monolayer dielectric leads to organic electronics: electronic materials
TRANSCRIPT
RESEARCH NEWS
APRIL 2007 | VOLUME 10 | NUMBER 4 15
Monolayer dielectric leads to organic electronicsELECTRONIC MATERIALS
Self-assembled monolayers are promising as gate
dielectric materials for making low-voltage organic
transistors. Studies have already shown how they can
be grown on Si and substrates coated with indium
tin oxide (ITO). Researchers from Germany have now
moved this work forward by growing high-quality,
monolayer dielectrics on patterned metal gates. [Klauk
et al., Nature (2007) 445, 745].
The key to making low-voltage transistors is to
find a gate dielectric that still shows small leakage
currents when deposited in very thin layers. “With
thick dielectrics, you have to apply a relatively large
voltage to induce enough charge carriers to get useful
transistor operation,” explains Hagen Klauk of the Max
Plank Institute for Solid State Research.
Klauk and colleagues used n-octadecylphosphonic
acid so that assembling molecules could anchor to a
20 nm thick Al surface with phosphonic acid groups.
The researchers recorded a large capacitance for the
monolayer dielectric [0.7 ± 0.05 µF/cm2] and a low
leakage current density [(5 ± 1) x 10-8 A/cm2].
The monolayer-based dielectric was then incorporated
into n-channel (pentacene) and p-channel (hexadeca-
fluorocopperphthalocyanine, F16CuPc) organic thin-
film transistors to give low-voltage complementary
circuitry. Static power consumption in complementary
inverters, NAND gates, and ring oscillators with supply
voltages of 1.5-3 V is just 1 nW per logic gate.
While the performance of the monolayer-based
dielectric was largely as expected, the team was
pleasantly surprised by the high level of reliability
and reproducibility. “These materials naturally self-
assemble into films that are of very high quality and
have a very low density of defects,” notes Klauk.
He regards this work as one small step toward the
viability of organic electronics in a wide range of
battery-powered applications.
Paula Gould
Many groups have measured the
conductance of single molecules
held between two electrodes with
the aim of developing molecular
electronic devices. Researchers at
the University of California, Berkeley
decided instead to investigate the
thermoelectricity of a molecular
junction [Reddy et al., Science (2007)
doi: 10.1126/science.1137149].
A number of approaches can measure
the current-voltage characteristics
of molecular junctions, but the
position of the Fermi level of the
electrical contacts with respect to
the molecular orbitals often remains
unknown. This means that even
whether the junctions are p- or n-
type can be unclear.
By measuring the Seebeck coefficient S of a benzenedithiol
(BDT) junction, the researchers can address unanswered
questions about its electronic structure and begin to explore
the possibility of cheap thermoelectric energy conversion
using organic molecules.
Thermoelectric converters can generate electric power
from waste heat. They make use of the Seebeck effect in
which a temperature difference across a material results in
the generation of an electrical potential. While S is usually
associated with bulk materials, it can also
be defined for a molecular junction. Since
the value of S depends on the junction
energy barrier, it can also be used to
determine the sign of the charge carriers
and the position of the Fermi level of the
Au electrodes.
The researchers. led by Arun Majumdar,
measured the voltage generated across
a BDT molecule trapped between the Au
tip of a scanning tunneling microscope
and a Au substrate held at different
temperatures. For the BDT junction,
S = 8.7 ± 2.1 µV/K , making it p-type with
a Fermi level ~1.2 eV from the highest
occupied molecular orbital.
“[The results imply] that these assemblies
can be used to garner additional electricity
from the hot steam exiting a turbine or from other heat-
releasing processes,” says Rachel Segalman. “This is an
important advance in thermoelectrics since organics are
comparably less expensive and more easily processed that
the current leading thermoelectric materials. We are currently
working to understand the relationship between molecular
structure and thermopower, and assembling molecules
and nanoparticles into larger scale devices for use as
thermoelectrics, capacitors, and photovoltaics.”.
Jonathan Wood
Molecular device may herald cheap thermoelectricityELECTRONIC MATERIALS
Heating one side of the molecular
junction generates current. (Courtesy
of Ben Utley.)
There can be a trade-off between
a metal’s strength and ductility.
Nanocrystalline metals show increased
strength (typically by a factor of five)
with added benefits in weight and
energy savings, but often at the price
of ductility. In recent years, nano-
twinned Cu with superior ductility
has been synthesized using pulsed
electrodeposition. Thanks to atomistic
simulations, a team from Georgia and
Massachusetts Institutes of Technology
(MIT) and Ohio State University
understand why [Zhu et al., Proc. Natl.
Acad. Sci. (2007) 104, 3031].
Atomic-level simulations, commonly
molecular dynamics (MD) combined
with experiment are routinely used to
study the mechanical deformation of
nanostructured materials. However,
such simulations cannot give
much information about changes
in microstructure arising from
deformation on the time scale of most
experiments. “To overcome the time
scale limitation, we [have] developed
a novel atomistic reaction pathway
exploration method that combines
the static atomic-scale calculation
with the theoretical analysis based
on the transition state theory,” says
Subra Suresh of MIT. This approach
has been applied to chemical reactions
and diffusion, but the researchers have
adapted it to handle nanomechanical
deformation mechanisms over longer
time periods and larger atomic
systems. “It turns out that… the twin
boundary plays a key role in both
enhancing the strength and retaining
significant ductility in the material,”
he says. The origin of ductility lies
in the interaction of line defects
(dislocations) with interfaces. Control
of plastic flow by planar defects,
i.e. twin boundary hardening, can
explain the improved ductility of
nanostructured Cu.
Cordelia Sealy
Defects define ductilityMECHANICAL PROPERTIES