Mo₆S₃Br₆: An Anisotropic 2D Superatomic Semiconductor

Abstract

Two-dimensional (2D) van der Waals materials with in-plane anisotropy are of great interest for directional transport of charge and energy, as exemplified by recent studies on black phosphorus and α-phase molybdenum trioxide (α-MO 3 ). Here, a layered van der Waals semiconductor with in-plane anisotropy built upon the superatomic units of Mo 6 S 3 Br 6 is reported. This material possesses robust 2D characteristics with a direct gap of 1.64 eV, as determined by scanning tunneling spectroscopy and first-principles calculations. Polarization-dependent Raman spectroscopy measurement and density functional theory calculation reveal strong in-plane anisotropy. These results suggest an effective strategy to explore anisotropic 2D electronic and optoelectronic properties from superatomic building blocks with multifunctionality, emergent properties, and hierarchical control.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201902951.for plasmon polaritons. [1][2][3] Likewise, inplane anisotropy in α-phase molybdenum trioxide (α-MoO 3 ) has been demonstrated to yield ultra-low-loss phonon polaritons. [4] Besides, in rhenium dichalcogenides, [5] the in-plane anisotropy [6] can be used to fabricate unique photo detectors and filedeffect transistors. [7] When compared to traditional 2D atomic solids, however, the structural diversity combined with in-plane anisotropy is limited, which has restricted their use.Beyond atomic solids, there is a growing interest in designing 2D materials from complex, hierarchical, and tunable building blocks, [8] as exemplified by colloidal nanoparticle 2D superlattices. [9][10][11] The use of zero-dimensional building blocks formed by atomic clusters, known as superatoms, is particularly attractive and advantageous due to their atomic precision, as well as the tunability of intercluster coupling. [12][13][14] Their assembly into strongly coupled hierarchical lattices has been shown to produce unique emergent material properties. For instance, threedimensional (3D) cluster-assembled solids such as Chevrel phases [15] and endohedral gallide cluster phases [16] are wellknown for their superconducting behaviors. In view of this, 2D structures with in-plane anisotropy promise unique physical properties and functions. However, there are very few examples of layered cluster-based structures [17][18][19] that can be exfoliated, and the existing ones typically have isotropic in-plane structures (e.g., Re 6 Se 8 Cl 2 ). [20][21][22] Here, we investigate Mo 6 S 3 Br 6 , a superatomic van der Waals material with strong in-plane anisotropy derived from the Chevrel phase in which [Mo 6 ] octahedral cluster subunits are covalently linked into layers. [19,23] Each [Mo 6 ] octahedron is tilted, enclosed in a cube of sulfur (S) and bromine (Br), and connected to two neighboring clusters along the c-axis via a shared S atom located at the apical positions as well as via an additional two bridging Br atoms (Figure 1a). The resulting corner-sharing ...

Review of Anisotropic 2D Materials: Controlled Growth, Optical Anisotropy Modulation, and Photonic Applications

“…[ 71 ] Semiconductive Mo 6 S 3 Br 6 with a similar super atomic structure has also been proved to have high in‐plane anisotropy in Raman scattering. [ 72 ] The metal chalcogenide halides, MXX′ (M = Sc, Ti, V, Cr, Fe, Ir, etc. ; X = O, S, Se, Te; X′ = Cl, Br, I) possess an orthorhombic or monoclinic structure with two atomic layers of halogen sandwiching the M–X mesh.…”

Coexistence of Incoherent and Ultrafast Coherent Exciton Transport in a Two-Dimensional Superatomic Semiconductor

“…Atomically precise clusters − are unique superatomic building blocks for designing semiconductors with diverse electronic, magnetic, and chemical properties. − These superatoms can be synthetically and postsynthetically modified to introduce desired functionality, and linked into ordered networks, enabling control over the resulting collective properties with exquisite precision. − The realization of macroscopic single crystals of van der Waals (vdW) superatomic semiconductors that can be exfoliated to the two-dimensional (2D) limit provides an additional tuning knob for controlling electronic transport and light–matter interactions through dimensionality reduction, − providing a robust route to accelerate the development of new optoelectronic technologies.…”

Charge carrier scattering and ultrafast Auger dynamics in two-dimensional superatomic semiconductors

“…1), respectively. The possibility to tune the cluster compositions and their interactions in superatom solids allows for great variation in properties, such as superconductors (e.g., PbMo 6 S 8 and currentannealed Re 6 S 8 Cl 2 ), 5,6 semiconductors (e.g., Mo 6 S 3 Br 6 and Re 6 Se 8 Cl 2 ), 7,8 and insulators (e.g., PbMo 6 Cl 14 or Re 6 Se 4 Cl 10 ). 9 Understanding the variety of electronic and magnetic properties derived from the common cluster framework could provide insight into further applications of this type of material.…”

“…Still, ultrafast carrier dynamics in many of these materials are largely unexplored. 1,7 Here, we study two prototypical 2D superatomic solids: 10 Re 6 Se 8 Cl 2 and Mo 6 S 3 Br 6 single crystals 7,8,11 to investigate the ultrafast charge carrier dynamics using time-resolved optical-pump terahertz (THz)-probe spectroscopy. Re 6 Se 8 Cl 2 is an indirect bandgap semiconductor with a bandgap of 1.49 7 eV and the estimated exciton binding energy of 100 meV.…”

“…Re 6 Se 8 Cl 2 is an indirect bandgap semiconductor with a bandgap of 1.49 7 eV and the estimated exciton binding energy of 100 meV. Mo 6 S 3 Br 6 , on the other hand, has a direct bandgap of 1.65 eV, 8 and the exciton binding energy is unknown. Analogous to the 2D transition metal dichalcogenides, [12][13][14] these materials have potential applications in light emitters, photodetectors, and solar cells.…”