发信人: maodouzi (毛豆子), 信区: NanoST
标 题: Applying a 250-year old discovery to nanotechnology fabrica
发信站: BBS 未名空间站 (Mon May 21 13:26:43 2007), 转信
Back in 1756, the German physicist Johann Gottlob Leidenfrost published a
manuscript titled De Aquae Communis Nonnullis Qualitatibus Tractatus ("A
Tract About Some Qualities of Common Water") in which he described a
phenomenon in which a liquid, in near contact with a mass significantly
hotter than its boiling point, produces an insulating vapor layer which
keeps that liquid from boiling rapidly. This effect came to be called the "
Leidenfrost Effect" and the associated temperature point the "Leidenfrost
Temperature." An everyday example of this can be seen in your own kitchen:
sprinkle a drop of water in a hot skillet — if the skillet's temperature is
at or above the Leidenfrost Temperature, the water skitters across the
metal and takes longer to evaporate than it would in a skillet that is hot,
but at a temperature below the Leidenfrost point. Researchers in Germany
have used this effect for a novel, template-free synthesis and patterning
method of nanostructures.
While nanoscale self-assembly is one of the core concepts of Nature,
scientists are only beginning to scratch the surface of the potential that
self-assembly holds for materials engineering. The list of self-assembly
fabrication methods for the formation of nanocluster structures seems to get
longer and longer but they all feature different timescales, complexities
and versatilities. The one thing they usually have in common is the
requirement for two steps – the formation of nanoparticles from precursors
in the liquid, solid, or gas phases employing either chemical or physical
deposition processes and, in a subsequent step, the organization of these
particles into useful structures.
Nanoscientists have recognized that template-free approaches of self-
organization would be the simplest and most effective way of building
nanostructures from the bottom up, as just recently witnessed by IBM's
announcement of the first-ever application of a breakthrough self-assembling
nanotechnology to conventional chip manufacturing.
"Wet-chemical strategies utilizing fluid mechanics appear to be the simplest
and most effective way of template-free, self-assembling nanostructuring"
Dr. Rainer Adelung explains to Nanowerk. "However, structuring of
nanocluster arrays or wirelike morphologies from a droplet still faces
Adelung, a researcher at the Department of Multicomponent Materials at the
University of Kiel, Germany, tells the story of his colleague Mady Elbahri
experimenting with new routes for ZnO synthesis and discovering a novel way
for a droplet-disposition-based nanostructuring technique.
"Inspired by kitchen experiments with his wife Julia, he discovered that it
might be possible to ignore the typical temperature limit of 100°C for
water-based synthesis. He found that at temperatures of 250°C, well above
the boiling point, it is possible to use water droplets that contain a small
amount of chemicals for nanostructuring – thanks to the Leidenfrost effect
What the researchers in Kiel ultimately developed is a droplet-deposition-
based, template-free, and rapid (only a few seconds) approach for
fabricating nanostructures without the use of any surfactant.
"Our general setup can be understood as the so-called “anti-Lotus effect”"
says Adelung. "The Lotus effect is well known for its removal of dust
particles from the surface of a lotus leaf by gathering them into a droplet
that is moving over the surface, thus cleaning it. The effect is based on
the ability of certain surfaces to form spherical droplets with contact
angles near 180° (i.e., superhydrophobic), enabling the incorporation of
surface particles as well as a reduction in friction. In contrast to this,
our work makes use of an anti-Lotus effect, in which the droplet delivers
material while moving over the surface."
It seems that, similar to room temperature droplets on superhydrophobic
surfaces, a Leidenfrost droplet can move with highly reduced friction over
an arbitrary surface such as plain silicon and deposit nanoparticles. These
can be nanoparticles already dispersed in the water droplet or formed in the
overheated steam from a mix of chemicals. In this way, reactions leading to
nanoparticle synthesis can be performed that do not occur at room
temperature (which the Kiel researchers have come to call the "Elbahri
Combining this method with a top down approach enables the formation of
regular patterns. A grid with regular pattern of openings is indeed enough
to force the self organized structures into a reproducible shape.
The left side of the figure shows such a pattern in a large scale AFM image
(baseline is 90 µm). Silver rings that form inside the opening made from
silver nitrate are stacked into each other. The larger the diameter is, the
smaller the ring cross section down to the nanoscale. The image on the right
shows a three dimensional representation from an AFM image of a silver
nanowire. (Images: Kahled Hirmas, Rainer Adelung)
Adelung points out that a random deposition of a solution drop would be
totally ineffective in fathering any technologically useful nanoscale
structures, such as arrangements of well-separated patterns prepared from
"In order to deposit material from a droplet in an organized manner, a
deeper insight into the underlying mechanism of the fluid drop is necessary"
The researchers prepared an extremely dilute solution of the desired
material powder in water. Then they gently placed a droplet from this
solution on a substrate, such as a silicon wafer, that was maintained at a
temperature of 230°C, which is the estimated Leidenfrost temperature for
any suspension droplet. They allowed the drop to stay on the hot substrate
for approximately 5 seconds before the substrate was tilted ca. 30° to the
horizontal to exploit gravity for the propagation of the loaded droplet
across the substrate surface. When a water droplet loaded with silver
nanoparticles was subjected to this procedure, wires or cluster chains were
"Apart from straight, parallel lines in series, nanoparticles can also be
arranged in concentric circles with our Leidenfrost structuring" says
Adelung. "We attribute this patterning to a different and interesting
phenomenon that also occurs at the Leidenfrost temperature, under slightly
modified fluid dynamics, where droplet impact is emphasized. Interfacial,
viscous, and capillary forces are the three basic forces governing the
behavior of a droplet on a surface. At temperatures below the Leidenfrost
temperature, we observed a spreading behavior that results in the formation
of a disk like thin film upon impact."
In a next step in their research, Adelung and his colleagues are now
planning to measure the electronic properties of the nanostructures for
possible sensor applications.
A recent paper in Advanced Materials describes this new nanopatterning
method ("Anti-Lotus Effect for Nanostructuring at the Leidenfrost
By Michael Berger, Copyright 2007 Nanowerk LLC
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