Transport Properties of a Two-Dimensional PbSe Square Superstructure in an Electrolyte-Gated Transistor.

Self-assembled nanocrystal solids show promise as a versatile platform for novel optoelectronic materials. Superlattices composed of a single layer of lead-chalcogenide and cadmium-chalcogenide nanocrystals with epitaxial connections between the nanocrystals, present outstanding questions to the community regarding their predicted band structure and electronic transport properties. However, the as-prepared materials are intrinsic semiconductors; to occupy the bands in a controlled way, chemical doping or external gating is required. Here, we show that square superlattices of PbSe nanocrystals can be incorporated as a nanocrystal monolayer in a transistor setup with an electrolyte gate. The electron (and hole) density can be controlled by the gate potential, up to 8 electrons per nanocrystal site. The electron mobility at room temperature is 18 cm2/(V s). Our work forms a first step in the investigation of the band structure and electronic transport properties of two-dimensional nanocrystal superlattices with controlled geometry, chemical composition, and carrier density.

Harvesting the photoexcited holes on a photocatalytic proton reduction metal-organic framework.

The highly porous titanium based metal-organic framework NH2-MIL-125(Ti) has recently attracted significant attention in the field of photocatalysis as a promising material for H+ reduction. This work reveals charge transfer upon visible light illumination from this MOF to two different charge acceptors, as an alternative to sacrificial electron donors. Charge transfer is demonstrated through a combined spectroscopic study between this MOF and: (1) 2-(1H-pyrazol-3-yl)phenol, a molecule that functionally mimics the tyrosine-histidine pair, responsible for shuttling the holes to the oxygen evolving centre in natural photosynthesis, and (2) TEMPO, a well known and stable radical. Charge transfer of the holes from the MOF to these occluded molecules takes place on the picosecond time scale. This work suggests that, by coupling a stable and recyclable charge acceptor to the photogenerated holes, the charges can be utilised for oxidation reactions and, thus, link the reduction to the oxidation reactions in water splitting.

Efficient photogeneration of charge carriers in silicon nanowires with a radial doping gradient.

by performing electrodeless time-resolved microwave conductivity measurements, the efficiency of charge carrier generation, their mobility, and the decay kinetics on photoexcitation were studied in arrays of Si nanowires grown by the vapor-liquid-solid mechanism. Large enhancements in the magnitude of the photoconductance and charge carrier lifetime are found depending on the incorporation of impurities during the growth. They are explained by the internal electric field that builds up, due to higher doped sidewalls, as revealed by detailed analysis of the nanowire morphology and chemical composition.

Wide energy-window view on the density of states and hole mobility in poly(p-phenylene vinylene).

Using an electrochemically gated transistor, we achieved controlled and reversible doping of poly(p-phenylene vinylene) in a large concentration range. Our data open a wide energy-window view on the density of states (DOS) and show, for the first time, that the core of the DOS function is Gaussian, while the low-energy tail has a more complex structure. The hole mobility increases by more than 4 orders of magnitude when the electrochemical potential is scanned through the DOS.