First-principles study of phase transition in theα-In2Se3/metal heterostructures.

With the discovery of two-dimensional (2D) ferroelectric materials such as CuInP2S6andα-In2Se3, the ferroelectric field effect transistors (Fe-FETs) based on these materials have entered a rapid-development period. The metal/semiconductor contact is an unavoidable topic in the construction of devices. In this paper, heterostructuresα-In2Se3/metals (Pd, Pt, Cu, Ag and Au) are discussed. According to different stacking types, the structures and energy of 160 heterostructures are calculated and compared. Whenα-In2Se3contacts with the Pd, Pt and Cu, theα-In2Se3may transforms intoß-In2Se3. This phenomenon has hardly been mentioned or analyzed in previous reports. Contacting with the Au and Ag, theα-In2Se3maintains the original structure. The internal physical mechanism of phase transition is explained from the binding energy and the charge transfer. The paper provides sufficient theoretical support for research and development of the Fe-FETs based onα-In2Se3.

Theoretical study on the tunable electronic band structure of Cs2PbI2Cl2/CsPbBr3 halide perovskite heterostructure driven by ferroelectric polarization modulation.

Ferroelectric polarizationhas been considered to be an key factor to tune the structural and photoelectric properties of perovskites and their heterostructures. While there has been growing researches made in the novel phenomena originating from interface formed between oxide perovskites, the effects of ferroelectric polarization on the electronic properties of halide perovskites and their heterostructures are rarely studied. Herein, by using first-principles calculations, all-inorganic halide perovskite heterostructure composed of 3D perovskite tetragonal CsPbBr3 and 2D Ruddlesden-Popper (RP) perovskite Cs2PbI2Cl2 is constructed for disclosing the relationship between the intrinsic polarization of tetragonal CsPbBr3 and electronic band structure of heterostructure. Cs atoms and Pb atoms of tetragonal CsPbBr3 in heterostructure are artificially moved away from the equivalent centers to simulate increased polarization. Our results show that with the spontaneous polarization of tetragonal CsPbBr3 increasing, the bandgap of heterostructure decreases, and the band alignment switches from staggered type-II to broken-gap type-III. Moreover, large cation-anion displacements along z-direction in tetragonal CsPbBr3 can be observed when tensile strains (≥5%) are applied, indicating a increased ferroelectric polarization, which also facilitates the decreasing of bandgap in heterostructure and the type-II-type-III transition of band alignment. Our study suggests that control over the polarization of ferroelectric materials is of great importance to tune the photoelectric properties of perovskite-based devices.

Theoretical study on the effect of the optical properties and electronic structure for the Bi-doped CsPbBr3.

Metal doping, including Bi, Yb, Eu, Sb and so on, are important means to improve the photoelectric properties and stability of metal halide perovskite materials. Among these works, Bi-doped CsPbBr3 especially has attracted much attention for both experimental and theoretical investigation. But there are still some arguments to be solved. One view thinks that Bi doping in CsPbBr3 not only influences the band structure, but also improves the charge transfer (Raihana et al 2017 J. Am. Chem. Soc. 139 731-7). The other supported the points that there are no changes in the valence band structure of Bi-doped CsPbBr3 and the concept of the band-gap engineering in Bi-doped CsPbBr3 halide perovskite is not valid (Olga et al 2018 J. Phys. Chem. Lett. 9 5408-11). They also have different opinions for the reason of the red-shift phenomenon caused by Bi-doped CsPbBr3. In this work, the density functional theory (DFT) based first-principles methods is adopted to investigate the effect of the optical properties and electronic structure for Bi doping CsPbBr3. The calculated results clarify that the red-shift phenomenon is caused by the slight reduction of band gap and the transition levels of Bii and BiPb defects. The values of red-shift also were estimated about 150 meV for Bii defects, which is close the experimental value of about 140 meV. Moreover, our studies also show that the Bi doping does not affect the valence bands, but Bii defects change the electron distribution of the conduction band. Our work and experimental results support and confirm each other, which provides a useful reference for the study of Sb-doped CsPbBr3, Eu-doped CsPbBr3 and so on.

The energy band engineering for the high-performance infrared photodetectors constructed by CdTe/MoS2 heterojunction.

Recently, the traditional infrared photodetectors (PDs) shows limited application in various areas, due to the narrow band-gap, high cost and even complex manufacturing process. In this situation, scientist have paid much attention to achieve the ultra broadband PDs from the deep ultraviolet to the near infrared. The energy band engineering for two-dimensional (2D) van der Waals heterojunction with free chemical dangling bonds is an effective method to fabricate High-performance Photodetectors. In this work, we employ density functional calculation to construct a type-II CdTe/MoS2 heterostructure and calculate its electronic properties. The results reveal that the CdTe/MoS2 has the narrow band gap of 0.64 eV and electrons transfer from the CdTe to MoS2 layer, which promotes the separation of photogenerated carriers and enhance the photoelectron conversion efficiency. Driven by the smaller band gap, it can respond to near infrared, visible and ultraviolet light, demonstrating it the promising application for solar cell. Furthermore, the analysis of molecules adsorption and band edge alignment indicates that the CdTe/MoS2 is prone to capture H2O and release the H2 molecules, which is conductive to the photocatalytic water splitting for hydrogen generation. Our work suggests that the CdTe/MoS2 heterostructure is a potential candidate as a solar cell and even photocatalyst, and also provides a new sight for experimental and theoretical research to design a highly efficient device.

The influence of electrode for electroluminescence devices based on all-inorganic halide perovskite CsPbBr3.

Electroluminescence devices based on all-inorganic halide perovskite material with excellent luminescence performance have been studied extensively in recent years. However, the important role for the electrodes of electroluminescence devices is payed few attention by theoretical and experimental studies. Appropriate electrodes can reduce the Schottky barrier height to decrease the energy loss, and prevent the metal impurities from diffusing into the perovskite material to generate deep traps levels, which improves the luminous efficiency and lifetime of devices. In this paper, not only the interface effects between CsPbBr3 and common metal electrode (Ag, Au, Ni, Cu and Pt) are studied by first-principle calculations, but also the diffusion effects of metal electrode atom into the CsPbBr3 layer are also explored by nudged elastic band calculations. The calculated results show the metal Ag is more suitable for the cathode for CsPbBr3 electroluminescence devices, while the metal Pt is more applicable for the anode. Based on the overall consideration about the interface effects and diffusion effects of the CsPbBr3-metal electrode junctions, the essential principle is analyzed. The work provides theoretical guidance for how to select the right electrode for the electroluminescence performance of all-inorganic halide perovskite. The critical factor of Schottky barrier height between the electrode and the light-emitting semiconductor, and transition level generated by metal impurities also provide a valuable reference how to select the suitable electrodes for other electroluminescence devices.

Band alignment engineering: ultrabroadband photodetection with SnX2 (X = S, Se)/ZnS heterostructures.

Ultrabroadband mid-frequency infrared photodetectors have important applications in surveillance, medical diagnosis, bioimaging and navigation aids. Thus, researchers hope to detect mid-infrared radiation with larger wavelength. However, due to the limitation of room temperature, it is difficult for these detectors to detect mid-infrared with 4 µm or larger wavelength. Therefore, at room temperature, how to realize mid-infrared detection in a wide range has become an urgent problem to be solved. In this paper, the band structure of SnX2 (X = S,Se)/ZnS and SnS2(1-ŋ)Se2ŋ /ZnS was studied by the density functional theory based first-principles methods. Under the specific stacking procedure, changing the [Formula: see text] of SnS2(1-ŋ)Se2ŋ , the band gap of heterojunctions can be continuously tuned from 0 to 0.97 eV. Amazingly, the band structure maintains the characteristics of a type-II heterojunction. The photodetection in our work is estimated for wavelengths from 1.2 µm to 10 µm, covering a wide wavelength range of mid-infrared. Such a wide range is considerable in current research. The characteristics of type-II band structure and the wide detection range imply that SnX2 /ZnS has great potential in mid-frequency infrared detection. Our work may provide some breakthroughs for the research of multiband photodetectors at room temperature.

Characterization of glycerophospholipid molecular species in muscles from three species of cephalopods by direct infusion-tandem mass spectrometry.

More than 200 molecular species of glycerophospholipids (GP) including glycerophosphocholine (GPC), glycerophosphoethanolamine (GPE), glycerophosphoserine (GPS), lysoglycerophosphocholine (LGPC), lysoglycerophosphoethanolamine (LGPE) and lysoglycerophosphoserine (LGPS), as well as 18 kinds of sphingomyelin (SM) were characterized by using a direct infusion-tandem mass (MS/MS) spectrometry method for lipids from the muscles of cephalopods Sepiella maindroni, Octopus ocellatus and Loligo chinensis for the first time. The majority of the GP molecular species contained long-chain omega-3 polyunsaturated fatty acids (n-3 LC-PUFA), especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Therefore, cephalopods can be a good possible source of dietary GP carrying n-3 LC-PUFA. The total lipids were composed of phospholipid (PL, 72.29-83.32 wt% of total lipids), cholesterol (12.70-23.60 wt% of total lipids), triacylglycerol (1.86-2.93 wt% of total lipids), diacylglycerol (0.15-1.09 wt% of total lipids), monoacylglycerol (0.06-0.18 wt% of total lipids) and free fatty acid (0.72-1.86 wt% of total lipids). For PL, phosphatidylcholine (44.47-62.30 mol%), phosphatidylethanolamine (22.57-39.08 mol%), phosphatidylserine (6.15-10.18 mol%), phosphatidylglycerol (0.68-3.11 mol%), phosphatidylinositol (2.41-7.15 mol%) and lysophosphatidylcholine (1.84-5.24 mol%) were detected. Furthermore, the total lipids from the muscles of cephalopods Sepiella maindroni, Octopus ocellatus and Loligo chinensis contained 41.80-50.02 mol% of saturated fatty acids, 11.53-21.54 mol% of monounsaturated fatty acids and 36.67-40.82 mol% of PUFA, whilst DHA (15.25-26.71 mol%) and EPA (6.29-16.57 mol%) were found to account for the majority of the PUFA. With these data presented, cephalopod muscle can be considered as a healthy food for humans.

Pressure-induced strong ferroelectric polarization in tetra-phase perovskite CsPbBr3.

Ab initio simulations combined with the Berry phase method are employed to investigate ferroelectric polarization of tetragonal CsPbBr3 crystals by applying hydrostatic pressure varying from 0 to 19 GPa; we find that the object research belongs to the P4mm space group. The calculated results show that the materials undergo a paraelectric-ferroelectric phase transition when the pressure increases to a critical value 15 GPa. The polarization is strongly enhanced and attains a high value of about 23 µC cm-2, owing to the increase in the ionic and electric contributions to the polarization under compressive strain. We present a detailed theoretical investigation to analyze the origin of polarization. The ionic polarization is mainly ascribed to the central displacements of Pb2+ cations and Br- anions induced by a highly distorted octahedral PbBr6- framework. Electronic structure calculations suggest that asymmetric hopping p orbital electrons of Br(3) ions are responsible for the enhancement in electric polarization. These discoveries suggest that tetragonal CsPbBr3 has significant potential in future ferroelectric applications, and this can broaden the application field from optoelectronics to ferroelectrics.

Layer-dependent transport and optoelectronic property in two-dimensional perovskite: (PEA)2PbI4.

Recently, two-dimensional (2D) layered organic-inorganic hybrid perovskites have attracted a huge amount of interest due to their unique layered structure, and potential optical properties. However, amongst researchers it has long been disputed as to whether it is suitable for use as a photovoltaic material or light-emitting device. Here, we present a detailed theoretical investigation to discuss the photovoltaic and optoelectronic properties of a novel synthetic 2D layered perovskite (PEA)2PbI4. Based on the calculated geometric and electronic structure, charge carrier mobilities of the 2D layered (PEA)2PbI4 are predicted theoretically. In addition, the linear dichroism and exciton binding energies are also calculated. We found that the carrier mobilities of the 2D layered (PEA)2PbI4 reach the same order of magnitude as those of the optoelectronic material MoS2, but smaller than those of the photovoltaic material MAPbI3 and Si crystal, whereas exciton binding energies (Eb) enlarge with the thinning layers, being obviously higher than MAPbI3 and Si crystal. Moreover, the system exhibits a strong linear dichroism, suggesting weak absorption along the c axis in the visible spectrum, which is detrimental to photovoltaics. Our work provides a theoretical basis to prove that ultrathin two-dimensional (2D) materials may be potential candidates for optoelectronic detection devices, rather than solar absorbers.

Tuning the Schottky rectification in graphene-hexagonal boron nitride-molybdenum disulfide heterostructure.

It was still a great challenge to design high performance of rectification characteristic for the rectifier diode. Lately, a new approach was proposed experimentally to tune the Schottky barrier height (SBH) by inserting an ultrathin insulated tunneling layer to form metal-insulator-semiconductor (MIS) heterostructures. However, the electronic properties touching off the high performance of these heterostructures and the possibility of designing more efficient applications for the rectifier diode were not presently clear. In this paper, the structural, electronic and interfacial properties of the novel MIS diode with the graphene/hexagonal boron nitride/monolayer molybdenum disulfide (GBM) heterostructure had been investigated by first-principle calculations. The calculated results showed that the intrinsic properties of graphene and MoS2 were preserved due to the weak van der Waals contact. The height of interfacial Schottky barrier can be tuned by the different thickness of hBN layers. In addition, the GBM Schottky diode showed more excellent rectification characteristic than that of GM Schottky diode due to the interfacial band bending caused by the epitaxial electric field. Based on the electronic band structure, we analyzed the relationship between the electronic structure and the nature of the Schottky rectifier, and revealed the potential of utilizing GBM Schottky diode for the higher rectification characteristic devices.