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

Chapter 6 Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 119 Chapter 6 Field Emission Study of Hydrogenated Tetrah

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

Nanotubes Core-Shell Nanostructures

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Chapter 6

Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated

Nanostructures

In this chapter, the field emission (FE) properties of a triple layered nanocomposite consisting of a hydrogenated layer on the surface of a core/shelled carbon nanotube/tetrahedral amorphous carbon (CNT/ta-C) structure will be studied The deposition of the ta-C coating films with various thicknesses was carried out by the pulsed laser deposition (PLD) technique After that, the ta-C coated CNT specimens with coating film thicknesses of 50 and 100 nm were treated by hydrogen plasma for

10, 20 and 30 s respectively The effects of the coating film thickness and the hydrogen plasma treatment duration on the FE properties of these samples will be specified and the underlying principles for their FE performances will be discussed as well

6.1 Introduction

Diamond is a form of carbon It is composed of sp3 hybridized carbon bonding corresponded to the tetrahedral configuration in which a carbon atom binds to 4

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neighbors giving rise to three-dimensional interconnected structure of carbon atoms [1] This structure grants excellent mechanical properties for diamond, making it the hardest natural material Diamond films can be produced by vacuum deposition but the optimum substrate temperature for coating is as high as 900 °C, which severely limits the range of substrates to which diamond can be applied [2] Fortunately, near room temperature, an amorphous carbon film can be produced in which a proportion of the carbon atoms are bonded as in diamond This amorphous carbon is called diamond-like carbon (DLC) DLC resembles diamond in many ways, such as high mechanical hardness, wear and chemical resistance and optical transparency [3] The hardest, strongest and slickest DLC is known as tetrahedral amorphous carbon, or ta-C, which generally contains little or no hydrogen but high sp3 content (> 80%) [4-6] DLC is applicable in many areas, within which the most prominent application is using it as a coating material to reduce the abrasive wear and extraordinarily increase the lifetime of components [7]

Diamond and DLC are potential in FE applications due to their low-field electron emission, hardness to withstand ion bombardment, and good thermal and electrical conductivity to endure high current Since the first report of FE phenomenon of diamond film in 1991, great attention has been attracted in this research area [8-10] It was found that suitable doping or surface treatment of diamond, such as hydrogen or oxygen plasma etching could lead to low or negative electron affinity (NEA) for the diamond surface, i.e., the conduction band minimum

of diamond can be higher than the vacuum level [11, 12] The NEA property could

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make electrons eject at a pretty low applied field and thereby saving the consumed energy of electronic devices As diamond or DLC is easy to be deposited as a thin film, it could act as a coating material on some nanoscale field emitters and lowered threshold fields of the emitters were observed after doing so [13-15]

In this project, we attempted to combine the advantages of the DLC and CNTs, i.e., the low electron affinity and strong mechanical properties of DLC and the one-dimensional free-standing geometry of CNTs by coating the DLC thin films directly onto the surface of the vertically-aligned CNTs In the DLC thin film coating process, the pulsed laser deposition (PLD) technique was chosen for its simple procedure, room temperature deposition and capability of producing high sp3 content DLC films

6.2 Preparation of Hydrogenated Ta-C Coated CNT Nanostructures

6.2.1 Setup of the PLD System Used

A custom-designed PLD system was used in this project The PLD system consists of 4 parts: the laser system, optics system, vacuum deposition chamber and the pumping system Fig 6.1 shows the schematic diagram of the PLD system used in this study and its laser route is shown in Fig 6.2

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Fig 6.1 Schematic illustration of the custom

Fig 6.2 The laser

Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon

Schematic illustration of the custom-degisned PLD system

The laser-route of the custom-designed PLD system

122 degisned PLD system

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The laser system used was a class 4 Compex Lambda Physik pulsed laser excimer laser with a wavelength of 248 nm that uses a KrF gas The maximum power it could attain was 50 W The specifications are as shown in Table 6.1

Table 6.1 Specifications of the PLD laser system used

Wavelength 248 nm Maximum pulse energy 600 mJ

Maximum repetition rate 50 Hz

Pulse duration (nominal) 25 ns (FWHM)

Beam dimensions 24 × 8 mm2 Average power 25 W

As the laser system emits a substantial amount of heat when in use, an exhaust pipe was connected so the heat emitted could be channeled out of the laboratory In addition, a water chiller was used to cool the laser system in order to prevent overheating A series of optics such as mirrors and focusing lenses were strategically put in place to reflect, guide and focus the laser beam into the vacuum chamber with minimum energy loss The focusing lens was used to focus the laser beam to a minimum possible size attainable so as to obtain a maximum intensity for that given reduced spot size After the series of optics were in place, the laser was then aligned to direct the laser beam into the vacuum chamber

A two-stage pumping system was used to achieve high vacuum level The first stage of pumping was done with a rotary pump to bring the pressure in the chamber

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down to about 10-3 Torr before the second stage of pumping started That involved a turbo molecular pump manually started to further pump down the vacuum within the chamber to a pressure of about 10-6 - 10-7 Torr

6.2.2 Preparation Procedures of the Samples

The preparation procedures of the composite samples are schematically illustrated in Fig 6.3 High density vertically-aligned CNTs with the length of around

14 µm were used as the substrates These substrates were fixed on a metallic holder and placed vertically in the PLD chamber, facing to the carbon target with a constant distance of 50 mm in between The carbon target was prepared of high purity carbon powder (99.9%) with the particle size of 325 meshes Next, the system was pumped down to 5 × 10-6 Torr for deposition During the deposition, the target was rotated with

a speed of around 6 rpm (round per minute) while being ablated by the laser with the energy density of around 20 J cm-2 The substrates were deposited for varied durations

In order to measure the thickness of these films, simultaneous deposition on bare silicon substrates was used as reference The film thickness was measured by a standard surface profiler

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Fig 6.3 Illustration of the preparation procedures of the s

silicon substrate (b) DLC thin films

post-treatment on the surface of

After deposition, the 5

hydrogen plasma for 10, 20 and 30 s respectively via

coupled with a 2.45 GHz microwave power supply

was applied for the hydrogenation The chamber pressure was set to be 15 Torr and the

H2 flow rate was set to be 300

6.3 Thickness Effect of

Composite Emitters

6.3.1 Confirmation of C

Emitters

The high resolution TEM image in Fig 6.4 show

composite emitters It confirms that the

core-shell nanostructures, consisting of core CNTs and shell

Illustration of the preparation procedures of the samples (a) CNT growth on the

DLC thin films coating on the CNTs (c) Hydrogen plasma reatment on the surface of DLC coated CNTs

After deposition, the 50 and 100 nm DLC deposited samples were treated hydrogen plasma for 10, 20 and 30 s respectively via a microwave plasma CVD facility

was applied for the hydrogenation The chamber pressure was set to be 15 Torr and the flow rate was set to be 300 sccm

6.3 Thickness Effect of Ta-C Films on FE Properties of Composite Emitters

6.3.1 Confirmation of Core-shell Nanostructures of the

The high resolution TEM image in Fig 6.4 shows a typical structure of these

confirms that these DLC thin film coated CNT composites are shell nanostructures, consisting of core CNTs and shell DLC films

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s (a) CNT growth on the coating on the CNTs (c) Hydrogen plasma

samples were treated with microwave plasma CVD facility Microwave power of 500 Watt was applied for the hydrogenation The chamber pressure was set to be 15 Torr and the

FE Properties of

shell Nanostructures of the

a typical structure of these

CNT composites are films The average

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diameter of the composite tube is approximately 4

CNT with the thickness of

DLC coating was obtained

the angle between the target and the

Fig 6.4 Core-shell structure

6.3.2 Confirmation of High sp

Films

To confirm the sp3 content,

High resolution XPS was used to analyze the sp

scan spectrum (not shown)

carbon Fig 6.5 shows the

with the binding energy (BE) of 284.5 and 285.2 eV confirm a high sp

diameter of the composite tube is approximately 45 nm, and the hollow part of the

the thickness of about 10 nm still can be observed A slightly nonuniform

ed around the CNT, and the nonuniformity is probably due to the angle between the target and the substrate during deposition

shell structure of a DLC thin film coated CNT confirmed by

6.3.2 Confirmation of High sp3 Content of the

content, a DLC film was directly deposited on silicon substrate High resolution XPS was used to analyze the sp3 content of the DLC film

(not shown) indicated the film surface was primarily c

the carbon 1s core level XPS spectrum of the film

with the binding energy (BE) of 284.5 and 285.2 eV confirm a high sp

126

and the hollow part of the

slightly nonuniform

s probably due to

confirmed by TEM

Content of the Coating

film was directly deposited on silicon substrate

C film The wide indicated the film surface was primarily composed of

of the film Two peaks with the binding energy (BE) of 284.5 and 285.2 eV confirm a high sp3 content of

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around 80% for the DLC film This result is similar to that previously reported by Tay [16] The high sp3 content of the DLC film suggests that the coating material is actually a ta-C thin film

The peak located at 283.9 eV is attributed to C-H bonding and the one with the BE

of 286.4 eV is due to ambient C-O oxidation The comparative peak intensities among the sp3, sp2, C-H and C-O peaks imply that the C-H and C-O content is much lower at the film surface

Fig 6.5 Carbon 1s core level XPS spectrum confirms high sp3 content of the DLC films

SP 2

Binding energy (eV)

SP 3

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6.3.3 Surface Morphology of the Composite Emitters

Low and High resolution SEM images of the composite emitters are shown in Fig 6.6 and 6.7 Films with thickness of 20, 50, 100, 200, 500 and 1000 nm were deposited respectively upon the CNT surface It can be observed from the low resolution images that the ta-C thin films were uniformly coated on the CNT substrates for all the samples The 20 nm film coated CNTs seem quite similar with the pristine CNTs shown previously With the increase of the film thickness, the nanotubes become thicker and more compact From the high resolution images it can

be observed that with the coating film thickness below 100 nm, the shape of the one-dimensional nanotubes is still remained The coating basically occurred at the upper portion of the CNTs The entire coating of the CNT walls is highly likely prevented because of the high density of CNTs When these films are thicker than 200

nm, the tips of these nanotubes begin to coalesce with each other and form a thick canopy composed of clustered particles on the top surface of CNTs eventually

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Fig 6.6 Low magnification top view SEM images of the composite emitters with varied coating film thicknesses: (a) 20 nm; (b) 50 nm; (c) 100 nm; (d) 200 nm; (e) 500 nm; (f) 1000

Low magnification top view SEM images of the composite emitters with varied coating film thicknesses: (a) 20 nm; (b) 50 nm; (c) 100 nm; (d) 200 nm; (e) 500 nm; (f) 1000

nm

129 Low magnification top view SEM images of the composite emitters with varied coating film thicknesses: (a) 20 nm; (b) 50 nm; (c) 100 nm; (d) 200 nm; (e) 500 nm; (f) 1000

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