Femtosecond laser-induced oxide formation on molybdenum thin films in varying oxygen environments

Master of Science, Graduate Program in Mechanical Engineering
University of California, Riverside, June 2015
Dr. Guillermo Aguilar, Chairperson

Pulsed  laser  processing  is  a  technique  to  induce  optical  and  structural  changes  in  materials,  such  as oxide line and ring structures, when metallic samples are irradiated with high repetition rate lasers in oxygen  and  mixed  atmospheres.  A  study  of  femtosecond  laser  processing  on  thin  films  of  the transition metal molybdenum in air and oxygen is presented. The molybdenum oxides, which can have very different electrical and optical properties, are gaining momentum in novel technical applications such as electrochromic devices, electrodes in microbatteries and solar cells, gas sensors, and catalysts, among others.

These  latest  short-pulsed,  ultrafast  lasers  have  only  recently  begun  to  find  more  widespread  uses  in research institutions. The novelty of the femtosecond lasers is that the pulse width is shorter than the electron cooling time, requiring advanced physical models to couple the electron and lattice heating. Because  of  this,  femtosecond  lasers  have  found  advancement  in  the  fields  of  physics,  chemistry, engineering,  medicine,  and  material  science.  When  a  femtosecond  laser  irradiates  a  transition  metal thin  film,  the  thin  film  reacts  with  the  gaseous  species  producing  different  chemical  and  structural phases.  These  oxides  and  phases  are  dependent  on  the  localized  atmosphere  and  laser  irradiance characteristics, e.g. the fluence, polarization, pressure, and duration of exposure.

The aim of this thesis is to determine the correlation between all the different tunable variables and the respective oxides formed on the thin film when irradiated by a femtosecond laser, both in ambient air and in varying pressurized oxygen atmospheres. The inputs varied in this study are the following: laser fluence/output  power,  irradiation  time,  gas,  and  gas  pressure.  Raman  spectroscopy,  atomic  force microscopy, profilometry, and scanning electron microscopy are used in characterizing the oxidation and phase shifting. It is shown that, with the addition of a pure (>99.9%) oxygen gaseous environment, the resulting molybdenum trioxide (MoO3) area is both much larger (~50%) and oxidized with much lower  fluences  and  timescales  than  the  corresponding  laser  irradiations  done  in  ambient  air environments.  Small  variances  in  oxygen  pressure  show  small  but  present  increases  in  trioxide  formation as the pressure is slightly increased. Changes in the other different oxides formed (MoO2 and Mo4O11) are also reported based on gas and pressure changes.