Sergio Marras

Hierarchical self-assembly of suspended branched colloidal nanocrystals into superlattice structures

Self-assembly of molecular units into complex and functional superstructures is ubiquitous in biology. The number of superstructures realized by self-assembly of man-made nanoscale units is also growing. However, assemblies of colloidal inorganic nanocrystals are still at an elementary level, not only because of the simplicity of the shape of the nanocrystal building blocks and their interactions, but also because of the poor control over these parameters in the fabrication of more elaborate nanocrystals. Here, we show how monodisperse colloidal octapod-shaped nanocrystals self-assemble, in a suitable solution environment, on two sequential levels. First, linear chains of interlocked octapods are formed, and subsequently the chains spontaneously self-assemble into three-dimensional superstructures. Remarkably, all the instructions for the hierarchical self-assembly are encoded in the octapod shape. The mechanical strength of these superstructures is improved by welding the constituent nanocrystals together.

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.

17.6% stabilized efficiency in low-temperature processed planar perovskite solar cells

We present here a planar perovskite solar cell with a stabilized power conversion efficiency (PCE) of 17.6% at the maximum power point and a PCE of 17% extracted from quasi-static J–V with an open-circuit voltage of 1.11 V. Such excellent figures of merit can be achieved by engineering a solution-processed electron buffer layer that does not require high temperature steps. A compact thin film of perovskite absorber is grown onto a PCBM-based electron extraction layer by implementing a novel two-step procedure which preserves the soluble organic interlayer during the deposition of successive layers. We demonstrate that efficient charge extraction is the key for high steady state efficiency in perovskite solar cells with a highly integrable architecture.

The Impact of the Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskite Films for Photovoltaics.

We investigate the relationship between structural and optical properties of organo-lead mixed halide perovskite films as a function of the crystallization mechanism. For methylammonium lead tri-iodide, the organic cations rearrange within the inorganic cage, moving from crystals grown in a mesoporous scaffold to larger, oriented crystals grown on a flat substrate. This reduces the strain felt by the bonds forming the cage and affects the motion of the organic cation in it, influencing the electronic transition at the onset of the optical absorption spectrum of the semiconductor. Moreover, we demonstrate that in mixed-halide perovskite, though Cl(-) ions are not present in a detectable concentration in the unit cell, they drive the crystallization dynamics. This induces a preferential order during crystallization, from a molecular, i.e., organic-inorganic moieties arrangement, to a nano-mesoscopic level, i.e., larger crystals with anisotropic shape. Finally, we show that while Cl is mainly expelled from flat films made of large crystals, in the presence of an oxide mesoporous scaffold they are partially retained in the composite.

Role of microstructure in the electron–hole interaction of hybrid lead halide perovskites

Solar cells based on hybrid inorganic-organic halide perovskites have demonstrated high power conversion efficiencies in a range of architectures. The existence and stability of bound electron-hole pairs in these materials, and their role in the exceptional performance of optoelectronic devices, remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modeling as a function of the degree of polycrystallinity and temperature, that the electron-hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycrystalline disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphology strongly influence perovskite behaviour, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.

Epitaxial CdSe-Au Nanocrystal Heterostructures by Thermal Annealing

The thermal evolution of a collection of heterogeneous CdSe-Au nanosystems (Au-decorated CdSe nanorods, networks, vertical assemblies) prepared by wet-chemical approaches was monitored in situ in the transmission electron microscope. In contrast to interfaces that are formed during kinetically controlled wet chemical synthesis, heating under vacuum conditions results in distinct and well-defined CdSe/Au interfaces, located at the CdSe polar surfaces. The high quality of these interfaces should make the heterostructures more suitable for use in nanoscale electronic devices.

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.

Cu3-xP Nanocrystals as a Material Platform for Near-Infrared Plasmonics and Cation Exchange Reactions

Synthesis approaches to colloidal Cu3P nanocrystals (NCs) have been recently developed, and their optical absorption features in the near-infrared (NIR) have been interpreted as arising from a localized surface plasmon resonance (LSPR). Our pump–probe measurements on platelet-shaped Cu3-xP NCs corroborate the plasmonic character of this absorption. In accordance with studies on crystal structure analysis of Cu3P dating back to the 1970s, our density functional calculations indicate that this material is substoichiometric in copper, since the energy of formation of Cu vacancies in certain crystallographic sites is negative, that is, they are thermodynamically favored. Also, thermoelectric measurements point to a p-type behavior of the majority carriers from films of Cu3-xP NCs. It is likely that both the LSPR and the p-type character of our Cu3-xP NCs arise from the presence of a large number of Cu vacancies in such NCs. Motivated by the presence of Cu vacancies that facilitate the ion diffusion, we have additionally exploited Cu3-xP NCs as a starting material on which to probe cation exchange reactions. We demonstrate here that Cu3-xP NCs can be easily cation-exchanged to hexagonal wurtzite InP NCs, with preservation of the anion framework (the anion framework in Cu3-xP is very close to that of wurtzite InP). Intermediate steps in this reaction are represented by Cu3-xP/InP heterostructures, as a consequence of the fact that the exchange between Cu+ and In3+ ions starts from the peripheral corners of each NC and gradually evolves toward the center. The feasibility of this transformation makes Cu3-xP NCs an interesting material platform from which to access other metal phosphides by cation exchange.

Colloidal synthesis of cuprite (Cu2O) octahedral nanocrystals and their electrochemical lithiation.

We report a facile colloidal route to prepare octahedral-shaped cuprite (Cu2O) nanocrystals (NCs) of ∼40 nm in size that exploits a new reduction pathway, i.e., the controlled reduction of a cupric ion by acetylacetonate directly to cuprite. Detailed structural, morphological, and chemical analyses were carried on the cuprite NCs. We also tested their electrochemical lithiation, using a combination of techniques (cyclic voltammetry, galvanostatic, and impedance spectroscopy), in view of their potential application as anodes for Li ion batteries. Along with these characterizations, the morphological, structural, and chemical analyses (via high-resolution electron microscopy, electron energy loss spectroscopy, and X-ray photoelectron spectroscopy) of the cycled Cu2O NCs (in the lithiated stage, after ∼50 cycles) demonstrate their partial conversion upon cycling. At this stage, most of the NCs had lost their octahedral shape and had evolved into multidomain particles and were eventually fragmented. Overall, the shape changes (upon cycling) did not appear to be concerted for all the NCs in the sample, suggesting that different subsets of NCs were characterized by different lithiation kinetics. We emphasize that a profound understanding of the lithiation reaction with NCs defined by a specific crystal habit is still essential to optimize nanoscale conversion reactions.

Influence of Chloride Ions on the Synthesis of Colloidal Branched CdSe/CdS Nanocrystals by Seeded Growth

We studied the influence of chloride ions (Cl(-)), introduced as CdCl(2), on the seeded growth synthesis of colloidal branched CdSe(core)/CdS(pods) nanocrystals. This is carried out by growing wurtzite CdS pods on top of preformed octahedral sphalerite CdSe seeds. When no CdCl(2) is added, the synthesis of multipods has a low reproducibility, and the side nucleation of CdS nanorods is often observed. At a suitable concentration of CdCl(2), octapods are formed and they are stable in solution during the synthesis. Our experiments indicate that Cl(-) ions introduced in the reaction reduce the availability of Cd(2+) ions in solution, most likely via formation of strong complexes with both Cd and the various surfactants. This prevents homogeneous nucleation of CdS nanocrystals, so that the heterogeneous nucleation of CdS pods on top of the CdSe seeds is the preferred process. Once such optimal concentration of CdCl(2) is set for a stable growth of octapods, the pod lengths can be tuned by varying the relative ratios of the various alkyl phosphonic acids used. Furthermore, at higher concentrations of CdCl(2) added, octapods are initially formed, but many of them evolve into tetrapods over time. This transformation points to an additional role of Cl species in regulating the growth rate and stability of various crystal facets of the CdS pods.