What are fullerenes and carbon nanotubes?
Common graphite is composed of sheets of carbon atoms joined together in hexagons. If one of these sheets of carbon atoms is rolled up into a cylinder a mere millionth of a millimetre across, a nanotube is formed. Alternatively, a football of carbon atoms, can be produced. This football is known as a fullerene, or 'buckyball', after Buckminster Fuller, who pioneered geodesic domes (structures based on the same mixture of hexagons and pentagons) in the U.S. The simplest fullerene is composed of 60 carbon atoms, and is therefore known simply as C60. There are many other fullerenes of different shapes and sizes, such as C70, C82 etc. A model of a C60 molecule is shown on the left. A small island of Nd@C82 on a Ag/Si(111) surface is shown on the right. Nanotubes and fullerenes occur naturally in soot. In order to make them in a slightly more controlled fashion, an electric arc is struck between carbon electrodes, or else graphite is vapourised using a laser. The resulting carbon powder, which comprises a mixture of amorphous carbon, fullerenes and nanotubes, is then collected and purified. An example of a nanotube is shown below. Usually the ends of a nanotube are capped with a hemisphere, just like half a fullerene.
What is a peapod?
If the nanotube is big enough, there is room for metal atoms, molecules, and even fullerenes to fit inside. A nanotube filled with fullerenes is known as a "peapod". A model of a peapod is shown below. By trapping atoms inside the buckballs, and then trapping those inside the nanotube, we can achieve some interesting properties. For example, we can encapsulate many types of atom which have a magnetic moment, and act like tiny bar magnets. By changing the orientation of each of these magnets, we can represent data in much the same way regions of a magnetic hard drive do. In a "pea-pod", we have a neat row of these atoms, allowing us to store and manipulate data at the atomic level. Because of the tiny size of these data units, we are also able to enter the realm of Quantum Computing, where superposed states are necessary. More detailed and specialist descriptions of our work are given below.
What are we trying to do?
Our objective is to fill the fullerenes with nitrogen, in order to use the spin on the nitrogen atom as a qubit. The nanotube acts to encapsulate the filled fullerenes to make a multi-qubit quantum computer. Interactions between the quibits are assumed to take place either between the fullerenes within the nanotube directly, or else via the nanotube. The type, properties and controllability of the interactions have yet to be determined. We aim to use experimental techniques, both microscopy (TEM, AFM/UFM, STM) and spectroscopy (especially Electron-Spin Resonance), and state-of-the-art computational modelling to determine the electronic structure of this system, so as develop a working quantum computer.Transmission Electron Microscopy
Transmission Electron Microscope (TEM) pictures of some peapods are shown below. The first picture shows a pair of peapods. The TEM looks through the nanotubes, so that the tube is seen in cross-section, and the fullerenes look like circles. In the second picture, dark dots can be made out on the edges of the fullerenes; these are Cerium (Ce) atoms which have been inserted into the fullerenes. These dots can be observed moving around on the surface of the fullerene. It is not known if the Ce atoms is mobile, or if the whole fullerene is rotating inside the nanotube.
Atomic Force Microscopy
An AFM is best described as something akin to an old LP stylus. A sharp tip in the shape of a pyramid moves over the contours of the surface, at near-atomic precision, and as it does so, the cantilever at the back of the tip moves up and down. This motion is detected using the deflection of a laser beam, which bounces off the back of the cantilever, and thus a map of any surface is obtained, at a resolution of less than 1nm vertically. There are different modes - contact mode, in which the tip is scraping across the surface, noncontact mode or tapping mode, in which the tip vibrates up and down, and responds to the proximity of the surface, without actually touching it, and ultrasonic mode, or UFM, in which the sample is vibrated at very high frequencies (1-2 MHz), which causes the tip to sense the relative softness of various features on the surface in addition to a high resolution.In our work, we find that contact mode tends to drag the nanotubes across the surface, so we don't see them. As a result, we mainly use tapping mode AFM, and UFM. In AFM, the nanotubes show up as protusions, while in the UFM mode, they show up as dark depressions as they are less hard than the silicon substrate. Some pictures of what we are trying to do are shown below.
The first two images demonstrate the different imaging modes of AFM. The one on the left is a topographic image, which just shows the nanotubes as bright lines. The one on the right is the error signal image, which detects edges much better, and gives the appearance of a landscape lighted from one side. These nanotubes are double-walled nanotubes, with an outer diameter of 2.5 nm, which is a bit easier to see than single-walled nanotubes, which are about 1.6 nm wide.
The second pair of images show the ultrasonic imaging mode. The image on the left is a standard topographic image, and the nanotubes show up white, while in the left-hand image, the nanotubes show up dark. This is because the nanotubes are softer than the silicon oxide substrate.
What we are trying to do is to locate individual nanotubes on a silicon surface, so that we can attach metal wires at each end, and measure the current through the nanotubes. Therefore we have made sample with gold letters on them in a grid 20 um apart. An example of a tube near a letter is shown below. In the next image, a nanotube has been located near a 'P', and then metal wires have been deposited across the nanotube. These wires are very thin, and look more like a string of metal crystals joined together.
Scanning Tunnelling Microscopy
Here are some Scanning Tunnelling Microscope(STM) pictures of nanotubes. The picture on the left shows a large bundle of tubes on a gold surface. In the right-hand picture, we have zoomed in on a single nanotube and obtained an atomic-resolution image.
Recently, we have been studying the properties of endohedral fullerenes by STM. In particular, we have looked at Nd@C82, Sc@C82, C82, and Er3N@C80. We have looked at two aspects of these fullerenes - their arrangement as islands on the Ag/Si(111) surface, and their molecular orbital structure. To do this, we have been using a low-temperature STM, which lets us image at -200 C, or 77 K. This low temperature reduces the motion of the fullerenes almost completely, so that we are able to image them while stationary.
Electron Spin Resonance
Electron Spin Resonance is one of the many methods we use to characterize the states of the atoms, buckyballs and nanotubes. A bulk sample is placed in a microwave cavity, and continuous irradiation by microwaves is carried out. A magnetic field is then applied and swept through a range of values, splitting the magnetic energy levels of the sample in proportion to the applied field. When the splitting between two of the sample's energy levels is the same as the energy of the microwave field photons, the system absorbs energy and there is a drop in intensity in the microwave cavity, which can be measured. In this way, by considering a broad range of fields, all of the magnetic energy levels of the material can be sampled and theoretical models created to describe its behaviour. More advanced experiments utilise pulses of microwave energy to create transients in the sample which we can observe directly.Our ESR work is of mostly the former, Continuous Wave, type, although we are planning some pulsed work in the near future. Our interest is in characterising N@C60, N@C70 and other endofullerene energy levels, and mechanisms for broadening and coupling in the solid state, including inside nanotubes. Other experiments investigate the magnetic properties of nanotubes, and of other filled fullerenes, such as La- and Y- complexes.
An ESR spectrum from the endofullerene N@C60 (this notation indicates a nitrogen atom inside a C60 cage) is shown below.
