Silane and Germane Molecular Electronics

Abstract

This Account provides an overview of our recent efforts to uncover the fundamental charge transport properties of Si-Si and Ge-Ge single bonds and introduce useful functions into group 14 molecular wires. We utilize the tools of chemical synthesis and a scanning tunneling microscopy-based break-junction technique to study the mechanism of charge transport in these molecular systems. We evaluated the fundamental ability of silicon, germanium, and carbon molecular wires to transport charge by comparing conductances within families of well-defined structures, the members of which differ only in the number of Si (or Ge or C) atoms in the wire. For each family, this procedure yielded a length-dependent conductance decay parameter, β. Comparison of the different β values demonstrates that Si-Si and Ge-Ge σ bonds are more conductive than the analogous C-C σ bonds. These molecular trends mirror what is seen in the bulk. The conductance decay of Si and Ge-based wires is similar in magnitude to those from π-based molecular wires such as paraphenylenes However, the chemistry of the linkers that attach the molecular wires to the electrodes has a large influence on the resulting β value. For example, Si- and Ge-based wires of many different lengths connected with a methyl-thiomethyl linker give β values of 0.36-0.39 Å, whereas Si- and Ge-based wires connected with aryl-thiomethyl groups give drastically different β values for short and long wires. This observation inspired us to study molecular wires that are composed of both π- and σ-orbitals. The sequence and composition of group 14 atoms in the σ chain modulates the electronic coupling between the π end-groups and dictates the molecular conductance. The conductance behavior originates from the coupling between the subunits, which can be understood by considering periodic trends such as bond length, polarizability, and bond polarity. We found that the same periodic trends determine the electric field-induced breakdown properties of individual Si-Si, Ge-Ge, Si-O, Si-C, and C-C bonds. Building from these studies, we have prepared a system that has two different, alternative conductance pathways. In this wire, we can intentionally break a labile, strained silicon-silicon bond and thereby shunt the current through the secondary conduction pathway. This type of in situ bond-rupture provides a new tool to study single molecule reactions that are induced by electric fields. Moreover, these studies provide guidance for designing dielectric materials as well as molecular devices that require stability under high voltage bias. The fundamental studies on the structure/function relationships of the molecular wires have guided the design of new functional systems based on the Si- and Ge-based wires. For example, we exploited the principle of strain-induced Lewis acidity from reaction chemistry to design a single molecule switch that can be controllably switched between two conductive states by varying the distance between the tip and substrate electrodes. We found that the strain intrin...

2D Oligosilyl Metal–Organic Frameworks as Multi‐state Switchable Materials

“…Herein, we present two examples of MOFs capable of reversible structural transformations between multiple crystalline or amorphous phases. Oligosilanes, molecules with Si−Si bonds, are a class of hybrid inorganic‐organic materials with remarkable light absorption and charge transport properties that uniquely combine flexibility and strong light absorption, a property that inspired us to construct 2D MOFs based on oligosilyl linkers. While organosilicon species have been previously used in MOF formation, the organic linker usually contains a single Si atom, which precludes access to properties inherent to σ‐conjugated extended oligosilanes.…”

Investigating the Microstructure of Poly(cyclosilane) by 29Si Solid-State NMR Spectroscopy and DFT Calculations

“…The synthesis of silicon-based soft matter is a growing field, with representative achievements in colloidal nanocrystals 1-5 , integrated nanocrystal-polymer hybrids, [5][6][7] donoracceptor chromophores, [8][9][10][11][12][13] molecular electronics [14][15][16][17][18][19] and conjugated polymers, [20][21][22][23][24][25][26] with potential applications as photovoltaic and photonic devices. [5][6][27][28][29][30][31][32] Recently, the dehydrocoupling polymerization of bifunctional cyclic precursors to provide polycyclosilanes has emerged as a route to functionalized polysilanes decorated with a periodic array of Si-Me and Si-H bonds (Scheme 1).…”

Synthesis of silacyclopent-2-en-4-ols via intramolecular [2 + 2] photocycloaddition of benzoyl(allyl)silanes

“…Notwithstanding the role of silicon in drug discovery, 2 organosilicon compounds have also gained notoriety in molecular electronics 10 and polymer science. 11 Conjugated organosilicon materials are particularly promising for the organic electronics and photonics field.…”