Chapter II Literture review…
2.6 Applications of SWNTs
Many researchers and engineers have been devoted to combine the CNTs with living. There are a lot of possible applications of CNTs products such as FED, field effect transistor (FET), hydrogen storage, etc. Until now, lots of prototypes of these applications have been published. Thus, it is believed that more and more commercial products will be published soon in the feature.
(a) Electron field emission elements:
The electron field emission elements, as implied by the name, utilized the field emission properties of CNTs. Among all of them, the closest to our life is FED. It is a next generation display after plasma display panel (PDP) and liquid crystal display (LCD) technologies. The theorem of formation of image is to use CNTs as cathode, then applies the potential between cathode and anode.
Electrons emits from cathode to anode with phosphors which generate illumination. Ultra thin, wider view angle, superior brightness and low operation power are main advantages of FED. Samsung corporation had been public the
4.5” FED prototype in 1999 [Fig. 2.27].[Choi-1999-3129] And the electron source like SEM filament[Chow-1992-1] or X-ray tube[Yue-2002-355] can also employ the CNTs as electron emitters, which possess longer life, small energy spreading and power-saving significantly. Another field emission application related to general public is cathode-ray tube (CRT) lighting elements. The original has been published in 1998 by Ise Electronics corporation, Japan.[Saito-1998-L346] The fabricated CRTs are of a triode type, consisting of a cathode (nanotubes field emitter arrays), a grid and an anode (phosphor screen) [Figs. 2.28(a) and 2.28(b)]. The maxima brightness with anode at 200μA is 64000 cd/cm2. Stable electron emission, adequate luminance and long life (over 10000 hours) are demonstrated. It can be applied to a giant outdoor display or ultra-high quality color CRT displays.
Fig. 2.27 FED display at color mode with red, green, and blue phosphor column.
[Choi-1999-3129]
(a) (b)
Fig. 2.28 Schematic drawing (a) and physical object (b) of a longitudinal cross section of a CRT fluorescent display with a field emission cathode composed of carbon nanotubes. [Saito-1998-L346]
(b)FET
FET is a very important electronic device in history. The overwhelming majority of FET is silicon or Ⅲ-Ⅴ based just because these materials are semiconductors. But some of CNTs also have semiconducting properties, it makes researchers want to fabricate the CNTs based FET. In 1998, Sander reported the room-temperature transistor based on a single SWNTs FET.
[Sander-1998-49] Fig. 2.29 shows the I-V curve of the CNT-FET. In 2001, Derycke in IBM corporation prepared both p-type and n-type nanotubes transistors to build the first nanotubes-based logic gates: voltage inverters [Figs. 2.30(a) and
2.30(b)].[Derycke-2001-453] Surely, it still have lots of complicated problems to mass production above-mention devices, but these results have told that the nano-electronics is not hollow words any more.
Fig. 2.29 Two probe I-Vbias curve for various values of the gate voltage from a CNTs-based FET. [Sander-1998-49]
(a) (b)
Fig. 2.30 (a) Atomic Force Microscopy (AFM) image shows the design of the voltage inverter. (b) Characteristics of the resulting intra-molecular voltage inverter. [Derycke-2001-453]
(c) Lithium intercalation
The basic principle of rechargeable lithium batteries is electrochemical intercalation and de-intercalation of Li in both electrodes. An ideal battery has a high-energy capacity, fast charging time and long cycle time. The capacity is determined by the Li saturation concentration of the electrode material. The SWNTs have shown to possess both high reversible and irreversible capacities
[Gao-1999-153].
(d) Hydrogen storage material
Face to possible energy-crisis of gasoline, people has started to find the substitution methods for many years. Fuel cell was considered to have potential among all of solutions. Once it does be generated, its use as a fuel that creates neither air pollution nor greenhouse gas emissions. But it needs a huge hydrogen storage capability material. SWNTs just can play this role.SWNTs can absorb higher hydrogen than conventional materials. A H2 uptake of 4.2 weight %, which corresponds to a H/C atom ratio of 0.52, was obtained by these SWNTs with an estimated purity of 50 weight %. Also, ~80% of the adsorbed H2 can be released at room temperature. These results indicate that SWNTs are highly promising for H2 adsorption even at room temperature [Liu-1999-1127]. The hydrogen storage mechanisms of CNTs are still not well known yet, and these properties usually occur at high pressure or low temperature environment. It remains
impossible to apply on commercial product so far.
(e) Composite materials
The SWNTs may be used as reinforcements in high strength, Low weight, and high performance composites due to their excellent mechanical properties. A main advantage of using SWNTs for structural polymer composites is that SWNT reinforcements will increase the toughness of the composites by absorbing energy during their highly flexible elastic behavior. Other advantages are the low density of the nanotubes, an increased electrical conduction and better performance during compressive load, or induced high thermal conductivity reinforced material.
(f) Other applications
Atomic Force Microscope (AFM) is employed to obtain the surface morphologies and roughness. It uses a probe scanning the surface of sample, and an incident laser beam irradiates the arm of probe reflecting to a detector which passes signals to computer and draws the images. In order to obtain a high resolution images, the tip must be ultra thin, extremely sharp and high strength.
General type of AFM tip is made of Si3N4. The first article that utilized SWNTs as AFM tip was reportedin 1996[Fig. 2.31]. [Dai-1996-147] From Fig. 2.32, one can clearly see the SWNTs tip shows the better image resolution. At the same time, SWNTs with excellent mechanical properties can make the damage ratio of tip
decrease as low as possible. It has been some commercial products of SWNTs AFM tip at present.
In 1998, Wong demonstrated that CNTs tip can be used for chemical and biological discrimination.[Wong-1998-52] Another possible application applied to biotechnology of medicine carriers are developing as well. In the feature, people will easily get to know what disease that we get. Also, people can use CNTs filled with drags injecting into body, then induce it to the proper position relaxing the medicine to destroy the etiology without hurting normal cell nearby.
Fig. 2.31 SWNT attached to the pyramidal tip of a silicon cantilever for AFM.
[Dai-1996-147]
(a) (b)
Fig. 2.32 (a)Tapping mode AFM image of a 400-nm-wide, 800-nm-deep trench taken with a bare pyramidal tip. (b) The image taken with a nanotubes attached to the pyramidal tip with the same specimen.[Dai-1996-147]
Chapter III
Experimental Methods
3.1 Flow chart
Fig. 3.1 Flow Chart of the experiment p-type Si (100) substrate
Catalyst precusor coating by PVD (CoCrPtOx, CoCrOx, Co)
H-plasma pretreatment by MPCVD
AlON buffer layer coating by PVD
Field emission
XPS EDX
Scanning local laser heating pretreatment
Substrate recycling
Figure 3.1 shows the experimental flowchart for the fabrication and analyses of the catalyst precursor-assisted SWNTs. First, the CoCrPtOx films to act as catalyst precursor were deposited by physical vapor deposition (PVD) and buffer layers were prepared by DC reactive sputter. Then, two type of pretreatment were used. (1) H-plasma pretreatment was performed in microwave plasma chemical vapor deposition (MP-CVD) system to carry out reduction of oxidized CoCrPtOx film on silicon wafer. The specimens were subsequently heated up to grow SWNTs in an appropriate CH4/H2 atmospheres for several minutes at appropriate chamber pressure. (2) Local laser heating pretreatment was operated using 659 nm pump laser to heat numerous spots on substrate surface up to ~700 oC, and then transferred specimens to MPCVD chamber to synthesize SWNTs on Si substrate. The morphologies of the pretreated catalyst precursor were studied by scanning electron microscopy (SEM). The size and distribution conditions of catalyst particles after pretreatment on the silicon wafer were characterized from grinding cross-section and plane view samples by transition electron microscopy (TEM). Additionally, X-ray photoelectron spectroscopy (XPS) was employed to characterize binding energy of CoCrPtOx
layer at as-deposited and after H-plasma pretreatment to analyze the self-assembly mechanism. The morphologies, microstructures and bonding structures of the as-grown SWNTs were investigated by SEM, TEM and Raman
spectroscopy with a 632.8 nm He-Ne laser, etc. The field emission measurements (J-E) of the specimens were conducted by the simple diode configuration and performed in high vacuum. The oxidation resistance properties of as-grown SWNTs was measured by thermal gravimetric analysis (TGA).
3.2 Raw materials (a) Substrates:
Silicon wafer [P-type (100)]
(b) Source gases:
Hydrogen gas (purity 99.9995%) Jian Ren Chemical Co.
Methane gas (purity 99.999%) San Fu Chemical Co.
Oxygen gas (purity 99.9995%) Jian Ren Chemical Co.
Nitrogen gas (purity 99.998%) Jian Ren Chemical Co.
Argon gas (purity 99.9995%) San Fu Chemical Co.
(c) Target:
CoCrPt Co 57.08 %, Cr 10.97 %, Pt 31.95 % CoCr Co 67.19%, Cr 32.81%
Al purity 99.999%
3.3 Strategy of self-assembly CoCrPtOx catalyst precursor
As shown in Fig. 2.12, the oxidized phase of PtOx is unstable and can be easily reduced back to metallic state with very fine size when temperature is approximately 500 °C [Kim-2003-1701]. Hence, it has been recently used in optical storage media and nonvolatile memory, as shown in section 2.2. Additionally, Co is the good element for carbon species dissolubility and Cr2O3 are revealed to suppress the grain growth of Ni-Cr alloy effectively [Shaijmon-2005-192]. Combining these unique properties, oxidized film of CoCrPt deposited by PVD is acted as the catalyst precursor to fabricate vertically well-aligned SWNTs and synthesize SWNTs with low process temperature on Si substrate in this study.
3.4 Catalyst precursor and buffer layer deposition procedures (a) Catalyst precursor
The CoCrPtOx thin films which were as catalysts were coated on a (100)-oriented p-silicon wafer with and without buffer layer using a pure CoCrPt target by PVD (Helix 6-gun) in a mixed argon and oxygen atmosphere. The argon and oxygen ratio for depositing was 10:30 (sccm/sccm) and the deposited thickness of CoCrPtOx films were 1, 2, 3, 5, 10 nm.
(b) Buffer layer
AlON which was reported as the most efficient buffer layer [Wang-2005-1906]
was used in this study. AlON film was deposited on a (100)-oriented p-silicon wafer by DC reactive sputtering (Unaxis Cube Trio) using an Al target in a gas mixture of oxygen, nitrogen and argon. The deposited thickness of AlON film was 10 nm which was the best thickness to assist catalyst particles in distributing, as shown in section 2.2[Wang-2005-1906].
3.5 Microwave plasma chemical vapor deposition system (MPCVD)
The schematic diagram of MPCVD system is shown in Fig. 3.2. The main components of the system can be divided into six parts: the microwave generator, wave guides, reaction chamber, gas flow controller, gas pressure controller and pumping system. The microwave generator of microwave source system (Frequency 2.45 GHz, Power 1.3 kW) was produced by Tokyo electronic Corp.
Ltd. The reaction chamber contains quartz tube (inner: 47 mm, outer: 50 mm, China Quartz Corp. Ltd), stainless chamber, stainless holder and rotary pump (Hitachi Corp. Ltd). As Fig. 3.3 shown, sample holder is manufactured by stainless steel, it can bear high working temperature and reduce vacuum pollutions while plasma working. The upper electrode that was made by stainless steel is connected to the DC power supply output. The upper electrode (ground) and substrate holder (negative) were employed to applying the substrate negative bias. The substrate temperature is measured by thermal couple
which equipped in the holder. Mass flow control (MKS model 247) system is used to regulate the flow rate of reacting gas while depositing. Besides, the flow rate controller with different range (1-10 sccm, 10-100 sccm) of flow rate of mass flow control system can be properly adjusted for different gas (Ar, NH3, H2, CH4, C2H2). The low pressure (0.1~100 Torr) of chamber can be detected by thermal couple of vacuum gauge and absolute pressure gauge (MKS Baratron).
The work pressure of chamber can be regulated stably by throttling valve. The degree of the throttling valve was controlled by APC controller (MKS model 263). There is no external heater system equipped on MPCVD. The plasma is used to heat the substrate as the heat source. Cooling cycle system is made up of the refrigerator with closed cooling water and the conduit.
Fig. 3.2 Schematic drawing of MPCVD system
Fig. 3.3 Schematic drawing of MPCVD reactor
3.6 Pretreatment methods
3.6.1 H-plasma pretreatment by MPCVD
After catalyst precursor and buffer layer deposition, the specimen was transferred in air to the chamber of MPCVD which was pumped down to its base pressure. Hydrogen plasma was utilized to activate the CoCrPtOx film and change surface morphology of it. The purpose of this part is to find out the optimum conditions of H-plasma pretreatment to obtain the small average size distribution and appropriate density of catalyst nano-particles. The most optimum CoCrPtOx catalyst precursor pretreatment conditions were: microwave power 600 W, working pressure 30 Torr, sample temperature 580 oC, H2 flow 100 sccm and process time 10 minutes. Finally, we draw three straight lines on SEM micrograph and calculate the average particle size by the ratio of line length to particle numbers.
3.6.2 Scanning local laser heating pretreatment
For scanning local laser heating pretreatment, catalyst precursor-deposited sample was transferred to the chamber of two laser static tester (Tueopicts, 633-nm-wavelength cw model diode laser and 659-nm-wavelenth duration mode diode laser). The 633-nm laser was used to monitor the reflectivity change to focus another 659-nm laser. Then using 659-nm pump-laser to heat numerous spots on substrate surface simultaneously to carry out reduction of oxidized
CoCrPtOx film up to ~ 700 oC and then well-distributed fine nano-particles were formed on transparent or Si substrate. Figure 3.4 shows the schematic diagram of local laser heating pretreatment. The most optimum local laser heating pretreatment conditions were: laser peak power 3 mW, process time 20 ns.
Fig. 3.4 Schematic drawing of scanning local laser heating pretreatment
3.7 Specimen stacking methods and growth procedures of SWNTs Design of specimen stacking sequences
The important parameters of carbon nanostructure growth procedures are gas ratio, gases flow rate, microwave power, working pressure, sample temperature, process time and specimen stacking sequences, etc. These parameters may affect the morphology, uniformity, quality of the SWNTs. The
specimen stacking sequences were shown in Fig. 3.5. Four the same specimens are placed one time. Sample A is placed upside down to face sample B upside.
Sample C is placed backside to face sample D upside.
Two experiments are studied as follows:
(a) Synthesize high quality well-aligned SWNTs (H-plasma pretreatment):
Due to high quality and high aspect ratio SWNTs can enhance electrical properties, thermal conductivity, mechanical properties and field emission properties, thus H-plasma pretreatment and buffer materials were utilized to approach this target. H-plasma pretreatment can etch catalyst film to become the finest nano-particles with uniform size and buffer layer can make nano-particles distribute uniformly and densely. Moreover, reduction reaction of oxidized CoCrPtOx film can form well-distributed fine nano-particles on silicon wafer.
Thus, high quality well-aligned SWNTs can be synthesized by MPCVD. The optimum deposition conditions for the SWNTs growth were: microwave power 750 W, working pressure 24 Torr, sample temperature 600 oC, H2/CH4 ratio = 50/4 (sccm), and deposition time 1~6 minutes for length control.
(b) Low temperature synthesis of SWNTs (Laser ablation pretreatment):
SWNTs have recently been considered as a promising candidate material for application as field emitters, nano-electronic devices (e.g., single electron transistor), et al. For such applications, the deposition temperature must be
lowered, particularly for compatibility with IC processes, and structural manipulation becomes an important issue. Therefore, in order to fabricate SWNTs with desired morphology at low temperatures, laser ablation pretreatment was utilized. Laser can heat a number of spots on substrate surface up to 700 oC in nano-seconds, and then sink heat rapidly to whole sample. From this, transparent PC substrate won’t be damaged, and PtOx can be reduced to nano-particles. Thus, SWNTs can be synthesized on transparent PC substrate by MPCVD at low temperature. The optimum deposition conditions for the SWNTs growth were: microwave power 300 W, working pressure 11 Torr, sample temperature 373 oC, H2/CH4 ratio = 50/4 (sccm), and deposition time 1~4 minutes for length control.
Fig. 3.5 Schematic diagram of specimen stacking sequences
Substrate recycling:
Substrate recycling process was studied by removing the as-deposited nanostructures from the substrates in ultrasonic bath, and the substrates were then pretreated in H-plasma to reactivate the catalyst particles on the substrate.
The well-aligned SWNTs can be obtained by substrate recycling for several times using the same procedures and conditions described in section 3.7(a).
Our specimen designation is shown in Table 3.1.
Table 3.1 Specimen designation and pretreatment and growth conditions
HP: H-plasma pretreatment, microwave power 600 W, working pressure 30 Torr , sample temperature 580°C, H2
flow 100 sccm, process time 10 minute.
SP: Scanning local laser heating pretreatment, laser peak power 3 mW, process time 20 ns.
MPCVD - H: Microwave power 750 W, working pressure 24 Torr. sample temperature 600°C, H2/CH4 ratio = 50/4 (sccm/sccm), deposition time 6 minutes.
MPCVD - L: Microwave power 300 W, working pressure 11 Torr, sample temperature 373 oC, H2/CH4 ratio = 50/4 (sccm/sccm), deposition time 4 minutes.
Specimen designation
Catalyst precursor thickness (nm)
Buffer layer
thickness (nm) Pretreatment Growth method A1 10 nm CoCrPtOx HP MPCVD - H
3.8 Structure analyses
3.8.1 Scanning electron microscopy (SEM)
SEM is a very useful tool for observing surface morphology of specimen.
SEM has secondary electrons (SE) or backscattered electrons (BSE) detectors passing the signal to computer and forming image. In this study, the surface morphology of as-pretreated catalyst precursor and SWNTs were characterized by focused ion beam & electron beam (FIB/SEM) system. The cross-section view of as-grown SWNTs was investigated by field-emission SEM (FE-SEM) (JEOL 6300) operating at 15 kV accelerating voltage.
3.8.2 Transmission electron microscopy (TEM)
The TEM image is the result of electron transmitting through the sample, and it reveals the interior microstructure of the specimen, and it can give the high-resolution lattice image and the electron diffraction pattern as well. In the experimental, the microstructure of as-grown SWNTs were characterized by JEOL, JEM-2010F TEM operating at 200 kV accelerating voltage. The size and distribution conditions of catalyst nano-particles after pretreatment on the silicon wafer were characterized from grinding cross-section and plane view samples by Philips, TECNAI 20 TEM/EDX. TEM cross-sectional specimens for the TEM analyses were prepared by mechanical polishing and subsequent argon ion
milling.
3.8.3 Raman spectroscopy (Raman)
Raman scattering was discovered by Raman in 1928. If an incident photon occurs inelastic scatter with specimen molecules and causes the energy change of the photon called Raman scattering. By this mechanism, one can measure the difference between incident and scattering light by a spectrometer to obtain the information of element and bonding structure of the specimen. In particular, Raman spectroscopy is useful in identifying carbon-based materials. There are two obvious bands located at about 1330 cm-1 (D band) and 1590 cm-1 (G band) which correlate with the vibration of sp3-bonded and sp2-bonded carbon atoms, respectively. Additionally, the special radial breathe mode (RBM) is a unique feature in the Raman spectrum of SWNTs and involves a collective vibrational movement of the carbon atoms towards and away from the central axis of a SWNT. Consequently, the associated RBM Wavelength and frequency are directly related to the perimeter of the nanotube. Base on this relationship, as the diameter of the nanotube increases, the RBM frequency shifts to lower wave numbers, as shown in section 2-5. In order to study the structural characterization of the SWNT samples, a Jobin Yvon LABRAM HR Micro-Raman system with a He-Ne laser (wavelength: 632.8 nm) was utilized in
the experiments.
3.8.4 X-ray photoelectron spectroscopy (XPS)
Surface analysis by XPS involves irradiating a solid in vacuum with mono-energetic soft X-rays and analyzing the emitted electrons by energy. The spectrum is obtained as a plot of the number of detected electrons per energy interval versus their kinetic energy. Quantitative data can be obtained from peak height or peak areas, and identification of chemical states often can be made from exact measurement of peak positions and separations. In this study, XPS was employed to characterize binding energy and chemical state of the of the
Surface analysis by XPS involves irradiating a solid in vacuum with mono-energetic soft X-rays and analyzing the emitted electrons by energy. The spectrum is obtained as a plot of the number of detected electrons per energy interval versus their kinetic energy. Quantitative data can be obtained from peak height or peak areas, and identification of chemical states often can be made from exact measurement of peak positions and separations. In this study, XPS was employed to characterize binding energy and chemical state of the of the