Graphene and its derivatives: switching ON and OFF

As the thinnest material ever known in the universe, graphene has been attracting tremendous amount of attention in both materials science and condensed-matter physics since its successful isolation a few years ago. This one-atom-thick two-dimensional pseudo-infinite nano-crystal consists of sp(2)-hybridized aromatic carbon atoms covalently packed into a continuous hexagonal lattice. Graphene exhibits a range of unique properties, viz., high three-dimensional aspect ratio and large specific surface area, superior mechanical stiffness and flexibility, remarkable optical transmittance, extraordinary thermal response and excellent electronic transport properties, promising its applications in the next generation electronics. To switch graphene and its derivatives between ON and OFF states in nanoelectronic memory devices, various techniques have been developed to manipulate the carbon atomic sheets via introducing the valence-conduction bandgap and to enhance their processability. In this article, we review the utilization of electrically, thermally and chemically modified graphene and its polymer-functionalized derivatives for switching and information storage applications. The challenges posed on the development of novel graphene materials and further enhancements of the device switching performance have also been discussed.     

From nanographene and graphene nanoribbons to graphene sheets: chemical synthesis

Graphene, an individual two-dimensional, atomically thick sheet of graphite composed of a hexagonal network of sp(2) carbon atoms, has been intensively investigated since its first isolation in 2004, which was based on repeated peeling of highly oriented pyrolyzed graphite (HOPG). The extraordinary electronic, thermal, and mechanical properties of graphene make it a promising candidate for practical applications in electronics, sensing, catalysis, energy storage, conversion, etc. Both the theoretical and experimental studies proved that the properties of graphene are mainly dependent on their geometric structures. Precise control over graphene synthesis is therefore crucial for probing their fundamental physical properties and introduction in promising applications. In this Minireview, we highlight the recent progress that has led to the successful chemical synthesis of graphene with a range of different sizes and chemical compositions based on both top-down and bottom-up strategies.     

石墨烯的功能化及其在储能材料领域中的应用

石墨烯是由sp~2杂化的碳原子紧密堆积成的单原子层二维碳材料,由于其优异的物理和化学性质被视为最有前景的新型材料之一。但由于石墨烯片层之间在范德华力的作用下易发生不可逆团聚,丧失其单层二维纳米片的结构特性,以及石墨烯表面呈现惰性状态,致使其与其他介质的相互作用较弱,难以均匀分散在极性或非极性的溶剂中,因而石墨烯的应用受到限制。对石墨烯进行功能化可以调控其分子结构、电子能级和化学性质,不仅可以有效抑制石墨烯的团聚而且能够改善其在溶剂中的分散性和稳定性,从而实现石墨烯基材料的多元化应用。本文综述了近年来共价键和非共价键功能化石墨烯以及其复合材料在储能领域的研究进展,并对功能化石墨烯的发展前景进行了展望。     

Deformation Behavior of Three-Dimensional Carbon Structures Under Hydrostatic Compression

The paper presents the results of numerical simulation aimed at studying the deformation behavior of carbon structures containing carbon atoms with various coordination numbers and, consequently, various electronic configurations and properties. Namely, the method of molecular dynamics was used to study the deformation behavior of two different structures of crumpled graphene (sp²-material formed by graphene flakes bonded by Van der Waals forces) and carbon diamond-like phases (rigid sp³-structures) under hydrostatic compression. Stress-strain curves have been obtained, structural features have been shown to affect mechanical properties of three-dimensional carbon structures.     

Electronic structures and bonding of graphyne sheet and its BN analog

Using density functional theory and generalized gradient approximation for exchange and correlation, we present theoretical analysis of the electronic structure of recently synthesized graphyne and its boron nitride analog (labeled as BN-yne). The former is composed of hexagonal carbon rings joined by C-chains, while the latter is composed of hexagonal BN rings joined by C-chains. We have explored the nature of bonding and energy band structure of these unique systems characterized by sp and sp(2) bonding. Both graphyne and BN-yne are found to be direct bandgap semiconductors. The bandgap can be modulated by changing the size of hexagonal ring and the length of carbon chain, providing more flexibilities of energy band engineering for device applications. The present study sheds theoretical insight on better understanding of the properties of the novel carbon-based 2D structures beyond the graphene sheet.     

Structural principles of semiconducting Group 14 clathrate frameworks

We have performed a comprehensive theoretical investigation of the structural principles of semiconducting clathrate frameworks composed of the Group 14 elements carbon, silicon, germanium, and tin. We have investigated the basic clathrate frameworks, together with their polytypes, intergrowth clathrate frameworks, and extended frameworks based on larger icosahedral building blocks. Quantum chemical calculations with the PBE0 hybrid density functional method provided a clear overview of the structural trends and electronic properties among the various clathrate frameworks. In agreement with previous experimental and theoretical studies, the clathrate II framework proved to be the energetically most favorable, but novel hexagonal polytypes of clathrate II also proved to be energetically very favorable. In the case of silicon, several of the studied clathrate frameworks possess direct and wide band gaps. The band structure diagrams and simulated powder X-ray patterns of the studied frameworks are provided and systematic preliminary evaluation of guest-occupied frameworks is conducted to shed light on the characteristics of novel, experimentally feasible clathrate compositions.     

聚合物/无机纳米粒子复合膜在气体分离中的研究进展

由于聚合物膜具有可高度设计、机械性能好、易于加工 等优点,是理想的气体分离材料。然而,聚合物膜在气体选择性和渗透性方面存在平衡限制,在聚合物中引入纳米粒子,是提高气体分离性能的一种有效手段。本文基于聚合物/无机纳米粒子复合膜在气体分离领域的研究现状,重点阐述了零维纳米粒子（二氧化硅、二氧化钛）、一维纳米粒子（碳纳米管）、二维纳米粒子（氧化石墨烯、二维过渡金属氧化物）、三维纳米粒子（金属有机框架、沸石）对气体分离性能的影响,并展望了聚合物复合分离膜的发展趋势,为未来高效分离膜的研发提供了参考。     

Investigation of possible structures of silicon nanotubes via density-functional tight-binding molecular dynamics simulations and ab initio calculations

We show, computationally, that single-walled silicon nanotubes (SiNTs) can adopt a number of distorted tubular structures, representing respective local energy minima, depending on the theory used and the initial models adopted. In particular, "gearlike" structures containing alternating sp(3)-like and sp(2)-like silicon local configurations have been found to be the dominant structural form for SiNTs via density-functional tight-binding molecular dynamics simulations (followed by geometrical optimization using Hartree-Fock or density function theory) at moderate temperatures (below 100 K). The gearlike structures of SiNTs deviate considerably from, and are energetically more stable than, the smooth-walled tubes (the silicon analogues of single-walled carbon nanotubes). They are, however, energetically less favorable than the "string-bean-like" SiNT structures previously derived from semiempirical molecular orbital calculations. The energetics and the structures of gearlike SiNTs are shown to depend primarily on the diameter of the tube, irrespective of the type (zigzag, armchair, or chiral). In contrast, the energy gap is very sensitive to both the diameter and the type of the nanotube.     

Two-dimensional MOFs: Design & Synthesis and Applications

For the past few years, two-dimensional materials have attracted widespread attention owing to their special properties and potential applications. It is well-known that graphene, transition metal disulfide compounds (TMDC), carbon nitride, transition metal carbonitrides (Mxenes), silene and hexagonal boron nitride are typical two-dimensional materials. Compared with these traditional two-dimensional materials, two-dimensional MOF is favored by numerous researchers because of its unique structure. Based on the unique metal ion and organic ligand coordination of MOF and two-dimensional layered structure, the applications of two-dimensional MOF were getting serious, including catalysis, supercapacitor, gas adsorption/separation, sensors and so on. This review presents a relatively comprehensive summary of the design & synthesis and applications of two-dimensional MOF over the past few years. Furthermore, the opportunities and challenges have been discussed to supply a promising prospect to this field.     

Graphene and graphene-based nanomaterials: the promising materials for bright future of electroanalytical chemistry

Similar to its popular older cousins of fullerene and carbon nanotubes (CNTs), the latest form of nanocarbon, graphene, is inspiring intensive research efforts in its own right. As an atomically thin layer of sp(2)-hybridized carbon, graphene possesses spectacular electronic, optical, magnetic, thermal and mechanical properties, which make it an exciting material in a variety of important applications. In this review, we present the current advances in the field of graphene electroanalytical chemistry, including the modern methods of graphene production, and graphene functionalization. Electrochemical (bio) sensing developments using graphene and graphene-based materials are summarized in more detail, and we also speculate on their future and discuss potential progress for their applications in electroanalytical chemistry.     

Structural, electronic, and optical properties of 9-heterofluorenes: a quantum chemical study

Density-functional theory studies were applied to investigate the structural, electronic, and optical properties of 9-heterofluorenes achieved by substituting the carbon at 9 position of fluorene with silicon, germanium, nitrogen, phosphor, oxygen, sulfur, selenium, or boron. These heterofluorenes and their oligomers up to pentamers are highly aromatic and electrooptically active. The alkyl and aryl substituents of the heteroatom have limited influence, but the oxidation of the atom has significant influence on their molecular structures and properties. The highest occupied molecular orbital (HOMO)-lowest occupied molecular orbital (LUMO) interaction theory was successfully applied to analyze the energy levels and the frontier wave functions of these heterofluorenes. Most heterofluorenes belong to type B of interaction with low-lying LUMO and have the second kind of wave function. Carbazole and selenafluorene have type C of interaction with high-lying HOMO and the third kind of wave function. Types C and D of heterofluorenes, such as carbazole, oxygafluorene, sulfurafluorene, and selenafluorene also have high triplet state energies. The extrapolated HOMO and LUMO for polyheterofluorenes indicate that polyselenonafluorene has the lowest LUMO; polycarbazole has the highest HOMO; polyselenafluorene has the highest bandgap (E(g)); and polyborafluorene has the lowest E(g). Heterofluorenes and their oligomers and polymers are of great experimental interests, especially those having extraordinary properties revealed in this study.     

Room Temperature Incorporation of Arsenic Atoms into the Germanium (001) Surface**

Germanium has emerged as an exceptionally promising material for spintronics and quantum information applications, with significant fundamental advantages over silicon. However, efforts to create atomic-scale devices using donor atoms as qubits have largely focused on phosphorus in silicon. Positioning phosphorus in silicon with atomic-scale precision requires a thermal incorporation anneal, but the low success rate for this step has been shown to be a fundamental limitation prohibiting the scale-up to large-scale devices. Here, we present a comprehensive study of arsine (AsH₃) on the germanium (001) surface. We show that, unlike any previously studied dopant precursor on silicon or germanium, arsenic atoms fully incorporate into substitutional surface lattice sites at room temperature. Our results pave the way for the next generation of atomic-scale donor devices combining the superior electronic properties of germanium with the enhanced properties of arsine/germanium chemistry that promises scale-up to large numbers of deterministically placed qubits.     

Template effect in the competition between Haeckelite and graphene growth on Ni(111): quantum chemical molecular dynamics simulations

Quantum chemical molecular dynamics (QM/MD) simulations of ensembles of C(2) molecules on the Ni(111) terrace show that, in the absence of a hexagonal template or step edge, Haeckelite is preferentially nucleated over graphene as a metastable intermediate. The nucleation process is dominated by the swift transition of long carbon chains toward a fully connected sp(2) carbon network. Starting from a pentagon as nucleus, pentagons and heptagons condense during ring collapse reactions, which results in zero overall curvature. To the contrary, in the presence of a coronene-like C(24) template, hexagonal ring formation is clearly promoted, in agreement with recent suggestions from experiments. In the absence of step edges or molecular templates, graphene nucleation follows Ostwald's "rule of stages" cascade of metastable states, from linear carbon chains, via Haeckelite islands that finally anneal to graphene.     

Multilayer DNA origami packed on hexagonal and hybrid lattices

"Scaffolded DNA origami" has been proven to be a powerful and efficient approach to construct two-dimensional or three-dimensional objects with great complexity. Multilayer DNA origami has been demonstrated with helices packing along either honeycomb-lattice geometry or square-lattice geometry. Here we report successful folding of multilayer DNA origami with helices arranged on a close-packed hexagonal lattice. This arrangement yields a higher density of helical packing and therefore higher resolution of spatial addressing than has been shown previously. We also demonstrate hybrid multilayer DNA origami with honeycomb-lattice, square-lattice, and hexagonal-lattice packing of helices all in one design. The availability of hexagonal close-packing of helices extends our ability to build complex structures using DNA nanotechnology.     

Carbon-Based Material-Supported Palladium Nanocatalysts in Coupling Reactions: Discussion on their Stability and Heterogeneity

Scientific interest in carbon-based materials (CBMs) has grown dramatically over the past few decades. Due to a variety of atomic orbital hybrid forms (sp, sp² and sp³ hybridization), carbon can form a variety of materials with diverse structures and characteristics. CBMs used as efficient catalyst supports show extensive promise in organic reactions, which is attributed to their structural similarity with organics, large specific surface area, chemical stability, and photocatalytic properties. This review presents the synthesis of CBM-supported palladium nanocatalysts based on impregnation, template methods, etc. The CBMs include activated carbon (AC), graphene, carbon nanotubes (CNTs), and their functionalized products, as supports for improving the activity and recyclability of simple Pd nanocatalysts. After surveying the literature where these catalysts have been utilized for carbon–carbon coupling reactions, there is a particular emphasis on Suzuki, Heck, and Sonogashira reactions. The catalytic mechanism of these Pd nanocatalysts (surface heterogeneous catalysis or homogeneous catalysis caused by Pd leaching) is discussed in detail, especially the effect of Pd leaching on the stability of the catalyst.     

Study of local lattice relaxation of substitutional impurities in silicon and germanium

Local lattice relaxation of substitutional donors in silicon investigated using self‐consistent multiple scattering Xα (MSXα) method within the framework of the standard muffin‐tin potential approximation is extended to substitutional donors in germanium and substitutional acceptors in both silicon and germanium. Incorporating the effect of lattice relaxation surrounding the impurity makes the model suitable for both shallow and deep levels. Chemical trends of some aspects of impurity states, such as local lattice relaxation and charge transfer, of the impurities both in silicon and germanium are inferred. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004     

Densely packed overlayer of iron phthalocyanine molecules grown on single-layer graphene

The FePc molecules form a series of order superstructures on single-layer graphene grown on Ru(0001) with increasing molecular coverage.     

Isoelectronic doping of graphdiyne with boron and nitrogen: stable configurations and band gap modification

Graphdiyne, consisting of sp- and sp(2)-hybridized carbon atoms, is a new member of carbon allotropes which has a natural band gap ~1.0 eV. Here, we report our first-principles calculations on the stable configurations and electronic structures of graphdiyne doped with boron-nitrogen (BN) units. We show that BN unit prefers to replace the sp-hybridized carbon atoms in the chain at a low doping rate, forming linear BN atomic chains between carbon hexagons. At a high doping rate, BN units replace first the carbon atoms in the hexagons and then those in the chains. A comparison study indicates that these substitution reactions may be easier to occur than those on graphene which composes purely of sp(2)-hybridized carbon atoms. With the increase of BN component, the band gap increases first gradually and then abruptly, corresponding to the transition between the two substitution motifs. The direct-band gap feature is intact in these BN-doped graphdiyne regardless the doping rate. A simple tight-binding model is proposed to interpret the origin of the band gap opening behaviors. Such wide-range band gap modification in graphdiyne may find applications in nanoscaled electronic devices and solar cells.     

The electronic structure of nonpolyhex carbon nanotubes

Generalizing the folding method to any periodic two-dimensional planar carbon structures we have calculated the corresponding electronic structures in the framework of the one orbital one site tight-binding (Bloch-Hückel) method by solving the eigenvalue problems in a numerical way. We discussed the metallic or the nonmetallic behavior of the nanotubes by applying the folding vectors of parameters (m, n). We extended the topological coordinate method to two-dimensional periodic planar structures as well. Nearly regular hexagonal, pentagonal, and heptagonal polygons were obtained. The curvatures of the final relaxed structures can be read from the sizes of the polygons. Thus relying only on the topological information we could describe the shape of the tubular structures and their conductivity behaviors.     

Theoretical study of the geometric and electronic structure of neutral and anionic doped silver clusters, Ag5X with X = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni

Density functional theory (DFT) has been applied to investigate the low-lying electronic states of neutral and anionic transition metal doped silver clusters Ag₅X^0,− with X = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni using the B3LYP functional with the Stuttgart SDD basis sets. The structural features, frontier orbital energy gaps (HOMO and LUMO), vertical detachment energies, and vertical and adiabatic electronic affinities are evaluated. For all doped silver clusters, both in neutral and anionic states, two-dimensional and three-dimensional low-energy isomers are found to coexist. For neutral clusters, dopant Sc, Ti, V, and Mn atoms largely decrease the frontier orbital energy gaps, while they are markedly increased by Sc and Fe atoms in the anionic clusters. A completely quenched dopant magnetic moment is found in Ag₅Sc, while high spin magnetic moments are located on the other dopant atoms in Ag₅X^0,−.