Characterization of Chromium Carbide Capped Carbon

In field emission theory, field enhancement factor, ß, is a term that further improved the field emission efficiency by geometric appearance. For a material that used as an emitter, ß would be larger when the emitter stands vertical from the substrate or parallel to the electric field because of the higher aspect ratio and smaller tip radius. Approaches have been made to aligned carbon nanotubes.^[161-163] In particular we have recently synthesized a new material^[164] using bias-assisted microwave plasma chemical vapor deposition, which grow perpendicular to the substrate with amazing uniformity.

Fig. 4.24(a) shows the surface morphology of chromium carbide capped carbon nanotips from the edge part of silicon substrate. The specimen was grown with a bias of 150V for 30min and the H2/CH4=30/10. The image demonstrates the great uniformity and the selective growth of the nanotips, which are important issues in field emission applications. Fig. 4.24(b) shows the magnified view of the nanotips. It has to be noticed that there are white spots on top of the nanotips for each one and the tip-like structure with their root connect to each other.

The average size of the nanoparticle on the tip is 50nm with its neck, the as-grown carbon nanotip of 40nm and increasing it’s diameter to the root part.

Fig. 4.25(a) shows the cross-section TEM image of the chromium carbide capped carbon nanotip. It is also found that the dark spots exit only on top of the nanotips without any residual at the bottom, this is quite distinct from carbon nanotubes where tip growth model and base growth model co-exist.^[165] Analysis from diffraction pattern shows that the specific composition of the crystalline structure to be Cr7C3.^[164] Fig. 4.25(b) shows individual crystalline chromium carbide embedded on a carbon nanotip with lots of defects such as twins or stacking faults. The phenomenon is believed due to the low temperature process and rapid formation of chromium carbide relative to the typical high temperature and log time for hard

coatings of Cr7C3.^[166] In fact, in the XRD analysis, several minor peaks indicate other carbides which include Cr3C2 and Cr23C6. Report^[167] has shown that chromium forms three stable carbides, Cr3C2, Cr23C6 and Cr7C3. Below 1000℃ no significant homogeneity has been detected for either the three carbides. This indicates that the three carbides are inevitable exit at the same time. The phenomenon makes it hard to determine the electronic property, for example, work function, that is important for the field emission. The inset in Fig. 4.25(b) shows the lateral section of the carbon nanotips with tip end points to the right. It shows the parallel graphitic layers stretch along the growth direction just like carbon nanofiber^[168] but accompany with decreasing shorter layer around them.

Fig 4.26 shows the surface morphologies of nanotips grew with different H2/CH4 ratio.

As can be seen, the nanotip structure grows when the H2/CH4 ratio is higher than 30/10 [Fig.

4.26(b), (c)], otherwise only carbon film is formed [Fig. 4.26(a)]. When the H2/CH4 ratio reaches 100/10, the structure starts to lose its shape [Fig. 4.26(d)]. Fig. 4.27 shows the morphologies of nanotips grew at different biases. No nanotips would grow [Fig. 4.27(a)]

unless there’s sufficient bias. It is found that only when the bias is larger than 100V, the nanotips would grow [Fig. 4.27(b)]. With the increasing bias, the nanotips change their appearance. With high bias of 200V or 300V [Fig. 4.27(c), (d)], an “asparagus-like” structure is formed.

The graphitization of the nanotips is shown in Fig. 4.28 and is expressed by ID/IG ratio obtained from Raman spectroscopy. The curve of ID/IG ratio on the top-right indicates the trend of graphitization with the same bias of 150V. Similar to carbon nanotubes, the ID/IG ratio decrease with the increasing hydrogen flow that represents the higher degree of graphitization due to selective etching between amorphous carbon and graphite. Another curve represents the graphitization degree with different biases that grow with H2/CH4=40/10. The increasing bias corresponding to a lower ID/IG ratio shows that the applied bias effectively assists the deposition of graphite and removes the amorphous carbon. Fig. 4.29 shows the growth of tip

length with time under bias of 150V and 300V. The results show that the high bias of 300V is effective in assisting the growth of nanotips at initial stage; however, the ultimate length is about 0.5µm, much shorter than the 150V case, which is about 1.5µm.

The surface morphologies change with growth time is demonstrated in Fig 4.30. Fig.

4.31 represents the schematic diagram of the corresponding growth stages showed in Fig. 4.30 for the growth of chromium carbide capped carbon nanotips. The substrate was first treated with H2 plasma to remove the passive layer and created certain roughness [Fig. 4.30(a) and Fig. 4.31(a)]. The reaction gas mixtures were then flow into the chamber to join the deposition process [Fig. 4.30(b) and Fig. 4.31(b)]. As the deposition keeps on, the carbon tends to deposit on the root of the nanotips and the nanotips therefore connect to each other on the root part [Fig. 4.30(c) and Fig. 4.31(c)]. After the catalyst lost its activity, the nanotips stop to grow and the following carbon therefore forms a continuous film [Fig. 4.30(d) and Fig.

4.31(d)]. Analogous to carbon nanotubes, chromium act as catalyst for the precipitation of carbon atoms, which may be a vapor-liquid-solid growth mechanism similar to the growth of carbon nanotubes^[169] or carbon nanofiber.^[168] But one thing is that the chromium did carburize during deposition, which the carbide is very stable. This suggests that surface diffusion^[170] become the main process for the growth. The carbon nanotips also increase its diameter during growth by vapor-solid process which means the carbon species deposited directly onto the tip body. The applied bias larger than 100V is essential for the growth of chromium carbide capped carbon nanotips. This is probably because the bias provides a higher reactant concentration around the chromium with relatively higher ion energy, and also the sharpness of the tips induces strong charge to attract positive ions. The high activation energy of chromium to diffuse carbon also leads to the growth of carbon nanotubes using chromium as catalyst been rarely seen.

Interestingly, bias helps the growth of the tips, but also destroys it, too. Accompany with the growth of nanotips, ion bombardment effect is drastic due to the strong field applied

by the DC bias^[171]. The growth of chromium carbide capped carbon nanotip is a competition between carbon deposition and ion bombardment. Under bias of 150 volts, the diameter of carbon nanotips increases from the as-grown diameter of 40nm with increasing time. But under high bias of 300V, the diameter of nanotip maintains about 40nm or less and some even breakdown from its neck, which is revealed in Fig. 4.27(c), (d). This is also direct evidence that the chromium carbide is efficiently resistant to ion bombardment and provides shield the carbon nanotip, which might indicate a way to improve field emission life time for applications. According to the phenomenon we observed, unlike carbon nanotubes of carbon nanofiber, a length limit exists. This might due to the fully carburization of chromium that leads to a result of deficiency in diffusing carbon. Bias in microwave plasma chemical vapor deposition for most of cases increases the deposition rate.^[151] The higher bias contributes to the faster formation of chromium carbide that limits the length of the nanotip. The following carbon thus starts to deposit around the chromium carbide, where the high field induced by tip sharpness and a higher carbon supplement become the preferential sites and makes them look like asparagus.

In fact, some similar phenomenon were also been observed for other elemental catalyst.

In this way, from Fowler–Nordheim model,^[109] the work function, φ, can be easily tuned for further improvement in field emission or use the special property of materials in some specific applications.

(a) (b)

Fig. 4.24 (a) Low magnification SEM image showing the uniformity of the vertical aligned chromium carbide capped carbon nanotips and (b) higher magnification top view image.

(b) (a)

Fig. 4.25 TEM images showing (a) the cross-section view of chromium carbide capped carbon nanotips and, (b) high magnification view of an individual carbon nanotip and the inset shows the chromium carbide head.^[164]

(b) (a)

(c) (d)

Fig. 4.26 SEM images of surface morphology change with H2/CH4 of (a) 10/10, (b) 30/10, (c) 50/10 and (d) 100/10.

(c) (d) (a) (b)

Fig. 4.27 SEM images of surface morphologies with applied biases: (a) 100V, (b) 150V, (c) 200V and (d) 300V.

100 150 200 250 300

Fig. 4.28 Ratios of ID/IG with methane concentrations and biases.

0 10 20 30 40 50 60

Fig. 4.29 Tip length variation with growth time under applied bias of 150V and 300V.

(b) (a)

(c) (d)

Fig. 4.30 Surface morphologies of chromium carbide capped carbon nanotips with different growth time.

H2 Plasma CH4, H2

Adsorption of carbon Diffusion of carbon

(a) (b)

(c) (d)

Fig. 4.31 Schematic diagram of the proposed growth model: (a) formation of the nucleation process; (b) cap growth; (c) deposition of carbon; (d) asparagus-like structure forms.

4.4.2 Field Emission Properties of Chromium Carbide Capped Carbon Nanotips and