Continuous Tuning of Band Gap for π‑Conjugated Ni Bis(dithiolene) Complex Bilayer

Based on density functional calculations in combination with empirical corrections for the van der Waals interaction, we show that a recently synthesized two-dimensional sheet of π-conjugated Ni bis­(dithiolene) complex (NiC₄S₄) exists in the form of bilayers with specific pairs of interlayer covalent bonds rather than in the form of isolated single layers. When one of the layers is slid relative to the other in the direction diagonal to two primitive vectors by applying a shear stress of ∼135 pN between the two layers brings about a gradual decrease in the band gap from 0.15 to 0 eV. Unlike in the case of an isolated single layer, the band gap of the bilayer is always found to be direct during the change. Therefore, the (NiC₄S₄)₂ bilayer will be quite useful in nanoelectromechanical devices as well as in optoelectronic devices

Electronic Structure and Carrier Mobility of Two-Dimensional α Arsenic Phosphide

Using first-principles calculations, we investigate electronic structures of α arsenic phosphide under strain. It is a two-dimensional monolayer composed of an equimolar mixture of phosphorus and arsenic, whose multilayer correspondents were synthesized very recently. According to structure optimizations and phonon calculations, the α phase branches into three distinct allotropes. Monolayers of the α₁ and α₃ phases are direct-gap semiconductors with band gaps that are similar to that of the α phosphorene. They exhibit anisotrpic carrier mobility. Specifically, the α₃ phase exhibits the electron mobility of ∼10 000 cm² V^–1 s^–1, which is 1 order of magnitude larger than that for the α phosphorene. Likewise, their electronic structures display highly anisotropic behavior under strain different from that of the α phosphorene. The complex response under strain can be mostly understood in terms of the relative alignment of bonding and antibonding As–P states under a specific strain

Electronic Structures and Li-Diffusion Properties of Group IV–V Layered Materials: Hexagonal Germanium Phosphide and Germanium Arsenide

Based on density functional theory (DFT) calculations that include an empirical van der Waals interaction, we propose a layered hexagonal phase of bulk GeP and GeAs that is marginally less stable than the monoclinic phase experimentally observed. Both types of monolayers are dynamically stable semiconductors. Application of 2% isotropic stretching along two in-plane directions practically transforms the GeAs monolayer into a direct-gap (= 1.60 eV) material, rendering it useful in optoelectronics. In addition, comparison of effective masses shows that the GeAs monolayer can function better as <i>n</i>-type materials, especially when it is subject to the in-plane strain. Furthermore, a detailed comparison of the activation barriers for the rate-determining steps along the different paths on the GeP surface indicates that the Li atom can diffuse on the surface ∼1000 times faster than on graphene. Another comparison of the barriers allows us to identify a preferred diffusion path in the interlayer region of bulk hexagonal GeP. The diffusion is expected to occur as fast as in graphite, suggesting that its bulk can be useful as an anode material in lithium ion battery

Electronic and Quantum Transport Properties of Heterobilayers of Graphene Nanoribbons and Zinc-Porphyrin Tapes

Using the first-principles calculation, we have shown that heterobilayers can be formed between armchair graphene nanoribbons (GNRs) and zinc-porphyrin tapes (Zn-PPTs). The PPTs investigated include triply lined (TL) and doubly linked (DL) PPTs. In addition, we have also investigated electronic structures and conductances of these heterobilayers. The bilayer involving the DL Zn-PPT is more stable than its TL correspondents due to stronger electronic coupling, which can be ascribed to the similar dispersion relations of the free-standing GNR and the DL PPT around the Fermi level. Consequently, the bilayer formation of TL Zn-PPT with GNR turns it into a metal, while its DL correspondent remains semiconducting but exhibits an increased on-current at an appropriate gate voltage. Our calculation of the band gap of the GNR as a function of the ribbon width also shows that the band-gap oscillation is reduced upon bilayer formation with DL Zn-PPT

Effect of Si–Si Bonds in Silicon-Doped α‑Phosphorene Bilayers: Two-Dimensional Layers and One-Dimensional Nanoribbons

We investigate the geometrical and electronic structures of various configurations of 2Si-doped two-dimensional (2D) bilayers of black phosphorene (αP)₂, in which two P atoms are substituted by Si atoms. Our first-principles calculations suggest that doping is cooperative, which is clearly manifested in the formation of Si–Si bonds in the two most stable configurations. As a result, both configurations become indirect-gap semiconductors, which differ from that of the pristine 2D bilayer. On the one hand, 2Si-doped armchair phosphorene nanoribbon (APNR) bilayers possess pseudodirect band gaps in the most stable configuration, which are one-dimensional materials cut with armchair edges saturated with hydrogen atoms. Comparisons of the deformation energy and the activation barrier suggest that Stone–Wales (SW) deformation can occur substantially more easily in the doped APNR than in carbon nanotubes, and molecular dynamics simulations show that the SW defect will be kinetically stable. This is because the deformation brings about shortening and strengthening of weak Si–Si bonds. As a result, the APNRs turn into real direct-gap materials

Mechanical and Electronic Properties of π‑Conjugated Metal Bis(dithiolene) Complex Sheets

Using first-principles calculations, we have investigated the mechanical properties and electronic structures of π^–conjugated metal bis­(dithiolene) complex sheets (MC₄S₄), where M = Ni and Pd. First, the sheets are much softer than graphene due to their large porosity. At zero strain, NiC₄S₄ is a semiconductor with an indirect gap, while PdC₄S₄ is a metal. Under either biaxial or uniaxial strain, our band structure analysis demonstrates that the band gap of the NiC₄S₄ slowly decreases to zero with increased strain, which can be attributed to the gradual weakening of π-bonds of the sheet. However, the PdC₄S₄ becomes a magnetic system beyond the deformation threshold that causes a plastic deformation along the <i>X</i>-axis. In addition, we also observe that both two-dimensional sheets undergo different types of nonreversible plastic changes under the uniaxial strains along the <i>X</i>- and <i>Y</i>-axes

Hepatitis B virus–triggered autophagy targets TNFRSF10B/death receptor 5 for degradation to limit TNFSF10/TRAIL response

<p>Death receptors of TNFSF10/TRAIL (tumor necrosis factor superfamily member 10) contribute to immune surveillance against virus-infected or transformed cells by promoting apoptosis. Many viruses evade antiviral immunity by modulating TNFSF10 receptor signaling, leading to persistent infection. Here, we report that hepatitis B virus (HBV) X protein (HBx) restricts TNFSF10 receptor signaling via macroautophagy/autophagy-mediated degradation of TNFRSF10B/DR5, a TNFSF10 death receptor, and thus permits survival of virus-infected cells. We demonstrate that the expression of the TNFRSF10B protein is dramatically reduced both in liver tissues of chronic hepatitis B patients and in cell lines transfected with HBV or HBx. HBx-mediated downregulation of TNFRSF10B is caused by the lysosomal, but not proteasomal, degradation pathway. Immunoblotting analysis of LC3B and SQSTM1, and microscopy analysis of tandem-fluorescence-tagged LC3B revealed that HBx promotes complete autophagy. Inhibition of autophagy with a pharmacological inhibitor and <i>LC3B</i> knockdown revealed that HBx-induced autophagy is crucial for TNFRSF10B degradation. Immunoprecipitation and GST affinity isolation assays showed that HBx directly interacts with TNFRSF10B and recruits it to phagophores, the precursors to autophagosomes. We confirmed that autophagy activation is related to the downregulation of the TNFRSF10B protein in liver tissues of chronic hepatitis B patients. Inhibition of autophagy enhanced the susceptibility of HBx-infected hepatocytes to TNFSF10. These results identify the dual function of HBx in TNFRSF10B degradation: HBx plays a role as an autophagy receptor–like molecule, which promotes the association of TNFRSF10B with LC3B; HBx is also an autophagy inducer. Our data suggest a molecular mechanism for HBV evasion from TNFSF10-mediated antiviral immunity, which may contribute to chronic HBV infection.</p

Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin

Germanium and tin sulfide nanostructures are considered the most promising candidates for useful alternative materials in commercial Li–graphite anodes of lithium ion batteries. Selenides have received less attention, but the electrochemical reaction mechanism is still being debated. We report the novel synthesis of GeSe_<i>x</i> and SnSe_<i>x</i> (<i>x</i> = 1 and 2) nanocrystals by a gas-phase laser photolysis reaction and their excellent reversible capacity for lithium ion batteries. The capacity was 400–800 (mA h)/g after 70 cycles, which is close to the theoretical capacity (Li_4.4Ge or Li_4.4Sn). Remarkably, SnSe_<i>x</i> exhibited higher rate capabilities than GeSe_<i>x</i>. Ex situ X-ray diffraction and Raman spectroscopy revealed the <i>cubic</i>–<i>tetragonal</i> phase conversion of Ge and Sn upon lithiation/delithiation to support their distinctive lithium ion battery capacities. First-principles calculations of the Li intercalation volume change indicate that the smallest volume expansion in the cubic Sn phase can guarantee the enhanced cycling capability of the Sn compounds

Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide

Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multilayer nanosheets of GaS–GaSe alloy are found to have a band gap (<i>E</i>_g) of 2.0–2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this <i>E</i>_g to 3.3–3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the <i>E</i>_g of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect <i>E</i>_g decrease significantly; the indirect <i>E</i>_g approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect <i>E</i>_g of GaS to converge, resulting in monolayers with a strong emission. This wide <i>E</i>_g tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays

Phase Evolution of Tin Nanocrystals in Lithium Ion Batteries

Sn-based nanostructures have emerged as promising alternative materials for commercial lithium–graphite anodes in lithium ion batteries (LIBs). However, there is limited information on their phase evolution during the discharge/charge cycles. In the present work, we comparatively investigated how the phases of Sn, tin sulfide (SnS), and tin oxide (SnO₂) nanocrystals (NCs) changed during repeated lithiation/delithiation processes. All NCs were synthesized by a convenient gas-phase photolysis of tetramethyl tin. They showed excellent cycling performance with reversible capacities of 700 mAh/g for Sn, 880 mAh/g for SnS, and 540 mAh/g for SnO₂ after 70 cycles. Tetragonal-phase Sn (β-Sn) was produced upon lithiation of SnS and SnO₂ NCs. Remarkably, a cubic phase of diamond-type Sn (α-Sn) coexisting with β-Sn was produced by lithiation for all NCs. As the cycle number increased, α-Sn became the dominant phase. First-principles calculations of the Li intercalation energy of α-Sn (Sn₈) and β-Sn (Sn₄) indicate that Sn₄Li_<i>x</i> (<i>x</i> ≤ 3) is thermodynamically more stable than Sn₈Li_<i>x</i> (<i>x</i> ≤ 6) when both have the same composition. α-Sn maintains its crystalline form, while β-Sn becomes amorphous upon lithiation. Based on these results, we suggest that once α-Sn is produced, it can retain its crystallinity over the repeated cycles, contributing to the excellent cycling performance