184 research outputs found
Temperature-Triggered Sequential On-Surface Synthesis of One and Two Covalently Bonded Porous Organic Nanoarchitectures on Au(111)
No full textInternational audienceSubtle variations of surface temperature can drastically influence the on-surface synthesis of two-dimensional covalent graphene nanoarchitectures. The structure of the engineered nanoarchitectures not only results from the temperature-activation of the catalytic process, but it is also governed by the temperature-dependent geometry of intermolecular assembly. The sequential engineering of porous organic nanoarchitectures based on the covalent Ullmann coupling of star-shaped 1,3,5-tris(3,5-dibromophenyl)benzene molecules on Au(111) in vacuum is investigated using scanning tunneling microscopy and X-ray photoemission spectroscopy. This molecule can form one-covalent-bond or two-covalent-bonds with neighboring molecules. At room temperature, the molecules self-assemble into a porous halogen-bonded network stabilized by two types of X 3 synthons. One-covalent-bond dimers appear on the surface after annealing at 145 °C. One-covalent-bond chains are created after annealing at 170 °C. Most of the molecules are bonded to two neighbors. One-covalent-bond hexagons as well as two-covalent-bond dimers are appearing on the surface after annealing at 175 °C. Annealing at 275 °C leads to the formation of a porous 2D hexagonal two-covalent-bond nanoarchitecture. STM images show that the number of intermolecular covalent bonds as well as the number of covalently bonded molecular neighbors increases as the temperature rises. Core level spectroscopy shows that the molecules are fully dehalogenated after annealing at 260 °C. These observations show that dibromophenyl-based molecules are promising organic compounds to hierarchically and selectively engineer covalent porous graphene nanoarchitectures having different structures
X synthon geometries in two-dimensional halogen-bonded 1,3,5-tris(3,5-dibromophenyl)benzene self-assembled nanoarchitectures on Au(111)-(22 x )
No full textInternational audienceSelf-assembly of star-shaped 1,3,5-tris(3,5-dibromophenyl)benzene molecules on Au(111)-(22 x ) in a vacuum is investigated using scanning tunneling microscopy and core-level spectroscopy. Scanning tunneling microscopy shows that the molecules self-assemble into a hexagonal porous halogen-bonded nanoarchitecture. This structure is stabilized by X synthons composed of three type-II halogen-interactions (halogen-bonds). The molecules are oriented along the same direction in this arrangement. Domain boundaries are observed in the hcp region of the herringbone gold surface reconstruction. Molecules of the neighboring domains are rotated by 180°. The domain boundaries are stabilized by the formation of X synthons composed of two type-II and one type-I halogen-interactions between molecules of the neighboring domains. Core-level spectroscopy confirms the existence of two types of halogen-interactions in the organic layer. These observations show that the gold surface reconstructions can be exploited to modify the long-range supramolecular halogen-bonded self-assemblie
High resolution and time resolved photoemission spectroscopy for developing more efficient materials to reduce energy consumption and increase renewable energy production
No full textDue to the increase of energy consumption and the resulting ecological challenge, a collective awareness leads to the development of renewable energies and more efficient materials to increase the green energy production. Development of efficient photovoltaic materials is very closely related to their chemical and electronic properties. A better knowledge of these imbricated properties is needed, in addition to a better comprehension of their interplay with charge transport mechanisms. Exciton creation and recombination processes, charge transfer and charge collection processes take place at the surface and interface of the photoactive materials. Photoemission spectroscopy as chemical specific and surface sensitive spectroscopic technique is a method of choice on the study of physical phenomena at the origin of photoconversion efficiency. Time resolved photoemission spectroscopy has been recently renewed interest covering time scale from fs to more than seconds. It permits to probe the dynamics of relaxation of photoexcited charges and determine their lifetime. It finds application in various materials used in solar photovoltaics. In this paper, we define the physical and chemical properties determined by the combination of high resolution and time resolved photoemission spectroscopy. We show examples dealing with the development of renewable energy and energy consumption reduction in agreement with the current ecological trend for a better future
Internal Structure of InP/ZnS Nanocrystals Unraveled by High-Resolution Soft X-ray Photoelectron Spectroscopy
No full textInternational audienc
β-Diketonato Complex Film on a Graphite Surface
No full textInternational audienceThe structural and magnetic properties of a drop-cast film of flat C54H34Br4CuO4, a β-diketonato complex functionalized with bromine atoms, on a graphite surface are investigated using scanning tunneling microscopy, synchrotron X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. Experimental measurements reveal that the Cu-complexes preferentially lay flat on the graphite surface. The magnetic hysteresis loops show that the organic thin film remains paramagnetic at 2 K with an easy axis of magnetization perpendicular to the graphite surface and is therefore perpendicular to the plane of the Cu-complex skeleton
Evidence of Mixed-Valence Hydrated Europium-Chloride Phase in Vacuum by Means of Optical and Electronic Spectroscopies
No full textInternational audienceStudying the optical and electronic properties of commercial hydrated EuCl 3 salts, we took advantage of the manifestation of the unfilled 4f valence shell in photo-luminescence and core-level photoemission spectroscopies to unambiguously identify contributions from both Eu 3+ and Eu 2+ configurations. Lanthanide ion salt exhibits sharp spectral lines in good agreement with Eu 3+ oxidized state. Surprisingly, the photoemission spectrum of europium 4d core level presents a singular asymmetric shape component attributed to Eu 2+ species, thus revealing a significant amount of divalent europium. X-ray absorption spectroscopy confirmed the presence of Eu 2+ species in proportions that depend on the origin of the salt and on the applied material processing
Do syllables play a role in German speech perception? Behavioral and electrophysiological data from primed lexical decision.
No full textCopyright © 2015 Bien, Bölte and Zwitserlood. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The
use, distribution or reproduction in other forums is permitted, provided the original
author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.We investigated the role of the syllable during speech processing in German, in an auditory-auditory fragment priming study with lexical decision and simultaneous EEG registration. Spoken fragment primes either shared segments (related) with the spoken targets or not (unrelated), and this segmental overlap either corresponded to the first syllable of the target (e.g., /teis/ - /teisti/), or not (e.g., /teis/ - /teistləs/). Similar prime conditions applied for word and pseudoword targets. Lexical decision latencies revealed facilitation due to related fragments that corresponded to the first syllable of the target (/teis/ - /teisti/). Despite segmental overlap, there were no positive effects for related fragments that mismatched the first syllable. No facilitation was observed for pseudowords. The EEG analyses showed a consistent effect of relatedness, independent of syllabic match, from 200 to 500 ms, including the P350 and N400 windows. Moreover, this held for words and pseudowords that differed however in the N400 window. The only specific effect of syllabic match for related prime-target pairs was observed in the time window from 200 to 300 ms. We discuss the nature and potential origin of these effects, and their relevance for speech processing and lexical access
Azobenzene as Light-Activable Carrier Density Switches in Nanocrystals
No full textInternational audienceControl of carrier density in colloidal quantum dots is a major challenge for their integration into optoelectronic devices. Several chemical methods have been proposed to reach this goal including: introduction of impurities, non-stoichiometric compounds, introduction of redox molecules as ligands and surface gating obtained by tuning the dipole associated with surface ligands. None of these techniques allows post synthesis tunability. Alternatively, optical pumping requires high excitation power which may heat and finally damage the sample. Here, we propose a new procedure based on grafting of azobenzenes (AZBs) on the nanocrystal surface. The AZBs have two conformations (cis and trans), which are associated with strongly different dipole moments. The transition from one conformation to the other can be activated using UV or visible light at low intensities (<100 mW.cm-2). Grafting the AZBs on the nanocrystal surface leads to a light-tunable surface dipole, which shifts the nanocrystal bands and lead to a tunable carrier density. We apply this method to p-type HgTe and degenerately n-doped HgSe nanocrystals. We demonstrate, thanks to transport measurements, a change of the carrier density corresponding to a band shift up to 40 meV
Multiphoton k -resolved photoemission from gold surface states with 800-nm femtosecond laser pulses
No full textInternational audienceWe measure direct multiphoton photoemission of the Au(111) surface state with 800-nm laser pulses. We observe the parabolic dispersion in the angular distribution of photoelectrons having absorbed between four and seven photons. The k dispersion we measure can be explained in terms of Shockley-state replicas, with a nascent hot electrons distribution at k above the Fermi level. Moderate laser power densities, of the order of 100 GW/cm 2 , resulted in large electron yields, indicating the importance of multiphoton excitations to define the electronic and magnetic properties of matter in the first hundred femtoseconds after laser excitation
A colloidal quantum dot infrared photodetector and its use for intraband detection
No full textInternational audienceWavefunction engineering using intraband transition is the most versatile strategy for the design of infrared devices. To date, this strategy is nevertheless limited to epitaxially grown semiconductors, which lead to prohibitive costs for many applications. Meanwhile, colloidal nanocrystals have gained a high level of maturity from a material perspective and now achieve a broad spectral tunability. Here, we demonstrate that the energy landscape of quantum well and quantum dot infrared photodetectors can be mimicked from a mixture of mercury selenide and mercury telluride nanocrystals. This metamaterial combines intraband absorption with enhanced transport properties (i.e. low dark current, fast time response and large thermal activation energy). We also integrate this material into a photodiode with the highest infrared detection performances reported for an intraband-based nanocrystal device. This work demonstrates that the concept of wavefunction engineering at the device scale can now be applied for the design of complex colloidal nanocrystal-based devices
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