Maosheng Miao

Band gap engineering of graphenylene by hydrogenation and halogenation: a density functional theory study

Graphenylene, a new form of two-dimensional (2D) carbon allotrope consisting of non-delocalized sp2-carbon atoms, has aroused considerable interest recently due to its thermodynamic stability and porous structure. In this work, density functional theory is used to investigate the hydrogenation and halogenation of graphenylene. The adsorption stability of hydrogen and halogen atoms on graphenylene is discussed at different concentrations of adsorbate atoms. The electronic structures of functionalized graphenylenes show that by controlling the concentration of adsorbate atoms, the band gap of graphenylene could be tuned over a wide range, from 0.075 to 4.98 eV by hydrogenation and 0.024 eV to 4.87 eV by halogenation.

A CNH monolayer: a direct gap 2D semiconductor with anisotropic electronic and optical properties

Recently, two-dimensional materials have received significant attention due to their superior transport and optical properties and their potential roles in future nanoscale devices. Compared to three-dimensional materials, there is still a lack of variety of 2D materials, especially with desired band gap. The number of wide gap 2D materials is quite limited. A good candidate is the well-known h-BN. However, this material has a gap of 5.56 eV, which is too high for a semiconductor, and its fabrication involves boron and nitrogen precursors, which are usually hard to process. In this study, using first principles calculations, we proposed a new 2D material (α-CNH) consisting of C, N, and H. It consists of array of polyethylene chains connected by N atoms in the perpendicular direction. Because of its framework formed by C–C and C–N bonds, α-CNH shows excellent stability and mechanical properties. It is a direct gap semiconductor with a band gap of 3.03 eV, as calculated by the hybrid functional, and exhibits interesting electronic and optical properties that are very anisotropic, as determined via its structure. The mobilities of both electrons and holes in this material are very anisotropic. The mobility along easy direction is 3 to 5 times higher than that along the hard direction. Interestingly, the high mobility directions of electrons and holes are different; this allows to design novel devices in which the high conducting directions can be altered by changing the carriers by applying gate voltage.

Atomic and electronic structure of CdTe/metal (Cu, Al, Pt) interfaces and their influence to the Schottky barrier

Schottky barrier heights (SBHs) and other features of the interfaces are determining factors for the performance of the CdTe based high-energy photon detectors. Although known for long time that SBH is sensitive to surface treatment and metal contact growth method, there is a lack of understanding of the effect of the atomic and electronic structures of CdTe/metal interface on the SBH. Employing first-principles electronic structure calculations, we have systematically studied the structural stability and electronic properties of a number of representing structures of Cd Terminated CdTe/metal (Cu, Pt, and Al) interfaces. Comparison of the total energies of the various optimized structural configurations allows to identify the existence of Te-metal alloy formation at the interface. The SBHs of Cu, Pt, and Al metal contacts with a number of stable interface structures are determined by aligning the band edges of bulk CdTe with the Fermi level of the metal/CdTe system. We find that the metal-induced states i...

A novel two-dimensional material B2S3 and its structural implication to new carbon and boron nitride allotropes

Two-dimensional (2D) semiconductor materials and the fabrication of related devices have become a new focus of electronics and materials science recently. Compared with three-dimensional (3D) semiconductors, the choice of 2D materials is very limited. Recently, the emerging goal of fabricating functional heterojunctions of 2D semiconductors has spurred a strong need to search for 2D materials that have a large variety of band gaps and band edges. Here, we propose a single layer of B2S3 as a new potential 2D material, conceived directly from its existing layered 3D crystal. Using an advanced hybrid functional method, we demonstrated that 2D B2S3 has a gap of 3.75 eV, filling a missing energy range for 2D materials. Furthermore, by adding extra B atoms at the ‘vacancy’ sites of the B2S3 structure to give a 1 : 1 stoichiometry, we constructed new 2D BN and graphene allotropes that show large variation in the electronic structure. The BN allotrope exhibits a gap that is 0.99 eV lower than h-BN. Although the structure is significantly different to graphene, the new C allotrope contains a Dirac cone. However, the Dirac point is slightly lower than the Fermi level because of the electron transfer from an adjacent valence band to the Dirac cone states, resulting in a metallic state with both ‘massless’ electrons and massive holes.

Molecular dynamics growth modeling of InAs1−xSbx-based type-II superlattice

Abstract. Type-II strained-layer superlattices (T2SL) based on InAs1−xSbx are a promising photovoltaic detector material technology for thermal imaging; however, Shockley–Read–Hall recombination and generation rates are still too high for thermal imagers based on InAs1−xSbx T2SL to reach their ideal performance. Molecular dynamics simulations using the Stillinger–Weber (SW) empirical potentials are a useful tool to study the growth of tetrahedral coordinated crystals and the nonequilibrium formation of defects within them, including the long-range effects of strain. SW potentials for the possible atomic interactions among {Ga, In, As, Sb} were developed by fitting to ab initio calculations of elastically distorted zinc blende and diamond unit cells. The SW potentials were tested against experimental observations of molecular beam epitaxial (MBE) growth and then used to simulate the MBE growth of InAs/InAs0.5Sb0.5 T2SL on GaSb substrates over a range of processes parameters. The simulations showed and helped to explain Sb cross-incorporation into the InAs T2SL layers, Sb segregation within the InAsSb layers, and identified medium-range defect clusters involving interstitials and their induction of interstitial-vacancy pairs. Defect formation was also found to be affected by growth temperature and flux stoichiometry.

Epitaxial strain controlled magnetocrystalline anisotropy in ultrathin FeRh/MgO bilayers

Using ab initio electronic structure calculations we have investigated the effect of epitaxial strain on the magnetocrystalline anisotropy (MCA) of ultrathin FeRh/MgO heterostructures. Analysis of the energy- and k-resolved distribution of the orbital character of the band structure reveals that MCA largely arises from the spin-orbit coupling (SOC) between dx2−y2 and dxz/dyz orbitals of Fe atoms at the FeRh/MgO interface. We demonstrate that the strain has significant effects on the MCA: It not only affects the value of the MCA but also induces a switching of the magnetic easy axis from perpendicular to in-plane direction. The mechanism is the strain-induced shifts of the SOC d-states. Our work demonstrates that strain engineering can open a viable pathway towards tailoring magnetic properties for antiferromagetic spintronic applications.

Electric field control of magnetization direction across the antiferromagnetic to ferromagnetic transition

Electric-field-induced magnetic switching can lead to a new paradigm of ultra-low power nonvolatile magnetoelectric random access memory (MeRAM). To date the realization of MeRAM relies primarily on ferromagnetic (FM) based heterostructures which exhibit low voltage-controlled magnetic anisotropy (VCMA) efficiency. On the other hand, manipulation of magnetism in antiferromagnetic (AFM) based nanojunctions by purely electric field means (rather than E-field induced strain) remains unexplored thus far. Ab initio electronic structure calculations reveal that the VCMA of ultrathin FeRh/MgO bilayers exhibits distinct linear or nonlinear behavior across the AFM to FM metamagnetic transition depending on the Fe- or Rh-interface termination. We predict that the AFM Fe-terminated phase undergoes an E-field magnetization switching with large VCMA efficiency and a spin reorientation across the metamagnetic transition. In sharp contrast, while the Rh-terminated interface exhibits large out-of-plane (in-plane) MA in the FM (AFM) phase, its magnetization is more rigid to external E-field. These findings demonstrate that manipulation of the AFM Néel-order magnetization direction via purely E-field means can pave the way toward ultra-low energy AFM-based MeRAM devices.

Asymmetric passivation of edges: a route to make magnetic graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to carry interesting properties beyond graphene, such as finite and variable band gaps. More interestingly, the edges of ZGNRs are magnetic due to single occupation of carbon dangling bonds (DB). However, the magnetic moments at two different edge sides couple antiferromagnetically, leading to a zero global moment for ZGNRs. Furthermore, the application of ZGNRs is limited by the high chemical activity of their edges that can be easily oxidized while exposed in air. It has been proposed and intensively studied to protect the edges by passivating them by hydrogenation or adsorption of other molecules such as CO2. In this work, we systematically studied the stability, the structures and the effect of CO2 adsorption at the edges of ZGNRs. Our calculations confirm the experimental observation that the CO2 molecules can be easily absorbed by the ZGNR edges. More interestingly, our calculations show that the asymmetric CO2 adsorption at two edges of ZGNR yields a ferrimagnetic state of ZGNRs that presents a finite global moment. Furthermore, considering the strong bonding between CO2 groups and ZGNRs, we propose that it can be utilized to stitch arrays of ZGNRs together to form new types of 2D materials that inherit the advantageous properties of the nanoribbons, such as finite gaps and novel magnetic properties.

Electronic Structure Change in DNA Caused by Base Pair Motions and Its Effect on Charge Transfer in DNA Chains

One important aspect of carrier transfer in DNA is its coupling with atomic motions. The collective motion of the base pairs can either improve the charge transfer by enhancing the π stacking between the bases, or trap the carriers due to strong coupling. By utilizing a pseudo-helical base pair stack model, we systematically studied the electronic structure and its dependence to geometry changes that represent the important DNA motions, including the translation, the twist and the torsion of the base pairs. Our calculations reveal that the above motions may significantly change the electron structure and affect their transport properties. In order to improve the transport of carriers in DNA so that it can become a prospective material in future electronics, it is necessary to make large changes to the atomic structure. Our calculations of the electronic structure under large geometry variation, including large base pair stacking deformation and the insertion of phenyl rings in the bases, can provide good guidelines for such structural modifications of DNA.