Quinten A. Akkerman

Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions.

We demonstrate that, via controlled anion exchange reactions using a range of different halide precursors, we can finely tune the chemical composition and the optical properties of presynthesized colloidal cesium lead halide perovskite nanocrystals (NCs), from green emitting CsPbBr3 to bright emitters in any other region of the visible spectrum, and back, by displacement of Cl– or I– ions and reinsertion of Br– ions. This approach gives access to perovskite semiconductor NCs with both structural and optical qualities comparable to those of directly synthesized NCs. We also show that anion exchange is a dynamic process that takes place in solution between NCs. Therefore, by mixing solutions containing perovskite NCs emitting in different spectral ranges (due to different halide compositions) their mutual fast exchange dynamics leads to homogenization in their composition, resulting in NCs emitting in a narrow spectral region that is intermediate between those of the parent nanoparticles.

Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control

We report a colloidal synthesis approach to CsPbBr3 nanoplatelets (NPLs). The nucleation and growth of the platelets, which takes place at room temperature, is triggered by the injection of acetone in a mixture of precursors that would remain unreactive otherwise. The low growth temperature enables the control of the plate thickness, which can be precisely tuned from 3 to 5 monolayers. The strong two-dimensional confinement of the carriers at such small vertical sizes is responsible for a narrow PL, strong excitonic absorption, and a blue shift of the optical band gap by more than 0.47 eV compared to that of bulk CsPbBr3. We also show that the composition of the NPLs can be varied all the way to CsPbBr3 or CsPbI3 by anion exchange, with preservation of the size and shape of the starting particles. The blue fluorescent CsPbCl3 NPLs represent a new member of the scarcely populated group of blue-emitting colloidal nanocrystals. The exciton dynamics were found to be independent of the extent of 2D confinement in these platelets, and this was supported by band structure calculations.

X-ray Lithography on Perovskite Nanocrystals Films: From Patterning with Anion-Exchange Reactions to Enhanced Stability in Air and Water

Films of colloidal CsPbX3 (X = I, Br or Cl) nanocrystals, prepared by solution drop-casting or spin-coating on a silicon substrate, were exposed to a low flux of X-rays from an X-ray photoelectron spectrometer source, causing intermolecular C=C bonding of the organic ligands that coat the surface of the nanocrystals. This transformation of the ligand shell resulted in a greater stability of the film, which translated into the following features: (i) Insolubility of the exposed regions in organic solvents which caused instead complete dissolution of the unexposed regions. This enabled the fabrication of stable and strongly fluorescent patterns over millimeter scale areas. (ii) Inhibition of the irradiated regions toward halide anion exchange reactions, when the films were exposed either to halide anions in solution or to hydrohalic vapors. This feature was exploited to create patterned regions of different CsPbIxBryClz compositions, starting from a film with homogeneous CsPbX3 composition. (iii) Resistance of the films to degradation caused by exposure to air and moisture, which represents one of the major drawbacks for the integration of these materials in devices. (iv) Stability of the film in water and biological buffer, which can open interesting perspectives for applications of halide perovskite nanocrystals in aqueous environments.

From Binary Cu2S to ternary Cu-In-S and quaternary Cu-In-Zn-S nanocrystals with tunable composition via partial cation exchange.

We present an approach for the synthesis of ternary copper indium sulfide (CIS) and quaternary copper indium zinc sulfide (CIZS) nanocrystals (NCs) by means of partial cation exchange with In(3+) and Zn(2+). The approach consists of a sequential three-step synthesis: first, binary Cu2S NCs were synthesized, followed by the homogeneous incorporation of In(3+) by an in situ partial cation-exchange reaction, leading to CIS NCs. In the last step, a second partial exchange was performed where Zn(2+) partially replaced the Cu(+) and In(3+) cations at the surface, creating a ZnS-rich shell with the preservation of the size and shape. By careful tuning reaction parameters (growth and exchange times as well as the initial Cu(+):In(3+):Zn(2+) ratios), control over both the size and composition was achieved. This led to a broad tuning of photoluminescence of the final CIZS NCs, ranging from 880 to 1030 nm without altering the NCs size. Cytotoxicity tests confirmed the biocompatibility of the synthesized CIZS NCs, which opens up opportunities for their application as near-infrared fluorescent markers in the biomedical field.

Polymer-Free Films of Inorganic Halide Perovskite Nanocrystals as UV-to-White Color-Conversion Layers in LEDs

Being considerably more efficient than traditional incandescent bulbs or fluorescent tubes, light-emitting diodes (LEDs) are becoming the main technology for general lighting applications.1,2 In the last years, a variety of white-LEDs have been demonstrated employing, for example, organic molecules (OLEDs)3−7 or inorganic quantum dots (QDs) of different composition.8 A common way of achieving a white-light emission is to couple a near-UV or blue LED with down-conversion materials such as phosphors.9 More recently, QDs have been incorporated into LEDs, replacing conventional phosphors to tune finely the emission spectrum.10 Among the different available QDs, lead halide-based perovskite nanocrystals have recently emerged as very promising candidates for many optoelectronic applications.11−15 Colloidal perovskite nanocrystals can be synthesized and/or transformed postsynthesis, so that samples emitting at different wavelengths throughout the whole visible spectrum, with high photoluminescence quantum yield (PLQY), can be prepared easily. This can be achieved either by changing the chemical composition (by anion-exchange for example)16,17 or the shape (cubes, platelets, sheets, wires).18−21 Also, electroluminescence has been observed from bulk perovskite films,22−25 blended perovskite-in-polymer films,26 as well as from nanocrystals-only films27,28 leading to the first perovskite-based LEDs.

Nearly Monodisperse Insulator Cs4PbX6 (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX3 Nanocrystals

We have developed a colloidal synthesis of nearly monodisperse nanocrystals of pure Cs4PbX6 (X = Cl, Br, I) and their mixed halide compositions with sizes ranging from 9 to 37 nm. The optical absorption spectra of these nanocrystals display a sharp, high energy peak due to transitions between states localized in individual PbX64– octahedra. These spectral features are insensitive to the size of the particles and in agreement with the features of the corresponding bulk materials. Samples with mixed halide composition exhibit absorption bands that are intermediate in spectral position between those of the pure halide compounds. Furthermore, the absorption bands of intermediate compositions broaden due to the different possible combinations of halide coordination around the Pb2+ ions. Both observations are supportive of the fact that the [PbX6]4– octahedra are electronically decoupled in these systems. Because of the large band gap of Cs4PbX6 (>3.2 eV), no excitonic emission in the visible range was observed. The Cs4PbBr6 nanocrystals can be converted into green fluorescent CsPbBr3 nanocrystals by their reaction with an excess of PbBr2 with preservation of size and size distributions. The insertion of PbX2 into Cs4PbX6 provides a means of accessing CsPbX3 nanocrystals in a wide variety of sizes, shapes, and compositions, an important aspect for the development of precisely tuned perovskite nanocrystal inks.

Self-Assembly of Colloidal Hexagonal Bipyramid- and Bifrustum-Shaped ZnS Nanocrystals into Two-Dimensional Superstructures

We present a combined experimental, theoretical, and simulation study on the self-assembly of colloidal hexagonal bipyramid- and hexagonal bifrustum-shaped ZnS nanocrystals (NCs) into two-dimensional superlattices. The simulated NC superstructures are in good agreement with the experimental ones. This shows that the self-assembly process is primarily driven by minimization of the interfacial free-energies and maximization of the packing density. Our study shows that a small truncation of the hexagonal bipyramids is sufficient to change the symmetry of the resulting superlattice from hexagonal to tetragonal, highlighting the crucial importance of precise shape control in the fabrication of functional metamaterials by self-assembly of colloidal NCs.

In Situ Transmission Electron Microscopy Study of Electron Beam-Induced Transformations in Colloidal Cesium Lead Halide Perovskite Nanocrystals

An increasing number of studies have recently reported the rapid degradation of hybrid and all-inorganic lead halide perovskite nanocrystals under electron beam irradiation in the transmission electron microscope, with the formation of nanometer size, high contrast particles. The nature of these nanoparticles and the involved transformations in the perovskite nanocrystals are still a matter of debate. Herein, we have studied the effects of high energy (80/200 keV) electron irradiation on colloidal cesium lead bromide (CsPbBr3) nanocrystals with different shapes and sizes, especially 3 nm thick nanosheets, a morphology that facilitated the analysis of the various ongoing processes. Our results show that the CsPbBr3 nanocrystals undergo a radiolysis process, with electron stimulated desorption of a fraction of bromine atoms and the reduction of a fraction of Pb2+ ions to Pb0. Subsequently Pb0 atoms diffuse and aggregate, giving rise to the high contrast particles, as previously reported by various groups. The diffusion is facilitated by both high temperature and electron beam irradiation. The early stage Pb nanoparticles are epitaxially bound to the parent CsPbBr3 lattice, and evolve into nonepitaxially bound Pb crystals upon further irradiation, leading to local amorphization and consequent dismantling of the CsPbBr3 lattice. The comparison among CsPbBr3 nanocrystals with various shapes and sizes evidences that the damage is particularly pronounced at the corners and edges of the surface, due to a lower diffusion barrier for Pb0 on the surface than inside the crystal and the presence of a larger fraction of under-coordinated atoms.

Changing the Dimensionality of Cesium Lead Bromide Nanocrystals by Reversible Postsynthesis Transformations with Amines

by Reversible Postsynthesis Transformations with Amines Francisco Palazon,† Guilherme Almeida,†,§ Quinten A. Akkerman,†,§ Luca De Trizio,† Zhiya Dang,† Mirko Prato,‡ and Liberato Manna*,† †Nanochemistry Department and ‡Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy Dipartimento di Chimica e Chimica Industriale, Universita ̀ degli Studi di Genova, Via Dodecaneso, 31, 16146 Genova, Italy

Genesis, challenges and opportunities for colloidal lead halide perovskite nanocrystals

Lead halide perovskites (LHPs) in the form of nanometre-sized colloidal crystals, or nanocrystals (NCs), have attracted the attention of diverse materials scientists due to their unique optical versatility, high photoluminescence quantum yields and facile synthesis. LHP NCs have a ‘soft’ and predominantly ionic lattice, and their optical and electronic properties are highly tolerant to structural defects and surface states. Therefore, they cannot be approached with the same experimental mindset and theoretical framework as conventional semiconductor NCs. In this Review, we discuss LHP NCs historical and current research pursuits, challenges in applications, and the related present and future mitigation strategies explored.Lead-halide perovskites have entered the family of colloidal nanocrystals, showing excellent optical properties and easy synthesizability. This Review provides an insight into their chemical versatility, stability challenges and use in optoelectronics.