氧空位对钴掺杂氧化锌半导体磁性能的影响

从实验和理论上阐述了氧空位对Co掺杂ZnO半导体磁性能的影响.采用磁控溅射法在不同的氧分压下制备了Zn_095Co_005O薄膜,研究了氧分压对薄膜磁性能的影响.实验结果表明,高真空条件下制备的Zn_095Co_005O薄膜具有室温铁磁性,提高氧分压后制备的薄膜铁磁性逐渐消失.第一性原理计算表明,在Co掺杂ZnO体系中引入氧空位有利于降低铁磁态的能量,铁磁态的稳定性与氧空位和Co之间的距离密切相关. 关键词： Co掺杂ZnO 稀磁半导体 第一性原理计算 氧空位缺陷     

The electronic and magnetic properties with intrinsic defects in ZnO nanosheets: First-principles prediction

Using full-potential linearized augmented plane wave (FLAPW) method, we investigated the effects of intrinsic vacancies on electronic and magnetic properties in graphene-like ZnO nanosheets. The results show that the oxygen vacancy (V_O) has no influence on magnetism in ZnO nanosheet, whereas the Zn vacancy (V_Zn) lead to spin polarization of the nanostructures with a total magnetic moment of 2.0μ_B due to O-2p and Zn-3d hybridization. When the distance of two V_Zn defects increases to 6.499 Å, the system shows an intriguing half-metallic character with 100% spin-polarized carriers due to O(2p)–Zn(3d)–O(2p) coupling chain between two V_Zn defects.     

A first-principles study of the magnetic properties in boron-doped ZnO

First-principles calculations based on density functional theory are performed to study the origin of ferromagnetism in boron-doped ZnO. It is found that boron atoms tend to reside at Zn sites. The induced Zn vacancy is a key factor for ferromagnetism in Zn1-xBxO (0相似文献   

Transitions of microstructure and photoluminescence properties of the Ge/ZnO multilayer films in certain annealing temperature region

The Ge/ZnO multilayer films have been prepared by rf magnetron sputtering. The effects of annealing on the microstructure and photoluminescence properties of the multilayers have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectrometry and photoluminescence (PL) spectrometry. The investigation of structural properties indicates that Zn₂GeO₄ has been formed with (2 2 0) texture and Zn deficiency from Ge/ZnO multilayer films in the process of annealing. However, lower Zn/Ge ratio can be improved by annealing. The annealed multilayers show three main emission bands at 532, 700, and 761 nm, which originate from the transition between oxygen vacancy () and Zn vacancies (V_Zn), the radiative recombination of quantum-confined excitons (QCE) in Ge nanocrystals, and the optical transition in the GeO color centers, respectively. Finally, the fabrication of thin film Zn₂GeO₄ from Ge/ZnO multilayer films by annealing at low temperature provides another approach to prepare the green-emitting oxide phosphor film:Zn₂GeO₄:Mn.     

Vacancy defects in (Zn, Mn)O

We have used positron annihilation spectroscopy to study the vacancy defects in (Zn, Mn)O crystals grown by chemical vapor transport (CVT). Our results show that Zn vacancies are present in both as-grown and high temperature annealed ZnO and Zn_0.985Mn_0.015O. In addition, we observe O vacancies in ZnO with no Mn. After annealing in O₂ at 1000 C, there is no change in the vacancy distribution in ZnO, while the Zn vacancy concentration increases by an order of magnitude in Zn_0.985Mn_0.015O.     

Magnetic and optical properties of Cu-doped ZnO nanosheet: First-principles calculations

We performed first-principles calculations within density-functional theory to study the magnetic and optical properties of Cu-doped ZnO nanosheet (NS). We found that Cu atom prefers to substitute for Zn site and can induce a local magnetic moment of 1.00 μ_B per unit in ZnO NS. When two Zn atoms are substituted by two Cu dopants, they tend to form a cluster and ferromagnetic (FM) ordering becomes energetically more favorable. In addition, localized states appear within the band gap due to the introduction of Cu dopant to ZnO NS. With increasing Cu concentrations, both the imaginary part of dielectric function and the absorption spectrum exhibit a red-shift behavior, which are in good agreement with the recent experimental results. The ferromagnetic coupling can be attributed to the p–d hybridization mechanism. The intriguing properties of Cu-doped ZnO NS may be promising for designing novel multifunctional nanodevice.     

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.     

Effects of temperature and atmosphere on the magnetic properties of Co-doped ZnO rods

Co-doped ZnO (Zn_0.95Co_0.05O) rods are fabricated by co-precipitation method at different temperatures and atmospheres. X-ray diffraction, Energy dispersive X-ray spectroscopy and Raman results indicate that the samples were crystalline with wurtzite structure and no metallic Co or other secondary phases were found. Raman results indicate that the Co-doped ZnO powders annealed at different temperatures have different oxygen vacancy concentrations. The oxygen vacancies play an important role in the magnetic origin for diluted magnetic semiconductors. At low oxygen vacancy concentration, room temperature ferromagnetism is presented in Co-doped ZnO rods, and the ferromagnetism increases with the increment of oxygen vacancy concentration. But at very high oxygen vacancy concentration, large paramagnetic or antiferromagnetic effects are observed in Co-doped ZnO rods due to the ferromagnetic-antiferromagnetic competition. In addition, the sample annealed in Ar gas has better magnetic properties than that annealed in air, which indicates that O₂ plays an important role. Therefore, the ferromagnetism is affected by the amounts of structural defects, which depend sensitively on atmosphere and annealing temperature.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Preparation,structural, photoluminescence and magnetic studies of Cu doped ZnO nanoparticles co-doped with Ni by sol–gel method

Zn_0.96−xNi_0.04Cu_xO nanoparticles have been synthesized by varying different Cu concentrations between 0% and 4% using simple sol–gel method. X-ray diffraction studies confirmed the hexagonal structure of the prepared samples. The formation of secondary phases, CuO (111) and Zn (101) at higher Cu content is due un-reacted Cu²⁺ and Zn²⁺ ions present in the solution which reduces the interaction between precursor ions and surfaces of ZnO. Well agglomerated and rod-like structure noticed at Cu=4% greatly de-generate and enhanced the particle size. The nominal elemental composition of Zn, Cu, Ni and O was confirmed by energy dispersive X-ray analysis. Even though energy gap was increased (blue-shift) from Cu=0–2% by quantum size effect, the s–d and p–d exchange interactions between the band electrons of ZnO and localized d electrons of Cu and Ni led to decrease (red-shift) the energy gap at Cu=4%. Presence of Zn–Ni–Cu–O bond was confirmed by Fourier transform infrared analysis. Ultraviolet emission by band to band electronic transition and defect related blue emission were discussed by photoluminescence spectra. The observed optical properties concluded that the doping of Cu in the present system is useful to tune the emission wavelength and hence acting as the important candidates for the optoelectronic device applications. Ferromagnetic ordering of Cu=2% sample was enhanced by charge carrier concentration where as the antiferromagnetic interaction between neighboring Cu–Cu ions suppressed the ferromagnetism at higher doping concentrations of Cu.     

Structural,optical and magnetic properties of Cu and V co-doped ZnO nanoparticles

Zn_0.98−xCu_xV_0.02O (x=0, 0.01, 0.02 and 0.03) samples were synthesized by the sol–gel technology to dope up to 3% Cu in ZnO. Investigations of structural, optical and magnetic properties of the samples have been done. The results of X-ray diffraction (XRD), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS) indicated that the V and Cu ions were incorporated into the crystal lattices of ZnO. With Cu doping concentration increasing up to 2 at%, the XRD results showed that all diffraction peaks corresponded to the wurtzite structure of ZnO. Photoluminescence (PL) measurements showed that Zn_0.98−xCu_xV_0.02O powders exhibited that the position of the ultraviolet (UV) emission peak of the samples showed an obvious red-shift and the green emission peak enhanced significantly with Cu doping in ZnVO nanoparticle. Magnetic measurements indicated that room temperature ferromagnetism (RTFM) of Zn_0.98−xCu_xV_0.02O was an intrinsic property when Cu concentration was less than 3 at%. The saturation magnetization (M_s) of Zn_0.98−xCu_xV_0.02O (x=0, 0.01 and 0.02) increased with the increase of the Cu concentration.     

掺银氧化锌纳米棒的水热法制备研究

利用水热法在直流磁控溅射制备的掺铝氧化锌 (AZO) 种子层上制备了不同形貌和光学性能的掺银ZnO纳米棒, 并采用XRD、扫描电镜、透射谱、光发射谱和EDS谱详细研究了Ag离子与Zn离子的摩尔百分比 (R_Ag/Zn) 及AZO种子层对掺银ZnO纳米棒的结构和光学性质的影响. 随着R_Ag/Zn的增加, 掺银ZnO 纳米棒的微结构和光学性质的变化与银掺杂诱导的纳米棒的端面尺寸变化有关. 平均端面尺寸的变化归结于种子层颗粒大小和颗粒数密度不同导致掺入的Ag离子的相对比例不同. 溅射15 min的AZO种子层上生长的ZnO纳米棒由于缺陷增多导致在可见光区的发光峰明显强于溅射10 min 的AZO种子层上、相同R_Ag/Zn 条件下生长的ZnO纳米棒. Ag掺杂产生的点缺陷增多导致可见光区PL波包较宽. 纯ZnO纳米棒的微结构与种子层厚度导致的结晶度和颗粒大小有关. 关键词： ZnO纳米棒 水热法 Ag掺杂 直流磁控溅射     

First-principles study of single intrinsic vacancy formation and its effect on the electronic density states and magnetic moment of V-doped ZnO

We calculated the formation energy of single vacancy in V-doped ZnO in different conditions (oxygen or zinc rich) by first principles. Effect of an intrinsic vacancy on the electronic density of states and magnetic moment of V-doped ZnO (Zn₁₅VO₁₆) with and without single vacancy was also calculated. Our calculation was performed by the CASTEP program within spin-polarized GGA approximation implemented in materials studio software. The formation energy showed that oxygen vacancy inclined to stay far from vanadium (V) and zinc vacancy preferred to stay at a position near V. The calculated formation energy also showed that a zinc vacancy may automatically occur but an oxygen vacancy may not appear automatically. Vanadium doping introduced spin-polarization around Fermi level. For an energy favorable vacancy, an oxygen vacancy had little effect on the electronic density of states. A zinc vacancy made the spin-polarization peaks around Fermi level broaden and decreased their magnitude. For the magnetic moment in energy favorable configurations, an oxygen vacancy had little effect on the magnetic moment; a zinc vacancy significantly decreased the magnetic moment (as high as 63.7%). Changes in magnetic moments were consistent with electronic density of states. Our calculation may interpret various experimental magnetic moment values. Our work also provided a reference for preparing V-doped ZnO-based dilute magnetic semiconductors.     

Properties of Cu and V co-doped ZnO nanoparticles annealed in different atmospheres

Zn_0.97Cu_0.01V_0.02O nanoparticles have been successfully prepared by the sol–gel method and sintered at 600 °C in argon and air atmosphere, respectively. The effects of annealing atmosphere on the structural, optical and magnetic properties of the obtained samples were studied. The X-ray diffraction result showed that the Zn_0.97Cu_0.01V_0.02O was single phase with the wurtzite structure of ZnO. The sample annealed in air had much better crystallization. Photoluminescence shows an increase in green emission when annealing in argon. The two Zn_0.97Cu_0.01V_0.02O samples exhibited ferromagnetism at room temperature. The ferromagnetism in this study was itself property of Cu, V co-doped ZnO and not originated from the secondary phase.     

Formation of ZnO and Zn1−xCdxO films on CdTe/CdZnTe single crystals

In this work we report on the properties of ZnO and Zn_1−xCd_xO films formed on top of CdTe and CdZnTe single crystals. The films have been obtained by thermal evaporation of Zn metal films and further oxidation in atmospheric conditions. The structural and compositional characteristics of the films have been analysed by means of scanning electron microscopy and energy-dispersive X-ray analysis. The chemical composition of the films as a function of growth parameters has been obtained. It has been possible to demonstrate by Raman spectroscopy the formation of both ZnO and Zn_1−xCd_xO films. The possible inter-diffusion effects between the films and the substrate, derived from the oxidation process, have been discussed. It has been possible to check by means of photoluminescence, the optical quality of the ZnO and Zn_1−xCd_xO films, also regarding to the presence of local changes. Differences between the optical spectra obtained from various ZnO films grown on top of CdTe and CdZnTe substrates enabled the determination of compositional differences introduced by the substrate when the deposition parameters are modified.     

Possible origin of ferromagnetism in un-doped ZnO: First-principles calculations

We performed first-principles calculations to investigate the electronic structures and magnetic properties of neutral and charged intrinsic defects of wurtzite ZnO. Our results show that an isolated single charged Zn vacancy can introduce a magnetic moment of 1.00μB, and may be responsible for the unexpected ferromagnetism in the system.     

p型K:ZnO导电机理的第一性原理研究

基于密度泛函理论,利用局域密度近似的第一性原理平面波赝势方法,对掺K以及含有氢填隙(H_i)、氧空位(V_O)、锌填隙(Zn_i)和锌空位(V_Zn)的K:ZnO电子结构分别进行了研究.结果表明,1) 单独掺K可引入浅受主,但系统总能增高;2) K与H共掺可降低系统总能,提升稳定性;3) V_O在K+H:ZnO中的形成比Zn_i困难得多,二者都是 关键词： 氧化锌 p型 第一性原理 电子结构     

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.     

Study of magnetic properties of ZnO nanoparticles codoped with Co and Cu

Here, we report on systematic studies of the magnetic properties of Co and Cu codoped ZnO nanoparticles prepared by sol–gel technique. The effect of hydrogenation and shape on the magnetic properties of Zn_0.93Co_0.05Cu_0.02O nanoparticles is presented. The Zn_0.93Co_0.05Cu_0.02O nanoparticles and well-aligned Zn_0.93Co_0.05Cu_0.02O nanorod array display ferromagnetic behavior at room temperature. Our results demonstrate the influence of shape and hydrogenation on the ferromagnetic properties of Zn_0.93Co_0.05Cu_0.02O nanoparticles at room temperature.     

Study on the structure and optical property of Zn1−xCuxO sol–gel thin films on quartz substrate

Zn_1−xCu_xO thin films (x=0, 1.0, 3.0, 5.0%) are prepared on quartz substrate by sol–gel method. The structure and morphology of the samples are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results show that Cu ions were effectively penetrated into the ZnO crystal lattices with substitutional and interstitial impurities to form stable solid solutions without changing the polycrystalline wurtzite structure. Two peaks at 420 nm (2.95 eV, violet), 485 nm (2.56 eV, blue) have been observed from the photoluminescence (PL) spectra of the samples. It is concluded that the violet peak may correspond to the exciton emission; the blue emission corresponds to the electron transition from the bottom of the conduction band to the acceptor level of zinc vacancy. The optical test shows that the optical band gap E_g is decreased with the increase amount of Cu doping in ZnO. The band gap decrease from 3.40 eV to 3.25 eV gradually. It is also found that the transmission rate is increased rapidly with the increase of Cu ions concentration.