Electron and X-ray-Induced Polymerization of Adsorbed Aromatic Molecules

Abstract

Experiments conducted during the last several decades demonstrate that multilayers of aromatic molecules cryogenically condensed on surfaces may be polymerized by X-rays, low-energy electrons, and in some cases ultraviolet (UV) light. The majority of studies indicate that photoelectrons and secondary electrons from the substrate are responsible for the process, as opposed to direct interaction of the photons or electrons with the aromatic molecules. UV photoelectron spectroscopy shows that π conjugation exists in the irradiated films, which may be up to several microns in thickness, and that the films are oligomeric, consisting of chains that are likely less than six units long. Thiophene and its derivatives have been the most widely studied class of aromatic molecules. In the case of 3-hexylthiophene, fluorescence measurements of beam-formed films support the conclusion that they are oligomeric. Some cross-linking is indicated by Fourier transform infrared (FTIR) spectroscopy of the films, and they appear similar to those obtained by electrochemical polymerization. Extensive electron or X-ray irradiation of thiophene, benzene, and other aromatic molecules eventually results in graphitic carbon, and forming graphene from a monolayer or a few layers of condensed molecules may be achieved. Patterns of conjugated oligomers may be formed either by using a focused electron beam or, for nanoscale lithography, a scanning tunneling microscope (STM) operating in field-emission mode. An STM tip operating in tunneling mode may also induce oligomerization by multiple-quantum transitions, such as vibrational excitation and CH bond cleavage. In the case of self-assembled monolayers with aromatic tail groups, electron irradiation may lead to cross-linking, with applications in resist formation. Possible mechanisms for photon- and electron-induced oligomerization are also discussed.