Bistable phases of diazodiphenylmethane on Cu(111): revelations from Inelastic Tunneling Spectroscopy and ab initio calculations

In the ever-changing field of information technology, molecular-scale electronics can benefit from molecules with switchable stable states whose transitions can be controlled. We have found diazodiphenylmethane (Ph₂CN₂) adsorbed on Cu(111)  to display this bistable configuration. The transition between the states occurs thermally or can be voltage-induced via scanning tunneling microscopy (STM).  With the use of STM, inelastic tunneling microscopy (IETS), and density functional theory (DFT) based calculations, the two distinctly different configurations of the molecule are observed and identified. While both the symmetrical and non-symmetrical orientations of the molecule with respect to the carbon rings are shown to be stable on the Cu surface with a binding energy of -1.58 eV and -1.72 eV respectively, their binding with the surface is not the same. The non-symmetrical phase has a stronger surface interaction and has its frontier orbitals shifted to the fermi level exhibiting metallic characteristics. The frontier orbitals of the symmetrical phase maintain their gas phase characteristics. A red shift in the IR spectra is seen from the symmetrical phase relative to the non-symmetrical phase, while IETS captures features only from the symmetrical phase.