Manipulation and Spectroscopy of Individual Phthalocyanine Molecules on InAs(111)A with a Low-Temperature Scanning Tunneling Microscope

Abstract

Single free-base phthalocyanine (H2Pc), copper phthalocyanine (CuPc), and naphthalocyanine (NPc) molecules on the III-V semiconductor surface InAs(111)A-(2 × 2) were investigated by cryogenic scanning tunneling microscopy (STM) at 5 K. STM imaging of largely unperturbed frontier orbitals of NPc on InAs(111)A reveals that the molecule is physisorbed. In the adsorbed state, the molecular electronic structure of NPc is preserved to a large extent, indicating a weak electronic coupling to the underlying substrate surface. As a free molecule, H2Pc is bistable because of an internal hydrogen transfer reaction (tautomerization). When adsorbed on the InAs(111)A surface, H2Pc experiences rotational fluctuations about its center because of excitations induced by inelastic electron tunneling (IET). STM-based atom and molecule manipulation techniques were used to sterically hinder the molecular rotation by assembling In adatom-molecule complexes, facilitating to probe the hydrogen transfer reaction in a controlled way. STM imaging of the Inad-H2Pc-Inad complex clearly reveals the presence of a left-handed and a right-handed conformer, indicating that the H2Pc tautomerization is left unperturbed by the substrate and the pinning adatoms, and that it can be triggered by the tunneling electrons.