Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.12 High resolution TEM images of (a) 50 nm ta C coated CNT sample, (b) 50 nm ta ta-C ta-C coated CNTs with a 10 s hydrogenation treatment, and (c) 50 nm ta C coated CNTs with a 30 ta-C s hydrogenation treatment 140 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 -2 ln (J/E2) Current density, J (mA/cm ) -4 -6 -8 -10 -12 -14 0.2 0.3 0.4 0.5 0.6 0.7 1/E 50 nm ta-C ta-C with 10s H ta-C with 20s H ta-C with 30s H Pristine CNTs 0 Applied electric field, E (V/µm) 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 141 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 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 φ1  Slope1   = φ2  Slope2    2/3 (6.1) Substituting the slope values from the F-N plots gives their emission barrier height ratios, which are φcnt : φtac : φh = 1.45 : 1.39 : (6.2) where Øcnt, Øtac and Øh 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, 142 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 143 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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] µe = −4.44 + (−1)(+1.229) + (0.0592 / 4)[4 pH − log10 ( pO2 )] (6.3) 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 ≈ Substituting the partial pressure pO2 ≈ 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 (EF) at the sample surface is 4.73 eV below the vacuum level (Evac) 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 144 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.14 Illustration of the band bending of the ta-C film in equilibrium with the absorbed water layer in air Evac represents the vacuum level, Ec refers to the conduction band minimum and Ev is the valence band maximum Driven by the potential difference of the ta Fermi ta-C energy level (EF) and absorbed water electrochemical potential ( e), electrons at the ta (µ ta-C surface would transfer to the water layer until these two energy levels aligned ce With surface hydrogenation treatments longer than 10 s, the FE performance of performances these composite emitters deteriorate with the increase of the treatment duration This deteriorated 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 transpo which is fundamentally good for FE However, transport, sp2 carbon bonds possess lower etching resistance than sp3 carbon bonds Pristine bonds CNTs are essentially compo composed of sp2 carbon bonds, hence longer hydrogen plasma treatments tend to etch CNT as shown in Fig 6.15 that the sp2 content detected at the CNTs surface of the composite emitters decreases from around 52% to 46% after the 10 s % hydrogen plasma treatment This finding is consistent with that previously reported by Hong et al [32] The etching of CNTs makes the electron transport along the emitter emitters become difficult, thereby resulting in poor FE performance performances 145 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.15 Carbon 1s core level XPS spectra of the (a) 50 nm ta C coated CNTs and (b) 10 s ta-C hydrogenated 50 nm ta-C coated CNT samples, indicating an increased sp3 content after C hydrogen plasma treatment The J-E curves of the pristine CNTs, 100 nm ta C coated CNTs and 100 nm ta ta-C ta-C coated CNTs with 10, 20 and 30 s hydrogenation samples are shown in Fig 6.16 The same features can be observed in this figure with that of the 50 nm ta coated s ta-C samples, confirming the reliability of the surface hydrogenation effect on FE properties of the ta-C coated CNTs Using the same analyzing method, the barrier C 146 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 : -2 ln (J/E2) Current density, J (mA/cm ) -4 -6 -8 -10 0.20 0.25 0.30 0.35 0.40 1/E 100 nm ta-C ta-C with 10s H ta-C with 20s H ta-C with 30s H Pristine CNTs 0 Applied electric field, E (V/µm) 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 147 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 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Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures Fig 6.15 Carbon 1s core level XPS spectra of the (a) 50 nm ta C coated. .. trend of the threshold fields exhibited by these samples during FE process, 142 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures... C 146 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures height ratios of the pristine CNTs, 100 nm ta-C coated CNTs and