RESEARCH NEWS

JUN-AUG 2008 | VOLUME 3 | NUMBER 3-410

RESEARCH NEWS

Randomness inherent in the

synthesis of metal nanowires has

prevented these structures from

being incorporated into high-density

electronic and optoelectronic devices.

Researchers from the University of

Illinois at Urbana-Champaign have

now demonstrated how a low-

temperature, catalyst-free technique

can produce copper nanowires

that could be suitable for display

applications [Kim et al., Adv. Mater.

(2008) 20, 1859].

Changwook Kim and colleagues use

chemical vapor deposition with a

Cu(ethyl-3-oxobutanoate)[triethyl

phosphate]2 precursor to grow Cu

nanowires on a variety of surfaces.

The diameters and lengths of

the nanowires are controlled by

altering the substrate material, the

temperature, deposition time and

precursor feeding rate.

Structural analysis reveals the Cu

nanowires have a fivefold twinned

structure, terminating in a sharp,

pentagonal tip and current-voltage

investigations indicate that they are

also promising electron emitters.

An array of Cu nanowires was grown

on a patterned Si substrate to test the

potential of this growth process in a

practical application. Electrons emitted

from the nanowire tips were used to

activate the letters ‘UI’ on a phosphor-

coated glass anode. “Packing a large

number of emitters into a patterned

area may be an attractive option

for electron emitters that may help

achieve a long-lasting field emission

display,” the authors wrote.

The catalyst-free synthetic procedure

is compatible with contemporary

silicon-processing protocols, adds

coauthor Hyungsoo Choi. The low

processing temperature makes this

method suitable for the use of metal

nanowires in plastic electronic displays.

Paula Gould

Nanowires go catalyst-free NANOFABRICATION

Post-fab melt perfects microchip detailsNANOFABRICATION

Improved fabrication techniques have made it possible

to control the size and shape of nanostructures with

greater precision. However as the required feature

size shrinks, defects caused by physical limits in the

nanofabrication process will begin to dominate the

structures produced, compromising performance. It

may also become impossible to reduce the size of

features any further.

Tackling this issue requires a paradigm shift, says

Stephen Y. Chou, professor of engineering at Princeton

University. Instead of struggling to improve existing

nanofabrication methods, imperfections to desired

components should be dealt with as a second, separate

step, he suggests.

Chou’s proposed method, developed with graduate

student Qiangfei Xia, involves selectively melting

flawed nanostructures for a short period of time

(hundreds of nanoseconds) while guiding the

molten material into the required shape prior to re-

solidification [Chou and Xia, Nature Nanotech. (2008)

doi: 10.1038/nnano.2008.95]

The pair tested this method, termed self-perfection by

liquefaction (SPEL), using a 20-ns excimer (λ = 308 nm)

laser pulse to perform the melt. With open SPEL

(no guide) they reduced line-edge roughness on

70 nm-wide chromium grating lines from 8.4 nm to

less than 1.5 nm. Placing a quartz plate in contact with

the top surface (capped SPEL) kept the sidewalls and

top surface flat during the melt. On adding spacers

between the surface and the plate (guided SPEL),

they reduced the width of a silicon line from 285 nm

to 175 nm, while increasing its height from 50 nm

to 90 nm.

Donald Tennant, director of operations at the

NanoScale Science at Cornell University, notes

that the techniques: “may be a way forward when

nanofabricators bump up against the limits of

lithography and pattern transfer.”

The authors point out that SPEL cannot be applied

when the dimensions of the defect are comparable

with the nanostructure’s dimensions, and it cannot fix

defects without sufficient total material. Applicability

to complex structures has yet to be studied. A series of

investigations using large (~20 cm) wafers are planned.

Paula Gould

Scanning electron micrographs of nanostructures before

(left) and after (right) treatment with a single excimer

laser pulse. (© 2008 Nature Publishing Group.)

Interacting dots hold key to information processingOPTICAL MATERIALS

Quantum dots (QDs) and the electrons that can be trapped in

their discrete energy levels are of great interest for quantum

information processing. The spin state of these trapped

electrons could act as carriers of quantum information or

‘qubits’.

Researchers at ETH in Zürich, Switzerland have taken an

important step toward quantum information processing by

demonstrating conditional dynamics for two coupled quantum

dots [Robledo et al., Science (2008) 320, 772].

“To process quantum information, an essential ingredient is a

system of two qubits that are coupled to each other,” explains

Atac Imamoglu of ETH. “More specifically, what is required is

‘conditional dynamics’, where the evolution on the state of

one qubit is controlled by the state of the other.”

Using a GaAs device containing two layers of self-assembled

InGaAs QDs separated by a 15 nm potential barrier, the

researchers show that the probability of whether a quantum

dot makes a transition to an optically excited state is

determined by the presence (or absence) of optical excitation

in a neighboring QD. The interaction between the neighboring

QDs relies on quantum mechanical tunneling between the

optically excited states. This enables the coupling to be tuned

and also means that the effects are much larger than in other

coupling approaches based on dipole-dipole interactions.

Applying a laser field of a particular frequency, which can be

done on a very fast timescale, can turn the interaction on or

off.

“This is a huge step toward realizing a solid-state two qubit

gate,” comments Richard J. Warburton of Heriot Watt

University in the UK.

The researchers suggest that the next steps will be to

demonstrate the general conditional interaction mechanism

for two coupled spins. Eventually, they hope to show that

their approach can be used to control the quantum state of

two coupled QDs in a time-resolved way.

Cordelia Sealy

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