School of Chemistry - Research Publications
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ItemSilver Nanoparticle Gradient Arrays: Fluorescence Enhancement of Organic Dyes(AMER CHEMICAL SOC, 2019-07-02)Noble metal nanoparticles show pronounced extinction peaks in the visible wavelength range due to their localized surface plasmon resonances. The excitation of these resonances leads to strong confinement of electromagnetic energy at nanometer scales, which is critical for ultrasensitive, fluorescence-based detection of analytes. The strength and spatial distribution of this near-field zone depend on particle size, shape, and composition. To determine how these near-field effects depend on the particle size, we have prepared nanoparticle gradients on centimeter-scale substrates using a colloid-based approach. This plasmonic gradient is used to study the steady-state emission and fluorescence lifetime of a common organic dye that was embedded into the monolayer.
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ItemA PTFE helical capillary microreactor for the high throughput synthesis of monodisperse silica particles(ELSEVIER SCIENCE SA, 2020-12-01)We propose a simple and inexpensive SiO2 submicron particle synthesis method based on a PTFE helical capillary microreactor. The device is based on Dean flow mediated, ultrafast mixing of two liquid phases in a microfluidic spiral pipe. Excellent control of particle size between 100 nm and 600 nm and narrow polydispersity can be achieved by controlling the device and process parameters. Numerical simulations are performed to determine the optimal device dimensions. In the mother liquor the silica particles exhibit zeta potentials < −60 mV, rendering them very stable even at high particle volume fractions. The current device typically produces around 0.234 g/h of the silica particles.
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ItemAqueous Synthesis of Cu2ZnSnSe4 Nanocrystals(American Chemical Society, 2019-03-26)Copper zinc tin selenide (CZTSe) nanocrystal inks show promise as a candidate for developing cheap, scalable, efficient, and nontoxic photovoltaic devices. They also present an important opportunity to controllably mix copper zinc tin sulfide (CZTS) with CZTSe to produce directly spectrally tunable Cu 2 ZnSn(S/Se) 4 (CZTSSe) solid-solutions using low-temperature processes. Herein, we describe a one-pot, low-temperature, aqueous-based synthesis that employs simultaneous redox and crystal formation reactions to yield CZTSe nanocrystal inks stabilized by Sn 2 Se 76- and thiourea. This versatile CZTSe synthesis is understood through the use of inductively coupled plasma mass spectrometry, Raman spectroscopy, Fourier transform infrared spectroscopy, and powder X-ray diffraction. It is further shown that stoichiometrically mixed CZTSe and CZTS nanocrystal powders yield a single CZTSSe phase at annealing temperatures between 200 and 250 °C. This facile and low-temperature process offers a low-energy alternative for the deposition of pure CZTSe/SSe thin films and enables the band gap to be readily tuned from 1.5 down to 1.0 eV by simple solution chemistry.
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ItemEffects of Hydrostatic Pressure on the Surface Plasmon Resonance of Gold Nanocrystals(American Chemical Society, 2019-01)The surface plasmon resonances of gold nanospheres and nanorods have been measured as a function of hydrostatic pressure up to 17 GPa in methanol-ethanol 4:1 solvent and up to 10 GPa in paraffin. Both the sphere resonance and the longitudinal rod resonance exhibit redshifts, whereas the transverse rod mode shows an extremely weak redshift or blueshift depending on the nanorod aspect ratio. Solidification of the solvent around 11 GPa causes some aggregation of the particles, readily identified through broadening of the surface plasmon band and further redshifting. Spectra collected during loading and unloading cycles exhibit only minimal hysteresis if the pressure remains below 11 GPa. The surface plasmon shifts are the result of two competing effects. Compression of the conduction electrons in the metals increases the bulk plasma frequency, which causes a blueshift. However, the increase in the solvent density under hydrostatic load leads to an increase in the solvent refractive index, which in turn leads to a redshift. We find that after accounting for the solvent contribution, we can spectroscopically determine the bulk modulus of the gold nanoparticles with a precision of 10%. The value obtained of K0 = 190 GPa is significantly higher than the value for bulk gold (167 GPa). Furthermore, we show that pressure-induced solidification causes a significant broadening and anomalous shift of the surface plasmon band that we attribute to aggregation and nanorod deformation.
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ItemUltrafast Nanodrum-on-Chip Pixels(AMER CHEMICAL SOC, 2024-03-15)Environmentally friendly, ultrafast display pixels of micrometer sizes are fabricated with nanometer-thick gold films and Si/SiO2 wafers. The color displayed is due to both the plasmon response of the gold film and the optical interference from the Fabry-Peerot cavity formed by the underlying silicon substrate, the semitransparent gold film and the air gap between them. When an electric potential is applied to the gold film, the electrostatic force induces an attraction between the gold film and the silicon wafer. Due to the flexibility of the film, the size of the air gap changes, resulting in a changing color. By applying different driving signals, we have achieved cyan, magenta, and yellow reflected colors. The maximum switching rate of the pixel is primarily determined by the thickness dependence of the metal drum and its Young's modulus and is typically in the MHz regime.
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ItemSurface Lattice Resonances in Self-Assembled Gold Nanoparticle Arrays: Impact of Lattice Period, Structural Disorder, and Refractive Index on Resonance Quality(American Chemical Society, 2020-11-17)Surface lattice resonances are optical resonances composed of hybridized plasmonic and diffractive modes. These collective resonances occur in periodic arrays of plasmonic nanoparticles with wavelength-scale interparticle distances. The appearance and strength of surface lattice resonances strongly depend on the single particle localized surface plasmon resonance and its spectral overlap with the diffractive modes of the array. Coupling to in-plane orders of diffraction is also strongly affected by the refractive index environment and its symmetry. In this work, we address the impact of the interparticle distance, the symmetry of the refractive index environment, and structural imperfections in self-assembled colloidal monolayers on the plasmonic–diffractive coupling. For this purpose, we prepared hexagonally ordered, nonclose packed monolayers of gold nanoparticles using a fast and efficient, interface-mediated, colloidal self-assembly approach. By tuning the thickness and deformability of the polymer shells, we were able to prepare monolayers with a broad range of interparticle distances. The optical properties of the samples were studied experimentally by UV–Vis spectroscopy and theoretically by finite difference time domain simulations. The measured and simulated spectra allow a comprehensive analysis of the details of electromagnetic coupling in periodic plasmonic arrays. In particular, we identify relevant criteria required for surface lattice resonances in the visible wavelength range with optimized quality factors in self-assembled monolayers.
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ItemSedimentation of C60 and C70: Testing the Limits of Stokes' Law(AMER CHEMICAL SOC, 2018-11-01)Virtually all dynamic methods for determining particle size on the nanoscale use the Stokes-Einstein-Sutherland (SES) equation to convert the diffusion coefficient into a hydrodynamic radius. The validity of this equation on the nanoscale has not been rigorously validated by experiment. Here we measure the sedimentation rates and diffusion coefficients of C60 and C70 in toluene using analytical ultracentrifugation and compare the results to the SES equation. We find that the SES equation for the drag force (nonslip boundary condition) works down to 1 nm length scales.
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ItemFabrication of a Three-Dimensional Plasmon Ruler Using an Atomic Force Microscope(AMER CHEMICAL SOC, 2019-08-15)We have assembled a three-dimensional (3D) plasmon ruler using an atomic force microscope (AFM) tip to manipulate single gold nanocrystals on top of electron beam lithography fabricated base layers. The 3D structures exhibit several polarization-dependent surface plasmon scattering peaks, including symmetric and asymmetric Fano resonances. We map these resonances as a function of the degree of asymmetry of the structure. We show that the coupled surface plasmon resonances are extremely sensitive to the position of the upper particle and that the resonances can be engineered and tuned using an AFM tip to move the upper nanocrystal just a few Angstroms.
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ItemGrowth of Gold Nanorods: A SAXS Study(AMER CHEMICAL SOC, 2021-09-16)Using simultaneous, in situ optical spectroscopy and time-resolved, small-angle X-ray scattering (SAXS), we have directly monitored the seeded growth of nearly monodisperse gold nanorods using hydroquinone as the reductant. Growth of the rods is much slower than with the ascorbate ion, allowing the rate of growth along both the longitudinal and transverse directions to be independently determined. The thickness of the stabilizing CTAB layer (3.2 ± 0.3 nm) has also been extracted. We find that increasing the hydrogen tetrachloroaurate(III) concentration produces longer rods, while conversely, increasing the hydroquinone concentration reduces the final aspect ratio. The final number of gold rods is smaller than the initial number of seed particles and decreases in the presence of larger concentrations of HAuCl4. The SAXS data reveal an early transition from a spherical morphology to an ellipsoidal one and then to spherically capped cylinders. The growth curve exhibits at least three distinct regimes: an initial phase comprising spherical seed growth, followed by symmetry breaking and slow elongation. A third phase is marked by rapid rod growth and increases in the aspect ratio. This process is temporally well resolved from the initial symmetry breaking but typically occurs when the rods are around 6 nm in diameter using hydroquinone as the reductant. The results provide qualitative support for the “popcorn model” proposed by Edgar et al. [ Formation of Gold Nanorods by a Stochastic “Popcorn” Mechanism. ACS Nano 2012, 6, 1116 1125 ].
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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.