Self-organization of pentacene grown on Cu (119): electronic states of aligned molecules for monolayer and thicker films (original) (raw)
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Electronic structure at highly ordered organic/metal interfaces: Pentacene on Cu(110)
Physical Review B, 2007
The electronic structure at highly ordered pentacene monolayer prepared on Cu͑110͒ substrate was studied by angle-resolved ultraviolet photoemission spectroscopy. The valence-level photoemission line shape showed the evidences of ͑i͒ formation of the interface states and ͑ii͒ two-dimensional energy-band dispersion of the resultant interface states. The lattice constant deduced from the observed energy-band dispersion is consistent with the reported one based on the low-energy electron diffraction experiments. Thus, the observed energyband dispersion can be ascribed to the in-plane intermolecular energy-band dispersion in the pentacene monolayer on Cu͑110͒. These phenomena may originate from the hybridization between the molecular orbital and the wave function of the substrate surface. Furthermore, work-function change of about −0.9 eV by adsorption of pentacene was observed from the shift of the secondary-electron cutoff. Such a decrease of the work function indicates the formation of a dipole layer at the interface with the molecule positively charged. This direction is opposite to the naive expectation from the electron transfer from the substrate to the molecule, which was suggested from the previous work of core-level photoemission spectroscopy ͓McDonald et al., Surf. Sci. 600, 1909 ͑2006͔͒. This unexpected result may originate from the charge redistribution at the interface due to the induced image charge in the metal and the push back of electrons spilled out from the metal surface by the adsorbed molecules, which may overwhelm the effect of electron transfer.
Physical Review B, 2008
A single layer ͑SL͒ of pentacene molecules deposited on the Cu͑119͒ surface and on an organic selfassembled monolayer ͑SAM͒ has been investigated by near-edge x-ray absorption fine structure ͑NEXAFS͒ at the C K edge, and by photoemission at the C 1s core level. The lowest unoccupied molecular orbitals ͑LU-MOs͒ of the pentacene SL on the SAM are basically unaffected with respect to that of pentacene in the gas phase, indicating a weak interaction of pentacene with the SAM, while a strong redistribution of the LUMOrelated final states is observed when the molecules are deposited on the Cu͑119͒ substrate, sign of an electronic mixing between molecular and metal electronic states, in agreement with recent theoretical predictions. The strong dichroic response of the NEXAFS signal indicates upstanding pentacene molecules oriented 16°off normal for the organic-organic heterostructure. The rehybridization of pentacene orbitals with Cu states at the pentacene/Cu interface hinders an accurate determination of the molecule orientation, which is, however, compatible with molecules lying flat on the Cu͑119͒ terraces.
Physical Review B, 2008
The interaction of pentacene molecules in contact with the Cu͑119͒ stepped surface has been directly imaged by scanning tunneling microscopy and analyzed by angle resolved photoemission spectroscopy. Interacting molecules, which are in contact with copper, generate dispersive electronic states associated with a perturbed electron charge density distribution of the molecular orbitals. In contrast, the electron charge density of molecules of the pentacene on top of the first layer, which is not in direct contact with the Cu surface, shows an intramolecular structure very similar to that of the free molecule. Our results indicate that the delocalization of the molecular states in the pentacene/Cu system is confined to the very first molecular layer at the interface.
Symmetry lowering of pentacene molecular states interacting with a Cu surface
Physical Review B, 2007
Pentacene adsorbed on the Cu͑119͒ vicinal surface forms long-range ordered chain structures. Photoemission spectroscopy measurements and ab initio density functional theory simulations provide consistent evidences that pentacene molecular orbitals mix with the copper bands, giving rise to interaction states localized at the interface. Angular-resolved and polarization dependent photoemission spectroscopy shows that most of the pentacene derived intensity is strongly dichroic. The symmetry of the molecular states of the free pentacene molecules is reduced upon adsorption on Cu͑119͒, as a consequence of the molecule-metal interaction. Theoretical results show a redistribution of the charge density in molecular states close to the Fermi level, consistent with the photoemission intensities ͑density of states͒ and polarization dependence ͑orbital symmetry͒.
The Journal of Chemical Physics, 2010
In order to clarify factors determining the interface dipole, we have studied the electronic structures of pentacene adsorbed on Cu͑111͒, Ag͑111͒, and Au͑111͒ by using first-principles density functional theoretical calculations. In the structural optimization, a semiempirical van der Waals ͑vdW͒ approach ͓S. Grimme, J. Comput. Chem. 27, 1787 ͑2006͔͒ is employed to include long-range vdW interactions and is shown to reproduce pentacene-metal distances quite accurately. The pentacene-metal distances for Cu, Ag, and Au are evaluated to be 0.24, 0.29, and 0.32 nm, respectively, and work function changes calculated by using the theoretically optimized adsorption geometries are in good agreement with the experimental values, indicating the validity of the present approach in the prediction of the interface dipole at metal/organic interfaces. We examined systematically how the geometric factors, especially the pentacene-substrate distance ͑Z C ͒, and the electronic properties of the metal substrates contribute to the interface dipole. We found that at Z C Ն 0.35 nm, the work function changes ͑⌬'s͒ do not depend on the substrate work function ͑ m ͒, indicating that the interface level alignment is nearly in the Schottky limit, whereas at Z C Յ 0.25 nm, ⌬'s vary nearly linearly with m , and the interface level alignment is in the Bardeen limit. Our results indicate the importance of both the geometric and the electronic factors in predicting the interface dipoles. The calculated electronic structure shows that on Au, the long-range vdW interaction dominates the pentacene-substrate interaction, whereas on Cu and Ag, the chemical hybridization contributes to the interaction. Toyoda et al. J. Chem. Phys. 132, 134703 ͑2010͒ 134703-3 Pentacene/noble metal interfaces J. Chem. Phys. 132, 134703 ͑2010͒ 134703-5 Pentacene/noble metal interfaces J. Chem. Phys. 132, 134703 ͑2010͒ FIG. 6. The density of states projected onto the molecular orbitals of pentacene ͑PDOS͒ on ͑a͒ Cu͑111͒, ͑b͒ Ag͑111͒, and ͑c͒ Au͑111͒ at Z C = 0.53 nm. The energy zero is taken to be the Fermi energy of the adsorbed system. The HOMO and the LUMO parts of the PDOS near the Fermi energy are magnified in the insets. 134703-7 Pentacene/noble metal interfaces J. Chem. Phys. 132, 134703 ͑2010͒ 134703-9 Pentacene/noble metal interfaces J. Chem. Phys. 132, 134703 ͑2010͒
Lateral band formation and hybridization in molecular monolayers: NTCDA on Ag(110) and Cu(100)
Physical Review B, 2013
The adsorption of aromatic molecules on metal surfaces leads to a complex reorganization of the molecular and metal wave functions. Various processes such as charge transfer, hybridization between molecular and metallic states, and the formation of dispersing bands within the interface have been demonstrated for organometallic interface systems. For the model molecule 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA), we compare highly ordered monolayers on Ag(110) and Cu , which allows us to identify changes of the interfacial electronic structure when altering the coupling strength with the substrate by means of angle-resolved photoelectron spectroscopy. The stronger coupling to the Ag(110) substrate goes along with a shorter photohole lifetime and a stronger hybridization of the NTCDA lowest unoccupied molecular orbital with metal states. Supported by ab initio calculations, we show that the observed band dispersion is greatly enhanced due to the interaction with Ag(110) while the laterally denser adsorption geometry of NTCDA on Cu(100) entails a larger intermolecular wave-function overlap, and the presence of the substrate results in no further bandwidth enhancement.
Multimorphism in molecular monolayers: Pentacene on Cu(110)
Physical Review B, 2009
The architecture of the contacting interface between organic molecular semiconductors and metallic or insulating substrates determines its cooperative properties such as the charge injection and the charge-carrier mobility of organic thin-film devices. This paper contributes a systematic approach to reveal the evolution of the different structural phases of pentacene on Cu͑110͒ while using the same growth conditions. Complementary measurement techniques such as scanning tunneling microscopy and low-energy electron diffraction together with ab initio calculations are applied to reveal the complex multiphase behavior of this system at room temperature. For coverages between 0.2 and 1 monolayer ͑ML͒ a complex multiphase behavior comprising five different phases is observed, which is associated to the interplay of molecule/molecule and molecule/substrate interactions. Multimorphism critically depends on the thermodynamics and kinetics determined by the growth parameters as well as the system itself and arises from shallow energy minima for structural rearrangements. In consequence, the multimorphism affects the interface structure and therefore the interface properties.
Surface Science, 2003
Scanning tunneling microscopy has been used to study the ordering of pentacene (C 22 H 14 ) molecules on the Ag/ Si(1 1 1)-( p 3 Â p 3)R30°surface at room temperature. Two solid phases, S1 and S2, are observed at coverages of 0.35monolayer(ML)and0.35 monolayer (ML) and 0.35monolayer(ML)and1.0 ML respectively. It is shown that the solid phase S1 has a high-order commensurate lattice, Ag/Si(1 1 1)-(25 Â 25)-pentacene, containing 75 molecules. The structure of this phase is determined from STM measurements at very low coverages where it is possible to image both the pentacene molecules and the structure of the Ag/ Si(1 1 1) substrate. Two adsorption sites are identified, a three-fold hollow site at the centre of a Ag-trimer (CA-site) and a six-fold hollow site at the centre of the hexagonal arrangement of silver atoms (CB-site). A higher pentacene coverage of $1 ML lead to a molecular reorganization and forms a new commensurate structure Ag/Si(1 1 1)-(2 Â 3)-pentacene, containing two molecules per unit cell. Because low energy electron diffraction patterns were not obtainable for this system, the structure of this second phase is determined by using the bias voltage as a tunable parameter to ''focus'' on either the molecular film or on the substrate. In this phase adsorption takes place exclusively on the Ag-trimer (CA) site and the CB-site is lost because of strong lateral molecule-molecule interactions. The role of competition between intermolecular and molecule-substrate interactions and the nature of the adsorption sites in determining the structure of the pentacene layers is discussed.
The Journal of Physical Chemistry C, 2014
The atomic structure of the pentacene/Cu(110) interface for coverages at and just below one monolayer has been determined by surface X-ray diffraction (SXRD), supported by state-of-the-art density functional theory (DFT) calculations. The in-depth sensitivity of SXRD to atom positions allows tracking the adsorption-induced distortions down to the fifth substrate layer. The main feature of the DFT model, namely, the buckling induced in the substrate, is fully confirmed by the experiment. The considerable atomic displacements which are the same for the two coverages under investigation are a signature of the strong molecule−substrate interaction, indicative of an adsorption mechanism of chemisorption type.