Sorption-Induced Fiber Optic Plasmonic Gas Sensing via Small Grazing Angle of Incidence.

Sensing technologies based on plasmonic nanomaterials are of interest for various chemical, biological, environmental, and medical applications. In this work, an incorporation strategy of colloidal plasmonic nanoparticles (pNPs) in microporous polymer for realizing distinct sorption-induced plasmonic sensing is reported. This approach is demonstrated by introducing tin-doped indium oxide pNPs into a polymer of intrinsic microporosity (PIM-1). The composite film (pNPs-polymer) provides distinct and tunable optical features on the fiber optic (FO) platform that can be used as a signal transducer for gas sensing (e.g., CO2 ) under atmospheric conditions. The resulting pNPs-polymer composite demonstrates high sensitivity response on FO in the evanescent field configuration, provided by the dramatic response of modes above the total-internal-reflection angle. Furthermore, by varying the pNPs content in the polymer matrix, the optical behavior of the pNPs-polymer composite film can be tuned to affect the operational wavelength by over several hundred nanometers and the sensitivity of the sensor in the near-infrared range. It is also shown that the pNPs-polymer composite film exhibits remarkable stability over a period of more than 10 months by mitigating the physical aging issue of the polymer.

Assessing the Effects of Temperature and Oxygen Vacancy on Band Gap Renormalization in LaCrO3-δ: First-Principles and Experimental Corroboration.

Understanding the temperature dependence of functional properties in high-temperature gas sensors is vital for applications in combustion environments. Temperature effect on the electronic structure due to electron-phonon coupling is a key property of interest as this influences other responses of sensors. In this work, we assess the impact of temperature on band gap renormalization of pristine and oxygen-vacant LaCrO3-δ perovskite employing Allen-Heine-Cardona theory with first-principles simulations and corroborate with experimental observation. Antiferromagnetic cubic LaCrO3 shows a direct ground-state band gap of 2.62 eV that is reduced by over 1 eV due to the presence of oxygen vacancies, which can form endothermically. We find excellent agreement in temperature-dependent band gap shift in LaCrO3 between theory and an in-house experiment, proving that the theory can adequately predict renormalization on the band gap in a magnetic system. Band gaps in cubic LaCrO3-δ are found to monotonically narrow by 1.13 eV in pristine and by around 0.62 eV in oxygen-vacant structures as temperature increases from 0 to 1500 K. The predicted band gap variations are rationalized using an analytical model. The experimental zero-temperature band gaps are extracted from the model fits that can provide useful insights on the simulated band gaps.

Fiber Coupled Near-Field Thermoplasmonic Emission from Gold Nanorods at 1100 K.

Nanostructured gold has attracted significant interest from materials science, chemistry, optics and photonics, and biology due to their extraordinary potential for manipulating visible and near-infrared light through the excitation of plasmon resonances. However, gold nanostructures are rarely measured experimentally in their plasmonic properties and hardly used for high-temperature applications because of the inherent instability in mass and shape due to the high surface energy at elevated temperatures. In this work, the first direct observation of thermally excited surface plasmons in gold nanorods at 1100 K is demonstrated. By coupling with an optical fiber in the near-field, the thermally excited surface plasmons from gold nanorods can be converted into the propagating modes in the optical fiber and experimentally characterized in a remote manner. This fiber-coupled technique can effectively characterize the near-field thermoplasmonic emission from gold nanorods. A direct simulation scheme is also developed to quantitively understand the thermal emission from the array of gold nanorods. The experimental work in conjunction with the direct simulation results paves the way of using gold nanostructures as high-temperature plasmonic nanomaterials, which has important implications in thermal energy conversion, thermal emission control, and chemical sensing.

Combined plasmonic Au-nanoparticle and conducting metal oxide high-temperature optical sensing with LSTO.

Fiber optic sensor technology offers several advantages for harsh-environment applications. However, the development of optical gas sensing layers that are stable under harsh environmental conditions is an ongoing research challenge. In this work, electronically conducting metal oxide lanthanum-doped strontium titanate (LSTO) films embedded with gold nanoparticles are examined as a sensing layer for application in reducing gas flows at high temperature (600-800 °C). A strong localized surface plasmon resonance (LSPR) based response to hydrogen is demonstrated in the visible region of the spectrum, while a Drude free electron-based response is observed in the near-IR. Characteristics of these responses are studied both on planar glass substrates and on silica fibers. Charge transfer between the oxide film and the gold nanoparticles is explored as a possible mechanism governing the Au LSPR response and is considered in terms of the corresponding properties of the conducting metal oxide-based matrix phase. Principal component analysis is applied to the combined plasmonic and free-carrier based response over a range of temperatures and hydrogen concentrations. It is demonstrated that the combined visible and near-IR response of these films provides improved versatility for multiwavelength interrogation, as well as improved discrimination of important process parameters (concentration and temperature) through application of multivariate analysis techniques.

Theoretical and experimental study of temperature effect on electronic and optical properties of TiO2: comparing rutile and anatase.

To gain fundamental understanding of the high-temperature optical gas-sensing and light-energy conversion materials, we comparatively investigate the temperature effects on the band gap and optical properties of rutile and anatase TiO2 experimentally and theoretically. Given that the electronic structures of rutile and anatase are fundamentally different, i.e. direct band gap in rutile and indirect gap in anatase, it is not clear whether these materials exhibit different electronic structure renormalizations with temperature. Using ab initio methods, we show that the electron-phonon interaction is the dominant factor for temperature band gap renormalization compared to the thermal expansion. As a result of different contributions from the acoustic and optical phonons, the band gap is found to widen with temperature up to 300 K, and to narrow at higher temperatures. Our calculations suggest that the band gap is narrowed by about 147 meV and 128 meV at 1000 K for rutile and anatase, respectively. Experimentally, for rutile and anatase TiO2 thin films we conducted UV-Vis transmission measurements at different temperatures, and analyzed band gaps from the Tauc plots. For both TiO2 phases, the band gap is found to decrease for temperature above 300 K quantitatively, agreeing with our theoretical results. The temperature effects on the dielectric functions, the refractive index, the extinction coefficient as well as the optical conductivity are also investigated. Rutile and anatase show generally similar optical properties, but differences exist in the long wavelength regime above 600 nm, where we found that the dielectric function of rutile decreases while that of anatase increases with temperature increase.

Anharmonicity Explains Temperature Renormalization Effects of the Band Gap in SrTiO3.

Soft phonon modes in strongly anharmonic crystals are often neglected in calculations of phonon-related properties. Herein, we experimentally measure the temperature effects on the band gap of cubic SrTiO3, and compare with first-principles calculations by accounting for electron-phonon coupling using harmonic and anharmonic phonon modes. The harmonic phonon modes show an increase in the band gap with temperature using either Allen-Heine-Cardona theory or finite-displacement approach, and with semilocal or hybrid exchange-correlation functionals. This finding is in contrast with experimental results that show a decrease in the band gap with temperature. We show that the disagreement can be rectified by using anharmonic phonon modes that modify the contributions not only from the significantly corrected soft modes, but also from the modes that show little correction in frequencies. Our results confirm the importance of soft-phonon modes that are often neglected in the computation of phonon-related properties and particularly in electron-phonon coupling.

Nanostructured sapphire optical fiber embedded with Au nanorods for high-temperature plasmonics in harsh environments.

Sensors for harsh environments must exhibit robust sensing response and considerable thermal and chemical stability. We report the exploration of a novel all-alumina nanostructured sapphire optical fiber (NSOF) embedded with Au nanorods (Au NRs) for plasmonics-based sensing at high temperatures. Temperature dependence of the localized surface plasmon resonance (LSPR) of Au NRs was studied in conjunction with numerical calculations using the Drude model. It was found that LSPR of Au NRs changes markedly with temperature, red shifting and increasing in transmission amplitude as the temperature increases. Furthermore, this variation is highly localized through tunneling by overlapping the near-field of thin cladding and sapphire optical fiber. The NSOF embedded with Au NRs has the potential for sensing in advanced energy generation systems.

Flexible nanograss with highest combination of transparency and haze for optoelectronic plastic substrates.

Transparent polymer substrates have recently received increased attention for various flexible optoelectronic devices. Optoelectronic applications such as solar cells and light emitting-diodes would benefit from substrates with both high transparency and high haze, which increase how much light scatters into or out of the underlying photoactive layers. In this letter, we demonstrate a new flexible nanograss plastic substrate that displays the highest combination of transparency and haze in the literature for polyethylene terephthalate (PET). As opposed to other nanostructures that increase haze at the expense of transparency, our nanograss demonstrates the potential to improve both haze and transparency. Furthermore, the monolithic nanograss may be fabricated in a facile scalable maskless reactive ion etching process without the need for additional lithography or synthesis of nanostructures. Our 9 µm height nanograss sample exhibits a transparency and haze of 92.4% and 89.4%, respectively, and our 34 µm height nanograss displays a transparency and haze of 91.0% and 97.1%, respectively. We also performed durability experiments that demonstrate these nanostructured PET substrates are robust from bending and show similar transmission and haze values after 5000 cycles of bending.