[tt] Physics News Update 876

[Eugen Leitl]

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From: physnews at aip.org
Date: Thu, 23 Oct 2008 16:22:33 -0400
To: eugen at LEITL.ORG
Subject: Physics News Update 876
Reply-to: physnews at aip.org

INSIDE SCIENCE RESEARCH---PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Research News
Number 876  October 23, 2008      www.aip.org/pnu

USING SUNLIGHT MORE EFFICIENTLY.  Researchers at the National
Renewable Energy Laboratory (NREL) in Golden, Colo have developed a
way for low-cost solar cells to more efficiently convert sunlight
into electricity.  The research, which increases the "lifetime" of
electrons created in a solar cell so they can make more electricity,
is a possible step in the direction of bringing down the relatively
high cost of solar cells. Reducing cost while sustaining efficiency
is the big factor in determining how soon solar power will become a
major player in the energy business.  Generally you could have good
efficiency or low cost but not both.  Efficiency refers to the
fraction of the sunlight falling on the solar panel that actually
gets converted into useable electricity.  And cost refers to the
expense of mass-producing the panels in large sheets.  Solar cells
have been used in niche markets, such as for powering remote sensors
or spacecraft, and are increasingly used for homes and utility
applications.
Most of these solar cells are made from crystalline silicon. But for
large-scale adoption to occur, the price will have to come down.
Currently the cost-per-kilowatt-hour for solar-generated power is
several times higher than for generating that power with fossil
fuels.  Solar cells mimic nature in the way that it converts
sunlight into useful energy.  In a green leaf, for example, the
incoming sunlight liberates an electron in a molecule of
chlorophyll.  The electron (and its energy) gets passed from one
molecule, eventually being incorporated into building up larger
molecules such as a carbohydrate.  In a solar cell the incoming
sunlight liberates an electron from a piece of semiconductor.  This
"excited" electron, if it stays excited, can be incorporated into an
electrical current feeding into an external circuit, where it can
flow into a battery or the electric grid.  The longer the lifetime
of the excited electron, the better the efficiency of the solar
cell.  Unfortunately, electrons tend to lose their energy when they
meet a defect or boundary in the crystals that make up a solar cell.
Until now to get a better excitation lifetime and better efficiency,
solar cells needed to be made of higher-priced single crystal
materials like silicon or gallium arsenide.  These solar cells need
lots of complex processing to build, and these costs are not likely
to be reduced. Meanwhile, lower-priced solar cells made from thin
layers of multi-crystalline materials, such as compounds made of the
atoms copper, indium, gallium, and selenium (CIGS), haven't been
nearly as efficient.
The research focused on improving electron lifetimes in solar cells
made from multi-crystalline CIGS, and in their research paper, NREL
scientists Wyatt Metzger, Ingrid Repins, and Miguel Contreras
announced they have achieved an electron lifetime of 250 billionths
of a second.  It sounds like a short time, but it is long enough for
more electrons to contribute to the cell's electricity, making it
dramatically more efficient, yet still low in cost when compared to
the high-efficiency silicon solar cells.  The results were recently
published in the journal Applied Physics Letters.  (Phillip F.
Schewe)

BUCKY BEAMS.  Once nanochip manufacturers have made their
multi-layered structures it is necessary also for them to verify
precisely that the layers are composed in the proper way.  One way
of doing this is to shoot beams of ions which, like meteorites
striking the Moon, eject material from below, providing information
about subsurface layering.  The ejected material is characterized
using mass spectrometry.  It seems that to do this large molecules
or clusters of atoms are better than single-atom ions since the
clusters excavate more cleanly and provide more unambiguous signs of
deep structure in the sample being imaged. The lab of Nick Winograd
(nxw at psu.edu) of Penn State has pioneered the use of beams of
carbon-60 molecules (buckyballs).  (See this site for pictures
illustrating the difference between single atom probes and C60
beams: http://nxw.chem.psu.edu/nxw/pdf%5C327.pdf ).  Recently
Winograd and his students have greatly improved the sensitivity of
detection of the ejected material by using an infrared laser for
photoionization prior to analysis by the mass spectrometer. The
infrared laser is effective since electrons can be removed from
molecules with high efficiency via tunneling and without significant
photofragmentation.  (Results presented this week at the AVS meeting
in Boston,  http://www.avssymposium.org/overview.asp, Paper
AS-TuM10)

TRAPPING SINGLE MOLECULES, at room temperature, and studying their
properties has been accomplished by Adam Cohen and his colleagues at
Harvard.  Pinning down one molecule at a time is difficult at low
temperatures, much less at warm temperatures, where the molecules
are more agitated.  The feat was carried off by using an
Anti-Brownian Electrokinetic (ABEL) trap.  In this device the
fluorescently labeled molecule is tracked in a fluorescence
microscope and its instantaneous motion slowed by the application of
carefully timed bits of electricity applied to electrodes that
surround the sample. Actually, the electrodes are kept at some
distance from the molecule, the better not to pollute the local
aqueous environment with chemical effects. The electric kicks are
imparted to the molecule along micro-channels in an underlying chip.
The faster this feedback process can be applied the better the
trapping. An ABEL trap can hold smaller samples at room temperature
than any other trap scheme.  To hold a molecule to the same tiny
volume of solution with laser light alone, enormous power would be
needed, and
this would “cook” the object rather than trap it. The ABEL trap is
gentle, and requires mere microwatts of laser power.  Cohen
(cohen at chemistry.harvard.edu) talked about the application of this
process to the dynamics of membrane proteins at this week’s AVS
meeting.  (http://www.avssymposium.org/overview.asp  Cohen website
https://www2.lsdiv.harvard.edu/labs/cohen/  Paper IPF-MoM1 )
		
CORRECTION: In PNU 875, the work of Jun Ye was incorrectly reported
as the world’s best atomic clock.  In fact Ye's strontium clock is
one of the most accurate clocks ever produced. It is the most
accurate neutral atom clock, but two NIST ion clocks (mercury and
aluminum) are better.

***********
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