Effect of Lattice Mismatch on Luminescence of ZnO/Si Hetero-Structure

The photoluminescence （PL） and Raman spectra of undoped ZnO films deposited directly on Si substrate （sample A）,on Si substrate through a SiC buffer layer （sample B）,and on a ZnO crystal wafer （sample C） are investigated. There are emission peaks centered at 3.18eV （ultraviolet,UV） and 2.38eV （green） in these sampies. Comparing the Raman spectra and the variation of the PL peak intensities with annealing atmosphere, we conclude that the luminescence of the samples is related to the tensile strain in the ZnO film due to the lattice mismatch between the film and the substrate. In particular, the tensile strain reduces the formation energy of OZn antisite oxygen defects,which generate the green emission center. After annealing in oxygen-rich atmosphere, many OZn defects are generated. Thus, the intensity of green emission in ZnO/Si hetero-structure materials increases due to tensile strain in ZnO films.

Lattice Mismatch Induced Tunable Dimensionality of Transition Metal Dichalcogenides

Low-dimensional materials have excellent properties which are closely related to their dimensionality.However,the growth mechanism underlying tunable dimensionality from 2D triangles to 1D ribbons of such materials is still unrevealed.Here,we establish a general kinetic Monte Carlo model for transition metal dichalcogenides(TMDs) growth to address such an issue.Our model is able to reproduce several key findings in experiments,and reveals that the dimensionality is determined by the lattice mismatch and the interaction strength between TMDs and the substrate.We predict that the dimensionality can be well tuned by the interaction strength and the geometry of the substrate.Our work deepens the understanding of tunable dimensionality of low-dimensional materials and may inspire new concepts for the design of such materials with expected dimensionality.

Monte Carlo simulation of lattice analysis of complex LaCeTh0.1CuOy using ion bombardment technique

Ion bombardment analysis of perovskite materials is challenging owing to their peculiar structure.This shortcoming renders the reliability on the technique somewhat questionable.In this research,three structured modifications(i.e.,scan angle,low energy,and large ion bombardment)were adopted to improve the ion bombardment analysis of 99,999 ions using Monte Carlo simulations.The modified technique was used to analyze the effects of a chemically pressurized‘‘A’’site in the perovskite lattice system.The LaCeTh0.1Cu2Oy compound was used in this experiment.Despite the low probing energy,it was observed that the high number of ions bombarding the material resulted in external pressure on the lattice structure of the material.Moreover,the chemically pressurized‘‘A’’site perovskite material was characterized by lattice mismatch,lattice fluctuations,grain boundary collapse,and oxygen displacement.The novel discovery of this research is the inter-and intra-extended lattice mismatches that are likely to connect.Hence,further investigation of the connection between inter-and intraextended lattice mismatches is recommended as they may enable fabrication of room-temperature superconductors.

Direct calculations on the band offsets of large-latticemismatched and heterovalent Si and III-V semiconductors

Band offset in semiconductors is a fundamental physical quantity that determines the performance of optoelectronic devices.However,the current method of calculating band offset is difficult to apply directly to the large-lattice-mismatched and heterovalent semiconductors because of the existing electric field and large strain at the interfaces.Here,we proposed a modified method to calculate band offsets for such systems,in which the core energy level shifts caused by heterovalent effects and lattice mismatch are estimated by interface reconstruction and the insertion of unidirectional strain structures as transitions,respectively.Taking the Si and III-V systems as examples,the results have the same accuracy as what is a widely used method for small-lattice-mismatched systems,and are much closer to the experimental values for the large-lattice-mismatched and heterovalent systems.Furthermore,by systematically studying the heterojunctions of Si and III-V semiconductors along different directions,it is found that the band offsets of Si/InAs and Si/InSb systems in[100],[110]and[111]directions belong to the type I,and could be beneficial for silicon-based luminescence performance.Our study offers a more reliable and direct method for calculating band offsets of large-lattice-mismatched and heterovalent semiconductors,and could provide theoretical support for the design of the high-performance silicon-based light sources.

Molecular dynamics simulations of La_2O_3 thin films on SiO_2

Classical molecular dynamics is used to investigate the equilibrium state of the surface region and interface of heteroepitaxial La2O3 thin films.Due to the lattice mismatch,heteroepitaxial thin films are subject to very large stress.For this reason the behavior of La2O3 thin films at SiO2interface becomes an important concern.Our result indicates that La2O3 can uniformly wet SiO2 surface.The properties of the simulated films are analyzed and the lack of any discernible crystalline phase in epitaxial La2O3 on SiO2 indicates that the lattice mismatch between SiO2 and La2O3 is sufficiently large to prevent the formation of even short-range orders in La2O3 film.

Gas Source Molecular Beam Epitaxy Growth of Si_(1-x)Ge_x/Si Alloys

Strained SiGe alloys are successfully grown on Si(100) substrate in gas source molecular beam epitaxy system using disilane (Si 2H 6) and germane (GeH 4). The surface reconstructions are monitored by in situ reflection high energy electron diffraction during epitaxy. At a fixed substrate temperature, the Ge composition in the alloys increase with GeH 4/(GeH 4+2Si 2H 6) gas flow rate ratio. Surface morphologies of the grown samples show a strong dependence on the Ge composition, substrate temperature and SiGe epilayer thickness. Results indicate that high substrate temperature and large Ge composition are favorable for the growth mode transition from two dimensional to three dimensional growth.

Effect of Solid Solution Supersaturation on Precipitation of γ′ in Rapidly Quenched Ni-Al Binary Alloys

The effect of solid solution supersaturation on the precipitation of γ＇ in rapidly quenched Ni-AI binary alloys containing 11.6, 14.2 and 16.5 at. pct AI was investigated. The samples were solutioned at 1250℃, quenched in iced brine, and then analyzed by electron microscopy and XRD （X-ray diffraction） techniques. In Ni-11.6 at. pct Al alloy, ordering and phase separation took place simultaneously, resulting in a uniform distribution of γ＇. A transition from uniform to bimodal γ＇ phase distribution occurred in the composition range between 14.2 and 16.5 at. pct Al. This transition was accompanied by changes in the morphology of γ＇ precipitates. The microstructural observations were discussed in view of both kinetics and crystallographic considerations.

Strain engineering in electrocatalysis:Strategies,characterization,and insights

Strain engineering,as a cutting-edge method for modulating the electronic structure of catalysts,plays a crucial role in regulating the interaction between the catalytic surface and the adsorbed molecules.The electrocatalytic performance is influenced by the electronic structure,which can be achieved by introducing the external forces or stresses to adjust interatomic spacing between surface atoms.The challenges in strain engineering research lie in accurately understanding the mechanical impact of strain on performance.This paper first introduces the basic strategy for generating the strain,summarizes the different strain generation forms and their advantages and disadvantages.The progress in researching the characterization means for the lattice strains and their applications in the field of electrocatalysis is also emphasized.Finally,the challenges of strain engineering are introduced,and an outlook on the future research directions is provided.

Vacuum brazing TiAl intermetallic to K4169 alloy using amorphous filler metals Ti_(56.25–x)Zr_(x)Ni_(25)Cu1_(8.75)

A series of Ti_(56.25-x)Zr_(x)Ni_(25)Cu1_(8.75)(x=0–25,at.%) filler metals were designed based on a cluster-plus-glue-atom model to vacuum braze TiAl intermetallic to K4169 alloy. The impact of Zr content on the interfacial microstructure and shear strength of joints was examined. And the relationship between the interfacial lattice structure and the fracture behavior of the joint was investigated. The findings reveal a sectionalized characteristic with three reaction zones (Zone I, Zone II and Zone III) in the microstructure of the TiAl intermetallic to K4169 alloy joint. As the Zr content in filler metals increased, the diffusion of Ti transitioned from long-distance to short-distance in Zone I, changing the initial composition from TiNi_(3) /TiNi/NiNb/(Cr, Fe, Ni)SS to NiCrFe/(Cr, Fe, Ni)SS /TiNi. In Zone II, the initial composition altered from TiNi_(3) /TiNi to TiNi/Ti_(2) Ni/TiNi_(3) /TiCu/TiNi. The interface between Zones II and III altered from a non-coherent and semi-coherent interface of TiNi/TiAl/Ti_(3) Al with significant residual stress to a semi-coherent interface of TiNi/TiNi_(3) /TiAl_(2) /Ti_(3) Al with a gradient distribution. The shear strength of the joint initially decreased and then increased. When the Zr content of filler metal was 25 at.%, the shear strength of the joint reached 288 MPa. The crack initiation position changed from non-coherent TiNi/TiAl interface with high angle grain boundaries (HAGBs) and lattice mismatch of 65.86 at.% to a semi-coherent Ti3 Al/TiAl2 interface with a lattice mismatch of 20.07 at.% when the Zr content increased. The brittle fracture was present on the fracture surfaces of all brazed joints.

通过共晶工程的选择性外延生长有机异质结构实现光信号定向转换

分层有机纳米结构(HONs)是一种将具有不同功能的多个成分集成在一个系统中的纳米结构,其在光电子应用中引起了极大的关注.然而,由于不同材料之间结构不相容性的难题,构建具有超低晶格失配率(η)的HONs仍然面临巨大挑战.共晶工程提供了探索合理制备HONs的有力手段,但迄今为止尚未得到系统的证明.在这里,我们展示了一种用于HONs的超低晶格失配异质外延的共晶工程策略.该策略具备足够的通用性,可实现多种材料的集成从而得到不同空间取向的异质结构.通过实验合成了一系列典型HONs,包括三嵌段(η_(1)=0.7%)、分支(η_(2)=0.8%)和核/壳(η_(3)=0.6%)纳米结构,其晶格失配率远低于先前报道的HONs(5%-10%).此外,我们选择性地合成了核/壳的子结构,包括三嵌段和三明治状纳米线,揭示了界面能在HONs形成过程中的诱导作用.作为概念验证,制备所得的不同异质结构成功实现了光子信号的定向转换,为构建下一代集成光电器件奠定了坚实的基础.

Organic heterostructures composed of one-and two-dimensional polymorphs for photonic applications

Organic heterostructures(OHSs)consist of organic micro/nanocrystals are of essential importance for the construction of integrated optoelectronics in the future.However,the scarcity of materials and the problem of phase separation still hinder the fine synthesis of OHSs.Herein,based on theαphase one-dimensional(1D)microrods and theβphase 2D microplates of one organic compound 3,3′-((1 E,1′E)-anthracene-9,10-diylbis(ethane-2,1-diyl))dibenzonitril(m-B2BCB),we facilely synthesized the OHSs composed of these two polymorph phases,whose growth mechanism is attributed to the low lattice mismatch rate of5.8%between(001)plane ofαphase(trunk)and(010)crystal plane ofβphase(branch).Significantly,the multiport in/output channels can be achieved in the OHSs,which demonstrates the structure-dependent optical signals with the different output channels in the OHSs.Therefore,our experiment exhibits the great prospect of polymorphism in OHSs,which could provide further applications on multifunctional organic integrated photonics circuits.

Oxygen vacancy-mediated WO_(3) phase junction to steering photogenerated charge separation for enhanced water splitting

Effective charge separation and transfer is deemed to be the contributing factor to achieve high photoelectrochemical(PEC)water splitting performance on photoelectrodes.Building a phase junction structure with controllable phase transition of WO_(3) can further improve the photocatalytic performance.In this work,we realized the transition from orthorhombic to monoclinic by regulating the annealing temperatures,and constructed an orthorhombic–monoclinic WO_(3)(o-WO_(3)/m-WO_(3))phase junction.The formation of oxygen vacancies causes an imbalance of the charge distribution in the crystal structure,which changes the W–O bond length and bond angle,accelerating the phase transition.As expected,an optimum PEC activity was achieved over the o-WO_(3)/m-WO_(3) phase junction in WO_(3)-450 photoelectrode,yielding the maximum O_(2) evolution rate roughly 32 times higher than that of pure WO_(3)-250 without any sacrificial agents under visible light irradiation.The enhancement of catalytic activity is attributed to the atomically smooth interface with a highly matched lattice and robust built-in electric field around the phase junction,which leads to a less-defective and abrupt interface and provides a smooth interfacial charge separation and transfer path,leading to improved charge separation and transfer efficiency and a great enhancement in photocatalytic activity.This work strikes out on new paths in the formation of an oxygen vacancy-induced phase transition and provides new ideas for the design of catalysts.

An InGaAs graded buffer layer in solar cells

This paper uses an InGaAs graded buffer layer to solve the problem of lattice mismatch and device performance degradation. In the graded buffer layer, we choose the ＂transition layer＂ and the ＂cover layer＂ to accommodate the 3.9% mismatch. No threading dislocations were observed in the uppermost part of the epitaxial layer stack when using a transmission electron microscope （TEM）. We analyze the factors which influence the saturation current. Simulation data shows that the cells grown by metal organic vapor phase epitaxy （MOVPE） have considerable open circuit voltage, short circuit current, and photoelectric conversion efficiency. Finally we propose that InP may have great development potential as a substrate material.

The first principles investigation of ferrite magnetic response with mismatch stress

The quasi-ferrite model is proposed and an appropriate PBE exchange functional with the spin density functional theory （SDFT） is selected for the calculation of the relation between magnetic moment and residual stress in ferrite using a quantum mechanics code. The relationship between ferrite magnetism and the carbon content is determined, and then a ferrite interstitial solid solution （ISS） model in a low carbon concentration state is replaced with an a-Fe model in the case of majority magnetic calculation. The band structure of the loaded α-Fe is compared with that of the unloaded a-Fe. The comparison shows that the energy of Fe atomic 3d orbital changes a little, while the energy of electron orbital of iron core below 3d almost keeps un- changed. The relationship between the magnetic moment and the stress appears intermittent due to the Bragg total reflection. The change in the magnetic moment due to lattice mismatch is much larger than that caused by mechanical loading.

Preparation of Bi-2212/YBCO heterostructure and their interfacial characters

YBa_(2)Cu_(3)O_(7‐x)(YBCO)thin films were prepared on LaAlO_(3)(LAO)substrates by a sol–gel method,and the epitaxial growth of Bi‐2212 thin films on YBCO thin films was investigated.Both YBCO and Bi_(2)Sr_(2)Ca1Cu_(2)O_(8)+δ(Bi‐2212)bilayer films exhibit good biaxial texture and superconducting properties.Afterward,a cross‐shaped Bi‐2212/YBCO heterostructure was fabricated,and its interfacial atomic arrangement and I‐V characteristics were analyzed.Atomic‐resolution STEM images obtained from a spherical‐aberration‐corrected transmission electron microscope show that two superconducting films exhibit a layered structure and the atoms inside the films are artfully arranged.Moreover,the order of the seven atomic layers between Bi‐2212 and YBCO layers with a thickness of about 1.32 nm is misarranged.Among them,the Y‐O layer of YBCO and the Sr‐O layer of Bi‐2212 share a CuO_(2) layer.The I‐V curves of Bi‐2212/YBCO bilayer films show that the seven misarranged atomic layers at Bi‐2212/YBCO interface acts as a barrier layer,which means that a Josephson junction can be fabricated using this interface characters.