RESUMO
Structural polymorphism is known for many bulk materials; however, on the nanoscale metastable polymorphs tend to form more readily than in the bulk, and with more structural variety. One such metastable polymorph observed for colloidal Ag2Se nanocrystals has traditionally been referred to as the "tetragonal" phase. While there are reports on the chemistry and properties of this metastable polymorph, its crystal structure, and therefore electronic structure, has yet to be determined. We report that an anti-PbCl2-like structure type (space group P21/n) more accurately describes the powder X-ray diffraction and X-ray total scattering patterns of colloidal Ag2Se nanocrystals prepared by several different methods. Density functional theory (DFT) calculations indicate that this anti-PbCl2-like Ag2Se polymorph is a dynamically stable, narrow-band-gap semiconductor. The anti-PbCl2-like structure of Ag2Se is a low-lying metastable polymorph at 5-25 meV/atom above the ground state, depending on the exchange-correlation functional used.
RESUMO
The gram-scale preparation of Fe2(WO4)3 by a new solution-based route and detailed characterization of the material are presented. The resulting Fe2(WO4)3 undergoes a reversible electrochemical reaction against lithium centered around 3.0 V with capacities near 93% of the theoretical maximum. Evolution of the Fe2(WO4)3 structure upon lithium insertion and deinsertion is probed using a battery of characterization techniques, including in situ X-ray diffraction, neutron total scattering, and X-ray absorption spectroscopy (XAS). A structural transformation from monoclinic to orthorhombic phases is confirmed during lithium intercalation. XAS and neutron total scattering measurements verify that Fe2(WO4)3 consists of trivalent iron and hexavalent tungsten ions. As lithium ions are inserted into the framework, iron ions are reduced to the divalent state, while the tungsten ions are electrochemically inactive and remain in the hexavalent state. Lithium insertion occurs via a concerted rotation of the rigid polyhedra in the host lattice driven by electrostatic interactions with the Li+ ions; the magnitude of these polyhedral rotations was found to be slightly larger for Fe2(WO4)3 than for the Fe2(MoO4)3 analog.
RESUMO
Colloidally synthesized quantum dots of CsPbBr3 are highly promising for light-emitting applications. Previous reports based on benchtop diffraction conflict as to the crystal structure of CsPbBr3 quantum dots. We present X-ray diffraction and PDF analysis of X-ray total scattering data that indicate that the crystal structure is unequivocally orthorhombic (Pnma).
RESUMO
Photocurrent measurements on devices containing perovskite (CH3NH3)PbI3 show two distinct spectral responses when deposited in a mesoporous oxide matrix, compared with one response for planar perovskite alone. With a TiO2 matrix, the shorter wavelength response has an inverted temperature response with increasing performance on cooling.
RESUMO
Polycrystalline thin films of tellurium and organic semiconductor molecules are paired in heterostructured field-effect transistors built on Si/SiO2 substrates. While charge carrier mobilities can exceed 1 cm(2)/(V s), there is only a limited gate voltage range over which the current is modulated. We employ continuous and pulsed measurements on transistors to explore the influence of charge equilibration time on device behavior, finding that pulsed gating improves output characteristics. We also use surface potential measurements to investigate the interfacial vacuum level offset between materials, and we modify the interlayer potential profile by interposing statically charged dielectric layers on the silicon dioxide. We show that interfacial fields determine the gate voltage range over which Te shows a field effect in heterostructures with organic semiconductors and that modification of these fields can extend this range.
