Study of field emission characteristics of ultrathin film coated carbon nanotubes core shell structures 7

Metal oxides such as molybdenum oxide and tungsten oxide ultrathin films were attempted to coat the pristine CNT substrates by a custom-designed MOCVD system at varying temperatures and

Chapter 7

Conclusions and Future Works

7.1 Conclusions

The primary objective of this study was to explore appropriate semiconductor

materials that could enhance the FE characteristics of pristine CNTs through coating them onto the surface of CNTs In order to achieve this goal, high density vertically-aligned CNT substrates were synthesized by PECVD technique at 700°C with 1.2 Torr C2H2 as the feed gas A layer of Fe thin film with the thickness of around

7 nm was pre-deposited as the catalyst for CNT growth The Fe catalyst has played an important role in CNT growth process and the growth mechanism was attributed to the tip growth mechanism The average growth rate for the CNTs was around 0.80 µm min-1

Metal oxides such as molybdenum oxide and tungsten oxide ultrathin films were attempted to coat the pristine CNT substrates by a custom-designed MOCVD system

at varying temperatures and the FE performances of these samples were investigated Core-shell nanostructures were successfully obtained for these samples For the molybdenum oxides coated CNTs, enhanced FE performance was observed for the sample obtained at 400 °C while for the tungsten oxide coated CNTs, the FE

performance of pristine CNTs was significantly improved with tungsten oxide coating

at 400 °C and further enhanced with coating at 600 °C At 200 °C, the deposition was failed because the temperature was lower than the decomposition temperature of the precursors The FE enhancement is probably due to the Schottky junction formed at the interface of the coating films and the CNTs, which induced electron injection from the Fermi level of the CNTs to the conduction band of metal oxides followed by the electron emission from the conduction band minimum of metal oxides to the vacuum The considerably lowered overall electron emission barrier height has increased the electron tunneling probability and therefore enhanced the FE performances of such composite emitters In addition, novel cactus-shaped nanostructures were obtained for the 600 °C tungsten oxide sample and their growth mechanism may be attributed to the dendritic growth The numerous branches perpendicularly aligned along the main stems may distort the applied electric field and remarkably enlarge the local field on the emission sites, thereby enhancing the FE performances of the composite emitters The ta-C films with different thicknesses were used to coat the pristine CNTs followed by various durations of hydrogen plasma treatments FE characteristics of these specimens were tested to study whether these steps could enhance the FE properties of the pristine CNTs Results showed that the FE properties of the CNTs would be affected both by the thickness of the ta-C coating films and by the treatment duration of the hydrogen plasma For the ta-C coated CNTs with varying film thicknesses, the optimum FE performance was obtained for the 50 nm ta-C coated

sample With films either thinner or thicker than that value, the effective emission potential barrier and the electron transport would be affected, and surface work function may be changed as well due to the substrate-induced effect In addition, with

a 10 s hydrogenation treatment, the sample exhibited significantly enhanced FE properties whereas with longer duration of hydrogenation treatments, the enhancement was degraded The FE enhancement of the slightly hydrogenated sample

is highly likely 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 severely damage the structures of the composite emitters and make the electron transport within the emitters become difficult, thus degrading the FE performances of the emitters

The primary contribution of this study is that it has explored alternative materials, i.e., the ta-C material and metal oxides such as molybdenum oxides and tungsten oxides to modify the surface of the pristine CNTs, leading to significant enhancement

on their FE characteristics In other words, new field emitters with low threshold field have been successfully fabricated and these emitters may be considered as alternative choices for the present commercial FE devices Moreover, the results and discussion of this study have provided a better understanding of the FE enhancement mechanism of

the multilayer structured field emitters The main factors influencing the FE properties

of the composite emitters have been determined as well To enhance the FE

characteristics of pristine field emitters, the following approaches could be employed: (1) Lower the effective potential emission barrier either by decreasing the barrier height or reducing the barrier width The former way could be achieved through inducing the energy band bending on the emitter surface by coating the pristine emitters with lower work function materials while the latter one could be realized by appropriately adjusting the width of the coating films

(2) Enlarge the field enhancement factor β by coating the pristine emitters with materials of special geometry such as the cactus-shaped tungsten oxides in this study The unique geometry would distort the applied electric field and lead to concentrated local field on the emission sites thus lowering the threshold field

(3) Promote the electron transport within the emitters by optimizing the thickness

of the coating films There would be little net charge transfer from the substrate to the emitter if the film is too thin whereas the scattering effect would become severe if the film is too thick

FE is a complicated process, which is not merely determined by one or two factors above Good field emitters can only be produced by the optimum combination

of all these factors

7.2 Recommendation for the Future Works

In this dissertation, the ta-C films and metal oxides such as molybdenum oxides

and tungsten oxides have been attempted in CNT coating to investigate the influence

on the FE performance A direct extension of this work is to explore some other materials for CNT coating For instance, Boron Nitride (BN) has been found to be one

of the most promising candidates in FE application due to its excellent properties such

as negative electron affinity, hardness and thermal conductivity, as well as chemical stability [1-3] It is speculated that an interesting composite emitter with novel FE properties will be generated by combining the good features of CNTs and the BN material

Second, as this dissertation is mainly concentrated on the film coating effect on

FE performance, the optimization of the preparation process of the CNT substrates is not included It is known that the state-of-art CNT arrays with desired tube length, diameter and adjacent nanotube distance can be easily fabricated by lithography technique [4] Future research could be attempted to further enhance the FE properties

of these emitters by coating films on the CNT substrates with the optimum morphologies

Third, the current work is largely focused on the improvement of material properties while the fabrication process of FE devices is not taken into account since it

is very complicated and not pertinent to the topic of this study However, to achieve the mass production of this kind of emitters, material assembly and integration are also required More specifically, these emitters should be integrated into FE devices and the reliability, stability and lifetime of the devices should be tested before the practical

application [5] In the cathode fabrication process, the adhesion problem of the CNTs

to the silicon substrate should be resolved Additionally, when applying in flat panel displays, large area uniform arrays of the emitters should be successfully produced

In summary, there are still some important issues needed to be addressed before the CNT-based composite emitters can be practically employed However, great progress has been made in this research area in the past few years And it is promising that the CNT-based emitters will be commercially utilized in the FE devices in the near future

References

1 F Song, L Zhang, M Han, H Huang, F Li, J Ge, J Wan, and G Wang, Physica B

366, 200 (2005)

2 H Chen, H Zhang, L Fu, Y Chen, J S Williams, C Yu, and D Yu, Appl Phys

Lett 92, 243105 (2008)

3 S Komatsu, A Okudo, D Kazami, D Golberg, Y Li, Y Moriyoshi, M Shiratani, and K Okada, J Phys Chem B 108, 5182 (2004)

4 D Kim, S H Lim, A J Guilley, C S Cojocaru, J E Bourée, L Vila, J H Ryu,

K.C Park, and J Jang, Thin Solid Films 516, 706 (2008)

5 Y A Kim, H Muramatsu, T Hayashi, M Endo, M Terrones, and M S

Dresselhaus, Chem Phys Lett 398, 87 (2004)

List of Publications

Journal papers with main contribution

1 J Yu, Y M Foong, A T T Koh, and Daniel H C Chua “Hydrogenation and its effects on the field emission characteristics of tetrahedral amorphous carbon coated carbon nanotubes” Journal of Physical Chemistry C (Accepted)

2 Jun Yu, Kuan He Choo, Lifang Niu, and Daniel H C Chua “Synthesis of

three-dimensional tungsten oxide cactus-shaped nanostructures supported on

carbon nanotubes template” Electrochemical and Solid-State Letters (Accepted)

3 Jun Yu, Pruthvi Anetab, Jovan Hsieh, Angel Ting Ting Koh and Daniel Hock

Chuan Chua “Enhanced electron emission from integrated carbon nanotubes/ diamond-like amorphous carbon core-shell nanostructures” Submitted to

Diamond and Related Materials

4 Jun Yu, and Daniel H C Chua “Effective electron emitters by molybdenum

oxide-coated carbon nanotubes core-shell nanostructures” Journal of Materials

Science 46, 4858 (2011)

5 Jun Yu, and Daniel H C Chua “Enhanced field emission properties of

hydrogenated tetrahedral amorphous carbon/carbon nanotubes nanostructures”

Electrochemical and Solid-State Letters 13, K80 (2010) (selected for publication

in Virtual Journal of Nanoscale Science & Technology in the July 19, 2010 issue)

6 J Yu, C H Sow, Andrew T S Wee, and Daniel H C Chua “Enhanced field

emission of vertically-aligned core-shell carbon nanotubes with molybdenum

oxide encapsulation” Journal of Applied Physics 105, 114320 (2009) (selected

for publication in Virtual Journal of Nanoscale Science & Technology in the June

22, 2009 issue)

Other journal papers

1 A T T Koh, Y M Foong, J Yu, Daniel H C Chua “Emission barrier height

comparison of amorphous carbon films containing nano-graphene clusters”

Submitted to Applied Physics Letters

2 H Y Wang, Z Tan, J Yu, and Daniel H C Chua “Enhancement of field

emission by coating carbon nanotubes with wide bandgap semiconductors”

Submitted to Journal of Materials Research

Conferences

1 Oral presentation Jun Yu and Daniel H C Chua “Enhanced Field Emission

Properties of Hydrogen Plasma Post-Treatment on Tetrahedral Amorphous Carbon Coated Carbon Nanotubes”, IEEE International NanoElectronics Conference (INEC) 2010, City University of Hong Kong, Hong Kong, China, Jan 3-8, 2010

2 Poster presentation accepted Y M Foong, A T T Koh, J Yu, M Zamri, M

Tanemura, and D H C Chua “Field emission of copper doped carbon composite

on nanoconical substrate”, Diamond 2011, Garmisch-Partenkirchen, Bavaria, Germany, Sep 4-8, 2011