Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

Abstract

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant X-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope-based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) that presents Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data.

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

“…The molecules are oriented along the [112] direction, but they are now in a cis-configuration to facilitate the formation of the N-H…N hydrogen bonds (of 2.25 Å) between neighboring molecules. As in the monomer phase, the molecules in the linear chains also bind preferentially to a single Au atom but with a slightly shorter N-Au bond of 2.82 Å and a molecule-substrate angle of 16.6°, consistent with the experimental value of 24°10°4 [18]. The optimized geometries of the linear chains are unchanged as a function of coverage and inter-chain spacing, as shown in the SI.…”

“…The benzenediamine (BDA)-Au(111) system has been well studied as a prototypical metal-organic interface [18][19][20][21]. 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].…”

“…[13,[18][19][20] However, recently scanning tunneling microscope (STM) experiments [21] have provided new information about the adsorption geometry, revealing the formation of self-assembled linear chains of BDA molecules on Au(111) and raising questions about the origin of this assembly and its effects on electronic structure (relative to the monomeric phase assumed in prior works).…”

“…Due to the limitations of Kohn-Sham DFT for describing quasiparticle states, [46] the calculated DFT-PBE HOMO resonance energies (EHOMO,DFT) are underestimated, as is well known [6,18,19,47]. To include exchange and correlation effects missing from DFT orbital energies, we correct the EHOMO,DFT values via the DFT+ method, a model GW approach first established for BDA-Au junctions but extended to adsorbates on surfaces.…”

Broadening of the derivative discontinuity in density functional theory

“…Over the years the density functional theory (DFT) developed into an important tool to study transport properties of nano-systems and single molecules. [1][2][3][4][5] This development occured despite of the fact, that often the results are quantitatively sensitive to the approximations made for the exchange correlation (XC) functional, V XC [n], underlying such calculations. [6][7][8][9][10] In principle, dc-transport calculations should combine either long-time evolution of wavepackets or a KS-based quasistationary formalism, in both cases with special dynamical XC-functionals.…”