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

作　　者：Hongfei Liu Yongqing Cai Mingyong Han Shifeng Guo Ming Lin Meng Zhao Yongwei Zhang Dongzhi Chi 

机构地区：[1]Institute of Materials Research and Engineering (IMRE), A＊STAR (Agency for Science, Technology and Research), 2 Eusionopolis Way, Singapore 138634, Singapore [2]Institute of High Performance Computing (IHPC), A ＊STAR (Agency for Science, Technology and Research), 1 Fusionopolis Way, Singapore 138632, Singapore

出　　处：《Nano Research》2018年第3期1193-1203,共11页纳米研究（英文版）

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.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.

关 键 词：α-MoO3 two-dimensionalmaterials exfoliations lattice vibrationaldynamics micro-Raman scattering 

分 类 号：O4-09[理学—物理] TQ426.8[化学工程]