A polarization-sensitive,self-powered,broadband and fast Ti3C2Tx MXene photodetector from visible to near-infrared driven by photogalvanic effects

Broadband, self-power, and polarization-sensitivity are desirable qualities for a photodetector. However, currently few photodetectors can fulfill these requirements simultaneously. Here, we propose a Ti₃C₂T_x (MXene) photodetector that is driven by the photogalvanic effect with impressive performances. A polarization-sensitive photocurrent is generated at zero bias under the illumination of linearly polarized laser light of 1064 nm, with an extinction ratio of 1.11. Meanwhile, a fast response with a 32/28 ms rise/decay time and a large on/off switching ratio of 120 are achieved. Besides, a robust zero-bias photocurrent is also generated in the photodetector under the illumination of 940 and 620 nm light, as well as the white light, showing a broadband photoresponse from the near-infrared to visible. Moreover, quantum transport simulations indicate that the photogalvanic effect plays an important role in the generation of the polarized photocurrent at zero bias due to the broken space inversion symmetry of the stacked few-layer Ti₃C₂T_x. Our results shed light on a potential application of the Ti₃C₂T_x–MXene in the low-power photodetection with high performances.     

Observations on the role of MgCl2 in the Weissler reaction

Aqueous solutions of Nal containing CCl₄ and MgCl₂ at various concentrations were irradiated under air with 1 MHz ultrasound and the yield of I₃⁻ was determined. The yield was not affected by MgCl₂ at concentrations up to 0.1 M. This contrasts with the finding of Lepoint and co-workers, who reported a sharp minimum in the yield at a MgCl₂ concentration of 2.5 × 10⁻³M, the yield decreased to 60% at 1 M MgCl₂, the reason being the lower solubility of CCl₄ at high MgCl₂ concentrations. In the absence of CCl₄, another dependence on the MgCl₂ concentration was observed: the yield was not affected up to 1 M, and at higher MgCl₂ concentrations the yield rapidly decreased owing to the increased viscosity of the solution. On the basis of these observations, there is no strong reason to postulate an electrical mechanism for the initiation of chemical reactions in the cavitation bubbles.     

Towards the preparation of realistic model Ziegler-Natta catalysts: XPS study of the MgCl2/TiCl4 interaction with flat SiO2/Si(1 0 0)

Despite of the wide use of supported Ti based Ziegler-Natta catalysts in the olefin polymerization industry, questions concerning the role of each one of the catalyst components in the polymerization process, have not found a satisfactory answer yet. This is mainly because of the high sensitivity of these systems to oxygen and atmospheric moisture that makes their study in an atomic level rather complicated. Realistic surface science models of the pre-activated SiO₂ supported MgCl₂/TiCl₄ and TiCl₄ Ziegler-Natta catalysts were prepared by spin coating on flat conductive SiO₂/Si(1 0 0) supports under inert atmosphere. This preparation technique resembles the wet chemical impregnation which is the industrial method of the catalyst preparation. XPS analysis showed that the catalyst precursor anchors on the silica surface through bonding of the Ti atoms with surface silanes or siloxanes, while Mg is attached to the Ti through chlorine bridges. Thermal treatment of the catalysts at 723 K leads to total Cl desorption when MgCl₂ is not present while a significant amount of the Ti atoms is reduced to the Ti³⁺ state.     

碱金属Li,Na,K,Rb,Cs,Fr的ns2S1/2,np2P1/2,3/2, nd2D3/2,5/2 和nf2F5/2,7/2 里德伯能级理论研究

本文在多通道量子数亏损理论(MQDT)框架下,利用相对论多通道理论(RMCT),分别在冻结实近似、 考虑Δl=-1的偶极极化效应、Δl=+1的偶极极化效应、Δl=± 1的偶极极化效应、伸缩模效应以及同时考虑偶极极化效应和伸缩模效应等不同层次近似下,系统地计算了碱金属Li, Na, K, Rb, Cs和Fr七个里德伯系列的能级,即ns²S_1/2, np²P_1/2, np²P_3/2, nd²D_3/2, nd²D_5/2, nf²F_5/2 和nf²F_7/2.计算结果表明,电子关联效应对碱金属原子的里德伯能级的影响很大.总的来说,偶极极化效应比伸缩模效应重要,而在偶极极化效应中, Δl = + 1的偶极极化效应比Δl = - 1的偶极极化效应重要.但对于Na的ns²S_1/2,(nd²D_3/2,nd²D_5/2)里德伯系列的能级,和Li的(np²P_1/2,np²P_3/2)里德伯系列的能级,是伸缩模效应比较重要.     

BiVO4/TiO2 heterojunction with rich oxygen vacancies for enhanced electrocatalytic nitrogen reduction reaction

The large-scale production of ammonia mainly depends on the Haber–Bosch process, which will lead to the problems of high energy consumption and carbon dioxide emission. Electrochemical nitrogen fixation is considered to be an environmental friendly and sustainable process, but its efficiency largely depends on the activity and stability of the catalyst. Therefore, it is imperative to develop highefficient electrocatalysts in the field of nitrogen reduction reaction (NRR). In this paper, we developed a BiVO₄/TiO₂ nanotube (BiVO₄/TNT) heterojunction composite with rich oxygen vacancies as an electrocatalytic NRR catalyst. The heterojunction interface and oxygen vacancy of BiVO₄/TNT can be the active site of N₂ dynamic activation and proton transition. The synergistic effect of TiO₂ and BiVO₄ shortens the proton transport path and reduces the over potential of chemical reaction. BiVO₄/TNT has high ammonia yield of 8.54 μg·h⁻¹·cm⁻² and high Faraday efficiency of 7.70% in −0.8 V vs. RHE in 0.1 M Na₂SO₄ solution.     

(Cu,N)共掺杂TiO2/MoS2异质结的电子和光学性能:杂化泛函HSE06

在密度泛函理论的基础上,系统地研究了Cu/N（共）掺杂的TiO₂/MoS₂异质结体系的几何结构、电子结构和光学性质.计算发现,TiO₂/MoS₂异质结的带隙相比于纯的TiO₂（101）表面明显变小,Cu/N（共）掺杂TiO₂/MoS₂异质结体系的禁带宽度也明显地减小,这导致光子激发能量的降低和光吸收能力的提高.通过计算Cu/N（共）掺杂TiO₂/MoS₂的差分电荷密度,发现光生电子与空穴积累在掺杂后的TiO₂（101）表面和单层MoS₂之间,这表明掺杂杂质体系可以有效地抑制光生电子-空穴对的复合.此外,我们计算了在不同压力下TiO₂/MoS₂异质结的几何、电子和光学性质,发现适当增加压力可以有效提高异质结的光吸收性能.本文结果表明,Cu/N（共）掺杂TiO₂/MoS₂异质结和对TiO₂/MoS₂异质结加压都能有效地提高材料的光学性能.     

A theoretical investigation of the binding of TiCln to MgCl2

The structure of the (0 0 0 1) surface of the -MgCl₂ crystal has been investigated using DFT-GGA periodic calculations. The calculated surface relaxation is in agreement with LEED measurements. Motivated for the use of MgCl₂ as support for the Ziegler–Natta reaction, we have studied the adsorption of the catalyst (titanium chlorides as monomers or dimers) on the (1 0 0) and (1 1 0) MgCl₂ surfaces. The structures of adsorbed species are close to those previously found on cluster models: bridging chlorine atoms connect the Ti to the Mg atoms and the systems remain in high spin states. The (0 0 0 1) surface is the most stable face of the -MgCl₂ crystal; however it is Cl-terminated and henceforth poorly reactive; it had been suggested to deposit metallic Mg in order to improve its reactivity. Our modelling explains the failure of this tentative; the interaction between the deposited metal and the surface is repulsive and uncharged Mg atom does not bind.     

Bi2S3/BiOCl复合光催化剂的水热合成及其高活性

采用一种简便的水热法在433 K的温度下成功合成了具有不同Bi₂S₃质量分数的Bi₂S₃/BiOCl复合光催化剂,利用各种技术对其进行了表征.在紫外光照射下,以甲基橙水溶液的光催化降解为模型反应,评价了Bi₂S₃/BiOCl复合光催化剂的活性.研究结果表明:与纯Bi₂S₃和纯BiOCl相比,Bi₂S₃/BiOCl样品明显具有更高的光催化性能,尤其当Bi₂S₃在Bi₂S₃/BiOCl中的质量分数为26.5%时,Bi₂S₃/BiOCl复合催化剂的光催化活性与商业P25(TiO₂)的活性非常接近,而这种商业P25在紫外光照射下是公认的高效光催化剂.这种明显提高的光催化活性主要归功于光生电子和空穴在Bi₂S₃和BiOCl形成异质结界面上的有效转移,降低了电子-空穴对的复合.     

High efficiency giant magnetoresistive device based on two-dimensional MXene (Mn2NO2)

Due to the unique electronic structure of half-metals, characterized by the conductivity of majority-spin and the band gap of minority-spin, these materials have emerged as suitable alternatives for the design of efficient giant magnetoresistive (GMR) devices. Based on the first-principles calculations, an excellent GMR device has been designed by using two-dimensional (2D) half-metal Mn₂NO₂. The results show that Mn₂NO₂ has sandwiched between the Au/nMn₂NO₂ (n = 1, 2, 3)/Au heterojunction and maintains its half-metallic properties. Due to the half-metallic characteristics of Mn₂NO₂, the total current of the monolayer device can reach up to 1500 nA in the ferromagnetic state. At low voltage, the maximum GMR is observed to be 1.15 × 10³¹ %. Further, by increasing the number of layers, the ultra-high GMR at low voltage is still maintained. The developed device is a spintronic device exhibiting the highest magnetoresistive ratio reported theoretically so far. Simultaneously, a significant negative differential resistance (NDR) effect is also observed in the heterojunction. Owing to its excellent half-metallic properties and 2D structure, Mn₂NO₂ is an ideal energy-saving GMR material.     

基于AAO模板的TiO2微纳结构及其光电特性

以多孔氧化铝（AAO）和导电玻璃FTO为基底,用射频（RF）磁控溅射法和溶胶凝胶（Sol-gel）法分别制备了微纳结构的TiO₂,两种TiO₂沉积技术各自体现出其特质。从XRD衍射光谱和拉曼散射光谱可以看到,在相同退火条件下,RF和Sol-gel法制备的TiO₂晶相结构不同。在以导电玻璃FTO为基底时,RF制备的薄膜颗粒分布均匀,没有团簇现象;而Sol-gel制备的薄膜由密实的颗粒构成。在以AAO为模板时,Sol-gel制备的TiO₂溶胶粒子由于胶体溶液的流动性使其对孔的填充率保持较高的水平;而RF制备的TiO₂对孔的填充率则降低。光电流实验的结果表明,前者具有更好的光电特性。     

CO2水合物在电解质溶液中的相平衡预测

关于二氧化碳水合物基础物性的研究,对二氧化碳置换开采天然气水合物和工业废气二氧化碳的深海埋藏具有重要意义。本文在Van der Waals-Platteeuw理论和Pizter的电解质模型的基础上,考虑了电解质和温度对二氧化碳在水中溶解度的影响,从理论上对水合物相平衡模型进行了修正,提高了预测的精度。本文预测了二氧化碳水合物在NaCl、KCl、CaCl₂、MgCl₂的一元及二元溶液体系中的相平衡条件,预测的最大压力误差7.92%,平均压力误差在1%～3%之间。结果表明,本文的理论预测结果和实验数据吻合较好。     

Electronic conduction properties of Au/C60/p-Si and C60/Au/p-Si sandwich structures: I–V and transducer characteristics

Gold-fullerite [C₆₀]-silicon (p-type) sandwich structures have been fabricated in order to investigate intrinsic cross-sectional and planar electronic conductive properties, in particular the C₆₀/p-Si p–n heterojunction. The turn-on voltage of this p–n heterojunction lies in the range 0.25–0.27 V. The I–V characteristics of the Au/C₆₀/p-Si structure are mostly defined by the bulk specific resistance of the fullerite crystal film itself (6×10⁷ Ω cm). I–V curves in the C₆₀/Au/p-Si structure are shown to be ohmic. Au/C₆₀/p-Si sandwiches irradiated with swift (300 MeV) heavy ions, (⁸⁴Kr¹⁴⁺) to a total fluence 10¹⁰ ion/cm² yield structures which are sensitive to ambient air pressure, specifically in the case of a transverse contact configuration, and if one of the contacts is located on the irradiated part of the fullerite film. The sandwich-structure sensitivity to pressure is 5×10⁻⁶ Pa⁻¹. This exceeds the sensitivity of conventional silicon pressure transducers by almost three orders of magnitude.     

Composite protonic solid electrolytes in the CsHSO4-SiO2 system

Transport, thermal and structural properties of the composite solid electrolytes (1 −x)CsHSO₄---xSiO₂ (where x = 0–0.8) were investigated. The composites were prepared by mechanical mixing of components followed by heating at temperatures near CsHSO₄ melting point (483 K). The dependence of low temperature phase conductivity on x has a maximum with a value 2.5 orders of magnitude higher than that of pure CsHSO₄ and conductivity is governed by protons. Heterogeneous doping is shown to change markedly the thermodynamic parameters of the ionic component. The phase transition temperature CsHSO₄ in the composites decreases from 414 to 350 K with the increase of the content of heterogeneous additive SiO₂ from 0 to 0.7. As x raises CsHSO₄ the amorphization takes place and the relative change of ionic conductivity at phase transition diminishes, the phase transition becomes diffusive and disappears for the 0.2CsHSO₄---0.8SiO₂ composite.     

悬空氧化铟纳米线晶体管制备与光电性能表征

一维(1D)半导体纳米线在纳米电子学和纳米光子学中表现出色。然而,纳米线晶体管的电特性对纳米线与衬底之间的相互作用非常敏感,而优化器件结构可以改善纳米线晶体管的电学和光电检测性能。本文报道了通过一步式光刻技术制造的悬浮式In₂O₃纳米线晶体管,显示出54.6 cm²V^?1s^?1的高迁移率和241.5 mVdec^?1的低亚阈值摆幅。作为紫外光电探测器,光电晶体管显示出极低的暗电流(~10?13 A)和高响应度1.6×10⁵ A?W^?1。悬浮晶体管的沟道材料的这种简单而有效的制备方法可广泛用于制造高性能微纳米器件。     

Spectroscopic studies on the effect of doping with CoBr2 and MgCl2 on some physical properties of polyvinylalcohol films

Filled and unfilled polyvinylalcohol (PVA) films were prepared by the casting technique. Films of equal amounts with various concentrations of two fillers (CoBr₂ and MgCl₂) were prepared. Spectroscopic, structural and some physical properties of these films were studied with different techniques. Fourier-transform infrared (FTIR) revealed that the syndiotacticity structure of the PVA samples causes dense molecular packing in the crystal and also stronger intermolecular hydrogen bonds, which are responsible for the disappearance of the molecular motion. X-ray diffraction (XRD) scans evidenced the presence of some semicrystalline structure of PVA films. The optical absorption spectra suggested the presence of an optical gap (E_g), which depends on filler concentration for all the filling levels. Differential thermal analysis (DTA) suggests that the segmental mobility of an amorphous pure PVA increases as a result of the addition of mixed fillers, becoming less-rigid segments. This indicates that the mixed fillers act as plasticizers.     

In2O3/CuO纳米异质结的制备和光催化性质研究

通过溶胶-凝胶结合静电纺丝过程制备了In₂O₃/CuO纳米异质结。XRD和Raman光谱的研究表明,随着前驱物中Cu/In比例的增加,CuO相的含量逐渐增加。吸收光谱研究表明,随着CuO含量的增加,复合纳米结构的可见光吸收明显增强。光催化实验研究结果表明,In₂O₃/CuO纳米异质结具有比单一相的In₂O₃和CuO更强的光催化性能,其主要来源于异质结所导致的增强的光生电子和空穴的分离效率。     

Anomalous changes in CuO2 plane in YBa2Cu4O8 under pressure

We report the results of an atomistic simulation study of the pressure-induced structural changes in YBa₂Cu₄O₈ from 0 to 20 GPa. It is found that the crystal has similar compressibilities in the a- and b-directions over the whole pressure range, whereas in the c-direction there are quite different compressibilities at different pressures. Our results suggest that there exists a correlation between the charge transfer and the change in the Cu(2)---O(1) bond length, but we do not support the suggestion of a direct dependence of T_c on the length of this bond. However, we predict that there are anomalous changes in the CuO₂ plane with pressure, which we propose may lead to a charge carrier redistribution on in-plane copper and in-plane oxygens, which in turn may be related to the optimum carrier concentration in the CuO₂ plane.     

Ba2(Cu0.7Cd0.3)O3: a new double-center perovskite structure

A new phase of Ba₂(Cu_0.7Cd_0.3)O₃ has been discovered. The composition of the phase was examined by energy dispersive X-ray spectrometry. The phase exhibits a tetragonal structure with lattice constants a=4.077 Å and c=7.970 Å, and its space group is P4/mmm. We propose a double-center-perovskite structure model, which is similar to that of (Ba, Sr)₂CuO₂(CO₃) [1, 2]. The simulated XRD is close to the observed one. The analysis results of elemental analyzer and Fourier transform infrared spectrometer reveal that the phase does not contain CO . The cuprate is an insulator, but its new cell structure and the large size of the CuO₂ planes seem to suggest a good reason for researching new superconductors. The roles of CdO in the solid state reaction and the formation of the new phase are discussed.     

Two dimensional GeO2/MoSi2N4 van der Waals heterostructures with robust type-II band alignment

Constructing two-dimensional (2D) van der Waals heterostructures (vdWHs) can expand the electronic and optoelectronic applications of 2D semiconductors. However, the work on the 2D vdWHs with robust band alignment is still scarce. Here, we employ a global structure search approach to construct the vdWHs with monolayer MoSi₂N₄ and wide-bandgap GeO₂. The studies show that the GeO₂/MoSi₂N₄ vdWHs have the characteristics of direct structures with the band gap of 0.946 eV and type-II band alignment with GeO₂ and MoSi₂N₄ layers as the conduction band minimum (CBM) and valence band maximum (VBM), respectively. Also, the direct-to-indirect band gap transition can be achieved by applying biaxial strain. In particular, the 2D GeO₂/MoSi₂N₄ vdWHs show a robust type-II band alignment under the effects of biaxial strain, interlayer distance and external electric field. The results provide a route to realize the robust type-II band alignment vdWHs, which is helpful for the implementation of optoelectronic nanodevices with stable characteristics.     

Double exchange model for RuSr2(Eu,Gd)Cu2O8

We propose a double exchange model to describe the RuO₂ planes of RuSr₂(Eu,Gd)Cu₂O₈. The Ru⁺⁵ ions are described by localized spins, and additional electrons provided by the superconducting CuO₂ planes are coupled ferromagnetically to them by Hund rules coupling. We calculate the spin structure factor, magnetic susceptibility and magnetization as a function of magnetic field and temperature, using a Monte Carlo algorithm in which the Ru⁺⁵ spins are treated as classical. Several experiments which seemed in contradiction with one another are explained by the theory.