Engineered carbon nanotube field emission devices

CHAPTER 

Engineered carbon nanotube field emission devices

5

Matthew T. Cole1,2, Mark Mann1, Kenneth B.K. Teo1,2, and William I. Milne1 1

Department of Engineering, University of Cambridge, UK, 2Aixtron Ltd., Buckingham Business Park, Swavesy, UK

CONTENTS 5.1 Introduction........................................................................................................126 5.1.1 Synthesis....................................................................................... 128 5.1.2 Positional Control........................................................................... 139 5.1.3 Alignment Control........................................................................... 142 5.2 Field Emission....................................................................................................144 5.2.1 Electron Microscopy........................................................................ 155 5.2.2 Parallel Electron Beam Lithography.................................................. 157 5.2.3 X-Ray Sources................................................................................ 159 5.2.4 Microwave Sources......................................................................... 162 5.2.5 Displays......................................................................................... 163 5.2.6 Gas Ionization Sensors and Gauges................................................... 166 5.2.7 Interstellar Propulsion..................................................................... 169 5.3 Conclusion.........................................................................................................169 Acknowledgments......................................................................................................170 References................................................................................................................170

Contrary to popular belief, carbon nanotubes (CNTs) were not discovered by Iijima in 1991 [1], but rather by Radushkevich and Lukyanovich in 1952 [2,3], who published clear transmission electron micrographs of 50 nm diameter tubes made of carbon. Unfortunately, due to the political tensions of the time this work went largely unnoticed. In fact, with the discovery that CNTs were responsible for the high strength of Damascus steel from the seventeenth century [4], it became clear that their advantageous mechanical properties have been employed for some time. Today their novel properties have found use in a number of innovative electronic devices. This chapter details some of the more prominent applications based on their excellent field emission behavior. Emerging Nanotechnologies for Manufacturing. DOI: http://dx.doi.org/10.1016/B978-0-323-28990-0.00005-1 Copyright © 2015 Elsevier Inc. All rights reserved.

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CHAPTER 5  Engineered carbon nanotube field emission devices

5.1  INTRODUCTION Iijima [1], using high-resolution transmission electron microscopy (HR-TEM), showed the connection between CNTs and the carbon fullerenes of Nobel Laureate H. Kroto [5]. It was revealed that CNTs were conjoined and concentrically nested graphene cylinders of varying lengths and ultrahigh-aspect ratio (>10,000). Since then there has been extensive research into the properties, synthesis, and possible myriad applications of CNTs. Single-wall CNTs (SWCNTs) are sp2 covalently bonded, hexagonally latticed single-atom-thick carbon cylinders. The graphene walls are rolled cylindrically to form tubes [Figure 5.1(a)]. The tube ends can either remain open, which is thermodynamically unstable, bond to a secondary surface that is not necessarily carbon, or adjust their edge atomic arrangement to form fullerene-like hemispherical caps [6]. SWCNTs are either semiconducting or metallic depending on the way in which they are rolled—their “chirality,” as illustrated in Figure 5.1(b). Three forms of SWCNTs exist, the armchair, zigzag, and chiral types [Figure 5.1(c)]. Figure 5.1(d) is a chiral (a)

(b) A A’ Roll-up Zigzag (n,0)

O

SWNT

Graphene sheet

a2

θ

a1

O’

Ch = 5a1+2a2=(5,2) Armchair (n,n)

(c)

(d) Zigzag

Armchair (0,0)

(1,0)

(2,0)

(1,1)

(3,0)

(2,1)

Zigzag

(4,0)

(3,1)

(2,2)

Ar

(5,0)

(4,1)

(3,2)

(5,1)

(4,2)

(3,3)

mc

ha

ir

(6,0)

(5,2)

(4,3)

(7,0)

(6,1)

(6,2)

(5,3)

(4,4)

(8,0)

(7,1)

(7,2)

(6,3)

(5,4)

Metallic

Semiconducting

(10,2)

(9,3)

(8,4)

(7,0)

(6,6)

(10,1) (11,1)

(9,2)

(8,3)

(7,4)

(6,5)

(10,0) (11,0) (12,0)

(9,1)

(8,2)

(7,3)

(6,4)

(5,5)

Chiral

(9,0)

(8,1)

(8,5)

(7,6)

(10,3)

(9,4)

(7,7)

(11,3)

(10,4)

(9,5)

(8,6)

(12,1)

(11,2)

(10,5)

(9,6)

(8,7)

(9,7)

FIGURE 5.1 (a) A graphene sheet rolled up to obtain a single-walled CNT. (b and d) Description and actual chiral map showing the different SWCNT configurations possible, namely, armchair, zigzag, and chiral, as shown in (c). If the graphene sheet rolls up in such a way that the atom at (0,0) would also be the atom at (6,6), the CNT is metallic. Likewise, if the CNT wraps up such that the C at (0,0) is also at (6,5), the CNT is semiconducting. The small green circles denote semiconducting CNTs; the large red circles denote quasi-metallic CNTs. Two-thirds of CNTs are semiconducting and one-third are metallic.

5.1  Introduction

map showing the various chiral vectors that give rise to metallic or semiconducting SWCNTs. Multiwalled CNTs (MWCNTs) are semimetallic and like graphite have a low band gap on the order of 40 meV. Their diameters can range from 2 to 500 nm, and lengths from 50 nm to a few millimeters. The longest reported CNTs are 18 mm, grown by the University of Cincinnati in 2007. MWCNTs contain several concentric, coaxial graphene cylinders—each with its own unique chirality—at an interlayer spacing of 0.34–0.39 nm, where this intershell spacing decreases with increasing tube diameter as a result of curvature-induced spreading of the interlayer spacing [7,8]. This is slightly larger than the interplane spacing in graphite (0.335 nm). As each cylinder has a different radius, the carbon atoms composing each graphene cylinder remain unaligned to the next radial carbon atom in the adjacent shell. Unlike crystalline Bernal or ABAB stacked graphite, MWCNTs tend to exhibit properties more akin to those of turbostratic graphite, whose layers are largely uncorrelated. Such misalignment can reduce intershell interactions, resulting in separated electron systems that, if contacted externally, allow current to pass only through the outermost shell [9]. CNTs have many interesting properties, such as a Young’s modulus approximately 10 times that of steel [10] and an electrical conductivity many times that of copper [11]. For brevity, some of the more important properties, compared to Al and graphene, are given in Table 5.1. These once enigmatic and challenging one-dimensional nanomaterials, principally resigned to the world of high-tech research and development facilities, have finally been incorporated into some practical devices, albeit after a rather significant delay following the initial hype. Semiconducting SWCNTs have been investigated as transistors and digital logic elements [40–42], whereas MWCNTs have found application in saturable absorbers in photonic systems [43,44] and novel composite structural reinforcement [45–47]. Though their chemical structure makes them largely chemically inert, CNTs have shown promise as chemical-sensing elements, since their lattice defects can be highly reactive. The electronic properties of CNTs can vary greatly in specific atmospheres, opening up the field of high-accuracy nanoscale sensors [48,49]. Electromechanical sensors, whose electrical characteristics change upon mechanical deformation, have also been demonstrated [50]. In highly crystalline CNTs, coherent electron transport has been shown in the form of electron spin devices [51]. CNTs have been used as flexible, transparent electrodes in electrochemical supercapacitors [52–54] and dye-sensitized solar cells [55,56]. The large surface area-to-bulk ratio gives rise to multiple charge transfer pathways, resulting in high electrical conductivity and greater charge storage capabilities [11]. Hydrogen storage has also been investigated [57–60], though the achieved storage capacity is much less than original expectations [60]. CNTs mechanically deflect upon electric stimulation, allowing the development of nanoscale electromechanical cantilevers and actuators for high-spatial-density switching devices and memory cells [61]. Though myriad applications have been considered that principally exploit either the high aspect ratio of individual CNTs or the combined flexibility and transparency of bulk CNTs [62–65], few have made it to full commercialization.

127

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CHAPTER 5  Engineered carbon nanotube field emission devices

Table 5.1  Physical Properties Mechanical

Optical

Thermal

Electrical

Young’s modulus (GPa) Ultimate tensile strength (GPa) Failure strain (%) Structural anisotropy Band gap (meV) Absorption at 550 nm (%) Thermal conductivitya (W/mK) Specific heat (mJ/gK) at RT Coefficient linearization thermal expansion (K−1) Thermoelectric power (µV/K) Resistivity (Ωm) Maximum current density (A/cm2) Quantized conductance (kΩ−1) Mobility (cm2/ Vs)

Al

CNTs

Graphene

References

70

200–1200

>1000

[12–14]

0.09

11–150

130

[15]

60

12–23

12

[16,17]

Low (3D) 0

High (1D)

Medium (2D)

/

<100, Tuneable 2.3

[18,19]

/

<100, Tuneable 20

250

30–3000

5300

[21,22]

962

200–600

200–750

[23,24]

10−5

10−6

5 × 10−6

[25–27]

2.5

65

90

[26,28,29]

10−8 106

10−4 109

10−7 2 × 109

[30,31] [31–33]

/

(6.5)−1

(6.5)−1

[34]

/

/

1.5 ×104– 2 × 105

[35,36]

[20]

a

At room temperature. Comparison of Aluminum, CNTs, and graphene [37–39].

5.1.1  SYNTHESIS A detailed understanding of the synthesis process is of the utmost importance if CNTs are to be seriously considered for device applications. Indeed, for enhanced operation the ability to engineer the growth kinematics is essential in order to tailor the CNTs to suit various device architectures and functional specifications. There is an ever-increasing range of synthesis methods, each with its own relative advantages and disadvantages. The lack of CNT-based products available in the marketplace

5.1  Introduction

H2 He Pump

Graphite Cathode/CNT Collector

Micrometer c. 100 mbar

20 V,100 A

Inert gas

∼1 mm

GND

Controller

DC power source

Graphite (MWCNTs) or Catalyst (Fe, Ni, Co) doped Graphite (SWNTs) anode

FIGURE 5.2  Arc Discharge CNT Synthesis. Schematic of a typical arc discharge chamber.

clearly indicates the level of difficulty engineers and materials scientists have had to overcome in utilizing these various methods to produce stable, reliable, and, perhaps most importantly, reproducible CNTs. CNTs can be grown by laser ablation [66,67], electric arc discharge [68,69], highpressure carbon monoxide conversion [70], and catalytic chemical vapor deposition (CVD) [71–74]. Arc discharge and laser ablation are classified as high temperature (often >3000 °C), short timescale reactions (µs to ms), whereas catalytic CVD is a medium temperature (300–1200 °C) long timescale reaction (min to h) [75]. Other less common and certainly nonscalable approaches include ball milling [76] and flame synthesis [77,78]. This section provides a general overview of the more prominent synthesis methods. Iijima, while working at the NEC laboratories, employed a popular method used to synthesis C60 fullerenes: the arc discharge chamber (Figure 5.2) [1]. Bethune et al. [79] (then at IBM) expanded this work in 1993. Their arc discharge system consisted of two carbon rods, 1 mm apart, placed end on end [80]. For a bias of ~20 V an electric arc passed a current of 50–200 A that rapidly (<1 s) increased the temperature of the anode to >1000 °C in a vacuum, inert atmosphere (such as He, H2, or N2)

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CHAPTER 5  Engineered carbon nanotube field emission devices

[81–83], or aqueous environment (such as liquid nitrogen or water [84,85]). The electrode separation is carefully controlled to induce maintained electrical breakdown between the electrodes. The carbon source sublimes and the mobilized stream of free carbon atoms travels toward the cooler anode, upon which it condenses to form nanotubes [83], fullerenes [86], and graphene [87] in small, rod-shaped deposits. Either pure graphite- or catalyst (Fe, Ni, Co)-doped graphite electrodes can be used to selectively synthesize MWCNTs or SWCNTs [88]. Catalyst doping also increases yield. Deposits can consist of as much as 70%–80% amorphous carbon, as well as many other nanoscopic carbonaceous materials [89]. Arc discharge produces both SWCNTs and MWCNTs of varying lengths. The CNTs are highly crystalline, have few defects (as a result of the high synthesis temperatures) and can be as long as 50 µm. However, during synthesis CNTs become coated by amorphous carbon (a-C) detritus [90]. Purification is necessary before they can be put to any practical use. A typical method [91] is to thermally anneal the deposits in air at ~500 °C to stimulate oxygen etching of the a-C. This also removes away much of the other carbonaceous debris. After purification and centrifugation, extremely low (<1 wt%) CNT yields are not uncommon, with 5–35 wt% accounting for metal impurities [92]. Further immersion and filtering in aggressive acids, such as HCl, further removes remaining undesirable carbon conglomerates and catalyst particles. Other routes to separate the CNTs involve boiling the deposits in 30% nitric acid. The diameter of the CNTs, which can be controlled by altering the reaction temperature during growth, cannot be controlled in the arc discharge process. Smalley et al. [93] were, in 1995, some of the first to heavily develop and understand laser ablation/evaporation/vaporization. They predominantly focused on fullerene synthesis and modified an existing process used to create metal molecules. They substituted the metal target for a graphite one. The method was later refined when the graphite targets were changed to a graphite–catalyst composite, the best yields coming from a 50:50 mixture of Co (or Ni) to graphite [88]. Laser ablation is similar in many respects to arc discharge insofar as it is a transient sublimation process. In an inert atmosphere a high-powered (5 kW) laser is incident on a solid (or occasionally liquid) carbon-containing target. The carbon target rapidly heats to temperatures in excess of 1000 °C, which stimulates vaporization. The ablated material sublimes forming plasma, which is electrostatically attracted to a cooler counterelectrode. Here it precipitates to form various nanocarbons. Fullerenes [94], nanotubes [83], and (only recently) graphene [95] have all been demonstrated using this method. Laser ablation tends to produce high-quality, minimally defective structures accompanied by a wide variety of undesirable carbonaceous species, as per arc discharge; a-C being the most prominent. Aggressive purification and post-synthesis manipulation are frequently required to separate out the nanostructures from the “soot.” Even though this method only produces 30% detritus—the same purification and separation procedures must be observed as per arc discharge synthesis—it is the most expensive widely used method [89]. Arc discharge and laser ablation are the principal methods for synthesizing small quantities of extremely high-quality, low-defect CNTs. However, both methods

5.1  Introduction

(a) NH3

(b)

C 2 H2

GND

Shower head (anode)

NH3:C2H2(4:1) 200:50 sccm

N2

Plasma 0–1000 V 500–800˚C

Controller

Graphite heater (cathode)

(c)

c.3.2 mbar

DC V source Heater I source

Cap. pressure guage and TC

Pump

FIGURE 5.3  Plasma-Enhanced/Thermal CVD Apparatus for CNT Synthesis. (a) System schematic and (b and c) optical micrographs of the graphite, ohmically heated stage during plasma exposure. Samples are loaded onto the heated stage and exposed to the carbon precursor and etching gases through the shower head [98–100].

involve sublimation of a carbon source. The process is not scalable and mass production is not an option. As these methods produce tangled CNTs immersed in a-C, it is not practicable to make even small electrical devices due to the difficulty in reproducibility associated with cleaning and manipulating single CNTs. CVD differs from the preceding techniques due to its high controllability. The deposition of carbon from a hydrocarbon gas employing a catalyst material was first reported in 1959 [96], but CNTs were not synthesized by CVD until 1993 [97]. Figure 5.3 illustrates a typical CVD reactor. CVD systems can be either hot- or cold-walled. Hot-walled systems, such as tube furnaces, are globally heated, whereas in coldwalled systems a smaller, thermally isolated heater resides within a water-cooled chamber. Cold-walled systems offer rapid processing whereas hot walled equivalents can take some time to cool before the next sample run, often many hours. High quality graphene [101], nanotubes [102], and nanofibers [103] have been controllably synthesized via CVD. All CVD processes require two basic steps: (1) the preparation of a catalyst bed and (2) consequent growth by the decomposition of reactant gases. Rather than a transient solid sublimation process, carbon is sourced by the maintained thermal decomposition (at 450–1200 °C) of a carbon-containing gas (such as acetylene—C2H2 [104], methane—CH4 [105], ethylene—C2H4 [106], or carbon monoxide—CO [107]) combined with a carrier/etching gas (such as NH3 [108,109] or H2 [72,110]) to inhibit a-C deposition. Extremely wide ranging feedstock partial

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CHAPTER 5  Engineered carbon nanotube field emission devices

pressures, from 10−3 to 103 mbar, have been reported [111–113]. Growth occurs at the site of the metal catalyst. Diluted (i.e., low partial pressures, ~10−2 mbar [114]) CH4 and C2H4 are commonly used as they are kinetically stable and undergo minimal thermal decomposition at high temperatures (900 °C). Other hydrocarbons have been used, although with less success [115]. CNT length increases with deposition pressure, and linearly with deposition time until catalyst poisoning occurs and the growth saturates [116]. CNT diameter can be controlled by the thickness and type of catalyst thin film and support material. The position of the CNTs can be accurately controlled in a variety of ways, which we discuss later. CVD offers unprecedented control over the dimensions and position of CNTs. The quality of CVD-grown CNTs is subjective, and even though very high quality CNTs have been synthesized [117,118] the actual quality depends largely on the application-specific structures required. Some applications may require high purity and defect-free crystallinity, others tight dimensional control, whilst demand for high packing densities and/or alignment may very well be imperative elsewhere. CVD techniques demonstrate significant promise due to simple automation, high controllability, comparative cheapness, and perhaps most importantly, scalability; all of which are tantamount to successful commercialization. Plasma-enhanced chemical vapor deposition [PE-CVD, Figure 5.3(b,c)], a CVD variant, employs a plasma to reduce the growth temperature, assist in the feedstock decomposition, and facilitate accurate nanotube alignment by regulating the strength and direction of an applied electric field, or more correctly, the sheath field. A variety of plasma types have been studied, including direct current (DC) [37], hot-filament [108,119,120], aided DC, radio frequency (RF) [121], magnetically enhanced RF [122], microwave [123–125], and inductively coupled [126,127]. In a typical hot-filament DC plasma system, 600 V is applied between a grounded ohmically heated graphite stage (400–900 °C) and a gas inlet. This strikes the plasma. Not only does the plasma heat and modify the catalyst surface [128], but it also plays a number of important roles in nanotube alignment and a-C etching [129]. Combined ion bombardment and joule heating produce temperatures in excess of 700 °C, at least 200 °C of which can be attributed to the plasma [130]. PE-CVD is most often used for nanotube and nanofiber growth, though graphene growth has been reported recently [131,132]. Figure 5.4 illustrates typical CNT growth with and without plasma. PE-CVD synthesized CNTs are often larger in diameter than their thermal CVD counterparts and are more correctly referred to as nanofibers. PE-CVD favors Ni over Fe catalysts, because the latter is often degraded by plasma etching. The relatively low growth temperatures typically result in reduced graphitization [133], though this is not always problematic for particular applications. Milne et al. [108], Dai et al. [134], Chhowalla et al. [116], Robertson et al. [135], and Amaratunga [136] have all championed the use of CVD and variants thereof, due to its intrinsic controllability. To the best of the authors’ knowledge, some of the lowest-reported CVD growth temperature for nanofiber growth is in the region of 270–350 °C, demonstrated by Hofmann et al. [137] and Chen et al. [138]. With continued maturity, CVD is perhaps the most promising technique for compatible Complementary Metal Oxide Semiconductor (CMOS) processing temperatures

5.1  Introduction

FIGURE 5.4  Nanotube Catalyst-Bed Formation. (i) Generalized CNT growth. (ii) Routes to catalyst nanoisland formation; (a) Deposition of wet catalyst (e.g., ferritin, metallic salts, and ferrocene). (b) Plasma, or wet etching of materials such as steel, or physical vapor deposited (PVD) thin films. (c) Thermal coalescence of a PVD thin film. (iii) Catalyst wetting behavior. The degree of wetting determines the adopted root or tip growth mechanism. Illustrations of nanoisland: (a) wetting and (b) dewetting. (iv) Ostwald ripening: catalyst formation and coalescence occurring during increasing temperature and/or annealing duration. Gradient arrow indicates either increasing anneal duration or temperature. The orange central region, where uniform catalyst particles are formed, denotes the optimal case [37].

133

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CHAPTER 5  Engineered carbon nanotube field emission devices

(<300 °C) and a clear understanding of the growth dynamics and the underlying hetrogenous catalysis will play important roles in achieving this. The general process of nanotube growth is illustrated in Figure 5.4. Catalysts can be thermally annealed [103] and/or plasma-treated thin films [137], preformed nanoparticles [139], bulk materials that have had their surface topography modified [140], or even floating particles [141]. CNT catalysts include the transition metals (Fe, Ni, Co) [142], which have shown some of the highest catalytic activity; the noble metals (Au [143], Ag [143], Cu [143]); the poor metals (Pb [144], In [145]); as well as semiconductors (Ga [145], Ge [146]), oxides (SiO2 [147,148], ZrO2 [149]), and carbides (SiC [150]). Various metals and metal oxides, including MgO [144], Al2O3 [151], and TiO2 [144] nanoparticles, have all shown varied catalyzing capabilities. Binary and tertiary catalyst structures, such as Al2O3/Fe, Mo/Co [152,153], and Mo/ Al2O3/Fe [154], increase the yields and growth rates while reducing diameter and chiral variations. Graphene and CNT CVD has been demonstrated, both catalytically and noncatalytically, on a wide variety of metal foils and PVD thin films such as Cu, Ni, Co, Pt, Ru, and Ir [101,155,156]; numerous alloys [101,157] and metallic compounds like AlN [158] and ZnS [159]; and even more recently, oxides [160], nitrides [157], and semiconductors [161]. CVD growth of nanocrystalline graphene on Si/SiO2 [162], Si/Si3N4 [157], quartz [163], sapphire [164], and GaN [161] has also been reported. There is not as yet any clear unified mechanism as to why oxides catalyze nanocarbon growth. The proposed mechanisms are hotly debated. Many speculate that surface defects offer suitable nucleation centers, while others attribute the growth to chamber contamination. Many materials apparently catalyze nanocarbon growth with varied degrees of activity. For the most part, Fe, Ni, and Co are frequently employed and offer the highest growth rates. An important feature of CNT growth, second perhaps only to the catalyst selection, is the structural modification of the catalyst prior to carbon precursor exposure. Restructured catalyst films template the CNT growth and control the diameter, and possibly the chirality. There are several routes to produce catalyst nanoparticle beds. Nanoislands can be formed by wet deposition of nanoparticles in aqueous solutions [165], plasma etching [128], or thermal coalescence [166] [Figure 5.4(ii)]. Accurate catalyst positional control and sub-80 nm patterning have been demonstrated using electron beam lithography [103] and nanoimprint contact printing [134]. Soluble metal salts (acetates [167], nitrates [168]), bicarbonates [169], biologically derived catalysts (ferritin [170]), organometallic/metallocene compounds (ferrocene [141,171]), and metallic colloids [143] are all typical examples of wet catalysts, which are deposited by dipping [172], pipetting [173], spray coating [174], spin coating/casting [175], electrochemical deposition [176], inkjet printing [177], or microcontact printing [178]. Calcination (in ambient atmosphere) at 200–500 °C oxidizes the catalysts. The resulting metal oxides are then catalytically activated by reduction in H2 or inert atmospheres [179,180]. Often samples are finally exposed to mild O2 plasma to remove remaining organics. Catalyst bed formation by physical etching involves depositing a catalyst layer (<100 nm) by either evaporation or sputter coating and bombarding it with energetic

5.1  Introduction

plasma [181,182]. This approach has found little favor used on its own. Rather, thermal coalescence of sub-10 nm uniform catalyst PVD (sputtered/evaporated) thin films of Fe, Co, or Ni, is perhaps the most common approach [116] [Figure 5.4(iii)]. The catalyst dewets and clusters during thermal pretreatment, due to surface tension and compressive stresses [183,184]. This increased mobility minimizes the surface free energy and forms the characteristic globular structures, as depicted in Figure 5.5(a,b). In situ electron microscopy studies have evidenced that the catalyst is not liquid [185], though some debate still exists. Nanotubes grow at temperatures well below both the catalysts’ glass-transition temperature and carbide eutectic temperatures. Although these temperatures certainly have a size (i.e., Gibbs–Thompson) and pressure dependence, the extrapolated temperatures are still higher than the growth temperatures, suggesting that the catalysts are indeed solid during growth. Ostwald ripening, an inhibitory effect associated with thermal annealing, accounts for the variation in nanoisland size and is extremely notable on highly hydrophobic supports, such as SiO2. Higher energy states exist at the nanoparticle’s surface, relative to the bulk. It is energetically favorable for the nanoparticles to agglomerate to form ever larger particles, as depicted in Figure 5.4(iv). Lengthy annealing cycles enhance

FIGURE 5.5  Catalyst Balling and Nanotube Growth. When Ni nanoclusters [(a) and (b)] are on a 4 nm layer of SiO2 deposited onto a Si substrate, they exhibit weak interactions (“hydrophobic”) with their supports hence favoring tip growth [the Ni is the high contrast dot seen at the tip of the nanotube as in (c)] [37].

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CHAPTER 5  Engineered carbon nanotube field emission devices

FIGURE 5.6  Nanotube and Nanofiber Growth Modes. Left: Generalized growth showing (a) the catalyst thin film, (b) nanoisland formation, and (c) nanotube growth with (top) and without (bottom) plasma. Right: (a) Weak catalyst– support interaction leading to catalyst dewetting and tip growth. (b) Root growth caused by strong catalyst–support interactions, resulting from catalyst wetting upon thermal pretreatment.

this agglomeration and cause unsuitably large nanoislands; rapid anneals do not permit complete reduction or surface reconstruction. Recipe optimization is important in achieving well-defined, reproducible nanostructures. Figure 5.6 depicts nanotube tip and root growth modes. Catalyst particles stay either at the tip or at the base of the CNTs during growth, though some catalyst shedding does occur along the length of the CNT during tip growth depending on the surface hydrophobicity and implicit adhesion between the catalyst particle and the support. The exact growth mode adopted depends on the strength of the catalyst– support interaction. For example, Ni on SiO2 [186] is characterized by a small contact angle [Figure 5.4(iii)] which has a correspondingly weak interaction and therefore favors tip growth. Correspondingly, a hydrophilic interface, for example; Fe on Si [187], has a large contact angle and favors root growth [Figure 5.4(iii)]. Here the catalyst particle remains at the substrate interface during carbon extrusion. A vast number of temperature-dependent chemical interactions occur at the support– catalyst interface [185,188,189]. At high temperatures catalysts can be consumed by the substrate due to uncontrolled diffusion, whereas silicide and carbide formation as well as alloying inhibit growth and reduce yields. Insulating and conducting diffusion barriers such as SiO2, Al2Ox, and ITO, TiN, respectively, were developed to solve many of these problems [108,190,191] (Figure 5.7). Following catalyst restructuring, carbon precursors adsorb and dissociate on the catalyst surface and CNTs grow via carbon precipitation. The common growth model, depicted in Figure 5.8, accounts equally well for the dominant mechanisms in nanofiber, nanotube, and graphene synthesis [74,102,193,194]; which involves hydrocarbon adsorption, dissociation, carbon diffusion, and precipitation. The majority of the growth mechansims reported focus on surface and bulk C-diffusion-limited

5.1  Introduction

FIGURE 5.7  CVD Nanofibers, MWCNTs, and Graphene. Scanning electron micrographs of (a) vertically aligned nanofibers (~2 µm pitch) (Scale bar: 2 μm), (b) MWCNTs (Scale bar: 20 μm) (Insert: HR-TEM showing the nanotube’s graphitic walls. [Scale bar: 5 nm]), and (c) monolayer graphene (Scale bar: 2 µm). Graphene was grown for 5 min at (1) 800 °C, (2) 850 °C, (3) 900 °C, and (4) 950 °C and PMMA-transferred to Si/SiO2 (300 nm) [137]. (a)

(b)

Dissociation

Gas

Surface diffusion

Adsorption Bulk diffusion Catalyst

(c) Surface

Solid

Dissociation Adsorption

Precipitation Catalyst

Surface diffusion

Bulk diffusion Precipitation

Mass transfer (adsorption) dissociation

Precipitation

Bulk diffusion Surface diffusion

FIGURE 5.8  CVD Nanocarbon Growth. Both (a) nanotube and (b) graphene growth can be modeled in conceptually similar ways. In particular (i) hydrocarbon adsorption, (ii) dissociation, (iii) carbon diffusion, and (iv) precipitation. Adapted from [192]. (c) Scheme representing the various potential ratelimiting steps during CVD growth [137].

processes [137]. The widely accepted model by Baker et al. [195] proposed that carbon diffusion through the bulk catalyst was rate limiting, based on the similarities in the activation energies of nanofiber growth and bulk diffusion [102,135,137,196]. However, the validity of this mechanism is under some scrutiny for nanofibers, and the debate is ongoing as to whether this model strictly accommodates nanotube and graphene synthesis. Are these two seemingly similar carbon-latticed materials similarly rate limited? Chhowalla et al. [116], for C2H2:NH3/Ni nanotubes grown by PE-CVD, suggested bulk-diffusion-limited growth based on an activation energy of 1.4 eV. Vinten et  al. [197], Puretzky et  al. [198], and Zhu et  al. [199] reported activation energies of 2.1–2.4 eV, whereas Lee et al. [200], Kim et al. [201], and Liu et al. [202] measured 1.3–1.7 eV. Hofmann et al. [137], by PE-CVD, documented an extremely low activation energy of 0.25 eV, ascribing this reduction to plasma-assisted precursor dissociation. Knudsen diffusion has also been implicated [193], though a strong temperature dependence in the kinematics casts some doubt on its legitimacy.

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CHAPTER 5  Engineered carbon nanotube field emission devices

FIGURE 5.9  Activation Energy Map of CVD Synthesized CNT, Nanofibres and Graphene. Arrhenius plot of the growth rates for >400 samples. Least-squares fit. Ea estimated from gradient linear interpolation. ITO/Ni: 0.51 (±0.15) eV, ITO/Fe: 0.44 (±0.17) eV, Al/Fe: 1.94 (±0.96) eV, Al2Ox/Fe: 1.33 (±0.10) eV. For Cu-catalyzed graphene Ea =2.33 (±0.10) eV. Supporting data from Hofmann et al. [192,203], Ducati et al. [135], Bartsch et al. [204], Coraux et al. [205], and Li et al. [206]. Experimental uncertainty ±10 nm/min, unless otherwise stated.

Figure 5.9 shows the growth rate (R) as a function of reciprocal (absolute) temperature for various PE-CVD and T-CVD synthesized nanotubes, nanofibers, and graphene [135,192,204]. Data has been linearly fitted according to the Arhenius relation:  −E  R ∝ Do So exp  a   kBT 

where T is the growth temperature, kB is Boltzmann’s constant, and Do and So denote the carbon diffusivity and solubility prefactors [135]. Ea is estimated by linear interpolation and gradient extrapolation [200]. For ITO/Ni and ITO/Fe nanofibers (closed/open red squares) the activation energies are 0.51 ± 0.15 eV and 0.44 ± 0.17 eV, respectively. For Al2Ox/Fe and Al/Fe nanotubes (closed/open green squares) the activation energies are 1.33 ± 0.10 eV and 1.94 ± 0.96 eV. For Cu-catalyzed graphene (orange squares) the

5.1  Introduction

Ea = 2.33 ± 0.10 eV. Various early works determined the diffusion coefficients and solubility of carbon in Ni, Fe, and Cu [207–209]. The diffusion barriers can be extracted via linear interpolation of the diffusion coefficient against inverse absolute temperature. Majica and Levenson [209] considered the barrier potentials to surface diffusion in polycrystalline Ni (0.27 eV) using surface-sensitive Auger electron spectroscopy, whereas Diamond and Wert [210] focused on the bulk diffusion in Ni (1.52 eV). Berry [211] summarized various work on Ni, spanning multiple techniques, and provided a good estimate for the bulk diffusion activation energy (1.49 ± 0.33 eV). Wert, Emsley, Massaro, Smithells, Homan, and Lander [208,212–216] all reported on carbon solubility in α (BCC), γ (FCC), and polycrystalline Fe and cumulatively gave a bulk diffusion barrier of 0.81 ± 0.09 eV for α- and polycrystalline phases and 1.25–1.68 eV [217] for the γ-phase. Although no empirical data on the surface diffusion of carbon in Fe is available, the density functional theory calculations by Jiang and Carter [218] suggest a surface diffusion barrier of 0.64 eV. Evidently, nanofiber activation energies in both Ni (0.51 eV) and Fe (0.44 eV) are significantly less than the values associated with bulk diffusion (Ni ~1.50 eV [219], Fe ~1.00 eV [approximate bulk α- and γ-phases]) and are comparable to the surface diffusion of carbon on (polycrystalline) Ni (0.3 eV [209]), and Fe (0.64 eV [216,218]), suggesting dominant surface diffusion even at elevated temperatures. Indeed, the simulations of Yazyev and Pasquarello [74] support this hypothesis. The extrapolated activation energy also suggests that the Fe particle may very well be in its polycrystalline or α-phase, a rather puzzling result as the bulk γ-phase becomes increasingly favorable, due to its thermodynamic stability at elevated temperature (>750 °C) and increased carbon content [220]. Nonetheless it is highly probable that multiple phases occur during growth. The understanding of CNT catalysis is continuing to develop. Indeed, recent bursts in graphene CVD research have provided new insight into a once stagnated field. Nevertheless, many higher level issues remain if we are to integrate CNTs successfully into real-world field emission devices. We now continue by considering some of these more pragmatic issues, including accurate positional and alignment control.

5.1.2  POSITIONAL CONTROL Various positional control techniques exist. Many require time-consuming nanomanipulators operated by skilled individuals who accurately position individual, previously grown CNTs [221,222]. By combining PE-CVD with mass production facilitated by parallelized lithographic techniques, many millions of CNTs can be simultaneously grown, positioned, and even aligned with extremely high accuracy. Lithography is by far the most commonly employed technique used to control CNT position. By controlling the catalyst location prior to CVD, the position of the synthesized CNT can be controlled to high degrees of accuracy. Three lithographic techniques are broadly used for this; optical lithography, electron beam lithography (e-beam), and focused ion beam (FIB) lithography. Other more esoteric techniques are becoming increasingly popular; such as nanoinkjet deposition [223,224], e.g., though for brevity we will focus on the three dominant approaches.

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(a) 1. Si patterned with Mo

(b) Si

2. Etch Si with KOH etchant

3. Lift-off Mo with A1 etchant

(c)

(d)

FIGURE 5.10 (a) summarizes the etching process of silicon. Squares of molybdenum deposited by optical lithography act as a mask to produce an array of silicon pyramids shown in (b). By filling the etched gaps with PMMA, leaving just the apex of the pyramid exposed, a group of CNTs can be grown at the top as in (c), but if e-beam lithography is employed, single CNTs can be grown at the apex of each pyramid as shown in (d) [225].

Optical lithography is often the first step to define physical vapor deposited W or Mo alignment marks. Photolithography is a parallel process. Entire substrates are simultaneously patterned when illuminated through a patterned chrome oxide-plated quartz mask by an ultraviolet (UV) optical source. Deep UV offers extremely high resolutions, the current technological node of which lies at 34 nm. Experiments with extreme UV, for even finer features, are ongoing. Typically, micron- to millimeterscale patterns are achievable. Optical lithography is also widely used to deposit metal contacts or CNT catalysts. For instance, if arrays of Fe, 0.1 nm in thickness, are deposited a few microns apart, SWCNTs can be grown by CVD between these catalyst pads, thereby forming facile single SWCNT transistors. E-beam lithography offers spatial resolution from 10 nm to many millimeters. However, it is a serial process and is consequently rather time consuming and costly, as the electron beam can take some time to raster scan large patterns. Nevertheless, e-beam lithography has found significant use in CNT research. For example, it was employed to pattern CNT catalysts at the top of individual Si pyramids, as shown in Figure 5.10, which would be particularly challenging by standard photolithography.

5.1  Introduction

(b)

Diode

Anode

Vo

(glass/TCF/Phosp.)

e–

δ

Electron emitter

10–5 10–6 Emission current (A)

(a)

10–7 10–8 10–9

10–10

Cathode <10–5 mbar

10–11 10–12 10–13 5

10 15 20 Applied field (V/µm)

25

Vg

Va

Triode e–

Anode e–

Gate Dielectric Cathode

FIGURE 5.11 (a) The working principle of a diode and triode field emitter operating in a vacuum of <10−5 mbar, with an interelectrode spacing of δ, employing a transparent anode (e.g., quartz, glass) with a TCF and a light-emitting phosphor layer. (b) Highly uniform growth of individual nanotubes. Sample tilt 55°. The inset shows a typical logarithmic current–voltage plot for the CNT array depicted in a diode configuration [108,227] (Scale bar: 10 µm).

Combined with CVD, e-beam lithography has enabled the placement of individual CNTs with unprecedented accuracy [37,103,226] and has opened up a variety of applications requiring extremely uniform CNTs [108]. Perhaps the simplest e-beam structures for field emission applications are arrays of vertically aligned CNTs [37,226]. Arrays are fabricated by patterning a grid of spots 100 nm in diameter, the separation and diameter of which can be varied with ease. Following resist development, a conducting diffusion barrier, typically TiN or ITO, and catalyst layer are sputtered to thicknesses of 7–10 nm. After lift-off in acetone, regular dot arrays remain. CNTs are then be grown by PE-CVD that are vertically aligned perpendicular to the substrate, as illustrated in Figure 5.11(b). Lithography is well suited to flat substrates. Resists are typically spin- or spraycoated to produce a required film thickness; typically anywhere between 100 nm and 50 μm, where the resist thickness is approximately equivalent to the wavelength of light to which it is being exposed to and to the feature size it is patterning. Spin and spray coating techniques ensure uniformity in film thickness and pattern exposure. However, for nonplanar surfaces uniformity is reduced, and processing becomes challenging. For Si, pyramids can be fabricated by patterning a Mo hard mask prior to wet etching [Figure 5.10(a,b)]. Patterning a CNT catalyst at the pyramid apex now becomes a challenge, since it is not only difficult to uniformly coat thin films of photoresist but shadowing of the optical source during exposure further complicates the process. Bell et al. [225] circumvented this issue by filling the gap between the pyramids with (poly)methymethacrylate (PMMA) so that it just reaches their apex [Figure 5.10(c,d)]. Thus, an artificially planar surface is derived to accommodate

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lithographic processing, to derive a nanoscopic CNT-topped pyramidal structure well suited for field emission applications.

5.1.3  ALIGNMENT CONTROL The potential for directionally dependent functionality is extremely valuable in many optoelectronic applications, and in particular field emission applications [228–231]. As alluded to in the previous section, alignment control can be achieved using manual nanomanipulation of pregrown CNTs. However, this is far from a scalable technique. More scalable approaches can be achieved by: (i) post-synthesis alignment, where external forces such as electric fields (i.e., dielectrophoresis [232,233]) orient the CNTs once dispersed onto a substrate, often by means of a liquid medium; or (ii) aligned synthesis using magnetic fields [234], electric fields, gas flow, or graphoepitaxy [235] to align the CNTs during synthesis. In CVD, there is no intrinsic alignment. The grown CNTs are often randomly oriented and are spaghetti-like. However, under certain reaction conditions, even in the absence of plasma, closely spaced CNTs form ultrahigh-density forests that maintain a vertical growth direction [236]. Nevertheless, most CNT growths by CVD remain unaligned. Bulk alignment has been demonstrated, though significant intertube interactions reduced the degree of alignment. Here, chemically untreated CNTs were horizontally aligned by compression, rolling or shearing the growth substrates onto various flexible substrates [236]. Such techniques are largely inaccurate and suffer from poor reproducibility. Nanomanipulation has been reported, though the processing is serial and requires time-consuming electron microscopy monitoring [237]. Spin and drop casting of nanotube suspensions have shown moderate success [238,239]. Despite the CNTs’ in-plane alignment, they have little or no linear directionality, meaning there is no controllable way to define the mean azimuth and that they are merely randomly oriented within the plane. Wei et al. [240] demonstrated the use of dielectrophoresis, exploiting the weak nanotube dipole. An electric field on the order of ~10 V/µm [241–244] is required to align solution-dispersed nanotubes, whereas only ~1 V/µm [14,245,246] is required to align the nanotubes during synthesis. Figure 5.12 shows scanning electron microscope (SEM) of various in situ alignment techniques, including graphoepitaxy [250–252], gas flow [248,253–255], and electric field [14,243,246]. CNTs graphoepitaxially align to crystal surface nanofacets during thermal CVD, such as the (011) quartz R-face or sapphire A-face [256]. The technique is surface critical, which limits its broadness in application. Costly substrates are necessary. Though high degrees of alignment and uniformity have been evidenced [252,257] the process involves prohibitively time-consuming substrate preparatory processes. Rational substrate design, using elevated Si pillars for example, offers another possible approach [134]. Here the CNTs grow between elevated pillars. The micrometer pillars limit the practicality of the technique as well as restricting the maximum packing density. As a result, techniques based on electric field and gas flow alignment show perhaps the most promise. They are rapid, parallel processes that offer simplicity and the ability to fabricate high density arrays in a variety of directions.

5.1  Introduction

FIGURE 5.12 Nanotube alignment. Examples of in situ CNT alignment by (a and b) gas flow [247,248], (c) graphoepitaxy [249], and (d and e) electric fields [241,245].

In the case of gas flow alignment, the flow of the growth gases induces the alignment. Huang et al. [255], Xin and Woolley [258], and Jin et al. [248] demonstrated gas flow alignment with varying degrees of success. PE-CVD offers a potential route to dynamically controlling the growth orientation, which can be achieved by controlling the electric field direction. The challenge lies in controlling the plasma around the substrate such that the field points in the direction the CNTs are required to grow. Law et al. [243] found, for electric field alignment, that plasma-induced self-biasing and the resultant surface-charging effects on metallic electrodes were sufficient to align the CNTs. Zhang et al. [245] and Ural et al. [246] reported similar alignment effects ascribed to electric fields on the order of 0.5–4.0 V/µm. They argued that the CNTs highly anisotropic polarizability induces large dipole moments when they interact with the local electric field. This interaction produces large aligning torques that govern the growth orientation. Blaek et  al. [259] estimated the electric field aligning force to be of the order of a few nanonewtons, a value approximately four orders of magnitude greater than the weight of the catalyst particle. Hertel et al. [260] estimated that a 10-nm wide CNT experiences a Van der Waals surface binding force on the order of 35 nN (per unit length). Few comprehensive attempts have been made in the literature to explain the orientation mechanism involved. Chen et al. [261] proposed the so-called kite mechanism. Here the catalyst particle at the CNT’s apex is pulled in the direction of the prevailing

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electric field at an angle from the substrate. When growth terminates, the CNTs can relax and fall to the substrate, where they become strongly bound. Huang et al. [253] presented a similar mechanism, whereas Yu et al. [262] postulated that charged species form bonds along the electric field direction and that the nanotubes can only grow if they align to the electric field. Tanemura et al. [263] suggested that the alignment may be a result of an excess of electrostatically attracted positive charge ions at the CNT’s tip. They concluded that the plasma, composed of positive ions as opposed to radicals or excited molecules, plays a decisive role in the alignment by reducing the lateral mechanical stress. Although these techniques offer viable approaches to alignment, the demanding electrode and catalyst patterning processes limit scalability, both in terms of CNT length and mass production. Practical devices require larger scales. The development of a simple, low-cost approach capable of spanning significant lengths (mm) is necessary. To solve this, membranes formed from free-standing aligned MWCNTs have also been demonstrated by highly efficient mechanical/ solid-state pulling from dense CNT forests [65,264]. Many novel devices and architectures become viable when we consider exploiting CNT alignment. However, perhaps the most viable route to market employing such aligned CNTs comes in the form of electron sources. Though there is no prerequisite for electron sources to have vertically oriented emitters (indeed, spaghetti-like emitters do in fact emit particularly well), it has been shown that such engineered aligned emitters offer many advantages over chaotic spaghetti-like CNT thin films with regards to emission stability, uniformity, and determinism [227,265]. Thus, much research has been focused on the application of aligned CNTs in field emission sources.

5.2  FIELD EMISSION The following section reviews the field emission process and various field emission applications in which CNTs have found particular recognition, including displays, microwave and X-ray sources, parallel electron beam lithography systems, gas ionization sensors, and interstellar propulsion. Field emission is the liberation of electrons from an electron-dense solid into a vacuum under an intense electric field [266]. It is a wholly quantum mechanical phenomenon and can be succinctly described in adequate detail using fairly simplistic descriptors based on the free-electron model. Figure 5.13 illustrates the process. A potential difference is applied between an electron-emitting surface (cathode) and a counterelectrode (anode). The idealized potential profile is given in Figure 5.13. N(E) shows the variation in the electron density in the emitter. The majority of electrons are confined to the Fermi Sea. The combined effect of the applied electric field and the image potential produces a triangular barrier, as shown. Electrons tunnel through this barrier when it is less than a few nanometers across. Increasing the electric field increases the barrier’s gradient, therefore reducing the tunnel width; the narrower the barrier, the higher the emission current. Surface contaminants and engineered coatings, such as Cs (1.9 eV), Ba (2.3 eV), and ZrO (2.3 eV), produce Schottky rounding

5.2  Field emission

EF

N(E)

Potential energy, V(z)

Vacuum level

E

Vacuum

E

Metal

Image potential (=e2/16TTε0z)

∅

Electric field potential (=eElocalz)

Barrier

Position, z

Tunnelling electrons

V(z) = ∅ –eElocalz–(e2/16TTε0z) J(E)

FIGURE 5.13 Field emission potential diagram. Large electric fields induce barrier narrowing, which increases the number of electrons tunnelling from the Fermi level in the metallic, electronrich, surface into the vacuum. Variations in the density of occupied states, N(E), and current density, J(E), as a function of electron energy. Surface contamination and charging effects (Schottky rounding) can be seen to drastically alter the potential profile. Source: Adapted from [267] and [268].

and reduce the work function (ϕ) of the emitting surface, which thins the barrier further, thereby reducing the turn-on potential. The inset of Figure 5.11(b) shows standard emission characteristics of a vertically aligned CNT array [269]. To obtain stable performance, vacuums of 10−9–10−5 mbar are necessary to avoid tip damage by gas ionization, sputtering, and ion bombardment. The emitted current density (as a function of the global electric field, F), J(F), is given by:   A  φ3 / 2  J ( F ) =  2  F 2 exp −Bv(s) F    φt (s) 

where ϕ is the work function of the emitting surface, A =(e3/8πh) =1.54 ×10−6 A.eV/ V2, and B =(8π/3eh)(2m)1/2 =6.83 ×109 (eV)−3/2 V/m. t(s) and v(s) are the slowly varying, dimensionless Nordheim elliptical functions [270,271], and are approximated by: t (s ) = (3.79 × 10−5 )F 1 / 2 / φ v(s ) = (0.956 − 1.062s 2 ) where s is the slope correction factor. For simplicity t(s) and v(s) are often set to unity, with minimal loss in accuracy.

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CHAPTER 5  Engineered carbon nanotube field emission devices

A rather elegant modification was proposed by Brodie and Spindt [272]. By substituting the current density and emission area with the measured current (I), and the local electric field with the product of the applied bias (Vo) and a geometry dependent field factor, they determined that  −Bφ3 / 2δ   A * A    exp  I = Vo2 β 2   φδ 2   βVo  where A* is the effective emission area, δ is the constant electrode separation and β, the field enhancement factor, is proportional to the aspect ratio of the emitting surface and is given by: β =

Flocal F

Here Flocal is the tip-enhanced electric field; that is, the increased electric field surrounding the sharp emitter. The sharper the emitter is the greater the enhancement factor and the steeper the potential profile gradient, which narrows the corresponding tunnel barrier. The field enhancement factor is a standard performance metric that is empirically evaluated by measuring the I–V characteristics and plotting the so-called Fowler–Nordheim (FN) curve according to 2  I   −Bφ 3 / 2δ  1   + ln  A * Aβ  y = mx + c ⇒ ln  2  =    V  Vo    φδ 2  β o

Numerical estimates of β are extracted by linear interpolation of the gradient, where the B, ϕ and δ are known constants. For CNTs, ϕ is 4.9–5.0 eV. d( I / Vo2 ) dy −1 = = ( Bφ3 / 2δ ) dx d(1 / Vo ) β Similarly, the y-intercept specifies either ϕ or A*. The field enhancement factor has extremely strong geometry dependence. This dependence was succinctly illustrated through the theoretical work of Rohrbach and Wand and Loew [273,274]. Narrow, elongated emitters are preferable, since they enhance the local field substantially compared to short whiskers. Si and W tips are typically used, fabricated by anisotropic etching or deposition techniques. Carbon has a rich history in field emission. In 1972, graphite fibers were shown to offer enhanced emission stability in modest atmospheres compared to several common metallic emitters [275]. Wang et  al. [276] and Geis et  al. [277], using

5.2  Field emission

diamond-based cathodes in 1991, reported emission at surprisingly low threshold fields (<3 V/μm). Later that same year, Djubua and Chubun [278] supported these findings by evidencing emission from arrays of diamond-like carbon (DLC) cones at extraction potentials much lower than those of either Mo or Hf tips. It was believed at the time that carbon materials could be classified into two distinct groups based on their proposed emission mechanism. These were the structured graphites, which emitted as a result of field enhancement, and the diamond types, where band-bending effects were believed to be important [279–283]. In the latter, low and even negative electron affinities in wide band gap materials, as in diamond and its synthetic derivatives, were thought to account for the emission. Electrons that were injected into the conduction band encounter a reduced barrier and as a result were easily emitted into vacuum. It is now accepted that this is not the case. Band bending would occur homogenously over the entire surface resulting in spatially uniform emission, yet the observed emission was always confined within a few highly localized and randomly distributed sites. The widely accepted model and data were inconsistent. Spatial mapping of the emission characteristics hinted that there was something peculiar about the emission sites in relation to the rest of the (apparently) planar coating [98]. Simultaneous photo and field emission studies employing highly uniform CVD diamond films showed that the excited electrons were sourced from 5 to 6 eV below the vacuum level [98]. They were emitted at unexpectedly low fields due to field enhancement originating from crystal grain boundaries [284], a result that was largely supported by the superior emission characteristics of highly defective and interface-rich films. As a result, tetrahedral amorphous carbon (ta-C), a DLC derivative, was heavily studied. These ta-C films only ever emitted after activation, which is when a vacuum arc discharge event occurred with sufficient vigor to perturb the surface topography to form tip-like structures, typically at applied fields of 80–180 V/μm. Post-emission electron microscopy analysis revealed sharp topographic features with a propensity to function as suitable geometrically enhanced emission sites [285]. Prior to activation no surface perturbations existed and poor emission characteristics, solely due to the intrinsic grain boundaries, were measured. Further analysis led to the inference that what were originally believed to be smooth ta-C films were in fact covered in sub-micron-sized graphitic inclusions and particles [286] functioning as emission sites with field enhancements of up to 250—values far too high to be consistent with the film’s apparent planar nature. A new trend emerged. The production of intentionally diverse composite carbon films, rich in a variety of graphitic nanostructures, became commonplace. These films contained wide assortments of naturally occurring carbonaceous nanostructures, including but certainly not limited to carbon particles, onions, clusters, fullerenes, nanotubes, and nanofibers. These nanostructured carbon films performed significantly better than both the planar carbon and Spindt metallic emitters with turn-on fields as low as 1–5 V/μm. Moreover, it was determined that carbon has one of the lowest sputter coefficients [287], making its allotropes extremely stable when bombarded by positive ions associated with local liberated ionized gas species. Field emission has also been reported from DLC [65,264], CVD graphite [288], and various polymers [289]. Of these materials, those with the highest aspect ratios or

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CHAPTER 5  Engineered carbon nanotube field emission devices

particularly perturbed surfaces outperformed the more conformal coatings. Evidently sharp tips were preferable. Indeed, an idealized field emission tip should be whiskerlike. Less optimal emitter morphologies include sharpened pyramids and hemispheroidals [290]. CNTs have a high aspect ratio, a small cap radius of curvature, and are good conductors at room temperature, because the graphene walls run parallel to the filament axis [291]. The first electron emission from a CNT was demonstrated by de Heer et al. [292] in the mid-1990s, where they validated the general applicability of the FN model in these semimetallic electron emitters. The covalent bonds of CNTs are significantly stronger than the metallic bonds of the ubiquitous refractory metal tips. The energy barrier associated with surface atomic migration and diffusion is much larger than in metallic emitters, making CNTs highly stable. CNT tips can thus withstand the extremely intense electric fields (several V/nm) necessary for high current applications. CNTs remain exceptionally stable at temperatures of up to 2000 K [293] and are physically inert to sputtering and chemically inert to surface work-function adjusting absorbates. The combination of high temperature and high electric fields in metals induces field sharpening by surface diffusion. This dominant failure mechanism and cause of temporal instabilities in metal tips increases the local electric field, current, and temperature further, creating a positive feedback mechanism resulting in an unstable thermal runaway and eventual tip destruction. Metal emitters thus degrade, have poor longevity, and require regular replacement. CNTs do not suffer from electric-field-induced sharpening. Their resistance decreases with temperature, which limits the diffusionstimulated I2R heating. They can carry a huge current density of up to 109 A/cm2 with minor electromigration. To attain large current densities with reasonable temporal and spatial uniformities (<2%), ultra stabilities, and long lifetimes (>10,000 h continuous operation), it is necessary to prevent individual CNTs from dominating the emission and instigating damaging current runaway. Integrating ballast resistances with each individual electron-emitting tip is one way of preventing this. Figure 5.14 shows one such ballasted CNT array using Si-on-insulator (SOI) substrates (Si/SiO2/Si) [294]. The undoped Si between the W biasing electrode and the CNT emitter functions as the controllable current-limiting ballast resistance. Vertically aligned CNT arrays (50 nm in diameter, 10 µm pitch) were deposited through a combination of e-beam patterning, Ni catalyst deposition and PE-CVD synthesis in an NH3:C2H2 atmosphere at 700 °C. Each W grid is 2.5 µm × 2.5 µm. The W electrodes were 0.5 µm wide. Individual CNT emitters are located at the center of each grid and have a nominal length of 1.25 µm [upper right inset Figure 5.14(a)]. The lower left inset shows a schematic diagram of the diode-like measurement system. The ballasted structure integrates a single transistor with each emitting tip. The W grid functions as the source electrode, the CNT as the drain electrode, and the underlying bulk Si substrate as the channel-modulating gate electrode that controls the conductivity of the upper Si ballast channel. Figure 5.14(b) shows the variation in emission characteristics as a function of the gate voltage and the consequent FN plot. The observed characteristics are a result of two competing trends, at a fixed

5.2  Field emission

(a)

Anode

Va

CNTs (drain)

GND

Tungsten grid (source)

Vg

Semiconductor Si Insulator Doped Si (back gate) Cu plate

2 µm

1E-4 1E-5

(b) Vg = –25 V

1E-6

–16

In (I/V2)

I (A)

Vg = 25 V

1E-7 1E-8 20

40

–18 –20 –22 –24 –26 –28

1

2

80 60 Applied bias (V)

3

100/V

4

5

100

FIGURE 5.14  Hot Electron Emission from SOI Substrate-Ballasted CNT Arrays. (a) SEM of a substrate-ballasted SOI-CNT field emitter array. Lower left inset: Schematic showing the measurement setup. Upper right inset: High magnification SEM of six CNT emitters and the W source electrode. (b) Measured emission current as a function of the anodes applied extraction bias. Inset: Corresponding FN plot [294].

extraction potential. The Si ballast resistance reduces the potential at the CNT, thereby increasing the extraction potential difference (between the extraction anode and the CNT emitter), which increases the emission current. However, this is offset by the ballast resistance limiting the number of electrons that can be emitted. This second process dominates. Although an increased emission current is accessible, by virtue of the maximized extraction electric field only electrons in the comparatively electron-deficient CNTs are accessible, thus the emission current is severely inhibited by the ballast resistance. The CNT emitters can only emit electrons, not source them. Electron sourcing must come from the W drain electrodes through the ballast Si

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CHAPTER 5  Engineered carbon nanotube field emission devices

channel. As the emission current increases, the potential at the drain contact becomes increasingly positive and the gate-drain potential difference is reduced, leading to a “pinch-off ”-type current saturation. The devices exhibited p-type transistor behavior with an on/off ratio of ~104. Electrons accumulating at the drain (CNT) of the transistor are “hot” due to the potential drop across the Si channel. These hot electrons propagate through the CNT and are readily emitted. Typically the electron energy distribution is characterized by the ambient temperature and defects along the length of the CNT, which scatter the electrons being transmitted toward the tip, ensuring their thermalization. In this case, field emission is determined by the transmission probability of the electrons from the Fermi Sea in the tip of the CNT, through the triangular potential barrier, into the vacuum. The injection of hot electrons from the ballast transistor modifies the transport through the CNT. These electrons are scattered much less effectively and arrive at the tip with sufficient excess momentum to significantly enhance the transmission into the vacuum. The effect of the ballast transistor is to increase the current contribution of every tip in the low-field region and to limit the current in the high-field region due to the resistance of the channel. Combining these effects improves the emission uniformity and lifetime since reduced electric fields are required for a given total current. In such structures the W source limits the attainable CNT packing density, and therefore the maximum emission current. The W bus also imparts undesirable beam shielding and focusing effects. Furthermore, reducing the number of patterning steps, especially in the case of e-beam lithography, significantly increases the commercialization potential. Thus, in an attempt to remove the costly and time-consuming patterning processes necessary, the possibility of directly coating CNT arrays and thin films with ballast resistor-like structures without compromising their field-enhancing characteristics has been widely considered. CNT arrays, such as the one shown in Figure 5.11(b), were grown by PE-CVD and ZnO nanowires (NWs) were deposited using a hydrothermal approach [295] (Figure 5.15). Zinc acetate dehydrate in 1-propanol (spectroscopic grade) was spincoated onto the CNTs at 2000 rpm for 30 s. These substrates were then annealed at 100 °C for 2 min to promote adhesion. Prepared substrates were then dipped into an equimolar solution of 25 nM zinc nitrate hexahydrate (Zn(NO3)2·6H2O and hexamethylenetetramine in de-ionized water heated to 80 °C to synthesize the ZnO NWs. An important benefit of these structures compared to the substrate-ballasted emitters is the less demanding fabrication process [296, p. 143114 (3 pp.)]. The emission characteristics of a standard CNT array and a CNT/ZnO NW coated array is shown in Figure 5.16(b,c) [296, p. 143114 (3 pp.)]. The unexpectedly low turn-on field for the ZnO/CNT emitters is due to the increased field enhancement caused by the addition of the high-aspect-ratio ZnO NWs. The average field enhancement factors of the CNT array and CNT/ZnO were 820 and 2400, respectively [296, p. 143114 (3 pp.)]. The maximum emission current densities were a few mA/cm2 in both cases. The ZnO NWs tended to reduce the maximum emission current by approximately a factor of two, as was the case in the ZnO NW coated CNT thin films. One of the key effects of the ballasting ZnO NWs was to improve the lifetime and stability of the array—the

5.2  Field emission

(a)

(b)

(c)

(d) (e) 100

0.0

1.0

E (V/µm) 2.0

(f)

3.0

1E-7 mbar

Resistance limited field emission

Rolled

ZnO + CNT

10–5 10–6 10–7

0.6

1/E 0.8

1.0

1.2

1.4

–13

Resistance limited field emission

–15

In (I/E2)

I (mA)

–11

10–3 10–4

0.4

–9

10–1 10–2

0.2

–19

Pure field emission

10–8

Rolled

–17 ZnO + CNT

Pure field emission

–21 –23 –25

FIGURE 5.15  Ballasted CNT/ZnO NW Thin Films. (a) SEM of a rolled CNT thin film and (b) ZnO NW coated CNT film. Inset: Schematic cross section of the flexible emitter structures. Optical micrographs of (c) a patterned CNT emitter and (d) a ZnO NW coated CNT emitter on APTES pre-treated PET substrates. (e) J–E characteristics and (f) the corresponding FN plot.

key barrier to the successful development of many field emission-based devices. Accelerated lifetime measurements at 5 ×10−6 mbar with an initial emission current density of 1 mA/cm2 [296, p. 143114 (3 pp.)] [Figure 5.16(d)] suggest that the ZnO-coated arrays show a substantially enhanced lifetime, but little improvement in stability (i.e., noise margin). The limited carrier concentration in the ZnO restricts the emission current, and the hermetic sealing of the CNTs prevents local gas absorbates from adjusting the surface work function, thereby protecting the underlying CNT emitters from high-pressure degradation, opening up the possibility of modest pressure environmental and flat panel lighting, for example. The devices considered thus far have all operated as diode-like emitters. Figure 5.11(a) shows simple diode and triode configurations. In the diode case a voltage, Vo (typically anywhere from 10 V–5 kV) is applied between the emitter (attached to the cathode which is typically grounded), adjacent to a biased anode with an interelectrode spacing, δ, from 10 μm to 5 mm. Electrons are liberated from the emitter and impinge on the anode, formed from a glass/transparent conducting film (TCF)/phosphor stack. Triode structures differ as a third electrode is introduced in the interelectrode evacuated cavity. This gate electrode is biased such that a proportion of the emitted current flows into the gate, offering a viable means to control the emission

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1.6

(a)

CNT

ZnO Nws

(d)

1.4

Jc (mA/cm2)

1.2

Si

1.0

ZnO+CNT

0.8 0.6

CNT array

0.4

2 µm

0.2

0

20,000 40,000 60,000 80,000 100,000 120,000

Time (S) (b)

Resistance limited field emission

–8

(c)

Resistance limited field emission

–10 1E-3

Pure field emission

–12

Pure field emission

In (I/V2)

Jc (A/cm2)

152

ZnO+CNT

–14

ZnO+CNT 1E-4

–16

CNT array

CNT array

–18 1E-5

–20 2

3

4

5

6

7

8

9

0.1

E (V/µm)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1/V

FIGURE 5.16  ZnO NW Ballasted CNT Arrays (ZnO + CNT). (a) SEM of ZnO NW coated CNT array. Upper left Inset: Schematic cross section of a coated CNT. (b) J–E comparative plot and (c) corresponding FN plot for a standard CNT array and a ZnO NW coated CNT array. (d) Temporal stability comparison between the raw CNT array and the ZnO NW coated CNT array [296].

current using low voltages that can be switched rapidly. Though more complicated to fabricate, such triode configurations open up a variety of new applications. The fabrication process of one such triode structure is illustrated in Figure 5.17 [297]. An etching step creates a cavity in which the CNTs grow. The process is selfaligning. The CNT automatically grows in the center of the gate and is aligned along the gate axis. A sandwich structure containing a gate electrode on top of an insulator, which in turn is on top of an emitter electrode, is first deposited via PVD. An array of 300 nm diameter holes (approximately 2 ×104 in total), at a 5 µm pitch, is then patterned by e-beam lithography. An isotropic reactive ion etching step using SF6 removes the polysilicon gate to form an 800 nm aperture. The SiO2 dielectric was then similarly wet-etched in buffered hydrofluoric acid [Figure 5.17(b)]. Both the gate and insulator were over-etched to produce an undercut so as to prevent the CNTs from short-circuiting to the gate and the SiO2 from charging during electron emission. A 15 nm thick conductive TiN layer was then deposited by magnetron sputtering, followed by 7 nm of Ni [Figure 5.17(c)]. The resist holes define the gate, insulator, and emitter position, hence the term self-aligning. The unwanted TiN and Ni over the gate are removed by lifting off the resist in acetone. CNTs were subsequently grown by

5.2  Field emission

(a)

(c)

Resist Gate electrode

Catalyst and barrier layer (if needed)

Gate

Insulator Emitter electrode

Emitter electrode

(b)

(d) Nanotube/nanowire

Gate

Gate Isotropic underetch

Emitter electrode

Emitter electrode

FIGURE 5.17  Self-Aligned Individual Nanotube Triodes. (a) A resist hole is first patterned onto a gate electrode/insulator/emitter electrode sandwich. (b) The gate and insulator material are then isotropically etched. (c) A thin film of catalyst, and diffusion barrier (if required), are deposited on the structure. (d) A lift-off is then performed to remove the unwanted catalyst on top of the gate, followed by the nanotube/NW growth inside the gate cavity [297].

PE-CVD using C2H2 and NH3 (54:200 sccm, respectively) at 5 mbar, 675 °C, with a −600 V sample bias [Figure 5.17(d)]. This process typically produces straight, vertically aligned CNTs for a deposition time of around 15 min. This CNT-based gated nanocathode array, shown in Figure 5.18, has a low turn-on voltage of 25 V and a peak current of 5 µA at 46 V, with a gate current of 10 nA. These low operating voltage cathodes are potentially useful as electron sources for portable field emission equipment, such as display back lights or miniaturized electron beam-based instrumentation. Of course, this is not the only way to create a gated structure. FIB lithography can also be used. Indeed, it has been reported that FIB has the potential to give higher resolution features than e-beam lithography [299]. FIB lithography resembles e-beam lithography in many respects, but an ion beam is substituted for the electron beam. FIB provides more lithographic regimes including resist-based lithography, subtractive lithography (sputtering), or additive lithography (ion beam-assisted deposition). FIB also allows simultaneous observation of the treated surface. The ion source is often a liquid metal, such as Ga. The spatial resolution is very similar to e-beam (tens of nm). The main advantage of FIB is that it is a maskless process and does not require postpatterning, metal deposition, or lift-off. Catalysts can either be directly

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CHAPTER 5  Engineered carbon nanotube field emission devices

FIGURE 5.18 (a) Planar view of an integrated gate CNT cathode. (gate aperture pitch = 5 µm). The CNT appears as a bright dot in each gate aperture. (b) Cross section SEM of the integrated gate CNT cathode, showing the polysilicon gate electrode, SiO2 insulator, TiW/ Mo/ TiW emitter electrode, and a single central vertically aligned CNT (Scale bar: 1 μm) [99,297,298]. FIB milling Pt—120 nm ITO—15 nm CNT

154

SiO2—1 µm Ni—10 nm ITO—15 nm Si Subtrate (a)

(b)

(c)

FIGURE 5.19 (a) Material stack including a Pt gate, a SiO2 insulating layer, and an embedded Ni catalyst with an ITO adhesion layer. (b) shows the result of FIB milling and (c) resulting CNT following PE-CVD [299].

deposited at required positions, or the FIB can be used to mill through the substrate to the catalyst layers beneath. Wu et al. reported the fabrication of gatelike structures by sputtering using FIB. Metal-gated CNT field emitter arrays were fabricated using a multilayer structure with an embedded catalyst layer instead of depositing the catalyst into milled holes, as is common practice [300]. A 15 nm ITO diffusion barrier was deposited onto a Si substrate followed by 10 nm Ni, which served as the CNT catalyst. A 1 µm thick insulating SiO2 layer was deposited onto the Ni catalyst by PE-CVD at 280 °C. Following this, another 15 nm ITO was deposited on the SiO2, followed by a 120 nm Pt thin film. The Pt serves as the gate electrode. A schematic of the structure is shown in Figure 5.19. An FEI dual-beam SEM/FIB was then used to mill an array of holes (300 nm–1 µm in diameter) to expose the Ni catalyst for

5.2  Field emission

FIGURE 5.20  FIB-Fabricated Triodes. (a) 1 µm wide holes with CNTs growing from within. (b) 300 nm holes with single CNTs growing from within the gate structure [301].

CNT growth, as illustrated in Figure 5.20. Once the catalyst was exposed, vertically aligned CNTs were simultaneously synthesized in each gated cavity by PE-CVD. During synthesis, the substrates were heated to 725 °C and exposed to 50 sccm of C2H2 and 200 sccm of NH3 for 30 min. FIB has numerous advantages over other reported methods, including the use of photolithography to etch arrays of holes into Si followed by sputter coating Fe into the holes [302], as in this instance the catalyst island deposition often yields additional undesired catalyzed regions rhR require additional electrochemical removal procedures. Using FIB to expose the catalyst proves to be significantly easier. The serial nature and controlling the milling depth and geometry are the greatest challenges faced by FIB.

5.2.1  ELECTRON MICROSCOPY Electron sources for microscopy applications must be bright, stable, have a low kinetic energy spread, low noise, and good contrast. State-of-the-art microscopy tips come in the form of W cold cathode emitters, Schottky Barrier Diode emitters, Lanthanum Hexaboride (LaB6), and W thermal emitters. However, research has focused intensively on the use of CNTs to examine their ability to improve upon current standards. Perhaps the most detailed investigations were carried out by Dean et al. [303–305] at Motorola and De Jonge et al. from FEI [6,237,306]. In much of their work individual CNTs were attached to the end of standard KOH-etched W tips, with the aim of retrofitting these CNT/W hybrid electron sources into existing electron microscopes. Several different methods were attempted. De Jonge initially used carbon glue and nanomanipulators (Figure 5.21). However, it is critical that the interfacial adhesion between the wire and CNT is strong and that the CNT is well-aligned to the optical axis to prevent astigmatism. The interfacial bond was often weak and the emission lifetime suffered as a result. The attachment process was extremely awkward and

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FIGURE 5.21  Single CNT Electron Sources. Sequence of electron micrographs showing the attachment process of a CNT tip [307]. The tip apex radius is approximately 100 nm. In (a), the CNT is brought close to the tip. (b) A potential difference is applied between the CNT and the tip. The CNT attaches. (c) Attempts to snap the CNT. (d) The resultant hybrid CNT/W tip with the CNT attached 30° off the optical axis (Scale bar: 1 μm) [306].

demanding of highly skilled personnel. The process was also rather time consuming (>30 min) and produced relatively defective tips. Direct CVD on catalyst-coated W tips was also considered, due to its implicit reproducibility. Catalyst deposition was challenging. Significant difficulty lies in reproducibly depositing resist onto sharp tips. Added to this is the challenge of irradiating the tip precisely with an e-beam or FIB catalyst source, since the tip radius is so small. Initial attempts using thermal CVD failed. Several CNTs of similar height grew clustered on the tip. To provide optimum performance single CNTs are required. A modified PE-CVD growth system was reported by Mann et al. [98] to improve yield and reduce the number of on-tip CNTs. When applying a potential difference between two parallel plates, most of the applied voltage in the resultant plasma is dropped across the narrow sheath region. Under typical process PE-CVD conditions (600–800 °C, 2–5 mbar), this sheath extends 2–3 mm above the cathode [37]. To obtain vertically aligned CNTs the tip apex must sit within this region. W wires were KOH wet-etched to a sharp point (<100 nm in diameter) and the entire W wire was coated in catalyst. However, the tip sharpness meant that it was possible to grow only one CNT at the apex by carefully controlling the amount of catalyst. A single CNT yield of >50% has been demonstrated. CNTs consistently align along the primary optical axis and form a robust bond with the W wire. This method is a major advance in CNT growth and brings the prospect of CNT electron sources for

5.2  Field emission

FIGURE 5.22 (a) A KOH wet-etched W wire forming a sub-micron sharp field emission tip. (b) A single CNT, grown by PE-CVD where the W wire was embedded into the heated cathode to ensure that it is enveloped by the plasma sheath. Such hybrid CNT/W tips produce highly confined, enhanced single electron beams for electron microscopy applications (Scale bar: 1 μm).

microscopy applications much closer to market. Simultaneous growth of individual CNTs onto several W tips has been demonstrated using this method. An example of one such tip is shown in Figure 5.22(b). A comparison of the emitter performance is given in Table 5.2. Such CNT sources are extremely competitive with current sources, but high-frequency noise is still problematic.

5.2.2  PARALLEL ELECTRON BEAM LITHOGRAPHY There is an ongoing need for efficient, rapid low-cost techniques for sub-100 nm semiconductor lithography and mask production. Although e-beam direct write lithography is a tried and tested method that is capable of providing some of the finest feature sizes, the throughput remains a major concern due to the serial writing. Several electron beams acting in parallel could vastly decrease the writing time. The feasibility of this approach was demonstrated by Muray et al. [308] using an array of four miniature (2 × 2 cm) electron beam columns. To further reduce the size of each column to the sub-millimeter range, Binh et al. [309] studied the concept of the microgun. Here, a nanotip together with micron-sized electrostatic extraction, focus, and deflection lenses was fabricated on a single Si substrate to obtain an electron beam with nanometric spot size. This microgun was successfully used to write 80 nm lines at a pitch of 160 nm in PMMA in 1999 [310].

157

Table 5.2  Typical Operating Parametersa of Various Industry Pervading Electron Sources (Therm.[ionic], Schot.[tky], and C.C. [Cold cathode]) Compared with CNT Field Emitters

ds (nm) D (nm) ∆E (eV) β Imax (nA) τ (year) Φ (eV) T (°C) Stability (%) Jc (A cm−2) Op. Vac. (mbar) Eon (V µm−1) a

Therm. W

Therm. LAB6

Schot. W/ZrO

C.C. W

CNTs

10,000 60,000 1.5 106 103 0.1 ~4.5 ~2200 <1 1 10−7 101

10,000 10,000 1.0 107 103 0.2 ~2.7 1500 <1 102 10−8 101

20 <1000 0.7 108 102 >1 ~2.8 ~1700 <1 103 10−9 102

3 <100 0.2 109 0.2 >1 ~4.5 25 5 105 10−10 103

10 <50 0.3 109 5 ×103 1 ~5.0 25–400 0.5 107 10−7 5

Where ds is the virtual source size, D is the cathode radius, ∆E is the energy spread of the emitted electronics, β is the electron-optical brightness, Imax is the maximum probe current, τ is the lifetime, Φ is the work function, T is the typical operating temperature, Jc is the current density, Op. Vac. is the operating vacuum, and Eon is the turn-on electric field. Figure of merit polar plot. CNT-based field emitters are seen to outperform all of the state-of-the-art electron sources across most metrics.

5.2  Field emission

Researchers at Thales further investigated the feasibility of parallel e-beam lithography, with the aim of demonstrating the concept with 32 CNT field-emitting microguns. The long-term, and rather ambitious, objective was to achieve high throughput maskless direct writing capability with an array of 1 million microguns. Even if e-beam lithography does not meet the requirements for mass production of integrated circuits in the near future, parallel e-beam lithography may still find itself suitably poised for low-cost photomask fabrication (since low throughput single e-beam systems are currently in use to manufacture photomasks) and economical low-volume maskless prototype fabrication (since a standard CMOS photomask set costs ~2 million euros). The microgun, shown in Figure 5.23, integrates CNT-based electron sources with extraction and focusing lenses but does not include deflection capabilities. To address all the “pixels” within any given writing field, the e-beam writing head (or the sample stage) is translated in plane by UHV-compatible piezoelectric actuators [Figure 5.23(d)]. Scanning is performed in tandem with a control system to determine the required pixels to be exposed for a given pattern. This fixed-focus/fixed-beam position approach has the advantage that the lenses are simpler to design as it eliminates aberrations associated with beam deflection (pin cushioning). For a microgun pitch of 100 µm, the writing time for an entire 4 in mask in parallel would be the time to scan the 100 µm × 100 µm field which is on the order of 1–5 min. Note that the writing head can also be over-scanned to 200 µm × 200 µm, where four microguns would each cover a 100 µm × 100 µm field, thereby providing inherent redundancy in the system. For an array consisting of 100 ×100 electron sources the emitted current can reach up to 10 µA (~1 nA/emitter). Yang et al. [312] have developed similar microcathodes but, as yet, no systems are available on the market.

5.2.3  X-RAY SOURCES There is a continuing need for portable X-ray sources. International border control necessitates accurate internal analysis of unattended baggage, searching for narcotics and hidden contraband, as well as vehicle and food inspection and medical examination. In recent years X-ray microsources, with spot size of the order of few microns and improved brilliance, have opened up many new opportunities. These advanced systems have been developed mainly with two purposes: for nondestructive testing based on lens-less projection radiography, and for diffraction and fluorescence with enhanced spatial resolution, obtained by exploiting innovative optical elements, such as capillaries, polycapillaries, and multilayers. Such advanced X-ray sources are also expected to improve the efficiency of mammography systems in medical care by detecting cancer at its early progressive stages. However, microsources are limited in power and photon flux because of the thermal load on the anode. The limited brilliance of the electron guns based on traditional hot cathodes makes the task of focusing electrons into submicron spots, with good spatial stability, difficult. Moreover, standard hot cathode X-ray sources only operate in continuous, DC mode and have no simple means of pulsed mode operation. Consequently, a new generation of

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CHAPTER 5  Engineered carbon nanotube field emission devices

X

Y

(b)

(a) (c)

Sub-100 nm beam size

Resin

Chromium layer Vmask +1–2 kV

500 µm Post acceleration system (PAS)

500 µm

VPAS 60–70 V

200–500 µm φ = 0.5 µm

Vfocusing ~0 V Vgrid ~50 V

CNT

1 µm

SiO2 Si substrate

Vemitter 0 V = on 50 V = off

Addressing and dose control CMOS (d)

100 µm

160

Each gun exposes the desired pixels as the 100x100 µm area is scanned by piezo movement. 100 µm

100 µm

100 µm

FIGURE 5.23  Parallel Electron Beam Lithography. (a) Schematic depiction of a parallel electron beam lithography write head. (b) SEM of an integrated gate emitter for a parallel electron beam writing head. (c and d) Working principle of the write head. (e) Optical micrograph of a fabricated parallel beam electron source write head consisting of 32 electron guns with integrated driver circuitry. Source: Adapted from [227,311].

5.2  Field emission

(e)

Current comparator

HV transistor to CNT emitter

Charge comparator

FIGURE 5.23  (Continued)

FIGURE 5.24  CNT-based X-ray Source. (a) Schematic of a typical CNT-based X-ray system; (b) X-ray image of a fish carcass. Source: Adapted from [313].

CNT-triode cathodes is now emerging. In the 1990s Oxford Instruments, working together with NASA, produced CNT-based X-ray sources whose application was targeted on low-power space missions. More recently Zhou et  al. [249] at Xintek developed a fast response X-ray source with on-times of <0.5 ms delivering 28 mA peak current (Figure 5.24). Here the CNTs were randomly deposited by electrophoresis, with no specific control of size and dimensions [313].

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5.2.4  MICROWAVE SOURCES Most long-range telecommunication systems employ microwave links requiring high power transmitters, both terrestrial and satellite-based. These satellites are typically equipped with 50–100 traveling wave tubes (TWTs) based on slow thermionic electron sources that cannot be switched at high speeds, and that do not satisfy a power budget of tens of Watts. Present day TWTs are bulky and heavy. They consume valuable portions of a satellite’s volume/weight budget. Miniaturization would give substantial cost savings and aid the development of microsatellites. The rapid growth of global telecommunications and the saturation of the present frequency bands demands that new bands are allocated at ever greater frequencies (>30 GHz), bringing present technology close to its functional limit. Solid-state devices are not suitable in this high-frequency regime, since their maximum attainable power at 30 GHz is only around 1 W, far too low to be of any practical use. Manufacturers are looking for inexpensive, powerful, lightweight, high-frequency alternatives. Attempts were initially made to replace the thermionic cathode in TWTs with Spindt tip cathodes delivering a DC electron beam. However, the bulk of the TWT remains, since it is the tube in which the electron beam modulation takes place, which is physically large. The most effective way to reduce the size of a TWT is via direct modulation of the e-beam, e.g., in a triode configuration using a CNT electron source. Teo et al. developed a number of vertically aligned CNT-based alternatives. The simplest microwave amplifier incorporated a microwave diode based on a cold-cathode field-emitting electron source consisting of sixteen CNT arrays [314]. The sixteen arrays each occupied an area of 0.5 ×0.5 mm2 and consisted of 2.5 ×103 extremely uniform CNTs with an average diameter of 49 nm (±4%) and height of 5.5 μm (±6%) at a pitch of 10 µm [108]. A Class D (i.e., pulse mode/on-off) amplifier formed from an MWCNT array cathode was also successfully operated at 1.5 GHz, with a peak current density of 12 A/cm2 [314]. More recently, a 32 GHz directly modulated CNT array cathode under Class A (i.e., sine wave) operation with over 90% modulation depth has also been demonstrated. This unique ability to directly modulate or generate RF/GHz electron beams allows such emitters to directly replace hot cathodes and the associated modulation stage. These CNT cathodes have no heating requirement and can turn on or off nearly instantaneously, offering highly efficient operation. Because of their small size and their ability to modulate the beam directly, CNT cathodes embody a new generation of lightweight, efficient, and compact microwave devices for telecommunications. Nevertheless, the main problem plaguing such systems at present is the attainable bandwidth. To overcome this Hudanski et al. [315,316] investigated optically controlled field emission photocathodes based on vertically aligned CNT arrays. Figure 5.25 shows an illustration and scanning electron micrograph (inset) of one such photocathode. In contrast to the cathodes described above, the applied electric field is constant and the applied high voltage source operates in a purely DC fashion. Modulation of the emitter current is achieved through optical control. This offers an unprecedented speed of operation, much wider than that of conventional electronics. Such devices are thus compatible with high frequency and very large bandwidth operation. A photocathode based on Si tips was first proposed by Schroder et  al.

5.2  Field emission

FIGURE 5.25  High Bandwidth Electron Emitters. Schematic of an MWCNT-based field-emitting photocathode. The incident 532 nm optical source modulates the emission current via the embedded pin diode, while the CNTs impart high-field enhancement [241].

in the 1970s [317], and was revisited by Liu et al. [318] and Chiang et al. [319] in 2008. Their approach consisted of using bulk Si for photon–electron conversion and a sharp tip to obtain field emission. However, simulations have shown that the emitted current per Si tip cannot exceed 60 µA, which is lower than experimental currents obtained with MWCNTs (100–200 µA). In more recent work [315,316], arrays of MWCNTs constitute the electron source, in a new photocathode concept that separates the functions of photon–electron conversion and electron emission. The photocathode integrates the exceptional field emission properties of CNTs with the photon/electron conversion efficiency of p-i-n photodiodes. These p-i-n photodiodes are capable of exhibiting very high internal quantum efficiencies (close to 100%) and can be operated at various wavelengths. The p-i-n material is then chosen according to the targeted application and available optical sources. Using a green (532 nm) laser and a Si-photodiode the photocathode was shown to deliver 0.5 mA with an internal quantum efficiency of 10% and an Ion/Ioff ratio of 30. It is envisaged that by using a higher frequency photodiode (e.g., GaInAs) and driving this with a 1.55 μm laser, operation in the 10–30 GHz range is possible.

5.2.5  DISPLAYS CNT electron sources were thought to have their largest potential market in flat-panel field-emission displays (FEDs) [320,321]. In 2011 the flat-panel display market had an estimated net worth in excess of $135 billion—one of the largest global markets in human history. Figure 5.26(g) illustrates a simple CNT-based display pixel, where the CNTs are vertically deposited on a matrix of electrodes in a vacuum housing.

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5.2  Field emission

Theoretically, CNTs can be positioned in such a way as to maximize the field at their apex, while maintaining a high current density. However, such processing is prohibitively expensive given the current cost of large area CVD reactors. Industry has been researching ways in which to deposit pregrown CNTs to achieve a similar functionality. Many companies, most notably Samsung Advanced Institute of Technology (SAIT) [326], worked on the use of CNTs for TV and display applications. Figure 5.26(b) shows a Samsung 4.5-in. prototype FED based on CNT thin-film technology. In 2000 SAIT unveiled a demo 38-in. diagonal full-color FED display employing spaghetti-like CNT thin films [327] (Figure 5.26(h)). The technology was transferred to Samsung SDI for production, but thus far they have not produced any products. Philips, Teco Nanotech Co., and ISE Electronics have all worked on the development of CNT-FEDs. Teco Nanotech Co. marketed three basic CNT-FEDs, the largest of which was 8.9 in. [264]. J. Dijon, at CEA-Leti in Grenoble, reported in 2005 the development of both monochromatic and full-color FEDs where the emitters where formed from directly CVD pixel arrays of 20 μm spaghetti-like CNT forests [Figure 5.26(c)]. The inset of Figure 5.26(c) shows SEM of the patterned catalyst, synthesized spaghetti-like CNTs, and a subset of the pixel array [328]. Sony’s Field Emission Displays Inc.—a spin-off entity based on the intellectual assets acquired from Nanotechnologies Inc. and entirely focused on FED production—was charged with the continued development of their FEDs. However, Sony closed down its FED sector in 2009 in a companywide back-off from FED technologies. AU Optronic from Taiwan acquired assets that included patents, various intellectual property FIGURE 5.26  Next-Generation Nanomaterials-Based FEDs. (a) 2.55-in. fully vacuum-sealed FED (160 × 360 pixel) prototype by Motorola based on nitrogen-doped amorphous carbon emitters with a 45 V switching voltage [303,322]. (b) A screen-printed CNT-FED developed by Samsung SDI (1999) [320]. Inset: Cross section SEM of the CNT field emitters [320]. A 30-in. diagonal version of the technology was reported to be near completion in 2004 (Samsung Display Technology. Courtesy of Y. Choi). (c) Monochromatic, 350 μm pixel, CNT-FED developed by CEA-Leti. A full-color, 600 μm pixel, display has also been reported (Courtesy of CEA-LITEN, http://issuu.com/ phantoms_foundation/docs/e_nano_newsletter_issue20_21/24). (d) Samsung Advanced Institute of Technologies CNT-based 9-in., 576 ×240 pixel thick film display [303,323] (Courtesy of J. M Kim, SAIT). (e) Printed Field Emitters (PFE) Ltd. addressable display. The emitters are formed by conductive particles in an insulating matrix [303]. (f) A video running 5-in. CNT-FED by SAIT, operating at a gate potential of 100 V, an anode bias of 1.5 kV, and a drive frequency of 100 Hz [324]. (g) Typical structure of an FED. Benefits of FEDs include substantially thinner and lightweight displays, higher energy efficiency, simplified planar construction, thousands of simultaneously firing electron beams, rapid subpixel switching rates (<10 μs), wide viewing angles (160°), and high contrast ratios. The reduction in cavity volume improves vacuum quality, display lifetime, and stability. The insets show typical RGB phosphor masks for each technology. Adapted from [136,325]. (h) An optical micrograph of Samsung’s CNT-based full-color, 100 Hz, prototype 38-in. (diagonal) FED display.

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rights, processing equipment, and materials from Japan-based FET, which was 40% owned by Sony Corp. AU Optronic has maintained a concerted effort to develop the technology but has not as yet planned mass production. CNT thin films make excellent field emitters. Indeed, most display prototypes have been based on thin films of disordered CNTs, as shown above. Though these spaghetti-type networks function adequately well in displays, Nilsson et al. [184], Groening et al. [183], and Bonard et al. [329] showed that such close packing densities introduce significant nearest-neighbor electrostatic shielding effects, which reduce the film’s effective field enhancement factor. Moreover, spatially resolved scanning field emission mapping has also shown that such spaghetti networks have broad field enhancement factor distributions—a product of the rough films. Thus, these displays turn on at very low voltages, but are largely unstable and have poor lifetimes. To fully exploit the exceptionally high field enhancement of each nanotube, vertically aligned arrays of nanotubes [99,104,226] are necessary with a pitch equal to approximately twice their height and simultaneously with narrow distribution in β to enhance display longevity. In 2006 Motorola utilized this in their published work on the nano-emissive large area high-definition prototype displays using in situ CVD, selectively positioned vertically aligned CNTs on glass substrates, but has since discontinued all FED development [330,331]. Indeed, though significant research has been carried out industrywide and various CNT-FED prototypes have shown significant promise with comparable pixel-to-pixel nonuniformity (5%) to those of liquid crystal displays (LCDs) (3%) and cathode ray tubes (2%), currently no displays based on these technologies are on the market. Nevertheless, efforts continue toward applications in high switching speed backlit units, especially for 3D displays based on active matrix LCDs. The application of field-enhanced CNTs to augmented LCDs is also being considered at present. These advanced electrodes reduce the driving voltage of existing LCDs. The high aspect ratio CNTs, grown on transparent conducting substrates, reduce the required switching voltages due to tip electric field enhancement. Other such systems based on similar concepts have been proposed, such as CNT array microlenses [332]. Arrays of vertically aligned CNTs, coated with liquid crystal and sandwiched beneath a planar optically transparent top gate are one such example. The liquid crystals align themselves during the applications of an electric field. The refractive index is altered locally around a CNT and thus creates a lens. The electric field required is typically lower than that at which field emission initiates. Figure 5.27 depicts the system. Such a device could have many potential applications in adaptive optical systems, wavefront sensors, and optical diffusers.

5.2.6  GAS IONIZATION SENSORS AND GAUGES Sharp morphologies increase the local electric field. Though often related expressly to high vacuum field emission, such an effect can also be used for gas and pressure detection. The typical structure of a gas ionization sensor is depicted in Figure 5.28(a,b). Here, an MWCNT thin film forms the sensing element and the current

5.2  Field emission

FIGURE 5.27  Augmented LCDs. (a) Schematic diagram representing the MWCNT immersed in a liquid crystal (LC). (b) A voltage applied to the top gate causes the LC to align with the induced electric field, which is distorted by the MWCNT. (c) SEM of pairs of CNTs grown from single catalyst spots at a pitch of 10 µm. Source: Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission [332,333].

flowing between the electrodes gives a measure of the pressure, while the breakdown voltage within the cell (the voltage at which an electric arc event occurs) is a signature of particular gas species between the electrodes. Composite sensing films have also been reported, such as a ZnO/CNT bilayer [335] [Figure 5.28(b,c,e,f)]. Choi and coworkers [336,337] at the Korean Research Institute of Standards and Science are exploiting the field emission effect from CNTs to characterize a new type of ionization gauge for detecting low pressures. The gauge is based on a triode configuration similar to that of a conventional hot cathode ionization gauge but instead uses a cold emission source based on CNTs. They used a screen-printing method to produce the cathode and found that the ratio of the ionization current to the CNT cathode current changes according to the pressure in the chamber. CNTs have been touted as next-generation sensing agents in conventional gas sensors due to their high surface-to-bulk ratio, but their ability to produce high

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(a)

(d)

I Al plate V Si substrate

Glass insulator

MWNTs (µm)

(b)

120

(c)

20 ppm

(e)

120

20 ppm 20 ppm

100

(f)

100

20 ppm

80

Current (pA)

Current (pA)

168

20 ppm 20 ppm

60 40

20 ppm 20 ppm

80 60 40

20

20 0

300

600

900

1200 1500 1800

Time (s)

0

30 60 90 120 150 Concentration of NH3 (ppm)

180

FIGURE 5.28  Gas Ionization Sensors. (a) Schematic depiction of a typical gas ionization sensor adopting a standard diode configuration [334]. (b and c) SEMs of a ZnO/CNT gas ionization sensor thin film [335]. (d) A SEM of an MWCNT gas ionization sensor (Scale bar: 10 μm). Inset: Device structure [334]. (e and f) Current response of the sensor depicted in (d) showing a reproducible response to 20 ppm NH3 [335].

ionization currents makes them of particular interest in gas ionization sensors. Gas ionization sensors work by fingerprinting the ionization characteristics of various gases. Ajayan and coworkers [334] reported the fabrication and successful testing of ionization CNT microsensors featuring the electrical breakdown of a wide range of pure gases (He, Ar, CO2, N2, O2, and NH3) and gas mixtures (air). As in the case of field emission, the sharp tips generate very high electric fields at relatively low voltages, thus lowering the breakdown voltage of the gas to be sensed in comparison to traditional electrodes and enabling compact, battery-powered, safe, portable operation. The sensors show good sensitivity and selectivity and are unaffected by extraneous factors such as temperature, humidity, and gas flow. As such, these devices offer several practical advantages over previously reported nanotube sensor systems. The

5.3  Conclusion

sensors could be deployed for a variety of applications, such as environmental monitoring, sensing in chemical processing plants, and gas detection for counterterrorism.

5.2.7  INTERSTELLAR PROPULSION There is considerable interest in microspacecraft to support interstellar robotic exploration and near-Earth environment characterization. The principle challenge here is to develop miniature electric propulsion systems capable of operating at much lower power levels than conventional momentum exchange electric propulsion hardware, and which meet the unique mass, power, and size requirements. The Busek Company, Inc. (Natick, MA) [338] has developed field emission cathodes based on CNTs for such an application. These devices have turn-on voltages about an order of magnitude lower than devices that rely on diamond or DLC films, and they have shown significant promise. Various groups have focused on the study of colloid thrusters and are in the process of developing novel, earth-orbiting spacecraft formations [339,340]. The Rutherford Appleton Laboratory and Brunel University are also studying the field emission performance of macroscopically gated MWCNTs for spacecraft neutralizers [341]. Electron emission from aligned MWCNTs has been investigated to determine if the performance—defined by the power consumption, lifetime, and emission currents—is suitable for spacecraft field emission propulsion. CNTs are attractive for neutralizer devices because they have low emission threshold potentials, high current densities, stable emission over prolonged time periods, and fabrication simplicity. A CNT neutralizer for a colloidal micro-Newton thruster already exists, but it operates at a somewhat high 250–700 V. For a fixed emission current such requirements are incompatible with the 0.2 W/mA baseline defined by the LISA Pathfinder power supply subsystem. Because of their intrinsically high aspect ratio, their high current-carrying capability, and their inertness to sputtering, CNTs have been one of the most intensely researched materials for cold cathode applications. Here we have reported some of the more prominent examples; many more will surely follow in the near future.

5.3  CONCLUSION Processes are being continually developed to grow CNTs in new and exciting ways for increasingly novel applications. This chapter has described aspects of the underlying heterogeneous catalysis, the various synthesis routes, and some of the key fabrication issues. Some example field emission applications were given, though this is by no means an exhaustive list, to indicate the ways in which CNT technology is continuing to grow more than two decades after its inception. Indeed, research is ongoing in many of these fields, and commercially viable devices are tentatively approaching the market due to a number of seminal advances in our understanding and exploitation of these enigmatic nanostructures.

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ACKNOWLEDGMENTS The authors acknowledge the ongoing support of, and collaboration with Y. Zhang, C. Li, K. Qu, K. B. K. Teo, G. A. J. Amaratunga, P. Hiralal, D. Pribat, N. Rupesinghe, P. Legagneux, L. Gangloff, M. Chhowalla, E. Minoux, L. Hudanski, F. Peauger, N. de Jonge, D. Dieumgard, O. Groening, V.T. Binh, and V. Semet—without whom much of this work would not have been possible. The authors would like to thank E. Heeres for original contributions. The authors express their sincere apologies to all those individuals who have made important contributions to the field that, due to space constraints, they did not manage to mention.

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