Number 717, January 27, 2005
by Phil Schewe and 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@hp.com, 650-857-2125 or Alison Chaiken, chaiken@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. (Hauschildet al., Physical Review Letters, 28 January 2005; contact Stefan
Tautz, 49-421-200-3223, s.tautz@iu-bremen.de; lab website http://imperia.iu-bremen.de/ses/physics/tautz/30797/
)
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