Low-energy electron transmission through thin-film molecular and biomolecular solids

The study of electron transmission through thin organic films provides information unavailable from any other source on the electronic properties of the organic molecules as individuals and on the properties that emerge due to interactions among the molecules in the film. These properties relate to many technological applications varying from the insulation of electrical lines to radiation damage in biological tissues and futuristic molecular electronic applications. The two methods descr ibed in the present review are complementary in nature. In LEET studies, a monochromatic electron beam hits an adsorbed molecular layer from the vacuum side; the transmission is monitored via the current generated in the conducting substrate. The same experimental setup can be used to study reflection. Both transmission and reflection are studied as functions of the incident electron energy, substrate type, and characteristics of the molecular layer. In the LEPET experiments, photoelectrons are ejected from a conductive substrate and are transmitted through the organic film to the vacuum side. Here the signal is the (angle- and velocity-resolved) transmitted electron flux as a function of incident photon energy, molecular film thickness, adsorbate, and substrate types and temperature. The two methods, LEET a nd LEPET, are sensitive to the electronic states in the films that are above the vacuum level. Namely, unbound electron-molecule states. Only when the electrons lose some of their initial kinetic energy can they be trapped in states below the vacuum level. Hence, these techniques may also provide indirectly insight into the bound electronic states. Relevant information on these lower energy regimes may also be obtained by monitoring current vs voltage in contacts made of two metal electrodes separated by a molecular spacer or in scanning tunneling microscopy, STM, where a surface scan of the current versus bias voltage can be measured as a function of film thickness (i.e., tip-substrate separation). An older technique, inelastic tunneling spectroscopy, is commonly used to obtain information on nuclear motion in the barrier by observing their effect on the electron-tunneling process. In the near future, it is expected that ele ctron transmission studies will be used to obtain details on the properties of electronic excited films and on films composed from hybrid structures like nanoparticles self-assembled with organic molecules or biomolecules. These films are becoming important building blocks in biotechnology and electronics. Because of the sensitivity and depth of information that can be obtained on very thin layers, electron transmission studies have the potential to become an important tool for studies in the fast expanding field of research on thin films. © 2007 American Chemical Society.