Graphene-like hexagonal boron phosphide with its moderate band gap and high carrier mobility is considered to be a high potential material for electronics and optoelectronics. In this work, the tight-binding Hamiltonian of hexagonal boron phosphide monolayer and bilayer with two stacking orders are derived in detail. Including up to fifth-nearest-neighbor in plane and next-nearest-neighbor interlayer hoppings, the tight-binding approximated band structure can well reproduce the first-principle calculations based on the screened Heyd–Scuseria–Ernzerhof hybrid functional level over the entire Brillouin zone. The band gap deviations for monolayer and bilayer between our tight-binding and first-principle results are only 2 meV. The low-energy effective Hamiltonian matrix and band structure are obtained by expanding the full band structure close to the K point. The results show that the iso-energetic lines of maximum valence band in the vicinity of K point undergo a pseudo-Lifshitz transition from h-BP monolayer to AB_B-P or AB_B-B bilayer. The mechanism of pseudo-Lifshitz transition can be attributed to two interlayer hoppings rather than one.
On the basis of first-principles calculations, pure and doped C 32 clusters are studied. Among the nine structural isomers, the fullerene structure with D 3 symmetry is found to be the most stable. Due to the small size of the C 32 cage, Li and Na atoms can be stably encapsulated, while K and Be atoms are not. On encapsulation, the bond length of the H 2 molecule is reduced while the vibration frequency is increased. Substitutional doping is more favourable than endohedral doping for Si atoms. Because of the sp 2-bonding features of C atoms, the Si atom is also threefold coordinated in substitutional doping; however, the existence of one dangling bond in the Si atom makes this doped heterofullerene reactive at the Si site, and H termination can produce substantial energy gain.
Dozens of layered V2IV2VI6 (V=P, As, Sb, Bi; IV=Si, Ge, Sn, Pb; VI=S, Se, Te) materials are investigated, several of which have been successfully synthesized in experiment. Among them, nine strong topological insulators (TIs), two strong Z2 topological metals (TMs), and nearly twenty trivial insulators are predicted at their equilibrium structures. The TIs are in the (1;111) topological class, with energy gaps ranging from 0.04 to 0.2 eV. The strong Z2 TMs and the trivial insulators belong to the (1;111) and (0;000) topological classes, respectively. Small compressive strains easily turn some of the trivial insulators into strong TIs. This study enriches not only the family of topological materials but also the family of van der Waals layered materials, providing promising candidates for the future spintronic devices.
Asthma is a chronic disease characterized by recurrent
attacks
of breathlessness and wheezing, which vary in severity and frequency
from person to person. H2S is considered as the biomarker
of asthma. Here, an ultrasensitive chemiresistive H2S gas
sensor based on a γ-Bi2MoO6–CuO
heterostructure with a detection limit of 5 ppb has been fabricated.
It can distinguish asthmatic patients from healthy people roughly
by analyzing the exhaled breaths of 28 asthmatic patients and 28 healthy
people, suggesting that the sensor can be used to assist physicians
in the diagnosis of asthma. Pathologically, it is discovered by this
sensor that with the relief of asthma, the concentration of H2S in one’s exhaled breath gradually increases. This
subtle concentration variation of H2S can be accurately
detected, indicating that this sensor can be used in the asthma severity
monitoring too. Physical models have been built by first-principles
calculation to reveal the causes of the sensor’s ultrasensitivity.
The stable adsorption of H2S on the surface of CuO results
in massive charge transferring and the appearance of the defect states,
which play the major role in the ultrasensitivity of the sensor. Upon
integrating this sensor with circuits, the cheap, smart, and portable
H2S sensing device can be obtained, which can make asthmatic
patients’ access to this device easy and make the severity
monitoring of asthma convenient, especially for children and the aged.
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