Unprecedented Thermal Stability of Plasmonic Titanium Nitride Films up to 1400 °C

Abstract

Titanium nitride (TiN) has emerged as one of the most promising refractory materials for plasmonic and photonic applications at high temperatures due to its prominent optical properties along with mechanical and thermal stability. From a high temperature standpoint, TiN is a substitution for Au and Ag in the visible to near‐infrared wavelength range, with potential applications including thermophotovoltaics, thermoplasmonics, hot‐electron and high temperature reflective coatings. However, the optical properties and thermal stability of TiN films strongly depend on the growth conditions, such as temperature, partial pressure of the reactive ion gas, ion energy, and substrate orientation. In this work, epitaxial TiN films are grown at 835 °C on an Al2O3 substrate using a radio frequency sputtering method. The oxidization behavior of TiN is investigated at 1000 °C under a medium vacuum condition of 2 × 10–3 mbar, which is relevant for practical technical applications, and the thermal stability at 1400 °C under a high vacuum condition of 2 × 10–6 mbar. The TiN film structure shows an unprecedented structural stability at 1000 °C for a minimum duration of 2 h under a medium vacuum condition, and an exceptional thermal stability at 1400 °C, for 8 h under a high vacuum condition, without any protective coating layer. The work reveals, for the first time to the authors’ knowledge, that the TiN film structure with columnar grains exhibits remarkable thermal stability at 1400 °C due to low‐index interfaces and twin boundaries. These findings unlock the fundamental understanding of the TiN material at extreme temperatures and demonstrate a key step towards fabricating thermally stable photonic/plasmonic devices for harsh environments.

Designing two-dimensional temperature profiles using tunable thermoplasmonics

“…The long-term thermal stability of continuous TiN thin films has been the subject of intense investigations. 22,25,26,55 Despite the fact that epitaxial TiN thin films exhibit unprecedented thermal stability up to 1400 °C in vacuum, 56 as-grown TiN thin films produced through dc magnetron sputtering are prone to oxidation in air at lower temperatures.…”

Au–Ag Alloy Nanocorals with Optimal Broadband Absorption for Sunlight-Driven Thermoplasmonic Applications

“…Sunlight-driven thermoplasmonic applications demand for a set of requisites that are not easily satisfied by conventional plasmonic nanostructures. ,,, The solar spectrum extends from the near UV to the NIR, with 87.7% of energy comprised in the 350–1350 nm range and the 52.4% at wavelengths >700 nm. ,, Thus, panchromatic absorption in this wide range is a first important requisite, typically difficult to achieve in noble metal nanostructures without a simultaneous increase of size and scattering cross section. ,,, Photostability and chemical stability are other important features often limiting the exploitation of anisotropic metal nanoparticles obtained by chemical reduction with templating agents. ,, This is due to a generally high surface energy and the tendency to reshaping into compact spheroidal morphologies, either in the dark or at low illumination intensity. ,,, More inert plasmonic materials, like nitrides, have lower absorption cross section per unit volume compared to noble metals, and are seldom processable as a colloidal solution, as desirable for inclusion in matrixes and substrates. ,,, They also do not benefit of the easy surface chemistry of noble metals, which are functionalizable in one step with thiolated molecules. ,, The ability to conjugate metal nanoparticles with functional organic molecules is crucial for maintaining colloidal stability in complex liquid environments such as electrolyte solutions, biological fluids, or organic solvents. ,, Surface functionalization is key also for the addition of selectivity versus target chemical species and the formation of surface patterns or integration in specific matrixes. ,,, …”

“…24,25,35,65 More inert plasmonic materials, like nitrides, have lower absorption cross section per unit volume compared to noble metals, and are seldom processable as a colloidal solution, as desirable for inclusion in matrixes and substrates. 3,9,24,66 They also do not benefit of the easy surface chemistry of noble metals, which are functionalizable in one step with thiolated molecules. 1,24,35 The ability to conjugate metal nanoparticles with functional organic molecules is crucial for maintaining colloidal stability in complex liquid environments such as electrolyte solutions, biological fluids, or organic solvents.…”

Transition Metal Nitrides Are Heating Up the Field of Plasmonics

“…Eighty-nanometerthick TiN films grown epitaxially on an Al 2 O 3 substrate were demonstrated to be structurally stable at 1400 °C after an initial increase in the grain size when annealed under medium to high vacuum conditions. 83 Studies on laser-induced photothermal reshaping of single ZrN nanocubes showed thermal stability in the range of 1127−1827 °C for these nanostructures (Figure 3A and B), which is higher than that of Au NPs (307 °C). 84 TiN, ZrN, and HfN NPs dropcast onto an Si wafer showed minimal sintering upon annealing it to 1000 °C under an Ar atmosphere.…”

“…As such, it is crucial to experimentally establish the thermal stability of nanoscale metal nitrides. Eighty-nanometer-thick TiN films grown epitaxially on an Al 2 O 3 substrate were demonstrated to be structurally stable at 1400 °C after an initial increase in the grain size when annealed under medium to high vacuum conditions . Studies on laser-induced photothermal reshaping of single ZrN nanocubes showed thermal stability in the range of 1127–1827 °C for these nanostructures (Figure A and B), which is higher than that of Au NPs (307 °C) .…”