The search for graphene or few-layer graphene production methods that are simple, allow mass production, and yield good quality material continues to provoke intense investigation. The present work contributes to this investigation through the study of the aqueous exfoliation of four types of graphene sources, which are namely graphite and graphite nanoflakes with different morphologies and geographical origins. The exfoliation was achieved in an aqueous solution of a soluble pyrene derivative that was synthesized to achieve maximum interaction with the graphene surface at low concentration (5 × 10−5 M). The yield of bilayer and few-layer graphene obtained was quantified by Raman spectroscopic analysis, and the adsorption of the pyrene derivative on the graphene surface was studied by thermogravimetric analysis and X-ray diffraction. The whole procedure was rationalized with the help of molecular modeling.
A phenomenological methodology has been used to characterize the intrinsic diffusion of tin in GaAs based on the coupled motion of substitutional and interstitial Sn atoms. Both the rapid diffusivity of interstitial Sn atoms and their transformation to substitutionals by occupying gallium vacancies are processes that lead to an anomalous “double-profile” diffusion curve observed in highly doped samples. The model was developed for samples annealed over a range of 973–1123K. The model developed while developed and perhaps only applicable over this temperature range may be extendable to a larger temperature range provided that the underlying mechanism is unchanged. The methodology consists of using key aspects of the diffusion profiles along with the mass conservation criterion to predetermine bounds which prescribe the values of the parameters used to model the experimental data. This fitting procedure allowed for a more rapid assessment of the model parameters by providing physical bounds on their values as derived from the experimental profiles. The proposed diffusion mechanism combines the simultaneous diffusion of both interstitial and substitutional Sn atoms as well as key defect reactions characterizing their mutual equilibrium with the background vacancy concentration.
Graphene nanoribbons (GNR) were generated in ethanol solution by unzipping pyrrolidine-functionalized carbon nanotubes under mild conditions. Evaporation of the solvent resulted in regular few-layer stacks of graphene nanoribbons observed by transmission electron microscopy (TEM) and X-ray diffraction. The experimental interlayer distance (0.49–0.56 nm) was confirmed by computer modelling (0.51 nm). Computer modelling showed that the large interlayer spacing (compared with graphite) is due to the presence of the functional groups and depends on their concentration. Stacked nanoribbons were observed to redissolve upon solvent addition. This preparation method could allow the fine-tuning of the interlayer distances by controlling the number and/or the nature of the chemical groups in between the graphene layers.
The thermal interdiffusion of AlSb/ GaSb multiquantum wells was measured and the intrinsic diffusivities of Al and Ga determined over a temperature range of 823-948 K for 30-9000 s. The 77-K photoluminescence ͑PL͒ was used to monitor the extent of interdiffusion through the shifts in the superlattice luminescence peaks. The chemical diffusion coefficient was quantitatively determined by fitting the observed PL peak shifts to the solution of the Schrödinger equation, using a potential derived from the solution of the diffusion equation. The value of the interdiffusion coefficient ranged from 5.2ϫ 10 −4 to 0.06 nm 2 / s over the conditions studied and was characterized by an activation energy of 3.0± 0.1 eV. The intrinsic diffusion coefficients for Al and Ga were also determined with higher values for Al than for Ga, described by activation energies of 2.8± 0.4 and 1.1± 0.1 eV, respectively.
Positron annihilation spectroscopy has been used to investigate the role of vacancies in the interdiffusion of Al and Ga in AlSb/GaSb superlattices. The samples were grown by metalorganic vapor-phase epitaxy on undoped and Te doped GaSb and consisted of ten periods of GaSb quantum wells (thickness 13 nm) and AlSb barriers (thickness 2–3 nm) and an approximately 50 nm thick capping layer of GaSb. The superlattices were annealed at 908 K for up to 250 s, resulting in interdiffusion of Al and Ga between well and barrier. A secondary ion mass spectrometry study showed that the Te dopant diffused from the substrate through the superlattice structure in the annealing process. In the positron annihilation study we observe that the vacancy concentration clearly decreases with annealing for the samples grown on undoped substrates, whereas the samples grown on Te doped substrates show a different annealing behavior.
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