Pulsed laser deposition of nanostructured tungsten oxide thin films for optical gas sensor applications

Mehdi Ranjbar ,  Azam Iraji zad ,  Seyed Mohammad Mahdavi ,  Reza Motefaregheh tilaki 

Sharif University of Technology, Azadi Avenue, Tehran 14588, Iran

Abstract

Nanostructured ungsten oxide thin films were deposited by pulsed laser deposition (PLD) (Nd: YAG, λ=1064 nm) for optical hydrogen gas sensor applications. Deposited was performed onto heated glass substrate kept at 400 °C in HV (5×10-5Torr) and 100mTorr oxygen partial pressure. Deposited films in HV showed blue color with 3.1 eV optical band gap and amorphous structure. Samples turned to transparent films with band gap of 3.0 eV and crystalline structure after annealing in air at 400 °C. Films that were deposited in oxygen partial pressure were amorphous and transparent with 3.3eV band gap and changed to dark blue after annealing in vacuum at 400 °C. The structure of these films remained amorphous after annealing and the band gap of these films was reduced to 2.7eV due to creation of oxygen vacancy. To observe hydrogen gas sensing properties a very thin layer of palladium was coated by electro-less method. Optical switching effect due to hydrogen exposure was observed only for samples deposited in oxygen environment. XPS data of the samples present W6+/W5+ ratio higher than those deposited in vacuum. Regarding to Atomic Force Microscopy (AFM) data, deposited samples in oxygen environment showed RMS surface roughness of 13.6 nm and grain boundary of 60 nm while those prepared in HV showed smother surface (2.87nm) and grain boundary of 150nm. Higher surface roughness and lower oxygen vacancy of the films prepared at oxygen partial pressure made it suitable for gas sensing response.

Keywords: tungsten oxide, pulsed laser deposition (PLD), gasochromic, hydrogen sensors.

 

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Presentation: Poster at E-MRS Fall Meeting 2006, Symposium A, by Mehdi Ranjbar
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-14 15:08
Revised:   2009-06-07 00:44