Connecting light to electronics

University of Groningen chemist Ryan Chiechi has produced a device that can be switched by light from low to high conductance at the speed of picoseconds. In time, such a photogated device could use organic molecules to convert optical pulses to electronic signals. The proof of principle study was published on 8 June in Nature Communications.

For high-speed data transfer you need optical fibres, but this means converting the light pulses into electrical signals that your computer, TV or other appliances can handle. Semiconductors are currently used for this conversion. Light pulses excite the semiconductor, and a charge carrier diffuses out of the switch.

‘But this process relies on charge carriers’, says Ryan Chiechi, Associate Professor of Organic Materials Chemistry and Devices. The conversion speed is constrained by the dimensions of the switch. Chiechi’s research may lead the way to conversion that modulates conductivity with tunnelling rather than the present generation of charge carriers.

Chiechi didn’t set out to speed up his internet connection. ‘I’m interested in studying fundamental properties of molecules, like conductance’, he explains. ‘But I do like to work with devices.’ Most of his colleagues study the conductance of molecules in a ‘single molecule’ setup. ‘You study a molecule between two electrodes with atomic-sized tips. But you can never study one molecule for an extended period, because it will diffuse away from the electrodes.’

Some four years ago, he developed a method of producing stable devices that allows him to measure the properties of molecules. Simply put, a monolayer of the molecule is sandwiched between two gold electrodes. In a little more detail: Chiechi’s lab managed to produce devices that combine the nanometre, micrometre and millimetre scales. See this instruction video for more on how he makes these devices.

A typical device consists of gold electrodes that are several millimetres long, and some 100 nanometres wide and high. Two of these electrodes overlap by about 250 to 500 micrometres, and are separated in the overlap by a monolayer of molecules. By running a current through the electrodes, Chiechi and his team can study the conductivity of these molecules. ‘So instead of a single-molecule setup, we study these molecules in very large numbers.’ This allows repeated measurements of these molecules in the same device.

In the new paper, the molecules under investigation are a photosensitive dye that was previously developed for use in solar cells. The paper shows that simply switching on the light causes the molecules go from low to high conductance . ‘Molecular photoswitching is a well-known phenomenon, but what we saw was something different’, Chiechi explains. In normal molecular photoswitching, bonds are broken and formed and atoms rearrange. In semiconductors, light excitation creates a charge carrier, which has to diffuse away before it is registered. This takes time and is temperature dependent.

‘What we observed is photogating: the conductivity of the molecule changes by a redistribution of the electrons, which is a quantum mechanical effect.’ This takes only picoseconds, and can be more than a million times faster than photoswitching based on isomerization. ‘Furthermore, it is not temperature dependent.’

This is the first time that a device based on photogating has been clearly established on the molecular scale. It is therefore proof of principle, showing that a device could be made to convert light pulses into an electronic signal at very high frequencies. In other words: a device that can transduce the signal from an optical data fibre to a computer in a manner that is potentially more efficient than current systems.

But, Chiechi stresses, his proof of principle device is far removed from such an application. ‘My interest is in the fundamental processes occurring in these molecules.’ He is planning further experiments, in order to analyse the responses of the molecules in the monolayer with spectroscopy. ‘Another thing you can’t do in single molecule studies’, he smiles.

Reference: Parisa Pourhossein, Rateesh K. Vijayaraghavan, Stefan C.J. Meskers and Ryan C. Chiechi. Optical Modulation of Nano-gap Tunneling Junctions Comprising Self-Assembled Monolayers of Hemicyanine Dyes. Nature Communications , 8 June 2016 DOI 10.1038/NCOMMS11749.