T. Di Luccio

Effects of substrate treatment and growth conditions on structure, morphology, and luminescence of homoepitaxial ZnTe deposited by metalorganic vapor phase epitaxy

The structure, morphology, and luminescence of homoepitaxial ZnTe layers grown by metalorganic vapor phase epitaxy on (100)ZnTe:P wafers are reported as function of substrate surface treatment and growth conditions. Epilayers grown on substrates in situ H2 treated at temperatures above 240 °C exhibit long range crystalline perfection comparable to the substrate, but their structure rapidly degrades at lower temperatures, an effect ascribed to the incomplete removal of native oxides from the wafer surface. Instead, the material microscopic structure improves monotonically by annealing the wafer up to 350 °C. A nearly featureless surface morphology is obtained for epilayers grown within a narrow temperature interval around 350 °C, corresponding to the transition between surface kinetics and mass transport limited growth. Surface ridging along a 〈110〉 in-plane direction is observed at lower temperatures, while large pyramidlike hillocks occur randomly on the surface of samples grown above 350 °C. Besides the...

Properties of Bi2+xSr2−xCuO6+δ thin films obtained by MBE

Abstract We report on the fabrication of epitaxial, c-axis oriented Bi2+xSr2−xCuO6+δ (2201) thin films deposited by molecular beam epitaxy on MgO and SrTiO3 substrates, using co-evaporation method. reflection high energy electron diffraction (RHEED) has been employed to monitor the surface structure of the samples during the growth. The morphology and the electronic properties of the films surface have been analyzed after the deposition by scanning tunneling microscopy (STM) at room temperature in ultra high vacuum conditions. The presence of well defined strikes in the RHEED patterns indicates a 2D nucleation growth, confirmed by STM analysis which shows very smooth surfaces on micron scales with the presence of half or one unit cell high terraces. STM spectroscopic measurements shows that the 2201 terminating layer is formed by BiO plane. The RHEED patterns show the presence of a superstructure along the b direction, due to an excess of oxygen in the BiO surface layer, whose periodicity is about twice that usually observed for BSCCO compound. The dependence of the c-axis length, obtained by X-ray diffraction patterns, has been studied as a function of the sample stoichiometry. It has been observed that a change in the Bi/Sr ratio strongly influence both the c-axis length and the electrical properties of the samples. Superconductivity has been detected in the samples with a little excess of Bi respect to Sr independently on the oxygenation conditions.

Bi2Sr2CuO6+δ/ACuO2(A=Ca, Sr) superconducting multilayers obtained by Molecular Beam Epitaxy

Abstract Bi-based superconducting multilayers, consisting of Bi 2 Sr 2 CuO 6+ δ (2201) layers alternately stacked with CaCuO 2 or SrCuO 2 layers, have been deposited by Molecular Beam Epitaxy (MBE) using co-deposition and growth interruption techniques. In situ Reflection High Energy Electron Diffraction (RHEED) has been used to monitor the surface of the different layers giving evidence of a two-dimensional growth mode. The layered structure of the samples has been confirmed by X-ray diffraction analyses. Resistive measurements have shown superconducting 2201/CaCuO 2 samples with critical temperatures strongly depending on the thickness of the CaCuO 2 . The 2201/SrCuO 2 multilayers do not show a zero electrical resistance above 4.2 K, but the resistance vs. temperature curves present a sharp decrease around 60 K indicative of a superconducting onset.

Vocs sensors based on polyaniline/graphene-nanosheets bilayer

In this work chemical sensors based on polyaniline and on polyaniline/graphene-nanosheets bilayer were prepared by dip coating. Their morphology and volatile organic compounds (VOCs) sensing behavior were compared. In particular, the devices were investigated in presence of VOCs as limonene and ethanol. Furthermore the devices were characterized under relative humidity. Bilayer samples, exposed to limonene, show a higher relative response respect to only polyaniline, encouraging the graphene use in VOCs sensor devices.

Luminescent nanocomposites of conducting polymers and in-situ grown CdS quantum dots

Luminescent PVK:CdS and P3HT:CdS nanocomposites with enhanced electrooptical properties have been synthesized. The nucleation and growth of CdS nanoparticles have been obtained by the thermolysis of a single Cd and S precursor dispersed in the polymers. The size distribution and morphology of the nanoparticles have been studied by TEM analyses. Monodispersive and very small nanoparticles of diameter below 3 nm in PVK and 2 nm in P3HT, have been obtained. The application of such nanocomposites as emitting layers in OLED devices is discussed.

Structural morphological and optical properties of P3HT/CdSe/WS2 ternary composites for hybrid organic/inorganic photovoltaics

Abstract The development of complete organic/inorganic hybrid solar cells (HOSCs) is strictly related to the achievement of appealing electrical performances. In this context, the optimization of device architectures and of the active material’s structural and morphological properties is crucial. In this work, such highly critical issues are addressed for a novel hybrid nanocomposite: tungsten disulfide nanotubes (WS2 NTs) and cadmium selenide quantum dots (CdSe QDs) embedded into a Poly (3-hexylthiophene-2,5-diyl) (P3HT) polymeric matrix. Both binary and ternary compounds are produced and investigated by means of an unconventional energy-dispersive X-ray diffraction technique combined with atomic force microscopy and Raman spectroscopy. Additionally, the effects of low-temperature thermal treatments on the nanocomposite’s structure and morphology are disclosed, unraveling their connection of these features to the optical response of the materials, investigated by UV–Vis spectroscopy. Promising results are obtained in terms of enhanced crystallinity, phase separation, and the possibility of decorating the WS2 NTs with CdSe dots is demonstrated, opening new perspectives for further development of complete HOSCs based on this innovative ternary system.

Structural characterization of ZnSe/ZnMgSe MQWs grown on (1 0 0)GaAs by low pressure MOVPE

A detailed structural characterization of ZnSe/ZnMgSe multiple quantum wells (MQWs) grown on GaAs by low pressure metalorganic vapour phase epitaxy is presented. ZnSe/Zn0.83Mg0.17Se MQWs having between 6 and 12 periods were deposited at 3301C and 304 mbar reactor pressure on (1 0 0)GaAs after a 4.2 nm ZnSe buffer layer. The MQWs had nominal 4.4 nm thick ZnSe wells and 5.3 nm thick Zn0.83Mg0.17Se barriers. The MQW structural properties were investigated by high-resolution X-ray diffraction (HRXRD) and X-ray specular reflectivity (XSR) measurements. Besides the MQWs-substrate mismatch, simulation of the HRXRD and XSR patterns allowed to determine the MQW period, individual layer thickness and barrier composition. Between 8 and 10 periods the MQW structure begins to relax, its critical thickness on GaAs being between 92 and 113 nm. Furthermore, HRXRD showed broader zeroth and first-order satellite peaks with increasing MQW periods, a result ascribed to strain fluctuations induced by either inhomogeneous Mg incorporation in the ZnSe lattice and/or interface roughening. Comparison of experimental and simulated XSR patterns allowed to determine the rms roughness at each multilayer interface, which linearly increases along the growth direction due to a cumulative intrinsic roughening. r 2002 Elsevier Science B.V. All rights reserved.

Anisotropy and transport properties of (Bi2Sr2CuO6+σ)m/(CauCuo2)n multilayers obtained by molecular beam epitaxy

Abstract Structural and transport properties of (Bi 2 Sr 2 CuO 6+σ ) m /(CCaCuO 2 ) n superconducting multilayers, obtained by molecular beam epitaxy (MBE) alternately stacking m layers of (Bi 2 Sr 2 CuO 6+σ with n layers of CaCuO 2 , have been studied. Resistivity measurements and I–V characteristics in perpendicular and parallel magnetic fields have allowed to estimate the anisotropy parameters which results very low with respect most of the high temperature superconductors.