All-inorganic cesium lead halide (CsPbX3; X = Cl, Br, and I) perovskite nanocubes (NCs) exhibit fascinating optical and optoelectronic properties. Postsynthesis anion exchange by mixing NCs with reactive anion species has emerged as a unique strategy to control their composition and band gap. For example, we started with CsPbBr3 NCs with intense green emission, and then anion exchange with iodide ions yields CsPb(Br/I)3 mixed halides and CsPbI3 with emission color systematically varying in the green-red region. However, the internal structure of the anion-exchanged perovskite NCs is not probed. It is believed that the NCs possess a homogeneous alloyed composition, but X-ray diffraction pattern could not give evidence for such alloy formation, because the crystal structure also varies with anion composition. Here, we elucidate the internal heterostructure of anion-exchanged NCs using variable energy hard X-ray photoelectron spectroscopy. The results show that, in contrast to a homogeneous alloy, there is a significant inhomogeneity in the composition across the radius of NCs. The surface of CsPb(Br/I)3 NCs is rich with exchanged iodide ions, whereas the core is rich with native bromide ions. Even CsPbI3 NCs obtained after assumed complete anion exchange show a small amount of bromide ions in the core. This finding of gradient internal heterostructure inside the anion-exchanged NCs will be important for future understanding of electronic properties and stability-related issues of CsPbX3 NCs.
We prepared Fe- and Sn-codoped colloidal In2O3 nanocrystals (∼6 nm). Sn doping provides free electrons in the conduction band, originating localized surface plasmon resonance (LSPR) and electrical conductivity. The LSPR band can be tuned between 2000 and >3000 nm, depending on the extent and kind of dopant ions. Fe doping, on the other hand, provides unpaired electrons, resulting in weak ferromagnetism at room temperature. Fe doping shifts the LSPR band of 10% Sn-doped In2O3 nanocrystals to a longer wavelength along with a reduction in intensity, suggesting trapping of charge carriers around the dopant centers, whereas Sn doping increases the magnetization of 10% Fe-doped In2O3 nanocrystals, probably because of the free electron mediated interactions between distant magnetic ions. The combination of plasmonics and magnetism, in addition to electronic conductivity and visible-light transparency, is a unique feature of our colloidal codoped nanocrystals.
Perovskites based on organometal lead halides have attracted great deal of scientific attention recently in the context of solar cells and optoelectronic devices due to their unique and tunable electronic and optical properties. Herein, we show that the use of electrospray technique in conjunction with the antisolvent-solvent extraction leads to novel low-dimensional quantum structures (especially 2-D nanosheets) of CH3NH3PbI3- and CH3NH3PbBr3-based layered perovskites with unusual luminescence properties. We also show that the optical bandgaps and emission characteristics of these colloidal nanomaterials can be tuned over a broad range of visible spectral region by compositional tailoring of mixed-halide (I- and Br-based) perovskites.
Multifunctional Fe−Sn codoped In 2 O 3 colloidal nanocrystals simultaneously exhibiting localized surface plasmon resonance band, high electrical conductivity, and charge mediated magnetic coupling have been developed. Interactions between Sn and Fe dopant ions have been found critical to control all these properties. Sn doping slowly releases free electrons in the colloidal nanocrystals, after reduction of active complex between Sn 4+ and interstitial O 2− . Unexpectedly, Fe codoping reduces the free electron concentration. Our X-ray absorption fine structure spectroscopy (XAFS) results show that Fe 3+ and Sn 4+ substitutes In 3+ in the In 2 O 3 lattice for all Fe-doped In 2 O 3 NCs and Sn-doped In 2 O 3 NCs. Interestingly, for Fe−Sn codoped NCs, a smaller fraction of Fe 3+ gets reduced to Fe 2+ by consuming free electrons produced by Sn doping. Therefore, Fe doping can manipulate free electron concentration in Fe−Sn codoped In 2 O 3 nanocrystals, controlling both plasmonic band and electrical conductivity. Free electrons, on the other hand, facilitate magnetic coupling between distant Fe 3+ ions. Such charge mediated magnetic coupling is useful for spin-based applications.
The rapidly increasing global energy consumption utilizing conventional polluting fuels has been putting enormous stress on the health of our environment and thereby the long term sustainability of the animal and plant life on our planet. The world clearly needs to move rapidly to the alternative sources of environment-friendly, carbon-neutral, clean and renewable energy. To this end, amongst the promising strategies being pursued, one of the best approaches is to produce hydrogen energy from water by using sunlight, with unlimited available resource of both water and sunlight. Concurrently, there is also an emergent need to control CO2 emissions by reducing them to valuable fuels or chemicals using sunlight. For both these goals, it is essential to have efficient, robust and affordable photocatalysts. The early emphasis on semiconductor photocatalysts along with expensive noble-metals co-catalysts has prevented the speedy advance of this energy technology. Extensive efforts are now being expended on designing high-performance photocatalysts based on emergent functional materials endowed with a fascinating set of physical and chemical properties. Towards this end, two-dimensional (2D) materials and their heterostructures have been attracting significant attention lately as potentially viable candidates owing to their unique, and highly tunable optical and electronics functionalities, which are technically adequate for the efficient hydrogen production and conversion of CO2 to fuels. In this topical review, we address the recent progress made in the domain. We believe that by virtue of the uniquely distinct characteristics of their electronic density of states, surface states, high surface area, and diverse possibilities of innovative surface chemical engineering, the 2D materials hold a great promise for facilitating economically viable renewable/clean energy harvesting solution(s) on commercial-scale, thereby accomplishing the urgent task of ensuring the future energy security for the world.
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