Semiconductor-Molecule Hybrid States: Si(111)–Anthracene Probed by STM/STS and UPS/LEIPS

Hybridization between semiconductor surfaces and covalently bound organic molecules offers a pathway to engineer surface-specific electronic states. However, due to the nanoscale nature of molecular functionalization, resolving such hybrid orbitals spatially and energetically is experimentally challenging. Here, we investigate a model system comprised of a silicon(111) substrate functionalized with anthracene (Si–Anth) to elucidate semiconductor-molecule hybrid electronic structure. Si–Anth was functionalized with a surface coverage ≲ 7% anthracene sites and backfilled with methyl groups. Anthracene was selected due to the relative energetic proximity of its frontier molecular orbitals to the silicon band edges, rendering it amenable to near band-edge spectroscopic investigation. STM imaging of the Si–Anth surface revealed molecular structures consistent with anthracene molecules oriented as expected from the covalent attachment procedure, compared with a lack of such anthracene-like structures on methyl-only samples. Ultrahigh vacuum techniques of ultraviolet photoelectron spectroscopy (UPS), low-energy inverse photoemission spectroscopy (LEIPS), and scanning tunneling spectroscopy (STS) were performed to compare both ensemble and local (single-molecule specific) electronic structure near the semiconductor band edges. Intriguingly, the UPS/LEIPS measurements resulted in a wider experimentally observed band gap for Si–Anth (Eg ≈ 1.5 eV) relative to the methylated Si–CH3 control (Eg ≈ 1.1 eV) on both p- and n-type samples. STS supported the Eg trends and revealed HOMO-like and LUMO-like features corresponding to hybridized Si–Anth states. These results demonstrate experimentally detectable features that quantitatively confirm semiconductor-molecule hybridization and provide spectroscopic signatures for future studies of hybrid semiconductor-molecule states.