Where are hot carriers created in plasmonically enhanced semiconductor substrates?

  • By Burcu Ozden
  • 5 December 2017

PQI members Hrvoje Petek, Jin Zhao and their colleagues investigated a less known fact about the microscopic details of how the combined optical, electronic and chemical properties of metal/semiconductor interfaces define the coupling of light into the electronic reagents on their recent paper published in Nature Photonics.

It is a well-known fact that integration of plasmonic nanoparticles in semiconductor substrates enhances their photovoltaic activities through creation of hot carriers. However, where these carriers are created are still under the debate.  Many sentients believe that these carriers are generated by the transfer of plasmonically generated hot electrons from the metal. Some claims that they are created through photoinduced interfacial charge transfer and dephasing of the interfacial plasmonic field. Petek and his colleagues studied the effect of the combined optical, electronic and chemical properties of metal/semiconductor interfaces on creation of hot carriers to unravel the debates on where they are created.  In this study, they investigated the coherence and hot electron dynamics in a prototypical Ag nanocluster/TiO2 heterojunction via ultrafast two-photon photoemission (2PP) spectroscopy, scanning tunneling microscopy (STM) and density functional theory (DFT).  The silver nanoclustors used in this study were grown via e-beam evaporation of Ag on top of TiO2 surface.

First clean TiO2 samples were characterized by 2PP to compare with the changes after deposition of Ag. It was found out that deposition of Ag onto TiO2 enhances the 2PP yields and modifies the spectra for both s- and p-polarized excitation. They attributed these enhancements to the formation and excitation of the Mie plasmon resonances of Ag cluster/TiO2 interface. They further investigated the enhancements of the 2PP yields with respect to the laser wavelength, polarization and the crystal azimuth orientation. They showed that while p polarize light has two electric field components in vertical and parallel directions s polarized light has only one electric field component in parallel direction. Later they studied the energy dependence of 2PP for s and p polarized lights. They found out that the ratio of 2pp has single peak for s polarization at 3.1 eV and two peaks at 3.1 and 3.8 Ev for p polarization. They further tested how the plasmon enhancements depend on alignment of the [110] and [001] in-plane azimuths relative to the optical plane. They found out that perpendicular plasmon peak is higher for [001] than the [110] azimuth even though parallel and perpendicular plasmon energies are constant. They attributed this effect to the anisotropy of perpendicular plasmon enhancement which affects the distribution of the hot electrons.

 Later, they assigned components of 2PP spectra of Ag/TiO2 with p-polarized light on the basis of their wavelength dependence.  They assigned 2PP spectra of the bare to hot electrons predominantly in TiO2 and plasmonically activated graphite to interface state (IFS). Their spectra didn’t show that IFS is excited via real intermediate state. Further they confirmed the assignment of the IFS, which is not a feature of 2PP spectra of either the Ag or TiO2 surfaces, by calculating the electronic structure of Ag clusters on both the stoichiometric and reduced TiO2 (110) surfaces. Finally, they examined the dependence of 2PP on the  the polarization of the driving field and the TiO2 azimuthal orientation.  The rotation gradually abated excitation of the perpendicular plasmon such that the parallel plasmon remained as the dominant Response. For p polarization, the 2PP spectra showed negligible azimuth dependence but for s-polarized excitation, the 2PP yielded for the  [110] azimuth.

In short, they have shown that the plasmon excitation, dephasing and hot electron processes that are related to plasmonically enhanced photocatalysis involve complex physical and chemical interactions, with strong interfacial character involving the chemical and plasmonic coupling of Ag nanoclusters and the TiO2 substrate that cannot be predicted by the properties of the component materials, but rather require an understanding of their interactions. They found that the dephasing of the perpendicular and parallel plasmons by the dielectric screening response of the TiO2 substrate generates hot electrons with anisotropic and non-thermal distributions.

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