A Spontaneously Electrical State of Matter

• Published in: Accounts of chemical research 2023-07, Vol.56 (14), p.1909-1919
• Main Authors: Cassidy, Andrew, McCoustra, Martin R. S., Field, David
• Format: Article
• Language: English
• Subjects: Chemistry
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.3c00094

Conspectus Molecular deposition on solid surfaces forms crystalline or amorphous/glassy thin solid films. Intermolecular interactions govern the packing and dynamics of these films. The connection between molecular structure and intermolecular interactions is based on understanding electrostatic forces, dispersion forces and hydrogen bonding. Recently, an entire class of dipolar molecular species have demonstrated counterintuitive self-organization such that the dipole moments of individual molecules are oriented in thin films. This leads to the spontaneous generation of polarized molecular films manifesting a polarization charge equivalent to tens to hundreds of volts in strength at the film-vacuum interface, relative to the film-substrate interface. These voltages, and the corresponding electric fields present in such films, result from a collective and spontaneous orientation of molecular dipoles throughout the film during film growth and represent a metastable state of polarized material. The existence of these materials should encourage reconsideration of the importance of solid-state intermolecular electrostatic interactions. This account will detail observations of the spontaneous electric fields in molecular solids, provide insights into the dynamics and structure of molecular materials that the emergence of these electric fields can facilitate, and present a dipole-alignment based mean-field model that reproduces the temperature dependence of the electric field strength. Species as diverse as carbon monoxide, nitrous oxide, freons, simple alcohols, and cis-methyl formate have been demonstrated to spontaneously generate electric fields. We have reported electric fields more than 108 V m–1, have shown how field strength varies with the film deposition temperature, and have reported temperature-dependent Stark shifts observable in both infrared and ultraviolet absorption spectra. The latter has led to the reporting of large Wannier−Mott excitons in wide band gap molecular materials, such as solid carbon monoxide and ammonia. Changes in the field strength with time, at specific temperatures, can be related to the structural dynamics of glassy molecular solids. Measurement of surface potentials is a very sensitive technique by which to observe the rotation and translation of molecular species buried in thin films. This is particularly true for polarized, supercooled molecular glasses, where surface potentials have been used to report on secondary relax