The tunneling electroresistance effect in a van der Waals ferroelectric tunnel junction based on a graphene/In2Se3/MoS2/graphene heterostructure.

In recent years, α-In2Se3 has attracted great attention in miniaturizing nonvolatile random memory devices because of its room temperature ferroelectricity and atomic thickness. In this work, we construct two-dimensional (2D) van der Waals (vdW) heterostructures α-In2Se3/MoS2 with different ferroelectric polarization and design a 2D graphene (Gr)/In2Se3/MoS2/Gr ferroelectric tunnel junction (FTJ) with the symmetric electrodes. Our calculations show that the band alignment of the heterostructures can be changed from type-I to type-II accompanied by the reversal of the ferroelectric polarization of In2Se3. Furthermore, the ferroelectricity persists in Gr/In2Se3/MoS2/Gr vdW FTJs, and the presence of dielectric layer MoS2 in the FTJs enables the effective modulation of the tunneling barrier by altering the ferroelectric polarization of α-In2Se3, which results in two distinct conducting states denoted as "ON" and "OFF" with a large tunneling electroresistance (TER) ratio exceeding 105%. These findings suggest the importance of ferroelectric vdW heterostructures in the design of FTJs and propose a promising route for applying the 2D ferroelectric/semiconductor heterostructures with out-of-plane polarization in high-density ferroelectric memory devices.

Polarization-tunable interfacial properties in monolayer-MoS2 transistors integrated with ferroelectric BiAlO3(0001) polar surfaces.

With the explosion of data-centric applications, new in-memory computing technologies, based on nonvolatile memory devices, have become competitive due to their merged logic-memory functionalities. Herein, employing first-principles quantum transport simulation, we theoretically investigate for the first time the electronic and contact properties of two types of monolayer (ML)-MoS2 ferroelectric field-effect transistors (FeFETs) integrated with ferroelectric BiAlO3(0001) (BAO(0001)) polar surfaces. Our study finds that the interfacial properties of the investigated partial FeFET devices are highly tunable by switching the electric polarization of the ferroelectric BAO(0001) dielectric. Specifically, the transition from quasi-Ohmic to the Schottky contact, as well as opposite contact polarity of respective n-type and p-type Schottky contact under two polarization states can be obtained, suggesting their superior performance metrics in terms of nonvolatile information storage. In addition, due to the feature of (quasi-)Ohmic contact in some polarization states, the explored FeFET devices, even when operating in the regular field-effect transistor (FET) mode, can be extremely significant in realizing a desirable low threshold voltage and interfacial contact resistance. In conjunction with the formed van der Waals (vdW) interfaces in ML-MoS2/ferroelectric systems with an interlayer, the proposed FeFETs are expected to provide excellent device performance with regard to cycling endurance and memory density.

Remarkable ferroelectricity-modulated electronic and magnetic properties in a 2H-VS2/BiAlO3(0001) hybrid system.

In the present work, a 2H-VS2/BiAlO3(0001) hybrid system is constructed to perform first-principles density functional theory (DFT) calculations. The results reveal that, in addition to the ionic-vdW interface coupling, the ferromagnetic semiconductive 2H-VS2 monolayer on the ferroelectric BiAlO3(0001) substrate exhibits n-type or p-type doping behavior and even half-metal characteristics. Furthermore, the magnetoelectric coefficient (αS) for the 2H-VS2/BiAlO3(0001) structures can reach a value of 10-10 G cm2 V-1 with ferroelectric polarization reversal. The estimated Curie temperatures (Tc) of the 2H-VS2 monolayer on the BiAlO3(0001) Z+ (positive), Z+↓ (polarization-reversed Z+), Z- (negative), and Z-↑ (polarization-reversed Z-) polar surfaces were found to be 176, 276, 266, and 87 K, respectively. This indicates that the magnetic properties of the 2H-VS2 monolayer are remarkably tunable using a ferroelectric BiAlO3(0001) knob. These important findings provide a distinctive treatment option for controllable and adjustable nanoelectronic, photoelectronic, and spintronic devices based on a 2H-VS2 monolayer.

Interface coupling and charge doping in graphene on ferroelectric BiAlO3(0001) polar surfaces.

The ferroelectric field-effect transistors (FeFETs) based on graphene/ferroelectric (Gr/FE) hybrid systems have been attracting a lot of attention in recent years. The interface interaction and charge transfer between graphene and the ferroelectric substrates are important factors that determine the performance of graphene-based FeFETs. According to our intuitive sense, the electrostatically doped carriers in graphene on the ferroelectric positive and negative surfaces should be n-type and p-type, respectively. In the present work, however, by employing first-principles density functional theory (DFT) calculations, we reveal that an unusual charge doping effect occurs in graphene on the thermodynamically preferred ferroelectric BiAlO3(0001) polar surfaces. The graphene on the BiAlO3(0001) positive surface is electrostatically doped p-type, while the BiAlO3(0001) negative surface induces n-type carriers in the graphene overlayer. Further analysis demonstrates that, although the electrostatic doping effect in the Gr/FE system depends on the polarization direction of the ferroelectric substrate, the resultant carrier type and density in graphene are determined by the specific band arrangement between graphene and the ferroelectric polar surface. In addition to the graphene-based FeFETs, our results predict that the Gr/BiAlO3(0001) systems can be used to fabricate graphene p-n homojunctions by engineering the domain pattern in the ferroelectric substrate.

Charge doping in graphene on thermodynamically preferred BiFeO3(0001) polar surfaces.

For graphene/ferroelectric hybrid structures, the atomistic and electronic details of the interfaces are of crucial importance for charge doping in graphene. In this paper, we choose thermodynamically stable BiFeO3(0001) surfaces to explore the adsorption behavior and charge doping effect in a graphene/BiFeO3 system. By performing first-principles calculations, we find that both the adsorption behavior and charge doping effect show distinct characteristics for graphene adsorbed on the oppositely polarized BiFeO3(0001) surfaces. We predict that n-type doping and p-type charge doping occur in graphene on the positive and negative BiFeO3(0001) surfaces, respectively. The carrier density is estimated to be 1013 cm-2 orders of magnitude. Our results reveal that the graphene/BiFeO3 hybrid system is an intriguing candidate to make graphene-based field-effect transistors, whose p-n junctions can be made by patterning the domain structure of the BiFeO3 substrate. Moreover, the graphene/BFO hybrid structure may display an outstanding photovoltaic effect due to the combination of the bulk photovoltaic effect of the BFO substrate and the optical transparency of the graphene electrode.

Electrostatic Modulation and Mechanism of the Electronic Properties of Monolayer MoS2 via Ferroelectric BiAlO3(0001) Polar Surfaces.

In the present work, first-principles density functional theory calculations were carried out to explore the intrinsic interface coupling and electrostatic modulation as well as the effect of ferroelectric polarization reversal in the MoS2/BiAlO3(0001) [MoS2/BAO(0001)] hybrid system. In addition to the interaction mechanism of the large ionic-van der Waals (vdW) coupling, our results indicate that the electronic properties of monolayer MoS2 on the BAO(0001) polar surface can be effectively modulated by reversing the ferroelectric polarization and/or engineering the domain structures of the substrate. Due to the unusual charge transfer between the MoS2 overlayer and the down-polarized ferroelectric BAO(0001) substrate, in the final analysis, the physical mechanism determining the interfacial charge transfer in the MoS2/BAO(0001) hybrid system is attributed to the specific band alignment between the clean BAO(0001) surface and the freestanding monolayer MoS2. Furthermore, our study predicts that MoS2-based ferroelectric field-effect transistors and various types of seamless p-i, n-i, p-n, p+-p, and n+-n homojunctions possessing an extremely steep built-in electric field can be fabricated by reversing the ferroelectric polarization and/or patterning the domain structure of the BAO(0001) substrate.

First-Principles Study of Hydrogen Storage of Sc-Modified Semiconductor Covalent Organic Framework-1.

At present, the development of new carbon-based nanoporous materials with semiconductor properties and high hydrogen storage capacity has become a research hotspot in the field of hydrogen storage and hydrogen supply. Here, we pioneered the study of the hydrogen storage capacity of a scandium (Sc) atom-modified semiconductor covalent organic framework-1 (COF-1) layer. It was found that the hydrogen storage capacity of the COF-1 structure was significantly enhanced after the modification of the Sc atom. We found that each Sc atom of the modified COF-1 structure can stably adsorb up to four H2 molecules, and the average adsorption energy of the four hydrogen molecules is -0.284 eV/H2. Six Sc atoms are stably adsorbed most bilaterally on the cell of the COF-1 unit, which can adsorb 24 H2 molecules in total. In addition, we have further studied the adsorption and desorption behaviors of H2 molecules on the 6Sc-COF-1 surface at 300 and 400 K, respectively. It can be found that each Sc atom of the COF-1 unit cell can stably adsorb three H2 molecules with a hydrogen storage performance of 5.23 wt % at 300 K, which is higher than those of lithium-modified phosphorene (4.4 wt %) and lithium-substituted BHNH sheets (3.16 wt %). At 400 K, all of the adsorbed H2 molecules are released. This confirms the excellent reversibility of 6Sc-COF-1 in hydrogen storage performance. This research has great significance in the application of fuel cells, surpassing traditional hydrogen storage materials.

Identification of the pheromone biosynthesis genes from the sex pheromone gland transcriptome of the diamondback moth, Plutella xylostella.

The diamondback moth was estimated to increase costs to the global agricultural economy as the global area increase of Brassica vegetable crops and oilseed rape. Sex pheromones traps are outstanding tools available in Integrated Pest Management for many years and provides an effective approach for DBM population monitoring and control. The ratio of two major sex pheromone compounds shows geographical variations. However, the limitation of our information in the DBM pheromone biosynthesis dampens our understanding of the ratio diversity of pheromone compounds. Here, we constructed a transcriptomic library from the DBM pheromone gland and identified genes putatively involved in the fatty acid biosynthesis, pheromones functional group transfer, and ß-oxidation enzymes. In addition, odorant binding protein, chemosensory protein and pheromone binding protein genes encoded in the pheromone gland transcriptome, suggest that female DBM moths may receive odors or pheromone compounds via their pheromone gland and ovipositor system. Tissue expression profiles further revealed that two ALR, three DES and one FAR5 genes were pheromone gland tissue biased, while some chemoreception genes expressed extensively in PG, pupa, antenna and legs tissues. Finally, the candidate genes from large-scale transcriptome information may be useful for characterizing a presumed biosynthetic pathway of the DBM sex pheromone.

Thermodynamic Stability of BiFeO3 (0001) Surfaces from ab Initio Theory.

The relative stability of multiferroic BiFeO3 (0001) surfaces, which is the (111) facet in the pseudocubic notation, with different stoichiometry is systematically studied by using ab initio thermodynamic approach in order to obtain insights into the stable surface terminations. We predict that under most chemical potential conditions the thermodynamically favored terminations for the negative and positive surfaces are -Bi-O2 and -Fe-O3-Bi, respectively. The predicted difference in oxygen content between the negative and positive surfaces is consistent with experimental observations at the BiFeO3/metal interfaces ( Nat. Mater. , 2014 , 13 , 1019 , DOI: 10.1038/nmat4058 ; Adv. Mater. , 2015 , 27 , 6934 , DOI: 10.1002/adma.201502754 ). We determine the atomic geometries and electronic states as well as the magnetic properties for the negatively and positively polarized stable surfaces. Our results demonstrate that not only the stoichiometry and atomic geometries but also the electronic and magnetic properties of the BiFeO3 (0001) surfaces show strong dependence on the ferroelectric polarization direction. Therefore, we expect that the surface physical and chemical properties of the BiFeO3 (0001) surfaces can be easily tuned by an external electric field.

[Tritrophic system of tree-trunkborer-insect natural enemy association].

Trunkborers, mainly bark beetles and cerambycids, are the important insect pests of forests, which can cause serious damage to forest ecosystem. Taking bark beetle and long-horned beetle as the examples, this paper summarized the research progress on the tritrophic system of tree-trunkborer-insect natural enemy association, with the past ten years research results on semiochemicals release source, active compounds identification, and release dynamics of volatiles introduced. The chemical signals selection of natural enemies and their visual function in finding hosts or preys, as well as whether there were other biological active compounds beyond this tritrophic system, which could influence the natural enemy's behaviors, were also discussed.