Research Highlights

What could be a model system for nanocrystal superradiance? One possibility is a nanocrystal monolayer. Together with collaborators from France and the US, we have theoretically studied the effects of nanocrystal properties on single-excitation superradiance. In short, large superradiance in small nanocrystals suffers from the slightest size dispersion, while big nanocrystals offer a compromise. Featured on the cover of J. Chem. Phys. and a part of the 2023 Emerging Investigators Special Collection, the work is an Open Access publication. 

How does a crystal made of semiconductor nanocrystals respond under light of high intensity? In this work, collaborators from Brescia studied films of CsPbBr3 nanocrystal superlattices by ultrafast spectroscopy varying the concentration of photo-excited carriers. An interesting collective response was observed, similar to an insulator-to-metal transition, opening up applications of such supercrystals as simulators of other quantum phenomena. This work is an Open Access publication.

The extended talk covering collective x-ray diffraction and photoluminescence of perovskite nanocrystal superlattices was presented at the research program Nanoparticle Assemblies: A New Form of Matter with Classical Structure and Quantum Function in Kavli Institute of Theoretical Physics in Santa Barbara (March 27 - May 19, 2023). A pdf copy of the slides is available on the KITP website.

The effort to understand perovskite nanocrystal superlattices started from noticing an unexpected weak peak splitting in early 2018 and grew into a series of papers developing an entire quantitative structural analysis methodology. The paper "Collective Diffraction Effects in Perovskite Nanocrystal Superlattices" by Toso et al. in Accounts of Chemical Research summarizes that progress, discusses the perspectives outside of the metal halides, and contains new results of simulated diffraction patterns of single and binary nanocrystal superlattices. This work is an Open Access publication.

A 10-min presentation from the 2021 Internet NanoGe Conference on Nanocrystals (iNCNC, June 28th - July 2nd, 2021) that summarizes various results on the chemistry, transformations, and characterization of colloidal Cs4PbBr6 nanocrystals (e.g., their conversion to nanoheterostructures, CsPbBr3, and CsPbBr3@SiO2 nanoparticles). Many thanks to nanoGe for making the recording available as a part of a freely accessible educational research collection. 

Raisin bread is commonly seen as a dessert and rarely as a way to protect raisins from the environment. Replace bread with organic co-polymer polystyrene-block-poly(acrylic acid) and raisins with perovskite nanocrystals, and, voilà, you get spherical capsules that protect nanocrystals even if they have been left in water for a couple of years. I had a chance to contribute nanocrystal and mentoring expertise to this long-in-the-making project. Nanocrystal encapsulation on the bench and with a Nimbus robot, bioimaging, toxicity tests, and various characterizations was a result of a collaborative effort by several groups at IIT. Read about it in the paper "Highly Emitting Perovskite Nanocrystals with 2-Year Stability in Water through an Automated Polymer Encapsulation for Bioimaging" by Avugadda and Castelli et al. published in ACS Nano. This work is an Open Access publication.

A combination of novel semiconductor materials with different exciton energetics and confinement regimes is ought to demonstrate interesting energy exchange processes under illumination with light. I had a chance to contribute nanocrystal expertise to the study of energy transfer between perovskite nanocrystals and monolayer TMDC led by the Functional Nanosystems group at IIT. Read about it in the paper "Generation of Free Carriers in MoSe2 Monolayers Via Energy Transfer from CsPbBr3 Nanocrystals" by Asaithambi et al. published in Advanced Optical Materials. This work is an Open Access publication.

A few papers back we reported on an anhydride-amine condensation-induced transformation of non-emissive cesium lead bromide into an emissive one. Over time, we've been building up on that chemistry and turned it towards silica shell growth around perovskite cores. Despite our best efforts, the emission efficiency remained low, but the transformation and shell growth chemistry are cool nevertheless. Read about it in the paper "Exploiting the Transformative Features of Metal Halides for the Synthesis of CsPbBr3@SiO2 Core–Shell Nanocrystals" by Rossi et al. published in Chemistry of Materials. This work is an Open Access publication.

UPD: The work has been highlighted as one of the 35 most cited and read papers in Chemistry of Materials in 2022!

Without continuation you cannot score a point in an ultimate frisbee game nor can you test the robustness of the natural phenomenon and its interpretation. Our continuing investigations of the multilayer diffraction of nanocrystals led to an exciting discovery that if approached quantitatively, the thinness of the platelets is an advantage that allows refining both their structure and the surface passivation. Do you think it's magic? Nope, it is a cleverly modeled scattering of x-rays. Read on about it in the paper "Structure and Surface Passivation of Ultrathin Cesium Lead Halide Nanoplatelets Revealed by Multilayer Diffraction" by Toso et al. published in ACS Nano. This work is an Open Access publication (with codes and data provided along!). For the background on multilayer diffraction in nanocrystals catch up with this awesome Twitter thread by Stefano.

If the three elements that comprise a metal halide perovskite nanocrystal make life seem easy, why not try five elements?  Jokes aside, I had a chance to contribute photoluminescence characterization of Pb-free and Cd-full layered double perovskite nanocrystals that glow red (a brave and adventurous departure from most of my studies in green) and have a formula of Cs4MnxCd1-xSb2Cl12. Curious what happens as you tune the amount of Mn? Find it out in the paper "Red-Emissive Nanocrystals of Cs4MnxCd1-xSb2Cl12 Layered Perovskite" by Sartori et al. published in Nanoscale. The peer-reviewed version of the paper is behind a paywall, but an earlier (submitted) version is available as a ChemRxiv preprint to read for free.

One of the inorganic lead halide compounds, Cs4PbBr6, has been puzzling scientists with its optical properties. Sometimes samples of that material show green emission under higher energy excitation, and sometimes they do not shine green. I had a chance to contribute samples and temperature-dependent optical experiments to a theoretical investigation of this puzzle. Long story short, intrinsically Cs4PbBr6 does not emit in green. However, as soon as some of the [PbBr6]4- octahedra connect for one reason or another, the green emission switches on. To taste the controversy and learn about the results check out the work "Fast Intrinsic Emission Quenching in Cs4PbBr6 Nanocrystals" by Petralanda and Biffi et al. Published in Nano Letters, this work is an Open Access publication.

A highlight from Marie Skłodowska-Curie Actions social media page about RETAIN project (2018-2020) is where I briefly explain the project and share the challenges and rewards of being an MSCA fellow. 

In my opinion, the MSCA fellowship is the best pan-European postdoctoral funding opportunity, and IIT in Italy is one great place to carry it out. If you happen to contemplate applying for one, check out the "Proposal-writing" section of the Toolbox for a few helpful resources, connect with the MSCA fellows network on social media (e.g., Facebook, LinkedIn), or contact me if you have a specific question that I could help answering.

Pandemic altered the format of scientific exchange and communication by pushing seminars online. In summer 2020, a group of nanocrystal enthusiasts from MIT and CU Boulder launched an open Zoom webinar News in Nanocrystals which grew into a regular scientific event attracting participation from all over the world. As the summer and fall seasons of the webinar wrapped up, we felt it was timely and relevant to share our experience, some stats, and learned lessons about running it. As a result, a Nano Focus article "News in Nanocrystals Seminar: Self-Assembly of Early Career Researchers toward Globally Accessible Nanoscience" (free to read) came out in ACS Nano in July 2021.

Paraphrasing a famous movie quote, one may say that "An ensemble of nanocrystals is like a box of chocolates. You never know what you're gonna get." In the case of this work, "Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra," two-dimensional optical spectroscopy enabled us to figure out exactly what we had in an ensemble of colloidal lead sulfide (PbS) nanocrystals. Application of spectroscopic theory combined with the state-of-the-art multidimensional spectroscopy produced all-optical determination of the change in chemical potential upon creation of an exciton and uncovered a surprising relationship between the Stokes shift and optical inhomogeneity in a nanocrystal ensemble. Published in Science Advances, this work is an Open Access publication.

A 2-min promo clip for the upcoming Self-Organization symposium at nanoGe Spring Meeting 2021. Self-assembly is beautiful and challenging. There are a lot of building blocks that can be combined in many ways, just like pieces of a Lego set, forming structures of mesmerizing elegance and complexity. The challenge is to design or figure out a special property or function - a multi-dimensional Rubik's cube, no less. 

At last, the nanocrystal superlattices have something "super" to show for their structure - the order with which perovskite nanocubes and perovskite or lead sulfide nanoplatelets are packed is comparable to that of traditional bulk crystals. Thanks to multilayer diffraction experiments and fits (data and code are openly available), as well as collaboration with groups of Dr. Cinzia Giannini in Bari, Prof. Andrej Singer at Cornell, and Prof. Alivisatos in Berkeley. Check out the full paper in ACS Nano (open access). 

Nanocrystals are beautiful and messy. At a glance, nice electron microscopy images of uniform nanoparticles are pleasant to the eye, but their dispersions in organic solvents (like toluene) are full of stuff that is hard to account for. For example, the ligands that keep nanocrystals afloat in a solvent are often in the dynamic equilibrium with a surface, constantly detaching and re-attaching. If the nanocrystals are fragile and cannot be rigorously purified, as is common with metal halide nanocrystals, then the residue of precursors is likely to be carried out along with the nanocrystals after post-synthetic isolation. In a recent study led by Dr. Zhiya Dang and published in Nanoscale, we looked at the dispersion of Cs4PbBr6 nanocrystals in toluene by means of liquid cell transmission electron microscopy (LCTEM). To our surprise, we observed in situ nucleation of what appeared to be metal Pb nanoparticles of rounded and dendrite shapes. Their likely origin was traced to the Pb oleate/bromide precursor carried over from the synthesis.

A fun and reader-friendly review article discussing four dimensions (non-orthogonal) of chemical reactivity and transformations of cesium lead halide nanocrystals: structure, color, shape, and surface. How cesium lead halide nanocrystals different from other nanocrystals? Why so many reactions are possible? Is rich reactivity a challenge or an opportunity? You can find answers to these and other questions on the pages of the Accounts of Chemical Research (open access). 

Nanocrystals are a lot like J.K. Rowling's fantastic beasts: unpredictable, different from each other, and deeply fascinating. In a study out in ACS Nano (open access), we attempted to dissect how nanocrystals of promising light-emitting material, perovskite CsPbBr3, change over time under vacuum and in air, and how that affects their collective optical properties (specifically, superfluorescence in perovskite nanocrystals). Turns out, nanocrystals really like to merge together into bigger crystals, causing energy transfer and changes in emission properties that, especially at low temperatures, have a seductively similar appearance of collective emission. They are fantastic beasts, aren't they?

The poster summarizes our efforts to understand the structure and light-emitting properties of CsPbBr3 nanocrystal superlattices. It was presented at the 11th International Conference on Quantum Dots, QD2020, that took place online on December 7-11, 2020. 

In this talk I overview the lessons learned about stability of perovskite nanocrystal superlattices and what holds them from being used in future screens, quantum circuits, and artificial photosynthesis. The talk "Miniature Light Emitters Based on Self-Assembled CsPb(Br1-xIx)3 Nanocrystals: Barriers and Opportunities" was presented at the NanoGe Fall Meeting, October 20-23, 2020. 

Identifying patterns that explain reactivity of inorganic compounds is a powerful scientific method for understanding their chemistry and generating predictions. The principle of cesium cation substructure preservation weaves through the myriad of reports and like an Ariadne's thread fleshes out a simple concept that connects and explains interconversions between lead halide perovskites and related compounds. Feel free to learn more about it in the ACS Energy Letters viewpoint (open access). 

In this talk I discussed aging of perovskite nanocrystal superlattices and how that aging defines their optical response and energy transfer at cryogenic temperatures. The talk "The Hidden Role of Nanocrystal Reactivity in Photoluminescence of Self-Assembled CsPbBr3 Nanocubes" was presented at the NanoGe Internet Conference for Quantum Dots (iCQD), July 14-17, 2020.

Oftentimes nanocrystals are mixed with polymers to make blends, and any reactivity between the two is considered detrimental unless something useful comes of it. In this example, a serendipitously discovered reaction between colorless nanocrystals of tetracesium lead hexabromide and a copolymer of maleic anhydride, a reactive organic functional group, resulted in the formation of brightly emissive green nanocubes of cesium lead bromide perovskite. A one-step mix 'n' shake approach to a high-quality material useful for applications in LEDs and scintillators. Check out a brief explanation video or dive into a full paper in Chemical Science (open access) to learn about nuts and bolts of this discovery.

"Superlattices" are macroscopic assemblies of size- and shape-pure nanocrystals. By looking at individual superlattices, one can study properties of an "average" nanocrystal, bypassing the need for challenging single-nanocrystal experiments (up to a point). Together with a visiting Ph.D. student Mike Brennan, we used this perspective to investigate how mixed halide CsPb(Br:I)3 nanocrystals hold under UV light. It is essential to know that because such nanocrystals are considered promising for future solar cells, lasers, and color-converters, i.e., applications which involve prolonged operation under optical excitation. It turns out that under UV light, superlattices expel iodide while changing colors and brightening (see accompanying video). We hypothesized why it could be the case in the recent ACS Energy Letters paper, which resulted from this project. 

How do we know that nanocrystal superlattices are size- and shape-pure nanocrystal ensembles? Because they appear to be exceptionally well-ordered solids. The strikingly high degree of nanocrystal order in with respect to each other in superlattices is noticeable from the X-ray interference patterns. The initial observations and analysis of this phenomenon through the straightforward application of Bragg's law were carried out with a Ph.D. student Stefano Toso and a collaborator Dr. Cinzia Giannini and published as a brief paper in a newly-launched journal ACS Materials Letters.