2D NbOI₂: A Chiral Semiconductor with Highly In‐Plane Anisotropic Electrical and Optical Properties

Abstract

induced diodes, [23,24] polarization-sensitive photodetectors, [25,26] synaptic-neuromorphic devices, [27] etc. Although the research on anisotropic 2D materials is progressing rapidly, it is still in its infancy as a whole. Therefore, developing new materials and exploring their anisotropic physical properties and functional devices are of great significance to the development of science and technology.Recently, layered transition metal oxide dihalides MOX 2 (M = V, Nb, Ta, Mo; X = Cl, Br, I) have gained increasing attention among materials scientists due to their chiral crystal structure and unique physical properties. [28][29][30][31][32][33] Among them, NbOI 2 has a low-symmetry monoclinic structure (space group C2), in which Nb atoms shift away from the center of [NbO 2 I 4 ] octahedra connected by sharing I-I edges and cornered O atoms along the c-and b-axes. Various connection modes enable NbOI 2 strong anisotropic physical properties and highly dispersed band structure. The simulated optical absorption coefficient along the c-axis (α c ) is an order of magnitude higher than that along the b-axis (α b ). [34] Consequently, the NbOI 2 is expected to exhibit unique in-plane anisotropy. However, until now, the preparation and basic properties of 2D NbOI 2 crystals have not been thoroughly investigated, and in particular, the in-plane anisotropy caused by its low-symmetry structure is still unclear.Here, we introduce a kind of transition metal oxide dihalides, NbOI 2 , into the 2D materials family with experimental demonstration of strong in-plane anisotropy. Few-layer and monolayer NbOI 2 crystals were obtained through mechanical exfoliation, and these crystals showed large second harmonic generation (SHG) response, verifying its noncentrosymmetric crystal structure. The strong in-plane anisotropy of phonon vibration in 2D NbOI 2 crystals was observed by using angleresolved polarized Raman measurements. The anisotropic electronic dispersion around the Fermi surfaces leads to the strong anisotropy of optical absorbance with an anisotropic factor of 1.75. In addition, highly anisotropic in-plane electrical resistance with a factor of 1.34 and photoresponsivity with a factor of 1.7 were demonstrated in 2D NbOI 2 crystals. These findings indicate that the 2D NbOI 2 crystal can serve as a promising candidate in anisotropic electronic and optoelectronic devices. Results and DiscussionBulk NbOI 2 belongs to the monoclinic space group C2 (No. 5) with a = 15.18 Å, b = 3.92 Å, c = 7.52 Å, β = 105.5°. Figure 1a Exploring in-plane anisotropic 2D materials is of great significance to the fundamental studies and further development of polarizationsensitive optoelectronics. Herein, chiral niobium oxide diiodide (NbOI 2 ) is introduced into the intriguing anisotropic 2D family with the experimental demonstration of anisotropic optical and electrical properties. 2D NbOI 2 crystals exhibit highly anisotropic dispersed band structures around the Fermi surface and strong in-plane anisotropy of phonon vibrations owing to the diff...

Nanoscale Optical Imaging of 2D Semiconductor Stacking Orders by Exciton‐Enhanced Second Harmonic Generation

“…Further, we demonstrate the power of combining nonlinear and linear nano-optics for correlating local variations of atomic registration and interlayer stacking to changes in excitonic light-matter interaction, revealing intimate structure-property relationships at the length scales relevant to quantum materials. In the far field, SHG polarization patterns can reveal the anisotropic crystal orientation [5,54] and vectorial strain field. [55] In this work, the near-field polarization is constrained by tip geometry.…”

Strongly anisotropic electronic and magnetic structures in oxide dichlorides RuOCl$_2$ and OsOCl$_2$

“…In addition, Peierls distortions were found along the b-axis [31,34], resulting in a spin-singlet configuration for the d xy orbitals [34], due to the formation of molecular states in the d xy bands. Very recently, the FE and Peierls distortions were confirmed experimentally for NbOI 2 with 4d 1 electronic configuration [35]. With additional spin-orbit coupling (SOC), a spin texture was also found at the Y point along the M -X chain direction [36].…”

Strong In‐Plane Anisotropy and Giant Second Harmonic Generation Response of Organic Single‐Crystalline Microwire Arrays

“…Long‐range ordered single‐crystal materials can compensate for these defects and maximize the inherent potential of the material. [ 21–24 ] In addition, the unique optical anisotropy and cleavage surface of the crystal are essential for the nonlinear optics, optical waveguide, construction of laser output resonant cavity, and other applications. At the current stage, the growth methods of organic NLO crystals are mainly the solution and the physical vapor transport method.…”