Yonghai Sun

Nanotube field electron emission: principles, development, and applications

There is a growing trend to apply field emission (FE) electron sources in vacuum electronic devices due to their fast response, high efficiency and low energy consumption compared to thermionic emission ones. Carbon nanotubes (CNTs) have been regarded as a promising class of electron field emitters since the 1990s and have promoted the development of FE technology greatly because of their high electrical and thermal conductivity, chemical stability, high aspect ratio and small size. Recent studies have shown that FE from CNTs has the potential to replace conventional thermionic emission in many areas and that it exhibits advanced features in practical applications. Consequently, FE from nanotubes and applications thereof have attracted much attention. This paper provides a comprehensive review of both recent advances in CNT field emitters and issues related to applications of CNT based FE. FE theories and principles are introduced, and the early development of field emitters is related. CNT emitter types and their FE performance are discussed. The current situation for applications based on nanotube FE is reviewed. Although challenges remain, the tremendous progress made in CNT FE over the past ten years indicates the fields development potential.

The Design and Fabrication of Carbon-Nanotube-Based Field Emission X-Ray Cathode With Ballast Resistor

The incorporation of a ballast resistor layer underneath field emitters can improve the reliability and stability of field emission cathodes. However, it has not been successfully demonstrated in carbon-nanotube (CNT)-based field emitters. In this paper, we introduce a fabrication process of a low-density vertically aligned CNT array with a ballast resistor layer. Furthermore, an X-ray tube prototype based on this cathode is demonstrated.

Design and fabrication of carbon nanotube field-emission cathode with coaxial gate and ballast resistor.

A low density vertically aligned carbon nanotube-based field-emission cathode with a ballast resistor and coaxial gate is designed and fabricated. The ballast resistor can overcome the non-uniformity of the local field-enhancement factor at the emitter apex. The self-aligned fabrication process of the coaxial gate can avoid the effects of emitter tip misalignment and height non-uniformity.

Electromechanical interactions in a carbon nanotube based thin film field emitting diode

Carbon nanotubes (CNTs) have emerged as promising candidates for biomedical x-ray devices and other applications of field emission. CNTs grown/deposited in a thin film are used as cathodes for field emission. In spite of the good performance of such cathodes, the procedure to estimate the device current is not straightforward and the required insight towards design optimization is not well developed. In this paper, we report an analysis aided by a computational model and experiments by which the process of evolution and self-assembly (reorientation) of CNTs is characterized and the device current is estimated. The modeling approach involves two steps: (i) a phenomenological description of the degradation and fragmentation of CNTs and (ii) a mechanics based modeling of electromechanical interaction among CNTs during field emission. A computational scheme is developed by which the states of CNTs are updated in a time incremental manner. Finally, the device current is obtained by using the Fowler-Nordheim equation for field emission and by integrating the current density over computational cells. A detailed analysis of the results reveals the deflected shapes of the CNTs in an ensemble and the extent to which the initial state of geometry and orientation angles affect the device current. Experimental results confirm these effects.

A novel field emission microscopy method to study field emission characteristics of freestanding carbon nanotube arrays

Field emission (FE) uniformity and the mechanism of emitter failure of freestanding carbon nanotube (CNT) arrays have not been well studied due to the difficulty of observing and quantifying FE performance of each emitter in CNT arrays. Herein a field emission microscopy (FEM) method based on poly(methyl methacrylate) (PMMA) thin film is proposed to study the FE uniformity and CNT emitter failure of freestanding CNT arrays. FE uniformity of freestanding CNT arrays and different levels of FE current contributions from each emitter in the arrays are recorded and visualized. FEM patterns on the PMMA thin film contain the details of the CNT emitter tip shape and whether multiple CNT emitters occur at an emission site. Observation of real-time FE performance and the CNT emitter failure process in freestanding CNT arrays are successfully achieved using a microscopic camera. High emission currents through CNT emitters causes Joule heating and light emission followed by an explosion of the CNTs. The proposed approach is capable of resolving the major challenge of building the relationship between FE performance and CNT morphologies, which can significantly facilitate the study of FE non-uniformity, the emitter failure mechanism and the development of stable and reliable FE devices in practical applications.

Simulation of field emission current uniformity of low-density freestanding CNT array

Field electron emission (FEE) from carbon nanotube (CNT) array has been widely studied, but improving the uniformity of emission current from a CNT emitter array remains a challenge. In this paper, the uniformity of a freestanding CNT array is simulated. Field enhancement factor (β), electrical field strength at the tip and emission current density of CNTs is calculated. A simple ballast resistor array, like the resistor layer in conventional Spindt cathodes, is utilized. The voltage drops on the resistors and the emission currents form corresponding CNTs are also calculated. Simulation results show that to achieve a reasonable total emission current, emission current from individual CNTs with ballast resistor are much lower than the current from a favorable CNT without the ballast resistor.

PolyMethyl Methacrylate Thin-Film-Based Field Emission Microscope

A field emission microscope (FEM) is a useful tool for investigating molecular surface structures. Conventional FEMs suffer from poor image contrast level and low sensitivities when low-energy electron beams are applied. In this article, a new anode material is employed to improve the FEM imaging performance. We demonstrate that the device has the capability of clearly capturing images of facet boundaries of crystal structures at the tip of a zinc oxide (ZnO) nanowire as defect sites on a Polymethyl methacrylate (PMMA) film that is exposed to electron beams. The clear image of facet boundaries has not been reported in conventional FEM images.

PMMA/MWCNT nanocomposite for proton radiation shielding applications

Radiation shielding in space missions is critical in order to protect astronauts, spacecraft and payloads from radiation damage. Low atomic-number materials are efficient in shielding particle-radiation, but they have relatively weak material properties compared to alloys that are widely used in space applications as structural materials. However, the issues related to weight and the secondary radiation generation make alloys not suitable for space radiation shielding. Polymers, on the other hand, can be filled with different filler materials for reinforcement of material properties, while at the same time provide sufficient radiation shielding function with lower weight and less secondary radiation generation. In this study, poly(methyl-methacrylate)/multi-walled carbon nanotube (PMMA/MWCNT) nanocomposite was fabricated. The role of MWCNTs embedded in PMMA matrix, in terms of radiation shielding effectiveness, was experimentally evaluated by comparing the proton transmission properties and secondary neutron generation of the PMMA/MWCNT nanocomposite with pure PMMA and aluminum. The results showed that the addition of MWCNTs in PMMA matrix can further reduce the secondary neutron generation of the pure polymer, while no obvious change was found in the proton transmission property. On the other hand, both the pure PMMA and the nanocomposite were 18%-19% lighter in weight than aluminum for stopping the protons with the same energy and generated up to 5% fewer secondary neutrons. Furthermore, the use of MWCNTs showed enhanced thermal stability over the pure polymer, and thus the overall reinforcement effects make MWCNT an effective filler material for applications in the space industry.

Field Emission Properties of Carbon Nanotube Thin Films Grown on Different Substrate Materials

The electron field emission characteristics from multiwalled carbon nanotube (MWNT) films grown on two different substrate materials have been studied in this paper. The MWNT films were grown on a flat conductor surface (stainless steel) and a flat insulator surface (quartz). The field emission experiments were carried out under a diode configuration. It was found that the film on the quartz surface exhibits better emission capability, while the film on the stainless steel surface shows better emission stability. Energy-dispersive X-ray spectroscopy analysis revealed that carbon atoms evaporated and deposited on the surface of anode during field emission, which is in agreement to studies previously reported in literature. It is estimated that the degradation was due to local heating of tips of CNTs, leading to evaporation and deposition of carbon atoms during the field emission process.

Study of Freestanding carbon nanotube array field emission uniformity with field emission microscopy

Field emission microscopy (FEM) has been used in materials science to investigate molecular surface structures and their electronic properties. Since PMMA thin film is electron sensitive, it allows us to visualize and record the FE performance of the CNT arrays. In this article, we demonstrate a PMMA thin film based FEM to analyze the field emission (FE) uniformity of free standing carbon nanotube (CNT) array. The FE dose (product of FE current and time) and spatial distribution of CNT arrays of two different densities are recorded and the deviation of the FE performance of CNT emitters is visualized as different morphology of FEM patterns on the PMMA thin film. This method facilitates the study of FE non-uniformity and emitter degradation mechanism.