School of Chemistry - Research Publications
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ItemTuning Single Quantum Dot Emission with a Micromirror(AMER CHEMICAL SOC, 2018-02)The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.
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ItemThe Degradation and Blinking of Single CsPbl3 Perovskite Quantum Dots(AMER CHEMICAL SOC, 2018-06-28)We demonstrate using single molecule spectroscopy that inorganic CsPbI3 perovskite quantum dots (PQDs) undergo an irreversible, photoaccelerated reaction with water that results in a blue-shift of the photoluminescence (PL) and ultimately to complete quenching of the emission. We find that decomposition does not take place in the presence of oxygen alone but that it requires light and water. We also analyze the blinking for some stable PQDs and find a continuous distribution of emission states with a linear correlation between intensity and lifetime. We postulate that, in addition to charging and discharging processes, blinking arises from the activation and deactivation of nonradiative recombination centers in the PQDs.
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ItemA Tunable Polymer-Metal Based Anti-Reflective Metasurface(WILEY-V C H VERLAG GMBH, 2020-01)Anti-reflective surfaces are of great interest for optical devices, sensing, photovoltaics, and photocatalysis. However, most of the anti-reflective surfaces lack in situ tunability of the extinction with respect to wavelength. This communication demonstrates a tunable anti-reflective surface based on colloidal particles comprising a metal core with an electrochromic polymer shell. Random deposition of these particles on a reflective surface results in a decrease in the reflectance of up to 99.8% at the localized surface plasmon resonance frequency. This narrow band feature can be tuned by varying the pH or by application of an electric potential, resulting in wavelength shifts of up to 30 nm. Electrophoretic particle deposition is shown to be an efficient method for controlling the interparticle distance and thereby further optimizing the overall efficiency of the anti-reflective metasurface.
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ItemPhotoexcited Pd(ii) auxiliaries enable light-induced control in C(sp3)-H bond functionalisation(ROYAL SOC CHEMISTRY, 2020-03-07)Herein we report the photophysical and photochemical properties of palladacycle complexes derived from 8-aminoquinoline ligands, commonly used auxiliaries in C-H activation. Spectroscopic, electrochemical and computational studies reveal that visible light irradiation induces a mixed LLCT/MLCT charge transfer providing access to synthetically relevant Pd(iii)/Pd(iv) redox couples. The Pd(ii) complex undergoes photoinduced electron transfer with alkyl halides generating C(sp3)-H halogenation products rather than C-C bond adducts. Online photochemical ESI-MS analysis implicates participation of a mononuclear Pd(iii) species which promotes C-X bond formation via a distinct Pd(iii)/Pd(iv) pathway. To demonstrate the synthetic utility, we developed a general method for inert C(sp3)-H bond bromination, chlorination and iodination with alkyl halides. This new strategy in auxiliary-directed C-H activation provides predictable and controllable access to distinct reactivity pathways proceeding via Pd(iii)/Pd(iv) redox couples induced by visible light irradiation.
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ItemHot-Carrier Organic Synthesis via the Near-Perfect Absorption of Light(AMER CHEMICAL SOC, 2018-11-01)Photocatalysis enables the synthesis of valuable organic compounds by exploiting photons as a chemical reagent. Although light absorption is an intrinsic step, existing approaches rely on poorly absorbing catalysts that require high illumination intensities to afford enhanced efficiencies. Here, we demonstrate that a plasmonic metamaterial capable of near-perfect light absorption (94%) readily catalyzes a model organic reaction with a 29-fold enhancement in conversion relative to controls. The oxidation of benzylamine proceeds via a reactive iminium intermediate with high selectivity at ambient temperature and pressure, using only low-intensity visible irradiation. Control experiments demonstrated that only hot charge carriers produced following photoexcitation facilitate the formation of superoxide radicals, which, in turn, leads to iminium formation. Modeling shows that hot holes with energies that overlap with the highest-occupied molecular orbital (HOMO) of the reactant can participate and initiate the photocatalytic conversion. These results have important implications for hot-carrier photocatalysis and plasmon-hot-carrier extraction.
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ItemLarge-Area Nanofabrication of Partially Embedded Nanostructures for Enhanced Plasmonic Hot-Carrier Extraction(AMER CHEMICAL SOC, 2019-03-01)When plasmonic nanoparticles are coupled with semiconductors, highly energetic hot carriers can be extracted from the metal-semiconductor interface for various applications in light energy conversion. However, the current quantum yields for hot-electron extraction are generally low. An approach for increasing the extraction efficiency consists of maximizing the contact area between the surface of the metal nanostructure and the electron-accepting material. In this work, we developed an innovative, simple, and scalable fabrication technique that partially embeds colloidal plasmonic nanostructures within a semiconductor TiO2 layer without utilizing any complex top-down nanofabrication method. The successful embedding is confirmed by scanning electron microscopy and atomic force microscopy imaging. Using visible-pump, near-IR probe transient absorption spectroscopy, we also provide evidence that the increase in the surface contact area between the nanostructures and the electron-accepting material leads to an increase in the amount of hot-electron injection into the TiO2 layer.
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ItemLuminescence of a Transition Metal Complex Inside a Metamaterial Nanocavity(John Wiley & Sons, 2017-08-25)Modification of the local density of optical states using metallic nanostructures leads to enhancement in the number of emitted quanta and photocatalytic turnover of luminescent materials. In this work, the fabrication of a metamaterial is presented that consists of a nanowire separated from a metallic mirror by a polymer thin film doped with a luminescent organometallic iridium(III) complex. The large spin–orbit coupling of the heavy metal atom results in an excited state with significant magnetic-dipole character. The nanostructured architecture supports two distinct optical modes and their assignment achieved with the assistance of numerical simulations. The simulations show that one mode is characterized by strong confinement of the electric field and the other by strong confinement of the magnetic field. These modes elicit drastic changes in the emitter’s photophysical properties, including dominant nanocavity-derived modes observable in the emission spectra along with significant increases in emission intensity and the total decay rate. A combination of simulations and momentum-resolved spectroscopy helps explain the mechanism of the different interactions of each optical mode supported by the metamaterial with the excited state of the emitter.
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ItemNo Preview AvailablePhotoinduced Electron Transfer in the Strong Coupling Regime: Waveguide-Plasmon Polaritons(AMER CHEMICAL SOC, 2016-04)Reversible exchange of photons between a material and an optical cavity can lead to the formation of hybrid light-matter states where material properties such as the work function [ Hutchison et al. Adv. Mater. 2013 , 25 , 2481 - 2485 ], chemical reactivity [ Hutchison et al. Angew. Chem., Int. Ed. 2012 , 51 , 1592 - 1596 ], ultrafast energy relaxation [ Salomon et al. Angew. Chem., Int. Ed. 2009 , 48 , 8748 - 8751 ; Gomez et al. J. Phys. Chem. B 2013 , 117 , 4340 - 4346 ], and electrical conductivity [ Orgiu et al. Nat. Mater. 2015 , 14 , 1123 - 1129 ] of matter differ significantly to those of the same material in the absence of strong interactions with the electromagnetic fields. Here we show that strong light-matter coupling between confined photons on a semiconductor waveguide and localized plasmon resonances on metal nanowires modifies the efficiency of the photoinduced charge-transfer rate of plasmonic derived (hot) electrons into accepting states in the semiconductor material. Ultrafast spectroscopy measurements reveal a strong correlation between the amplitude of the transient signals, attributed to electrons residing in the semiconductor and the hybridization of waveguide and plasmon excitations.
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ItemNo Preview AvailableHot Carrier Extraction with Plasmonic Broadband Absorbers(American Chemical Society, 2016-04-01)Hot charge carrier extraction from metallic nanostructures is a very promising approach for applications in photocatalysis, photovoltaics, and photodetection. One limitation is that many metallic nanostructures support a single plasmon resonance thus restricting the light-to-charge-carrier activity to a spectral band. Here we demonstrate that a monolayer of plasmonic nanoparticles can be assembled on a multistack layered configuration to achieve broadband, near-unit light absorption, which is spatially localized on the nanoparticle layer. We show that this enhanced light absorbance leads to ∼40-fold increases in the photon-to-electron conversion efficiency by the plasmonic nanostructures. We developed a model that successfully captures the essential physics of the plasmonic hot electron charge generation and separation in these structures. This model also allowed us to establish that efficient hot carrier extraction is limited to spectral regions where (i) the photons have energies higher than the Schottky junctions and (ii) the absorption of light is localized on the metal nanoparticles.