Browsing by Author "Kumar, Anjli"

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    Micro-Extinction Spectroscopy (MExS): a versatile optical characterization technique
    (Springer, 2018) Kumar, Anjli; Villarreal, Eduardo; Zhang, Xiang; Ringe, Emilie
    Micro-Extinction Spectroscopy (MExS), a flexible, optical, and spatial-scanning hyperspectral technique, has been developed and is described with examples. Software and hardware capabilities are described in detail, including transmission, reflectance, and scattering measurements. Each capability is demonstrated through a case study of nanomaterial characterization, i.e., transmission of transition metal dichalcogenides revealing transition energy and efficiency, reflectance of transition metal dichalcogenides grown on nontransparent substrates identifying the presence of monolayer following electrochemical ablation, and scattering to study single plasmonic nanoparticles and obtain values for the refractive index sensitivity and sensing figure of merit of over a hundred single particles with various shapes and sizes. With the growing integration of nanotechnology in many areas, MExS can be a powerful tool to both characterize and test nanomaterials.
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    Spectroelectrochemistry of Nanomaterials
    (2018-11-12) Kumar, Anjli; Ringe, Emilie
    As the development and application of novel nanomaterials is continually expanding, so is the need for versatile characterization methods that can probe nanoscale processes as they occur. Such characterization techniques would permit non-destructive and parallel spectroscopy and imaging of nanomaterials. The objective of this work is to address the need for such techniques. First, a new spectroelectrochemical characterization technique, Micro-Extinction Spectroscopy (MExS), is presented, including instrumental design as well as data acquisition and analysis algorithms. Two nanomaterial systems are then studied with MExS: two-dimensional transmission metal dichalcogenides (TMDs) and plasmonic nanoparticles. For TMDs, in situ and hyperspectral reflectance studies coupled with cyclic voltammetry and chronocoulometry enable the time-resolved observation of the process of electroablation, an oxidative technique recently developed for the synthesis of monolayer TMDs. For plasmonic nanoparticles, dark-field microscopy is used in conjunction with chronocoulometry to observe, in a hyperspectral manner, single-particle electrodeposition, a technique developed for nanoparticle surface modification. Together, these two techniques position spectroelectrochemistry as a rich tool for optical nanomaterials, capable of both synthesis and concurrent characterization.
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    Structural and Optical Properties of Discrete Dendritic Pt Nanoparticles on Colloidal Au Nanoprisms
    (American Chemical Society, 2016) Leary, Rowan K.; Kumar, Anjli; Straney, Patrick J.; Collins, Sean M.; Yazdi, Sadegh; Dunin-Borkowski, Rafal E.; Midgley, Paul A.; Millstone, Jill E.; Ringe, Emilie
    Catalytic and optical properties can be coupled by combining different metals into nanoscale architectures where both the shape and composition provide fine-tuning of functionality. Here, discrete, small Pt nanoparticles (diameter = 3 - 6 nm) were grown in linear arrays on Au nanoprisms, and the resulting structures are shown to retain strong localized surface plasmon resonances. Multi-dimensional electron microscopy and spectroscopy techniques (energy dispersive X-ray spectroscopy, electron tomography and electron energy-loss spectroscopy) were used to unravel their local composition, 3D morphology, growth patterns, and optical properties. The composition and tomographic analyses disclose otherwise ambiguous details of the Pt-decorated Au nanoprisms, revealing that both pseudospherical protrusions and dendritic Pt nanoparticles grow on all faces of the nanoprisms (the faceted or occasionally twisted morphologies of which are also revealed), and shed light on the alignment of Pt nanoparticles. The electron energy-loss spectroscopy investigations show that the Au nanoprisms sustain multiple localized surface plasmon resonances despite the presence of pendant Pt nanoparticles. The plasmonic fields at the surface of the nanoprisms indeed extend into the Pt nanoparticles, opening possibilities for combined optical and catalytic applications. These insights pave the way towards comprehensive nano-engineering of multi-functional bimetallic nanostructures, with potential application in plasmon-enhanced catalysis and in-situ monitoring of chemical processes via surface-enhanced spectroscopy.