Nonlinear Refractory Plasmonics with Titanium Nitride Nanoantennas

283

204

absorption. Although photocatalysis is already used in small-scale abatement of both indoor and outdoor pollution, the low efficiency (5%-10%) of solar-to-chemical energy conversion limits further breakthroughs in strategic applications such as hydrogen production and photovoltaics (PVs). [1,2] The engineering of composite interfaces for optimized operation is a key step to increase the solar absorption, electron-hole separation, and collection, and finally to provide high reaction yield. [3,4] The use of plasmonic nanostructures in energy conversion applications has been recently demonstrated with the enhanced performance of solar-to-fuels and solar-to-electricity devices. [5][6][7][8][9][10][11] Surface plasmons concentrate light into nanoscale volumes at the interface of a dielectric or a semiconductor providing intense electromagnetic field localization and improved scattering. The possibility to harness hot electrons generated from the surface plasmon decay [12,13] has shown to be a promising route toward selective nanocatalysis, [14][15][16][17][18][19][20] full-spectrum solar water splitting, [21][22][23] and ultrafast photodetection. [24][25][26][27] The nonradiative decay of surface plasmons, occurring in the 40-150 fs timescale, [12] produces a population of highly energetic (hot) electrons that are stabilized through injection into the semiconductor conduction band across a Schottky barrier. [28] Plasmonic nanoparticles (NPs) enable the efficient conversion of solar light into electrons with an energy well below the band gap of the semiconductor but limited to energies higher than the potential barrier (φ B ). As of yet, these demonstrations have been limited to NPs made of plasmonic noble metals such as Ag and Au. Despite their good optical performance, several challenges remain for noble metal NPs including high cost, poor chemical (Ag) and thermal stability (Au, Ag), diffusion into surrounding structures, and CMOS incompatibility, which hinders the practical implementation of conventional plasmonic structures.Aluminum nanocrystals are alternative plasmonic photocatalysts that provide efficient hot electron generation at low cost (i.e., high abundance of Al on the earth crust). [29][30][31][32][33][34] Nevertheless, issues for large-scale applications are related to Al NPs preparation, due to explosive reactivity of Al molecular The use of hot electrons generated from the decay of surface plasmons is a novel concept that promises to increase the conversion yield in solar energy technologies. Titanium nitride (TiN) is an emerging plasmonic material that offers compatibility with complementary metal-oxide-semiconductor (CMOS) technology, corrosion resistance, as well as mechanical strength and durability, thus outperforming noble metals in terms of cost, mechanical, chemical, and thermal stability. Here, it is shown that plasmonic TiN can inject into TiO 2 twice as many hot electrons as Au nanoparticles. TiO 2 nanowires decorated with TiN nanoparticles show higher photocurrent enhancement than decorat...

Section: Introductionmentioning

confidence: 99%

Broadband Hot‐Electron Collection for Solar Water Splitting with Plasmonic Titanium Nitride

Naldoni

Guler

Wang

et al. 2017

283

204

“…These materials have gained much interest for plasmonic applications for the visible and near‐infrared region due to their tailorable optical properties and refractory quality . TiN has already been shown to have potential applications in photovoltaics, waveguiding, modulators, and nonlinear optical devices . Although some of these devices use films of ≈10 nm, there is additional interest in the formation of ultrathin metallic films (<10 nm) which may facilitate additional applications.…”

Section: Introductionmentioning

confidence: 99%

Optical Properties of Plasmonic Ultrathin TiN Films

Shah

Reddy

Kinsey

et al. 2017

107

101

Theoretical studies have been conducted to comprehend these characteristics in ultrathin gold films. [6,7] However, the fabrication of such thin films is generally quite difficult. For conventional plasmonic metals, such as silver and gold, continuous, smooth films with thicknesses lower than 10 nm are challenging to produce because of island formation and large defect concentrations which result from their high surface energy. [8][9][10] On the other hand, transition metal nitrides (TiN, ZrN, etc.) can be grown epitaxially on substrates such as c-sapphire and MgO, [11] enabling the formation of continuous ultrathin films down to 2 nm while maintaining high quality. [12] These materials have gained much interest for plasmonic applications for the visible and near-infrared region due to their tailorable optical properties and refractory quality. [13] TiN has already been shown to have potential applications in photovoltaics, [14,15] waveguiding, [16] modulators, [17,18] and nonlinear optical devices. [3,19] Although some of these devices use films of ≈10 nm, there is additional interest in the formation of ultrathin metallic films (<10 nm) which may facilitate additional applications. To support this, there have been efforts to grow ultrathin TiN films using atomic layer deposition (ALD). However, these films become dielectric below 5 nm and may not be useful for nanophotonic applications. [20] This is because TiN thin films grown using ALD with a TiCl 4 precursor are more prone to Cl impurities, which could result in a degradation of the metallic properties in thinner films. [21][22][23] In the present study, we have grown continuous, epitaxial, ultrathin TiN films that remain plasmonic in the optical range via DC reactive magnetron sputtering. Using sputtering to grow the film decreases the chance of contamination since only Ti, N, and Ar are introduced into the system, which helps maintain the metallic properties in ultrathin films. The dielectric permittivity of these films was extracted using spectroscopic ellipsometry, while Hall measurements give further insight into the optical behavior of the films. Results and DiscussionUltrathin TiN films with thicknesses of 2, 4, 6, 8, and 10 nm were grown on MgO using DC reactive magnetron sputtering. Due to the lattice matching between TiN (4.24 Å) and substrates such as MgO (4.21 Å) and sapphire (4.76 Å), high quality,Overcoming the challenge of growing ultrathin metallic films is of great importance for practical applications of nanoplasmonic structures. In the present work, epitaxial, ultrathin (<10 nm) films of plasmonic TiN are grown on MgO using DC reactive magnetron sputtering. The optical properties of the films are studied through variable angle spectroscopic ellipsometry and Hall measurements. As the film thickness decreases, they become less metallic and exhibit higher loss while still remaining plasmonic in the optical range. These trends are related to the decreasing carrier concentration in the thinner films and increased scattering, respectively. Howev...

“…Titanium nitride (TiN) has recently emerged as an alternative refractory plasmonic material and gained enormous interest due to high thermal stability, chemical stability, mechanical hardness, and CMOS compatibility . TiN provides unique optical properties in the visible to mid‐infrared regime, and the optical losses of TiN are strongly dependent on process temperature and deposition conditions .…”

mentioning

confidence: 99%

“…TiN provides unique optical properties in the visible to mid‐infrared regime, and the optical losses of TiN are strongly dependent on process temperature and deposition conditions . TiN is one of the highly investigated transition‐metal nitrides with optical properties similar to Au, and its potential applications are in solar heat transducers, solar water splitting, hot electron excitation, broadband photodetectors, waveguiding, modulators, nonlinear optical devices, and particularly in high‐temperature applications, such as, STPV (broadband absorber and thermal emitter) . However, the applicability of TiN material in plasmonics is significantly limited by the tedious high‐temperature fabrication procedure, and the use of lithography tools (e‐beam and laser interference lithography) is expensive and time‐consuming, and cannot currently be realized to wafer‐scale fabrication methods .…”

mentioning

confidence: 99%

Large‐Area Ultrabroadband Absorber for Solar Thermophotovoltaics Based on 3D Titanium Nitride Nanopillars

Chirumamilla

Yang

et al. 2017

136

Refractorymetal-based broadband absorber/narrowband emitters is a flourishing field in energy harvesting where the physical and chemical stability of the metals at high temperatures provide efficient absorption/emission of solar/ heat energy. [1][2][3] Advancements in solar/ thermophotovoltaics (S/TPV) must be accompanied by thermally stable devices in order to withstand extreme operating conditions. The fundamental limiting factor [Shockley Queisser (SQ) efficiency limit] in traditional single-junction solar cells is its inability in converting the broad solar spectrum into a narrow range of wavelengths defined by the PV cells. [4] Solar photons with energies below the bandgap of the PV cell are not converted, and the photons with energies higher than the bandgap lose their additional energy through a process known as thermalization. In solar thermophotovoltaics (STPV), an intermediate system composed of absorber and emitter is used to overcome the SQ limit by harvesting the solar energy followed by the emission of narrowband radiation. [5] The absorber should provide unitary absorption in the entire solar spectral range over a range of incidence angles with polarization insensitive nature, and minimum thermal reradiation to avoid near-infrared heat radiation at elevated temperatures. Through thermal conduction, the absorbed heat energy is transferred to the emitter, which is tailored to emit a narrowband radiation defined by the bandgap energy of a PV cell. Thus, the absorber needs to withstand high temperatures to transfer a large amount of heat energy to the emitter. The low-loss noble metals are successfully used in unitary absorbers so far, particularly Au and Ag. [6][7][8] However, noble metals are not compatible with hightemperature photovoltaic applications and standard silicon manufacturing processes (complementary metal oxide semiconductor, CMOS, technology), owing to low melting point and diffusion of noble metals into silicon. Annealing the substrates at high temperatures induce oxidation, surface diffusion, corrosion, cracking, and delamination of thin films from the substrate. [7] The situation is even worse in the case of the Broadband absorbers, with the simultaneous advantages of thermal stability, insensitivity to light polarization and angle, robustness against harsh environmental conditions, and large area fabrication by scalable methods, are essential elements in (solar) thermophotovoltaics. Compared to the noble metal and multilayered broadband absorbers, high-temperature refractory metal-based nanostructures with low-Q resonators are reported less. In this work, 3D titanium nitride (TiN) nanopillars are investigated for ultrabroadband absorption in the visible and near-infrared spectral regions with average absorptivities of 0.94, over a wide range of oblique angles between 0° and 75°. The effect of geometrical parameters of the TiN nanopillars on broadband absorption is investigated. By combining the flexibility of nanopillar design and lossy TiN films, ultrabroadband absorption in the vi...