Abstract
Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young’s modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far.
Highlights
Two-dimensional (2D) materials with an in-plane anisotropy, such as several transition metal chalcogenides, group-VA, black phosphorus and compounds made of two group-VA elements, have been extensively studied[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]
Calculations show that in-plane carrier mobility exhibit strong anisotropic behavior[44] and highly anisotropic propagation of phonon polaritons have been recently observed in α-MoO345–48
We found that the flakes show a strong birefringence, i.e., their refractive index depends on the alignment between the polarization of the incident light and the crystalline axis
Summary
Two-dimensional (2D) materials with an in-plane anisotropy, such as several transition metal chalcogenides, group-VA, black phosphorus and compounds made of two group-VA elements (so called V-V binary materials), have been extensively studied[1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21]. Among the available anisotropic 2D materials, molybdenum trioxide is a wide band gap semiconductor (>2.7 eV)[28], which makes it quasi-transparent in the visible spectrum while being still electrically conductive This nanomaterial has been proven useful for gas sensing[29], resistive memory technology[30], optoelectronic[31,32,33,34,35,36,37,38,39,40], electrochromic[41] and flexible[42,43] applications. In comparison with metallic plasmon polaritons, phonon polaritons can achieve reduced optical losses, improved light confinements and higher quality factors[49,50,51,52] These interesting optical properties are mainly caused by the anisotropy of the fundamental properties of molybdenum trioxide, which until now have been scarcely investigated.
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