-
[show abstract]
[hide abstract]
ABSTRACT: In conjugated organic molecules, the difference between the HOMO and LUMO Kohn–Sham eigenvalues is significantly smaller than the transport gap measured experimentally. We discuss here, within a local-orbital formulation of DFT, how this problem can be corrected using appropriate hybrid potentials, that add a fraction of Hartree–Fock exchange interaction in the DFT calculation. We illustrate this approach presenting calculations for two simple systems: H 2 and C 6 H 6 ; then, we discuss how to implement this hybrid approach in a general local-orbital calculation, adjusting the hybrid contribution to yield the correct experimental HOMO/LUMO energy gap for the molecule. We also consider the case of an organic molecule on a metal and analyze the effect of the molecule–metal interaction on the organic energy gap. In particular, we discuss how to introduce in this hybrid-potential scheme the effect of the image potential, and present results for the organic molecules PTCDA, TTF, benzene and pentacene on the metal surfaces Au(111), Ag(111) and Cu(111).
Journal of Physics Condensed Matter 02/2013; 25:094007. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The interaction between graphene layers is analyzed combining local orbital DFT and second order perturbation theory. For this purpose we use the linear combination of atomic orbitals-orbital occupancy (LCAO-OO) formalism, that allows us to separate the interaction energy as the sum of a weak chemical interaction between graphene layers plus the van der Waals interaction (Dappe et al 2006 Phys. Rev. B 74 205434). In this work, the weak chemical interaction is calculated by means of corrected-LDA calculations using an atomic-like sp(3)d(5) basis set. The van der Waals interaction is calculated by means of second order perturbation theory using an atom-atom interaction approximation and the atomic-like-orbital occupancies. We also analyze the effect of dynamical screening in the van der Waals interaction using a simple model. We find that this dynamical screening reduces by 40% the van der Waals interaction. Taking this effect into account, we obtain a graphene-graphene interaction energy of 70 ± 5 meV/atom in reasonable agreement with the experimental evidence.
Journal of Physics Condensed Matter 10/2012; 24(42):424208. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Recent Scanning Tunneling Hydrogen Microscopy (STHM) experiments on PTCDA/Au(111) have shown unprecedented intramolecular and intermolecular spatial resolution. The origin of this resolution is studied using an accurate STHM theoretical simulation technique that includes a detailed description of the electronic structure of both tip and sample. Our results show that H2 molecules are dissociated on the Au-tip; the adsorbed H atoms change the Density of States (DOS) at the Fermi level (EF) of the tip, increasing its p-orbital character and reducing the s-orbital contribution. Also, due to the interaction with the H-decorated tip, EF is shifted to the middle of the PTCDA LUMO peak, increasing dramatically the DOS of the sample at EF. These effects give rise to the enhanced STHM resolution.
Physical Review Letters 01/2012; · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In conjugated organic molecules, the difference between the HOMO and LUMO Kohn-Sham eigenvalues is significantly smaller than the transport gap measured experimentally. We discuss here, within a local-orbital formulation of DFT, how this problem can be corrected using appropriate hybrid potentials, that add a fraction of Hartree-Fock exchange interaction in the DFT calculation. We illustrate this approach presenting calculations for two simple systems: H2 and C6H6; then, we discuss how to implement this hybrid approach in a general local-orbital calculation, adjusting the hybrid contribution to yield the correct experimental HOMO/LUMO energy gap for the molecule. We also consider the case of an organic molecule on a metal and analyze the effect of the molecule-metal interaction on the organic energy gap. In particular, we discuss how to introduce in this hybrid-potential scheme the effect of the image potential, and present results for the organic molecules PTCDA, TTF, benzene and pentacene on the metal surfaces Au(111), Ag(111) and Cu(111).
Journal of Physics C Solid State Physics 01/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: A detailed density functional study is performed to analyze the TTF/Au (111) interface, including the effect of the molecular charging energy on the transport gap. Theoretical STM calculations are carried out, and compared with recent STM experimental evidence, for a dilute TTF/Au (111) structure in order to validate the interface geometry used in our calculations. We show that the alignment between the metal and the organic levels is mainly controlled by the charge transfer between the two materials, as determined by the difference between the molecule Charge Neutrality Level (CNL), and the initial Fermi level. The calculated transport gap is 4.1 eV, and the CNL is found close to the LUMO level, located about 0.7 eV from vacuum.
Organic Electronics 12/2011; 13:399. · 4.05 Impact Factor
-
L Chaput,
C Tournier-Colletta,
L Cardenas,
A Tejeda,
B Kierren,
D Malterre,
Y Fagot-Revurat,
P Le Fèvre,
F Bertran,
A Taleb-Ibrahimi,
D G Trabada, J Ortega,
F Flores
[show abstract]
[hide abstract]
ABSTRACT: Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.
Physical Review Letters 10/2011; 107(18):187603. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We analyze the pentacene/Au(111) interface by means of density functional theory (DFT) calculations using a new hybrid functional; in our approach we introduce, in a local-orbital formulation of DFT, a hybrid exchange potential, and combine it with a calculation of the molecule charging energy to properly describe the transport energy gap of pentacene on Au(111). Van der Waals forces are taken into account to obtain the adsorption geometry. Interface dipole potentials are also calculated; it is shown that the metal/pentacene energy level alignment is determined by the potential induced by the charge transfer between the metal surface and the organic material, as described by the induced density of interface states model. Our results compare well with the experimental data.
The Journal of chemical physics 08/2011; 135(8):084702. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We analyze the benzene/Au(111) interface taking into account charging energy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3521271]
JOURNAL OF CHEMICAL PHYSICS. 01/2011; 134:044701.
-
[show abstract]
[hide abstract]
ABSTRACT: A Density Functional calculation is performed to analyze the PTCDA/Au(111) interface, including the effect of the molecular charging energy on the transport gap. The calculated transport gap is 2.9 eV, and the lowest unoccupied molecular orbital, or LUMO, level is found at 4.25 eV below the vacuum level. We show that the alignment between the metal and the organic levels is controlled by the charge transfer between the two materials, as determined by the difference between the molecule Charge Neutrality Level, or CNL, (located 0.3 eV below the LUMO level) and the metal work function. We compare with inde-pendent General Gradient Approximation–Density Functional calculations, DFT–GGA, yielding an energy gap of 1.4 eV; we find, however, good agreement between both approaches for the CNL and the interface potential step. Good agreement is also found between our results and the experimental evidence.
Chemical Physics 01/2011; 390:14. · 1.90 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We analyze the benzene/Au(111) interface taking into account charging energy effects to properly describe the electronic structure of the interface and van der Waals interactions to obtain the adsorption energy and geometry. We also analyze the interface dipoles and discuss the barrier formation as a function of the metal work-function. We interpret our DFT calculations within the induced density of interface states (IDIS) model. Our results compare well with experimental and other theoretical results, showing that the dipole formation of these interfaces is due to the charge transfer between the metal and benzene, as described in the IDIS model.
The Journal of chemical physics 01/2011; 134(4):044701. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We analyze the nanogap organic molecular junction formed by a C(60) molecule in between an Au-tip and an Au(111)-surface. In our approach, we first calculate the atomic geometry of the tip/C(60)/Au(111) nanocontact as a function of the tip-surface distance and molecule adsorption site using a density functional theory-local density approximation (DFT-LDA) technique. The electronic structure and metal/organic barrier height formation (for a single molecule) are analyzed using the induced density of interface states model for metal/organic interfaces. From this analysis we calculate the charging energy U(eff) of the C(60) molecule at the nanocontact and determine self-consistently the transport energy gap and the organic molecule density of states. Our results are shown to be in good agreement with the experimental evidence.
Journal of Physics Condensed Matter 08/2010; 22(30):304007. · 2.55 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In this contribution we address the theoretical underst anding of weak chemical interactions and of the van der Waals forces,
in conjunction with the last developments in this area and selected applications to nanostructures. In the first section,
we highlight the importance of these interactions, in physics and chemistry and also in biology, and we recall early treatments
of these issues, as those by van der Waals and London. After a brief review of the existing methods to treat such interactions,
we present a model based on DFT (for each van der Waals-interacting independent system) and an intermolecular perturbation
theory that uses a localized orbitals basis set. We will first detail a weak overlap expansion (LCAO-S
2) as a perturbation treatment to determine the weak chemical interaction. Then we will show how to implement the van der Waals
interaction in the DFT solution, from the dipolar approximation in a perturbation theory. We apply this model to a reference
system for weak interactions, i.e., the interaction between two planes of graphene. In the framework of a minimal basis set
that describes each independent system and the weak chemical repulsion, we show that it is necessary to take into account
atomic dipole transitions involving high excited states like 3d orbitals to properly describe the van der Waals interaction. We demonstrate how the delicate balance between chemical repulsion
and van der Waals attractive interaction gives the equilibrium geometry and the binding energy of the system. Moreover, as
an extension of this work, we obtain the adsorption energy of a carbon nanotube on graphene, the adsorption energy of a C60 molecule on a carbon nanotube, and the energy of a C60 molecule encapsulated in a carbon nanotube. This gives us the opportunity to discuss incommensurable systems. A complete
study of C60 dimers is also presented with future perspective for the study of C60 molecular crystals. We will conclude with an overview of this work, discussing interaction and transport at metal–organics
interfaces from the point of view of applications in the field of molecular electronics.
01/2010: pages 45-79;
-
[show abstract]
[hide abstract]
ABSTRACT: A review of our theoretical understanding of the band alignment at organic interfaces is presented with particular emphasis on the metal/organic (MO) case. The unified IDIS (induced density of interface states) and the ICT (integer charge transfer) models are reviewed and shown to describe qualitatively and semiquantitatively the barrier height formation at those interfaces. The IDIS model, governed by the organic CNL (charge neutrality level) and the interface screening includes: (a) charge transfer across the interface; (b) the "pillow" (or Pauli) effect associated with the compression of the metal wavefunction tails; and (c) the molecular dipoles. We argue that the ICT-model can be described as a limiting case of the unified IDIS-model for weak interface screening. For a fully quantitative understanding of the band alignment at organic interfaces, use of DFT (density functional theory) or quantum chemistry methods is highly desirable. In this Perspective review, we concentrate our discussion on DFT and show that conventional LDA or GGA calculations are limited by the "energy gap problem of the organic materials", because the LDA (or GGA) Kohn-Sham energy levels have to be corrected by the self-interaction energy of the corresponding wavefunction, to provide the appropriate molecule transport energy gap. Image potential and polarization effects at MO interfaces tend to cancel these self-interaction corrections; in particular, we show that for organic molecules lying flat on Cu and Ag, these cancellations are so strong that we can rely on conventional DFT to calculate their interface properties. For Au, however, the cancellations are weaker making it necessary to go beyond conventional DFT. We discuss several alternatives beyond conventional LDA or GGA. The most accurate approach is the well-known GW-technique, but its use is limited by its high demanding computer time. In a very simple approach one can combine conventional DFT with a "scissor" operator which incorporates self-interaction corrections and polarization effects in the organic energy levels. Hybrid potentials combined with conventional DFT represent, probably, the best alternative for having a simple and accurate approach for analyzing organic interfaces. The problem then is to find an appropriate one for both the metal and the organic material in a plane-wave formulation; we show, however, how to overcome this difficulty using a local-orbital basis formulation. As examples of these alternatives, we present some DFT-calculations for several organic interfaces, using either the scissor operator or a hybrid potential, which can be interpreted in terms of the unified IDIS-model.
Physical Chemistry Chemical Physics 10/2009; 11(39):8658-75. · 3.57 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We present an experimental and theoretical study of the interaction between a tip and single-walled carbon nanotubes (SWCNT) lying on a SiO2 surface. Adhesion and jump-to-contact forces (JC) are measured in a high vacuum system for SWCNT with diameters (D) ranging from 1.4 to 4.5 nm; we find adhesion forces approximately 3.2 nN and JC forces approximately 2.4 nN. Simulations yield adhesion forces in agreement with experiment showing that for D approximately 1.4 nm half of this force is due to tip-SWCNT and tip-SiO2 van der Waals interactions.
Physical Review Letters 04/2009; 102(10):106801. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The ground state properties of indium atom chains on the Si(111) 8 x 2-In surface and the nature of their insulator-metal (IM) transition near 120 K are under intense dispute. We compare experimental scanning tunneling microscopy (STM) images of the low temperature (LT) 8 x 2 phase with STM image calculations from Density Functional Theory (DFT). Our LT studies clearly indicate the existence of a frozen shear distortion between neighboring atom chains, resulting in the formation of indium hexagons. Tunneling spectra furthermore indicate that the IM transition coincides with the collapse of a approximately 0.3 eV surface-state band gap at the Gamma point of the 4 x 2 Brillouin zone. This implies that the IM transition is driven by a shear phonon, not by Fermi surface nesting.
Physical Review Letters 04/2009; 102(11):115501. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A local orbital DFT-approach combined with a “scissor”-operator is used to obtain the Charge Neutrality Level and the screening
parameter in the benzene/Au(111) and C60/Au(111) interfaces. The “pillow” dipole and interface Fermi level are also calculated. The total dipole induced across the
interface is compared with the experimental evidences: while the agreement for C60/Au(111) is excellent, for benzene/Au(111), some discrepancies appear that are discussed in the light of other models.
Applied Physics A 01/2009; 95(1):119-124. · 1.63 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: A new embedding method to include local correlation in large systems is proposed. In this method the density of the whole system, calculated via density functional theory approaches, is partitioned in two pieces, one corresponding to the subsystem of interest and the rest to the environment. In the second step, an embedding potential is obtained iteratively using as a driving force the self-repulsion due to the density difference, in a similar form as proposed by Zhao et al. [Phys. Rev. A 50, 2138 (1994)], to obtain the "exact" exchange-correlation functional. Such potential is added to the Fock equation to build the localized molecular orbitals which are further used to include the local electronic correlation in the subsystem of interest. This method is an alternative to the previous DFT-based embedding methods first proposed by Wesolowski and Washell [J. Phys. Chem. 97, 8050 (1993)] and after enhanced by Govind et al. [J. Chem. Phys. 110, 7677 (1999)] and adapted to metal extended systems, which use density functionals to describe the kinetic energy contribution to the embedding potential, whose precise form has been largely treated in the literature and its crucial role is discussed here. The method is applied to hydrogen chains and its van der Waals interaction with H(2). The results obtained are in very good agreement with exact calculations performed on the whole system, which demonstrates that the method proposed is a very promising route to introduce correlation in large systems.
The Journal of chemical physics 12/2008; 129(18):184104. · 3.09 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Band alignment at metal/organic (MO) and organic/organic (OO) interfaces is discussed within a unified Induced Density of
Interface States (IDIS) model, which incorporates most of the effects that can be expected to operate at weakly interacting
organic interfaces: compression of the metal electron tails due to Pauli repulsion, orientation of molecular dipoles and electron
charge transfer between the two media. This last mechanism tends to align the Charge Neutrality Level (CNL) of the organic
material and the metal Fermi level (EF): electron charge transfer reduces the initial misalignment between the CNL and the metal work function (πM-CNL) to S(πM-CNL), where S is the interface screening parameter which is shown to also screen the ‘Pauli’ and molecular interface dipoles.
Results for several Au/organic and organic/organic interfaces are presented and discussed. PACS numbers: 79.60.Jv, 79.60.Dp,
73.40.Gk, 73.20.-r
11/2008: pages 207-218;
-
[show abstract]
[hide abstract]
ABSTRACT: We analyze theoretically the formation of stretched Pd-nanowires and their interaction with hydrogen. In our approach, we simulate the nanowire stretching process using a first-principles molecular-dynamics method to obtain realistic atomic geometries of the contact in its final stages before the nanowire breaks. The electrical conductance of the nanowire is also calculated at each point of the deformation path. For the clean Pd-nanowire in the last stages of the deformation process we find that the nanowire develops, first, a one-atom-neck and, at the end, a dimer whose bond is finally broken. For these atomic configurations, the calculated electrical conductances are in good agreement with the experimental evidence. The interaction with hydrogen is analyzed adsorbing one or two H atoms on the Pd-nanowire for different configurations along the stretching process. In the case of one H atom we obtain geometries with conductances in the range 0.8-1.4G(0), while for two H atoms we find conductance plateaus with values ∼0.5G(0) and ∼1.0G(0). These results are in excellent agreement with the experimental evidence for nanocontact breaking in an H(2) atmosphere and indicate that the conductance peak around 0.5G(0) observed experimentally is associated with nanowires where two H atoms have been adsorbed.
Nanotechnology 08/2008; 19(33):335711. · 3.98 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The energy level alignment at the metal-organic and organic-organic interfaces of the Cu(100)/benzenethiolate/pentacene heterostructure is studied by photoemission spectroscopy and discussed theoretically using a model that includes, in a consistent way, charge transfer, Pauli repulsion, intrinsic molecular dipoles, and interface screening as a function of coverage. Despite the different nature of the two interfaces, our model provides a unified explanation for the work-function changes at both junctions and enables us to determine the benzenethiolate orientation as a function of coverage.
Physical Review Letters 02/2008; 100(2):027601. · 7.37 Impact Factor
