Quasi-Fermi level splitting in nanoscale junctions from ab initio.

Area of Science:

• Condensed Matter Physics
• Materials Science
• Computational Chemistry

Background:

• Quasi-Fermi levels (QFLs) are crucial for understanding semiconductor device operation under bias.
• Atomic-scale characterization of QFLs in nanoscale junctions is challenging.

Purpose of the Study:

• To present nonequilibrium QFL profiles within single-molecule junctions using first-principles calculations.
• To establish the necessity of maintaining separate electrode-originated nonlocal QFLs during calculations.
• To investigate QFL behavior in both insulating and conducting molecular junctions.

Main Methods:

• First-principles multispace constrained-search density-functional formalism.
• Benchmarking against standard nonequilibrium Green's function calculations.
• Analysis of QFL profiles, electrostatic potential, and Landauer residual-resistivity dipoles.

Main Results:

• For insulating junctions, QFLs show discontinuities at interfaces with linear potential drops.
• For conducting junctions, QFLs penetrate the channel, exhibiting split QFLs and nonlinear potential drops.
• Split QFLs correlate with the highest occupied molecular orbital entering the bias window and asymmetric dipole formation.

Conclusions:

• First-principles extraction of QFLs is vital for nanoscale junctions.
• Understanding QFL behavior informs the computational design of advanced semiconductor devices.
• The study provides a framework for analyzing electronic transport in molecular systems.