Direct Observation of Band Structure Modifications in Nanocrystals of CsPbBr3 Perovskite.

We investigate the variation of the bandgap energy of single quantum dots of CsPbBr3 inorganic halide perovskite as a function of size and shape and upon embedding within an ensemble. For that purpose, we make use of valence-loss electron spectroscopy with Z-contrast annular dark-field (ADF) imaging in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope. In the experiment, energy absorption is directly mapped onto individual quantum dots, whose dimensions and location are simultaneously measured to the highest precision. In that way, we establish an intimate relation between quantum dot size and even shape and its bandgap energy on a single object level. We explicitly follow the bandgap increase in smaller quantum dots due to quantum confinement and demonstrate that it is predominantly governed by the smallest of the three edges of the cuboidal perovskite dot. We also show the presence of an effective coupling between proximal dots in an ensemble, leading to band structure modification. These unique insights are directly relevant to the development of custom-designed quantum structures and solids which will be realized by purposeful assemblage of individually characterized and selected quantum dots, serving as building blocks.

Photon Recycling in CsPbBr3 All-Inorganic Perovskite Nanocrystals.

Photon recycling, the iterative process of re-absorption and re-emission of photons in an absorbing medium, can play an important role in the power-conversion efficiency of photovoltaic cells. To date, several studies have proposed that this process may occur in bulk or thin films of inorganic lead-halide perovskites, but conclusive proof of the occurrence and magnitude of this effect is missing. Here, we provide clear evidence and quantitative estimation of photon recycling in CsPbBr3 nanocrystal suspensions by combining measurements of steady-state and time-resolved photoluminescence (PL) and PL quantum yield with simulations of photon diffusion through the suspension. The steady-state PL shows clear spectral modifications including red shifts and quantum yield decrease, while the time-resolved measurements show prolonged PL decay and rise times. These effects grow as the nanocrystal concentration and distance traveled through the suspension increase. Monte Carlo simulations of photons diffusing through the medium and exhibiting absorption and re-emission account quantitatively for the observed trends and show that up to five re-emission cycles are involved. We thus identify 4 quantifiable measures, PL red shift, PL QY, PL decay time, and PL rise time that together all point toward repeated, energy-directed radiative transfer between nanocrystals. These results highlight the importance of photon recycling for both optical properties and photovoltaic applications of inorganic perovskite nanocrystals.

Correction to "Hybridization of Single Nanocrystals of Cs4PbBr6 and CsPbBr3".

[This corrects the article DOI: 10.1021/acs.jpcc.7b05752.].

Extraordinary Interfacial Stitching between Single All-Inorganic Perovskite Nanocrystals.

All-inorganic cesium lead halide perovskite nanocrystals are extensively studied because of their outstanding optoelectronic properties. Being of a cubic shape and typically featuring a narrow size distribution, CsPbX3 (X = Cl, Br, and I) nanocrystals are the ideal starting material for the development of homogeneous thin films as required for photovoltaic and optoelectronic applications. Recent experiments reveal spontaneous merging of drop-casted CsPbBr3 nanocrystals, which is promoted by humidity and mild-temperature treatments and arrested by electron beam irradiation. Here, we make use of atom-resolved annular dark-field imaging microscopy and valence electron energy loss spectroscopy in a state-of-the-art low-voltage monochromatic scanning transmission electron microscope to investigate the aggregation between individual nanocrystals at the atomic level. We show that the merging process preserves the elemental composition and electronic structure of CsPbBr3 and takes place between nanocrystals of different sizes and orientations. In particular, we reveal seamless stitching for aligned nanocrystals, similar to that reported in the past for graphene flakes. Because the crystallographic alignment occurs naturally in drop-casted layers of CsPbX3 nanocrystals, our findings constitute the essential first step toward the development of large-area nanosheets with band gap energies predesigned by the nanocrystal choice-the gateway to large-scale photovoltaic applications of inorganic perovskites.

Efficient carrier multiplication in CsPbI3 perovskite nanocrystals.

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties-these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.

Color-stable water-dispersed cesium lead halide perovskite nanocrystals.

Cesium lead halide perovskite nanocrystals are being lately explored for optoelectronic applications due to their emission tunability, high photoluminescence quantum yields, and narrow emission bands. Nevertheless, their incompatibility with polar solvents and composition homogenization driven by a fast anion-exchange are still important drawbacks to overcome. Herein we report on a successful encapsulation of colloidal perovskite nanocrystals within solid-lipid structures mainly consisting of stearic acid. The product is water-stable for a period longer than 2 months and anion-exchange is fully arrested when mixing nanocrystals of different halide compositions. This strategy boosts the potential applications of all-inorganic perovskite nanocrystals for ink-printing.

Comparison of the Optical Properties of Graphene and Alkyl-terminated Si and Ge Quantum Dots.

Semiconductor quantum dots are widely investigated due to their size dependent energy structure. In particular, colloidal quantum dots represent a promising nanomaterial for optoelectronic devices, such as photodetectors and solar cells, but also luminescent markers for biotechnology, among other applications. Ideal materials for these applications should feature efficient radiative recombination and absorption transitions, altogether with spectral tunability over a wide range. Group IV semiconductor quantum dots can fulfill these requirements and serve as an alternative to the commonly used direct bandgap materials containing toxic and/or rare elements. Here, we present optical properties of butyl-terminated Si and Ge quantum dots and compare them to those of graphene quantum dots, finding them remarkably similar. We investigate their time-resolved photoluminescence emission as well as the photoluminescence excitation and linear absorption spectra. We contemplate that their emission characteristics indicate a (semi-) resonant activation of the emitting channel; the photoluminescence excitation shows characteristics similar to those of a molecule. The optical density is consistent with band-to-band absorption processes originating from core-related states. Hence, these observations strongly indicate a different microscopic origin for absorption and radiative recombination in the three investigated quantum dot systems.