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Vol 38  No 1
August 11, 2005


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Ligands: the molecular ties that bind metals

Chemist’s research at the molecular level may have future high tech applications

By Deborah Inkpen

Dr. Marty T. Lemaire, former research associate of Dr. Thompson, with the Quantum Design MPMS5S SQUID magnetometer. (Photo submitted)

Living in a technological age, we’ve come to take for granted the amount of data our computers can store. However, current computer systems do have storage limitations. But what if we could store information at the molecular level? The research being carried out by a professor in Memorial’s Department of Chemistry may hold the key to solving future data storage challenges.

Dr. Laurence Thompson and his team of graduate students and postdoctoral fellows have been conducting research using molecules containing large numbers of metal atoms (ions) bound to organic ligands to store information. Ligand donor atoms such as nitrogen, sulphur and oxygen actually bind to the metal atoms to form the polymetallic molecular assemblies. When this is done it is possible to make use of the magnetic and electronic properties of the bonded metals to store information.

“Inorganic transition metal ions, like manganese, iron, cobalt, nickel and copper all have interesting magnetic and electronic properties due to the presence of unpaired electrons,” said Dr. Thompson. “However when they are collected together in these polymetallic clusters we get novel magnetic and electronic properties as a result of their being close together. One of the things we are interested in is the molecular magnetism and the way metal ions communicate with one another in terms of their magnetic and electronic properties. The metals ‘talk to each other’ electronically through molecular circuitry, and they sense the magnetic and electronic properties of neighboring metal atoms. It’s like joining up parts of the molecule with wires, and allowing these electrons to interact with each other. The molecule itself acts like a conduit through which these communications can take place.”

Memorial is home to the National Magnetic Instrumentation facility which houses a Quantum Design MPMS5S SQUID magnetometer, which Dr. Thompson uses for his research. “It’s not unique in the country but there are very few, so we provide magnetic data for other institutions,” explained Dr. Thompson.

The SQUID Magnetometer measures magnetic properties of molecules at varying temperatures and in variable magnetic fields.

Dr. Thompson’s research involves nano-scale chemistry. “It’s in between the molecular and macro world and it’s in a realm of physics and chemistry that is largely untapped in terms of properties. We know that computers work on a small scale, but this is very large compared with an average molecule,” he said. “If we can get molecules to do things the chances are they are going to be much more efficient than the sort of ‘devices’ that are currently part of computers. So by gaining this size advantage, and by using the bottom-up approach to produce novel molecular assemblies, you can produce things which cannot be achieved using the ‘top-down’ used in developing new computer systems. The micro-electronics industry is still operating on this ‘top-down’ principle, which in many areas like computer technology leads to serious limitations regarding processing speed, and data storage capacity. The ‘bottom-up’ approach, using atoms or molecules, is seen as the way of the future; but harnessing the power of an individual molecule is a major challenge.”

Dr. Thompson points out the limitations of the approach of using a single ligand if you wish to produce systems with a large number of metals. “What we then get into is the area of supra-molecular self-assembly. This approach uses the metal itself to direct the molecular assembly, rather like a policeman organizing an orderly traffic flow at a busy intersection. The DNA sequence is the ultimate example of self-assembly in nature ­ it’s the concept of the perfect fit that allows a larger entity to assemble. In our case we adopt this approach and use the metal as a means of creating something of much larger molecular complexity. It’s like a Rubik’s Cube arrangement; the components fit together perfectly with the properties of the metal directing the self assembly. The nice thing is that the properties of the individual pieces can be modified to create new things.”

Dr. Laurence Thompson (Photo by Chris Hammond)

 

Dr. Thompson believes that this approach has the potential to provide new ways to encode information at the molecular level. “Producing systems with large numbers of metals creates molecules with novel magnetic and electronic properties, brought to bear by the communication between the metals. This can only occur if you have them close together and if they are linked. The key is for the molecule to be able to exist in two stable states, which can be accessed by a simple external stimulus to encode, for example a zero or one. We have achieved this in terms of the electronic properties of a series of nona-nuclear manganese complexes (nine manganese metal centers per molecule), which respond simply to an external voltage to change the states. The additional requirement is that you have to be able to address the molecule in order to switch it ‘on’ and ‘off’. This has been achieved at the molecular level by attaching the molecules to a gold or graphite surface in a monolayer assembly, and then probing them with a very thin electrode, and varying the voltage at the electrode. This is a technique called Scanning Tunneling Microscopy (STM). This also provides an image of the molecule bound to the surface, so we know that is there”, Dr. Thompson said.

The major challenge is going from this simple experiment, which involves components with nanometer dimensions (1 nanometer = 1x10-9 meter), to something practical in terms of everyday use. “This is where we hand over the challenge to the microelectronics industry.”

Dr. Thompson and his team are working on producing systems with larger number of metals to explore systems that have much more interesting properties than previously seen. He said that in the future the computer industry must look at molecules as the way to move forward.

“We will see the conventional magnetic particles found in modern computer components, for example hard drives and floppy drives, replaced by smaller molecular based components. These may then respond to external magnetic or electronic stimuli to store information. When this is achieved the storage capacity on a hard drive of the future could approach 150 Tb/sq. in.”

Dr. Thompson received the Alcan Lecture Award sponsored by the Chemical Society of Canada in June 2004 to mark his contributions to Inorganic Chemistry in Canada.

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