Valence and conduction band offset measurements in Ni0.07Zn0.93O/ZnO heterostructure

We report valence and conduction band offset measurements in a pulsed laser deposited Ni_0.07Zn_0.93O/ZnO heterostructure using X-ray photoelectron spectroscopy, valence band spectroscopy and ultraviolet visible spectroscopy. Neglecting the strain effect, the valence band offset was estimated to be 0.32 eV and the conduction band offset comes out to be −0.23 eV. Ratio between conduction band and valence band offset is 0.72. Core level shifting due to Ni doping has also been explained. Magnetotransport study of Ni_0.07Zn_0.93O film reveals that the charge carriers might be spin polarized at the interface of the heterojunction.     

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.     

Effect of doping with 3d elements (Co, Ni, Cu) on the intrinsic defect structure and photocatalytic properties of nanostructured ZnO with tubular morphology of aggregates

The precursor-derived nanostructured solid solutions Zn_0.95 M _0.05O (M = Co, Ni, Cu) with tubular aggregates have been investigated using optical absorption spectroscopy and electron paramagnetic resonance. The dependences of the concentration of intrinsic defects V _o ⁺ and the effective band gap on the dopant type have been determined. It has been shown using the oxidation reaction of hydroquinone dissolved in water as an example that an increase in the photocatalytic activity in the series ZnO → Zn_0.95Ni_0.05O → Zn_0.95Co_0.05O Zn_0.95Cu_0.05O in the ultraviolet and visible spectral regions correlates with a decrease in the band gap and with an increase in the concentration of oxygen vacancies V _O ⁺ .     

Fe掺杂对溶胶凝胶法制备的ZnO: Ni薄膜结构及发光特性的影响

采用溶胶凝胶法在玻璃基片上制备了ZnO及Ni, Fe共掺杂的Zn_0.95-xNi_0.05Fe_xO (x=0, 0.005, 0.01, 0.03, 0.05) 薄膜. 通过扫描电镜(SEM) 和X射线衍射(XRD) 研究了薄膜样品的表面形貌和晶体结构. 结果表明所有样品都具有(002) 择优取向, Fe掺杂导致ZnO: Ni薄膜的晶体质量变差, 晶粒尺寸减小, 但适当的Fe掺杂有利于获得致密、 均匀的薄膜. XPS测试结果表明样品中Ni离子的价态为+2价, Fe离子的价态为+2价和+3价.室温光致发光(PL) 测量表明, 所有样品均观察到较强的紫外发光峰, 蓝光双峰和绿光发光峰. ZnO: Ni薄膜的发光强度可以通过Fe掺杂进行有效调节. 进而我们讨论了Ni, Fe共掺杂ZnO样品的发光机理.     

Structural, magnetic and transport properties of Ni-doped ZnO films

In this work, Ni-doped ZnO (Zn_1−xNi_xO, x=0, 0.03, 0.06, 0.11) films were prepared using magnetron sputtering. X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), temperature dependence electrical resistance, Hall and magnetic measurements were utilized in order to study the properties of the Ni-doped ZnO films. XRD and XAS results indicate that all the samples have a ZnO wurtzite structure and Ni atoms incorporated into ZnO host matrix without forming any secondary phase. The Hall and electrical resistance measurements revealed that the resistivity increased by Ni doping, and all the Ni-doped ZnO films exhibited n-type semiconducting behavior. The magnetic measurements showed that for the samples with x=0.06 and 0.11 are room-temperature ferromagnetic having a saturation magnetization of 0.33 and 0.39 μ_B/Ni, respectively. The bound-magnetic-polaron mediated exchange is proposed to be the possible mechanism for the room-temperature ferromagnetism in this work.     

纤锌矿结构Zn1-xMgxO电子结构及吸收光谱的第一性原理研究

采用第一性原理计算方法,计算了纤锌矿结构Zn_1-xMg_xO(x=0,00625,0125,025)的电子结构及吸收光谱. 计算结果表明,Mg的掺入使ZnO的电子结构发生了较大的改变,与Mg邻近的O原子得到电子的数目明显增大,进而O原子返回部分电子给邻近Zn原子. Zn-O间相互作用减弱,禁带宽度变大,这也从同一合金中Zn4s上移的程度得到证实. 其吸收光谱也随着Mg的掺入出现蓝移现象,其吸收边对应波长分别为379, 关键词： 第一性原理计算 电子结构 吸收光谱 纤锌矿结构ZnO     

ZnO高掺杂Ga的浓度对导电性能和红移效应影响的第一性原理研究

采用基于密度泛函理论框架下的第一性原理平面波超软赝势方法,在相同环境条件下建立了浓度不同的由Ga原子取代Zn原子的Zn_1-xGa_xO模型.对低温高掺杂Ga原子的Zn_1-xGa_xO半导体的能带结构、态密度和吸收光谱进行了计算.结果表明:Ga原子浓度越大,进入导带的相对电子数越多,但是电子迁移率反而减小.通过对掺杂和未掺杂ZnO的电导率以及最小间隙带宽度分别进行了比较 关键词： ZnO高掺杂Ga 电导率 红移 第一性原理     

Structural, magnetic properties and photoemission study of Ni-doped ZnO

Nickel-doped ZnO (Zn_1−xNi_xO) have been produced using rf magnetron sputtering. X-ray diffraction measurements revealed that nickel atoms were successfully incorporated into ZnO host matrix without forming any detectable secondary phase. Ni 2p core-level photoemission spectroscopy confirmed this result and suggested Ni has a chemical valence of 2+. According to the magnetization measurements, no ferromagnetic but paramagnetic behavior was found for Zn_0.86Ni_0.14O. We studied the electronic structure of Zn_0.86Ni_0.14O by valence-band photoemission spectroscopy. The spectra demonstrate a structure at ∼2 eV below the Fermi energy E_F, which is of Ni 3d origin. No emission was found at E_F, suggesting the insulating nature of the film.     

Effect of La doping on the electronic structure and optical properties of ZnO

The electronic structure and optical properties of ZnO doped with La have been investigated using density functional theory based on first-principles ultrasoft pseudopotential method. The calculated results show that the La doping increases the bandgap of ZnO, in agreement with the experimental results; while the Fermi level shifts into the conduction band, revealing the so-called Burstein-Moss effect. In comparison to pure ZnO, a new peak appears in the imaginary part of dielectric function in the system doped with La and the optical absorption edge has been obviously changed. Moreover, the covalent property of Zn_1−xLa_xO is found to weaken with the increase of La concentration.     

Ferromagnetic half-metallic characteristic in bulk Ni0.5M0.5O (M=Cu,Zn and Cd): A GGA+U study

Ferromagnetic half metallicity with a high spin polarization of 100% was predicted in the bulk Ni_0.5Cu_0.5O using density-functional theory method. The band gap of majority spin is 3.45 eV for Ni_0.5Cu_0.5O. The density of states of minority spin at the Fermi level are mainly from Cu 3d and O 2p in the Ni_0.5Cu_0.5O. The magnetic moments are from Ni 3d states. Ni_0.5Zn_0.5O and Ni_0.5Cd_0.5O systems are ferromagnetic insulators, but the magnetic moment of Ni²⁺ ions is enhanced by the Zn and Cd incorporation. Therefore, Ni_0.5Cu_0.5O is the potential candidate for spintronics devices because of the predicted high spin polarization.     

Enhancement of optical properties and donor-related emissions in Y-doped ZnO

The Zn_1−xY_xO nanoparticles with good optical properties have been prepared by sol–gel method. The yttrium doping effect on the structures and optical properties were investigated by XRD, SEM, XPS and low temperature photoluminescence. The UV emission intensity of yttrium doped ZnO was over 300 times stronger than that of pure ZnO, which was an exciting result in enhancing the ultraviolet near band edge emission in photoluminescence from ZnO nanoparticles. The UV emission band of doped ZnO nanoparticles exhibits a red shift from 388 to 398 nm, indicating a shallow energy level near valence band has been formed due to the yttrium doping into ZnO lattices. The defect-related band is suppressed (I_D/I_UV = 1–0.83) considerably in Zn_1−xY_xO nanoparticles, revealing the quenching of the broad yellow-orange emission. The doping effect on the optical properties is investigated by temperature dependent photoluminescence. The experimental results indicated that the donor level of yttrium is deeper than that of undoped ZnO.     

Effects of In and Mg doping on properties of ZnO nanoparticles by flame spray synthesis

The Mg- and In-doped zinc oxide (Mg _x Zn_1−x O, In _y Zn_1−y O) nanoparticles were successfully prepared by flame spray synthesis method. According to the results obtained from X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Vis absorption spectra, it was concluded that the Mg or In doping induced the lattice constants to change to some extent; the band gap of Mg _x Zn_1−x O also increased with respect to the decreasing band gap of In _y Zn_1−y O. Moreover, the strong UV emission and weak visible emission were investigated by photoluminescence spectra, while the mechanisms of Mg or In doping on PL spectra have been discussed in detail.     

Optical characterization of band gap graded ZnMgO films

We investigated the optical properties of compositionally graded Zn_1?xMg_xO (g-ZnMgO) films using spectroscopic ellipsometry. The g-ZnMgO and ZnO films were grown on Pt/Ti/SiO₂/Si substrates by ultrasonic spray pyrolysis. We simulated a uniformly graded optical band gap layer on the Pt substrate to reproduce the experimental result. The band gap of the bottommost layer of the g-ZnMgO film was estimated to be ～3.22 eV, the same as the undoped ZnO film. Then we considered a linearly increasing band gap with the film composition, and obtained a band gap of ～3.56 eV for the topmost layer of the film. In addition, the exciton peak showed a strong increase for the topmost layer of the film suggesting an important role of doping.     

Structural and optical properties of cobalt doped ZnO nanocrystals

Zn_1−xCo_xO nanocrystals with nominal Co doping concentrations of x = 0–0.1 were synthesized through a simple solution route followed by a calcining process. The doping effects on the structural, morphological and optical properties were investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman, absorption and luminescence spectroscopy. The results indicated that a small amount of Co ions were incorporated into ZnO lattice structure, whereas the secondary phase of Co₃O₄ was segregated and precipitated at high Co doping concentrations, the solid solubility of Co ions in ZnO nanocrystals could be lower than 0.05. The spectra related to transitions within the tetrahedral Co²⁺ ions in the ZnO host crystal were observed in absorption and luminescence spectra.     

Room-temperature ferromagnetism and optical properties of Zn1−xMnxO nanoparticles

The room-temperature ferromagnetism is observed in Zn_0.98Mn_0.02O nanoparticles, which is related to the host-lattice defects induced by doping Mn. The ferromagnetism in Zn_0.95Mn_0.05O nanoparticles can be suppressed by Mn clusters. The additional peak at 519 cm^?1 is observed in Raman scattering spectra of the Zn_1?xMn_xO nanoparticles associated with intrinsic host-lattice defects, which become activated due to the Mn doping. The decrease in band gap and the weak intensity of absorption peak in the nanoparticles may be due to the sp–d interaction between transition metal and Zn anions.     

First-principles investigation of electronic and optical properties in wurtzite Zn<Subscript>1-x</Subscript>Mg<Subscript>x</Subscript>O

A first-principles study has been performed to evaluate the electronic and optical properties of wurtzite Zn_1-xMg_xO. Substitutional doping is considered with Mg concentrations of x = 0, 0.0625, 0.125, 0.1875 and 0.25, respectively. Mg incorporation can induce band gap widening due to the decrease of Zn 4s states. The imaginary part of the dielectric function shows that the optical transition from band edge emission decreases slightly with increasing Mg contents. The optical band gap also increases from 3.2 to 3.7 eV with increasing Mg contents from 0.0625 to 0.25. The calculated results suggest that relatively high Mg concentration is necessary for effective band gap engineering of wurtzite Zn_1-xMg_xO.     

Ga/N高共掺浓度对ZnO导电性能和红移影响的第一性原理研究

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, 建立了未掺杂ZnO单胞和两种不同浓度的Ga/N高共掺ZnO超胞模型, 分别进行了几何结构优化、总态密度分布和能带分布的计算. 研究表明, ZnO高共掺Ga/N的条件下, Ga/N高共掺浓度越大, 导电性能越弱, 并且高掺杂后高能区红移效应显著, 计算得到的结果与实验结果的变化趋势一致. 关键词： Ga/N高共掺ZnO 电导率 红移 第一性原理     

Optical and structural properties of Zn1−x Mg x O ceramic materials

This paper reports structural, optical and cathodoluminescence characterizations of sintered Zn_1?x Mg _x O composite materials. The effects of MgO composition on these film properties have been analyzed. X-ray diffraction (XRD) confirms that all composites are polycrystalline with prominent hexagonal wurtzite structure along two preferred orientations (002) and (101) for the crystallite growth. Above doping content x = 10 %, the formation of the hexagonal ZnMgO alloy phase and the segregation of the cubic MgO phase start. From reflectance and absorption measurements, we determined the band gap energy which tends to increase from 3.287 to 3.827 eV as the doping content increases. This widening of the optical band gap is explained by the Burstein–Moss effect which causes a significant increase of electron concentration (2.89 × 10¹⁸?5.19 × 10²⁰ cm^?3). The luminescent properties of the Zn_1?x Mg _x O pellets are studied by cathodoluminescence (CL) at room and liquid nitrogen temperatures under different electron beam excitations. At room temperature, the CL spectra of the Zn_1?x Mg _x O composites exhibit a dominant broad yellow-green light band at 2.38 eV and two ultraviolet emission peaks at 3.24 and 3.45 eV corresponding to the luminescence of the hexagonal ZnO and ZnMgO structures, respectively. For the doped ZnO samples, it reveals also new red peaks at 1.72 and 1.77 eV assigned to impurities’ emissions. However, the CL spectra recorded at 77 K show the presence of excitonic emission peaks related to recombination of free exciton (X _A), neutral donor-bound excitons (D⁰X) and their phonon replicas. The CL intensity and energy position of the green, red and ultraviolet emission peaks are found to depend strongly on the MgO doping content. The CL intensity of the UV and red emissions is more enhanced than the green light when the MgO content increases. CL imaging analysis shows that the repartition of the emitting centers in Zn_1?x Mg _x O composites is intimately connected to the film composition and surface morphology.     

Swift heavy ion induced modifications in cobalt doped ZnO thin films: Structural and optical studies

Modifications in the structural and optical properties of 100 MeV Ni⁷⁺ ions irradiated cobalt doped ZnO thin films (Zn_1−xCo_xO, x = 0.05) prepared by sol-gel route were studied. The films irradiated with a fluence of 1 × 10¹³ ions/cm² were single phase and show improved crystalline structure with preferred C-axis orientation as revealed from XRD analysis. Effects of irradiation on bond structure of thin films were studied by FTIR spectroscopy. The spectrum shows no change in bonding structure of Zn-O after irradiation. Improved quality of films is further supported by FTIR studies. Optical properties of the pristine and irradiated samples have been determined by using UV-vis spectroscopic technique. Optical absorption spectra show an appreciable red shift in the band gap of irradiated Zn_1−xCo_xO thin film due to sp-d interaction between Co²⁺ ions and ZnO band electrons. Transmission spectra show absorption band edges at 1.8 eV, 2.05 eV and 2.18 eV corresponding to d-d transition of Co²⁺ ions in tetrahedral field of ZnO. The AFM study shows a slight increase in grain size and surface roughness of the thin films after irradiation.     

Influence of Y doping concentration on the properties of nanostructured MxZn1−xO (M=Y) thin film deposited by nebulizer spray pyrolysis technique

Yttrium doped Zinc Oxide (Y_xZn_1−xO) thin films deposited at a substrate temperature 400 °C. The effect of substrate temperature on the structural, surface morphology, compositional, optical and electrical properties of Y_xZn_1−xO thin films was studied. X-ray diffraction studies show that all films are polycrystalline in nature with hexagonal crystal structure having highly textured (002) plane parallel to the surface of the substrate. The structural parameters, such as lattice constants (a and c), crystallite size (D), dislocation density (δ), microstrain (σ) and texture coefficient were calculated for different yttrium doping concentrations (x). High resolution scanning electron microscopy measurements reveal that the surface morphology of the films change from platelet like grains to hexagonal structure with grain size increase due to the yttrium doping. Energy dispersive spectroscopy confirms the presence of Y, Zn and O elements in the films prepared. Optical studies showed that all samples have a strong optical transmittance higher than 70% in the visible range. A slight shift of the absorption edge towards the large wavelengths was observed as the Y doping concentration increased. This result shows that the band gap is slightly decreased from 3.10 to 2.05 eV with increase of the yttrium doping concentrations (up to 7.5%) and then slightly increased. Room temperature PL measurements were done and the band-to-band emission energies of films were determined and reported. The complex impedance of the 10%Y doped ZnO film shows two distinguished semicircles and the diameter of the arcs got decreased in diameter as the temperature increases from 70 to 175 °C.