Carbon Nanotubes Turn Electrical Current into Light-emitting Quasi-particles

Strong light-matter coupling in these semiconducting tubes may hold the key to electrically pumped lasers

Light-matter quasi-partic­les can be generated electrically in semiconducting carbon nanotubes. Material scientists and physicists from Heidelberg University (Germany) and the University of St Andrews (Scotland) used light-emitting and extremely stable transistors to reach strong light-matter coupling and create exciton-polaritons. These particles may pave the way for new light sources, so-called electrically pumped polariton lasers, that could be manufactured with carbon nanotubes. These findings, published in “Nature Materials”, are the result of a cooperation between Prof. Dr Jana Zaumseil (Heidelberg) and Prof. Dr Malte C. Gather (St Andrews).

In recent years, research on organic, carbon-based semiconductors for optoelectronic components has led to a variety of applications. Among them are light-emitting diodes for energy-efficient, high-resolution smartphone and TV screens. Despite the rapid progress in this area, realising an electrically pumped laser from organic materials remains elusive. To get closer to this goal, researchers in Heidelberg and St Andrews are working on coupling light and matter in semiconducting carbon nanotubes – microscopically small, tube-shaped structures of carbon.


Artistic rendering of a light-emitting transistor with carbon nanotubes between two mirrors for electrical generation of polaritons.

Researchers from the University of Heidelberg (Germany) and the University of St Andrews (UK) demonstrated electrically pumped near-infrared exciton-polariton emission at room temperature, using a SWCNT-based ambipolar LEFET embedded in an optical microcavity.

The researchers disclose a seemingly simple to manufacture device with a strong potential for laser applications.

The authors explain that while organic semiconductors are actively researched to design exciton-polariton lasing devices, low charge carrier mobilities have been hindering the applicability of such devices. Here they used single-walled carbon nanotubes (SWCNTs) embedded in a polymer matrix as a planar semiconducting material between source and drain electrodes, topping it up with a gate doubling up as a top mirror to form a light-emitting field-effect transistor (LEFET). The carbon nanotubes are readily processed from solution and boast exceptionally high electron and hole mobilities.

They built the whole device within an optical cavity formed perpendicular to the direction of charge transport, by the top gate mirror and a semitransparent bottom mirror electrically isolated by a layer of aluminium oxide, on a glass substrate. Through multiple experiments, they were able to tune the narrow-band polariton electroluminescence (EL) from 1,060 nm up to 1,530, by simply adjusting the cavity through different spacer thicknesses.

Nature Materials – Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

15 pages of supplemental material

Abstract

Exciton-polaritons are hybrid light–matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (voer 10 kA cm−2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ~104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material–device combination paves the way to carbon-based polariton emitters and possibly lasers.

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