Beam-induced modifications of TCNQ multilayers

Abstract

We report on the investigation of electron and ion-beam-induced modifications of multilayer quantities of tetracyanoquinodimethane (TCNQ) adsorbed on a metal substrate. X-ray and ultraviolet photoelectron spectroscopy, thermal desorption spectroscopy, Auger electron spectroscopy and Raman spectroscopy have been used to study the beam-induced chemistry. The changes induced by electron or ion-beam exposure include: (i) stabilization against thermal decomposition as indicated by TDS, (ii) changes in the UPS and Raman spectra, (iii) binding energy shifts in the XPS carbon and nitrogen peak position as well as changes in the chemical reactivity of the beam exposed film. In particular the beam modified material is stable to temperatures of 625 K at which point the onset of decomposition is observed with evolution of HCN, H 2 and N 2. In comparison, unmodified TCNQ multilayers sublime at 350 K leaving behind a few layers which decompose beginning at 475 K. The integrity of the modified material below the decomposition temperature is verified by temperature dependent UPS measurements. The work function, derived from the UPS, increases upon modification. As the multilayer is modified the Raman spectrum characteristic of molecular TCNQ converts into a broad, featureless spectrum. The Raman and XPS spectra have been used to follow the effect of beam dosage. The N 1s peak of the XPS spectrum broadens and becomes a doublet with beam dosage while the carbon peaks narrow to become a singlet. These changes appear to be consistent with beam induced polymerization of TCNQ.