Electrically Conductive Copper Core-Shell Nanowires through Benzenethiol-Directed Assembly.

Ultrathin nanowires with <3 nm diameter have long been sought for novel properties that emerge from dimensional constraint as well as for continued size reduction and performance improvement of nanoelectronic devices. Here, we report on a facile and large-scale synthesis of a new class of electrically conductive ultrathin core-shell nanowires using benzenethiols. Core-shell nanowires are atomically precise and have inorganic five-atom copper-sulfur cross-sectional cores encapsulated by organic shells encompassing aromatic substituents with ring planes oriented parallel. The exact nanowire atomic structures were revealed via a two-pronged approach combining computational methods coupled with experimental synthesis and advanced characterizations. Core-shell nanowires were determined to be indirect bandgap materials with a predicted room-temperature resistivity of ∼120 Ω·m. Nanowire morphology was found to be tunable by changing the interwire interactions imparted by the functional group on the benzenethiol molecular precursors, and the nanowire core diameter was determined by the steric bulkiness of the ligand. These discoveries help define our understanding of the fundamental constituents of atomically well-defined and electrically conductive core-shell nanowires, representing significant advances toward nanowire building blocks for smaller, faster, and more powerful nanoelectronics.

Elastic and thermal expansion asymmetry in dense molecular materials.

The elastic modulus and coefficient of thermal expansion are fundamental properties of elastically stiff molecular materials and are assumed to be the same (symmetric) under both tension and compression loading. We show that molecular materials can have a marked asymmetric elastic modulus and coefficient of thermal expansion that are inherently related to terminal chemical groups that limit molecular network connectivity. In compression, terminal groups sterically interact to stiffen the network, whereas in tension they interact less and disconnect the network. The existence of asymmetric elastic and thermal expansion behaviour has fundamental implications for computational approaches to molecular materials modelling and practical implications on the thermomechanical strains and associated elastic stresses. We develop a design space to control the degree of elastic asymmetry in molecular materials, a vital step towards understanding their integration into device technologies.

The Effects of Terminal Groups on Elastic Asymmetries in Hybrid Molecular Materials.

An asymmetric elastic modulus is a recently discovered and unexpected property of hybrid molecular materials that has significant implications for their underlying thermomechanical reliability. Elastic asymmetries are inherently related to terminal groups in the molecular structure, which limit network connectivity. Terminal groups sterically interact to stiffen the network in compression, while they disconnect the network and interact significantly less in tension. Here we study the importance of terminal group molecular weight and size (OH, methyl, vinyl, and phenyl) on the resulting elastic asymmetries and find that increasing the terminal group size actually leads to even larger degrees of asymmetry. As a result, we develop a molecular design criterion to predict how molecular structure affects the mechanical properties, a vital step toward integrating hybrid molecular materials into emerging nanotechnologies.

Hyperconnected molecular glass network architectures with exceptional elastic properties.

Hyperconnected network architectures can endow nanomaterials with remarkable mechanical properties that are fundamentally controlled by designing connectivity into the intrinsic molecular structure. For hybrid organic-inorganic nanomaterials, here we show that by using 1,3,5 silyl benzene precursors, the connectivity of a silicon atom within the network extends beyond its chemical coordination number, resulting in a hyperconnected network with exceptional elastic stiffness, higher than that of fully dense silica. The exceptional intrinsic stiffness of these hyperconnected glass networks is demonstrated with molecular dynamics models and these model predictions are calibrated through the synthesis and characterization of an intrinsically porous hybrid glass processed from 1,3,5(triethoxysilyl)benzene. The proposed molecular design strategy applies to any materials system wherein the mechanical properties are controlled by the underlying network connectivity.