WE all know diamonds, or we think
we do. Diamonds are a girl’s best friend, sang Marilyn Monroe
in the movie Some Like it Hot. They appear on many a third finger,
brilliantly faceted and sparkling. On a more practical note, because
diamonds are one of the hardest substances on Earth, the industrial
sector makes extensive use of them.
would suspect then that the most miniscule bit of diamond, just
a few hundred atoms, would take on the exotic shape of a fullerene
or buckyball at the surface? At the nanoscale—a nanometer
is a billionth of a meter or 1,000 times smaller than the diameter
of a human hair—materials behave differently than they do
in their larger, bulk form. In this size regime, the laws of quantum
the recent revelation that the outer surface of a molecule of diamond,
a nanodiamond, is shaped like a soccer ball came as a real surprise.
The discovery was made by a Livermore team that for the first time
computationally modeled nanodiamonds to determine their optical
properties. They had previously modeled two similar semiconductor
materials, silicon and germanium, and expected about the same results.
Giulia Galli, who leads Livermore’s Quantum Simulations Group,
says, “At the nanoscale, the surface of silicon and germanium
rearranges its atomic geometry in a way that somehow compresses
the core of the nanostructure.” To their amazement, the team
found that nanodiamond expands, with a crystalline diamondlike
core and a fullerenelike structure around it at the surface.
first fullerene was a 60-atom carbon buckyball. Livermore’s
simulations are the first to reveal bucky diamonds, a new family
of carbon clusters. The discovery of the bucky diamond was just
one recent finding by a research team led by Galli and Tony van
Buuren, an experimental physicist. The team performs quantum molecular
simulations and scrutinizes materials experimentally as they seek
to better understand the properties of the semiconductor materials
silicon, germanium, and diamond at the nanoscale. As part of the
Group IV series of elements on the periodic table, these three
materials share some interesting properties, as described in the
The Periodic Table
semiconductor is a crystalline solid that in its pure
form exhibits a conductivity midway between that of
metals and insulators. The three semiconductor materials
that Livermore is studying for possible use as sensors
and detectors are silicon, germanium, and diamond.
Silicon accounts for almost 99 percent of all commercial
semiconductor products. Germanium became famous when
the transistor was invented but has since been replaced
largely by silicon. Diamond, a monocrystal of carbon,
has the physical properties of a wide-optical gap semiconductor,
but current technologies do not allow its use as a
three materials comprise some of the
Group IV elements on the periodic table,
as shown below. Tin, the fourth potential
semiconductor material in this group,
has the physical properties of a semiconductor
at low temperatures but at room temperature
behaves like a metal. These four materials
are elemental semiconductors.
in Groups II and VI and in Groups III
and V are often combined to form compound
semiconductors. Gallium–arsenide is a typical Group
III/V compound semiconductor often used in microwave
devices and optoelectonics. Most
experiments designed to explore the optical properties
of semiconductor nanoclusters have focused on such
Group II/VI compound semiconductors as cadmium–selenium.
contrast, the synthesis of covalently bonded nanoparticles
such as silicon has proven to be much more challenging.
Silicon and other Group IV semiconductor elements
are thus much less well characterized than Group
II/VI compounds, and the interplay of quantum confinement
and surface properties is less clear. Yet silicon
is the preferred material for biomarkers because
of its compatibility—at least in its bulk form—with
biological materials. Silicon nanoclusters could
also be integrated with existing silicon technologies
to create nanoscale optoelectronic devices. Germanium
and nanodiamond have been studied much less than
silicon, but their intriguing characteristics inspire
hope that they may be useful as well.
to physical chemist Lou Terminello, materials program leader for
the Defense and Nuclear Technologies Directorate, Livermore research
on semiconductor nanostructures—also known as nanodots or
quantum dots—is aimed primarily at using them in detectors
to reveal the presence of biological or chemical warfare agents.
A protein added to the surface of one of these nanoparticles would
change when exposed to a biological agent, serving as an indicator.
the nanoscale, silicon and germanium emit light when stimulated.
Nanodiamond, which has more recently come under the microscope,
may also change its optical properties as a function of size. In
bulk form, all three semiconductors are compatible with biological
materials and so could easily be linked with a protein. Whether
this biocompatibility still exists when the semiconductors are
reduced to the nanoscale remains to be determined. If these nanosemiconductors
are indeed biocompatible, their optical, or light-emitting, properties
could be exploited to detect specific molecules.
uses for light-emitting semiconductor nanoparticles include photonic
switches, tunable lasers, and nanocrystal solar cells. Terminello
adds, “Quantum dots will likely be some of the next-generation
materials for targets at the National Ignition Facility.” By
starting with nanostructures, scientists could dictate the target’s
precise design. Atom by atom, they could gradually build up the
targets in size from the nanoscale to macroscopic structures.
study of the very small—is fundamental to U.S. research.
Funding by the U.S. government for nanoscience and nanotechnology
is higher than ever, just behind defense spending and funding for
biological research. Before tiny bits of any material can be put
to use, their unique properties must be better understood. As the
recent discovery of the bucky diamond illustrates, the world of
Group IV semiconductor nanostructures is still a mystery.
or buckyballs, are soccer-ball-shaped molecules named for R.
Buckminster Fuller, whose popular geodesic dome is structurally
similar to a fullerene molecule. In first-principles simulations
of nanodiamond, (a) the surface of a 1.4-nanometer nanodiamond
with 275 atoms spontaneously rearranges itself into (b) a fullerene
at about 300 kelvins. These carbon clusters have a diamond
core (yellow) and a fullerenelike reconstructed surface (red).
(c) A classic 60-atom carbon buckyball.
Small Size, Big Change
any piece of material from a chunk that we might recognize to the
nanometer scale changes virtually all of its most basic properties
in a fundamental way. Its shape and crystalline structure change,
as do its melting and boiling temperatures. Its magnetic properties
may be different at the nanoscale. Its optical and electronic properties
a nanosemiconductor, an effect known as quantum confinement occurs
when electrons and “holes” in the material are confined.
(A hole is the absence of an electron; the hole behaves as though
it were a positively charged particle.) Typically, quantum confinement
causes the material’s optical gap—the energy difference
between filled states and empty states—to widen. A larger
optical gap prompts dramatic changes in electronic and optical
properties. Bulk silicon when stimulated does not emit visible
light, but in 1990, researchers found that nanoparticles of silicon
researchers and others have since determined that silicon nanoparticles
emit different colors of light depending on their diameter. In
1997, germanium nanoparticles were found to emit light. In the
last two years, other Livermore scientists have discovered that
the optical gap of nanodiamond does not change until its size is
reduced to less than 2 nanometers.
are also different from the bulk form of the material in that the
percentage of atoms at or near the surface of the particle is far
greater. The surface of nanoparticles thus plays a large role in
determining the particle’s electronic and optical properties.
An image of silicon nanocrystals
using an atomic force microscope. The nanocrystals, produced
using gas-phase vaporization, range from 1 to 4 nanometers.
Note how the nanoclusters gather at the step edges of the
graphite substrate or assemble into snowflakelike superclusters.
It Stared with Silicon
first work with Group IV semiconductor nanostructures took place
in the mid-1990s. The photoluminescence of silicon had only recently
been discovered, indicating that this element might be a promising
material for optical applications.
researchers used a gas-phase vaporization process, in which melted
silicon was heated and vaporized in the presence of a buffer gas,
to synthesize silicon particles ranging from 1 to 6 nanometers.
Numerous production techniques exist, but most of them allow only
limited size control of the resulting particles. They also produce
particles with a specific surface chemistry that is less useful
for investigations of precise electronic structure.
hydrogen or oxygen was then bonded to the surface of the tiny molecules
to “passivate” the dangling bonds of highly reactive
silicon. Using spectroscopic and x-ray absorption techniques to
probe the particles’ characteristics, the Livermore team
was the first to measure the band edges of the optical gap of silicon
and to determine that the gap changes as the nanoclusters become
smaller. These findings clearly indicated the importance of the
quantum confinement effect on the optical properties of silicon
fine-tuning of the synthesizing process made it possible to produce
silicon nanoclusters in an even narrower distribution of sizes
(±7 percent of average size) as measured using an atomic
force microscope. Work in the late 1990s definitively correlated
quantum confinement changes as a function of the size of silicon
nanoparticles, in agreement with quantum confinement theory.
Livermore and other research institutions worldwide experimented
further with semiconductor nanoclusters, their potential uses as
biological markers and nanostructure lasers became more evident.
With increased concerns about bioterrorism, van Buuren and Galli
obtained funding from the Laboratory Directed Research and Development
Program to develop atomically controlled nanostructures for biowarfare
detectors. Their team, composed of researchers from the Physics
and Advanced Technologies and the Chemistry and Materials Science
directorates, is relatively large. As interest in all things nano
has burgeoned, the number of nanoscience experts at Livermore has
The “idealized” structure of a 1.8-nanometer silicon
nanodot. When the molecule is removed from a larger piece of
bulk silicon, each silicon atom is ideally terminated with
a hydrogen atom. (b) But at the nanoscale, the molecule instead
reconstructs. Its electronic properties change considerably,
increasing the optical gap.
Simulations Verify and Surprise
traditional purpose of computerized simulations of physical phenomena
is to verify experimental findings. But simulations can also go
where an experiment cannot. This is especially true for examining
the surface of nanoclusters. The effects of quantum confinement
on semiconductor nanodots can be obtained experimentally; however,
the changes in the properties of the comparatively large surface
area of a nanostructure are difficult to determine in experiments.
First-principles simulations, which do not contain any input from
experimental data, are a valuable tool for discovering the dependence
of a nanostructure’s optical and mechanical properties on
its surface structure.
Livermore’s massively parallel supercomputers, Galli’s
group has undertaken several computational studies of the surface
chemistry of Group IV semiconductor nanoclusters. An early study
used density functional theory and quantum Monte Carlo codes
to perform first-principles calculations of the surfaces of silicon
nanoclusters. The group examined the effect of replacing one
or more atoms of a hydrogen-passivated silicon nanocluster with
other passivants. A remarkable change results when just two hydrogen
atoms are replaced by more reactive oxygen atoms. The electron
charge cloud is drawn toward the oxygen atom, dramatically changing
the optical properties of the silicon dot.
these and many similar calculations, the group has concluded that
quantum confinement is only one mechanism responsible for a semiconductor’s
light-emitting properties. For example, they have confirmed experimental
findings by researchers outside the Laboratory that oxygen passivation
of silicon dots reduces their optical gap while hydrogen passivation
study modeled spherical silicon clusters ranging from 53 to 331 atoms
(0.7 to 2.0 nanometers), the largest nanoparticles ever studied
with the highly accurate quantum Monte Carlo technique. A team
examined the process of surface reconstruction—in which unstable
dangling bonds on a nanoparticle’s surface spontaneously
rearrange themselves—and its effects on the particle’s
optical properties. In this study, the team found that reconstruction
of the surface of silicon nanostructures could have the effect
of compressing the nanoparticle. “Time and again, we have
found that the specific surface chemistry must be taken into account
if we want to quantitatively explain the optical properties of
semiconductor nanoparticles,” says Galli.
Livermore’s latest gas-phase
chamber for synthesizing nanosemiconductors is portable
so that nanodots can be prepared and deposited in situ.
This gas-phase condensation technique works for virtually
all elements, is ultra clean, and produces a wide range
of sizes of nanocrystals whose surface chemistry can be
Germanium Joins the Fray
germanium was used extensively in early semiconductor devices,
it has since been displaced by silicon as the substrate for most
devices. But the 1997 discovery that nanodots of germanium emit
light sparked a new interest in this element.
he was at Livermore as a graduate student of the University of
Hamburg, Germany, physicist Christoph Bostedt improved Livermore’s
earlier vaporization chamber for synthesizing semiconductor nanoparticles.
Among the many modifications he made, the chamber can now synthesize
nanoparticles composed of virtually any element. Using this chamber,
Bostedt found that by varying preparation parameters, he could
dictate the size of the resulting germanium particles.
a Livermore postdoctoral fellow, Bostedt is using synchrotron
radiation at Lawrence Berkeley National Laboratory’s Advanced
Light Source (ALS) for photoemission spectroscopy and x-ray absorption
studies of the electronic microstructure of germanium nanocrystal
films. “Using ALS, we have produced spectra for germanium
that are some of the best obtained anywhere,” he says.
recently, his team has shown in experiments with ALS that quantum
confinement effects are greater in germanium nanocrystals than
in silicon nanocrystals for particles smaller than 2 nanometers.
The strong confinement they observed and the fast opening of the
optical gap—which translate into a highly “tunable” material—indicate
germanium nanocrystals would be especially useful in detectors
and optoelectronic applications that require extreme sensitivity.
the theoretical community, others have made similar predictions
about the quantum confinement of germanium versus silicon, although
considerable controversy exists. The Livermore team is the first
to make this discovery experimentally using thin films of germanium
nanocrystal, finding that the behavior of germanium nanocrystals
is as sensitive to changes at the surface as silicon. “We
believe that disagreements between our experimental results and
some theoretical predictions are due to the structural details
of the nanocrystals,” says Bostedt. “The structure,
especially at the surface, of nanocrystals cannot be ignored.”
models that do not use sophisticated quantum simulations typically
use idealized nanocrystals isolated in space and not resting on
any surface. The nanodot’s atomic structure is almost always
ignored as well. In contrast, a quantum Monte Carlo investigation
at Livermore into the structure and stability of germanium nanoparticles
revealed the key role that structure plays. The simulations team
found that the surface of germanium nanodots reconstructs when
their diameters are smaller than 2.5 to 3 nanometers, a geometric
rearrangement that agrees with the Laboratory’s photoemission
experiments at ALS.
Most applications using nanomaterials
will require thick films of nanocrystals, such as these
germanium nanodots shown under an atomic force microscope.
The Surprising Nanodiamond
the most recent Group IV semiconductor to be examined at Livermore,
offers plenty of surprises. Livermore is one of the few research
groups in the world to perform quantum simulations of nanodiamond
data show that the size of nanodiamond must be reduced to less
than 2 nanometers before its optical gap increases beyond
that of the bulk form. This behavior differs dramatically from
that of silicon and germanium where quantum-confinement effects
persist in particles of up to 6 and 7 nanometers. These results
came from both computer simulations and x-ray absorption and emission
experiments using ALS and the Stanford Synchrotron Radiation Laboratory.
Both studies aimed to derive a structural model for nanodiamond.
bucky diamond appeared during calculations of surface reconstruction
of 1.4-nanometer diamond particles, which Galli performed with
physicist Jean-Yves Raty of Livermore and the University of Liege,
Belgium. These simulations started with bare, unpassivated nanodiamond.
At low temperature, the bucky diamond reconstruction occurred spontaneously.
The first faceted layer took on the properties of graphite, which
was followed by the formation of pentagons linking the graphene
fragments with atoms underneath. This change made the surface increasingly
curved, eventually resulting in an arrangement like half of a 60-atom
carbon molecule, the classic buckyball. Simulations showed similar
results for surface reconstructions of 2- and 3-nanometer clusters.
results point yet again to the importance of nanoparticle surfaces. “When
the calculations and measured spectra of nanodiamonds are compared,” says
van Buuren, “it becomes clear that the surface reconstruction
identified by computer simulations is consistent with the features
observed in absorption spectra.”
is interesting because it has been found in meteorites, interstellar
dusts, and protoplanetary nebulae, and it appears in residues of
detonation. (Nanoparticles of diamond for Livermore experiments
are obtained through synthesis from detonation.) And regardless
of whether they come from meteorites or detonation, most nanodiamond
particles fall in the 2- to 5-nanometer range. Other nanomaterials
display a much wider range of sizes even at this small scale.
and Galli used computational methods to explore the causes for
this size limitation. The team found that at about 3 nanometers—and
for a broad range of pressure and temperature conditions—particles
with bare, reconstructed surfaces become thermodynamically more
stable than those with hydrogenated surfaces, and hydrogenation
prevents the formation of larger grains.
Simulation of a silicon nanostructure in water. (b) and (c)
Monte Carlo simulations from first principles of nanodiamond
precursors in water. Both (b) methane and (c) silane are forms
of carbon and are hydrophobic; that is, they repel water, just
as oil repels water. Although the methane and silane are similar
structurally, they interact with water quite differently.
Prediction Is the Goal
how size and surface affect optical and electronic properties is
what our research is all about,” says van Buuren. Experimentalists
and quantum simulation experts are working together to establish
a basic knowledge of the structure and optical properties of semiconductor
nanostructures. Their goal is to match these two sets of data and
form an ability to predict the characteristics of nanoparticles.
Someday, a scientist will know exactly how to produce a nanowidget
to detect a deadly pathogen. Perhaps the widget must emit blue
light, and the scientist will know that using a nanoparticle of
a given size and density produces the desired wavelength.
the meantime, moving toward that goal, Livermore researchers are
beginning to observe the interaction among nanostructures. One
team recently performed quantum simulations of the interplay of
silicon quantum dots, an inorganic material, and organic molecules,
which will be essential in a semiconductor biodetector. In particular,
investigators simulated what occurs when organic molecules are
attached to silicon quantum dots. They found that the probability
of attaching an organic molecule to a nanodot is greatly increased
if light shines on the nanodot, a result that agrees with recent
experimental findings by others. Their simulations also indicated
a way to select silicon quantum dots with a specific optical gap
at the same time that organic molecules are being attached.
on Bostedt’s agenda is to make thick films on which germanium
particles are closer together and touching, which is how they will
be in real-world applications. Unfortunately, when they touch,
nanosemiconductor particles tend to lose some of their special
electronic properties. Bostedt has developed a surface passivation
technique that keeps the particles isolated, reducing the effect
of touching. Further experiments will examine the interface where
interactions occur between passivated layers to determine what
happens to the electronic properties of the entire device.
on the team are starting simulations and experiments to explore
the structural and optical properties of silicon and germanium
nanoparticles in solution. A new two-step cluster aggregation source
is under development that will allow for wet chemical modification
of the surface of crystalline silicon nanoparticles produced in
the gas phase. This work is a major step toward producing silicon
nanoparticles that are useful for biological applications.
the world faces ever-changing threats and often-unseen enemies,
research that enhances our ability to respond is critical. With
this research on nanosemiconductors for use in biodetectors,
Livermore brings its unique computational and experimental expertise
to bear on an issue of extreme importance.
Key Words: biodetector, buckyball, germanium, nanocluster,
nanocrystal, nanodiamond, nanoparticle, nanoscale, nanoscience, nanotechnology,
quantum dot, quantum molecular simulations, semiconductor, silicon.
For further information on experiments, contact Tony van
Buuren (925) 423-5639 (firstname.lastname@example.org), or on simulations,
contact Giulia Galli (925) 423-4223 (email@example.com).
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