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Mastrapa G.C.,Pontifical Catholic University of Rio de Janeiro | Da Costa M.E.H.M.,Pontifical Catholic University of Rio de Janeiro | Larrude D.G.,Pontifical Catholic University of Rio de Janeiro | Freire F.L.,Pontifical Catholic University of Rio de Janeiro | Freire F.L.,Brazilian Center for Physical Research
Materials Chemistry and Physics | Year: 2015

The synthesis of a single-layer graphene using a low-pressure Chemical Vapor Deposition (CVD) system with triphenylphosphine as precursor is reported. The amount of triphenylphosphine used as precursor was in the range of 10-40 mg. Raman spectroscopy was employed to analyze samples prepared with 10 mg of the precursor, and these spectra were found typical of graphene. The Raman measurements indicate that the progressive degradation of graphene occurs as the amount of triphenylphosphine increases. X-ray photoelectron spectroscopy measurements were performed to investigate the different chemical environments involving carbon and phosphorous atoms. Scanning electron microscopy and transmission electron microscopy were also employed and the results reveal the formation of dispersed nanostructures on top of the graphene layer, In addition, the number of these nanostructures is directly related to the amount of precursor used for sample growth. © 2015 Elsevier B.V. Source

Ospina C.A.,Brazilian Center for Physical Research | Ospina C.A.,Electronic Microscopy Laboratory | Terra J.,Brazilian Center for Physical Research | Ramirez A.J.,Electronic Microscopy Laboratory | And 3 more authors.
Colloids and Surfaces B: Biointerfaces | Year: 2012

High-resolution transmission electron microscopy (HRTEM) and ab initio quantum-mechanical calculations of electronic structure were combined to investigate the structure of the hydroxyapatite (HA) (0. 1. 0) surface, which plays an important role in HA interactions with biological media. HA was synthesized by in vitro precipitation at 37 °C. HRTEM images revealed thin elongated rod nanoparticles with preferential growth along the [0. 0. 1] direction and terminations parallel to the (0. 1. 0) plane. The focal series reconstruction (FSR) technique was applied to develop an atomic-scale structural model of the high-resolution images. The HRTEM simulations identified the coexistence of two structurally distinct terminations for (0. 1. 0) surfaces: a rather flat Ca(II)-terminated surface and a zig-zag structure with open OH channels. Density functional theory (DFT) was applied in a periodic slab plane-wave pseudopotential approach to refine details of atomic coordination and bond lengths of Ca(I) and Ca(II) sites in hydrated HA (0. 1. 0) surfaces, starting from the HRTEM model. © 2011 Elsevier B.V. Source

Larrude D.G.,Mackenzie Presbyterian University | Larrude D.G.,Pontifical Catholic University of Rio de Janeiro | Garcia-Basabe Y.,Federal University of Rio de Janeiro | Garcia-Basabe Y.,Iguacu University | And 3 more authors.
RSC Advances | Year: 2015

Graphene sheet layers were grown by chemical vapor deposition (CVD) under a polycrystalline copper substrate using methane (CH4) and triphenylphosphine (P(C6H5)3) as carbon and phosphorous precursors, respectively. The films obtained from the CH4 and P(C6H5)3 chemical precursors were labeled as G/Cu and GP/Cu, respectively. Electronic structure investigation was performed on these two graphene samples combining different spectroscopic techniques. Raman spectroscopy shows the presence of single and multilayers in G/Cu and GP/Cu, respectively. A blue shift of 30 cm-1 of the 2D band in the GP/Cu film with respect to G/Cu is evidence of the p-type doping of GP/Cu. X-ray photoelectron and reflection electron energy loss spectroscopy (REELS) confirm the bilayer formation in the GP/Cu film. REELS also shows that the presence of phosphorous does not open the electronic band gap of the graphene film. The work function of 4.3 eV for G/Cu and 4.8 eV for GP/Cu films were determined by ultraviolet photoelectron spectroscopy. The increase of the work function is attributed to the electron transfer to the Cu substrate. The partially unoccupied densities of states in phosphorous doped graphene (GP/Cu) were evaluated by X-ray photoabsorption spectroscopy. The core-hole clock approach using resonant Auger spectroscopy was employed for investigating the charge transfer dynamics around the P K-edge in GP/Cu. Ultrafast charge transfer delocalization on a time scale of femtoseconds was observed, demonstrating a strong electronic coupling between unoccupied states of the phosphorous and the conduction band of the copper substrate. The combined spectroscopic results suggest p-type doping in GP/Cu by the electron transfer mechanism. © 2015 The Royal Society of Chemistry. Source

Ospina C.A.,Brazilian Center for Physical Research | Ospina C.A.,Electronic Microscopy Laboratory | Terra J.,Brazilian Center for Physical Research | Ramirez A.J.,Electronic Microscopy Laboratory | And 2 more authors.
Key Engineering Materials | Year: 2012

Hydroxyapatite (HA, Ca10(PO4)6(OH) 2) is one of the most important biomaterials used in bone regeneration therapies due to their chemical properties are very similar to the inorganic phase found in bone tissues. The direct observation of the ultrastructure of HA is very important in the comprehension of their nucleation and interactions with the molecules involved in bone formation. High-resolution Transmission Electron Microscopy (HRTEM) is a currently technique used for this task. However, the interpretation of the images is not straightforward and needs the use of softwares dedicated to high-resolution images simulations. This work presents the applicability of MEGACELL software in the analysis of HRTEM images of HA nanoparticles. MEGACELL is the most newly software, developed to construct nanocrystals models for HRTEM multislice simulations. The output files generated by MEGACELL are raw data format (.xyz), containing all the atomic positions, as well as input files compatible with JEMS (Java Electron Microscopy Software) format files. High-resolution images were acquired using a JEM 3010 URP microscope, with a LaB6 thermionic electron gun operating at 300 kV, with a point-to-point resolution of 0.17 nm and a CCD Gatan 794SC multiscan digital camera, attached to the DigitalMicrographTM software for recording and image processing. Electron microscopy samples were prepared by dropping HA powder on copper TEM grids. HRTEM experimental images of HA particles, orientated along different zone axes, were interpreted applying the MEGACELL software to construct HA nanocrystal models and the multislice method to simulated them. MEGACELL improves the extraction of the ultrastructural features and facilitates a better interpretation of the phase-contrast images. © (2012) Trans Tech Publications. Source

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