monolayer dielectric leads to organic electronics: electronic materials

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