The Fowler–Nordheim plot behaviour from virtual field emitters arrays of zinc oxide nanorods mixed with carbon nanotubes

The plasma-induced emission properties of ZnO nanorod and carbon nanotube (CNT) arrays were investigated under the pulse electric field. The formation of plasma on the array surface was found and high intensity electron beams were obtained from the two kinds of arrays. The plasma-induced emission properties of the ZnO nanorod and CNT arrays have big differences. Under the same electric field, the CNT arrays have higher emission current than the ZnO nanorod arrays. With the emission currents changing, the electron emissions of the ZnO nanorod arrays always are very uniform; but that of the CNT arrays are non-uniform. The plasma expansion velocity of the ZnO nanorod arrays is lower than that of the CNT arrays. Accordingly, the emission stability of the ZnO nanorod arrays is better than that of the CNT arrays.

The plasma-induced electron emission properties of ZnO nanorod and carbon nanotube (CNT) arrays were investigated under a pulsed electric field. The formation of plasma on the array surface was found and intense current electron beams were obtained from the two kinds of arrays. The plasma-induced emission properties of the ZnO nanorod and CNT arrays have big differences. Under the same electric field, the CNT arrays have higher emission current than the ZnO nanorod arrays. The uniformity of the plasma-induced electron emission was in situ characterized by the plasma radiation and Cerenkov radiation. With the emission currents changing, the emissions of the ZnO nanorod arrays are always very uniform; but that of the CNT arrays are non-uniform. Moreover, the plasma-induced electron emission stability of ZnO nanorod arrays is better than that of the CNT arrays. The emission uniformity and stability of the ZnO nanorod arrays is better than that of the CNT arrays.

Carbon nanotubes (CNTs) are ideal field emission sources because of combining a high aspect ratio with chemical inertness, good electron and thermal conductivity and mechanical strength. A stable field emission current of 1 A from one multi-walled CNT and a high emission current density of 1 A/cm/sup 2/ from CNTs emitter arrays have been reported. Vertically aligned, sparse CNTs are proposed as being the ideal field emitters. Well-aligned CNTs have been made with hot filament chemical vapor deposition (HFCVD), plasma enhanced chemical vapor deposition (PECVD) and microwave plasma chemical vapor deposition (MWPCVD). However, up to now it is difficult to obtain CNT arrays with uniform and large current density, because of the strong field shielding effect due to the high density of these well-aligned CNTs. We report in this paper a method to grow CNT arrays, which are freestanding, well-aligned, and vertically-oriented. Furthermore, these CNTs have a uniform length and diameter. Growth has been realized at lithographically defined sites on metallic, semi-conducting and glass substrates. A sandwiched catalyst structure and MWPCVD is utilized to form multi-walled and also single-walled carbon nanotubes. Figure 1a) and 1b) is a scanning electron microscope (SEM) image of our as-grown CNTs using sandwiched catalyst structure. It is feasible to grow CNTs between two terminals directly, as shown in figure 1a), and grow straight vertical CNT bundles with small diameter of less than 2 m, as shown in figure 1b). The transmission electron microscope (TEM) image of the as-grown CNTs shows typical multi-walled CNTs lattice structure with few defects, as shown in figure 1c). After treating the as-grown CNTs in a oxygen radio-frequency plasma, a highly stable field emission current density of more than 6A/cm/sup 2/ at a electric field of 7.4V/m with a total field emission current of >600 A was obtained, as shown in figure 3. The work functions of the CNTs before treatment and after treatment were 4.56 eV and 4.0 eV respectively, measured by XPS. Our experiments indicate a fabrication route for largely improving the field emission characteristics of CNT-based field emitters. Further more, this new technology offers a simple and scalable pathway to create nano-sized electronic circuits and devices with a 3D-structure.

Band Engineering of Carbon Nanotubes for Device Applications

Improving the emission properties of graphene–carbon nanotube hybrid nanostructures through functionalization with BaO nanoparticles and laser treatment

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.

A vacuum electron device requires a high-performance electron source that provides high current and current density. A carbon nanotube (CNT) field emission cold cathode is the optimal choice. To achieve its higher emission current capacity, its macroscale and microscale structures should be combined. Here, a two-dimensional fin-shaped CNT field emission structure is proposed, integrating a macroscale CNT fin with billions of nanoscale nanotubes. The fin contributes two-dimensional heat dissipation paths, and the nanotubes provide a high field enhancement factor, both of which enhance the high-current field emission characteristics. A model combining macro- and microstructures was simulated to optimize the structure and fin-shaped array parameters. The calculation of the field enhancement factor of the compound structure is proposed. It was also determined that the fin-shaped array configuration can be densely arranged without field screen effects, thereby enhancing the emission area efficiency. The fin-shaped CNT emitter and array emitters with different parameters were fabricated by laser ablation, which demonstrated superior field emission characteristics. A 16.55 mA pulsing emission current, 1103.33 A/cm2 current density, and 6.13% current fluctuation were achieved in a single fin-shaped CNT emitter. An 87.29 mA pulsing emission current, 0.349 A/cm2 current density, and 1.9% current fluctuation were achieved in a fin-shaped CNT array. The results demonstrate that the high-current field emission electron source can be realized in a well-designed emission structure that bridges the nanoscale emitter and macroscale structure.

Carbon nanotubes (CNTs) are one dimensional nanomaterials with unique functional properties, including electronic, mechanical and many other properties. CNTs are very promising for different nanoelectronic devices. Among them are CNT-based nano-transistors, generators, NEMS, single photon sources, components for nanophotonics, MEMS/NEMS, nanosensorics, and many others. One of the most outstanding functional properties of CNT with many future applications is electron cold field emission. The unique field emission properties of CNTs were first observed in 1994. Since 1994 there are many publications about field emission from CNTs, but till now the mechanism of its occurrence is still not clear. In many papers CNTs are considered as the thinnest metal-conducting cathode with the highest aspect ratio, i.e. the largest electric field amplification factor. In such papers the volt-ampere characteristics (VAC) obtained from CNT is considered in frame of the Fowler-Nordheim law. But in other recent papers it was shown, that in many cases experimental results show that VAC of CNT demonstrate deviations from Fowler Nordheim's law, at least at high and low currents. So the question raises about other possible mechanisms of electron cold field emission from CNT. In this report we use bottom up mechanical nano manipulation for creating nanodevices and set ups to study the electron cold field emission from CNTs. The analysis of many works on field emission from CNTs, as well as our experiments, convincingly show that the mechanism of CNT field emission is different from the emission of metal pointed cathodes. In the present report a new model of cold field emission from CNTs is suggested taking into account the Friedel oscillations of the charge density.

Macroscopic field emission properties of aligned carbon nanotubes array and randomly oriented carbon nanotubes layer

In carbon nanotube (CNT) arrays, unequal current contribution from each emitter to the overall emission current would lead to non-uniform field emission (FE) performance and emitter failure. Therefore, we propose a novel field emission microscopy (FEM) method that allows us to visualize and record the FE performance of each emitter in free standing CNT arrays. In this study, this method successfully leads us to record and observe that about 70% of the emitters participate in the FE and two of them make a very strong FE contribution in an 11×11 freestanding CNT array. This study proves that this method will facilitate the study of FE non-uniformity and emitter failure mechanism by overcoming the difficulty of observing and quantifying FE performance of each emitter in CNT arrays.

Modeling and simulation for the field emission of carbon nanotubes array

Field-emission electron sources can be used in microwave amplifiers, microthrusters, field-emission lamps, and X-ray tubes. However, both large cathode areas (multiple square millimeters) and large total currents ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$&gt;$</tex></formula> 10 mA) are required in these devices. It has been reported that the average field-emission current density can be very high when the emission area is quite small. However, the current density decreases a lot when the emission area is large. The nonuniform field emission from a large field emitter array and its influences on the total emission current are analyzed in this paper. From the results of the numerical simulations, the relations between the average field-emission current density and the size of the carbon nanotube (CNT) array are found. Due to the standard height deviation of CNTs in an array, some CNTs may be quite high if the array is large. To avoid the current overload from the highest CNT, the total field-emission current has a nonlinear relation with the area of the CNT array. Therefore, the field-emission current is limited even if the emission area is large. According to the results of the theoretical studies, a new way is proposed to enhance the field-emission current. In this method, the field emitter array is divided into a few isolated subregions. After the measurement of the field-emission characteristics of different isolated subregions, the balanced resistances are attached to these regions to improve emission uniformity. Finally, the field-emission current increases from 5 to 13.8 mA after the adoption of isolated subregions and balanced resistances, and the average current density is 1.76 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{A/cm}^{2}$</tex></formula> .

Carbon Nanotube (CNT) arrays and graphene have undergone several investigations to achieve efficient field emission (FE) owing to CNT’s remarkable large aspect ratio and graphene’s exceptional FE stability. However, when dense CNT arrays and planar graphene layers were used as field emitters, their field enhancement factor reduced dramatically. Therefore, in this paper, we numerically analyze the growth of a dense CNT array with planar graphene layers (PGLs) on top, resulting in a CNT-PGL hybrid and the associated field enhancement factor. The growth of the CNT array is investigated using Plasma Enhanced Chemical Vapor Deposition (PECVD) chamber in C2H2/NH3 environment with variable C2H2 flow, Ni catalyst film thickness, and substrate temperature followed by PGL precipitation on its top at an optimized cooling rate and Ni film thickness. The analytical model developed accounts for the number density of ions and neutrals, various surface elementary processes on catalyst film, CNT array growth, and PGLs precipitation. According to our investigation, the average growth rate of CNTs increases and then decreases with increasing C2H2 flow rate and catalyst film thickness. CNTs grow at a faster rate when the substrate temperature increases. Furthermore, as the chamber temperature is lowered from 750 °C to 250 °C in N2 environment and Ni film thickness grows, the number of the graphene layers increases. The field enhancement factors for the CNT array and hybrid are then calculated based on the optimal parameter values. The average height of the nanotubes, their spacing from one another, and the penetration of the electric field due to graphene coverage are considered while computing the field enhancement factor. It has been found that adding planar graphene layers to densely packed CNTs can raise its field enhancement factor. The results obtained match the current experimental observations quite well.

This paper describes a new approach to pressure sensors development using field emission and capacitive principles. Both sensors consist of two high doped silicon electrodes. Usually, for both pressure measurements, one electrode is anisotropic etched to obtain a sensitive membrane and the other one is solid with a carbon nanotubes (CNTs) array. The field emission sensor works on the principle that the field emission current is correlated with the electrical field intensity, i.e. the anode-emitter distance when the applied voltage is fixed. The capacitive sensor takes advantage of CNTs dimensions to increase the surface. This means that the CNTs array in the emission sensors serves as the emitter source of electrons between the cathode and the anode in the electric field and the CNTs arrays in the capacitive sensors increase the surface of the electrodes, which are similar to a plate capacitor.

Optimization for field emission from carbon nanotubes array by Fowler–Nordheim equation