Fast optical work-function tuning at an organic/metal interface

In a two-color experiment, we demonstrate how light can be used as an external control to continuously tune the work function of a gold substrate functionalized with a dilute azobenzene-based self-assembled monolayer (SAM). The work function is measured by two-photon photoelectron spectroscopy. While in the ground state the azobenzene moiety adopts the trans configuration, illumination with pulsed laser light at a wavelength of 368 nm results in a photostationary state (PSS) mainly comprising cis isomers. An additional 450 nm continuous-wave laser with tunable intensity serves to shift the PSS back towards the ground state. This way the work function is freely adjustable in real time over a range of ∼240 meV between the two PSS extrema. We furthermore relate the change in work function to the average change in dipole moment per azobenzene chromophore. Quantum-chemical calculations that take into account available structural data of the molecules in the SAM must consider at least two different trans and fo...