Browsing by Author "Zhao, Hangqi"

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    A Comparison of Plasmon-induced and Photoexcited Hot Carriers in Metallic Nanostructures
    (2015-12-02) Zhao, Hangqi; Halas, Naomi; Nordlander, Peter; Link, Stephan
    The incompressible oscillations of electrons in metallic nanostructures, known as surface plasmons, have provided a promising route to increasing light-matter coupling and boosting the efficiency of solar energy conversion in photovoltaic devices. When plasmons decay, energetic electron-hole pairs are created through a non-radiative channel. These hot electrons have found applications in photodetection and photocatalysis but remain poorly understood in terms of mechanisms. In this work1, we made a comprehensive comparison between plasmon-induced hot carrier generation and direct excitations of hot carriers by photon absorption. Using a gold nanowire based hot carrier device, which either forms a Schottky barrier or an Ohmic barrier between nanostructures and a wide-bandgap semiconductor substrate, we are able to distinguish between these two mechanisms of hot carrier generation. We show that plasmon-induced hot electrons have higher energies than directly excited carriers, and can be characterized by the integration of electrical field enhancement within the nanostructures, while photoexcited carriers are correlated with material absorption. Our work paves the way for increasing the energy conversion efficiency by decreasing the Schottky barrier and collecting both the plasmonic and interband photocurrent, which may find wide applications in future photovoltaic devices.
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    A room-temperature mid-infrared photodetector for on-chip molecular vibrational spectroscopy
    (AIP Publishing, 2018) Zheng, Bob; Zhao, Hangqi; Cerjan, Ben; Yazdi, Sadegh; Ringe, Emilie; Nordlander, Peter; Halas, Naomi J.; Laboratory for Nanophotonics
    Infrared (IR) photodetection is of major scientific and technical interest since virtually all molecules exhibit characteristic vibrational modes in the mid-infrared region of the spectrum, giving rise to molecular spectroscopy and chemical imaging in this wavelength range. High-resolution IR spectroscopies, such as Fourier Transform IR spectroscopy, typically require large, bulky optical measurement systems and expensive photodetector components. Here, we present a high-responsivity photodetector for the mid-IR spectral region which operates at room temperature. Fabricated from silicon and aluminum, the photodetection mechanism is based on free carrier absorption, giving rise to a photoresponse rivalling commercially available cooled IR photodetectors. We demonstrate that infrared spectra of molecules deposited on this detector can be obtained by a direct electrical read-out. This work could pave the way for simple, fully integrated chemical sensors and other applications, such as chemical imaging, which would benefit from the combination of mid-IR detection, room-temperature operation, and ultracompact portability.
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    Absorption Spectroscopy of an Individual Fano Cluster
    (American Chemical Society, 2016) Yorulmaz, Mustafa; Hoggard, Anneli; Zhao, Hangqi; Wen, Fangfang; Chang, Wei-Shun; Halas, Naomi J.; Nordlander, Peter; Link, Stephan; Laboratory for Nanophotonics
    Plasmonic clusters can exhibit Fano resonances with unique and tunable asymmetric line shapes, which arise due to the coupling of bright and dark plasmon modes within each multiparticle structure. These structures are capable of generating remarkably large local electromagnetic field enhancements and should give rise to high hot carrier yields relative to other plasmonic nanostructures. While the scattering properties of individual plasmonic Fano resonances have been characterized extensively both experimentally and theoretically, their absorption properties, critical for hot carrier generation, have not yet been measured. Here, we utilize single-particle absorption spectroscopy based on photothermal imaging to distinguish between the radiative and nonradiative properties of an individual Fano cluster. In observing the absorption spectrum of individual Fano clusters, we directly verify the theoretical prediction that while Fano interference may be prominent in scattering, it is completely absent in absorption. Our results provide microscopic insight into the nature of Fano interference in systems of coupled plasmonic nanoparticles and should pave the way for the optimization of hot carrier production using plasmonic Fano clusters.
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    Chiral and Achiral Nanodumbbell Dimers: The Effect of Geometry on Plasmonic Properties
    (American Chemical Society, 2016) Smith, Kyle W.; Zhao, Hangqi; Zhang, Hui; Sánchez -Iglesias, Ana; Grzelczak, Marek; Wang, Yumin; Chang, Wei-Shun; Nordlander, Peter; Liz-Marzán, Luis; Link, Stephan; Laboratory for Nanophotonics
    Metal nanoparticles with a dumbbell-like geometry have plasmonic properties similar to those of their nanorod counterparts, but the unique steric constraints induced by their enlarged tips result in distinct geometries when self-assembled. Here, we investigate gold dumbbells that are assembled into dimers within polymeric micelles. A single-particle approach with correlated scanning electron microscopy and dark-field scattering spectroscopy reveals the effects of dimer geometry variation on the scattering properties. The dimers are prepared using exclusively achiral reagents, and the resulting dimer solution produces no detectable ensemble circular dichroism response. However, single-particle circular differential scattering measurements uncover that this dimer sample is a racemic mixture of individual nanostructures with significant positive and negative chiroptical signals. These measurements are complemented with detailed simulations that confirm the influence of various symmetry elements on the overall peak resonance energy, spectral line shape, and circular differential scattering response. This work expands the current understanding of the influence self-assembled geometries have on plasmonic properties, particularly with regard to chiral and/or racemic samples which may have significant optical activity that may be overlooked when using exclusively ensemble characterization techniques.
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    Distinguishing between plasmon-induced and photoexcited carriers in a device geometry
    (Macmillan Publishers Limited, 2015) Zheng, Bob Y.; Zhao, Hangqi; Manjavacas, Alejandro; McClain, Michael; Nordlander, Peter; Laboratory for Nanophotonics (LANP)
    The use of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, to boost the efficiency of light-harvesting devices through increased light-matter interactions could drastically alter how sunlight is converted into electricity or fuels. These excitations can decay directly into energetic electron-hole pairs, useful for photocurrent generation or photocatalysis. However, the mechanisms behind plasmonic carrier generation remain poorly understood. Here we use nanowire-based hot-carrier devices on a wide-bandgap semiconductor to show that plasmonic carrier generation is proportional to internal field-intensity enhancement and occurs independently of bulk absorption. We also show that plasmon-induced hot electrons have higher energies than carriers generated by direct excitation and that reducing the barrier height allows for the collection of carriers from plasmons and direct photoexcitation. Our results provide a route to increasing the efficiency of plasmonic hot-carrier devices, which could lead to more efficient devices for converting sunlight into usable energy.
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    Hot Carrier Generation in Nanostructures for Efficient Photocatalysis and Photodetection
    (2017-09-28) Zhao, Hangqi; Halas, Naomi; Nordlander, Peter
    Surface plasmons are incompressible oscillations of conduction band electrons in metallic nanostructures and have provided a promising route for light-harvesting and light-driven catalysis. Energetic electron-hole pairs, known as hot carriers, are created when plasmons decay through a non-radiative channel and hold extraordinary potential for boosting the efficiency of both photocurrent generation in photovoltaic devices and plasmon-enhanced photocatalysis. In this thesis, the fundamentals and mechanisms of plasmon-induced hot carrier generation were firstly introduced. Then we demonstrated how hot carrier generation could facilitate chemical reactions with the antenna-reactor concept. In this picture, we showed that by directly combining plasmonic and catalytic nanoparticles, the plasmonic nanoantenna could couple strongly with light and induce a forced plasmon in the catalytic reactor, enabling significantly enhanced generation of hot carriers within the catalyst nanoparticles and dramatically increased chemical reaction rates consequently. This could overcome the weak light coupling of traditional transition metal catalysts and provide independent control of chemical and light-harvesting properties of the catalysts by modular design. This approach is investigated and demonstrated by various heterometallic antenna-reactor complexes, including Pd islands decorated Al nanocrystals, Al-Pd heterodimers and Al-Cu2O nanoshell structures. In the second part of the thesis, a novel device for Mid-infrared photodetection was introduced based on efficient collections of hot holes. Apart from its high responsivity rivalling commercially available IR detectors, this photodetector could work on room temperature, which is significantly advantageous over conventional IR detector that requires cryogenic cooling. The devices consists of a plasmonic Al grating that operates both as an electric contact and optical filter, and a p-doped silicon substrates acting as a MIR absorber through free carrier absorption, generally regarded as detrimental in IR detection. The photodetector achieves its high performance through a modulation of the carrier mobility in silicon. Direct electrical read-outs of the absorption spectra of two molecules were performed using this detector, demonstrating its great potential for on-chip molecular vibrational spectroscopy.
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    Infrared photodetectors
    (2023-05-16) Zheng, Bob Yi; Zhao, Hangqi; Cerjan, Benjamin; Tanzid, Mehbuba; Nordlander, Peter J.; Halas, Nancy J.; Rice University; William Marsh Rice University; United States Patent and Trademark Office
    An infrared photodetector includes: a p-type and highly-doped silicon substrate; a metal structure disposed on the silicon substrate; a first electric contact to the silicon substrate; and a second electric contact to the metal structure.
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    Multicomponent plasmonic photocatalysts consisting of a plasmonic antenna and a reactive catalytic surface: the antenna-reactor effect
    (2020-09-08) Halas, Nancy Jean; Nordlander, Peter; Robatjazi, Hossein; Swearer, Dayne Francis; Zhang, Chao; Zhao, Hangqi; Zhou, Linan; Rice University; United States Patent and Trademark Office
    A multicomponent photocatalyst includes a reactive component optically, electronically, or thermally coupled to a plasmonic material. A method of performing a catalytic reaction includes loading a multicomponent photocatalyst including a reactive component optically, electronically, or thermally coupled to a plasmonic material into a reaction chamber, introducing molecular reactants into the reaction chamber, and illuminating the reaction chamber with a light source.
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    Multicomponent plasmonic photocatalysts consisting of a plasmonic antenna and a reactive catalytic surface: the antenna-reactor effect
    (2024-04-16) Halas, Nancy Jean; Nordlander, Peter; Robatjazi, Hossein; Swearer, Dayne Francis; Zhang, Chao; Zhao, Hangqi; Zhou, Linan; Rice University; United States Patent and Trademark Office
    A method of making a multicomponent photocatalyst, includes inducing precipitation from a pre-cursor solution comprising a pre-cursor of a plasmonic material and a pre-cursor of a reactive component to form co-precipitated particles; collecting the co-precipitated particles; and annealing the co-precipitated particles to form the multicomponent photocatalyst comprising a reactive component optically, thermally, or electronically coupled to a plasmonic material.
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    Multicomponent plasmonic photocatalysts consisting of a plasmonic antenna and a reactive catalytic surface: the antenna-reactor effect
    (2024-10-08) Halas, Nancy Jean; Nordlander, Peter; Robatjazi, Hossein; Swearer, Dayne Francis; Zhang, Chao; Zhao, Hangqi; Zhou, Linan; Rice University; United States Patent and Trademark Office
    A multicomponent photocatalyst includes a reactive component optically, electronically, or thermally coupled to a plasmonic material. A method of performing a catalytic reaction includes loading a multicomponent photocatalyst including a reactive component optically, electronically, or thermally coupled to a plasmonic material into a reaction chamber; introducing molecular reactants into the reaction chamber; and illuminating the reaction chamber with a light source.