NEWS & VIEWS

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Stacey F. Bentis in the Department of Chemical Engineering, Stanford University, Stanford, California 94305, USA.

e-mail: sbent@stanford.edu

Nature 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.

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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.

references1. 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).

FUNDAMENTAL PHYSICS

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

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