Research Highlights

Like a Crystal (Part Two)

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.

Red Velvet Nanocrystal Cake

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.

To Green or not to Green

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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.

Fellow of the Week (Sept 24, 2021)

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.

News in Nanocrystals Revue

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.

2D Spectra Tell It All

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.

nanoGe Symposium Promo

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.

Like a Crystal (Part One)

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).

The Sea of Monsters

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.

Metamorphōseōn Librī

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).

Aging of Self-Assembled Lead Halide Perovskite Nanocrystal Superlattices

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?

QD2020 Poster: Structural Coherence, Energy Transfer, and Optical Gain in Assemblies of Perovskite Nanocrystals

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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.

NFM20 Talk: Perovskite Nanocrystal Superlattices

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.

Follow the Cesium

The authors are figuring out their way through the labyrinth of Cs-Pb-X nanostructures.

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).

iCQD Talk: Aging of Nanocrystal Superlattices

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.

(Nano) Transformers

Non-innocent oleylamine ligands from the ternary lead halide nanocrystals will happily engage with maleic anhydride groups of the organic co-polymer and trigger nanocrystal transformation into brightly emissive perovskite nanocubes.

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 Greener on the Other Side

"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.

Behind Diffraction Peaks

Interference of X-rays from well-ordered nanocrystals results in a distinct progression of superlattice peaks at high angles, as contrasted with the powder of randomly-oriented nanocrystals which produces a single broad peak.

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.