Abstract
A key quantity for molecule–metal interfaces is the energy level alignment of molecular electronic states with the metallic Fermi level. We develop and apply an efficient theoretical method, based on density functional theory (DFT) that can yield quantitatively accurate energy level alignment information for physisorbed metal–molecule interfaces. The method builds on the “DFT+Σ” approach, grounded in many-body perturbation theory, which introduces an approximate electron self-energy that corrects the level alignment obtained from conventional DFT for missing exchange and correlation effects associated with the gas-phase molecule and substrate polarization. Here, we extend the DFT+Σ approach in two important ways: first, we employ optimally tuned range-separated hybrid functionals to compute the gas-phase term, rather than rely on GW or total energy differences as in prior work; second, we use a nonclassical DFT-determined image-charge plane of the metallic surface to compute the substrate polarization term, rather than the classical DFT-derived image plane used previously. We validate this new approach by a detailed comparison with experimental and theoretical reference data for several prototypical molecule–metal interfaces, where excellent agreement with experiment is achieved: benzene on graphite (0001), and 1,4-benzenediamine, Cu-phthalocyanine, and 3,4,9,10-perylene-tetracarboxylic-dianhydride on Au(111). In particular, we show that the method correctly captures level alignment trends across chemical systems and that it retains its accuracy even for molecules for which conventional DFT suffers from severe self-interaction errors.
Highlights
Interfaces between molecules and metals are central components in emerging technologies such as organic and molecular electronics and third-generation photovoltaic cells
Many-body perturbation theory, typically using the GW approximation to Dyson’s equation, in principle allows for a rigorous determination of excitation energies[25,26] and, energy level alignment
In this Letter, we develop and apply a first-principles method within the Density functional theory (DFT) framework that allows for a quantitatively reliable calculation of energy level alignment in physisorbed molecule−metal systems
Summary
Letter numerically.[29,30] Density functional theory (DFT)[31] offers an excellent trade-off between accuracy and efficiency and became the workhorse methodology of contemporary molecule−metal interface calculations. Following the DFT+Σ framework,[33,34,43−46] we augment the PBE−PDOS, obtained for the full metal− molecule interface from a plane-wave based calculation (see Supporting Information), by shifting each molecular resonance individually using a “one-shot” nonself-consistent correction combining the above two steps This is a model self-energy correction, but extending previous work it is premised entirely on DFT quantities. This large correction is in good agreement with previously reported GW calculations, which were feasible for these moderately complex molecule−metal systems.[33,36] These findings show that the DFT+Σaxc approach correctly reflects the highly system-dependent nature of the physical effects that govern energy level alignment It correctly reproduces the experimentally determined chemical trends for the molecule−gold interfaces, which highlights its capabilities for reliable predictions of energy level alignment.
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