Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: El acceso a la versión del editor puede requerir la suscripción del recurso Access to the published version may require subscription Tuning the conductance of single walled carbon nanotubes by ion irradiation in the Anderson localization regimeC. Gómez-Navarro 1 , P.J. Carbon nanotubes 1,2 are a good realization of one-dimensional crystals where basic science and potential nanodevice applications merge 3 . Defects are known to modify the electrical resistance of carbon nanotubes 4 . They can be present in asgrown carbon nanotubes, but controlling externally their density opens a path towards the tuning of the nanotube electronic characteristics. In this work consecutive Ar + irradiation doses are applied to single-walled nanotubes (SWNTs) producing a uniform density of defects. After each dose, the room temperature resistance versus SWNT-length [R(L)] along the nanotube is measured. Our data show an exponential dependence of R(L) indicating that the system is within the strong Anderson localization regime. Theoretical simulations demonstrate that mainly di-vacancies contribute to the resistance increase induced by irradiation and that just a 0.03% of di-vacancies produces an increase of three orders of magnitude in the resistance of a 400 nm SWNT length.The traditional approximation to reduce the size and enhance the performance of electronic devices may not be applicable in the near future 5 . Electronic circuits based on molecules have created great expectation for their new foresighted properties. For the case of electronic circuits based on carbon nanotubes 6 , the influence of disorder and defects 4,7 is of fundamental relevance in the performance of the device. In particular, the density of defects would determine the transport in nanotubes from a ballistic regime 8,9 to either weak or strong localization regimes. Quantum theory dictates that for a one dimensional conductor of length L 10,11 , with a given density of defects, localization effects emerge when the "phase coherence length" L φ is larger than the localization length L 0 . If L is not too large (for L about 3-10 L 0 ) and the inelastic interaction is weak, the wire resistance is controlled by the phase-coherent electron propagation 12 , falling into the strong localization regime in which the resistance increases exponentially with the length of the wire. This regime has not been observed in single-walled nanotubes in spite of the many evidences for weak localization diffusive regime and quantum interference in multiwalled carbon nanotubes 13 . By changing the density of defects, L 0 can be modified allowing to control the resistance of the one dimensional conductor.Induced defects have been already used to modify different properties of carbon nanotubes. Indeed, electron-beam has been used to create in-situ nanotube junctions 14 and to enhance the mechanical response of nanotubes bundles by creating stable links among the tubes 15...

The Sn͞Ge(111) interface has been investigated across the 3 3 3 ! p 3 3 p 3 R30 ± phase transition using core level and valence band photoemission spectroscopies. We find, both above and below the transition, two different components in the Sn 4d core level and a band splitting in the surface state crossing the Fermi energy. Theoretical calculations show that these two effects are due to the existence of two structurally different kinds of Sn atoms that fluctuate at room temperature between two positions and are stabilized in a 3 3 3 structure at low temperature. [S0031-9007(98)

We present a mechanism that explains the energy-level alignment at organic-organic ͑OO͒ semiconductor heterojunctions. Following our work on metal/organic interfaces, we extend the concepts of charge neutrality level ͑CNL͒ and induced density of interface states to OO interfaces, and propose that the energy-level alignment is driven by the alignment of the CNLs of the two organic semiconductors. The initial offset between the CNLs gives rise to a charge transfer across the interface, which induces an interface dipole and tends to align the CNLs. The initial CNL difference is reduced according to the screening factor S, a quantity related to the dielectric functions of the organic materials. Good quantitative agreement with experiment is found. Our model thus provides a simple and intuitive, yet general, explanation of the energy-level alignment at organic semiconductor heterojunctions.

Correlation effects in the transport properties of a single quantum level coupled to electron reservoirs are discussed theoretically using a non-equilibrium Green functions approach. Our method is based on the introduction of a second-order self-energy associated with the Coulomb interaction that consistently eliminates the pathologies found in previous perturbative calculations. We present results for the current-voltage characteristic illustrating the different correlation effects that may be found in this system, including the Kondo anomaly and Coulomb blockade. We finally discuss the experimental conditions for the simultaneous observation of these effects in an ultrasmall quantum dot.

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.

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.Electronic materials made of molecular films are a fast developing field, with many potential applications in organicbased devices. Designing new organic-based materials requires a detailed understanding of the different processes occurring in these devices. In particular, metal/organic and semiconductor/organic interface barriers play a decisive role [1,2]. However, the formation of barriers is not yet well understood.In the Schottky-Mott model of metal/organic interfaces, it is assumed that no interface dipole is formed at the junction, and that the position of molecular levels with respect to the metal Fermi level is defined by vacuum level alignment. This situation was disproved by Narioka et al.[3] who, using ultra-violet photoemission spectroscopy (UPS), found large interface dipoles (∼ 0.5 − 1.0eV ) at several metal/organic interfaces. Independent data by Hill et al.[4] confirmed this conclusion. Various mechanisms are believed to operate simultaneously at these interfaces, and several models have been advanced [1,2]. Metal-molecule chemical reaction has been seen to create interface gap states that pin the Fermi level [5], a situation that is analogous to that described by the Unified Defect Model proposed for inorganic semiconductor/metal interfaces [6]. Compression of the metal surface electronic tail by adsorbed molecules, leading to vacuum level interface shift (the "pillow" effect), has also been proposed as a general metal/organic interface mechanism [7,8,9].In this letter, we explore the first application to a metal/organic interface of the Induced Density of Interface (or virtual) States (IDIS) Model [10]. We study a metal/PTCDA (3, 4, 9, 10-perylenetetracarboxylic dianhydride) interface and analyze how the chemical interaction between the organic molecule and the metal creates an IDIS in the organic energy gap. Our calculations show that, although the chemical interaction is weak, the IDIS is large enough that a Charge Neutrality Level (CNL) of the organic molecule can be defined. Our results show that the interface Fermi level E F is pinned at the CNL, a situation similar to that described for the formation of Schottky barriers at conventional semiconductor/metal junctions.In this theoretical analysis, we study the metal/PTCDA interact...

A unified model, embodying the "pillow" effect and the induced density of interface states (IDIS) model, is presented for describing the level alignment at a metal/organic interface. The pillow effect, which originates from the orthogonalization of the metal and organic wave functions, is calculated using a many-body linear combination of atomic orbitals Hamiltonian, whereby electron long-range interactions are obtained using an expansion in the metal/organic wave function overlap, while the electronic charge of both materials remains unchanged. This approach yields the pillow dipole and represents the first effect induced by the metal/organic interaction, resulting in a reduction of the metal work function. In a second step, we consider how charge is transferred between the metal and the organic material by means of the IDIS model: Charge transfer is determined by the relative position of the metal work function (corrected by the pillow effect) and the organic charge neutrality level, as well as by an interface parameter S, which measures how this potential difference is screened. In our approach, we show that the combined IDIS-pillow effects can be described in terms of the original IDIS alignment corrected by a screened pillow dipole. For the organic materials considered in this paper, we see that the IDIS dipole already represents most of the realignment induced at the metal/organic interface. We therefore conclude that the pillow effect yields minor corrections to the IDIS model.

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