Suppression of Secondary Electron Emission by Vertical Graphene Coating on Ni Microcavity Substrate.

Suppression of secondary electron emission (SEE) from metal surfaces is crucial for enhancing the performance of particle accelerators, spacecraft, and vacuum electronic devices. Earlier research has demonstrated that either etching the metal surface to create undulating structures or coating it with materials having low secondary electron yield (SEY) can markedly decrease SEE. However, the effectiveness of growing vertical graphene (VG) on laser-etched metal surfaces in suppressing SEE remains uncertain. This study examined the collective impact of these methods by applying nanoscale arrays of VG coating using plasma-enhanced chemical vapor deposition on Ni substrates, along with the formation of micrometer-sized microcavity array through laser etching. Comparative tests conducted revealed that the SEY of the samples subjected to VG coating on a microcavity array was lower compared to samples with either only a microcavity array or VG coating alone. Additionally, the crystallinity of VG grown on substrates of varying shapes exhibited variations. This study presents a new method for investigating the suppression of SEE on metal surfaces, contributing to the existing body of knowledge in this field.

Nonequilibrium Dynamics of Electron Emission from Cold and Hot Graphene under Proton Irradiation.

Characteristic properties of secondary electrons emitted from irradiated two-dimensional materials arise from multi-length and multi-time-scale relaxation processes that connect the initial nonequilibrium excited electron distribution with their eventual emission. To understand these processes, which are critical for using secondary electrons as high-resolution thermalization probes, we combine first-principles real-time electron dynamics with irradiation experiments. Our data for cold and hot proton-irradiated graphene show signatures of kinetic and potential emission and generally good agreement for electron yields between experiment and theory. The duration of the emission pulse is about 1.5 fs, which indicates high time resolution when used as a probe. Our newly developed method to predict kinetic energy spectra shows good agreement with electron and ion irradiation experiments and prior models. We find that the lattice temperature significantly increases secondary electron emission, whereas electron temperature has a negligible effect.

Tailoring Organic/Inorganic Interface Trap States of Metal Oxide/Polyimide toward Improved Vacuum Surface Insulation.

High-voltage and high-power devices are indispensable in spacecraft for outer space explorations, whose operations require aerospace materials with adequate vacuum surface insulation performance. Despite persistent attempts to fabricate such materials, current efforts are restricted to trial-and-error methods and a universal design guideline is missing. The present work proposes to improve the vacuum surface insulation by tailoring the surface trap state density and energy level of the metal oxides with varied bandgaps, using coating on a polyimide (PI) substrate, aiming for a more systematical workflow for the insulation material design. First-principle calculations and trap diagnostics are employed to evaluate the material properties and reveal the interplay between trap states and the flashover threshold, supported by dedicated analyses of the flashover voltage, secondary electron emission (SEE) from insulators, and surface charging behaviors. Experimental results suggest that the coated PI (i.e., CuO@PI, SrO@PI, MgO@PI, and Al2O3@PI) can effectively increase the trap density and alter the trap energy levels. Elevated trap density is demonstrated to always yield lower SEE. In addition, increasing shallow trap density accelerates surface charge dissipation, which is favorable for improving surface insulation. CuO@PI exhibits the most remarkable increase in shallow trap density, and accordingly, the highest flashover voltage is 42.5% higher than that of pristine PI. This study reveals the critical role played by surface trap states in flashover mitigation and offers a novel strategy to optimize the surface insulation of materials.

Electron-beam-induced charging of an Al2O3 nanotip studied using off-axis electron holography.

Electrostatic charging of specimens during electron, photon or ion irradiation is a complicated and poorly understood phenomenon, which can affect the acquisition and interpretation of experimental data and alter the functional properties of the constituent materials. It is usually linked to secondary electron emission, but also depends on the geometry and electrical properties of the specimen. Here, we use off-axis electron holography in the transmission electron microscope to study electron-beam-induced charging of an insulating Al2O3 nanotip on a conducting support. The measurements are performed under parallel electron illumination conditions as a function of specimen temperature, electron dose, primary electron energy and surface cleanliness. We observe a lack of reproducibility of charge density measurements after cycling the specimen temperature. Surprisingly, we find both positively and negatively charged regions in closely adjacent parts of the specimen.

First-Principle Calculation on Inelastic Electron Scattering in Diamond and Graphite.

In this work, we consider the inelastic scattering of incident electrons as a key process for analyzing the significant differences in secondary electron (SE) emission between diamond and graphite. Dielectric functions and energy- and momentum-dependent energy loss functions were obtained by first-principle calculations. These were then used to calculate the inelastic mean free path (IMFP) and stopping power in different directions. The results show that the properties of diamond are very close in different directions, and its IMFP is lower than that of graphite when the electron energy is higher than 30 eV. In graphite, the incident electrons may exhibit directional preferences in their motion. These results indicate that, in graphite, SEs are excited in deeper positions than in diamond, and more SEs move in a horizontal direction than in a vertical direction, which leads to the difference in secondary electron yield (SEY).

Effect of Isothermal Conditions on the Charge Trapping/Detrapping Parameters in e-Beam Irradiated Thermally Aged XLPE Insulation in SEM.

The effect of isothermal conditions on the trapping/detrapping process of charges in e-beam irradiated thermally aged XLPE insulation in scanning electron microscopy (SEM) has been investigated. Different isothermal conditions ranging from room temperature to 120 °C are applied on both unaged and aged XLPE samples (2 mm thick) by a suitable arrangement associated with SEM. For each applied test temperature, leakage, and influence currents have been measured simultaneously during and after e-beam irradiation. Experimental results show a big difference between the fresh and aged material regarding trapping and detrapping behavior. It has been pointed out that in the unaged material deep traps govern the process, whereas the shallow traps take part in the aged one. Almost all obtained results reveal that the trapped charge decreases and then increases as the temperature increases for the unaged sample. A deflection temperature corresponding to a minimum is observed at 50 °C. However, for the aged material, the maximum trapped charge decreases continuously with increasing temperature, and the material seems to trap fewer charges under e-beam irradiation at high temperature. Furthermore, thermal aging leads to the occurrence of detrapping process at high temperatures even under e-beam irradiation, which explains the decrease with time evolution of trapped charge during this period. The recorded leakage current increases with increasing temperature for both cases with pronounced values for aged material. The effect of temperature and thermal aging on electrostatic influence factor (K) and total secondary electron emission yield (σ) were also studied.

Transient Strain-Induced Electronic Structure Modulation in a Semiconducting Polymer Imaged by Scanning Ultrafast Electron Microscopy.

Understanding the optoelectronic properties of semiconducting polymers under external strain is essential for their applications in flexible devices. Although prior studies have highlighted the impact of static and macroscopic strains, assessing the effect of a local transient deformation before structural relaxation occurs remains challenging. Here, we employ scanning ultrafast electron microscopy (SUEM) to image the dynamics of a photoinduced transient strain in the semiconducting polymer poly(3-hexylthiophene) (P3HT). We observe that the photoinduced SUEM contrast, corresponding to the local change of secondary electron emission, exhibits an unusual ring-shaped profile. We attribute the observation to the electronic structure modulation of P3HT caused by a photoinduced strain field owing to its low modulus and strong electron-lattice coupling, supported by a finite-element analysis. Our work provides insights into tailoring optoelectronic properties using transient mechanical deformation in semiconducting polymers and demonstrates the versatility of SUEM to study photophysical processes in diverse materials.

The Design of the Emission Layer for Electron Multipliers.

The electron multipliers gain is closely related to the secondary electron emission coefficient (SEE) of the emission layer materials. The SEE is closely related to the thickness of the emission layer. If the emission layer is thin, the low SEE causes the low gain of electron multipliers. If the emission layer is thick, the conductive layer can't timely supplement charge to the emission layer, the electronic amplifier gain is low too. The electron multipliers usually choose Al2O3 and MgO film as the emission layer because of the high SEE level. MgO easy deliquescence into Mg(OH)2 Mg2(OH)2CO3 and MgCO3 resulting in the lower SEE level. The SEE level of Al2O3 is lower than MgO, but Al2O3 is stable. We designed a spherical system for testing the SEE level of materials, and proposed to use low-energy secondary electrons instead of low-energy electron beam for neutralization to measuring the SEE level of Al2O3, MgO, MgO/Al2O3, Al2O3/MgO, and precisely control the film thickness by using atomic layer deposition. We propose to compare the SEE under the adjacent incident electrons energy to partition the SEE value of the material, and obtain four empirical formulas for the relationship between SEE and thickness. Since the main materials that cause the decrease in SEE are Mg2(OH)2CO3 and MgCO3, we use the C element atomic concentration measured by XPS to study the deliquescent depth of the material. We propose to use the concept of transition layer for SEE interpretation of multilayer materials. Through experiments and calculations, we put forward a new emission layer for electron multipliers, including 2-3 nm Al2O3 buffer layer, 5-9 nm MgO main-body layer, 1 nm Al2O3 protective layer or 0.3 nm Al2O3 enhancement layer. We prepared this emission layer to microchannel plate (MCP), which significantly improved the gain of MCP. We can also apply this new emission layer to channel electron multiplier and separate electron multiplier.

Monitoring Carbon in Electron and Ion Beam Deposition within FIB-SEM.

It is well known that carbon present in scanning electron microscopes (SEM), Focused ion beam (FIB) systems and FIB-SEMs, causes imaging artefacts and influences the quality of TEM lamellae or structures fabricated in FIB-SEMs. The severity of such effects depends not only on the quantity of carbon present but also on its bonding state. Despite this, the presence of carbon and its bonding state is not regularly monitored in FIB-SEMs. Here we demonstrated that Secondary Electron Hyperspectral Imaging (SEHI) can be implemented in different FIB-SEMs (ThermoFisher Helios G4-CXe PFIB and Helios Nanolab G3 UC) and used to observe carbon built up/removal and bonding changes resulting from electron/ion beam exposure. As well as the ability to monitor, this study also showed the capability of Plasma FIB Xe exposure to remove carbon contamination from the surface of a Ti6246 alloy without the requirement of chemical surface treatments.

Understanding Surface Modifications Induced via Argon Plasma Treatment through Secondary Electron Hyperspectral Imaging.

Understanding the effects that sterilization methods have on the surface of a biomaterial is a prerequisite for clinical deployment. Sterilization causes alterations in a material's surface chemistry and surface structures that can result in significant changes to its cellular response. Here we compare surfaces resulting from the application of the industry standard autoclave sterilisation to that of surfaces resulting from the use of low-pressure Argon glow discharge within a novel gas permeable packaging method in order to explore a potential new biomaterial sterilisation method. Material surfaces are assessed by applying secondary electron hyperspectral imaging (SEHI). SEHI is a novel low-voltage scanning electron microscopy based characterization technique that, in addition to capturing topographical images, also provides nanoscale resolution chemical maps by utilizing the energy distribution of emitted secondary electrons. Here, SEHI maps are exploited to assess the lateral distributions of diverse functional groups that are effected by the sterilization treatments. This information combined with a range of conventional surface analysis techniques and a cellular metabolic activity assay reveals persuasive reasons as to why low-pressure argon glow discharge should be considered for further optimization as a potential terminal sterilization method for PGS-M, a functionalized form of poly(glycerol sebacate) (PGS).

Experimental research on influence of gas adsorption on secondary electron emission of copper surface.

Secondary electron (SE) emission is a superficial physical phenomenon. The range of SEs excited in materials which are finally emitted from the surface is generally within a few tens of nanometers. SE emission from materials by electron irradiation has great dependence on surface quality, such as gas adsorption, oxidation, contamination, and morphology. In this paper, the influence of surface adsorption of N2, O2, Ar and air on SE yield (SEY) and the SE spectrum (SES) of copper was investigated by experiment. The measured SEY and SES were compared before and after the copper surface was sputtered by Ar ions. The results show that gas adsorption could increase SEY, and air adsorption lead to the maximum increase, Ar adsorption the minimum. Meanwhile, the relationship between the SES and the SEY was deduced and validated by the experiment. It was also found in the experiment that the Ar ion sputtering or heating alone can effectively reduce the SEY, while heating the sample after Ar ion sputtering increases SEY. The reasons for the variation of the SEYs with gas adsorption, sputtering and heating are preliminarily explained with the help of in-situ XPS. Accordingly, the research helps to further reveal the mechanism of SEE influenced by complex superficial factors.

Probing Electrified Liquid-Solid Interfaces with Scanning Electron Microscopy.

Electrical double layers play a key role in a variety of electrochemical systems. The mean free path of secondary electrons in aqueous solutions is on the order of a nanometer, making them suitable for probing ultrathin electrical double layers at solid-liquid electrolyte interfaces. Employing graphene as an electron-transparent electrode in a two-electrode electrochemical system, we show that the secondary electron yield of the graphene-liquid interface depends on the ionic strength and concentration of the electrolyte and the applied bias at the remote counter electrode. These observations have been related to polarization-induced changes in the potential distribution within the electrical double layer and demonstrate the feasibility of using scanning electron microscopy to examine and map electrified liquid-solid interfaces.

Unravelling new principles of site-selective doping contrast in the dual-beam focused ion beam/scanning electron microscope.

Doping contrast using the secondary electron (SE) signal in the scanning electron microscope (SEM) can satisfy the International Roadmap for Semiconductors (ITRS) requisites for quantitative dopant profiling of next-generation integrated circuits and devices, but only if adopting a site-selective specimen preparation procedure. In this study, site-specific dopant profiling was performed on the trench side-wall cut by a 30-kV Ga+ focused ion beam (FIB) into silicon p-n junction specimens and milled using successively lower voltages in the dual-beam FIB/SEM. Although depositing the protective platinum strap on the surface effectively controls 'curtaining' effects at low final milling voltages, significantly reduced doping contrast from the side-wall compared to that from a cleaved surface subjected to the same ion-beam energy is ascribed to the material affected by a previous milling step, as well as the dissimilar geometries of milling and imaging. New principles underpinning the doping contrast mechanism were surveyed taking into account the depth and concentration of ion implantation and amorphization damage as a linear function of the final milling voltage. Patch fields are suppressed, but the bulk doping-dependent surface band-bending fields at the amorphous-crystalline interface is crucial for doping contrast. In general, as the milling voltage decreases the doping contrast increases, which although reaches up to only half that attainable from a freshly-cleaved specimen, is usable, and demonstrates the feasibility of site-specific dopant profiling in situ in the SEM.

Characterizing Cross-Linking Within Polymeric Biomaterials in the SEM by Secondary Electron Hyperspectral Imaging.

A novel capability built upon secondary electron (SE) spectroscopy provides an enhanced cross-linking characterization toolset for polymeric biomaterials, with cross-linking density and variation captured at a multiscale level. The potential of SE spectroscopy for material characterization has been investigated since 1947. The absence of suitable instrumentation and signal processing proved insurmountable barriers to applying SE spectroscopy to biomaterials, and consequently, capturing SE spectra containing cross-linking information is a new concept. To date, cross-linking extent is inferred from analytical techniques such as nuclear magnetic resonance (NMR), differential scanning calorimetry, and Raman spectroscopy (RS). NMR provides extremely localized information on the atomic scale and molecular scale, while RS information volume is on the microscale. Other methods for the indirect study of cross-linking are bulk mechanical averaging methods, such as tensile and compression modulus testing. However, these established averaging methods for the estimation of polymer cross-linking density are incomplete because they fail to provide information of spatial distributions within the biomaterial morphology across all relevant length scales. The efficacy of the SE spectroscopy capability is demonstrated in this paper by the analysis of poly(glycerol sebacate)-methacrylate (PGS-M) at different degrees of methacrylation delivering new insights into PGS-M morphology.

Making Sense of Complex Carbon and Metal/Carbon Systems by Secondary Electron Hyperspectral Imaging.

Carbon and carbon/metal systems with a multitude of functionalities are ubiquitous in new technologies but understanding on the nanoscale remains elusive due to their affinity for interaction with their environment and limitations in available characterization techniques. This paper introduces a spectroscopic technique and demonstrates its capacity to reveal chemical variations of carbon. The effectiveness of this approach is validated experimentally through spatially averaging spectroscopic techniques and using Monte Carlo modeling. Characteristic spectra shapes and peak positions for varying contributions of sp2-like or sp3-like bond types and amorphous hydrogenated carbon are reported under circumstances which might be observed on highly oriented pyrolytic graphite (HOPG) surfaces as a result of air or electron beam exposure. The spectral features identified above are then used to identify the different forms of carbon present within the metallic films deposited from reactive organometallic inks. While spectra for metals is obtained in dedicated surface science instrumentation, the complex relations between carbon and metal species is only revealed by secondary electron (SE) spectroscopy and SE hyperspectral imaging obtained in a state-of-the-art scanning electron microscope (SEM). This work reveals the inhomogeneous incorporation of carbon on the nanoscale but also uncovers a link between local orientation of metallic components and carbon form.

Substrate Temperature Dependent Microstructure and Electron-Induced Secondary Electron Emission Properties of Magnetron Sputter-Deposited Amorphous Carbon Films.

For special instruments or equipments including particle accelerators, space microwave devices and spacecrafts, the suppression for electron-induced secondary electron emission (SEE) occurring on the component surfaces is of great significance due to a negative influence caused by SEE on their normal operations. In this paper, amorphous carbon (a-C) films were prepared on stainless-steel substrates by radio frequency magnetron sputtering, and the effects of substrate temperature (Ts) and continuous electron bombardment on the microstructure and secondary electron emission yield (SEY) of a-C film were investigated in order to achieve a better inhibition for SEE. The experimental results show that a rise of Ts during the a-C film preparation is conducive to a SEY reduction and an increase of multipactor threshold due to the increases of surface roughness and sp2 bond content. In addition, although the SEY of a-C film has a slight increase with the rise of electron bombardment time, the a-C film sample with a lower SEY keeps its lower SEY all the time during continuous electron bombardment. The a-C film prepared at Ts of 500 °C has the lowest SEY peak value of 1.09 with a reduction of 30.6% in comparison with the stainless-steel substrate.

Three-Dimensional Imaging of Beam-Induced Biasing of InP/GaInP Tunnel Diodes.

Electron holographic tomography was used to obtain three-dimensional reconstructions of the morphology and electrostatic potential gradient of axial GaInP/InP nanowire tunnel diodes. Crystal growth was carried out in two opposite directions: GaInP-Zn/InP-S and InP-Sn/GaInP-Zn, using Zn as the p-type dopant in the GaInP but with changes to the n-type dopant (S or Sn) in the InP. Secondary electron and electron beam-induced current images obtained using scanning electron microscopy indicated the presence of p-n junctions in both cases and current-voltage characteristics measured via lithographic contacts showed the negative differential resistance, characteristic of band-to-band tunneling, for both diodes. Electron holographic tomography measurements confirmed a short depletion width in both cases (21 ± 3 nm) but different built-in potentials, Vbi, of 1.0 V for the p-type (Zn) to n-type (S) transition, and 0.4 V for both were lower than the expected 1.5 V for these junctions if degenerately doped. Charging induced by the electron beam was evident in phase images which showed nonlinearity in the surrounding vacuum, most severe in the case of the nanowire grounded at the p-type Au contact. We attribute their lower Vbi to asymmetric secondary electron emission, beam-induced current biasing, and poor grounding contacts.

Imaging photoinduced surface potentials on hybrid perovskites by real-time Scanning Electron Microscopy.

We introduce laser-assisted Time-Resolved SEM (TR-SEM), joining Scanning Electron Microscopy and laser light excitation, to probe the long-term temporal evolution of optically excited charge distributions at the surface of Metal Ammonium Lead Triiodide (MAPbI3) hybrid perovskite thin films. Laser-assisted TR-SEM relies on the optically induced local modification of Secondary Electron (SE) detection yield to provide mapping of photoexcited potentials and charge dynamics at surfaces, and qualifies as a complementary approach to near-field probe microscopies and nonlinear photoemission spectroscopies for photovoltage measurements. Real-time imaging of evolving field patterns are provided on timescales compatible with SEM scanning rates, so that temporal resolution in the millisecond range can be ultimately envisaged. MAPbI3 is an outstanding light-sensitive material candidate for applications in solar light harvesting and photovoltaics, also appealing as an active system for light generation. In this work, the real time temporal evolution of optically induced SE contrast patterns in MAPbI3 is experimentally recorded, both under illumination by a 405 nm blue laser and after light removal, showing the occurrence of modifications related to photoinduced positive charge fields at surface. The long term evolution of these surface fields are tentatively attributed to ion migration within the film, under the action of the illumination gradient and the hole collecting substrate. This optical excitation is fully reversible in MAPbI3 over timescales of hours and a complete recovery of the system occurs within days. Permanent irradiation damage of the material is avoided by operating the SEM at 5 keV of energy and 1-10 pA of primary current. Optical excitation is provided by intense above-bandgap illumination (up to 50 W/cm2). TR-SEM patterns show a strong dependence on the geometry of SE collection. Measurements are taken at different axial orientations of the sample with respect to the entrance of the in-column detection system of the SEM and compared with numerical modeling of the SE detection process. This enables to single out the information regarding the local potential distribution. Results are interpreted by combining data about the spectral distribution of emitted SEs with the configuration of the electric and magnetic fields in the specimen chamber. The present modeling sets a robust basis for the understanding of photoinduced SE electron contrast.

Sensitivity-Selectivity Trade-Offs in Surface Ionization Gas Detection.

Surface ionization (SI) provides a simple, sensitive, and selective method for the detection of high-proton affinity substances, such as organic decay products, medical and illicit drugs as well as a range of other hazardous materials. Tests on different kinds of SI sensors showed that the sensitivity and selectivity of such devices is not only dependent on the stoichiometry and nanomorphology of the emitter materials, but also on the shape of the electrode configurations that are used to read out the SI signals. Whereas, in parallel-plate capacitor devices, different kinds of emitter materials exhibit a high level of amine-selectivity, MEMS (micro-electro-mechanical-systems) and NEMS (nanowire) versions of SI sensors employing the same kinds of emitter materials provide significantly higher sensitivity, however, at the expense of a reduced chemical selectivity. In this paper, it is argued that such sensitivity-selectivity trade-offs arise from unselective physical ionization phenomena that occur in the high-field regions immediately adjacent to the surfaces of sharply curved MEMS (NEMS) emitter and collector electrodes.

Roughness Evolution and Charging in Plasma-Based Surface Engineering of Polymeric Substrates: The Effects of Ion Reflection and Secondary Electron Emission.

The interaction of plasma with polymeric substrates generates both roughness and charging on the surface of the substrates. This work, toward the comprehension and, finally, the control of plasma-induced surface roughness, delves into the intertwined effects of surface charging, ion reflection, and secondary electron-electron emission (SEEE) on roughness evolution during plasma etching of polymeric substrates. For this purpose, a modeling framework consisting of a surface charging module, a surface etching model, and a profile evolution module is utilized. The case study is etching of a poly(methyl methacrylate) (PMMA) substrate by argon plasma. Starting from an initial surface profile with microscale roughness, the results show that the surface charging contributes to a faster elimination of the roughness compared to the case without charging, especially when ion reflection is taken into account. Ion reflection sustains roughness; without ion reflection, roughness is eliminated. Either with or without ion reflection, the effect of SEEE on the evolution of the rms roughness over etching time is marginal. The mutual interaction of the roughness and the charging potential is revealed through the correlation of the charging potential with a parameter combining rms roughness and skewness of the surface profile. A practical implication of the current study is that the elimination or the reduction of surface charging will result in greater surface roughness of polymeric, and generally dielectric, substrates.