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

Department of Systems Design Engineering, University of Waterloo, Waterloo, ON, N2L3G1, Canada.
Nanotechnology (Impact Factor: 3.82). 01/2008; 19(2):025701. DOI: 10.1088/0957-4484/19/02/025701
Source: PubMed


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.

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    • "Field-emission was among the first applications considered for carbon nanotubes, and pioneering studies go back to the early days of nanotube research [72] [73] [74]. Field-emission experiments have involved single-walled [75] [76] [77], double-walled [78] and multiwalled [79] [80] [81] [82] [83] [84] [85] [86] carbon nanotubes and included devices made of individual nanotubes or collections of nanotubes, sometimes mixed with other materials. A large number of the studies have primarily focused on current-voltage characteristics, while others have also investigated the emission spots using field-emission microscopy (Figures 4(b) and 4(c)) [87] [88]. "
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    ABSTRACT: Carbon nanotubes have a host of properties that make them excellent candidates for electron emitters. A significant amount of research has been conducted on nanotube-based field-emitters over the past two decades, and they have been investigated for devices ranging from flat-panel displays to vacuum tubes and electron microscopes. Other electron emission mechanisms from carbon nanotubes, such as photoemission, secondary emission, and thermionic emission, have also been studied, although to a lesser degree than field-emission. This paper presents an overview of the topic, with emphasis on these less-explored mechanisms, although field-emission is also discussed. We will see that not only is electron emission from nanotubes promising for electron-source applications, but also its study could reveal unusual phenomena and open the door to new devices that are not directly related to electron beams.
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    • "The alternative explanation of electromechanical rearrangement of individual CNTs, in contrast, is expected to show no dependence on ambient pressure. Furthermore, the fluctuations due to electromechanical effects have been shown theoretically to occur on a time scale of several seconds [18], which may correspond to the low-frequency switching observed at early times for LP1 (Figure 5(b)). The observed telegraph noise diminishes after 15 minutes of testing, consistent with the system reaching an electromechanical equilibrium. "
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    ABSTRACT: In recent years, carbon nanotubes (CNTs) have emerged as one of the best field emitters for a variety of technological applications. The field emitting cathodes have several advantages over the conventional thermionic cathodes: (i) current density from field emission would be orders of magnitude greater than in the thermionic case, (ii) a cold cathode would minimize the need for cooling, and (iii) a field emitting cathode can be miniaturized. In spite of good performance of such cathodes, the procedure to estimate the device current is not straight forward and the required insight towards design optimization is not well understood. In addition, the current in CNT-based thin film devices shows fluctuation. Such fluctuation in field emission current is not desirable for many biomedical applications such as x-ray devices. The CNTs in a thin film undergo complex dynamics during field emission, which includes processes such as (i) evolution, (ii) electromechanical interaction, (iii) thermoelectric heating, (iv) ballistic transport, and (v) electron gas flow. These processes are coupled and nonlinear. Therefore, they must be analyzed accurately from the stability and long-term performance point of view. In this research, we develop detailed physics-based models of CNTs considering the aspects mentioned above. The models are integrated in a systematic manner to calculate the device current by using the Fowler-Nordheim equation. Using the models, we were able to capture the fluctuations in the field emission current, which have been observed in actual experiments. 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. In addition, investigations on the influence of defects and impurities in CNTs on their field emission properties have been carried out. By inclusion of defects and impurities, the field emission properties of CNTs can be tailored for specific device applications in future. For stable performance of CNT-based field emission devices, such as x-ray generators, design optimization studies have been presented. It has been found that the proposed design minimizes transience in field emission current. In this thesis, it has been demonstrated that phonon-assisted control of field emission current in CNT based thin film is possible. Finally, experimental studies pertaining to crosstalk phenomenon in a multi-pixel CNT array are presented.
    Preview · Article · Jan 2008
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