School of Chemistry - Research Publications

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    Silver Nanoparticle Gradient Arrays: Fluorescence Enhancement of Organic Dyes
    Sindram, J ; Volk, K ; Mulvaney, P ; Karg, M (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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    A PTFE helical capillary microreactor for the high throughput synthesis of monodisperse silica particles
    Yang, H ; Akinoglu, EM ; Guo, L ; Jin, M ; Zhou, G ; Giersig, M ; Shui, L ; Mulvaney, P (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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    Aqueous Synthesis of Cu2ZnSnSe4 Nanocrystals
    Ritchie, C ; Chesman, ASR ; Jasieniak, J ; Mulvaney, P (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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    Effects of Hydrostatic Pressure on the Surface Plasmon Resonance of Gold Nanocrystals
    Martin-Sanchez, C ; Antonio Barreda-Argueso, J ; Seibt, S ; Mulvaney, P ; Rodriguez, F (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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    Photophysical Identification of Three Kinds of Low-Energy Green Band Defects in Wide-Bandgap Polyfluorenes
    Bo, Y-F ; Liu, Y-Y ; Soleimaninejad, H ; Yu, M-N ; Xie, L-H ; Smith, TA ; Ghiggino, KP ; Huang, W (AMER CHEMICAL SOC, 2019-04-04)
    Blue-light-emitting semiconductors based on polyfluorenes often exhibit an undesired green emission band. In this report, three well-defined oligofluorenes corresponding to three types of "defects" attributed to aggregation, keto formation, and chain entanglement, respectively, are systemically investigated to unveil the origins of the green emission band in fluorene-based materials. First, the optical properties of defect molecules in different states are studied. The defect associated with aggregation is absent in dilute solutions and in films doped at 0.01 wt % with poly(methyl methacrylate). Second, the dependence of the emission spectra on the solvent was monitored to compare the effects of the "keto-" and "chain-entanglement defect" molecules. The green emission of keto defects exhibited a strong dependence on solvent polarity, whereas this cannot be observed in case of chain-entanglement defect. Third, energy transfer between poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]- co-[5-(octyloxy)-9,9-diphenyl-fluoren-2,7-diyl] and the keto or chain-entanglement defect molecules is illustrated. Compared to those of the chain-entanglement defect, the spectra of the keto defect molecule (1:10-3) show signs of defect emission at lower proportions. These investigations not only provide insight into the photophysics of oligofluorenes but also supply a new strategy to explore defects in semiconductor polymers, which will aid in the development of effective approaches to obtain stable, pure blue organic light-emitting diodes based on polyfluorenes.
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    Chemical vapor deposition growth of phase-selective inorganic lead halide perovskite films for sensitive photodetectors
    Xu, W ; Niu, M ; Yang, X ; Chen, H ; Cai, X ; Smith, TA ; Ghiggino, KP ; Hao, X (ELSEVIER SCIENCE INC, 2021-01)
    Inorganic lead halide perovskites are attractive optoelectronic materials owing to their relative stability compared to organic cation alternatives. The chemical vapor deposition (CVD) method offers potential for high quality perovskite film growth. The deposition temperature is a critical parameter determining the film quality owing to the melting difference between the precursors. Here, perovskite films were deposited by the CVD method at various temperatures between 500−800 °C. The perovskite phase converts from CsPb2Br5 to CsPbBr3 gradually as the deposition temperature is increased. The grain size of the perovskite films also increases with temperature. The phase transition mechanism was clarified. The photoexcited state dynamics were investigated by spatially and temporally resolved fluorescence measurements. The perovskite film deposited under 750 °C condition is of the CsPbBr3 phase, showing low trap-state density and large crystalline grain size. A photodetector based on perovskite films shows high photocurrent and an on/off ratio of ∼2.5 × 104.
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    Time-resolved emission microscopy of light-induced aggregation of luminescent polymers
    Xu, Y ; Zhou, J ; Smith, TA (IOP Publishing, 2019-12-23)
    Photon pressure has been used to induce the aggregation from solution of a series of photoluminescent conjugated polyelectrolytes containing tetraphenylethene units. These polymers show steady-state and time-resolved emission properties that are dependent on the local chromophore environment that can be influenced by the degree of intra- and inter-molecular interactions, which enables the photoaggregation process to be monitored by time-resolved fluorescence imaging techniques. Structural differences in the polymer lead to variations in the photo-induced aggregation behaviour.
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    Light losses from scattering in luminescent solar concentrator waveguides
    Breukers, RD ; Smith, GJ ; Stirrat, HL ; Swanson, AJ ; Smith, TA ; Ghiggino, KP ; Raymond, SG ; Winch, NM ; Clarke, DJ ; Kay, AJ (OPTICAL SOC AMER, 2017-04-01)
    The reductions in the transmission of emission originating from a fluorophore dissolved in a polymer matrix due to light scattering were compared in two forms of planar waveguides used as luminescent solar concentrators: a thin film of poly(methylmethacrylate) (PMMA) spin-coated on a glass plate and a solid PMMA plate of the same dimensions. The losses attributable to light scattering encountered in the waveguide consisting of the thin film of polymer coated on a glass plate were not detectable within experimental uncertainty, whereas the losses in the solid polymer plate were significant. The losses in the solid plate are interpreted as arising from light-scattering centers comprising minute bubbles of vapor/gas, incomplete polymerization or water clusters that are introduced during or after the thermally induced polymerization process.
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    Excitonic Processes in a Conjugated Polyelectrolyte Complex
    Nitneth, DT ; Hutchison, JA ; Ghiggino, KP (CSIRO Publishing, 2020)
    In aqueous solution, a di-sulfonated phenylenevinylene polymer (DPS-PPV) forms a complex with non-ionic poly(vinyl alcohol) (PVA) leading to absorption spectroscopic shifts and a dramatic (6-fold) increase in DPS-PPV fluorescence intensity. Spectroscopic investigations demonstrate that the complexation with PVA and other neutral polymers results in conformational changes in the DPS-PPV chains that lead to the removal of non-fluorescent energy traps and results in the observed increase in fluorescence in the bulk solution. Single molecule fluorescence measurements of DPS-PPV chains dispersed on glass and in PVA films confirm that efficient exciton energy transfer occurs within each photo-excited DPS-PPV chain and that the observed increase in fluorescence intensity in the PVA film environment is also associated with fewer quenching sites. The results highlight the importance of conjugated polyelectrolyte conformation on exciton relaxation pathways.
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    Revealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells
    Zheng, F ; Zuo, C ; Niu, M ; Zhou, C ; Bradley, SJ ; Hall, CR ; Xu, W ; Wen, X ; Hao, X ; Gao, M ; Smith, TA ; Ghiggino, KP (American Chemical Society, 2020-06-10)
    Layered perovskite films, composed of two-dimensional (2D) Ruddlesden–Popper perovskites (RPPs), show improved stability compared to their conventional three-dimensional (3D) counterparts in perovskite solar cells (PSCs). However, 2D PSCs exhibit a lower power conversion efficiency (PCE), which has been attributed to compositional inhomogeneity and nonuniform alignment of the 2D perovskite phases. Methylammonium chloride (MACl) has been adopted as an additive to improve the PCE and the operational stability of 2D PSCs, although the role of MACl in performance enhancement is unclear. In this work, time- and spatially resolved fluorescence and absorption techniques have been applied to study the composition and charge carrier dynamics in MACl-doped BA2MA4Pb5I16 (⟨n⟩ = 5) layered perovskite films. The inhomogeneous phase orientation distribution in the direction orthogonal to the substrate for undoped layered perovskite films undergoes reorganization upon MACl doping. Based on structural and crystallographic analyses, it is revealed that MACl can facilitate the crystallization of small-n 2D perovskite phases at the cost of consuming an increased amount of BA cations. Consequently, an increase in the thickness of large-n 2D perovskite phases accompanies their enhanced perpendicular alignment ([101] crystalline orientation) to the substrate, which facilitates charge carrier transport and collection by electrodes. The defect passivation of the MACl-doped layered perovskite film provided by the small-n phase is also beneficial to the photovoltaic performance of the PSC device. A maximum PCE (∼14.3%) was achieved at 6 mol % MACl doping, with this optimum level influenced by the increased interfacial roughness of the layered perovskite film caused by the edges of small-n perovskite flakes emerging on the front surface.