1,720,982 research outputs found
Validation of SERS enhancement factor measurements
No full textThe huge signal enhancement that motivates the rapid spreading of surface-enhanced Raman scattering (SERS) in a wide range of applications is commonly quantified by measuring the enhancement factor (EF). Concerning this important parameter, in this paper, we tackle 2 points: (a) We validate the use of a Macro Raman configuration with line focus to measure the EFs, against the more common Micro Raman one. The validation is carried out by comparing the EFs measured in the 2 configurations with the well-established methodology, on the same set of commercial substrates. Macro Raman with line focus can be advantageous over the Micro Raman because it can provide a higher signal-to-noise ratio at equal laser intensity impinging on the sample (useful in sensing or analytical applications). (b) It is a common practice for practitioners validating an experiment by measuring a commercial compound, acting as a “standard,” whose properties have been already determined in the literature: To our knowledge, an equivalent “standard” does not exist for EF measurements, probably due to the reproducibility and aging issues that prevent them from being identically reproduced in different laboratories. Because the EF is the cross-section ratio of a test molecule under SERS and normal Raman conditions, if one replaces the SERS substrate and the reference liquid with 2 commercial materials in the EF measurement procedure, this leads to the determination of another cross-section ratio: We propose then to use the cross-section ratio of suitable commercial materials for the validation of the instrumental procedure used to measure EFs
Large third-order nonlinear optical response of porphyrin J-aggregates oriented in self-assembled thin films
No full textThe preparation and characterization of a self-assembled material showing a high nonlinear response and good photostability to ultrashort laser pulses is presented. The material is built by alternate deposition of tetrakis(4-sulfonatophenyl) porphyrin diacid (H4TPPS2-) and poly(diallyldimethylammonium chloride) (PDDA) forming electrostatically self-assembled multilayers (ESAMs). UV-visible absorption and emission experiments show that in this matrix H4TPPS2- is present mainly in its J-aggregated form. Furthermore, linear dichroism experiments on a 3 bilayer film show a preferential alignment of the porphyrin aggregate with the J-band transition dipole moment parallel to the film surface. The two photon absorption (TPA) properties of these films are investigated with the Z-scan technique at 806 nm, employing 130 fs pulses. The samples exhibit strong nonlinearities with a very large two-photon absorption coefficient beta(TPA) of 50 cm GW(-1). The origin of this large response is investigated. It has been already demonstrated that aggregation enhances the molecular TPA cross section of H4TPPS2- from 30 to 1000 GM in water solution thanks to cooperative effects. In a 20 bilayer film a further increase by a factor of 1.7 is observed and explained in terms of preferential alignment of J-aggregates in the multilayers
MULTIRESONANCE FWM AS A PROBE OF SPECTRAL BROADENING MECHANISMS - INFLUENCE OF EXCITED-STATE DYNAMICS ON RESONANCE CARS PROCESSES
No full textCOUPLING OF ELECTRONS TO INTERMOLECULAR PHONONS IN MOLECULAR CHARGE-TRANSFER DIMERS - A RESONANCE RAMAN-STUDY
No full textWe report resonance Raman scattering RRS spectra and Raman excitation profiles REP of a system containing dimers of identical molecular radical ions measured with laser excitation in resonance with the charge transfer CT transition. A Peierls–Hubbard PH Hamiltonian has been used to model the investigated system and to calculate its optical and RRS properties. Results are reported for two polyoxometallate salts of tetrathiafulvalene TTF, namely TTF2W6O19) and TTF2 Mo6 O19) whose structures contain almost isolated TTF ) 2 dimers. The RRS spectra of TTF2W6O19), measured in resonance with the CT absorption band centered at 832 nm, show 1 three phonon modes located at 55, 90, and 116 cm modes have been associated to the out-of-phase combinations of the translational motions of the two molecules composing the dimer. Such modes are effective in modulating the intradimer transfer integral, thus providing an efficient mechanism for coupling with the electronic system and for enhancement of the scattering intensity at resonance with the CT transition. The REP for the three strongly coupled modes of TTF2W6O19) have been measured with laser excitation wavelengths ranging from 740 to 930 nm. Quantitative analysis of the REP data has been performed based on a perturbative solution of the PH model to second order in the electron-molecular-vibration EMV and electron-intermolecular-phonon EIP interactions. The CT absorption profile and the REP’s have been calculated using a time correlator technique and the model parameters have been optimized in order to fit the experimental REP data. Infrared vibronic absorptions of TTF2W6O19), originated by the EMV coupling, have been measured and independent information on the electronic parameters of the PH model have been derived. This has made the choice of the fitting parameters used for the REP calculations rather unambiguous and has allowed us to obtain, for the first time, reliable experimental estimates of the EIP coupling constants
COUPLING OF ELECTRONS TO LOW-FREQUENCY PHONONS IN CATION-RADICAL SALTS OF TTF STUDIED BY RESONANCE RAMAN-SCATTERING
No full textOptical excitations dynamics at hetero-interfaces fullerene/quantum dots
No full textEmbedding Semiconductor Quantum Dots (QDs) into hybrid organic-inorganic solar cell holds promises for improving photovoltaic performances. Thanks to their strong coupling with electro-magnetic radiation field, QDs represent paradigmatic photon absorbers. Nevertheless, the quest for suitable charge separating hetero-interfaces is still an open challenge. Within this framework, the excited state interactions between QDs and fullerene derivatives are of great interest for ternary solar cells (polymer:QDs:fullerene). In this work, we investigated the exciton dynamics of core/shell CdSe/CdS QDs both in solution and in blends with fullerene derivative (PCBM). By means of transient optical techniques, we aimed to unveil the dynamics of the QDs-PCBM interaction. Indeed, the observed excited state depopulation of QDs in blends is compatible with an excited state interaction living on picosecond timescale. Through electron paramagnetic resonance, we delved into the nature of this interaction, identifying the presence of charge separated states. The concurrence of these observations suggest a fast electron transfer process, where QDs act as donors and PCBM molecules as acceptors, followed by effective charge separation. Therefore, our experimental results indicate the QDs-PCBM heterointerface as suitable exciton separating interface, paving the way for possible applications in photovoltaics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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