Organic molecules at metal surfaces: the role of functional groups in self-assembly and charge transfer

2007/2008The understanding of the interaction of organic molecules with metal surfaces is crucial for tailoring the desired properties of future devices that can be employed for molecular electronics or biomedical applications. Self-assembly of complex supramolecular structures and charge transfer through molecular films or even through single molecules are some of the properties that have recently attracted much interest both for possible applications and for more fundamental studies. The molecule-surface interaction takes place thanks to the functional groups that constitute the molecule. The choice of appropriate functional groups of the molecules allows their use as building blocks in the fabrication of complicate architectures [1]. In fact, the functional entities can influence molecule-molecule and molecule-surface interactions, governing the self-assembly of the molecules on the surface. In particular, in the thesis I will report on the characterization by means of Helium Atom Scattering (HAS), X-ray Photoemission Spectroscopy (XPS), Near Edge X-ray Absorption Fine Structure (NEXAFS) and Scanning Tunneling Microscopy (STM) of the self-assembly in ultra high vacuum (UHV) conditions of L-methionine molecules on different metal substrates (Ag(111), Cu(111), Au(111), Au(110)). L-methionine is an amino acid with three functional groups which can interact with the substrate or with other molecules: the amino (-NH2), the carboxyl (-COOH) and the thioether (-S-). Moreover, the first two can change their charge state in a protonated amino group (-NH+ 3 ) and a deprotonated carboxyl group (-COO−): the molecules are called zwitterionic and it is allowed the formation of hydrogen bonds between them. Hydrogen bonding between zwitterionic molecules is responsible for the crystallization in the solid state. In the thesis I have studied how, depending on the choice of the substrate and the growth conditions, L-methionine molecules form assemblies with different morphologies and different chemical states of the building blocks. L-methionine molecules deposited on Ag(111) and Au(111) are in the zwitterionic state and interact strongly via hydrogen bonding forming dimers of molecules. The weak interaction with the substrate allows the organization of these dimers in extended bidimensional nanogratings composed of chains of length extending in the micrometer range and with tunable periodicity across the chains. At temperatures below 270K, L-methionine on Cu(111) forms short aggregates of zwitterionic dimers. By increasing the substrate temperature above 300K the charge state of the amino group changes and also the interaction with the surface. Molecules are anionic (-NH2 and -COO−) and form again charged nanogratings. The anionic state of the molecules can also be obtained on the Au(110) surface, where the interaction of the amino and thioether groups with the gold inhibits the formation of zwitterionic dimers via hydrogen bonding. The functional groups in the molecules can also influence their transport properties. The final goal of miniaturization in molecular electronics research is the formation and characterization of a nanoelectronic device in which a molecule between two electrodes plays the role of an active conducting element. In such a device the interaction between the functional groups anchoring the molecule to the electrodes and the electrodes is a crucial element in order to understand and control the conduction. Recent STM-break junction experiments [2] have shown that Au-molecule-Au contacts with amino (- NH2) terminated molecules are better defined than Au-molecule-Au contacts formed with thiol (-SH) terminated molecules [3]. The strong interaction of thiols with gold surfaces is well known in literature and the self-assembly of thiolated molecules is widely employed in many applications. In contrast, the weak interaction of amino terminated molecules with gold is poorly studied. Theoretical calculations suggest that the amine lone pair is responsible for bonding and that it prefers to bind to undercoordinated gold atoms. Within this framework, in the thesis I report on the study of growth of thin films of 1,4-benzenediamine and p-toluidine on two different Au surfaces, where the atoms present different coordination: Au(111) and Au(110). Both molecules interact more strongly with the low coordination (110) surface. By means of Resonant Photoemission Spectroscopy (RPES) it has been possible to disentangle molecular orbitals and determine the ones involved in the charge transfer at the surface. In both cases the charge transfer involves states localized also on the nitrogen atoms indicating a possible interaction of the molecule with the surface through nitrogen atoms. I also studied the assembly of three benzene substituted diamines on Au(111). These results complement very well the results obtained from conduction experiments of different amine-terminated molecules and combined with theoretical investigations can help understanding the basics of the molecular charge transport mechanism. [1] Barth J.V., Costantini G., Kern K., Nature, 437 (2005) 671 [2] Venkataraman L., Klare J. E., Nuckolls C., Hybertsen M. S., Steigerwald M. L., Nature, 442 (7105), 904 (2006) [3] Schreiber F., Progress in Surface Science, 65 (5-8) (2000) 151Lo studio dell’interazione di molecole organiche con superfici metalliche è di fondamentale importanza per la progettazione di futuri dispositivi che possiedano proprietà ben controllabili in modo tale che possano essere usati per l’elettronica molecolare o per applicazioni biomediche. L’autoassemblaggio di complesse strutture ”supramolecolari” e il trasferimento di carica attraverso film molecolari o anche attraverso singole molecole sono alcune delle proprietà che hanno attratto di recente grande interesse sia per le possibili applicazioni future che per studi di tipo più fondamentale. L’interazione molecola-superficie avviene attraverso i gruppi funzionali che costituiscono le molecole. Molecole con appropriate funzionalizzazioni possono essere usate come mattoni elementari nella fabbricazione di architetture complesse [1]. Infatti, tali gruppi funzionali possono influenzare le interazioni del tipo molecola-molecola e molecola-superficie che governano l’autoassemblaggio delle molecole sulla superficie. In particolare, in questa tesi riporter`o circa la caratterizzazione mediante diffrazione di atomi di elio (HAS), spettroscopia di fotoemissione di raggi X (XPS), misure di assorbimento di raggi X (NEXAFS) e microscopia ad effetto tunnel (STM) dell’autoassemblaggio in condizioni di ultra alto vuoto (UHV) di molecole di L-metionina su diversi substrati metallici (Ag(111), Cu(111), Au(111), Au(110)). La molecola di L-metionina `e un amminoacido che presenta tre gruppi funzionali i quali possono interagire con il substrato o con altre molecole: il gruppo amminico (-NH2), il gruppo carbossilico (- COOH) e il gruppo tioetere (-S-). I primi due possono inoltre cambiare il loro stato di carica originando un gruppo amminico protonato (-NH+ 3 ) e un gruppo carbossilico deprotonato (COO−): in tal caso le molecole sono dette zwitterioniche ed è permessa la formazione di legami a idrogeno tra esse. I legami a idrogeno tra molecole zwitterioniche sono responsabili della loro cristallizzazione nello stato solido. In questa tesi ho studiato come, a seconda della scelta del substrato e delle condizioni di cescita, le molecole di L-metionina formino strutture assemblate che presentano diverse morfologie e diversi stati chimici delle molecole costituenti. Le molecole di L-metionina depositate su Ag(111) e Au(111) sono zwitterioniche e interagiscono fortemente tra di loro tramite legami a idrogeno a formare dimeri di molecole sulla superficie. La debole interazione con il substrato permette l’organizzazione di questi dimeri in estesi reticoli bidimensionali di dimensione nanometrica composti da catene di lunghezza nel range micrometrico e con spaziatura tra le catene controllabile. A temperature sotto 270K, le molecole di L-metionina su Cu(111) formano corti aggregati di dimeri zwitterionici. Aumentando la temperatura del substrato oltre 300K lo stato di carica del gruppo amminico cambia e quindi l’interazione con la superficie. Le molecole sono anioniche (-NH2 e COO−) e formano di nuovo reticoli carichi. Lo stato anionico delle molecole si può ottenere anche sulla superficie di Au(110) dove l’interazione dei gruppi amminico e tioetere con l’oro inibisce la formazione di dimeri zwitterionici via legami a idrogeno. I gruppi funzionali nelle molecole possono anche influenzare le loro proprietà di trasporto. Lo scopo finale della miniaturizzazione nella ricerca nel campo dell’elettronica molecolare è la formazione e caratterizzazione di un dispositivo nanoelettronico in cui una molecola immobilizzata tra due elettrodi gioca il ruolo di elemento conduttivo attivo. In tale dispositivo il controllo dell’interazione tra i gruppi funzionali che tengono la molecola attaccata gli elettrodi e gli elettrodi è un elemento cruciale per la comprensione e il controllo della conduzione. Recenti esperimenti del tipo STM break junction [2] hanno motrato che contatti del tipo Au-molecola-Au con molecole con terminazioni amminiche (NH2) sono meglio definiti che contatti del medesimo tipo con molecole con terminazione tiolica (-SH) [3]. La forte interazione dei tioli con superfici d’oro è ben nota in letteratura e l’autoassemblaggio di molecole con terminazione tiolica è largamente utilizzato in molte applicazioni. In contrasto, la debole interazione di molecole con terminazione amminica con superfici d’oro è stata poco studiata. Recenti calcoli teorici hanno previsto che le molecole si legano alla superficie d’oro attraverso il ”lone pair” localizzato sull’azoto e che sono preferiti i legami con atomi di oro di bassa coordinazione. In particolare, nella tesi riporterò i risultati dello studio della crescita di film sottili di 1,4-benzenediamina e p-toluidina su due diverse superfici d’oro, i cui atomi di superficie presentano diversa coordinazione: Au(111) e Au(110). Ambedue le molecole interagiscono più fortemente con la superficie di bassa coordinazione (110). Tramite la tecnica di fotoemissione risonante (RPES) è stato possibile individuare gli orbitali molecolari e determinare quelli coinvolti nel trasferimento di carica all’interfaccia. In ambedue i casi il trasferimento di carica coinvolge stati che sono localizzati anche sull’atomo di azoto, il che indica una possibile interazione della molecola con la superficie attraverso i gruppi amminici. Ho anche studiato l’assemblaggio su Au(111) di tre diverse benzene-diamine con vii diversi sostituenti legati all’anello. Questi risultati sono un complemento ai risultati ottenuti da esperimenti di conduzione di molecole con diverse terminazioni amminiche e combinati con le investigazioni teoriche possono aiutare nella comprensione dei fondamenti dei meccanismi di trasporto di carica nelle molecole. [1] Barth J.V., Costantini G., Kern K., Nature, 437 (2005) 671 [2] Venkataraman L., Klare J. E., Nuckolls C., Hybertsen M. S., Steigerwald M. L., Nature, 442 (7105), 904 (2006) [3] Schreiber F., Progress in Surface Science, 65 (5-8) (2000) 151XXI Ciclo198

Broadening of the Derivative Discontinuity in Density Functional Theory

We clarify an important aspect of density functional theories, the broadening of the derivative discontinuity (DD) in a quantum system, with fluctuating particle number. Our focus is on a correlated model system, the single level quantum dot in the regime of the Coulomb blockade. We find that the DD-broadening is controlled by the small parameter $\Gamma/U$, where $\Gamma$ is the level broadening due to contacting and $U$ is a measure of the charging energy. Our analysis suggests, that Kondoesque fluctuations have a tendency to increase the DD-broadening, in our model by a factor of two.Comment: 4 pages, 2 figure

Basic pathophysiology and options of treatment for surgical management of functional tricuspid regurgitation: a systematic review

Background: Functional tricuspid regurgitation (TR) appears frequently in the presence of left-sided heart valve diseases, combined with symptoms of heart failure, worsens if left untreated, and is associated with poor patient survival. Correct indications for surgery and the choice of suitable technique, which should be based on pathophysiology of disease are of utmost importance to ensure longevity and durability of repair; particularly given the risky nature of reoperations due to residual/recurrent TR. Methods: A systematic review was performed using Embase, Ovid Medline, Cochrane, Web of Science, and Google to deepen knowledge of major and controversial aspects of the subject. Results: A total of 1,579 studies were reviewed, and 32 of these were enclosed in the final review: 13 studies were primarily focused on pathophysiology and preoperative assessment of functional TR; 19 studies on surgical treatment of functional TR. A total of 15,509 patients were included. Conclusions: Indications for treatment of TR are based on the severity of regurgitation (grading), as well as on the presence of signs and symtoms of right-sided heart failure and on the extent of tricuspid annular dilation, leaflet tethering, and pulmonary hypertension (staging of disease). Despite improved knowledge of the underlying pathophysiology of TR, issues regarding indications for treatment and options of repair remain present. There is no consensus within the scientific community, for the preferred method to quantify the severity of TR; the recently introduced 5-grade TR classification based on objective quantitative parameters has not yet become common practice. The assessment of TR during stress exercise is rarely performed, though it takes into account the changes in severity of regurgitation that occur under different physiological conditions. Magnetic resonance imaging, which is the gold standard for the right heart evaluation is occasionally carried out before surgery. The threshold beyond which the tricuspid annular dilation should be repaired is unclear and recent studies put forward the idea that it may be lower than current recommendations. Tricuspid valve annuloplasty is the most adopted surgical option today. However, the ideal annuloplasty device remains elusive. In addition, as severe leaflet tethering cannot be addressed by annuloplasty alone, the addition of new techniques further increasing leaflet coaptation might optimize long-term valve continence. Further investigations are needed to address all these issues, alongside the potential of percutaneous options

Ultrafast Ge-Te bond dynamics in a phase-change superlattice

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.M.M. acknowledges the support of the BACH beamline staff during the synchrotron experiments and Roberta Ciprian for insightful discussions. This work was supported by EU within FP7 project PASTRY [GA 317764]

Relating Energy Level Alignment and Amine-Linked Single Molecule Junction Conductance

Using photoemission spectroscopy, we determine the relationship between electronic energy level alignment at a metal-molecule interface and single-molecule junction transport data. We measure the position of the highest occupied molecular orbital (HOMO) relative to the Au metal Fermi level for three 1,4-benzenediamine derivatives on Au(111) and Au(110) with ultraviolet and resonant x-ray photoemission spectroscopy. We compare these results to scanning tunnelling microscope based break-junction measurements of single molecule conductance and to first-principles calculations. We find that the energy difference between the HOMO and Fermi level for the three molecules adsorbed on Au(111) correlate well with changes in conductance, and agree well with quasiparticle energies computed from first-principles calculations incorporating self-energy corrections. On the Au(110) which present Au atoms with lower-coordination, critical in break-junction conductance measurements, we see that the HOMO level shifts further from the Fermi level. These results provide the first direct comparison of spectroscopic energy level alignment measurements with single molecule junction transport data

Phase separation in the non-equilibrium Verwey transition in magnetite

We present equilibrium and out-of-equilibrium studies of the Verwey transition in magnetite. In the equilibrium optical conductivity, we find a step-like change at the phase transition for photon energies below about 2 eV. The possibility of triggering a non-equilibrium transient metallic state in insulating magnetite by photo excitation was recently demonstrated by an x-ray study. Here we report a full characterization of the optical properties in the visible frequency range across the non-equilibrium phase transition. Our analysis of the spectral features is based on a detailed description of the equilibrium properties. The out-of-equilibrium optical data bear the initial electronic response associated to localized photo-excitation, the occurrence of phase separation, and the transition to a transient metallic phase for excitation density larger than a critical value. This allows us to identify the electronic nature of the transient state, to unveil the phase transition dynamics, and to study the consequences of phase separation on the reflectivity, suggesting a spectroscopic feature that may be generally linked to out-of-equilibrium phase separation

Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy

We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100  fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes

Ultrafast adsorbate excitation probed with sub-ps resolution XAS

We use a pump-probe scheme to measure the time evolution of the C K-edge X-ray absorption spectrum (XAS) from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Due to the short duration of the X-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first ps after the pump can be resolved with unprecedented time resolution. By comparing with theoretical (DFT) spectrum calculations we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the ps regime. The ~100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e. g. electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to non-thermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.Comment: 16 pages, 16 figures. To be published in Physical Review Letters: https://journals.aps.org/prl/accepted/c1070Y74M8b18063d9cd0221b000631d50ef7a24

Atom-Specific Probing of Electron Dynamics in an Atomic Adsorbate by Time-Resolved X-ray Spectroscopy

The electronic excitation occurring on adsorbates at ultrafast time scales from optical lasers that initiate surface chemical reactions is still an open question. Here, we report the ultrafast temporal evolution of X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) of a simple well known adsorbate prototype system, namely carbon (C) atoms adsorbed on a nickel (Ni(100)) surface, following intense laser optical pumping at 400 nm. We observe ultrafast (~100 fs) changes in both XAS and XES showing clear signatures of the formation of a hot electron-hole pair distribution on the adsorbate. This is followed by slower changes on a few ps time scale, shown to be consistent with thermalization of the complete C/Ni system. Density functional theory spectrum simulations support this interpretation.Comment: 33 pages, 12 figures. Submitted to Physical Review Letter