High quality CVD diamond: a Raman scattering and photoluminescence study

High quality synthetic diamonds were grown on single-crystal silicon by microwave plasma enhanced chemical vapour deposition (CVD). A careful optimisation of both the experimental setup and the growth parameters was necessary before that the achievement of the best results was made possible. The films were deposited using a CH₄-H₂ gas mixture at methane concentrations variable in the range 0.6-2.2%, while the substrate temperature was fixed at 750 ^°C. Raman spectroscopy and photoluminescence (PL) were utilised to monitor the quality of the deposited films and to study the spatial distribution of defects, respectively. Micro-Raman analysis shows that linewidths of the diamond peak lower than 2.4 cm^-1 can be easily measured at the growth surface, indicating that the crystalline quality of individual grains is comparable to that of the best natural diamonds. The excellent phase purity of the diamond microcrystals at the growth surface is witnessed by the complete absence of any non-diamond carbon feature and by a very weak luminescence background in the 1.6-2.4 eV spectral range. A worsening of the quality of the diamond particles is found moving from the growth surface towards the film-substrate interface. A photoluminescence feature at about 1.68 eV, commonly associated to Si impurities, is distinctly observed as the exciting laser beam is focused close to the interface. A progressive degradation of the global quality of the films is found with increasing methane concentration in the gas mixture, as witnessed by an increased PL background in the films grown at higher methane concentrations. Received 24 November 2000     

Bogomol'nyi,Prasad, and Sommerfield configurations in smectics

It is typical in smectic liquid crystals to describe elastic deformations with a linear theory when the elastic strain is small. In smectics, certain essential nonlinearities arise from the requirement of rotational invariance. By employing the Bogomol'nyi, Prasad, and Sommerfield decomposition and relying on boundary conditions and geometric invariants, we have found a large class of exact solutions. We introduce an approximation for the deformation profile far from a spherical inclusion and find an enhanced attractive interaction at long distances due to the nonlinear elasticity, confirmed by numerical minimization.     

Inorganic cluster syntheses of TM2+-doped quantum dots (CdSe, CdS, CdSe/CdS): physical property dependence on dopant locale

A series of colloidal transition-metal-doped chalcogenide semiconductor nanocrystals (TM2+:CdSe, TM2+:CdS, etc.) has been prepared by thermal decomposition of inorganic cluster precursors. It is shown through extensive spectroscopic and structural characterization that the nanocrystals prepared following literature procedures for synthesis of TM2+:CdSe nanocrystals actually possess an unintended CdSe/TM2+:CdS core/shell morphology. The conditions required for successful formation of TM2+:CdSe and TM2+:CdS by cluster decomposition have been determined. Magneto-optical and photoluminescence spectroscopic results for this series of doped nanocrystals reveal major physical consequences of dopant localization within the shell and demonstrate the capacity to engineer dopant-carrier exchange interactions via core/shell doping strategies. The results presented here illustrate some of the remarkable and unexpected complexities that can arise in nanocrystal doping chemistries and emphasize the need for meticulous characterization to avoid false positives.     

Effect of nature and location of defects on bandgap narrowing in black TiO2 nanoparticles

The increasing need for new materials capable of solar fuel generation is central in the development of a green energy economy. In this contribution, we demonstrate that black TiO(2) nanoparticles obtained through a one-step reduction/crystallization process exhibit a bandgap of only 1.85 eV, which matches well with visible light absorption. The electronic structure of black TiO(2) nanoparticles is determined by the unique crystalline and defective core/disordered shell morphology. We introduce new insights that will be useful for the design of nanostructured photocatalysts for energy applications.     

Micro‐Raman and photoluminescence analysis of composite vanadium oxide/poly‐vinyl acetate fibres synthesised by electro‐spinning

Composite vanadium oxide (VO_x)‐based fibres were synthesised by the electro‐spinning method combined with conventional sol–gel processing using polyvinyl acetate (PVAc) as a polymeric binder and vanadium oxytriisopropoxide as a vanadium oxide precursor. The microstructure and composition of as‐spun and calcined (300–500 °C) VO_x–PVAc fibres were systematically investigated by scanning electron microscopy, thermogravimetry, reflectance infrared Fourier transform, micro‐Raman spectroscopy and photoluminescence in view of their possible use in gas sensor fabrication. The comparative discussion of the characterization results indicates that V₂O₅–PVAc fibres are obtained. Calcination gradually removes PVAc and promotes structural rearrangement with consequent fibre‐morphology changes. With increasing calcination temperature, the crystallinity degree of V₂O₅ improves and a more oxygen‐deficient substoichiometric surface layer forms. Calcination at 400 °C preserves the fibre integrity. Indeed, fibres calcined at this temperature appear as the most suitable ones for use as the active layer in gas‐sensing devices. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of sulphuric–nitric acid mixture composition on surface chemistry and structural evolution of liquid‐phase oxidised carbon nanotubes

Liquid phase functionalisation of carbon nanotubes is carried out via a H₂SO₄ + HNO₃ mixture, and the effect of the sulphuric to nitric acid volume ratio (1:3–3:1) is systematically investigated by means of complementary techniques, observing the expected progressive downgrade of the crystalline quality, along with the increase of oxygenated functionality concentration. In addition, in contrast with common expectations, the results obtained demonstrate that the concentration of carboxylic groups (acids and anhydrides) never exceeds that of all other functionalities (lactones, phenols, quinones/carbonyls and sulphonic groups) introduced by chemical oxidation. Only by using equal volumes of sulphuric and nitric acids the concentrations of carboxylic and non‐carboxylic groups become comparable. Raman analysis reveals that a change in the sample homogeneity accompanies the variations of the relative proportions of the various oxygenated groups, by the typology of which the vibration modes of carbon pairs and carbon rings appear to be affected to different extents. Copyright © 2012 John Wiley & Sons, Ltd.     

Monte Carlo analysis of electron scattering in microstructure processes in the 0.2 μm region

Summary Theoretical understanding of electron scattering phenomena is essential to the implementation of X-ray lithography. In this paper we theoretically investigate electron scattering effects in X-ray mask-making, by means of a subtractive process, in the 0.2 μm region. The results of Monte Carlo simulation are utilized in order to calculate radial absorbed energy distributions and resist development profiles with respect to some cases of practical interest. Relevant process variables are considered, such as electron-beam energy (10 to 30 keV) and spot sizes. The results show how, by choosing appropriate working conditions, high-resolution patterns can be obtained. Favourable process conditions are found at the low electron energy limit. However, it is shown that suitable electron-beam spot sizes are required in order to take fully advantage of the favourable properties of the scattering. Research supported by Finalized Project MA.D.E.S.S. of Consiglio Nazionale delle Ricerche.     

Hydric precipitation as joint statistical process: Dry or wet day and amount of water

Summary The long time series of daily hydric precipitations in Modena, Italy, allows homogeneity tests which do not involve the assumption of particular models. The tests show that the number of dry days which have just preceded any wet day and the amount of precipitated water are independent. This result, which is likely to be valid for the Po Valley, implies that the hydric precipitation considered as a joint process can be factorized into two independent processes. The first concerns the probability that the particular day considered is dry or wet; the second concerns the probability of a certain amount of precipitated water in that wet day. The probabilities of each process have been often studied in the literature, whereas studies on the joint process are absent. Paper presented at the 1° Congresso del Gruppo Nazionale di Fisica dell' Atmosfera e dell'Oceano, June 19–22, 1984, Rome.     

Bimodal bond-length distributions in cobalt-doped CdSe, ZnSe, and Cd1-xZnxSe quantum dots

Electronic absorption spectroscopy has been used to study changes in Co2+ ligand-field parameters as a function of alloy composition in Co2+-doped Cd(1-x)Zn(x)Se nanocrystals. A shift in the energy of the 4T1(P) excited-state with alloy composition is observed. Analysis reveals that Co2+-Se2- bond lengths change relatively little as the host is varied continuously from CdSe to ZnSe, generating a large difference between microscopic and average cation-anion bond lengths in Co2+-doped CdSe nanocrystals but not in Co2+-doped ZnSe nanocrystals. The bimodal bond-length distributions observed here are shown to cause a diameter-dependent enthalpic destabilization of doped semiconductor nanocrystals.