Simon M. Fairclough

Nanojunction‐Mediated Photocatalytic Enhancement in Heterostructured CdS/ZnO, CdSe/ZnO, and CdTe/ZnO Nanocrystals

A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis.

Influence of Luminescence Quantum Yield, Surface Coating, and Functionalization of Quantum Dots on the Sensitivity of Time-Resolved FRET Bioassays

In clinical diagnostics, homogeneous time-resolved (TR) FRET immunoassays are used for fast and highly sensitive detection of biomarkers in serum samples. The most common immunoassay format is based on europium chelate or cryptate donors and allophycocyanin acceptors. Replacing europium donors with terbium complexes and the acceptors with QDs offers large photophysical advantages for multiplexed diagnostics, because the Tb-complex can be used as FRET donor for QD acceptors of different colors. Water-soluble and biocompatible QDs are commercially available or can be synthesized in the laboratory using many available recipes from the literature. Apart from the semiconductor material composition, an important aspect of choosing the right QD for TR-FRET assays is the thickness of the QD coating, which will influence the photophysical properties and long-term stability as well as the donor-acceptor distance and FRET efficiency. Here we present a detailed time-resolved spectroscopic study of three different QDs with an emission maximum around 605 nm for their application as FRET acceptors (using a common Tb donor) in TR-bioassays: (i) Invitrogen/Life Technologies Qdot605, (ii) eBioscience eFluorNC605 and iii) ter-polymer stabilized CdSe/CdS/ZnS QDs synthesized in our laboratories. All FRET systems are very stable and possess large Förster distances (7.4-9.1 nm), high FRET efficiencies (0.63-0.80) and low detection limits (0.06-2.0 pM) within the FRET-bioassays. Shapes, sizes and the biotin/QD ratio of the biocompatible QDs could be determined directly in the solution phase bioassays at subnanomolar concentrations. Both commercial amphiphilic polymer/lipid encapsulated QDs and self-made ligand-exchanged QDs provide extremely low detection limits for highly sensitive TR-FRET bioassays.

Charge dynamics at heterojunctions for PbS/ZnO colloidal quantum dot solar cells probed with time-resolved surface photovoltage spectroscopy

Time-resolved laser-pump X-ray-photoemission-probe spectroscopy of a ZnO ( 101¯0) substrate with and without PbS quantum dots (QDs) chemically linked to the surface is performed, using laser photon energies resonant with and below the band gap energy of the substrate (λ = 372 and 640 nm, hν = 3.33 and 1.94 eV). Charge injection from the photoexcited QDs to ZnO is demonstrated through the change in the surface photovoltage of the ZnO substrate observed when the heterojunction is illuminated with 1.94 eV radiation. The measured carrier dynamics are limited by the persistent photoconductivity of ZnO, giving dark carrier lifetimes of the order of 200 μs in a depletion layer at the interface. The chemical specificity of soft X-rays is used to separately measure the charge dynamics in the quantum dots and the substrate, yielding evidence that the depletion region at the interface extends into the PbS QD layer.

A New Class of Tunable Heterojunction by using Two Support Materials for the Synthesis of Supported Bimetallic Catalysts

Finely dispersed Rh or Pd nanoparticles are decorated with a small quantity of Fe atoms, originating from the controlled reduction of two supports, the mixed oxides of ZnFe2O4–Fe2O3, to form supported RhFe or PdFe bimetallic nanoparticles without significant change in particle size. The selectivity of ethylene glycol hydrogenolysis can be manipulated by adjusting the compositions of the catalysts. This reveals the important underlying principle for rational design and synthesis of new supported bimetallic nanoparticles by using two mixed oxides with a tailored heterojunction that exerts tuning catalytic properties.

Importance of the structural integrity of a carbon conjugated mediator for photocatalytic hydrogen generation from water over a CdS–carbon nanotube–MoS2 composite

Incorporation of CdS quantum dots is shown to significantly promote photocatalytic hydrogen production from water over single-layer MoS2 in a remote manner via their dispersions on a carbon nanotube as a nanocomposite: the hydrogen evolution rate is found to be critically dependent on the content and structural integrity of the carbon nanotube such that the double-walled carbon nanotube shows superior H2 production to a single-walled carbon nanotube because the inner carbon tubules survive from the structural damage during functionalization.

Hydrophobin-Encapsulated Quantum Dots.

The phase transfer of quantum dots to water is an important aspect of preparing nanomaterials that are suitable for biological applications, and although numerous reports describe ligand exchange, very few describe efficient ligand encapsulation techniques. In this report, we not only report a new method of phase transferring quantum dots (QDs) using an amphiphilic protein (hydrophobin) but also describe the advantages of using a biological molecule with available functional groups and their use in imaging cancer cells in vivo and other imaging applications.