Anthony Ayari

This study investigates the potential of commercially available single-walled carbon nanotube (SWNT) papers as high-current field emission cathodes from both macroscopic and microscopic perspectives. By employing an optimized cutting strategy designed to maximize the density of emitting bundles, we demonstrate that macroscopic emission currents of up to 15 mA at 3 kV can be achieved using a 1 cm long sample. To better understand the emission behavior and performance limits of these cathodes, in situ field emission experiments were carried out inside a scanning electron microscope on both individual bundles and groups of bundles. These experiments enabled the characterization of the maximum emission currents as well as the identification of the dominant degradation mechanisms, including mechanical tearing, field evaporation, and vacuum breakdown events. Interestingly, the SWNT papers exhibit a remarkable resilience to vacuum breakdown and maintain excellent emission properties after exposure to ambient air. These results highlight the potential of SWNT papers as practical and robust cathodes for the next-generation vacuum electronic devices.

In this paper, we theoretically and experimentally investigate the specificities of single-walled carbon nanotubes (SWNTs) for field electron emission (FE) and field ion evaporation (FI). For FE, the small radii of curvature of these nanotubes lead to a significant widening of the tunnel barrier, and thus, for a given emission current, the fields at the surface of the SWNTs are significantly higher than commonly observed in standard large radius FE. For currents in the μA range, for example, fields ranging from 7–8 V/nm for large nanotubes to 17–18 V/nm for the thinnest are required. These strong fields, in turn, have repercussions on FI and electrostatic forces. The electric fields and longitudinal electrostatic forces during FE and FI as a function of nanotube radius are presented. The evolution of the nanotube during field evaporation was also studied in an environmental transmission electron microscope as a function of field strength and polarity. A strong dissymmetry between positive and negative polarity is observed. For negative polarity, the nanotubes can gradually shorten as the voltage is increased, whereas in positive polarity, they are almost systematically torn off before apex evaporation occurs. A model to explain this dissymmetry for our samples is presented. These results can be used to optimize nanotube cathode geometries.

The performance of field emitters is usually analyzed by linear fitting of a Fowler–Nordheim plot. It has sometimes been observed that the fitted slopes and intercepts show a strong correlation, but no convincing explanation has been provided. We propose a simple model showing that this correlation is due to fluctuations in the slope and the fact that the experiments are carried out over a defined range of voltage and current. By performing a meta-analysis, we show that this explanation correctly predicts the results of other groups in this field.

The manuscript1 used several methods of analysis of field emission data. One of the methods, the seventh one, used the full Murphy-Good formula at 300 K an The manuscript1 used several methods of analysis of field emission data. One of the methods, the seventh one, used the full Murphy-Good formula at 300 K an

Optimizing the synthesis of carbon nanotubes (CNTs) for applications like field emission (FE) sources requires a fundamental understanding of the growth kinetics of individual CNTs. In this article, we explore how applying electric fields during CNT synthesis influences the as-grown nanotubes and their FE performance. We observe growth and undertake FE measurements in real time using an environmental transmission electron microscope. This is achieved through a polarizable capacitor gap within a microchip sample heater specifically designed for this purpose. Individual nanotubes are easily resolved and are predominantly single-wall CNTs. At low-applied fields, the growing nanotubes can span the gap and link with the opposite electrode, albeit with some loss due to mechanical failure. With a high-applied field and positive bias for FE, we continue to observe the oriented growth of nanotubes. However, this growth is constrained within the gap due to the possibility of FE occurring during the growth process, which can result in either saturation or damage. At any given time, we have the flexibility to halt the growth process and conduct in situ FE experiments. This approach enables us to comprehensively track the complete development of the CNTs and gain insights into the various mechanisms responsible for limiting the performance of CNT cathodes. Interestingly, we report an original self-oscillation induced destruction mechanism that has not been reported before.

Representing field emission data on a Fowler–Nordheim plot is both very common and strongly not recommended. It leads to a spurious estimation of the emitter parameters despite a very good data fit. There is a lack of a reliable method of analysis and a proper estimation of the uncertainty in the extracted parameters. In this article, we show that the uncertainty in the estimation of the field enhancement factor or the emission area can be as high as ±50% even for a tungsten single emitter in good ultrahigh vacuum conditions analyzed by the Murphy–Good model. Moreover, the choice of the exact Murphy–Good method can have a noticeable impact. We found that advanced analysis methods, based on the measurement of the differential conductance of the emitter, are so demanding in terms of emitter stability that up to now its requirements are probably out of reach in any field emission laboratory.

Field emission (FE) from semiconducting nanowires (NWs) is studied for expanding electron gun performances and functionality in terms of stability, brightness, and pulsed emission. Here, we report on a pronounced and robust double negative differential resistance (NDR) in the FE IV characteristics measured during photoassisted field emission experiments on highly crystalline p-type silicon NWs. The main feature is a double NDR in the current saturation regime, which can be modulated by both temperature and light intensity. These results contrast with previous FE studies in which only a barely noticeable single NDR was reported. Several mechanisms for the physical explanation of the NDR are currently under consideration: photogenerated carrier instabilities in the depletion region, which give rise to a pulsed space-charge current in the nanowire or tunneling through a double quantum well formed by confinement at the NW apex. Because NDRs are signatures of pulsed currents, these results suggest new functionalities for which pulsed electron sources can potentially be achieved at high repetition rates.

Field emission data are often represented on a Fowler–Nordheim plot but a new empirical equation has been recently proposed to better analyze experiments. Such an equation is based on approximations of the Murphy and Good model and predicts that a constant parameter κ, depending only on the work function of the emitter, can be extracted from the data. We compared this empirical equation with simulations of the Murphy and Good model in order to determine the range of validity of the approximations and the robustness of the relationship between κ and the work function. We found that κ is constant only over a limited range of electric fields and so depends significantly on the field enhancement factor. This result calls into question the usefulness of the new empirical equation.