Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

利用单能慢正电子束流,对原生的和经过电子辐照的6H-SiC内的缺陷形成及其退火行为进行研究．发现在n型6H-SiC中,经过退火后缺陷浓度降低．这主要是因为在退火过程中缺陷和间隙子的相互作用所引起．n型6H-Si经过1400 oC、30 min真空退火后,在SiC表面形成一个大约20 nm的Si层,这是在高温退火过程中Si原子向表面逸出的有力证明．在高温退火中,在样品的近表面区域有一个明显的表面效应,既在这些区域的S参数整体较大,这种现象与高温退火中Si不断向表面逸出有关．经过10 MeV的电子辐照,在n型6H-SiC中,正电子有效扩散长度从86.2 nm减少至39.1 nm,说明在样品中由于电子辐照产生大量缺陷．但是对p型6H-SiC,经过10 MeV电子辐照后有效扩散长度变化不大,这与其中缺陷的正电性有关．同时还对n型6H-SiC进行了1.8 MeV电子辐照后的300 oC退火实验,发现退火后缺陷浓度不减反增,这主要是因为在退火过程中,一些双空位缺陷和Si间隙子互相作用从而产生了VC缺陷的缘故．     

Structural, defect and optical properties of ZnO films grown under various O2/Ar gas ratios

Thin wurtzite (0 0 2) textured ZnO thin films were deposited on glass substrates by radio frequency magnetron sputtering under O₂/Ar ratios R varying from 0.05 to 1.0 at room temperature. The structure of, and defects in, the films were investigated by XRD, SEM and slow positron beam techniques. The XRD spectra showed that ZnO thin films were polycrystalline with hexagonal structure and a good c-axis orientation perpendicular to the substrate. The thickness, grain size and the crystalline quality of the films strongly depended on R; the larger grain size and thicker ZnO films were grown when R was lower. Positron beam Doppler broadening measurements showed that in low R films additional vacancy-type defects (e.g. Zn-related vacancy complexes or clusters) were formed. Photoluminescence spectra found that the film with R = 0.4 had the highest luminescence efficiency, in good agreement with the best c-axis preferential orientation. The transmittance spectra of the films decreased with decreasing R, due to the thickness effect. Correlations between microstructure, defect and optical properties are discussed.     

Effect of KOH molarity and annealing temperature on ZnO nanostructure properties

In this work, ZnO nanorods (NRs) were fabricated using a low cost chemical bath deposition (CBD) method. The effect of the potassium hydroxide concentration on the fabricated ZnO nanostructures was studied in depth. The optical, structure, and surface morphology properties of the fabricated ZnO nanostructures were investigated using Uv-vis spectroscopy, XRD, and SEM. The results indicate that the formation of hexagonally structured ZnO nanorods with different lengths and diameters was dependent on the KOH concentration. The sample prepared with 2 M of KOH was the best one for optoelectronic applications, since it has the smallest diameters. This sample was annealed at different temperatures (473 K–1073 K). Positron Annihilation Lifetime Spectroscopy was used to determine the defects in the ZnO nanorods. The results show that the positron mean lifetime (τ_m) decreased as the annealing temperature increased, which means that the overall defects in the ZnO nanorods decreased with increasing temperature. Consequently, higher performance semiconductor devices based on ZnO nanorods could be fabricated after high annealing of the ZnO nanorods.     

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.     

Strain engineering for ZnO nanowires: First-principle calculations

We employ density-functional theory to investigate the strain engineering for infinitely long [0001] ZnO nanowires with rectangular cross sections. The structural and electronic properties of ZnO nanowires with uniaxial, lateral and shear strain are systemically calculated. The results show that the band-gaps of ZnONWs will decrease (increase) with increasing (decreasing) tensile (compressive) uniaxial strain. The tensile (compressive) lateral strain on {10 1̅0} surfaces will improve (reduce) the band-gaps for ZnONW with clearly nonlinear characteristic, while the change trend of band-gaps for ZnONW with lateral strain on {1 2̅10} surfaces is basically opposite. When we enhance shear strain on ZnONWs, the band-gaps are reduced. The increasing shear strain along [10 1̅0] direction will sharply reduce the band-gap and the curve is nonlinear, while the band-gap decreases nearly linearly with the increase of shear strain along [1 2̅10] direction.     

Evolution of native point defects in ZnO bulk probed bypositron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600℃ annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600℃ annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.     

Ferromagnetism and microstructure in Fe-implanted ZnO

Fe ions were implanted into ZnO single crystals with multiple energies between 50 and 380 keV up to a total fluence of 12.5×10¹⁷ cm⁻². The crystal quality of Fe⁺ implanted ZnO was investigated by X-ray diffraction 2θ scans and ω-rocking curve measurements. The appearance of Fe related diffraction peaks after C annealing of the implanted sample indicates possibly formation of Fe nanoparticles. Superconducting quantum interference device measurements revealed ferromagnetic behavior below 250 K for both the as-implanted and post-annealed ZnO. Photoluminescence and Raman scattering as well as X-ray rocking curve measurements all indicate introduction of structural defects after Fe implantation. Some of the defects act as nonradiative recombination centers, and suppress the visible and ultraviolet (UV) emission in ZnO. These defects also break the Raman selection rule, and lead to the activation of some silent phonon modes. Annealing of the implanted sample at C causes partial recovery of the defects, however, the X-ray diffraction results of the anneal ZnO show even an improvement of the crystal quality compared with the un-implanted one. This could be attributed to the recovery of the grown-in defects in ZnO.     

Defect structure studies using positron annihilation spectroscopy

The positron annihilation method is a new addition to the range of sensitive complementary nuclear techniques available for materials’ research. The preferential sensitivity of positrons towards micro-defect domains which are not assessable by other techniques makes it an attractive tool for many materials science problems. The present paper is intended as a brief introduction on the principle of measurements and its potential is exemplified with the help of results on some metallic and ceramic systems.     

Slow positron beam study of corrosion-related defects in pure iron

Corrosion-related defects of pure iron were investigated by measuring Doppler broadening energy spectra (DBES) of positron annihilation and positron annihilation lifetime (PAL). Defect profiles of the S-parameter from DBES as a function of positron incident energy up to 30 keV (i.e. ∼1 μm depth) were analyzed. The DBES data show that S-parameter increases as a function of positron incident energy (mean depth) after corrosion, and the increase in the S-parameter is larger near the surface than in the bulk due to corrosion. Furthermore, information on defect size from PAL data as a function of positron incident energy up to 10 keV (i.e. ∼0.2 μm depth) was analyzed. In the two-state trapping model, the lifetime τ₂ = 500 ps is ascribed to annihilation of positrons in voids with a size of the order of nanometer. τ₁, which decreases with depth from the surface to the bulk, is ascribed to the annihilation of positrons in dislocations and three-dimensional vacancy clusters. The corroded samples show a significant increase in τ₁ and the intensity I₂, and near the surface the corroded iron introduces both voids and large-size three-dimensional vacancy clusters. The size of vacancy clusters decreases with depth.     

Morphology engineering of ZnO nanostructures

Nanosized ZnO structures were grown by atmospheric pressure metalorganic chemical vapor deposition (APMOCVD) in the temperature range 200–500 °C at variable precursor pressure. Temperature induced evolution of the ZnO microstructure was observed, resulting in regular transformation of the material from conventional polycrystalline layers to hierarchically arranged sheaves of ZnO nanowires. The structures obtained were uniformly planarly located over the substrate and possessed as low nanowires diameter as 30–45 nm at the tips. The observed growth evolution is explained in terms of ZnO crystal planes free energy difference and growth kinetics. For comparison, the convenient growth at constant precursor pressure on Si and SiC substrates has been performed, resulting in island-type grown ZnO nanostructures. The demonstrated nanosized ZnO structures may have unique possible areas of application, which are listed here.     

Control of polarity of heteroepitaxial ZnO films by interface engineering

Control of polarity of heteroepitaxial ZnO films has been examined by interface engineering. ZnO films were grown by plasma-assisted molecular beam epitaxy on Ga-polar GaN template and c-plane sapphire substrates. Polarity of all the samples is determined by coaxial impact collision ion scattering spectroscopy. Zn- and O-polar ZnO films have successfully grown by Zn- and O-plasma pre-exposures on Ga-polar GaN templates prior to ZnO growth. High-resolution transmission electron microscopy revealed formation of a single-crystalline monoclinic Ga₂O₃ interface layer by O-plasma pre-exposure on Ga-polar GaN templates, while no interface layer was observed for Zn pre-exposed ZnO films. The polarity of ZnO films grown under oxygen ambient on c-plane sapphire with MgO buffer is revealed as O-polar. Fabrication of polarity inverted ZnO heterostructure has been studied: polarity of ZnO films on Ga-polar GaN templates was changed from Zn-polar to O-polar by inserting a MgO layer. High-resolution transmission electron microscopy revealed atomically flat interfaces at both lower and upper ZnO/MgO interfaces and no inversion domain boundaries were detected in the upper ZnO layer.     

Positron annihilation study of the electronic structure of CeCu6 and LaCu6

Summary The results of a prelimianry study of the electronic structure of the heavy-fermion system CeCu₆ via the two-dimensional angular correlation of electron-positron annihilation radiation (2D ACAR) technique were compared with those obtained for the isostructural reference non-f-electron material LaCu₆. The spectra relating to the 2 compounds displayed extremely similar anisotropic structures, with features related to the symmetry of the Brillouin zone. At present, these structures cannot however be attributed to Fermi-surface or positron wave function effects or to anisotropies of some full valence band. In the case of LaCu₆, data processing prior to the LCW procedure via new filtering techniques revealed a reasonably good agreement between the LCW data and LAPW Fermi-surface calculations while in the case of CeCu₆ it appeared difficult to relate the results, still of controversial interpretation, to any theoretical model.     

Co掺杂纳米ZnO微结构的正电子湮没研究

利用正电子湮没技术研究了10 at.% Co掺杂的Co₃O₄/ZnO纳米复合物中退火对缺陷的影响. 利用X射线衍射(XRD)测量了Co₃O₄/ZnO纳米复合物的结构和晶粒尺寸. 随着退火温度升高,Co₃O₄相逐步消失,ZnO晶粒尺寸也有显著增加. 经过1000 ℃以上退火后,Co₃O₄相完全消失,并出现了CoO的岩盐结构. 正电子湮没寿命测量显示出Co₃O₄ /ZnO纳米复合物中存在大量的Zn空位和空位团. 这些空位缺陷可能存在于纳米复合物的界面区域. 当退火温度达到700 ℃后Zn空位开始恢复,空位团也开始收缩. 900 ℃以上退火后,所有空位缺陷基本消失,正电子寿命接近ZnO完整晶格中的体态寿命值. 符合多普勒展宽谱测量也显示Co₃O₄ /ZnO纳米复合物经过900 ℃以上退火后电子动量分布与单晶ZnO基本一致,表明界面缺陷经过退火后得到消除. 关键词： ZnO 界面缺陷 正电子湮没     

Effect of carbon co-doping on the magnetic properties of bulk Mn doped ZnO

We report the systematic study on the effect of C-doping and annealing atmosphere (air, O₂, N₂ and Ar) on the magnetic properties of pure ZnO as well as Zn_0.98Mn_0.02O (ZMO). We found diamagnetic behavior for pure ZnO samples in all the annealing atmospheres whereas all ZMO samples are paramagnetic except the one, which is annealed in argon. While maintaining the same annealing atmospheres and varying the C content we found that on increasing the C-doping concentration the ferromagnetic fraction in the sample increases. The observed RTFM in C-doped ZMO is possibly due to carriers introduced into the samples due to oxygen vacancies and the substitution of C at O sites.     

Angular correlation of scattered annihilation photons, to test the possibility of hidden variables in quantum theory

Angular correlations of the annihilation photons, Compton scattered by plastic scintillators and detected by means of NaI (T1) crystals, have been measured in order to test the possibility of deviations of the experimental results from the predictions of the quantum theory. In fact, Jauch and Bohm, starting with different motivations, both arrive at the possibility of a lower correlation ratio between the two orthogonal polarization states of the two photons than predicted by quantum theory. This in turn should give a lower azimuthal anisotropy in the angular correlations. Our experimental results compared with the theoretical predictions, after correction for finite geometry by means of a Montecarlo method, do not confirm quantum theory and exclude the hypotheses of Jauch and of Bohm. We are continuing the experiment in order to test wether the breakdown in the polarization correlation depends on the distance (spatial and/or temporal) between the two correlated scattering events, as suggested by Jauch. Paper A 34 presented at 3 rd Internat'l Conf. Positron Annihilation, Otaniemi, Finland (August 1973).     

An EPR study of Ni+ centers in SrF2:Ni and BaF2:Ni

An EPR study of RT X-irradiated SrF2:Ni and BaF2:Ni has been performed. Different kinds of Ni⁺ centers showing small deviations with respect to a basic structure have been observed. This basic structure consists of a Ni⁺ ion displaced along a <100> direction from the cation substitutional position toward the center of a face of the cube of fluorines. These results are similar to those previously reported on CaF2:Ni.

The spin Hamiltonian parameters of the different centers obtained from the analysis of the corresponding rotational diagrams are also reported.     

氧化锌薄膜的缺陷发光和它作为白光光源的潜在应用

ZnO发光二极管(LEDs)在照明应用方面有着巨大的潜力。需要解决的主要问题是光的产生和对辐射的控制,这个问题来自LED波长的变化和组合。发现缺陷发光的ZnO有着各种波长范围,适合LED在白光产生方面的应用。同时展示了在实验和理论上可以用于ZnO系统的缺陷辐射。这种类型的缺陷相较于传统的掺杂材料和其他材料,其优点在于不需要广泛和昂贵的生产系统。不仅提出了ZnO薄膜在白色平面LED光源本征缺陷发光的潜在应用,同时也利用一些方法一个特定的中心位置和ZnO薄膜在初期发射谱带的宽分布来控制缺陷的产生。根据不同的制备方法和特定的实验条件,不同的白色,如稍白色和青白色等原本的和重要的颜色-蓝光波段(455, 458 nm),绿光波段(517, 548 nm),红光波段(613, 569 nm)分别被获得。从而说明了这是一种制作白光LED更好的办法-利用ZnO材料。在对ZnO薄膜电学性质的调查研究中,通过薄膜表面的额电子插入和正离子的湮灭已经证明了的观点,随着质子的植入、正离子的湮没、电子的插入和ZnO表面的电学性质的研究,表述结果被进一步的证实。研究人员对单晶ZnO的已经有了一定的研究,PL质子植入ZnO以后呈现橘红色,并且在700 ℃退火后仍然存在,清楚的可以看出PL缺陷的存在。在植入粒子方面最近的文章也有报道,例如在ZnO缺陷表层中注入离子和电子来改变PL性能。V_Zn也发现了氧化锌薄膜的主要缺陷之一是正电子湮没,同样的,Vlasenko和Watkins也发现了氧化锌表面由于电子辐射产生的缺陷。导致绿色透光率的减少,增加PL致600～700 nm。之后分析和解释ZnO薄膜电阻率的缺陷。由霍尔系数的迹象表明ZnO表现为N型传导,这样做的原因是因为把V_O和Zn原子联系在一起,使Zn具有较低的电阻率。试验中氧气退货可以增加ZnO的电阻率,其电阻率的增加是由于V_O的减少。另外,在200 ℃条件下准备的样品导电率很低,说明了V_O的作用很大。退火氧化锌薄膜电导率下降表明, 看到了主要的缺陷。     

Structural and magnetic studies on transition metal (Mn, Co) doped ZnO nanoparticles

We report on the structural and magnetic properties of the nanocrystalline samples of Zn_1−x(TM)_xO (TM=Mn, Co and x=0.02, 0.05, 0.10) synthesized by chemical vapor deposition (CVD) method using different carrier gases i.e., Argon (Ar), Oxygen (O₂) and Nitrogen (N₂). X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies reveal wurtzite structure of pure ZnO in all the samples and particle sizes in the range of 15-40 nm. No evidence of any secondary phases having room temperature ferromagnetic behavior has been observed through XRD and TEM studies. Magnetic measurements reveal presence of mixed magnetic phases in the samples, which may be the reason for the low saturation magnetization in the nanoparticles.     

质子辐照ZnO白漆光学退化的慢正电子湮没分析

采用慢正电子湮没光谱研究低能质子辐照下ZnO白漆的光学退化。研究结果表明,随质子辐照注量的增加, 多普勒展宽谱的S参数逐渐减小,W参数逐渐增大。质子辐照下S-W参数拟合曲线的斜率发生改变。S参数的减小可以归结为锌空位含量的减少以及准正电子素的形成。准正电子素{单电离氧空位（捕获一个电子）+正电子}的形成,能够降低正电子湮没的速率,导致S参数减小。S参数的减小证实了质子辐照导致ZnO白漆中单电离氧空位数量的增加。S-W参数拟合曲线斜率的变化可以归结于质子辐照下双电离氧空位向单电离氧空位的转变。