1 Scientific RepoRts | 7: 3445 | DOI:10.1038/s41598-017-03186-x www.nature.com/scientificreports Indium selenide: an insight into electronic band structure and surface excitations A. politano 1,10 , D. Campi 2 , M. Cattelan 3,9 , I. Ben Amara 4 , S. Jaziri 4,5 , A. Mazzotti 1 , A. Barinov 6 , B. Gürbulak 7 , S. Duman 8 , S. Agnoli 3 , L. S. Caputi 1 , G. Granozzi 3 & A. Cupolillo 1 We have investigated the electronic response of single crystals of indium selenide by means of angle- resolved photoemission spectroscopy, electron energy loss spectroscopy and density functional theory. The loss spectrum of indium selenide shows the direct free exciton at ~1.3 eV and several other peaks, which do not exhibit dispersion with the momentum. the joint analysis of the experimental band structure and the density of states indicates that spectral features in the loss function are strictly related to single-particle transitions. these excitations cannot be considered as fully coherent plasmons and they are damped even in the optical limit, i.e. for small momenta. The comparison of the calculated symmetry-projected density of states with electron energy loss spectra enables the assignment of the spectral features to transitions between specifc electronic states. Furthermore, the efects of ambient gases on the band structure and on the loss function have been probed. Two-dimensional (2D) van der Waals semiconductors 1–4 , combining fnite band gaps 5, 6 and fexibility 7, 8 , are emerging in recent years as the most promising materials for nanoelectronics 9, 10 . Te presence of a band gap, absent in graphene 11 , is crucial for achieving a high ON/OFF ratio in nanode- vices 9 . Furthermore, a direct band gap also allows the use of materials in optoelectronics 12 . Four requisites are crucial for a suitable use of 2D materials in nanotechnology: (i) high mobility of charge carriers; (ii) the possibility to achieve highly crystalline samples via mechanical/liquid exfoliation; (iii) ambient stability; (iv) high fexibility together with a sufciently high fracture toughness. Several classes of materials fail to fulfl the above-mentioned conditions for diferent motivations: silicene 13 and germanene 14 cannot be exfoliated; transition-metal dichalcogenides are characterized by a relatively low value of the mobility of charge carriers 15 ; black phosphorus sufers of rapid oxidation in ambient conditions 16 ; while bismuth chalcogenides have a poor fracture toughness 17 . A suitable candidate for nanoelectronics is represented by InSe, which is a layered semiconductor made of stacked layers of Se-In-In-Se atoms with van der Waals bonding between quadruple layers 18, 19 . Recently, many works reported the superb performance of InSe-based optoelectronic devices 20, 21 . Field-efect transistors with an active channel of InSe are characterized by an electron mobility near 10 3 cm 2 /(V s) 20 and, moreover, excellent fexibility 22, 23 and ambient stability 24 , in spite of the presence of a p-type doping arising from water decomposition at Se vacancies 24 . Furthermore, InSe is also a promising material for strain engineering 25 , nonlinear optics 26 , and photovoltaics 22 . 1 Department of Physics, University of Calabria, via ponte Bucci, cubo 31/C, I-87036, Rende, Italy. 2 theory and Simulation of Materials (THEOS), and National Centre for Computational Design and Discovery of Novel Materials (MARVEL), Ecole Polytechnique Federale de Lausanne, CH-1015, Lausanne, Switzerland. 3 Department of Chemical Sciences, University of Padova, via Marzolo 1, I-35131, Padova, Italy. 4 Laboratoire de Physique de la Matière Condensée, Faculté des Sciences de Tunis, Université de Tunis El Manar, Tunis, 2092, Tunisia. 5 Laboratoire de Physique des Matériaux, Faculté des Sciences de Bizerte, Université de Carthage, 7021, Zarzouna, Tunisia. 6 Elettra-Sincrotrone Trieste S.C.p.A., S.S. 14, km 163.5, I-34149, Trieste, Italy. 7 Department of Physics, Faculty of Sciences, Atatürk University, 25240, Erzurum, Turkey. 8 Department of Basic Sciences, Faculty of Sciences, Erzurum Technical University, 25050, Erzurum, Turkey. 9 Present address: School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom. 10 Present address: Fondazione Istituto Italiano di Tecnologia, Graphene Labs, Via Morego 30, 16163, Genoa, Italy. Correspondence and requests for materials should be addressed to A.P. (email: antonio. politano@iit.it) or A.C. (email: anna.cupolillo@fs.unical.it) Received: 31 October 2016 Accepted: 20 February 2017 Published: xx xx xxxx opeN