Direct thermal oxidization evaporation growth, structure, and optical properties of single-crystalline nanobelts of molybdenum trioxide

Single-crystalline nanobelts of molybdenum trioxides were grown by direct thermal oxidization evaporation of metal molybdenum foils. Their structures, defects, and optical properties were investigated via x-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), atomic force microscopy, micro-Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and ultraviolet-visible spectroscopy (UV-VIS). Single-crystalline nanobelts were identified as an orthorhombic structure with an average stoichiometry of MoO3.02 analyzed by energy dispersive spectroscopy of x-rays. The length, width, and thickness of a nanobelt were determined to be parallel to the b, c, and a axis of the MoO3 unit cell, respectively. The thickness of the nanobelt increased by integer multiples of 0.5a in a layer-by-layer fashion during growth. A density of dislocations as high as about 1.2 x 10(13) cm(-2) was formed, which may be attributed to relaxation of large strains during cooling. A special dislocation configuration was observed by HRTEM, which was well reproduced by image simulations based on the proposed model. The resulting morphology of nanobelts was proposed to be governed by growth kinetics. Micro-Raman and FTIR spectra were successfully analyzed on the basis of vibration of MoO6 octahedra. It was found that micro-Raman spectra were quite dependent on the size of the nanobelts. A band gap energy of 3.04 eV derived from UV-VIS measurements was observed to be red shifted relative to the previously reported experimental values, which may be due to the presence of a high density of defects.