Observation of an Orientational Glass in a Superlattice of Elliptically-Faceted CdSe Nanocrystals

Abstract

Extensive prior work has shown that colloidal inorganic nanocrystals coated with organic ligand shells can behave as artificial atoms and, as such, form superlattices with different crystal structures and packing densities. Although ordered superlattices present a high degree of longrange positional order, the relative crystallographic orientation of the inorganic nanocrystals with respect to each other tends to be random. Recent works have shown that superlattices can achieve orientational alignment through combinations of nanocrystal faceting and ligand modification, as well as selective metal particle attachment to particular facets. These studies have focused on the assembly of high-symmetry nanocrystals, such as cubes and cuboctahedra.Here, we study the assembly of elliptically-faceted CdSe/CdS core/shell nanocrystals with one unique crystallographic orientation along the major elliptical axis. We show that the nanocrystals form an unexpectedly well-ordered translational superlattice, with a degree of order comparable to that achieved with higher-symmetry nanocrystals. Additionally, we show that, due to the particles' faceted shape, the superlattice is characterized by an orientational glass phase in which only certain orientations are possible due to entropically-frustrated crystallization. In this phase, the nanocrystals do not exhibit a local orientational ordering but rather have distinct orientations that emerge at different locations within the same domain. The distinct orientations are a result of a facet-to-facet lock-in mechanism that occurs during the self-assembly process. These facet-tofacet alignments force the nanocrystals to tilt on different lattice planes forming different projections that we termed apparent polydispersity. Our experimental realization of an orientational glass phase for multifaceted semiconducting nanocrystals will allow further 3 investigation of how this phase is formed and how it can be utilized for potential optical, electrical, and thermal transport applications.

Porous Magneto-Fluorescent Superparticles by Rapid Emulsion Densification

“…Their Feret diameter 76 is 13.1 nm ±22%, where the different orientations of the faceted NCs with respect to the electron beam likely contribute to their apparent higher size polydispersity. 77 We combine these two types of NCs into superparticles by using our recently developed source-sink emulsion templated assembly procedure. 48 Specifically, we use droplet microfluidics to prepare a monodisperse emulsion of toluene-inwater, the source emulsion, containing the binary dispersion of NCs at a total volume fraction of 0.01%, volumetric ratio of the two NC types of 1:1 or ϕ = 0.5, and NC size ratio γ = 0.70.…”

“…CdSe/CdS NCs appear faceted likely because of the slow epitaxial growth of the CdS shell on the CdSe core. Their Feret diameter is 13.1 nm ±22%, where the different orientations of the faceted NCs with respect to the electron beam likely contribute to their apparent higher size polydispersity …”

“…The field of nanoscale self-assembly has experienced tremendous growth over the last three decades, fueled by advances in the synthesis of complex nanocrystals (NCs) as well as increased control over their organization into complex superstructures. NCs can be assembled into amorphous, , crystalline, − and quasicrystalline − superstructures (superlattices) by careful tuning of experimental parameters such as NC shape, − stoichiometry, , ligand design, − and grafting density. − These experimental handles influence the magnitude , and length scale , of the emergent interparticle interactions between NCs that ultimately dictate the self-assembled morphologies. Currently, NC superlattices are the prime candidates for applications across a wide range of fields, including optoelectronic, , catalytic, − and smart/adaptive materials .…”

Quantitative Comparison of Recombination Rates of Core/Shell Quantum Dots in Colloidal Solutions and Self-Assembled Monolayer Superlattices

“…QD solids and films are useful in device fabrication and technology. There has been significant progress to control the structure of self-assembled 2D or 3D QD superlattices − for optimized device performance. ,− Among a range of different self-assembly mechanisms, the self-assembly of QDs at the liquid–air interface has been shown to reproducibly form well-ordered superlattices with high packing densities . Although the structure of these superlattices can be well-controlled, understanding of the photoluminescence properties of these thin films is limited due to the challenge of determining the PLQY and hence Γ r and Γ nr of low volume materials with sufficiently high signal-to-noise ratios.…”

Quaternization-Assisted Assembly of Polymer-Tethered Gold Nanoparticles into Superlattices with a Tunable Structure

“…Hierarchical assembly of functional inorganic nanoparticles (NPs) into ordered superstructures has attracted increasing attention because the bottom-up strategy provides a robust and promising pathway for the artificial construction of new functional materials and devices with advanced applications in bioimaging, cancer therapy, energy, and information storage. − Until now, a rich variety of exquisite and multidimensional superstructures with a highly ordered arrangement or crystalline packing of NPs over sufficiently long distances has been elaborately prepared. − Particularly, the fabrication of a three-dimensional (3D) multilayer superlattice film and hollow structures has received significant interest. − For instance, Murray et al assembled multicomponent NPs into free-standing binary superlattices via the cocrystallization of two types of NPs, giving a general method of growing centimeter-scale membranes for nanocrystal-based devices . Chen et al reported the development of plasmonic-magnetic hollow vesicles assembled from iron oxide-gold Janus nanoparticles that could generate reactive oxygen species (ROS) and release doxorubicin (DOX) .…”