Bias-polarity dependent electroluminescence from a single platinum phthalocyanine molecule

By using scanning tunneling microscope induced luminescence (STML) technique, we investigate systematically the bias-polarity dependent electroluminescence behavior of a single platinum phthalocyanine (PtPc) molecule and the electron excitation mechanisms behind. The molecule is found to emit light at both bias polarities but with different emission energies. At negative excitation bias, only the fluorescence at 637 nm is observed, which originates from the LUMO→HOMO transition of the neutral PtPc molecule and exhibits stepwise-like increases in emission intensities over three different excitation regions. Strong fluorescence in region (I) is excited by the carrier injection (CI) mechanism with holes injected into the HOMO state first; moderate fluorescence in region (II) is excited by the inelastic electron scattering (IES) mechanism; and weak fluorescence in region (III) is associated with an up-conversion process and excited by a combined CI and IES mechanism involving a spin-triplet relay state. At positive excitation bias, more-than-one emission peaks are observed and the excitation and emission mechanisms become complicated. The sharp molecule-specific emission peak at ~911 nm is attributed to the anionic emission of PtPc− originated from the LUMO+1→LUMO transition, whose excitation is dominated by a CI mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.