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Materials science

Nanotubes see the light

When it comes to having their conduction properties tweaked, carbon nanotubes are bothersome customers. One way to do it is to incorporate a photosensitive dye into the nanotubes' walls.

One of the great challenges of nanotechnology today is scaling electronic and mechanical devices down to atomic dimensions. A notably successful example has been the field-effect transistor (FET), in which a weak electric field is used to switch on and off a flow of electricity in a nearby semiconductor material. FETs are now used in their millions to, for example, amplify wireless signals. Writing in Physical Review Letters, Simmons et al.1 report a further refinement of such nanoscale devices: the incorporation of a photosensitive dye into a single-walled carbon nanotube, so that its electrical conductivity can be controlled by light.

Single-walled carbon nanotubes — long, thin carbon 'wires' just a nanometre or so across, but up to many thousands of times longer — have exciting mechanical, optical and electrical properties that would seem to make them ideal nanoscale materials2. But despite this great promise, materials scientists have encountered huge problems in actually working with nanotubes. This is partly because all methods used to synthesize them in reasonable quantities produce mixtures of metallic and semiconducting species. In addition, very strong attractive potentials build up between individual nanotubes, leading them to stick together in ropes or bundles.

But transistor applications, for instance, require semiconducting nanotubes, and these will be overpowered by their metallic counterparts when stuck together. So far, there is no practical way, beyond individual manipulation of the nanotubes, to separate the two species, or to arrange them into molecular transistor circuits.

Some of these limitations can be overcome through the controlled covalent functionalization of the nanotubes' side walls3,4: in other words, tweaking their properties by attaching covalently linked molecular 'handles'. The formation of the covalent linkages guarantees the structural integrity of the nanotube skeleton, but it also fundamentally changes the electronic structure of the individual carbon atoms. The most notable effect is that the inherent conductivity of the nanotubes is destroyed altogether.

Simmons and colleagues1 demonstrate an alternative strategy that bypasses this problem by facilitating the integration of a molecular handle — a photosensitive dye — while on the other hand preserving the carbon atoms' original electron-orbital structure. They achieve this through a strategy of non-covalent, 'supramolecular' functionalization. Supramolecular interactions involve, for example, hydrophobic, van der Waals and electrostatic forces, and are efficient when used at short range between molecular building-blocks, but are too weak to cause intermolecular changes or form actual bonds.

Such interactions implicitly require the physical adsorption of suitable molecules onto the side walls of the nanotubes. The non-covalent immobilization of numerous polycyclic aromatic species — most notably, conjugated polymers5 and small molecules such as pyrene6,7 — onto a nanotube surface has been at the forefront of research so far. Such additions have emerged as versatile building-blocks that can be used to modify nanotubes' solubility, fine-tune their electronic properties and exfoliate individual nanotubes from the initial bundles.

Simmons and colleagues' functionalizing addition is the commercially available Disperse Red 1 dye. Like many other azo-based dyes (that is, dyes centred around a nitrogen double bond), Disperse Red 1 undergoes a highly reversible 'cis–trans' molecular reconfiguration, or isomerization, when exposed to light of different wavelengths. This photoisomerization involves the rearrangement of the dye's outlying molecular groups around the double bond, which cannot itself rotate. Ultraviolet light of wavelength 254 nm initiates the conversion from the trans to the cis form, whereas blue light of 365 nm brings about the back-conversion from cis to trans (Fig. 1).

Figure 1: Light bending and stretching.
figure1

Simmons et al.1 make the conductivity of carbon nanotubes responsive to light by adding molecules of the azo-based Disperse Red 1 dye to the nanotube walls. These dye molecules undergo photoisomerization, with their molecular conformation shifting around their central nitrogen double bond: from the trans to the cis form under ultraviolet light of wavelength 254 nm, and back again under blue light of 365 nm. The changes cause significant, reversible shifts in the molecules' electrical dipole moments (unit: debye, D; arrows indicate direction), and thus in the electrical conductance of a nanotube transistor as a whole.

Importantly, the reconfiguration of the dye molecule causes a significant change in the electrical dipole moment along its principal axis. Simmons et al. document impressively that this shift also modifies the local electrostatic potential in the nanotube and, in turn, modulates its conductance by shifting the threshold voltage at which current flows. With a dye coverage of 1–2%, the photoisomerization induced a shift in the threshold voltage of up to 1.2 V. In particular, a carbon-nanotube transistor exhibiting p-type behaviour (that is, the flow of holes, carriers of positive electric charge) saw its threshold voltage increase by 1 V to higher, positive gate voltages.

As far as integrating single-walled carbon nanotubes into transistors is concerned, the technology described by Simmons and colleagues2 has several advantages. First, the functionalized transistors show repeatable switching for many cycles and a modest switching time of around 2 seconds. The authors believe that decreasing the dye concentration could further accelerate the switching. Second, the low light intensities (about 100 µW cm−2) needed to modulate the functionalized transistors are in stark contrast to those required to induce intrinsic nanotube photoconductivity (typically 1 kW cm−2). Finally, the authors' synthesis technique is versatile enough to immobilize different dyes onto the nanotubes. That should allow the tuning both of the wavelengths that initiate the photoisomerization and the magnitude of the switching, independently of the nanotube's electronic structure.

In that light, the work is surely a breakthrough in implementing carbon nanotubes in optoelectronic devices — technologies from photovoltaic cells to flat-panel displays that have a bright future.

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Guldi, D. Nanotubes see the light. Nature 447, 50–51 (2007). https://doi.org/10.1038/447050a

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