Three-Dimensional Colloidal Nanocrystal Metamaterials

Colloidal nanocrystals (NCs) have inorganic cores and organic or inorganic ligand shells. They are prized for their size- and shape-dependent properties and serve as building blocks of artificial materials and unconventional devices. Here, we describe NC-based, three-dimensional optical metamaterials constructed using imprinting techniques from single- and multiple-types of metal and metal oxide NCs. We focus on the chemical and thermal addressability of NCs, i.e., the ability to select, exchange, strip, or add atoms, ions, and molecules during or post-deposition, that is not accessible in bulk materials, and allows the control of metamaterial structure and properties. Through ligand engineering we tailor the dielectric function of metal NC assemblies through an insulator-to-metal transition.¹ By juxtaposing NC assemblies and bulk thin films to make bilayer heterostructures, we exploit ligand exchange to trigger folding of two- into three-dimensional structures,² which we use to achieve broadband^3,4 and reconfigurable 3D chiral optical metamaterials. These structures can also be released from the surface to create and study active matter and its interface with microscale robotics.⁵<br/>(1) Fafarman, A. T.; Hong, S.-H.; Caglayan, H.; Ye, X.; Diroll, B. T.; Paik, T.; Engheta, N.; Murray, C. B.; Kagan, C. R. Chemically Tailored Dielectric-to-Metal Transition for the Design of Metamaterials from Nanoimprinted Colloidal Nanocrystals. <i>Nano Lett.</i> <b>2013</b>, <i>13</i> (2), 350–357.<br/>(2) Zhang, M.; Guo, J.; Yu, Y.; Wu, Y.; Yun, H.; Jishkariani, D.; Chen, W.; Greybush, N. J.; Kübel, C.; Stein, A.; Murray, C. B.; Kagan, C. R. 3D Nanofabrication via Chemo Mechanical Transformation of Nanocrystal/Bulk Heterostructures. <i>Adv. Mater.</i> <b>2018</b>, <i>30</i> (22), 1800233.<br/>(3) Guo, J.; Kim, J.-Y.; Zhang, M.; Wang, H.; Stein, A.; Murray, C. B.; Kotov, N. A.; Kagan, C. R. Chemo- and Thermomechanically Configurable 3D Optical Metamaterials Constructed from Colloidal Nanocrystal Assemblies. <i>ACS Nano</i> <b>2020</b>, <i>14</i> (2), 1427–1435.<br/>(4) Guo, J.; Kim, J.-Y.; Yang, S.; Xu, J.; Choi, Y. C.; Stein, A.; Murray, C. B.; Kotov, N. A.; Kagan, C. R. Broadband Circular Polarizers via Coupling in 3D Plasmonic Meta-Atom Arrays. <i>ACS Photonics</i> <b>2021</b>, <i>8</i> (5), 1286–1292.<br/>(5) McNeill, J. M.; Choi, Y. C.; Cai, Y.-Y.; Guo, J.; Nadal, F.; Kagan, C. R.; Mallouk, T. E. Three-Dimensionally Complex Phase Behavior and Collective Phenomena in Mixtures of Acoustically Powered Chiral Microspinners. <i>ACS Nano</i> <b>2023</b>, <i>17</i> (8), 7911–7919.