Fabrication of Tungsten Oxide Nanostructured Films Using Anodic Porous Alumina and Application in Gas Sensing Submitted by See Yeow Hoe, Godwin Department of Electrical and Computer Engineering In partial fulfillment of the Requirements for the Degree of Master of Engineering National University of Singapore ABSTRACT ABSTRACT Anodization, a self-ordering technique for creating nano-channels in alumina, is a simple and cheap method for creating highly ordered nanoporous film The dimensions of the nanochannels, including pore diameter and pore depth can be controlled accurately through appropriate anodization conditions In this thesis, we examined the suitability of using porous alumina as a supporting substrate for creating a textured metal oxide semiconducting (MOX) nanofilm to be used as a gas sensor in argon ambient A measurement system that can be used to characterize a gas-sensing device with respect to sensitivity, response time and recovery time was designed and set up A chemical vapour deposition (CVD) system for CVD of tungsten was designed and set up as well The textured film was deposited using low pressure chemical vapour deposition (LPCVD) of tungsten Tungsten hexacarbonyl W(CO)6 was used as the precursor Electrical and structural characterization were performed on the deposited films Comparison of oxygen sensing characteristics were made between the textured film deposited on porous alumina and that of a thin film deposited on glass substrate using the measurement system It was found that the non-textured film performed better than the textured film in terms of sensitivity, response time and recovery time Possible explanations for the observed phenomena were given Lastly, a novel honeycomb nanostructure was fabricated using pyrolysis of tungsten hexacarbonyl on pore-widened anodic porous alumina This structure has potential applications in gas-sensing LIST OF FIGURES ACKNOWLEDGMENTS The author would like to thank the following people, who have helped in one way or another through the course of this project: Supervisors A/Prof Chim Wai Kin and A/Prof John Thong for patience, guidance and invaluable suggestions throughout the course of the project, and for imparting many life application skills; Mrs Ho Chiow Mooi and Mr Goh Thiam Pheng for their assistance in obtaining the resources required for experiments; Jayson Koh for providing sound advice and technical support, and for making his stay in CICFAR II a comfortable and enjoyable one; Chiam Sing Yang for helping with X-ray photon spectroscopic analysis; Tan Soon Leng, Alfrad Quah, Goh Szu Huat, Ho Heng Wah, Yao Guhua, Yan Jian, Li Qi, Luo Tao, You Guofeng, Wong Kin Mun for working together in CICFAR and reducing the stress of working on the project by having meals and breaks together; Parents Mr and Mrs See for providing strong moral support and lastly; Friends from Baptist Fellowship Church who labored with him in prayer on his project LIST OF FIGURES LIST OF FIGURES Figure 2.1: Simplified Diagram of Anodization 15 Figure 2.2: Schematic Microstructure of an Anodic Film [Henley 1982] .16 Figure 2.3: Ideal Hexagonal Pore Array 17 Figure 2.4: Migration of Al3+ and O2- ions during Anodization [Jessensky 1998]20 Figure 2.5: Empirical Trend of Interpore Distance and Pore Diameter vs Anodizing Voltage [Sullivan et al 1970] 25 Figure 2.6: Wire grid type polarizer made of anodized alumina film The film transmits the light polarized vertically to the metal columns, and attenuates light polarized horizontally to the columns 26 Figure 2.7: A typical MISFET gas sensor It is similar to a MOSFET except that different gate metals may be used to sense different gases For example, to sense hydrogen, a palladium gate may be used [Bergveld et al 1998] 28 Figure 2.8: A typical acoustic wave gas sensor device It consists of two sets of interdigital transducers One transducer converts electric field energy into mechanical wave energy; the other converts the mechanical energy back into an electric field (Extracted from http://www.sensorsmag.com/articles/1000/68/main.shtml) 28 Figure 2.9: Schematic diagram of the Taguchi sensor and are electrical contacts indicates a porous ceramic body and represents a semiconductor material filling the pores in the ceramic body 30 Figure 2.10: Physical model and associated band model of the grains of a MOX sensing layer [Hoel 2004] 31 Figure 2.12: Schematic illustration of the ZrO2 HEGO sensor 37 Figure 2.13: Typical response of a commercial ZrO2 oxygen sensor to changes in air-fuel ratio of an engine .40 Figure 2.14: Atomic force microscopy image of a metal/S-SWNT/metal sample used for the experiments conducted by Kong’s group [Kong et al 2000] The diameter of the nanotube is 1.8 nm The metal electrodes consist of 20-nmthick Ni, with 60-nm-thick Au on top 42 Figure 2.15: Schematic model of crystalline WO3 in the undistorted cubic phase 43 Figure 2.16: Current response of a palladium nanowire-based H2 sensor under exposure to hydrogen/nitrogen mixtures (concentration of H2 as shown) [Walter et al 2002] 47 Figure 2.17: Resistance response of annealed titanium oxide film following a step change in composition from air to 50ppm NH3 in air [Manno et al 1997] 47 Figure 2.18: FTIR spectra of SnO nanopowder film at 300 oC (a) under 50 mbar oxygen; (b) after addition of 10 mbar CO; (c) after evacuation [Baraton et al 2002] 49 LIST OF FIGURES Fig 2.19: Variations of the infrared energy transmitted by SnO powder film versus gas exposures: (a) at 300 oC; (b) at 150 oC [Baraton et al 2002] .49 Figure 3.1: Schematic diagram of the LPCVD setup used to deposit tungsten via pyrolysis of tungsten hexacarbonyl .54 Figure 3.2: Schematic diagram showing process of fabricating (a) flat substrate device and (b) textured substrate device 59 Figure 3.3: Schematic representation of the setup used to characterize the gas sensing characteristics of devices .61 Figure 4.1: SEM micrographs of samples obtained by anodizing at (a) 40V, (b) 45V, (c) 50V and (d) 55V in 0.3M oxalic acid 66 Figure 4.2: SEM micrographs of a typical sample of anodized alumina anodized at 50V showing (a) three-dimensional view and (b) bottom view .67 Figure 4.3: Deposition rate of tungsten by pyrolysis of tungsten hexacarbonyl on flat alumina substrate .68 Figure 4.4: Tungsten film deposited by pyrolysis of tungsten hexacarbonyl (W(CO)6) on alumina substrate for a duration of (a) 10min, (b) 5min, (c) 2min, (d) 1min and (e) 30s 69 Figure 4.5: SEM micrographs showing three-dimensional views of films deposited by pyrolysis of tungsten hexacarbonyl with durations of (a) and (b) 30s .70 Figure 4.6: AFM images of films deposited by pyrolysis of tungsten hexacarbonyl with durations of (a) 30 s and (b) .71 Figure 4.7: XPS Depth Spectra of tungsten film deposited by pyrolysis of tungsten hexacarbonyl The time in the legend indicates the sputtering duration before each XPS spectrum was acquired .72 Figure 4.8: Arrhenius plot (ln R vs 1/T) of deposited film before and after oxidation 73 Figure 4.9: SEM micrographs showing the surfaces of fabricated samples using (a) front-side deposition and (b) reverse-side deposition 76 Figure 4.10: Typical (a) flat substrate device and (b) textured substrate device that were used for performing gas sensing experiments .77 Figure 5.1: Response graph of a typical device to nitrogen The test was conducted at 473K (200oC) 30% nitrogen was flowed into the chamber at 500s The nitrogen flow was switched off at 3500s 78 Figure 5.2: Graph showing the compensation method for small temperature fluctuations during gas sensing experiments The black line is the corrected current (Ac), after compensating for the small temperature fluctuations, and the grey line is the actual current taken during the experiment (I) 80 Figure 5.3: Ideal response curve in the presence of the test gas and subsequent removal of the gas 81 LIST OF FIGURES Figure 5.4: Time variation of the current of flat substrate device under a DC bias of 5V at 473K (200oC) and at a ammonia concentration of (a) 30%, (b) 20%, (c) 10%, (d) 5% and (e) 2% The line at the top indicates temperature (oC) The grey line indicates the actual current and the black line indicates the compensated current 84 Figure 5.5 Sensitivity of flat substrate device to various concentrations of ammonia at test temperatures ranging from 433K to 573K 85 Figure 5.6: (a) Response time and (b) recovery time of flat substrate device to various concentrations of ammonia at test temperatures ranging from 433K to 573K .86 Figure 5.7: Time variation of the current of textured substrate device under a DC bias of 5V at 473K and at an ammonia (NH3) concentration of (a) 30%, (b) 20% and (c) 10% 87 Figure 5.8: Sensitivity of textured substrate device to various concentrations of ammonia at test temperatures ranging from 433K to 573K 88 Figure 5.9: (a) Response time and (b) recovery time of textured substrate device to various concentrations of ammonia at test temperatures ranging from 433K to 573K .88 Figure 5.10: Time variation of the current of flat substrate device under a DC bias of 5V at 473K and at oxygen (O2) concentration of (a) 30%, (b) 20%, and (c) 10% 90 Figure 5.11: Sensitivity of flat substrate device to various concentrations of oxygen at test temperatures ranging from 433K to 573K 92 Figure 5.12: (a) Response time and (b) recovery time of flat substrate device to various concentrations of oxygen at test temperatures ranging from 433K to 573K .93 Figure 5.13: Time variation of the current of textured substrate device under a DC bias of 5V at 473K and at oxygen (O2) concentration of (a) 30%, (b) 20%, (c) 10% and (d) 5% 94 Figure 5.14: Sensitivity of textured substrate device to various concentrations of oxygen at test temperatures ranging from 433K to 673K 95 Figure 5.15: (a) Response time and (b) recovery time of textured substrate device to various concentrations of oxygen at test temperatures ranging from 433K to 673K 95 Figure 5.16: Time variation of the current of textured substrate device under a DC bias of 5V at 473K and at gas concentration of (a) 30% oxygen (O2) after hr of annealing, (b) 30% ammonia (NH3) after hr of annealing, (c) 30% oxygen after hrs of annealing, (d) 30% oxygen after 19 hrs of annealing and (e) 30% ammonia after 19 hrs of annealing The annealing temperature was 973K 98 LIST OF FIGURES Figure 5.17: SEM micrographs (a) tilted cross-section view and (b) direct crosssection view of tungsten film on template after 30s deposition of tungsten by pyrolysis of tungsten hexacarbonyl Tungsten covers the top of the template The pore walls pore walls are also covered with tungsten to a depth of about 500nm (bright regions) 101 Figure 5.18: I-V characteristics of a typical gas sensor device at 473K Poff indicates the I-V characteristic before the power supply was turned on and Pon indicates the I-V characteristic after the power supply was turned on 104 Figure 5.19: Porous alumina anodized at 55V and subsequently pore-widened by immersion in 5% wt phosphoric acid for (a) 0min, (b) 15min, (c) 30min and (d) 45min 106 Figure 5.20: (a) SEM micrograph of an anodic porous alumina template sample after pore-widening to form thin walls and after deposition of tungsten by LPCVD of tungsten hexacarbonyl (b) SEM micrograph showing columnar structures nucleating on the porous alumina template .107 LIST OF TABLES LIST OF TABLES Table 2.1: Relationship between Physical Characteristics of Pore Arrays and Anodizing Conditions 22 Table 2.2: Types of Acid used for Different Anodizing Voltages 23 Table 2.3: Known polymorphs of tungsten trioxide [Gallardo 2003] 43 CONTENTS CONTENTS ABSTRACT ACKNOWLEDGMENTS LIST OF FIGURES LIST OF TABLES CONTENTS INTRODUCTION 10 1.1 Motivation and Objective 12 1.2 Organization of Thesis 13 LITERATURE REVIEW 15 2.1 Anodization 15 2.1.1 The Anodization Process 15 2.1.2 Terminologies Used 17 2.1.3 Mechanism for Formation of Hexagonal Pore Arrays 19 2.1.4 Known Dependencies in Anodization 22 2.1.5 Some Applications of Anodic Porous Alumina 25 2.2 Metal Oxide Semiconductor Gas Sensors 27 2.2.1 Theory of MOX Gas Sensing 29 2.2.2 Factors Affecting the Performance of Gas Sensors 34 2.2.3 Enhancing Performance of MOX Sensors Through the Use of Catalytic Additives 35 2.2.4 An Application of the Oxygen Sensor-Heated Exhaust Gas Oxygen (HEGO) Sensor 37 2.2.5 Using Nanostructures to Enhance Gas Sensing 40 2.2.6 Structural Properties of Tungsten Trioxide 42 2.2.7 Methods of Depositing Tungsten Trioxide Thin Films for Gas Sensing Applications 44 2.2.8 Methods of Characterizing Gas Sensors 46 2.3 Summary 50 EXPERIMENTAL SETUP 51 3.1 Preparing the Samples for Film Deposition 51 3.2 Depositing Tungsten Oxide Thin Film by Pyrolysis of Tungsten Hexacarbonyl 52 3.3 Characterizing the Metal Oxide Sensor 60 3.4 Structural and Electrical Characterization 63 3.5 Summary 63 CONTENTS ELECTRICAL AND STRUCTURAL CHARACTERIZATION OF FABRICATED DEVICES 65 4.1 Results of Anodization 65 4.2 Characterizing Tungsten Film Deposited by Pyrolysis of Tungsten Hexacarbonyl 67 4.3 Obtaining a Continuous Tungsten Oxide Film on Porous Alumina 75 4.4 Summary 77 GAS SENSING CHARACTERISTICS OF FABRICATED DEVICES78 5.1 Compensation for Temperature Fluctuations 79 5.2 Terms and Definitions 81 5.3 Ammonia Sensing 82 5.4 Oxygen Sensing 89 5.4.1 Explanation for Unusual Behavior in Oxygen Sensing 96 5.5 Performance Differences between Flat and Textured Substrate Sensors 100 5.6 Sources of errors 103 5.7 A Novel Honeycomb Nanostructure Using Porous Alumina Template 105 5.8 Summary 108 CONCLUSION 109 6.1 Summary of Thesis 109 6.2 Possible Future Developments 110 REFERENCES 112 CHAPTER FIVE 5.6 Sources of errors It can be seen that the actual current versus time graphs had some noise fluctuations These fluctuations reduced the precision of the extracted gas sensing performance parameters of sensitivity, response and recovery times There are a few possible sources of noise Three of the biggest sources are described here Compensation for temperature fluctuations In Section 4.1, the method of compensating differences in current due to temperature fluctuations in section 4.1, a linear relationship between changes in current and temperature was assumed In reality, however, the relationship between the conductivity (equivalent to the measured current) of a semiconductor and temperature is as follows: σ α exp(-Eg/2kT) σ –conductivity, (5.1) Eg—bandgap of the material, k—Boltzmann constant, T— temperature In other words, the relationship is exponential However, it would be a tedious and complex task to make use of this exponential relationship due to various factors involved, such as changes in carrier mobility, presence of carbon in the samples and adsorption/desorption of gases Since the method is Section 4.1 gave reasonable results and was more efficient, it was used for this thesis Moreover, the temperature fluctuations in the experiments were kept to a narrow range (i.e 103 CHAPTER FIVE within ±2K) so that the linear assumption between current and temperature is reasonable Gas flow control The gas flow rate was controlled by a fine valve, as shown in figure 3.2 in Chapter There was no active feedback control to change the size of the orifice of the valve to respond to changes in flow conditions, so that the gas flow rate could be maintained at a precise level Hence there were small fluctuations in the gas flow rate during gas sensing experiments, resulting in some noise in the response curve Noise from power supply Figure 5.18 shows the current-voltage characteristics of a typical gas sensor device (i.e., typical of a flat substrate and textured substrate device) I (A) 3.50E-08 3.00E-08 2.50E-08 2.00E-08 1.50E-08 Poff 1.00E-08 Pon 5.00E-09 0.00E+00 -5.00E-09 V (V) Figure 5.18: I-V characteristics of a typical gas sensor device at 473K Poff indicates the I-V characteristic before the power supply was turned on and Pon indicates the I-V characteristic after the power supply was turned on 104 CHAPTER FIVE With the power supply to the heating stage off (Poff), a smooth curve was observed With the power supply on (Pon), some noise fluctuation in the I-V characteristic was observed This noise resulted in current fluctuations when measuring the response of the devices to the test gases 5.7 A Novel Honeycomb Nanostructure Using Porous Alumina Template Different kinds of nanostructures can be fabricated using anodic porous alumina as a mask or template Such structures can be used for various applications, as explained in chapter one In 2003, Ng et al reported on the growth of epitaxial ZnO nanowires at the junctions of nanowalls using the vapor-liquid-solid mechanism [Ng et al 2003] Epitaxial growth of ZnO occurred along grain boundaries of the sapphire substrate as these regions were the most thermodynamically active sites (probably having higher surface energy as well) This resulted in the formation of ZnO nanowalls In this thesis, pore-widening was done to create very thin pore walls on anodic alumina Such thin walls would have high surface energy at the sharp edges on top of the walls It would then be possible that when LPCVD of tungsten hexacarbonyl was performed using anodic porous alumina as a supporting substrate, the tungsten hexacarbonyl precursor would decompose preferentially on the top of the walls, forming a tungsten honeycomb nanostructure Figure 5.19 shows the surface of a typical sample of anodic porous alumina after various durations (0 to 45 minutes) of pore-widening 105 CHAPTER FIVE (a) (b) (c) (d) Figure 5.19: Porous alumina anodized at 55V and subsequently pore-widened by immersion in 5% wt phosphoric acid for (a) 0min, (b) 15min, (c) 30min and (d) 45min Figures 5.20 show the sample after pore-widening for a duration of 45min to form thin walls and subsequent deposition of tungsten by LPCVD of tungsten hexacarbonyl Columnar structures were seen to rise from the pore walls forming a honeycomb structure Figure 5.20(b) shows clearly the columnar structure nucleating on the porous alumina template 106 CHAPTER FIVE (a) (b) Figure 5.20: (a) SEM micrograph of an anodic porous alumina template sample after pore-widening to form thin walls and after deposition of tungsten by LPCVD of tungsten hexacarbonyl (b) SEM micrograph showing columnar structures nucleating on the porous alumina template It may be possible to use this structure as a gas sensing layer after further work has been done Some possible exploratory work is described here A systematic study of the fabrication process in order to understand the mechanism involved can be performed This may include varying the pore-widening conditions to 107 CHAPTER FIVE determine the thickness of the pore walls at which honeycomb structures begin to form It was observed from figure 5.20(b) that the honeycomb nanostructure was not continuous In order to obtain a continuous honeycomb structure, argon sputtering can be performed to remove surface contaminants on the alumina template This may induce uniform nucleation of tungsten islands on the template during LPCVD of tungsten hexacarbonyl and hence facilitate the formation of a continuous film Such a structure, in addition to being suitable for gas sensing, may be useful in other applications , e.g., gas storage 5.8 Summary In this chapter, the gas sensing capability of both the flat substrate device and textured substrate device were characterized with respect to sensitivity, response time, and recovery time Both devices were compared and it was concluded that the flat substrate device performed better than the textured substrate device in terms of better sensitivity, faster response and recovery times Possible reasons were given In addition, it was found that the fabricated devices (both flat and textured substrate devices) exhibited an unexpected response to oxygen Experiments were conducted and possible explanations were given to account for the response Lastly, a novel nanostructure using anodic porous alumina template was proposed as a suitable device for gas sensing In the next chapter, an overall summary of the thesis will be given and possible future developments of this work are discussed 108 REFERENCES Conclusion In this chapter, a summary of the work done for this thesis is presented Some possible future developments for this project are discussed 6.1 Summary of Thesis Two types of gas sensing devices were fabricated, using low pressure chemical vapor deposition of tungsten (LPCVD) by pyrolysis of tungsten hexacarbonyl (W(CO)6) The equipment needed to perform LPCVD was designed and set up in-house The first type of device was a flat substrate device using glass as the supporting substrate and the second device was a textured substrate device using anodic porous alumina as the supporting substrate The fabricated devices were then characterized electrically and structurally A novel honeycomb tungsten nanostructure was also fabricated by pyrolysis of tungsten hexacarbonyl on porewidened anodic porous alumina This structure has potential applications in gas storage and gas-sensing However, since the fabricated structure was discontinuous, optimization of the fabrication process is needed to obtain a continuous film so that gas sensing experiments can be conducted The gas sensing behavior of the flat substrate and textured substrate devices was characterized using a gas chamber setup that was designed and built in-house Comparisons between the two devices in terms of their gas-sensing performance to ammonia and oxygen were made In general, the flat substrate device performed better than the textured substrate device in all three performance indicators of sensitivity, response time and recovery time The reasons could be because of the thicker film and diffusion barrier effect of the alumina template for 109 REFERENCES the textured substrate sensor The unusual response of the devices to oxygen was explained as well The reason could be due to the presence of surface contaminants, especially CO molecules 6.2 Possible Future Developments A systematic study of the physical properties of films deposited by pyrolysis of tungsten hexacarbonyl on a flat substrate can be performed to shed some light on the gas-sensing mechanism Characterization techniques to be used for such a study may include the following: • X-Ray Photon Spectroscopy (XPS) to obtain the chemical composition as well as chemical bonding nature of the atoms in the film; • X-Ray Diffraction (XRD) to obtain information on the crystalline structure of the film as well as qualitative information of the grain size; • Atomic Force Microscopy (AFM) to obtain quantitative information on the grain sizes of the film as well as the surface roughness; • Scanning Electron Microscopy (SEM) to obtain information on the film morphology By depositing thin films that possess different chemical composition, crystalline structure, grain size, and morphology, and testing their response to various gases, the optimal conditions for fabricating a flat substrate gas sensing layer can be obtained Anodic porous alumina can then be used to further optimize the sensing capability of the sensor further by creating a larger surface area to the gas to be sensed For this thesis, although the surface area exposed to the ambient was larger for the textured substrate device, the performance of the device was worse 110 REFERENCES than the flat substrate device in general This could be because the tungsten oxide film was thicker for the textured substrate device due to decomposition of tungsten hexacarbonyl on the pore walls of the device Moreover, the alumina pore walls could have been a diffusion barrier to gas species To solve this problem, a very thin anodic alumina template of thickness less than 200nm can be fabricated so that the sensing layer will be thin The template can be mounted onto a flat insulating 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