Identifying Atomic-Level Correlation between Geometric and Electronic Structure at a Metal–Organic Interface

Abstract

The interface of π-conjugated organic molecules with metal surfaces plays a crucial role in determining the efficiency of organic-based electronic devices. Because the interface is buried under the molecule, it is often experimentally inaccessible. Here, we propose a way of simultaneous structural and electronic characterization of the interface of the metal−organic system. Using the molecular manipulation technique of scanning tunneling microscope, we conducted a systematic study of geometric and electronic structure on the same footing and identified the atomic-scale correlation between them for copper phthalocyanine molecule on the Au( 110). The energy level of the lowestunoccupied molecular orbital (LUMO) is found to be linked with the number of underlying Au atoms (n = 2−10) at the interface. The LUMO decreases with n in energy and reaches close to the Fermi level, demonstrating explicitly that the electronic property of the molecule is affected by the atomic-scale variation of the interface. The density functional theory calculations reproduced the energy-level shift as a function of n and identified the origin of the correlation.

Intermixing of Unoccupied States of Metal Phthalocyanine Chains Assembled on Au(110)

“…Recently, STS results [45] indicated that the LUMO CuPc on Au(110), in a very lowcoverage regime (≤0.3 SL), is always located above EF for the Au surface, in agreement with our assignment at 0.3 SL and confirming the weak interaction, even for coverages below the molecular chain formation and a negligible substrate/molecule CT.…”

“…Recently, STS results [45] indicated that the LUMO CuPc on Au(110), in a very lowcoverage regime (≤0.3 SL), is always located above E F for the Au surface, in agreement with our assignment at 0.3 SL and confirming the weak interaction, even for coverages below the molecular chain formation and a negligible substrate/molecule CT. The IPES data of the CuPc/Au(110) system (Figure 3b) show that the emission close to EF is dominated by the metallic substrate, while the molecular state intensities become prominent at about 0.5-0.7 SL, corresponding to the (5 × 5)-symmetry reconstruction of the interface [12].…”

CuPc Adsorption on Au(110)-(1 × 2): From a Monomer to a Periodic Chain

“…We schematically illustrate the adsorption structure for A-CuPc in Figure 2(g), where the trough is locally filled with four Au adatoms, onto which the molecule adsorbs. Note that the number of Au adatoms was typically four but distributed between two to six, as we reported previously [25]. Figure 2(f ) shows the atomic-resolution STM image of the "dark" area.…”

“…The results presented here indicate that surface reconstruction occurs from the initial CuPc adsorption stage. We assume that such a surface reconstruction is inherent in the system of π-conjugated planar molecules (e.g., CuPc) on "corrugated" surfaces [e.g., Au(110)-(1 × 2)]; the molecule binds to the surface via the electronic coupling between the organic ligand (isoindole rings) and substrate [7,25,29]. Therefore, the molecule maximizes the contact area, i.e., the overlap between the molecular orbitals and surface electronic states.…”

“…In this study, we used STM to investigate the CuPc adsorption on Au(110)-(1 × 2), from the initial (monomeric) adsorption to chain formation. We employed an STM manipulation technique [25], which enabled us to move CuPc molecules and facilitate evaluation of the local structure at CuPc/Au(110) interface. We found that surface reconstruction commences under CuPc even at the initial adsorption stage, i.e., monomeric adsorption.…”