Measuring the transport of electrons through a graphene sheet necessarily involves contacting it with metal electrodes. We study the adsorption of graphene on metal substrates using firstprinciples calculations at the level of density functional theory. The bonding of graphene to Al, Ag, Cu, Au and Pt(111) surfaces is so weak that its unique "ultrarelativistic" electronic structure is preserved. The interaction does, however, lead to a charge transfer that shifts the Fermi level by up to 0.5 eV with respect to the conical points. The crossover from p-type to n-type doping occurs for a metal with a work function ∼ 5.4 eV, a value much larger than the work function of free-standing graphene, 4.5 eV. We develop a simple analytical model that describes the Fermi level shift in graphene in terms of the metal substrate work function. Graphene interacts with and binds more strongly to Co, Ni, Pd and Ti. This chemisorption involves hybridization between graphene pz-states and metal d-states that opens a band gap in graphene. The graphene work function is as a result reduced considerably. In a current-in-plane device geometry this should lead to n-type doping of graphene.

We study the dipole formation at the surface formed by -CH 3 and -CF 3 terminated shortchain alkyl-thiolate monolayers on Au(111). In particular, we monitor the change in work function upon chemisorption using density functional theory calculations. We separate the surface dipole into two contributions, resulting from the gold-adsorbate interaction and the intrinsic dipole of the adsorbate layer, respectively. The two contributions turn out to be approximately additive. Adsorbate dipoles are defined by calculating dipole densities of free-standing molecular monolayers. The gold-adsorbate interaction is to a good degree determined by the Au-S bond only. This bond is nearly apolar and its contribution to the surface dipole is relatively small. The surface dipole of the self-assembled 2 monolayer is then dominated by the intrinsic dipole of the thiolate molecules. Alkyl-thiolates increase the work function of Au(111), whereas fluorinated alkyl-thiolates decrease it.

Using first-principles calculations we show that the work function of noble metals can be decreased or increased by up to 2 eV upon the adsorption of self-assembled monolayers of organic molecules. We identify the contributions to these changes for several (fluorinated) thiolate molecules adsorbed on Ag(111), Au(111) and Pt(111) surfaces. The work function of the clean metal surfaces increases in this order, but adsorption of the monolayers reverses the order completely. Bonds between the thiolate molecules and the metal surfaces generate an interface dipole, whose size is a function of the metal, but it is relatively independent of the molecules. The molecular and bond dipoles can then be added to determine the overall work function. PACS numbers: 73.30.+y, Recent advances in molecular electronics, where organic molecules constitute active materials in electronic devices, have created a large interest in metal organic interfaces [1]. Transport of charge carriers across the interfaces between metal electrodes and the organic material often determines the performance of a device [2]. Organic semiconductors differ from inorganic ones as they are composed of molecules and intermolecular forces are relatively weak. In a bulk material this increases the importance of electron-phonon and electron-electron interactions [3]. At a metal organic interface the energy barrier for charge carrier injection into the organic material is often determined by the formation of an interface dipole localized at the first molecular layer. The interface dipole can be extracted by monitoring the change in the metal surface work function after deposition of an organic layer [1,4].Atoms and molecules that are physisorbed on a metal surface usually decrease the work function, as the Pauli repulsion between the molecular and surface electrons decreases the surface dipole [5,6]. Chemisorption can give an increase or a decrease of the work function, and can even lead to counterintuitive results [7,8]. Selfassembled monolayers (SAMs) are exemplary systems to study the effect of chemisorbed organic molecules upon metal work functions [9]. More specifically, alkyl thiolate (C n H 2n+1 S) SAMs on the gold (111) surface are among the most extensively studied systems [10,11,12,13,14]. The sulphur atoms of the thiolate molecules form stable bonds to the gold surface and their alkyl tails are close packed, which results in a well ordered monolayer. SAMs with similar structures are formed by alkyl thiolates on a range of other (noble) metal surfaces [10,14,15].Often the change in work function upon adsorption of a SAM is interpreted mainly in terms of the dipole moments of the individual thiolate molecules, whereas only a minor role is attributed to the change induced by chemisorption [9,11,12]. This assumption turns out to be reasonable for adsorption of methyl thiolate (CH 3 S) on Au(111) [13], but for CH 3 S on Cu(111) it is not [14]. In this paper we apply first-principles calculations to study the interface dipoles and the work function change ...

The formation of interface dipoles in self-assembled monolayers (SAMs) of -CH3 and -CF3 terminated short-chain alkanethiolates on Ag(111) is studied by means of density functional theory calculations. The interface dipoles are characterized by monitoring the change in the surface work function upon adsorption of the SAM. We compare results obtained for SAMs in structures with a different packing density of molecules, i.e. (• , and p(2×2). The work function of alkanethiolate SAMs on silver depends weakly on the packing density; that of fluorinated alkanethiolates shows a stronger dependance. The results are analyzed in terms of two nearly independent contributions to the interface dipole. These originate respectively from the molecular dipoles and from a charge transfer between the metal surface and the molecules. The charge transfer is determined by the silver-sulfur bond and it is independent of the electronegativity of the molecules.

Substantial dipoles are formed at interfaces between metals and organic molecules, even in case of relatively weak interactions. We monitor interface dipoles by first-principles calculations of work function changes caused by adsorption of perylene and 3,4,9,10-perylene-tetra-carboxylic-dianhydride monolayers. These changes are the result of two competing effects. Pauli repulsion pushes electrons into the surface, which decreases the work function. If the metal work function is sufficiently low, electrons are donated back from the surface to the molecule. In this regime the work function is effectively determined by pinning of the Fermi level at a molecular energy level.

We study the dipole layer formed at metal-organic interfaces by means of first-principles calculations. Interface dipoles are monitored by calculating the work function change of Au, Ag, Al, Mg and Ca surfaces upon adsorption of a monolayer of PTCDA (3,4,9,10-perylene-tetra-carboxylic-dianhydride), perylene or benzene molecules. Adsorption of PTCDA leads to pinning of the work function for a range of metal substrates. It gives interface dipoles that compensate for the difference in the clean metal work functions, leading to a nearly constant work function. In contrast, adsorption of benzene always results in a decrease of the work function, which is relatively constant for all metal substrates. Both effects are found in perylene, where adsorption on low work function metals gives work function pinning, whereas adsorption on high work function metals gives work function lowering. The work function changes upon adsorption are analyzed and interpreted in terms of two competing effects. If the molecule and substrate interact weakly, the molecule pushes electrons into the surface, which lowers the work function. If the metal work function is sufficiently low with respect to the unoccupied states of the molecule, electrons are donated into these states, which increases the binding and the work function.