Orbital-resolved visualization of single-molecule photocurrent channels

Area of Science:

• Molecular Biophysics
• Nanotechnology
• Physical Chemistry

Background:

• Photoinduced electron transfer (PET) is crucial for light energy utilization but lacks molecular-level spatial resolution.
• Microscopic photocurrent measurements correlate efficiency with local features but are spatially limited.
• Scanning tunnelling microscopy (STM) combined with plasmon fields allows single-molecule excitation and probing.

Purpose of the Study:

• To visualize photocurrent channels at the molecular orbital level in a single molecule.
• To investigate the influence of bias voltage on photocurrent direction and distribution.
• To explore the competition between PET and photoluminescence and methods for controlling them.

Main Methods:

• Utilizing STM combined with localized plasmon fields driven by a tunable laser.
• Exciting and probing single free-base phthalocyanine (FBPc) molecules.
• Detecting electrons from the first excited state via STM tip tunneling.

Main Results:

• Direct visualization of photocurrent channels through molecular orbitals with atomic resolution.
• Identification of bias voltage-dependent photocurrent direction and spatial distribution.
• Observation of counter-flowing photocurrents even near zero average voltage.
• Evidence of competition between PET and photoluminescence.
• Demonstration of control over PET versus photoluminescence relaxation via STM tip positioning.

Conclusions:

• Photocurrent channels are sensitive to bias voltage and molecular orbital coupling.
• Atomic-scale control over PET and photoluminescence is achievable.
• New strategies for improving energy-conversion efficiencies through atomic-level interface engineering are proposed.