Orientational Ordering of Nonplanar Phthalocyanines on Cu(111): Strength and Orientation of the Electric Dipole Moment

Abstract

In order to investigate the orientational ordering of molecular dipoles and the associated electronic properties, we studied the adsorption of chlorogallium phthalocyanine molecules (GaClPc, Pc = C32N8H16 −2 ) on Cu(111) using the X-ray standing wave technique, photoelectron spectroscopy, and quantum chemical calculations. We find that for sub-monolayer coverages on Cu(111) the majority of GaClPc molecules adsorb in a 'Cl-down' configuration by forming a covalent bond to the substrate. For bilayer coverages the XSW data indicate a co-existence of the 'Cl-down' and 'Cl-up' configuration on the substrate. The structural details established for both cases and supplementary calculations of the adsorbate system allow us to analyze the observed change of the work function.PACS numbers: 68.49. Uv, 68.43.Fg, 79.60.Fr The adsorption of organic semiconductor molecules has been in the focus of numerous experimental and theoretical investigations -many of them addressing the subtle interplay of electronic and structural properties. Early studies [1], which show that the energy levels of organic semiconductor/metal interfaces can exhibit large deviations from the Schottky-Mott relation, conveyed the significance of interface dipoles. Until today and despite the ubiquity of this concept in the field of organic materials, the origin of the interface dipole often remains vague.To establish a better understanding of the energy level alignment at the interface one should not neglect effects related to the molecular structure of organic adsorbates: Planar molecules such as F 16 CuPc [2], PTCDA [3,4] or pentacene derivatives [5], for example, can distort upon adsorption due to the interaction with the substrate and therefore exhibit an induced molecular dipole. Non-planar molecules such as TiOPc [6], SnPc [7][8][9], SubPc [10, 11] and VOPc [12], which may adsorb in different orientations, form layers with at least partially aligned dipole moments. Hence, for this class of systems the orientational order on the surface is a quantity which strongly influences the interface dipole. In particular, it has been shown that depending on the orientation a layer of molecular dipoles p with an area density N dip can shift the vacuum level (VL) in either direction and, therefore, increase or decrease the work function Φ of the sample according to [13] where ǫ is the effective dielectric constant of the monolayer. An experimentally and theoretically challenging model system of non-planar organic molecules with a significant dipole moment, for which these effects can be directly studied, is chlorogallium phthalocyanine (GaClPc, Fig. 1) [14].In this letter, we present a detailed study on the bonding and orientational ordering of GaClPc on Cu(111) surfaces using the X-ray standing wave (XSW) technique [15], ultra-violet photoelectron spectroscopy (UPS) and density functional theory (DFT) based calculations. While XSW data are taken to determine exact the atomic positions along the surface normal and, thereby, also the orientation of the molecul...

Exploring the bonding of large hydrocarbons on noble metals: Diindoperylene on Cu(111), Ag(111), and Au(111)

“…Model simulations similar to those presented in Ref. 37 show that the DFT-derived adsorption geometry on Cu(111) results in a relatively small decrease of the coherent fraction ( f H = −0.07), which lies within the standard deviation of our XSW measurements.…”

Control of the dipole layer of polar organic molecules adsorbed on metal surfaces via different charge-transfer channels

“…12 Dependence of the ELA on the polar-molecule orientation and packing density has also been reported [13][14][15][16][17][18][19][20][21][22] , and connection to device applications has been demonstrated. [23][24][25] Hosokai et al, who compared the as-grown (AG) and annealed (AN) chlorogallium-phthalocyanine (ClGaPc) films grown on highly oriented pyrolytic graphite (HOPG), reported large band bending of the HOMO state for the AG films with a stacked bilayer structure, but this band bending could be suppressed in the AN films because of the formation of a uniform Cl-up configuration after the thermal treatment; 13 the substrate, HOPG, was however fairly inert in this case, and charge transfer at the organic/metal interface was expected to be minimal.…”

Orientation of molecular interface dipole on metal surface investigated by noncontact atomic force microscopy

“…Chin Sci Bull, 2013Bull, , 58: 36303635, doi: 10.1007 Interface dipoles induced by self-assembly of organic molecules on metal surfaces play an important role in controlling the surface and/or interface properties, especially in the modification of work function of the metal substrate and the energy-level alignment of organic semiconductor with the metal Fermi level [1,2]. Interface dipole is strongly influenced by the permanent dipole of the organic molecule and the "bonding" dipole induced by the molecule-metal interaction [3][4][5][6]. To understand the energy-level alignment at the interface and design interfaces with desired properties, atomic level investigations on the molecular structures of organic adsorbates and interfacial electronic structures are highly required and challenging.…”

“…Metal phthalocyanines (MPcs), together with their derivatives, have attracted great interest over the past years because of their unique optical and electrical properties [7]. Non-planar MPcs, such as titanyl phthalocyanine (TiOPc), tin phthalocyanine (SnPc) [8], vanadyl phthalocyanine (VOPc) [9,10], chloroaluminum phthalocyanine (ClAlPc) [5] and chlorogallium phthalocyanine (GaClPc) [3], may adsorb on metal surface with two opposite molecular orientations, which give rise to different dipole moments ( Figure 1(b)). Different adsorption configurations have significant influence on the functionalities of molecules and surface and/or interface properties.…”

“…Interface dipole is strongly influenced by the permanent dipole of the organic molecule and the "bonding" dipole induced by the molecule-metal interaction [3][4][5][6]. To understand the energy-level alignment at the interface and design interfaces with desired properties, atomic level investigations on the molecular structures of organic adsorbates and interfacial electronic structures are highly required and challenging.…”