Molecular Electronic Level Alignment at Weakly Coupled Organic Film/Metal Interfaces

Abstract

Electronic level alignment at interfaces of molecular materials with inorganic semiconductors and metals controls many interfacial phenomena. How the intrinsic properties of the interacting systems define the electronic structure of their interface remains one of the most important problems in molecular electronics and nanotechnology that can be solved through a combination of surface science experimental techniques and theoretical modeling. In this article, we address this fundamental problem through experimental and computational studies of molecular electronic level alignment of thin films of C(6)F(6) on noble metal surfaces. The unoccupied electronic structure of C(6)F(6) is characterized with single molecule resolution using low-temperature scanning tunneling microscopy-based constant-current distance-voltage spectroscopy. The experiments are performed on several noble metal surfaces with different work functions and distinct surface-normal projected band structures. In parallel, the electronic structures of the quantum wells (QWs) formed by the lowest unoccupied molecular orbital state of the C(6)F(6) monolayer and multilayer films and their alignment with respect to the vacuum level of the metallic substrates are calculated by solving the Schrödinger equation for a semiempirical one-dimensional (1D) potential of the combined system using input from density functional theory. Our analysis shows that the level alignment for C(6)F(6) molecules bound through weak van der Waals interactions to noble metal surfaces is primarily defined by the image potential of metal, the electron affinity of the molecule, and the molecule surface distance. We expect the same factors to determine the interfacial electronic structure for a broad range of molecule/metal interfaces.

Revealing the Buried Metal–Organic Interface: Restructuring of the First Layer by van der Waals Forces

“…19 Figure 1(a) gives an overview of the surface morphology. The first 6P wetting layer consists of flat and planar molecules forming a dense c(22×2) structure, where the molecules are aligned with their long molecular axis in the [1][2][3][4][5][6][7][8][9][10] direction. 10 For sub-monolayer exposures, the molecules repel each other due to Coulomb repulsion when the LUMO is occupied on hybridization.…”

“…Yet although the second layer molecules have begun to approach their bulk configuration of twisted and tilted phenyl rings 20 , the second layer spacing remains significantly larger than the bulk configuration due to interactions with the first layer. The second layer is always decorated by quasi-periodic bright chains of molecules running along the [1][2][3][4][5][6][7][8][9][10] direction. At room temperature these chains of molecules are very mobile, but at low temperature they become immobilized and can be imaged in the STM.…”

“…17,18 PEEM results indicate enhanced 1-dimensional diffusion of 6P molecules on formation of 3D crystallites that occurs after deposition of the third layer, which was postulated to be the result of a highly corrugated film forming on dewetting of metastable layers. Indeed STM images show a highly corrugated molecular film with troughs aligned in [1][2][3][4][5][6][7][8][9][10] direction. 18 Here the nature of the instigator for this corrugated film morphology has been explored using molecular manipulation in a cryogenic STM.…”

Model potential for the description of metal/organic interface states

“…We recently used it for the description of image-potential states in graphene/metal systems29. Unlike other, more adsorbate-specific model potentials that have been used to describe interfacial electronic states,3031 its main parameter is simply the distance of the carbon plane from the metal substrate. We will show that the same model potential not only predicts the energy of the interface state in various graphene/metal systems, but can be applied to large class of flat lying molecular layers with a similar π - π interaction as in graphene.…”

Time-resolved photoemission study of the electronic structure and dynamics of chemisorbed alkali atoms on Ru(0001)

“…DFT calculations are not expected to predict accurately the resonance energies of alkali atoms on metal surface because the image potential is not included. 11,72 Because the image potential is homogeneous within the 2D surface, however, we expect the errors in predicting band dispersions to be insignificant. Therefore, DFT calculations provide useful information on the comparative tendency of the σ-and π-resonances to form bands on the Ru(0001) and Cu(111) surfaces.…”