Organic light-emitting devices have deficiencies that include instability in air. Scientists from the Hong Kong University of Science & Technology (HKUST) and Zhejiang University1 have now developed a procedure that enables lasing from light-emitting molecules embedded within a stable matrix of polymer nanowires.

Nanowires are being explored for their use in optoelectronic devices because of their improved emission properties and the unique geometry which enables the fabrication of novel device architectures. Given the huge choice of possible functional molecules, organic nanowire light-emitters are particularly promising for such applications.

Fig. 1: TPE-doped PMMA nanowires. a, SEM image of the nanowires embedded within the AAO template. b, nanowires after the AAO removal.

However, most organic light-emitters are sensitive to oxygen and moisture. Therefore, the HKUST group developed a fabrication method that incorporates functional light-emitting molecules—such as tetraphenylethylene (TPE)—directly within polymer nanowires (Fig. 1).

“Our results show that nanowire lasers can be made by simply doping functional molecules into a PMMA matrix, which is highly processable and air stable,” says Kam Sing Wong from HKUST’s Physics Department.

The researchers used anodic aluminum oxide (AAO) membranes containing nanochannels of only 200 nm in diameter. To achieve a complete filling of the nanowire channels care was taken so that the PMMA was polymerized only when placed within the nanochannels. Once the PMMA/TPE mixture was polymerized, the nanowires were stable and the AAO template was removed.

In solution the TPE molecules were not very efficient light emitters, because molecular vibrations and torsional motions compete with the light emission process. Once embedded into the polymerized nanowires, however, the situation was different. Then, non-radiative processes based on molecular motions were restricted and instead a strong so-called ‘aggregate-induced emission’ (AIE) was observed.

Consequently, light emission from the nanowires was very efficient, and even room-temperature lasing was achieved from optically-pumped nanowires that were still embedded within the AAO template. For single, individual nanowires with the AAO removed, however, lasing was only observed at low temperatures. This degradation may have been due to damage to the nanowire surfaces during the etching process, which led to increased losses.

This approach of using AAO templates to produce air-stable nanowire lasers offers many opportunities. “Applications for these polymer nanowires are of course as light-emitting diodes, but they may also be useful as chemosensors and bioprobes,” says Wong.