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ABSTRACT: A scrupulous cleaning and degreasing of the deposition chamber allows to make a ZrN film with stoichiometry of 1.3 and to achieve a level of oxygen contamination equal to 5%. This film exhibits a low number of carriers estimated at N* = 3 × 1021 cm− 3 and with a very high electrical resistivity value of about 105 μΩ·m. This result points the way for further improvements in the quality of the material.
This amount of oxygen contamination may be reduced in conditions in which only the oxygen will be removed without disturbing the zirconium presence in the film. A bias voltage value between 8 eV and 20 eV reduces the oxygen contamination.
The ionic assistance is often proposed as a mean to minimize the oxygen contamination. However, one must consider the negative phenomena as re-sputtering, ion implantation, atom displacement and stress generation that introduce defects in the film and affect its properties. This work proposes a very low bias voltage value to control oxygen contamination. The bias voltage value is chosen higher than the nitrogen sputtering threshold energy and lower than the argon sputtering threshold energy. The re-sputtering phenomenon, far from being a problem, can be used to achieve the stoichiometry if one starts from a nitrogen-rich compound. In this way, ZrNx is grown with x about 1, with an effective free electron concentration N* = 8.9 × 1021 cm− 3. Furthermore its resistivity value is about 2 μΩ·m and the oxygen Secondary Ion Mass Spectrometry (SIMS) signal is similar to the noise signal.
Surface and Coatings Technology 01/2012; 206:2711. · 1.87 Impact Factor
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Journal of Physical Chemistry C. 01/2012; 116(47):25119-25125.
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ABSTRACT: Zr–N films were grown on glass substrates via radio-frequency magnetron sputtering using an Ar + N2 + H2 mixture. Hydrogen was employed in order to reduce oxygen contamination coming from background pressure, as confirmed by secondary ion mass spectroscopy analysis. The tuned process parameter was the nitrogen flux percentage (RN2) in the mixture. The crystallographic structure of the films was studied using x-ray diffraction. The measurements show that the films deposited at low RN2 (lower than or equal to 50%) crystallize in the rocksalt ZrN structure. As RN2 exceeds 50%, the film exhibits the co-presence of ZrN and Zr3N4 (denoted as o-Zr3N4) phases. When the deposition is performed in only nitrogen atmosphere (RN2 = 100%), a broad peak located at 2θ ≈ 32.2° is mainly attributed to the contribution coming from (320) planes of the o-Zr3N4. An envelope method, based on the optical reflection and transmission spectra taken at normal incidence, has been applied for the optical characterization of the nitride films. Such a method allows the determination of the samples’ average thickness and optical constants (refractive index n and extinction coefficient k) in the ultraviolet-visible-near infrared regions. The evaluated thickness was about 2500 nm, which is in good agreement with the value obtained from profilometry. The absorption coefficient α was calculated from reflectance and transmittance spectra. The energy bandgap ranges from 2.3 eV to 2.4 eV. Electrical characterization was performed using capacitance-voltage measurements, which showed that the films evolve from insulating to semiconductor behavior when the nitrogen content in the sputtering atmosphere is decreased, confirming structural and optical results.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 10/2011; 29:61507. · 1.25 Impact Factor
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ABSTRACT: Efficient solar energy conversion is strongly related to the development of new materials with enhanced functional properties. In this context, a wide variety of inorganic, organic or hybrid nanostructured materials have been investigated. In particular, in hybrid organic-inorganic nanocomposites are combined the convenient properties of organic polymers, such as easy manipulation and mechanical flexibility, and the unique size dependent properties of nanocrystals (NCs). However, applications of hybrid nanocomposites in photovoltaic devices require a homogeneous and highly dense dispersion of NCs in polymer in order to guarantee not only an efficient charge separation but also an efficient transport of the carriers to the electrodes without recombination. In previous works we demonstrated that cadmium thiolate complexes are suitable precursors for the in-situ synthesis of nanocrystalline CdS. Here, we show that the soluble [Cd(SBz)2]2.(1-methylimidazole) complex can be efficiently annealed in a conjugated polymer obtaining a nanocomposite with a regular and compact network of NCs. The proposed synthetic strategies require annealing temperatures well below 200°C and short time for the thermal treatment, i.e. less than 30 min. We also show that the same complex can be used to synthesize CdS NCs in mesoporous TiO2. The adsorption of cadmium thiolate molecule in TiO2 matrix can be obtained by using chemical bath deposition technique and subsequent thermal annealing. The use of NCs, quantum dots, as sensitizers of TiO2 matrices represents a very promising alternative to common dye-sensitized solar cells and an interesting solution for heterogeneous photocatalysis.
Journal of Nanoparticle Research 03/2011; · 3.29 Impact Factor
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ABSTRACT: Nanocrystalline anatase titania thin films were prepared by using two different precursor solutions, a highly acid solution (Sol-1) and a polymer-like solution (Sol-2), via the dip-coating technique on different substrates (<100>-Si wafer, fused silica and soda lime glass). The influence of the two sol-gel titania precursor solutions and of the substrate type on the film morphology, coating porosity, surface roughness, crystalline phases and grain size of the titania films were investigated. Our experimental results clearly indicate that the sol - composition and substrate type remarkably influence the microstructural/morphological properties of the titanium dioxide. They consequently modify the optical response and hydrophilic performances of the samples. The photocatalytic oxidations of the methylene blue in water of the samples grown on the glass substrate were monitored to investigate the influence of the sol-gel precursor solution on the photocatalytic activity of the titania coatings, and the results were put in relation with the hydrophilic and optical properties of the films. The outcome demonstrates that the optical properties and the hydrophilic and photocatalytic performances of nanocrystalline titania can be opportunely tailored tuning the size dimension of the crystalline domain according to the specific coating applications.
MRS Proceedings. 12/2010; 1352.
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ABSTRACT: The optical functions (complex dielectric function, complex index of refraction, and complex conductivity) of sputtered zirconium nitride films are derived starting from optical reflectance measurements. Their evolution with the different bias voltages applied during the films growth is used to deduce information about the variations in the electronic structure influenced by a different oxygen and nitrogen content in the films. Improvement in the electrical conductivity is observed at increasing bias voltage due to a reduction in both oxygen contamination and nitrogen content. The separation of the different contributions (free conduction electrons and different electronic transitions) in the optical functions is achieved through the Drude–Lorentz model, allowing the detection of an unusual low-energy electronic transition in films grown at low bias voltages. Through considerations about the electronic structure and about the results coming from other characterization techniques, this transition can be ascribed to the presence of defects/impurities, nitrogen superstoichiometry, or oxygen contamination.
Journal of Applied Physics 11/2010; · 2.17 Impact Factor
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ABSTRACT: Ion bombardment during thin film growth is known to cause structural and morphological changes in the deposited films, thus affecting their physical properties. In this work zirconium nitride films have been deposited by the ion assisted magnetron sputtering technique. The ion energy is controlled by varying the voltage applied to the substrate in the range 0–25 V. The deposited ZrN films are characterized for their structure, surface roughness, oxygen contamination, optical reflectance and electrical resistivity. With increasing substrate voltage crystallinity of the films is enhanced with a preferential orientation of the ZrN grains having the (1 1 1) axis perpendicular to the substrate surface. At the same time, a decrease in electrical resistivity and oxygen contamination content is observed up to 20 V. A higher substrate voltage (25 V) causes an inversion in the observed experimental trends. The role of oxygen contamination decrease and generation of nitrogen vacancies due to ionic assistance have been considered as a possible explanation for the experimental results.
Journal of Physics D Applied Physics 05/2010; 43:225401. · 2.54 Impact Factor
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ABSTRACT: Nanostructured titania thin films were prepared under controlled atmospheric conditions by the sol–gel dip-coating technique on glass, fused silica and (1 0 0)-silicon substrates. Two different sol–gel routes were employed by using different precursor solutions, a highly acid solution and a polymer-like solution. The influence of sol composition and of the substrate type on the morphology, coating porosity, surface roughness, crystalline phases and grain size of the titania films were investigated in detail. In addition, the relationship between microstructural/morphological properties and the optical properties (energy gap, refractive index and extinction coefficient) and the hydrophilic performance of the coatings were evaluated. Our experimental results clearly indicate that the sol composition and substrate type remarkably influence the films' morphology and microstructure; moreover, they consequently modify the optical response and hydrophilic performances of the samples, showing that superhydrophilic titania coatings can be obtained opportunely by choosing the composition of the precursor sol–gel solution. Blue shift of the band gap energy and a band structure mutation from indirect to direct were also revealed. The hydrophilic properties and the change in the band gaps transition can be attributed to oxygen vacancies on the surface of the titania nanocrystallites that gives rise to Ti3+ sites and, consequently, to structural changes/defects of the anatase nanoarchitecture. These findings allow us to design and tailor the optical and hydrophilic properties of the titania coatings.
Journal of Physics D Applied Physics 02/2010; 43(9):095301. · 2.54 Impact Factor
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Polymer Composites 01/2010; 31:1075. · 1.23 Impact Factor
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The Journal of Physical Chemistry C 01/2010; 114:13985. · 4.80 Impact Factor
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ABSTRACT: Polymer films embedding cadmium thiolate precursors have been irradiated with ultraviolet laser pulses resulting in the formation of cadmium sulfide crystalline nanoparticles through a macroscopically non-destructive procedure for the host matrix. Controlling the number of the incident laser pulses, the gradual increase of the size of the nanoparticles is accomplished, and consequently the progressive change of the emission characteristics of the formed nanocomposites. The X-ray diffraction and transmission electron microscopy measurements were used for the full characterization of the nanoparticles. This study compares two polymer matrices, poly(methyl methacrylate) and a cyclic olefin copolymer, and reveals the importance of each one for the emission characteristics of the formed cadmium sulfide nanocrystals. It is found that the poly(methyl methacrylate) matrix contributes to the increase of the trap states on the surface of the formed nanocrystals, causing the broadening of their emission. On the other hand the cadmium sulfide nanoparticles, grown into the cyclic olefin copolymer matrix, exhibit narrower emission spectra.
Journal of Nanoscience and Nanotechnology 01/2010; 10(2):1267-1272. · 1.56 Impact Factor
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Science and Technology of Advanced Materials 01/2010; 66:17-22. · 3.51 Impact Factor
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ABSTRACT: Zirconium oxynitride (ZrNO) films were deposited by RF reactive magnetron sputtering in water vapour–nitrogen atmosphere varying the deposition temperature from RT to 600 °C. Optical analysis, x-ray diffraction, x-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are the employed characterization techniques to investigate the influence of the substrate temperature on the films physical properties. It was found that the variation of the substrate temperature from RT to 600 °C caused a structural transition from cubic phase of Zr2ON2 to ZrN one, as confirmed by TEM observations too. In particular, Forouhi–Bloomer dispersion equations for optical parameters (n and k) and deconvolution of XPS spectra allowed further chemical properties be elucidated. They also permitted identification of two oxynitride phases, γ phase (Eg = 1.94 eV) and β phase (Eg = 1.7 eV), and an over-stoichiometric nitride one. The use of [Ec − Ev] values helped to confirm further the distinction between (γ, β)-phases and N-rich zirconium nitride compound, which is unachievable by using only Eg values.
Journal of Physics D Applied Physics 11/2009; 42(23):235401. · 2.54 Impact Factor
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ABSTRACT: The aim of this study was the development and characterization of transition metal oxynitride multilayers for optical applications. The reactive RF magnetron sputtering technique in rotation mode was used for stacking of zirconium oxynitride (ZrNO) and titanium oxynitride (TiNO) nanolayers. The depositions were carried out in a reactive Ar+N2+O2 atmosphere by sputtering titanium and zirconium targets. By means of different substrate rotation speeds, the bilayer period has been changed in the range 11–20 nm. A multilayer deposition rate increasing with the bilayer period decreasing has been evaluated. Structural, compositional, mechanical and optical analyses have been performed. The x-ray diffraction spectra confirmed the formation of a multilayer structure with a nitride formation prevalence. Non-abrupt interfaces between the layers and non-uniform chemical composition (chemical intermixing) have been detected by transmission electron microscope (TEM) observations. The gradient interface structure turns out to be an advantage for the improvement of the mechanical properties. Higher hardness values were calculated by the Chicot–Lesage and Jonsson–Hogmark models for TiNO/ZrNO multilayer compared with monolayer TiNO and ZrNO coatings. Also SIMS analysis has confirmed a compositional interface grading but also an increase in oxygen content with decreasing substrate rotation speed or similarly with decreasing deposition rate. Moreover, a tuning of the optical properties, going from metallic behaviour to dielectric with the decrease in the substrate rotation speed has been gained. The variation of the deposition rate allows a sort of 'regulation' of the oxygen incorporation with a precise tailoring of the optical properties. This result can be employed with the aim of depositing graded composition multilayer systems with a precise control of their optical selective wavelength properties. The improvement in the mechanical performance in graded oxynitride multilayer coatings would also allow an increase in the optical device lifetime.
Journal of Physics D Applied Physics 05/2009; 42(11):115406. · 2.54 Impact Factor
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ABSTRACT: A method of in situ formation of patterns of size controlled CdS nanocrystals in a polymer matrix by pulsed UV irradiation is presented. The films consist of Cd thiolate precursors with different carbon chain lengths embedded in TOPAS polymer matrices. Under UV irradiation the precursors are photolyzed, driving to the formation of CdS nanocrystals in the quantum size regime, with size and concentration defined by the number of incident UV pulses, while the host polymer remains macroscopically/microscopically unaffected. The emission of the formed nanocomposite materials strongly depends on the dimensions of the CdS nanocrystals, thus, their growth at the different phases of the irradiation is monitored using spatially resolved photoluminescence by means of a confocal microscope. X-ray diffraction measurements verified the existence of the CdS nanocrystals, and defined their crystal structure for all the studied cases. The results are reinforced by transmission electron microscopy. It is proved that the selection of the precursor determines the efficiency of the procedure, and the quality of the formed nanocrystals. Moreover it is demonstrated that there is the possibility of laser induced formation of well-defined patterns of CdS nanocrystals, opening up new perspectives in the development of nanodevices.
Nanotechnology 05/2009; 20(15):155302. · 3.98 Impact Factor
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Microelectronic Engineering 01/2009; 86:816. · 1.56 Impact Factor
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Nanotechnology. 01/2009; 20:155302.
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ABSTRACT: Nanostructured gadolinia thin layers with small sized crystalline grains (∅ ≈ 3–13 nm) were successfully prepared by the sol–gel process on fused silica and silicon substrates. The film growth, the chemical composition, the morphology and the microstructure of the films were investigated in detail by thermo-gravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy, field emission scanning electron microscopy and x-ray diffraction. The experimental results clearly show that the morphological and structural properties of the films are not influenced by the substrate properties and the Gd2O3 cubic crystalline phase (symmetry group ) is enhanced when the films are free from organic residuals and decompose to Gd-oxides. FTIR analysis of gadolinia layers grown on silicon suggest that the formation of Gd2O3 grains may favour the diffusion of oxygen through the coatings. The optical spectroscopy measurements evidence a blue shift of the energy gap and suggest a quantum confinement effect in nano-sized Gd2O3. In addition, two absorption features are observed, which can be attributed to a simultaneous presence of direct and indirect electronic transition indicating a modification of the band structure when the grain size changes from very small to larger Gd2O3 grains.
Journal of Physics D Applied Physics 10/2008; 41(22):225408. · 2.54 Impact Factor
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ABSTRACT: Homogeneous nanocomposite silica films uniformly doped with size-selected gold nanoparticles (AuNPs) have been prepared by a combined use of colloidal chemistry and the sol-gel process. For this purpose, stable thiol-functionalized AuNPs (DDT-AuNPs) were first synthesized by a two-phase aqueous/organic system and, subsequently, dispersed in an acid-catalysed sol-gel silica solution. The microstructural morphology of the samples was investigated by x-ray diffraction and field emission scanning electron microscopy. X-ray photoelectron spectroscopy (XPS) and UV-vis optical spectrophotometry were instead employed to investigate the elemental chemical behaviour and the evolution of the surface plasmon resonance (SPR) band of the AuNPs from their synthesis up to the formation of the Au-doped silica films. The results show that the size, shape and crystalline domains of the AuNPs remain unchanged during the entire preparation process, indicating that their aggregation or decomposition was prevented. XPS results show that the DDT-AuNPs lose the capping shells and oxidize themselves when dispersed in acid-catalysed sol-gel solutions, and that bare AuNPs are embedded in the SiO(2) films. A large broadening of the SPR band, observed for systems with DDT-AuNPs, suggests the presence of interface effects which cause a surface electron density lowering. Thiol chain detachment from the AuNPs determines an increase of the SPR peak intensity while the oxidation of the Au surfaces causes a red shift of its position. The latter is no longer observed in doped films, suggesting that no interfacial effects between bare AuNPs and the host medium are present.
Nanotechnology 09/2008; 19(36):365706. · 3.98 Impact Factor
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ABSTRACT: We report the metalorganic vapour phase epitaxy (MOVPE) of CdTe layers on detector-grade travelling heater method (THM)-grown
(111)B-CdTe substrates, a technological step towards the fabrication of homoepitaxial p–i–n diodes as nuclear radiation detectors.
CdTe layers were grown at 330°C using dimethylcadmium (Me2Cd) and di-isopropyltelluride. A quantitative analysis of the substrate X-ray surface reflectivity after a sequence of treatment
steps, i.e. (i) Br2–methanol etching, (ii) in situ H2 heat cleaning at 350°C, and (iii) H2 heat cleaning and annealing in H2+Me2Cd atmosphere, demonstrated that the rms roughness of the (111)B surface steadily decreases after each treatment, the smoothest
surface being obtained after annealing in H2+Me2Cd. The growth of (111)-oriented homoepitaxial layers depends critically on a combination of in situ substrate treatment and
precursor stoichiometry during the MOVPE process. CdTe layers grown under a 1:1 molar flow ratio between Te and Cd precursors
on H2 heat cleaned substrates show a polycrystalline structure and a rough surface morphology, an effect ascribed to poor material
nucleation on the Te-rich (111)B surface, as left upon Br2–methanol etching of the THM-grown crystal. Further annealing of the substrates in H2+Me2Cd shifts the Te:Cd stoichiometry of the crystal surface closer to the ideal 1:1 bulk value. Layers grown on such Cd-annealed
substrates show a more (111)-oriented crystalline texture and substantially improved surface morphologies, but also the occurrence
of a tetragonal secondary phase. Fully epitaxial (111)-oriented layers were instead obtained on H2 heat cleaned substrates by growing under Cd-rich vapour conditions.
Applied Physics A 03/2008; 91(1):23-28. · 1.63 Impact Factor
