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

6.12 High resolution TEM images of a 50 nm ta coated CNTs with a 10 s hydrogenation treatment, and c 50 nm ta Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon N

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

Fig 6.12 High resolution TEM images of (a) 50 nm ta

coated CNTs with a 10 s hydrogenation treatment, and (c) 50 nm ta

Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

Nanotubes Core-Shell Nanostructures

High resolution TEM images of (a) 50 nm ta-C coated CNT sample, (b) 50 nm ta

10 s hydrogenation treatment, and (c) 50 nm ta-C coated CNTs with

s hydrogenation treatment

140

C coated CNT sample, (b) 50 nm ta-C

C coated CNTs with a 30

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141

6.4.2 FE Properties of the Hydrogenated Composite Emitters

The FE J-E curves of the pristine CNTs, 50 nm ta-C coated CNTs and 50 nm

ta-C coated CNTs with 10, 20 and 30 s hydrogenation samples are shown in Fig.6.13

It is clear that the 10 s hydrogenation sample has exhibited the best FE performance, suggesting a remarkable enhancement of FE properties with respect to the pristine CNTs and the ta-C coated CNTs However, longer hydrogenation treatments gradually reduce this enhancement (20 s hydrogenation treatment) or even make it worse than the

Fig 6.13 The FE J-E characteristics of the pristine CNTs substrate and the 50 nm ta-C coated

composite emitters with varied hydrogenation durations (10, 20 and 30 s) The corresponding

F-N plots are shown in the insert

0

2

4

6

8

0.2 0.3 0.4 0.5 0.6 0.7 -14

-12 -10 -8 -6 -4 -2 0

2)

1/E

50 nm ta-C ta-C with 10s H ta-C with 20s H ta-C with 30s H Pristine CNTs

2 )

Applied electric field, E (V/µµµµm)

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142

original ta-C coated one (30 s hydrogenation treatment) Here, the threshold field,

which can be obtained from the J-E curve, is defined as the electric field where

emission current density arrives at 1 mA cm-2 [30] The threshold field values for the pristine CNTs, ta-C coated CNTs and the coated CNTs with 10, 20 and 30 s hydrogenation samples are 4.45, 4.01, 2.64, 3.59 and 4.11 V µm-1, respectively For the pristine CNTs, 50 nm ta-C coated CNTs and 10 s hydrogenated ta-C coated CNT samples, their threshold field values exhibit a decreasing trend, suggesting that the electron emission behavior was easier to take place for the coated CNTs than the pristine CNTs and even easier for the slightly hydrogenated sample In order to investigate the enhanced FE mechanism, F-N theory was employed to estimate the emission barrier heights of the three kinds of samples As the ta-C coated CNTs and the hydrogenated samples are merely the pristine CNTs with ultrathin film

coating on the tube surface, the β values of these three kinds of samples can be assumed

to be the same Thus, according to Eq (2.4), their emission barrier height ratios can be estimated by

3 / 2

2 1

2

1













=

Slope

Substituting the slope values from the F-N plots gives their emission barrier height ratios, which are

1 : 39 1 : 45 1 :

where Ø cnt , Ø tac and Ø h represent the barrier heights for the pristine CNTs, ta-C coated CNTs and the 10 s hydrogenated samples This result is exactly consistent with the decreasing trend of the threshold fields exhibited by these samples during FE process,

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143

confirming the proportional influence of the barrier height to the commencement of FE behavior In other words, the lower the barrier height for the electron tunneling, the easier for the launch of electron emission

In order to further confirm the estimated barrier height ratios of these samples, UPS technique was employed to measure their work function values at room temperature The work function values for the pristine CNTs, ta-C coated CNTs and the hydrogenated samples were measured to be 4.81, 4.73 and 4.38 eV, respectively The trend of these values obtained through UPS measurement matches well with the

barrier height ratios obtained via calculation, suggesting that the assumption of equal β

values is reasonable Namely, the geometry of these three kinds of samples has nearly equal contribution to the electron emission

The mechanism of the FE enhancement of the 10 s hydrogenated sample is probably due to the C-H dipole formed at the ta-C surface [27, 31] As hydrogen possesses a lower electronegativity than carbon, the C-H bond would be polarized with a positive charge on the H atom, resulting in a C-H dipole pointing from the sample surface toward the film The generated potential would help to pull the vacuum level down and to lower the electron affinity or emission barrier height of the sample surface

On the other hand, the FE enhancement mechanism of the surface hydrogenated sample could also be rationalized considering the hydrogenation effect on the diamond Recently, researchers have found that hydrogenation termination would absorb a water layer in air and result in a raised valence band maximum of diamond, in other words, a

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144

reduced barrier height for the electron emission [12, 27, 28] The hydrogenation treatment may have played a similar role on the ta-C surface.According to this theory, after hydrogenation, the ta-C surface would absorb a thin water layer when exposed to atmosphere as indicated in Fig 6.14 The electrochemical potential (µe) value of the absorbed water layer can be calculated via transformation of Nerst’s equation:[12]

)]

( log 4

)[

4 / 0592 0 ( ) 229 1 )(

1 ( 44

here, the electrochemical potential of electrons is -4.44 eV under the standard hydrogen electrode (SHE) conditions while the standard electrode potential of the reactions is +1.229 eV versus SHE Due to the CO2 content in air, the standard atmospheric

conditions lead to pH ≈ 6 Substituting the partial pressure pO 2 ≈ 0.21 bar into Eq (6.3) gives µe = -5.3 eV The work function value of the ta-C coated sample without hydrogen

termination is Ø = 4.73 eV, suggesting the Fermi energy level (E F) at the sample surface

is 4.73 eV below the vacuum level (E vac) and also (-4.73 eV) - (-5.3 eV) = 0.57 eV above the electrochemical potential level of the water layer Driven by this potential difference, electrons in the emitters would transfer to the water layer through the aqueous redox couple below until these two energy levels aligned:

O2 + 4H＋ + 4e－ 2H2O

In this process, the barrier height for electron emission would reduce resulted from the band bending as shown in Fig 6.14 This explains the FE enhancement of the hydrogenated triple layered nanocomposite specimen

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Fig 6.14 Illustration of the band bending of

water layer in air E vac represents the vacuum level,

and E v is the valence band maximum Driven by the potential difference of the ta

energy level (E F) and absorbed water electrochemical potential (

surface would transfer to the water layer until these two energy levels aligned

With surface hydrogenation treatments longer than 10 s, the FE performance these composite emitters deteriorate

deterioration is probably due to the severe damage of the nanostructures caused by the plasma etching effect as shown in the TEM images

excellent route for electron transpo

sp2 carbon bonds possess

CNTs are essentially compo

treatments tend to etch CNT

surface of the composite emitters

hydrogen plasma treatment This finding is consistent with that

Hong et al [32] The etching of CNTs

become difficult, thereby resulting in poor FE performance

Illustration of the band bending of the ta-C film in equilibrium with the absorbed

represents the vacuum level, E c refers to the conduction band minimum

is the valence band maximum Driven by the potential difference of the ta

) and absorbed water electrochemical potential (µe), electrons at the ta

ce would transfer to the water layer until these two energy levels aligned

With surface hydrogenation treatments longer than 10 s, the FE performance these composite emitters deteriorated with the increase of the treatment duration This deterioration is probably due to the severe damage of the nanostructures caused by the plasma etching effect as shown in the TEM images It is well known that CNT is an excellent route for electron transport, which is fundamentally good for

carbon bonds possess lower etching resistance than sp3 carbon bonds

composed of sp2 carbon bonds, hence longer hydrogen plasma tend to etch CNTs as shown in Fig 6.15 that the sp2 content detected at surface of the composite emitters decreases from around 52% to 46% after

hydrogen plasma treatment This finding is consistent with that previously

The etching of CNTs makes the electron transport along the emitter

resulting in poor FE performances

145

in equilibrium with the absorbed refers to the conduction band minimum

is the valence band maximum Driven by the potential difference of the ta-C Fermi

), electrons at the ta-C

ce would transfer to the water layer until these two energy levels aligned

With surface hydrogenation treatments longer than 10 s, the FE performances of

with the increase of the treatment duration This deterioration is probably due to the severe damage of the nanostructures caused by the

It is well known that CNT is an

for FE However, carbon bonds Pristine hydrogen plasma content detected at the

% after the 10 s previously reported by electron transport along the emitters

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Fig 6.15 Carbon 1s core level XPS spectra of the (a) 50 nm ta

hydrogenated 50 nm ta-C coated CNT samples, indicating an increased sp

The J-E curves of the pristine CNTs, 100 nm ta

coated CNTs with 10, 20 and 30 s hydrogenation samples are shown in Fig

same features can be observed in this figure with that of

samples, confirming the

properties of the ta-C coated CNTs Using the same analyzing method, the barrier

Carbon 1s core level XPS spectra of the (a) 50 nm ta-C coated CNTs and (b) 10 s

C coated CNT samples, indicating an increased sp3 hydrogen plasma treatment

curves of the pristine CNTs, 100 nm ta-C coated CNTs and 100 nm ta coated CNTs with 10, 20 and 30 s hydrogenation samples are shown in Fig

s can be observed in this figure with that of the 50 nm ta

reliability of the surface hydrogenation effect on FE

C coated CNTs Using the same analyzing method, the barrier

146

C coated CNTs and (b) 10 s

3 content after

C coated CNTs and 100 nm ta-C coated CNTs with 10, 20 and 30 s hydrogenation samples are shown in Fig.6.16 The

50 nm ta-C coated reliability of the surface hydrogenation effect on FE

C coated CNTs Using the same analyzing method, the barrier

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147

height ratios of the pristine CNTs, 100 nm ta-C coated CNTs and the 10 s hydrogenated ta-C coated CNT samples were calculated to be approximately 2.10 : 2.01 : 1

Fig 6.16 The FE J-E characteristics of the pristine CNT substrate and the 100 nm ta-C coated

composite emitters with varied hydrogenation durations (10, 20 and 30 s) The corresponding

F-N plots are shown in the insert

6.5 Summary

In this chapter, the core-shell CNT/ta-C nanostructures have been successfully fabricated The ta-C film thickness effect and the hydrogen plasma treatment duration effect on the FE properties of the composite emitters have been thoroughly investigated Results show that the coating film thickness correlates with the FE

0

2

4

6

8

-10 -8 -6 -4 -2 0

2)

1/E

100 nm ta-C ta-C with 10s H ta-C with 20s H ta-C with 30s H Pristine CNTs

2 )

Applied electric field, E (V/µµµµm)

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148

performance of the emitters and there exists an optimum thickness of the ta-C coating film, i.e., 50 nm in this case With the change of the ta-C film thickness, not only would the surface work function change due to the substrate-induced effect, but also the effective emission potential barrier and the electron transport would be affected In addition, a slight hydrogen plasma treatment, i.e., 10 s hydrogenation would significantly enhance the FE properties of the composite emitters due to the positive C-H dipoles generated at the sample surface and the reduced surface barrier height resulted from the energy band bending caused by the charge transfer between the ta-C and the absorbed water layer on its surface However, longer hydrogen plasma treatments (> 10 s) would degrade the FE performance by severely damaging the structures thereby making electron transport within the emitters become difficult

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