A. Zappettini

Adsorption effects of NO2 at ppm level on visible photoluminescence response of SnO2 nanobelts

The visible photoluminescence (PL) of tin oxide nanobelts is quenched by nitrogen dioxide at ppm level in a fast (time scale order of seconds) and reversible way. Besides, the response seems highly selective toward humidity and other polluting species, such as CO and NH3. We believe that adsorbed gaseous species that create surface states can quench PL by creating competitive nonradiative paths. A comparison between conductometric and PL response suggests that the two responses are ascribable to different adsorption processes.

Third order optical characterisation of a π-conjugated polydiacetylene by Maker fringes technique

Abstract Conjugated polymers are promising photonic materials due to the high values of non-linear optical coefficients when compared to inorganic compounds. A key point in view of applications is the figure of merit defined as the ratio between the non-linear optical and the absorption coefficients. We report on the NIR measurement of the third order optical susceptibility ( χ (3) ) by third harmonic generation in polydiacetylene (poly-[1,6-bis-(3,6-dihexadecyl- N -carbazolyl)-2,4-hexadiyne], polyDCHD-HS). Samples are prepared as thin films by spin-coating technique and the χ (3) is evaluated by Maker fringes method in the spectral range from 1200 to 2000xa0nm. The absorption coefficient has been derived from measurements in CS 2 solution. Spectra of χ (3) show typical resonant behaviour due to the multi-photon processes and are strictly related to the absorption spectra in the visible range. Values of χ (3) are in the range of 10 −11 xa0esu. The absorption coefficient is as low as 0.25xa0cm −1 at 1.5xa0μm showing the good figure of merit of polyDCHD-HS.

Titania inverse opals for infrared optical applications

Photonic crystals have gathered great importance in recent years. In particular macroporous materials (inverse opals) show interesting properties as photonic crystals. Ordered macroporous titanium dioxide (TiO2) is made using polystyrene spheres as a template. Titania is chosen for its high refractive index (>2.5). Following an already known technique [E.G. Judith, J. Wijnhoven, W.L. Vos, Science 281 (1998) 802; B.T. Holland, C.F. Blanford, A. Stein, Science 281 (1998) 538; B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chem. Mater. 11 (1999) 795] large-scale order in macroporous TiO2 is obtained both using the 778 and the 3190 nm beads as documented by optical microscope and SEM images. These structures would lead to applications in the mid-infrared range.

A new process for synthesizing high-purity stoichiometric cadmium telluride

In the frame of a research project aimed at preparing high-resistivity CdTe single crystals, the authors have developed as a first step, a new, low cost, fast technique which allows to synthesize from the elements polycrystalline CdTe with very low stoichiometric deviations and with a level of background impurities much lower than usually available in commercial materials. The material obtained by this technique as will be described and discussed, appears especially suitable for preparing source charges for physical vapour transport growth experiments.

Sputtered stoichiometric TeO2 glass films: Dispersion of linear and nonlinear optical properties

We report on the optical characterization of TeO2 amorphous films deposited by radio-frequency (rf) reactive sputtering. X-ray diffraction, electronic microscopy, and Rutherford backscattering analysis have been performed in order to assess the structural and compositional properties of the samples. The linear optical characterization has been performed by variable angle spectroscopic ellipsometry at wavelengths between 260 and 1700 nm, leading to determination of the dispersion of the complex refractive index for sputtered tellurium dioxide. In the spectral range of transparency, between 1700 and 500 nm, refractive index values ranging from 2.05 to 2.16 have been obtained. The dispersion of the complex third-order nonlinear optical coefficient of the sample has been evaluated in the near-infrared spectral range by third harmonic generation measurements. The out-of-resonance value of the χ(3) coefficient for TeO2 is in the range between 1.2×10−12 and 1.4×10−12u2009esu. The conjugation of optical transparency,...

Growth and Deep Level Characterisation of Undoped High Resistivity CdTe Crystals

High resistivity CdTe crystals were grown by Bridgman and by physical vapour transport techniques without intentional doping. Novel synthesis and pre-growth treatment procedures were developed for controlling background impurities concentration and stoichiometry of the source material. A detailed characterisation of the deep levels responsible for the high resistivity behaviour is reported.

Heat treatment in semi-closed ampoule for obtaining stoichiometrically controlled cadmium telluride

Abstract The electrical behaviour of CdTe crystals is controlled by background impurity concentrations and stoichiometric deviations. The stoichiometry control is particularly important in vapour growth, in that even extremely reduced deviations in the starting polycrystalline charge give rise to large increases in excess-component partial pressure, thus limiting the crystal growth rate. A novel procedure was developed for preparing polycrystalline CdTe with highly reduced off-stoichiometry. The method is based on a heat treatment of the CdTe charge and on the control of the flux of the Cd and Te 2 vapours through an effusion hole. In situ monitoring of the vapour pressures, and thus of the stoichiometry of the charge was demonstrated. It was shown that the monitoring of the Te 2 -partial pressure with time allows to interrupt the process when the desired stoichiometric ratio (Cd/Te≈1 or ≠1) has been reached. Nominally undoped single crystals could be grown from vapour and melt, with a resistivity of 10 8 –10 9 xa0Ω×cm, when the Cd/Te atomic ratio was close to unity. A correlation between resistivity and Te 2 -partial pressure is reported and discussed.

Stoichiometric deviations and partial-pressure measurements in solid–vapour cadmium telluride system

Abstract In the frame of a research project aimed at developing a single-crystal-growth technology of cadmium telluride (CdTe) for electrooptics and photorefractive applications, the authors have implemented a laser absorption technique by which the partial pressure of the vapours (Te2(g), Cd(g)) over solid CdTe can be monitored as a function of temperature in the temperature range 500–900°C. By testing polycrystalline materials of different origins, this technique proved to be a reliable tool for selecting the most suitable polycrystalline CdTe to be used as a source in the physical vapour transport (PVT) growth of CdTe single crystals.

Characterization of electro-optic shielding effect in bulk CdTe : In crystals

Abstract We have characterized the role of electro-optic field shielding effect in bulk CdTexa0:xa0In rods at 1550xa0nm. Different temperatures, modulating frequencies and probe beam power have been tested. Experimental results agree with a dielectric relaxation explanation, when taking into account the high injection regime and the contribution of collection of photo-generated free excess carriers at the contacts. The lowering in electro-optic yield can be minimized by a suitable reduction in operating temperature and sample dimensions, having defined the optical power of the signal to be processed.

Crystal growth of undoped semi-insulating CdTe

In the frame of a research project aimed at obtaining high-resistivity CdTe crystals, the authors report on the preparation of (nominally) undoped semi-insulating (SI) CdTe crystals. The method is based on the use of polycrystalline CdTe charges synthesized from 7 N pure Cd and Te elements, in such a way as to ensure an atomic stoichiometric ratio (Cd/Te) equal to 1 within the weighing error. The charges are further stoichiometrically adjusted by subliming off residual excess components through effusion in semi-open ampoules vented to vacuum. By making use of these charges, undoped SI CdTe ingots, made up of large single-crystal grains, have been grown from both vapour, by closed-ampoule physical vapour transport and melt, by liquid encapsulation Bridgman technique. The yields in SI material turned out to be 100% for vapour-grown samples and about 50% for melt-grown ones. Preliminary results of this research work are presented here and briefly discussed.