Topological phases, local magnetic moments, and spin polarization triggered by C558-line defects in armchair graphene nanoribbons.

The topological and magnetic properties induced by topological defects in graphene have attracted attention. Here, we study a novel topological defect structure for graphene nanoribbons interspersed with C558-line defects along the armchair boundary, which possesses topological properties and is tritopic. Using strain engineering to regulate the magnitude of hopping at defects, the position of the energy level can be easily changed to achieve a topological phase transition. We also discuss the local magnetic moment and the ferromagnetic ground state in the context of line defects. This leads to spin polarization of the whole system. Finally, when C558 graphene nanoribbons are controlled by a nonlocal exchange magnetic field, spin-polarized quantum conductivity occurs near the Fermi level. Consequently, spin filtering can be achieved by varying the incident energy of the electrons. Our results provide new insights into realizing topological and spin electronics in low-dimensional quantum devices.

Effects of V and Gd doping on novel positive colossal electroresistance and quantum transport in PbPdO2 thin films with (002) preferred orientation.

In this work, PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 thin films with (002) preferred orientation were prepared using a pulsed laser deposition technique. The temperature dependence of resistivities ρI(T) was investigated under various applied DC currents. Colossal electroresistance (CER) effects were found in PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2. It was found that the positive CER values of PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 reach 3816% and 154% for I = 1.00 µA at 10 K, respectively. In addition, the ρI(T) cycle curves of PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 thin films showed a critical temperature similar to that of PbPdO2 (Tc = 260 K). Particularly, charge transfer between O1- and O2- was confirmed by in situ XPS. Additionally, based on first-principles calculations and internal electric field models, the CER and magnetic sources in PbPd0.9V0.1O2 and PbPd0.9Gd0.1O2 can be well explained. Finally, it was found that thin film samples doped with V and G ions exhibit weak localization (WL) and weak anti-localization (WAL) quantum transport properties. Ion doping leads to a transition from WAL to WL. The study results indicate that PbPdO2, one of the few oxide topological insulators, can exhibit novel quantum transport behavior after ion doping.

Effects of Mn doping on electronic and quantum transport in PbPdO2 thin films.

PbPdO2, a gapless semiconductor, can be transformed into a spin gapless semiconductor structure by magnetic ion doping. This unique band-gap structure serves as the foundation for its distinctive physical properties. In this study, PbPd1-xMnxO2 (x = 0.05, 0.1, 0.15) thin films with (002) preferred orientation were prepared by laser pulse deposition (PLD). The structural, electroresistive and magnetoresistive properties were systematically characterized, and the results suggest that films with different Mn doping ratios exhibit a current-induced positive colossal electroresistance (CER), and the CER values of PbPd1-xMnxO2 thin films increase with the increase of Mn doping concentration. The CER values are several fold magnitudes higher compared to those of the previously reported PbPdO2 films possessing identical (002) orientation. Combined with first-principles calculation, the underlying influence mechanism of Mn doping on CER is elucidated. In situ X-ray photoelectron spectroscopy (XPS) demonstrates a close correlation between the positive CER and the band gap change induced by oxygen vacancies in PbPd1-xMnxO2. Additionally, it is observed that Mn-doped films exhibit weak localization (WL) and weak anti-localization (WAL) quantum transport. Moreover, it is found that Mn doping can lead to a transition from WAL to WL; a small amount of Mn doping significantly enhances the weak anti-localization effect. However, with increasing Mn concentration, the WAL effect is conversely weakened. The results of studies suggest strongly that PbPdO2, one of the few oxide topological insulators, can display novel quantum transport behavior by ion doping.

The Zintl phase compounds AEIn2As2 (AE = Ca, Sr, Ba): topological phase transition under pressure.

AEIn2As2 (AE = Ca, Sr, Ba), as a new crucial nonmagnetic thermoelectric candidate, needs to be understood in terms of its potential electronic structure properties and topological characteristics in both experimental and theoretical studies. Here we report that AEIn2As2 with Zintl phases will undergo insulator-metal phase transition and topological quantum phase transition under pressure modulation based on first-principles calculations. Firstly, band inversion occurred between the In(As)-s and As(In)-p states in the structures of AEIn2As2 with the P63/mmc space group in the absence of pressure and identified that they are all non-trivial topological insulators. Next, Bader charge and AIM topology analysis elucidate the nature of pressure-induced chemical bond enhancement. Lastly, we have discovered pressure-controllable band gap closure while the topologically protected surface states disappear, realizing insulator-metal phase transition and topological quantum phase transition. Our research not only enriches the family of topological insulators but also provides a good platform for the study of thermoelectric properties.

First-principles study on the heterostructure of twisted graphene/hexagonal boron nitride/graphene sandwich structure.

In recent years, the discovery of 'magic angle' graphene has given new inspiration to the formation of heterojunctions. Similarly, the use of hexagonal boron nitride, known as white graphene, as a substrate for graphene devices has more aroused great interest in the graphene/hexagonal boron nitride heterostructure system. Based on the first principles method of density functional theory, the band structure, density of states, Mulliken population, and differential charge density of a tightly packed model of twisted graphene/hexagonal boron nitride/graphene sandwich structure have been studied. Through the establishment of heterostructure models twisted bilayer-graphene inserting hBN with different twisted angles, it was found that the band gap, Mulliken population, and charge density, exhibited specific evolution regulars with the rotation angle of the upper graphene, showing novel electronic properties and realizing metal-insulator phase transition. We find that the particular value of the twist angle at which the metal-insulator phase transition occurs and propose a rotational regulation mechanism with angular periodicity. Our results have guiding significance for the practical application of heterojunction electronic devices.

Zn(O,S) Buffer Layer for in Situ Hydrothermal Sb2S3 Planar Solar Cells.

Hydrothermal deposition is emerging as a highly potential route for antimony-based solar cells, in which the Sb2(S,Se)3 is typically in situ grown on a common toxic CdS buffer layer. The narrow band gap of CdS causes a considerable absorption in the short-wavelength region and then lowers the current density of the device. Herein, TiO2 is first evaluated as an alternative Cd-free buffer layer for hydrothermally derived Sb2S3 solar cells. But it suffers from a severely inhomogeneous Sb2S3 coverage, which is effectively eliminated by inserting a Zn(O,S) layer. The surface atom of sulfur in Zn(O,S) uniquely provides a chemical bridge to enable the quasi-epitaxial deposition of Sb2S3 thin film, confirming by both morphology and binding energy analysis using DFT. Then the results of the first-principles calculations also show that Zn(O,S)/Sb2S3 has a more stable structure than TiO2/Sb2S3. The resultant perfect Zn(O,S)/Sb2S3 junction, with a suitable band alignment and excellent interface contact, delivers a remarkably enhanced JSC and VOC for Sb2S3 solar cells. The device efficiency with the TiO2/Zn(O,S) buffer layer boosts from 0.54% to 3.70% compared with the counterpart of TiO2, which has a champion efficiency of Cd-free Sb2S3 solar cells with a structure of ITO/TiO2/Zn(O,S)/Sb2S3/Carbon/Ag by in situ hydrothermal deposition. This work provides a guideline for the hydrothermal deposition of antimony-based films upon a nontoxic buffer layer.

Co-construction of sulfur vacancies and carbon confinement in V5S8/CNFs to induce an ultra-stable performance for half/full sodium-ion and potassium-ion batteries.

The construction of anode materials for sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) with a high energy and a long lifespan is significant and still challenging. Here, sulfur-defective vanadium sulfide/carbon fiber composites (D-V5S8/CNFs) are designed and fabricated by a facile electrospinning method, followed by sulfuration treatment. The unique architecture, in which V5S8 nanoparticles are confined inside the carbon fiber, provides a short-range channel and abundant adsorption sites for ion storage. Moreover, enlarged interlayer spacings could also alleviate the volume changes, and offer small vdW interactions and ionic diffusion resistance to store more Na and K ions reversibly and simultaneously. The DFT calculations further demonstrate that sulfur defects can effectively facilitate the adsorption behavior of Na+ and K+ and offer low energy barriers for ion intercalation. Taking advantage of the functional integration of these merits, the D-V5S8/CNF anode exhibits excellent storage performance and long-term cycling stability. It reveals a high capacity of 462 mA h g-1 at a current density of 0.2 A g-1 in SIBs, while it is 350 mA h g-1 at 0.1 A g-1 in PIBs, as well as admirable long-term cycling characteristics (190 mA h g-1/17 000 cycles/5 A g-1 for SIBs and 165 mA h g-1/3000 cycles/1 A g-1 for PIBs). Practically, full SIBs upon pairing with a Na3V2(PO4)3 cathode also exhibit superior performance.