Monitoring Morphological Changes in 2D Monolayer Semiconductors Using Atom-Thick Plasmonic Nanocavities

Daniel O. Sigle NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK Jan Mertens NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK Lars Herrmann NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK Richard W. Bowman NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK Sandrine Ithurria Laboratoire de Physique et d’Etude des Matériaux, Universite Paris Sud, Paris, France Benoit Dubertret Laboratoire de Physique et d’Etude des Matériaux, Universite Paris Sud, Paris, France Yumeng Shi Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore, Singapore Hui Ying Yang Pillar of Engineering Product Development, Singapore University of Technology and Design, Singapore, Singapore Christos Tserkezis Donostia International Physics Center (DIPC) and Centro de Física de Materiales, Centro Mixto CSIC-UPV/EHU, Donostia-San Sebastián, Spain Javier Aizpurua Donostia International Physics Center (DIPC) and Centro de Física de Materiales, Centro Mixto CSIC-UPV/EHU, Donostia-San Sebastián, Spain Jeremy J. Baumberg NanoPhotonics Centre, Cavendish Laboratory, University of Cambridge, Cambridge, UK

Nanometre-sized gaps between plasmonically-coupled adjacent metal nanoparticles enclose extremely-localised optical fields which are strongly enhanced. [1] This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. [1-3] Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor.[4] The gold faces are separated by either monolayers of molybdenum disulphide (MoS₂) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with hundred-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures. This spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.

Figure 1: a Geometry utilised: a gold nanoparticle is placed on a semiconductor monolayer on a gold mirror. b Electron microscopy images of a faceted gold nanoparticle, MoS₂ platelet and darkfield image showing a gold nanoparticle on a MoS₂ platelet.

[1] D.O. Sigle, et al. Phys. Rev. Lett., 113, 1-5 (2014).

[2] J. Mertens, et al. Nano Lett., 13, 5033–5038 (2013).

[3] F. Benz, et al. Nano Lett. accepted, doi:10.1021/nl5041786 (2014).

[4] D.O. Sigle, et al. ACS Nano. accepted, doi: 10.1021/nn5064198 (2014).

jm806@cam.ac.uk