Chun-Yao Niu

Anomalous doping effect in black phosphorene using first-principles calculations

Using first-principles density functional theory calculations, we investigate the geometries, electronic structures, and thermodynamic stabilities of substitutionally doped phosphorene sheets with group III, IV, V, and VI elements. We find that the electronic properties of phosphorene are drastically modified by the number of valence electrons in dopant atoms. The dopants with an even number of valence electrons enable the doped phosphorenes to have a metallic feature, while the dopants with an odd number of valence electrons retain a semiconducting feature. This even-odd oscillating behavior is attributed to the peculiar bonding characteristics of phosphorene and the strong hybridization of sp orbitals between dopants and phosphorene. Furthermore, the calculated formation energies of various substitutional dopants in phosphorene show that such doped systems can be thermodynamically stable. These results propose an intriguing route to tune the transport properties of electronic and photoelectronic devices based on phosphorene.

Dilute Magnetic Semiconductor and Half-Metal Behaviors in 3d Transition-Metal Doped Black and Blue Phosphorenes: A First-Principles Study.

AbstractWe present first-principles density-functional calculations for the structural, electronic, and magnetic properties of substitutional 3d transition metal (TM) impurities in two-dimensional black and blue phosphorenes. We find that the magnetic properties of such substitutional impurities can be understood in terms of a simple model based on the Hund’s rule. The TM-doped black phosphorenes with Ti, V, Cr, Mn, Fe, and Ni impurities show dilute magnetic semiconductor (DMS) properties while those with Sc and Co impurities show nonmagnetic properties. On the other hand, the TM-doped blue phosphorenes with V, Cr, Mn, and Fe impurities show DMS properties, with Ni impurity showing half-metal properties, whereas Sc- and Co-doped systems show nonmagnetic properties. We identify two different regimes depending on the occupation of the hybridized electronic states of TM and phosphorous atoms: (i) bonding states are completely empty or filled for Sc- and Co-doped black and blue phosphorenes, leading to nonmagnetic; (ii) non-bonding d states are partially occupied for Ti-, V-, Cr-, Mn-, Fe- and Ni-doped black and blue phosphorenes, giving rise to large and localized spin moments. These results provide a new route for the potential applications of dilute magnetic semiconductor and half-metal in spintronic devices by employing black and blue phosphorenes. PACS numbers: 73.22.-f, 75.50.Pp, 75.75. + a

Multivalency-Driven Formation of Te-Based Monolayer Materials: A Combined First-Principles and Experimental study

1 International Laboratory for Quantum Functional Material s of Henan, and School of Physics and Engineering, Zhengzhou Universit y, Zhengzhou 450001, China 2 Department of Physics, Hanyang University, 17 Haengdang-D ong, Seongdong-Ku, Seoul 133-791, Korea 3 Department of Chemistry, University College London, Londo n WC1E 6BT, United Kingdom 4 Department of Materials Science and Engineering, Universi ty of Utah, Salt Lake City, Utah 84112, USA 5 ICQD, Hefei National Laboratory for Physical Sciences at th e Microscale, and Synergetic Innovation Center of Quantum Information an d Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China (Dated: February 1, 2017)Contemporary science is witnessing a rapid expansion of the two-dimensional (2D) materials family, each member possessing intriguing emergent properties of fundamental and practical importance. Using the particle-swarm optimization method in combination with first-principles density functional theory calculations, here we predict a new category of 2D monolayers named tellurene, composed of the metalloid element Te, with stable 1T-MoS_{2}-like (α-Te), and metastable tetragonal (β-Te) and 2H-MoS_{2}-like (γ-Te) structures. The underlying formation mechanism is inherently rooted in the multivalent nature of Te, with the central-layer Te behaving more metal-like (e.g., Mo), and the two outer layers more semiconductorlike (e.g., S). We also show that the α-Te phase can be spontaneously obtained from the magic thicknesses divisible by three layers truncated along the [001] direction of the trigonal structure of bulk Te, and both the α- and β-Te phases possess electron and hole mobilities much higher than MoS_{2}. Furthermore, we present preliminary but convincing experimental evidence for the layering behavior of Te on HOPG substrates, and predict the importance of multivalency in the layering behavior of Se. These findings effectively extend the realm of 2D materials to group-VI elements.

Atomically thin binary V–V compound semiconductor: a first-principles study

Finding novel 2D semiconductors is crucial to develop next-generation low-dimensional electronic devices. Using first-principles calculations, we propose a class of unexplored binary V–V compound semiconductors (PN, AsN, SbN, AsP, SbP and SbAs) with monolayer black phosphorene (α) and blue phosphorene (β) structures. Our phonon spectra and room-temperature molecular dynamics (MD) calculations indicate that all compounds are very stable. Moreover, most of compounds are found to present a moderate energy gap in the visible frequency range, which can be tuned gradually by in-plane strain. Especially, α-phase V–V compounds have a direct gap, while β-SbN, AsN, SbP, and SbAs may be promising candidates for 2D solar cell materials due to a wide gap separating acoustic and optical phonon modes. Furthermore, vertical heterostructures can be also built using lattice matched α(β)-SbN and phosphorene, and both vdW heterostructures are found to have intriguing direct band gaps. The present investigation not only broadens the scope of layered group V semiconductors but also provides an unprecedented route for the potential applications of 2D V–V families in optoelectronic and nanoelectronic semiconductor devices.

H18 Carbon: A New Metallic Phase with sp2-sp3 Hybridized Bonding Network.

Design and synthesis of three-dimensional metallic carbons are currently one of the hot issues in contemporary condensed matter physics because of their fascinating properties. Here, based on first-principles calculations, we discover a novel stable metallic carbon allotrope (termed H18 carbon) in () symmetry with a mixed sp2-sp3 hybridized bonding network. The dynamical stability of H18 carbon is verified by phonon mode analysis and molecular dynamics simulations, and its mechanical stability is analyzed by elastic constants, bulk modulus, and shear modulus. By simulating the x-ray diffraction patterns, we propose that H18 carbon would be one of the unidentified carbon phases observed in recent detonation experiments. The analysis of the band structure and density of states reveal that this new carbon phase has a metallic feature mainly due to the C atoms with sp2 hybridization. This novel 3D metallic carbon phase is anticipated to be useful for practical applications such as electronic and mechanical devices.

Tunable electronic properties of GeSe/phosphorene heterostructure from first-principles study

Vertical integration of two-dimensional materials has recently emerged as an exciting method for the design of electronic and optoelectronic devices. In this letter, first principles calculations are employed to explore the structural and electronic properties of the GeSe/phosphorene van der Waals (vdW) p-n heterostructure. Our results suggest that this heterostructure has an intrinsic type-II band alignment and indirect band gap. Moreover, we also find that an intriguing indirect-direct and insulator-metal transition can be induced by strain. In addition, spontaneous electron-hole charge separation is expected to occur, implying that the GeSe/phosphorene heterostructure is a good candidate for applications in optoelectronics. These results provide a route for applications of the GeSe/phosphorene vdW heterostructure in future flexible electronics, optoelectronics, and semiconductor devices.

C 20 − T carbon: a novel superhard sp 3 carbon allotrope with large cavities

Through first-principles calculations, we predict a new superhard carbon allotrope named C 20  -  T, which possesses a cubic T symmetry with space group No.198(P213). This new carbon allotrope has an all-sp (3) hybridized bonding network with 20 atoms in its primitive unit cell. The dynamic, mechanical, and thermal stabilities of this new carbon phase at zero pressure are confirmed by using a variety of state-of-the-art theoretical calculations. Interestingly, despite the fact that C 20  -  T carbon has a porous structure with large cavities, our calculations identify its superhard properties with the Vickers hardness of 72.76 Gpa. The ideal tensile and shear strength of C 20  -  T carbon are calculated to be 71.1 and 55.2 GPa respectively, comparable to that of c-BN. Electronic band calculations reveal that this new carbon allotrope is a transparent insulator with an indirect band gap of 5.44 eV. These results broaden our understanding of superhard carbon allotropes.

Computational prediction of the diversity of monolayer boron phosphide allotropes

We propose previously unrecognized allotropes of monolayer boron phosphorus (BP) based on ab initio density functional calculations. In addition to the hexagonal structure of h-BP, four types of boron phosphide compounds were predicted to be stable as monolayers. They can form sp2 hybridized planar structures composed of 6-membered rings, and buckled geometries including 4–8 or 3–9 membered rings with sp3 like bonding for P atoms. The calculated Bader charges illustrate their ionic characters with the charge transfers from B to P atoms. The competing between the electrostatic energy and the bonding energy of sp2 and sp3 hybridizations reflected in P atoms results in multiple structures of BP. These 2D BP structures can be semiconducting or metallic depending on their geometric structures. Our findings significantly broaden the diversity of monolayer BP allotropes and provide valuable guidance to other 2D group-III-V allotropes.

Strain induced quantum spin Hall insulator in monolayer β-BiSb from first-principles study

Topological insulator (TI) is a peculiar phase of matter exhibiting excellent quantum transport properties with potential applications in lower-power-consuming electronic devices. Searching for inversion-asymmetric quantum spin Hall (QSH) insulators persists as an effect for realizing new topological phenomena. Using first-principles density functional theory calculations, we investigate the geometry, dynamic stability, and electronic structures of monolayer β-BiSb. We find that it presents QSH state under biaxial tensile strain of 14%. The nontrivial topological situation in the strained system is confirmed by the identified band inversion, Z2 topological invariant (Z2 = 1), and an explicit presence of the topological edge states. Owning to the asymmetric structure, remarkable Rashba spin splitting is produced in both the valence and conduction bands of the strained system. These results provide an intriguing platform for applications of monolayer β-BiSb in future alternative quantum Hall spintronic devices.

Origin of Symmetric Dimer Images of Si(001) Observed by Low-Temperature Scanning Tunneling Microscopy

It has been a long-standing puzzle why buckled dimers of the Si(001) surface appeared symmetric below ~20 K in scanning tunneling microscopy (STM) experiments. Although such symmetric dimer images were concluded to be due to an artifact induced by STM measurements, its underlying mechanism is still veiled. Here, we demonstrate, based on a first-principles density-functional theory calculation, that the symmetric dimer images are originated from the flip-flop motion of buckled dimers, driven by quantum tunneling (QT). It is revealed that at low temperature the tunneling-induced surface charging with holes reduces the energy barrier for the flipping of buckled dimers, thereby giving rise to a sizable QT-driven frequency of the flip-flop motion. However, such a QT phenomenon becomes marginal in the tunneling-induced surface charging with electrons. Our findings provide an explanation for low-temperature STM data that exhibits apparent symmetric (buckled) dimer structure in the filled-state (empty-state) images.