– Take something no wider than a human hair and shrink
it a thousand fold to a few nanometers across, and
its electronic and other properties change radically.
But whether the crystal structure of these nanoparticles
remains basically the same is a matter scientists
continue to debate.
Now, a new report by scientists at the University
of California, Berkeley, and Lawrence Berkeley National
Laboratory (LBNL) shows that's far from the case.
Zinc sulfide nanoparticles a mere 10 atoms across
have a disordered crystal structure that puts them
under constant strain, increasing the stiffness of
the particles and probably affecting other properties,
such as strength and elasticity, according to the
"In this material, disorder and a kind of strain
is pervasive throughout the whole particle,"
said Benjamin Gilbert, a postdoctoral fellow at UC
Berkeley. "That is an important observation,
because it emphasizes that the assumption of bulk
structure is not good enough. We would expect to find
this kind of behavior in a wide range of semiconducting
The disordered structure could have an effect on properties
other than stiffness, Gilbert said. He and colleagues
in the laboratory of Jillian F. Banfield, UC Berkeley
professor of earth and planetary science, also are
looking at potential changes in optical and electronic
Aside from helping researchers understand these submicroscopic
nanoparticles, the new findings could help scientists
better predict the properties of new nanoparticles
and custom design them with specified properties.
The report by Gilbert, Banfield and their colleagues
was published July 1 on Science magazine's online
Web site, Science Express.
Nanoparticles are one of many nanometer-size materials
that are the focus of university and industry research
today, with potential applications as sensors, solar
power generators, electronic circuit elements, lasers
and numerous other nanodevices. Nanoparticles also
are akin to nanometer-size minerals produced naturally
by some bacteria, which are of interest as possible
telltale signs of life on other planets.
Last year, Gilbert, Banfield, research scientist Hengzhong
Zhang and their colleagues reported that the crystal
structure of nanoparticles made of the semiconducting
material zinc sulfide (or zinc sulphide, ZnS) changes
when it gets wet, which means surface interactions
have a large effect throughout the nanoparticle.
The scientists' latest findings reinforce that message.
"The presence of the surface is a dominant structural
perturbation that makes its effects felt throughout
the nanoparticle and in the material's properties,"
The researchers studied particles of ZnS containing
only 700 atoms and a mere three to four nanometers
across, which means the center is only about five
atoms away from the surface.
In similar semiconducting nanoparticles, such as those
made of cadmium selenide, slight differences in size
lead to absorption and emission of different wavelengths
of light, making them useful as fluorescent tracers.
The dominant cause of such properties is quantum mechanical
confinement of the electrons in a small package. But
the disordered crystal structure now found in nanoparticles
could affect light absorption and emission also.
"The point is that a nanomaterial's properties
are a direct consequence of the structural changes
and hence not simply related to the bulk materials'
properties," Gilbert said. "If you make
nanoparticle, say a small piece of zinc sulfide, the
idea that in the middle it's basically bulk-like with
maybe a bit of relaxation on the surface doesn't work."
These results come after Banfield's team, in collaboration
with Glenn A. Waychunas at LBNL, developed a new way
of analyzing X-ray diffraction images of nanoparticles
so as to separate the effects of size from those of
disordered structure, which are similar and have been
difficult to distinguish. The X-ray experiments were
conducted with the Advanced Photon Source at Argonne
National Laboratory and with an X-ray beamline at
the Stanford Synchrotron Radiation Laboratory.
Gilbert noted that X-ray diffraction of single nanoparticles
is not yet possible, but their technique using collections
of similar nanoparticles provides a quantitative description
of disorder and strain within nanoparticles.
The Science paper is coauthored by Gilbert, Banfield,
Zhang, Waychunas and Feng Huang, a postdoctoral researcher
in the UC Berkeley Department of Earth and Planetary
Sciences. The work is funded by the United States
Department of Energy, the National Science Foundation