Exciton Condensation in an Atomically-thin MoS2 Semiconductor

Condensation of a dilute Bose gas of excitons (coupled electron-hole pairs) in a direct bandgap semiconductor was first theoretically predicted in 19681. This exotic state of matter is expected to exhibit spectacular non-linear properties, such as superradiance and superfluidity. However, direct experimental observation of condensation of optically active excitons in conventional semiconductors has been hindered by their short lifetimes and weak collective excitonic interactions. Here, we have experimentally realized the condensation of short-lived excitons in a direct-bandgap, atomically-thin MoS2 semiconductor. The signature is the anomalous transport of the fast-expanding exciton density, originating from a thermalized dilute gas generated under the laser spot. Below the critical temperature Tc~150 K, the exciton liquid propagates over ultra-long distances (at least 60 micrometers) with record speed in a solid-state system of 1.8*10^7 m/s (~6% the speed of light), fuelled by the unconventionally strong repulsions among excitons. The condensation is controlled by many-body interactions in the gas mixture of excitons (bosons) and free-carriers (fermions) via an electrical backgate. Our results demonstrate electrostatic doping as a simple approach for the investigation of correlated states of matter at high-temperatures, excitonic circuitry and spin-valley Hall devices mediated by exciton superfluids in semiconducting monolayers.

Structural and Optoelectronic Properties of Two-Dimensional Ruddlesden–Popper Hybrid Perovskite CsSnBr3

Ultrathin inorganic halogenated perovskites have attracted attention owing to their excellent photoelectric properties. In this work, we designed two types of Ruddlesden–Popper hybrid perovskites, Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2, and studied their band structures and band gaps as a function of the number of layers (n = 1–5). The calculation results show that Csn+1SnnBr3n+1 has a direct bandgap while the bandgap of CsnSnn+1Br3n+2 can be altered from indirect to direct, induced by the 5p-Sn state. As the layers increased from 1 to 5, the bandgap energies of Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2 decreased from 1.209 to 0.797 eV and 1.310 to 1.013 eV, respectively. In addition, the optical absorption of Csn+1SnnBr3n+1 and CsnSnn+1Br3n+2 was blue-shifted as the structure changed from bulk to nanolayer. Compared with that of Csn+1SnnBr3n+1, the optical absorption of CsnSnn+1Br3n+2 was sensitive to the layers along the z direction, which exhibited anisotropy induced by the SnBr2-terminated surface.

Theoretical Study of Abnormal Thermal Expansion of CuSCN and Effect on Electronic Structure

CuSCN, as a new type of inorganic hole-transporting semiconductor with a wide bandgap (>3.4 eV), is attracting much attention in the fabrication of perovskite solar cells. In this article, by using first-principles density functional theory (DFT) and the quasi-harmonic approximation (QHA) approach, we have studied lattice dynamics and abnormal thermal expansion of the system, including α- and β-CuSCN phases. The influence of the abnormal thermal expansion of the lattice on the electronic structure, especially on the bandgap of the system, was explored and discussed. We found that due to the shearing modes and the three acoustic modes along the direction of the c-axis, the α- and β-CuSCN show a negative thermal expansion (NTE) in the direction of the c-axis. The torsion modes of the Cu–N–C–S atomic chains in the α-CuSCN may lead to an NTE in the directions of the a, b-axes of the α-phase. As a result, our theoretical results demonstrated that the α-CuSCN exhibits an anisotropic bulk NTE. While the β-CuSCN displays a strong uniaxial negative thermal expansion in the direction of the c-axis, in the directions of the a, b-axes, it exhibits positive thermal expansion. Our DFT calculations also predicted that the α-CuSCN has a direct bandgap, which increases slightly with increasing temperature. However, the β-CuSCN has an indirect bandgap at low temperature, which converts to a direct bandgap near the temperature of 375 K due to the strong positive expansion in the ab plane of the phase. Our work revealed the mechanisms of the abnormal thermal expansion of the two phases and a strong coupling between the anisotropic thermal expansion and the electronic structures of the system.

Special Issue “1D, 2D, and 3D ZnO: Synthesis, Characterization, and Applications”

Zinc Oxide (ZnO) is a well-known II–VI semiconductor with a direct bandgap around 3 [...]