Nanoparticle Sheets

Self-assembly is a powerful tool to precisely construct periodic arrays on the nanoscale. Working in 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 ~10^12 close-packed particles per square centimeter (Figure 1) [1]. Our early work used these nanoparticle sheets as platforms to study electronic transport [2]. Ongoing work below focuses on the fascinating mechanical properties of these nanoparticle sheets and their applications as ultrathin ion transport or water filtration membranes.

[1] Bigioni et al., Nature Mater. 5, 265-270 (2006). link; [2] Parthasarathy et al., Phys. Rev. Lett. 92, 076801 (2004). link
Fig. 1 (a) Sketch of nanoparticle monolayer self-assembly on air–water interface and the formation of freestanding monolayer on a TEM grid after water has evaporated. (b) SEM image of freestanding nanoparticle monolayers on carbon-coated TEM grid with array of circular holes. Inset: zoomed in detail of region within freestanding membrane measured by TEM.

Ultrathin Film Mechanics

Despite only being held together by weak van der Waals forces, nanoparticle monolayer sheets are remarkably strong. In fact, our group was the first to demonstrate that these could be freely suspended over micron-sized holes and possess Young's moduli in the GPa range [3-6]. Though traditionally underappreciated, the humble ligand molecules attached to the nanoparticle surface actually underlie these surprising mechanical properties [7-8] and can even be used to direct curling and bending upon e-beam irradiation [9].

Our current work seeks to understand how the mechanical properties of these nanoparticle sheets differs from those of other ultrathin materials like graphene, such as their ability to conform to a curved surface [10], and also explores novel methods to actuate these ultrathin sheets.

[3] Mueggenburg et al. Nature Mater. 6, 656-660 (2007). link; [4] He et al. Small 6, 1449-1456 (2010). link; [5] Wang et al. Nano Lett. 14, 826-830 (2014). link; [6] Wang et al. Faraday Discuss. 181, 325-338 (2015). link; [7] Wang et al. Nano Lett. 15, 6732-6737 (2015). link; [8] Wang et al. ACS Nano 11, 8026-8033 (2017). link ; [9] Jiang et al. Nature Mater. 14, 912-918 (2015). link; [10] Mitchell et al. Soft Matter, 14, 9107-9117 (2018) link
When a nanoparticle sheet is pressed down onto a curved substrate, it is caught between two competing forces. The sheet seeks to resist bending and cracking but also experiences an adhesive pull to conform to the substrate. The balance between these two extremes is determined by the Gaussian curvature of the substrate (increasing curvature from a --> d above).

Unconventional Membrane Materials

An ideal membrane has an imposing set of design criteria: simple and scalable fabrication, high permeability, high selectivity, chemical stability, and mechanical integrity. Based on their ultrathin nature and surprising mechanical strength, we have investigated nanoparticle sheets as novel water filtration membranes [11]. These thin sheets exhibited much higher thickness-normalized permeabilities 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 and used as effective ion transport membranes [12]. Our most recent work inverted this paradigm, instead using the NP cores to polymerize the surrounding ligand matrix and then removing the NP cores to yield a nanoporous hydrocarbon film [13].

[11] He et al., Nano Lett. 11, 2430–2435 (2011) link; [12] Barry et al., Nature Commun. 5, 5847 (2014). link[13] Jackson et al. Nano Lett., 21, 166-174 (2021) link
Electron beam irradiation of 2D or 3D NP arrays chemically crosslinks the surrounding ligand matrix. Subsequent chemical etching removes NP cores and yields nanoporous hydrocarbon membranes.