Browsing by Author "Ostovar, Behnaz"

Now showing 1 - 4 of 4
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    Light emission from plasmonic nanostructures
    (AIP Publishing LLC, 2021) Cai, Yi-Yu; Tauzin, Lawrence J.; Ostovar, Behnaz; Lee, Stephen; Link, Stephan
    The mechanism of light emission from metallic nanoparticles has been a subject of debate in recent years. Photoluminescence and electronic Raman scattering mechanisms have both been proposed to explain the observed emission from plasmonic nanostructures. Recent results from Stokes and anti-Stokes emission spectroscopy of single gold nanorods using continuous wave laser excitation carried out in our laboratory are summarized here. We show that varying excitation wavelength and power change the energy distribution of hot carriers and impact the emission spectral lineshape. We then examine the role of interband and intraband transitions in the emission lineshape by varying the particle size. We establish a relationship between the single particle emission quantum yield and its corresponding plasmonic resonance quality factor, which we also tune through nanorod crystallinity. Finally, based on anti-Stokes emission, we extract electron temperatures that further suggest a hot carrier based mechanism. The central role of hot carriers in our systematic study on gold nanorods as a model system supports a Purcell effect enhanced hot carrier photoluminescence mechanism. We end with a discussion on the impact of understanding the light emission mechanism on fields utilizing hot carrier distributions, such as photocatalysis and nanothermometry.
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    Light Emission in Plasmonic Nanostructures
    (2019-12-06) Ostovar, Behnaz; Link, Stephan
    Recently, photoinduced light emission from plasmonic nanoparticles has attracted considerable interest within the scientific community because of its potential applications in novel sensing, imaging and most recently in nanothermometry. The light emission from plasmonic nanoparticles has been assigned to the radiative recombination of hot carriers through inter- and intra-band transitions enhanced by surface plasmons. To precisely study the effects of the size on the light emission of nanoparticles, we conducted a systematic size dependent study of gold nanorods with similar aspect ratios and varying widths. Using our single particle photoluminescence and scattering spectroscopy along with correlated SEM images and FDTD simulations, we calculated the quantum yield and Purcell enhancement factors of individual gold nanorods. Our results register a strong size dependence of quantum yield in gold nanorods suggesting higher emission efficiency in smaller gold nanorods compared to that of large ones. Furthermore, our calculations allowed us to separate the contribution of inter- and intra-band transitions into the emission spectra of individual gold nanorods. We observed an increase in the contributions of the geometry-assisted intraband transitions in the emission of 20-30 nm wide gold nanorods. Our FDTD simulations of the electric field distribution of gold nanorods in the near field also demonstrates that electric field confinement is 2.3 times stronger in smaller gold nanorods. Such confinement can provide the necessary momentum mismatch to excite electrons efficiently through intraband transitions. This prediction agrees well with our experimental observations.
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    Mechanism for plasmon-generated solvated electrons
    (PNAS, 2023) Al-Zubeidi, Alexander; Ostovar, Behnaz; Carlin, Claire C.; Li, Boxi Cam; Lee, Stephen A.; Chiang, Wei-Yi; Gross, Niklas; Dutta, Sukanya; Misiura, Anastasiia; Searles, Emily K.; Chakraborty, Amrita; Roberts, Sean T.; Dionne, Jennifer A.; Rossky, Peter J.; Landes, Christy F.; Link, Stephan; Center for Adapting Flaws into Features
    Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions.
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    Plasmon-Mediated Carrier Dynamics in Metal Nanoparticles and Hybrid Nanostructures
    (2022-05-03) Ostovar, Behnaz; Link, Stephan
    Novel nanomaterials have attracted considerable attention in nanophononics, aiming to advance the development of light-driven applications. Metallic plasmonic nanoparticles offer highly tunable optical properties and create energetic electron-hole pairs, making them ideal for building blocks of light-harvesting applications such as solar cells and photocatalysis. To efficiently use these unique properties and engineer plasmon-based devices, it is essential to understand the optical properties and dynamics of such plasmonic nanostructures and their hybrid heterostructures. In this thesis, I studied the optical properties and ultrafast dynamics of novel nanomaterials, including emerging aluminum nanostructures, gold nanorods, and novel gold nanorod-semiconductor core-shell heterostructures. I utilized single-particle microscopy and transient ultrafast spectroscopy techniques that allow for detailed investigation of transient dynamics and optical properties of individual nanoobjects. In the first part of this thesis, aluminum nanostructures are studied as a great plasmonic candidate due to their broad wavelength tunability and high natural abundance. I investigated the ultrafast dynamics of single aluminum nanocrystals and discovered the effects of crystallinity on their coherent phonon modes, resulting from substantially varying degrees of nanoparticle crystallinity. Based on the strong dependence on crystallinity, I established that highly crystalline aluminum nanoparticles are the best candidates for applications based on the optomechanical properties of plasmonic nanoparticles. Next, I investigated the radiative optical properties of plasmonic gold nanorod using correlated single-particle dark-field scattering and photoluminescence spectroscopy techniques. I demonstrated the effect of size on the photoluminescence of gold nanorods and determined that enhanced intraband transitions in the smallest gold nanorods yield the largest photoluminescence quantum yields. I established that single-particle photoluminescence spectroscopy can be used to probe hot carrier distributions and their relaxation pathway. In the third part of the thesis, I investigated the charge transfer mechanism in metal-semiconductor heterostructures by combining their ultrafast transient dynamics and steady-state optical properties. The results acquired in this part revealed the critical details regarding electron transfer pathways from metal to the semiconductor and their injection efficiencies. I quantified the relative contribution of the sequential transfer of hot electrons and direct metal-to-semiconductor interfacial charge transfer pathways. I conclusively showed the direct electron transfer pathway through chemical interface damping and found that about 50% of the total injected electrons are transferred through this direct pathway. These results are crucial for advancing our understanding of electron transfer in metal-semiconductor heterostructures, holding a great promise for improving the efficiency of light-harvesting systems towards generating currents or driving chemical reactions with visible illumination.