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A Small, Small, Smaller World
    As everyone who follows cutting-edge technology is aware, the next big thing is going to be very small--as in a few billionths of a meter, or less than one-hundred millionths of an inch. The Nano-Age is coming, and Berkeley Lab researchers are engaged in multiple research projects to help speed its arrival. Among the significant developments this past year were the creation of hybrid solar cells that combine nanotechnology with plastic electronics, and the fabrication of the first nanowires to be composed of two different semiconductors.
  Above: Chemist Paul Alivisatos, director of Berkeley Lab's Molecular Foundry, in his laboratory where he and his team developed a hybrid semiconductor and plastic solar cell (top). The tiny solar cell assemblies were created using nanoscale rods of cadmium selenide and the P3HT polymer. Such cells will be cheaper and easier to make than their semiconductor counterparts.

The hybrid solar cells were developed by Paul Alivisatos, a chemist who holds a joint appointment with Berkeley Lab's Materials Science Division (MSD) and UC Berkeley's Chemistry Department, along with Janke Dittmer, an MSD staff scientist, and UC Berkeley graduate student Wendy Huynh. The researchers believe their hybrid cell will be cheaper and easier to make than its semiconductor counterparts and offer the same nearly infinite variety of shapes as pure polymers.

"We have demonstrated that semiconductor nanorods can be used to fabricate readily processed and energy-efficient hybrid solar cells together with polymers," says Alivisatos, director of the Molecular Foundry, a center for nanoscience now being established at Berkeley Lab.

At the heart of all photovoltaic devices are two separate layers of materials, one with an abundance of electrons that functions as a "negative pole," and one with an abundance of electron holes (vacant, positively-charged energy spaces) that functions as a "positive pole." When photons from the sun or some other light source are absorbed, their energy is transferred to the extra electrons in the negative pole, causing them to flow to the positive pole and creating new holes that start flowing to the negative pole. This electrical current can then be used to power electronic devices.

In a typical semiconductor solar cell, the two poles are made from n-type and p-type semiconductors. In a plastic solar cell, they're made from hole-acceptor and electron-acceptor polymers. In their new hybrid solar cell, Alivisatos and his colleagues used the semicrystalline polymer known as poly(3-hexylthiophene) or P3HT for the hole acceptor or negative pole, and nanometer-sized cadmium selenide (CdSe) rods as the positive pole.
"With CdSe rods measuring 7 by 60 nanometers, our hybrid solar cells achieved a monochromatic power conversion efficiency of 6.9 percent, one of the highest ever reported for a plastic photovoltaic device," says Alivisatos.

Even so, Alivisatos says that many engineering tricks can be applied to make future versions of the hybrid solar cells much more efficient.

Growing Striped Nanowires

Development of nanowires composed of two different semiconductors was led by Peidong Yang, a chemist who also holds a joint appointment with Berkeley Lab's Materials Sciences Division and UC Berkeley's Chemistry Department. These nanowires are called "striped" or "superlatticed" because their semiconductors (silicon and a silicon/germanium alloy) are arranged in discrete alternating segments. They have potential because they can function as a transistor, light-emitting diode, biochemical sensor, heat-pumping thermoelectric device, or all of the above, along the same length of wire.

Above: Chemist Peidong Yang led the development of a new "striped" nanowire.
The image at top, taken with a scanning transmission electron microscope, reveals two nanowires featuring alternating bands of silicon (light) and a silicon germanium alloy (dark), which form interfaces that could be made into transistors, LEDs, and other types of electronic devices. Each wire is less than a hundredth the diameter of a human hair.

"This is a major advancement in the field of one-dimensional nanostructure research," says Yang. "It gives us the ability to create various functional devices, such as a p-n junction, a coupled quantum dot structure, or a bipolar transistor, on a single nanowire--which can then be used as a building block for constructing more complex systems."

Microchips have been likened to sandwiches in that they are made by depositing layers of different types of semiconductors with different electrical properties on a wafer of silicon. The interfaces between these different layers control the flow of electrons and enable transistors and other electronic components to function. Striped nanowires offer the same electronic diversity as a two-dimensional microchip in a one-dimensional nanoscale platform.

Yang and his research team grow their striped wires using a hybrid "pulsed laser ablation/chemical vapor deposition" process. A silicon wafer coated with a thin layer of gold is heated in a furnace so that the gold film forms a liquid alloy with the silicon and spontaneously breaks up into nanometer-sized droplets. Vapors of the two semiconductors will then condense around the gold droplets as deposits which become nanowire segments. Chemicals are used as the source of the silicon vapor and a laser is used to vaporize the germanium. When the laser is off, only silicon is deposited on the gold particles; when the laser is on, both silicon and germanium are deposited.

"By periodically turning the laser on and off--and this can be readily programmed--we can form a silicon and silicon/germanium superlattice on every individual nanowire in a block-by-block fashion," says Yang. "The entire growth process resembles the living polymerization synthesis of a block copolymer." The technique is efficient and cheap. In just one hour, millions of striped nanowires can be made at relatively small cost.

Working with Yang on the striped nanowire project were UC Berkeley graduate students Yiying Wu and Rong Fan.

-- Lynn Yarris

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