Study by EELS and EPES of the stability of InPO4/InP system

The goal of this research is to highlight the effectiveness of associating the spectroscopic methods EELS and EPES in the study of thin film grown on substrates. We use the great sensitivity of the Electron Energy Loss Spectroscopy (EELS) and the Elastic Peak Electron Spectroscopy (EPES) to study native InPO₄ oxide of thin thickness (10 Å) grown on InP by UV/ozone oxidation. By varying the primary energy of the electron beam and the incidence angle, we give interesting results related to the chemical and the physical analyses of InPO₄/InP system. These spectroscopic methods reveal the homogeneity of the chemical composition of InPO₄ on the surface. Furthermore, the electron irradiation of InPO₄/InP leads to the breaking of chemical bonds between the species of InPO₄ and InP to form a new oxide In₂O₃ on the surface. We show that the heating of InPO₄/InP at 450 °C in UHV allows a good reconstruction of the surface with elimination of defects on the surface and at the interface. Thus, the surface becomes more stable to impede all oxidation processes due to the electron beam irradiation even for a time as long as 30 min.     

A study of the charging and dissociation of SiO2 Surfaces by AES

AES is used to determine the initial spectrum of a vacuum-broken SiO₂ surface and to follow its dissociation under the electron beam probe. Both Auger peaks heights and energies are affected by the irradiation. The change in stoichiometry is accompanied by a decrease of the surface charge by 5–8 V. The relation between stoichiometry and charge is explained by the influence of radiation-induced defects on secondary electron emission. The reduction of SiO₂ is characterized in terms of irradiation dose, dissociation cross-section and electron impact efficiency. Resistance to radiation damage is increased by surface carbon contamination. The chemical contribution to the Auger peak energy can be distinguished from the charging effect leading to a shift between element and compound of 12 eV for the silicon peak.     

电介质/半导体结构样品电子束感生电流瞬态特性

电子束照射下电介质/半导体样品的电子束感生电流(electron beam induced current,EBIC)是其电子显微检测的重要手段.结合数值模拟和实验测量,研究了高能电子束辐照下SiO2/Si薄膜的瞬态EBIC特性.基于Rutherford模型和快二次电子模型研究电子的散射过程,基于电流连续性方程计算电荷的输运、俘获和复合过程,获得了电荷分布、EBIC和透射电流瞬态特性以及束能和束流对它们的影响.结果表明,由于电子散射效应,自由电子密度沿入射方向逐渐减小.由于二次电子出射,净电荷密度呈现近表面为正、内部为负的特性,空间电场在表面附近为正而在样品内部为负,导致一些电子输运到基底以及一些出射二次电子返回表面.SiO2与Si界面处俘获电子导致界面附近负电荷密度高于周围区域.随电子束照射样品内部净电荷密度逐渐降低,带电强度减弱.同时,负电荷逐渐向基底输运,EBIC和样品电流逐渐增大,电场强度逐渐减小.由于样品带电强度较弱,表面出射电流和透射电流随照射基本保持恒定.EBIC、透射电流及表面出射电流均随束流呈现近似正比例关系.对于本文SiO2/Si薄膜,透射电流随束能的升高逐渐增大并接近于束流值,EBIC在束能约15 keV时呈现极大值.     

The influence of UV irradiation on the surface of chitosan films

The surface properties of chitosan films before and after UV-irradiation (λ = 254 nm and 248 nm, respectively) were investigated using the technique of scanning electron microscopy (SEM) and by means of contact angle measurements allowing the calculation of surface free energy. Moreover, in order to determine the film mass changes, quartz crystal microbalance (QCM) measurements were performed. Measurements of the contact angle for diiodomethane (D), formamide (F) and glycerol (G) on the surface of chitosan films were made. The chemical and structural changes during UV irradiation were studied by FTIR-ATR spectroscopy.The contact angle and the surface free energy were altered by UV irradiation of chitosan films. The microscopy images have shown that the KrF excimer laser irradiation caused visible damages on the surface in comparison with the surface exposed to the mercury UV lamp. The surface modification of chitosan films can be achieved using both, the low intensity UV lamp and the excimer laser.     

真空中电子辐照下聚四氟乙烯沿面闪络电压特性

为了研究聚四氟乙烯材料(PTFE)在空间粒子环境中放电规律及其影响因素,通过实验获得了高真空低能电子辐照下PTFE高压直流沿面闪络电压,并采用等温电位衰减法测试了PTEE在辐照前及辐照后的陷阱密度,分析了影响PTEE沿面闪络电压的因素。研究结果表明:相比于无辐照时PTFE沿面闪络电压,当辐照电子能量为19~25 keV时,闪络电压明显更高;在电子束流密度不变的情况下,电子能量越高,材料表面正电荷密度越小,陷阱密度与电导率越大,电场畸形程度越小,因此闪络电压升高;当电子能量一定时,束流密度越高,初始电子数量和二次电子数量越多,因此闪络电压降低。     

低能电子束照射接地绝缘薄膜的负带电过程

为揭示低能电子束照射接地绝缘薄膜的负带电过程及其机理,建立了同时考虑电子散射与电子输运的计算模型,综合Monte Carlo方法和有限差分法进行了数值模拟,获得了内部空间电荷、泄漏电流和表面电位随电子束照射的演化规律.结果表明,入射电子因迁移、扩散效应会超越通常的散射区域产生负空间电荷分布,并经过一定的渡越时间后到达接地基板,形成泄漏电流,负带电暂态过程则随着泄漏电流的增加而趋于平衡.在平衡状态下,泄漏电流随电子束能量和电流而增大;薄膜净负电荷量和表面电位随膜厚而增加、随电子迁移率的增大而降低,随着电子束 关键词： 绝缘薄膜 电子束照射 带电效应 数值模拟     

Low-energy electron transmission through dielectric capillaries

Transmission of electrons with energies from 2 to 10 keV through glass polycapillaries, tubes, and cones was experimentally studied. The dependence of the transmittance on the electron energy, beam current, capillary diameter, and irradiation time was measured. The electron transmittance through polycapillaries decreases with the beam current and measurement time. The results of measurements of the electron and X-ray spectrum at the quartz tube output suggest that a fraction of electrons incident on the wall charge it and ionize wall atoms; other electrons pass through the channel without collision with the wall, since their energy is almost unchanged. It follows from the measurement results that the electron transmission through capillaries is controlled by the potential at the inner channel surface induced by the charge injected during irradiation.     

Surface free energy of ultra-high molecular weight polyethylene modified by electron and gamma irradiation

Surface free energy of biocompatible polymers is important factor which affects the surface properties such as wetting, adhesion and biocompatibility. In the present work, the change in the surface free energy of ultra-high molecular weight polyethylene (UHMWPE) samples, which is produced by electron beam and gamma ray irradiation were, investigated. Mechanism of the changes in surface free energy induced by irradiations of doses ranging from 25 to 500 kGy was studied. FTIR technique was applied for sample analysis. Contact angle measurements showed that wettability and surface free energy of samples have increased with increasing the irradiation dose, where the values of droplet contact angle of the samples decrease gradually with increasing the radiation dose. The increase in the wettability and surface free energy of the irradiated samples are attributed to formation of hydrophilic groups on the polymer surface by the oxidation, which apparently occurs by exposure of irradiated samples to the air.     

Surface-induced magnetism of the solids with impurities and vacancies

Using the quantum-mechanical approach combined with the image charge method we calculated the lowest energy levels of the impurities and neutral vacancies with two electrons or holes located in the vicinity of flat surface of different solids. Unexpectedly we obtained that the magnetic triplet state is the ground state of the impurities and neutral vacancies in the vicinity of surface, while the nonmagnetic singlet is the ground state in the bulk, for e.g. He atom, Li⁺, Be⁺⁺ ions, etc. The energy difference between the lowest triplet and singlet states strongly depends on the electron (hole) effective mass μ, dielectric permittivity of the solid ε₂ and the distance from the surface z₀. For z₀=0 and defect charge ∣Z∣=2 the energy difference is more than several hundreds of Kelvins at μ=(0.5−1)m_e and ε₂=2-10, more than several tens of Kelvins at μ=(0.1−0.2)m_e and ε₂=5-10, and not more than several Kelvins at μ<0.1m_e and ε₂>15 (m_e is the mass of a free electron). Pair interaction of the identical surface defects (two doubly charged impurities or vacancies with two electrons or holes) reveals the ferromagnetic spin state with the maximal exchange energy at the definite distance between the defects (∼5-25 nm). We estimated the critical concentration of surface defects and transition temperature of ferromagnetic long-range order appearance in the framework of percolation and mean field theories, and RKKY approach for semiconductors like ZnO. We obtained that the nonmagnetic singlet state is the lowest one for a molecule with two electrons formed by a pair of identical surface impurities (like surface hydrogen), while its next state with deep enough negative energy minimum is the magnetic triplet. The metastable magnetic triplet state appeared for such molecule at the surface indicates the possibility of metastable ortho-states of the hydrogen-like molecules, while they are absent in the bulk of material. The two series of spectral lines are expected due to the coexistence of ortho- and para-states of the molecules at the surface. We hope that obtained results could provide an alternative mechanism of the room temperature ferromagnetism observed in TiO₂, HfO₂, and In₂O₃ thin films with contribution of the oxygen vacancies. We expect that both anion and cation vacancies near the flat surface act as magnetic defects because of their triplet ground state and Hund's rule. The theoretical forecasts are waiting for experimental justification allowing for the number of the defects in the vicinity of surface is much larger than in the bulk of as-grown samples.     

Photoconductivity and photoemission studies of diamond irradiated by ultrashort VUV pulses

We investigated relaxation of free charge carriers in pure crystalline diamond exposed to VUV irradiation of high order harmonics of femtosecond Ti:Sa laser in the spectral range 17–32 eV. Electron–hole pairs, possessing a significant kinetic energy, are generated in the material via direct interband transitions, relaxation of which is monitored by means of induced conductivity in the bulk and photoemission from the surface of the material. The experimental data provided by these complementary techniques are compared and discussed in terms of the competition between ionization and conductivity looking for evidences of multiplication of free charge carriers due to impact ionization. PACS 42.65.Re; 72.20.Jv; 72.40.+w; 79.60.-i     

Analysis of avalanche mechanisms in short-pulses laser-induced damage

A theory based on the rate equation of free electron is used to analyze the process of free electron multiplication in optical materials under the laser irradiation, specially researching on the effect of avalanche ionization on the electron multiplication and material damage threshold. Numerical investigation with SiO₂ is processed using this theoretical model, and damage thresholds under different avalanche models are analyzed. The result shows that during research on the energy charge between electron and electromagnetic field, the probability of avalanche ionization should be considered. Under this assumption, the numerical threshold gets well with the experimental result.     

Image contrast enhancement in field-emission scanning electron microscopy of single-walled carbon nanotubes

The image contrast enhancement in scanning electron microscopy of single-walled carbon nanotubes (SWNTs) on SiO₂ surfaces was experimentally investigated using a field-emission scanning electron microscope (FESEM) using a wide range of primary electron (PE) voltages. SWNT images of different contrasts were obtained at different PE voltages. Image contrast enhancement of SWNTs was investigated by charging SiO₂ surfaces at different PE voltages. The phenomena are ascribed to the surface potential difference and charge injection between SWNTs and SiO₂ substrates induced by the electron-beam irradiation.     

Observing the effect of water vapor on post-irradiated surface morphology of SiO2 and Si3N4 insulators by atomic force microscopy

In this work, we investigated the effect of water-vapor treatment on the surface morphology of SiO₂ and Si₃N₄ insulators before and after Co⁶⁰ gamma-ray irradiation by using the atomic force microscopy (AFM) operated under non-contact mode. Before irradiation, no apparent surface morphology change was found in SiO₂ samples even they were water vapor treated. However, bright spots were found on post-irradiated water-vapor-treated SiO₂ sample surfaces but not on those without water-vapor treatment. We attributed the bright spots to the negative charge accumulation in the oxide due to charge balancing between hydroxyl (OH⁻) ions adsorbed on SiO₂ surface and electron-hole pairs (ehps) generated during irradiation since they can be annealed out after low temperature annealing process. On the contrary, no bright spots were observed on post-irradiated Si₃N₄ samples with and without water-vapor treatment. This result confirms that Si₃N₄ is a better water-resist passivation layer than SiO₂ layer.     

高能电子辐照绝缘厚样品的表面电位动态特性

采用数值计算和实验测量相结合的方法, 阐明了高能电子束照射下绝缘厚样品的表面电位和电子产额动态特性. 结果表明: 由于电子在样品内部的散射和输运, 沿着深度方向, 空间电位先缓慢下降到最小值, 然后逐渐升高并趋近于零; 随着电子束照射, 样品的表面电位逐渐下降, 可至负千伏量级, 电子总产额逐渐增大至一个接近于1的稳定值; 电子束停止照射后, 长时间放置下, 表面电位将逐渐升高, 但带电并不会消除; 表面电位随电子束能量的升高近似线性下降, 随入射角的增大而升高, 而随样品厚度的增大仅略有下降.     

Mechanism of electron multiplication due to charging for a SiO2 sample with a buried microstructure in SEM: A simulation analysis

This study investigates the mechanism of electron redistribution and multiplication for a SiO₂ sample with a buried structure in scanning electron microscopy by numerical simulation. The simulation involved electron scattering and internal charge transport in the sample, the tracking of emitted secondary electrons (SEs), and the generation of tertiary electrons (TEs) produced by returned SEs due to charging of the sample. The results show that a buried grounded structure causes a non-uniform distribution of surface potential, and an electric field above the surface. As a result, although the number of escaped SEs above the margin of the buried structure decreases, the number of generated TEs increases more, leading to a final current of electrons that include escaped SEs and increased TEs. This multiplication of SEs might make a crucial contribution to the abnormal negative-charging contrast in SEM. During the electron beam irradiation, the variation in the number of total escaped electrons presents an obvious increase after an initial slight decrease, which corresponded to the transient characteristics of gray levels in SEM images from dark to abnormally bright.     

Analysis of microscopic parameters of surface charging in polymer caused by defocused electron beam irradiation

The relationship between microscopic parameters and polymer charging caused by defocused electron beam irradiation is investigated using a dynamic scattering-transport model. The dynamic charging process of an irradiated polymer using a defocused 30 keV electron beam is conducted. In this study, the space charge distribution with a 30 keV non-penetrating e-beam is negative and supported by some existing experimental data. The internal potential is negative, but relatively high near the surface, and it decreases to a maximum negative value at z = 6 μm and finally tend to 0 at the bottom of film. The leakage current and the surface potential behave similarly, and the secondary electron and leakage currents follow the charging equilibrium condition. The surface potential decreases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. The total charge density increases with increasing beam current density, trap concentration, capture cross section, film thickness and electron–hole recombination rate, but with decreasing electron mobility and electron energy. This study shows a comprehensive analysis of microscopic factors of surface charging characteristics in an electron-based surface microscopy and analysis.     

Surface energy of M2AC(0 0 0 1) determined by density functional theory (M = Ti, V, Cr; A = Al, Ga, Ge)

We have studied the correlation between the valence electron configuration and the electronic structure of M₂AC(0 0 0 1) surfaces (M = Ti, V, Cr; A = Al, Ga, Ge) by density functional theory. The A surface termination is the most stable configuration for all systems studied according to our surface energy data. As the M valence electron population is increased, the surface energy increases by 22% and 12% for A = Al and Ga, respectively, while it decreases by 29% for A = Ge. This can be understood by evaluating the valence electron concentration induced changes in the surface density of states. Antibonding surface Md-Ap states are present as Ti is substituted by Cr in M₂AC(0 0 0 1) for A = Al and Ga, while antibonding surface Md-Ap states are not present as Ti is substituted by Cr in M₂GeC(0 0 0 1).     

Phase change of CoTi and Co3Ti induced by MeV-scale electron irradiation

This study investigates equilibrium-to-nonequilibrium solid phase transitions induced by MeV-scale electron irradiation in B2-CoTi and L1₂-Co₃Ti intermetallic compounds by means of high-voltage electron microscopy. Under MeV-scale electron irradiation, B2-CoTi transforms into a body-centered cubic solid solution through chemical disordering and eventually transforms into an amorphous phase. The critical temperature for amorphisation is found to be 110 K. L1₂-Co₃Ti also exhibits chemical disordering at temperatures below 700 K. However, its amorphisation does not occur even at a low temperature of 20 K. The dominant factor in these solid phase transitions is discussed in terms of the Gibbs free energy.     

Microscopic theory of some surface effects in a dense electron gas

We consider interacting electrons confined to a slab of finite thickness or a half space in the random phase approximation. The electronic charge is assumed to be neutralized by a homogeneous positive background. A linear response function is introduced from which it is possible to calculate the induced density variation of the electron gas, caused by an external field. The linear response function also determines the Helm-holtz free energy and, by a sum rule, the density of the undisturbed system. The interaction of a classical point charge with the electrons of a half-space is discussed. In addition, a formula for the surface energy is given.     

Nanotitania-coated multi-walled carbon nanotube composite by facile colloidal processing route for photocatalytic applications

Multi-walled carbon nanotubes (MWCNTs) and titanium dioxide nanocomposites (MWCNTs/TiO₂) were fabricated by a simple novel colloidal processing route and tested as a photocatalyst for degradation of methylene blue under UV irradiation. The novel idea behind this work is to make MWCNTs and TiO₂ nanoparticle suspensions separately highly oppositely charged and utilize the electrostatic force of attraction between two entities to deposit nanotitania onto MWCNTs surface. Particle charge detector, scanning electron microscopy, transmission electron microscope, energy dispersive X-rays, X-rays diffraction (XRD), and Raman spectroscopy were used to characterize the composite. XRD and Raman spectroscopic analysis showed the crystalline structure of deposited TiO₂ over MWCNTs surface structure as anatase phase. It was found that MWCNTs/TiO₂ composite structure have much higher photocatalytic activity compared to TiO₂ nanoparticles. The composite material developed may find potential applications in the degradation of organic pollutants in aqueous medium under UV irradiation.