Mechanistic investigation of plasmon-induced charge and energy transfer in metal/semiconductor nano-heterostructures
Date
2024
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Plasmonic materials such as pure Au, Ag, and Ag/Au alloy have been the subject of intense investigations due to their intriguing optical and electronic properties. They possess a unique ability to concentrate light into sub-wavelength volume thereby enhancing a range of linear and non-linear dynamics. The cynosure of their distinctive characteristics lies in the localized surface plasmon resonance excitations (LSPR); characterized by an exceptionally large absorption cross-section. In general, plasmonic excitations are very short-lived and decay in the order of tens of femtoseconds. The exceptionally short lifetime hampers the direct measurement and practical application of the LSPR. One approach to expand the application scope of these materials is to convert plasmonic excitations into other forms of useful excitations, such as excitons in a semiconductor. This strategy entails growing semiconductor shells on a precise array of plasmonic metal or alloy nanoparticle cores to form metal-semiconductor nano-heterostructures (MSNH) or alloy-semiconductor hybrids. The precise arrays of MSNH serve as model systems for studying the fundamental physics of the charge or energy transfer processes. Different mechanisms that contribute to charge carrier and energy transfer in MSNH have been identified; these mechanisms include: a) hot charge-carrier transfer (HCT), b) plasmon-induced resonant energy transfer (PIRET), c) direct interfacial charge transfer transitions (DICTT), and d) plasmon-induced interfacial electron transfer transition (PICTT). Transfer mechanisms such as HCT are inefficient, therefore direct energy or charge transfer such as PIRET, PICTT or DICTT is preferred. The inefficiency of HCT transfer process is a result of the competition between electron-electron scattering in the metal core that occurs at a similar time scale as HCT. Also, the energy barrier at the metal-semiconductor interface reduces the population of carriers that can be transferred. Consequently, it is expedient to investigate the conditions necessary for plasmon-induced charge and/or energy transfer; also, parameters that govern the underlying processes need to be understood. ☐ As mentioned earlier, direct measurement of LSPR dynamics in plasmonic materials is challenging due to the ultrashort nature of the plasmon (10 fs). An additional difficulty arises due to the strategies adopted to fabricate or synthesize plasmonic materials which inhibit the achievable high time resolution to measure the materials. The majority of wet chemistry methods involve synthesizing plasmonic materials either in suspension or by drop-casting them onto solid substrates. These methods lead to colloidal agglomeration and introduce dispersion in measurements. Therefore, we are presented with synthesis and spectroscopic challenges. ☐ In this thesis, novel functional plasmonic materials with well-tailored optical properties are fabricated. The novel plasmonic materials include arrays of nanoparticles such as Au, Ag, and Ag/Au alloy nanoparticles; also, Au/Cu2O, and Au/CdS MSNH. These materials are adapted to enable a systematic variation of certain parameters such as plasmon resonance frequency in the metal or alloy core, semiconductor band gap, and utilization of different intrinsically doped semiconductors for a wide range of relative band and energy alignment between the metal and semiconductor domains. The choice of p-type and n-type semiconductors offers information about the transfer of hot holes and hot electrons, respectively, during HCT process. In addition, the option of having a wide band gap and a narrow band gap semiconductor that is excited within and out of plasmon resonance provides information about the influence of band alignment on charge or energy transfer. The materials fabricated are characterized after fabrication by employing techniques such as electron microscopy, X-ray diffraction, and X-ray photo-electron spectroscopy. The plasmon bands of the materials are obtained using UV-Vis absorption spectroscopy. Further, the excited state dynamics are investigated by employing a femtosecond ultrafast spectroscopic method called transient absorption spectroscopy.
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Keywords
Energy transfer, Plasmonic materials, Semiconductors, Nano-heterostructures, Metal nanoparticles