Unoccupied Interface and Molecular States in Thiol and Dithiol Monolayers

Area of Science:

• Surface science
• Materials science
• Physical chemistry

Background:

• Self-assembled monolayers (SAMs) are crucial for modifying surfaces.
• Understanding their electronic structure is key to controlling interfacial properties.
• Thiol and dithiol molecules form well-ordered SAMs on gold.

Purpose of the Study:

• To characterize the electronic structure of SAMs formed by thiols and dithiols on Au(111).
• To investigate the influence of molecular length and bonding on electronic properties.
• To elucidate the nature of interface states and molecular orbitals.

Main Methods:

• Inverse photoemission spectroscopy (IPES) for experimental electronic structure determination.
• Density functional theory (DFT) for computational modeling of the molecule/Au system.
• Characterization of SAMs with varying thiol and dithiol chain lengths.

Main Results:

• A consistent signal for the lowest unoccupied system orbital (LUSO) was observed ~3 eV above the Fermi level for all SAMs.
• Evidence of interface states localized at the Au-substrate junction was found.
• An antibonding Au-S state was identified, with its energy differing between thiol and dithiol SAMs.

Conclusions:

• The electronic structure of thiol and dithiol SAMs on Au(111) is well-defined by IPES and DFT.
• Interface states and molecular orbitals contribute significantly to the observed electronic properties.
• Differences in the antibonding Au-S state energy highlight variations between thiol and dithiol SAMs.