Theoretical study of the magnetic interaction of Cr-doped ZnO with and without vacancies

First-principles density-functional theory (DFT) calculations have been performed to study the magnetic properties of ZnO:Cr with and without vacancies. The results indicate that the doping of Cr in ZnO induces obvious spin polarization around the Fermi level and a total magnetic moment of 3.77μ_B. The ferromagnetism (FM) exchange interaction between Cr atoms is short-ranged and decreases with increasing Cr separation distance. It is suggested that the FM state is not stable with low concentration of Cr. The presence of O vacancies can make the half-metallic FM state of the system more stable, so that higher Curie temperature ferromagnetism may be expected. Nevertheless, Zn vacancies can result in the FM stability decreasing slightly. The calculated formation energy shows that V_Zn+Cr_Zn complex forms spontaneously under O-rich conditions. However, under Zn-rich conditions, the complex of V_O+Cr_Zn forms more easily. Thus, ZnO doped with Cr may exhibit a concentration of vacancies that influence the magnetic properties.     

The effects of Zn vacancies on ferromagnetism in Cu-doped ZnO films controlled by oxygen pressure and Li doping

Zn0.99Cu0.01O films were studied experimentally and theoretically.The films were prepared by pulsed-laser deposi tion on Pt(111)/Ti/SiO2/Si substrates under various oxygen pressures to investigate the growth-dependence of the ferromag netic properties.The structural,magnetic,and optical properties were studied,and it was found that all the samples possess a typical wurtzite structure,and that the films exhibit room-temperature ferromagnetism.The sample deposited at 600℃and an oxygen pressure of 10 Pa showed a large saturation magnetization of 0.83μB/Cu.The enhanced ferromagnetism in the(Cu,Li)-codoped ZnO is attributable to the existence of Zn vacancies(VZn),as shown by first-principles calcu lations.The photoluminescence analysis demonstrated the existence of V Zn in both Zn0.99Cu0.01O and(Cu,Li)-codoped ZnO thin films,and this plays an important role in the increase of ferromagnetism,according to the results of first-principles calculations.     

纤锌矿ZnO：Cu体系空位缺陷的电磁特性理论研究

ZnO:Cu体系具有p型导电性并出现室温铁磁性,但是对于其磁性来源还颇有争议.用Cu掺杂ZnO晶体容易增加空位缺陷产生的几率,从而使ZnO:Cu体系产生磁性.因此,本文采用基于密度泛函理论的第一性原理平面波超软赝势法对ZnO:Cu及其本征空位缺陷体系进行了理论研究,分别计算分析了ZnO:Cu超晶胞中相对Cu为近邻、次近邻、远近邻位置锌空位和氧空位的出现后体系的晶格结构、形成能、能带结构、态密度以及磁矩,以便准确合理地对其电磁特性进行判定.结果表明,ZnO:Cu远近邻VZn容易形成且其费米能级附近态密度较无缺陷体系增大,导电性增强;而含VO的缺陷体系禁带远远增大且变为间接带隙半导体,其费米能级处的态密度几乎不变或微弱减小,导电性无增强.Cu近邻VZn和VO的引入会导致ZnO:Cu掺杂系统的磁性相几乎或完全消失,但较远VO的出现无法显著改变磁性,较远VZn的出现使体系磁性增强.因此,在实验过程中要实现ZnO:Cu掺杂体系的良好电磁特性,应尽量避免Cu近邻VZn和VO的出现,而有效利用远近邻锌空位缺陷.     

Modulation effects of Cu doping on magnetic properties of Zn(Ni)O: First-principle calculations

Based on the density functional theory, we explore the electronic and magnetic properties of (Ni,Cu)-codoped ZnO systems (Zn(Ni,Cu)O). It is shown that dopant Cu has a long-range modulation for the Ni-Ni ferromagnetic coupling and enhances the stability of ferromagnetism. Furthermore, we demonstrate that the path of Cu modulation is through the Cu-Ni magnetic interaction.     

运用第一性原理研究氧化锌电阻阀片中氧空位与掺杂条件下ZnO晶体电学性质

为了比较Nb_2O_5、MnO_2、MgO三种添加剂对氧化锌电阻阀片电学性能影响,在微观层面模拟Nb、Mn、Mg三种元素分别掺杂ZnO完整超晶胞和带有氧空位缺陷的ZnO超晶胞,并运用第一性原理分析掺杂晶胞的特性.本文计算了晶体结构、掺杂形成能、氧空位形成能、能带结构、态密度、载流子迁移率、电导率等.结果表明,掺入Nb原子的掺杂体系晶格体积最大,Mg掺杂体系的形成能最大,稳定性最弱,Nb掺杂氧空位形成能最低,更容易引入氧空位.Nb掺杂的ZnO超晶胞禁带宽度最小,氧空位缺陷增大掺杂晶体的禁带宽度.在相同掺杂浓度和同等条件下,Mn掺杂的晶体电导率最高.     

不同位置的Mn掺杂对ZnO量子点的电磁特性影响

采用基于态密度泛函理论的第一性原理赝势法,分析了直径为1.2nm的ZnO量子点体系(Zn45O45H72,并经H钝化)在中心、中间、表面三种不同位置Mn掺杂情况下的晶体结构、能带结构、态密度分布和磁性.模拟结果表明：中间位置掺杂时,体系的结合能最低,同时导电性能最好;中心位置掺杂略劣于中间位置掺杂;而表面位置掺杂时,结合能明显偏高,同时导电性能明显减弱.在磁性方面,随着掺杂位置变化,体系中各原子的总磁矩和磁矩分布会发生明显变化,但体系的总磁矩基本不变.     

First-principles study of the co-effect of carbon doping and oxygen vacancies in ZnO photocatalyst

Although tuning band structure of optoelectronic semiconductor-based materials by means of doping single defect is an important approach for potential photocatalysis application,C-doping or oxygen vacancy(Vo)as a single defect in ZnO still has limitations for photocatalytic activity.Meanwhile,the influence of co-existence of various defects in ZnO still lacks sufficient studies.Therefore,we investigate the photocatalytic properties of ZnOx C0.0625(x=0.9375,0.875,0.8125),confirming that the co-effect of various defects has a greater enhancement for photocatalytic activity driven by visible-light than the single defect in ZnO.To clarify the underlying mechanism of co-existence of various defects in ZnO,we perform systematically the electronic properties calculations using density functional theory.It is found that the coeffect of C-doping and Vo in ZnO can achieve a more controllable band gap than doping solely in ZnO.Moreover,the impact of the effective masses of ZnO_xC_0.0625(x=0.9375,0.875,0.8125)is also taken into account.In comparison with heavy Vo concentrations,the light Vo concentration(x=0.875)as the optimal component together with C-doping in ZnO,can significantly improve the visible-light absorption and benefit photocatalytic activity.     

The study on the ZnO and Zn0.95Mn0.05O added YBCO system: Investigation of microstructure and transport properties

The effect of nano-size Zn_0.95Mn_0.05O and ZnO (30 nm) addition on the microstructure and the normal state transport properties of polycrystalline YBa₂Cu₃O_y (YBCO) was systematically studied. Samples were synthesized in air using a standard solid state reaction technique by adding nano-sized particles up to 10 wt.%. When Zn_0.95Mn_0.05O and ZnO are added to the YBCO the orthorhombic structure maintained even at the highest concentration. TEM and EDS analyses show the presence of inhomeginities embedded in the superconducting matrix. To interpret the normal state properties of the samples, the percolation theory based on localized states is applied. A cross-over between variable-range hopping and Coulomb gap mechanisms is observed as a result of increasing the nano-particles concentration. The ZnO addition modifies the electrical behavior of samples from metallic to insulating with a much lower concentration comparatively to Zn_0.95Mn_0.05O addition. The calculated values of the localization length, d, are greater in the case of Zn_0.95Mn_0.05O addition. This result can be interpreted by the internal structure defects.     

Theoretical investigations of the electronic and optical properties of wurtzite and metastable rock-salt ZnO

Based on the density function pseudopotential method, the electronic structures and the optical properties for wurtzite and metastable rock-salt ZnO are comparatively investigated in detail. The differences in electronic structures between the two polymorphs lead to remarkable differences in their optical properties. A negative differential conduction effect is predicted for the metastable rock-salt ZnO. The stronger electron–photon coupling and the wider optical response region in the metastable rock-salt ZnO make it more suitable for optical applications especially in the extremely short-wave region (25–35 nm).     

Ni掺杂对ZnO磁光性能的影响

在掺杂浓度范围为2.78%—6.25%(物质的量分数)时,Ni掺杂ZnO体系吸收光谱分布的实验结果存在争议,目前仍然没有合理的理论解释.为了解决存在的争议,在电子自旋极化状态下,采用密度泛函理论框架下的第一性原理平面波超软赝势方法,构建不同Ni掺杂量的ZnO超胞模型,分别对模型进行几何结构优化和能量计算.结果表明,Ni掺杂量越大,形成能越高,掺杂越难,体系稳定性越低,掺杂体系带隙越窄,吸收光谱红移越显著.采用LDA(局域密度近似)+U方法调整带隙.结果表明,掺杂体系的铁磁性居里温度能够达到室温以上,磁矩来源于p-d态杂化电子交换作用.Ni掺杂量越高,掺杂体系的磁矩越小.另外还发现Ni原子在ZnO中间隙掺杂时,掺杂体系在紫外光和可见光区的吸收光谱发生蓝移现象.     

Excitons and photoluminescence in ZnO and Zn0.99Mn0.01O nanocrystals

The photoluminescence and photoluminescence excitation spectra for Zn_1?x Mn _x O nanocrystals are presented. After annealing of powders in air, the intensity of the bands attributable to manganese decreases noticeably. This suggests that the oxygen vacancies affect the Zhang-Rice-like states appearing due to strong d-p-hybridization, which is confirmed by an increase in the band gap of Zn_1?x Mn _x O for low x. The origin of the 2.9-eV peak and the shape of its excitation spectrum are discussed qualitatively. For Zn_1?x Mn _x O nanocrystals, the shape of the excitation spectrum is as unusual as the intense absorption in the range (2.2–3.0) eV.     

Synthesis and magnetic properties of Mn doped ZnO nanowires

Mn doped ZnO nanowires have been synthesized using a simple autocombustion method. The as-synthesized Mn doped ZnO nanowires were characterized by X-ray diffraction and transmission electron microscopy. An increase in the hexagonal lattice parameters of ZnO is observed on increasing the Mn concentration. Optical absorption studies show an increment in the band gap with increasing Mn content, and also give evidence for the presence of Mn²⁺ ions in tetrahedral sites. All Zn_1−xMn_xO (0≤x≤0.25) samples are paramagnetic at room temperature. However, a large increase in the magnetization is observed below 50 K. This behavior, along with the negative value of the Weiss constant obtained from the linear fit to the susceptibility data below room temperature, indicate ferrimagnetic behavior. The origin of ferrimagnetism is likely to be either the intrinsic characteristics of the Mn doped samples, or due to some spinel-type impurity phases present in the samples that could not be detected.     

Effect of Mn doping on the structural and optical properties of sol-gel derived ZnO nanoparticles

Un-doped and Mn-doped ZnO nanoparticles were successfully synthesized in an ethanolic solution by using a sol-gel method. Material properties of the samples dependence on preparation conditions and Mn concentrations were investigated while other parameters were controlled to ensure reproducibility. It was observed that the structural properties, particle size, band gap, photoluminescence intensity and wavelength of maximum intensity were influenced by the amount of Mn ions present in the precursor. The XRD spectra for ZnO nanoparticles show the entire peaks corresponding to the various planes of wurtzite ZnO, indicating a single phase. The diffraction peaks of doped samples are slightly shifted to lower angles with an increase in the Mn ion concentration, signifying the expansion of the lattice constants and increase in the band gap of ZnO. All the samples show the absorption in the visible region. The absorbance spectra show that the excitonic absorption peak shifts towards the lower wavelength side with the Mn-doped ZnO nanoparticles. The PL spectra of undoped ZnO consist of UV emission at 388 nm and broad visible emission at 560 nm with varying relative peak intensities. The doping of ZnO with Mn quenches significantly the green emission while UV luminescence is slightly affected.     

Effect of Mn doping on the microstructures and photoluminescence properties of CBD derived ZnO nanorods

Mn-doped ZnO nanorods were synthesized from aqueous solutions of zinc nitrate hexahydrate, manganese nitrate and methenamine by the chemical solution deposition method (CBD). Their microstructures, morphologies and optical properties were studied in detail. X-ray diffraction (XRD) results illustrated that all the diffraction peaks can be indexed to ZnO with the hexagonal wurtzite structure. Scanning electron microscope (SEM) results showed that the average diameter of Mn-doped ZnO nanorods was larger than that of the undoped one. Photoluminescence (PL) spectra indicated that manganese doping suppressed the emission intensity and caused the blue shift of UV emission position compared with the undoped ZnO nanorods. In the Raman spectrum of Mn-doped ZnO nanorods, an additional mode at about 525 cm⁻¹ appeared which was significantly enhanced and broadened with the increase of Mn doping concentration.     

Effect of iron doping concentration on magnetic properties of ZnO nanoparticles

The ZnO:Fe nanoparticles of mean size 3-10 nm were synthesized at room temperature by simple co-precipitation method. The crystallite structure, morphology and size estimation were performed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM). The wurtzite structure of ZnO gradually degrades with the increasing Fe doping concentration. The magnetic behavior of the nanoparticles of ZnO with varying Fe doping concentration was investigated using a vibrating sample magnetometer (VSM). Initially these nanoparticles showed strong ferromagnetic behavior, however at higher doping percentage of Fe, the ferromagnetic behavior was suppressed and paramagnetic nature was observed. The enhanced antiferromagnetic interaction between neighboring Fe-Fe ions suppressed the ferromagnetism at higher doping concentrations of Fe. Room-temperature Mössbauer spectroscopy investigation showed Fe³⁺ nature of the iron atom in ZnO matrix.     

First-principles study of the effect of heavy Ni doping on the electronic structure and absorption spectrum of wurtzite ZnO

The band structures, densities of states and absorption spectra of pure ZnO and two heavily Ni doped supercells of Zn_0.9722Ni_0.0278O and Zn_0.9583Ni_0.0417O have been investigated using the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory. The calculated results showed that the band gap is narrowed by Ni doping in ZnO; this, is because the conduction band undergoes a greater shift toward the low-energy region than the valence band and because heavier doping concentrations lead to, narrower band gaps. Moreover, the optical absorption edge exhibits a redshift due to the narrowing of the band gap. Heavier doping concentrations leads to more significant redshifts, which is in agreement with the experimental results.     

Electronic structures and optical properties of IIIA-doped wurtzite Mg0.25Zn0.75O

The energy band structures, density of states, and optical properties of IliA-doped wurtzite Mg0.25Zn0.75O （IIIA= A1, Ga, In） are investigated by a first-principles method based on the density functional theory. The calculated results show that the optical bandgaps of Mg0.25Zn0.75O：IIIA are larger than those of Mg0.25Zn0.75O because of the Burstein-Moss effect and the bandgap renormalization effect. The electron effective mass values of Mg0.25Zn0.75O：IIIA are heavier than those of Mgo.25Zno.750, which is in agreement with the previous experimental result. The formation energies of MgZnO：Al and MgZnO：Ga are smaller than that of MgZnO：In, while their optical bandgaps are larger, so MgZnO：Al and MgZnO：Ga are suitable to be fabricated and used as transparent conductive oxide films in the ultra-violet （UV） and deep UV optoelectronic devices.     

Effect of Zn and Mg doping on structural,dielectric and magnetic properties of tetragonal CuFe2O4

The Cu_1?xA_xFe₂O₄ (A = Zn, Mg; x = 0.0, 0.5) ferrites were successfully synthesized by chemical co-precipitation method. X-ray diffraction and Raman spectroscopy reveals that all the ferrite samples are in single-phase with tetragonal structure for CFO and cubic spinel structure for CZFO and CMFO samples. SEM micrograph shows the variation of grain size with Zn and Mg doping in parent CFO sample. Frequency dependent dielectric response confirms the dielectric polarization and electrical conduction mechanism in the present series with a maximum value of dielectric constant and loss tangent for CZFO sample. The anomaly ～493 K in temperature dependent dielectric constant and dielectric loss is assigned to tetragonal to cubic phase transition in CFO sample. The magnetic measurement explored that the saturation value (M_s) is maximum for CZFO as compared to CFO and CMFO ferrites samples.     

Strain effects on valence bands of wurtzite ZnO

Based on the k.p theory of Luttinger-Kohn and Bir-Pikus,analytical E-k solutions for the valence band of strained wurtzite ZnO materials are obtained.Strain effects on valence band edges and hole effective masses in strained wurtzite ZnO materials are also discussed.In comparison with unstrained ZnO materials,apparent movement of valence band edges such as light hole band,heavy hole band and crystal splitting band at Γ point is found in strained wurtzite ZnO materials.Moreover,effective masses of light hole band,heavy hole band and crystal splitting band for strained wurtzite ZnO materials as the function of stress are given.The analytical results can provide a theoretical foundation for the understanding of physics of strained ZnO materials and its applications with the framework for an effective mass theory.