Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Fabrication of a Three-Dimensional Plasmon Ruler Using an Atomic Force Microscope Jianing Lu,† Shaoding Liu,§ Sean S. E. Collins,† Linlong Tang,‡ Xingzhan Wei,*,†,‡ and Paul Mulvaney*,† †
ARC Centre of Excellence in Exciton Science, School of Chemistry, University of Melbourne, Parkville, VIC 3010, Australia Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, P. R. China § Key Lab of Advanced Transducers and Intelligent Control System of Ministry of Education, and Department of Physics and Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, P. R. China Downloaded via NOTTINGHAM TRENT UNIV on August 12, 2019 at 05:10:39 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: We have assembled a three-dimensional (3D) plasmon ruler using an atomic force microscope (AFM) tip to manipulate single gold nanocrystals on top of electron beam lithography fabricated base layers. The 3D structures exhibit several polarization-dependent surface plasmon scattering peaks, including symmetric and asymmetric Fano resonances. We map these resonances as a function of the degree of asymmetry of the structure. We show that the coupled surface plasmon resonances are extremely sensitive to the position of the upper particle and that the resonances can be engineered and tuned using an AFM tip to move the upper nanocrystal just a few Angstroms.
1. INTRODUCTION Noble-metal nanoparticles exhibit morphology-dependent absorption and scattering spectra due to the presence of localized surface plasmon resonances (LSPR).1,2 Under a darkfield microscope, individual metal nanoparticles can easily be observed because they scatter light intensely at wavelengths where LSPRs occur. These depend greatly on the particle properties (dielectric function, size, and shape) as well as the dielectric constant of the host medium.1−6 When two nanoparticles are placed in close proximity, their plasmon resonances couple and generate a scattering spectrum, which depends strongly on the interparticle separation.7−11 The coupling effect has been termed a plasmon ruler because nanometer-scale changes in separation can be monitored by the color shifts of the dimer resonance peak.12 The onedimensional dimer plasmon ruler has been successfully used to monitor disordered nanoparticle aggregates (DNA) bending,13 right-hand nanodisks (RNA) cleavage,14 and biological activities in living cells.15 Beyond dimers, complex plasmonic superstructures, such as one-dimensional linear arrays,16 two-dimensional (2D) trimers and tetramers,17,18 and laterally assembled nanorod arrays,19 have also been created, which enable different sensing geometries to be utilized. More interestingly, Liu et al. have reported a complicated three-dimensional (3D) plasmon ruler composed of five nanorods precisely arranged in three layers.20 The 3D structure was fabricated by combining high-precision, electron beam lithography (EBL), and layer-by-layer stacking techniques. Their optical measurements revealed a double © XXXX American Chemical Society
plasmon-induced transparency, and this was used to monitor conformational changes during complex macromolecular and biological processes. However, the shortest distance between two adjacent rods in their EBL-fabricated 3D structures was 30 nm, which led to relatively weak coupling and electric field enhancement, compared to superstructures created by the DNA assembly method.21 To obtain strong coupling, separations
