Study of field emission characteristics of ultrathin film coated carbon nanotubes core shell structures 6b

Chapter 6 Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Fig.. The deviation of the F commonly observed for semiconductor field overheating of the emitte

Chapter 6 Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

Fig 6.9 Schematic illustration of effective potential area (shadowed parts) of electron tunneling varying with the change

represents the vacuum level,

CBM means the conduction band minimum, and VBM is the valence band maximum

In addition, it is noted that the F

regions with a knee point in between, one in the lower field region and the other in the higher field region The deviation of the F

commonly observed for semiconductor field

overheating of the emitter tips and

Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

Nanotubes Core-Shell Nanostructures

Schematic illustration of effective potential area (shadowed parts) of electron tunneling varying with the change of the thickness of the coating ultrathin film

represents the vacuum level, E F donates the Fermi energy, Ø is the work function of CNT,

CBM means the conduction band minimum, and VBM is the valence band maximum

noted that the F-N plots of some samples comprise regions with a knee point in between, one in the lower field region and the other in the

deviation of the F-N plot in the high electric field region is commonly observed for semiconductor field emitters and it is probably due to overheating of the emitter tips and the space charge effect [20-23] More specifically,

Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

134

Schematic illustration of effective potential area (shadowed parts) of electron

thickness of the coating ultrathin film E vac

is the work function of CNT, CBM means the conduction band minimum, and VBM is the valence band maximum

comprise two linear regions with a knee point in between, one in the lower field region and the other in the

N plot in the high electric field region is emitters and it is probably due to the

More specifically,

with the increase of ambient temperature during emission process, the work function of the emitter would change such that the emitter exhibited enhanced FE performance On the other hand, space charge would be generated around the emitter tips during emission, which sharply reduced the local electric field on the emission sites As the ta-C mostly consists of sp3 carbon bonds while CNTs are rich in sp2 bonds, deposition

of ta-C onto the CNT surface would result in a decreased conductivity, which explains the more severe space charge effect of the ta-C coated samples during emission process

6.4 Hydrogenation Effect on FE Properties of the Composite Emitters

Hydrogenation treatment is a commonly used method to improve the surface conductivity of diamond [24-28] Recently, surface hydrogenation has been found to

be capable of significantly reducing the work functions of DLC thin films [29] Theoretically, higher conductivity and lower surface work function for diamond or ta-C films would enhance the FE properties of the ta-C coated CNT emitters Therefore, in this project, hydrogen plasma treatments with varied durations (10, 20 and 30 s) were conducted on the 50 nm ta-C coated CNT composite emitters in order

to investigate the hydrogenation effect on their FE performances The same

Chapter 6 Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

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were used as substrate in this project

6.4.1 Characterization by SEM and TEM

Fig 6.10 shows the top and cross-sectional view SEM images of the 50 nm ta-C coated composite emitters with and without hydrogen plasma post-treatment From the top view it can be clearly observed that the nanotubes become increasing thinner

as the duration of hydrogen plasma treatments was increased From the cross-sectional view it is obvious that without hydrogenation treatment, the CNT tips are wrapped with thin films, which look like a layer of whiskers formed with the average diameter larger than that of the pristine CNTs and the whiskers extended from the tips of the CNTs With a 10 s surface hydrogenation treatment, the sample appears similar with the one without hydrogenation treatment in cross-sectional image However, the tips of these whiskers seem to protrude out and stand freely rather than exhibiting curly shape at the tips like the pristine ta-C coated CNTs do With 20 s hydrogenation, the surface of the sample becomes very flat, without many random protrusions of individual nanotubes Moreover, little or no thick whiskers can be observed from this image With an even longer hydrogen plasma treatment, i.e., 30 s hydrogenation duration, strictly no thick whiskers can be found on the CNT tips The surface is rather flat and the average diameter of these nanotubes is comparable to that

Fig 6.10 Top and cross-sectional view SEM images of composite emitters (a) and (b) 50 nm sectional view SEM images of composite emitters (a) and (b) 50 nm sectional view SEM images of composite emitters (a) and (b) 50 nm

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of the pristine CNTs Hence, it is reasonable to assume that the coated tips of these nanotubes were fully etched away by the hydrogen plasma This assumption was further confirmed by the emitters’ average length reduce, which was around 0.5 µm comparing the 30 s hydrogenation sample to the 20 s sample The same features can

be extracted as well from the SEM images of the 100 nm ta-C coated composite emitters with and without hydrogen plasma post-treatment as shown in Fig 6.11 High resolution TEM images of the composite emitters with and without hydrogenation treatment are shown in Fig 6.12 Fig 6.12(a) confirms the core-shell structure of the composite emitter After the 10 s hydrogen plasma treatment, the edge

of the composite nanotube was slightly etched as shown in Fig 6.12(b) With the 30 s hydrogenation treatment, the nanotube was severely etched at the surface such that the CNT can be hardly observed, which was believed to be entirely etched away by plasma as shown in Fig 6.12(c)

Fig 6.11 Top and cross-sectional view SEM images of composite emitsectional view SEM images of composite emitters (a) and (b) 100 nm ters (a) and (b) 100 nm