The optical properties of few-layer InSe

Abstract

Few-layer InSe draws tremendous research interests owing to the superior electronic and optical properties. It exhibits high carrier mobility up to more than 1000 cm 2 /Vs at room temperature. The strongly layer-tunable band gap spans a large spectral range from near-infrared to the visible. In this perspective, we systematically review the optical properties of few-layer InSe. Firstly, the intrinsic optical and electronic properties are introduced. Compared to other two-dimensional (2D) materials, the light-matter interaction of few-layer InSe is unusual. The band gap transition is inactive or extremely weak for in-plane polarized light, and the emission light is mainly polarized along the out-of-plane direction. Secondly, we will present several schemes to tune the optical properties of few-layer InSe such as external strain, surface chemical doping and van der Waals (vdW) interfacing.Thirdly, we survey the applications of few-layer InSe in photodetection and heterostructures.Overall, few-layer InSe exhibits great potential not only in fundamental research, but also in electronic and optoelectronic applications. / 42Furthermore, the applications of few-layer InSe in optoelectronics will be discussed in section Ⅳ.Finally, we present an outlook on the future study of the optical properties in section Ⅴ. II. The intrinsic optical and electronic properties of few-layer InSe A. Layer-dependent band structuresThe band structures of 2D materials can exhibit strong layer dependence due to the quantum confinement in the out-of-plane direction. For instance, there exists an indirect-to-direct bandgap transition for the TMDCs with thickness decreasing from bilayer to monolayer 18,19 . Another wellknown example is BP, possessing largely layer-tunable direct bandgap from the visible (the optical band gap is 1.73 eV for monolayer) to mid-infrared range (0.35 eV for bulk) [30][31][32][33] . The band structures of few-layer InSe can be largely modified by changing the layer number according to the DFT and/or tight binding (TB) calculations [40][41][42][43][44][45][46][47] (for a book see 48 ) as well as PL experiments 34,35,[49][50][51][52][53][54] . Fig. 1(a) shows the DFT band structure of monolayer InSe 43 . There are two interband transitions near the Γ point in Brillouin zone in which we are interested. One is the band gap transition between the valence band maximum (Se-pz orbital dominated) and the conduction band minimum (In-s orbital dominated), marked as transition-A. Another one is the interband transition between the deeper valence bands (degenerate Se-px/y orbitals dominated) and the conduction band (In-s orbital dominated), which is labeled as transition-B. For few-layer InSe, the valence/conduction band splits into subbands due to the interlayer interactions, resulting in a reduced band gap. Bandurin et al.systematically performed PL measurements on monolayer and few-layer InSe encapsulated by hexagonal boron nitride (hBN) 35 . Note that the PL peak is originated from the light emission of excitons, whose energy ...

Strain-Induced Bandgap Enhancement of InSe Ultrathin Films with Self-Formed Two-Dimensional Electron Gas

“…1 , E 2g 1 , and A 1g 2 , respectively. 9,13,14 The single-phonon vibrations of A 1g 1 and A 1g 2 originate from the breathing and opposite out-of-plane vibrations of In−Se layers, respectively, and can induce an anomalous magneto-optic effect revealed by recent magneto-Raman measurements. 30 Such spectrum characteristics are highly distinguished between InSe and In 2 Se 3 samples, as indicated by the signals of vertical broken cyan lines.…”

“…36 The values and inverse trend reasonably match well with our first-principles calculations with an overall fluctuation of ∼0.2 eV (Supporting Information (SI) Figure S1), as well as agreeing with previous experimental and theoretical results reported in literature. 9,10,27,37 The sensitive tunability of the band gap via film thickness is commonly observed in 2D vdW materials. In contrast to the indirect-to-direct band gap transition in TMDs and black phosphorus with decreased film thickness, 7,8,38 here the band gap in InSe film experiences a crossover from direct to indirect upon reducing layers, as revealed by spatially resolved angular resolved photoemission spectroscopy (ARPES) studies.…”

“…6−8 Among the tremendous studied candidates, indium selenide represents a rapid-growing family with progressive attentions endowing intriguing mechanical, electronic and optical properties. 9 Particularly, layered indium selenide belongs to a large family of complex compounds with various stoichiometries, involving InSe, In 2 Se 3 , In 3 Se 4 , In 4 Se 3 , In 6 Se 7 , etc. Even for the same stoichiometry, distinct phases and crystal structures may stably coexist.…”

“…It belongs to a typical III−V semiconductor that can be stablized down to the 2D limit, thus becomes widely exploited with exotic properties. Few-layer InSe exhibits some superior prospects over the transition metal chalcogenides or black phosphorus, 1,8,9 such as strong photoresponsivity, fast response time, 15 characters 16,17 and tunable current-induced spin polarization by spin−orbital coupling effect. 18,19 Besides, by engineering InSe with other 2D vdW materials, it is highly compelling to build type-II heterostructures with radiative optical transitions irrespective of lattice mismatches.…”

“…The inspiring discovery of graphene has triggered a wide range of upsurge in 2D van der Waals (vdW) materials for decades, which consist of a layered structure with covalently bonded intralayer and weakly vdW interacted interlayer. Enthusiasm has been ignited not only by their exotic emerging physics with diverse dimensionalities, − but also by their promising potentials in the areas of next-generation optoelectronic and photovoltaic devices. − Among the tremendous studied candidates, indium selenide represents a rapid-growing family with progressive attentions endowing intriguing mechanical, electronic and optical properties . Particularly, layered indium selenide belongs to a large family of complex compounds with various stoichiometries, involving InSe, In 2 Se 3 , In 3 Se 4 , In 4 Se 3 , In 6 Se 7 , etc .…”