Patterned Partially Hydrogenated Graphene (C₄H) and Its One-Dimensional Analogues: A Computational Study

By means of density functional theory (DFT) computations, we systematically studied the structural and electronic properties of the experimentally just achieved new two-dimensional (2D) hydrocarbon  the patterned partially hydrogenated graphene with formula C₄H (Adv. Mater 2011, 23, 4497), and in particular its one-dimensional (1D) analogues. The C₄H layer is a stable 2D crystal featured with periodic Clar sextet aromatic rings and is semiconducting with a wide band gap; however, this single-sided patterned partially hydrogenated C₄H layer can only be obtained when the possibility of double-sided hydrogenation is excluded, since the double-sided graphane-embedded structure is energetically more favorable. The 1D C₄H nanotubes, rolled up by the C₄H layer, exhibit excellent thermodynamic properties and all have a wide band gap regardless of the tube diameter and chirality. In contrast, cutting the C₄H layer into 1D C₄H nanoribbons can result in rich electronic characteristics: they can be metallic or semiconducting depending on the chirality and edge configuration

XH/π (X = C, Si) Interactions in Graphene and Silicene: Weak in Strength, Strong in Tuning Band Structures

The lack of a band gap has greatly hindered the applications of graphene in electronic devices. By means of dispersion-corrected density functional theory computations, we demonstrated that considerable CH/π interactions exist between graphene and its fully (graphane) or patterned partially (C₄H) hydrogenated derivatives. Due to the equivalence breaking of two sublattices of graphene, a 90 meV band gap is opened in the graphene/C₄H bilayer. The band gap can be further increased to 270 meV by sandwiching graphene between two C₄H layers. By taking advantage of the similar SiH/π interactions, a 120 meV band gap also can be opened for silicene. Interestingly, the high carrier mobility of graphene/silicene can be well-preserved. Our theoretical results suggest a rather practical solution for gap opening of graphene and silicene, which would allow them to serve as field effect transistors and other nanodevices

Tuning Electronic Properties of Germanane Layers by External Electric Field and Biaxial Tensile Strain: A Computational Study

Comprehensive density functional theory computations with van der Waals (vdW) correction demonstrated that there exists strong hydrogen bonding between two-dimensional (2D) germanane layers. Especially, germanane layers all have a direct band gap, irrespective of stacking pattern and thickness. The band gap of germanane bilayer can be flexibly reduced by applying an external electronic field (E-field), leading to a semiconducting-metallic transition, whereas the band gap of germanane monolayer is rather robust in response to E-field. In contrast, the band gaps of both germanane monolayer and bilayer can be reduced to zero when subjected to a biaxial tensile strain. These results provide many useful insights for the wide applications of germanane layers in electronics and optoelectronics

Graphane/Fluorographene Bilayer: Considerable C–H···F–C Hydrogen Bonding and Effective Band Structure Engineering

Systematic density functional theory (DFT) computations revealed the existence of considerable C–H···F–C bonding between the experimentally realized graphane and fluorographene layers. The unique C–H···F–C bonds define the conformation of graphane/fluorographene (G/FG) bilayer and contribute to its stability. Interestingly, G/FG bilayer has an energy gap (0.5 eV) much lower than those of individual graphane and fluorographene. The binding strength of G/FG bilayer can be significantly enhanced by applying appropriate external electric field (E-field). Especially, changing the direction and strength of E-field can effectively modulate the energy gap of G/FG bilayer, and correspondingly causes a semiconductor–metal transition. These findings open new opportunities in fabricating new electronics and opto-electronics devices based on G/FG bilayer, and call for more efforts in using weak interactions for band structure engineering

Self-Modulated Band Structure Engineering in C₄F Nanosheets: First-Principles Insights

Density functional theory (DFT) computations with van der Waals (vdw) correction revealed the existence of considerable C^δ+F^δ−···C^δ+F^δ− dipole–dipole interactions between two experimentally realized C₄F monolayers. The dipole–dipole interactions induce a subtle interlayer polarization, which results in a significantly reduced band gap for C₄F bilayer as compared to the individual C₄F monolayer. With increasing the number of stacked layers, the band gap of C₄F nanosheets can be further reduced, leading to a semiconducting–metallic transition. Moreover, the band gap of C₄F nanosheets can be feasibly modulated by applying an external electric field. Our results provide new insights on taking advantage of nonbonding interactions to tune the electronic properties of graphene materials

Electronic and Magnetic Properties of Hybrid Graphene Nanoribbons with Zigzag-Armchair Heterojunctions

Inspired by the experimentally observed zigzag-armchair graphene nanoribbon heterojunctions, we constructed a new class of infinitely long hybrid graphene nanoribbons (HGNRs), and systematically investigated their electronic and magnetic properties by means of spin-polarized first-principles computations. HGNRs are converted from nonmagnetic semiconductors to magnetic semiconductors by increasing the length of zigzag segments. In particular, half metallicity can be achieved in HGNRs under external transverse electric fields. These results suggest that the introduction of armchair “impurity” will not affect the desired electronic and magnetic properties of zigzag graphene nanoribbons

GeP₃: A Small Indirect Band Gap 2D Crystal with High Carrier Mobility and Strong Interlayer Quantum Confinement

We propose a two-dimensional crystal that possesses low indirect band gaps of 0.55 eV (monolayer) and 0.43 eV (bilayer) and high carrier mobilities similar to those of phosphorene, GeP₃. GeP₃ has a stable three-dimensional layered bulk counterpart, which is metallic and known from experiment since 1970. GeP₃ monolayer has a calculated cleavage energy of 1.14 J m^–2, which suggests exfoliation of bulk material as viable means for the preparation of mono- and few-layer materials. The material shows strong interlayer quantum confinement effects, resulting in a band gap reduction from mono- to bilayer, and then to a semiconductor–metal transition between bi- and triple layer. Under biaxial strain, the indirect band gap can be turned into a direct one. Pronounced light absorption in the spectral range from ∼600 to 1400 nm is predicted for monolayer and bilayer and promises applications in photovoltaics

FeB₆ Monolayers: The Graphene-like Material with Hypercoordinate Transition Metal

By means of density functional theory (DFT) computations and global minimum search using particle-swarm optimization (PSO) method, we predicted three FeB₆ monolayers, namely α-FeB₆, β-FeB₆ and γ-FeB₆, which consist of the Fe©B_<i>x</i> (<i>x</i> = 6, 8) wheels with planar hypercoordinate Fe atoms locating at the center of six- or eight-membered boron rings. In particular, the α-FeB₆ sheet constructed by Fe©B₈ motifs is the global minimum due to completely shared and well delocalized electrons. The two-dimensional (2D) boron networks are dramatically stabilized by the electron transfer from Fe atoms, and the FeB₆ monolayers have pronounced stabilities. The α-FeB₆ monolayer is metallic, while the β-FeB₆ and γ-FeB₆ sheets are semiconductors with indirect band gaps and significant visible-light absorptions. Besides the novel chemical bonding, the high feasibility for experimental realization, and unique electronic and optical properties, render them very welcome new members to the graphene-like materials family

Dirac State in the FeB₂ Monolayer with Graphene-Like Boron Sheet

By introducing the commonly utilized Fe atoms into a two-dimensional (2D) honeycomb boron network, we theoretically designed a new Dirac material of FeB₂ monolayer with a Fermi velocity in the same order of graphene. The electron transfer from Fe atoms to B networks not only effectively stabilizes the FeB₂ networks but also leads to the strong interaction between the Fe and B atoms. The Dirac state in FeB₂ system primarily arises from the Fe d orbitals and hybridized orbital from Fe-d and B-p states. The newly predicted FeB₂ monolayer has excellent dynamic and thermal stabilities and is also the global minimum of 2D FeB₂ system, implying its experimental feasibility. Our results are beneficial to further uncovering the mechanism of the Dirac cones and providing a feasible strategy for Dirac materials design