A computational study of molecular desorption from the Si(111)-7x7 surface

Abstract

Computational calculations using a plane wave implementation of density functional theory are employed to investigate the desorption of aromatic molecules (benzene, chlorobenzene, toluene) from the Si(111)-7x7 surface. The surface is modelled by a non-periodic Si21H34 cluster and periodic slabs with unit cell size 5x5 and 7x7.

Binding energy calculations are performed to identify the energetically preferred geometries of the molecule/cluster systems. The finite size of these systems also enables simulating and examining the effect of localised charge, which is present in the real molecule/Si(111)-7x7 system immediately following charge-injection from a scanning tunneling microscope (STM) tip positioned directly above the chemisorbed molecule.

The molecule/cluster systems are also used in vibrational phonon mode analysis to account for the previously reported low-temperature activation energies of STM charge induced molecular desorption. A phonon mode at 421 cm 1 is identified as the likely mode responsible for assisting hole-induced desorption from the Si(111)-7x7 surface.

The same energetically preferred geometry is then used as the starting point for more detailed investigations employing the molecule/slab model. Compared to the binding energy found using the molecule/cluster system (1.7 eV), a more accurate value of 1.4 eV is calculated using the periodic system. Calculations requiring a larger (or the full) unit cell size of the Si(111)-7x7 surface are performed, such as investigating the influence of intermolecular interactions when three molecules are chemisorbed on the surface in close proximity with each other. Results show that the molecular binding preference for the unfaulted half of the Si(111)-7x7 unit cell does not change when increasing the surface coverage from a single adsorbed molecule per unit cell to three adsorbed molecules per half-unit-cell.

The effect of the close proximity of the STM tip with the chemisorbed molecule is also investigated by positioning a model tip above the molecule/slab system and quasistatically moving the tip towards the surface to replicate the scenario in STM desorption experiments. By comparing density of states and isosurfaces of the electronic wavefunction, the existence of a tip-derived state is confirmed which is proposed to be responsible for the significant reduction in the probability of hole-induced desorption of toluene molecules from the Si(111)-7x7 surface in recent STM experiments.

Date of Award	23 Mar 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Simon Crampin (Supervisor), Daniel Wolverson (Supervisor) & Kristina Rusimova (Supervisor)