Huge Absorption Edge Blue shifts of Layered α-MoO₃ Crystals upon Thickness Reduction Approaching 2D Nanosheets

Abstract

Recent theoretical studies suggest none or minor changes in the band gap of two-dimensional (2D) α-MoO3 nanosheets as compared with that of the bulk because of the weak interlayer electronic interactions. Unfortunately, this suggestion is lacking positive support in the literature. Herein, we report experimental observations of huge blue shifts in the absorption edge of layered α-MoO3 as its thickness t is reduced approaching atomic layers. When t > 10 nm, every order of magnitude of thickness reduction gives rise to a blue shift of ∼0.29 eV without causing any Raman mode shifts. This blue shift, in terms of finite difference time domain calculations, is attributable to optical interferences at the crystal surfaces. However, when t is further reduced below ∼10 nm, an even larger blue shift, accompanied by a mode softening of the most strengthened Mo–O–Mo stretching phonon (Ag), has been observed. This observation is consistent with those of 2D α-MoO3 nanosheets produced by aqueous exfoliations and, based on our calculations of the electronic structures, can be explained as anisotropic in-plane strain relaxations/redistributions. A gas-phase layer-by-layer etching of the layered α-MoO3 single crystals has also been demonstrated for consequent fabrications of novel electronic devices, as well as their integrations, based on α-MoO3 and other 2D nanosheets.

Structural and electronic properties of bulk and ultrathin layers of V2O5 and MoO3

“…This result contrasts with a recent study by Liu et al 7 based on the preparation of micro-to nanosized crystals of MoO 3 . According to this study, the optical band-gap shows a clear blue shift (~1.5 eV) going from 4 to 8 layers, and to the bulk phase of MoO 3 .…”

“…12 As for MoO 3 , Molina-Mandoza et al 6 have studied the dependence of the properties on the thickness but no major change in band-gap and band-structure has been observed going from the monolayer to the bulk. This result contrasts with a recent study by Liu et al 7 based on the preparation of micro-to nanosized crystals of MoO 3 . According to this study, the optical band-gap shows a clear blue shift (~1.5 eV) going from 4 to 8 layers, and to the bulk phase of MoO 3 .…”

“…On the other hand, layered oxides like vanadium pentoxide (V 2 O 5 ) or molybdenum trioxide (MoO 3 ) exist in nature, and exfoliation of these layered oxides can in principle be used to obtain new materials with novel properties and applications. 6,7 For instance, a recent study has shown the potential of liquid exfoliation of MoO 3 nanosheets to obtain new materials for supercapacitors energy storage applications. 8 The thermodynamically stable orthorhombic phase (also known as α-phase) of V 2 O 5 and MoO 3 , two transition metal oxides with d 0 character, have been widely studied both experimentally and theoretically due to their excellent opto-electronic, photo-and electrochemical properties, also useful for catalysis.…”

“…Moving to MoO 3 , PBE-D2 DFT calculations have been performed introducing compressive and tensile strain to explain some experimental findings. 7 In a study of bulk, monolayer and nano-ribbons of MoO 3 (PBE-D2+U DFT functional), the authors have reported a tiny change of the indirect band-gap of bulk (1.71 eV) compared to monolayer (1.73 eV). 20 Lutfalla et al have discussed the formation enthalpy from the possible redox pairs of V 2 O 5 /VO 2 /V 2 O 3 and MoO 3 /MoO 2 .…”

New Insights into Planar Defects in Layered α-MoO3 Crystals

“…The setup and processing procedures for the plasma treatments have been reported in an earlier publication. 14 It is seen that darkening of the α-MoO 3 has been introduced by APT at its surficial defective areas where the present cracks and/or atomic layer steps as those seen in Figure 2a before the plasma treatments. Because of the APT-induced darkening, the surficial defects of α-MoO 3 became dim and are hardly seen in Figure 2b.…”