A Hybrid Nanofabrication Route for the Synthesis of Highly Ordered Large-Area Arrays of Planar and Chiral Nanostructures

Plasmonic noble metal nanostructures exhibit unique light-matter interactions which are dependent on their nanometer-scale dimensions and which cannot be realized on the macro-scale. In recent years, applications have been found that capitalize on these properties in the development of optoelectronic components, highly sensitive biological and chemical sensors, metasurfaces, and photochemistry. These applications have a strong dependence on nanostructure geometry and size, as well as the ability to place structures on a surface in an organized manner for the fabrication of functional devices. Electron beam lithography has long been the most powerful method for fabricating surfaces of plasmonic nanostructures and is capable of generating complex shapes with a high degree of orientational and placement control on the substrate surface. However, it exhibits notable weaknesses in (i) a highly time-intensive and technically demanding serial writing process, (ii) the relative scarcity and high expense of equipment, (iii) a difficulty to produce monocrystalline structures when positive resists are used, (iv) limits in resolution that present challenges in the fabrication of plasmonic structures with sharp tips and nanogaps, and (v) an inability to produce structures with anything but straight-walled two-dimensional profiles without increasingly complicated processes and instrumentation. Alternative low-cost lithographic processes have been devised in attempts to improve the accessibility and throughput of array-based nanostructures, but likewise fall short in the ability to generate complex structures in organized arrays with true long-range order. Other hybrid processes capitalize on the ability of colloidal syntheses to produce complex, highly crystalline nanostructures, followed by the use of a variety of techniques that place structures at lithographically defined locations on a substrate surface. While impressive, these methods are limited by the homogeneity of the colloid, exhibit a reliance on electron beam lithography, and are challenged in providing consistent alignment for asymmetric structures. Herein a low-cost, rapid processing route is put forth to address the shortcomings of existing processes that provides (i) a high degree of placement control over 1 cm^2 areas through the use of nanoimprint lithography, (ii) orientational control and a high crystallinity of nanostructures through the establishment of an epitaxial relationship between the seed and crystalline substrate using high-temperature vapor-phase assembly, and (iii) geometric control of sophisticated structures through the use of liquid-phase chemical syntheses. This process is demonstrated through the synthesis of large-area arrays of Au structures on sapphire substrates using plasmon-mediated liquid-phase syntheses to grow hexagonal and triangular nanoplates on the substrate surface, as well as chiral nanostructures exhibiting a spiral geometry. As shown via TEM cross-sectional and top-down analysis, it is demonstrated that Au seeds can be formed on the substrate with stacking faults running parallel to the substrate surface. This crystalline structure favors highly crystalline planar growth along the substrate surface which can also be directed along additional symmetry-breaking pathways to produce asymmetric three-dimensional structures. This work represents a first-of-its kind demonstration of an on-substrate liquid-phase synthesis of oriented arrays of highly crystalline Au nanoplates and chiral nanostructures, which have the potential to forward viable nanomanufacturing technologies for the development of functional plasmonic devices.