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
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. By introducing this single-molecule
memory element, IBM researchers have demonstrated
that molecular electronics is also a valid post-CMOS
candidate and made another big leap toward reaching
the ultimate limits in miniaturization.
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