Nano Focus
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ergy Focus ZnO nanoforest delivers high-efficiency solar cell
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nspired by the network of branches that trees use to gather sunlight, an innovative team of researchers recently grew a “nanoforest” of light-absorbing nanowires to ramp up productivity in dye-sensitized solar cells (DSSC). As detailed in the January 5th online edition of Nano Letters (DOI: 10.1021/nl1037962), S.H. Ko and co-workers at the Korea Advanced Institute of Science and Technology and University of California, Berkeley developed a multi-stage seeded growth process to grow intricately branched structures of ZnO nanowires that resemble a tightly packed forest of pine trees. The team then fabricated the nanoforests into a DSSC and demonstrated a significant jump in the solar cell’s efficiency. In a DSSC, a wide-bandgap semiconductor works in conjunction with a sensitizing dye to absorb incident sunlight and convert it to electrical energy. ZnO is
Nano Focus Functionalization of graphene leads to enhanced hydrogen adsorption
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he on-board, high-capacity, facile, and reversible storage of hydrogen fuel is one of several significant challenges for hydrogen-fueled vehicles. While gas-adsorbant metal-organic framework structures display high H2 uptake, the volume of H2 they can hold
14 days at 1000°C, which resulted in a highly concentrated (Fe concentration 2 × 1019 cm-3) gain element. High gain in the developed active medium ensured reduced oscillation build-up time, improved temporal overlap of the pump pulse (20 ns) and output oscillation (15 ns), and effective absorption of the pump pulse, leading to increased output energy. A fourfold increase in the output energy of the gain-switched Fe:ZnSe
laser was obtained. The maximum laser output energy was 4.7 mJ at 4.3 μm and 236 K, and 3.6 mJ at 4.37 μm and 300 K (limited by available pump energy) with maximum obtained power of 0.3 MW at 4.3 μm. The researchers said that additional improvements can be achieved by further optimization of the laser cavity and also by using a pump with higher energy. Mousumi Mani Biswas
an effective semiconductor for this purpose and is appealing due to the ease with which it can be grown, but the resulting solar cells suffer from low efficiencies. However, this nanoforest approach could be the crucial step to making ZnO a viable DSSC component. To achieve the forest-like structure, the researchers first grow nanowires by a hydrothermal growth process, depositing seed particles (ZnO quantum dots) on a substrate and immersing it in a solution containing zinc nitrate hydrate and the polymers hexamethylenetetramine (HMTA) and polyethylenimine (PEI). Then they wash off the polymer, bake the sample, and deposit a new layer of seed particles over the pillar-like nanowires. Next they repeat the hydrothermal growth step, which now causes nanowires to form along the length of the original wires, like branches on a tree. They perform additional cycles to create higher orders of branching, leading to a dense network of hierarchically branched nanowires.
The team demonstrated t
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