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Honeycomb Carbon: A Review of Graphene

Electrolytes and interphases in Li-ion batteries and beyond.

To realize graphene-based electronics, various types of graphene are required; thus, modulation of its electrical properties is of great importance. Theoretic studies show that intentional doping is a promising route for this goal, and the doped graphene might promise fascinating properties and widespread applications. However, there is no experimental example and electrical testing of the substitutionally doped graphene up to date. Here, we synthesize the N-doped graphene by a chemical vapor deposition (CVD) method. We find that most of them are few-layer graphene, although single-layer graphene can be occasionally detected. As doping accompanies with the recombination of carbon atoms into graphene in the CVD process, N atoms can be substitutionally doped into the graphene lattice, which is hard to realize by other synthetic methods. Electrical measurements show that the N-doped graphene exhibits an n-type behavior, indicating substitutional doping can effectively modulate the electrical properties of graphene. Our finding provides a new experimental instance of graphene and would promote the research and applications of graphene.

Synthesis of N-Doped Graphene by Chemical Vapor Deposition and Its Electrical Properties

Graphitic carbon nitride, g-C3N4, can be made by polymerization of cyanamide, dicyandiamide or melamine. Depending on reaction conditions, different materials with different degrees of condensation, properties and reactivities are obtained. The firstly formed polymeric C3N4 structure, melon, with pendant amino groups, is a highly ordered polymer. Further reaction leads to more condensed and less defective C3N4 species, based on tri-s-triazine (C6N7) units as elementary building blocks. High resolution transmission electron microscopy proves the extended two-dimensional character of the condensation motif. Due to the polymerization-type synthesis from a liquid precursor, a variety of material nanostructures such as nanoparticles or mesoporous powders can be accessed. Those nanostructures also allow fine tuning of properties, the ability for intercalation, as well as the possibility to give surface-rich materials for heterogeneous reactions. Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions, such as for the activation of benzene, trimerization reactions, and also the activation of carbon dioxide. Model calculations are presented to explain this unusual case of heterogeneous, metal-free catalysis. Carbon nitride can also act as a heterogeneous reactant, and a new family of metal nitride nanostructures can be accessed from the corresponding oxides.

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Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts

Polymeric graphitic carbon nitride materials (for simplicity: g-C(3)N(4)) have attracted much attention in recent years because of their similarity to graphene. They are composed of C, N, and some minor H content only. In contrast to graphenes, g-C(3)N(4) is a medium-bandgap semiconductor and in that role an effective photocatalyst and chemical catalyst for a broad variety of reactions. In this Review, we describe the "polymer chemistry" of this structure, how band positions and bandgap can be varied by doping and copolymerization, and how the organic solid can be textured to make it an effective heterogenous catalyst. g-C(3)N(4) and its modifications have a high thermal and chemical stability and can catalyze a number of "dream reactions", such as photochemical splitting of water, mild and selective oxidation reactions, and--as a coactive catalytic support--superactive hydrogenation reactions. As carbon nitride is metal-free as such, it also tolerates functional groups and is therefore suited for multipurpose applications in biomass conversion and sustainable chemistry.

Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry

We report the development of a simple chemical route to growing Ge1−xSnx semiconductors using ultrahigh-vacuum chemical vapor deposition and the molecular precursor (Ph)SnD3 as the source of Sn atoms. Thin films were deposited on oxidized and oxide-free Si by reactions of (Ph)SnD3 with Ge2H6 at 350 °C. The composition, microstructure, and bonding properties of the films were characterized by Rutherford backscattering, high-resolution analytical electron microscopy, and Raman spectroscopy. As-deposited Ge1−xSnx on oxidized Si displayed good crystallinity which improved significantly by annealing at 400 °C. High-resolution electron microscopy and diffraction indicated a diamond-cubic structure with lattice constants intermediate to those of Ge and α-Sn. As-deposited Ge1−xSnx on pure Si was monocrystalline and epitaxial. Nanoprobe analysis in plan view and cross section revealed that the as-deposited and annealed materials were homogeneous with good chemical purity. The Raman spectra showed bands correspondi...

Simple chemical routes to diamond-cubic germanium-tin alloys

The compositional dependence of the cubic lattice parameter in Ge1-ySny alloys has been revisited Large 1000-atom supercell ab initio simulations confirm earlier theoretical predictions that indicate a positive quadratic deviation from Vegard's law, albeit with a somewhat smaller bowing coefficient, θ = 0047 A, than found from 64-atom cell simulations (θ = 0063 A) On the other hand, measurements from an extensive set of alloy samples with compositions y < 015 reveal a negative deviation from Vegard's law The discrepancy with earlier experimental data, which supported the theoretical results, is traced back to an unexpected compositional dependence of the residual strain after growth on Si substrates The experimental bowing parameter for the relaxed lattice constant of the alloys is found to be θ = -0066 A Possible reasons for the disagreement between theory and experiment are discussed in detail

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Nonlinear structure-composition relationships in the Ge1-ySny/Si(100) (y<0.15) system

Gallium nitride films, epitaxially grown on Si(111) via a lattice-matched ZrB(2) buffer layer by plasma-assisted molecular beam epitaxy, have been studied in situ by noncontact atomic force microscopy and also in real time by reflection high-energy electron diffraction. The grown films were determined to be always N-polar. First-principles theoretical calculations modeling the interface structure between GaN(0001) and ZrB(2)(0001) clarify the origin of the N polarity.

Surface and Interface Studies of GaN Epitaxy on Si(111) via ZrB 2 Buffer Layers

The emission properties of GeSn heterostructure pin diodes have been investigated The devices contain thick (400–600 nm) Ge1−ySny i-layers spanning a broad compositional range below and above the crossover Sn concentration yc where the Ge1−ySny alloy becomes a direct-gap material These results are made possible by an optimized device architecture containing a single defected interface thereby mitigating the deleterious effects of mismatch-induced defects The observed emission intensities as a function of composition show the contributions from two separate trends: an increase in direct gap emission as the Sn concentration is increased, as expected from the reduction and eventual reversal of the separation between the direct and indirect edges, and a parallel increase in non-radiative recombination when the mismatch strains between the structure components is partially relaxed by the generation of misfit dislocations An estimation of recombination times based on the observed electroluminescence intensi

Electroluminescence from GeSn heterostructure pin diodes at the indirect to direct transition

The compositional dependence of the Ge–Ge Raman mode in SnGe alloys has been measured in samples grown on Si substrates using a chemical vapor deposition technique. The experimental result, Δω(s)=(−68±5)s (where s is the Sn concentration), is in very good agreement with a theoretical prediction from a simple model with parameters adjusted to the compositional dependence of Raman frequencies in GeSi alloys.

John Kouvetakis

Papers

Simple chemical routes to diamond-cubic germanium-tin alloys

Nonlinear structure-composition relationships in the Ge1-ySny/Si(100) (y<0.15) system

Surface and Interface Studies of GaN Epitaxy on Si(111) via ZrB 2 Buffer Layers

Electroluminescence from GeSn heterostructure pin diodes at the indirect to direct transition

Scaling law for the compositional dependence of Raman frequencies in SnGe and GeSi alloys