Abstract text (incl. figure legends and references)
A charge transfer plasmon (CTP) emerges via a direct charge transfer between linked nanoparticles or via quantum tunneling between nearly touching nanoparticles. [1,2] CTPs find applications in single-molecule sensing and functional devices including ultrafast nanoswitches and terahertz frequency photonic devices. [3] Unlike conventional plasmonic materials such as gold and silver, aluminum (Al) supports plasmon excitations in visible and ultraviolet regions of the spectrum. A natural oxide covering the surface of metallic Al protects not only its further oxidation, but also induces an energy red shift of the plasmon modes. [4] In addition, a support film underlying the plasmonics can induce a mode mixing and a red shift in plasmon modes. [5] Although electron-beam lithography (EBL) enables fabricating robust Al plasmonic devices, producing patterned Al nanostructures with a sub 5 nm gap by EBL is challenging due to a 3-5 nm-thick oxide layer surrounding metallic Al.
Here we monitor the evolution of low-energy plasmon modes in linked Al nanostructures produced by EBL on graphene via a gradual manipulation of their junctions. Moreover, we expect to reduce substrate-induced mode mixing and redshift by locating plasmonics on an ultra-thin substrate such as suspended monolayer graphene.
To overcome the difficulties in fabricating patterned Al nanostructures with a sub 5 nm gap, we manipulate the conductive junctions of linked Al nanoprims via a focused electron beam. The gradual manipulation of junctions enables monitoring the evolution of low-energy plasmon modes by low-loss EELS measurements in a monochromated scanning transmission electron microscope (STEM). The plasmon modes measured by EELS are further analyzed and confirmed by an LC circuit model and full electromagnetic simulations performed with boundary-element and frequency-domain methods.
The EELS measurements indicate that the CTP resonances of linked nanoprisms depend strongly on the kinetic inductance of the conductive junction. With the increase of junction width, the CTP resonances of linked nanoprisms blueshift, whereas their lifetimes decay monotonically on the femtosecond time-scale. In contrast to CTPs, their 3λ/2 and λ resonances display a redshift. After shrinking the junction width via electron-beam irradiation, the CTP, 3λ/2 and λ modes explicitly redshift. When the nanoprisms are separated completely by a focused electron–beam, the CTP vanishes and a new bonding-type gap mode appears. In addition, the 3λ/2 and λ resonances evolve into bonding dipolar and antibonding dipolar localized surface plasmon resonances (LSPRs).
References
[1] F. Wen et al., ACS Nano 2015, 9 (6), 6428-6435.
[2] O. Pérez-González et al., Nano Letters 2010, 10 (8), 3090-3095.
[3] J. Gu, et al., Nature Communications 2012, 3 (1), 1151.
[4] M. W. Knight et al., ACS Nano 2014, 8 (1), 834-840.
[5] C. Cherqui et al., The Journal of Physical Chemistry Letters 2018, 9 (3), 504-512.