Nanostructured Carbon Electron Emitters and Their Applications

Yahachi Saito, Yahachi Saito

• Publisher: Jenny Stanford Publishing
• Year: 2022
• ISBN:
  9781000333879
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Carbon forms a variety of allotropes due to the diverse hybridization of s- and p-electron orbitals, including the time-honored graphite and diamond as well as new forms such as C 60 fullerene, nanotubes, graphene, and carbyne. The new family of carbon isotopes—fullerene, nanotubes, graphene, and carbyne—is called "nanostructured carbon" or "nanocarbon." These isotopes exhibit extreme properties such as ultrahigh mechanical strength, ultrahigh charge–carrier mobility, and high thermal conductivity, attracting considerable attention for their electronic and mechanical applications as well as for exploring new physics and chemistry in the field of basic materials science. Electron sources are important in a wide range of areas, from basic physics and scientific instruments to medical and industrial applications. Carbon nanotubes (CNTs) and graphene behave as excellent electron-field emitters owing to their exceptional properties and offer several benefits compared to traditional cathodes. Field emission (FE) produces very intense electron currents from a small surface area with a narrow energy spread, providing a highly coherent electron beam—a combination that not only provides us with the brightest electron sources but also explores a new field of electron beam–related research.

This book presents the enthusiastic research and development of CNT-based FE devices and focuses on the fundamental aspects of FE from nanocarbon materials, including CNTs and graphene, and the latest research findings related to it. It discusses applications of FE to X-ray and UV generation and reviews electron sources in vacuum electronic devices and space thrusters. Finally, it reports on the new forms of carbon produced via FE from CNT.

Chapter 1 FEM and FIM of Carbon Nanotubes

Yahachi Saito

Toyota Physical and Chemical Research Institute, Yokomichi 41-1, Nagakute, Aichi 480-1192, Japan

[email protected]

Carbon nanotube (CNT) possesses beneficial properties as field electron emitters. In this chapter, structural, electrical, mechanical, and thermal properties of CNT are briefly described, and their field-emission phenomena observed by field-emission microscopy (FEM) and field ion microscopy (FIM) are reviewed.

1.1 Structure of CNT and Electron Emission Properties

CNT is made of seamlessly rolled graphene (honeycomb lattice of carbon atoms) with outer diameters ranging from abouti nm to over 50 nm, depending on the number of walls comprising the nanotube. CNTs composed of one sheet of graphene are called single-wall CNT (SWCNT) [1–3], and those made of more than two sheets, multiwall CNT (MWCNT) [4]. Among MWCNTs, those made of two walls and three walls are called double-wall CNT (DWCNT) and triple-wall CNT (TWCNT), respectively. The length of nanotubes exceeds 10 μm, and the longest ones are reportedly on the order of a centimeter [5, 6]. Thus, CNT exhibits quasi-one-dimensional properties; van Hove singularities (steep maxima) in electronic density of states (DOS) are induced, which brings about characteristic optical absorption and photoluminescence [7]. In addition, ballistic transport of electrons (i.e., no scattering of carriers) and quantum conductance are observed [8].

For mass production of CNTs, catalyst-assisted chemical vapor deposition (CVD) method is mainly employed now, though arc-discharge and laser evaporation of carbon were used at a pioneering stage of CNT studies. The arc-discharge technique is a unique one in the production of high-quality MWCNTs without metal catalysts. These methods of CNT preparations are briefly reviewed in Ref. [9].

CNTs possess various extreme properties as summarized in Tables 1.1 and 1.2 in addition to its geometrical shape with extremely high aspect ratio (length to diameter). Especially, the following properties make CNTs most favorable electron field emitters: (1) needle-like shape with a sharp tip, (2) high electrical and thermal conductivity, (3) high chemical and thermal stability, (4) high mechanical strength, and (5) low surface diffusion of carbon atoms. When an electric potential is applied to a conductor with a sharp tip, electric field concentrates at the sharp point. The field strength (F) at the tip surface is inversely proportional to the radius of curvature (r) of the tip:

$F=F/kr$ (1.1)

where V is an electric potential of the emitter and k is a constant depending on the emitter shape being approximately 5 in the presence of a cylindrical or conical shank [10]. Therefore, the sharper the tip of the emitter is, the stronger the field strength at the tip is even with the same applied voltage. The first advantage of CNTs, needle-like shape with a tip of an extremely small radius of curvature, realizes easily high electric field required for field emission (FE). Owing to the second advantage, surfaces of CNTs are inert and stable against reactions with residual gas molecules in vacuum. Therefore, the CNT emitters operate stably even at a vacuum of 10⁻⁴ Pa, in contrast to metal emitters such as tungsten and molybdenum needles that need ultrahigh vacuum to operate stably. High thermal conductivity facilitates to dissipate the Joule heat generated by current flowing along a CNT during FE operation to the cathode substrate. The third advantage of CNT emitters enables them to endure the high tensile stress due to electrostatic force (Maxwell tension). Together with this robustness, the last property of CNTs (low surface diffusion) keeps their original shapes (slender and sharp tip) even under a high field for prolonged usage. Needless to say, since CNTs (especially MWCNTs formed by arc discharge) are a good electrical conductor, they can transfer and emit high density of electron current (10⁸ A/cm²) through their length.

Table 1.1 Electrical properties of CNT

	Value	Reference
Work function
SWCNT	4.7 eV (contact pot. difference)	X. Cui et al., Nano Lett. 3 (2003), 783
	4.8 eV (photoelectron emission)	S. Suzuki et al., Appl. Phys. Lett. 76 (2000), 4007
MWCNT	4.3 eV (photoelectron emission)	H. Ago et al., J. Phys. Chem. B, 103 (1999), 8116
	4.6–4.8 eV (contact pot. difference)	R. Gao et al., Appl. Phys. Lett. 78 (2001), 1757
	4.51–4.78 eV (field emission)	Z. Xu et al. Appl. Phys. Lett. 87 (2005), 163106
SWCNT, DWCNT, MWCNT bundles	4.7–4.9 eV (thermionic emission from side wall)	P. Liu et al., Nano Lett. 8 (2008), 647
Electrical resistance
Individual SWCNT	~ 7 kΩ/μm (bridged single tube)	S. Li et al., Nano Lett. 4 (2004), 2003
Electrical resistivity
Aligned SWCNT film	≲ 5.5 × 10^−4 Ω-cm (// alignment)	J. E. Fisher et al., J. AppL Phys. 93 (2003), 2157
	≲ 3.4 × 10^−3 Ω-cm (⊥)
Aligned SWCNT rope	0.825 × 10^−3 Ω-cm (// rope) at RT	J. Hone et al., Appl. Phys. Lett. 77 (2000), 667
	5.0 × 10^−3 Ω-cm (⊥) at RT
Individual MWCNT	Longitudinal (// c-plane)
	Resistivity ρ = 10^−5–10^−4 Ω-cm	T. W. Ebbesen et al., Nature 382 (1996), 54
	Transversal (⊥ c-plane)
	Intershell conductance 4.5 mS/μm for d = 33 nm	A. Stetter et al., Phys. Rev. B 82 (2010), 115451
	13–19 mS/μm for d = 13–18 nm (270–350 mS/μm^2)	Y. Shinomiya et al., Surf Interface Anal. 48 (2016), 1206
	Intershell resistivity
	≈ 3 Ω-cm for d = 30 nm	A. Stetter et al., Appl. Phys. Lett. 93 (2008) 172103
	= (0.8–1.1) × 10^−3 Ω-cm at high temp.	Y. Shinomiya et al., unpublished
cf. Graphite	In-plane resistivity ~5 × 10^−5 Ω-cm	K. Matsubara et al., Phys. Rev. B 41 (1990), 969
	c-axis resistivity ~10^−3 Ω-cm for natural and synthetic single crystal
	~10^−2–100 Ω-cm for HOPG
Max. current
SWCNT in vacuum	>20 μA (10^9 A/cm^2) on SiO2/Si	Z. Yao et al., Phys. Rev. Lett. 84 (2000), 2941
MWCNT in vacuum	> 10^9 A/cm^2 on SiO2/Si	B. Q. Wei et al., Appl. Phys. Lett. 79 (2001), 1172
	~2 × 10^9 A/cm^2 on Si3N4 membrane	G. E. Begtrup et al., Phys. Rev. Lett. 99 (2007), 155901
	~0.7–0.35 × 10^9 A/cm^2 suspended	K. Yamauchi et al., unpublished
	(decrease with the increase of layer number from 6 to 13)
Carrier mobility (FET) of SWCNT	≈ 100 cm^2/Vs (isolated on SiO2)	F. Nihey et al., Jpn. J. AppL Phys., 42 (2003), L128
	~10...