bijesh kafle - Academia.edu (original) (raw)

Papers by bijesh kafle

2017 International Conference on Optical MEMS and Nanophotonics (OMN), 2017

We studied selectively catalyzed plasmon driven photochemical reactions of paraaminothiophenol (P... more We studied selectively catalyzed plasmon driven photochemical reactions of paraaminothiophenol (PATP) as model molecule placed in cavity between gold film (AuF) and different ligands coated gold nanorods (AuNR) by surface enhanced Raman spectroscopy (SERS).

Using Government drawings, specifications, or other data included in this document for any purpos... more Using Government drawings, specifications, or other data included in this document for any purpose other than Government procurement does not in any way obligate the U.S. Government. The fact that the Government formulated or supplied the drawings, specifications, or other data does not license the holder or any other person or corporation; or convey any rights or permission to manufacture, use, or sell any patented invention that may relate to them. This report is the result of contracted fundamental research which is exempt from public affairs security and policy review in accordance with AFI 61-201, paragraph 2.3.5.1. This report is available to the general public, including foreign nationals. Copies may be obtained from the Defense Technical Information Center (DTIC) (http://www.dtic.mil).

Compared to the highly sensitive silicon based affordable visible light detectors, infrared photo... more Compared to the highly sensitive silicon based affordable visible light detectors, infrared photodetectors require significant improvement. Localized surface plasmon resonances of metal nanoparticles can be utilized for increasing the absorption efficiency of semiconductors suited for detection of infrared radiation. In this work, plasmonic gold nanorods (AuNRs) are used to enhance generation of charge carriers and photon emission by InAs/InGaAs/GaAs quantum dots-in-a-well semiconductor heterostructures. Comparison of measured and calculated scattering spectra reveals that the AuNRs on GaAs exhibit red to green colors depending on their proximity to the GaAs surface. On the other hand, theoretical and experimental near-field optical characterization show that the electric field is tightly localized at the AuNR-GaAs interfacial regions, creating a convenient platform for investigating localized carrier generation and diffusion by monitoring the emission of InAs QDs. The carrier gener...

Active Photonic Platforms XIII, 2021

In-situ observation of light-induced physico-chemical changes is a long sought experimental appro... more In-situ observation of light-induced physico-chemical changes is a long sought experimental approach to gain mechanistic understanding of the underlying fundamental processes. Here, we present experimental results that demonstrate the possibility of using plasmonic nanocavity for inducing and observing physical and chemical changes. This is demonstrated by monitoring photons scattered by a plasmonic nanocavity, in which materials of interest (organic and inorganic systems) are embedded.

The Journal of Physical Chemistry C, 2020

Quantifying the sensitivity limit of scattering-type scanning near-field optical microscopy (s-SN... more Quantifying the sensitivity limit of scattering-type scanning near-field optical microscopy (s-SNOM) in vibrational infrared imaging requires assembly of molecular systems with continuous variation of height across lateral displacements, which has not been available to date. In this work, we fabricate a film of poly(4-vinylpyridine) (P4VP) with about 7°angle of elevation on gold and silicon substrates and compare the chemical contrast due to the ring stretching vibration of P4VP as a function of sample thickness. We observe that the near-field contrast starts to change at the same time as the sample height, which increases at a rate of a nanometer per 10 nm lateral displacement crossing from the bare substrates to the P4VP-coated region. Saturation of phase contrast is observed for a thicker than 100 nm sample on silicon, while it continues to increase even beyond 200 nm on gold. The presaturation regime on silicon appears to coincide with the higher net field enhancement in the sample on silicon than on gold, in spite of the higher overall scattering amplitude on gold. Although the chemical contrasts are observable as the sample thickness decreases to a subnanometer scale on both substrates, important distinctions are observed because of the roughness of the as-prepared gold film. The variation in the local tip−sample geometry results in signal fluctuation that creates uncertainty in the chemical contrast when the sample thickness is comparable to the roughness. The results presented here provide clarity to advance s-SNOM chemical imaging to the molecular "finger print" region of electromagnetic radiation.

ACS Photonics, 2018

Understanding how the plasmonic response of a metallic nanoparticle is modified by its coupling w... more Understanding how the plasmonic response of a metallic nanoparticle is modified by its coupling with a metallic film is a fundamental research problem relevant for many applications including sensing, solar energy harvesting, spectroscopy, and photochemistry. Despite significant research effort on this topic, the nature of the interaction between colloidal nanoparticles and metallic films is not fully understood. Here, we investigate, both experimentally and theoretically, the optical response of surface ligand-coated gold nanorods interacting with gold films. We find that the scattering cross section of these systems is dominated by a charge transfer plasmon mode, for which charge flows between the particle and the film. The properties of this mode are dictated by the 1 Page 1 of 25 ACS Paragon Plus Environment ACS Photonics characteristics of the particle-film junction, which makes the frequency of this charge transfer plasmon far less sensitive to the nanoparticle size and geometry than a typical dipolar plasmon mode excited in similar nanorods placed directly on a purely dielectric substrate. The results of this work serve to advance our understanding of the interaction between metallic nanoparticles and metallic films, as well as provide a method for creating more robust plasmonic platforms that are less affected by changes in the size of individual nanoparticles.

The Journal of Physical Chemistry C, 2019

Photochemistry that can be driven at low incident photon flux on optically excited plasmonic nano... more Photochemistry that can be driven at low incident photon flux on optically excited plasmonic nanoparticles is attracting increasing research interest because of the fundamental reason to combine surface reaction and in situ spectroscopy as well as the opportunity that plasmon-driven reaction may offer a pathway for efficient conversion of solar energy into fuel. In mechanistic studies of plasmon-driven reactions to date, a great deal of emphasis is given to hot electron transfer. The results summarized in this featured article indicate that photochemistry on plasmonic nanoparticles can be induced by hot electron transfer from the nanoparticle to an unoccupied orbital of the adsorbate and/or by plasmon-pumped electron transition from an occupied molecular orbital to an unoccupied molecular orbital of the adsorbate. The branching photochemical reaction of para-aminothiophenol on plasmonic gold surface depending on the presence of cetyltrimethylammonium bromide surface ligand that influences the hot electron concentration is used to highlight reactions driven by hot carriers. The importance of plasmonpumped electronic excitation of adsorbates in initiating surface photochemistry is demonstrated based on the N-demethylation of methylene blue (MB) on gold nanostructures depending on

Scientific Reports, 2017

Understanding the enhancement of charge carrier generation and their diffusion is imperative for ... more Understanding the enhancement of charge carrier generation and their diffusion is imperative for improving the efficiency of optoelectronic devices particularly infrared photodetectors that are less developed than their visible counterpart. Here, using gold nanorods as model plasmonic systems, InAs quantum dots (QDs) embedded in an InGaAs quantum well as an emitter, and GaAs as an active mediator of surface plasmons for enhancing carrier generation and photon emission, the distance dependence of energy transfer and carrier diffusion have been investigated both experimentally and theoretically. Analysis of the QD emission enhancement as a function of distance reveals a Förster radius of 3.85 ± 0.15 nm, a near-field decay length of 4.8 ± 0.1 nm and an effective carrier diffusion length of 64.0 ± 3.0 nm. Theoretical study of the temporal-evolution of the electron-hole occupation number of the excited states of the QDs indicates that the emission enhancement trend is determined by the c...

The Journal of Physical Chemistry C, 2017

Using methylene blue (MB) as a model system, we demonstrate surface plasmon-enhanced resonant exc... more Using methylene blue (MB) as a model system, we demonstrate surface plasmon-enhanced resonant excitation that leads to N-demethylation reaction under visible light irradiation (λ = 633 nm) at low photon flux. The chemical changes are monitored by detecting the vibrational signatures of the reactant and product species in situ using surface-enhanced Raman scattering (SERS) spectroscopy. Drastic temporal evolution of SERS spectra has been observed upon continuous irradiation. While the SERS spectra acquired immediately after irradiation are the same as the Raman spectrum of MB solid powder, the spectra recorded a few seconds later are remarkably similar to that of thionine solid powder, indicating N-demethylation of MB. No demethylation reaction has been observed under resonant excitation (λ = 633 nm) of MB adsorbed on nonplasmonic surfaces. Similarly, excitations of plasmon resonances at 532 and 808 nm wavelengths that do not overlap with the MB electronic transition do not lead to transformation of MB to thionine. The reaction mechanism is discussed in terms of resonant excitation of MB and hot electron transfer to adsorbed species. Considering that both MB and thionine have large SERS signal due to the combination of resonance Raman and electromagnetic enhancement effects that provide high detection sensitivity, we suggest that this demethylation reaction serves as a convenient model system for future mechanistic studies.

The Journal of Physical Chemistry Letters, 2017

Contrary to the general expectation that surface ligands reduce the reactivity of surfaces by blo... more Contrary to the general expectation that surface ligands reduce the reactivity of surfaces by blocking the active sites, we present experimental evidence that surface ligands can in fact increase reactivity and induce important reaction pathways in plasmon-driven surface photochemistry. The remarkable effect of surface ligands is demonstrated by comparing the photochemistry of p-aminothiophenol (PATP) on resonant plasmonic gold nanorods (AuNRs) in the presence of citrate, hexadecyltrimethylammonium bromide (CTAB), and no surface ligands under visible light irradiation. The use of AuNRs with citrate and no surface ligand results in the usual azocoupling reaction. In contrast, CTAB-coated AuNRs oxidize PATP primarily to pnitrothiophenol (PNTP). Strong correlation has been observed between the N−O and Au−Br vibration band intensities, suggesting that CTAB influences the reaction pathway through the Br − counterions that can minimize the electron−hole recombination rate by reacting with the hole and hence increasing the concentration of hot electrons that drive the oxidation reaction.

The Journal of Physical Chemistry C, 2016

We report the transformation of near-infrared CdSeTe/ZnS quantum dots (QDs) that are exposed to w... more We report the transformation of near-infrared CdSeTe/ZnS quantum dots (QDs) that are exposed to water. When the colloidal QDs with 840 nm emission wavelength and 75 nm spectral line width are self-assembled on water surface and transferred to an oxide-coated silicon wafer using a Langmuir–Blodgett (LB) procedure, two prominent relatively sharp photoluminescence (PL) bands are observed at ∼630 and ∼660 nm peak wavelengths with line width of ∼23 and ∼39 nm, respectively. On the other hand, the PL spectrum of the QDs as they are assembled on the water surface is essentially the same as that of the solution phase. Structural analysis of the LB films shows that the QDs are stripped off the stabilizing excess surfactant molecules by the preferential interaction at the water–air interface. After the film is transferred, the QDs are interfaced with each other and with the substrate directly, while covered with the stack of surfactant molecules from the top. Based on analysis of the chemical composition using X-ra...

We present nanoplasmonic imaging of exosome using antibody-functionalized undercut arrayed gold n... more We present nanoplasmonic imaging of exosome using antibody-functionalized undercut arrayed gold nanodisks. We show a nanoplasmonic shift map that allows the detection and quantitation of exosome binding.