Xiaochuan Hu

冷大气压等离子体（cold atmospheric plasma,CAP）在癌症治疗中展现出重要潜力，但其诱导的脂质过氧化在电穿孔形成前后对活性氧物种（reactive oxygen species,ROS）跨膜行为的微观影响尚不明确.本文采用分子动力学模拟方法，研究脂质过氧化对膜结构、电穿孔形成及ROS分布和自由能剖面的影响.结果表明，脂质过氧化会增大单脂质面积、降低膜厚度，并缩短细胞膜电穿孔形成时间；在0.7 V/nm电场下，40%过氧化膜的平均穿孔时间较原生膜缩短约51%.在未穿孔膜中，脂质过氧化虽会改变亲水性ROS在水-膜界面附近的分布，但仍不足以使其进入膜中心；电穿孔形成的水通道为其进入膜中心提供了途径，脂质过氧化会进一步调节ROS的孔内分布.在非反应性模拟条件下，HO₂在高过氧化穿孔膜的中心区域表现出较高密度分布，并伴随HO₂-POPCOOH氢键增加和局部自由能势阱加深.上述结果揭示了脂质过氧化对电穿孔形成及ROS局部行为的阶段性调控作用，为理解CAP相关氧化损伤模型中膜组成变化与穿孔动力学及ROS分布之间的内在联系提供了分子尺度参考.

The interaction of low-energy electrons with solid surfaces is fundamentally governed by atomic-scale surface conditions, critically impacting multipactor discharge in high-power radio-frequency (RF) components. While adsorption-induced secondary electron yield (SEY) variations have been widely explored, concurrent changes in the secondary electron spectrum (SES) remain largely neglected, leading to inaccurate multipactor process predictions. Here, oxygen-free copper samples with controlled adsorption coverage were prepared via in-situ thermal treatment and Ar ⁺ sputter cleaning under ultra-high vacuum. Their SEY and SES were systematically characterized, and adsorption-dependent parameters were extracted using a refined Furman model and incorporated into a particle-in-cell/Monte Carlo framework. The results show that increasing the coverage fraction from 0 to 0.6 raises the maximum SEY from 1.26 to 2.21. Simultaneously, the SES narrows and shifts toward lower energies, with the most probable energy decreasing from 4.4 to 1 eV. The area of the multipactor susceptibility zone increases from 1055.84 to 13 188.32 GHz·mm·V, indicating that the increase in coverage fraction significantly increases the risk of multipactor. These findings clarify surface-mediated multipactor mechanisms and provide quantitative guidance for surface engineering of high-power RF components.

Secondary electron emission (SEE) critically limits the performance of high-power microwave components and particle accelerators. Amorphous carbon (a-C) films featuring a low secondary electron yield (SEY) and surface grooves offer promising applications for suppressing the SEE effects. However, the efficiency of SEE suppression and the underlying microscopic mechanisms remain subjects of debate. This paper employs the Monte Carlo method to develop a novel electron scattering and an SEE model for copper surfaces coated with a-C films and patterned with rectangular grooves. The model accurately simulates electron transport within the film and substrate, explicitly accounting for the influences of the film/substrate interface energy barrier and the groove walls on electron scattering trajectories. Results demonstrate that increasing the a-C film thickness from 0 to 5 nm reduces the maximum SEY from 1.18 to 0.82. The SEE suppression effect is significantly enhanced on grooved a-C surfaces. Specifically, with a groove depth of 30 nm and a film thickness of 5 nm, the maximum SEY reaches a minimum value of 0.47. These findings provide important insights into the mechanism by which grooves and a-C films suppress SEE and offer practical guidance for designing high-performance, low-SEY surfaces for the next generation of high-power devices.

Amorphous carbon films have attracted much attention in the field of abnormal discharge of vacuum microwave devices and equipment due to their extremely low secondary electron yields (SEYs). However, the dynamic process and microscopic mechanism of the effect of amorphous carbon film on secondary electron emission are still poorly understood. In this work, a numerical simulation model of the secondary electron emission of amorphous carbon film on copper surface is developed by the Monte Carlo method, which can accurately simulate the dynamic processes of electron scattering and emission of the film and the substrate. The results show that the maximum SEY decreases by about 20% when the film thickness increases from 0 to 1.5 nm. Further increasing the thickness, the SEY no longer decreases. However, when the film is thicker than 0.9 nm, the SEY curve exhibits a double-hump form, but with the thickness increasing to 3 nm, the second peak gradually weakens or even disappears. The electron scattering trajectories and energy distribution of secondary electrons indicate that this double-hump phenomenon is caused by electron scattering in two different materials. Compared with previous models, the proposed model takes into account the change of work function and the effect of interfacial barrier on electron scattering path. Our model can explain the formation of the double-hump of SEY curve and provides theoretical predictions for suppressing the SEY by amorphous carbon film.

The secondary electron spectrum (SES) contains complex surface information and is widely used in surface analysis technology. A theoretical model of the SES for metals was proposed to reveal the effect of the surface potential barrier on the SES. In our model, an improved inner secondary electron energy distribution was established according to the free electron gas model. The transmission coefficient of inner secondary electrons was determined by solving the density of electronic states. Finally, the model was verified by our experiments, and the effects of the coverage fraction and potential barrier height, width, and form on the SES were comprehensively analyzed. The results show that H2O adsorption lowers the barrier height and enhances the tunneling effect, resulting in the most probable energy shifting toward the low-energy end and full width at half maximum narrowing. In addition, increasing or broadening the barrier can weaken the secondary electron tunneling effect, reduce the secondary electron number, and increase the most probable energy and the full width at half maximum. The model in this paper provides a reliable analysis method for studying the SES under different surface barriers for metals. Relevant results have significant value for the analysis of scanning electron microscopy images and optimization of secondary electron detection systems.

We developed an adsorption model to examine secondary electron (SE) emission from a surface adsorbed by N ₂ molecules. In this model, the scattering processes of electrons and N ₂ molecules are simulated by Monte Carlo method. The traces of both primary electrons (PEs) and SEs are tracked by combining the electron scattering in the material and adsorbed layer. The SE yield (SEY) is then calculated from the final states of electrons that can emit out from the adsorbed layers. Moreover, the effects of the work function variation induced by adsorption are also taken into account in the model. The SE emission properties of the surface under different pressures and surface coverage are examined by our adsorption model. We find that the SEY reaches its minimum for an intermediate coverage of 1 ~ 2 × 10 ¹⁵ /cm ² for the Cu substrate. Further investigations on returning and outgoing ratios and energy distributions of SEs indicate that an adsorbed layer can generate new electrons but weaken the electron energy, especially at a high PE energy. The competing mechanism of ‘SE generation event’ and ‘energy losing event’ controls the variation of SEY. Accordingly, our model could serve as a powerful tool and our results can provide comprehensive insight into the SE emission of the surface covered by gas molecules, thereby helping us to find a rationale for the behaviour of SEs under various surface conditions.

The GaN-based heterostructures and related HEMTs (High Electron Mobility Transistors) were investigated by 1MeV neutrons at fluences up to 10¹⁵ cm⁻², yielding an increase of the densities of screw dislocations and edge dislocations for GaN-based heterostructures. It gave the result that neutron irradiation-induced structural defects into GaN-based materials, and the irradiation-induced dislocations would propagate to the material surface causing surface morphology deterioration. However, the GaN-based material strain was robust to neutrons, and the more initial dislocations, the easier to generate irradiation defects and thus, more strongly affecting the electrical property degradations of materials and devices. Meanwhile, the reduction of the two-dimensional electron gas (2DEG) concentration (n_s) caused by irradiation-induced defects led to the reducing the drain current. Moreover, the significant degradation of the reverse gate leakage current at fluences ranging from 10¹⁴ to 10¹⁵ cm⁻² could be attributed to the irradiation-induced deep defects. The neutron induced damage was more difficult to anneal recovery than other particles, due to the neutron irradiation-induced deep levels and defect complexes such as defect clusters.