Exciton-Trion Conversion Dynamics in a Single Molecule

Abstract

Charged optical excitations (trions) generated by charge carrier injection are crucial for emerging optoelectronic technologies as they can be produced and manipulated by electric fields. Trions and neutral excitons can be efficiently induced in single molecules by means of tip-enhanced spectromicroscopic techniques. However, little is known of the exciton-trion dynamics at single molecule level as this requires methods permitting simultaneous subnanometer and subnanosecond characterization. Here, we investigate exciton-trion dynamics by phase fluorometry, combining radio frequency modulated scanning tunnelling luminescence with time-resolved single photon detection. We generate excitons and trions in single Zinc Phthalocyanine (ZnPc) molecules on NaCl/Ag(111), and trace the evolution of the system in the picosecond range. We explore the dependence of effective lifetimes on bias voltage and describe the conversion mechanism from neutral excitons to trions, via charge capture, as the primary pathway to trion formation. We corroborate the dynamics of the system by a causally deterministic four-state model.

Internal Stark effect of single-molecule fluorescence

“…Based on a comparison with time-dependent density functional theory (TD-DFT) simulations, these shifts can be traced back to the radial electric field generated by charges confined to the σ-orbitals of the deprotonated chromophores whereas their π-orbitals remain unchanged. This is in contrast with the scanning probe experiments in which charging a molecule alters its π-orbital structure [19][20][21][22][23][24] and demonstrate the iso-electronic nature of the three compounds. The deprotonation procedure also affects the vibronic emission of the molecule, inducing measurable frequency shifts for several modes, an effect that is discussed in terms of a vibrational Stark effect 25 and mass changes similar to isotopic shifts.…”

“…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).…”

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.…”

Constant amplitude driving of a radio frequency excited plasmonic tunnel junction

“…https://doi.org/10.1063/5.0048476 Radio frequency (RF) modulated bias in scanning tunneling microscopy (STM) opens the way for new kinds of spectroscopies with atomic resolution. [1][2][3][4][5][6][7][8][9][10] Operation in the frequency-and timedomain improves the temporal resolution of conventional STM which is usually limited by the small bandwidth of transimpedance amplifiers. 2 In addition, nuclear and electron spin resonance (ESR) spectroscopy 3,11 can achieve energy resolution on the order of neV compared to conventional tunneling spectroscopies limited by thermal broadening of electronic states to several leV at mK temperatures.…”