Plasmonic nanoparticle superlattice sheets
There is an increasing interest harnessing the unique properties and tremendous potential of nanoscale metals to revolutionize the ways in which materials and products are created. The past decades have witnessed encouraging progress in manufacturing metal in solutions with controlled sizes and shapes, allowing for the formulation of so-called artificial periodic tables [1]. Organization of these artificial ‘meta-atoms’ over multiple length scales into ordered superlattices offers a powerful route towards the creation of metamaterials with properties different from those displayed in isolated or bulk phases. As a result of coupling effects between nanoparticles, these emergent collective properties are desirable to foster innovative applications in a variety of fields, including optoelectronics, nanophotonics, catalysis, and chemical sensing.
Nonetheless, it remains challenging to precisely manipulate metal nanoparticles into reliable large-scale, hierarchical assemblies due to the complex nanoscale interaction forces at different spatial and temporal scales [2]. Engineering of surface ligands, in particular soft organic capping ligands such as surfactant molecules, polymer and biomolecules, appears to be an effective approach towards regulating these nanoscale forces. The soft ligand–ligand interactions including steric hindrance, hydrogen bonding interactions, electrostatic forces and specific DNA Watson–Crick base-pair interactions have been successfully demonstrated to balance the dominant core-to-core van der Waals attractive forces. This allows directing of superlattice growth to a certain degree, leading to a variety of one-, two- and three-dimensional nanoparticle superlattices [3], [4],[5], [6], [7] and [8].
Recently, we have demonstrated a general and simple yet efficient methodology comprising the use of polystyrene-capped nanoparticles in conjunction with evaporation-mediated self-assembly at the air–liquid interface to fabricate plasmonic nanoparticle superlattice sheets. These nanosheets, which we termed as ‘Plasmene’, are highly ordered and free-standing in nature with ultimate thickness limit and macroscopic lateral dimensions, hence representing the thinnest version of two-dimensional plasmonic nanoparticle superlattices. Plasmene nanosheets were found to demonstrate several unique fundamental properties, such as high mechanical compliance, resonance properties, gap mode plasmonics and propagating plasmonics [9]. Moreover, the superior robustness and flexibility of plasmene allows it to be patterned into the first cut-shaped one-dimensional nanoribbon superlattice with width-dependent plasmonics and folded into the first geometrically well-defined three-dimensional superlattice origami nanostructures with folding-induced plasmonics.
Based on these unique features, plasmene nanosheets have been successfully utilized for several promising optical applications. The free-standing plasmene nanosheets can act as a mechanical membrane resonator, integrating plasmonic resonance and mechanical resonance properties into a single device which allows interesting investigation of fundamental science in opto-mechanics [9]. The propagating plasmonics of plasmene enabled the integration of plasmene into a superlattice-fiber waveguide coupler with transverse magnetic (TM) polarizing selectivity [9]. The coupled plasmons between the nanoscale interstitial gaps of plasmene gives rise to a concentrated distribution of enhanced electromagnetic field (hotspots) across a large area that induces enhancement of Raman signal. The near-field confinement strength is programmable by systematically engineering the shapes and sizes of the plasmene nanosheet [10]. Coupled with the softness and optical transparency of plasmene, these nanosheets can be used as a soft surface-attachable adhesive for reproducible and ultrasensitive multiphase detection in aqueous and vapour phase, as well as direct detection on topologically complex surfaces which are otherwise difficult to achieve with traditional rigid/opaque surface enhanced Raman scattering (SERS) substrates. These interesting findings triggered potential real-world applications such as overdose drug quantification, direct trace detection of drugs on banknotes [11] and creation of a plasmene-specific dual plasmonic and SERS coded anti-counterfeit security label [12].
This issue's cover of Materials Today shows a scanning electron microscope (SEM) image of a rhombic dodecahedral-based plasmene diamond origami fabricated from a combination of bottom-up self-assembly and top-down focused ion beam (FIB) lithography. The origami folding approach is based on a ‘gentle’ FIB milling in which bombardment of high energy gallium ions resulted in localized heating and partial etching of the surface binding polystyrene ligands. This destabilizes the balanced interactive nanoscale forces, leading to differences in localized stress that resulted in an automatic self-folding effect. With a strategic induction of localized heating and stress build-up, the self-folding process can be programmed in a variety of ways with sophisticated control over the topological and geometrical design features of the origami nanostructure. In addition to diamond origami, other intricate origami structures such as cubes, pentagons, hexagons, hearts and even a ‘flying bird’ effect have been accomplished [2], each with their own intriguing plasmonic functionalities. Such origami-inspired self-folding plasmene sheets would undoubtedly deepen our fundamental understanding of nanoparticle assembly and nanoparticle plasmonic interactions, as well as catalyze important developments of next-generation devices and sensors with exotic functions.
Acknowledgements
The authors would like to acknowledge the Australian Research Council’s Discovery projects funding schemes DP120100170 and DP140100052 for financial support. This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Y. Chen acknowledges the support from the National Natural Science Foundation of China(21501027) and Collaborative Innovation Center of Suzhou Nano Science and Technology.
Further reading
[1] S.J. Tan, et al.
Nat. Nanotechnol., 6 (5) (2011), pp. 268–276
[2] K.J.M. Bishop, et al.
Small, 5 (14) (2009), pp. 1600–1630
[3] W. Cheng, et al.
Nat. Mater., 8 (5) (2009), pp. 519–525
[4] J.J. Urban, et al.
Nat. Mater., 6 (2) (2007), pp. 115–121
[5] R.J. Macfarlane, et al.
Science, 334 (6053) (2011), pp. 204–208
[6] A. Dong, et al.
Nature, 466 (7305) (2010), pp. 474–477
[7] K.E. Mueggenburg, et al.
Nat. Mater., 6 (9) (2007), pp. 656–660
[8] Z. Nie, et al.
J. Am. Chem. Soc., 130 (11) (2008), pp. 3683–3689
[9] K.J. Si, et al.
ACS Nano, 8 (11) (2014), pp. 11086–11093
[10] Y. Chen, et al.
Adv. Opt. Mater., 3 (7) (2015), pp. 919–924
[11] K.J. Si, et al.
Anal. Chem., 87 (10) (2015), pp. 5263–5269
[12] K.J. Si, et al.
Adv. Opt. Mater. (2015) http://dx.doi.org/10.1002/adom.201500335