Low-Energy Electron Irradiation Damage in Few-Monolayer Pentacene Films

Abstract

Crystalline films of pentacene molecules, two to four monolayers in thickness, are grown via in situ sublimation on silicon substrates in the ultrahigh vacuum chamber of a low-energy electron microscope. It is observed that the diffraction pattern of the pentacene layers fades upon irradiation with low-energy electrons. The damage cross section is found to increase by more than an order of magnitude for electron energies from 0 to 10 eV and by another order of magnitude from 10 to 40 eV. Close to 0 eV, damage is virtually nil. Creation of chemically reactive atomic centers after electron attachment or impact ionization is thought to trigger chemical reactions between neighboring molecules that gradually transform the layer into a disordered carbon nanomembrane. Additionally, diminishing spectroscopic features related to the unoccupied band structure of the layers, accompanied by loss of definition in real-space images, and an increase in the background intensity of diffraction images during irradiation point to chemical changes and formation of a disordered layer.

Critical role of electronic states above the vacuum level in photoelectron and secondary electron emission in few-monolayer pentacene films

“…In a previous study [22], we reported a gradual diminishing of LEEM-IV features as a result of continued exposure of pentacene layers to an electron beam, attributed to beam damage and loss of crystalline order in the layers. Here, we use this change as an independent tool to correlate SE emission and unoccupied states.…”

“…Additionally, we probe the yield and energy distributions of SEs from pentacene, for a series of electron-beam energies [21]. Here, we deliberately use electron-beam damage to create chemical, structural, and electronic changes in the layers [22]. Doing this in a controlled manner allows us to correlate changes in LEEM spectra and SE energy distribution curves.…”

“…We note that the measurements of the LEEM-IV spectra themselves (taken in-between measurements of the electron energy spectra) are expected to cause only minimal damage. This is due to the negligible damage cross section of pentacene films for electrons of energies up to ∼5.5 eV [22], i.e., the energy up to which the LEEM-IV spectra in Fig. 3(a) were obtained.…”