Interfacial Electronic Structure in Thiolate Self-Assembled Monolayers:  Implication for Molecular Electronics

Abstract

Thiolate self-assembled monolayers (SAMs) on metal surfaces have been explored recently to address the assembly and connection issue in molecular electronics. In these systems, the molecule-metal contact is detrimental to electron transport. This is manifested not only in contact resistance, but also in the nature of the molecular device, which depends on the extent of wave function mixing between the molecule and the metal surface. We probe interfacial electronic structure, particularly unoccupied electronic states, in thiolate SAMs on Cu(111) using laser two-photon photoemission spectroscopy, in conjunction with ab initio calculations of model molecules. We find that the interfacial electronic structure is dominated by two virtual orbitals localized to the thiolate anchor and strongly coupled to the metal substrate. The shapes and energies of these interfacial σ*-like orbitals are independent of the nature of the hydrocarbon group (conjugated aromatics or saturated alkyls). As low-lying acceptor orbitals, they may play important roles in electron transport through self-assembled molecular wires.

Two-photon photoemission from ex-situ prepared butanethiol SAMs on Au (111)

“…The authors assigned this feature to the unoccupied r ⁄ resonance of the sulfur-gold bond, following the interpretation of prior 2PPE studies of thiolates on Cu (111) [19] and Ag (111) [20]. The spectral asymmetry was not considered in this work and consequently not related to any additional electron orbital.…”

Site-specific photocatalytic splitting of methanol on TiO2(110)

“…This method has also been used for the study of hot electron dynamics of metals 21 and semiconductors, 22 and to probe the electronically excited states and the dynamics of atoms and molecules adsorbed metal surfaces. [23][24][25][26][27][28] In recent years, 2PPE has been applied to investigate the surface electron dynamics of molecular adsorbed metal oxide surfaces. 29 In this work, we have used a recently developed femtosecond 2PPE apparatus to study the methanol/ TiO 2 (110) photochemistry.…”

Molecules and Electronic Materials

“…[7] In a quite different (experimental) context, Vondrak et al concluded from their two-photon photoelectron spectroscopy results, backed up by theoretical calculations, that while S ªalligator clipsº may be conducting for hole transport across conjugated (p-bonded) molecules, they can be insulating for electron transport. [78] …”