AP4 QUANTUM ELECTRONICS PROJECT CARBON NANOTUBES JUSTIN MCMORROW & MICHAEL PENDER

Transcription

1 AP4 QUANTUM ELECTRONICS PROJECT CARBON NANOTUBES JUSTIN MCMORROW & MICHAEL PENDER

2 1.Introduction to Carbon nanotubes Carbon nanotubes are expected to be the equivalent for the 21 st century for what silicon semiconductors were for the 20 th. Both single-walled and muilt-walled tubes are essentially rolled up sheets of graphite (carbon) and can behave as either a semiconductor or a metal. Due to there fascinating electrical and mechanical properties, either as a molecular transistor, field emitter or an STM tip. The Single Walled NanoTube Graphite has a 2D-honeycomb structure; each atom of carbon is bonded to 3 other atoms in the same 2D plane. The only force that weakly attaches each sheet is Van der Waals forces, making it easy to peel off a sheet. Figure 1. carbon nanotube molecular structure [1] The semiconductor or metal behaviour of the carbon nanotube is a result of the direction it is rolled, because of the carbon bonding some directions contain stronger then others. If rolled in most directions the electrons propagating down the tube will be back scattered be the atoms in the lattice, this forms an energy band gape similar to that of a semiconductor. If rolled along a particular direction then these scattering electrons case destructive interference with each other thus minimising the back scattering, the band gap diagram of this tube will be like that of a metal. It can be shown that electron waves can only be set up or exist when the tube circumference is a multiple of the electron wavelength. Conduction is limited any structural defects of atoms that were absorbed on to the tube, the conductivity is limited these as barriers and the highest of these barriers dominates the resistance. Single walled nanotubes can be produced by laser ablation of a metal-doped graphite target. In such a production by Guo et. al.[chem. Phys. Lett. 234, 49()1995)] a graphite pettet with a 1-2% content of Co-Ni as a catalyst, was laser ablated in a furnace containing Argon gas at 1200 Degrees Celcius giving a yield of about 80% [5]. 2. Properties of carbon nanotubes.

3 Conduction properties Before we can examine these electrical properties we must attach them to metallic electrodes. Usually made by electron beam lithography and then to drop the tubes onto the electrodes. That is to drop the tubes between the source and the drain as shown, then the conductive properties can be measured and the gate can be used as a capacitor, where the two plates of the capacitor are the tube are the gate. When the conductance is measured as a function of the gate voltage, two types of behaviour are observed hence you have a way to distinguish between metal and semiconductor type tubes. The electron conductance of the tube is ballistic, this means that every electron entering the tube comes out the other end. There is some resistance but it is independent of length so ohm s law does not apply. The only better conductor is a superconductor. Theory has predicted that for perfect ballistic tubes the conductive is At low tempters the conductivity oscillates. These are coulomb oscillations ; they re caused when electrons are added to the tube. Because they re regular and periodic it shows that the electron states exist over the enter length of the tube. Coiling the tube back onto itself making a loop that can be used as a very small solenoid to create magnetic fields, these can be used to investigate the quantum interference phenomena. Or stretching the tube between two contacts and allowing it to vibrate like a guitar string can be used to investigate the interactions between mechanical and electronic degrees of freedom in the tube. Semiconductor properties As Transistors the set up is shown in the diagram above and works on the same principle as a MOSFET. When the gate is negative, holes are induced into the tube and the tube conducts, it is ON. When the gate is positive you get depletion of holes and the gate is OFF. The resistance of the OFF State is some million times that of the ON State. The doping level of the semiconductor like tube is dependent on external conditions that remove electrons thus leaving the remaining mobile carrier responsible for conduction. These semiconductor devices are 1nm wide, any silicon device of this size suffers from surface states, that s when states arising from the 3D lattice meet the surface. In a nanotube there is no 3D lattice for these effect to form from. Multiwall carbon nanotubes Are composed of concentric rings of single wall tubes and the outer diameter of these structures is typically 10-50nm. And are currently being grown up to a length greater then 100 micrometers [3]. Mechanical properties It a sample of graphite it is very easy to peal a sheet away, not so with multiwall tubes. The young modules can be measured for these tubes by arranging them vertically so as one end is stationary and the other free to move, then measuring the thermal vibration on the free moving end using a scanning tunnelling microscopy. A young modulus of about a TerraPascal is measured, that s about 5 times the young modules of steel and the modulus has been measured of a value up to 1.25 TerraPascal [6]. Other methods of measuring the young modulus are using a Scanning Force Microscopy (SFM) to bend the vertical tube or to use the SFM to bend a tube suspended between two stationary points. All these methods show that the tubes produced by arc discharge are of an order of magnitude stronger then those grown by catalytic decomposition of hydrocarbons. In comparison to other very strong material such as silicon nano-rods, The rods brake when certain force is applied tubes and when kinks form due to stress the are plastic not elastic so the do not disappear when the force is removed. Multiwalled nanotubed on the otherhand deform rather then break, can bend to a very severe angle and when kinks form they are elastic so disappear once the force is removed. Imagine a car that would after a crash reform itself to it s original shape, it practice the reforming would propel the car in the opposite direction at the same speed it impacted on.

4 Electronic properties You would expect that the interactions between the multiply nanotubes layers would cause the electrical properties of the milti-walled nanotube to be very complex. In 1998 Walt de Harr in Georgia Institute of Technology tried to measure the conductivity of these tubes. A fibre was attached to a SFM and lowered into a drop of metal in a controlled fashion. The SFM held many tubes and the conductivity would change as each tip entered the metal but as the movement was very small the conductivity as a function of the tube length could also be investigated. They found that the each tube had the same conductivity, about and the dependence on the length of the tube was very small. So the Multiwalled nanotube appeared also to be a ballistic conductor despite the interactions expected between the layers of the tube. It was also found that the tubes could carry more then 10 million A per cm squared, any convention resistor would vaporise at these levels of current. 3. Controlling Carbon Nanotube Growth. For carbon nanotubes to become the useable in the world of nanoscale electronic devices there first has to be a way of controlling nanotube growth. Another important factor for nanoscientists to consider is that for nanotubes to become an everyday technology, then the nanotubes and nanometre-sized devices have to be grown and fabricated on a large scale. From this point onwards, there will be a description of the obstacles overcome so far by the many groups involved worldwide in nanotube technology and the many that still face them as more and more is being learnt about their electrical and mechanical properties. It was only nine years ago, in 1991, that Sumio Iijima of the NEC Fundamental Research Laboratory in Japan discovered multiwall carbon nanotubes. The discovery was made in the soot that was produced by the electric arc discharge between graphite electrodes in a helium atmosphere [1]. This discovery set the ball rolling for others to find a way of producing larger amounts of multiwall nanotubes. In 1992, Richard Smalley at Rice University used a high energy pulsed laser focussed on a carbon target to produce large amounts of high quality single wall nanotubes. However the most successful method so far of growing large amounts of carbon nanotubes is by chemical vapour deposition (CVD). This method of deposition is widely used in the semiconductor industry to deposit thin films onto substrates. To grow carbon nanotubes by this method, a hydrocarbon gas is passed over a heated catalyst ( C) which has the effect of separating the hydrogen and carbon atoms allowing for carbon nanotube growth. The hydrocarbon gas is normally ethylene or acetylene, and either iron, nickel or cobalt nanoparticles as the catalyst, which determine the diameters of the nanotubes. At the above temperatures the carbon atoms dissolve into the metal catalysts which eventually become saturated with them. The problem with this CVD method is that the nanotubes do not crystallize perfectly and have large numbers of structural defects as a result, which makes electrical and mechanical studies of pure nanotubes impossible. In 1998 however, a group of nanoscientists at Stanford University were successful in producing perfectly structured single-wall nanotubes using the CVD method. The hydrocarbon gas used to deposit the carbon was methane at temperatures around C. These temperatures allowed the defect free nanotubes to have small diameters and high strain energies for maximum strength. Also the catalyst that was used was another key factor in the defect free structures. It was iron-oxide nanoparticles dispersed on aluminum oxide. Yet again, as good as these results were, there was still the fact that there still remained some of the catalyst left after the growth of the nanotubes, almost 60% of the material used. Jie Liu overcame this problem at Duke University, USA. Jie used a highly porous catalyst, which meant that 100% of the catalyst atoms were replaced by carbon atoms in the CVD process.

5 Now nanotechnology was at a stage where it could produce kilograms of perfectly formed nanotubes and it was up to the nanoscientists to come up with ideas for using them in nanotube devices. The group in Stanford University has manipulated multiwall carbon nanotubes to grow on silicon tower like structures at equal distances from each other (figure 1a). They also grew a network of nanotubes between four silicon (Si) pillars in a square formation (figure 1b). [2] These structures are grown using the methane CVD method. The methane gas keeps the nanotubes afloat and so they do not start to form on the substrate. Also the strong van de Waals bonds between the top Si atoms and the carbon atoms insure that the nanotubes do not start to grow anywhere else but on top of the pillars. This was one of the first structures built with carbon nanotubes and since then more and more ideas are been spun out every time nanotubes are mentioned. Some are feasible and some are not, but the excitement generated by these nanotubes in the technology area in the last ten years is quite remarkable. When all things are considered, it has only been ten years after all since they were first discovered and the area has developed in leaps and bounds since. Carbon nanotubes Figures 1a & 1b. Carbon nanotubes suspended between silicon pillars [2] 4. Applications of Carbon Nanotubes. As the electrical properties of carbon nanotubes are beginning to be understood, it is the obvious next step to start manipulating these unique conductors into miniature electrical devices. As nanotubes allow electrons to be transported through them unimpeded at room temperatures (if they are slightly modified); industry immediately recognized that there is endless possibilities for their everyday use. Some however may be too far-fetched and expensive to develop, but here are some applications of carbon nanotubes which are underway or which have already been developed.

6 1.The nanoscale transistor: To-day s microelectronic industries are striving for smaller and smaller electronic devices and carbon nanotubes could be their answer for devices on a nanometre scale. Because of their size, nanotubes may become invaluable as interconnects in electronic devices. One such example is the Field effect transistor (FET) (figure2). Cees Dekker and companions at Delft University of technology in Holland, Charles Leiber at Harvard University, Alex Zettl and Paul McEuen at University of California at Berkley and nanoscientists at IBM have all researched this nanoscale transistor. Researchers at IBM have investigated, using both single walled and multi-walled carbon tubes, all the possible characteristics of the nanoscale FET and they have shown that the hole mobility, which is proportional to the speed of the device, is matching up with its silicon counterpart. Hence, for an electronics company such as IBM if you can get more transistors onto a chip and if you know that they will match its rivals, then it could mean big bucks! If this FET could compete with the state of the art silicon FET then the age of molecular electronics could be upon us. However it is not all plain sailing for the nanoscale FET. There is still no method of reliably synthesizing nanoscale devices. For this FET there is no way of achieving a low contact resistance between the nanotube and the source and drain which effects the device speed. Figure 2. The nanoscale FET. [1] 2.Nano-tweezers: Imagine being able to pick up objects in the nanometric scale. This is now possible due the remarkable nanotweezers (figure 3). This means that one day it will be possible for researchers to manipulate biological cells, perform microsurgery, build nano-robots or machines, even to grip individual molecules. A group of nanoscientists in Harvard University le by Charles Lieber and Phillip Kim at Berkley successfully designed, built and worked the first nanotweezer. They used the tool to pick up a cluster of polystyrene spheres of around 500nm in diameter. They were also able to pick out a 20nm diameter semiconductor wire from a bunch of other entangled wires using the same tweezers. The tweezers were made by depositing gold electrodes on opposite sides of a glass rod (100nm diameter) which tapered to the tweezers arm end. The arms themselves are each a bundle of carbon nanotubes, one which was then glued to each electrode using conductive adhesive.

7 Figure 3. The nanotweezers [4] The tweezers are operated by applying a voltage across the electrodes, causing the arms to develop a positive and negative charge of equal magnitude which attractive force makes the tweezers close. A voltage of 8.5V was enough to completely close the arms but a gradual grip can be achieved by gradually increasing the voltage. To release the grip on the miniature object, the voltage is removed from the electrodes causing the arms to again repel each other. It is hoped that one day such a tweezers will be able to pick up individual molecules. Soon maybe it will be move over doctor and let the nanoengineer perform the surgery! [4] Other major areas of nanotube applications are: The carbon nanotube field emission lamp, which has already been investigated, has shown great energy efficiency and hence industrial interest. On the same principle is the nanotube flat-panel displays which Samsung have developed into a 9 inch full colour display that can work at video speed rates (100 images per second). Many others include hydrogen energy storage batteries and in the area of scanning probe microscopy. Summary and Conclusions It has been shown here the possible structures of carbon nanotubes, their electrical and mechanical properties, the worldwide attempts by nanotube researchers to control their growth and formation and the possible applications that these unique conductors have or may become part of. All of the above topics have enthralled physicists and chemists over the past ten or so years who are learning more and more about carbon nanotubes every time a new idea is brought to light. But this may also be one of the reasons why nanotubes will not suddenly explode into our everyday lives. It takes a large amount of research in one area for another area to proceed with its development. The nanoscientists themselves would be the first to admit that not everything about carbon nanotubes is understood, far from it. Their electronic transport, fieldemission properties, their electrochemistry and even some of their electrical and mechanical properties are still not properly understood. However the future is not bleak for these quasi one-dimensional conductors. The amount of interest in this field of nanotechnology is not just going to die away and there is plenty of new discoveries around the corner to keep this interest alive and well for a long time to come. As the electronics industry tries to produce smaller, faster and higher quality devices; you can be

8 sure that carbon nanotube interest will get larger and more intense. Lets hope they stick the pace! References: [1] [2] [3] Physics World 2000.page 43 Articles by Paul McEven, Christian Schonenberger, Laszlo Forro, Hongjie Dai, Walt de Heer and Richard Martel. [4] [5] [6] [7] Carbon Nanotubes, Physics World, june 2000, Volume 13 No 6.