Transition metal nitrides (TMNs) have emerged as promising alternative materials for plasmonic and optoelectronic applications in the visible and near-IR spectral ranges. However, the deposition of TMNs with optical properties suitable for plasmonic applications typically requires high temperatures (>800 °C). In this work, we use high-power impulse magnetron sputtering (HIPIMS) to deposit plasmonic TiN thin films at room temperature, without intentional heating. HIPIMS increases the energies of metal ions and dissociated nitrogen (N1+) in the flux, enabling the low-temperature deposition of high-quality TiN thin films on a range of industrially relevant substrates, including flexible polymer substrates. We demonstrate that the room-temperature deposition method can be used to produce plasmonic TiN nanoarrays via colloidal lithography, with tunable shapes and dimensions, creating features on the order of 100-500 nm. We characterize the optical response and plasmonic performance of these nanoarrays, demonstrating tailorable resonances in the visible and NIR spectral range, in agreement with optical simulations reported here.

Plasmonic nanostructures are widely utilized in fields such as chemical sensing, photothermal therapy, and optoelectronics due to their ability to confine and enhance electromagnetic fields at the nanoscale. Among these, nanorings offer unique advantages owing to their hollow-core geometry, which supports multiple plasmonic modes and enables efficient analyte access. However, conventional plasmonic materials such as gold, silver, or copper suffer from poor thermal stability due to either oxidation or morphological degradation, rendering them unsuitable for high temperatures or chemically harsh environments. In this study, we investigate titanium nitride (TiN) as a robust alternative for the fabrication of thermally stable plasmonic nanorings. TiN combines metallic optical behavior in the visible to near-infrared range with excellent thermal, mechanical, and chemical stability, making it particularly suitable for harsh-environment sensing applications. Using Hole-mask Colloidal Lithography (HCL), a scalable and cost-effective bottom-up technique, we successfully fabricate well-defined TiN nanorings over large substrate areas. We further evaluated their structural and spectral resilience through annealing experiments conducted up to 400 °C in air. The results demonstrate that TiN nanorings maintain their morphology and localized surface plasmon resonance (LSPR) characteristics under elevated temperatures, in stark contrast to their noble-metal counterparts. This work establishes a reproducible, scalable route for producing refractory plasmonic nanostructures and highlights the potential of TiN nanorings for robust operation in high-temperature sensing platforms.

Gold nanoparticle (AuNP) decoration is a commonly used method to enhance the optical responses in many applications such as photocatalysis, biosensing, solar cells, etc. The morphology and structure of AuNPs are essential factors determining the functionality of the sample. However, tailoring the growth mechanism of AuNPs on an identical surface is not straightforward. In this study, AuNPs were deposited on the surface of a perovskite thin film, strontium niobate (SNO), using pulsed laser deposition (PLD). AuNPs exhibited a dramatic variation in their growth mechanisms, depending on whether they were deposited on SNO thin films grown on magnesium oxide (SNO/MgO) or strontium titanate (SNO/STO) substrates. On SNO/MgO, the Au aggregates form large NPs with an average size of up to 3500 nm2. These AuNPs are triangular with sharp edges and corners. The out-of-plane direction of growth is favored, and the surface coverage ratio by AuNPs is low. When deposited on SNO/STO, the average size of AuNPs is much smaller, i.e., ∼250 nm2. This reduction in the average size is accompanied by an increase in the number density of NPs. AuNPs on SNO/STO have a round shape and high coverage ratio. Such an impact from the substrate selection on the AuNP structure is significant when the sandwiched SNO film is below 80 nm thickness and is weakened for 200 nm of SNO films. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize all samples. Strain analysis was used to explain the growth mechanism of AuNPs. The average height of AuNPs was measured by using atomic force microscopy (AFM). Ellipsometry in the visible-near-infrared (vis-NIR) region was used to characterize the optical response of all samples. AuNP-decorated SNO/MgO and SNO/STO thin films exhibit different optical properties, with only gold-decorated SNO/MgO samples showing a size-dependent epsilon-near-zero behavior of nanoparticles. These results provide an additional route to control the structure of AuNPs. They can be used for various plasmonic applications like the design and development of strain-engineered gold-nanoparticle-decorated devices for surface-enhanced Raman spectroscopy (SERS) and photocatalysis.

Titanium nitride (TiN) nanostructures were manufactured by depositing a thin film on a needle shaped structure of black silicon. The resulting TiN needle structure was characterized by scanning electron microscopy and UV-vis spectroscopy. The surface enhanced Raman scattering (SERS) properties of the created substrate was assessed using a probe molecule (Rhodamine 6G) showing strong enhancement of the original Raman signal on the order of 10 2 . The created TiN surfaces showed to have promising Raman enhancement properties to create SERS substrates for sensing applications and being alternative for the most used materials such as gold or silver.

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where sub-bandgap photons can be excited and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO2 and titanium oxynitride/TiO2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO2, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO0.5N0.5) exhibits the highest carrier extraction efficiency, slowest trapping and an appreciable hot electron population reaching the surface oxide. Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and elongating electron lifetimes providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Refractory plasmonic materials are of interest for high-temperature plasmonic applications due to their increased thermal stability when compared to gold and silver. Titanium nitride (TiN) has been highlighted as a promising refractory material, offering both strong plasmonic and thermal performance. In this work, we analyze the stability of both the structural and optical response of individual plasmonic nanodiscs of various diameters subjected to elevated temperature conditions in air. Using cathodoluminescence spectroscopy, we trace the resonance spectra and shape modifications of the same single TiN and Au discs annealed at increasing temperatures up to 325 °C. TiN discs display greater morphological stability, but the optical properties of both materials deteriorate from 200 °C, although the mechanisms of degradation are different. The results are essential for optimizing nanostructured materials for high temperature nanophotonic applications.

The performance of graphene devices is often limited by defects and impurities induced during device fabrication. Polymer residue left on the surface of graphene after photoresist processing can increase electron scattering and hinder electron transport. Furthermore, exposing graphene to plasma-based processing such as sputtering of metallization layers can increase the defect density in graphene and alter the device performance. Therefore, the preservation of the high-quality surface of graphene during thin-film deposition and device manufacturing is essential for many electronic applications. Here, we show that the use of self-assembled monolayers (SAMs) of hexamethyldisilazane (HMDS) as a buffer layer during the device fabrication of graphene can significantly reduce damage, improve the quality of graphene, and enhance device performance. The role of HMDS has been systematically investigated using surface analysis techniques and electrical measurements. The benefits of HMDS treatment include a significant reduction in defect density compared with as-treated graphene and more than a 2-fold reduction of contact resistance. This surface treatment is simple and offers a practical route for improving graphene device interfaces, which is important for the integration of graphene into functional devices such as electronics and sensor devices.

Refractory transition metal nitrides and oxynitrides are of interest for plasmonic applications due to their increased thermal stability, tunability and comparatively low loss. Presented here is an experimental investigation of reactively sputtered niobium oxynitride thin films, demonstrating screened plasma wavelengths tunable over a similar to 90 nm spectral range. The optical response is correlated with resistivity and Hall measurements. Additionally, an explanation is provided for the double epsilon-near-zero (2ENZ) behavior observed and it is compared with other material systems. Experimental analysis of film composition and structure are combined with computational modelling, based on density functional theory, to explain the dispersion behavior observed and indicate that 2ENZ behavior arises due to the incorporation of oxygen within the films during deposition.

Hot carrier based methods constitute a valuable approach for efficient and silicon compatible sub-bandgap photodetection. Although, hot electron excitation and transfer have been studied extensively on traditional materials such as Au and Ti, reports on alternative materials such as titanium nitride (TiN) are rare. Here, we perform hot hole photodetection measurements on a p-Si/metal thin film junction using Ti, Au and TiN. This material is of interest as it constitutes a refractory alternative to Au which is an important property for plasmonic applications where high field intensities can occur. In contrast to Au, a TiN/Si junction does not suffer from metal diffusion into the Si, which eases the integration with current Si-fabrication techniques. This work shows that a backside illuminated p-Si/TiN system can be used for efficient hot hole extraction in the IR, allowing for a responsivity of 1 mA/W at an excitation wavelength of 1250 nm and at zero bias. Via a comparison between TiN and other commonly used materials such as Au, the origin of this comparably high photoresponse can be traced back to be directly linked to a thin TiO_2-x interfacial layer allowing for a distinct hot-hole transfer mechanism. Moreover, the fabrication of TiN nanodisk arrays is demonstrated which bears great promise to further boost the device efficiency.

Transition metal nitrides have received significant interest for use within plasmonic and optoelectronic devices. However, deposition temperature remains a significant barrier to the integration of transition metal nitrides as plasmonic materials within CMOS fabrication processes. Binary, ternary and layered transition metal nitride thin films based on titanium and niobium nitride are deposited using High Power Impulse Magnetron Sputtering (HIPIMS). The increased plasma densities achieved in the HIPIMS process allow high quality plasmonic thin films to be deposited at CMOS compatible temperatures of less than 300C. Thin films are deposited on a range of industrially relevant substrates and display tunable plasma frequencies in the ultraviolet to visible spectral ranges. The thin film quality, combined with the scalability of the deposition process, indicates that HIPIMS deposition of nitride films is an industrially viable technique and can pave the way towards the fabrication of next generation plasmonic and optoelectronic devices.

Transition-metal nitrides have received significant interest for use within plasrnonic and optoelectronic devices because of their tunability and environmental stability. However, the deposition temperature remains a significant barrier to widespread adoption through the integration of transition-metal nitrides as plasmonic materials within coinplementary metal oxide-semiconductor (CMOS) fabrication processes. Binary, ternary, and layered plasmonic transition-metal nitride thin films based on titanium and niobium nitride are deposited using high-power impulse magnetron sputtering (HIPIMS) technology. The increased plasma densities achieved in the HIPIMS process allow thin films with high plasmonic quality to be deposited at CMOS-compatible temperatures of less than 300 degrees C. Thin films are deposited on a range of industrially relex'ant substrates and display-tunable plasma frequencies in the ultraviolet to visible spectral ranges. Strain-mediated tunability is discovered in layered films compared to that in ternary films. The thin film quality, combined with the scalability of the deposition process, indicates that HIPIMS deposition of nitride films is an industrially viable technique and can pave the way toward the fabrication of next-generation plasmonic and optoelectronic devices.

Titanium oxynitride enables a range of plasmonic and optoelectronic functionality using long-lived photo-generated hot carriers. We explore the time scale of hot carriers in TiN and their use in photochemical reduction and Schottky detectors.

This Conference Presentation, "Oxide-enhanced IR hot-carrier-based photo detection in metal thin-film Si junctions" was recorded at Photonics West 2020 held in San Francisco, California, United States.

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 similar to 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.

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 nanostructures under high-power laser illumination. To investigate this, we fabricate TiN strips, in which the lateral confinement allows for the polarization-selective excitation of the LSPR. Using ultrafast pump-probe spectroscopy, optical characterization, and Raman vibrational spectroscopy, we relate the differences and changes observed in the electron behavior 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 degrees C. Characterizations by AFM, XRD, and spectroscopic ellipsometry after each anneal identify a loss of metallic behaviour after annealing at 500 degrees 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's surface, which suppresses oxidation at elevated temperatures. Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License.

Materials such as W, TiN, and SrRuO3 (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, SrRuO3 and SrNbO3 are investigated to assess their viability for robust refractory plasmonic applications. These results are further compared to the performance of SrMoO3 reported in literature. Films ranging in thickness from 50 to 105 nm are deposited on MgO, SrTiO3 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 SrRuO3 retains metallic behaviour after annealing at 800° C, while SrNbO3 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.

Materials such as W, TiN, and SrRuO3 (SRO) have been suggested as promising alternatives to Au and Ag in plasmonic applications owing to their refractory properties. However, investigation of the reproducibility of the optical properties after thermal cycling at high operational temperatures is so far lacking. Here, thin films of W, Mo, Ti, TiN, TiON, Ag, Au, and SrRuO3 are investigated to assess their viability for robust refractory plasmonic applications. Films ranging in thickness from 50 - 180 nm are deposited on MgO and Si substrates by RF magnetron sputtering and, in the case of SrRuO3, 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 - 1000 C for one hour, to establish the maximum cycling temperature and potential longevity at temperature for each material. It is found that SrRuO3 retains metallic behaviour after annealing at 800 C, however, importantly, the optical properties of TiN and TiON are degraded as a result of oxidation. Nevertheless, both TiN and TiON may be better suited than Au or SRO for high temperature applications operating under vacuum conditions.