Methods
The Sputtering Process
The Rapid Thermal Processor
Chemical Vapour Deposition
Growth Conditions
The Sputtering Process: Creating the Platform
The magnetron sputtering system is one of two sputtering machines currently in use at Dalhousie University, and is unique in its design, specialized for depositing one or more types of materials onto over twenty samples (per run). The sputtering machine is used to deposit the atoms of an iron catalyst onto a thin, oxidized, 11cm x 1cm strip of silicon oxide (SiO2). This makes a platform that we use to grow the nanotubes on. The magnetron sputtering system uses energetic particles to remove a material (Fe, in our case) from a target, making atoms of that material that build up and layer on any surface they land on (i.e. the silicon strips). The deposition source becomes an electrode to which a large negative voltage is applied. Deposition occurs when the sputtering chamber is filled with argon gas at a very low pressure. The low pressure is obtained by using a turbo pump (much like a jet engine) to remove the chamber’s gases. The argon gas is ionized by the strong voltage, causing the argon to exist as both a gas and a liquid, producing glowing discharge plasma which is confined in a ring over the target by strong permanent magnets behind the electrode (the deposition source). The positive ions of the charged argon plasma strike the electrode with several hundred electron volts (eV) of kinetic energy, smashing the material out of the target. The atoms then pass through a mask that covers the target and allows for a controlled pattern of iron deposition on the silicon strips.
The magnetron sputtering machine in the Dahn Research Lab, Sir James Dunn Building, Rm 316
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The Rapid Thermal Processor: Beading the Iron
The rapdi thermal processor (RTP) is a high temperature furnace that is used excessively by multiple research groups in the Physics department to anneal metals on the molecular level for study. For our research, the RTP was used to bead the iron catalyst into ‘islands’ of nanoscale size by submitting the iron particles on the silicon substrate to intense light and heat, annealing the metal to within a fraction of its melting point and changing it into an iron oxide. Up to five or six samples are placed onto silicon plates, and then set onto a quartz glass tray which slides into a quartz tube inside the RTP. Silicon is used because it has a high tolerance for heat and a low coefficient of thermal expansion (3 x 10-6 K-1: units are length (cm) / width (cm) / temperature in Kelvin (K)), meaning that the silicon doesn’t expand and contract very much during heating and cooling (Giancoli, 2000). Inside the RTP chamber, pure oxygen gas surrounds the samples. The samples are heated from above and below by two halogen lamp banks, which can raise the temperature to around 1100°C, although we scarcely go above 550°C in a standard RTP run (assume that we are annealing iron, as the temperature is diferent for other substances).
Rapid Thermal Processor, used to anneal metals
Example graph of an RTP process run, demonstrating the fluctuations in temperature and oxygen flow rate
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Chemical Vapour Deposition: Growing the Nanotubes
The chemical vapour deposition process takes place in a Lindberg Blue quartz tube furnace, which allows a sample exposed to flowing gases to be heated within a temperature range of room temperature (25°C) to about 1000°C. The carbon nanotubes are actually ‘grown’ via carbon deposits during this step. One sample at a time is placed into a quartz tube and heated in the furnace, while different gases are blown through the tube and over the sample: ammonia (NH3) to purify and get rid of the oxidant; ethylene (C2H2) to deposit carbon onto the iron ‘islands’; and argon (Ar) to purify the run and cool the sample at the end. This is called chemical vapour deposition (CVD). The sample is first exposed to NH3 and then heated up to around 720°C for a standard run. After the temperature has remained steady for approximately three minutes, the flow rate of the NH3 is reduced and ethylene gas is blown over the sample for twenty minutes (in a standard run). The ethylene is then turned off and argon is blown through the tube while the furnace cools down to a manageable temperature: when the temperature reads 150°C, it is referring to the heating element itself, but the tube is actually cool enough to handle.
Lindberg Blue Quartz Tube Furnace
This is an example of a chemical vapour deposition run. Time for the different run processes is shown on the x-axis, with a total time of about 45 minutes.
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Changes in Growth Conditions
With our research project, we experimented with variations of the growth conditions in the different processes. These variations can be seen in the table below.