Secondary electron emission from single-walled carbon nanotubes

Heat removal in metal structures is often the limiting factor in high power, high frequency microwave tube devices. As higher critical heat flux cooling techniques are found, the operating power of the microwave device may be increased if other limiting factors are not of importance. Larger output powers may be realized for brief periods in time at the expense of operating in a pulsed power and/or pulsed cooling mode. During these brief periods, temporary temperature fluctuations on the walls of the microwave tube will be realized. It is hypothesized that the temperature fluctuations and to a much lesser extent temperature gradients may influence secondary electron emission resulting from primary electrons colliding with the microwave wall. Low energy primary electrons, that penetrate the microwave wall, experience the collective effects of the electrons (plasmons) in the conduction band of the metal. It is anticipated that the surface and bulk temperature of the metal wall influences the ability for plasmons to interact with the primary electron in the scattering process. Further, aging effects resulting from heat stress cycling of the metal cavity may alter the state of the cavity walls leading to changes in the secondary electron emission properties. Pulsed, low energy (1 keV), low beam currents are used to examine collectively the change in the spatial distribution profile of secondary electrons as the sample cools without significantly modifying or stressing the surface of the sample under test. SEE distribution profiles will be presented for a metal target at room temperature and in transient cooling mode with peak temperatures on the order of 390° K.

This paper is an attempt to analyse most of the complicated mechanisms involved in charging and discharging of insulators investigated by scanning electron microscopy (SEM). Fundamental concepts on the secondary electron emission (SEE) yield from insulators combined with electrostatics arguments permit to reconsider, first, the widespread opinion following which charging is minimised when the incident beam energy E0 is chosen to be equal to the critical energy E(o)2, where the nominal total yield delta(o) + eta(o) = 1. For bare insulators submitted to a defocused irradiation, it is suggested here that the critical energy under permanent irradiation EC2 corresponds to a range of primary electrons, R, and nearly equals the maximum escape depth of the secondary electrons, r. This suggestion is supported by a comparison between published data of the SEE yield delta(o) of insulators (short pulse experiments) and experimental results obtained from a permanent irradiation for EC2. New SEE effects are also predicted at the early beginning of irradiation when finely focused probes are used. Practical considerations are also developed, with specific attention given to the role of a contamination layer where a negative charging may occur at any beam energy. The role of the various time constants involved in charging and discharging is also investigated, with special attention given to the dielectric time constant, which explains the dose rate-dependent effects on the effective landing energy in the steady state. Numerical applications permit to give orders of magnitude of various effects, and several other practical consequences are deduced and illustrated. Some new mechanisms for the contrast reversal during irradiation or with the change of the primary electron (PE) energy are also suggested.

The plasma-wall interaction in the presence of strong secondary electron or thermionic emission has been studied theoretically and experimentally both as a basic phenomenon and in relation to numerous plasma applications such as, for example, fusion devices and plasma propulsion. For Hall thrusters, existing fluid models predict that secondary electron emission (SEE) is strong enough to enhance electron energy losses at the walls. According to the kinetic simulations, the electron velocity distribution function in a collisionless thruster plasma is non-Maxwellian, anisotropic, and features beams of secondary electrons emitted from the walls. Under such conditions, the effects of SEE on the plasma can be substantially weaker than predicted by the fluid models. This talk will review previous and recent experimental results, including probe measurements in the Hall thruster with various wall materials, which support the predictions of the kinetic studies. It is also shown that the wall material properties affect the electron cross-field transport in the thruster discharge. We will also discuss how these results can help in implementation of highly efficient and stable plasma regimes of the Hall thrusters.

Summary form only given. Secondary electron emission (SEE) can potentially lead to beam instabilities, material degradation and r.f. breakdown. The past SEE studies have deduced secondary electron yield by invoking a conservation of charge without the need to experimentally account for all measurables. In the spatial distribution studies of SEE, one may not invoke conservation of charge without some knowledge leading to wall loss effects. An electron gun in the SEE test stand at UNLV directs the primary electron beam through a beam drift tube piercing the center of a particle position detector with controlling grid towards the sample under test. Secondary electrons emitted by the sample either drift freely or are drawn by a grid potential to the particle position detector. The location and number of electrons impinging on the detector records the number of electrons and the locations of electron impact. Low primary beam currents are necessary in order to perform these measurements within the resolution of the detector. Because the detector surface is flat and finite in dimension, some loss to the chamber walls will result. Consequential SEE loss to the walls is not measurable in a noise environment even if the experimental apparatus (the SEE test stand) is isolated from the earth ground. To monitor the primary beam current, a C shaped ferromagnetic core coil fitted to the beam drift tube was designed to divert the primary beam to the beam drift tube for measurement with minimal SEE loss from the tube. Assuming successive shots are similar, the total number of incident charge can be deduced statistically. The coil current is then changed to partially null the Earth's magnetic field when experiments are performed. Beam drift tube, grid, and sample currents are monitored and integrated to yield the number of charges collected by each component over time. Along with time resolved, spatially distributed, SEE count from the detector, wall loss may be deduced by invoking conservation of charge. Spatial distributions of secondary electron yield and secondary' electron emission wall loss counts will be reported. Such distributions offer insights in determining the initial conditions on the emitted secondary electrons.

Secondary electron emission (SEE) coefficients of alumina ceramics with three different surface finishes have been measured using a scanning electron microscope with a single-pulse electron beam (100 pA, 1 ms). SEE coefficients of those aluminas with annealing process became lower after mechanical grinding operations even though its average roughness was almost same as those of as-sintered ones. SEE coefficients of mirror-finished samples were the smallest among the samples. Changes of SEE coefficient with incident angle of primary electrons for smooth and rough surfaces are also discussed.

This study aims to understand how secondary electron emissions influence the behaviour of dusty plasma sheaths. The analysis is carried out within the framework of a non-extensive Tsallis distribution in the presence of dynamically charged dust particles and an oblique external magnetic field. Two main types of secondary emission are considered: those emitted by the dynamically charged dust grains, which are treated using the Sternglass model, and those emitted from the wall bounding the sheath. These are accelerated within the sheath by its electric potential, and modeled using a half-Maxwellian distribution. The objective of this study is to improve our understanding of the mechanisms that govern complex plasma sheaths. It is observed that the combined effect of these secondary emissions significantly alters the properties of the sheath, particularly the dynamics of the dust, the sheath potential, and the density distribution of charged species. This influence is particularly pronounced at lower values of the non-extensive parameter q of the primary electrons.

Secondary electron emission (SEE) induced by the positive ion is an essential physical process to influence the dynamics of gas discharge which relies on the specific surface material. Surface charging has a significant impact on the material properties, thereby affecting the SEE in the plasma-surface interactions. However, it does not attract enough attention in the previous studies. In this paper, SEE dependent on the charged surface of specific materials is described with the computational method combining a density functional theory (DFT) model from the first-principle theory and the theory of Auger neutralization. The effect of κ-Al2O3 surface charge, as an example, on the ion-induced secondary electron emission coefficient (SEEC) is investigated by analyzing the defect energy level and band structure on the charged surface. Simulation results indicate that, with the surface charge from negative to positive, the SEEC of a part of low ionization energy ions (such as E i = 12.6 eV) increases first and then decreases, exhibiting a nonlinear changing trend. This is quite different from the monotonic decreasing tendency observed in the previous model which simplifies the electronic structure. This irregular increase of the SEEC can be attributed to the lower escaped probability of orbital energy. The results further illustrate that the excessive charge could cause the bottom of the conduction band close to the valence band, thus leading to the decrease of the orbital energy occupied by the excited electrons. The nonlinear change of SEEC demonstrates a more realistic situation of how the electronic structure of material surface influences the SEE process. This work provides an accurate method of calculating SEEC from specific materials, which is urgent in widespread physical scenarios sensitive to surface materials, such as increasingly growing practical applications concerning plasma-surface interactions.

Microwave (mw) breakdown of atmospheric pressure microgaps is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions numerical model. The effect of both field electron emission and secondary electron emission (due to electron impact, ion impact, and primary electron reflection) from surfaces on the breakdown process is considered. For conditions where field emission is the dominant electron emission mechanism from the electrode surfaces, it is found that the breakdown voltage of mw microdischarge coincides with the breakdown voltage of direct-current (dc) microdischarge. When microdischarge properties are controlled by both field and secondary electron emission, breakdown voltage of mw microdischarge exceeds that of dc microdischarge. When microdischarge is controlled only by secondary electron emission, breakdown voltage of mw microdischarge is smaller than that of dc microdischarge. It is shown that if the interelectrode gap exceeds some critical value, mw microdischarge can be ignited only by electrons initially seeded within the gap volume. In addition, the influence of electron reflection and secondary emission due to electron impact is studied.

Structural modification of MgO/Au thin films by aluminum Co-doping and related studies on secondary electron emission

Secondary electron emission (SEE), a common physical phenomenon, exhibits both prominent applications in various fields and concomitant adverse impacts. Secondary electron yield (SEY) is a parameter for characterizing SEE, which determines the effect of the application and is very sensitive to the surroundings, particularly the magnetic field. Previous studies mainly focused on the influence of the magnetic field on secondary electron trajectories rather than the SEY itself. In this work, a specialized experimental platform was established to measure SEY, through which the influences caused by primary electrons and secondary electron trajectories were eliminated. The research findings demonstrate that as the magnetic field intensity increases, the SEY decreases rapidly and then gradually stabilizes. Meanwhile, a self-developed Monte Carlo simulation program was utilized to verify the above results, and it was concluded that the influence of magnetic field on SEY stems from the increased number of collisions of secondary electrons within the material. These results possess significant guiding significance and reference value for certain application scenarios where SEY suppression is required as well as for the design of multiplier devices aiming to enhance the output by utilizing magnetic fields.

On the analysis of secondary electron emission spectra

Hybrid nanomaterials with unique physiochemical properties have received a lot of attention, making them attractive for application in different fields like cancer treatment. This study was designed to investigate the combined effects of single-walled carbon nanotubes (SWCNTs) hybridized with silver titanium dioxide composite (SWCNTs@Ag–TiO 2 ). Transmission electron microscopy and field emission scanning electron microscopy images demonstrated the accumulation of SWCNTs with Ag–TiO 2 due to an increased main grain size with functionalization to 40 nm. The D and G bands in SWCNTs @Ag–TiO 2 shifted to 1,366 and 1,534 cm −1 , respectively. SWCNTs@Ag-TiO 2 were assessed for their cytotoxicity and autophagy induction in liver cancer cells (Hep-G2) using the lactate dehydrogenase assay, MTT assay, and flow cytometry methods. The results showed that SWCNTs and SWCNTs@Ag–TiO 2 exhibited strong anti-cancer activity in vitro against Hep-G2 cells by inducing apoptosis and autophagy in liver cancer cells via controlling the AKT and JNK mitogen-activated protein kinase pathways. The results show that SWCNTs and SWCNTs coated with silver/titanium dioxide (SWCNTs@Ag–TiO 2 ) reduce the cells’ viability and proliferation. It was shown that an excessive amount of reactive oxygen species was a crucial mediator of both the cell death caused by SWCNTs and the cell death caused by SWCNTs combined with Ag–TiO 2 . Based on these findings, it appears that SWCNTs and SWCNTs@Ag–TiO 2 have the potential to be developed as nanotherapeutics for the treatment of liver cancer cells.

Enhanced secondary electron emission properties of sputter-deposited MgO–Au composite film via Cr doping

Particle-in-cell/Monte Carlo collision (PIC/MCC) simulations are performed to investigate the asymmetric secondary electron emission (SEE) effects when electrons strike two different material electrodes in low pressure capacitively coupled plasmas (CCPs). To describe the electron-surface interactions, a realistic model, considering the primary electron impact energy and angle, as well as the corresponding surface property-dependent secondary electron yields, is employed in PIC/MCC simulations. In this model, three kinds of electrons emitted from the surface are considered: (i) elastically reflected electrons, (ii) inelastically backscattered electrons, and (iii) electron induced secondary electrons (SEs, i.e., δ-electrons). Here, we examined the effects of electron-surface interactions on the ionization dynamics and plasma characteristics of an argon discharge. The discharge is driven by a voltage source of 13.56 MHz with amplitudes in the range of 200–2000 V. The grounded electrode material is copper (Cu) for all cases, while the powered electrode material is either Cu or silicon dioxide (SiO2). The simulations reveal that the electron impact-induced SEE is an essential process at low pressures, especially at high voltages. Different electrode materials result in an asymmetric response of SEE. Depending on the instantaneous local sheath potential and the phase of the SEE, these SEs either are reflected by the opposite sheath or strike the electrode surface, where they can induce δ-electrons upon their residual energies. It is shown that highly energetic δ-electrons contribute significantly to the ionization rate and a self-bias forms when the powered electrode material is assumed to be made of SiO2. Complex dynamics is observed due to the multiple electron-surface interaction processes and asymmetric yields of SEs in CCPs.

Secondary ion emission from dielectric films as a function of primary ion velocity