Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedented capabilities in optoelectronic devices, medical treatments and nanoscale chemistry, due to a strong enhancement of light-matter interactions. As we continue to explore novel applications, we require a systematic quantitative method to compare suitability across different geometries and a growing library of materials. In this work, we propose application-specific figures of merit constructed from fundamental electronic and optical properties of each material. We compare 17 materials from four material classes (noble metals, refractory metals, transition metal nitrides, and conductive oxides) considering eight topical LSPR applications. Our figures of merit go beyond purely electromagnetic effects and account for the materials’ thermal properties, interactions with adjacent materials, and realistic illumination conditions. For each application we compare, for simplicity, an optimized spherical antenna geometry and benchmark our proposed choice against the state-of-the-art from the literature. Our propositions suggest the most suitable plasmonic materials for key technology applications and can act as a starting point for those working directly on the design, fabrication, and testing of such devices.

We investigate titanium nitride (TiN) thin film coatings on silicon for CMOS-compatible sub-bandgap charge separation upon incident illumination, which is a key feature in the vast field of on-chip photodetection and related integrated photonic devices. Titanium nitride of tunable oxidation distributions serves as an adjustable broadband light absorber with high mechanical robustness and strong chemical resistivity. Backside-illuminated TiN on p-type Si (pSi) constitutes a self-powered and refractory alternative for photodetection, providing a photoresponsivity of about ∼1 mA/W at 1250 nm and zero bias while outperforming conventional metal coatings such as gold (Au). Our study discloses that the enhanced photoresponse of TiN/pSi in the near-infrared spectral range is directly linked to trap states in an ultrathin TiO2–x interfacial interlayer that forms between TiN and Si. We show that a pSi substrate in conjunction with a few nanometer thick amorphous TiO2–x film can serve as a platform for photocurrent enhancement of various other metals such as Au and Ti. Moreover, the photoresponse of Au on a TiO2–x /pSi platform can be increased to about 4 mA/W under 0.45 V reverse bias at 1250 nm, allowing for controlled photoswitching. A clear deviation from the typically assumed Fowler-like response is observed, and an alternative mechanism is proposed to account for the metal/semiconductor TiO2–x interlayer, capable of facilitating hole transport.

Materials such as W, TiN, and SrRuO (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their stability at high operational temperatures. However, investigation of the reproducibility of the optical properties after thermal cycling between room and elevated temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, SrRuO and SrNbO are investigated to assess their viability for robust refractory plasmonic applications. These results are further compared to the performance of SrMoO reported in literature. Films ranging in thickness from 50 to 105 nm are deposited on MgO, SrTiO and Si substrates by e-beam evaporation, RF magnetron sputtering and pulsed laser deposition, prior to characterisation by means of AFM, XRD, spectroscopic ellipsometry, and DC resistivity. Measurements are conducted before and after annealing in air at temperatures ranging from 300 to 1000° C for one hour, to establish the maximum cycling temperature and potential longevity at elevated temperatures for each material. It is found that SrRuO retains metallic behaviour after annealing at 800° C, while SrNbO undergoes a phase transition resulting in a loss of metallic behaviour after annealing at 400° C. Importantly, the optical properties of TiN and TiON are degraded as a result of oxidation and show a loss of metallic behaviour after annealing at 500° C, while the same is not observed in Au until annealing at 600° C. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions.

Titanium nitride (TiN) continues to prove itself as an inexpensive, robust, and efficient alternative to gold in plasmonic applications. Notably, TiN has improved hot electron harvesting and photocatalytic abilities compared to gold systems, which we recently attributed to the role of oxygen in TiN and its native semiconducting TiO2-x surface layer. Here, we explore the role of localized surface plasmon resonances (LSPRs) on electron harvesting across the TiN/TiO2-x interface and probe the resilience of TiN nanoparticles under high-power laser illumination. To investigate this, we fabricate titanium nitride strips where the lateral confinement allows for polarization-selective excitation of the LSPR. Using ultrafast pumpprobe spectroscopy, optical characterization and Raman vibrational spectroscopy we relate differences and changes observed in the electron behaviour to specific material properties. We observe plasmon-enhanced electron harvesting beyond what is expected resulting from the enhanced absorption of the plasmonic mode. We accredit this to the surface oxide damping the plasmon resonance providing additional nonradiative loss channels. Subsequently, we show that low-power annealing of the surface oxide layer reduces the trap density at the interface and increases the initial harvested electron concentration. The unique properties of TiN make it important in the future development of plasmonic electron harvesting applications.

In the search for alternative plasmonic materials SrMoO3 has recently been identified as possessing a number of desirable optical properties. Owing to the requirement for many plasmonic devices to operate at elevated temperatures however, it is essential to characterize the degradation of these properties upon heating. Here, SrMoO3 thin films are annealed in air at temperatures ranging from 75 -500° C. Characterizations by AFM, XRD, and spectroscopic ellipsometry after each anneal identify a loss of metallic behaviour after annealing at 500° C, together with the underlying mechanism. Moreover, it is shown that by annealing the films in nitrogen following deposition, an additional crystalline phase of SrMoO4 is induced at the film surface, which suppresses oxidation at elevated temperatures. ExperimentalThe SrMoO3 PLD target material was prepared from SrMoO4 powder (99.9% purity) supplied by Alfa Aesar. The powder was placed in propan-2-ol and ball milled at 300 rpm for 20 hrs before evaporating the propan-2-ol by placing the powder in an oven overnight at 60° C. The powder was then reduced in a furnace under 100 mL min -1 gas flow of 5% H2 / 95% N2 at 1400° C for 10 hrs. The powder was then pressed into a target with a density of approximately 4 g cm -3 before sintering under the same gas flow conditions at 1500° C for 12 hours.The SrMoO3 target material was rotated throughout each pulsed laser deposition process and held 60 mm from the SrTiO3 substrate. A KrF excimer laser (240 nm) was used for the deposition of all samples with a repetition rate of 8 Hz and a 10 s relaxation period after every 20 pulses. A laser fluency of 1.2 J cm -2 was used. Vacuum conditions, approximately 1×10 -7 Torr, were used for the deposition of all samples and the substrate temperature was 650° C. The samples were cooled to room temperature after each deposition process at a rate of 10° C min -1 , either in vacuum or following annealing in 500 Torr N2 (6N purity, supplied by BOC), prior to removal from the vacuum chamber. Single side polished 5x5 mm (100) oriented STO substrates with a thickness of 0.5 mm were used.An IONTOF ToF-SIMS 5 instrument was used for SIMS depth profiling of the samples. An area of 100x100 μm 2 was analysed using a 25 keV Bi+ LMIG in high current bunch mode with a beam current of approximately 1 pA. Only negative secondary ions were collected. For depth profiling, a 1 keV Cs+ ion beam with a current of 75 nA was used, giving a sputter crater area of 300x300 μm 2 .The surfaces of the SMO films before and after annealing were characterised using X-ray photoelectron spectroscopy (XPS). The spectra were recorded on a Thermo Scientific K-Alpha+ spectrometer operating at a base pressure of 2x10 -9 mbar. This system incorporates a monochromated, microfocused Al Kα X-ray source (hν = 1486.6 eV) and a 180° double focusing hemispherical analyser with a 2D detector. The X-ray source was operated a 6 mA emission current and 12 kV anode bias. Data were collected at pass energies of 200 eV for survey, and 20 eV for core le...