While traditional "bottom-up" chemical approaches enable exquisite control over atomic-level structures and "top-down" fabrication strategies excel at patterning macroscopic lengthscales, few approaches can reliably and efficiently decorate surfaces with periodic features in the 10-100 nanometer regime. Self-assembly is a powerful tool to circumvent this limitation and precisely construct useful materials from simple, nanometer-sized building blocks.
Utilizing a robust drop casting technique pioneered by our research group (Figure 1) [1-2] and working in close collaboration with Dr. Xiao-Min Lin (Center for Nanoscale Materials, Argonne National Laboratory), we study the self-assembly of monodisperse, ligand-stabilized nanocrystals from solution into monolayer sheets comprising more than 10^8 close-packed particles. These metal-organic hybrid materials preserve the electrical, optical, or magnetic properties of the nanoparticles while the embedding matrix of organic molecules imbues remarkable mechanical properties. Our early work used monolayer and multilayer nanoparticle sheets as well-defined platforms to study the role of structural order on electronic transport properties [3-5]. As summarized below, ongoing work focuses on exploring the fascinating mechanical properties of these nanoparticle sheets and advancing their use as ultrathin ion transport or water filtration membranes.
 Bigioni, T. P. et al., “Kinetically-Driven Self-Assembly of Highly-Ordered Nanocrystal Monolayers”, Nature Mater. 5, 265-270 (2006). link Mueggenburg, K. E. et al., "Elastic Membranes of Close-packed Nanoparticle Arrays", Nature Mater. 6, 656-660 (2007). link Parthasarathy, R. et al., "Electronic Transport in Metal Nanocrystals Arrays: The Effect of Structural Disorder on Scaling Behavior", Phys. Rev. Lett. 87, 186807 (2001). link Parthasarathy, R. et al., “Percolating Through Networks of Random Thresholds: Finite Temperature Electron Tunneling in Metal Nanocrystal Arrays”, Phys. Rev. Lett. 92, 076801 (2004). link Tran, T. B. et al., “Sequential tunneling and inelastic cotunneling in nanoparticle arrays”, Phys. Rev. B 7, 075437 (2008). link
Film Mechanics in the Ultra-Thin Limit
If you pressed on a nanoparticle monolayer, would it respond more like paper or more like an elastic rubber sheet?
In our initial studies of nanoparticle monolayers, we demonstrated that these sheets could be freely suspended over micron-sized holes . In addition to the relative flexibility of these sheets, AFM indentation tests revealed a large Young's modulus in the GPa range. In subsequent studies, we have investigated the generality of this result for various nanoparticle systems with different ligands , employed ion and e-beam radiation to investigate strain gradients , and stretched monolayers deposited on elastomeric substrates to determine the mechanism of tensile failure [4,5].
Ongoing work is exploring the role of the organic ligands on the mechanical properties [6,8,9]. We have demonstrated that the nanoparticles develop an asymmetric ligand distribution during the self-assembly process, which we have exploited to induce curling and bending upon e-beam irradiation .
 Mueggenburg, K. E. et al., "Elastic Membranes of Close-packed Nanoparticle Arrays", Nature Mater. 6, 656-660 (2007). link He, J. et al. "Fabrication and Mechanical Properties of Large‐Scale Freestanding Nanoparticle Membranes", Small 6, 1449-1456 (2010). link Pongsakorn Kanjanaboos et al., “Strain Patterning and Direct Measurement of Poisson’s Ratio in Nanoparticle Monolayer Sheets”, Nano Letters 11, 2567–2571 (2011). link Wang, Y. et al. "Fracture and Failure of Nanoparticle Monolayers and Multilayers", Nano Lett. 14, 826-830 (2014). link Wang, Y. et al. "Mechanical Properties of Self-assembled Nanoparticle Membranes: Stretching and Bending", Faraday Discuss. 181, 325-338 (2015). link Jiang, Z. et al. "Subnanometre Ligand-shell Asymmetry Leads to Janus-like Nanoparticle Membranes", Nature Mater. 14, 912-918 (2015). link Wang, Y. et al. "Strong Resistance to Bending Observed for Nanoparticle Membranes", Nano Lett. 15, 6732-6737 (2015). link Wang, Y. et al. "Thermomechanical Response of Self-Assembled Nanoparticle Membranes", ACS Nano 11, 8026-8033 (2017). link  Griesemer, S. D. et al., "The Role of Ligands in the Mechanical Properties of Langmuir Nanoparticle Films," Soft Matter 13, 3125-3133 (2017). link Mitchell, N. et al. "Conforming Nanoparticle Sheets to Surfaces with Gaussian Curvature", arXiv:1808.03552 (2018)
Unconventional Membrane Materials
An ideal membrane has an imposing set of design criteria: simple and scalable fabrication, high flux, high selectivity, chemical stability, and mechanical integrity. Our prior work had already established that low-defect, large scale nanoparticle sheets could be readily formed through drying-mediated self-assembly and possess remarkable mechanical strength. Inspired by our observation that a water droplet will continue to evaporate even when encased by a nanoparticle monolayer, we experimentally and theoretically investigated these materials as novel water filtration membranes . These thin sheets exhibited much higher fluxes than conventional polymer membranes while maintaining high selectivity due to the well-defined <2 nm pores at the nanoparticle interstices. Subsequent work established that the pores of these nanoparticle membranes could be coated with ionic functionalities through ligand-exchange and used as effective ion transport membranes .
 He, J. et al., “Diffusion and Filtration Properties of Self-Assembled Gold Nanocrystal Membranes”, Nano Lett. 11, 2430–2435 (2011). link Barry, E. et al., “Ion Transport Controlled by Nanoparticle-Functionalized Membranes”, Nature Commun. 5, 5847 (2014). link