基于从头算法的TM金属共掺杂的ZnO基稀磁半导体的磁性研究(英文)

本文采用从头计算的方法研究了基于过渡性金属共掺杂Ⅱ-Ⅵ族稀释半导体的磁性和电子结构.并系统的研究了氧化锌基的稀释半导体铁磁态的稳定性和对其材料设计.在所有的共掺杂体系中,发现(Mn,Co),(Co,Ni)和(Mn,Ni)共掺杂体系是铁磁态的,而(Fe,Ni)共掺杂体系是自旋玻璃态.另一方面,Fe-,Co-和Ni掺杂ZnO基系统的稳态是铁磁态.同时,本文研究了ZnO基稀释半导体的载流子传导铁磁性,计算分析了电子态密度,铁磁态的稳定性.结合双交换和超交换理论解释共掺杂稀释半导体的磁性机理.     

过渡金属掺杂SnO_2超晶格磁学和光学性质的研究

基于密度泛函理论的第一性原理,采用全势线性缀加平面波方法 (FPLAPW)和广义梯度近似(GGA)来处理相关能,计算了n层过渡金属Cr掺杂SnO2超晶格的电子态密度、能带结构和光学性质(n=1,2,3)。结果表明,掺杂使材料均表现出了半金属性,并且导电性和磁矩随着n的增大而增强。掺杂后材料磁矩主要来源于Cr3d态,Cr的加入使O的态密度产生了自旋极化现象。在0~1.8eV处掺杂所形成的第一吸收峰的位置随着n的增加而向高能方向推移,在1.8~7.0eV处所形成峰的峰值随着n的增大而增加,在7.0~17.0eV处所形成的峰的峰值随着n的增大而减小。     

Mg掺杂AlN的磁性

采用基于密度泛函理论(Density function theory,DFT)的第一性原理方法,计算研究了AlN中掺杂Mg后的电子结构和磁性。我们从理论上给出了AlN∶Mg的晶体结构参数和电子结构,计算了铁磁相和反铁磁相的总能,发现AlN∶Mg具有半金属特性,并且其铁磁相更稳定。我们通过分析比较AlN和AlN∶Mg的电子结构,解释了非磁性杂质Mg掺杂在AlN中产生磁性的原因是Mg取代Al后,会在自旋向下的价带顶部引入空穴,导致费米能级移入价带中,从而使自旋向上和自旋向下的态密度产生不对称的分布,从而导致材料中有净磁矩。另外,通过比较AlN∶Mg的多种构型的形成能,发现在纤锌矿结构AlN中掺入Mg比较容易实现,在闪锌矿结构中次之,但对于其它几何构型,由于形成能太高,Mg很难掺入到材料中。这种材料可作为制备非磁性掺杂的半导体自旋电子器件的备选材料。     

Cr替代Mn对La0.4Ca0.6MnO3磁性质的影响

用固相反应法制备了La0.4Ca0.6Mn1-xCrxO3(x=0.00,0.02,0.04,0.06,0.08,0.10,0.15)多晶样品,通过X射线衍射(XRD)图谱、磁化强度-温度(M-T)曲线、电子自旋共振(ESR)谱线,研究了Cr替代Mn对La0.4Ca0.6MnO3磁性质的影响.实验结果表明:La0.4Ca0.6MnO3存在复杂的磁结构,在258 K出现电荷有序相,从175 K到50 K,产生强关联CO-AFM(电荷有序?反铁磁)相,温度降到41 K左右出现自旋玻璃态;当替代量x≥0.06时,电荷有序相被融化;当x≥0.10时,自旋玻璃态被融化.对实验结果进行了初步解释:电荷有序相被融化主要原因是Cr3+与Mn4+具有相同的电子结构,Cr3+替代Mn3+破坏了CE型反铁磁的自旋序,从而引起电荷序的坍塌,实验证明了电荷序CE型反铁磁体系中,电荷序和自旋序之间存在强耦合相互作用;自旋玻璃态的融化,是由于Cr替代Mn破坏了自旋玻璃态的生成条件,即反铁磁背景下有少量铁磁成分.     

Ga掺杂前后LaCoO_3的电子结构研究

采用基于密度泛函理论(DFT)的第一性原理平面波超软赝势方法,结合局域自旋密度近似和Hubbard U修正,对不同Co离子自旋态下及Ga掺杂LaCoO_3的超晶胞体系进行了几何结构优化,计算并分析了它们的电子结构。结果表明,当Co离子处于低自旋态时,LaCoO_3为非磁绝缘性的绝缘体;当Co离子被激发到中间自旋态时,由于强烈的p-d轨道杂化作用,LaCoO_3转变成一个有磁性的半金属体;当Co离子处于高自旋态时,体系呈现金属铁磁性,磁矩由中间自旋态的0.91μB增大到高自旋态的2.2μB。Ga掺杂后,体系的Co3d态电子和Ga4p态电子以及O2p态电子在费米能级附近发生p-d轨道杂化,引入杂质带,形成受主能级,使体系的导电能力增强,体系呈现半金属铁磁性,其净磁矩为4.01μB。     

Cr掺杂对ZnO最小光学带隙和吸收光谱的影响

当Cr掺杂ZnO的摩尔数为0.0313~0.0625的范围内,掺杂体系的最小光学带隙宽度和吸收光谱分布随Cr掺杂浓度的变化出现了两类相反的实验结果。为了解决本问题,采用密度泛函理论(DFT)框架下的广义梯度近似(GGA+U)平面波超软赝势方法,用PBE泛函的计算方案来描述电子间的交换关联能,对未掺杂ZnO和3种不同浓度Cr掺杂ZnO超胞模型进行了能带结构、态密度、差分电荷密度、布居值以及吸收光谱的计算。结果表明,当Cr掺杂摩尔数为0.0313~0.0625的范围内,随着Cr掺杂量增加,掺杂体系的晶格常数和体积增大,总能量和形成能减小,结构更稳定,掺杂更容易,最小光学带隙宽度增大,吸收光谱显著蓝移。计算结果与实验结果相一致,并合理解释了存在的问题。这对制备Cr掺杂ZnO中实现短波长光学器件有一定的理论指导作用。     

Cr掺杂AlN半导体电磁性质的第一性原理研究

基于密度泛函理论(DFT)的第一性原理方法,在广义梯度近似(GGA)下分别计算了无掺杂和掺杂过渡金属Cr原子的AlN半导体的电磁性质,结果表明,掺杂后AlN由半导体转变为半金属,并具有较宽的半金属能隙.当掺杂浓度为25%、12.5%、5.6%时,半金属能隙分别为0.8eV、1.1eV、1.2eV.以掺杂浓度为12.5%的Cr-AlN(2×2×1)为例,详细分析了其能带结构、态密度分布和电子布居数以及磁矩等.     

新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO电子结构与光学性质的第一性原理计算

采用基于密度泛函理论的第一性原理赝势平面波方法,对新型稀磁半导体(La_(0.75)Ba_(0.25))(Ag_(0.75)Mn_(0.25))SO的电子能带结构、自旋极化态密度和光学性质进行了计算与分析。计算结果表明La_(0.75)Ba_(0.25)Ag_(0.75)Mn_(0.25)SO为直接带隙半导体,能隙约为1.2 eV,掺杂后体系带隙明显减少;费米面附近态密度发生劈裂,出现了明显的自旋有序排列,呈明显的铁磁性,磁性的产生是通过Mn3d和相邻S3p原子之间的p-d杂化作用实现的;La_(0.75)Ba_(0.25)Ag_(0.75)-Mn_(0.25)SO反射主峰与吸收曲线峰值都在紫外波段,而且集中在100~300 nm的范围。但在近红外和可见光区,此材料反射和吸收都较低,可用于可见光高透明的领域。     

V,Cr,Mn掺杂对单层WS_2磁性的影响

采用密度泛函理论框架下的第一性原理平面波赝势方法,研究了过渡金属V,Cr和Mn原子掺杂单层WS2的磁性和稳定性。计算结果表明,Cr掺杂时体系不显示磁性,V和Mn掺杂均能产生一定的磁矩,而且磁矩主要集中在掺杂的V和Mn原子上。进一步讨论V或Mn双原子掺杂WS2体系中掺杂原子之间的耦合作用发现,Mn掺杂的体系在室温下显示出稳定的铁磁性,而V掺杂则表现出非自旋极化基态。形成能的计算表明Mn掺杂的WS2体系比V和Cr掺杂结构更稳定。     

Ti掺杂β-Ga2O3电子结构和光学性质的第一性原理计算

采用基于密度泛函理论框架下的第一性原理计算方法,研究了Ti掺杂β-Ga2O3系统的电子结构和光学性质。计算结果表明,Ti替代八面体的Ga(2)时系统形成能最低,容易在实验上合成;Ti掺杂在导带底附近引入了浅施主能级,极大地提高了β-Ga2O3系统的导电性。Ti掺杂时稳定体系倾向于自旋极化态,且费米面处自旋极化率接近100%。光学性质的计算结果显示,Ti掺杂β-Ga2O3是极具潜力的n型紫外透明的半导体。     

Electronic structure and spectroscopy of cadmium telluride quantum wires

The size-dependent electronic structure of CdTe quantum wires is determined by density functional theory using the local density approximation with band-corrected pseudopotential method. The results of the calculations are then used to assign the size-dependent absorption spectrum of colloidal CdTe quantum wires synthesized by the solution-liquid-solid mechanism. Quantitative agreement between experiment and theory is achieved. The absorption features comprise transitions involving the highest 25-30 valence-band states and lowest 15 conduction-band states. Individual transitions are not resolved; rather, the absorption features consist of clusters of transitions that are determined by the conduction-band energy-level spacings. The sequence, character, and spacing of the conduction-band states are strikingly consistent with the predictions of the simple effective-mass-approximation, particle-in-a-cylinder model. The model is used to calculate the size dependence of the electron effective mass in CdTe quantum wires.     

Control of exciton spin relaxation by electron-hole decoupling in type-II nanocrystal heterostructures

The electron spin flip relaxation dynamics in type II CdSe/CdTe nanorod heterostructures are investigated by an ultrafast polarization transient grating technique. Photoexcited charge separation in the heterostructures suppresses the electron-hole exchange interaction and their recombination, which reduces the electron spin relaxation rate in CdSe nanocrystals by 1 order of magnitude compared to exciton relaxation. The electron orientation is preserved during charge transfer from CdTe to CdSe, and its relaxation time constant is found to be approximately 5 ps at 293 K in the CdSe part of these nanorods. This finding suggests that hole spin relaxation determines the exciton fine structure relaxation rate and therefore control of exciton spin relaxation in semiconductor nanostructures is possible by delocalizing or translating the hole density relative to the electron.     

Luminescent CdTe and CdSe semiconductor nanocrystals: preparation, optical properties and applications

The novel optical and electrical properties of luminescent semiconductor nanocrystals are appealing for ultrasensitive multiplexing and multicolor applications in a variety of fields, such as biotechnology, nanoscale electronics, and opto-electronics. Luminescent CdSe and CdTe nanocrystals are archetypes for this dynamic research area and have gained interest from diverse research communities. In this review, we first describe the advances in preparation of size- and shape-controlled CdSe and CdTe semiconductor nanocrystals with the organometallic approach. This article gives particular focus to water soluble nanocrystals due to the increasing interest of using semiconductor nanocrystals for biological applications. Post-synthetic methods to obtain water solubility, the direct synthesis routes in aqueous medium, and the strategies to improve the photoluminescence efficiency in both organic and aqueous phase are discussed. The shape evolution in aqueous medium via self-organization of preformed nanoparticles is a versatile and powerful method for production of nanocrystals with different geometries, and some recent advances in this field are presented with a qualitative discussion on the mechanism. Some examples of CdSe and CdTe nanocrystals that have been applied successfully to problems in biosensing and bioimaging are introduced, which may profoundly impact biological and biomedical research. Finally we present the research on the use of luminescent semiconductor nanocrystals for construction of light emitting diodes, solar cells, and chemical sensors, which demonstrate that they are promising building blocks for next generation electronics.     

Aqueous synthesis of Au:CdTe nanocrystals for noninvasive fluorescence imaging in living cells

The gold-doped cadmium telluride (Au:CdTe) nanocrystals were synthesized by aqueous solution route using L-glutathione and L-cysteine as stabilizers. As-prepared Au:CdTe nanocrystals have good monodispersity and a zinc-blende structure. Compared with undoped CdTe nanocrystals, the Au:CdTe nanocrystals exhibited improved photostability, higher cellular affinity, and lower cytotoxicity. The Au:CdTe nanocrystals were used as probes for long-term noninvasive fluorescence imaging in living cells (The human lung epithelial carcinoma A549 cells). They could be endocytic uptaken by A549 cells and stably labeled the cytoplasm for over a week. By transmission electron microscopy (TEM) analysis, the Au:CdTe NCs could be observed in vesicles after being uptaken by A549 cells. Doping semiconductor nanocrystals with gold has the potential to engineer the photostability and biocompatibility for extensive biomedical applications. This work developed a facile aqueous solution route to synthesize gold-doped semiconductor nanocrystals and may assist in the design of doped nanobiomaterials.     

碲化镉薄膜太阳能电池及其溅射制备

简单综述了化舍物半导体碲化镉太阳能电池的发展历史、基本结构和核心问题,在此基础上重点总结了用溅射法制备的多晶碲化镉薄膜太阳能电池的优缺点、面临问题、发展现状,展望了它的发展趋势,并讨论了用溅射法制备渐变带隙碲化镉薄膜太阳能电池以提高转化效率的可能性.     

Dynamics of melting and crystallization processes in CdTe under laser irradiation

Numerical modeling results indicate that cadmium vaporization caused by nanosecond pulses of a ruby laser has a significant effect on the dynamics of phase transitions in the near-surface region of CdTe and leads to surface cooling of the material, resulting in a nonmonotonic temperature profile, with a maximum at a depth of about 20 nm. At incident energy densities above the threshold for CdTe melting, the molten zone forming below the surface layer extends both toward the surface and into the bulk of the semiconductor. Cd vaporization and the diffusion of Cd and Te in the melt give rise to tellurium enrichment in the near-surface region. Taking into account the dependences of the crystallization temperature and the latent heat of the phase transition on the Cd and Te concentrations in the melt, we achieved reasonable agreement with experimental data on the effect of incident energy density on the time during which a molten layer is present in CdTe.     

Ultrafast synthesis of CdTe/ZnSe semiconductor QDs by microwave method and investigation of structural,optical, and photocatalytic properties of CdTe/ZnSe QDs

Journal of Materials Science: Materials in Electronics - CdTe/ZnSe semiconductor quantum dots were synthesized by the ultrafast microwave method in an aqueous solution. X-ray diffraction results...     

Aqueous synthesis of highly monodispersed thiol-capped CdTe quantum dots through electrochemical approach

A facile green approach has been developed to control the growth regime in the aqueous synthesis of CdTe semiconductor Quantum dots (QDs) via the electrochemistry method. The Low growth temperature and slow injection of Te precursors are used to prolong the diffusion controlled stage and thus suppress Ostwald ripening during nanocrystal growth. The experimental results showed that a low concentration of Te precursor would definitely influence the growth procedure. The narrow absorption peaks in the UV-visible absorption spectra, as well as transmission electron microscopy images indicated that the as-prepared CdTe QDs had a good monodispersity. The high-resolution transmission electron microscopy (HRTEM) images and powder X-ray diffraction (XRD) pattern suggested that the as-prepared QDs have high crystallinity and cubic structure. The QDs exhibited high fluorescence QYs about 50% and the best of QY 67% without any postpreparative treatment over a broad spectral range of 516-609 nm, which could be further broadened by long-term refluxing. The current work suggested that electrochemical method was an attractive approach to the synthesis of high-quality II-VI semiconductor QDs at a large scale.     

Electrochemical deposition of CdSe/CdTe multilayer nanorods for hybrid solar cell

CdSe and CdTe are composite semiconductor materials used in hybrid solar cell due to their high absorption coefficients. CdSe and CdTe have different band gaps, 1.74 eV and 1.45 eV respectively and then they can absorb solar energy in a wider range of wavelength compare to the silicon solar cell. In this research, CdSe and CdTe nanorods were fabricated using electrochemical deposition in an anodic aluminum oxide template. The electrodeposition behaviors of CdSe and CdTe were investigated using cyclic voltammetric technique. The deposition potentials of CdSe and CdTe were obtained through cyclic voltammetric technique. The effects of Te and Se ion concentration in the electrolyte on the composition of the deposits were investigated to obtain 1:1 atomic ratio. Structures of layered CdSe/CdTe nanorods were analyzed with FESEM and EDS.