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ScienceWeek
SCIENCEWEEK
ScienceWeek
April 18, 2003
Vol. 7 Number 16
An Online Digest of Research in the Sciences
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Science keeps moving us away from the Apes.
Of course, if one wants to be an ape, one
objects to the movement.
-- Anonymous
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Section 1
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Symposium: Nanotechnology
1. Introduction
2. Nanotubes
3. Nanoelectromechanical Systems
4. Nanowires and Nanocircuits
5. Nanofabrication
6. Nanofluidics
7. Nanocavities
Notices and Subscription Information
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Section 2
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1. INTRODUCTION
ON THE NEW SCIENCE OF NANOTECHNOLOGY
"Technology is getting smaller. The average size of
microelectronic components has been decreasing steadily for the
past few decades, as has the bit size of memory devices. Between
1985 and 1995, for example, the storage density of commercial
disc drives increased by almost two orders of magnitude. The rule
of thumb coined by Gordon Moore, cofounder of Intel, is that the
power of commercial computers doubles every year and a half.
Since the earliest days of computing in the 1950s, this power has
increased by a factor of around ten billion. The driving force
behind these advances has been miniaturization. Today you can
hold in your hand a 20-cm silicon wafer containing more
electronic components than there are people in the world. So far,
the reduction in scale has been made possible by
photolithographic procedures that use photochemical patterning
followed by selective etching to carve microscopic structures
into semiconductor wafers. However, this approach has fundamental
limits of resolution, set by the wave optics of the patterning
beams: at present, a resolution of 0.1 microns for commercial
photolithography remains the Holy Grail. Electron-beam, hard
ultra-violet and X-ray lithographies offer still finer
resolution, but currently at greater cost; these are not yet
routine industrial techniques. Yet, if current trends prevail,
the scale of miniaturization will approach the scale of large
molecules -- a nanometre or so -- in just a few decades. Once
this happens, completely new technologies will be needed.
"As an alternative to the "top-down" techniques of conventional
semiconductor engineering, it now seems expedient to take
seriously the option of "bottom-up" approaches that build
nanostructures from molecular components. That is, rather than
building devices and patterning materials by "reduction" -- by
carving them from larger, monolithic blocks -- we can think of
achieving this goal by synthesis, putting the structures together
molecule by molecule, atom by atom. The methods of standard
chemical synthesis provide one option; but by exploiting self-
assembly and self-organization, we might attain the same ends in
a spontaneous, pre-programmed and less labor-intensive manner.
The creation of organized molecular structures and devices is the
objective of the nascent field of nanotechnology, whose adherents
propose to conduct engineering on the scale of nanometres."
J-M. Lehn and P. Ball: in The New Chemistry, Nina Hall (ed.).
Cambridge University Press 2000, p.347.
CARBON AND THE STRUCTURE OF FULLERENES
In the periodic table of the elements, carbon is listed as
crystallizing in the hexagonal structure. This structure consists
of planar layers of carbon atoms arranged in honeycomb lattices
called graphene sheets. Within a sheet, 3 of the 4 valence
electrons of a carbon atom form 3 strong trigonal bonds to 3
equidistant neighbors 0.14 nanometers away. The fourth valence
electrons from different carbon atoms interact to form weak pi-
bonds perpendicular to successive sheets that are loosely piled
up on top of each other every 0.34 nanometers in an alternating
ABAB ... sequence producing a 3-dimensional hexagonal unit cell.
Although there are various other stacking arrangements, this
allotrope, known as "graphite", is the most stable and most
abundant solid form of pure carbon found in nature. A slightly
less stable and vastly less abundant crystallographic form is
diamond, which has a cubic structure in which each atom is
covalently bound to 4 neighbors at the apexes of a regular
tetrahedron.
Until approximately 15 years ago, such were the only known
crystalline forms of solid carbon. In 1985, the science of carbon
was unexpectedly enlarged by the discovery of an entirely new
class of structures "fullerenes". The fullerenes first discovered
are spheroidal molecules, and such molecular clusters are
sometimes called "curved graphite" because of their obvious
appearance as curved sheets of graphene, with the typical 3-fold
coordination of each atom in a honeycomb lattice. However,
occasional pentagonal rings occur in the hexagonal network, and
these cause the curvature and eventual closure of the graphene
sheets. Fullerene molecules are in turn able to crystallize in a
variety of 3-dimensional structures.
Fullerenes were discovered serendipitously in the soot formed
when a hot carbon vapor (several thousand degrees) cools off and
condenses into clusters in an inert gas atmosphere. The most
abundant and most celebrated such molecule, C(sub60), comprises
60 carbon atoms, all equivalent, regularly arranged in 12
pentagonal and 20 hexagonal rings, in a soccer ball arrangement.
The 1996 Nobel Prize in Chemistry was awarded to R. Curl, H.
Kroto, and R. Smalley, the discoverers of this molecule. Beyond
C(sub60), the next most abundant fullerene in the condensed
carbon vapor is C(sub70). This molecule can be conceived of being
constructed by addition of a ring of 10 atoms at one of the 5-
fold equators of C(sub60). By adding successively (n) such
parallel rings, while maintaining the graphite 3-fold
coordination, one can theoretically produce a series of cigar-
shaped molecules C(sub60+10n). In the limit of large (n), these
are particular members of a subfamily of fullerenes called
"single wall nanotubes". In general, single-wall nanotubes can be
conceived as any such long strip of graphene rolled up into a
seamless cylinder. The latter can be left open or can be capped
by hemifullerenes.
Adapted from: A.A. Lucas et al: Revs. Mod. Phys. 2002 74:1
ON NANOFABRICATION AND NANOTECHNOLOGY
"'Make it small!' is a technological edict that has changed the
world. The development of microelectronics -- first the
transistor and then the aggregation of transistors into
microprocessors, memory chips and controllers -- has brought
forth a cornucopia of machines that manipulate information by
streaming electrons through silicon. Microelectronics rests on
techniques that routinely fabricate structures almost as small as
100 nanometers across... This size is tiny by the standards of
everyday experience -- about one thousandth the width of a human
hair -- but it is large on the scale of atoms and molecules. The
diameter of a 100-nanometer-wide wire would span about 500 atoms
of silicon.
"The idea of making 'nanostructures' that comprise just one or a
few atoms has great appeal, both as a scientific challenge and
for practical reasons. A structure the size of an atom represents
a fundamental limit: to make anything smaller would require
manipulating atomic nuclei -- essentially, transmuting one
chemical element into another. In recent years, scientists have
learned various techniques for building nanostructures, but they
have only just begun to investigate their properties and
potential applications. The age of nanofabrication is here, and
the age of nanoscience has dawned, but the age of nanotechnology
-- finding practical uses for nano-structures -- has not really
started yet."
G.M. Whitesides and J.C. Love: Scientific American 2001 September
ScienceWeek http://www.scienceweek.com
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2. NANOTUBES
STRUCTURE AND PROPERTIES OF CARBON NANOTUBES
H.F. Bettinger et al (Rice University, US) discuss carbon
nanotubes, the authors making the following points:
1) Multiwall carbon nanotubes, a new tubular form of carbon, were
discovered in 1991 by S. Iijima. Carbon nanotubes are comprised
of a rolled graphite sheet ("graphene") and closed by fullerene-
like caps. Depending on the way the graphene is rolled, different
chiralities are possible, and are commonly distinguished by their
chiral vector (n,m). The (n,n) tubes are called "armchair" and
the (n.0) tubes are called "zigzag" nanotubes. A simple analysis
imposing appropriate boundary conditions on the graphene band
structure predicts that the armchair tubes are metallic (i.e.,
the band gap is zero due to band crossing), whereas the zigzag
tubes are either semimetals or semiconductors, depending on the
value of (n). The computations of the band structures using
density functional theory (plane-wave pseudopotential local
density approximation) indicate that these simple rules need to
be refined, at least for narrow zigzag tubes. It was found by
these calculations that the rehybridization of the carbon states
due to the strong curvature of small tubes introduces low-lying
conduction band states into the band gap of insulating tubes.
2) Whereas Iijima's multiwall carbon nanotubes consist of at
least 2 concentric tubes, single wall carbon nanotubes are
produced by laser vaporization of a metal-graphite (Co, Ni)
target. These single-wall nanotubes are comprised of a single
rolled graphene sheet, but have a tendency to form "ropes", i.e.,
bundles of single-wall nanotubes. The tubes produced by the
laser-oven technique have very uniform diameters of approximately
1.38 +- 0.02 nanometers, according to x-ray diffraction, and an
intertube distance of 0.315 nanometers in the ropes, similar to
that in crystalline C(sub60). It has been concluded that the
ropes are made up of (10,10) armchair single-wall nanotubes.
J. Am. Chem. Soc. 2001 123:12849
Related Background:
CARBON NANOTUBES -- THE ROUTE TOWARD APPLICATIONS
R.H. Baughman et al (University of Texas Dallas, US) discuss
carbon nanotubes, the authors making the following points:
1) There are two main types of carbon nanotubes that can have
high structural perfection. Single-walled nanotubes (SWNTs)
consist of a single graphite sheet seamlessly wrapped into a
cylindrical tube. Multiwalled nanotubes (MWNTs) comprise an array
of such nanotubes that are concentrically nested like rings of a
tree trunk.
2) Despite structural similarity to a single sheet of graphite,
which is a semiconductor with zero band gap, SWNTs may be either
metallic or semiconducting, depending on the sheet direction
about which the graphite sheet is rolled to form a nanotube
cylinder. This direction in the graphite sheet plane and the
nanotube diameter are obtainable from a pair of integers (n,m)
that denote the nanotube type (1). Depending on the appearance of
a belt of carbon bonds around the nanotube diameter, the nanotube
is either of the armchair (n = m), zigzag (n = 0 or m = 0), or
chiral (any other n and m) variety. All armchair SWNTs are
metals; those with n - m = 3k, where k is a nonzero integer, are
semiconductors with a tiny band gap; and all others are
semiconductors with a band gap that inversely depends on the
nanotube diameter (1).
3) The electronic properties of perfect MWNTs are rather similar
to those of perfect SWNTs, because the coupling between the
cylinders is weak in MWNTs. Because of the nearly one-dimensional
electronic structure, electronic transport in metallic SWNTs and
MWNTs occurs ballistically (i.e., without scattering) over long
nanotube lengths, enabling them to carry high currents with
essentially no heating (2,3). Phonons also propagate easily along
the nanotube: The measured room temperature thermal conductivity
for an individual MWNT (>3000 W/m.K) is greater than that of
natural diamond and the basal plane of graphite (both 2000 W/m.K)
(4). Superconductivity has also been observed, but only at low
temperatures, with transition temperatures of about 0.55 K for
1.4-nm-diameter SWNTs (5) and ~5 K for 0.5-nm-diameter SWNTs
grown in zeolites.
4) In summary: Many potential applications have been proposed for
carbon nanotubes, including conductive and high-strength
composites; energy storage and energy conversion devices;
sensors; field emission displays and radiation sources; hydrogen
storage media; and nanometer-sized semiconductor devices, probes,
and interconnects. Some of these applications are now realized in
products. Others are demonstrated in early to advanced devices,
and one, hydrogen storage, is clouded by controversy. Nanotube
cost, polydispersity in nanotube type, and limitations in
processing and assembly methods are important barriers for some
applications of single-walled nanotubes.
References (abridged):
1. S. G. Louie, Top. Appl. Phys. 80, 113 (2001)
2. W. Liang, et al., Nature 411, 665 (2001)
3. S. P. Frank et al., Science 280, 1744 (1998)
4. P. Kim et al, Phys. Rev. Lett. 87, 215502 (2001)
5. M. Kociak, et al., Phys. Rev. Lett. 86, 2416 (2001)
Science 2002 297:787
Related Background:
POLYMER NANOTUBES BY WETTING OF ORDERED POROUS TEMPLATES
M. Steinhart et al (Philipps University Marburg, DE) discuss
polymer nanotubes, the authors making the following points:
1) The authors report they have developed a simple technique for
the fabrication of polymer nanotubes with a monodisperse size
distribution and uniform orientation. When either a polymer melt
or solution is placed on a substrate with high surface energy, it
will spread to form a thin film, known as a precursor film,
similar to the behavior of low molar mass liquids (1,2). Similar
wetting phenomena occur if porous templates are brought into
contact with polymer solutions or melts: A thin surface film will
cover the pore walls in the initial stages of wetting. This is
because the cohesive driving forces for complete filling are much
weaker than the adhesive forces. Wall wetting and complete
filling of the pores thus take place on different time scales.
The latter is prevented by thermal quenching in the case of melts
or by solvent evaporation in the case of solutions, thus
preserving a nanotube structure.
2) If the template is of monodisperse size distribution, aligned
or ordered, so are the nanotubes, and ordered polymer nanotube
arrays can be obtained if the template is removed. Any melt-
processible polymer, such as polytetrafluoroethylene (PTFE),
blends, or multicomponent solutions can be formed into nanotubes
with a wall thickness of a few tens of nanometers. The authors
suggest that owing to its versatility, this approach should be a
promising route toward functionalized polymer nanotubes.
3) The authors used ordered porous alumina and oxidized
macroporous silicon templates with narrow pore size distribution
(3). Extended regular pore arrays were prepared by lithography.
The pores are well-defined, straight, with a smooth inner surface
and with diameters DP between 300 and 900 nm. To process melts,
the authors placed the polymer on a pore array at a temperature
well above its glass transition temperature, in the case of
amorphous polymers, or its melting point, in the case of
partially crystalline polymers. The liquid polymer forms a thin
wetting film covering the entire pore surface on a time scale
ranging from a few minutes to half an hour. Polymer solutions
were dropped on the templates at ambient conditions. The
resulting nanotubes obtained from either method had wall
thicknesses between 20 and 50 nm and lengths of up to 100 æm.(4)
References (abridged):
1. P. G. de Gennes, Rev. Mod. Phys. 57, 827 (1985)
2. S. F. Kistler, in Wettability, Surfactant Science Series, vol.
49, J. C. Berg, Ed. (Dekker, New York, 1993), chap. 6.
3. R. B. Wehrspohn and J. Schilling, MRS Bull. 8, 623 (2001)
4. M. Bognitzki, et al., Adv. Mater. 12, 637 (2000)
Science 2002 296:1997
Related Background:
ENTROPICALLY DRIVEN SELF-ASSEMBLY OF MULTICHANNEL ROSETTE
NANOTUBES
Hicham Fenniri et al (Purdue University, US) discuss nanotubes,
the authors making the following points:
1) Unidimensional nanotubular objects have captivated the minds
of the scientific community over the past decade because of their
boundless potential in nanoscale science and technology. The
strategies developed to achieve the synthesis of these materials
spanned the areas of inorganic (1-5) and organic chemistry and
resulted in, for instance, carbon nanotubes (1), peptide, and
rosette nanotubes, as well as surfactant-derived tubular
architectures. Although inorganic systems benefit from the vast
majority of the elements of the periodic table and the rich
physical and chemical properties associated with them, organic
systems inherited the power of synthetic molecular and
supramolecular chemistry. As such, the latter approach offers
limitless possibilities in terms of structural, physical, and
chemical engineering.
2) A heteroaromatic bicyclic base (G^C), possessing the Watson-
Crick donor-donor-acceptor of guanine and acceptor-acceptor-donor
of cytosine, was recently reported in the context of the self-
assembly of helical rosette nanotubes. Because of the disymmetry
of its hydrogen bonding arrays, their spatial arrangement, and
the hydrophobic character of the bicyclic system, G^C undergoes a
hierarchical self-assembly process under physiological conditions
to form a six-membered supermacrocycle maintained by 18 H-bonds.
The resulting and substantially more hydrophobic aggregate then
undergoes a second level of organization to produce a stack. The
architecture thus generated defines an unoccluded central pore
running the length of the stack with tunable inner and outer
diameters. The inner space is directly related to the distance
separating the H-bonding arrays within G^C, whereas the
peripheral diameter and its chemistry are dictated by the choice
of the functional groups conjugated to this motif.
3) In summary: Rosette nanotubes are a new class of organic
nanotubes obtained through the hierarchical self-assembly of low
molecular weight synthetic modules in water. The authors
demonstrate that these materials can serve as scaffolds for the
supramolecular synthesis of multichannel nanotubular
architectures and report on the discovery of their entropy-driven
self-assembly process.
References (abridged):
1. Iijima, S. (1991) Nature (London) 354, 56-58
2. Hamilton, E.J. et al. (1993) Science 260, 659-661
3. Brumlik, C. and Martin, C. (1991) J. Am. Chem. Soc. 113, 3174-
3175
4. Shenton, W., et al. (1999) Adv. Mater. 11, 253-256
5. Chopra, N.G., et al. (1995) Science 269, 966-967
Proc. Nat. Acad. Sci. 2002 99:6487
Related Background:
HELICAL ROSETTE CHIROPTICAL NANOTUBES
H. Fenniri et al (Purdue University, US) discuss nanotubes, the
authors making the following points:
1) Nonrandom symmetry breaking in supramolecular systems may be
induced by the following three general processes: (a) The
"sergeant and soldiers principle"(1), initially formulated for
covalent polymeric systems(2) to describe the inductive effect of
a small population of chiral components on the chiroptical
outcome of macromolecular systems.(3) This effect was also
recently reported as a means to transfer molecular chirality into
oligomeric(4) systems as well as in a variety of supramolecular
assemblies.(5) (b) Molecular recognition induced chirality, a
well-studied phenomenon encountered in host-guest chemistry which
ensues from the specific recognition of a chiral guest by an
achiral host and can be rationalized on the basis of steric and
electronic factors. (c) External means such as chiral vortex
forces, photoinduced electron transfer, and redox- and photo-
switches.
2) A distinct advantage of these systems over those in which
supramolecular chirality results from spontaneous resolution is
that the chiroptical outcome is predictable and reproducible. As
a result, it can be utilized in chirotechnology for the design of
sensors,(3), chiral cholesteric phases, catalysts, asymmetric
synthesis of materials with electromagnetic and optoelectronic
applications,(3a) information storage, display systems,(3b-d) and
photochromic materials, and for the design of materials with
unique chiral light-emitting and nonlinear optical properties.
3) The authors describe two supramolecular processes for the
self-assembly of helical rosette nanotubes with adjustable
chiroptical properties. The first is the result of symmetry
breaking in a preexisting racemic mixture of M- and P-helical
rosette nanotubes. The second is the result of the triggering
effect of a chiral promoter on a prochiral molecular module that
leads to the hierarchical self-assembly of the module-promoter
complex into said rosette nanotubes. In both cases, the promoter
assumes the dual role of transferring its molecular chirality,
physical, and chemical properties to the supramolecular ensemble,
and contributing to the stabilization of the resulting
nanotubular assembly.
References (abridged):
1. (a) Green, M. M.; Peterson, N. C.; Sato, T.; Teramoto, A.;
Cook, R.; Lifson, S. Science 1995, 268, 1860-1866. (b) Green, M.
M.; Reiddy, M. P.; Johnson, R. J.; Darling, G.; O'Leary, D. J.;
Wilson, G. J. Am. Chem. Soc. 1989, 111, 6452-6454.
2. (a) Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 1998,
37, 63-68. (b) Ferringa, B. L.; van Delden, R. A. Angew. Chem.,
Int. Ed. 1999, 38, 3419-3438. (c) Yashima, E.; Matsushima, T.;
Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345-6359.
3. (a) Akagi, K.; Piao, G.; Kaneko, S.; Sakamaki, S.; Shirakawa,
H.; Kyotani, M. Science 1998, 282, 1683-1686. (b) Feringa, B. L.;
Huck, N. P. M.; van Doren, H. A. J. Am. Chem. Soc. 1995, 117,
9929-9930. (c) Huck, N. P. M.; Jager, W. F.; de Lange, B.;
Feringa, B. L. Science 1996, 273, 1686-1688. (d) Feringa, B. L.;
Huck, N. P. M.; Schoevaars, A. M. Adv. Mater. 1996, 8, 681-684.
(e) Seebach, D.; Beck, A. K.; Heckel, A. Angew. Chem., Int. Ed.
2001, 40, 93-138. (f) Oda, R.; Huc, I.; Schmutz, M.; Candau, S.
J.; MacKintosh, F. C. Nature 1999, 399, 566-569. (g) Nakano, T.;
Okamoto, Y. Chem. Rev. 2001, 101, 4013-4038. (h) Cornelissen, J.
J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M.
Chem. Rev. 2001, 101, 4039-4070.
4. (a) Prince, R. B.; Moore, J. S.; Brunsveld, L.; Meijer, E. W.
Chem.-Eur. J. 2001, 7, 4150-4154. (b) Hill, D. J.; Mio, M. J.;
Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev. 2001, 101,
3893-4011.
5. (a) Palmans, A. R. A.; Vekemans, J. A. J. M.; Havinga, E. E.;
Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2648-2651.
(b) Hirschberg, K. J. H. K.; Brunsveld, L.; Ramzi, A.; Vekemans,
J. A. J. M.; Sijbesma, R. P.; Meijer, E. W. Nature 2000, 407,
167-170. (c) Brunsveld, L.; Lohmeijer, B. G. G.; Vekemans, J. A.
J. M.; Meijer, E. W. Chem. Commun. 2000, 2305-2306. (d) Prins, L.
J.; Timmmerman, P.; Reinhoudt, D. N. J. Am. Chem. Soc. 2001, 123,
10153-10163. (e) Brunsveld, L.; Meijer, E. W.; Prince, R. B.;
Moore, J. S. J. Am. Chem. Soc. 2001, 123, 7978-7984. (f) Prins,
L. J.; De Jong, F.; Timmerman, P.; Reinhoudt, D. N. Nature 2000,
408, 181-184. (g) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.;
Sijbesma, R. P. Chem. Rev. 2001, 101, 4071-4097.
J. Am. Chem. Soc. 2002 124:11064
Related Background:
NANOTUBES: ON ATTACHING METAL IONS TO SIDEWALLS
H.C. Choi et al (Stanford University, US) discuss the chemistry
of nanotube sidewalls, the authors making the following points:
1) There has been much recent interest in the covalent and
noncovalent sidewall chemistry of single-walled carbon nanotubes
(SWNTs).(1-5) The research activities are motivated by the idea
of modifying the inert nanotube sidewalls to impart solubility in
solvents or to immobilize various organic, inorganic, or
biological species to afford nanotube "macromolecules" with
chemical functionality. Covalent modification of nanotube
sidewalls includes oxidation, fluorination, and formation of
amide bonds.(1,2) Noncovalent approaches utilize pi-stacking or
van der Waals interactions between aromatic molecules or polymers
and nanotubes.(4,5)
2) Attaching metal nanoparticles to nanotube sidewalls is of
interest for obtaining nanotube/nanoparticle hybrid materials
with useful properties,(3) and for forming metal nanowires on
nanotube templates. It has been shown, for instance, that
functionalization of SWNTs by Pd nanoparticles imparts
sensitivity to molecular hydrogen for nanotube electrical
detectors.(3) Previous approaches to metal nanoparticle
functionalization of nanotubes include physical evaporation,(3)
attachment after oxidation of nanotubes, solid-state reaction
with metal salts at elevated temperatures, and electroless
deposition from salt solutions with the aid of reducing agents or
catalyst.
3) In summary: The authors report nanotube/nanoparticle hybrid
structures were prepared by forming Au and Pt nanoparticles on
the sidewalls of single-walled carbon nanotubes. Reducing agent
or catalyst-free electroless deposition, which purely utilizes
the redox potential difference between Au3+, Pt2+, and the carbon
nanotube, is the main driving force for this reaction. It is also
shown that carbon nanotubes act as a template for wire-like metal
structures. The successful formation of the hybrid structures was
monitored by atomic force microscopy (AFM) and electrical
measurements.
References (abridged):
1. Chen, J.; Hammon, M. A.; Hu, H.; Chen, Y. S.; Rao, A. M.;
Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95.
2. Boul, P.; Liu, J.; Mickelson, E.; Huffman, C.; Ericson, L.;
Chiang, I.; Smith, K.; Colbert, D.; Hauge, R.; Margrave, J.;
Smalley, R. Chem. Phys. Lett. 1999, 310, 367.
3. Kong, J.; Chapline, M.; Dai, H. Adv. Mater. 2001, 13, 1384.
4. Chen, R.; Zhang, Y.; Wang, D.; Dai, H. J. Am. Chem. Soc. 2001,
123, 3838.
5. Star, A.; Stoddart, J.; Steuerman, D.; Diehl, M.; Boukai, A.;
Wong, E.; Yang, X.; Chung, S.; Choi, S.; Heath, J. R. Angew.
Chem., Int. Ed. 2001, 40, 1721.
J. Am. Chem. Soc. 2002 124:9058
Related Background Brief:
FUNCTIONALIZATION OF CARBON NANOTUBES BY ELECTROCHEMICAL
REDUCTION OF ARYL DIAZONIUM SALTS: A BUCKY PAPER ELECTRODE.
Small-diameter (ca. 0.7 nm) single-wall carbon nanotubes are
predicted to display enhanced reactivity relative to larger-
diameter nanotubes due to increased curvature strain. The
derivatization of these small-diameter nanotubes via
electrochemical reduction of a variety of aryl diazonium salts is
described. The estimated degree of functionalization is as high
as one out of every 20 carbons in the nanotubes bearing a
functionalized moiety. The functionalizing moieties can be
removed by heating in an argon atmosphere. Nanotubes derivatized
with a 4-tert-butylbenzene moiety were found to possess
significantly improved solubility in organic solvents. The
authors demonstrate functionalization of the nanotubes with a
molecular system that exhibits switching and memory behavior, and
they suggest this represents the marriage of wire-like nanotubes
with molecular electronic devices. J.L. Bahr et al: J. Am. Chem.
Soc. 2001 123:6536.
Related Background Brief:
SIDEWALL FUNCTIONALIZATION OF CARBON NANOTUBES. The authors
report covalent sidewall functionalization reactions and the
detailed characterization of the resulting organo-molecules.
Three types of reactions for direct addition to the unsaturated
pi-electron system of the nanotubes are selected. These types of
reactions are cycloaddition of nitrenes, the addition of
nucleophilic carbenes, and the addition of radicals. The authors
suggest that with three different methods of functionalization of
the side walls of carbon nanotubes, the first steps towards
general routes to a wide variety of new-nanotube derivatives have
been made. With appropriate addends, carbon nanotubes that
possess better solubility are easier to characterize and feature
more straightforward processibility for technological
applications. M. Holzinger et al: Ang. Chem. Int. Ed. 2001
40:4002.
Related Background:
ON CHARGE-INDUCED DISTORTION OF CARBON NANOTUBES
Y.N. Gartstein et al (Xerox Corporation, US) discuss distortion
of carbon nanotubes, the authors making the following points:
1) Carbon nanotubes are particularly interesting nanoscopic
systems [1] whose electronic and mechanical properties have been
the subject of numerous studies and are attractive for diverse
applications [2,3]. One of the proposals is to use carbon single-
wall nanotubes (SWNTs) as electrochemically driven
electromechanical actuators. In these demonstrated devices, large
electrochemical charge injection can result from the high surface
area of nanotube assemblies [4]. The charge injection produces
the electromechanical actuation. Actuator strains of above 1 %
have been observed [3], which is about 10 times that of
ferroelectrics. This high strain indicates the potential for
obtaining order of magnitude advantages over any prior-art
actuator technologies for directly converting electrical energy
to mechanical energy. Currently available nanotube sheets and
long fibers comprise bundles of SWNTs, each bundle containing
from 30 to 100 of SWNTs of various internal geometries, or chiral
vectors (N,M) [1]: from zigzag (N,0) to armchair (N,N) tubes. The
observed actuation is likely to be an average from different
SWNTs. Improved synthetic methods are expected to eventually make
it possible to use SWNTs of selected types in actuators [5].
2) The authors report an analysis designed to predict the
actuator strains that would result for different types of SWNTs
by studying a simplified electron-lattice model. Suppose one adds
delta(n) extra electrons per carbon atom to a SWNT. How would
interatomic distances be affected? The authors studied the
contribution to bond length changes arising from the modulation
of electron hopping integrals by lattice distortions. Since
Coulombic effects are ignored in this model, the results are
restricted to low charge injection levels. The authors
demonstrate that SWNTs exhibit quite a unique picture of
electromechanical actuation that strongly depends on (N, M). The
magnitude of the actuator response of individual carbon nanotubes
can be appreciably larger than that of graphite, presenting an
exciting opportunity of enhanced actuation.
3) In summary: To accommodate extra electrons or holes injected
into a single-wall carbon nanotube, carbon-carbon bonds adjust
their lengths. Resulting changes in carbon-nanotube length as a
function of charge injection provide the basis for
electromechanical actuators. The authors demonstrate that a key
mechanism at low injection levels, modulation of electron kinetic
energy, provides nanotube deformations that are both anisotropic
and strongly dependent on nanotube structure. Nanotubes can
exhibit both expansion and contraction, as well as nonmonotonic
size changes. The magnitude of the actuation response of
semiconducting carbon nanotubes may be substantially larger than
that of graphite.
References (abridged):
1. R. Saito, G. Dresselhaus, and M. Dresselhaus, Physical
Properties of Carbon Nanotubes (Imperial College Press, London,
1998)
2. Carbon Nanotubes: Synthesis, Structure, Properties and
Applications, edited by M. Dresselhaus, G. Dresselhaus, and P.
Avouris (Springer, Berlin, 2000)
3. R. Baughman, A. Zakhidov, and W. deHeer, Science (to be
published).
4. R. Baughman et al., Science 284, 1340 (1999)
5. R. Schlittler, Science 292, 1136 (2001)
Phys. Rev. Lett. 2002 89:045503
Related Background:
CONDUCTION OF WATER THROUGH HYDROPHOBIC NANOTUBES
G. Hummer et al (NIH, US) discuss water conduction through
nanotubes, the authors making the following points:
1) Confinement of matter on the nanometer scale can induce phase
transitions not seen in bulk systems. In the case of water, so-
called "drying transitions" occur on this scale as a result of
strong hydrogen-bonding between water molecules, which can cause
the liquid to recede from nonpolar surfaces to form a vapor layer
separating the bulk phase from the surface.
2) The authors report molecular dynamics simulations
demonstrating spontaneous and continuous filling of a nonpolar
carbon nanotube with a 1-dimensional ordered chain of water
molecules. Although the molecules forming the chain are in
chemical and thermal equilibrium with the surrounding bath, the
authors report they observe pulse-like transmission of water
through the nanotube. These transmission bursts result from the
tight hydrogen-bonding network inside the tube, which ensures
that density fluctuations in the surrounding bath lead to
concerted and rapid motion along the tube axis. The authors also
find that a minute reduction in the attraction between the tube
wall and water dramatically affects pore hydration, leading to
sharp two-state transitions between empty and filled states on a
nanosecond timescale.
3) The authors suggest their observations indicate that carbon
nanotubes, with their rigid nonpolar structures, might be
exploited as unique molecular channels for water and protons,
with the channel occupancy and conductivity tunable by changes in
the local channel polarity and solvent conditions.
Nature 2001 414:188
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3. NANOELECTROMECHANICAL SYSTEMS
ON MICRO- AND NANOELECTROMECHANICAL SYSTEMS
H. G. Craighead (Cornell University, US) discusses
nanoelectromechanical systems, the author making the following
points:
1) Microelectromechanical systems (MEMS) have been studied for
decades (1,2), with interest increasing recently because of
growing commercial applications. Arrays of micromechanical
mirrors for optical crossbar switches, for example, recently
caused a stir in the optical communications industry (3). Similar
technologies have been used in projection displays with an array
of metal mirrors used to modulate light beams (4). Ink-jet
printers, using control of fluid jets, represent a major use of
micromachined integrated electromechanical systems (5).
Accelerometers, used as sensors for deploying automobile air
bags, are also in wide use, and a range of MEMS sensors and
actuators are in various stages of development. Many of the
devices in practical use today are made with silicon-based
fabrication technology, because of the well-developed methods
created for use by the microelectronics industry. Typical
dimensions of MEMS devices are in the several micrometers to
hundreds of micrometers range. The importance of MEMS technology
is not so much the size, but rather the use of planar processing
technologies, related to those used in the fabrication of
electronic integrated circuits, to simultaneously "machine" large
numbers of relatively simple mechanical devices in an integrated
fashion.
2) Nanoelectromechanical systems, or NEMS, are characterized by
small dimensions, where the dimensions are relevant for the
function of the devices. Critical feature sizes may be from
hundreds to a few nanometers. New physical properties, resulting
from the small dimensions, may dominate the operation of the
devices, and new fabrication approaches may be required to make
them. Microelectronics fabrication technologies are driving
relentlessly to manufacture smaller transistors packed with
increasing density on integrated circuit chips. The economic
driving forces for this miniaturization are strong and have
driven transistor minimum feature sizes down to the 100-nm
regime. The miniaturization of commercial electronics has been
taking place with an allied physics-motivated study of electron
transport and magnetic properties of mesoscopic and nanoscale
devices.
3) In summary: Nanoelectromechanical systems are evolving, with
new scientific studies and technical applications emerging.
Mechanical devices are shrinking in thickness and width to reduce
mass, increase resonant frequency, and lower the force constants
of these systems. Advances in the field include improvements in
fabrication processes and new methods for actuating and detecting
motion at the nanoscale. Lithographic approaches are capable of
creating freestanding objects in silicon and other materials,
with thickness and lateral dimensions down to about 20
nanometers. Similar processes can make channels or pores of
comparable dimensions, approaching the molecular scale. This
allows access to a new experimental regime and suggests new
applications in sensing and molecular interactions.
References (abridged):
1. W. E. Newell, Science 161, 1320 (1968)
2. K. E. Peterson, IEEE Trans. Electron Devices 25, 1241 (1978)
3. L. J. Hornbeck, Texas Instruments Tech. J. 15, 7 (1998) ; U.S.
Patent 5,583,668 (multilevel digital micromirror device); P. F.
Van Kessel, L. J. Hornbeck, R. E. Meier, M. R. Douglass, Proc.
IEEE 86, 1687 (1998)
4. E. Bassous, H. H. Taub, L. Kuhn, Appl. Phys. Lett. 31, 135
(1977)
5. L. M. Roylanceans and J. B. Angell, IEEE Trans. Electron
Devices 26, 1911 (1979)
Science 2000 290:1532
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4. NANOWIRES AND NANOCIRCUITS
Nanowires (and nanotubes) are nanoscale [of the order of 10^(-9)
meters] structures that are effectively one-dimensional, and such
structures have great potential importance in many applications
ranging from probe microscopy to nanoelectronics. There are a
number of methods of producing one-dimensional nanoscale
structures, but so far none of them has allowed enough control of
structure parameters to be satisfactory. Vapor-liquid-solid
growth is a method of forming crystalline wire-like structures
with a liquid metal cluster or catalyst acting as the energetic-
ally favored site for absorption of gas-phase reactants. What
happens is that the cluster supersaturates and grows a one-
dimensional structure of the material, and the lower diameter
limit of the grown one-dimensional structure is apparently the
diameter of the liquid metal cluster starting locus. In the
context of this report, the term "laser ablation cluster
formation" refers to the use of a laser to reduce (ablate) a
cluster to nanoscale dimensions.
FUNCTIONAL NANOSCALE ELECTRONIC DEVICES ASSEMBLED USING SILICON
NANOWIRE BUILDING BLOCKS
Y. Cui and C.M. Lieber (Harvard University, US) discuss
nanowires, the authors making the following points:
1) Miniaturization of silicon electronics is being intensely
pursued (1), although fundamental limits of lithography may
prevent current techniques from reaching the deep nanometer
regime for highly integrated devices (2). The use of nanoscale
structures as building blocks for self-assembled (3-5) structures
could potentially eliminate conventional and costly fabrication
lines, while still maintaining some concepts that have proven
successful in microelectronics. One-dimensional structures, such
as nanowires (NWs) and carbon nanotubes (NTs), could be ideal
building blocks for nanoelectronics, because they can function
both as devices and as the wires that access them. Electrical
transport studies of NTs have yielded data supporting the
possibility of field-effect transistors, low-temperature single-
electron transistors, intramolecular metal-semiconductor diodes,
and intermolecular-crossed NT-NT diodes. However, the use of NT
building blocks is limited, because the selective growth and/or
assembly of semiconducting or metallic NTs is not currently
possible.
2) Successful implementation of a building-block approach for the
assembly of nanodevices and device arrays will require that the
electronic properties of the blocks be defined and controlled.
The authors recently demonstrated that the carrier type
(electrons, n-type; holes, p-type) and carrier concentration in
single-crystal silicon NWs (SiNWs) could be controlled during
growth, using phosphorous and boron dopants. The authors now
report the rational assembly of these well-defined SiNW building
blocks into functional electronic devices in which critical
junctions are formed by the assembly of one or more SiNW/SiNW
crosses using p- and n-type materials.
3) In summary: Because semiconductor nanowires can transport
electrons and holes, they could function as building blocks for
nanoscale electronics assembled without the need for complex and
costly fabrication facilities. The authors report that boron- and
phosphorous-doped silicon nanowires were used as building blocks
to assemble three types of semiconductor nanodevices. Passive
diode structures consisting of crossed p- and n-type nanowires
exhibit rectifying transport similar to planar p-n junctions.
Active bipolar transistors, consisting of heavily and lightly n-
doped nanowires crossing a common p-type wire base, exhibit
common base and emitter current gains as large as 0.94 and 16,
respectively. In addition, p- and n-type nanowires have been used
to assemble complementary inverter-like structures. The authors
suggest that the facile assembly of key electronic device
elements from well-defined nanoscale building blocks may
represent a step toward a "bottom-up" paradigm for electronics
manufacturing.
1. P. Peercy, Nature 406, 1023 (2000)
2. G. Timp, Nanotechnology (Springer-Verlag, New York, 1999), pp.
161-206
3. C. B. Murray, C. R. Kagan, M. G. Bawendi, Science 270, 1335
(1995)
4. C. A. Mirkin, R. L. Letsinger, R. C. Mucic, J. J. Storhoff,
Nature 382, 607 (1996)
5.A. P. Alivisatos, et al., Nature 382, 609 (1996)
Science 2001 291:851
ON NANOWIRE BUILDING BLOCKS
Y. Huang et al (Harvard University, US) discuss nanowire building
blocks, the authors making the following points:
1) Fundamental physical constraints and economics are expected to
limit continued miniaturization in electronics by conventional
top-down manufacturing during the next one or two decades and
have thus motivated efforts worldwide to search for new
strategies to meet expected computing demands of the future.
Bottom-up approaches to nanoelectronics, where the functional
electronic structures are assembled from well-defined nanoscale
building blocks, such as carbon nanotubes, molecules, and/or
semiconductor nanowires, have the potential to go far beyond the
limits of top-down manufacturing. For example, single-walled
carbon nanotubes have been used as building blocks to fabricate
room-temperature field-effect transistors, diodes, and an
inverter that represents a key component for logic operations.
2) However, the inability to control whether such nanotubes are
semiconducting or metallic makes specific device fabrication
largely a random event and poses a serious issue for integration
beyond the single-device element level. A potential solution to
the problem of coexisting metallic and semiconductor nanotubes
involves selective destruction of metallic tubes, although such
an approach requires extensive top-down lithography and
subsequent processing to implement.
3) The authors report a bottom-up approach in which functional
device elements and element arrays have been assembled from
solution through the use of electronically well-defined
semiconductor nanowire building blocks. The authors demonstrate
that crossed nanowire p-n junctions and junction arrays can be
assembled with over 95 percent yield and with controllable
electrical characteristics, and in addition, that these junctions
can be used to create integrated nanoscale field-effect
transistor arrays with nanowires as both the conducting channel
and gate electrode. Nanowire junction arrays have been configured
by the authors as key OR, AND, and NOR logic-gate structures with
substantial gain and have been used to implement basic
computation.
Science 2001 294:1313
Related Background:
BANDGAP MODULATION OF CARBON NANOTUBES
J. Lee et al (Seoul National University, KR) discuss carbon
nanotubes, the authors making the following points:
1) Technical and economic difficulties in further miniaturizing
silicon-based transistors with present fabrication technologies
have motivated a strong effort to develop alternative electronic
devices, including devices based on single molecules. Carbon
nanotubes have been successfully used for nanometer-sized devices
such as diodes, transistors, and random access memory cells, with
such nanotube devices usually very long compared to silicon-based
transistors.
2) The authors report a method for dividing a semiconductor
nanotube into multiple *quantum dots with lengths of
approximately 10 nanometers by inserting Gd@C(sub82) *endohedral
fullerenes. The spatial modulation of the nanotube electronic
*bandgap is observed with a low-temperature scanning tunneling
microscope. The authors report that a bandgap of approximately
0.5 electronvolts is narrowed down to 01. eV at sites where
endohedral metallofullerenes are inserted. The authors suggest
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-electronics and nano-optoelectronics.
Nature 2002 415:1005
Notes:
*quantum dots: 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.
*endohedral: "In-the-cage"; describing a chemical entity
contained within a molecular cage.
*bandgap: In general, a forbidden energy band. In this context, a
"band" is a closely spaced group of energy levels in atoms, in
particular a range of energies that electrons can have in a
solid. Each band represents a large number of allowed quantum
states. The outermost electrons of the atoms form the "valence
band" of the solid. In order for electrons to move through a
solid, there must exist empty quantum states with the same
energy, and this can occur only in an unfilled band, the
"conduction band". In general, so-called "metals" are good
conductors because the partly filled conduction band overlaps
with a filled valence band, and vacant energy states in the
conduction band are thus readily available to electrons. In
"insulators", the conduction band and valence band are separated
by a wide forbidden band, and electrons do not have enough energy
to jump from one band to another. In intrinsic "semiconductors",
the forbidden gap is narrow, and at normal temperatures some
electrons at the top of the valence band can move by thermal
agitation into the conduction band. In a so-called "doped"
semiconductor, the doping impurities essentially create one or
more thin separate conduction bands in the forbidden band. In
this context, the "gap" refers to the gap between energy bands,
i.e., from the upper boundary of the valence band to the lower
boundary of the conduction band.
Related Background:
ON MAGNETORESISTANCE OF CARBON NANOTUBES
S. Roche and R. Saito (Atomic Energy Commission Grenoble, FR)
discuss magnetoresistance of carbon nanotubes, the authors making
the following points:
1) Single-walled carbon nanotubes exhibit either a metallic or a
semiconducting character depending on their helicity. Metallic
tubes have micron-long mean free paths and behave as long
ballistic conductors. Conversely, the intrinsic properties of
conducting multi-walled nanotubes are elusive. Indeed, reported
ballistic, diffusive, or insulating experimental behaviors remain
difficult to relate to the number and helicities of constitutive
shells and interlayer coupling. It is usually assumed that the
outermost shell, in contact with metallic electrodes, determines
the metallic or semiconducting character of multi-walled carbon
nanotubes.
2) Two fundamental issues concerning these systems are the role
of magnetic fields in transport mechanisms, and the sign and
oscillations of "magnetofingerprints". In general, measuring
magnetoresistance in mesoscopic systems is a formidable tool to
investigate quantum coherent phenomena beyond classical effects.
An elegant perturbation theory of localization has been developed
for metallic cylinders when the mean free path is smaller than
the cylinder circumference. By applying a magnetic field, time
reversal invariance is broken, and dephasing of the electronic
pathways reduces the enhancement of the probability of "return to
the origin" (weak localization). Accordingly, a decrease of
resistance with magnetic field results (negative
magnetoresistance). If the mean free path becomes much larger
than the circumference, the electronic conduction is called
"quasiballistic", and a larger magnetic field is needed to
suppress weak localization. The conduction is finally called
"ballistic" when the mean free path is larger than the distance
between electrodes.
3) In a study of magnetoresistance in these systems, the author
demonstrate that for small nanotube diameters, the location of
the chemical potential and the orientation of the magnetic field
are parameters that enable tuning from positive to negative
magnetoresistance, a phenomenon not related to weak localization.
For larger diameters (=> 10 nanometers), the conventional
mesoscopic behavior of magnetotransport is recovered.
Phys. Rev. Lett. 2001 87:246803
Related Background:
LIMITS OF SILICON NANOELECTRONICS
J.D. Meindl et al (Georgia Institute of Technology, US) discuss
the limits of silicon nanoelectronics, the authors making the
following points:
1) Silicon technology has advanced at exponential rates in both
performance and productivity throughout the past four decades.
From 1960 to 2000, the energy transfer associated with a binary
switching transition -- the canonical digital computing operation
-- decreased by approximately 5 orders of magnitude and the
number of transistors per chip increased by approximately 9
orders of magnitude. But such exponential advances must
eventually come to a halt imposed by a hierarchy of physical
limits. The 5 levels of this hierarchy are defined as:
fundamental, material, device, circuit, and system.
2) A coherent analysis of the key limits of each of these levels
reveals that silicon technology has the enormous remaining
potential to achieve terascale integration of more than 1
trillion transistors per chip, with critical device dimensions or
channel lengths in the 10 nanometer range. This potential
represents more than a 3-decade increase in the number of
transistors per chip and more than a 1-decade reduction in
minimum transistor feature size compared with the state of the
art in 2001.
3) Fundamental physical limits that are independent of the
characteristics of any particular material, device structure,
circuit configuration, or system architecture will be virtually
impenetrable barriers to additional advances of terascale
integration. But limited terascale integration on a massive scale
is feasible assuming the development and economical mass
production of double-gate metal-oxide-semiconductor field effect
transistors with gate oxide thickness of approximately 1
nanometer, silicon channel thickness of approximately 3
nanometers, and channel length of approximately 10 nanometers.
The development of interconnecting wires for these transistors
presents a major current challenge to the achievement of
nanoelectronics involving terascale integration.
Science 2001 293:2044
Related Background:
FABRICATION OF SILVER NANOWIRES
B.H. Hong et al (Pohang University of Science and Technology, KR)
discuss silver nanowires, the authors making the following
points:
1) Nanowires have attracted extensive interest in recent years
because of their unusual quantum properties and potential use as
nanoconnectors and nanoscale devices. In order to have enhanced
physical properties, the wires must be of small diameter, high
aspect ratio (i.e., ratio of length to thickness), and uniformly
oriented. The stability of the nanowires is also a concern, since
metal nanowires of approximately 1 nanometer diameter are
reported to have existed only transiently (less than 10 seconds
in ultrahigh vacuum). The authors report the synthesis of single-
crystalline silver nanowires of atomic dimensions. The ultrathin
silver wires with 0.4 nanometer width grow up to micron-scale
length inside the pores of self-assembled calix[4]hydroquinone
nanotubes by electro-/photochemical redox reaction in an ambient
aqueous phase. The authors report these subnanowires are very
stable under ambient air and aqueous environments, unlike
previously reported metal wires of approximately 1 nanometer
diameter, which existed only transiently in ultrahigh vacuum. The
present wires exist as coherently oriented 3-dimensional arrays
of ultrahigh density and thus could be used as model systems for
investigating 1-dimensional phenomena and as nanoconnectors for
designing nanoelectronic devices.
Science 2001 294:348
Related Background:
ON NANOWIRE NANOSENSORS
Y. Cui et al (Harvard University, US) discuss nanowire
nanosensors, the authors making the following points:
1) Planar semiconductors can serve as the basis for chemical and
biological sensors in which detection can be monitored
electrically and/or optically. For example, a planar field effect
transistor (FET) can be configured as a sensor by modifying the
gate oxide (without the gate electrode) with molecular receptors
or a selective membrane for the analyte of interest: binding of a
charged species then results in depletion or accumulation of
carriers within the transistor structure. An attractive feature
of such chemically sensitive FETs is that binding can be
monitored by a direct change in conductance or some related
electrical property, although the sensitivity and potential for
integration are limited. The physical properties limiting sensor
devices fabricated in planar semiconductors can be readily
overcome by exploiting nanoscale FETs.
2) First, binding to the surface of a nanowire or nanotube can
lead to depletion or accumulation of carriers in the bulk of the
nanoscale structure (as opposed to only the surface region of a
planar device) and increase sensitivity to the point that single-
molecule detection is possible. Second, the small size of
nanowires and nanotube building blocks and recent advances in
assembly suggest that dense arrays of sensors could be prepared.
Indeed nanotube FETs have recently been demonstrated by J. Kong
et al (Science 2000 287:622) to function as gas sensors, with
calculations suggesting that direct binding of electron-
withdrawing or electron-donating gas molecules to the nanotube
surface chemically gated these devices.
Science 2001 293:1289
Related Background:
PHOTOLUMINESCENT NANOWIRES
J. Wang et al (Harvard University, US) discuss photoluminescence
and photodetection from single nanowires, the authors making the
following points:
1) Optical studies of 1-dimensional nanostructures have in the
past focused primarily on lithographically and epitaxially
defined quantum wires embedded in a semiconductor medium. Free-
standing nanowires have several attractive differences from these
systems, including a large variation in the dielectric constant
between the nanowire and the surrounding medium, and a
cylindrical and strongly confined potential for both electrons
and holes.
2) The authors report optical studies of individual free-standing
indium-phosphide nanowires that demonstrate giant polarization
anisotropy in photoluminescence measurements, and the use of
these indium-phosphide nanowires as photoconductivity-based
photodetectors. Polarization measurements reveal a striking
anisotropy in the photoluminescence intensity recorded parallel
and perpendicular to the long axis of a nanowire. The order-of-
magnitude polarization anisotropy was quantitatively explained in
terms of the large dielectric constant contrast between these
free-standing nanowires and the surrounding environment, as
opposed to an explanation in terms of quantum confinement
effects. This intrinsic anisotropy was used to create
polarization-sensitive nanoscale photodetectors that may prove
useful in integrated photonic circuits, optical switches and
interconnects, near-field imaging, and high-resolution detectors.
Science 2001 293:1455
Related Background:
FABRICATION OF METAL NANOWIRES
J.H. Song et al (University of California Berkeley, US) discuss
the fabrication of metal nanowires, the authors making the
following points:
1) Recent research in the field of nanoscale electronics has
focused on two fundamental issues: a) the operating principles of
small-scale devices, and b) schemes that lead to their
realization and eventual integration into useful circuits. The
availability of a nanoscale toolbox is the key for this field of
research, and among the many potential building blocks within
this nanoscale toolbox, nanowires are considered one of the key
components because they can be used as interconnects and other
functional devices in nanoelectronics.
2) Unfortunately, although several processes have been developed
for the synthesis of semiconductor nanowires, few methods have
been developed for preparing free-standing uniform metal
nanowires. Among these methods, template synthesis (in porous
matrices such as porous Al(sub2)O(sub3) films and mesoporous
silica) and step-edge decoration are considered to be effective
approaches. The step-edge decoration method was recently used to
produce molybdenum nanowires of 100 nanometers thickness. Metal
nanowires have also been prepared using DNA and carbon nanotubes
as templates.
3) The authors report a simple chemical process for synthesizing
long free-standing metal nanowires. In their method,
LiMo(sub3)Se(sub3) nanowires are used as both reducing agents and
sacrificial templates in these experiments to yield continuous
metal nanowires (Au, Ag, Pt, and Pd) generally having diameters
10 to 100 nanometers and lengths of several microns. The authors
report these metal nanowires display small ohmic resistances at
room temperature, indicating that these wires could prove useful
as interconnects in nanoelectronic circuits.
J. Am. Chem. Soc. 2001 123:10397
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5. NANOFABRICATION
ON BUILDING NANOSTRUCTURES FROM THE BOTTOM UP
N.C. Seeman and A.M. Belcher (University of Texas Austin, US)
discuss building nanostructures, the authors making the following
points:
1) We hear continually that nanoscience and nanotechnology are
frontier areas. Everyone is aware that nanotechnology and
nanoscience involve the construction and analysis of objects and
devices that are very small on the macroscopic scale.
Nevertheless, if the ultimate feature sizes of nanoscale objects
are approximately a nanometer or so, we are talking about
dimensions an order of magnitude larger than the scale exploited
by chemists for over a century. Synthetic chemists have
manipulated the constituents, bonding, and stereochemistry of
vast numbers of molecules on the angstrom scale, and physical and
analytical chemists have examined the properties of these
molecules. So what is so special about the nanoscale?
2) There are many answers to this question, possibly as many as
there are people who call themselves nanoscientists or
nanotechnologists. A particularly intriguing feature of the
nanoscale is that this is the scale on which biological systems
build their structural components, such as microtubules,
microfilaments, and chromatin. The associations maintaining these
and the associations of other cellular components seem relatively
simple when examined by high-resolution structural methods, such
as crystallography or NMR -- shape complementarity, charge
neutralization, hydrogen bonding, and hydrophobic interactions.
3) A key property of biological nanostructures is molecular
recognition, leading to self-assembly and the templating of
atomic and molecular structures. For example, it is well known
that two complementary strands of DNA will pair to form a double
helix. DNA illustrates two features of self-assembly. The
molecules have a strong affinity for each other and they form a
predictable structure when they associate. Those who wish to
create defined nanostructures would like to develop systems that
emulate this behavior. Thus, rather than milling down from the
macroscopic level, using tools of greater and greater precision
(and probably cost), they would like to build nanoconstructs from
the bottom up, starting with chemical systems.(1-5)
References (abridged):
1. Seeman, N. C. (1999) Trends Biotechnol. 17, 437-443
2. Seeman, N. C. (1982) J. Theor. Biol. 99, 237-247
3. Qiu, H. , Dewan, J. C. & Seeman, N. C. (1997) J. Mol. Biol.
267, 881-898
4. Robinson, B. H. & Seeman, N. C. (1987) Protein Eng. 1, 295-300
5. Petrillo, M. L. et al. (1988) Biopolymers 27, 1337-1352
Proc. Nat. Acad. Sci. 2002 99:6451
Related Background:
SEGMENTED NANOFIBERS OF SPIDER DRAGLINE SILK: ATOMIC FORCE
MICROSCOPY AND SINGLE-MOLECULE FORCE SPECTROSCOPY
E. Oroudjev et al (University of California Santa Barbara, US)
discuss nanofibers, the authors making the following points:
1) The last decade has seen a significant increase in the
scientific literature on spider dragline silk. This interest is
due to the impressive mechanical properties of spider dragline
silk, at a time when biomaterials and biomimetics are both
exciting interest in the rapidly growing field of materials
research. The viscoelastic fibers of spider dragline silk combine
both a high tensile strength that is comparable to steel and is
only slightly inferior to Kevlar (2/3 of its tensile strength),
and a high elasticity (30% elongation before failure) that is
comparable to rubber (1-4). This unique combination makes spider
dragline silk mechanically superior to almost any other natural
or man-made material. It is apparent that the mechanical
properties of the dragline silk protein's intramolecular
structure as well as the intermolecular organization of these
proteins in the fiber are critical for spider silk performance
(2,5).
2) Spider dragline silk can be pictured as a composite material
consisting of a semiamorphous matrix filled with tiny (<10 nm)
nano-crystalline-like particles. The amino acid sequence for
spider dragline silk proteins is comprised of poly(A)
[poly(alanine)], for some silks substituted by poly(GA)
[poly(glycyalanine)], and glycine-rich sequences (2). Despite
intensive structural studies on spider dragline silk proteins,
their exact structural organization remains to be solved. The 4-
to 10-residue-long poly(A) and poly(GA) motifs are thought to be
involved in the formation of beta-sheet nano-crystalline-like
particles. Glycine-rich sequences are thought to fold into some
non-alpha-helical helical structure for GGX or into beta-turns
for GPGGX, thus forming the semiamorphous matrix (2). On the
other hand, a few reports suggest that at least part of these
glycine-rich motifs can also fold into beta-sheets and/or form an
interphase between crystalline-like objects and a semiamorphous
matrix.
3) Despite its remarkable materials properties, the structure of
spider dragline silk has remained unsolved. Results from two
probe microscopy techniques provide new insights into the
structure of spider dragline silk. A soluble synthetic protein
from dragline silk spontaneously forms nanofibers, as observed by
atomic force microscopy. These nanofibers have a segmented
substructure. The segment length and amino acid sequence are
consistent with a slab-like shape for individual silk protein
molecules. The height and width of nanofiber segments suggest a
stacking pattern of slab-like molecules in each nanofiber
segment. This stacking pattern produces nano-crystals in an
amorphous matrix, as observed previously by NMR and x-ray
diffraction of spider dragline silk. The possible importance of
nanofiber formation to native silk production is discussed. Force
spectra for single molecules of the silk protein demonstrate that
this protein unfolds through a number of rupture events,
indicating a modular substructure within single silk protein
molecules. A minimal unfolding module size is estimated to be
around 14 nm, which corresponds to the extended length of a
single repeated module, 38 amino acids long. The structure of
this spider silk protein is distinctly different from the
structures of other proteins that have been analyzed by single-
molecule force spectroscopy, and the force spectra show
correspondingly novel features.
1. Hinman, M. B. , Jones, J. A. & Lewis, R. V. (2000) Trends
Biotechnol. 18, 374-379
2. Hayashi, C. Y., Shipley, N. H. & Lewis, R. V. (1999) Int. J.
Biol. Macromol. 24, 271-275
3. Tirrell, D. A. (1996) Science 271, 39-40
4. Cunniff, P. M., et al. (1994) Polym. Adv. Technol. 5, 401-410.
5. Vollrath, F. & Knight, D. (2001) Nature (London) 410, 541-548
Proc. Nat. Acad. Sci. 2002 99:6460
Related Background:
MICROFABRICATING CONJUGATED POLYMER ACTUATORS
W. Edwin et al (Linkopings University, SE) discuss
microfabrication of actuators, the authors making the following
points:
1) The miniaturization of electronic and optical devices has
fueled the information technology revolution. During the past
decade, a similar miniaturization has been going on for sensors
and actuators for mechanical, chemical, and biological
applications. The integrated gas chromatograph was an early
example (1); today, an integrated analysis system for sample
handling for biological characterization has recently been
developed (2).
2) Microstructures promise to be of great importance for the
coming biotechnology revolution. There is currently a tremendous
increase in both academic and corporate research on micromachined
laboratories-on-a-chip, or micro-total analysis systems (3).
These devices will find applications in areas such as genomic and
proteomic studies, which will require extensive parallelism to
allow many small simultaneous experiments. The integration of
multiple experiments on a single carrier requires a miniature
format. To minimize the chance of cross contamination when
handling biological samples, a single-use device is preferred.
Therefore, these devices should be disposable and thus be
produced with inexpensive materials and patterning techniques.
Polymers might be an option.
3) Polymers can be patterned by inexpensive methods such as hot
embossing and imprinting and therefore make attractive carrier
materials. Imprint methods allow submicrometer patterning with
dimensions smaller than 100 nm, as has been demonstrated with
hard (4) and soft (5) imprint materials. Polymers can also
deliver active functions. Polymer surfaces can be chemically
modified in a variety of ways, and this property is important in
microstructures, which have a high surface-to-volume ratio. For
example, surface-bound processes may be used to alter
biomolecular function. Some polymers even allow the formation of
electronic devices. Field effect transistors with useful carrier
mobility have been made. Because thin polymer films may be easily
prepared by spin coating, they can be integrated into functional
systems. This capability may be important in the development of
inexpensive, disposable chemical detectors for genomics and
proteomics.
4) In summary: Conjugated polymer actuators can be operated in
aqueous media, which makes them attractive for laboratories-on-a-
chip and applications under physiological conditions. One of the
most stable conjugated polymers under these conditions is
polypyrrole, which can be patterned by means of standard
photolithography. Polypyrrole-gold bilayer actuators that bend
out of the plane of the wafer have been microfabricated in the
laboratory of the authors. These can be used to move and position
other microcomponents. The authors review the current status of
these microactuators, outlining the methods used to fabricate
them, and describe the devices that have been demonstrated as
well as some potential future applications.
References (abridged):
1. S. C. Terry, J. H. Jerman, J. B. Angell, IEEE Trans. Electron.
Devices 26, 1880
2. S. Ekstrom, et al., Anal. Chem. 72, 286 (2000)
3. A. v. d. Berg, W. Olthuis, P. Bergveld, Eds., Micro Total
Analysis Systems 2000 (Kluwer, Dordrecht, Netherlands, 2000)
4. S. Y. Chou, P. R. Krauss, P. J. Renstrom, Appl. Phys. Lett.
67, 3114 (1995)
5. Y. Xia and G. M. Whitesides, Angew. Chem. Int. Ed. 37, 550
(1998)
science 2000 290:1523
Related Background:
SELF-ASSEMBLY AND MINERALIZATION OF NANOFIBERS
J.D. Hartgerink et al (Northwestern University, US) discuss
nanofibers, the authors making the following points:
1) Self-assembly and biomineralization are used in biological
systems for natural fabrication of many composite materials. Bone
tissue is a particularly complex example of such a composite
because it contains multiple levels of hierarchical organization.
At the lowest level of his hierarchy is the organization of
collagen fibrils with respect to hydroxyapatite crystals. The
collagen fibrils are formed by self-assembly of collagen triple
helices and hydroxyapatite crystals grow within these fibrils in
such a way that their (c) axes are oriented along the long axes
of the fibers.
2) The preparation of any material with structure on the
nanoscale is a challenging problem. Fabrication of materials that
resemble bone, even at the lowest level of hierarchical
organization, is even more difficult because it involves two
dissimilar organic and inorganic nanophases that have a specific
spatial relation with respect to one another. One way to
accomplish this in an artificial system is to prepare an organic
nanophase designed to exert control over crystal nucleation and
growth of the inorganic component.
3) The authors report they have used the pH-induced self-assembly
of a peptide-amphiphile to make a nanostructured fibrous scaffold
reminiscent of the extracellular matrix of living tissue. The
design of this peptide-amphiphile allows the nanofibers to be
reversibly cross-linked to enhance or decrease mineralization of
hydroxyapatite to form a composite material in which the
crystallographic (c) axes of hydroxyapatite are aligned with the
long axes of the fibers. This alignment is the same as that
observed between collagen fibrils and hydroxyapatite crystals in
bone.
Science 2001 294:1684
Related Background:
SELF-ASSEMBLY OF METAL NANOSTRUCTURES ON POLYMER SCAFFOLDS
W.A. Lopes and H.M. Jaeger (University of Chicago, US) discuss
self-assembly of metal nanostructures, the authors making the
following points:
1) Self-assembly is emerging as an elegant "bottom-up" method for
fabricating nanostructured materials. This approach becomes
particularly powerful when the ease and control offered by the
self-assembly of organic components is combined with the
electronic, magnetic, or photonic properties of inorganic
components.
2) The authors report a demonstration of a versatile hierarchical
approach for the assembly of organic-inorganic copolymer-metal
nanostructures in which one level of self-assembly guides the
next. In a first step, ultrathin diblock copolymer films form a
regular scaffold of highly anisotropic stripe-like domains.
During a second assembly step, differential wetting guides
diffusing metal ions to aggregate selectively along the scaffold,
producing highly organized metal nanostructures.
3) The authors report that in contrast to the usual requirement
of near-equilibrium conditions for ordering, the metal arranged
on the copolymer scaffold produces the most highly ordered
configurations when the system is far from equilibrium. The
authors delineate two distinct assembly modes of the metal
component -- each mode characterized by different ordering
kinetics and strikingly different current-voltage
characteristics. The authors suggest these results therefore
demonstrate the possibility of guided large-scale assembly of
laterally nanostructured systems.
Nature 2001 414:735
Related Background:
ON THE DESIGN OF NANOSCALE MATERIALS
C.N. Fleming et al (University of North Carolina Chapel Hill, US)
discuss the design of nanoscale materials, the authors making the
following points:
1) One strategy for designing functional nanoscale materials is
to organize molecular constituents into assemblies that perform
complex functions. A critical factor in the development of such
materials is the ability to control the spatial arrangement of
the molecular components, especially if intermolecular energy-
and charge-transfer processes are at the core of the function of
the material.
2) Spatial organization of molecular components can be achieved
through the design of covalently bonded supramolecules in which
molecular subunits are linked together so that their relative
geometries (i.e., separations and orientations) are well defined.
Structures of this kind are demanding to synthesize, but they
offer the greatest degree of control over structural parameters.
3) An alternative method of spatial organization of components
utilizes disordered supports to organize the necessary
components. Derivatized polymers are attractive for the
positioning of molecular constituents because they offer
flexibility and simplicity in the degree of multicomponent
assemblies. For example, controlled positioning of chromophores
and other components along a polymer backbone such as that of
derivatized polystyrene has been demonstrated by several
laboratories.
J. Am. Chem. Soc. 2001 123:10336
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6. NANOFLUIDICS
FORMATION OF GEOMETRICALLY COMPLEX LIPID NANOTUBE-VESICLE
NETWORKS OF HIGHER-ORDER TOPOLOGIES
M. Karlsson et al (Chalmers University of Technology, SE) discuss
formation of lipid nanotubes, the authors making the following
points:
1) The last two decades have witnessed a tremendous development
in miniaturization of fluidic devices. The rapid progress in
processing hard materials such as silicon and metals (1),
polymeric materials such as polydimethylsiloxane (2), and
parylenes (3) together with advancements in flow regulation (4,
5) have made it possible to manufacture complex chip structures
for a wide range of applications, including chemical kinetics,
computations, and chemical analysis.
2) The ultimate fluidic device is one that can handle single
molecules and colloid particles. Such devices require
unprecedented control over transport and mixing behaviors, and to
advance current fluidics into the single-molecule regime, we have
to develop systems having physical dimensions in the nanometer
scale. To create such devices, we can draw much knowledge from
biological systems. For example, the Golgi-endoplasmic reticulum
network in eukaryotic cells has many attractive features for
sorting and routing of single molecules, such as ultra-small-
scale dimension, transport control, and capability to recognize
different molecular species, and for performing chemical
transformations in nanometer-sized compartments with minimal
dilution. It is, however, extremely difficult to mimic these
biological systems by using traditional microfabrication
technologies and materials because of their small scale, complex
geometries, and advanced topologies. Furthermore, it is difficult
to implement traditional flow regulation methods on nanoscale
systems.
3) In summary: The authors present a microelectrofusion method
for construction of fluid-state lipid bilayer networks of high
geometrical complexity up to fully connected networks with genus
= 3 topology. Within networks, self-organizing branching nanotube
architectures could be produced where intersections spontaneously
arrange themselves into three-way junctions with an angle of 120
degrees between each nanotube. Formation of branching nanotube
networks appears to follow a minimum-bending energy algorithm
that solves for pathway minimization. It is also demonstrated
that materials can be injected into specific containers within a
network by nanotube-mediated transport of satellite vesicles
having defined contents. Using a combination of
microelectrofusion, spontaneous nanotube pattern formation, and
satellite-vesicle injection, complex networks of containers and
nanotubes can be produced for a range of applications in, for
example, nanofluidics and artificial cell design. In addition,
this electrofusion method allows integration of biological cells
into lipid nanotube-vesicle networks.
References (abridged):
1. Brittain, S. T. , Schueller, O. J. A. , Wu, H. K. ,
Whitesides, S. & Whitesides, G. M. (2001) J. Phys. Chem. B 105,
347-350
2. McDonald, J. C. , Duffy, D. C. , Anderson, J. R. , Chiu, D. T.
, Wu, H. K. , Schueller, O. J. A. & Whitesides, G. M. (2000)
Electrophoresis 21, 27-40
3. Vaeth, K. M. , Jackman, R. J. , Black, A. J. , Whitesides, G.
M. & Jensen, K. F. (2000) Langmuir 16, 8495-8500
4. Jorgenson, J. W. & Lukacs, K. D. (1981) Anal. Chem. 53, 1298-
1302
5. Desmet, G. & Baron, G. V. (2000) Anal. Chem. 72, 2160-2165
Proc. Nat. Acad. Sci. 2002 99:11573
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7. NANOCAVITIES
CREATING NANOCAVITIES OF TUNABLE SIZES: HOLLOW HELICES
B. Gong et al (State University of New York Buffalo, US) discuss
nanocavities, the authors making the following points:
1) Based on the folding of biopolymers, nature has developed
astonishingly efficient and sophisticated strategies for
generating various nanostructures. Of particular interests is the
availability of a wide variety of nanosized cavities and holes
that are responsible for numerous biological processes and
functions. In recent years there has been intense interest in
developing folding oligomers and polymers (foldamers) with
unnatural backbones that adopt well-defined structures (1-3),
which may eventually lead to protein-like molecular objects with
sizes in the nanometer range. Many foldamer systems have been
described (4,5).
2) Despite the progress made so far, the foldamer field is still
in its infancy. One daunting challenge involves the design of
foldamers with cavities and holes of tunable sizes in the
nanometer range, the realization of which will have far-reaching
significance for not only fundamental understanding but also
important applications. While cavities and holes are mostly seen
at the tertiary and quaternary structural levels of biopolymers,
almost all foldamers reported so far fold into secondary
structures.
3) In summary: The authors present a general strategy for
creating nanocavities with tunable sizes based on the folding of
unnatural oligomers. The backbones of these oligomers are
rigidified by localized, three-center intramolecular hydrogen
bonds, which lead to well-defined hollow helical conformations.
Changing the curvature of the oligomer backbone leads to the
adjustment of the interior cavity size. Helices with interior
cavities of 10 angstroms to >30 angstroms across, the largest
thus far formed by the folding of unnatural foldamers, are
generated. Cavities of these sizes are usually seen at the
tertiary and quaternary structural levels of proteins. The
ability to tune molecular dimensions without altering the
underlying topology is seen in few natural and unnatural foldamer
systems.
References (abridged):
1. Gellman, S. H. (1998) Acc. Chem. Res. 31, 173-180
2. Rowan, A. E. & Nolte, R. J. M. (1998) Angew. Chem. 37, 63-68
3. Hill, D. J. , Mio, M. J. , Prince, R. B. , Hughes, T. S. &
Moore, J. S. (2001) Chem. Rev. 101, 3893-4011
4. Appella, D. H. , Christianson, L. A. , Klein, D. A. , Powell,
D. R. , Huang, X. L. , Barchi, J. J. & Gellman, S. H. (1997)
Nature (London) 387, 381-384
5. Seebach, D. , Abele, S. , Gademann, S. K. & Jaun, B. (1999)
Angew. Chem. 38, 1595-1597
Proc. Nat. Acad. Sci. 2002 99:11583
Related Background:
ON NANOSCALE ENCAPSULATION
I.G. Loscertales et al (University of Malaga, ES) discuss
encapsulation, the authors making the following points:
1) Production and control of droplets and particles of micrometer
or even nanometer size with a narrow size distribution are of
interest for many applications in science and technology.
Usually, these particles are formed as either an aerosol or a
hydrosol phase. Aerosols and hydrosols of compound particles,
such that each particle is made of a small amount of a certain
substance surrounded by another substance, are of particular
importance for encapsulation of food additives, targeted drug
delivery, and special material processing, among other
technological fields. In all of these cases, encapsulation is
used to provide compound particles in an appropriate size range.
2) One of the most widely adopted methods to obtain micrometer or
nanometer capsules is based on emulsion technology. Two
immiscible fluids, one carrying the substance to be encapsulated
and the other fluid carrying the polymer for the shell, are
stirred to form an emulsion. This emulsion is stabilized by
pouring it into a third solution ("double emulsion process"),
thereby extracting the polymer solvent and solidifying the
polymer as a capsule. Related methods also incorporate phase
separation and similar physical or chemical phenomena.
3) Other approaches for encapsulation resort to the formation and
control of liquid jets with diameters in the micrometer/nanometer
range. In the electrospray technique, a conducting liquid is
slowly injected through an electrified capillary tube. When the
electric potential between the liquid and its surroundings rises
to a few kilovolts, the meniscus at the tube exit develops a
conical shape, commonly referred to as the "Taylor cone". A thin
microthread of liquid is issued from the tip of the Taylor cone,
which eventually fragments to form a spray of highly charged
droplets. Its most well-known application has been in mass
spectrometry, where it has been successfully exploited as a way
to produce multiply-charged gas phase ions of huge biomolecules
present in the liquid phase.
4) The authors report a method to generate steady coaxial jets of
immiscible liquids with diameters in the range of
micrometer/nanometer size. This compound jet is generated by the
action of electro-hydrodynamic forces with a diameter that ranges
from tens of nanometers to tens of micrometers. The eventual jet
breakup results in an aerosol of monodisperse compound droplets
with the outer liquid surrounding or encapsulating the inner
liquid. The authors report they have produced monodisperse
capsules with diameters varying between 0.15 and 10 microns,
depending on the running parameters.
Science 2002 295:1695
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