RESEARCH NEWS

JULY-AUGUST 2007 | VOLUME 10 | NUMBER 7-8 13

Despite many years of research, high-temperature

superconductors remain an enigma. In conventional

superconductors, electrons pair up at the

superconducting transition temperature (Tc), creating

an energy gap in the electronic density of states.

However, in high-Tc superconductors, a partial gap

exists above Tc. The question is, is this energy gap

associated with pairing and, if so, at what temperature

do pairs form? Recent research provides a more

detailed picture of this behavior.

A team from Princeton University and the Central

Research Institute of Electric Power Industry in

Japan have performed the first spatially resolved

measurements of energy gap formation in the high-Tc

superconductor, Bi2Sr2CaCu2O8+δ, as a function of

temperature and doping [Gomes et al., Nature (2007)

447, 569]. “We have developed a unique ability to

perform spectroscopic measurements at a specific

atomic site as a function of temperature,” explains

Ali Yazdani of Princeton University. The group used

a specially designed, variable temperature, ultrahigh

vacuum scanning tunneling microscope (STM) to probe

the evolution of electron or Cooper pairs as a function

of temperature (and doping) in real space. By varying

the energy of the tunneling electrons, the STM can

break apart electron pairs. “The key discovery is that

pairs appear not to break up until temperatures well

above the critical temperature and survive in small

puddles (1-3 nm) up to very high temperatures,” says

Yazdani. The finding that Cooper pairs persist in small

regions, even when the entire sample is too warm

to exhibit superconductivity, is key to understanding

superconductivity. “If we can figure out the details of

what is happening at these local patches within the

samples, it might be possible to construct a material

that performs better overall,” says Yazdani.

Cordelia Sealy

Growth of metal nanoparticles without an organic shell is of

importance in the fabrication of conductive nanowires that

require intimate electrical contacts.

Metal nanostructures can now be fabricated chemically

on surfaces with lyophilic and lyophobic patterns by a

technique called wetting driven self-assembly (WDSA), say

researchers from The Weizmann Institute of Science in Israel

[Chowdhury et al., Nano Lett. (2007) doi: 10.1021/nl070842x].

The new approach can be used to immobilize discrete

particles of metal, 2.2 ± 0.5 nm high and 27 ± 6 nm wide

on pre-defined surface sites. The metal features obtained are

stable, suggesting that the route could be used to confine a

wide range of metal species.

In a process known as constructive nanolithography (CN),

Jacob Sagiv and colleagues self-assemble silane monolayers

onto Si substrates, then use a biased scanning tunneling

microscope (STM) tip to oxidize -CH3 groups located at the

surface of the silane electrochemically. Oxidation gives rise

to narrow lines of -COOH groups, creating lyophilic patterns

on a lyophobic background. The modified substrate is then

retracted rapidly through a thiol melt at temperatures well

above the molecule’s melting point. Under these conditions,

wettability drives selective self-assembly of the melt onto the

lyophilic areas. No traces of melt are found on the lyophobic

background. The melt solidifies on the sample upon exposure

to ambient temperature.

Self-assembly of melts is not limited to thiols. Any nonvolatile

material that has an appropriate melting temperature for the

technique and exhibits surface wetting properties could be

used.

Formation of a clean product facilitates further chemical

processing, including oxidation or photoreaction reactions

that may be required to produce immobilization sites for the

metal. A solution of AgCH3COO is used as a source of metal

ions. Once immobilized, the Ag+ ions are reduced to Ag(0),

creating elemental nanoparticles. If required, the nanoparticles

can be treated with a Ag enhancer to increase their height.

Katerina Busuttil

NANOTECHNOLOGY

Map of electron pairs (shown in red) as they form in

Bi2Sr2CaCu2O8+δ. From top left, the same 30 nm2 area

is shown with decreasing temperature. Even at 10°C

above Tc, electron pairs still exist in small regions.

(Courtesy of Ali Yazdani.)

Superconductors find breaking up hard to doMAGNETIC BEHAVIOR

The separation of different kinds of

carbon nanotubes is important

because their electrical, mechanical,

and optical properties are closely

related to their structure. Now a team

of researchers in Japan has taken a

significant step toward the preparation

of a single type of carbon nanotube by

separating nanotube optical isomers

for the first time [Peng et al., Nat.

Nanotechnol. (2007) doi:10.1038/

nnano.2007.142].

Chromatography can be used to

separate nanotubes by size, but little

attention has been paid to the fact

that the resulting samples have equal

distributions of left- or right-handed

helices, says Naoki Komatsu from

Shiga University of Medical Science,

who worked with colleagues at

Kyoto University and Osaka Electro-

Communication University. These

mirror image forms, or optical isomers,

may display different chemical or

optical properties.

To separate left- from right-handed

nanotubes, the team used a chiral

surfactant, meta-phenylene-bridged

zinc(II) diporphyrins. Mirror image

forms of these ‘chiral nanotweezers’

are first synthesized separately. One

version is then introduced into a

suspension of nanotubes in methanol,

where the chiral surfactant forms a

soluble complex with the nanotubes.

After removing the insoluble

component, the surfactant is removed,

leaving a solution enriched with

one chiral form. Circular dichroism

experiments confirm that the resulting

sample is optically active.

Future work will concentrate on

obtaining an ‘optically pure’ sample

of nanotubes with only one chirality.

Komatsu believes this will enable

determination of reliable and precise

physical data of the structure of

carbon nanotubes.

Pauline Rigby

Nanotubes: left or right?NANOTECHNOLOGY

How to create tiny metal patterns