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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

High-quality, tensile-strained Ge layers with variable thickness (>30nm) have been deposited at low temperature (350–380°C) on Si(100) via fully relaxed Ge1−ySny buffers. The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable tensile strains up to 0.25% for y=0.025. Combined Raman analysis and high resolution x-ray diffraction using multiple off-axis reflections reveal unequivocally that the symmetry of tensile Ge is perfectly tetragonal, while the strain state of the buffer (∼200nm thick) remains essentially unchanged. A downshift of the direct gap consistent with tensile strain has been observed.

Perfectly tetragonal, tensile-strained Ge on Ge1−ySny buffered Si(100)

A direct absorption edge tunable between 0.8 and approximately 1.4 eV is demonstrated in strain-free ternary Ge_{1-x-y}Si_{x}Sn_{y} alloys epitaxially grown on Ge-buffered Si. This decoupling of electronic structure and lattice parameter-unprecedented in group-IV alloys-opens up new possibilities in silicon photonics, particularly in the field of photovoltaics. The compositional dependence of the direct band gap in Ge_{1-x-y}Si_{x}Sn_{y} exhibits a nonmonotonic behavior that is explained in terms of coexisting small and giant bowing parameters in the two-dimensional compositional space.

Tunable optical gap at a fixed lattice constant in group-IV semiconductor alloys.

Raman scattering in Ge1−ySny alloys

A semiconductor structure including a single quantum well Ge 1−x1−y Si x1 Sn/Ge 1−x2 Si x2 heterostructure grown strain-free on Si(100) via a Sn 1−x Ge x buffer layer is shown.

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Materials and optical devices based on group IV quantum wells grown on Si-Ge-Sn buffered silicon

Electroluminescence spectra from Si/Ge1−ySny heterostructure diodes are reported. The observed emission is dominated by direct gap optical transitions and displays the expected compositional dependence of the peak energy. Weaker indirect gap emission is also observed, and their energies are consistent with a closing of the indirect-direct separation as the Sn concentration is increased. The intensity of the EL spectra shows a superlinear dependence on the injection current, which is modeled using a Van Roosbroeck–Shockley expression for the emission intensity. The model assumes quasiequilibrium conditions for the electrons populating the different valleys in the conduction band of Ge.

John Kouvetakis

Papers

Perfectly tetragonal, tensile-strained Ge on Ge1−ySny buffered Si(100)

Tunable optical gap at a fixed lattice constant in group-IV semiconductor alloys.

Raman scattering in Ge1−ySny alloys

Materials and optical devices based on group IV quantum wells grown on Si-Ge-Sn buffered silicon

Direct gap electroluminescence from Si/Ge1−ySny p-i-n heterostructure diodes