Browsing by Author "Samaimongkol, Panupon"

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    Launching low-energy surface plasmons in purple gold (AuAl2)
    Samaimongkol, Panupon; Robinson, Hans D. (Springer, 2019-03-01)
    We confirm that the unusual purple color of the intermetallic compound AuAl 2 is of a plasmonic origin by launching surface plasmons (SPs) in thin AuAl 2 films. We measure the SP dispersion relation and also use the films to measure the index of refraction of sucrose solutions using standard SP resonance sensing. We find that the SP energy in planar AuAl 2 is approximately 2.1 eV, about 0.4 eV lower than in gold, and the material is highly resistant to oxidation. This is close to what is expected from previously reported measurements of the dielectric function of AuAl 2 . On this basis, we predict that AuAl 2 nanoparticles will a have very strong, spectrally nearly uniform light absorbance about an order of magnitude greater than standard carbon black. Such particles may therefore find applications as obscurants or as an alternative to more complex light-absorbing gold structures in areas such as photothermal therapy or solar steam generation, or in plasmonic catalysis.
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    Surface plasmon resonance study of the purple gold (AuAl₂) intermetallic, pH-responsive fluorescence gold nanoparticles, and gold nanosphere assembly
    Samaimongkol, Panupon (Virginia Tech, 2018-07-31)
    In this dissertation, I have verified that the striking purple color of the intermetallic compound AuAl₂, also known as purple gold, originates from surface plasmons (SPs). This contrasts to a previous assumption that this color is due to an interband absorption transition. The existence of SPs was demonstrated by launching them in thin AuAl2 films in the Kretschmann configuration, which enables us to measure the SP dispersion relation. I observed that the SP energy in thin films of purple gold is around 2.1 eV, comparable to previous work on the dielectric function of this material. Furthermore, SP sensing using AuAl₂ also shows the ability to measure the change in the refractive index of standard sucrose solution. AuAl₂ in nanoparticle form is also discussed in terms of plasmonic applications, where Mie scattering theory predicts that the particle bears nearly uniform absorption over the entire visible spectrum with an order magnitude higher than a lightabsorbing carbonaceous particle. The second topic of this dissertation focuses on plasmon enhanced fluorescence in gold nanoparticles (Au NPs). Here, I investigated the distance-dependent fluorescence emission of rhodamine green 110 fluorophores from Au NPs with tunable spacers. These spacers consist of polyelectrolyte multilayers (PEMs) consisting of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at pH 8.4. The distance between Au NPs and fluorophores was varied by changing the ambient pH from 3 to 10 and back, which causes the swelling and deswelling of PEM spacer. Maximum fluorescence intensity with 4.0-fold enhancement was observed with 7-layer coated Au NPs at ambient pH 10 referenced to pH 3. The last topic of this dissertation examines a novel approach to assemble nanoparticles, in particular, dimers of gold nanospheres (NSs). 16 nm and 60 nm diameter NSs were connected using photocleavable molecules as linkers. I showed that the orientation of the dimers can be controlled with the polarization of UV illumination that cleaves the linkers, making dipolar patches. This type of assembly provides a simple method with potential applications in multiple contexts, such as biomedicine and nanorobotics.
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    Tunable Gap Plasmons in Gold Nanospheres Adsorbed into a pH-Responsive Polymer Film
    Jao, Chih-Yu; Robinson, Hans D.; Samaimongkol, Panupon (Elsevier, 2019-06)
    Hypothesis Plasmon nanorules are exquisitely sensitive distance sensors that are based on the electromagnetic interaction between metal nanoparticles and surfaces. We hypothesize that nanorulers can act as quantitative probes of processes such as particle aggregation and adsorption, and deploy them to investigate particle adsorption onto stimulus-responsive polymer films. While such systems have previously been qualitatively investigated with plasmon nanorulers, our quantitative analysis should provide deeper insights. Experiment Gold nanospheres are adsorbed from solution onto pH-responsive, amine-rich polyelectrolyte multilayer (PEM) films that are either directly deposited on a gold substrate or onto an intermediate self-assembled monolayer (SAM) of charged thiols. Fitting the optical scattering spectrum to a full-wave calculation, we quantify the sphere-substrate gap distance with good accuracy. Findings We find that the gold spheres partially embed into the PEMs rather than ride on top of them, and that although the amount of actuation of the spheres afforded by tuning the pH is well controlled, it is significantly smaller than the corresponding thickness changes in unstrained films. Further, the presence of a SAM below the PEM increases the amount of polymer in the PEM, except for the thickest and most highly charged films, where the SAM instead appears to displace from the area below the nanospheres.