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
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Crystalline α-MoO3 belts consisting of nanosheets stacked along their  axes were synthesized via thermal vapor transport of MoO3 powders at elevated temperatures. The MoO3 belts were millimeters in length along their  axes and tens to hundreds of micrometers in width along their  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.
Keywordsα-MoO3 two-dimensional materials exfoliations lattice vibrational dynamics micro-Raman scattering
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The authors would like to acknowledge B. Li for his setting-up of the tube-furnace and Coryl J. J Lee for collecting the SEM/EDX/XRD data. This research is supported by A*STAR Science and Engineering Research Council Pharos 2D Program (SERC Grant No. 152-70-00012).
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