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Research Highlights

January 2019

Forces for Good

PNNL research reveals the interplay of attraction and repulsion between nanoparticles as they assemble


TEM observations
PNNL researchers used real-time liquid cell transmission electron microscopy (TEM) observations of gold nanoparticles in toluene (left), combined with a high speed camera, to analyze and quantify the various forces (right) at work during nanoparticle self-assembly. Enlarge Image.

A superlattice is an ordered structure formed from repeating building units, such as nanoparticles. Superlattice structures formed by the self-assembly of nanoparticles are emerging as a potentially important class of nanomaterials due to their desirable physical and chemical properties. However, a better understanding of the relationship between the dynamics of individual nanoparticles and superlattice patterns is key to controlling the structure of superlattice patterns and properties.

To address the knowledge gap, researchers from Pacific Northwest National Laboratory provide new details about the complex competition between forces at different scales in their study, "Mechanistic Understanding of the Growth Kinetics and Dynamics of Nanoparticle Superlattices by Coupling Interparticle Forces from Real-Time Measurements," published in ACS Nano.

Why it matters

The study builds a framework to understand particle self-assembly at a fundamental level. In particular, it provides in-depth insights of the interplay among Van der Waals, Brownian, hydrodynamic, and steric hindrance forces that control superlattice assembly, crystal growth, and particle sedimentation.

Designing superlattice patterns with defined separation distances can play an important role in controlling properties such as fluorescence absorption and lifetime. Fluorescence is important for biomedicine applications such as imaging guided therapy.

Summary

Researchers at PNNL are working to develop a predictive understanding of how interparticle forces drive the assembly of nanosized building blocks into tailored superlattice structures. In this experiment, the research team used liquid cell TEM equipped with a high speed camera to capture and measure the fast movements of gold nanoparticles. They examined the kinetics and dynamics of cluster growth in three stages: initial slow assembly, subsequent fast assembly, and a final post-assembly state.

Initially, the gold nanoparticles aggregated via Brownian coagulation. However, strong, short-range repulsive interactions between particles led to a slow assembly process characterized by reaction-limited particle assembly. In other words, cluster growth was largely determined by particle interactions rather than diffusion.

As particles became closer to each other, Van der Waals attraction force played a dominating role and increased the assembly rate. After assembly, competition between Van der Waals and steric hindrance repulsion forces led to clusters of particles that continued to move together, but did not touch. This TEM video shows the particle self-assembly process.

By combining detailed analyses of the experiments with initial aggregation kinetics and population balance models, this research uncovered the critical role of particle interactions during self-assembly. The results demonstrated that cluster growth kinetics can be reasonably modeled by reaction-limited Brownian aggregation.

Researchers also successfully resolved the detailed contributions and competition between the various forces and correlated these forces to the structure and dynamics of nanoparticle self-assembly.

What's next?  

New understanding of the interactions between surface coating materials can be extended to a wide range of ligands with different chain lengths, functionalized groups, or biodegradable polymers. These advances can also be applied to the fields of material synthesis, bio-mineralization, bio-sensors, or electronic devices.

Acknowledgments

This work aligns with the DOE Basic Energy Science goal of achieving "energy-efficient synthesis of revolutionary new forms of matter with tailored properties" through an understanding of the molecular-to-mesocale principles controlling the superlattice organization.

Sponsors

Research design, experiment, and image and theoretical force analysis were supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), Early Career Research program under award # 67037. Data interpretation and theoretical analyses for the interparticle forces were also supported by the DOE-BES, Synthesis Science and Processing Program.

Initial experiments were supported by the Materials Synthesis and Simulations across Scales Initiative, a Laboratory Directed Research and Development Program at Pacific Northwest National Laboratory (PNNL). Image analysis was also supported by the Laboratory Directed Research and Development Program at PNNL, through the Linus Pauling Distinguished Postdoctoral Fellowship program.

Research team

Jaewon Lee, Elias Nakouzi, Miao Song, Jaehun Chun, and Dongsheng Li (Pacific Northwest National Laboratory); Bin Wang (Pacific Northwest National Laboratory and Xi'an Jiaotong University).  

Reference

Lee J, Nakouzi E, Song M, Wang B, Chun J, Li D.  "Mechanistic Understanding of the Growth Kinetics and Dynamics of Nanoparticle Superlattices by Coupling Interparticle Forces from Real-Time Measurements." ACS Nano, 2018;12(12): 12778-12787. DOI: 10.1021/acsnano.8b07880


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