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Register a new userBarrier formation at metal/organic interfaces: dipole formation and the Charge Neutrality Level
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Added by
H\\'ector V\\'azquez
Publication date
2004
fields
Physics
and research's language is
English
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The barrier formation for metal/organic semiconductor interfaces is analyzed within the Induced Density of Interface States (IDIS) model. Using weak chemisorption theory, we calculate the induced density of states in the organic energy gap and show that it is high enough to control the barrier formation. We calculate the Charge Neutrality Levels of several organic molecules (PTCDA, PTCBI and CBP) and the interface Fermi level for their contact with a Au(111) surface. We find an excellent agreement with the experimental evidence and conclude that the barrier formation is due to the charge transfer between the metal and the states induced in the organic energy gap.
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The formation of a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface barrier is analyzed using weak-chemisorption theory. The electronic structure of the uncoupled PTCDA molecule and of the metal surface is calculated. Then, the induced density of interface states is obtained as a function of these two electronic structures and the interaction between both systems. This induced density of states is found to be large enough (even if the metal/PTCDA interaction is weak) for the definition of a Charge Neutrality Level for PTCDA, located 2.45 eV above the highest occupied molecular orbital. We conclude that the metal/PTCDA interface molecular level alignment is due to the electrostatic dipole created by the charge transfer between the two solids.
Three driving forces control the energy level alignment between transition-metal oxides and organic materials: the chemical interaction between the two materials, the organic electronegativity and the possible space charge layer formed in the oxide. This is illustrated in this letter by analyzing experimentally and theoretically a paradigmatic case, the TiO2(110) / TCNQ interface: due to the chemical interaction between the two materials, the organic electron affinity level is located below the Fermi energy of the n-doped TiO2. Then, one electron is transferred from the oxide to this level and a space charge layer is developed in the oxide inducing an important increase in the interface dipole and in the oxide work function.
Adsorption of organic molecules on well-oriented single crystal coinage metal surfaces fundamentally affects the energy distribution curve of ultra-violet photoelectron spectroscopy spectra. New features not present in the spectrum of the pristine metal can be assigned as interface states having some degree of molecule-substrate hybridization. Here it is shown that interface states having molecular orbital character can easily be identified at low binding energy as isolated features above the featureless substrate sp-plateau. On the other hand much care must be taken in assigning adsorbate-induced features when these lie within the d-band spectral region of the substrate. In fact, features often interpreted as characteristic of the molecule-substrate interaction may actually arise from substrate photoelectrons scattered by the adsorbates. This phenomenon is illustrated through a series of examples of noble-metal single-crystal surfaces covered by monolayers of large pi-conjugated organic molecules.
The performance of GaN-on-Silicon electronic devices is severely degraded by the presence of a parasitic conduction pathway at the nitride-substrate interface which contributes to switching losses and lower breakdown voltages. The physical nature of such a parasitic channel and its properties are however, not well understood. We report on a pronounced thickness dependence of the parasitic channel formation at AlN/Si interfaces due to increased surface acceptor densities at the interface in silicon. The origin of these surface acceptors is analyzed using secondary ion mass spectroscopy measurements and traced to thermal acceptor formation due to Si-O-N complexes. Low-temperature (5K) magneto-resistance (MR) data reveals a transition from positive to negative MR with increasing AlN film thickness indicating the presence of an inversion layer of electrons which also contributes to parasitic channel formation but whose contribution is secondary at room temperatures.
The interaction of the strong electron-acceptor tetracyanoethylene (TCNE) with the Cu(100) surface has been studied with scanning tunneling microscopy experiments and first-principles density functional theory calculations. We compare two different adsorption models with the experimental results and show that the molecular self-assembly is caused by a strong structural modification of the Cu(100) surface rather than the formation of a coordination network by diffusing Cu adatoms. Surface atoms become highly buckled and the chemisorption of TCNE is accompanied by a partial charge-transfer.
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