We investigate the structural and physical properties of the AgSn m SbSe m+2 system with m = 1–20 (i.e., SnSe matrix and ∼5–50% AgSbSe2) from atomic, nano, and macro length scales. We find the 50:50 ...composition, with m = 1 (i.e., AgSnSbSe3), forms a stable cation-disordered cubic rock-salt p-type semiconductor with a special multi-peak electronic valence band structure. AgSnSbSe3 has an intrinsically low lattice thermal conductivity of ∼0.47 W m–1 K–1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on Se sites creates a quinary high-entropy NaCl-type solid solution AgSnSbSe3‑x Te x with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ∼0.32 W m–1 K–1 at 723 K and higher hole carrier concentration than AgSnSbSe3. Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe1.5Te1.5, the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ∼9.54 μW cm–1 K–2 (400–773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K, and a high average ZT of ∼1.0 over a wide temperature range of 400–773 K in AgSnSbSe1.5Te1.5.
Recent findings about ultrahigh thermoelectric performance in SnSe single crystals have stimulated related research on this simple binary compound, which is focused mostly on its polycrystalline ...counterparts, and particularly on electrical property enhancement by effective doping. This work systematically investigated the thermoelectric properties of polycrystalline SnSe doped with three alkali metals (Li, Na, and K). It is found that Na has the best doping efficiency, leading to an increase in hole concentration from 3.2 × 1017 to 4.4 × 1019 cm–3 at room temperature, accompanied by a drop in Seebeck coefficient from 480 to 142 μV/K. An equivalent single parabolic band model was found adequate to capture the variation tendency of Seebeck coefficient with doping levels within a wide range. A mixed scattering of carriers by acoustic phonons and grain boundaries is suitable for numerically understanding the temperature-dependence of carrier mobility. A maximum ZT of ∼0.8 was achieved in 1% Na- or K-doped SnSe at 800 K. Possible strategies to improve the mobility and ZT of polycrystals were also proposed.
Abstract
Gamma-ray detection and spectroscopy is the quantitative determination of their energy spectra, and is of critical value and critically important in diverse technological and scientific ...fields. Here we report an improved melt growth method for cesium lead bromide and a special detector design with asymmetrical metal electrode configuration that leads to a high performance at room temperature. As-grown centimeter-sized crystals possess extremely low impurity levels (below 10 p.p.m. for total 69 elements) and detectors achieve 3.9% energy resolution for 122 keV
57
Co gamma-ray and 3.8% for 662 keV
137
Cs gamma-ray. Cesium lead bromide is unique among all gamma-ray detection materials in that its hole transport properties are responsible for the high performance. The superior mobility-lifetime product for holes (1.34 × 10
−3
cm
2
V
−1
) derives mainly from the record long hole carrier lifetime (over 25 μs). The easily scalable crystal growth and high-energy resolution, highlight cesium lead bromide as an exceptional next generation material for room temperature radiation detection.
The unique hybrid nature of 2D Ruddlesden–Popper (R–P) perovskites has bestowed upon them not only tunability of their electronic properties but also high-performance electronic devices with improved ...environmental stability as compared to their 3D analogs. However, there is limited information about their inherent heat, light, and air stability and how different parameters such as the inorganic layer number and length of organic spacer molecule affect stability. To gain deeper understanding on the matter we have expanded the family of 2D R–P perovskites, by utilizing pentylamine (PA)2(MA) n−1Pb n I3n+1 (n = 1–5, PA = CH3(CH2)4NH3 +, C5) and hexylamine (HA)2(MA) n−1Pb n I3n+1 (n = 1–4, HA = CH3(CH2)5NH3 +, C6) as the organic spacer molecules between the inorganic slabs, creating two new series of layered materials, for up to n = 5 and 4 layers, respectively. The resulting compounds were extensively characterized through a combination of physical and spectroscopic methods, including single crystal X-ray analysis. High resolution powder X-ray diffraction studies using synchrotron radiation shed light for the first time to the phase transitions of the higher layer 2D R–P perovskites. The increase in the length of the organic spacer molecules did not affect their optical properties; however, it has a pronounced effect on the air, heat, and light stability of the fabricated thin films. An extensive study of heat, light, and air stability with and without encapsulation revealed that specific compounds can be air stable (relative humidity (RH) = 20–80% ± 5%) for more than 450 days, while heat and light stability in air can be exponentially increased by encapsulating the corresponding films. Evaluation of the out-of-plane mechanical properties of the corresponding materials showed that their soft and flexible nature can be compared to current commercially available polymer substrates (e.g., PMMA), rendering them suitable for fabricating flexible and wearable electronic devices.
Using in situ electrical biasing transmission electron microscopy, structural and chemical modification to n–i–p‐type MAPbI3 solar cells are examined with a TiO2 electron‐transporting layer caused by ...bias in the absence of other stimuli known to affect the physical integrity of MAPbI3 such as moisture, oxygen, light, and thermal stress. Electron energy loss spectroscopy (EELS) measurements reveal that oxygen ions are released from the TiO2 and migrate into the MAPbI3 under a forward bias. The injection of oxygen is accompanied by significant structural transformation; a single‐crystalline MAPbI3 grain becomes amorphous with the appearance of PbI2. Withdrawal of oxygen back to the TiO2, and some restoration of the crystallinity of the MAPbI3, is observed after the storage in dark under no bias. A subsequent application of a reverse bias further removes more oxygen ions from the MAPbI3. Light current–voltage measurements of perovskite solar cells exhibit poorer performance after elongated forward biasing; recovery of the performance, though not complete, is achieved by subsequently applying a negative bias. The results indicate negative impacts on the device performance caused by the oxygen migration to the MAPbI3 under a forward bias. This study identifies a new degradation mechanism intrinsic to n–i–p MAPbI3 devices with TiO2.
Using an in situ biasing TEM experiment, a new intrinsic degradation mechanism of methylammonium lead triiodide (MAPbI3) solar cells with a titanium dioxide (TiO2) electron‐transporting layer is identified: oxygen migration from the TiO2 layer to the MAPbI3 under forward biasing, which leads to severe structural modification of the MAPbI3 and the process is pseudo‐reversible.
Thermoelectric materials can directly generate electrical power from waste heat but the challenge is in designing efficient, stable and inexpensive systems. Nanostructuring in bulk materials ...dramatically reduces the thermal conductivity but simultaneously increases the charge carrier scattering, which has a detrimental effect on the carrier mobility. We have experimentally achieved concurrent phonon blocking and charge transmitting via the endotaxial placement of nanocrystals in a thermoelectric material host. Endotaxially arranged SrTe nanocrystals at concentrations as low as 2% were incorporated in a PbTe matrix doped with Na(2)Te. This effectively inhibits the heat flow in the system but does not affect the hole mobility, allowing a large power factor to be achieved. The crystallographic alignment of SrTe and PbTe lattices decouples phonon and electron transport and this allows the system to reach a thermoelectric figure of merit of 1.7 at ~800 K.
Development of low‐cost, high‐performance, and bifunctional electrocatalysts for water splitting is essential for renewable and clean energy technologies. Although binary phosphides are inexpensive, ...their performance is not as good as noble metals. Adding a third metal element to binary phosphides (Ni‐P, Co‐P) provides the opportunity to tune their crystalline and electronic structures and thus their electrocatalytic properties. Here, ternary phosphide (NiCoP) films with different nickel to cobalt ratios via an electrodeposition technique are synthesized. The films have a triple‐layered and hierarchical morphology, consisting of nanosheets in the bottom layer, ≈90–120 nm nanospheres in the middle layer, and larger spherical particles on the top layer. The ternary phosphides exhibit versatile activities that are strongly dependent on the Ni/Co ratios and Ni0.51Co0.49P film is found to have the best electrocatalytic activities for both hydrogen evolution reactions and oxygen evolution reactions. The high performance of the ternary phosphide film is attributed to enhanced electric conductivity so that reaction kinetics is accelerated, enlarged surface area due to the hierarchical and three‐layered morphology, and increased local electric dipole so that the energy barrier for the water splitting reaction is lowered.
Bimetallic phosphide (Ni0.51Co0.49P) films with a triple‐layered and hierarchical morphology with superior performance toward overall water splitting are successfully synthesized. The phosphides present versatile activities that are strongly dependent on the Ni/Co ratios. The improvement in performance is mainly ascribed to the alloying effect between Ni and Co atoms.
Black phosphorus (BP) with unique 2D structure enables the intercalation of foreign elements or molecules, which makes BP directly relevant to high‐capacity rechargeable batteries and also opens a ...promising strategy for tunable electronic transport and superconductivity. However, the underlying intercalation mechanism is not fully understood. Here, a comparative investigation on the electrochemically driven intercalation of lithium and sodium using in situ transmission electron microscopy is presented. Despite the same preferable intercalation channels along 100 (zigzag) direction, distinct anisotropic intercalation behaviors are observed, i.e., Li ions activate lateral intercalation along 010 (armchair) direction to form an overall uniform propagation, whereas Na diffusion is limited in the zigzag channels to cause the columnar intercalation. First‐principles calculations indicate that the diffusion of both Li and Na ions along the zigzag direction is energetically favorable, while Li/Na diffusion long the armchair direction encounters an increased energy barrier, but that of Na is significantly larger and insurmountable, which accounts for the orientation‐dependent intercalation channels. The evolution of chemical states during phase transformations (from LixP/NaxP to Li3P/Na3P) is identified by analytical electron diffraction and energy‐loss spectroscopy. The findings elucidate atomistic Li/Na intercalation mechanisms in BP and show potential implications for other similar 2D materials.
The intercalation of lithium and sodium in black phosphorus with orientation‐dependent channels and distinct anisotropic pathways is discovered using in situ transmission electron microscopy combined with density functional theory calculations. The atomic structure evolution along zigzag and armchair directions and the relevant changes in chemical states are elucidated, which offers a fundamental understanding of intercalation mechanisms.
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