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1. Perspective on functional metal-oxide plasmonic metastructures

   https://doi.org/10.1063/5.0134141  

   Sadeghi, Seyed M.; Wing, Waylin J.; Gutha, Rithvik R. (February 2023, Journal of Applied Physics)

   Plasmonic nanostructures and metasurfaces are appealing hosts for investigation of novel optical devices and exploration of new frontiers in physical/optical processes and materials research. Recent studies have shown that these structures hold the promise of greater control over the optical and electronic properties of quantum emitters, offering a unique horizon for ultra-fast spin-controlled optical devices, quantum computation, laser systems, and sensitive photodetectors. In this Perspective, we discuss how heterostructures consisting of metal oxides, metallic nanoantennas, and dielectrics can offer a material platform wherein one can use the decay of plasmons and their near fields to passivate the defect sites of semiconductor quantum dots while enhancing their radiative decay rates. Such a platform, called functional metal-oxide plasmonic metasubstrates (FMOPs), relies on formation of two junctions at very close vicinity of each other. These include an Au/Si Schottky junction and an Si/Al oxide charge barrier. Such a double junction allows one to use hot electrons to generate a field-passivation effect, preventing migration of photo-excited electrons from quantum dots to the defect sites. Prospects of FMOP, including impact of enhancement exciton–plasmon coupling, collective transport of excitation energy, and suppression of quantum dot fluorescence blinking, are discussed. 

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2. Laser-controlled projection of quantum dot dipoles using metal-oxide plasmonic metastructures: maintaining spin polarization memory

   https://doi.org/10.1039/d1tc02532e  

   Sadeghi, Seyed M.; Wing, Waylin; Gutha, Rithvik R.; Sharp, Christina; Roberts, Dustin; Mao, Chuanbin (October 2021, Journal of Materials Chemistry C)

   It is known that the spontaneous emission of semiconductor quantum dots is mostly unpolarized when they are excited off-resonantly. The complete loss of polarization memory is associated with the ultrafast carrier scattering, leading to complete spin polarization relaxation. We study the application of metal-oxide plasmonic double-junction structures to transfer the excitation polarization memory of quantum dots to their spontaneous emission. These structures consist of arrays of metallic nanoantennas in the presence of heterostructures consisting of Au/Si Schottky junctions and Si/Al-oxide charge barriers. Our results show that by using such double-junction structures, one can control the states of polarization and intensity of the emission of quantum dots using the state of polarization of an off-resonant laser field. For achieving this, we explore the optical control of exciton–plasmon coupling using optical lattice modes caused by the arrays of metallic nanoantennas, and the application of the electrostatic field generated by the hot electrons captured at the Au/Si Schottky junction. 

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3. Plasmonic Nickel–TiO ₂ Heterostructures for Visible‐Light‐Driven Photochemical Reactions

   https://doi.org/10.1002/anie.201901987  

   He, Shuai; Huang, Jiawei; Goodsell, Justin L.; Angerhofer, Alexander; Wei, Wei David (March 2019, Angewandte Chemie International Edition)

   Abstract Plasmon‐mediated carrier transfer (PMCT) at metal–semiconductor heterojunctions has been extensively exploited to drive photochemical reactions, offering intriguing opportunities for solar photocatalysis. However, to date, most studies have been conducted using noble metals. Inexpensive materials capable of generating and transferring hot carriers for photocatalysis via PMCT have been rarely explored. Here, we demonstrate that the plasmon excitation of nickel induces the transfer of both hot electrons and holes from Ni to TiO₂in a rationally designed Ni–TiO₂heterostructure. Furthermore, it is discovered that the transferred hot electrons either occupy oxygen vacancies (V_O) or produce Ti³⁺on TiO₂, while the transferred hot holes are located on surface oxygens at TiO₂. Moreover, the transferred hot electrons are identified to play a primary role in driving the degradation of methylene blue (MB). Taken together, our results validate Ni as a promising low‐cost plasmonic material for prompting visible‐light photochemical reactions. 

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4. Photochemistry of ferritin decorated with plasmonic gold nanoparticles

   https://doi.org/10.1039/C8EN01000E  

   Cerkez, Elizabeth B.; Dutton, Kaitlyn G.; Ghidey, Yonatan G.; Kukulka, Mark A.; Valentine, Ann M.; Strongin, Daniel R. (January 2019, Environmental Science: Nano)

   The photochemistry of a plasmonic biomaterial that consisted of gold nanoparticles (AuNP) on the exterior of the iron sequestration protein, ferritin (Ftn), was investigated. The light driven photochemistry of the hybrid system was studied mechanistically and for the reduction of the high priority pollutant, chromate, Cr( vi ) as CrO 4 2− . In the absence of aqueous Cr( vi ), but in the presence of a sacrificial electron donor, the Fe( iii ) oxyhydroxide semiconducting core of Ftn underwent a photoreaction to release Fe( ii ) when exposed to light having wavelengths, λ < 475 nm. AuNP grown on the exterior of the Ftn produced plasmonic heterostructures (Au/Ftn) that allowed similar photochemistry to occur at longer wavelengths of light ( i.e. , λ > 475 nm). Au/Ftn also facilitated the reduction of Cr( vi ) to Cr( iii ) in the presence of visible light ( λ > 475 nm), a reaction that was not observed if AuNP were not attached to the Ftn cage. Results also indicated that AuNP need to be intimately bound to Ftn to extend the photochemistry of Au/Ftn to longer light wavelengths, relative to Au-free Ftn. 

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5. Optical spectroscopic detection of Schottky barrier height at a two-dimensional transition-metal dichalcogenide/metal interface

   https://doi.org/10.1039/D3NR05799B  

   Chen, Du; Anantharaman, Surendra B.; Wu, Jinyuan; Qiu, Diana Y.; Jariwala, Deep; Guo, Peijun (February 2024, Nanoscale)

   Atomically thin two-dimensional transition-metal dichalcogenides (2D-TMDs) have emerged as semiconductors for next-generation nanoelectronics. As 2D-TMD-based devices typically utilize metals as the contacts, it is crucial to understand the properties of the 2D-TMD/metal interface, including the characteristics of the Schottky barriers formed at the semiconductor-metal junction. Conventional methods for investigating the Schottky barrier height (SBH) at these interfaces predominantly rely on contact-based electrical measurements with complex gating structures. In this study, we introduce an all-optical approach for non-contact measurement of the SBH, utilizing high-quality WS2/Au heterostructures as a model system. Our approach employs a below-bandgap pump to excite hot carriers from the gold into WS2 with varying thicknesses. By monitoring the resultant carrier density changes within the WS2 layers with a broadband probe, we traced the dynamics and magnitude of charge transfer across the interface. A systematic sweep of the pump wavelength enables us to determine the SBH values and unveil an inverse relationship between the SBH and the thickness of the WS2 layers. First-principles calculations reveal the correlation between the probability of injection and the density of states near the conduction band minimum of WS2. The versatile optical methodology for probing TMD/metal interfaces can shed light on the intricate charge transfer characteristics within various 2D heterostructures, facilitating the development of more efficient and scalable nano-electronic and optoelectronic technologies. 

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