Charge transfer between a molecule and a solid surface lies at the heart of a number of really important devices such as solar cells and organic LEDs.
Charge transfer between a molecule and a solid surface lies at the heart of a number of really important devices such as solar cells and organic LEDs. To make a dye-sensitised solar cell for example you can take TiO2 which doesn't absorb visible light (this is evident by the fact that it's white) and coat it with a single layer of dye molecules similar to those shown in the figure to the right. The dye can absorb a photon of visible light, exciting an electron into a previously unoccupied molecular orbital, which can then tunnel into the conduction band of the oxide to which it is bound. Not only does this provide electrons in the conduction band, but the electron and hole are immediately separated.
Clearly, the interaction of the dye molecule with the oxide surface is key to our understanding of the underlying physics.
We use a variety of synchrotron radiation techniques (at MAXlab in Sweden and SOLEIL in France), including photoemission (PES) and X-ray absorption spectroscopy (XAS) to investigate the chemical nature of the adsorbed molecules and their bonding geometry. Going beyond this, using the 'core-hole clock' implementation of resonant photoemission (RPES) we can actually measure how quickly an electron tunnels from the molecule to the substrate on the timescale of the lifetime of core-hole, on the order of a few femtoseconds.
Basically, when a core-electron is excited into a previously unoccupied molecular orbital it leaves the atom in a core-excited state. This state is unstable and will decay on the timescale of femtoseconds. The mechanism by which it decays can either be radiative (it releases a photon) or non-radiative (it releases an electron) as depicted in the figure to the right. In the case of non-radiative decay, a valence electron falls down to fill the core-hole and sufficient energy is given to another valence electron so that it can escape the atom. We can tell whether the originally excited electron was involved in this process by measuring the kinetic energy of all the electrons that are emitted, and infer from this how long the excited electron remained on the molecule before tunnelling into the conduction band of the oxide surface.
Using this technique we have measured the charge transfer dynamics of a range of complex molecules on titanium dioxide related to dye-sensitised solar cells and water splitting devices (see publications). In the case of radiative decay, a soft x-ray photon is emitted from the atom. Measuring the energy of this photon is not so easy and the numbers of photons emitted from the carbon and nitrogen atoms that we are interested in have a very low abundance. However, using a technique known as resonant inelastic x-ray scattering (RIXS) we can measure the energy of the emitted photons and compare the number emitted at the same energy as the incoming photon (elastic scattering) to those at a lower energy (inelastic scattering).
In another beautifully complicated scheme this information can also tell us how long the originally excited electron remained on the molecule before tunnelling, and so this technique is a complementary method to RPES for measuring ultra-fast charge transfer dynamics in adsorbed molecules. Its advantage is that it is a photons-in-photons-out technique and can therefore be applied to buried interfaces and high-pressure systems. We have recently provided the first experimental evidence for this method from our work at MAXlab, and continue to develop this technique at SOLEIL.
Applying these techniques to study the complex molecules used in real solar cells and water splitting devices requires a method to controllably deposit them onto a surface under the UHV conditions required by the relevant spectroscopic techniques. To this end we have been developing a portable system for in-situ UHV-compatible electrospray deposition (see Complex Molecules at Surfaces) at synchrotron beamlines and other UHV systems.