Determination of Energy Level Alignment and Coupling Strength in 4,4′-Bipyridine Single-Molecule Junctions

Abstract

We measure conductance and thermopower of single Au-4,4'-bipyridine-Au junctions in distinct low and high conductance binding geometries accessed by modulating the electrode separation. We use these data to determine the electronic energy level alignment and coupling strength for these junctions, which are known to conduct through the lowest unoccupied molecular orbital (LUMO). Contrary to intuition, we find that, in the high-conductance junction, the LUMO resonance energy is further away from the Au Fermi energy than in the low-conductance junction. However, the LUMO of the high-conducting junction is better coupled to the electrode. These results are in good quantitative agreement with self-energy corrected zero-bias density functional theory calculations. Our calculations show further that measurements of conductance and thermopower in amine-terminated oligophenyl-Au junctions, where conduction occurs through the highest occupied molecular orbitals, cannot be used to extract electronic parameters as their transmission functions do not follow a simple Lorentzian form.

The energy level alignment at metal–molecule interfaces using Wannier–Koopmans method

“…In system (b), a BP molecule is interfaced on both sides by Au electrodes, as in a single-molecule break-junction experiment. Using a DFT+Σ calculation as in previous work [20,21], corroborated by experiments [22], predicted that the lowest unoccupied molecular consists of only one surface and features a HOMO resonance near E F , while the second is a junction and features a LUMO resonance near E F . In addition, in the first system, the molecule is adsorbed flat on the surface, while in the second system, the molecule is bound on either side to a motif consisting of a Au trimer.…”

“…System (a) has been well studied by experiment [17], GW [18], and DFT+Σ [18,19]; and system (b) has been studied in the context of charge transport through molecular junctions [20][21][22], with both break-junction experiments and DFT+Σ. The atomic geometries [23] of these two systems are first optimized using DFT with the Perdew-Burke-Ernzerhof (PBE) functional [24] as implemented in SIESTA [25], shown in Fig.…”

“…3(b)]. However, the transport measurements suggest [20,22] that the LUMO level should be about 1.2-1.6 eV above the Au Fermi energy [ Fig. 3(b)].…”

Energy level alignment of self-assembled linear chains of benzenediamine on Au(111) from first principles

“…Additionally, BDA has been a "fruit-fly" molecule for the study of single-molecule junction transport properties and energy level alignment, leading to significant progress in both experiment [14,15] and theory [7,16,17]. The highest occupied molecular orbital (HOMO) resonance energies of the adsorbed BDA molecules have been experimentally measured with ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS).…”

Single-Molecule Electrochemical Transistor Utilizing a Nickel-Pyridyl Spinterface

“…7 Au-pyridyl contacts, such as the Au-44BP bond, have been shown to provide reproducible junctions for which two conductance values can be distinguished due to different binding geometries. [8][9][10] However, despite significant progress investigating different chemical linker groups 9,[11][12][13][14][15][16][17][18] there have been few previous attempts to broaden the range of metal electrodes studied.…”