Excitons and Electron–Hole Liquid State in 2D γ‐Phase Group‐IV Monochalcogenides

Abstract

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first-principles calculations, it is demonstrated that a new phase of group-IV monochalcogenides (γ-MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ-MXs are shown to possess a unique band dispersion-that is, camel's back like structure-in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest-energy exciton render it advantageous for the generation of electron-hole liquid state. After careful evaluation of the electron-hole dissociation temperature and the Mott critical density, it is predicted that a high-temperature exciton gas to electron-hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.

Tunable electronic and magnetic properties of monolayer and bilayer Janus Cr2Cl3I3: a first-principles study

“…For example, the successive discoveries of integer 1,2 and fractional 3 quantum Hall effects and superconductivity have introduced new physics. Since the discovery of graphene, 4,5 the family of 2D materials has grown explosively, [6][7][8][9][10] including many famous members such as transition metal dichalcogenides, 11 hexagon boron nitride, 12 carbon boron nitride, 6,8 stanene, 9 etc. It is well known that strong fluctuations prohibit long-range magnetic order at finite temperature in a 2D system, as depicted by the Mermin-Wagner theorem.…”

Quasiparticle band structures, spontaneous polarization, and spin-splitting in noncentrosymmetric few-layer and bulk $γ$-GeSe

“…1(a). Previous theoretical work [21] showed that this ABCA-sequence quadruple layer is stable and is energetically preferred to other stacking sequences such as ABAB, ABAC, and ABBA. For bulk γ-GeSe, one has to consider how quadruple layers are stacked, and it was reported that AB stacking of quadruple layers is energetically preferred to AA stacking [21].…”

“…Recently, a new atomic structure of group-IV monochalcogenide γ-MX having a double-layer honeycomb lattice (DLHL) has been predicted theoretically [21]. The predicted monolayer and AB-stacked bulk γ-GeSe are centrosymmetric, so the intrinsic ferroelectricity does not appear.…”

Thickness and strain engineering of structural and electronic properties for 2D square-octagon AlN

“…Inspired by the successful of graphene [1,2], a great deal of research efforts are devoted toward the investigation of two-dimensional (2D) materials in last decade [3,4]. The 2D materials not only exhibit unique and fascinating physical features by comparing their bulk counterpart [5][6][7][8][9][10], but also support a new dimension to tune the properties of by engineering the interlayer coupling [11][12][13]. The linear dispersion at K point endows novel phenomena in monolayer graphene, including the ultra-high mobility, fantastic thermal and electronic conduction and the anomalous room-temperature quantum Hall effect.…”

“…The graphene-based devices have also been explored [5][6][7]. Transition from indirect to direct bandgap is revealed in transition-metal chalcogenides (TMCs) by reducing the thickness to monolayer [8][9][10]. Moreover, a large number of new 2D materials have been theoretically designed and experimentally realized, like monoelemental materials (silicene, germanene, phosphorene, borophene), MXenes, organic materials and nitrides.…”