Positron annihilation studies of mesoporous silica films using a slow positron beam

Positron annihilation lifetime spectra were measured for mesoporous silica films, which were synthesized using triblock copolymer (EO₁₀₆PO₇₀EO₁₀₆) as a structure-directing agent. Different positron lifetime spectra for the deposited and calcined films indicated the formation of meso-structure after calcination, which was confirmed by Fourier transform infrared (FTIR) spectra and field emission-scanning electron microscopy (FE-SEM) observation. Open porosity or pore interconnectivity of a silica film might be evaluated by a two-dimensional positron annihilation lifetime spectrum of an uncapped film. Pore sizes and their distributions in the silica films were found to be affected by thermal treatments.     

Mesoporous titania films investigated by positron annihilation based on a slow positron beam

Tunable mesoporous titania (TiO₂) thin films were synthesized via a sol-gel method using an amphiphilic triblock copolymer F38 as the structural template. The dependence of crystalization, pore morphology and interconnectivity of TiO₂ films on the weight ratio of F38 was studied by wide-angle X-ray diffraction, field emission scanning electron microscopy and Doppler broadening of positron annihilation radiation spectroscopy based on a slow positron beam. By loading more F38, the crystallization of TiO₂ films is enhanced, accompanied by a decrement in oxygen vacancies/grain boundaries. Smaller and isolated mesopores are formed in the films prepared with F38 less than 15?wt%. The pore percolation occurs when the weight ratio of F38 is up to 20?wt% and larger and interconnected worm-like pores are formed.     

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.     

Effect of vacancy charge state on positron annihilation in silicon

The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseu- dopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simu- lation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.     

多孔硅薄层的正电子湮灭寿命研究

用阳极氧化法按不同腐蚀条件制备的多孔硅，其薄层厚度仅为几十纳米，远小于^22Na源的正电子平均射程．文章提出了一种用控Na的正电子测量体寿命谱并扣除基片贡献的方法，来确定多孔硅薄层的平均孔径。     

Design of a new type positron beam system

A new type positron beam system is being constructed in Wuhan university. The goal of this project is to build a positron beam which can measure positron lifetimes and has high moderation efficiency. The system utilizes a magnetically guided incident positron beam and the sample is biased to a high negative potential to achieve the desired implantation energies. A conventional tungsten moderator is replaced by a solid Ne moderator with high moderation efficiency (about 1%). A multi-functional target chamber for slow beam studies is designed, which can be used for positron annihilation lifetime spectroscopy (PALS), Doppler broadening (DB) and coincidence Doppler broadening (CDB) measurements.     

水蒸气退火多孔硅发光性能的正电子谱学研究

使用正电子湮没寿命谱和正电子寿命-动量关联谱对水蒸气和真空条件下退火的多孔硅样品的微观缺陷结构进行表征,结合发射光谱测量结果,对影响多孔硅发光性能的因素进行了讨论.实验结果表明,水蒸气退火后样品孔壁表面的悬挂键减少,并出现新的E′γ和EX类缺陷.水蒸气退火后样品中两种缺陷数量发生变化是导致多孔硅样品发光增强的直接原因;真空退火未使样品中发光相关缺陷发生变化,样品的发光性能没有显著改变.     

Microstructure characterization of porous silicon as studied by positron annihilation measurements at low temperatures and high vacuum

Atomic scale properties of thin porous silicon (PSi) layers, characterized by the formation of positronium, are investigated using positron annihilation lifetime spectroscopy in the temperature range 20-300 K under 10⁻⁷ Torr vacuum. The longest orthopositronium as well as the shortest parapositronium components are found to have quite low intensities in the thin layer at room temperature. It is also found that at temperatures ≤240 K, these two components do not show up in the spectrum. The reason for this absence of the longest lifetime component is suggested.     

质子辐照空间级硅橡胶的正电子淹没寿命谱研究

 用正电子淹没寿命谱方法（PALS）研究了质子辐照对空间级硅橡胶KH-L-Y微观结构的影响。试验结果表明，PALS谱所揭示的最长寿命成分的t₃, I₃及自由体积分数Vf随辐照剂量的增加开始明显下降；而当辐照剂量大于10¹⁵cm^-2后，随剂量的增加平缓上升。辐照剂量小于10¹⁵cm^-2时，质子辐照使硅橡胶自由体积减小，分子链间堆砌紧密；辐照剂量大于10¹⁵cm^-2时，质子辐照使硅橡胶自由体积增大。交联密度及DMA测试结果同样表明，质子辐照在剂量较小时硅橡胶的交联密度及玻璃化转变温度增加，辐照以交联效应为主；而剂量较大时辐照降解占优势。     

Slow positron beam study of nitrogen implanted CZ-Si subjected to rapid thermal processing

In this experiment, nitrogen ions were implanted into CZ-silicon wafer at 100 keV at room temperature with the fluence of 5 × 10¹⁵ N₂⁺/cm², followed by rapid thermal processing (RTP) at different temperatures. The single detector Doppler broadening and coincidence Doppler broadening measurements on slow positron beam were carried out to characterize the defects in the as-implanted silicon and RTP-treated samples. It is found that both nitrogen-vacancy complexes (N-Vsi) and oxygen-vacancy complexes (O-Vsi) produced by nitrogen implantation diffuse back to the sample surface upon annealing. But the N-Vsi and the O-Vsi complete with each other and give a summed effect on positron annihilation characteristics. It is shown that the N-Vsi win out the O-Vsi in as-implanted sample and by RTP at 650 °C, 750 °C, which make the S-parameter increase; O-Vsi plays a dominant role after annealing above 850 °C, which makes the S parameter decrease.     

Computational studies of the effect of hydrogen on the thermalized positron state in amorphous silicon

The two-component density functional theory is applied to the study of the thermalized positron state in simulated structures of hydrogenated amorphous silicon. Results show that positron properties in bulk and ad hoc defect structures are sensitive to the defect free-volume. Using the normalized positron density, it is determined that the thermalized positron state is weakly localized at hydrogen-decorated vacancy-like complexes, and not at microscopic open volume defects. These defect complexes form as clusters of hydrogen-passivated dangling bonds. It is also found that hydrogen enhances the delocalization of positron density in the simulated structures. The relevance of the present results to the interpretation of actual positron lifetime spectroscopy in real materials is discussed.     

Effect of rare earth substitutions on some physical properties of Mn-Zn ferrite studied by positron annihilation lifetime spectroscopy

Mn-Zn ferrite substituted with rare earth ions have been investigated by means of positron annihilation lifetime spectroscopy (PALS). The variations of lifetime parameters τ_av, I₂, and κ with ionic radius of rare earth ions, grain size and electrical resistivity for all samples have been studied. The intergranular pores increase with increasing the ionic radius of rare earth ions and grain size of the samples.     

A comparative study of electron and positron penetration in silicon dioxide

Many researches are devoted to the study of silicon dioxide, a material of great interest for its use in the micro-electronics industry. This paper aims to compare the behavior of electrons and positrons when impinging on silicon dioxide targets in order to investigate the differences and the similarities. In particular, the inelastic mean free path, the stopping power, the differential elastic scattering cross-section and the total and transport elastic scattering cross-section of electrons and positrons penetrating in silicon dioxide targets are compared in order to better understand their influence in determining the implantation profiles shapes, the mean range of penetration, the maximum range of penetration and the backscattering coefficient as a function of the primary energy of the incident particles.     

镍掺杂硅纳米线电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理计算,对镍掺杂硅纳米线的结构稳定性、电子与光学性质进行了研究.结果表明:Ni容易占据硅纳米线表面的替代位置.镍掺杂后的硅纳米线引入了杂质能级,杂质能级主要来源于Ni的3d电子的贡献.由于Ni的3d态和Si的3p态的耦合作用,使禁带宽度变窄.掺杂后的硅纳米线在低能区出现了一个较强的吸收峰,且吸收带出现宽化现象. 关键词： 硅纳米线 掺杂 电子结构 光学性质     

A comparison study on the electronic structures,lattice dynamics and thermoelectric properties of bulk silicon and silicon nanotubes

In order to investigate the mechanism of the electron and phonon transport in a silicon nanotube(SiNT),the electronic structures,the lattice dynamics,and the thermoelectric properties of bulk silicon(bulk Si)and a SiNT have been calculated in this work using density functional theory and Boltzmann transport theory.Our results suggest that the thermal conductivity of a SiNT is reduced by a factor of 1,while its electrical conductivity is improved significantly,although the Seebeck coefficient is increased slightly as compared to those of the bulk Si.As a consequence,the figure of merit(ZT)of a SiNT at 1200 K is enhanced by 12 times from 0.08 for bulk Si to 1.10.The large enhancement in electrical conductivity originates from the largely increased density of states at the Fermi energy level and the obviously narrowed band gap.The significant reduction in thermal conductivity is ascribed to the remarkably suppressed phonon thermal conductivity caused by a weakened covalent bonding,a decreased phonon density of states,a reduced phonon vibration frequency,as well as a shortened mean free path of phonons.The other factors influencing the thermoelectric properties have also been studied from the perspective of electronic structures and lattice dynamics.     

中频磁脉冲处理Fe52Co34HfTB6Cu1非晶合金的正电子湮没研究

对熔体急冷法制备的非晶合金Fe52Co34HfTB6Cu1进行了不同频率的中频磁脉冲处理,用透射电子显微镜、穆斯堡尔谱、正电子湮没寿命谱等方法研究了处理前后试样的微观结构及结构缺陷变化．结果表明,经中频磁脉冲处理后,样品发生了部分纳米晶化,晶化量随磁脉冲频率增加而增加,当磁脉冲频率为2000Hz时,晶化量达33．1％：在淬态非晶样品中,正电子在类单空位中的湮没寿命t1为150．5ps,强度,1为77．7％,在微孔洞中的湮没寿命仡为349．7ps,强度I2为22．3％;随磁脉冲频率的增加,t1,t2值呈现减小的变化趋势,与淬态非晶相比,I1有所增加,如下降,t1,t2的平均值亍大幅降低．     

Surface investigation of Si(1 0 0), Cu, Cu on Si(1 0 0), and Au on Cu with positron annihilation induced Auger-electron spectroscopy

The surfaces of polycrystalline Cu, Au-coated Cu, Si(1 0 0) and of Si(1 0 0) coated with 1.5 monolayer Cu were investigated with positron annihilation induced Auger-electron spectroscopy (PAES). Since the electron background has been reduced considerably we observed the Cu M_2,3VV-Auger transition on a copper surface within only three hours which is the shortest acquisition time reported so far for PAES. In order to demonstrate PAES’ high potential the Auger-yield, the signal-to-background ratio as well as the surface selectivity were compared with accompanying EAES-measurements quantitatively. A more efficient electron energy analyzer for the present PAES setup would lead to an additional efficiency gain of more than two orders of magnitude. The presented measurements were performed at the low-energy positron beam of high intensity NEPOMUC at the research reactor FRM II.     

不同浓度Er掺杂Si纳米晶粒电子结构和光学性质的第一性原理研究

利用基于密度泛函理论的第一性原理,对不同浓度Er掺杂Si纳米晶粒的结构稳定性、电子和光学性质进行了研究.结果表明: Si纳米晶粒中Er掺杂浓度越低,结构越稳定;Er掺杂后的Si纳米晶粒引入了杂质能级,导致禁带宽度变窄;掺杂后的Si纳米晶粒在低能区出现了一个较强的吸收峰,随着浓度的降低,吸收峰峰值逐渐减小,甚至消失. 这为Si基发光材料的设计提供了理论依据. 关键词： Si纳米晶粒 掺杂 电子结构 光学性质     

Study on the Fermi surface and the spin-dependent momentum space density of ferromagnetic Gd by positron annihilation experiments

The Fermi surface (FS) and spin-dependent momentum space density distribution of ferromagnetic Gd was studied via longitudinally polarised positrons. The measurements were performed using a 2D angular correlation of the annihilation radiation experiments with the reversal magnetic field direction parallel and anti-parallel to the polarisation direction of the positron. It was found that the minority-spin states were concentrated in the basal plane and majority-spin states were concentrated around the A, L and H points. The analysis confirmed that the main contributions to the FS of Gd were influenced by the mixing of both the 5d–6s and the 4f–5d hybrid bands. The general layout of this FS was observed as two hole-like surfaces running along the [ΓA] axis and one electron-like surface running along the [MK] direction. In general, the experimental results showed good agreement with earlier investigations.     

Vacancy effect on the doping of silicon nanowires:A first-principles study

The influence of vacancy defect on the doping of silicon nanowires is systematically studied by the first-principles calculations. The atomic structures and electronic properties of vacancies and vacancy–boron(vacancy–phosphor) complexes in H-passivated silicon nanowire with a diameter of 2.3 nm are explored. The results of geometry optimization indicate that a central vacancy can exist stably, while the vacancy at the edge of the nanowire undergoes a local surface reconstruction, which results in the extradition of the vacancy out of the nanowire. Total-energy calculations indicate that the central vacancy tends to form a vacancy–dopant defect pair. Further analysis shows that n-type doping efficiency is strongly inhibited by the unintentional vacancy defect. In contrast, the vacancy defect has little effect on p-type doping.Our results suggest that the vacancy defect should be avoided during the growth and the fabrication of devices.