Program Highlights

Search for Ferroelectricity in Two-Dimensional Molecular Crystals

Paolo Costa, Jacob Teeter, Alexander Sinitskii and Axel Enders  
 Nebraska MRSEC

Molecular ferroelectrics are a special class of organic materials that exhibit a bistable electric polarization that can be reversed by applying an external electric field. Molecular ferroelectrics can be useful in ways most inorganic materials cannot. They are inherently more flexible and they do not require epitaxy or other constraining structures for fabrication, and therefore can be made on nearly any surface – including flexible sheets and fabrics. Imidazole-based crystals are a promising class of molecular ferroelectrics. These organic materials are based on molecular chains with hydrogen bonds of the NH^...N type that can be bistable in electric polarity and electrically switchable through field-induced proton migration along the hydrogen bond and tautomerization. 

This MRSEC team investigated the formation of two-dimensional assemblies of two-types of imidazole-based molecules: benzimidazole and 2-trifluoromethyl benzimidazole, deposited on gold single crystal substrates. Scanning tunneling microscopy images reveal that in both cases the two-dimensional assemblies of molecules consist of hydrogen-bonded molecular chains. This observation is consistent with the model established for bulk imidazole-based crystals, so that ferroelectricity could potentially exist in those chains along the chain axes. This first direct visualization of NH^...N-bonded assemblies of the two-dimensional molecular ferroelectrics of this class opens new opportunities to engineer new low-dimensional structures of molecular ferroelectrics.

These programs are supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 1420645.

Scanning tunneling microscopy images of molecular chains of benzimidazole (left) and 2-trifluoromethyl benzimidazole (right) molecules on Au(111). Bottom: structure model of benzimidazole chains, highlighting the tautomerization that is at the origin of ferroelectricity in benzimidazole crystals.