Electron impact on benzene layers on W(110)

Electron impact on benzene chemisorbed on W(110) and on benzene multilayers physisorbed on top of chemisorbed benzene was carried out. The only observable desorption products were H+ and traces of H2. There was no loss of C from chemisorbed benzene and less than 10% from benzene multilayers, as determined by Auger and XPS measurements. The decay cross sections thus correspond to C-H bond breaking only and are 3 × 10−17 cm2 for chemisorbed benzene and 2 × 10−17 cm2 for low coverage of physisorbed benzene. For thicker deposits, 5 langmuir or more, a second decay regime with a cross section of 2 × 10−18 cm2 is also seen. On heating, the H+ yield from chemisorbed benzene (measured at 90 K) increases up to 800 K, and then decreases. H+ can still be detected at 1400 K, indicating that C-H bonds still exist at this temperature. A loss at 6 eV is seen on heating multilayers after electron impact to T ≥ 650 K and is attributed to a graphite plasmon. Work function measurements give a value of 4.4 eV after electron impact at 90 K; this increases to 4.5 eV after heating to 1000 K. These values are close to that reported for bulk graphite. A spot, 1.8 mm in diameter was prepared by irradiating a multilayer deposit with a 2 keV electron beam from an Auger gun. After heating to 200 K to remove unreacted physisorbed benzene the carbon spot was found to be stable to > 1100 K; two adjacent spots were found to be unchanged after exposure to air, and bakeout at 200°C. Retardation measurements indicate that carbon layers formed by electron impact on benzene multilayers do not charge up at current densities up to 25 microampere/cm2. The present results indicate that it should be possible to make carbon nanostructures by electron beam writing on benzene layers, physisorbed at 77–120 K, with unreacted benzene subsequently removed by heating to 200 K.