Two- and three-dimensional lead-halide perovskite (LHP) materials are novel semiconductors that have generated broad interest owing to their outstanding optical and electronic properties. ...Characterization and understanding of their atomic structure and structure–property relationships are often nontrivial as a result of the vast structural and compositional tunability of LHPs as well as the enhanced structure dynamics as compared with oxide perovskites or more conventional semiconductors. Nuclear magnetic resonance (NMR) spectroscopy contributes to this thrust through its unique capability of sampling chemical bonding element-specifically (1/2H, 13C, 14/15N, 35/37Cl, 39K, 79/81Br, 87Rb, 127I, 133Cs, and 207Pb nuclei) and locally and shedding light onto the connectivity, geometry, topology, and dynamics of bonding. NMR can therefore readily observe phase transitions, evaluate phase purity and compositional and structural disorder, and probe molecular dynamics and ionic motion in diverse forms of LHPs, in which they can be used practically, ranging from bulk single crystals (e.g., in gamma and X-ray detectors) to polycrystalline films (e.g., in photovoltaics, photodetectors, and light-emitting diodes) and colloidal nanocrystals (e.g., in liquid crystal displays and future quantum light sources). Herein we also outline the immense practical potential of nuclear quadrupolar resonance (NQR) spectroscopy for characterizing LHPs, owing to the strong quadrupole moments, good sensitivity, and high natural abundance of several halide nuclei (79/81Br and 127I) combined with the enhanced electric field gradients around these nuclei existing in LHPs as well as the instrumental simplicity. Strong quadrupole interactions, on one side, make 79/81Br and 127I NMR rather impractical but turn NQR into a high-resolution probe of the local structure around halide ions.
Graphite dual-ion batteries represent a potential battery concept for large-scale stationary storage of electricity, especially when constructed free of lithium and other chemical elements with ...limited natural reserves. Owing to their non-rocking-chair operation mechanism, however, the practical deployment of graphite dual-ion batteries is inherently limited by the need for large quantities of electrolyte solutions as reservoirs of all ions that are needed for complete charge and discharge of the electrodes. Thus far, lithium-free graphite dual-ion batteries have employed moderately concentrated electrolyte solutions (0.3-1 M), resulting in rather low cell-level energy densities of 20-70 Wh kg
. In this work, we present a lithium-free graphite dual-ion battery utilizing a highly concentrated electrolyte solution of 5 M potassium bis(fluorosulfonyl)imide in alkyl carbonates. The resultant battery offers an energy density of 207 Wh kg
, along with a high energy efficiency of 89% and an average discharge voltage of 4.7 V.
The efficient conversion between thermal and electrical energy by means of durable, silent and scalable solid-state thermoelectric devices has been a long standing goal. While nanocrystalline ...materials have already led to substantially higher thermoelectric efficiencies, further improvements are expected to arise from precise chemical engineering of nanoscale building blocks and interfaces. Here we present a simple and versatile bottom-up strategy based on the assembly of colloidal nanocrystals to produce consolidated yet nanostructured thermoelectric materials. In the case study on the PbS-Ag system, Ag nanodomains not only contribute to block phonon propagation, but also provide electrons to the PbS host semiconductor and reduce the PbS intergrain energy barriers for charge transport. Thus, PbS-Ag nanocomposites exhibit reduced thermal conductivities and higher charge carrier concentrations and mobilities than PbS nanomaterial. Such improvements of the material transport properties provide thermoelectric figures of merit up to 1.7 at 850 K.
Understanding the structure and dynamics of newcomer optoelectronic materials - lead halide perovskites APbX3 A = Cs, methylammonium (CH3NH3+, MA), formamidinium (CH(NH2)2+, FA); X = Cl, Br, I - has ...been a major research thrust. In this work, new insights could be gained by using 207Pb solid-state nuclear magnetic resonance (NMR) spectroscopy at variable temperatures between 100 and 300 K. The existence of scalar couplings 1JPb-Cl of ca. 400 Hz and 1JPb-Br of ca. 2.3 kHz could be confirmed for MAPbX3 and CsPbX3. Diverse and fast structure dynamics, including rotations of A-cations, harmonic and anharmonic vibrations of the lead-halide framework and ionic mobility, affect the resolution of the coupling pattern. 207Pb NMR can therefore be used to detect the structural disorder and phase transitions. Furthermore, by comparing bulk and nanocrystalline CsPbBr3 a greater structural disorder of the PbBr6-octahedra had been confirmed in a nanoscale counterpart, not readily captured by diffraction-based techniques.
At present, the technical progress of secondary batteries employing metallic magnesium as the anode material has been severely hindered due to the low oxidation stability of state-of-the-art Mg ...electrolytes, which cannot be used to explore high-voltage (>3 V versus Mg2+/Mg) cathode materials. All known electrolytes based on oxidatively stable solvents and salts, such as Mg(ClO4)2 and Mg bis(trifluoromethanesulfonimide), react with the metallic magnesium anode, forming a passivating layer at its surface and preventing the reversible plating and stripping of Mg. Therefore, in a near-term effort to extend the upper voltage limit in the exploration of future candidate Mg-ion battery cathode materials, bismuth anodes have attracted considerable attention due to their efficient magnesiation and demagnesiation alloying reaction in such electrolytes. In this context, we present colloidal Bi nanocrystals (NCs) as a model anode material for the exploration of cathode materials for rechargeable Mg-ion batteries. Bi NCs demonstrate a stable capacity of 325 mAh g–1 over at least 150 cycles at a current density of 770 mA g–1, which is among the most-stable performance of Mg-ion battery anode materials. First-principles crystal structure prediction methodologies and ex situ X-ray diffraction measurements reveal that the magnesiation of Bi NCs leads to the simultaneous formation of the low-temperature trigonal structure, α-Mg3Bi2, and the high-temperature cubic structure, β-Mg3Bi2, which sheds insight into the high stability of this reversible alloying reaction. Furthermore, small-angle X-ray scattering measurements indicate that although the monodispersed, crystalline nature of the Bi NCs is indeed disturbed during the first discharge step, no notable morphological or structural changes occur in the following electrochemical cycles. The cost-effective and facile synthesis of colloidal Bi NCs and their remarkably high electrochemical stability upon magnesiation make them an excellent model anode material with which to accelerate progress in the field of Mg-ion secondary batteries.
Lead-halide perovskites increasingly mesmerize researchers because they exhibit a high degree of structural defects and dynamics yet nonetheless offer an outstanding (opto)electronic performance on ...par with the best examples of structurally stable and defect-free semiconductors. This highly unusual feature necessitates the adoption of an experimental and theoretical mindset and the reexamination of techniques that may be uniquely suited to understand these materials. Surprisingly, the suite of methods for the structural characterization of these materials does not commonly include nuclear magnetic resonance (NMR) spectroscopy. The present study showcases both the utility and versatility of halide NMR and NQR (nuclear quadrupole resonance) for probing the structure and structural dynamics of CsPbX3 (X = Cl, Br, I), in both bulk and nanocrystalline forms. The strong quadrupole couplings, which originate from the interaction between the large quadrupole moments of, e.g., the 35Cl, 79Br, and 127I nuclei, and the local electric-field gradients, are highly sensitive to subtle structural variations, both static and dynamic. The quadrupole interaction can resolve structural changes with accuracies commensurate with synchrotron X-ray diffraction and scattering. It is shown that space-averaged site-disorder is greatly enhanced in the nanocrystals compared to the bulk, while the dynamics of nuclear spin relaxation indicates enhanced structural dynamics in the nanocrystals. The findings from NMR and NQR were corroborated by ab initio molecular dynamics, which point to the role of the surface in causing the radial strain distribution and disorder. These findings showcase a great synergy between solid-state NMR or NQR and molecular dynamics simulations in shedding light on the structure of soft lead-halide semiconductors.
In this work colloidal quantum dots double gate transistors are introduced. A high‐k (k = 43) relaxor ferroelectric polymer is used as a dielectric material for the top gate in a device where the ...other gate is fabricated from SiO2. The device in double gate configuration is characterized by reduced hysteresis in the transfer curves measured by separately sweeping the voltage of the SiO2 and of the polymer gate. Gating with the relaxor polymer leads to mobility values of μe = 1.1 cm2 V−1s−1 and μh = 6 × 10−3 cm2 V−1s−1 that exceed those extracted from the SiO2 gating: μe = 0.5 cm2 V−1s−1 and μh = 2 × 10−3 cm2 V−1s−1. Measurements under double gating conditions prove that the device works in a single channel mode that is delocalized over the whole film thickness. Double gating allows for shifting the threshold voltage into a desired position and also allows increasing the on‐current of the devices.
The first double gate colloidal quantum dot field‐effect transistor is demonstrated. With respect to a SiO2 gate, when using a high‐k polymer as gate dielectric, the operational voltage of the transistor decreases and the hysteresis and the mobility are improved. When the two gates are simultaneously active, the threshold voltage, the carrier type, and the on‐current can be tuned.
Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a long-standing goal of paramount importance for all their potential optoelectronic applications, ...which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size- and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.
The quest for low-cost and large-scale stationary storage of electricity has led to a surge of reports on novel batteries comprising exclusively highly abundant chemical elements. Aluminum-based ...systems, inter alia, are appealing because of the safety and affordability of aluminum anodes. In this work, we examined the recently proposed aluminum–ionic liquid–graphite architecture. Using 27Al nuclear magnetic resonance, we confirmed that AlCl4 – acts as an intercalating species. Although previous studies have focused on graphitic cathodes, we analyzed the practicality of achievable energy densities and found that the AlCl3-based ionic liquid is a capacity-limiting anode material. By focusing on both the graphitic cathode and the AlCl3-based anode, we improved the overall energy density. First, high cathodic capacities of ≤150 mAh g–1 and energy efficiencies of 90% at high electrode loadings of at least 10 mg cm–2 were obtained with natural, highly crystalline graphite flakes, which were subjected to minimal mechanical processing. Second, the AlCl3 content in the ionic liquid was increased to its maximal value, which essentially doubled the energy density of the battery, resulting in a cell-level energy density of ≤62 Wh kg–1. The resulting batteries were also characterized by high power densities of at least 489 W kg–1.
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