Loading...

Synthesis, Characterization and Field Emission Study of Molybdenum Oxide (MoOx)Nanostructures

Khademi, Ali | 2009

689 Viewed
  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 39748 (04)
  4. University: Sharif University of Technology
  5. Department: Physics
  6. Advisor(s): Moshfegh, Alireza
  7. Abstract:
  8. Different molybdenum oxide nano/micro structure thin films were synthesized by thermal evaporation method under various growth conditions such as deposition time, temperature and evaporation source-substrate distance. Molybdenum oxide nanostructures grown on Si(100) such as nanowalls, nanostars and nanowires were characterized with scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). Shape and type of nanostructures were determined with SEM, AFM and HRTEM analysis. XPS analysis showed that the synthesized nanostructures contained Mo6+, Mo5+, Mo4+ and Moδ+ (where 0 < δ < 4) states. Mo4+ is the main oxidation state in the XPS spectrum and this state produces crystalline MoO2 structure. In addition, the mixture of Mo5+ and Mo6+ states with their corresponding percentages obtained from XPS results produces a compound that is nearly the same as the crystalline phase of Mo4O11. According to XRD analysis, the synthesized nanostructures are composed of both crystalline Mo4O11 and MoO2 structures. TEM observation indicated the nanostars consist of MoO2 nanorods. Therefore, Mo4O11 crystalline peaks seen in XRD pattern can be indexed to thin layer located beneath the MoO2 nanostars which was also observed in SEM cross section image. Field-emission measurements were performed in an indigenously constructed set-up under UHV conditions. Experimental were carried out with different cathode (synthesized nanostructures)-anode (aluminum probe) spacing distances. Plots of the field emission current density, J, versus the applied electric fields, E, and theirs corresponding logarithmic or Fowler- Nordheim (FN) plots at different anode–sample separations (vacuum gap) were obtained and their field emission properties such as turn-on field, pre-threshold field, field enhancement factor (β) were determined. The field emission (FE) measurements in different vacuum gaps (d) indicate that the relationship between 1/β and 1/d is a linear function and follows a two-region FE model. It was found that the field emission from Molybdenum oxide nanostructures followed the FN theory, but had different parameters at low-field and high-field emission ranges. The turn-on emission field and the enhancement factor are found to be 1.9 V/μm and 8120 at the vacuum gap of 300 μm for the MoO2 nanostars. These value were obtained 1.2 V/μm and 83077 for MoO2 nanowires with large tips and 0.2 V/μm and 42991 for MoO2 nanowires. These values indicate that nanowires are better field emitter as compared with nanostars. These measurements demonstrate that the synthesized molybdenum oxide nanostructures are good candidates for the applications as an electron emitting source in various vacuum electronic devices due to their relatively low turn-on field and high enhancement factor
  9. Keywords:
  10. Nanowire ; Molybdenum Oxide ; Nanostars ; Field Emission ; Field Emission Setup

 Digital Object List

 Bookmark

No TOC