Field Emission Properties of Chromium Carbide Capped Carbon

The previous section has shown the uniformity and nanosize of the chromium carbide capped carbon nanotips. The characters make it a good candidate for field emission applications. In our team’s previous work, an original type of carbon nanotips was synthesized on bare silicon. Due to their similarities of structure (which both consist of graphite body) and dimension (which are about 1µm, relatively short compare to the CNTs), we compare the two materials to figure out the effects of the chromium carbide on the field emission properties.

Fig. 4.32(a) and (c) show the surface morphology of the carbon nanotips and chromium carbide capped carbon nanotips, respectively by SEM observation. The growth parameters were chosen to grow the two kinds of nanotips to about 0.5µm in their length, as to ensure the similar conditions for field emission measurements. The tip end of the carbon nanotips consist of several graphene layers which were less than 0.1 nm [Fig. 4.32(b)]. The d-spacing of the graphite is 3.57Å which showed a deviation from perfect crystalline graphite (002) plane of 3.395Å and a large difference from Si(100) of 3.290Å. The chromium carbide capped carbon nanotips ends with crystalline nanoparticles which was about 40nm and the diameter of the graphite right beyond the chromium carbide was a few nanometers smaller than the chromium carbide [Fig. 4.32(d)]. The chromium film deposited for the growth of the capped carbon nanotip was used as catalyst and was carburized into chromium carbide which was gradually lifted-off during growth. The diameter of the two kinds of nanotips gradually increased toward the substrate via vapor-solid growth and may reach to about a hundred nanometers.

The graphene layers of the nanotips body were perpendicular to the silicon substrate for both nanotips and were important to electron conduction.

Relationship between field emission current density with electric field of the as grown materials was shown in Fig. 4.33. Due to the ultra-sharpness of the bare carbon nanotips, a

low turn-on field (defined as the field required to emit a current density of 10µA/cm²) of 1.40V/µm can be reached. With chromium carbide, the tip structure exhibited a much higher turn-on field of 3.50V/µm. Basically, the results differed from the surface appearances of which the field enhancement factor was greatly affected by the geometrical distribution. The sharp and less dense of the carbon nanotips also comparatively reduced screening effect [10, 11] due to a larger tip-to-tip spacing. The insets of Fig. 4.33(a) and (b) show respectively the Fowler-Nordheim (FN) relations. The linearity of the plot at the initial applied voltages before turn-on suggested the behavior of FN tunneling of the electron emission process.

Fig. 4.34 shows the applied field as a function of emission time under constant current density which was set to 1mA. Total emission time was 36,000 seconds, which was 10 hours.

The degree of stability of field emission was obviously quite different. The mean electric field for chromium carbide capped carbon nanotips was 8.02V/µm with fluctuation less than 5 percent can be achieved. On the contrary, although the carbon nanotips showed a superior turn-on and thresholds field on the beginning of the test, the situation became chaotic after about ten thousand seconds. The oscillation was mainly due to the physical damage of the tip, which was confirmed by post observation of SEM. Instead, the chromium carbide capped carbon nanotips showed a smoother surface after the test without distinguishable major change. This phenomenon suggests that chromium carbide, known for its excellent oxidation resistance and hardness, effectively shield the carbon nanotips from destruction, which may mainly due to ion bombardment, heating and oxidation. Here we had assumed that the field emission current arise from the chromium carbide nanoparticles since the ends of the nanotips suffer a larger electric field and field enhancement, in contrast to the graphite body.

On the other hand, the extremely sharp of the carbon nanotips would cause a great amount of current density arise over the tip and generate a lot of heat due to ohmic heating.

The uniformity of the capped carbon nanotips also benefited to the lifetime experiment while the disorderliness of the uncapped nanotips causes un-equivalent current contribution and as a

result of breakdown sequentially.

Chromium carbide has been synthesized and used for long period of time, but were focus on the bulk applications such as hard coatings for tribological use. Therefore, the electric properties have not been totally explored so far. Reports have shown that Cr3C2 with conductivity of 6.8×10⁶Ω^-1cm^-1 was used as conductive ceramics^[172] and Cr7C3, which was the main structure of the chromium carbide nanoparticles in this study, may exhibit metallic behavior proposed from the chemical bonding structure.^[173] In fact, the defects in the chromium carbide may also provide a low energy conduction path.^[174]. In this case the situation may be more complex since we were dealing with more than one material. The difference in work function between the carbides or between carbide and graphite could form ohmic and Schottky junctions which may limit the current. Even though the existence of the chromium carbide may not be favorable for electron conduction, the steady emission had shown importance for practical application.

Fig. 4.35 shows the image that was taken by digital camera from the vacuum chamber during the field emission measurement for chromium carbide capped carbon nanotips. The image shows the phosphor being light up by the field emission current with an acceptable uniformity.

(a) (b)

3.57Å

(c) (d)

Fig. 4.32 SEM images showing the surface morphology of (a)(b) carbon nanotips and (c)(d) chromium carbide capped carbon nanotips.

0 1 2 3 0.00

0.05 0.10

Curent Density (mA/cm2 )

Electric Field V/µm

(a)

0.2 0.4 0.6 0.8 1.0 1.2 1.4

1/V ln(I/V2 )

0 1 2 3 4 5

0.00 0.05 0.10

Curent Density (mA/cm2 )

Electric Field V/µm

(b)

0.0 0.2 0.4 0.6 0.8

1/V ln(I/V2 )

Fig. 4.33 The field emission current density as a function of the electric field for (a) carbon nanotips and (b) chromium carbide capped carbon nanotips. The insets show the Fowler-Nordheim plot for each material.

0 10000 20000 30000 0

2 4 6 8 10 12

Chromium carbide capped Carbon nanotips

Carbon nanotips

Field (V/µm)

Emission time (seconds)

Fig. 4.34 Plot of electric field with emission time for carbon nanotips and chromium carbide capped carbon nanotips. The current was set to a constant value of 1mA.

Fig. 4.35 Image of the lighted phosphor in vacuum chamber during field emission measurement test which is taken by digital camera.

4.5 Synthesis and Field Emission of Chromium Carbide capped Carbon