PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 717 January 27, 2005
by Phillip F. Schewe, Ben Stein

A PHASE CHANGE IN HIGH-DENSITY DATA STORAGE.  A new approach to storing
bits of information in a rewritable medium substitutes electron beams for
optical beams.  Scientists at Hewlett Packard create individual bits in the
form of tiny amorphous regions inside a thin indium-selenium layer.  That
layer, along with another layer beneath (gallium-selenium) and a silicon
substrate, form the principal parts of a pn-junction diode.  The read-write
cycle goes like this: short, high-power bursts from an electron beam are
used to write a "1" by melting a tiny portion of the InSe layer,
turning it into a glassy blob.  Alternatively the blob can be erased by the
use of a longer, low-power beam pulse, which recrystallizes the material. 
With the help of an even lower-power beam pulse the bit can be read out as
either a 1 (the amorphous blob yields little or no detectable current in
the pn-junction diode ) or a 0 (the crystalline material yields a high
diode current).  Electron-beam storage can potentially reach higher
 densities than optical storage due to the shorter wavelength of
high-energy electrons.  Ultimately, it may also enable faster data access
through electrostatic deflection of the electron-beams. The HP tests so far
have used a laser beam rather than an electron beam to do the writing part
(their electron beam isn't yet strong enough) but employ an e-beam
(essentially a scanned electron microscope) to do the reading.  The
response of the diode storage medium is fast enough to allow reading rates
of at least a million bits per second per electron-beam and more than 100
write/erase/rewrite cycles have been carried out successfully.  The bit
size right now is about 150 nm in lateral extent (for an area density of
about 29 gigabits per square inch), but this will probably be made far
smaller, maybe down to 10 nm.  (Gibson et al., Applied Physics Letters, 31
January 2005; contact Gary Gibson, gary.gibson{at}hp.com, 650-857-2125 or
Alison Chaiken, chaiken{at}hpl.hp.com, 650-23 6-2231)
        
ORGANIC MOLECULES ON THE REBOUND. Scientists at the International
University of Bremen and the University of Bonn have recently determined
the precise structure of a large organic molecule after its interaction
with a metal surface. The group of scientists also used the structure
information to decipher clues about the chemical bond between the molecule
and the surface.  The organic-metallic interface is very important in
science, especially in the fields of catalysis (chemical reactions between
two species proceeding in the presence of a third species), bio-sensing,
and molecular electronics (where signals are processed through circuit
elements consisting, in some cases, of single molecules or arrays of
molecules). In this regard, larger molecules are harder to study because of
their size, their tortuous shape, and many internal modes of vibration.  In
the Bremen-Bonn experiment the starting point is a super-clean silver
surface in ultrahigh vacuum.  Next the molecule is allowed to fall onto the
surface where it reacts chemically with surface atoms and is slightly
distorted thereby.  Next, x rays from a synchrotron are brought to bear on
the adsorbed molecule.  By the scattering of the x rays the researchers can
deduce, in some cases atom for atom, where the component parts of the
molecule are relative to the nearby metal surface.  The worked-out
structure of the reacted molecule can then be compared to the structure for
the same type of molecule in the free (gaseous) state.  In this way the
distortion of the molecule, whose full name is
perylene-tetracarboxylic-dianhydride (PTCDA), can be worked out.  It is
notable that the x-ray scattering technique used here was not the normal
Bragg scattering in use for decades.  Because the sample was so thin, the
approach employed here was based on standing x-ray waves.  The x rays
reflected from the silver crystal formed standing waves when they
interfered with incoming x rays.  The ensuing atomic-scale
"ruler" can be used to map the organic molecule by slightly
grading the energy of the incoming x rays.  This normal incidence x-ray
standing wave technique has been used before but very rarely on large
organic adsorbates where it has great potential. What happened as the
normally planar molecule approached the surface?  Surprisingly, there was
some bending, mostly because of the readiness of some oxygen atoms (which
weren't supposed to play much of a chemical role) to form bonds with the
surface silver atoms.  Another discovery: the molecule forms not a single
bond but a hierarchy of two types of bonds.  (Hauschild et al., Physical
Review Letters, 28 January 2005; contact Stefan Tautz, 49-421-200-3223,
s.tautz{at}iu-bremen.de;
lab website http://imperia.iu-bremen.de/ses/physics/tautz/30797/ )

---