Stable, one-dimensional suspended and supported monatomic chains of pnictogens: a metal-insulator framework.

Group-VA elements P, As, Sb, and Bi can construct free-standing, stable zigzag monatomic chain structures, which show unusual properties. They are normally semimetals with bands crossing at the Fermi level, but a very narrow gap opens due to spin-orbit coupling. They attain one quantum of conductance under a small bias potential; Bi, being an exception, attains two quanta of conductance. Finite size chains are magnetic semiconductors; their magnetic moments and the order of spin states show an even-odd disparity depending on the number of chain atoms. Variations of the HOMO-LUMO band gaps depending on the spin polarization and the size of the finite chains offer critical tunability. In the periodic, zigzag compound chains, a small band gap opens at the Fermi level. The mysterious zigzag geometry, cohesion, stability and band order of all these chains are well-explained by a simple bond model. When placed on the parent or other monolayers like graphene, h-BN and GaSe, these chains become weakly bound and construct a 1D metallic channel. The artificial grids or networks of these metallic chains on the insulating substrates can constitute metal-insulator frameworks of desired geometry. The zigzag phosphorene chain, having the highest stability, remains stable even at full coverage of adsorbates like H and OH, whereas other chains dissociate. While P-chains can be synthesized on GaSe and graphene substrates, phosphorene nanoribbons can transform into suspended chains under excessive tensile strain. Additionally, we showed that As, Sb, and Bi zigzag chains are weakly bound to their parent monolayers and remain stable. Nitrogen monatomic chains, on the other hand, are prone to instability. The diverse properties unveiled in this study based on the density functional method offer tunability through electric fields and strain.

Electronic and magnetic properties of monolayer α-RuCl3: a first-principles and Monte Carlo study.

Recent experiments revealed that monolayer α-RuCl3 can be obtained by a chemical exfoliation method and exfoliation or restacking of nanosheets can manipulate the magnetic properties of the materials. In this paper, the electronic and magnetic properties of an α-RuCl3 monolayer are investigated by combining first-principles calculations and Monte Carlo simulations. From first-principles calculations, we found that the spin configuration of FM corresponds to the ground state for α-RuCl3, however, the other excited zigzag oriented spin configuration has an energy of 5 meV per atom higher than the ground state. The energy band gap is found to be 3 meV using PBE functionals. When the spin-orbit coupling effect is taken into account, the corresponding energy gap is determined to be 57 meV. We also investigate the effect of the Hubbard U energy terms on the electronic band structure of the α-RuCl3 monolayer and revealed that the band gap increases approximately linearly with increasing U value. Moreover, spin-spin coupling terms (J1, J2, and J3) have been obtained using first-principles calculations. By benefiting from these terms, Monte Carlo simulations with a single site update Metropolis algorithm have been implemented to elucidate the magnetic properties of the considered system. Thermal variations of magnetization, susceptibility and also specific heat curves indicate that monolayer α-RuCl3 exhibits a phase transition between ordered and disordered phases at the Curie temperature of 14.21 K. We believe that this study can be utilized to improve two-dimensional magnetic materials.

Influence of chalcogen composition on the structural transition and on the electronic and optical properties of the monolayer titanium trichalcogenide ordered alloys.

Based on first-principles spin-polarized density functional theory, we investigate the effects of chalcogen composition on the structural, electronic, and optical properties of monolayer (where X and X' = S, Se, Te) ordered alloys with values of x of 0, 0.167, 0.333, 0.500, 0.667, 0.833, and 1. We determine the optimized geometry for all possible substitutional adsorptions of chalcogen atoms for each x composition, and identify the energetically most stable allotropes as a function of composition. Our extensive analysis reveals that the structural and electronic properties depend on the chemical composition of the monolayers, and the band gap of TiX3 nanosheets can be tuned by adjusting the ratio of chalcogen compositions. While substitutional doping of tellurium atoms into TiS3 or TiSe3 monolayers results in a semiconductor-metal transition, the alloys remain a semiconductor under the transition from TiS3 to TiSe3 with band gaps which are very suitable for optical devices and infrared detectors. We also find that each TiS3(1-x)Se3x structure has an anisotropic dielectric function. Because of the anisotropy of the dielectric function, they can be useful materials for application in the transition metal trichalcogenide-based nanoelectronics industry in the future.

Electronic structure of BSb defective monolayers and nanoribbons.

In this paper, we investigate two- and one-dimensional honeycomb structures of boron antimony (BSb) using a first-principles plane wave method within the density functional theory. BSb with a two-dimensional honeycomb structure is a semiconductor with a 0.336 eV band gap. The vacancy defects, such as B, Sb, B + Sb divacancy, and B + Sb antisite disorder affect the electronic and magnetic properties of the 2D BSb sheet. All the structures with vacancies have nonmagnetic metallic characters, while the system with antisite disorder has a semiconducting band structure. We also examine bare and hydrogen-passivated quasi-one-dimensional armchair BSb nanoribbons. The effects of ribbon width (n) on an armchair BSb nanoribbon and hydrogen passivation on both B and Sb edge atoms are considered. The band gaps of bare and H passivated A-Nr-BSb oscillate with increasing ribbon width; this property is important for quantum dots. For ribbon width n = 12, the bare A-Nr-BSb is a nonmagnetic semiconductor with a 0.280 eV indirect band gap, but it becomes a nonmagnetic metal when B edge atoms are passivated with hydrogen. When Sb atoms are passivated with hydrogen, a ferromagnetic half-metallic ground state is observed with 2.09µB magnetic moment. When both B and Sb edges are passivated with hydrogen, a direct gap semiconductor is obtained with 0.490 eV band gap with disappearance of the bands of edge atoms.

Routine colposcopic evaluation of patients with persistent inflammatory cellular changes on Pap smear.

OBJECTIVE: To assess the rate of undetected dysplasia in patients with two consecutive reports of inflammatory cellular changes without atypia on Pap smears despite anti-inflammatory therapy. METHOD: A prospective randomized study. RESULT: 2798 premenopausal non-pregnant patients were evaluated by Pap smears of the cervix. Of these, 397 (14.2%) were reported as 'inflammatory cellular changes'. A total of 238 (8.5%) had persistent inflammatory changes without atypia despite the anti-inflammatory therapy. Fourteen patients refused colposcope. The mean age and parity of the remaining 224 patients were 30.2 +/- 6.3 (18-46) and 1.7 +/- 2.3 (0-6), respectively. When these patients underwent colposcopically-directed biopsies of the cervix, in 51 (22.7%) patients human papillomavirus lesions, dysplasia and in situ cancer were noted. Mean age, parity, age of marriage, prevalence of smoking and contraceptive methods of the two groups of patients (173 vs. 51) did not show statistically significant differences. CONCLUSION: Colposcopically-directed biopsies of the cervix are indicated even when inflammatory cellular changes without atypia persist despite therapy.