Superionic Colloidal Crystals: Ionic to Metallic Bonding Transitions

Abstract

Size-asymmetric binary charged colloidal solutions can assemble into ionic colloidal crystals. These are often stabilized by ionic-type bonding, where the components with smaller size and charge sit at fixed points within the lattice of large particles. Here, we study the transition termed ionic to metallic bonding transition, by which the lattice of the smaller component melts while the crystal of the large particles is preserved, as in metallic bonding. We simulate a charged colloidal crystal in equilibrium with a solution containing small colloidal particles and counterions using the Coulomb interaction between the finite-size components. We find ionic to metallic first-order transitions by increasing either the temperature or the concentration of the small particles in the solution. The transition is accompanied by a lattice expansion and increased absorption of small particles into the crystal. We compute the free energies of the ionic and metallic states using the Madelung constant and Wigner–Seitz cell approaches, respectively, combined with the quasi-harmonic lattice model. The calculation reproduces the simulated transition and reveals that the enthalpic gain is more pronounced than the entropic gain in the transition from ionic to metallic bonding when material is exchanged with the solution.

Supramolecular Semiconductivity through Emerging Ionic Gates in Ion–Nanoparticle Superlattices

“…Experiments and molecular simulations showed how the electron equivalents can break their distribution symmetry around the atom equivalents and move in the superlattice in a way resembling the behavior of electrons in metals. 38 , 67 − 73 Similar intralattice mobility has also been observed in aggregates of gold NPs decorated with a high density of positively charged groups. Cl – counterions coassembled with NPs decorated with a high density of (11-mercaptoundecyl)- N,N,N -trimethylammonium (TMA) groups generated colloidal superclusters with semiconductive properties useful for building chemoelectronic circuits.…”

“…Stimulated by recent reports on the quasi-metallicity of colloidal superlattices, − we have designed an ad hoc computational approach to investigate the supramolecular conductive behavior of colloidal crystals coassembled from TMA-functionalized Au NPs and CIT ions (Figure ) in which the current is carried by the CIT ions. We, thus, aim at investigating whetherand to what extentthe mobility of ionic species and the superlattice’s responsiveness can mimic the conductivity behavior typical of metals, semiconductors, or insulators.…”

“…Pioneering works have shown how the selectivity of DNA interactions can be exploited to assemble NPs functionalized with single-stranded DNA into a variety of superstructures with symmetries typical of a metallic crystal. ,− As “atom equivalents,” the NPs in such supercrystals are surrounded by the smaller particles, which keep them together as superelectron equivalents. Experiments and molecular simulations showed how the electron equivalents can break their distribution symmetry around the atom equivalents and move in the superlattice in a way resembling the behavior of electrons in metals. ,− Similar intralattice mobility has also been observed in aggregates of gold NPs decorated with a high density of positively charged groups. Cl – counterions coassembled with NPs decorated with a high density of (11-mercaptoundecyl)- N,N,N -trimethylammonium (TMA) groups generated colloidal superclusters with semiconductive properties useful for building chemoelectronic circuits. ,, More recently, static and dynamic superlattices have been also obtained via mediated assembly of TMA-NPs (atom analogues, AAs) in water using citrate (CIT) or ATP as counterions.…”

“…The ability of the EAs to move in AA superlattices may suggest intriguing hierarchical analogies with atomic crystalline materials. For example, metal-like transitions , can be associated with variations in the EA distribution upon temperature change. Interesting similarities also arise with superionic conductance, in which components of ionic crystals can diffuse across emerging defects in the lattice, which similarly results in significant charge transport. − However, it is worth noting that the conductive behavior of metals, semiconductors, or superionic conductors exhibits specific fingerprints, which are nontrivial to ascertain in such higher scale supercrystals.…”

Colloidal superionic conductors

“…The superionic behavior in colloidal crystals was subsequently predicted to exist in various three-dimensional crystal structures ( 20 , 21 ) as well as two-dimensional lattices ( 22 ) and is observed experimentally via electrostatics in a face-centered cubic ( fcc ) colloidal crystals ( 23 ). Computer simulations of binary colloidal crystals that are highly asymmetric in size and charge ( 24 , 25 ) showed electrostatics provide sufficient cohesive energy to stabilize fcc structures. As the temperature is increased, a first-order ionic to superionic transition that resembles an IMT in atomic superionics was observed ( 24 , 25 ).…”