Study of the Emission Characteristics of Single-Walled CNT and Carbon Nano-Fiber Pyrograf III

Field emission microscopy measurements from Single-Walled Carbon Nanotubes (SWCNTs) and Carbon Nano-Fibers Pyrograf III PR-1 (CNF) were performed. Details of the materials employed in the experiments are as follows: (a) Carbon Nano-Fibers Pyrograf III PR-1 (CNF), having an average fiber diameter that is ranging between (100-200) nm with a length of (30-100) μm. (b) Single walled Carbon Nanotubes were produced by high-pressure CO over Fe particle (HiPCO: High-Pressure Carbon Monoxide process), having an average diameter ranging between (1-4) nm with a length of (1-3) μm. The experiments were performed under vacuum pressure value of (10−7 mbar). The research work reported here includes the field electron emission current-voltage (I-V) characteristics and presented as Fowler-Nordheim (FN) plots and the spatial emission current distributions (electron emission images) obtained and analyzed in terms of electron source features. For both the SWCNT and the CNF a single spot pattern for the electron spatial; distributions were observed.


Introduction
It is known that Pyrograf-III can be uniformly dispersed in a polymer in order to provide maximum conductive network using mixing and compounding equipment [1]. Electron emission (FE) refers to the release of electrons from the surface of a material into vacuum [2], which is occurring when the electric field strength is at the scale of (10 9 V.m -1 ) [3]. FE has a long history up to the derivation of the original Fowler-Nordheim (FN) equation in 1928. In early days of FE experiments, metal was used as field emitters. However, because of the active nature of materials the residual gases and ion bombardment during FE working limited the lifetime of metal field emitters, impeding the development of FE [4]. Since discovering of Carbon Nanotubes (CNTs) from 1991 to 1993 with both types Multi-Walled Carbon Nanotubes (MWCNTs) and Single Wall Carbon Nanotubes (SWCNTs) by Sumio Iijima [5] [6], it has a growing attentions due to their perfect properties that make it the best candidate to be the next generation of field emitters such as, low turn-on fields, high emission current, stable emission current, long lifetime, low energy spreading, and high brightness [7].  The history of Carbon Nano-Fibers (CNFs) goes back more than a century. In a patent published in 1889 [8], it is reported that Carbon filaments are grown from Carboncontaining gases using a metallic crucible as the catalyst [9]. Carbon nanofibers (CNFs), also known as Stacked-Cup Carbon Nanotubes (SCCNTs), is with in the class of materials termed Multi-Walled Carbon Nanotubes and are produced by the floating catalyst method. CNFs are discontinuous and highly graphitic, but there are differences in the structure as figure (1) shows. (SCCNTs) or (CNFs) have a unique morphology in that graphene planes are canted from the fiber axis, resulting in exposed edge plane on the interior and exterior surfaces of the fiber; CNTs, on the other hand, typically resemble an assembly of concentric cylinders of graphene [10]. Fowler-Nordheim (FN) first derived a semiclassical theory of FE based on physical assumptions and mathematical approaches; the theory has been modified many times over years to describe Cold Field Emission (CFE) from bulk metal where it is known as Fowler-Nordheim (FN) equations.  The simplest of these is the so-called elementary FN-type equation Where ݅ is the current emission, ߶ local work function, ܸ is the applied voltage, ܽ and ܾ are the first and second FN constants, as usually defined in Ref [11], ߚ is the local voltageto-surface field conversion factor at some specific reference point on the emitter surface, and ‫ܣ‬ is the national emission area defined by considering the local emission current density at the reference point. This equation and related equations that use macroscopic field as an independent variables have often been used to describe CFE from Carbon Nanotubes (CNTs) including closed single walled (CNTs) of small apex radius, particularly in technological context [12].

Materials and Methods
The apparatus are used to create a prepared glass micro-point emitter where the CNTs inserted in it, so they would protrude at the tip. Figure (2) and (3) shows the produced sample. This study uses two types of CNTs, (a) Single-Walled Carbon Nanotubes (SWCNTs) were produced by high-pressure CO over Fe particles (HiPCO: High-Pressure Carbon Monoxide process), having an average diameter ranging between (1-4) nm with length (1-3) µm [13].

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The Both tips prepared by employing a drawing technique using a glass puller, figure (4) shown the apparatus that are used to produce such tips, then the emitters are mounted in a standard Field Emission Microscopy (FEM) with an emitter-screen distance of ~10 mm [14], and current limiting resistor of (100 MΩ) is used. The system was evacuated to pressure ~10 -8 mbar after baking at ~150⁰C overnight.
The FEM applied voltage V is increased slowly until a "switch-on voltage" VSW is reached, at which point the emission current suddenly "switch-on", at VSW the current increase rapidly from about a Nanoampere to a much greater saturated value ISW, by slowly decreasing the applied voltage to a lower value the emission current starts to be decreasing until limit reached, beyond which the emission current falls smoothly to zero as the applied voltage is decreased to the threshold value VTH [15].

Results and Discussion
This section describes the results of experiments on the emission characteristics of the Carbon Nano-Fibers and Single-Walled Carbon Nanotubes. These results include the I-V characteristics, Fowler-Nordheim (FN) plot, and emission images. Emission current measurements are made using a pico-ammeter connected between the cathode and earth.
As the applied electric field is slowly applied, the emission is only observed after a threshold field is reached, and emission suddenly jumps to a constant saturated value ISAT, further increasing the applied electric field the VMAX will be reached, after that the tip explode, as the voltage slowly decreasing, the emission current fall to nearly zero, reaching the threshold value VTH. Figure (2) shows an optical image of the first tip to be discussed. As we increase the applied voltage slowly, "switch-on" phenomena has been occurring at applied voltage value VSW = 800 V, with emission current value ISW = 4 µA, then the applied voltage was reduced, the VSAT remained constant to the saturated voltage value of VSAT = 400 V, with emission current value ISW = 1.06 µA, further voltage reduction, the emission current falls smoothly  The experiment was repeated under the same condition and the results were as follows, the electron emission is initiated at an applied voltage of (320 V) with emission current value (2 pA), and by continue increasing the applied voltage, a point was reached, at VSW =650 V, where the emission current suddenly "switched-on" to a stable saturated value ISAT, and by decreasing the applied voltage the saturated region extends down to value VSAT =380 V, with emission current value (1.06 µA). Further voltage reduction causes the emission current to fall to nearly zero, reaching the threshold value VTH =140 V, with emission

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The The second type of CNTs have been tested under the same conditions and the results were as follows, as the applied voltage increased slowly, the emission current initiated at applied voltage value (200 V), with emission current value (20 pA), then the emission current suddenly "switched-on" at VSW =1100 V, ISW=1.2 µA, then as we reduce the applied voltage, the saturation region extends down to VSAT =400 V, with emission current value of (1.09 µA), further reduction, the emission current falls down to (10 pA) at applied voltage value (150 V). Figure (9) shows the I-V behavior during increasing and decreasing cycle, as well as FN plot.
As we did for the first type of CNTs, we did the same for the second type, the test has been repeated for another cycle of increasing and decreasing the applied voltage, under the same conditions, and the results were as follows, as we increase the applied voltage the emission current start from (20 pA) at applied voltage value (350 V), then the current emission suddenly "switch-on" to (3.6 μA) at applied voltage value (750 V), we continue increasing the applied voltage until we reached (1900 V), and the emission current was (12.3 µA), we

Conclusions
Both types of the CNT emitters have been prepared by employing a drawing technique using glass puller, and the I-V characteristics and FN plot has been investigated, also the emission images have been studied, and it can be concluded by comparing their characteristics that SWCNTs have the lower VSW than CNFs Pyrograf Ⅲ PR-1 and larger emission current value at same applied voltage, the possible reason for this result is that the amount of field penetration in SWCNTs is larger than CNFs, which effects on the effective work function value. And the charge distribution on the tip apex [16].