Tsz-Wai Ng

Green Synthesis of Bifunctional Fluorescent Carbon Dots from Garlic for Cellular Imaging and Free Radical Scavenging

Nitrogen and sulfur codoped carbon dots (CDs) were prepared from garlic by a hydrothermal method. The as-prepared CDs possess good water dispersibility, strong blue fluorescence emission with a fluorescent quantum yield of 17.5%, and excellent photo and pH stabilities. It is also demonstrated that the fluorescence of CDs are resistant to the interference of metal ions, biomolecules, and high ionic strength environments. Combining with low cytotoxicity properties, CDs could be used as an excellent fluorescent probe for cellular multicolor imaging. Moreover, the CDs were also demonstrated to exhibit favorable radical scavenging activity.

Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (Voc) of organic photovoltaic devices has been interpreted with either the metal-insulator-metal (MIM) model or the energy offset between highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMOD-LUMOA). To elucidate the relation between Voc and the two models, we have used electrodes of a wide range of work functions to connect the CuPc/C60 organic photovoltaic devices. We found that when the work function difference (Δϕelectrodes) between ITO and Al electrode is in the range −3 and 0 eV, Voc increases linearly with Δϕelectrodes as prescribed by the MIM model. Outside this range, Voc saturates with values close to that given by the HOMOD-LUMOA less the exciton binding energy.

Formation chemistry of perovskites with mixed iodide/chloride content and the implications on charge transport properties

Although Cl-doping is a common technique for achieving high photovoltaic (PV) performance, the Cl content is negligibly small and cannot easily be tuned. Therefore, we herein study the formation chemistry of Cl-doped perovskites by examining the chemical interactions between thermally evaporated MAI and PbCl2 through X-ray photoemission spectroscopy (XPS). We show that PbCl2 is not stable at the MAI/PbCl2 contact surface. The Cl atom readily detaches from the PbCl2, which subsequently initiates electron transition from Pb to MAI. Via thermal-evaporation, a perovskite with a high PbCl2 content can be prepared and examined. We found that the presence of metallic Pb, associated with increased Cl content, can quench the photogenerated exciton in PV devices. By optimizing the ratio of MAI : PbCl2, a perovskite solar cell with ∼6% efficiency was obtained.

Facile synthesis of laminate-structured graphene sheet–Fe3O4 nanocomposites with superior high reversible specific capacity and cyclic stability for lithium-ion batteries

A facile one pot hydrothermal method has been developed to synthesize laminate-structured graphene sheet–Fe3O4 nanocomposites (GNS–Fe3O4). Fe3O4 nanoparticles were decorated densely and homogeneously in the graphene matrix. Galvanostatic charge–discharge cycling of the GNS–Fe3O4 nanocomposites exhibited a reversible specific capacity over 1200 mAh g−1 at 100 mA g−1 without palpable fading for 50 cycles in the voltage range 0.01–3.0 V. A cell for the rate capacity test indicated a high current density of 946 mAh g−1 at a cycling rate of 1000 mA g−1, which could be fully recovered to 1359 mAh g−1 at 100 mA g−1 after 50 cycles. The superior electrochemical performance of the nanocomposites can be attributed to the following factors: (i) the thermal expanded graphene oxide (TEGO) under atmosphere could attach more oxygen functional groups than the hydrogen reduced graphene, which benefited the adsorption and fastness of the nano-sized Fe3O4; (ii) annealing of TEGO–Fe3O4 nanocomposites further improved the conductivity of the graphene matrix, providing a high electron transport rate at the electrode–electrolyte interface; (iii) the laminated structure of nanocomposites could prevent the agglomeration of Fe3O4 nanoparticles and the restacking of graphene sheets, and effectively release the strain caused by the volume expansion of the Fe3O4 nanoparticles, facilitating ion/electron transportation within the electrode and at the electrode–electrolyte interface.

Ambient effects on fullerene/copper phthalocyanine photovoltaic interface

Effects of ambient-air exposure on the energy levels at photovoltaic interface of fullerene (C60)/copper phthalocyanine (CuPc) were studied using ultraviolet photoemission spectroscopy. The junction prepared in ultrahigh vacuum showed flat energy levels with little vacuum level offset, while exposure to ambient air at 10−5 Torr induced p-type doping of C60 with energy levels bend up for 0.27 eV. The energy difference between HOMOCuPc−LUMOC60, describing the theoretical maximum open-circuit voltage, increased from 0.64 to 0.81 eV. The exposure moved the LUMOC60 away from the Fermi level, leading to reduction in carrier concentration and film conductivity.

Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.

Iron(II) molybdate (FeMoO4) nanorods as a high-performance anode for lithium ion batteries: structural and chemical evolution upon cycling

FeMoO4 nanorods were synthesized by a one-step solvothermal method and demonstrated to have attractive performance as an anode material in lithium ion batteries (LIBs). The specific capacity of the electrode exhibited an initial fading in the first 50 cycles and subsequently recovered to 1265 mA h g−1 at about the 500th cycle at a rate of 1C, after that, the capacity remained stable around 1110 mA h g−1 until the 1000th cycle. Based on comprehensive analysis of the structural and chemical evolution at each stage of capacity variation, we illustrated that the FeMoO4 nanorods were converted to a Fe2O3/MoO3 mixture after the first cycle and they experienced gradual structural variation of grain refinement and amorphization with their morphology transformed from nanorods to nanosheets upon cycling. Such changes in the chemical composition and microstructure of nanorods led to larger effective surface area, improved electrochemical reaction kinetics, and capacity retention capability. As a similar tendency of the specific capacity upon cycling has been widely observed for metal oxide anodes, studies on structural and chemical evolution of electrode materials during the whole cyclic life will be helpful for understanding their electrochemical reaction mechanism and provide guidance to material design and structural optimization of electrodes.

Synthesis of 1T-MoSe2 ultrathin nanosheets with an expanded interlayer spacing of 1.17 nm for efficient hydrogen evolution reaction

In this work, we report for the first time a simple solvothermal method to synthesize assembled 1T-MoSe2 nanosheets, which possess expanded (002) interlayer spacings as large as 1.17 nm with an 81% expansion as compared to that (0.646 nm) of the bulk counterpart. The 1T-MoSe2 nanosheets exhibit striking kinetic metrics for the hydrogen evolution reaction (HER) with a low onset potential of 60 mV and a small Tafel slope of 78 mV dec−1, which are better than those of the 2H-MoSe2 counterpart with a normal interlayer spacing of 0.64 nm. The outstanding electrocatalytic activity is attributed to the high concentration of the metallic 1T phase as well as the expanded interlayer distance, contributing to increasing the number and catalytic activity of the active sites.

Copper substituted P2-type Na0.67CuxMn1−xO2: a stable high-power sodium-ion battery cathode

While sodium-ion batteries (SIBs) are considered as a next-generation energy storage device because of the higher abundance and lower cost of sodium compared to those of lithium, developing high-power and stable cathode materials remains a great challenge. Here, micron-sized plate-like copper-substituted layered P2-type Na0.67CuxMn1−xO2 is demonstrated to rapidly charge and discharge within 5 minutes while giving a capacity of more than 90 mA h g−1, corresponding to a half-cell energy density of 260 W h (kg cathode)−1 at a power density of 3000 W (kg cathode)−1, which is comparable to that of high-power lithium-ion cathodes. The materials show excellent stability, retaining more than 70% of the initial capacity after 500 cycles at 1000 mA g−1. The good cycle and rate performances of the materials are attributed to copper in the lattice, which stabilizes the crystal structure, increases the average discharge potential and improves sodium transport. This makes Na0.67CuxMn1−xO2 an ideal choice as a cathode for high-power sodium-ion batteries.

Charge-Transfer Complexes and Their Role in Exciplex Emission and Near-Infrared Photovoltaics

Charge transfer and interactions at organic heterojunctions (OHJs) are known to have critical influences on various properties of organic electronic devices. In this Research News article, a short review is given from the electronic viewpoint on how the local molecular interactions and interfacial energetics at P/N OHJs contribute to the recombination/dissociation of electron-hole pairs. Very often, the P-type materials donate electrons to the N-type materials, giving rise to charge-transfer complexes (CTCs) with a P(δ+) -N(δ-) configuration. A recently observed opposite charge-transfer direction in OHJs is also discussed (i.e., N-type material donates electrons to P-type material to form P(δ-) -N(δ+) ). Recent studies on the electronic structures of CTC-forming material pairs are also summarized. The formation of P(δ-) -N(δ+) -type CTCs and their correlations with exciplex emission are examined. Furthermore, the potential applications of CTCs in NIR photovoltaic devices are reviewed.