Lawrence Livermore National Laboratory



Christine Orme

Overview

Three-dimensional nanocrystal assemblies represent a new form of solid material that, if well-coupled, possesses properties derived from both its individual building blocks as well as cooperative interactions. One of the requirements for successfully developing these novel materials is the ability to control the crystalline quality of the assemblies using a method that allows the film microstructure to be deliberately tuned. We demonstrated next-generation control over the microstructure of superlattice films and showed that we could create highly ordered three-dimensional superlattices. We were able to control the nucleation density and the growth rate because we used an electrical field to adjust the flux. We also used our electrical-field-driven approach to develop better in situ diagnostics to study the assembly process. This resulted in a comprehensive quantitative description of the depositional process. We demonstrated that the electrical field created size and polydispersity gradients that led to a size-selection effect that could be used to tune the superlattice's lattice constant. In addition, the electrical field reduced the polydispersity at the electrode interface by 21 percent, which enabled the creation of higher quality crystals. We also developed a modeling framework to describe the depositional process and found that the measured concentration profiles could not be explained without invoking non-equilibrium effects. We also demonstrated that the superlattice structure could be altered by changing the hydrophobic nature of the substrate.

Background and Research Objectives

Colloidal nanocrystals are the building blocks for new and improved electronic devices due to their tunable properties and their potential for low-cost solution processing. Nanocrystals within devices form ensembles whose collective properties (such as charge carrier mobility) rely heavily on the individual nanocrystals and the way that they are arranged. Ordered nanocrystal ensembles (known as superlattices) will have better electronic transport properties (due to higher density) and the potential for mini-band formation. In practice, however, few devices built from ordered nanocrystal superlattices have been developed. This situation could be improved if the crystallization process were better controlled and if the nanocrystal assembly process was more quantitatively described.

Our project had two primary objectives. The first was to develop simple techniques to make controlled nanocrystal arrays that outperform current synthesis methods. The second was to develop a more comprehensive quantitative understanding of the depositional process through in situ observation and modeling. Toward these ends, we realized four major accomplishments:

  1. We demonstrated next-generation control over the microstructure of superlattice films.
  2. We compiled a comprehensive quantitative description of the depositional process (Yu et al. 2018).
  3. We developed a modeling framework to describe the deposition process.
  4. We demonstrated that the superlattice structure could be altered by changing the hydrophobic nature of the substrate.

Impact on Mission

The science of electrophoretic deposition is an enabler for several programs at Lawrence Livermore National Laboratory, including laser target fabrication, armor development, and exchange spring magnets. Furthermore, by employing base materials and techniques developed within the Laboratory's manufacturing initiative and adding new functionality, our research supports the Laboratory’s core competencies in advanced materials and manufacturing, as well as high-performance computing, simulation, and data science. Finally, this project also provided the preliminary data for technology commercialization funding managed by the NNSA’s Office of Technology Transfer.

Conclusion

We have developed a simple and efficient method to grow superlattices that offers superior depositional control compared to standard evaporative techniques and that will benefit applications such as solar cells, light-emitting diodes, and photodetectors. Certain fundamental questions need to be answered before our approach can be applied to a wider variety of materials. There are also unanswered questions regarding the depositional process that require more detailed modeling. We intend to continue the in situ experiments and modeling. Our research attracted the interest of an industrial partner and we have acquired technology commercialization funding to demonstrate the viability of our approach in the realm of infrared sensors.

Publications and Presentations

Orme, C. 2017. "Reversible, Tunable, Electric-Field Driven Assembly of Silver Nanocrystal Superlattices." Nano Letters 17:3862–3869. doi: 10.1021/acs.nanolett.7b01323. LLNL-JRNL-710158.

Orme, C., et al. 2016. "Using Electrophoretic Deposition to Assemble Nanocrystal Superlattices." LLNL-PRES-704412.

——— . 2017. "Reversible, Field-Driven Superlattice Formation." LLNL-POST-740786.

Yu, Y. and C. Orme. 2017. "Electric Field-Driven Assembly of Silver Nanocrystal Superlattices." MRS Spring Meeting and Exhibit, Phoenix, AZ, April 2017. LLNL-PRES-729482.