Abstract
Using a coarse-grained molecular dynamics model, we simulate the self-assembly of PbSe nanocrystals (NCs) adsorbed at a flat fluid-fluid interface. The model includes all key forces involved: NC-NC short-range facet-specific attractive and repulsive interactions, entropic effects, and forces due to the NC adsorption at fluid-fluid interfaces. Realistic values are used for the input parameters regulating the various forces. The interface-adsorption parameters are estimated using a recently introduced sharp-interface numerical method which includes capillary deformation effects. We find that the final structure in which the NCs self-assemble is drastically affected by the input values of the parameters of our coarse-grained model. In particular, by slightly tuning just a few parameters of the model, we can induce NC self-assembly into either silicene-honeycomb superstructures, where all NCs have a facet parallel to the fluid-fluid interface plane, or square superstructures, where all NCs have a facet parallel to the interface plane. Both of these nanostructures have been observed experimentally. However, it is still not clear their formation mechanism, and, in particular, which are the factors directing the NC self-assembly into one or another structure. In this work, we identify and quantify such factors, showing illustrative assembled-phase diagrams obtained from our simulations. In addition, with our model, we can study the self-assembly dynamics, simulating how the NCs’ structures evolve from few-NCs aggregates to gradually larger domains. For example, we observe linear chains, where all NCs have a facet parallel to the interface plane as typical precursors of the square superstructure, and zigzag aggregates, where all NCs have a facet parallel to the interface plane as typical precursors of the silicene-honeycomb superstructure. Both of these aggregates have also been observed experimentally. Finally, we show indications that our method can be applied to study defects of the obtained superstructures.
- Received 28 October 2018
- Revised 28 January 2019
DOI:https://doi.org/10.1103/PhysRevX.9.021015
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Nanomaterials—materials with geometric features no longer than a few hundred nanometers—are emerging as a vital component of many new technologies such as biosensing, high-quality filters, and advanced lubricants. One promising method to create nanomaterials is the so-called bottom-up approach, where nanoparticles are synthesized with properties (such as shape and chemical interactions) necessary to drive self-assembly into the desired structure. Researchers, therefore, need to know how to tune these properties to correctly program this self-assembly. Here, we simulate the self-assembly of nanocrystals into square or honeycomb arrangements, showing which parameters are key for directing self-assembly into one structure or another.
Square and honeycomb “superlattices”—lattices of atomic lattices—are of great interest for semiconductor applications thanks to their optoelectronic properties. Honeycomb superlattices, in particular, are expected to combine properties of graphene with those of semiconductors. While both superlattices have been observed experimentally, researchers do not fully understand how they form.
For our simulations, we introduce a new coarse-grained molecular dynamics model, which includes all the relevant forces involved in self-assembly. We find that tuning just a few parameters directs the final outcome. In particular, the assembled structure depends on the interplay between interface-adsorption forces and short-range electrostatic forces between the nanocrystals.
Current synthesized honeycomb superlattices are afflicted by a large amount of disorder. Our results will help researchers control this synthesis, allowing them to finally observe the expected optoelectronic properties. Our new molecular dynamics model can also easily be extended to study the self-assembly of nanoparticles at fluid-fluid interfaces into different structures.
