2D Octagon-Structure Carbon and Its Polarization Resolved Raman Spectra.

We predict a new phase of two-dimensional carbon with density functional theory (DFT). It was found to be semimetal with two Dirac points. The vibrational properties and the polarization resolved Raman spectra of the carbon monolayer are predicted. There are five Raman active modes: 574 cm-1 (Eg), 1112 cm-1 (B1g), 1186 cm-1 (B2g), 1605 cm-1 (B2g) and 1734 cm-1 (A1g). We consider the incident light wave vector to be perpendicular and parallel to the plane of the carbon monolayer. By calculating Raman tensor of each Raman active mode, we obtained polarization angle dependent Raman intensities. Our results will help materials scientists to identify the existence and orientation of octagon-structure carbon monolayer when they are growing it.

Nanoassembly and Multiscale Computation of Multifunctional Optical-Magnetic Nanoprobes for Tumor-Targeted Theranostics.

Gold nanorods, mesoporous silica, gadolinia, folic acid, and polyethylene glycol (PEG) derivatives have been investigated due to their own advantages in cancer theranostics. However, it remains a great challenge to assemble these components into a stable unity with the diverse and enhanced functionality for more potential applications. Herein, as inspired by the first-principles calculation, a highly stable and safe all-in-one nanoprobe is fabricated via a novel nanoassembly strategy. Multiscale calculations were performed to address the atomistic bonding of a nanoprobe, heat necrosis of a tumor adjacent to the vasculature, and thermal diffusion in a photothermal circumstance, respectively. The nanoprobe gains an 8-fold increase in magnetic resonance imaging (MRI) relaxivity compared to the clinical gadolinium diethylenetriaminepentaacetate, achieving a significant MRI signal in vivo. Conjugated with folate-PEG, the nanoprobe can be effectively absorbed by tumoral cells, obtaining a vivid two-photon cell imaging. A specific multisite scheme for photothermal therapy of a solid tumor is proposed to improve low photothermal efficacy caused by thermal diffusion in a large tumor, leading to the successful cure of the mice with xenograft tumor sized 10-12 mm. In vitro and in vivo toxicity, long-term excretion data, and the recovery of the treated mice demonstrate that the theranostic nanoprobe possesses good biocompatibility and metabolism efficacy.

Topological phase transition induced by magnetic proximity effect in two dimensions.

We study the magnetic proximity effect on a two-dimensional topological insulator in a CrI3/SnI3/CrI3 trilayer structure. From first-principles calculations, the BiI3-type SnI3 monolayer without spin-orbit coupling has Dirac cones at the corners of the hexagonal Brillouin zone. With spin-orbit coupling turned on, it becomes a topological insulator, as revealed by a non-vanishing Z 2 invariant and an effective model from symmetry considerations. Without spin-orbit coupling, the Dirac points are protected if the CrI3 layers are stacked ferromagnetically, and are gapped if the CrI3 layers are stacked antiferromagnetically, which can be explained by the irreducible representations of the magnetic space groups [Formula: see text] and [Formula: see text], corresponding to ferromagnetic and antiferromagnetic stacking, respectively. By analyzing the effective model including the perturbations, we find that the competition between the magnetic proximity effect and spin-orbit coupling leads to a topological phase transition between a trivial insulator and a topological insulator.

Stabilities and novel electronic structures of three carbon nitride bilayers.

We predict three novel phases of the carbon nitride (CN) bilayer, denoted α-C2N2, ß-C2N2 and γ-C4N4, respectively. All of them consist of two CN sheets connected by C-C covalent bonds. The phonon dispersions reveal that all these phases are dynamically stable, because no imaginary frequency is present. The transition pathway between α-C2N2 and ß-C2N2 is investigated, which involves bond-breaking and bond-reforming between C and N. This conversion is difficult, since the activation energy barrier is 1.90 eV per unit cell, high enough to prevent the transformation at room temperature. Electronic structure calculations show that all three phases are semiconductors with indirect band gaps of 3.76/5.22 eV, 4.23/5.75 eV and 2.06/3.53 eV, respectively, by PBE/HSE calculation. The ß-C2N2 has the widest band gap among the three phases. All three bilayers can become metallic under tensile strain, and the indirect gap of γ-C4N4 can turn into a direct one. γ-C4N4 can become an anisotropic Dirac semimetal under uniaxial tensile strain. Anisotropic Dirac cones with high Fermi velocity of the order of 105 m/s appear under 12% strain. Our results suggest that the three two-dimensional materials have potential applications in electronics, semiconductors, optics and spintronics.

Anode distance effect on field electron emission from carbon nanotubes: a molecular/quantum mechanical simulation.

Field electron emission from single-walled (5,5) carbon nanotubes was simulated with a quantum chemistry method, emphasizing the effect of distance between the anode and apex. The emission probability and the field enhancement factor were obtained for different anode-apex separations with two representative applied macroscopic fields. The quantum chemistry simulation was compared to the classical finite element calculation. It was found that the field enhancement factor was overestimated by about a factor 2 in the classical calculation (for the capped carbon nanotube). The effective work function lowering due to the field penetration into the apex has important contribution to the emission probability. A peculiar decrease of the effective work function with the anode-apex separation was found for the capped carbon nanotube, and its quantum mechanical origin is discussed.