Gd掺杂ZnO纳米线磁耦合性质的第一性原理研究

本文利用基于密度泛函理论的第一性原理方法计算了钆(Gd)掺杂氧化锌(ZnO)纳米线的磁耦合特性. 讨论了两个Gd原子替换ZnO纳米线中不同位置Zn原子的各种可能情况. 计算发现, ZnO中掺杂的Gd原子处于相邻的位置时它们之间的相互作用是铁磁性的, 并且体系的铁磁性可以通过注入合适数目的电子来得到加强. 同时发现Gd掺杂ZnO纳米线后s-f耦合作用变得显著, 使得体系的铁磁性变得更加稳定, 这也是Gd掺杂ZnO纳米线呈现铁磁性的原因. 这些结果为实验上发现的Gd掺杂ZnO纳米线呈铁磁性提供了理论依据.     

Ferromagnetism of V-doped ZnO nanowires

The geometry structures,electronic structures,and magnetic properties of Zn46V2O48 nanowires are studied by density functional theory(DFT) calculations.We find that the ferromagnetic(FM) coupling is more stable for six configurations of Zn46V2O48 nanowires,and is mediated by neighboring O as evidenced from the strong hybridization of V 3d and O 2p states,exhibiting strong spin polarization.The spin polarization is found to be 100% in the Zn46V2O48 nanowires,which confirms that it is a half-metallic ferromagnet and very suitable for the injection of the spin carriers,which shows that Zn46V2O48 nanowire is one of the ideal materials to realize spin electronic devices.At the same time,the magnetic coupling mechanisms of Zn46V2O48 nanowires are analyzed with V 3d and O 2p orbitals and their magnetic moments mainly come from the contributions of the unpaired electrons of V 3d orbitals.The above results provide a theoretical basis for the preparation of 3d transition metal-doped ZnO nanowire materials.     

Multi-photon-pumped stimulated emission from ZnO nanowires: A time-resolved study

In this work, we study temporal evolution of multi-photon-pumped stimulated emission from ZnO nanowires. In addition to second harmonic generation, ultraviolet stimulated emission is observed in ZnO nanowires under femtosecond pulse excitation at 800 nm. Sharp emission peaks appear when excitation flux reaches a threshold of 80 mJ/cm², which can be interpreted as lasing action in self-formed nanowire microcavities. Temporal evolution of the emission captured by Kerr shutter technique shows strong excitation-power dependence. The dynamic trace of stimulated emission exhibits a fast decay with a lifetime about 4.5 ps at intermediate excitation (∼100 mJ/cm²) and a lifetime about 2 ps at high excitation (>160 mJ/cm²). The difference in the lifetime can be attributed to different gain mechanisms related to excitonic interaction and electron-hole plasma, respectively.     

ANSYS仿真激光切割氧化锌纳米线

 以脉冲高斯激光为光源,直径在500 nm～1 000 nm、长度10 μm的一维氧化锌材料为对象,采用有限元分析软件ANSYS建立了激光切割氧化锌纳米线的热力学仿真模型,并采用生死单元技术对超过熔点的单元进行处理,得到不同激光工作参数和氧化锌纳米线直径下的温度分布场和切割形貌。讨论了纳米线直径和聚焦光斑相对纳米线的位移对切割的影响,纳米线直径越大允许的激光离焦量越小;当无离焦或负离焦较小时,切割纳米线产生的形貌较为理想。     

Modifying optical properties of ZnO nanowires via strain-gradient

We conduct systematical cathodolumiuescence study on red-shift of near-band-edge emission energy in elastic bent ZnO nanowires with diameters within the exciton diffusion length （- 200 nm） in liquid nitrogen temperature （81 K）. By charactering the emission spectra of the nanowires with different; local curvatures, we find a linear relationship between strain-gradient and the red-shift of near-band-edge emission photon energy, an elastic strain-gradient effect in semiconductor similar to the famous flexoelectric effect in liquid crystals. Our results provide a new route to understand the inhomogeneous strain effect on the energy bands and optical properties of semiconductors and should be useful for designing advanced nano-optoelectronic devices.     

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet (UV) band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.     

单晶ZnO、TiO_2纳米线拉伸力学特性的分子动力学研究

采用分子动力学方法,模拟了不同加载速度、不同温度下单晶ZnO、TiO_2纳米线的拉伸破坏过程.通过模拟结果,对比、分析了两种单晶金属氧化物纳米线拉伸力学特性的差异.研究表明,1)ZnO纳米线的断裂机制为:表面微裂纹-微孔-微裂纹与微孔贯穿-断裂,而TiO_2纳米线的断裂机制为:局部屈服-颈缩-断裂;2)TiO_2纳米线的承载能力优于ZnO纳米线,而承受变形的能力劣于ZnO纳米线;3)温度较低的情况下,纳米线的抗拉性能较好;加载速度越高,纳米线的抗载性能越好,而抗变形能力越差.     

Defect engineering of ZnO

The defect responsible for the transparent to red color change of nominally undoped ZnO bulk single crystals is investigated. Upon annealing in the presence of metallic Zn as reported by Halliburton et al. and also Ti and Zr a native defect forms with an energy level about 0.7 eV below the conduction band. This change is reversible upon annealing in oxygen. Optical transmission data along with positron depth profiles and annealing studies are combined to identify the defect as oxygen vacancies. Vacancy clustering occurs at about 500 °C if isolated zinc and oxygen vacancies. In the absence of zinc vacancies, clusters form at about 800 °C.     

Structure-controllable synthesis of ZnO nanowires using water vapor in an atmospheric-pressure microwave plasma system

Although the use of atmospheric-pressure microwave plasma to synthesize ZnO nanowires addresses various issues associated with conventional synthesis methods, it is difficult to obtain nanowires with controlled diameters and lengths. Herein, we investigated the effects of microwave input power and water vapor on the structure of ZnO nanowires grown using an atmospheric-pressure microwave plasma system. It was found that the aspect ratio of ZnO nanowires could be controlled by adjusting the plasma temperature and the amount of OH radicals. This method successfully produced ZnO nanowires in a structure-controllable manner with a wide range of aspect ratios from 10 to 340. Interestingly, we found that the sensitivity of the ZnO nanowires to UV irradiation was closely related to the morphology. In particular, the ZnO nanowires with high aspect ratios and densities showed excellent sensing characteristics.     

Nanoindentation of vertical ZnO nanowires

We report the experimental observations of buckling instabilities of vertical well-aligned single-crystal ZnO nanowires prepared on ZnO:Ga/glass templates. It was found that critical buckling load and buckling energy of the ZnO nanowires were 215 μN and 3.69×10⁻¹¹ J, respectively. It was also found that Young's modulus of the ZnO nanowires were 232 and 454 GPa while critical buckling strains were 0.35% and 0.18% when fixed–fixed column mode and fixed–pinned column mode were used, respectively.     

Be掺杂纤锌矿ZnO电子结构的第一性原理研究

采用密度泛函理论结合投影缀加波方法,对Be掺杂导致ZnO禁带宽度增加的机理进行了研究.通过对掺杂前后电子能带结构、总态密度以及分态密度的计算和比较,发现导带底(CBM)是由Be 2s电子与Zn 4s电子共同控制;而Be_xZn_1-xO价带顶 (VBM)始终由O 2p电子占据.随着掺杂量的增加,决定带隙宽度的CBM的位置上升,同时VBM的位置下降,从而导致了带隙的变宽,出现了蓝移现象.此外,Be掺杂会使晶胞发生压缩,这种压应变也是导致Be 关键词： 密度泛函理论 电子结构 Be掺杂ZnO     

First-principles study of ferromagnetism in Ti-doped ZnO with oxygen vacancy

Using first-principles density functional calculations, we have investigated the electronic structures of Ti-doped ZnO (Ti is in 4+ oxidation state) with and without oxygen vacancy. The ferromagnetic property is identified in the presence of oxygen vacancy despite Ti being nonmagnetic in its natural phase. The ferromagnetism originates from the charge transferring from donor derived-defect band to unoccupied Ti-3d states and the hybridization between Ti-3d and O-2p band near the Fermi level. On increasing the oxygen vacancy concentration, a transition from a long-ranged magnetic order to a short-ranged interaction is found and the oxygen vacancies prefer to distribute non-uniformly in Ti-doped ZnO.     

First-principle calculation for the phonon structure on iron-based superconductors

We perform first-principle phonon calculations for three typical iron-based superconductors, i.e., LaFeAsO,BaFe₂As₂, and FeSe. Though those crystals have different structures, we find that the optical modes associated with Fe vibration have almost similar characters. Moreover, we examine the pressure effect on phonons in FeSe. By increasing the external pressure, the phonon mode frequency related to Fe vibration effectively rises up and the electronic density of states at Fermi level also increases. These results may correlate to the critical temperature enhancement under high pressure.     

Tuning the growth of ZnO nanowires

ZnO nanowires (NWs) with different diameters were obtained by controlling the particles of ZnO sub-layer (SL) exploring hydrothermal method; the diameter of the epitaxial NWs could be tuned from 60 to 146 nm when using SL with a thickness of 70 nm. The thickness of the SL would influence the orientation of the NWs. The top agglomerate NWs could be formed on the SL with a thickness of 10 nm, and the NWs with better orientation were obtained using SL with a thickness of 70 nm. Well aligned ZnO NWs grew perpendicular to the completely stress released SL. The diameter of the NWs was also greatly influenced by the solution concentration; thus ultra fine (diameter∼11 nm) ZnO NWs were obtained through adjusting the solution concentration to 0.001 mol/L. Through our research, we also found that the growth rate of the NWs could also be influenced by the different polarity surface of the SL. In other words, the size of the ZnO NWs could be tuned exactly under optimal conditions.     

Co和Mn共掺杂ZnO铁磁性的第一性原理

基于密度泛函理论(DFT)的总体能量平面波超软赝势方法,结合广义梯度近似(GGA),对Co单独掺杂ZnO和(Co,Mn)共掺杂ZnO的32原子超原胞体系进行了几何结构优化,计算了纤锌矿结构ZnO、Co-ZnO及(Co,Mn)共掺杂ZnO的能带结构、电子态密度,并对此进行了详细的分析。计算结果表明,ZnO中单独掺杂Co元素显示出铁磁性行为,Mn的引入减弱了Co-ZnO的铁磁性。     

Structural, Raman, and photoluminescence characteristics of ZnO nanowires coated with Al-doped ZnO shell layers

Al-doped ZnO (AZO) shell layers were coated on core ZnO nanowires to fabricate ZnO/AZO core–shell nanowires. The energy-dispersive X-ray spectra confirmed the presence of Al element in the shell layers, and the lattice resolved transmission electron microscopy image revealed that these layers corresponded to the hexagonal ZnO structure. The X-ray diffraction pattern exhibited a shift of the ZnO peaks, suggesting the substitutive incorporation of Al into the ZnO lattice. The A₁(LO) mode line in the Raman spectra was enhanced by the AZO coating. In the photoluminescence measurements, the AZO coating enhanced the intensity ratio of the UV to green emission.     

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.     

银修饰的ZnO纳米线的发光和光响应性质

采用不同浓度的银溶液对水热方法生长的ZnO纳米线进行了银修饰,并对其形貌、光致发光和光响应特性进行了研究.实验结果表明,随着修饰用银浓度的加大,光致发光的强度减弱.对原生的和用浓度为10mmol/L的银修饰的ZnO纳米线的光响应进行了测试和比较,发现银修饰的ZnO纳米线在380 nm的响应度达4.6 mA/W,其值是未...     

海胆状ZnO纳米线阵列的制备及其光学性能

采用一种低成本的有效方法制备出了有序排列的海胆状ZnO纳米线阵列。首先利用自组装的方法得到了单层的聚苯乙烯(PS)小球,以其为模板用水热法在小球表面生长ZnO纳米线,得到了由PS小球和ZnO纳米线构成的海胆状结构。纳米线的直径均一,长度可通过水热反应时间进行控制。利用这种方法制备的一维ZnO纳米结构在传感器、太阳能电池及光催化领域有潜在的应用价值。     

ZnO纳米线二极管发光器件制备及特性研究

运用液相法生长成ZnO纳米线薄膜,并利用肖特基型异质结的发光原理,构造成功肖特基型ZnO纳米线二极管发光器件.在大于6V直流电压驱动下,观察到近紫外波段392nm处和可见光波段525nm的发射谱带.从单向导电特性及ZnO纳米线材料的能带结构等方面探讨了该种器件的电致发光机理. 关键词： ZnO纳米线 肖特基二极管 电致发光