Scanning Probe Tips made of Plasmonic Germanium for Resonant Scattering Imaging of ZnO Nanowires Embedded in a PDMS Matrix

Emilie Sakat Department of Physics, Politecnico di Milano, Milano, Italy Valeria Giliberti Department of Physics, Sapienza University of Rome, Rome, Italy Institute for Photonics and Nanotechnologies, National Research Council of Italy, Rome, Italy Paolo Biagioni Department of Physics, Politecnico di Milano, Milano, Italy Michele Celebrano Department of Physics, Politecnico di Milano, Milano, Italy Marco Finazzi Department of Physics, Politecnico di Milano, Milano, Italy Andrea Notargiacomo Institute for Photonics and Nanotechnologies, National Research Council of Italy, Rome, Italy Marialilia Pea Institute for Photonics and Nanotechnologies, National Research Council of Italy, Rome, Italy Monica Bollani Institute for Photonics and Nanotechnologies, National Research Council of Italy, Como, Italy LNESS, Politecnico di Milano, Como, Italy Jacopo Frigerio LNESS, Politecnico di Milano, Como, Italy Giovanni Isella LNESS, Politecnico di Milano, Como, Italy Maria Virginia Altoe Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, USA Stefano Cabrini Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, USA Mauro Melli Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, USA Alexander Weber-Bargioni Molecular Foundry, Lawrence Berkeley National Labs, Berkeley, USA Leonetta Baldassarre Center for Life and Nano Sciences, Italian Institute of Technology, Rome, Italy Michele Ortolani Department of Physics, Sapienza University of Rome, Rome, Italy Institute for Photonics and Nanotechnologies, National Research Council of Italy, Rome, Italy

The fabrication of novel scanning probe tips based on electromagnetic design of their plasmonic response has allowed unprecedented resolution and sensitivity in near-field optical microscopy both in the visible [1] and in the mid-infrared (mid-IR) [2] ranges.

In this work, we aim at the high-contrast nanoimaging of composite materials employing mid-IR radiation to achieve substance-specificity. Instead of using metals to obtain the desired plasmonic behavior in the mid-IR, we fabricated tips made of epitaxial semiconductor material with ultra-high doping level above 10¹⁹ cm^-3. The material of choice was n-doped germanium grown on silicon wafers. The tip fabrication consisted in gluing pyramids of epitaxial material onto flat-hat AFM cantilevers using a focused-ion beam apparatus. TEM images confirm that the crystal quality was preserved during the fabrication steps.

Scattering scanning near-field mid-IR microscopy was performed on ZnO nanowires embedded in a PDMS matrix, a prototype for novel nano-piezo generators, at several electromagnetic frequencies, both above and below the plasma frequency of the Ge tip. Numerical electromagnetic simulations allow understanding the details of the absorption and scattering processes involved.

Work at the Molecular Foundry was performed under User Proposal # 1773. The research leading to these results has received funding from the European Union’s Seventh Framework Programme under grant agreement n°613055. We acknowledge support from Neaspec Gmbh.

[1] Wei Bao, M. Melli, N. Caselli, F. Riboli, D. S. Wiersma, M. Staffaroni, H. Choo, D. F. Ogletree, S. Aloni, J. Bokor, S. Cabrini, F. Intonti, M. B. Salmeron, E. Yablonovitch, P. J. Schuck and A. Weber-Bargioni Science 338, 1317 (2012)

[2] F. Huth, A. Chuvilin, M. Schnell, I. Amenabar, R. Krutokhvostov, S. Lopatin and R. Hillenbrand, Nano Lett. 13, 1065 (2013).

emilie.sakat@polimi.it