Single-molecule photo-spintronics

Spintronics is like electronics except that it uses the spin of the electron (a quantum mechanical property that behaves like angular momentum and is closely linked to magnetism) as well as the electron's electric charge. Using spin and charge together could lead to computers that use much less energy, for example. Photo-spintronics adds light to the mix. This is very useful because light can easily carry information over long distances (think of optic fibres). Light and spin are also key to future quantum technologies such as quantum computing and quantum information.

Our research is to find ways of using organic molecules, based on chains and rings of carbon atoms, in photo-spintronics. This is an exciting prospect because carbon has a low atomic number which reduces the chances of losing spin information, and because there are so many different organic molecules and ways of linking them that the opportunities to find new and useful phenomena are practically endless. Our plan is to study single molecules linking a semiconductor and a magnetic metal. Single molecule experiments are difficult but not impossible, and we have made them successfully in the past using a modified scanning tunnelling microscope. Single molecule studies have helped greatly in understanding molecular electronics because studying molecules individually reveals information that is lost when they are measured in a large group.

Ours will be the first single molecule studies in photo-spintronics. We will create a population of excited electrons in the semiconductor by illuminating it and use the polarization of the light to control the spin of the electrons. We will then measure the current between the semiconductor and the ferromagnetic metal. If the current depends on the polarization of the light and the direction in which the metal is magnetized, that will be evidence that spin is being transported through the molecules. Once we show that we can make photo-spintronic measurements through a single molecule, we will investigate how the spin transport depends on the type of semiconductor, the metal, the voltage between the two (known as the bias), and the types of chemical bond between the molecule and the semiconductor and metal. This will show us how best to use organic molecules in future spintronic and photo-spintronic devices.

The exploitation of spin degrees of freedom, which underpins the discipline of spintronics or spin-electronics, is expected to make a growing impact. Contemporary challenges include encoding information in spins, manipulating spins for computation, transmitting spin encoded information and preserving spin coherence.

Information encoded in spins can be written and processed with low energies. The development of new ultra low power devices which facilitate the processing and encoding of spin would be transformative for the whole electronics industry. They would lead to significantly lower energy consumption and operating costs, and reduce the emission of greenhouse gases. This would result in major environmental benefits as well as important economic opportunities, with the emergence of new high technology manufacturing sectors. By carrying out fundamental research in this field, the UK will be in an excellent position to benefit from its growth, whether through start-ups or inward investment attracted by the skills developed nationally.

Inorganic materials, notably metals, oxides and semiconductors, have been at the forefront of such research but organic materials are receiving increasing attention. In spintronics there are many good reasons to investigate organic molecules, including the possibility of transmitting spin with lower loss than in other materials. Although the employment of organic materials within a practical spintronic device is still probably some years away, for this promising technology to realize its potential the underpinning research is needed now. The UK is especially well-placed to exploit this opportunity because it can draw on its excellent track record in molecular electronics.

The distinctive feature of this application over and above other research on organic spintronics is that we will use optical excitation to inject spin-polarized charge carriers into the molecule. If successful this approach will likely convey technical advantages since it will be easy to integrate with proven III-V semiconductor processing technologies. Even more importantly, it will bridge the gap between molecular spintronics and photonics. This link could have a major impact, leading to the attractive prospect of efficient optical signal transfer between spintronic devices or new approaches in quantum information processing and communications, for example.