Computational Prediction of a Novel Superhard sp^(3) Trigonal Carbon Allotrope with Bandgap Larger than Diamond

Searching for new carbon allotropes with superior properties has been a longstanding interest in material sciences and condensed matter physics.Here we identify a novel superhard carbon phase with an 18-atom trigonal unit cell in a full-sp^(3) bonding network,termed tri-C_(18) carbon,by first-principles calculations.Its structural stability has been verified by total energy,phonon spectra,elastic constants,and molecular dynamics simulations.Furthermore,tri-C_(18) carbon has a high bulk modulus of 400 GPa and Vickers hardness of 79.0 GPa,comparable to those of diamond.Meanwhile,the simulated x-ray diffraction pattern of tri-C_(18) carbon matches well with the previously unexplained diffraction peaks found in chimney soot,indicating the possible presence of tri-C_(18) carbon.Remarkably,electronic band structure calculations reveal that tri-C_(18) carbon has a wide indirect bandgap of 6.32 eV,larger than that of cubic diamond,indicating its great potential in electronic or optoelectronic devices working in the deep ultraviolet region.

Tellurene:An elemental 2D monolayer material beyond its bulk phases without van der Waals layered structures

Due to the quantum confinement effect,atomically thin two-dimensional(2D)monolayer materials possess distinct characteristics from their corresponding bulk materials,which have received wide attention from science and industry.Among all the 2D materials,elemental 2D materials with the simplest components are most striking.As an emerging group-VIA elemental 2D monolayer material,tellurene exhibits many exciting fundamental properties,such as chemical and mechanical stabilities,bandgap and high carrier mobilities compared to phosphorene,graphene and MoS2,respectively.Besides,in further exploration,it was found that tellurene or tellurene-based device presents excellent thermoelectric properties,piezoelectric properties,quantum Hall effects,and superb optical properties especially nonlinear optics characteristics,etc.The properties of tellurene can be modulated by virtue of strain,defects,edges,and heterojunction effects.In view of so many unique properties,it has drawn significant interest since tellurene was predicted and fabricated successfully in 2017.In this paper,we review the 2D tellurene allotropes,experimental preparation,excellent properties,performance modulation and future development.

In-plane grain boundary induced defect state in hierarchical NiCo-LDH and effect on battery-type charge storage

Domain boundaries are regarded as the effective active sites for electrochemical energy storage materials due to defects enrichment therein.However,layered double hydroxides(LDHs)tend to grow into single crystalline nano sheets due to their unique two-dimentional(2D)lattice structure.Previously,much efforts were made on the designing hierarchical structure to provide more exposed electroactive sites as well as accelerate the mass transfer.Herein,we demonstrate a strategy to introduce low angle grain boundary(LAGB)in the flakes of Ni/Co layered double hydroxides(NiCo-LDHs).These defect-rich nano flakes were self-assembled into hydrangea-like spheres that further constructed hollow cage structure.Both the formation of hierarchical structure and grain boundaries are interpreted with the synergistic effect of Ni2+/Co2+ratio in an“etching-growth”process.The domain boundary defect also results in the preferential formation of oxygen vacancy(Vo).Additionally,density functional theory(DFT)calculation reveals that Co substitution is a critical factor for the formation of adjacent lattice defects,which contributes to the formation of domains boundary.The fabricated battery-type Faradaic NiCo-LDH-2 electrode material exhibits significantly enhanced specific capacitance of 899 C·g^(−1)at a current density of 1 A·g^(−1).NiCo-LDH-2//AC asymmetric capacitor shows a maximum energy density of 101.1 Wh·kg^(−1)at the power density of 1.5 kW·kg^(−1).