Synthesis of sele heating gas sensor based on tin oxide nanowire material

INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE --- HÀ MINH TÂN SYNTHESIS OF SELF-HEATING GAS SENSOR BASED ON TIN OXIDE NANOWIRE MATERIAL MASTER’S THESIS ELECTRONIC MATERIALS

INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS

SCIENCE -

HÀ MINH TÂN

SYNTHESIS OF SELF-HEATING GAS SENSOR BASED ON TIN

OXIDE NANOWIRE MATERIAL

MASTER’S THESIS ELECTRONIC MATERIALS SCIENCE AND ENGINEERING

Hanoi - October 2014

HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

INTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCIENCE

MASTER’S THESIS

Synthesis of self-heating gas sensor based on

tin oxide nanowire material

HA MINH TAN

Student ID: CB120169 Advisors: PhD NGUYEN VAN DUY

A thesis submitted to Ta Quang Buu library, Hanoi University of Science and Technology in partial fulfillment of the requirement for the degree of Master of

Science

HA NOI – OCTOBER 2014

Declaration of Originality

“I, the candidate, hereby certify that the thesis comprises only my original work except where indicated; due acknowledgment has been made in the text to all materials used.”

Acknowledgements

Firstly, I would like to express the deepest appreciation to my supervisors, Professor Nguyen Van Hieu and PhD Nguyen Van Duy for guiding me to do my project They gave me valuable guidance and advice

Besides, I would like to thank to all members in Gas Sensor Group at ITIMS for helping me during all the time I do my work

Finally, I thank to all my friends and family for caring and inspiring me all the time I do project

Table of Contents

Declaration of Originality 1

The Comment of Advisor 1

Acknowledgements 3

Table of Contents 4

Table of Figures 6

List of Tables 9

Acronyms and Abbreviations 10

INTRODUCTION 1

1 Study motivation 1

2 Objective of the study 1

3 New features of the thesis 2

4 Methodology 2

CHAPTER I – OVERVIEW 3

I.1 Brief history of the evolution of gas sensors 3

I.2 Brief knowledge about gas sensing devices and SnO 2 nanowires materials 9

I.2.1 Microstructure and sensing mechanism of SnO 2 nanowires 9

I.2.2 Characteristics of gas sensing devices 13

I.3 Joule heating and its application in gas sensing devices 17

I.3.1 Joule heating and heat transfer 17

I.3.2 Overview about gas sensing devices based on self-heating effect 20

I.4 Research’s approach 25

CHAPTER II – EXPERIMENTAL 26

II.1 Preparation of Interdigitated Electrode (IDE) 26

II.1.1 Designing of electrodes 26

II.1.2 Electrode fabrication 26

II.2 SnO 2 nanowires growth 28

II.2.1 Equipment, apparatus and chemical preparation 28

II.2.2 Growth procedure of SnO 2 NWs at 800 o C 28

II.3 Material characterization 29

II.4 Gas sensing properties investigation 30

II.4.1 Measurement system 30

II.4.2 Gas sensing characterization of sensor using self-heating effect 33

CHAPTER III – RESULTS AND DISCUSSION 34

III.1 Structure and morphology of grown nanowires 34

III.2 Existence of self-heating effect of nanowire networked sensors 37

III.3 Gas sensing performance comparison between sensors heated by external heater and self-heating 41 III.3.1 Sample b1 44

III.3.2 Sample b2 50

III.3.3 Sample b3 55

III.3.4 Sample b4 60

III.3.5 Sample b5 65

III.4 Evaluation of working temperature of self-heated sensor using gas mixing measurement and thermal emission microscopy 70

III.4.1 Gas mixing measurement 70

III.4.2 Temperature evaluation using thermal emission microscope 72

III.5 Self-heating rate 75

III.6 Capability of gas sensor using self-heating effect 77

III.6.1 Selectivity 77

III.6.2 Stability and repeatability 79

III.6.3 Significance of self-heating effect in gas sensing applications 79

CONCLUSION 82

References 83

Table of Figures

Figure I.1: a) Davey’s lamp – b) Jiro Tsuji and his gas detector using ligh-wave interference – c) Johnson – Williams’s founder 4 Figure I.2: The schematic of the platinum catalyst type of gas sensor _ 4 Figure I.3: a) The structure of resistive gas sensor – b) The working principle using voltage meter, Vh, Vc, Vout, and R L , which represent the heating voltage, circuit voltage, signal voltage, and load resistor,

respectively – c) The working principle using current meter _ 5 Figure I.4: a, b) The sensors use thin film material on the silicon substrate – b) The cross at the center of sensor is the micro heater – c) Sensor after packaged 6 Figure I.5: a) the nanowires are printed on the substrate – b) the first integrated nanowire sensor circuitry _ 7 Figure I.6: Microstructure of tin oxide _ 9 Figure I.7: The transform of Oxygen on the surface of SnO 2 NWs _ 9 Figure I.8: Physisorption and chemisorption steps involved in forming oxygen ion species on SnO 2 surface 10 Figure I.9: The depletion zone at the surface of nanowires and nanobelts _ 10 Figure I.10: SnO 2 is exposed in NO 2 gas: low temperature (a), high temperature (b) _ 11 Figure I.11: Direct contact among NW and metal electrode 11 Figure I.12: NWs junctions and potential barrier at the junction _ 12 Figure I.13: Equivalent circuit of total resistance of one networked nanowires _ 12 Figure I.14: VLS mechanism 13 Figure I.15: Changing of resistance of sensor when gas is in 14 Figure I.16: An example graph of the sensitivity versus temperature _ 16 Figure I.17: Heat losses to metal contacts, environment gas and irradiation[1] _ 18 Figure I.18: Model of a gas sensor using micro heater as thermal source[2] _ 20 Figure I.19: a) power and b) temperature of sensor depend on applied AC voltage[5] _ 21 Figure I.20: Temperature versus provided power[5] _ 21 Figure I.21: SEM image of a SnO 2 nanowire connected to two Pt microelectrodes fabricated with focused ion beam[6] _ 22 Figure I.22: The sensor setup and principal thermal losses in the suspended nanowire heated by the Joule heat SEM image of the suspended SnO 2 chemiresistor[13] 22 Figure I.23: Heat loss depends on temperature and dimension of the nanowires[13] 23 Figure I.24: Schematic description of surface modification by self-heating of a nanowire: nanoparticles were formed by hydrothermal reactionvia Joule heating of a Si nanowires [12] _ 24 Figure I.25: Network structured nanowire on the chip _ 25 Figure II.1: The structure of the interdigitated electrode array 26 Figure II.2: Processes to fabricate the IDE _ 27 Figure II.3: The CVD system 28 Figure II.4: Thermal cycle for fabrication SnO 2 _ 29

Figure II.5: The chamber for gas sensing investigation 30 Figure II.6: Schematic of gas-mixing part _ 31 Figure II.7: The SourceMeter, Keithley model K2602A _ 31 Figure II.8: Flow control of the resistance measurement by loading constant power 32 Figure III.1: Image of samples and be marked from b1 to b5 ( left to right) 34 Figure III.2: XRD pattern of SnO 2 NWs at 800 o C 34 Figure III.3: Typical FE-SEM images of junction structured tin oxide nanowires oh the 60 μm spacing PIEs a) sample b1 – b) sample b2 – c, d) sample b3 – e, f) sample b4 – g, h) sample b5 35 Figure III.4: The higher magnification SEM image of samples _ 36 Figure III.5: The I-V curve of sample b2 _ 37 Figure III.6: a) The dependence of sensor resistance on temperature and b) loaded power _ 38 Figure III.7: The response of sample b3 to 2.5 ppm NO 2 a)using external heater b) and loading constant electrical power _ 39 Figure III.8: Response time and recovery time of sample b3 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 40 Figure III.9: The resistance of samples at various temperatures _ 42 Figure III.10: Schematics of equivalent circuit of samples with different number of nanowires junction _ 43 Figure III.11: Resistance of sample b1 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm of NO 2 gas using external heater and by loading constant powers a) 150 o C – a’) 16 mW – b) 200 o C – b’) 18 mW – c)

250 o C – c’) 20 mW– d) 300 o C – d’) 22 mW _ 44 Figure III.12: The base resistance of sample b1, a) using external heater, b) using self-heating and c) linear fitting of resistance using external heater and self-heating effect _ 45 Figure III.13: The response of sample b1 using external heater and self-heating 46 Figure III.14: 3D graphs in different angles of response of sample b1 to various concentrations of NO 2 gas at several temperature or loading power _ 47 Figure III.15: Response time and recovery time of sample b1 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 47 Figure III.16: Exponential fitting of a) response time and b) recovery time of sample b1 using external heater and self-heating effect 48 Figure III.17: Relation between loading power (mW) and temperature ( o C) of sample b1 by four comparing methods: base resistance, response, response time and recovery time _ 49 Figure III.18: Resistance of sample b2 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm of NO 2 gas using external heater and by loading constant powers a) 150 o C – a’) 20 mW – b) 200 o C – b’) 25 mW – c)

250 o C – c’) 30 mW– d) 300 o C – d’) 35 mW _ 50 Figure III.19: The base resistance of sample b2, a) using external heater, b) using self-heating and c) linear fitting of resistance using external heater and self-heating effect _ 51 Figure III.20: The response of sample b2 using external heater and self-heating 52

Figure III.21: Response time and recovery time of sample b2 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 52 Figure III.22: Exponential fitting of a) response time and b) recovery time of sample b2 using external heater and self-heating effect 53 Figure III.23: Relation between loading power (mW) and temperature ( o C) of sample b2 by four comparing methods: base resistance, response time and recovery time _ 53 Figure III.24: Resistance of sample b3 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm of NO 2 gas using external heater and by loading constant powers a) 150 o C – a’) 30 mW – b) 200 o C – b’) 40 mW – c)

250 o C – c’) 50 mW– d) 300 o C – d’) 60 mW _ 55 Figure III.25: The base resistance of sample b3, a) using external heater, b) using self-heating and c) linear fitting of resistance using external heater and self-heating effect _ 56 Figure III.26: The response of sample b3 using external heater and self-heating 57 Figure III.27: 3D graphs in different angles of response of sample b3 to various concentrations of NO 2 gas at several temperature or loading power _ 57 Figure III.28: Response time and recovery time of sample b3 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 58 Figure III.29: Exponential fitting of a) response time and b) recovery time of sample b3 using external heater and self-heating effect 58 Figure III.30: Relation between loading power (mW) and temperature ( o C) of sample b3 by four comparing methods: base resistance, response, response time and recovery time _ 59 Figure III.31: Resistance of sample b4 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm of NO 2 gas using external heater and by loading constant powers a) 150 o C – a’) 90 mW – b) 200 o C – b’) 120 mW – c)

250 o C – c’) 150 mW– d) 300 o C – d’) 180 mW _ 60 Figure III.32: The base resistance of sample b4, a) using external heater, b) using self-heating and c) linear fitting of resistance using external heater and self-heating effect _ 61 Figure III.33: The response of sample b4 using external heater and self-heating 62 Figure III.34: Response time and recovery time of sample b4 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 63 Figure III.35: Exponential fitting of a) response time and b) recovery time of sample b4 using external heater and self-heating effect 63 Figure III.36: Relation between loading power (mW) and temperature ( o C) of sample b4 by four comparing methods: base resistance, response time and recovery time 64 Figure III.37: Resistance of sample b1 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm of NO 2 gas using external heater and by loading constant powers a) 150 o C – a’) 300 mW – b) 200 o C – b’) 350 mW – c)

250 o C – c’) 400 mW– d) 300 o C – d’) 450 mW _ 65 Figure III.38: The base resistance of sample b5, a) using external heater, b) using self-heating and c) linear fitting of resistance using external heater and self-heating effect _ 66 Figure III.39: The response of sample b5 using external heater and self-heating 67

Figure III.40: 3D graphs in different angles of response of sample b5 to various concentrations of NO 2 gas at several temperature or loading power _ 67 Figure III.41: Response time and recovery time of sample b5 to 2.5 ppm NO 2 a)using external heater b)and loading constant electrical power _ 68 Figure III.42: Exponential fitting of a) response time and b) recovery time of sample b5 using external heater and self-heating effect 68 Figure III.43: Relation between loading power (mW) and temperature ( o C) of sample b5 by four comparing methods: base resistance, response, response time and recovery time _ 69 Figure III.44: The response of sample b3 to mixed gases at different temperature using external heater 70 Figure III.45: The response of sample b3 to mixed gases using self-heating effect at different loading powers 71 Figure III.46: The response of sample b3 to mixed gases using external heater at different loading powers and 300 o C background using external heater _ 72 Figure III.47: The loading power to sample b2 versus measured temperature; and the temperature mapping

of the sample at 40, 114, 199.5 and 253 o C _ 73 Figure III.48: The relation between loading power to sample b2 and temperature obtained by four methods: response time, themal emission, base resistance and recovery time _ 74 Figure III.49: The relation between loading power and actual temperature of samples from b1 to b5 using fitting recovery time _ 75 Figure III.50: The response of loaded 20 mW sample b2 to several gases: H 2 , NH 3 , H 2 S, Ethanol and NO 2 77 Figure III.51: The response of sample b2 to various gases H 2 , NH 3 , H 2 S and Ethanol at different loading power (left side – 40 mW, right side – 50 mW) 78 Figure III.52: The response of sample b2 to reducing gases versus loading power _ 78 Figure III.53: The repeatability of sample b2 to 2.5 ppm NO 2 using self-heating at 20 mW 79 Figure III.54: A small and long lasting gas sensor device fed by a battery 80

List of Tables

Table 1: Example applications and markets for gas sensors 3 Table 2: Self-heating rate of samples 76 Table 3: Compare power consumption and price of some electrodes 80

Acronyms and Abbreviations

In the previous studies, the sensing layer of gas sensor is typically based on nanostructured semiconducting metal oxides, such as SnO2, In2O3, ZnO, WO3, TiO2,

et cetera In particular, SnO2 materials have many advantages such as high-sensitivity capability, low resistance; thus, the applicability of SnO2 is much greater than other materials The sensitivity of sensor based on SnO2 nanowires (NWs) is investigated, and commercial products are available on the market; however, there are some disadvantages in those products such as low selectivity, high working temperature, large power consumption and most of them work as stationary devices

In this study, the performance of sensor based on SnO2 to analyte gases were improved, and the power consumption of the devices were decreased by self-heating effect of nanowires The provided power for self –heating nanowires is much smaller than the external heater, thus it saves tons of power and makes possibility handy or mobile gas sensor devices The removing external heater also reduces the difficulty

of device’s synthetic and takes the cost down The replacement of external heater by self-heated nanowires reduces the gas sensor’s cost due to using plain structure of sensor as well as simple fabrication process

The high performance, low energy consumption, cheap gas sensors based heating NWs give us a better monitoring and detecting gas tool to protect and control the quality of air and environment in urbanization and factory zones

self-2 Objective of the study

The study aims to investigate the equivalent performance of gas sensors working at room temperature and powered by specific electrical energy in comparison

to ones working at high temperature Under this determination, the suitable applied powers are found to drive directly the gas sensors without external heater Every measuring gas has a most reactive temperature; therefore, the gas sensor device can

be adjusted quickly to appropriate temperature by changing the supplied power to detect or monitor the presence of identical gases Moreover, decreasing power consumption of the gas sensor is also a object of this thesis

3 New features of the thesis

Recent researches about the self-heating effect only focus on single nanowires, but my study investigates the self-heating effect on network-structured nanowires They were obtained on the thermal isolated glasses by simple CVD process It helped the nanowires rich high temperature by small supplied power (milli Watts)

4 Methodology

By inheriting the previous researches and information from international articles, the tin oxide was chosen as materials and NO2 as analyte gas Then the growth of tin oxide nanowires on the glass was repeated to find out the standard procedure After that, the sample was loaded a constant power to test the maximum power they can withstand Then the gas sensing characteristics of samples was investigated in several conditions The morphology of the nanowires on the glass substrates was investigated by SEM imaging After all, the best performance gas sensor was reassessed the gas sensing properties such as the stability, the selectivity,

to name a few

CHAPTER I – OVERVIEW

I.1 Brief history of the evolution of gas sensors

Generally, sensors can be divided into three major categories: physical sensors, chemical sensors, and biosensors Gas sensors belong to both physical and chemical sensors group Gas sensors are widely used in many different applications, and gas sensing technology has become a basic enabling technology in many fields, which is described in Table 1

Water treatment Cl2, CO2, O2, O3, H2S

Steel production O2, H2, CO, conventional pollutants

Table 1: Example applications and markets for gas sensors

Gas detection is developed early in the beginning of the Industrial Revolution, when the fuel became very important Then the safety in coal mine is critical People working in these areas was threatened by methane This gas especially is dangerous because it cannot be seen or smelled, and appears naturally from the ground It makes workers hard to breath or causes an explosion When miners began to realize how dangerous methane was, they started exploring ways to detect it The first gas detector was the flame safety lamp, also known as Davey’s lamp, which was invented by Sir Humphry Davey in 1815 The flame in the lamp indicates the level of methane and

oxygen in atmosphere; higher flame means methane gas presents; lower flame means low oxygen However, this kind of lamp was not capable of detecting other hazardous environments

Figure I.1: a) Davey’s lamp – b) Jiro Tsuji and his gas detector using ligh-wave

interference – c) Johnson – Williams’s founder

In the 1920’s a number of significant advancement in the field of gas detection came into play In Japan, Dr Jiro Tsuji developed a method of detecting combustible gases using light-wave interference in 1925 Dr Tsuji later went on to found Riken Keiki Co., Ltd, currently represented in North America by RKI Instruments In 1927,

Dr Oliver Johnson of the Standard Oil Company developed a method of detecting combustible gases using a platinum catalyst in a Wheatstone bridge electronic circuit

Dr Johnson later went on to found Johnson-Williams or J-W, one of the first gas detection companies in the United States

Figure I.2: The schematic of the platinum catalyst type of gas sensor

Late years of 1960’s are mile stones for a new chapter of gas detection EC Sensor Ecolyzer company pioneered the development of electrochemical sensors and gas monitors in 1969 J-W company introduced world’s first portable Oxygen meter using galvanic cell (Clark Cell) in 1970 Since 1969, Taguchi from Japan also had been developed new technology sensor type, based on metal oxides, permitted low cost detection of many gases and vapors The sensors which are studied in this thesis belong to this kind of gas sensors; therefore, this type of sensors was focused in this thesis despite of there are many others kind of gas sensors based on different methods and mechanism such as Photo Ionization Detectors, Flame Ionization Detectors, Infrared or Ultraviolet Absorption, Thermal Conductivity, Colorimetric, Light Scattering, to name a few

Figure I.3: a) The structure of resistive gas sensor – b) The working principle using voltage meter, Vh, Vc, Vout, and R L , which represent the heating voltage, circuit voltage, signal voltage, and load resistor, respectively – c) The working

principle using current meter

The gas adsorption influence on semiconductor conductance that is researched

by Brattain W and Bardeen J from 1953, but who play the main role in semiconductor gas sensors those are Taguchi N and Seiyama T They established Figaro company, which produces gas sensors based on metal oxide semiconductors only Their main products based on ZnO and SnO2 materials, from bulk or thick film

to thin film Generally, the working principle of this type of gas sensors is described

in the Figure I.3 The sensors still work in high temperature so an external heater like Ni-Cr is an important part Then, the resistance of the sensors (gas sensitive layer of semiconductor material) are measured and calculated by voltage meter (Figure I.3b)

or current meter (Figure I.3c) Following the changing of the resistance of sensors in the presence of analyte gases, the concentration of gases can be determined However, this generation of gas sensors using bulk semiconductor material have some disadvantage such as low sensitivity and require a large amount of power to the external heater

Figure I.4: a, b) The sensors use thin film material on the silicon substrate – b) The cross at the center of sensor is the micro heater – c) Sensor after packaged

Come up with the development of micro-fabrication technology, the next generation of semiconductive gas sensor bases on thin film material The size of material downs to nano scale so it raises the response of sensors remarkably and improves the stability of sensors Additionally, the micro fabrication processing helps the sensors be resized smaller so the micro external heater consumes much smaller energy than the previous generation still provide indispensable temperature The Figure I.4 shows the examples of gas sensor which use thin film material and be fabricated by micro technology

However, the gas sensors based on thin films materials still present some weak-point such as slow response and recovery time, poor long-term stability anf gas selectivity Therefore, nanowires materials and one-dimensional materials are main characters in the development of gas sensors in recent years Inheriting the advantages

of previous generation, these gas sensors based on nanowires require small energy and give better performance Besides, there is a problem with the transferring nanowire materials to the substrate of devices Using some method such as drop-coating, spin-coating and so on will not give good adhesion between the nanowires and the substrate

Figure I.5: a) the nanowires are printed on the substrate – b) the first integrated

nanowire sensor circuitry

Ali Javey et al as known as the one fabricated successfully the first integrated nanowire sensor circuitry by using contact printing method His research opened a new way to transfer nanowires to the circuit substrate and miniature size of the sensors Another approach is growing the nanowires directly on the substrate This

method promises the very well contacting between the nanowires and the substrate will be obtained and reducing the damage while transferring

However, the sensors based on nanowires and micro heater have a weakness that they consume high power, and self-heating effect on the nanowires is a solution for this problem Gas sensors based on self-heating nanowires consume very small power then working duration of gas sensor is increased and the cost for energy, maintenance or electric recharge (for portable devices) also reduced Thus, self-heating effect promises to be a key role for the near future of gas sensors, and the first devices based on it can be announced next few years

I.2 Brief knowledge about gas sensing devices and SnO 2 nanowires materials

Figure I.6: Microstructure of tin oxide

SnO2 has crystal structure, which is the tetragonal structure in rutile phase SnO2 is n-type semiconductor and the band gap Eg = 3.6 eV SnO2 is promising for gas sensing applications due to its suitable physicochemical properties, including high stability and reactivity to reducing gases such as hydrogen, carbon monoxide Recently, macrostructure forms of SnO2 have been used for gas sensing applications

When SnO2 is exposed to the air, oxygen molecules are adsorbed on the surface The adsorbed oxygen molecules extract electrons from SnO2, forming oxygen ions on the surface

Figure I.7: The transform of Oxygen on the surface of SnO 2 NWs

At the low temperature, under 200 oC, Oxygen is in form of O2 At the higher temperature, Oxygen is in form of O and 2

O  like equations below:

Figure I.8: Physisorption and chemisorption steps involved in forming oxygen ion

species on SnO 2 surface

Since SnO2 is known to be a native n-type semiconductor, the extraction of electrons makes a depletion region on the surface that leads to the increase in the resistance of the nanowires/nanobelts

Figure I.9: The depletion zone at the surface of nanowires and nanobelts

When SnO2 is exposed in NO2, the mechanism is also the same NO2 gas is adsorb on the surface of SnO2 and makes a depletion zone At the lower temperature, this equation below occurs:

The processes is demonstrate like figure below

Figure I.10: SnO 2 is exposed in NO 2 gas: low temperature (a), high temperature

(b)

Because the mechanism is quite the same, the resistance is slightly increased, especially in case of “short-cut” – the direct contact among nanowires/nanobelt and metal electrode

Figure I.11: Direct contact among NW and metal electrode

However, in case of junctions bridged, there is an additional mechanism for the higher sensitivity In other words, there are nanowire/nanowire junctions at the networking points, a feature available in this network-structured sensor but missing

in the single-nanowire devices Such a junction, when exposed to NO2, should form

a depleted layer around the intersection and thus block the electron flow in a way which is more efficient than the surface depletion of single nanowires with metal contacts

Figure I.12: NWs junctions and potential barrier at the junction

Thus, the enhanced sensitivity of our sensor can be attributed to the changes

in the resistance of the gas sensor due to both a surface depletion region of each nanowire and the potential barrier height in the junction The Figure I.13 below demonstrates the equivalent circuit of total resistance of one networked nanowires It shows the high effect of potential barrier

Figure I.13: Equivalent circuit of total resistance of one networked nanowires

c) Mechanism for the growth of nanowires

The vapor–liquid–solid method (VLS) is a mechanism for the growth of dimensional structures, such as NWs The VLS mechanism consists of three stages which are illustrated in Figure I.14 below:

one-Figure I.14: VLS mechanism

First, a metal particle absorbs semiconductor material and forms an alloy In this step, the volume of the particle increases and the particle often transitions from a solid to a liquid state Second, the alloy particle absorbs more semiconductor material until it is saturated The saturated alloy droplet becomes in equilibrium with the solid phase of the semiconductor, and nucleation occurs (i.e solute/solid phase transition) During the final phase, a steady state is formed in which a semiconductor crystal grows at the solid/liquid interface The precipitated semiconductor material grows as

a wire because it is energetically more favorable than extension of the solid-liquid interface

There are some typical properties associated with gas sensors based on metal oxide semiconductors, a kind of chemical sensor, as follows:

 A sensitive layers or materials are in chemical contact with the analyte gases and a change in the chemistry of the sensitive layers or materials (a reaction) is produced after exposure to the analyte

 The sensitive layers or materials are on a platform or device that allows transduction of the change to electric signals And they operate in real time

 The sensors devices are physically small and typically less expensive and more convenient than an equivalent instrument for the same measurements

Besides, quality and performance of a gas sensor device also are evaluated by some parameters like: sensitivity, response time and recovery time, selectivity, optimal working temperature, and stability

a) Response and Sensitivity

Sensitivity is the ability of a sensor to detect a gas with an individual concentration value of this gas Response of a sensor is the changing in physical characteristic (normally resistance) between absence and presence of a certain concentration of analyte gas Usually, the sensitivity is determined by the slope of response versus the concentration of gas

Response is denoted S and is defined as the ratio:

gas or p-type sensor, oxidant gas

or p-type sensor, reducing gas

Where, Rair is stable resistance of the sensor in air (Ra)

Rgas is stable resistance of the sensor in mixture of gas that includes target gas (Rg)

b) Response time and recovery time

Response time is the changing time since the target gas appeared until the resistance of sensor reached a stable value For calculating, response time is the changing time to 90% (or 10%, depend on the kind of sensor material and target gas are n-type or p-type) of absolute final value of sensor resistance

Recovery time is the changing time from when the target gas was cut off until the resistance of sensor returns to its initial value For calculating, recovery time is the changing time to 10% (or 90%, depend on the kind of sensor material and target gas is n-type or p-type) of absolute initial value of sensor resistance

For a gas sensor, the response time and recovery time are smaller; the performance of the sensor is higher

c) Selectivity

Selectivity is the sensing ability of the sensor for an individual gas in the gas mixture The presence of other gases has no effect or little effect on the change of the sensor Selective ability of the sensor depends on several factors such as manufacturing materials, types of impurities, impurity concentration and working temperature of the sensor

d) Optimal working temperature

Temperature is a factor that has a huge influence to the sensitivity of a sensor For each sensor, there is always a temperature at which sensitivity reaches the maximum value This temperature are optimal working temperature, denote as TM Sensitivity depends on the temperature graph usually takes the form shown in Figure I.16

Figure I.16: An example graph of the sensitivity versus temperature

e) Stability

Stability is the ability of the sensor to work stability after prolonged use The measurement gives the same value in the same environmental conditions for a long time and after a large number of cycles

I.3 Joule heating and its application in gas sensing devices

Because the active energy of reaction between nano materials and almost gas targets are quite high so the working temperature of gas sensor around 100 to 400 oC

To provide such lofty temperature, an external heater is required as a part of gas sensor devices A weakness of the micro heater that it consumes a large power, usually from hundreds of milli Watts to few Watts It is not a problem for a gas sensing station, but a big headache for a handy device because battery runs out fast Therefore, self-heating of nano material and optimization of devices to reduce heat dissipate, which are keys to make low power consumption gas sensing devices

Firstly, principal of self-heating is considered Self-heating effect is based on Joule’s first law, also known as Ohmic heating and resistive heating, is the process

by which the passage of an electric current through a conductor releases heat The amount of heat released is proportional to the square of the current which is described

by following equation:

Where Q is released heat (Joule)

I is electric current (Ampere)

R is resistance of conductors (Ohm) The nanowires or thin films are conductors themselves then released heat by conducting heat them up to high working temperature Thus, the devices can work without external heater; that is a big advantage It helps the fabrication process of devices is easier and save tons of cost However, the heat produced by Joule’s effect

is small due to tiny power that nano materials consume so thermal insulation between materials and other parts of devices are critical

According to Equation 1, electric current is the most important factor to the Joule heating Furthermore, diameter of nanowires or grain size of thin film (D), and width of depletion zone (W) impact directly to the electric current It is demonstrated

by follow equation:

of nanowires Heat from the center must warm up to depletion zone outside to react with target gas If 𝐷 ≫ 𝑊, there is enough heat to rise whole nanowires to lofty temperature, and changing of depletion zone depth is not a big deal to cross section area so temperature and heat transfer in nanowires itself is more stable However, the response of gas sensors may not be high because the changing of depletion zone depth

is incomparable to the diameter of nanowires If 𝐷 ≈ 𝑊, the nanowires are not heated

up to working temperature; and whenever the reaction between nanowires and gases occurred, the temperature in the center of nanowires is greatly changed due to fast decreasing of conduction channel This case is more suitable for devices, which use the external heater because it does not care about suddenly changing of temperature when exposing to gases, and it gives the better response than the previous case

Figure I.17: Heat losses to metal contacts, environment gas and irradiation[1]

Heat from nanowires also transfers to other parts of the device such as metal pads, environment gas and irradiation[1] which is illustrated like Figure I.17 The losing heat to the metal contacts is demonstrated by following equation:

Where S is contact area between nanowires and metal pads

k is heat conductivity of nanowires

T is temperature at the center of nanowires

To is temperature of environment The heat loses to environment gas is described by following equation:

𝑃𝑅 ≈ 𝜀𝜎𝑆∗(𝑇4+ 𝑇3𝑇0+ 𝑇2𝑇02 + 𝑇𝑇03− 4𝑇04)/5 (Equation 5)

Where 𝜎 is Boltzmann constant

𝜀 is emission coefficient Losing heat cannot be avoided, but can be minimized Reducing heat loses by irradiation is impossible, decreasing heat loses to environment gas is negligible, but lessening heat loses to metal pads is considerable Therefore, trying to isolate the nanowires and metal pads will reduce heat losing significantly

Woo-Jin Hwang and his colleagues fabricated a gas sensor which uses an optimized micro heater[2] All the sensor is built on a silicon substrate The material

was used to make heater and sensing membrane are poly silicon and silicon nitride, respectively

Figure I.18: Model of a gas sensor using micro heater as thermal source[2]

The silicon volume under the sensing area is removed to avoid heat dissipate

to this volume Note that, silicon is a good thermal conductivity material, but the thermal capacity of this material also is high Therefore, hanging the sensing materials

is a good strategy to reduce heat loss

There are much research and publication about the self-heating effect on nano structured, which apply to the gas sensor Most of them are based on thin film materials or single nanowire

Earlier years, gas sensors based on the self-heating effect on nano thin film was investigated and achieved lots of succeeds Several kinds of material layers were used such as SnO2, WO3[3] and NiO[4] to detect CO, benzene and formaldehyde gas, respectively

In 2003, Salehi published his research about highly sensitive self heated SnO2

carbon monoxide sensor[5] Figure I.19 demonstrates obtained power and temperature when applied AC voltage to the gas sensor From that figure, the self heated sensor has a heating efficiency of about 87oC/W

Figure I.19: a) power and b) temperature of sensor depend on applied AC

voltage[5]

Thus, the sensor needed to be provided a large power to reach the high temperatures, about 1.5 W to rise up to 150 oC, which is illustrated by Figure I.20 below:

Figure I.20: Temperature versus provided power[5]

Recent years, gas sensors based on nanowires are drawn the attentions of researchers There were some publications about the self-heating effect on gas sensors, which used nanowires, and most of the authors studied Joule heating on

single nanowires[1], [6]–[12] In 2008, J D Prades and his colleagues published their research about model of a SnO2 nanowire connected to two Pt microelectrodes fabricated with focused ion beam[6] like Figure I.21 They compared the response to

NO2 gas of this device using external heater and using self-heating, then concluded that this device operated under optimal conditions for NO2 sensing with less than 20

μW to both bias and heat them, which is significantly lower than the 140 mW required for the external micro heater to obtain 175 oC on the nanowires An important note

by these authors, undesired aging and failure of the nanowires are considerable issue

Figure I.21: SEM image of a SnO 2 nanowire connected to two Pt microelectrodes

fabricated with focused ion beam[6]

In 2009, Evgheni Strelcov calculated the heat loses to environment (air), metal contacts and irradiation in a self-heated suspended nanowires like Figure I.22 According to his research[13], the heat loses most to the air, and the irradiation is eligible, which is described in Figure I.23 When the nanowires is longer, the lesser heat loss to the metal contacts

Figure I.22: The sensor setup and principal thermal losses in the suspended nanowire heated by the Joule heat SEM image of the suspended SnO 2

chemiresistor[13]

Figure I.23: Heat loss depends on temperature and dimension of the

heating Lower sensitivity and faster response were observed as higher voltage was applied, that is a truly evidence of self-heating effect in nanowires They expected the temperature is 135 oC when applied 25 V to the nanowires in air environment

Figure I.24: Schematic description of surface modification by self-heating of a nanowire: nanoparticles were formed by hydrothermal reactionvia Joule heating

of a Si nanowires [12]

I.4 Research’s approach

In this thesis, we suppose to investigate the self-heating effect on the networked nanowires for the gas sensor By using the networked NWs grown on chip, the difficulties of single NW device fabrication could be solved Thermal CVD process was selected to deploy tin oxide nanowire material on to the desired area on the gas sensor chip This method was preferred due to the low cost and simple processing Meanwhile the networked NW sensor inherits the advantages of using self-heating effect by neglecting extra microheater and lowering power consumption Additionally, the junction structure among NWs would bring the higher gas response The illustration for fabricated sensor is shown in Figure I.25

Figure I.25: Network structured nanowire on the chip

Obviously, comparing to the single NW device, the networked one consumes

a greater electric power The lager NWs area would require the higher applied electric power Hence, the sensor size should be optimized depending on the fabrication technique

CHAPTER II – EXPERIMENTAL

II.1 Preparation of Interdigitated Electrode (IDE)

II.1.1 Designing of electrodes

The structure of the IDE was made by CorelDRAW software The design of the IDE is illustrated in Figure II.1 below The IDE includes five smaller parts; each part is a sensor which is named from b1 to b5 The space between the digits is 60 µm The difference between sensors that is the length of gaps From b1 to b5, the lengths are 30 µm, 90 µm, 270 µm, 810 µm and 2.43 mm respectively This difference serves

a purpose that investigates the resistance and power consumption between sample (b1

to b5) to find out the best design for further application The real IDE includes four layers: Pt, SnO2, Pt and SnO2 on the glass substrate

Figure II.1: The structure of the interdigitated electrode array

II.1.2 Electrode fabrication

The IDE was fabricated on a glass substrate, and all the processes are performed in clean room at ITIMS The processes to fabricate IDE is demonstrated

as Figure II.2 below Firstly, a layer of photoresist was coated on the substrate Then printed on the substrate using photolithography technique Next step, a sequence of SnO2, Pt then SnO2 and Pt layers were sputtered on the wafer The bottom SnO2 layer ensures the contacting to the first Pt layer and the silicon oxide The first Pt layer is the electric conducting channel The second SnO2 layer protects the conducting Pt

layer from damaging in nanowires growth progress The top Pt layer acts as a catalytic for nanowires growth in VLS mechanism Finally, the wafer is soaked in acetone to lift off photoresist layer

Figure II.2: Processes to fabricate the IDE

After liftoff

Sputtered Sputtering 4 layers

After developed After photolithography

After coated a layer of photoresist The substrate

II.2 SnO 2 nanowires growth

II.2.1 Equipment, apparatus and chemical preparation

Used equipments and chemicals to grow tin oxide nanowire include the CVD system (like Figure II.3), quartz tubes, alumina boat, tin powder, Ar gas and O2 gas

Figure II.3: The CVD system

The apparatuses must be clean before do the experiment to prevent contamination and obtain higher vacuum while is in CVD process The quartz tube and alumina was cleaned boat by soaking in HF solution 1% on one day Then use de-ionization water in the last washing and dry them by heater Next, 20 mg tin powder was put into the alumina boat and put the boat together with the electodes at the center of the quartz tube Then, the quartz tube was put in the furnace like Figure II.3

To grow SnO2 nanowires, the 4-step thermal CVD process as the following description was employed:

Step 1: Close the O-ring; turn on the vacuum pump to exhaust air in the quartz

tube When the pressure in the tube comes to 2-3x10-1 torr, close pump valve and open

Ar gas valve and adjust the flow of about 300 sccm until the pressure up to atmosphere After that, close Ar valve and open pump valve Repeat this process about three times, it will be exhausted nearly all the oxygen in the tube At the end of