Aqueous and mechanical exfoliation, unique properties, and theoretical understanding of MoO3 nanosheets made from free-standing α-MoO3 crystals: Raman mode softening and absorption edge blue shift

摘要

Crystalline α-MoO3 belts consisting of nanosheets stacked along their [010] axes were synthesized via thermal vapor transport of MoO3 powders at elevated temperatures.The MoO3 belts were millimeters in length along their [001] axes and tens to hundreds of micrometers in width along their [100] axes.Mechanical and aqueous exfoliations of the belts to form two-dimensional (2D) nanosheets were processed via the scotch-tape and bovine serum albumin (BSA) assisted methods,respectively.Upon scotch-tape exfoliation,the Raman features of MoO3 exhibited monotonic decreases in intensity as the thickness was gradually fell to approach that of a 2D nanosheet.Most Raman features eventually disappeared when a monolayer nanosheet was produced,except for the Mo-O-Mo stretching mode (Ag) at ～ 818 cm-1,which was accompanied by mode-softening of up to 5 cm-1.This mode softening,hitherto not reported for 2D α-MoO3 nanosheets,can be attributed to lattice relaxations that are validated here via theoretical density functional perturbation theory calculations.The BSA-assisted exfoliation products exhibited a blueshift in the α-MoO3 nanosheet absorption edge;they also revealed an absorption peak at 3.98 eV that can be attributed to their intrinsic exciton absorptions.These observations,together with the facile synthesis of high-purity α-MoO3 crystals,illuminate the possibility of further 2D α-MoO3 nanosheet production and lattice dynamic studies.