High Quality Ordered Titanium Nitride Nanotriangles and Nanowires for Plasmonic Applications

Novel and cost-efficient fabrication techniques of alternative plasmonic nanostructures pave the pathway for practical applications in various fields such energy harvesting, microelectronics and medicine. Transition metals nitrides (TMNs) emerge as alternative plasmonic nanomaterials suitable for a wide range of applications from photovoltaics to photonics and medicine. The TMNs are conductive ceramics that combine exceptional properties such as substantial electronic conductivity, high melting point (&gt;3000 K) and tunable work function, while the TMNs are particularly stable in hostile chemical environments, high temperature, and strong electric fields, such as in lasers. Among them, Titanium Nitride (TiN) is emerging as significant candidate material for practical plasmonic applications (biosensors, catalysis and photochemistry, solar energy harvesting, photo-detection, and optical storage of information).<br/>In this work, we focus on the low-cost fabrication of TiN nanotriangles with controlled spacing and tunable dimensions (thickness and lateral dimensions) using a combination of Nanosphere Lithography (NSL) and several reactive magnetron sputtering (MS) deposition techniques such as DC, Closed-Field Unbalanced MS or Highly Power Impulse MS (HIPMS) on rigid and flexible substrates and with the aim to study the fundamentals that will unlock the fabrication of high quality TMNs nanotriangles and nanowires for plasmonic applications.<br/>NSL appears as a very promising approach, due to its rapid implementation and compatibility with wafer-scale processes, combines the advantages of both top-down and bottom-up approaches and includes: (a) development of the nanospheres monolayer colloidal mask, (b) deposition of the desired material in the empty space between the nanospheres and (c) removal/lift-off of the nanosphere colloidal mask to “reveal” the deposited material that keeps the ordered patterning of the mask interstices. Specifically, a suspension of monodisperse polystyrene nanospheres (diameter, d=552 nm or d=175 nm) was spin coated on a substrate (such as Si (001), glass, flexible PET) to form the colloidal mask. A UV ozone process was used to confine the triple-junction vias of the polystyrene mask. Subsequently, the selective growth of TiN was made by the above mentioned MS in Ar/N atmosphere by varying the TiN thickness from 10 to 30 nm, while the MS process parameters were also fine-tuned to increase the directionality of deposited species, and the TiN crystal quality (low concentration of point defects). In particular, the substrate-to-target distance was maximized to improve geometrical directionality, and a negative bias voltage during the growth of the TiN was used to “guide” the ionic species deep into the vias between the nanospheres.<br/>The arrays of ordered TiN nanostructures and nanowirers appear after the lift-off of the mask. Atomic Force Microscopy characterization of the samples showed the fabrication of TiN nanotriangles, with low concentration of point defects, similar structure with the continuous TiN films of high electrical conductivity and plasmonic performance, and durability at least up to 400^o C.<br/><br/>Refs.: P. Patsalas, N. Kalfagiannis, S. Kassavetis, G. Abadias, D.V. Bellas, Ch. Lekka, E. Lidorikis, Materials Science and Engineering R 123 (2018) 1–55.<br/>Panos, S., Tselekidou, D., Kassavetis, S., Fekas, I., Arvanitidis, J., Christofilos, D., Karfaridis, D., Dellis, S., Logothetidis, S. and Patsalas, P., Phys. Status Solidi B, 258 (2021) 2000573