Bag Boundaries for Quasispinor Confinement Within Nanolanes on a Graphene Sheet

The problem of bag boundary conditions within a field-theoretic approach is revisited to study confinement of massless Dirac quasispinors in monolayer graphene. While no-flux bag boundaries have previously been used to model lattice termination sites in graphene nanoribbons, a generalized setting is considered in which the confining boundaries are envisaged as arbitrary straight lines drawn across a graphene sheet and the quasispinor currents are allowed to partially permeate (leak) through such boundaries. Specifically focus is on rectangular nanolanes defined as areas confined between a pair of parallel lines at arbitrary separation on an unbounded lattice. It is shown that such nanolanes exhibit a considerable range of bandgap tunability depending on their widths and armchair, zigzag, or intermediate orientation. The case of nanoribbons can be derived as a special limit from the nanolane model. In this case, certain inconsistencies are clarified in previous implementations of no-flux bag boundaries and show that the continuum approach reproduces the tight-binding bandgaps accurately (within just a few percent in relative deviation) even as the nanoribbon width is decreased to just a couple of lattice spacings. This accentuates the proper use of boundary conditions when field-theoretic approaches are applied to graphene systems.     

Growth behaviors and emission properties of Co-deposited MAPbI3 ultrathin films on MoS2

Hybrid organic-inorganic perovskite thin films have attracted much attention in optoelectronic and information fields because of their intriguing properties. Due to quantum confinement effects, ultrathin films in nm scale usually show special properties. Here, we report on the growth of methylammonium lead iodide (MAPbI₃) ultrathin films via co-deposition of PbI₂ and CH₃NH₃I (MAI) on chemical-vapor-deposition-grown monolayer MoS₂ as well as the corresponding photoluminescence (PL) properties at different growing stages. Atomic force microscopy and scanning electron microscopy measurements reveal the MoS₂ tuned growth of MAPbI₃ in a Stranski-Krastanov mode. PL and Kelvin probe force microscopy results confirm that MAPbI₃/MoS₂ heterostructures have a type-II energy level alignment at the interface. Temperaturedependent PL measurements on layered MAPbI₃ (at the initial stage) and on MAPbI₃ crystals in averaged size of 500 nm (at the later stage) show rather different temperature dependence as well as the phase transitions from tetragonal to orthorhombic at 120 and 150 K, respectively. Our findings are useful in fabricating MAPbI₃/transition-metal dichalcogenide based innovative devices for wider optoelectronic applications.     

Computational analysis of geometric structures and edge-termination effects of boron-nitride and edge-termination boron-nitride nanoribbons

In this work, we examined geometry, electronic structure, and edged termination effect of boron-nitride nanoribbons (BNNRs) by employing localized Gaussian-type orbital, periodic-boundary condition, density functional theory (LGTO-PBC-DFT) calculations. Armchair (ABNNR) and zigzag (ZBNNR)-type BNNRs are obtained and then bond lengths variation in two types of BNNRs are analyzed. We find that the B-N bond length variation of ZBNNR is less diverse than that of ABNNR, and decreases with an increase in the ribbon width, monotonically. For those in ABNNR, it appears as an oscillatory convergence as a function of the ribbon width. The energy gap of edged termination ABNNR and ZBNNRs are also calculated using OH and SH as electron donating groups and CN-, and Cl- as electron-withdrawing groups. The introduction of SH- and Cl-terminator significantly reduce the energy gap to the semiconducting region for ZBNNR, but for ABNNR. These calculated results may be exploited for nano-electronic applications. To examine the surface behavior of the BNNRs, we also calculated their Raman shifts with the edge termination effects.     

Electronic Structure of the Low-Lying States of the Triatomic MoS2 Molecule: The Building Block of 2D MoS2

Molybdenum disulfide (MoS₂) is the building component of 1D-monolayer, 2D-layered nanosheets and nanotubes having many applications in industry, and it is detected in various molecular systems observed in nature. Here, the electronic structure and the chemical bonding of sixteen low-lying states of the triatomic MoS₂ molecule are investigated, while the connection of the chemical bonding of the isolated MoS₂ molecule to the relevant 2D-MoS₂, is emphasized. The MoS₂ molecule is studied via DFT and multireference methodologies, i. e., MRCISD(+Q)/aug-cc-pVQZ(−PP)_Mo. The ground state, ³B₁, is bent (Mo−S=2.133 Å and ϕ(SMoS)=115.9°) with a dissociation energy to atomic products of 194.7 kcal/mol at MRCISD+Q. In the ground and in the first excited state a double bond is formed between Mo and each S atom, i. e., . These two states differ in which d electrons of Mo are unpaired. The Mo−S bond distances of the calculated states range from 2.108 to 2.505 Å, the SMoS angles range from 104.1 to 180.0°, and the Mo−S bonds are single or double. Potential energy curves and surfaces have been plotted for the ³B₁, ⁵A₁ and ⁵B₁ states. Finally, the low-lying septet states of the triatomic molecule are involved in the material as a building block, explaining the variety of its morphologies.     

High thermoelectric figure of merit in rhombic porous carbon nitride nanoribbons

By performing extensive density functional theory calculations combined with non-equilibrium Green's function technique, we predict the rhombic porous carbon nitride nanoribbon (rPCNNR) and the vertical rPCNNR junction exhibiting high thermoelectric figure of merit (ZT) values of 0.57 and 2.1 at room temperature respectively. Theoretical results reveal that the ZT value of rPCNNR is significantly larger than that of armchair graphene nanoribbon with the almost same width (~0.035) due to the large Seebeck coefficients and the significantly decreased thermal conductance of rPCNNR, where the phonon states are blocked by the built-in porous structure and rhombic edge in rPCNNR. The ZT value is further enhanced to be 2.1 in the vertical rPCNNR junction, which is achieved by the synergy effect between the dramatically suppressed thermal conductance in in-plane direction due to the weak van der Waals interaction between two rPCNNRs, the almost unchanged Seebeck coefficients, and the good electron conductivity provided by the strong overlapping of delocalized VB- and CB-derived states in the scattering region. These presented findings highlight rPCNNR as a promising candidate in building flexible devices with high thermoelectric performance.     

配位聚合物[HgI_2(bpea)]_n[bpea=1,2-双(4-吡啶基)乙烷]的合成结构表征和光学折射效应(英文)

制备44连吡啶基二卤加和物MX244′bipyn仍然有较大困难。这类化合物数量不多且已报道的多为难处理的微晶材料。本文在室温下通过简单的操作合成了新的碘化汞连吡啶乙烷加和物HgI2bpean。X射线单晶衍射分析表明其中的汞原子通过与两个碘原子以及吡啶乙烷的两个氮原子配位形成扭曲的四面体构型。每个HgI2单元由bpea桥连形成锯齿状链。其非线性光学性质用8纳秒激光在532nm波长进行了研究。该化合物相对于入射光表现为一定的光学吸收和强的自聚焦效应,其三阶非线性吸收系数α2=1.1×10-11m·W-1折射系数n2=5.29×10-18m2·W-1。