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

Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell 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 119 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 120 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 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 6.2 121 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.1 Schematic illustration of the custom degisned PLD system custom-degisned Fig 6.2 The laser laser-route of the custom-designed PLD system 122 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures The laser system used was a class 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 × 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 123 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 × 10-6 Torr for deposition During the deposition, the target was rotated with a speed of around 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 124 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.3 Illustration of the preparation procedures of the s samples (a) CNT growth on the s silicon substrate (b) DLC thin films coating on the CNTs (c) Hydrogen plasma post-treatment on the surface of DLC coated CNTs reatment After deposition, the and 100 nm DLC deposited samples were treated with 50 hydrogen plasma for 10, 20 and 30 s respectively via a microwave plasma CVD facility coupled with a 2.45 GHz microwave power supply unit Microwave power of 500 Watt was applied for the hydrogenation The chamber pressure was set to be 15 Torr and the H2 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 C Core-shell Nanostructures of the shell Emitters The high resolution TEM image in Fig 6.4 show a typical structure of these shows composite emitters It confirms that the DLC thin film coated CNT composites are these core-shell nanostructures, consisting of core CNTs and shell DLC films The average shell films 125 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures diameter of the composite tube is approximately nm, and the hollow part of the 45 CNT with the thickness of about 10 nm still can be observed A slightly nonuniform DLC coating was obtained around the CNT, and the nonuniformity is probably due to ed s the angle between the target and the substrate during deposition Fig 6.4 Core-shell structure of a DLC thin film coated CNT confirmed by TEM shell 6.3.2 Confirmation of High sp3 Content of the Coating Films To confirm the sp3 content, a DLC film was directly deposited on silicon substrate High resolution XPS was used to analyze the sp3 content of the DLC film The wide C scan spectrum (not shown) indicated the film surface was primarily c composed of carbon Fig 6.5 shows the carbon 1s core level XPS spectrum of the film Two peaks with the binding energy (BE) of 284.5 and 285.2 eV confirm a high sp3 content of 126 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 Intensity (arb units) SP SP 280 282 284 286 288 290 Binding energy (eV) Fig 6.5 Carbon 1s core level XPS spectrum confirms high sp3 content of the DLC films 127 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 128 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 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 nm 129 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures Fig 6.7 High 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 Their tilted images are shown in the insert respectively 130 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 6.3.4 FE Performance of the Composite Emitters with Varied Thicknesses of Coating Films The FE properties of these composite core-shell nanostructural emitters were investigated Fig 6.8 shows the J-E characteristics of the pristine CNTs and the composite emitters with coating film thicknesses ranged from 20 to 200 nm, and their corresponding F-N plots are also shown in the insert The emitters with 500 and 1000 nm ta-C coating have not produced satisfactory emission results such that their plots are excluded from this figure It is obvious that the FE properties of pristine CNTs were significantly enhanced with ta-C thin film coating less than 200 nm thick However, the FE performances of these coated CNT emitters were evidently affected by the varied thicknesses of the coating films with the optimum film thickness of 50 nm For the emitters with coating film thickness below 50 nm, their FE properties were improved with the increase of film thickness whereas for those above 50 nm, their FE performances exhibited an opposite trend If defining the turn-on field as the applied electric field where µA cm-2 emission current is achieved, the turn-on fields for the pristine CNTs, 20, 50, 100 and 200 nm composite emitters were 1.89, 1.36, 0.98, 1.34 and 1.50 V µm-1 respectively In the F-N plots, the straight lines confirm the FE mechanism for these samples The slopes of all these composite emitters are similar except for the 50 nm sample, implying a possible lower emission barrier for this sample 131 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures 10 ln ( J/E2) Current density, J ( µ A/cm ) -2 -4 -6 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 1/E Pristine CNTs ta-C 20 nm ta-C 50 nm ta-C 100 nm ta-C 200 nm 0.0 0.5 1.0 1.5 2.0 Applied electric field, E ( V/µm) Fig 6.8 The FE J-E characteristics of the pristine CNT substrate and the composite emitters with varied ta-C film thicknesses The corresponding F-N plots are shown in the insert The optimum coating film thickness of 50 nm is probably due to several reasons First, the surface work function of an ultrathin film correlates with the thickness of the film Below certain point, the increase of the film thickness leads to a significant decrease in the surface work function due to the substrate-induced effects [17] In other words, with the increase of the thickness of the ta-C film, net charge transfer from the substrate to the ta-C film occurs, resulting in the generation of a dipole with an electric field directing toward the film side at the interface This dipole helps reduce the work function of the ultrathin film [18] Second, for multilayer thin film structures or core-shell structures, the FE 132 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core-Shell Nanostructures properties are influenced by their surface potential barrier height and width If defining the effective emission potential barrier as the total area of the potential barrier above the Fermi energy, the effective emission barrier can be found to be reduced with the increase of the coating film thickness by simulation (shown in Fig 6.9) [19] The lower the effective emission barrier, the higher the transmission probability for the electron tunneling, and thus the smaller the turn-on filed is needed for the commencement of electron emission Although the work function decreases with the increase of the coating film thickness, it does not mean that the thickness can be increased without a limit If the thickness is larger than a certain value, the electron transport within the film would be affected The electron scattering caused by impurities or the boundary of the quantum barrier would become severe, hence the electron transmission probability would be remarkably reduced Therefore, the combination of these factors, i.e., the work function or effective emission barrier, and the electron transport would determine a most suitable coating film thickness for the composite emitters, which is 50 nm in this case 133 ... these core- shell nanostructures, consisting of core CNTs and shell DLC films The average shell films 125 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes. .. multilayer thin film structures or core- shell structures, the FE 132 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell Nanostructures properties... 6.5 Carbon 1s core level XPS spectrum confirms high sp3 content of the DLC films 127 Chapter Field Emission Study of Hydrogenated Tetrahedral Amorphous Carbon Coated Carbon Nanotubes Core- Shell