid solutions of formula [QH](PF6)x(BF4)1−x for x = 0.9, 0.8, 0.7 have also been obtained. The crystalline materials have been investigated by a combination of solid-state techniques, including variable temperature XRD, thermal analyses, multinuclear (11B, 13C, 15N, 19F, and 31P) solid-state NMR spectroscopy, variable temperature wideline 19F T1 relaxation measurements, and micro-Raman spectroscopy to investigate their thermal stability and phase transition behaviors. It has been shown that the salts [QH]PF6 and [QH]BF4 undergo an order–disorder solid–solid phase transition to plastic phases, whereas [QH]2SO4·H2O and [QH]BPh4 do not display any plastic phase transition. Doping [QH]BF4 into the [QH]PF6 lattice up to 30% results in the formation of a solid solution that is plastic in an expanded thermal range, thanks to a phenomenon that we describe here for the first time as “reordering frustration”

Disentangling the Effects of Structure and Lone-Pair Electrons in the Lattice Dynamics of Halide Perovskites

Metal halide perovskites have shown great performance as solar energy materials, but their outstanding optoelectronic properties are paired with unusually strong anharmonic effects. It has been proposed that this intriguing combination of properties derives from the "lone pair" 6$s^2$ electron configuration of the Pb$^{2+}$ cations, and associated weak pseudo-Jahn-Teller effect, but the precise impact of this chemical feature remains unclear. Here we show that in fact an $ns^2$ electron configuration is not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in this class of materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and first-principles molecular dynamics calculations to directly contrast the lattice dynamics of CsSrBr$_3$ with those of CsPbBr$_3$, two compounds which bear close structural similarity but with the former lacking the propensity to form lone pairs on the 5$s^0$ octahedral cation. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint to detect anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This work highlights the key role of structure in perovskite lattice dynamics, providing important design rules for the emerging class of soft perovskite semiconductors for optoelectronic and light-harvesting devices

Disentangling the effects of structure and lone-pair electrons in the lattice dynamics of halide perovskites

Halide perovskites show great optoelectronic performance, but their favorable properties are paired with unusually strong anharmonicity. It was proposed that this combination derives from the ns2 electron configuration of octahedral cations and associated pseudo-Jahn–Teller effect. We show that such cations are not a prerequisite for the strong anharmonicity and low-energy lattice dynamics encountered in these materials. We combine X-ray diffraction, infrared and Raman spectroscopies, and molecular dynamics to contrast the lattice dynamics of CsSrBr3 with those of CsPbBr3, two compounds that are structurally similar but with the former lacking ns2 cations with the propensity to form electron lone pairs. We exploit low-frequency diffusive Raman scattering, nominally symmetry-forbidden in the cubic phase, as a fingerprint of anharmonicity and reveal that low-frequency tilting occurs irrespective of octahedral cation electron configuration. This highlights the role of structure in perovskite lattice dynamics, providing design rules for the emerging class of soft perovskite semiconductors

Similarity-based model for ordered categorical data

<p>In a large variety of applications, the data for a variable we wish to explain are ordered and categorical. In this paper, we present a new similarity-based model for the scenario and investigate its properties. We establish that the process is <i>ψ</i>-mixing and strictly stationary and derive the explicit form of the autocorrelation function in some special cases. Consistency and asymptotic normality of the maximum likelihood estimator of the model’s parameters are proven. A simulation study supports our findings. The results are applied to the Netflix data set, comprised of a survey on users’ grading of movies.</p