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Aerosol Assisted Chemical Vapour Deposition of Tungsten and Molybdenum Oxide Thin Films

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Aerosol Assisted Chemical Vapour Deposition of Tungsten and Molybdenum Oxide Thin Films Aerosol Assisted Chemical Vapour Deposition of Tungsten and Molybdenum Oxide Thin Films This thesis is submitted in partial fulfillment of the requirements for the Degree of Doctor of Philosophy (Chemistry) Sobia Ashraf UCL 2007 1 UMI Number: U591288 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U591288 Published by ProQuest LLC 2013. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 I, Sobia Ashraf, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 Abstract Tungsten oxide and molybdenum oxide thin films have been deposited by aerosol assisted chemical vapor deposition (AACVD) using polyoxometalates, large ionic clusters which would be unsuitable for use in conventional atmospheric pressure chemical vapor deposition due to their low volatility. AACVD reactions of polyoxotungstates resulted in the formation of either fully oxidized yellow or partially reduced blue tungsten oxide films. Raman spectroscopy, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis indicated that there is a correlation between the stoichiometry of the films and the randomisation of the crystallites; blue films have a partially reduced W(Xx stoichiometry and show preferred orientation along the < 0 1 0 > direction, whereas yellow films are fully oxidized WO 3 and composed of randomly orientated crystallites. Depositions using polyoxotungstates containing heteroatoms, such as niobium, tantalum and titanium afforded doped tungsten oxide films in which the W:heteroatom ratio of the precursor was retained. This was also observed for films deposited using mixtures comprising of a doped polyoxometalate in conjunction with an undoped polyoxometalate demonstrating that polyoxometalates provide a route to the deposition of films with a highly controlled level of doping. Polyoxometalates have also been used as precursors to molybdenum oxide films. The resulting films were comprised of either M 0 O 3 , M0 O2 or a mixture of the two oxides, with the M0 O2 phase predominating with increasing temperature and distance from the precursor inlet. Results are also presented on the AACVD reactions of tungsten hexacarbonyl, [W(CO) 6 j in a variety of organic solvents. The tungsten oxide films function as gas sensitive resistors for detecting traces of ethanol and nitrogen dioxide in air, showing responses exceeding those of commercially available screen printed sensors, with a faster speed of response and lower optimum temperature for nitrogen dioxide detection. 3 Acknowledgements I would like to begin by thanking my supervisor, Professor Ivan Parkin, for his guidance and support, during the course of this work. I also wish to thank Drs. Chris Blackman, Clara Piccirillo, Keith Pratt and Dominic Mann. I would especially like to thank Dr. Simon Naisbitt for all his assistance with the gas-sensing aspects of this project. I am very grateful to Dr. Geoff Hyett for teaching me the joys of Rietveld refinement and the many useful discussions. Thank you. I am, of course, indebted to the staff of the chemistry (and archaeology) departments. Most especially: Messrs. Kevin Reeves, Dave Knapp, Joe Nolan, Dave Ladd, Dave Morphett, Dick Waymark, Peter Mackie and Dr. Steve Firth. I would like to thank Miss Ayo Afonja, Mrs. Clare Bishop, Miss Amy Dee, Miss Victoria Lees, Dr. Siama Basharat and Mr. Zahi Sulaiman for their friendship. Above all I would like to thank Robert for his love and support and for XPS analysis. 4 Table of Contents Abstract 3 Acknowledgements 4 Table of Contents 5 List of Figures 9 List of Tables 16 Chapter 1: Introduction 18 1.1 General Introduction 18 1.2 Chemical Vapour Deposition (CVD) 19 1.2.1 Overview of CVD 19 1.2.2 Variants of the CVD Process 22 1.2.3 Aerosol Assisted CVD (AACVD) 23 1.2.4 Forces Acting on Gas Phase Precursors 27 1.2.5 Variants of the AACVD Process 29 1.3 Tungsten Oxide 31 1.4 CVD of Tungsten Oxide Thin Films 32 1.4.1 Dual-Source CVD Routes to Tungsten Oxide 32 1.4.2 Single-Source Precursors to Tungsten Oxide Thin Films 35 1.5 Molybdenum Oxide 38 1. 6 CVD of Molybdenum Oxide Thin Films 39 1.7 Polyoxometalates 41 1. 8 Semiconducting Gas Sensors 46 1.8.1 Introduction 46 1.8.2 Mechanisms for Resistance Change in Semiconducting Gas 46 sensors 1.8.3 Tungsten Oxide Gas Sensors 51 5 Chapter 2: Experimental 54 2.1 Introduction 54 2.2 Aerosol Assisted CVD 54 2.3 Physical Analysis Techniques 55 2.4 Functional Tests 57 Chapter 3: Aerosol Assisted CVD of Tungsten Oxide Thin Films from 58 Polyoxometalate Precursors 3.1 Introduction 58 3.2 Precursors 58 3.3 Depositions using Polyoxometalates 62 3.4 Conclusions 74 Chapter 4: Aerosol Assisted CVD of Tungsten Oxide Thin Films from 76 Tungsten Hexacarbonyl 4.1 Introduction 76 4.2 Precursors 76 4.3 Depositions using Tungsten Hexacarbonyl 77 4.4 Conclusions 8 6 Chapter 5: Aerosol Assisted CVD o f Tungsten Oxide Thin Films from 88 Heteropoly oxom etalates 5.1 Introduction 8 8 5.2 AACVD of Tungsten Oxide Films from ["Bu 4 N]3 [NbW5Om] 8 8 5.2.1 Precursors 8 8 5.2.2 Depositions using ["BmNbtNbW.sOi*] 89 5.3 AACVD of Tungsten Oxide Films from ["Bu-^NhiTaWsOm] 98 5.3.1 Precursors 98 5.3.2 Depositions using ["Bu 4 Nh[TaW5 0 ,9 ] 99 6 5.4 AACVD of Tungsten Oxide Films from Mixtures of ["Bu 4 N]2 [W6 0 i9 ] 106 and ["Bu 4 N]3 [NbW5 0 i9 ] 5.4.1 Introduction 106 5.4.2 Depositions using Mixtures of ["Bu 4 N]2 [W6Oi9 l and 106 ["Bu4 N]3 [NbW5O i9 ] 5.5 AACVD of Tungsten Oxide Films from Mixtures of ["B^NElWftO^l 114 and rB u 4 N]3 [TaW5 0 ,9 ] 5.6 AACVD of a Tungsten Oxide Films from a mixtures of 120 [,'Bu 4 N]2 [W6 0 19 ], rB u 4 N]3 [NbW5Oi9 ] and ["B^NhlTaWsO^] 5.7 AACVD of Tungsten Oxide Films from [CpTi(W 5 0 ix)H]["Bu4 N ]2 126 5.7.1 Precursors 126 5.7.2 Depositions using [CpTi(W 5 0 ix)H]["Bu4 N ]2 127 5.8 AACVD of Tungsten Oxide Films from [''B^N^F^fSiMoW^^o] 134 5.8.1 Precursors 134 5.8.2 Depositions using [”Bu 4 N]4 H3 [SiMoWn 0 4 o] 134 5.9 Conclusions 141 Chapter 6: Aerosol Assisted CVD o f Molybdenum Oxide Thin Films 147 from Polyoxometalate Precursors 6.1 Introduction 147 6.2 Precursors 147 6.3 Depositions using Polyoxometalates 151 6.4 Conclusions 166 Chapter 7: Gas Sensing Properties of Tungsten Oxide Thin Films 168 deposited by Aerosol Assisted CVD 7.1 Introduction 168 7.2 Tungsten Oxide Sensors Deposited from [W(CO)6] 168 7.3 Tungsten Oxide Sensors Deposited from ["Bu 4 N]2 [Wi0O 3 2 ] 204 7.4 Ethanol Testing 208 7.5 Conclusions 213 7 Chapter 8: Conclusions References List of Figures Chapter 1: Introduction Figure 1.1 A schematic representation of the steps involved in the CVD Process for a 20 single-source precursor. Figure 1.2 Idealised plots showing the influence of temperature and flow rate on film 21 growth. Figure 1.3 Influence of temperature on the AACVD process. 26 Figure 1.4 Structure of monoclinic Y-WO 3 31 Figure 1.5 Structures of orthorhombic M 0 O 3 and monoclinic M 0 O2 . 39 Figure 1.6 Representation of the three types of metal-oxygen bonds at each 42 hexavalent tungsten centre in [W 6 Oi9 ]2 . Figure 1.7 Structural representation of the anion, [M 6 Oi9 ]2~ (M= W, Mo). 43 Figure 1.8 Structural representation of the anion, [W 10O3 2 ]4’. 43 Figure 1.9 Structural representation of the anion, [Mox026]4 • 44 Figure 1.10 Structural representation of the Keggin anion, [PM 0 1 2 O4 0 ]3’. 44 Figure 1.11 Structural representation of the anion, [SiMoWii0 4 o]4. 45 Figure 1.12 Charge exchange associated with the adsorption of oxygen onto the 47 surface of an n-type semiconductor. Figure 1.13 Charge exchange associated with the adsorption of oxygen onto the 48 surface of a p-type semiconductor. Figure 1.14 Surface processes which occur on an n-type semiconductor. 49 Chapter 2: Experimental Figure 2.1: Schematic diagram of a CVD reactor 54 Chapter 3: Aerosol Assisted CVD of Tungsten Oxide Thin Films from Polyoxometalate Precursors Figure 3.1 Thermal gravametric analysis of [nBu 4 N]4 [Wio 0 3 2 ]. 61 9 Figure 3.2 Thermal gravametric analysis of [NH 4 ]6 [Wi2 0 3 9]. 61 Figure 3.3 Powder XRD patterns of the films deposited by the AACVD reaction of 65 ["Bu4 N]3 [W0 4] in acetonitrile as a function of substrate temperature. Figure 3.4 Powder XRD patterns of the films deposited from the polyoxometalate 66 precursors at 550°C. Figure 3.5 Raman spectra of the films deposited from the polyoxometalate precursors 68 at 550°C. Figure 3.6 Scanning electron micrographs of tungsten oxide films deposited from the 71 AACVD reactions of polyoxometalates at 550°C and their corresponding size distributions.
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