Thickness-dependent band gap inversion turns indium selenide on its head

Graphene Flagship partners at the University of Manchester and the University of Warwick, UK, together with researchers at Elettra Sincrotrone Trieste, Italy, have used submicrometer angle-resolved photoemission spectroscopy (μARPES) to visualise the band structure of few-atom thick indium selenide. Their measurements show that indium selenide changes between an indirect gap semiconductor in mono- and bilayer films to a direct gap semiconductor in films with six or more layers – a behaviour opposite to common transition metal dichalcogenide semiconductors.

Indium selenide can be exfoliated peel off few-atom-thick semiconducting films using mechanical methods like scotch tape exfoliation. Previous photoluminescence measurements revealed that indium selenide's band gap, and therefore the way it interacts with electrons and emits light, changes with thickness. The layer-number-dependent nature of indium selenide's band gap had been theorised before, the inversion of the band gap in flakes with six or more indium selenide layers was unknown.

Neil Wilson, from Graphene Flagship partner the University of Manchester, used a combination of μARPES, optical spectroscopy and computational calculations to measure how the band structure of indium selenide changes with increasing atomic thickness. Different to regular photoelectron spectroscopy, μARPES measures both the energy and the angle of emitted electrons, giving information on both electron energy and momentum. "There has been a lot of theoretical work and interest in these materials, so understanding their fundamental properties using μARPES is important," Wilson says.

The data from Wilson's study shows that indium selenide, when exfoliated to six or more layers, changes from an indirect to a direct band gap material – causing it to emit light along its axis, which means it can be used in fibre optics and LED lighting. This is because light emitted by the A-exciton is predominantly polarized perpendicular to the plane of the two-dimensional crystal. Furthermore, the intensity of light produced can be scaled up by increasing the atomic thickness of indium selenide films. "Now that we know the theory is correct, we can apply the materials in different ways," he continues.

Vladimir Fal'ko, Leader of the Graphene Flagship's Enabling Research Work Package, says: "We were delighted to get confirmation of the theoretically anticipated properties of few-layer InSe films."

Graphene Flagship researchers will now be able to confidently predict the band gap structure of indium selenide flakes and use that information to create new materials with a desired set of optoelectronic properties. Wilson says that indium selenide can be used to make new types of lighting and design more efficient electronic devices.

Andrea C. Ferrari, Science and Technology Officer of the Graphene Flagship and Chair of its Management Panel, comments: "The Graphene Flagship, since its inception, is committed to continue fundamental research and to explore the thousands of possible layered materials. This paper experimentally demonstrates an optical behaviour for InSe quite different from other semiconducting layered materials. This will guide the development of optimized optoelectronic devices based on multi-layer indium selenide."

References

'Indirect to Direct Gap Crossover in Two-Dimensional InSe Revealed by Angle-Resolved Photoemission Spectroscopy,' Matthew J. Hamer, Johanna Zultak, Anastasia V. Tyurnina, Viktor Zólyomi, Daniel Terry, Alexei Barinov, Alistair Garner, Jack Donoghue, Aidan P. Rooney, Viktor Kandyba, Alessio Giampietri, Abigail Graham, Natalie Teutsch, Xue Xia, Maciej Koperski, Sarah J. Haigh. Vladimir I. Fal'ko, Roman V. Gorbachev, Neil R. Wilson. ACS Nano, 13, 2, (2019).