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ScienceWeek
SCIENCEWEEK
ScienceWeek
March 21, 2003
Vol. 7 Number 12
An Online Digest of Research in the Sciences
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Even in the quite recent past, the physical processes taking
place in solids were assumed to take place on two length scales,
one being the microscopic level represented by individual atoms,
and the other being the macroscopic level more akin to our
everyday experience. We have now discovered that many interesting
effects occur on what has been called the mesoscopic scale, which
is somewhere between these two extremes... The future
applications of these effects may be far beyond anything we can
currently envisage.
-- Richard Turton
I believe there is no philosophical high-road in science, with
epistemological signposts. No, we are in a jungle and find our
way by trial and error, building our road behind us as we
proceed."
-- Max Born (1882-1970)
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Section 1
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Symposium: Quantum Dots
1. Introduction.
2. Quantum Dots and Semiconductors.
3. Quantum Dots and Photon Devices.
4. Quantum Dots and Computers.
5. Quantum Dots and Nanotubes.
6. Quantum Dots and Chemical Systems.
7. Quantum Dots and Biochemical Systems.
Notices and Subscription Information
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Section 2
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1. INTRODUCTION.
It is now possible to create extremely small crystals which
contain less than 1000 atoms, each crystal measuring a few
millionths of a millimeter across and thus in the nanoscale
domain. Certain of these nano-crystals, those of cadmium
selenide, for example, have peculiar attributes: crystals of
exactly the same composition but of different size exhibit quite
different properties, with the large nano-crystals of cadmium
selenide red in color, smaller crystals orange, and the smallest
(containing barely 100 atoms) yellow in color. The differences in
properties are due to quantum mechanical effects. These extremely
small atomic arrays are called "quantum dots", and there is a
current consensus that if quantum dots could be integrated onto a
chip, their unique electrical properties could be harnessed to
perform a function similar to a conventional transistor, while
requiring only a small fraction of the space. In consequence, the
creation of an appropriate regular array of quantum dots would
allow a computer processor many times more powerful than any
current supercomputer to be constructed on single chip.
ON QUANTUM DOTS
"Atomic physics progressed rapidly at the beginning of the last
century, thanks, in large part, to optical spectroscopy.
Quantization and spin were discovered through optical studies, as
were other fundamental atomic properties. With the advent of the
laser, physicists learned how to manipulate atomic wavefunctions
by applying coherent optical fields. More discoveries followed.
Now, at the beginning of the new century, optical techniques are
being used to explore a new scientific frontier: the atom-like
entities known as quantum dots (QDs).
"Measuring 1-100 nm across, QDs are semiconductor structures in
which the electron wavefunction is confined in all three
dimensions by the potential energy barriers that form the QD's
boundaries. A QD's electronic response, like that of a single
atom, is manifest in its discrete energy spectrum, which appears
when electron hole pairs are excited. Although the wavefunction
of a QD electron, and its corresponding hole, extends over many
thousands of lattice atoms, the pair -- termed an 'exciton' --
behaves in a quantized and coherent fashion. The coherence is
relatively easy to detect and control optically -- for two
reasons. First, the superposition of the ground and excited
states dephases more slowly in QDs than in higher-dimensional
semiconductor structures. Second, QDs have large dipole moments
(50-100 times larger than those of atoms). Thanks to these
advantages, it is possible to probe and manipulate the
wavefunction of a single QD.
"QDs possess another attractive property. Their size, shape, and
composition can all be tailored to create a variety of desired
properties. These "artificial atoms" can, in turn, be positioned
and assembled into complexes that serve as new materials.
[Researchers} who work on QDs anticipate that a host of complex,
customized QD-based materials will become available."
D. Gammon and D.G. Steel: Physics Today 2002 October
ON THE PRODUCTION OF QUANTUM DOTS BY ETCHING
"What length scales are required to produce a quantum dot? ...In
a quantum well the wavelike properties of the electrons only
become apparent for layer widths of about a hundredth of a
micron... To produce a quantum dot we need to achieve similar
length scales along the other two dimensions. Using present
techniques it is relatively easy to reduce the lateral dimensions
of the islands to about a tenth of a micron. With great effort
further reductions can be achieved, but producing a structure one
hundredth of a micron across presents an immense technical
challenge. Fortunately nature is on our side. Suppose that we
etch two parallel channels through the layered materials to leave
a ridge approximately one-tenth of a micron across. In the
etching process the atoms on the vertical surfaces of the ridge
are disturbed so that the properties of the material are changed
considerably. In fact, surfaces in general have quite different
properties to bulk material. Let us consider an example using a
crystal of silicon. We know that each silicon atom tends to bond
with four neighboring atoms to obtain a full complement of
electrons in its outer shell. However, atoms at a surface cannot
always find another four atoms with which to bond. This means
that surface atoms have a tendency to hold on to any nearby free
electrons in order to obtain a full shell. This process alone
tends to remove conduction electrons from the surface layers. In
addition, the accumulation of negative charges trapped by these
surface atoms creates an electric field which repels other
electrons from this region. As a result we find that the only
free electrons in our tiny ridge-like structure are confined to
an even smaller channel in the center. It is rather like a
coaxial cable in which the central copper core is surrounded by
an insulating layer. For a ridge which is a tenth of a micron
across, the electrons are confined to a region which is only
about a hundredth of a micron wide. This is just the right length
scale required to observe the wavelike nature of the electron. So
although at first sight the dimensions appear to be far too
large, we can observe lateral quantum confinement in these
structures. In this case we have created a quantum wire, since
the electrons are still free to travel along the length of the
ridge. From this stage the construction of a quantum dot is
relatively straightforward. If we etch another set of similarly
spaced grooves at right-angles to the first set, then the surface
effects in these directions are sufficient to cause the electrons
to be trapped in a tiny box. Using these techniques it is
possible to pattern a relatively large area to produce a regular
grid of quantum dots."
Richard Turton: The Quantum Dot: A journey into the Future of
Microelectronics. Oxford University Press 1996, p.147
ON FINITE FERMION SYSTEMS
"Low-dimensional nanometer-sized systems have defined a new
research area in condensed-matter physics within the last 20
years. Modern semiconductor processing techniques allowed the
artificial creation of quantum confinement of only a few
electrons. Such finite fermion systems have much in common with
atoms, yet they are man-made structures, designed and fabricated
in the laboratory. Usually they are called "quantum dots",
referring to their quantum confinement in all three spatial
dimensions. A common way to fabricate quantum dots is to restrict
the two-dimensional electron gas in a semiconductor
heterostructure laterally by electrostatic gates, or vertically
by etching techniques. This creates a bowl-like potential in
which the conduction electrons are trapped. In addition to the
many possible technological applications, what makes the study of
these "artificial atoms" or "designer atoms" interesting are the
far-reaching analogies to systems that exist in nature and have
defined paradigms of many-body physics: atoms, nuclei, and, more
recently, metallic clusters or trapped atomic gases. Quantum dots
added another such paradigm. Their properties can be changed in a
controlled way by electrostatic gates, changes in the dot
geometry, or applied magnetic fields. Their technological
realization gave access to quantum effects in finite low-
dimensional systems that were largely unexplored.
"After the initial success in the fabrication and control of
mesoscopic semiconductor structures, which are typically about
one hundred nanometers in size and confine several hundred
electrons, many groups focused on the further miniaturization of
such devices. A breakthrough to the "atomic" regime was achieved
with the experimental discovery of shell structure in
fluctuations of the charging energy spectra of small, vertical
quantum dots: the borderline between the physics of bulk
condensed matter and few-body quantum systems was crossed. Much
of the many-body physics that was developed for the understanding
of atoms or nuclei could be applied. In turn, measurements on
artificial atoms yielded a wealth of data from which a
fundamental insight into the many-body physics of low-
dimensional finite fermion systems was obtained. With further
progress in experimental techniques, artificial atoms will
continue to be a rich source of information on many-body physics
and undoubtedly will hold a few surprises."
S.M. Reimann and M. Manninen: Revs. Mod. Phys. 2002 74:1283
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2. QUANTUM DOTS AND SEMICONDUCTORS.
The "Kondo effect" is a large anomalous increase in the
resistance of certain dilute alloys of magnetic materials in
nonmagnetic hosts as the temperature is lowered. In general, the
Kondo effect occurs when an impurity atom with an unpaired
electron is placed in a metal, producing an interaction of
localized electrons with delocalized electrons. The "Kondo
temperature" is the temperature at which the Kondo effect
predominates.
In general, in this context, the term "Coulomb blockade" refers
to an effective blockade of quantum mechanical tunneling produced
by specific energy barrier constraints.
KONDO RESONANCE IN A SINGLE-MOLECULE TRANSISTOR
W. Liang et al (Harvard University, US( discuss single-molecule
transistors, the authors making the following points:
1) When an individual molecule(1), nanocrystal(2-4), nanotube(5),
or lithographically defined quantum dot is attached to metallic
electrodes via tunnel barriers, electron transport is dominated
by single-electron charging and energy-level quantization. As the
coupling to the electrodes increases, higher-order tunneling and
correlated electron motion give rise to new phenomena, including
the Kondo resonance. To date, all of the studies of Kondo
phenomena in quantum dots have been performed on systems where
precise control over the spin degrees of freedom is difficult.
Molecules incorporating transition-metal atoms provide powerful
new systems in this regard, because the spin and orbital degrees
of freedom can be controlled through well-defined chemistry.
2) The authors prepared devices by an extension of the methods
previously used in constructing single-C60 (ref. 1) and single-
nanocrystal transistors(3). Using electron-beam lithography, a
narrow gold bridge was fabricated on an aluminum pad with a 3-nm
oxide layer serving as a gate electrode. The electromigration-
induced break-junction technique(1,3) was then used to create two
closely spaced gold electrodes. Scanning electron microscope
imaging and tunnel current measurements reveal that the narrowest
gap between the two electrodes is consistently 1 nm.
3) In summary: The authors report the observation of the Kondo
effect in single-molecule transistors, where an individual
divanadium molecule serves as a spin impurity. The authors find
that the Kondo resonance can be tuned reversibly using the gate
voltage to alter the charge and spin state of the molecule. The
resonance persists at temperatures up to 30 K and when the energy
separation between the molecular state and the Fermi level of the
metal exceeds 100 meV. The authors suggest the present study
demonstrates that molecules can provide a Kondo system where
critical parameters of Kondo physics, such as the spin and
orbital degrees of freedom, are defined by chemical synthesis.
With the recent advances of synthetic methodology, the
preparation of molecular clusters possessing adjustable magnetic
properties is becoming feasible. Future investigations of such
species are expected to provide detailed insight into electron
transport through a molecular system where the spin and orbital
degeneracies are precisely controlled.
References (abridged):
1. Park, H. et al. Nano-mechanical oscillations in a single-C60
transistor. Nature 407, 57-60 (2000)
2. Klein, D. L. et al. A single-electron transistor made from a
cadmium selenide nanocrystal. Nature 389, 699-701 (1997)
3. Park, H. et al. Fabrication of metallic electrodes with
nanometer separation by electromigration. Appl. Phys. Lett. 75,
301-303 (1999)
4. Banin, U., Cao, Y., Katz, D. & Millo, O. Identification of
atomic-like electronic states in indium arsenide nanocrystal
quantum dots. Nature 400, 542-544 (1999)
5. Tans, S. J. et al. Individual single-wall carbon nanotubes as
quantum wires. Nature 386, 474-476 (1997)
Nature 2002 417:725
Related Background:
A TUNABLE KONDO EFFECT IN QUANTUM DOTS
Quantum dots are small electrically conducting regions, typically
less than 1 micron in diameter, that contain from one to a few
thousand electrons. Because of the small volume, the electron
energies within the dot are quantized, and the behavior of the
quantum dot is intermediate between that of an atom and that of a
classical macroscopic object. Such intermediate systems are
called "mesoscopic" systems, and in the past several years great
attention has been devoted to the physics of such systems, since
they apparently can provide insights into quantum systems in
general. The electronic states in quantum dots can be probed by
transport when a small *tunnel coupling is allowed between the
dot and nearby source and drain leads.
Cronenwett et al (3 authors at 2 installations, NL US) report the
realization of a tunable Kondo effect in small quantum dots, with
the capability of switching a dot from a Kondo system to non-
Kondo system as the number of electrons on the dot is changed
from odd to even. The Kondo temperature can be tuned by means of
a gate voltage as a single-particle energy state nears the Fermi
energy. Measurements of the temperature and magnetic field
dependence of a Coulomb-blockaded dot show good agreement with
prediction of both equilibrium and nonequilibrium Kondo effects.
Science 24 Jul 98 281:540
Related Background:
QUANTUM DOTS
F. Remacle and R.D. Levine (2 installations, BE IL) present a
theoretical discussion of assemblies of metallic quantum dots
with each dot considered as an "atom". The dots are taken as
being packed close enough to be interacting. The authors suggest
that the key point is that such dots are essentially "designer"
atoms, since their electronic properties can be controlled via
the synthetic method used to prepare the dots. Of direct
significance are the size of the dot and the nature of the
ligands used to prevent coalescence of the dots. The energy
required to remove or add an electron to the dot is determined by
the size of the dot. The ligands control how closely the dots can
be packed and hence the strength of the coupling between adjacent
dots. An important parameter is the energy cost of adding an
electron to a dot: because of the large size of the dots, the
Coulomb repulsion of the added electron is low. Unlike most
ordinary atoms, quantum dots have a high capacity for
accommodating an additional electron.
Proc. Nat. Acad. Sci. 2000 97:553
Related Background:
LOCALIZATION-DELOCALIZATION IN QUANTUM DOTS
N.B. Zhitenev et al (US) report a study of the electron
localization- delocalization transition in quantum dots. The
problem of electron localization has remained a prime focus of
experiment and theoretical research over the past 40 years.
Single-electron capacitance spectroscopy precisely measures the
energies required to add individual electrons to a quantum dot.
The spatial extent of electronic wave functions was probed by
investigating the dependence of these energies on changes in the
dot confining potential. For low electron densities, electrons
occupy distinct spatial sites localized within the dot. At higher
densities, the electrons become delocalized, and all wave
functions are spread over the full dot area. Near the
delocalization transition, the last remaining localized states
exist at the perimeter of the dot. Unexpectedly, these electrons
appear to bind with electrons in the dot center.
Science 1999 285:715
Related Background:
NANOMECHANICAL OSCILLATIONS IN A SINGLE-C60 TRANSISTOR
H. Park et al (University of California Berkeley, US) discuss C-
60 transistors, the authors making the following points:
1) The motion of electrons through quantum dots is strongly
modified by single-electron charging and the quantization of
energy levels(1,2). Much effort has been directed towards
extending studies of electron transport to chemical
nanostructures, including molecules(3-5), nanocrystals, and
nanotubes. The authors report the fabrication of single-molecule
transistors based on individual C60 molecules connected to gold
electrodes. The authors performed transport measurements that
provide evidence for a coupling between the center-of-mass motion
of the C60 molecules and single-electron hopping -- a conduction
mechanism that has not been observed previously in quantum dot
studies. The coupling is manifest as quantized nano-mechanical
oscillations of the C60 molecule against the gold surface, with a
frequency of about 1.2 THz. This value is in good agreement with
a simple theoretical estimate based on van der Waals and
electrostatic interactions between C60 molecules and gold
electrodes.
2) Single-C60 transistors were prepared by depositing a dilute
toluene solution of C60 onto a pair of connected gold electrodes
fabricated using electron-beam lithography. A break-junction
technique was then used to create a gap between these electrodes
by the process of electromigration. The typical lateral size of
the fabricated electrodes was of the order of 100 nm at the point
of the gap formation, and the height of the electrodes was 15 nm.
Scanning electron microscope images of fabricated electrodes
reveal that the gap between two electrodes is not uniform and
that the narrowest gap is formed only between small protrusions
(10 nm) of two gold electrodes. Current voltage measurements of
these electrodes at cryogenic temperatures without deposited C60
molecules show that the size of the gap is consistently about 1
nm. In a significant fraction of the C60 devices, the conductance
of the junction after initial breaking is substantially enhanced
compared to devices with no C60 deposited, indicating that C60
molecules reside in the junction. The entire structure was
defined on a SiO2 insulating layer on top of a degenerately doped
silicon wafer that serves as a gate electrode that modulates the
electrostatic potential of C60.
3) The authors suggest the transport measurements presented
demonstrate that single-electron-tunneling events can be used
both to excite and probe the motion of a molecule: indeed, the
single-C60 transistor behaves as a high-frequency nanomechanical
oscillator. Furthermore, the oscillations of the C60 molecule
must be treated in a quantized fashion, showing that this is a
true quantum "mechanical" system. The authors expect that the
coupling between the quantized electronic and mechanical degrees
of freedom will be generically important in electron transport
through nanomolecular systems.
References (abridged):
1. Grabert, H. & Devoret, M. H. Single Charge Tunneling (Plenum,
New York, 1992).
2. Sohn, L. L., Kouwenhoven, L. P. & Schn, G. Mesoscopic
Electron Transport (Kluwer Academic, Dordrecht, 1997).
3. Bumm, L. A. et al. Are single molecular wires conducting?
Science 271, 1705-1707 (1996).
4. Reed, M. A., Zhou, C., Muller, C. J., Burgin, T. P. & Tour, J.
M. Conductance of a molecular junction. Science 278, 252-254
(1997).
5. Datta, S. et al. Current-voltage characteristics of self-
assembled monolayers by scanning tunneling microscopy. Phys. Rev.
Lett. 79, 2530-2533 (1997).
Nature 2000 407:57
Related Background:
A SINGLE-ELECTRON TRANSISTOR MADE FROM A CADMIUM SELENIDE
NANOCRYSTAL
D.L. Klein et al (University of California Berkeley, US) discuss
single-electron transistors, the authors making the following
points:
1) The techniques of colloidal chemistry permit the routine
creation of semiconductor nanocrystals(1,2) whose dimensions are
much smaller than those that can be realized using lithographic
techniques(3-5). The sizes of such nanocrystals can be varied
systematically to study quantum size effects or to make novel
electronic or optical materials with tailored properties.
Preliminary studies of both the electrical and optical properties
of individual nanocrystals have been performed recently. These
studies show clearly that a single excess charge on a nanocrystal
can markedly influence its properties.
2) The authors present measurements of electrical transport in a
single-electron transistor made from a colloidal nanocrystal of
cadmium selenide. This device structure enables the number of
charge carriers on the nanocrystal to be tuned directly, and so
permits the measurement of the energy required for adding
successive charge carriers. Such measurements are invaluable in
understanding the energy-level spectra of small electronic
systems, as has been shown by similar studies of lithographically
patterned quantum dots and small metallic grains.
3) The authors suggest their work represents a new type of
spectroscopy for single nanocrystals. Unlike optical
measurements, where electron hole pairs are created, these
measurements probe the energy for adding a single type of charge
carrier. The authors hope that these results will stimulate new
calculations of the hole addition energies for a CdSe nanocrystal
that include the effects of exchange, correlations, and screening
by the metallic electrodes. Future measurements will investigate
how the ground-state and excited-state properties vary with the
size, shape and composition of nanocrystals as well as with the
composition of the leads.
References (abridged):
1. Brus, L. Quantum crystallites and nonlinear optics. Appl.
Phys. A 53, 465-474 (1991).
2. Alivisatos, A. P. Semiconductor clusters, nanocrystals, and
quantum dots. Science 271, 933-937 (1996).
3. Kastner, M. A. Artificial atoms. Phys. Today 46, 24-31 (1993).
4. Ashoori, R. C. Electrons in artificial atoms. Nature 380, 559
(1996).
5. Tarucha, S., Austing, D. G., Honda, T., van der Hage, R. J. &
Kouwenhoven, L. P. Shell filling and spin effects in a few
electron quantum dot. Phys. Rev. Lett. 77, 3613-3616 (1996).
Nature 1997 389:699
Related Background:
IDENTIFICATION OF ATOMIC-LIKE ELECTRONIC STATES IN INDIUM
ARSENIDE NANOCRYSTAL QUANTUM DOTS
U. Banin et al (Hebrew University Jerusalem, IL) discuss
electronic states in quantum dots, the authors making the
following points:
1) Semiconductor quantum dots, due to their small size, mark the
transition between molecular and solid-state regimes, and are
often described as "artificial atoms"(1-3). This analogy
originates from the early work on quantum confinement effects in
semiconductor nanocrystals, where the electronic wavefunctions
are predicted(4) to exhibit atomic-like symmetries. Spectroscopic
studies of quantum dots have demonstrated discrete energy level
structures and narrow transition linewidths(5), but the symmetry
of the discrete states could be inferred only indirectly.
2) The authors report they use cryogenic scanning tunneling
spectroscopy to identify directly atomic-like electronic states
with s and p character in a series of indium arsenide
nanocrystals. These states are manifest in tunneling current
voltage measurements as two- and six-fold single-electron-
charging multiplets respectively, and they follow an atom-like
Aufbau principle of sequential energy level occupation.
3) The authors suggest that the good correlation between optical
transitions and the spacing of levels observed in the tunneling
spectra indicates that the charging did not have a significant
effect on the QD level structure. The tunneling I V curves are
thus well described as an algebraic sum of single electron
charging energies and the QD level spacings.
References (abridged):
1. Alivisatos, A. P. Semiconductor clusters, nanocrystals, and
quantum dots.Science271, 933-937 (1996).
2. Kastner, M. A. Artificial atoms.Phys. Today46, 24-31 (1993).
3. Ashoori, R. C. Electrons in artificial atoms.Nature379, 413-
419 (1996).
4. Brus, L. E. Electron-electron and electron-hole interaction in
small semiconductor crystallites. The size dependence of the
lowest excited electronic state.J. Chem. Phys.80, 4403-4409
(1984).
5. Norris, D. J., Sacra, A., Murray, C. B. & Bawendi, M. G.
Measurement of the size dependent hole spectrum in CdSe quantum
dots.Phys. Rev. Lett.72, 2612-2615 (1994).
Nature 1999 400:542
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3. QUANTUM DOTS AND PHOTON DEVICES.
INDISTINGUISHABLE PHOTONS FROM A SINGLE-PHOTON DEVICE
C. Santori et al (Stanford University, US) discuss single-photon
devices, the authors making the following points:
1) Single-photon sources have recently been demonstrated using a
variety of devices, including molecules(1-3), mesoscopic quantum
wells(4), color centres(5), trapped ions and semiconductor
quantum dots. Compared with a Poisson-distributed source of the
same intensity, these sources rarely emit two or more photons in
the same pulse. Numerous applications for single-photon sources
have been proposed in the field of quantum information, but most
-- including linear-optical quantum computation -- also require
consecutive photons to have identical wave packets. For a source
based on a single quantum emitter, the emitter must therefore be
excited in a rapid or deterministic way, and interact little with
its surrounding environment.
2) When identical single photons enter a 50 50 beam splitter from
opposite sides, quantum mechanics predicts that both photons must
leave in the same direction, if their wave packets overlap
perfectly. This two-photon interference effect originates from
the Bose Einstein statistics of photons. This bunching effect was
first observed using pairs of highly correlated photons produced
by parametric downcoversion, but it should also occur with
single, independently generated photons. Most proposed
applications for single-photon sources in the field of quantum
information (with the notable exception of quantum cryptography)
involve two-photon interference. Such applications include
quantum teleportation, post-selective production of polarization-
entangled photons, and linear-optics quantum computation. It is
therefore important to demonstrate that consecutive photons
emitted by a single-photon source are identical and exhibit
mutual two-photon interference effects.
3) The authors report a test of the indistinguishability of
photons emitted by a semiconductor quantum dot in a microcavity
through a Hong Ou Mandel-type two-photon interference experiment.
The authors find that consecutive photons are largely
indistinguishable, with a mean wave-packet overlap as large as
0.81, making this source useful in a variety of experiments in
quantum optics and quantum information.
References (abridged):
1. De Martini, F., Di Giuseppe, G. & Marrocco, M. Single-mode
generation of quantum photon states by excited single molecules
in a microcavity trap. Phys. Rev. Lett. 76, 900-903 (1996)
2. Brunel, C., Lounis, B., Tamarat, P. & Orrit, M. Triggered
source of single photons based on controlled single molecule
fluorescence. Phys. Rev. Lett. 83, 2722-2725 (1999)
3. Lounis, B. & Moerner, W. E. Single photons on demand from a
single molecule at room temperature. Nature 407, 491-493 (2000)
4. Kim, J., Benson, O., Kan, H. & Yamamoto, Y. A single-photon
turnstile device. Nature 397, 500-503 (1999)
5. Beveratos, A. et al. Room temperature stable single-photon
source. Eur. Phys. J. D 18, 191-196 (2002)
Nature 2002 419:594
Related Background:
QUANTUM DOTS AND ELECTROLUMINESCENCE
T. Tsutsui (Kyushu University, JP) discusses quantum dots and
electroluminescence, the author making the following points:
1) Electroluminescence has taken a new turn: S. Coe et al(1)
report the fabrication of high-efficiency organic light-emitting
diodes (LEDs) in which the light-emitting centers are cadmium-
selenium (CdSe) nanocrystals, or quantum dots. Organic light-
emitting diodes (LEDs) bring with them the advantages of robust
fabrication technique and high performance, which, when coupled
with the excellent luminescent properties of nanocrystals, offer
exciting prospects for real, workable devices. Mobile phones with
small colour displays using organic LEDs are already commercially
available. The images are generated through fluorescence, as
electrons make transitions between orbital states of pi-
conjugated organic molecules (the pi-bond arises from the overlap
of the 2p orbitals of electrons in carbon atoms). As well as
having high quantum efficiency for electron-to-photon conversion,
pi-conjugated molecules in organic LEDs have the advantage of
colour tunability, so that they can be used to build full-color
displays of red green blue (RGB) emitters.
2) But there is also a drawback: the emission spectra of pi-
conjugated molecules are very broad, typically spanning 50 to 100
nm (the "full width at half-maximum", or FWHM). This range of
molecular fluorescence is caused by the vibrational and
rotational motion of atoms inside the pi-conjugated molecules. So
with an organic LED, it is difficult to get, for example, pure
red light emitted with high quantum efficiency. Nevertheless,
sharp RGB emission has been demonstrated from LEDs based on some
specific materials, such as europium chelates, aggregated
structures of cyanine dyes or layered inorganic organic
perovskite compounds. But these LEDs have not achieved the
emission efficiency or device durability needed for practical
display applications(2,3).
3) Some inorganic nanocrystals emit visible light with sharp
emission spectra that are less than 30 nm FWHM. The nanocrystals
are in effect quantum dots and confine charge so well within
their small volume that a high quantum efficiency, exceeding 50%,
is possible. It might be expected, then, that quantum dots
incorporated in organic LEDs would make excellent emission
centres4. In fact, electroluminescence has been observed by
simply mixing inorganic nanocrystals with -conjugated polymers,
but the emission efficiency was far lower than that of
conventional polymer LEDs(5). Coe et al(1) have fabricated an
organic LED with a single layer of CdSe quantum dots sandwiched
between organic thin films. Remarkably, the efficiency of their
device is about 25 times higher than that achieved so far with
quantum-dot LEDs. Thinking ahead to the design of device
architectures, there are two noteworthy aspects here: the
structure of this quantum-dot LED is already close to that for an
optimum device and the process of fabricating a layer of quantum
dots is simple.
References (abridged):
1. Coe, S., Woo, W.-K, Bawendi, M. & Bulovi, V. Nature 420, 800-
803 (2002).
2. Era, M., Hayashi, S., Tsutsui, T. & Saito, S. J. Chem. Soc.
Chem. Commun. 557-558 (1985).
3. Era, M., Morimoto, S., Tsutsui, T. & Saito, S. Appl. Phys.
Lett. 65, 676-678 (1994).
4. Hines, M. A. & Guyot-Sionnest, P. J. Phys. Chem. 100, 468-471
(1996).
5. Dabbousi, B. O., Bawendi, M. G., Onitsuka, O. & Rubner, M. F.
Appl. Phys. Lett. 66, 1316-1318 (1995).
Nature 2002 420:752
Related Background:
ON THE OPTICAL ACTIVITY OF QUANTUM DOTS
Daniel Gammon (Naval Research Laboratory, US) presents a
commentary on current research on electrons in artificial atoms,
the author making the following points:
1) An electron in a quantum dot can be described by a quantum
wavefunction that is similar to that used for an electron in a
single atom, although the energy of the electron in the quantum
dot is spread in a coordinated way (spread "coherently") over the
lattice of atomic nuclei. The electronic wave functions of
quantum dots are often labeled with atomic notation, but quantum
dots are very much solid-state nanostructures that can be
tailored into different shapes. Recent studies (M. Bayer et al:
Nature 405:923 2000; R.J. Warburton et al: Nature 405:926 2000)
describe the optical behavior of individual quantum dots and
quantum rings, and such behavior is of considerable interest
because quantum dots that emit light are expected to form the
basis of a new generation of lasers.
2) In these optical studies of quantum dots, the semiconductor
dots and rings are made from indium arsenide embedded in gallium
arsenide, and the structures were grown using techniques
developed within the past decade that allow much smaller
nanostructures to be created than were previously possible. In
these new experiments, electrons are introduced one by one into
individual quantum dots while the optical emission of the dots is
measured with great precision. These studies provide new
perspectives on the internal quantum-mechanical workings of
quantum dots; the ultimate goal is to create useful electronic
and optical nanomaterials that have been quantum-mechanically
engineered by tailoring the shape, size, composition, and
position of various quantum dots.
3) Concerning the physics of quantum dots, adding even a single
electron to such a system requires a significant amount of extra
energy because of the repulsion between the negatively charged
electrons as they are forced into a smaller volume. One result of
this, called the "Coulomb blockade", is to make possible a
greater laboratory control of the number of electrons in a
quantum dot, i.e., researchers can tune the number of electrons
by manipulating input energy.
4) In general, optical excitation of a semiconductor leads to the
creation of a quasi-particle known as an "exciton" -- a
negatively charged electron bound together with a positively
charged "hole". In contrast to the Coulomb blockade resulting
from electrical injection of electrons into a quantum dot, such
dots remain neutrally charged following optical excitation, and
the quantum dot exciton has been studied in detail by measuring
the light emitted when the hole and electron recombine.
5) The author concludes: "Quantum dots have great flexibility
because their properties can be artificially engineered, but this
comes at a price. Nature has given us atoms; scientists must make
quantum dots. Further advances in this exciting field of science
and technology will depend heavily on the creativity of
physicists, chemists, and materials scientists who make these
tiny structures."
Nature 2000 405:899
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4. QUANTUM DOTS AND COMPUTERS.
QUANTUM DOTS AND QUANTUM INFORMATION PROCESSING
Manfred Bayer (University of Dortmund, DE) discuss quantum dots
and quantum computations, the author making the following points:
1) Physicists are actively seeking to transfer the weird
phenomena of the quantum world from their laboratories to the
real world. In particular, the prospect of quantum-information
processing has attracted considerable attention for its potential
to improve the speed and reliability of data handling(1).
Information would be encoded in "quantum bits", and the search is
on for a physical system that could form a reliable, controllable
quantum bit.
2) In contrast to classical bits, which can be in either state 0
or state 1, quantum bits exist as a combination (a linear
superposition) of two quantum logic states, represented as |0>
and |1>. In a quantum computer, the quantum bits first have to be
controlled individually in order to initialize the quantum
register in which information is stored. Then a controllable
interaction between the quantum bits must be established so that
the quantum states become entangled. It is this "entanglement"
that is the key to a quantum computer's power: in effect, a rigid
coupling is introduced between the quantum bits, which can then
no longer be considered individually but are affected
simultaneously by a calculational operation. It is thanks to this
capacity for parallel processing that a quantum computer should
be able to perform calculations much faster than a classical
computer.
3) The original proposals for quantum computers were based on
atomic systems(1), such as atoms held in traps, where the quantum
bit is formed by two energy levels between which an atomic
electron can make transitions. Now that semiconductor quantum
dots have been synthesized, it opens up the possibility of
mimicking these approaches in a solid-state environment. Quantum
dots, tiny clusters of semiconductor material, are often called
"artificial atoms", because the charge carriers in these systems
(electrons or holes) can only occupy a restricted set of energy
levels, just like the electrons in an atom. In fact, quantum dots
offer a variety of two-level systems, based on charge or spin (or
both). One such two-level system is a coupled electron hole pair
-- an exciton. The absence (equivalent to the state |0>) and
presence (state |1>) of an exciton in a semiconductor quantum dot
could represent the two levels of a quantum bit.(2-5)
References (abridged):
1. Bouwmeester, D., Ekert, A. & Zeilinger, A. (eds) The Physics
of Quantum Information (Springer, Berlin, 2000).
2. Zrenner, A. et al. Nature 418, 612-614 (2002).
3. Stievater, T. H. et al. Phys. Rev. Lett. 87, 133603 (2000).
4. Kamada, H. et al. Phys. Rev. Lett. 87, 246401 (2001).
5. Htoon, H. et al. Phys. Rev. Lett. 88, 087401 (2002).
Nature 2002 418:597
Related Background:
COHERENT PROPERTIES OF A TWO-LEVEL SYSTEM BASED ON A QUANTUM-DOT
PHOTODIODE
A. Zrenner et al (Technical University of Munich, DE) discuss
excitons and quantum information processing, the authors making
the following points:
1) Present-day information technology is based mainly on
incoherent processes in conventional semiconductor devices(1). To
realize concepts for future quantum information technologies,
which are based on coherent phenomena, a new type of "hardware"
is required(2). Semiconductor quantum dots are promising
candidates for the basic device units for quantum information
processing. One approach is to exploit optical excitations
(excitons) in quantum dots. It has already been demonstrated that
coherent manipulation between two excitonic energy levels -- via
so-called Rabi oscillations -- can be achieved in single quantum
dots by applying electromagnetic fields(3-5).
2) The authors report they make use of this effect by placing an
InGaAs quantum dot in a photodiode, which essentially connects it
to an electric circuit. The authors demonstrate that coherent
optical excitations in the quantum-dot two-level system can be
converted into deterministic photocurrents. The authors find that
this device can function as an optically triggered single-
electron turnstile.
3) A coherently driven single-QD photodiode is an optically
triggered single-electron source, which offers, just like the
related single-electron turnstile device, single electrons (or
holes) on demand. Both devices have the potential to deliver
frequency-controlled currents according to I = fe. With
circularly polarized excitation, a single-QD photodiode has the
potential to provide deterministic streams of spin-polarized
charges. More subtle points concerning the absolute accuracy of
this device, like the influence of incomplete tunneling or
possible avalanche multiplication, will certainly be the subject
of future work. By means of single-QD photodiodes, it becomes
possible to transfer optical excitations of single quantum
systems into deterministic electric currents. This optoelectronic
functionality provides a link between coherent optical
excitations and electric currents on the single-electron level.
The authors believe that this will allow the electric readout of
excitonic quantum gates, and could have a substantial impact on
quantum information technology.
References (abridged):
1. Alferov, Z. I. Nobel Lecture: The double heterostructure
concept and its applications in physics, electronics, and
technology. Rev. Mod. Phys. 73, 767-782 (2001)
2. Bouwmeester, D., Ekert, A. & Zeilinger, A. (eds) The Physics
of Quantum Information (Springer, Berlin, 2000)
3. Stievater, T. H. et al. Rabi oscillations of excitons in
single quantum dots. Phys. Rev. Lett. 87, 133603-1-133603-4
(2001)
4. Kamada, H. et al. Exciton Rabi oscillation in a single quantum
dot. Phys. Rev. Lett. 87, 247401-1-247401-4 (2001)
5. Htoon, H. et al. Interplay of Rabi oscillations and quantum
interference in semiconductor quantum dots. Phys. Rev. Lett. 88,
087401-1-087401-4 (2002)
Nature 418:612
Related Background:
ALLOWED AND FORBIDDEN TRANSITIONS IN ARTIFICIAL HYDROGEN AND
HELIUM ATOMS
T. Fujisawa et al (NIT Corporation, JP) discuss artificial atoms,
the authors making the following points:
1) The strength of radiative transitions in atoms is governed by
selection rules that depend on the occupation of atomic orbitals
with electrons(1). Experiments have shown(2-5) similar electron
occupation of the quantized energy levels in semiconductor
quantum dots -- often described as artificial atoms. But unlike
real atoms, the confinement potential of quantum dots is
anisotropic, and the electrons can easily couple with phonons of
the material.
2) The authors report electrical pump-and-probe experiments that
probe the allowed and "forbidden" transitions between energy
levels under phonon emission in quantum dots with one or two
electrons (artificial hydrogen and helium atoms). The forbidden
transitions are in fact allowed by higher-order processes where
electrons flip their spin. The authors find that the relaxation
time is about 200 microseconds for forbidden transitions, 4 to 5
orders of magnitude longer than for allowed transitions. This
indicates that the spin degree of freedom is well separated from
the orbital degree of freedom, and that the total spin in the
quantum dots is an excellent quantum number. The authors suggest
this is an encouraging result for potential applications of
quantum dots as basic entities for spin-based quantum information
storage.
References (abridged):
1. Bethe, H. A. & Salpeter, E. E. Quantum Mechanics of One- and
Two-Electron Atoms (Springer, Berlin, 1957)
2. Kouwenhoven, L. P., et al. in Mesoscopic Electron Transport
NATO ASI series E 345 (eds Sohn, L. L., Kouwenhoven, L. P. &
Schn, G.) 105-214 (Kluwer, Dordrecht, 1997)
3. Ashoori, R. C. et al. Single-electron capacitance spectroscopy
of discrete quantum levels. Phys. Rev. Lett. 68, 3088-3091 (1992)
4. Tarucha, S., Austing, D. G., Honda, T., van der Hage, R. J. &
Kouwenhoven, L. P. Shell filling and spin effects in a few
electron quantum dot. Phys. Rev. Lett. 77, 3613-3616 (1996)
5. Kouwenhoven, L. P. et al. Excitation spectra of circular, few-
electron quantum dots. Science 278, 1788-1792 (1997)
Nature 2002419:278
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5. QUANTUM DOTS AND NANOTUBES.
BANDGAP MODULATION OF CARBON NANOTUBES BY ENCAPSULATED
METALLOFULLERENES
J. Lee et al (Seoul National University, KR) discuss quantum-dot
carbon nanotubes, the authors making the following points:
1) The technical and economic difficulties in further
miniaturizing silicon-based transistors with the present
fabrication technologies have motivated a strong effort to
develop alternative electronic devices, based, for example, on
single molecules(1,2). Recently, carbon nanotubes have been
successfully used for nanometer-sized devices such as
diodes(3,4), transistors(5), and random access memory cells. Such
nanotube devices are usually very long compared to silicon-based
transistors.
2) Since the discovery of carbon nanotube a decade ago,
researchers have synthesized a variety of forms of carbon
nanotubes. Recently, it has been shown experimentally that
fullerenes or endohedral metallofullerenes such as Gd
encapsulated inside C82 can be inserted into single-wall
nanotubes (SWNTs), forming a pea-pod-like structure. When the
diameter of the inserted fullerene is smaller than that of the
SWNT minus a certain length (0.7 nm, roughly twice the van der
Waals bond length), the fullerenes are inserted exothermally.
When the diameter of a fullerene slightly exceeds the length
described above, the fullerene can be inserted endothermally, but
the resultant SWNT is elastically strained. A theoretical study
has predicted that the electronic structure is severely modified,
including the positions of the van Hove singularities (VHS), when
an SWNT is uniaxially strained. Combining these ideas, one can
perform "local bandgap engineering" at the site where a fullerene
is endothermally inserted.
3) In summary: The authors report a method for dividing a
semiconductor nanotube into multiple quantum dots with lengths of
about 10 nm by inserting Gd-C82 endohedral fullerenes. The
spatial modulation of the nanotube electronic bandgap is observed
with a low-temperature scanning tunneling microscope. The authors
find that a bandgap of 0.5 eV is narrowed down to 0.1 eV at sites
where endohedral metallofullerenes are inserted. This change in
bandgap can be explained by local elastic strain and charge
transfer at metallofullerene sites. The authors suggest this
technique for fabricating an array of quantum dots could be used
for nano-electronics8 and nano-optoelectronics.
References (abridged):
1. Aviram, A. & Ratner, M. (eds) Molecular Electronics: Science
and Technology. (Annals of the New York Academy of Sciences, Vol.
852, New York, 1998).
2. Reed, M. A. & Tour, J. M. Computing with molecules. Sci. Am.
282(6), 86-93 (June, 2000).
3. Yao, Z., Postma, H. W. Ch., Balents, L. & Dekker, C. Carbon
nanotube intramolecular junctions. Nature 402, 273-276 (1999).
4. Zhou, C., Kong, J., Yenilmez, E. & Dai, H. Modulated chemical
doping of individual carbon nanotubes. Science 290, 1552-1555
(2000).
5. Tan, S. J., Verschueren, A. R. M. & Dekker, C. Room-
temperature transistor based on a single carbon nanotube. Nature
393, 49-52 (1998).
Nature 2002 415:1005
Related Background:
SCANNED PROBE IMAGING OF SINGLE-ELECTRON CHARGE STATES IN
NANOTUBE QUANTUM DOTS
M.T. Woodside and P.L. McEuen (University of California Berkeley,
US) discuss nanotube quantum dots, the authors making the
following points:
1) Single-electron charging phenomena are ubiquitous in atoms,
molecules, and small electronic devices, and their effects are
central to an understanding of the physics and technology of
nanoscale systems. Single-electron effects arise because the
number of electrons residing on a small, quasi-isolated,
conducting island is quantized. Adding an additional charge to
such a quantum dot costs an electrostatic energy on the order of
U = e2/C, where C is the capacitance of the dot and e is the
electronic charge (1). This charging energy suppresses charge
transport when U >> kBT, where kBT is the thermal energy, leading
to the Coulomb blockade of charge motion on and off the dot.
2) Although Coulomb blockade phenomena have been studied
extensively with transport measurements (2), such measurements
lack the spatial discrimination necessary to probe the interior
of a dot or to probe complex multidot systems. An alternative
approach is to detect single-charge motion using scanned probe
techniques, such as scanned capacitance microscopy (3,4), scanned
single-electron transistors (5), and atomic force microsopy
(AFM). The first two of these have excellent charge sensitivity
but are technically very difficult; moreover, they are not easily
able to image the topography of the device under study. AFM-based
techniques, on the other hand, can be used both to image the
sample and to interact with it in a variety of ways. For example,
electrostatic force microscopy (EFM), which measures the
electrostatic force between a sample and a metallized AFM tip,
has been used to detect the motion of single charges during
contact electrification of insulating surfaces and to image the
potential profile in carbon nanotubes. In addition, scanned gate
microsopy (SGM), in which the AFM tip is used to perturb the
conducting properties of a sample, has been used to image
electron trajectories and scattering centers in two-dimensional
electron gases and barriers in carbon nanotubes.
3) In summary: The authors report that an atomic force microscope
was used to study single-electron motion in nanotube quantum
dots. By applying a voltage to the microscope tip, the number of
electrons occupying the quantum dot could be changed, causing
Coulomb oscillations in the nanotube conductance. Spatial maps of
these oscillations were used to locate individual dots and to
study the electrostatic coupling between the dot and the tip. The
electrostatic forces associated with single electrons hopping on
and off the quantum dot were also measured. These forces changed
the amplitude, frequency, and quality factor of the cantilever
oscillation, demonstrating how single-electron motion can
interact with a mechanical oscillator.
References (abridged):
1. H. Grabert, M. H. Devoret, Eds., Single Charge Tunneling
(Plenum, New York, 1992).
2. L. P. Kouwenhoven et al., in Mesoscopic Electron Transport, L.
L. Sohn, L. P. Kouwenhoven, G. Schn, Eds. (Kluwer, Dordrecht,
Netherlands, 1997).
3. S. H. Tessmer, P. I. Glicofridis, R. C. Ashoori, L. S.
Levitov, M. R. Melloch, Nature 392, 51 (1998)
4. G. Finkelstein, P. I. Glicofridis, R. C. Ashoori, M. Shayegan,
Science 289, 90 (2000)
5. M. J. Yoo, et al., Science 276, 579 (1997)
Science 2002 296:1098
Related Background:
KONDO PHYSICS IN CARBON NANOTUBES
The "Kondo effect", first explained by the theoretical physicist
Jun Kondo, is observed when magnetic ions occur as dilute
impurities in nonmagnetic crystals. If the host crystal is a
metal (e.g., copper), the magnetic impurities (e.g., iron)
contribute to the electrical resistivity, with the conduction
electrons scattered by the magnetic impurity. The scattering is
strong at low temperatures, and increases slightly as temperature
decreases, an anomalous resistivity-temperature relation. The
effect essentially results from the interactions between the
localized spins of the magnetic impurities and the free electrons
involved in conduction. There is a characteristic temperature,
called the "Kondo temperature", which depends on the impurity and
on the host crystal, the resistivity increasing at low
temperature, starting near the Kondo temperature. The Kondo
temperature of the system iron-in-copper is 24 degrees kelvin.
Nearly all transition metal atoms are found as magnetic
impurities in copper or gold, and each system has a different
Kondo temperature, which varies from 1000 degrees kelvin to a
fraction of 1 degree kelvin.
Fullerenes are large molecules composed entirely of carbon, with
the chemical formula C(n), where n is any even number from 20 to
over 100. They apparently have the structure of a hollow
spheroidal cage with a surface network of carbon atoms connected
in hexagonal and pentagonal rings. Carbon nanotubes are similar
to fullerenes, except their shape is tubular. They were first
discovered by Sumio Iijima (NEC Laboratories, JP) in 1991, they
come in both multi-wall and single-wall versions, with single-
wall nanotubes having a diameter of approximately 1 nanometer and
multi-wall versions having diameters of the order of 10 to 30
nanometers. There have been rapid developments in understanding
the chemistry and physics of carbon nanotubes, and there is much
excitement in both the materials science and electronics
communities concerning possible applications of these unique
structures.
In general, the term "quantum dot" refers to an artificial atom.
As realized in the laboratory, quantum dots are small
electrically conducting regions, typically less than 1 micron in
diameter, that contain from one to a few thousand electrons.
Because of the small volume, the electron energies within the dot
are quantized, and the behavior of the quantum dot is
intermediate between that of an atom and that of a classical
macroscopic object. Electron transport in conductors is
usually well described by so-called "Fermi-liquid theory", which
assumes that the energy states of the electrons near the *Fermi
level are not qualitatively altered by Coulomb interactions. In
one-dimensional systems, however, even weak Coulomb interactions
cause strong perturbations, and the resulting system, known as a
"Luttinger liquid" (after Joaquim Luttinger, who did major work
on such systems in the 1950s) is predicted to be distinctly
different from its 2- and 3-dimensional counterparts. Single-
walled carbon nanotubes are model systems for the study of 1-
dimensional electron transport, and according to theoretical
predictions, such systems should be very sensitive to Coulomb
repulsion and become insulators at low temperatures. However,
researchers have demonstrated that it is possible to create
superconducting junctions when carbon nanotubes are connected to
superconducting contacts, a phenomenon called "proximity-induced
superconductivity". In the case of nanotubes, the junction
properties are apparently related to its highly 1-dimensional
character, and such junctions may have potential applications as
thermal detectors of weak infrared radiation (bolometers).
In this context, the term "ballistic" (as opposed to the term
"diffusive") refers to the passage of electrons through a
conductor whose length is less than the mean free path of
electrons in the conductor, with the result that most of the
electrons pass through the conductor without scattering.
J. Nygard et al (3 authors at 2 installations, DK UK) present a
report on Kondo physics in carbon nanotubes, the authors making
the following points:
1) The authors point out that the connection of electrical leads
to wire-like molecules is a logical step in the development of
molecular electronics, but such a system also allows studies of
fundamental physics. For example, metallic carbon nanotubes are
essentially quantum wires that have been found to act as 1-
dimensional quantum dots, Luttinger liquids, proximity-induced
superconductors, and ballistic and diffusive 1-dimensional
metals.
2) The authors report that electrically-contacted single-walled
carbon nanotubes can serve as powerful probes of Kondo physics,
demonstrating the universality of the Kondo effect. Arising in
the prototypical case from the interaction between a localized
impurity magnetic moment and delocalized electrons in a metallic
host, the Kondo effect has been used to explain enhanced low-
temperature scattering from magnetic impurities in metals, and
also occurs in transport through semiconductor quantum dots. The
far greater tunability of quantum dots (in this case, nanotubes)
compared with atomic impurities makes new classes of Kondo-like
effects accessible to study. The nanotube devices used by the
authors differ from previous systems in which Kondo effects have
been observed in that they are 1-dimensional quantum dots with 3-
dimensional metal (gold) reservoirs. This allows observations of
Kondo effects for very large electron numbers in the quantum dot,
numbers estimated as of the order of tens of thousands.
Nature 2000 408:342
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6. QUANTUM DOTS AND CHEMICAL SYSTEMS.
PROBING AND CONTROLLING THE BONDS OF AN ARTIFICIAL MOLECULE
A.W. Holleitner et al (Ludwig-Maximilians-Universitt Mnich, DE)
discuss bonds in artificial molecules, the authors making the
following points:
1) Coupled quantum dots in semiconductor materials are often
referred to as artificial molecules, because these electron boxes
allow us to precisely control the number of charge carriers and
their mutual interaction (1,2). For future quantum information
processing, entangled quantum states within such mesoscopic
systems need to be implemented in electronic circuits (3,4). This
requires precise control of the bonds of the artificial molecule
in situ. In this context, coupled dots can be used for
investigating the probing mechanism of quantum physics; that is,
how to attach leads to real molecules. Generally the interaction
of quantum systems with their environment induces decoherence and
the decay of quantum superpositions (5). Key questions are how
accurately the coupling of two artificial atoms can be adjusted
and how coherent coupling can be monitored noninvasively without
destroying the molecular quantum states.
2) It has already been shown in transport and microwave
spectroscopy that the transition from ionic to covalent binding
in quantum dot molecules can be traced. The detection mechanism
for those techniques relies on the transmission of a single
electron through the coupled dot system. In contrast, the authors
have used a co-tunneling mechanism, involving two electrons
tunneling, simultaneously, to probe the coupling between the
dots. Even though the probed quantum states are only weakly
coupled to adjacent reservoirs, the dynamics resemble the Kondo
effect, in which hybridization of electron states with source and
drain contacts is desired.
3) In summary: The authors demonstrate how molecular quantum
states of coupled semiconductor quantum dots are coherently
probed and manipulated in transport experiments. The applied
method probes quantum states by the virtual co-tunneling of two
electrons and hence resolves the sequences of molecular states
simultaneously. This result is achieved by weakly probing the
quantum system through parallel contacts to its constituting
quantum dots. The overlap of the dots' wave functions and, in
turn, the splitting of molecular states are adjusted by the
direct influence of coupling electrodes.
References (abridged):
1. M. Kastner, Phys. Today 46 (no. 1), 24 (1993)
2. C. Livermore, C. H. Crouch, R. M. Westervelt, K. L. Campman,
A. C. Gossard, Science 274, 1332 (1996)
3. M. Bayer, et al., Science 291, 451 (2001)
4. M. N. Leuenberger and D. Loss, Nature 410, 789 (2001)
5. C. J. Myatt, et al., Nature 403, 269 (2000)
Science 2002 297:70
Related Background:
ELECTROCHEMISTRY AND ELECTROGENERATED CHEMILUMINESCENCE FROM
SILICON NANOCRYSTAL QUANTUM DOTS
S. Ding et al (University of Texas Austin, US) discuss quantum
dots in electrochemistry, the authors making the following
points:
1) In a bulk semiconductor, electrons and holes move freely
throughout the crystal. However, in a nanocrystal, confinement of
the electrons and holes leads to a variety of optical and
electronic consequences, including size-dependent molecular-like
optical properties, greater electron/hole overlap for enhanced
photoluminescence (PL) efficiencies, and discrete single-
electron/hole charging. Because of their enormous surface area-
to-volume ratios, nanocrystals (NCs) are highly susceptible to
heterogeneous redox chemistry with the surrounding environment.
Depending on the semiconductor and the surface chemistry, this
chemical reactivity can lead to either fatal chemical degradation
or new useful properties, such as reversible photocatalytic and
electrochromic properties and redox reactivity.
2) Semiconductor NCs have been prepared with narrow size
distributions, controlled surface chemistry, and internal bulk
crystal structure (1,2), and adsorbed capping ligands are often
used to control size and prevent irreversible aggregation.
Although the electrochemical properties of monolayer-protected
metallic NCs have been well documented (3,4), reports concerning
the electrochemical properties of semiconductor NCs remain scarce
(5). Difficulties include the limited potential window available
in aqueous solutions, the limited solubility of many NCs in
nonaqueous solvents, and the need for highly pure, isolated,
monodisperse dots. Compound semiconductor NCs, such as CdS, are
also chemically unstable upon electron transfer.
3) In summary: Reversible electrochemical injection of discrete
numbers of electrons into sterically stabilized silicon
nanocrystals (NCs) ( approximately 2 to 4 nanometers in diameter)
was observed by the authors by differential pulse voltammetry
(DPV) in N,N'-dimethylformamide and acetonitrile. The
electrochemical gap between the onset of electron injection and
hole injection -- related to the highest occupied and lowest
unoccupied molecular orbitals -- grew with decreasing nanocrystal
size, and the DPV peak potentials above the onset for electron
injection roughly correspond to expected Coulomb blockade or
quantized double-layer charging energies. Electron transfer
reactions between positively and negatively charged nanocrystals
(or between charged nanocrystals and molecular redox-active
coreactants) occurred that led to electron and hole annihilation,
producing visible light. The electrogenerated chemiluminescence
spectra exhibited a peak maximum at 640 nanometers, a significant
red shift from the photoluminescence maximum (420 nanometers) of
the same silicon NC solution. The authors suggest these results
demonstrate that the chemical stability of silicon NCs could
enable their use as redox-active macromolecular species with the
combined optical and charging properties of semiconductor quantum
dots.
References (abridged):
1. A. P. Alivisatos, Science 271, 933 (1996)
2. S. S. Iyer and Y. H. Xie, Science 260, 40 (1993)
3. S. Chen et al., Science 280, 2098 (1998)
4. S. Chen, R. W. Murray, S. W. Feldberg, J. Phys. Chem. B 102,
9898 (1998)
5. P. Hoyer and H. Weller, Chem. Phys. Lett. 224, 75 (1994)
Science 2002 296:1293
Related Background:
MICROWAVE SPECTROSCOPY OF A QUANTUM DOT MOLECULE
Quantum dots are small conductive regions in a semiconductor, the
regions containing a variable number of electrons (from 1 to
1000) that occupy well-defined, discrete quantum states -- for
which reason they are often referred to as "artificial atoms".
Connecting quantum dots to current and voltage contacts allows
the discrete energy spectra of the system to be probed by charge-
transport measurements. Two quantum dots can be connected to form
an "artificial molecule", and depending on the strength of the
inter-dot coupling (which supports quantum-mechanical tunneling
of electrons between the dots), the two dots can form "ionic" or
"covalent" bonds. In the ionic bond case, the electrons are
localized on individual dots, and in the covalent bond case, the
electrons are delocalized over both dots.
T.H. Oosterkamp et al report a transition from ionic bonding to
covalent bonding in a quantum-dot "artificial molecule" probed by
microwave excitations. The authors suggest their results
demonstrate controllable *quantum coherence in single-electron
devices, an essential requirement for practical applications of
quantum-dot circuitry in the construction of quantum computers.
Nature 1998 395:873
Related Background:
INCORPORATION OF QUANTUM DOTS IN COLLOIDS
W. Wang and S.A. Asher (University of Pittsburgh, US) discuss
customized nanoparticles, the authors making the following
points:
1) Nanoscale metal and semiconductor particles are of current
interest because they mark a material transition range between
quantum and bulk properties. With decreasing particle size, bulk
properties are lost as the continuum of electronic states becomes
discrete (the quantum size effect) and as the fraction of surface
atoms becomes large. The electronic and magnetic properties of
metallic nanoparticles and nanoclusters show new characteristics
that can be utilized in novel applications in areas that range
from nonlinear optical switching and catalysis to high-density
information storage.
2) Numerous methods have been developed to synthesize metal
nanoparticles. A major difficulty with scale-up of these methods
is that the metal colloid stability is often controlled by
electrostatic interactions across the Debye double layer and
sterically through adsorption of steric stabilizing agents such
as polymers and surfactants. As a consequence, such metal
colloids are extremely sensitive to their environment.
3) One way to improve the stability of metal nanoparticles is to
coat them with silica, which is very resistant to coagulation,
even at high volume fractions. It has been reported that
particles of noble metals such as Ag and Au can be coated with
silica shells, but these procedures usually involve a multi-step
process, and only single metal particle cores could be coated.
4) The authors report the development of a new method to
fabricate nanocomposite silicon dioxide spheres (approximately
100 nanometers in diameter) containing homogeneously dispersed Ag
quantum dots (2 to 5 nanometers in diameter). The inclusion
morphology is controlled through the timing of the photochemical
reduction of silver ions during hydrolysis of tetraethoxysilane
in a microemulsion.
J. Am. Chem. Soc. 2001 123:12528
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7. QUANTUM DOTS AND BIOCHEMICAL SYSTEMS.
IN VIVO IMAGING OF QUANTUM DOTS ENCAPSULATED IN PHOSPHOLIPID
MICELLES
B. Dubertret et al (Rockefeller University, US) discuss quantum
dots and phospholipid micelles, the authors making the following
points:
1 Nanometer-scale semiconductor crystallites (known as
nanocrystals or quantum dots) (1-3) could dramatically improve
the use of fluorescent markers in biological imaging (4,5).
Because these colloidal particles act as robust broadly tunable
nanoemitters that can be excited by a single light source, they
could provide distinct advantages over current in vitro and in
vivo markers (e.g., organic dyes and fluorescent proteins).
However, before nanocrystals can be widely used as biolabels,
they must maintain three properties under aqueous biological
conditions: efficient fluorescence, colloidal stability, and low
nonspecific adsorption. Unfortunately, despite recent advances
(4,5), these conditions have not been simultaneously satisfied,
limiting the development of in vivo applications of nonaggregated
(or individual) semiconductor nanocrystals.
2) The main challenge is that the quantum dots (QDs), as
synthesized, have hydrophobic organic ligands coating their
surface (2,3). To make the QDs water soluble, these organophilic
surface species are generally exchanged with more-polar species,
and both monolayer (5) and multilayer (4) ligand shells have been
pursued. Although the monolayer method is reproducible, rapid,
and produces QDs with a regular, well-oriented, thin coating,
their colloidal stability is poor. In contrast, the multilayer
method yields QDs that are stable in vitro, but the coating
process is long and the coating is difficult to control. A more
serious concern is that both approaches still produce QDs that
tend to aggregate and adsorb nonspecifically. To resolve this
problem, researchers have explored two additional coatings.
First, the outer ligand shell of the QD has been overcoated with
proteins adsorbed through hydrophobic or ionic interactions.
Other layers can then be added to allow conjugation with specific
biomolecules. Indeed, this method has provided new reagents for
fluoroimmunoassays. Second, the outer ligand shell has been
overcoated with surfactants or polymers to prevent nonspecific
adsorption of biomolecules while still permitting bioconjugation.
For example, silica-coated QDs have been further modified with
small monomers of poly(ethylene glycol) to reduce nonspecific
adsorption. Despite these efforts, nonspecific adsorption and
aggregation still occur when QDs are used in biological
environments. Studies of cellular uptakes of QDs report large
aggregate formation inside the cell (5). The same aggregation
problems are reported when QDs are used for fluorescence in situ
hybridization, or as markers for molecular recognition on cell
surfaces. Consequently, the use of QDs in biological applications
is still limited and primarily confined to in vitro studies.
3) In summary: Fluorescent semiconductor nanocrystals (quantum
dots) have the potential to revolutionize biological imaging, but
their use has been limited by difficulties in obtaining
nanocrystals that are biocompatible. To address this problem, the
authors encapsulated individual nanocrystals in phospholipid
block-copolymer micelles and demonstrated both in vitro and in
vivo imaging. When conjugated to DNA, the nanocrystal-micelles
acted as in vitro fluorescent probes to hybridize to specific
complementary sequences. Moreover, when injected into Xenopus
embryos, the nanocrystal-micelles were stable, nontoxic (<5 109
nanocrystals per cell), cell autonomous, and slow to photobleach.
Nanocrystal fluorescence could be followed to the tadpole stage,
allowing lineage-tracing experiments in embryogenesis.
References (abridged):
1. A. P. Alivisatos, Science 271, 933 (1996)
2. C. B. Murray, D. J. Norris, M. G. Bawendi, J. Am. Chem. Soc.
115, 8706 (1993)
3. M. A. Hines and P. Guyot-Sionnest, J. Phys. Chem. 100, 468
(1996)
4. M. Bruchez, M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos,
Science 281, 2013 (1998)
5. W. C. W. Chan and S. Nie, Science 281, 2016 (1998)
Science 2002 298:1759
Related Background:
ORDERING OF QUANTUM DOTS USING GENETICALLY ENGINEERED VIRUSES
S-W. Lee et al (University of Texas Austin, US) discuss the
ordering of quantum dots, the authors making the following
points:
1) Building ordered and defect-free two- and three-dimensional
structures on the nanometer scale is essential for the
construction of next-generation optical, electronic, and magnetic
materials and devices (1-4). Traditional assembly approaches have
been based on hydrogen bonding, coulombic interactions, and van
der Waals forces (1,4). Although a bacterial synthetic method was
reported to make monodisperse modified polypeptides (5), it has
been difficult to tune the layer spacing and structure of
conventional synthetic polymers because of their polydisperse
chain lengths. Efforts have been directed toward the use of soft
materials to organize inorganic materials at the nanoscale.
2) Protein cages have been used as templates to synthesize
nanoscale materials in capsids. DNA recognition linkers have been
successfully used to construct specific gold nanocrystal
structures. ZnS and CdS have been nucleated in a lyotropic liquid
crystalline medium to make nanowires and nanocrystal superlattice
structures by a surfactant assembly pathway. However, these
methods have limitations with respect to length scale and type of
inorganic material.
3) Monodisperse biomaterials that have an anisotropic shape are
promising as components of well-ordered structures. Liquid
crystalline structures of wild-type viruses (Fd, M13, and TMV)
have been tunable by controlling the solution concentrations, the
solution ionic strength, and the external magnetic fields applied
to the solutions. The authors recently demonstrated that
engineered viruses can recognize specific semiconductor surfaces
through the method of selection by combinatorial phage display.
These specific recognition properties of the virus can be used to
organize inorganic nanocrystals, forming ordered arrays over the
length scale defined by liquid crystal formation.
4) In summary: The authors report that a liquid crystal system
was used for the fabrication of a highly ordered composite
material from genetically engineered M13 bacteriophage and zinc
sulfide (ZnS) nanocrystals. The bacteriophage, which formed the
basis of the self-ordering system, were selected to have a
specific recognition moiety for ZnS crystal surfaces. The
bacteriophage were coupled with ZnS solution precursors and
spontaneously evolved a self-supporting hybrid film material that
was ordered at the nanoscale and at the micrometer scale into
approximately 72-micron domains, which were continuous over a
centimeter length scale. In addition, suspensions were prepared
in which the lyotropic liquid crystalline phase behavior of the
hybrid material was controlled by solvent concentration and by
the use of a magnetic field.
References (abridged):
1. J. P. Mathias, E. E. Simanek, G. M. Whitesides, J. Am. Chem.
Soc. 116, 4326 (1994)
2. X. Duan, J. Wang, C. M. Lieber, Appl. Phys. Lett. 76, 1116
(2000)
3. D. J. Norris and M. G. Bawendi, Phys. Rev. B 53, 16347 (1996)
4. C. E. Fowler, W. Shenton, G. Stubbs, S. Mann, Adv. Mater. 13,
1266 (2001)
5. S. M. Yu, et al., Nature 389, 167 (1997)
Science 2002 296:892
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