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Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor


The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells1,2,3,4, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies4. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds5,6,7,8,9 for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution10,11 and intramolecular thermalization of excited states12,13,14 occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution15,16, to show that electron transfer from an aromatic adsorbate to a TiO2 semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes17.

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Figure 1: Geometry of the adsorption structure of bi-isonicotinic acid on TiO2(110).
Figure 2: Relevant processes.
Figure 3: Density-of-states for bi-isonicotinic acid on TiO2(110): experimental monolayer density-of-states, experimental clean substrate occupied density-of-states, theoretical substrate density-of-states, and XAS calculation.
Figure 4: Comparison of RPES and XAS for a bi-isonicotinic acid monolayer.


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We are grateful for financial support to the Consortium on Clusters and Ultrafine Particles and to the ATOMICS Consortium, which are funded by Stiftelsen for Strategisk Forskning, to Göran Gustafssons Stiftelse, and to Vetenskapsrådet. We acknowledge the Swedish National Supercomputer Centre (NSC) for computer time, and the Group for Numerically-intensive Computations of the IBM Research Laboratory Zürich and the Abteilung Parrinello of MPI Stuttgart for help with the calculations.

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Correspondence to Paul A. Brühwiler.

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Schnadt, J., Brühwiler, P., Patthey, L. et al. Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor. Nature 418, 620–623 (2002).

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