Strain-Driven In-plane Ordering in Vertically Aligned ZnO-Au Nanocomposites with Highly Correlated Metamaterial Properties.

Robynne L Paldi,Haiyan Wang,Xinghang Zhang,Xing Sun,Xuejing Wang

doi:10.1021/acsomega.9b03356

Abstract

Hyperbolic metamaterials demonstrate exotic optical properties that are poised to find applications in subdiffraction imaging and hyperlenses. Key challenges remain for practical applications, such as high energy losses and lack of hyperbolic properties in shorter wavelengths. In this work, a new oxide–metal (ZnO–Au) hybrid-material system in the vertically aligned nanocomposite thin-film form has been demonstrated with very promising in-plane two-phase ordering using a one-step growth method. Au nanopillars grow epitaxially in the ZnO matrix, and the pillar morphology, orientation, and quasi-hexagonal in-plane ordering are found to be effectively tuned by the growth parameters. Strong surface plasmon resonance has been observed in the hybrid system in the UV–vis range, and highly anisotropic dielectric properties have resulted with much broader and tunable hyperbolic wavelength regimes. The observed strain-driven two-phase in-plane ordering and its novel tunable hyperbolic metamaterial properties all demonstrate strong potential for future oxide–metal hybrid-material design toward future integrated hybrid photonics.

Highlights

Optical metamaterials with epsilon near-zero (ENZ) permittivity and hyperbolic dispersion can be created in nanostructures that consist of periodic arrangements of negative permittivity
Oxidation was determined not to occur, as there was no presence of Au−oxide peaks in any X-ray diffraction (XRD) scans in Figure 2e or 3e or any diffraction spots besides those from Au, ZnO, or sapphire in selective area electron diffraction (SAED) patterns in Supporting Information Figure S1a,b
When growth was performed in vacuum, this led to irregular morphology and films that would wipe away caused by poor adhesion, possibly due to high laser plume kinetic energy

Summary

■ INTRODUCTION

Optical metamaterials with epsilon near-zero (ENZ) permittivity and hyperbolic dispersion can be created in nanostructures that consist of periodic arrangements of negative permittivity. VANs have demonstrated their potential in nanoscale metamaterial design through robust material selection and highly anisotropic optical properties.[20,21] The unique vertical strain coupling along the two-phase interface has led to unique multifunctionalities.[22−26] extensive work on morphology tuning and strain engineering has been focused on oxide−oxide VAN systems and related properties.[22,27−29] The work on strain engineering and morphology tuning in oxide−metal systems is scarce.[30,31]. Through the morphology and inplane ordering control, it is expected to achieve tunable optical responses such as hyperbolic behavior and wavelength range, anisotropic complex dielectric properties, and plasmonic wavelength. X-ray (EDX) mapping are conducted to couple with the optical properties measured by optical transmittance and ellipsometry

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES