Dipole-directed self-assembly can be used to create robust one-dimensional nanostructures on silicon. It also provides new insights into interactions between molecules and this important technological material.

1. NEWS & VIEWS
nature nanotechnology | VOL 3 | APRIL 2008 | www.nature.com/naturenanotechnology 185
Stacey F. Bent
is in the Department of Chemical Engineering,
Stanford University, Stanford, California 94305, USA.
e-mail: sbent@stanford.edu
N
ature uses self-assembly processes
to construct large functional
structures — such as cell
membranes, protein aggregates and the
DNA double helix — from relatively
small molecular building blocks1
. Self-
assembly also offers an attractive bottom-
up approach for forming complex
nanostructures that may pave the
way for new technologies. This is a
particularly appealing alternative to
traditional top-down methods, because it
avoids the arduous task of carrying out
lithography at very small length-scales —
which can be both time-consuming
and expensive. On page 222 of this
issue, John Polanyi and co-workers
from the University of Toronto and the
University of Liverpool report a new
mechanism for surface-based assembly,
called dipole-directed assembly2
,
that can be used to make ordered
molecular lines — up to 10 nm long —
on silicon substrates.
Self-assembly processes are often
driven by the formation of a large
number of weak intermolecular
interactions — such as hydrogen
bonding, van der Waals forces and
the hydrophobic effect — between the
building blocks. These interactions
must be reversible, and the molecules
must remain mobile, to ensure that the
final structure has a thermodynamically
favoured configuration1
. On a surface,
a delicate balance between adsorbate–
adsorbate and adsorbate–surface
interactions must be achieved for
nanostructures to self-assemble3
.
Consequently, many of the systems in
which such self-assembled structures
have been demonstrated are those in
which the adsorbate–surface bonds
are relatively weak, including beautiful
examples on graphite and low-
temperature metals.
The microelectronics industry is
founded on silicon, so there exists a well-
established processing infrastructure
built around it. Silicon is therefore a very
attractive substrate for self-assembly
approaches, yet there are only limited
examples of molecular nanostructures
built up directly on this material.
Because silicon–adsorbate interactions
are usually strong, the surface directs
the organization of adsorbed molecules
rather than any self-assembly processes.
Moreover, bare silicon is so reactive that
many adsorbates have limited mobility on
the surface before they bond. Of course,
by using weakly bonding molecules and
cooling to low temperatures, one can
encourage self-assembly. Alternatively,
stepped or passivated silicon surfaces can
be used as a starting point. An example
is the formation of ordered molecular
assemblies on hydrogen-passivated
silicon surfaces through a radical
chain reaction4,5
.
Polanyi and co-workers now
describe a room-temperature self-
assembly process that enables the
formation of robust one-dimensional
nanostructures on bare silicon
surfaces and avoids the constraints of
passivation and low temperature. This
was achieved by choosing a molecule —
1,5-dichloropentane — that physically,
rather than chemically, adsorbs on a
Dipole-directed self-assembly can be used to create robust one-dimensional nanostructures
on silicon. It also provides new insights into interactions between molecules and this important
technological material.
Surface patterning
Silicon falls into line
Silicon dimer
row direction
Add 1,5-
dichloropentane
Molecular line
growth direction
Buckling induced by
adjacent adsorbate
Tilted Si-Si dimer 1,5-dichloropentane Dipole moment
Figure 1 An illustration of the dipole-directed assembly process. The unperturbed silicon surface consists of rows
of silicon–silicon dimers (blue) that are buckled in an alternating fashion. Adsorption of a 1,5-dichloropentane
molecule (red and green) on a pair of adjacent dimers induces a readjustment of the dimers in the next row,
generating a favourable site for another molecule to bind to. Consecutive adsorption events lead to the growth of
one-dimensional lines of 1,5-dichloropentane molecules.
© 2008 Nature Publishing Group

2. NEWS VIEWS
186 nature nanotechnology | VOL 3 | APRIL 2008 | www.nature.com/naturenanotechnology
silicon substrate at room temperature and
does not, therefore, form covalent bonds
with the surface that would cause the
molecule to break apart. Importantly, this
molecule was also chosen because it has
a large electric dipole moment — owing
to the chlorine atoms at each end of the
hydrocarbon chain — and a length that
is close to the Si–Si dimer spacing on the
Si(100)-(2 × 1) surface.
The 1,5-dichloropentane molecules
bond to the surface by bridging across
two dimers in the same row; both
chlorine atoms interact with adjacent
silicon atoms. Single 1,5-dichloropentane
adsorbates were not observed on the
surface, however, suggesting that the
individual molecules that did not form
part of a larger self-assembled structure
were weakly bound and highly mobile.
Rather, the molecule assembled into
one-dimensional molecular lines
that grew orthogonally to the silicon
dimer rows (Fig. 1). Interestingly, the
direction of line growth was always in
the direction opposite to the dipole
moment. Theoretical calculations
suggested that the energetic driving
force for assembly comes from a dipole-
induced charge readjustment at the
surface. This charge-transfer process
causes the adjacent silicon dimers in the
next row to buckle in a way that provides
a favourable binding site — relative
to unaffected dimers — for another
1,5-dichloropentane molecule to adsorb.
This effect is the origin of the dipole-
directed assembly.
An interesting element of this
study is that the molecular assembly
process involves an interplay between
the adsorbate dipole moment and the
electronic properties of the surface,
leading to self-assembled structures with
features influenced by the underlying
lattice. The study builds on earlier work
that explored the role of surface-charge
distribution on molecular bonding to
silicon. Theoretical calculations have
shown that donation of electronic charge
from a nucleophile bonded to the surface
spreads to adjacent surface dimers6–8
, and
experiments have confirmed that this
charge delocalization can be harnessed
to direct the assembly of adsorbates6–9
.
However, the present study is unusual in
that the directed assembly spans across
silicon rows, as opposed to the along-
row assembly examined in the earlier
studies. Moreover, the present study
seems to be the first in which criteria
for self-assembly are met sufficiently for
molecular lines of significant length (up
to 10 nm) to be formed.
What are the implications and
potential of the method for future
nanotechnologies? It may offer a new
way to ‘set’ a pattern by self-assembly
under reasonable temperatures on the
silicon substrates. However, this work
is just the first step along the road to
any future bottom-up nanofabrication
process. Subsequent steps are needed
to lock the two-dimensional pattern in
and propagate it into something more
functional, such as nanowires or other
three-dimensional structures. What
is more significant, however, is that it
provides new insight into molecular
bonding on silicon substrates and will
inspire further studies to determine
whether the dipole-directed assembly
process is a general one that applies to
other molecules, other surfaces, and
more complex systems. Undoubtedly,
however, silicon will be put firmly in
the spotlight to see whether other self-
assembly processes lurk on (or beneath)
its surface.
References
1. Whitesides, G. M. Grzybowski, B. Science 295,
2418–2421 (2002).
2. Harikumar, K. R. et al. Nature Nanotech. 3, 222–228 (2008).
3. Bent, S. F. ACS Nano 1, 10–12 (2007).
4. Lopinski, G. P., Wayner, D. D. M. Wolkow, R. A. Nature 406,
48–51 (2000).
5. Hossain, M. Z., Kato, H. S. Kawai, M. J. Phys. Chem. B 109,
23129–23133 (2005).
6. Queeney, K. T., Chabal, Y. J. Raghavachari, K. Phys. Rev. Lett.
86, 1046–1049 (2001).
7. Widjaja, Y. Musgrave, C. B. Surf. Sci. 469, 9–20 (2000).
8. Wang, Y. Hwang, G. S. J. Chem. Phys. 122, 164706 (2005).
9. Hossain, M. Z., Yamashita, Y. Mukai, K. Yoshinobu, J.
Phys. Rev. B 68, 235322 (2003).
FUNDAMENTALPHYSICS
A most unusual crystal
suspended between two platinum
electrodes (Nature Physics 4, 314–318;
2008). A Wigner crystal is more likely to
be seen in a semiconducting nanotube
with large bandgap, but such nanotubes
are also more prone to disorder. To
overcome this problem Deshpande
and Bockrath grew the nanotubes
directly from a catalyst on top of
one of the electrodes, and measured
their transport properties at room
temperature to identify devices suitable
for more detailed measurements at
low temperature. The charge carriers
in the nanotubes were ‘holes’ rather
than electrons, but the basic physics
In 1934, Eugene Wigner — the
theoretical physicist who went on to win
the Nobel Prize in 1963 — predicted
that electrons would enter a novel
crystalline phase at low densities when
the effects of long-range Coulomb forces
between the electrons overwhelmed
their kinetic energy. This phase, now
known as a Wigner crystal, has been
seen in two-dimensional systems but
never in one or three dimensions,
usually because defects and impurities
get in the way. Now Vikram Deshpande
and Marc Bockrath of Caltech have
seen a one-dimensional Wigner crystal
in a single-walled carbon nanotube
was the same. The experiment involved
cooling the devices to 1.4 K in an
applied magnetic field and measuring
the number of holes passing through the
nanotubes — which were about 500 nm
long — as a gate voltage (horizontal axis
in the figure above) and the magnetic
field strength (vertical axis) were
varied; low currents are shown in blue
and high currents in red. The Caltech
pair actually observed three distinct
regimes (separated by the yellow and red
lines) with different magnetic and spin
properties within the Wigner crystal as
the density of holes was varied.
Peter Rodgers
© 2008 Nature Publishing Group