Bias-polarity dependent electroluminescence from a single platinum phthalocyanine molecule

Abstract

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 increase in emission intensities over three different excitation-voltage regions. Strong fluorescence in region (I) is excited by the carrier injection mechanism with holes injected into the HOMO state first; moderate fluorescence in region (II) is excited by the inelastic electron scattering mechanism; and weak fluorescence in region (III) is associated with an up-conversion process and excited by a combined carrier injection and inelastic electron scattering 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 carrier injection mechanism with electrons first injected into the LUMO+1 or higher-lying empty orbitals.

Exciton-Trion Conversion Dynamics in a Single Molecule

“…It is also a key to develop efficient single photon quantum cryptography and quantum computing protocols. − Although most experiments rely on ensemble or bulk measurements, excitation and control of electron–hole bound states in single molecules and defects in molecular solids are possible for diluted emitters. , Recent developments in tip-enhanced spectroscopies, prominently in scanning tunneling microscopy luminescence (STML), made it possible to explore at atomic-scale the mechanisms generating singlet, , triplet, , and doublet molecular Frenkel excitons and the role of their nanoscopic environment. Emission from positively and negatively charged excitons (trions) was recently discovered for single zinc and platinum phthalocyanine (ZnPc, PtPc) emitters. , Despite important advances in measuring fast dynamics of optical excitations on the nanoscale by application of Hanbury Brown Twiss (HBT) interferometry − and time-resolved STML, − capturing the combined dynamics of the molecular excitons and trions in single molecules remained a challenge. Here we achieve time-resolved measurements of the excitons and trions of a single ZnPc molecule, which we excite by direct charge injection to induce photon emission.…”

“…Emission from positively and negatively charged excitons (trions) was recently discovered for single zinc and platinum phthalocyanine (ZnPc, PtPc) emitters. 12,13 Despite important advances in measuring fast dynamics of optical excitations on the nanoscale by application of Hanbury Brown Twiss (HBT) interferometry 14−19 and time-resolved STML, 20−22 capturing the combined dynamics of the molecular excitons and trions in single molecules remained a challenge. Here we achieve timeresolved measurements of the excitons and trions of a single ZnPc molecule, which we excite by direct charge injection to induce photon emission.…”

Internal Stark effect of single-molecule fluorescence

“…Similarly, we theoretically ruled out the impact of the static screening of the NaCl substrate on the energy shifts by performing DFT and TD-DFT calculations 44 (discussed in detail in Supplementary Note 2 ). On the other hand, charged chromophores as π -type phthalocyanine anions and cations discussed in previous reports 21 – 23 , 49 are systematically characterized by a strongly red-shifted emission (≈400 meV) compared to neutral compounds, reflecting important modifications of the π -orbitals involved in the optical transition. Similarly, if one assumes neutral dehydrogenated compounds, one is left with one unpaired electron in the HOMO (HPc) or with an empty HOMO (Pc) 24 , 32 .…”

“…Charged states of single molecules have been recently probed in a wide range of experimental schemes involving scanning probe approaches 19 – 23 , 35 , 49 . By simultaneously preserving the π -orbital structure of H 2 Pc and leaving an excess σ -electron within the chromophore, the deprotonation procedure reported here provides a unique opportunity to study the Stark effect generated by an internal charge on the fluorescence emission of an individual chromophore.…”

Internal Stark effect of single-molecule fluorescence

“…Similarly, we theoretically ruled out the impact of the static screening of the NaCl substrate on the energy shifts by performing DFT and TD-DFT calculations 44 (discussed in detail in Supplementary Note 2). On the other hand, charged chromophores as π-type phthalocyanine anions and cations discussed in previous reports [21][22][23]47 are systematically characterized by a strongly red-shifted emission (≈ 400 meV) compared to neutral compounds, reflecting important modifications of the π-orbitals involved in the optical transition. Similarly, if one assumes neutral dehydrogenated compounds, one is left with one unpaired electron in the HOMO (HPc) or with an empty HOMO (Pc).…”