All chemists love their toys, whether it's an upgraded modelling program, the latest spectrometer, or a laptop to take to conferences. But some chemists take that passion for the newest and shiniest to extremes - they like to invent their own fun and games. And it's even better if they can add in the feature of the ultrasmall. Annabel Wood takes a look at some of the latest tiny toys in the nano-Toy Story. In a recent coup, Chad Mirkin (Northwestern University, Illinois) reported the development of a 'nano- pen'. Tipped as a breakthrough which could, in the not-too-distant future, create electronic circuits far more compact than any today, the nano-pen has enormous potential. It 'writes' on 'paper' (flat gold surfaces), using a standard Atomic Force Microscope (AFM) tip as the 'nib' of the pen. The 'ink' is loaded onto the nib by evaporating sulfur-containing compounds onto the AFM tip. While this is not a new concept, the breakthrough came about when the researchers realised that water vapour is the key to the writing process. The technique relies on the formation of a meniscus of water between the tip and the gold surface, drawing the ink off the nib onto the paper. The humidity of the air surrounding the pen and paper controls the characteristics of the meniscus. Adjusting the relative humidity lets the artist control the flow of ink off the pen, and determines whether lines are thick, or thin and sharp. In fact, the lines are so well-controlled, they are only one molecule thick and a few molecules wide.
In a further development, they discovered that using different inks (1-octadecanethiol and 16- mercaptohexadecanoic acid) allows for overwriting of lines on the paper, similar to conventional plotters using several different colours to create complex diagrams. Using the 'nano-plotter', they were able to write out part of a speech by Nobel Prize-wining physicist Richard Feynman. Feynman is credited with the conception of nanotechnology as the ability to be able to precisely place and manipulate individual atoms. The nano-plotter was able to reproduce the speech in just a few minutes, comparing favourably with the time taken by an everyday colour printer.
The use of different colours, and the ability to write extremely sharp, precise lines is what has those interested in electronic circuits buzzing with excitement. With demand for computer memory continually increasing, along with the trend for smaller workstations and laptops, technology for increasing the computing power of electronic circuits is the Holy Grail of the computer industry. A potential rival for the nano-pen's future dominance of electronic circuit writing is based on the carbon nanotube. Most chemists are familiar with the concept of the buckyball (C60) and its cylindrical relative, the buckytube, which can be thought of as rolled-up sheets of carbon film. While it's expected that buckytubes will prove as useful as bricks in a Lego kit to chemists, it's taken a while to come up with real applications for them. Currently, the tip of a Scanning Tunneling Microscope (STM) can deposit single atoms on a surface, but the technique is limited by the discontinuous deposition, which makes it difficult to build 3D structures or draw continuous lines.
In a similar scheme to Mirkin's, David Tomanek and Peter Kral (Michigan State University and University of Toronto) have proposed the use of a buckytube as the inkwell in a 'nano-fountain pen'. They plan to fill the nanotube with 'ink', and move the atoms through the tube, using two lasers with the beams crossed. One laser will have double the frequency of the other. The lasers will control the flow of ink atoms out of the buckytube, allowing single atoms to be continually deposited in precise positions on the paper. The laser light hits the walls of the nanotubes, freeing electrons, which are moved along the tube by interference between the laser beams. The electrons force the trapped ink atoms to move down the buckytube, where they can be deposited once they reach the open end. Adjustments made to the laser beams can control the flow of the atoms in the desired speed and direction. "A single laser will create hot electrons, but half would move left and half would move right in the nanotube," Professor Tomanek explains. "We propose the use of two lasers. And by adjusting the phase difference between them, we can drive the electrons to the right or to the left. Since we need no [electrical] contacts, we can have 10,000 nanotubes in the laser spot, and in each of these nanotubes we will induce a current."
The nano-fountain pen could even be used to insert molecules into individual cells, for instance, for pharmaceutical testing. "The nanotubes are so small that the cell membrane is not damaged by the insertion of the nanotube."
Tomanek is enthusiastic about the potential applications of carbon nanotubes. "The most likely application is in the micro-electronic industry," he said. "Nanotube research is one of the five or 10 most exciting research directions presently in physics."
He cautions, "It's hard to estimate how long until those products will be available". It won't be long if Harvard University's Philip Kim and Charles Lieber have their way. Their welcome addition to the nano-tool-kit is the 'nano-tweezer'. Their nano-tweezers consist of carbon nanotubes attached to independent electrodes made on glass micropipettes. When a voltage is applied to the electrodes, the ends of the nanotubes can open or close, making it possible for the tweezers to pick up and move tiny objects. The researchers used the nano-tweezers to pick up fluorescently-labelled polystyrene spheres. They were able to grip a cluster of nano-spheres, using the tweezers to pull the cluster away from its support.
The grip of the nano-tweezers can be adjusted by altering the voltage. At 0 V, the tweezers are fully open. Applying a voltage gradually closes the tweezers, until at 8.5 V, they are closed. The ability to alter the nano-tweezers' grip introduces the possibility that they could be used to manipulate delicate objects, such as individual biological cells.
An added bonus is the ability to measure the electrical properties of the object gripped in the tweezers. This was investigated further when the researchers used the tweezers to tease out a single nano-wire of GaAs from a tangle of other wires, then measured its electrical properties. The potential to pick up and precisely place objects could help in manufacturing small-scale versions of existing devices. Improvements to the nano-tweezers may lead to the ability to manoeuvre single atoms. The story of nanotechnology has come a long way since the concept of being able to manipulate individual atoms was first discussed in 1959.
The nano-toys described here are but a few of the many devices that will appear in the next few years. As our ability to manipulate atoms and molecules increases, we may finally see some of the key predictions of nanotechnology, such as 'intelligent' devices small enough to implant in the human body, finally being realized.
1. Piner RD, Zhu J, Xu F, Hong SH and Mirkin CA,
Science 283 661 (1999)
2. Hong SH, Zhu J and Mirkin CA, Science 286 523 (1999)
3. Kral P and Tomanek D, Physical Review Letters 82 5373 (1999)
4. Kim, P and Lieber, CM, Science 286 2148 (1999)