Influence of Group-IVA Doping on Electronic and Optical Properties of ZnS Monolayer: A First-Principles Study.

Element doping is a universal way to improve the electronic and optical properties of two-dimensional (2D) materials. Here, we investigate the influence of group-ⅣA element (C, Si, Ge, Sn, and Pb) doping on the electronic and optical properties of the ZnS monolayer with a tetragonal phase by using first-principles calculations. The results indicate that the doping atoms tend to form tetrahedral structures with neighboring S atoms. In these doped models, the formation energies are all negative, indicating that the formation processes of the doped models will release energy. The formation energy is smallest for C-doped ZnS and gradually increases with the metallicity of the doping element. The doped ZnS monolayer retains a direct band gap, with this band gap changing little in other element doping cases. Moreover, intermediate states are observed that are induced by the sp3 hybridization from the doping atoms and S atoms. Such intermediate states expand the optical absorption range into the visible spectrum. Our findings provide an in-depth understanding of the electronic and optical properties of the ZnS monolayer and the associated doping structures, which is helpful for application in optoelectronic devices.

A first-principles study of electronic and optical properties of the tetragonal phase of monolayer ZnS modulated by biaxial strain.

Modulation of the electronic and optical properties of two-dimensional (2D) materials is of great significance for their practical applications. Here, by using first-principles calculations, we study a tetragonal phase of monolayer ZnS, and explore its associated electronic and optical properties under biaxial strain. The results from phonon dispersion and molecular dynamics simulation demonstrate that the tetragonal phase of monolayer ZnS possesses a very high stability. The monolayer ZnS has a direct band gap of 4.20 eV. It changes to an indirect band gap under both compression and tension, exhibiting a decrease in band gap. However, the band gap decreases more slowly under compression compared to the tension process such that the direct band gap remains within -8%, demonstrating excellent endurance under pressure. Fortunately, tetragonal ZnS exhibits a good absorption ability in the ultraviolet (UV) range regardless of strain. Our research results enrich the understanding of monolayer ZnS, which is helpful for the design and application of optoelectronic devices using the material.

Electronic and optical properties of hydrogen-terminated biphenylene nanoribbons: a first-principles study.

The electronic structures and optical properties of novel 2D biphenylene and hydrogen-terminated nanoribbons of different widths which are cut from a layer of biphenylene were explored via first-principles calculations. The findings of phonon computations demonstrate that such a biphenylene is dynamically stable and shows metallic properties. The crystal orbital Hamilton population analysis indicates that the tetra-ring local structure results in anisotropic mechanical properties. For 1D nanoribbons, their band gaps shrink, and a direct-indirect transition occurs in the band gap as the width increases, transforming the nanoribbon to endow them with metallic characteristics at a certain width. This is attributed to the weak coupling between the tetra-ring atoms, shrinking the direct band gap at the Y point in the Brillouin zone. Finally, the contribution of interband transitions to the dielectric function in 6-, 9-, and 12-armchair biphenylene nanoribbons (ABNRs) was identified. The lowest peak in the imaginary part of the dielectric function ε2 spectrum was mainly a contribution of a Γ-Γ transition. As the width of ABNR increases, the transitions in the x direction become stronger while the transition strength in the y direction is not significantly altered. This investigation extends the understanding of the electronic and optical properties of 2D biphenylene and 1D nanoribbons, which will benefit the practical applications of these materials in optoelectronics and electronics.

Electric Field-Tunable Structural Phase Transitions in Monolayer Tellurium.

Electronic properties of monolayer tellurium (Te) with three proposed atomic configurations under external electric field were investigated through first-principles calculations. The calculated results demonstrate that α-Te and γ-Te have indirect band gaps, whereas ß-Te, when no electric field is applied, can be considered as a direct semiconductor. An interesting structural change occurs in α- and γ-phase Te under a specific electric field strength, as does a change in structural chirality. In the presence of a perpendicular electric field, the band gaps can be modified and drawn close to 0 eV at a certain critical electric field strength. Before that, the band gaps of α-Te and γ-Te are nearly constant, while that of ß-Te shows a quadratic relationship to electric field strength. These findings not only enrich our understanding of the electronic properties of monolayer tellurium but also show that monolayer tellurium has tremendous potential in nanoscale electronic devices owing to its tunable band gaps.

A DFT study for silicene quantum dots embedded in silicane: controllable magnetism and tuneable band gap by hydrogen.

This paper presents a design for silicene quantum dots (SiQDs) embedded in silicane. The shape and size of an embedded SiQD are managed by hydrogen atoms. A first-principles method was used to evaluate the magnetism as well as the electronic and structural properties of embedded SiQDs of various shapes and sizes. The shape of the embedded SiQD determined its electronic structure as well as the dot size. Moreover, the magnetic properties of SiQDs in silicane were highly shape dependent. The triangular SiQDs were all magnetic, some small parallelogram SiQDs were nonmagnetic, and all others were antiferromagnetic; almost all hexagonal SiQDs were nonmagnetic. An unequal number of bare Si atoms at the A and B sites was identified as a critical factor for establishing magnetism in embedded SiQDs. The tip of a triangular SiQD enhanced the magnetic moment of the dot. The parallelogram SiQD with two tip atoms appeared as a magnetic needle and has potential for use in spintronic applications. SiQDs embedded in silicane can be used in the design of silicon-based nanoelectronic devices and binary logic based on nanoscale magnetism.

Electronic Structures of Clusters of Hydrogen Vacancies on Graphene.

Hydrogen vacancies in graphane are products of incomplete hydrogenation of graphene. The missing H atoms can alter the electronic structure of graphane and therefore tune the electronic, magnetic, and optical properties of the composite. We systematically studied a variety of well-separated clusters of hydrogen vacancies in graphane, including the geometrical shapes of triangles, parallelograms, hexagons, and rectangles, by first-principles density functional calculation. The results indicate that energy levels caused by the missing H are generated in the broad band gap of pure graphane. All triangular clusters of H vacancies are magnetic, the larger the triangle the higher the magnetic moment. The defect levels introduced by the missing H in triangular and parallelogram clusters are spin-polarized and can find application in optical transition. Parallelograms and open-ended rectangles are antiferromagnetic and can be used for nanoscale registration of digital information.

A CMOS cantilever-based label-free DNA SoC with improved sensitivity for hepatitis B virus detection.

This paper presents a highly-integrated DNA detection SoC, where several kinds of cantilever DNA sensors, a readout circuit, an MCU, voltage regulators, and a wireless transceiver, are integrated monolithically in a 0.35 µm CMOS Bio-MEMS process. The cantilever-based biosensors with embedded piezoresistors aim to transduce DNA hybridization into resistance variation without cumbersome labeling process. To improve detection sensitivity for low DNA concentration use, an oscillator-based self-calibrated readout circuit with high precision is proposed to convert small resistance variation ( of original resistance) of the sensor into adequate frequency variation and further into digital data. Moreover, its wireless capacity enables isolation of the sample solution from electrical wire lines and facilitates data transmission. To demonstrate the effectiveness of full system, it is applied to detect hepatitis B virus (HBV) DNA. The experimental results show that it has the capability to distinguish between one base-pair (1-bp) mismatch DNAs and match DNAs and achieves a limit of detection (LOD) of less than 1 pM.

Curvature effects on electronic properties of armchair graphene nanoribbons without passivation.

The geometric and electronic properties of curved armchair graphene nanoribbons without hydrogen atoms are investigated by first-principles calculations. The edge-atom bond length and ground state energy dramatically vary with the arc angle. The zipping or unzipping requirements for energy, arc angle, and interaction distance depend on the ribbon width. The increasing curvatures lead to drastic changes in electronic structures, such as energy gaps, energy dispersions, band-edge states, band mixing, band overlap and state degeneracy. There exist semiconductor-metal transitions during the variation of curvature. These are associated with the contribution of the edge atoms, the competition between the π and σ bonds, and hybridization of the 2p(y) and 2p(z) orbitals. The main features of the energy bands dominate the frequency, height, number, and structure of the prominent peaks in the density of states. The predicted results could be examined by experimental measurements.

Synthesis and mesomorphic behavior of a donor-acceptor-type hexaazatriphenylene.

[structure: see text] A new donor-acceptor, 1,4,5,8,9,12-hexaazatriphenylene HATCNOR(n), is described. The synthesis of HATCNOR1 and HATCNOR6 is achieved by the regioselective displacement of 1,4,5,8,9,12-hexaazatriphenylene hexacarbonitrile (HATCN) with an alkoxy group. The X-ray analysis revealed self-assembly of HATCNOR1 in the solid state. HATCNOR6 is the new difunctionalized hexaazatriphenylene discotic liquid crystal.