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Zurich, Switzerland, 4 August 2006—Scientists
at the IBM (NYSE: IBM) Zurich Research Laboratory have demonstrated
how a single molecule can be switched between two distinct conductive
states, which allows it to store data. As published today in SMALL,
these experiments show that certain types of molecules reveal intrinsic
molecular functionalities that are comparable to devices used in
today's semiconductor technology. This finding is yet another promising
result to emerge from IBM's research labs in their efforts to explore
and develop novel technologies for the post-CMOS era.
In the August 4 issue of SMALL, IBM researchers Heike
Riel and Emanuel Lörtscher report on a single-molecule switch
and memory element. Using a sophisticated mechanical method, they
were able to establish electrical contact with an individual molecule
to demonstrate reversible and controllable switching between two
distinct conductive states. This investigation is part of their
work to explore and characterize molecules to become possible building
blocks for future memory and logic applications. With dimensions
of a single molecule on the order of one nanometer (one millionth
of a millimeter), molecular electronics redefines the ultimate limit
of miniaturization far beyond that of today's silicon-based technology.
The results show that these molecules exhibit properties that can
be utilized to perform the same logic operations as used in today's
information technology. Namely, by applying voltage pulses to the
molecule, it can be controllably switched between two distinct "on"
and "off" states. These correspond to the "0"
and "1" states on which data storage is based. Moreover,
both conductive states are stable and enable non-destructive read-out
of the bit state—a prerequisite for nonvolatile memory operation—which
the IBM researchers demonstrated by performing repeated
write-read-erase-read cycles. With this single-molecule memory element,
Riel and Lörtscher have documented more than 500 switching
cycles and switching times in the microsecond range.
Crucial for investigating the inherent properties of molecules
is the ability to deal with them individually. To do this, Riel
and Lörtscher extended a method called the mechanically controllable
break-junction (MCBJ). With this technique, a metallic bridge on
an insulating substrate is carefully stretched by mechanical bending.
Ultimately the bridge breaks, creating two separate electrodes that
possess atomic-sized tips. The gap between the electrodes can be
controlled with picometer (one thousandth of a nanometer) accuracy
due to the very high transmission ratio of the bending mechanism.
In a next step, a solution of the organic molecules is deposited
on top of the electrodes. As the junction closes, a molecule capable
of chemically bonding to both metallic electrodes can bridge the
gap. In this way, an individual molecule is "caught" between
the electrodes, and measurements can be performed.
The molecules investigated are specially designed organic molecules
measuring only about 1.5 nanometers in length, approximately one
hundredth of a state-of-the-art CMOS element. The molecule was designed
and synthesized by Professor James M. Tour and co-workers of Rice
University, Houston, USA.
"The main advantage of exploiting transport capabilities at
the molecular scale is that the fundamental building blocks are
much smaller than today's CMOS elements," explains lead researcher
Heike Riel of the IBM Zurich Lab. "Furthermore, chemical synthesis
produces completely identical molecules, which, in principle, are
building blocks with no variability. This allows us to avoid a known
problem that CMOS devices face as they are scaled to ever smaller
dimensions. In addition, we hope to discover possibly novel, yet
unknown properties that silicon and related materials do not have."
Promising nanotechnologies for the post-CMOS era
The single-molecule switch is the most recent success in a series
of groundbreaking results achieved by IBM researchers in their efforts
to explore and develop novel technologies that will surpass conventional
CMOS technology. Miniaturizing the basic building blocks of microprocessors,
thereby achieving more functionality on the same area, is also referred
to as scaling, which is the main principle driving the semiconductor
industry. Known as "Moore's Law", which states that the
transistor density of semiconductor chips will double roughly every
18 months, this principle has governed the chip industry for the
past 40 years. The result has been the most dramatic and unequaled
increase in performance ever known.
However, CMOS technology will reach its ultimate limits in 10 to
15 years. As chip structures, which currently have dimensions of
about 40 nm, continue to shrink below the 20 nm mark, ever more
complex challenges arise and scaling appears not to be economically
feasible any more. And below 10 nm, the fundamental physical limits
of CMOS technology will be reached. Therefore, novel concepts are
needed.
In order to enhance computing performance beyond that of CMOS,
fundamentally different device concepts and architectures are being
investigated at IBM. Among the technologies closest to realization
are carbon nanotubes and semiconducting nanowires. Further research
is also being conducted in the field of spintronics.
The scientific paper entitled "Reversible and Controllable
Switching of a Single-Molecule Junction" by E. Lörtscher,
J. W. Ciszek, J. Tour, and H. Riel, was published in Small, Volume
2, Issue 8-9 , pp. 973-977 (04 August 2006).
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