Lilliputian light sticks

Building two different fluorescing dyes into a composite organic nanocrystal makes a tunable light generator. At just the right dye proportions, a low-cost, highly efficient source of white light is the result.

If you have ever wondered what makes that ghostly colour in the light sticks used by trick-or-treaters on a Hallowe'en night, wonder no more: a typical answer might be a fluorescing organic molecule such as 5,6,11,12-tetraphenyltetracene, also known as rubrene. Writing in Advanced Materials, Zhao et al.¹ describe how they used such organic molecules to make white-light-emitting composite nanocrystals that are visible only under a microscope. Cute as these lilliputian light sticks are, they might also point the way to a new generation of light sources.

A typical light stick contains two chemicals: hydrogen peroxide and a phenyl oxalate ester. When these two substances are mixed, energy is released. That energy excites a suitable fluorescent dye, causing it to emit a photon. The wavelength of the photon, and so the colour of the emitted light, depends on the structure of the dye (Fig. 1a). Rubrene, for example, is an orange-emitter; the related molecule 1,3,5-triphenyl-2-pyrazoline (TPP) is blue.

Figure 1: Great white hope.

VISUALS UNLIMITED/CORBIS

a, Commercial light sticks containing fluorescent dyes are available in various colours. b, Zhao et al.¹ engineer similar rods on a tiny scale (scale bar, 5 µm), incorporating two dyes, TPP and rubrene, so that the rods emit white light.

Zhao et al.¹ use a process called physical vapour deposition, which is a common way of making organic nanocrystals, to co-evaporate rubrene and TPP at 200–300 °C and condense them onto a substrate at a lower temperature. The result is a collection of uniform organic nanorods with diameters of hundreds of nanometres and lengths of several micrometres (Fig. 1b). Each nanorod functions as a tiny light stick, with its colour determined by the molecular ratio of the two organic dyes.

The energy-transfer mechanism in these nanorods is known as intermolecular fluorescence resonance energy transfer (IFRET), and is fundamentally different from that of a typical light stick. In a typical IFRET process², energy is absorbed by one fluorescent molecule (blue-emitting TPP in Zhao and colleagues' case), and transferred non-radiatively to another fluorescent molecule (orange-emitting rubrene). Consequently, the emission of the first molecule is quenched, and the emission of the second is enhanced. The efficiency of this process depends sensitively on the degree of electronic coupling between the two molecules. In an amorphous thin film, for example, where TPP and rubrene molecules are intimately mixed, the emission of TPP can be almost completely quenched with a very small amount of rubrene.

The nanorods prepared by Zhao et al. show an intermediate degree of IFRET owing to an unusual structural feature: as X-ray diffraction studies reveal, the rubrene nanocrystals are uniformly embedded in a crystalline TPP matrix. This results in incomplete quenching of the blue emission from TPP, even with decent levels of rubrene 'doping', leading to colour mixing of the orange and blue emissions. Importantly, at the proper molecular ratio, it becomes possible to generate stable white light.

Among researchers investigating comparable lighting devices based on inorganic semiconductors, this kind of colour tunability is often achieved by using homogeneous mixtures of different compounds. A good example is recent research into indium gallium nitride (InGaN) materials, which are considered excellent candidates for solid-state lighting applications. Here, a mixture of indium nitride and gallium nitride is used to systematically shift the emission of the materials from ultraviolet wavelengths to the near-infrared³. Similarly, doping has commonly been used in organic light-emitting diodes (OLEDs), both to tune their emission colour and to improve their luminescence efficiency⁴.

Many of these doping studies have used amorphous thin films, in which charge carriers have low mobility. Zhao and colleagues' composite organic nanorods not only represent an unusual source of stable white light, but, because of their ordered crystalline natures, should offer better transport properties, and hence better optoelectronic performance. Single crystalline nanowires of the aromatic hydrocarbon hexathiapentacene have been shown to have charge-carrier mobilities almost ten times those of more disordered thin-film structures⁵. To sound a note of caution, however, the emission efficiency of these composite nanorods has yet to be determined. Their integration into a functional electroluminescent device must also be demonstrated.

Because of the great tunability of both their crystal and their electronic structures, inorganic semiconductor nanowires have proved to be workhorses of nanoscale science and engineering⁶, finding applications in various electronic, photonic and sensing devices. Considering the vast number of optically and electronically active organic molecules, the organic version of the nanowires could reasonably be expected to have similar potential — and indeed, they are already being considered for devices such as transistors, LEDs and photovoltaic cells. Their one-dimensional nanostructures offer the additional advantage of being mechanically flexible, making them particularly appealing for flexible optoelectronic applications: a hexathiapentacene nanowire transistor, for instance, suffers no significant loss in performance when placed under mechanical stress⁵.

Just as with their inorganic counterparts, several important optical properties have already been demonstrated in organic nanowires, including lasing⁷, waveguiding⁸, nonlinear optical mixing⁹ and polarized emission¹⁰. With Zhao et al.¹ adding colour tunability to the list, the nanowires seem to have a bright, white future ahead.