A.M. Mezzasalma

Optical properties of the semiconductor 7, 7′, 8, 8′, tetracyanoquinodimethane (TCNQ)

SummaryThe reflectivity and absorption spectra of 7, 7′, 8, 8′ tetracyanoquinodimethane thin films, in the visible and near-ultraviolet range, have been measured. Samples have been grown and characterized by means of elemental microanalysis and differential scanning calorimetry (DSC) in order to obtain the purity degree. The experimental optical spectra are interpreted on the basis of a calculation of the electronic molecular energy levels by the complete neglect of differential-overlap method in a new parametrization scheme.RiassuntoSono stati misurati gli spettri di riflettività e di assorbimento di film sottili di 7, 7′, 8, 8′ tetracianochinodimetano nella regione spettrale del visibile e del primo ultravioletto. Il grado di purezza dei campioni è stato determinato mediante microanalisi elementale e calorimetria differenziale a scansione (DSC). Gli spettri sperimentali sono stati interpretati sulla base di un calcolo di livelli elettronici con il metodo CNDO seguendo un nuovo schema di parametrizzazione.РезюмеБыли измерены спектры отражательной способности и поглощения тонких пленок 7, 7′, 8, 8′ тетрацианоквинодиметана в видимой области и в области ближнего ультрафиолета. Проведено выращивание образцов. Проведены элементный микроанализ и дифференциальная сканирующая калориметрия, чтобы определить степень чистоты. Экспериментальные оптические спектры интерпретируются на основе вычисления электронных молекулярных энергетических уровней в новой схеме параметризации.

Fluorescence spectra and electronic-energy levels of the semiconductor 7, 7′, 8, 8′ tetracyanoquinodimethane (TCNQ)

SummaryThe luminescence of solutions of TCNQ together with their absorption spectra have been measured and compared with the optical properties of powders, as deduced by diffuse reflectance spectroscopy. The experimental results are interpreted on the basis of the electronic-energy levels in the molecule obtained by a CNDO calculation scheme.RiassuntoSono stati misurati gli spettri di luminescenza e di assorbimento di soluzioni di TCNQ e confrontati con le proprietà ottiche delle polveri cosí come dedotte da misure di riflettanza diffusa. I risultati sperimentali sono stati interpretati sulla base di un calcolo di livelli di energia elettronici effettuato con il metodo CNDO.

Characterization of pulsed laser deposited a-C films by means of reflection electron energy loss spectroscopy

Abstract Amorphous carbon films have been deposited by ablating a high purity graphite target using a KrF excimer pulsed laser. Both the laser energy density (up to 15 J/cm 2 ) and the substrate temperature (up to 900 K) were varied. Reflection electron energy loss spectra have been carried out in an ultra high vacuum surface analysis apparatus. The loss spectra were processed by removing the elastic peak contribution and the secondary electron background. The optical functions were deduced using a procedure based on the assumption that surface effects are negligible with respect to the bulk ones. The loss spectra are dominated by the presence of the π and π+σ plasma resonances, whose energy positions and shapes are significantly affected by the different preparation conditions. In particular, it has been made possible to determine, on the basis of reflection electron energy loss (REEL) data only, the sp 2 /sp 3 bonding ratio. Overall, these results are also relevant for the correlation between the details of the preparation method and the material electronic properties.

Optical constants of CNx thin films from reflection electron energy loss spectroscopy

Abstract Electron energy loss spectroscopy (EELS), in both transmission (TEELS) and reflection (REELS) geometries, can provide a powerful tool to obtain the complex dielectric constant of a material over a very wide energy range. The procedures used are generally complicated having to deconvolve the experimental spectra from a number of different contributions in order to extract the bulk term which is proportional to Im(−1/ e ). In this work we have adopted a simplified method to analyse REELS data under the assumption of a weak surface contribution to the plasmon losses due to the large mean free path for inelastic scattering of the investigated samples. The so deduced dielectric constant is presented, up to 50 eV, for some a-CNx, thin films deposited by pulsed laser ablation of graphite targets in a controlled nitrogen atmosphere. The results are compared with the experimental data obtained by conventional optical reflectivity and also with those obtained from pure amorphous carbon.

sp2 and sp3 bonding configurations in low nitrogen content a-CNx thin films

The results of an electron spectroscopy study carried out on a set of pulsed laser deposited CNx films are reported. A progressive degree of graphitization, deduced from the continuous increase in the sp2 bond fraction, has been found for x values up to 25%. The mass density values, deduced by a proper treating of both the x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy data, are consistent with a material made up by two phases, namely a sp2 threefold (graphite-like) and a sp3 fourfold (diamond-like) coordinated one. The behaviour of the density of the samples as a function of the N content does not show any abrupt change in going from lower to higher nitrogen concentrations as found by other authors, this certainly being due to differences in the starting sp3/sp2 ratio of the films at zero N concentration and to their different preparation parameters.

Palladium-TCNQ-based charge-transfer complexes (TCNQ = 7,7′-8,8-tetracyanoquinodimethane)

The reaction between the palladium(0) complex [Pd(dba)2] [dba = dibenzylidenacetone) with TCNQ proceeds via an electron-transfer process leading to a solid having the empirical formula [Pd(TCNQ)2(CH3CN)x] (x<1). The same reaction performed in the presence of a variety of ligands leads to compounds whose nature is dependent on the geometrical and electronic properties of the ligands. [Pd(dba)2] reacts with TCNQ in the presence of 2,2′-bipyridine (bipy) affording, depending on the experimental conditions, two charge-transfer (CT) solids of general formula [Pd(bipy)2 (TCNQ)2], one of which exhibits spectral features of conducting TCNQ complexes. When the reaction is performed in the presence of the more steric demanding ligand, 2,9-chelating ligand, 2,9-dimethylphenanthroline (dmphen), the palladium complex [Pd(dmphen)(π-TCNQ)] is obtained. A complex with a similar stoichiometry and spectroscopic properties is obtained by reacting [Pd(dba)2] with TCNQ in the presence of PPh3, while by using the bis-chelating 1,2-bis(diphenylphosphino)ethane (dppe) ligand the CT salt [Pd(dppe)2][TCQM]2 is obtained. The CT complex [Pd(terpy)(η1-TCNQ][TCNQ] is the product of the reaction of [Pd(dba)2] and TCNQ in the presence of the ter-chelating 2,2′,6′,2″-terpyridine ligand (terpy). All compounds have been characterized by UV-Vis/ near-IR and IR spectroscopic techniques. X-ray photoelectron (XP) and Auger electron (XAE) spectroscopy have been employed to determine the formal charge on the palladium metal.

An investigation of the electronic and structural properties of pulsed laser-deposited a-C films

Abstract The electronic and structural properties of amorphous carbon films have been studied by means of Raman and X-ray photoelectron spectroscopy. The films were deposited, under vacuum, by KrF excimer laser ablation of graphite targets. Both the substrate temperature and laser fluence were increased up to 900 K and 15 J cm −2 , respectively. The Raman spectra of the films deposited at room temperature showed the presence of a broad band peaked at nearly 1550 cm −1 with a shoulder in the 1300–1400 cm −1 region, with an sp 3 content ranging from 60 to 70%. Upon varying the laser fluence, only slight modifications in the Raman spectra were observed. On the contrary, increasing the substrate temperature increased the low-frequency component, suggesting an increase in the sp 2 content. This result is also supported by the lineshape modifications observed for the C 1s photoemission peak and by a possible reduction in the film density.

LAMQS and XRF analyses of ancient Egyptian bronze coins

A set of Egyptian bronze coins, dating back to the sixth or seventh century AD, has been studied by different experimental techniques in order to compare their composition and surface morphology, the process of coinage and, possibly, to also identify the place of production. The measurements have been performed by laser ablation with mass quadrupole spectrometry and energy dispersed X-ray fluorescence. Both analyses are non-invasive and can be safely used according to the integrity requirements of the analyzed pieces. Owing to the poor number of available samples, this work, more than to solve a numismatic question, has been carried out in order to test the validity of the above experimental techniques in view of further analyses on the same coins, based on better quality statistics. The preliminary results, presented in this paper, indicate significant differences in the chemistry of the coins’ patina, i.e. composition and isotopic species content. This seems to support, in agreement with the archaeological expectations, the hypothesis of the existence of a local mint in Antinoopolis, never before considered in Egyptian numismatics.

Electronic and fluorescence properties of 2,3-diphenyl indole

Abstract The electronic properties of the new organic compound 2,3-diphenyl indole have been studied. Fluorescence decay measurements using a cross correlation phase and a modulation fluorometer have been performed and the data analyzed using different minimisation techniques. The absorption experimental spectra obtained by REELS and the reflectivity data have been interpreted on the basis of a CNDO calculation.

Disorder and optical characterization of evaporation deposited a-Si: H films

SummaryWe have found, from optical transmission and photoconductivity measurements, that the width of the Urbach’s tail and the optical-band gap value depend on the amount of disorder present in the network of a-Si: H films. The nature of the disorder, being it thermal or structural, affects in the same way the behaviour of the absorption coefficient and supports the hypothesis that the value of the optical-band gap depends only indirectly on the overall hydrogen content.RiassuntoMediante misure di assorbimento ottico e di fotoconducibilità si è trovato che la larghezza della coda di Urbach e il valore del gap ottico dipendono fortemente dal disordine presente nella matrice dei film di silicio amorfo idrogenato. Inoltre si è osservato che il disordine termico e quello strutturale influenzano allo stesso modo il coefficiente di assorbimento avvalorando l’ipotesi che il valore del gap ottico dipende solo indirettamente dalla concentrazione totale di idrogeno.