Luận văn thạc sĩ synthesis of sele heating gas sensor based on tin oxide nanowire material

Vaut, and R,, which represent the heating voltage, circuit voltage, signal valrage, and load resistor, respectively - ¢ The working principle using current meter 5 Figure 1.4: a, b The s

SYNTHESIS OF SELF-HEATING GAS SENSOR BASED ON TIN

OXIDE NANOWIRE MATERIAL

MASTER’S THESIS ELECTRONIC MATERIALS SCIENCE AND ENGINEERING

Hanoi - October 2014

NTERNATIONAL TRAINING INSTITUTE FOR MATERIALS SCTENCE

MASTER’S THESIS

Synthesis of self-heating gas sensor based on

tin oxide nanowire material

HA MINH TAN

Student 1): CB120169 Advisors: PhD NGUYEN VAN DUY

A thesis submitted to Ta Quang Buu library, [anoi 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

“T, the candidate, hereby certify that the thesis comprises only my origimal

work except where indicated; due acknowledgment has been made in the text to all

Acknowledgements

Firstly, T would like (o express the deepest appreciation Lo 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

Resides, T wontd like to thank (o all members in Gas Sensor Group at TTIMS:

for helping me during all the time 1 do my work

Finally, thank to all my friends and family for caring, and inspiring me all the

time 1 do project

13 Joule heating and its application in gas sensing devices cine we dF

13.2 Overview about gas sensing devices based on self-heating effect 20

I2 SnÔ;nanowires growth

IL2.1 Equipment, apparatus and chemical preparation

14 60s sensing properties investigation

Ik4.2 Gas sensing characterization of sensor using self-heating effect 38

CHAPTER Ill RESULTS AND DISCUSSION

1.1 Structure and morphology of grown nanowires

W2 Existence of self-heating effect of nanowire networked sensors a7 1.3 Gas sensing performance comparison between sensors heated by external heater ond

0.4.2 Temperature evaluation using thermat emission microscope we TB JW5 - Se-heating rate TH mi 75 L6 _ Capobilty of gas sensar using self-heating effect 77

1.6.3 Significance of self-heating affect in gas sensing applicatians 9

CONCLUSION

References

Vaut, and R,, which represent the heating voltage, circuit voltage, signal valrage, and load resistor,

respectively - ¢) The working principle using current meter 5 Figure 1.4: a, b) The sensors use thin jm material on tne silicon substrate ~ b) The cross at the center of

Sensor is the micro heater ~ ¢} Sensor after packaged 6 Figure 1.5: a) the nanowires are printed on the substrate — b) the first integrated nanowire sensar circuitry_ 7

Figure 1.7: The transfarm of Oxygen an the surface pƒ Šn0; NWWS 3 Figure 1.8: Physisorption and chemisorption steps involved in forming axygen ion species on Sn surface 20 Figure 1.9: The depletion zone at the surface of nanowires and nanobelts 10 Figure 1.20: SnO> 1s exposed in NO> gos: low temperature (a), high temperature {b) 1 Figure 21: Direct cantact among NW end metal electrode 4 Figure 1.22: NWs Junctions and potentie barrler at the junction, 1 Figure 1.24: Equivalent circuit of total resistonce of ane networked nanowires 12

Figure 1.15: Changing of resistance of sensor when gas is in 4 Figure 1.26: An example graph of the sensitivity versus temperature 16 Figure 1.27: Heat fosses to metal contacts, environment gas and irradiiation[1J 38 Figure 1.28: Mode! of a gas sensor using micro heter as thermal source[2] 20 Figure 1.19: a) power and b} temperature of sensor depend on applied AC voltggel5] 2

Figure 1.21: SEM image of a SnG: nanawire connected to twa Pt microelectrades fabricated with focused ian

Figure 1.22: The sensor setup and principal thermal lasses in the suspended nanowire heated by the Joule heot, SEM image of the suspended SnO; chemirasistor[13] 22 igure 1.23: Heat loss depends on temperature and dimension of the nanowires[13] 23 Figure 24: Schematic description of surface modification hy self-heating af a nanowire: nanaparticles were Jormed by hydrothermal reactionvia Joule heating of a Si nanowises [12) 2 Figure 1.25: Network structured nanowire on the chip, 25 Figure 1.1: The structure of the interdighated efectrode array 26

Figure 11.5: The chamber for gas sensing investigation 3ñ

Figure 11.7: The SoureeMeter, Keithley model K2602A 31 Figure 11.8: Flow control of the resistence measurement by loading constant power 32 Figure 1.2: image of samples and be marked fram b1 ta b5 { left to right) 34

Figure 11.3: Typical FE-SEM images of junctian structured tin oxide nanowires oh the 60 jim spacing PIES a) Sample bã —b) sample 62 —c, d} sample b3 — e, f} sample b4 — g, h) sample E5 35 Figure 11.4: The higher magnification SEM image of samples 36

Figure 1.6: a) The dependence of sensor resistance on temperature and b} loaded power 38 Figure 11.7: The response of sample b3 to 2.5 ppm NO: ajusing external heater b) and loading constant

Figure 1N.8: Response time and recovery time of sample b3 to 2.5 ppm NO2 ajusing external heater bland

Figure 11.9: The resistance of samples at variaus temperatures az Figure i1t.10: Schematics of equivalent circuit of samples with different number of nanowires junction 43 Figure 11.12: Resistance of sample b1 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 pam of NO2 gas using external heater and by foading constant powers a) 250°C — a") 16 mW ~ bị 200°C - b') 18 mw —e}

Figure II.12: The base resistance of sample b1, o) using external heater, b} using self-heating and c} finear fitting af resistance using extemal heater and self-heating effect 45 Figure 11.13: The response of sample hi using external heater and self-heating, 46 Figure 1.18: 3D graphs in different angles of response of somple bi ta various concentrations of NOz gas at

Figure 11.15: Response time and recovery time of sample b2 ta 2.5 ppm NOe ajusing externaf heater band

Figure (1.16: Exponential fitting of a) response time and bj recovery time of sample bi using external

Figure II.17: Relation between loading power (mW) and temperature (°C) of sample b2 by four comparing methods: base resistance, response, response time and recovery time 49 Figure i118: Resistance of sample b2 in the presence of 2.5 ppm, 5 ppm, 10 ppm ond 20 ppm af NO2 gas using external heater and by loading constant powers a) 150 °C — 0") 20 mW- bj 200 °C- b') 25 mW —c}

Figure [IL18: The base resistance of sample b2, a) using external heater, b} using self-heating and ¢} finear fitting of resistance using external heater and self-heating effect SI Figure II.20: The response of sample b2 using external heater and self-heating, 52

Figure 11.21: Response time and recovery time of sampie b2 ta 2.5 ppm NOz ajusing externat heater band

Figure 11.22: Exponential fitting of a] respanse time and bj recovery time of somple b2 using external

Figure II.23: Relation between loading power (mW) and temperature (°C) of sample b2 by four comparing methods: base resistance, response time and recovery time 53 Figure 11.24: Resistance of sample b3 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 ppm af NO2 gas using external heater and by loading constant powers a) 150 °C - 0°) 30 mW - bj 200 °C — b’) 40 mW —c)

250°C —c'} 50 mW di} 300 %C — đ”] 6D mW 55

Figure 11.25: The base resistance of sample b3, a) using external heater, b) using self-heating and ¢} hnear Jitting of resistance using extemal heater and self-heating effect 56 Figure 11.26: The respanse of sample b3 using external heater and self-heating 57 Figure 11.27: 3D graphs in different angles of response of sample bi ta various concentrations of NOz gas at

Figure (IL.28: Response time and recovery time of sampie b3 to 2.5 pm NO: ø]using external heater bland

Figure 11.29: Expanential fitting of a) respanse time and b} recovery time of somple b3 using external

Figure 11.30: Relation between loading power (mW) and temperature (°C) of sample b3 by fous comparing methods: bose resistance, response, response time and recovery time sa Figure II.31: Resistance of sample b4 in the presence of 2.5 ppm, 5 ppm, 10 ppm and 20 pam af NO» gas using external heater and by foading canstant powers a) 150°C a} 90 mW - bj 200°C - b’) 120 mw-¢)

Figure 11.32: The base resistance of sample b4, o) using externol heater, b} using self-heating and c} linear fitting of resistance using extemal henter and self-heating effect 41 Figure 11.33: The response af sample ba using external heater and self-heating, “ Figure II.34: Response time and recovery time of sample ba to 2.5 ppm NO» ajusing externat heater bland

Figure 1.35: Exponential fitting of a] response time and bj recovery time of sample bd using external

Figure II.36: Relation between foading power (mW) and temperature (°C) of sample b4 by four comparing methods: base resistonce, response time and recovery time, 64 Figure 11.37: Resistance of sample BJ in the presence of 2.5 ppm, 5 ppm, 20 ppm and 20 ppm af NO2 gas using external heater and by loading constant powers a) 150 °C— ø'} 300 mW — b] 200 "€ — b'J 350 mW — c]

Figure 11.38: The base resistance of sample b5, 0} using externol henter, b} using self-heating and c} hnear Sitting of resistance using extemal heater and self-heating effect %6 Figure 11.39: The response of sample b5 using external heater and self-heating, a7

Figure 1.40: 3D graphs in different angles of response of samaple 5 ta various concentrations of NO: gas at

Figure 11.41: Response time and recovery time of sample hS to 2.5 ppm NOz ausing externat heater bland

Figure 11.42: Exponential jiteing of a) response time and b} recovery time of sample b5 using external

Figure 111.43: Relation between loading power (mW) and temperature (°C) of sample bS by fous comparing methods: bose resistance, response, response tlme and recovery time 69

Figure ill.44: The response of sample b3 to mixed gases ot different temperature using externaf heater _ 70

Figure (1.45: The respanse of sample b3 to mnfxed gases using self-heating effect at different louding powers

”

Figure 11.46: The respanse of sample b3 to mixed gases using external heater at different loading powers

Tigure II.47: The londing power to sample b2 versus measured temperature; and the temperature mopping

Figure 11.48: The relation between foading power to sample b2 and temperature obtained by four methods: response time, themal emission, base resistance and recovery time 74 Figure 11.49: The relation between loading power and actual temperature of samples from bh? ta bS using

Figure 11.50: The response dj looded 20 mW sample b2 to severe! gases: Hs, NMs, H2S, Ethanof and NO; _— 77 Figure 1.52: The respanse of sample b2 to varlous gases H„ NH›, HS and Ethanof at different loading power

Figure 111.52: The response of sample b2 to reducing gases versus loading power 78 Figure 11.53: The repeatability of sample b2 to 2.5 ppm NO2 using self-heating at 20 mW 7 Figure 11.54: 4 small and long fasting gas sensor device fed by ơ battery, 30

List of ‘l'ables Table 1: Example npplications and mmurkets [0r gas sEHS0V5 2s.zec.ecsce we

Table 2: Seif-heating rate of samples

Table 3: Compare power consumption and pric# 0ƒ some elertrodes ~ 8a

cvD Chemical Vapor Deposition

FE-SEM_| Ficld Emission Scanning Electron Microscopy

10L Interdigitated Ulectrode

PIE Patterned-Interdigital Electrodes

SEM Scanning Electron Microscopy

VIS Vapor — Táqmd — Solid

XRD X-Ray Diffraction

as CO, COa, NOx, S02, NHa, has increased from few to several tens of times higher

than the level allowed by intemational standards The measuring, monitoring and assessment of enviromental pollution are necessary; thus, gas sonsor plays a very

important role in this field

In the previous studies, the sensing layer of gas sensor is typically based on

nanostructured sciniconducling metal oxides, such as SuOz, InzO3, ZnO, WOs, Ti0s,

et cetera In particular, SnO, materials have many advantages such as high-sensitivity

capabilily, low resislance, thus, the apphealality of SiOz is much greater than other

materials ‘the sensitivity of sensor based on SuOz nanowires (NWs) is investigated, and commercial products are available on the market, however, there are some

disadvantages in those produets such as low seleclivily, high working temperature, large power consumption and most of them work as stationary devices

In this study, the performance of sensor based on SnQ) 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 sell-healed nanowires reduces the gas sensor’s vost due lo using plain structure of sensor as well as simple fabrication process

The high performance, low energy consumption, cheap gas sensors based self-

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

2 Objective af the study

consumption of the gas sensor is also a object of this Lhesis

3 New features of the thesis

Recent rescarelies about the scli-hoating offect only focus on single nanowires, but my study investigates the self-heating effect on network-structured nanowires They were obtained on the thermal isolaled glasses by simple CVD process Tt helped

the nanowires rich high temperature by small supplied power (milli Watts)

4 Methodology

Ly inheriting the previous researches and information from intemational

arucles, the tin oxide was chosen as malenals and NO2 as analyte gas Thon 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 withsland Then the gus sensing characicnstics of samples was investigated in several conditions ‘I'he morphology of the nanowires on the glass substrates was investigated by SEM imaging After all, the best performance gas sensor was reassessed Ilie gas sensing properties such as the stability, the sclectivily,

toname a few

CHAPTER I- OVERVIEW L1 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

Aulomotive On, Ha, CO, NOx, Hydrocarbons

Indoor air quality CO, CHu, humidity, CO2

Water treatment Ch, CO2, O2, Or, 11:8

Industrial safety Indoor air quality, toxic gases, combustible gases, O2

Petrochemical Hydrocarbons, conventional pollutants

Steel production Ox, Ta, CO, conventional pollutants

Military Explosive, chemical weapons deteclion, propellants

Aerospace Th, O2, COs, humidity

Table 1: Example applications and markets for gus sensors

Gas detection is developed early in the beginning of the Industrial Revolution, when the furl beeame very imporlanL Then the safety in coal mine is critioal People working in these areas was threatened by methane This pas especially is dangerous because il cannot be seen or smelled, aud appears naturally frou the ground TLnakes 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 (lame sufety 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

[DIEISTIV2IEU ii

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

interference — c) Johnson — Williams's founder

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

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

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 SnO; materials, from bulk or thick film

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

in the Figure 1.3 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

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 1.4 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 scnsors 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-

healing effect promises to bea key role for the near Culure of gas sensors, and the first

devices based on it can be announced next few years

L2 Brief knowledge about gas sensing devices and SuO2 nanowires materials

12.1 Microstructure and sensing mechanism of SnO, nanowires

a) Microstructure ot SnQ: material

Figure I.6: Microstructure of tin oxide

SnO, has crystal structure which is the tetragonal structure in rutile phase

SnOz is n-type semiconductor and the band gap Ke = 3.6 eV SnOa is promising for

gas sensing applications due to its suitable physicochemical properties, including high

stability and reactiviy to reducing gases such as hydrogen, carbon monoxide

Recently, macrostructure forms of SnO2 have been used for gas sensing applications

b) Sensing mechanism of Sn; nanowires

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

Oyigas) <=> Op ind) <= GF (ad) O- [nd) <= OP (adi + ŒT (attice

Figure 1.7: The transform of Oxygen on the surface of SnOz NWs

At the low temperature, under 200 °C, Oxygen is in form of O;, At the higher lemperalure, Oxygen is in form of o- and O% like equations below

Tigure I.8: Physisorption and chemisorption steps invelved in forming oxygen ion

species on SnOz 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

across grain boundaries

Figure 1.9: The depletion zone at the surface of nanowires and nanobelts

When SnO) is exposed in NO2, the mechanism is also the same NO? gas is

adsorb on the surface of SnO2 and makes a depletion zone At the lower temperature,

this equation below occurs:

NO

At higher temperature:

NO, 5.5) +€ > NO + Ở z2)

The processes is demonstrate like figure below

) +O; +2e > NO; +20"

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

Depletion

region

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 1.13 below

demonstrates the equivalent circuit of total resistance of one networked nanowires It

shows the high effect of potential barrier

c) Mechanism for the growth of nanowires

The vapor—liquid-solid method (VLS) is a mechanism for the growth of one-

dimensional structures, such as NWs The VLS mechanism consists of three stages

which are illustrated in Figure 1.14 below:

ly vly Td

Figure 1.14: 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:

gases and a change in the chemistry of the sensitive layers or materials (a reaction) is

produced after exposure to the analyte

transduction of the change to electric signals And they operate in real time

® The sensors devices are physically small and lypically less expensive and

more conveniem than an equivalent instrument for the same measurements

Besides, quality and performance of a gas sensor device also arc evaluated by

some parameters like: sensitivity, response time and recovery time, selectivity,

optimal working temperature, and stability

a) Response and Sensitivity

Sensilivily is the ability of a sensor to detect a gas wilh 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

Sea or R for n-type sensor, oxidant gas

o p-type sensor, reducing gas

Where, Rairis stable vesistanec of the sensor in air (Ra)

Ras is stable resistance of the sensor in mixture of gas that includes target gas (Re)

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 Vor calculating, response time is the changing, time to 90% (or 10%, depend on the kind of sensar material and target gas are n-lype or pelype) of absolute [inal value of sensor resistance

Recovery time is the changing time from when the target gas was cut off until

the resistance of sensor retums 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 2as is n-type or p-type) of absolute initial value of sensor resistance

For a gas scnsor, Ihe response time and recovery lime are smaller, he

performance of the sensor is higher

©) 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 hitle effect on the charye 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

time and after a large number of cycles

L3 Joule heating and its application in gas sensing devices

Because the active energy of reaction between nano materials and almost gas

1argels are quite Ingh so the working temperature of gas sensor around 100 to 400 °C

‘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 Walis lo [ew Watis Tl is mol a problem for a gas

sensing station, but a big headache for a handy device because battery rans out fast

Therefore, seH-healing of nano material and optimization of devices tn reduce heat

dissipate, which are keys to make low power consumption gas sensing, devices

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

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

by following equation

Where Q is released heal (Joule)

Lis electric current (Ampere) Ris resistance of conductors (Ohm)

The nanowires or thin films are conductors themselves them released heat by

conducting heat them up to high working temperature ‘Thus, the devices can work

wilhoul external heater, thal is a big advantage Tt 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 Liquation 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 ame (W) impacl dircetly lo the clectric curren TLis 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

occured, 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

Tleat from nanowires also transfers to other parts of the device such as metal pads, environment gas and irradiation[1] which is illustrated like Figure 117 The

losing heat to the metal contacts is demonstrated by following equation:

Where S is contact arca 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:

‘Another important factor is the irradiation of nanowires which is illustrated by following equation:

Pa CoS (P41 y + 1773 +713 —41G)/5 (Equation 3) Where ¢ is Boltzmann constant

¢ 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 ramowires and mclal pads will reduce heal, lasing significantly

Woo-Iin IIwang 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

Figure 1.18: 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

13.2 Overview about gas sensing devices based on self-heating effect

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, WOs[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 119 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 87°C/W

‘Thus, the sensor needed to be provided a large power to reach the high

temperatures, about 1.5 W to rise up to 150 °C, which is illustrated by Figure 1.20

Figure 1.20: Temperature versus provided power{5}

Revent years, gas sensors based on nanowires are drawn the allentions of

researchers There were some publications about the self-heating effect on gas

sensors, which used wanowires, and mast of the authors studied Joule healing on

HA MINH TAN —ITIMS - K2013

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 121 They compared the response to NO) gas of this device using external heater and using self-heating, then concluded

that this device operated under optimal conditions for NO) sensing with less than 20

1.W to both bias and heat them, which is significantly lower than the 140 mW required

for the external micro heater to obtain 175 °C on the nanowires An important note

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

CN ic

Figure I.21: SEM image of a SnOz 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 uradiation in a self-heated suspended nanowires like Figure [22 According to his research[13], the heat loses most to the air, and the irradiation is eligible, which is described in Figure 1.23 When the nanowires is longer, the lesser

heat loss to the metal contacts

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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 °C when applied 25 V to the nanowires in air environment,

SINW devik NYY device

Metal

precursor

solution

nanowire: nanoparticles were formed by hydrothermal reactionvia Joule heating

of a Si nanowires [12]

1.4 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 1.25

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 IL — EXPERIMENTAL IL.1 Preparation of Interdigitated Electrode (IDE)

ILL1 Designing of electrodes

‘The structure of the IDK was made by CorelDRAW software ‘The design of the IDE is illustrated in Figure II.1 below The IDE includes five smaller parts; each

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part is a sensor which is named from bl to bS The space between the digits

The difference between sensors that is the length of gaps From bl to bS, the lengths

a purpose thal investigates the

to b5) to find out the best design for further application ‘Ihe real LDH includes four

csistarice and power consumplion between sample (b1

layers: Pl, SuO2, PLand SnO©2 on the glass substrate

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Figure IL1: The structure of the interdigitated electrode array

IL1.2 Electredc fabrication

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

as Figure 11.2 below Firstly, a layer of photoresisl was coated on the substrale Then ptinted on the substrate using photolithography technique Next step, a sequence of SnOz, Pt then SnOz and Ft layers were sputtered on the wafer The bottom SnOz layer

onsures the conlacting to dhe first PL layer and the silicon oxide The first PL layer is

the electric conducting channel The second SnOz 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

IL.2 SnO2 nanowires growth

12.1 Equipment, apparatus and chemical preparation

Used equipments and chemicals to grow tin oxide nanowire include the CVD system (like Figure IL.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 yacuum 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

1L3

12.2 Growth procedure of SnO2 NWs at 800 °C

To grow SnO; 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 2-3:1 0" 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