Pulsed Laser Deposition: A novel approach to synthesize catalyst for growth of carbon nanotube (CNT) arrays
Pulsed laser deposition (PLD) process, also known as laser ablation, is used to develop a nanocatalyst. This catalyst bed is later used to grow multi-wall carbon nanotube (CNT) arrays using thermal CVD process. Effect of annealing of the catalytic substrate on the growth of CNT arrays is studied. It is noticed that the size and distribution of the metal nanoparticles play a vital role on the length and alignment of the nanotube arrays.
Long, well-aligned and impurity-free carbon nanotube (CNT) growth has been a challenging task to researchers over the past few years. This is due to various factors that influence the growth of CNTs. Among the parameters that influence nanotube growth, catalyst plays a vital role. The size, distribution and pre-treatment of catalyst and the oxide buffer layer directs the growth and alignment of nanotube arays. Several methods are used to synthesize catalyst such as e-beam evaporation, sol-gel methods. In recent years, pulsed laser deposition (PLD) method has been successfully used by the researchers for growth of nanostructured thin films, nanoparticles and other multi-phase nanocomposite materials. We attempted to use PLD method to synthesize the nanoparticulated Fe catalyst and CNT arrays are grown on this catalyst using thermal CVD method. Using PLD method, a combinatorial substrate with Fe catalyst is prepared. Substrate with varying thickness of Fe and Al2O¬3 is prepared at 200 °C on Si substrate with SiO2 layer.
In a PLD process, commonly known as laser ablation, high-power laser pulses hit the target material and ablate material from the target surface, resulting in the formation of a plume. The plume consisting of a mixture of energetic species of atoms, molecules, electrons, ions and micron-sized solid particulates, is ejected normal to the target surface. A typical schematic of the PLD process is shown in Figure. The laser radiation is absorbed by the solid target surface during which electromagnetic energy is first converted to electronic excitation and then to thermal, chemical, and even mechanical energies, causing evaporation, ablation, excitation, and plasma formation. The plume expands away from the target with a strong forward-directed velocity distribution of the different particles. The ablated species condense and deposit on the substrate placed opposite to the target. The temperature of the substrate is controlled by a heater, which is also used as a substrate mount. Substrates with Fe nanoparticles for nanotube growth were prepared by PLD process.
In this method, an excimer KrF laser (248 nm, 25 ns/pulse) was used for sequential ablation of Al2O3 and Fe targets of 99.99% purity. Depositions were carried out at ~2 × 10-5 Torr vacuum, 10 Hz repetition rate and 4 J/cm2 energy density. In all samples Al2O3 was deposited on Si/SiO2 substrates (silica scale layer had thickness of 150 nm) and a thin layer of iron was deposited on the top of alumina layer. Deposition growth rates of alumina and iron were determined from the calibration experiments. The thickness of the films in calibration depositions was measured by WYKO RST-500 optical profilometer and verified by profiling on MTS Nanoindenter XP. Combinatorial approach has been used for optimization of catalytic properties of the multilayered system. For this, three different thicknesses of Al2O3 (10, 20 and 30 nm) and Fe (1, 2 and 3 nm) on Si/SiO2 substrate developing 16 combinations of Si/SiO2/Al2O3/Fe system on one sample were prepared using moving mask technique. Samples were annealed in the air at 450°C for 4 hours before CNT growth. CNT arrays were grown on these substrates using thermal CVD method using a horizontal EasyTubeTM nano furnace from First Nano Inc. Ethylene is used as carbon precursor and hydrogen is used for hydrocarbon decomposition. Argon creates an inert atmosphere during the purging and cooling process steps. Argon purges the reactor until temperature is raised to 700 ºC. Then gas flows are switched to ethylene, hydrogen for expected growth times. Nanotube synthesis occurs during this stage when the supply of argon is stopped and ethylene is run for 30 minutes along with a flow of hydrogen. After nanotube arrays are synthesized they are cooled to ambient temperature, which completes the last process step. The synthesized CNTs are characterized using scanning electron microscope (SEM) to determine the height of the nanotubes.
Smart Structures and Biomaterials Laboratory, University of Cincinnati, Cincinnati, OH. Center for Advanced Materials and Smart Structures, North Carolian A&T; State University, Greensboro, NC.