School of Chemistry - Research Publications
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ItemNo Preview AvailableResolving conjugated polymer film morphology with polarised transmission and time-resolved emission microscopy(IOP Publishing, 2024-07-01)The alignment of chromophores plays a crucial role in determining the optoelectronic properties of materials. Such alignment can make interpretation of fluorescence anisotropy microscopy (FAM) images somewhat ambiguous. The time-resolved emission behaviour can also influence the fluorescence anisotropy. This is particularly the case when probing excitation energy migration between chromophores in a condensed phase. Ideally information concerning the chromophoric alignment, emission decay kinetics and fluorescence anisotropy can be recorded and correlated. We report on the use of polarised transmission imaging (PTI) coupled with both steady-state and time-resolved FAM to enable accurate identification of chromophoric alignment and morphology in thin films of a conjugated polydiarylfluorene. We show that the combination of these three imaging modes presents a comprehensive methodology for investigating the alignment and morphology of chromophores in thin films, particularly for accurately mapping the distribution of amorphous and crystalline phases within the thin films, offering valuable insights for the design and optimization of materials with enhanced optoelectronic performance.
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ItemChemical vapor deposition growth of phase-selective inorganic lead halide perovskite films for sensitive photodetectors(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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ItemRevealing the Role of Methylammonium Chloride for Improving the Performance of 2D Perovskite Solar Cells(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.
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ItemMorphological Requirements for Nanoscale Electric Field Buildup in a Bulk Heterojunction Solar Cell(AMER CHEMICAL SOC, 2021-01-14)The morphology of organic semiconductors is critical to their function in optoelectronic devices and is particularly crucial in the donor-acceptor mixture that comprises the bulk heterojunction of organic solar cells. Here, energy landscapes can play integral roles in charge photogeneration, and recently have been shown to drive the accumulation of charge carriers away from the interface, resulting in the buildup of large nanoscale electric fields, much like a capacitor. In this work we combine morphological and spectroscopic data to outline the requirements for this interdomain charge accumulation, finding that this effect is driven by a three-phase morphology that creates an energetic cascade for charge carriers. By adjusting annealing conditions, we show that domain purity, but not size, is critical for an electro-absorption feature to grow-in. This demonstrates that the energy landscape around the interface shapes the movement of charges and that pure domains are required for charge carrier buildup that results in reduced recombination and large interdomain nanoscale electric fields.
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ItemSpectroscopic study of L-DOPA and dopamine binding on novel gold nanoparticles towards more efficient drug-delivery system for Parkinson's disease(PERGAMON-ELSEVIER SCIENCE LTD, 2022-03-05)Nano-drug delivery systems may potentially overcome current challenges in the treatment of Parkinson's disease (PD) by enabling targeted delivery and more efficient blood-brain penetration ability. This study investigates novel gold nanoparticles (AuNPs) to be used as delivery systems for L-DOPA and dopamine by considering their binding capabilities in the presence and absence of a model protein, bovine serum albumin (BSA). Four different AuNPs were prepared by surface functionalization with polyethylene glycol (PEG), 1-adamantylamine (Ad), 1-adamantylglycine (AdGly), and peptidoglycan monomer (PGM). Fluorescence and UV-Vis measurements demonstrated the strongest binding affinity and L-DOPA/dopamine loading efficiency for PGM-functionalized AuNPs with negligible impact of the serum protein presence. Thermodynamic analysis revealed a spontaneous binding process between L-DOPA or dopamine and AuNPs that predominantly occurred through van der Waals interactions/hydrogen bonds or electrostatic interactions. These results represent PGM-functionalized AuNPs as the most efficient at L-DOPA and dopamine binding with a potential to become a drug-delivery system for neurodegenerative diseases. Detailed investigation of L-DOPA/dopamine interactions with different AuNPs was described here for the first time. Moreover, this study highlights a cost- and time-effective methodology for evaluating drug binding to nanomaterials.
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ItemNo Preview AvailableA simple and ubiquitous device for picric acid detection in latent fingerprints using carbon dots(Royal Society of Chemistry, 2020-05-07)This work addresses the synthetic optimization of carbon dots (CDs) and their application in sensing picric acid from latent fingerprints by exploiting a smartphone-based RGB tool. The optimization of the synthesis of CDs is investigated towards achieving shorter reaction time, better product yield and fluorescence quantum efficiency. Precursors such as citric acid and thiourea were chosen for the synthesis of CDs. Among the various synthetic methodologies, it is found that the pyrolysis method offers ∼50% product yield within 15 min. The morphology and optical properties of the prepared CDs are characterized using the typical microscopic and spectroscopic techniques, respectively. The synthesized CDs exhibit quasi-spherical shape with an average particle size of 1.7 nm. The excitation dependent emissive properties of CDs are investigated by time resolved fluorescence spectroscopy. Furthermore, the excellent fluorescence properties (ϕ = 11%) of CDs are explored as a fluorescent fingerprint powder for the identification of latent fingerprints on various substrates. In addition, the presence of picric acid in latent fingerprints was detected. Furthermore, this study is extended to perform real time detection of fingerprints and harmful contaminants in fingerprints by utilizing a smartphone-based RGB color analysis tool. Based on these investigations, the prepared CDs could be a prospective fluorescent material in the field of forensics.
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ItemTriplet fusion upconversion using sterically protected 9,10-diphenylanthracene as the emitter(ROYAL SOC CHEMISTRY, 2020-03-21)Improving the efficiency of triplet fusion upconversion (TF-UC) in the solid-state is still challenging due to the aggregation and phase separation of chromophores. In this work, two 9,10-diphenylanthracene (DPA) derivatives based on the modification of the 9,10-phenyl rings with bulky isopropyl groups (bDPA-1 and bDPA-2) were used as emitters. By using platinum octaethylporphyrin (PtOEP) as the sensitizer, TF-UC performance was comprehensively investigated in 3 media: toluene solution, polyurethane thin film and nano/micro-crystals in a polyvinyl alcohol matrix. Only a small difference in upconversion efficiency between the bulky DPAs and the DPA reference was observed in toluene solution and polyurethane thin film. However, a large improvement of TF-UC quantum yield was achieved in bDPA-2/PtOEP crystals (ΦUC = (0.92 ± 0.05)%) with a low excitation intensity threshold (52 mW cm-2) compared to that of DPA/PtOEP crystals (ΦUC = (0.09 ± 0.03)%). This difference was largely attributed to improved dispersibility of the PtOEP sensitizer in the bDPA-2 emitter crystals. The bulky DPAs also show excellent stability under UV irradiation with exposure to oxygen compared to DPA. These results provide a strategy for developing efficient solid-state TF-UC systems based on nano/micro-particles of emitter-sensitizer mixtures.
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ItemCompetitive Triplet Formation and Recombination in Crystalline Films of Perylenediimide Derivatives: Implications for Singlet Fission(AMER CHEMICAL SOC, 2020-05-28)Developing photostable compounds that undergo quantitative singlet fission (SF) is a key challenge. As SF necessitates electron transfer between neighboring molecules, the SF rate is highly sensitive to intermolecular coupling in the solid state. We investigate SF in thin films for a series of perylenediimide (PDI) molecules. By adding different substituents at the imide positions, the packing of the molecules in the solid state can be changed. The relationship between SF parameters and the stacked geometry in PDI films is investigated, with two-electron direct coupling found to be the main SF mechanism. Time-resolved emission and transient absorption data show that all of the PDI films undergo SF although with different rates and yields varying from 35 to 200%. The results show that PDI1 and 2, which are stacked PDI pairs twisted out of alignment along the highest occupied molecular orbital to lowest unoccupied molecular orbital transition, exhibit faster and more efficient SF up to 200% yield. We demonstrate that both triplet formation and decay rates are highly sensitive to the ordering of the molecules within a film. The results of this study will assist in the design of optimized structures with a fast SF rate and low recombination rate that are required for useful light harvesting applications.
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ItemFRET-enhanced photoluminescence of perylene diimides by combining molecular aggregation and insulation(Royal Society of Chemistry, 2020-07-14)The photoluminescence quantum yield (ϕPL) of perylene diimide derivatives (PDIs) is often limited by aggregation caused quenching (ACQ) at high concentration or in the neat solid-state. Energy transfer in high dye concentration systems is also a key factor in determining ϕPL as a result of energy funneling to trap sites in the sample. By tuning the substituents, we present two classes of PDIs with aggregation and insulation of the PDI core. By combining these fluorophores in a polymer film, we demonstrate highly emissive samples (85% ϕPL) at high concentration (140 mM or 20% w/w). Experimental and theoretical studies provide insight into why such a combination is necessary to achieve high ϕPL. While insulated fluorophores maintain respectable ϕPL at high concentration, an improved ϕPL can be achieved in the presence of appropriately oriented fluorophore aggregates as emissive traps. The theoretical calculations show that the relative orientation of aggregated monomers can result in energetic separation of localized states from the charge-transfer and bi-excitonic states thereby enabling high ϕPL.
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ItemBilirubin analogues as model compounds for exciton coupling(ROYAL SOC CHEMISTRY, 2020-07-21)A series of phycobilin analogues have been investigated in terms of coupled excitonic systems. These compounds consist of a monomer, a tetrapyrrole structurally similar to bilirubin (bR), and two conjugated bR analogues. Spectroscopic and computational methods have been used to investigate the degree of interchromophore coupling. We find the synthesised bR analogue shows stronger excitonic coupling than bR, owing to a different molecular geometry. The excitonic coupling in the conjugated molecules can be controlled by modifying the bridge side-group. New computed energy levels for bR using the DFT/MRCI method are also presented, which improve on published values and re-assign the character of excited singlet states.