Doctor of Philosophy / Department of Physics / Itzhak Ben-Itzhak / Patrick Richard / This work focuses on the ultrafast dynamics of electrons and phonons in graphitic materials. In particular, we experimentally investigated the factors which influence the transport properties of graphite and carbon nanotubes. In the first part of this dissertation, we used Time-resolved Two Photon photoemission (TR-TPP) spectroscopy to probe the dynamics of optically excited charge carriers above the Fermi energy of double-wall carbon nanotubes (DWNTs). In the second part of this study, time-resolved anti-Stokes Raman (ASR) spectroscopy is applied to investigating in real time the phonon-phonon interactions, and addressing the way the temperature affects the dynamics of single-wall carbon nanotubes (SWNTs) and graphite. With respect to the first part, we aim to deeply understand the dynamics of the charge carriers and electron-phonon interactions, in order to achieve an as complete as possible knowledge of DWNTs. We measured the energy transfer rate from the electronic system to the lattice, and we observed a strong non-linear increase with the temperature of the electrons. In addition, we determined the electron-phonon coupling parameter, and the mean-free path of the electrons. The TR-TPP technique enables us to measure the above quantities without any electrical contacts, with the advantage of reducing the errors introduced by the metallic electrodes. The second investigation uses time-resolved ASR spectroscopy to probe in real time the G-mode non-equilibrium phonon dynamics and the energy relaxation paths towards the lattice by variation of the temperature in SWNTs and graphite. The lifetime range of the optically excited phonons obtained is 1.23 ps to 0.70 ps in the lowest (cryogenic temperatures) and highest temperature limits, respectively. We have also observed an increase in the energy of the G-mode optical phonons in graphite with the transient temperature. The findings of this study are important since the non-equilibrium phonon population has been invoked to explain the negative differential conductance and current saturation in high biased transport phenomena.

Cette thèse présente une étude des propriétés électroniques de systèmes de basse dimension à base de métaux et de semiconducteurs. La première partie de l'étude traite le confinement de l'état de Shockley dans des nanostructures tridimensionnelles d'Ag(111), par des mesures STM/STS à très basse température (5 K). Nous avons d'abord analysé en détail la structure en énergie et la distribution spatiale des modes confinés. Nous avons ensuite mis à profit la nature discrète du spectre en énergie pour étudier le temps de vie des quasiparticules. Un comportement typique de liquide de Fermi est mis en évidence, et nous montrons que le mécanisme de diffusion dominant est associé au couplage électron-phonon. La contribution extrinsèque provenant du confinement partiel de l'onde électronique a également été obtenue. Une loi d'échelle est observée avec la taille des nanostructures, ce qui permet d'extraire un coefficient de réflexion plus important que dans de simples ilôts monoatomiques. La seconde partie de l'étude est consacrée aux couches ultra-minces semiconductrices obtenues par dépôts d'alcalins (K, Rb, Cs) sur la surface Si(111):B-[racine]3. Ce travail résout la controverse concernant la nature de l'état fondamental de ce système, et notamment l'origine de la reconstruction 2[racine]3 obtenue à la saturation du taux de couverture. La compréhension en amont de la structure cristallographique permet d'élucider les propriétés électroniques. Nous montrons qu'une approche à un électron, conduisant à un isolant de bandes, décrit le système de manière convaincante, malgré l'indication de forts effets polaroniques. Ce résultat est le fruit d'une étude approfondie combinant des techniques diverses et complémentaires (LEED, ARPES, XPS, STM/STS et calcul DFT) / This thesis is devoted to the electronic properties of low-dimensional systems based on metal and semiconducting materials. The first part deals with the Shockley state confinement in Ag(111) nanostructures, by means of very-low temperature (5 K) STM/STS measurements. We study the electronic structure and spatial distribution of the confined modes. Then the discrete nature of the electronic spectrum allows one to yield the quasiparticule lifetime. A Fermi-liquid behaviour is evidenced and we show that the dominant decay mechanism is attributed to the electron-phonon coupling. The extrinsic contribution arising from the partial confinement of the electronic wave is obtained as well. A scaling law with the nanostructure width is demonstrated, from which we deduce a higher reflection amplitude than in monoatomic islands. In the second part of the thesis, we study semiconducting ultra-thin films produced by alkali (K, Rb, Cs) deposition on the Si(111):B-[root of]3 surface. This work solves the controversy concerning the ground state of this system, and especially the nature of the 2[root of]3 surface recontruction obtained at saturation coverage. Prior understanding of the crystallographic structure allows to elucidate the electronic properties. We show that a one-electron picture, leading to a band insulator scenario, gives a good description of the system, in spite of strong polaronic effects. This conclusion results from an in-depth, combined study of complementary techniques (LEED, ARPES, XPS, STM/STS and DFT calculations).

Cette thèse vise à améliorer la méthode d'analyse du fond continu inélastique afin de l'appliquer à des cas qui présentent un intérêt technologique. En effet, ces améliorations sont cruciales car elles portent sur des critères de précision et de gain de temps, plus particulièrement pour l’étude de dispositifs présentant plusieurs couches profondément enterrées de matériaux bien distincts. Ainsi, l'analyse du fond continu inélastique associée à la spectroscopie de photoélectrons à rayons X durs (HAXPES) présente un grand intérêt car l’HAXPES permet de sonder plus profondément dans un échantillon qu'avec la spectroscopie de photoélectrons à rayons X classique (XPS). Ce présent travail porte sur des échantillons technologiquement pertinents, principalement des transistors à haute mobilité d'électrons (HEMTs), à certaines étapes cruciales de leur processus de fabrication, tels que des recuits. Il est donc très important que ces analyses soient effectuées de manière non destructive afin de préserver les interfaces enterrées. Ce sont souvent l'emplacement de phénomènes complexes qui sont critiques pour les performances du dispositif et une meilleure compréhension est une condition préalable à l’amélioration des dispositifs. Dans ce travail, les phénomènes de diffusion en profondeur sont étudiés grâce à l’analyse du fond continu inélastique associée à l’HAXPES (en utilisant le logiciel QUASES) pour des profondeurs allant jusqu'à 60 nm. Les résultats de distribution en profondeur présentent des écarts par rapport aux mesures TEM inférieures à 5%. Le choix des paramètres d'entrée de la méthode est discuté pour une large gamme d'échantillons et des règles simples en sont issues qui rendent l'analyse réelle plus facile et plus rapide à effectuer. Enfin, il a été montré que la spectromicroscopie faite avec la technique HAXPEEM peut fournir des spectres à chaque pixel utilisables pour l’analyse du fond continu inélastique. Cela peut fournir une cartographie 3D de la distribution en profondeur des éléments de manière non-destructive. / This thesis aims at improving the inelastic background analysis method in order to apply it to technologically relevant samples. Actually, these improvements are utterly needed as they concern criteria of accuracy and time saving particularly for analysis of devices presenting deeply buried layers with different materials. For this purpose, the interest of the inelastic background analysis method is at its best when combined with hard X-ray photoelectron spectroscopy (HAXPES) because HAXPES allows to probe deeper in the sample than with conventional X-ray photoelectron spectroscopy (XPS). The present work deals with technologically relevant samples, mainly the high-electron mobility transistor (HEMT), at some crucial steps of their fabrication process as annealing. Actually, it is very important that these analyses shall be performed non-destructively in order to preserve the buried interfaces. These are often the location of complex phenomena that are critical for device performances and a better understanding is often a prerequisite for any improvement. In this thesis, the in-depth diffusion phenomena are studied with the inelastic background analysis technique (using the QUASES software) combined with HAXPES for depth up to 60 nm. The depth distribution results are determined with deviations from TEM measurements smaller than a typical value of 5%. The choice of the input parameters of the method is discussed over a large range of samples and simple rules are derived which make the actual analysis easier and faster to perform. Finally, it was shown that spectromicroscopy obtained with the HAXPEEM technique can provide spectra at each pixel usable for inelastic background analysis. This is a proof of principle that it can provide a 3D mapping of the elemental depth distribution with a nondestructive method. / Denne afhandling har til formål at forbedre den uelastiske baggrundsanalysemetode til anvendelser i den til teknologiske industri. Faktisk er disse forbedringer absolut nødvendige, for at opnå nøjagtighed og tidsbesparelse, især for analyse af prøver med dybt begravede lag af forskellige materialer. Til det formål er interessen for den uelastiske baggrundsanalysemetode bedst i kombination med hård røntgenfotoelektron-spektroskopi (HAXPES), fordi HAXPES gør det muligt at probe dybere i prøven end med konventionel røntgenfotoelektron-spektroskopi (XPS). Dette arbejde beskæftiger sig med teknologisk relevante prøver, hovedsagelig høj-elektron mobilitetstransistor (HEMT), på nogle afgørende trin i deres fremstillingsproces som fx annealing. Faktisk er det meget vigtigt, at disse analyser udføres på en ikke-destruktiv måde for at bevare de begravede grænseflader. Det er ofte her de komplekse fysiske fænomener opstår, som er kritiske for fuktionaliteten, og en bedre forståelse af grænsefladerne er ofte en forudsætning for at kunne forbedre denne. I denne afhandling studeres de dybdegående diffusionsfænomener med den uelastiske baggrundsanalyse teknik (ved hjælp af QUASES software) kombineret med HAXPES for dybder op til 60 nm. Dybdestributionsresultaterne har afvigelser fra TEM-målinger mindre end en typisk værdi på 5%. Valget af input parametre for metoden er diskuteret på bagground af et stort udvalg af prøver samt omfattende simuleringer og enkle regler er udledt, hvilket gør den praktiske analyse nemmere og hurtigere at udføre. Endelig blev det vist, at spektromikroskopi opnået med HAXPEEM-teknikken kan tilvejebringe spektre ved hver enkelt pixel som kan anvendes til uelastisk baggrundsanalyse. Dette viser at i princippet kan en 3D-billeddannelse af den elementære dybdefordeling bestemmes ikke destruktivt.

Cette étude porte sur des oxydes TiO2, SrTiO3 et SrZrO3 déficients en oxygène ou dopés par des ions de métaux de transition. Nous avons préparé des échantillons sous forme de polycristaux, de monocristaux et de films minces. Leurs propriétés structurelles, physiques et électroniques ont été mesurées à l’aide de techniques sensibles aux volumes (diffraction des rayons X, magnétométrie SQUID, résonance paramagnétique électronique) ou sensibles aux surfaces (spectroscopie de photoémission, spectroscopie d’absorption X). Les mesures de RPE et au SQUID permettent non seulement d’obtenir leurs propriétés magnétiques mais également la valence des ions Cr dopant. Nous avons ainsi pu établir les paramètres clés qui contrôlent la valence des ions chrome lors de la synthèse. Des phases secondaires telles que SrCrO4 peuvent se former quand les échantillons sont synthétisés dans des atmosphères riches en oxygène. Les propriétés de films SrZrO3 dopés au chrome sont également discutées. Leurs conditions de préparation influencent non seulement le comportement des ions chrome mais également celui de la commutation de résistivité. Ce dernier semble dépendre de la chimie de surface des films. L’accumulation d’ions Cr3+ au voisinage de la surface fournit une interface propre exempte d’oxydes non stœchiométriques. Cette terminaison nette de l’interface a pour résultat de bonnes performances de la commutation de résistivité. / In this study, the oxygen-deficient TiO2, SrTiO3 systems and transition metal ion (Cr or V) doped TiO2, SrTiO3 and SrZrO3 systems have been investigated. We prepared samples as polycrystals, single crystals and thin films for various desires. Their structural, physical and electronic properties were measured by bulk-sensitive techniques (X-Ray Diffraction, SQUID and Electro Paramagnetic Resonance) or surface-sensitive techniques (Photoemission spectroscopy and X-ray absorption spectroscopy). The measurement of SQUID and EPR showed not only their magnetic properties but also the valence state of Cr dopant. We verified the valence state of Cr ions in oxides and found the key parameters of sample synthesis which control the valence state of Cr ions. Segregated phases such as SrCrO4 were formed when the samples were synthesized under O2 rich environment. The surface properties of Cr doped SrZrO3 films are also discussed. We found the synthesis conditions which influence on not only the behavior of Cr ions but also the resistive-switching behaviors. Various resistive-switching behaviors seem to depend on the surface chemistry of films. We found that the accumulation of Cr3+ on film surface provides a clean interface without any non-stoichiometric oxides and that this sharp interface termination results in a good performance of resistive-switching.

Cette thèse présente une étude des interfaces métal de transition ferromagnétique/oxyde et leur importance dans les mécanismes de transport tunnel polarisé en spin. Le travail a porté sur des bicouches ultraminces NiMnSb/MgO(001), Fe/ MgO(001), Co/ MgO(001) et Mn/ MgO(001) élaborées par épitaxie par jets moléculaires. L'originalité de ce travail réside dans l'étude approfondie des propriétés électroniques du matériau magnétique en contact avec la barrière d'oxyde : hybridation, polarisation et magnétisme à l'interface ont été étudiés en utilisant les techniques de laboratoire ainsi que le rayonnement synchrotron. Parallèlement, l'étude des effets magnétorésistifs dans des jonctions tunnel totalement épitaxiées Fe/MgO/Fe(001) a apporté des preuves du filtrage par la barrière MgO des ondes de Bloch en fonction de leur symétrie. Enfin, nous avons mis en évidence l'influence de la qualité structurale et chimique de l'interface Fe/MgO et des électrodes sur ces mécanismes.

In this work, multi-band (multi-color) detector structures considering different semiconductor device concepts and architectures are presented. Results on detectors operating in ultraviolet-to-infrared regions (UV-to-IR) are discussed. Multi-band detectors are based on quantum dot (QD) structures; which include quantum-dots-in-a-well (DWELL), tunneling quantum dot infrared photodetectors (T-QDIPs), and bi-layer quantum dot infrared photodetectors (Bi-QDIPs); and homo-/heterojunction interfacial workfunction internal photoemission (HIWIP/HEIWIP) structures. QD-based detectors show multi-color characteristics in mid- and far-infrared (MIR/FIR) regions, where as HIWIP/HEIWIP detectors show responses in UV or near-infrared (NIR) regions, and MIR-to-FIR regions. In DWELL structures, InAs QDs are placed in an InGaAs/GaAs quantum well (QW) to introduce photon induced electronic transitions from energy states in the QD to that in QW, leading to multi-color response peaks. One of the DWELL detectors shows response peaks at ∼ 6.25, ∼ 10.5 and ∼ 23.3 µm. In T-QDIP structures, photoexcited carriers are selectively collected from InGaAs QDs through resonant tunneling, while the dark current is blocked using AlGaAs/InGaAsAlGaAs/ blocking barriers placed in the structure. A two-color T-QDIP with photoresponse peaks at 6 and 17 µm operating at room temperature and a 6 THz detector operating at 150 K are presented. Bi-QDIPs consist of two layers of InAs QDs with different QD sizes. The detector exhibits three distinct peaks at 5.6, 8.0, and 23.0 µm. A typical HIWIP/HEIWIP detector structure consists of a single (or series of) doped emitter(s) and undoped barrier(s), which are placed between two highly doped contact layers. The dual-band response arises from interband transitions of carriers in the undoped barrier and intraband transitions in the doped emitter. Two HIWIP detectors, p-GaAs/GaAs and p-Si/Si, showing interband responses with wavelength thresholds at 0.82 and 1.05 µm, and intraband responses with zero response thresholds at 70 and 32 µm, respectively, are presented. HEIWIP detectors based on n-GaN/AlGaN show an interband response in the UV region and intraband response in the 2-14 µm region. A GaN/AlGaN detector structure consisting of three electrical contacts for separate UV and IR active regions is proposed for simultaneous measurements of the two components of the photocurrent generated by UV and IR radiation.

Ever since the discovery of exotic phenomena like high temperature (Tc) superconductivity in the cuprates and colossal magnetoresistance in the manganites, strongly correlated electron systems have become the center of attention in the field of condensed matter physics research. This renewed interest has been further kindled by the rapid development of sophisticated experimental techniques and tremendous computational power. Computation plays a pivotal role in the theoretical investigation of these systems, because one cannot explain their complicated phase diagrams by simple, exactly solvable models. Among the plethora of experimental techniques, various kinds of high energy electron spectroscopies are fast gaining importance due to the multitude of physical properties and phenomena which they can access. However the physical processes involved and the interpretation of the spectra obtained from these spectroscopies are extremely complex and require extensive theoretical modelling. This thesis is concerned with the theoretical modelling of a certain class of high energy electron spectroscopies, viz. the core-level electron spectroscopies, for strongly correlated systems of various kinds. The spectroscopies covered are Auger electron spectroscopy (AES), core-level photoemission spectroscopy (core-level PES) and X-ray absorption spec- troscopy (XAS), which provide non-magnetic information, and also X-ray magnetic circular and linear dichroism (XMCD and XMLD), which provide magnetic information. .

In this work systematic studies of the organic semiconductor CuPc have been presented. In general the investigation can be devided in three parts. In the first one we have studied the electronic structure of clean CuPc thin film. The next two parts are devoted to organic-organic and metal–organic interface formation, where one of the interface components is CuPc thin film. The main results of this thesis are: - The electronic structure of the pristine organic semiconductor CuPc (valence band and empty states) has been obtained by a combination of conventional and resonant photoemission, near-edge X-ray absorption, as well as by theoretical ab initio quantum-chemical calculations. A qualitative assignment of different VB structures has been given, or in other words the contributions of different atomic species as well as sites of the CuPc molecule to the electronic DOS has been established. In particular, it was shown, that the HOMO is mainly comprised of the spectral weights from the orbitals of carbon pyrolle atoms. Additional contributions to the HOMO stems from the benzene atoms. A combined experimental and theoretical study of the unoccupied electronic density of states of CuPc was presented. Our study allows identifying the contributions from different parts of the molecule to the unoccupied DOS and the measured spectra, which lays grounds for future studies of the evolution of the CuPc electronic states upon e.g. functionalization or doping. Application of similar studies to other organic semiconductors will also provide significant insight into their unoccupied electronic states. - The electronic properties of the organic heterointerfaces between fullerite and pristine copper phthalocyanine were studied. Both interfaces, CuPc/C60 and C60/CuPc, were found to be non-reactive with pronounced shifts of the vacuum level pointing to the formation of an interfacial dipole mainly at the CuPc side of the heterojunctions. The dipole values are close to the difference of the work functions of the two materials. Important interface parameters and hole-injection barriers were obtained. The sequence of deposition does not influence the electronic properties of the interfaces. - CuPc doped with potassium was studied by means of photoemission and photoabsorption spectroscopy. A detailed analysis of the core-level PE spectra allows one to propose possible lattice sites, which harbor the potassium ions. Contrasting to a few results reported in the literature, the films prepared in this thesis showed no finite electronic density of states at the Fermi level. - Two stages of the In/CuPc interface formation have been distinguished. The low-coverage stage is characterized by a strong diffusion of the In atoms into the organic film. Metal ions occupy sites close to the pyrolle nitrogen and strongly interact with molecules transferring negative charge to CuPc. Indium diffusion into the organic films saturates at a stoichiometry of In2CuPc. Subsequently, in the second stage the formation of a metallic indium film occurs on the top of the In2CuPc film. - Upon deposition on CuPc film Sn and Ag atoms do not diffuse into the organic film forming metallic clusters and/or thin metallic overlayer. Sharp metal-organic film interface is formed, in contrast to indium and potassium deposition. Presented experimental results also give evidence for absence of noticeable chemical reaction of Sn and Ag with CuPc thin film. - The systematic investigation of interface formation between CuPc thin film and various metals gives us the possibility to summarize all results with demonstrating similarities and differences for all systems studied.

Au cours de cette thèse, nous nous sommes intéressés à l'effet Kondo moléculaire au sein de réseaux supramoléculaires bidimensionnels en contact avec des surfaces métalliques monocristallines. Les techniques mises en oeuvre sont la diffraction d'électrons lents (LEED) et la microscopie à effet tunnel (STM) pour l'étude de la croissance ainsi que la spectroscopie à effet tunnel (STS) et la photoémission pour l'étude des propriétés électroniques. Les molécules employées dans le cadre de ce travail sont composées de macrocycles de phtalocyanine et tétraphénylporphyrine. Elles ont été choisies sur la base de l'orbitale contenant le spin moléculaire. Dans un premier cas, une phtalocyanine non métallée 2HPc a été déposée sur une surface monocristalline d'Ag(111). Un transfert de charge du métal vers la molécule conduit à l'apparition d'un effet Kondo à base température observé par STS sur des molécules individuelles ainsi que sur des auto-assemblages. Dans le deuxième cas, nous avons choisi un composé hôte à double plateaux contenant un ion cérium de type 4f. Nous avons mis en évidence l'influence du substrat sur l'effet Kondo. En effet, tandis que nous observons par STS, l'apparition d'une résonance Kondo lorsque la molécule est auto-assemblée sur Ag(111), aucun effet n'est observé sur Cu(111). Les résultats sont discutés en terme d'interactions molécule-molécule et molécule-substrat / In this work, we investigated molecular Kondo effect in two-dimensional supramolecular lattices adsorbed on metallic single crystalline surfaces by means of low-energy electron diffraction (LEED), scanning tunneling microscopy/spectroscopy (STM/STS) and photoemission (PES). The molecular compounds used in this PhD thesis have been chosen in regards to their spin orbital. The first molecule under investigation is a metal free phthalocyanine adsorbed on Ag(111). A Kondo effect has been evidenced by STS for single molecules as well as for self-assembly up to one monolayer coverage at low temperature. In that case, the spin originates from charge transfer from the Ag surface to the molecule. The second molecular compound is a double-decker molecule hosting a cerium 4f-ion. In that case, a Kondo resonance is evidenced by STS when it is self-assembled on Ag(111) whereas it is absent on Cu(111). These results are discussed in terms of intermolecular and molecule-metal interactions

Dans ce travail nous démontrons, au travers de deux études, l'intérêt fondamental du couplage des techniques de photoémission résolue en angle (ARPES) et de spectroscopie tunnel (STS) dans l'analyse des propriétés électroniques d'interfaces nanostructurées. Dans la première partie, nous présentons une méthodologie permettant de déduire le potentiel de surface induit par la reconstruction triangulaire d'une monocouche d'Ag/Cu(111). Cette méthode est basée sur la mesure des gaps caractérisant la structure de bande de l'état de Shockley du système aux points de haute symétrie de la zone de Brillouin. L'évaporation d'adatomes de potassium permet d'augmenter le nombre de gaps accessibles en photoémission en décalant les bandes vers les états occupés. Dans un modèle d'électrons presque libres, leur amplitude nous donne accès aux premières composantes de Fourier du potentiel. La reconstruction de ce dernier dans l'espace direct nous permet ensuite de calculer la densité d'états locale que nous comparons aux mesures de conductance STS. La seconde partie est consacrée à l'étude de la croissance et des propriétés électroniques des molécules de 1,4-dibromobenzène (DBB) et 1,4-diiodobenzène (DIB) évaporées sur Cu(110). Leur dépôt à température ambiante sur la surface entraîne la déshalogénation des molécules et la formation de phases organométalliques. A 200°C, le système polymérise pour former des chaînes unidimensionnelles de poly(p-phénylène) parfaitement alignées. Les mesures ARPES révèlent l'existence d'une bande pi unidimensionnelle d'états HOMOs dispersant sous le niveau de Fermi. En STS, nous observons également, pour des petites chaînes, le confinement des états LUMOs dans la partie inoccupée du spectre. Le déconfinement de ces états pour les grandes chaînes conduit à la formation d'une bande continue croisant le niveau de Fermi, conférant au polymère un caractère métallique 1D. Le gap HOMO-LUMO est alors mesuré à 1.15 eV / In this work, through two different studies, we demonstrate the fundamental interest in the coupling of angle resolved photoemission (ARPES) and scanning tunneling spectroscopy (STS) to investigate the electronic properties of nanostructured interfaces. In the first part we present a methodology to determine the surface potential of the triangular reconstructed one monolayer of Ag/Cu(111) interface from ARPES. This method is based on the measurement of the Shockley state band structure’s gaps at the high symmetry points of the Brillouin zone. Deposition of potassium adatoms allows us to shift the surface state towards higher binding energies in order to increase the number of accessible gaps in photoemission. From the magnitude of these gaps we deduce the two first Fourier components of the potential felt by electrons using the nearly free electron model. Then we reconstruct it and calculate the local density of states in order to compare it with the conductance maps probed by STS. In the second part we report the study of the growth and the electronic properties of the two molecules 1,4-dibromobenzene (DBB) and 1,4-diiodobenzene (DIB) evaporated on Cu(110). For room temperature deposition, we first observe their deshalogenation and the formation of an intermediate organometallic phase. Then, above 200°C, the system polymerizes into a long-range ordered array of one dimensional poly(p-phenylene) polymer. ARPES intensity maps allowed us to identify a one dimensional graphene-like strongly dispersive pi-band below the Fermi energy. By STS we also observed LUMOs confined states for small chains over the Fermi level. The loss of confinement for long chains induces the formation of a continuous dispersive band which crosses the Fermi energy, conferring a 1D metallic character to the polymer. The HOMO-LUMO gap is found to be 1.15 eV