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" 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
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.
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
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
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.
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
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.