Chapter 2 Literature Review
2.5 Analysis methods of the CNSs
2.5.3 Electrical, optical and thermal properties measurements
Field emission (FE) is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields. In this phenomenon, electrons with energies below the Fermi level tunnel through the potential barrier at the surface, which the high electric field sufficiently narrows for the electrons to have a non-negligible tunneling probability. Variations in the emitted current are primarily due to the field dependence of this surface potential barrier. The measurements were conducted by the simple diode configuration and carried out in a high vacuum chamber pumped down to a pressure of about 10-6 Torr. A high voltage source-measure unit (Keithley 237) was used for providing the sweeping electric field (E) and monitoring the emission current density (J).
The thermal properties are often measured by thermogravimetric analysis (TGA), which is a type of testing that is performed on samples to determine changes in weight in relation to change in temperature. Such analysis relies on a high degree of precision in three measurements: weight, temperature, and temperature change. As many weight loss curves look similar, the weight loss curve may require transformation before results may be interpreted. A derivative weight loss curve can be used to tell the point at which weight loss is most apparent.
Photoluminescence (PL) is a process in which a substance absorbs photons (electromagnetic radiation) and then re-radiates photons. Quantum mechanically, this can be described as an excitation to a higher energy state and then a return to a lower energy state accompanied by the emission of a photon. A basic understanding of the principles involved can be gained by studying the electron configurations and molecular orbitals of simple atoms and molecules. More complicated molecules and advanced subtleties are treated in the field of computational chemistry.
Chapter 3
Experimental Methods
Three processes were developed to selectively deposit the oriented CNSs with the given number density on the desired positions to form CNSs patterns. The processes include the AAO template-catalyst-assisted, electroless plating catalyst-assisted, and CNSs- electrophoresis-assisted methods. The experimental flowcharts, the detail experimental procedures and structure-property-analyses methods for each process will be illustrated separately in the following Sections.
3.1 Experimental flowcharts
Fig. 3-1 illustrates the experimental flowchart of the AAO template-catalyst-assisted process. The vertically-aligned carbon nanostructures with tunable density are grown using AAO as a template and applied for field emitter arrays.
The experimental flowchart in Fig. 3-2 shows fabrication steps and analyses of the electroless plating catalyst-assisted process. In this process, the catalyst assisted CNTs by electroless plating are examined and then horizontally CNTs are grown across the patterned substrate where the catalysts are selectively deposited by electroless plating.
Flowchart of the CNSs-electrophoresis-assisted process is shown in Fig. 3-3.
Electrophoresis method was used for depositing carbon nanostructure films on various substrates from stable SWNTs suspensions and their field emission properties are measured.
Fig. 3-1 Experimental flowchart for the AAO template-catalyst-assisted process.
Fig. 3-2 Experimental flowchart for the electroless plating catalyst-assisted process.
Fig. 3-3 Experimental flowchart for the CNSs-electrophoresis-assisted process.
3.2 AAO template-catalyst-assisted process
3.2.1 AAO-template preparation by anodization process
In this study, AAO template is directly fabricated on the Si wafer by a two-step anodization process, and the corresponding schematic diagram is sketched in Fig. 3-4. It begins with the deposition of an aluminum film on a (100)-oriented p-silicon wafer by a high vacuum thermal evaporator (ULVAC EBX-6D) with a base pressure of < 4×10-6 Torr.
Aluminum ingots with a purity of 99.999 % are used as aluminum source after cleaning in a 10 vol.% hydrogen chloride (HCl) solution for 60 s. Tungsten boats are cleaned in a mixed solution of 20 vol.% hydrogen fluoride (HF) and 80 vol.% nitric acid (HNO3) for 15 sec and then are used to carry and melt the aluminum ingots.
Experimental setup for anodization is shown in the Fig. 3-5, including a tank, electrodes, electrolyte and a power supply. A platinum foil is used as the cathode and the anode was the aluminum film specimen which only a circle with an area of about 1.3 cm2 is exposed to the electrolyte. First anodizing step is carried out in a 0.3 M oxalic acid (H2C2O4) solution at room temperature under a constant polarization voltage of 40 V for 5 min (Fig. 3-4(b)). Power is applied by a source-measure unit (Keithley Model 2400) and controlled by the computer program. The thus formed nanoporous AAO about 1 μm thick is removed by wet chemical etching at 60°C with a mixture solution of H3PO4 and CrO3, thereby leaving a relatively ordered indent pattern on the surface of the Al film (Fig. 3-4(c)). The second anodizing step is carried out on the indented Al film for 4 min under the same anodization condition as the first anodizing step (Fig. 3-4(d)). AAO barrier layer etching and pore widening are preformed by immersing the specimens in a 5 wt.% H3PO4 solution at 30oC for 40 min. This results in highly uniform and periodic nanoporous channels in the AAO layer, which are then used as the template for the growth of carbon nanostructures.
Fig. 3-4 Schematic diagram of the two-step anodization process.
Fig. 3-5 Schematic diagram of experimental setup for the Al anodization.
3.2.2 Catalyst deposition in the nanopores of AAO template by electroplating
A metallic Al layer ~300 nm thick is left on the wafer surface after the AAO fabrication, and is used as the bottom electrode. Cobalt is used as the catalyst for CNS growth, and is electrochemically deposited at the AAO pore bottom in the mixture electrolyte of CoSO4 and H3BO3. Co electroplating is carried out in the electrolyte mixture of CoSO4 and H3BO3 by applying an AC voltage of 12.3 Vrms. The size of the Co particle in AAO pore channels is tuned by varying the plating time (0 ~ 1200 s).
3.2.3 CNSs deposition by ECR-CVD
The schematic diagram of the ECR-CVD system is shown in Fig. 3-8. Microwave power (2.45 GHz) is supplied to a plasma chamber through a quartz dome entrance window. A magnetic field of 875 Gauss for ECR plasma excitation is generated by the coils surrounding the resonance volume and is applied perpendicularly to the substrate surface. A negative dc bias is applied to the substrate holder. The plasma stream is introduced into the deposition chamber, which has previously been pumped down to a base pressure of about 10-6 Torr by a diffusion pump system. It takes the advantages of low temperature, high dissociation efficiency and wider deposition area.
To deposit carbon nanostructures in the AAO template, the Co catalysts are first treated with the H2 plasma at 600oC for 10 min in the ECR-CVD system. Then, the CNS growth is carried out at 600oC in an ECR-CVD system using a gas mixture of 20% CH4 and 80% H2 (5 sccm/ 25 sccm) as the plasma source for 30 min. During the CNS growth, the ECR-CVD system is under the following operation conditions: magnetic field strength 875 G, microwave power 750 W, substrate bias -100 V, working pressure ~ 2×10-3 Torr, deposition temperature 600oC.
3.2.4 Fabrications of the emitter arrays on the AAO-assisted CNSs
The fabrication scheme of the emitter arrays from AAO-assisted CNSs is shown in the Fig. 3-9. After the preparation of the AAO-assisted CNS emiiters, tetraethoxysilane (TEOS) oxide is deposited on the CNSs as the dielectric layer by plasma enhanced CVD (PE-CVD), and followed by the evaporation-deposition of an Al layer as the gate electrode. The pattern with a diameter of 6.5 μm is photolithographically defined by a mask aligner (Karl-Suss MJB-3). The Al top layer is etched by a high density plasma reactive ion etching (HDP-RIE) system using the gas mixture of BCl3 and Cl2 as the etchant source. The oxide dielectric is then HDP-RIE etched using a gas mixture of CHF3 and Ar as the plasma source. After the HDP-RIE process, buffered oxide etchant (BOE) is used to remove the remnant SiO2.
Fig. 3-6 Schematic diagram of the ECR-CVD system.
Fig. 3-7 The fabrication scheme of the emitter arrays on the AAO-assisted CNSs: (a) preparation of the AAO layer on the Si wafer, (b) CNS growth by ECR-CVD deposition, (c) SiO2 dielectric and Al gate electrode depositions on the CNSs, and (d) RIE and BOE etches to open the field-emission area.
Table 3-1 Specimen designations of the AAO-assisted CNSs and their deposition conditions.
CH4/H2=5/25 sccm/sccm, 750 W microwave power, substrate bias -100 V, under ~ 2×10-3 Torr, and ~ 600oC in the ECRCVD system.
☆Other emitter arrays preparation conditions:
The pattern with a diameter of 6.5 μm is photo-lithographically defined and etched by a HDP-RIE system and then BOE is used to remove the remnant SiO2.
3.3 Electroless plating catalyst-assisted process
3.3.1 Substrate preparation for selective deposition of catalyst
Two substrates are used for electroless plating catalyst-assisted process. For examining the characteristics of the catalysts and carbon nanotubes by electroless plating, substrate condition 1 (SC 1) is prepared by coating a 150 nm-thick a:Si layer on the Si substrate by vertical furnace system. The other substrate (SC 2) is the trench patterned Si substrate designed for growing horizontally- oriented CNTs across the trenches and the process is schematically shown in Fig. 3-8.
Fig. 3-8 Schematic diagrams showing the electroless plating catalyst-assisted CNTs grown across trenches of a pattern.
For film depositing step in Fig. 3-8(a), a 100 nm-thick Si3N4 layer is coated by LPCVD and a 100 nm-thick SiO2 layer is first coated on the Si substrate by furnace to prevent crack formation from large tensile stress of Si3N4 layer. Then, the film depositing step is followed by a 50 nm-thick a:Si layer coating in the vertical furnace system and accomplished after another 100 nm-thick Si3N4 layer coating. After photolithography, the 400 nm-wide and 800 nm-wide trenches on the wafer were then created by RIE process as shown in Fig. 3-8(b) and the top-view of the designed pattern is shown in the right of Fig. 3-8(b).
3.3.2 Catalyst deposition by electroless plating
Three different solutions (Ni, Co, and Fe Bath) for electroless plating are prepared and their compositions are listed in the Table 3-2. There are no external electrodes present and electroless plating reaction begins when the specimens are immersed into the solution at 100oC. Instead of an anode, the metal is supplied by the metal salt and a substrate serving as the cathode, and the presence of a chemical reducing agent in solution to reduce metallic ions to the metal state. For the Ni alkaline bath with 100 ml, it consists of a metallic salt 0.5g
Table 3-2 The compositions of the electroless solutions.
0.02M NiSO4⋅6H2O, a reducing agent 10 ml 10% N2H4, 0.5g 0.1M NH4Cl, 20 ml 1.4%
NH4OH and 140 ml de-ionized water. For the Co bath or Fe bath, the metallic salt is changed to Co(NO3)2‧6H2O or Fe(NO3)3‧9H2O, respectively. In order to selectively deposit on the pattern, the patterned substrate is immersed into the solution and the catalyst are coated on the catalytic surface a:Si layer as shown in Fig. 3-8(c).
3.3.3 CNTs deposition by MPCVD
The schematic diagram of MPCVD system is shown in Fig.3-9. 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. 3-10 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 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. The work pressure of chamber can be regulated stably by throttling valve. 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.
The catalyst films are pretreated with H plasma under the following conditions : H2 flow rate 100 sccm, microwave power 400 W, pressure 9 Torr, and deposition temperature 550 ~ 580oC in the MPCVD system and then followed by CNTs deposition using CH4 and H2 as source gases. The CNTs deposition conditions are CH4/H2 = 1/100 sccm/sccm, 800 W
microwave power, under 16 Torr and 660~680oC.
3.3.4 RTA heat treatment
Some as-deposited horizontally CNT are annealed at 760oC by rapid thermal annealing (RTA). Table 3-3 shows the specimen designations of the electroless plating catalyst-assisted method and clearly describes the substrate conditions catalyst material and their plating time.
Fig. 3-9 Schematic diagram of the MPCVD system.
Table 3-3 Specimen designations of the electroless plating catalyst-assisted CNTs and their
* Substrate conditions (as described in 3.2.1):
SC 1: Si substrate with a:Si coating.
SC 2: Si substrate with SiO2 100 nm / Si3N4 100 nm /a:Si 50 nm /Si3N4 100 nm stacks patterned by Si3N4 / a:Si and with 400 nm trench width.
SC 3: Substrate preparation similar with SC2 but with 800 nm trench width.
# Other disposition conditions: 660~680oC in the MPCVD system.
3.4 CNSs-electrophoresis-assisted process
3.4.1 Substrate preparation for selective deposition
Various substrates are used in this method such as Al films on Si substrate, ITO glass, Al foil and the square-shaped pattern made of Al and SiO2 components by lithography process.
For the Al/Si substrate, an aluminum film is coated on silicon wafer by a high vacuum thermal evaporator (ULVAC EBX-6D) with a base pressure of < 4×10-6 Torr and aluminum ingots with a purity of 99.999% were used as aluminum source. For the Al-SiO2 pattern, the SiO2 film is first coated by plasma-enhanced chemical vapor deposition (PECVD, STS Multiplex Cluster System) using TEOS and O2 process gas under plasma power 300W;
process pressure: 300~800 mTorr and temperature 300oC. After photolithography, the photo resist is remained on the dark region in Fig. 3-10 and the region is designed for the SiO2 part.
The aluminum film is also coated by thermal evaporator and the aluminum on the photo resist is lifted off by sonification in acetone.
Fig. 3-10 Square-shaped pattern made of Al and SiO2 components for EPD process.
3.4.2 Suspensions preparation for CNTs dispersion
The raw SWNTs in this work were prepared by arc discharge method and provided by ITRI (Industrial Technology Research Institute, Taiwan). The SWNT suspensions were made by adding 0.001 g of SWNTs and 0.2 g of the surfactant into 20 ml of solvent (de-ionized water or butyl alcohol) and sonicated for 3 hours, where the surfactants (acting as dispersants) include SDS (sodium dodecyl sulfate), CTAB (hexadecyl trimethyl ammonium bromide) and TOPO (trioctylphosphine oxide). The suspension solutions were then centrifuged for approximately 15 min and discarded the sedimentation. Suspension designations and their compositions are listed in the Table 3-4.
Table 3-4 Suspension designations and their compositions.
Suspension
# Other composition: 0.001g CNT category are used in each suspension.
* SDS : Sodium dodecyl sulfate
CTAB : Hexadecyl trimethyl ammonium bromide TOPO : Trioctylphosphine oxide
3.4.3 The electrophoretic processes with CNT suspensions
Experimental setup for electrophoretic depositions included a tank, electrodes, electrolyte and a power supply. Different EPD parameters such as applying dc voltage (30 ~ 50 V), anode-cathode distance (10 mm) and deposition time (60 ~ 120 s) are examined.
3.4.4 Post-annealing
The as-deposited CNSs films by electrophoresis were then annealed under air
environment on a hot plate at 100~300oC for 1 ~ 30 min. Table 3-5 shows the specimen designations for the EPD-prepared CNSs and their deposition parameters.
Table 3-5 Specimen designations of the CNSs-EPD-assisted films and their deposition parameters.
EPD parameters at RT Post air annealing
# The preparation of suspensions is described in detail in Table 3-4.
* Electrodes of anode and cathode consist of the Si substrate coated with different electrode materials, except ITO and Al foil which are electrode materials without Si substrate.
＆Al-SiO2: The square-shaped pattern consists of Al and SiO2 components, which is fabricated by lithography technique.
3.5 Structures and properties characterization
3.5.1 SEM, TEM, EDS and HRTEM
SEM is a very useful tool for observing surface morphology of specimen. SEM has secondary electrons or backscattered electrons detectors passing the signal to computer and forming image. TEM image is the result of electron transmitting through the specimen. TEM reveals the interior microstructure of the specimen, and it can give the high-resolution lattice image and the electron diffraction pattern as well. In this study, the morphologies and microstructures of the deposited structures were characterized by a field-emission SEM (FE-SEM) (Hitachi S-4000, JEOL JSM-6500F, and JSM-6700F), TEM (Philips Tecnai 20 (200 keV) and FETEM (JEOL-2100 (200 keV)). Specimens for the TEM analysis were cutting by the FIB technique or by ultrasonic agitation and dispersed onto Cu TEM grids.
Moreover, EDS analyses were also performed in the TEM system to identify the chemical composition of the specimens.
3.5.2 AES
AES analysis technique employs an electron beam (2-30 keV) irradiating the specimen surface to excite Auger electrons which possess specific energy. Through assaying the kinetics energy of the Auger electrons by an electron energy analyzer, one can get to know the element composition and chemical state of the specimen. Because the incident electrons with low-energies (1-3 keV) have very short inelastic mean free paths (5-20 Å) inside the solid phase materials, AES technique is usually used to obtain the information within 50 Å away from the surface (surface analysis). In this study, AES was employed to investigate the chemical composition of the catalyst on the pattern. The AES analyses were performed using a Physical Electronics Auger 670 PHI Xi system with a Schottky field emission electron source.
3.5.3 Raman spectroscopy
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. In order to study the structural characterization of the CNS samples, a Jobin Yvon LABRAM HR Micro-Raman system with a He-Ne laser (wavelength: 632.8 nm) was utilized in the experiments.
3.5.4 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 analysis the composition of the catalyst films by electroless plating. XPS analyses were performed on a ULVAC-PHI 1600 ESCA system with Al-Kα (1486.6 eV) excitation. X-ray emission energy was 400 W with 15 kV accelerating voltage. Argon ion with ion energy of 5 keV was used for sputter profiling.
3.5.5 Electrical properties measurement
Fig. 3-11 shows the instrument setup and the test configuration used during field emission characterization. The measurements were conducted by the simple diode configuration and carried out in a high vacuum chamber pumped down to a pressure of about 10-6 Torr with a turbo molecular pump, backed up by a rotary mechanical pump.
A steel probe with a diameter of 2 mm as anode was used for the measurement. The distance between the specimen and anode was about 100 μm controlled by a precision screw meter.
The specimen (cathode) was biased with a voltage swept positively from 0 to 1000 V at room temperature to extract electrons from emitters. A high voltage source-measure unit (Keithley 237) was used for providing the sweeping electric field (E) and monitoring the emission current density (J). The measurement instruments are auto-controlled by the computer.
The I-V curves of the horizontally-oriented CNTs were examined by precision semiconductor parameter analyzer (HP 4156). The patterns were designed as shown in Fig.
3-8.
3.5.6 UV-Vis spectroscopy and Zeta potential measurement
Ultraviolet-visible (UV-Vis) absorption spectroscopy involves the spectroscopy of photons in the UV-visible region. It uses light in the visible and adjacent near ultraviolet and near infrared (NIR) ranges. UV-Vis spectrophotometer measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (Io). The ratio I / Io is called the transmittance, and is usually expressed as a
Ultraviolet-visible (UV-Vis) absorption spectroscopy involves the spectroscopy of photons in the UV-visible region. It uses light in the visible and adjacent near ultraviolet and near infrared (NIR) ranges. UV-Vis spectrophotometer measures the intensity of light passing through a sample (I), and compares it to the intensity of light before it passes through the sample (Io). The ratio I / Io is called the transmittance, and is usually expressed as a