Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron–hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials’ properties.
Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119 Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity ( C p ) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ 0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b , c , and a directions, respectively.
A record high zT of 2.2 at 740 K is reported in Ge0.92Sb0.08Te single crystals, with an optimal hole carrier concentration ≈4 × 1020 cm−3 that simultaneously maximizes the power factor (PF) ≈56 µW cm−1 K−2 and minimizes the thermal conductivity ≈1.9 Wm−1 K−1. In addition to the presence of herringbone domains and stacking faults, the Ge0.92Sb0.08Te exhibits significant modification to phonon dispersion with an extra phonon excitation around ≈5–6 meV at Γ point of the Brillouin zone as confirmed through inelastic neutron scattering (INS) measurements. Density functional theory (DFT) confirmed this phonon excitation, and predicted another higher energy phonon excitation ≈12–13 meV at W point. These phonon excitations collectively increase the number of phonon decay channels leading to softening of phonon frequencies such that a three‐phonon process is dominant in Ge0.92Sb0.08Te, in contrast to a dominant four‐phonon process in pristine GeTe, highlighting the importance of phonon engineering approaches to improving thermoelectric (TE) performance.
In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge 1−x Bi x Te crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level ( E F ) to an eligible region. Experimentally, with moderate 6–10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 10 20 cm −3 to 3–5 × 10 20 cm −3 and the Seebeck coefficient is boosted three times to 75 μVK −1 . In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E F starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m −1 K −1 at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge 1−x Bi x Te specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.
We report the phase evolution and thermoelectric properties of a series of Co(Ge0.5Te0.5)3–x Sb x (x = 0–0.20) compositions synthesized by mechanical alloying. Pristine ternary Co(Ge0.5Te0.5)3 skutterudite crystallizes in the rhombohedral symmetry (R3̅), and Sb doping induces a structural transition to the cubic phase (ideal skutterudite, Im3̅). The Sb substitution increases the carrier concentration while maintaining a high thermopower even at higher doping levels owing to an increased effective mass. The exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier transport from both the primary and secondary conduction bands, as shown by theoretical calculations. The enhanced electrical conductivity and high thermopower increase the power factor by more than 20 times. Because the dominant phonon propagation modes in binary skutterudites are associated with the vibrations of pnictogen rings, twisting the latter through the isoelectronic replacement of Sb4 rings with Ge2Te2 ones, as done in this study, can effectively reduce the thermal conductivity. This leads to an increase in the dimensionless figure-of-merit (zT) by a factor of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.
A thin metal layer is introduced into a slab waveguide grating with guided-mode resonances to reduce the switching intensity in its bistable operational mode. A dielectric grating put on top of the metal layer plays the role of a coupling element between the normally incident light and the guided mode in the slab waveguide grating. The presence of the metal layer increases the reflectivity of the optical device that as a consequence exhibits high-reflection side bands and close-to-zero reflectivity drops. Several structures are designed by changing the grating depths and the metal layer thicknesses, and their influences on linear and nonlinear characteristics are analysed using the finite-difference time-domain method. We found that the metal layer increases the quality factor of guided-mode resonance filters. Numerical results show that the quality factor improves 5.6 times and the switching intensity is reduced 45 times when these devices are compared to typical slab waveguide gratings in the same working conditions, in terms of polarization and operating wavelength.
Implication of the coexistence of intrinsic ferromagnetism and superconductivity in monolayer CoO2.
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