graduation thesis ha minh tan final v2

7/31/2019 Graduation Thesis Ha Minh Tan Final v2

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HANOI UNIVERSITY OF SCIENCE AND TECHNOLOGY

SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GRADUATION THESIS

Density-controllable growth of SnO 2 nanowire

junctions bridged across electrodes for high

performance NO 2 gas sensor

HA MINH TAN

Student ID: 20072525

Class: Electronics & Nano Materials K52

Advisors: Assoc.Prof. NGUYEN VAN HIEU

PhD. NGUYEN VAN DUY

HA NOI - 06/ 2012


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The Comment of Advisor


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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 my lecturers in Hanoi University of

Science and Technology, who taught me knowledge to complete my project. I 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.


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Table of Contents

Abstract ..................................................................................................................1

CHAPTER I: OVERVIEW ...................................................................................2

I.1. Microstructure and properties of SnO 2........................................................2

I.2. Sensing mechanism of nanostructured SnO 2 ...............................................2

I.3. Characteristics of gas sensing devices ..........................................................6

I.3.1. Sensitivity ........................................................................................6

I.3.2. Response time and recovery time ..................................................7

I.3.3. Selectivity ...................................................................................... 11

I.3.4. Optimal working temperature ..................................................... 11

I.3.5. Stability ......................................................................................... 12

I.4. Some methodologies to fabrication gas sensor devices .............................. 12

I.4.1. Nanowire-Printing 7 ....................................................................... 12

I.4.2. Dielectrophoresis 8 ......................................................................... 15

I.4.3. Polydimethylsiloxane (PDMS) patterning and solution

deposition 9.................................................................................................... 17

I.4.4. On-chip fabrication 10 .................................................................... 19

a) VLS mechanism .................................................................... 19

b) Fabrication of gas sensor based on ZnO nanowires by on-

chip growth method ...................................................................... 20

I.5. Motivation ................................................................................................... 23

I.5.1. Historical survey for NO 2 sensor based on metal oxide nanowires....................................................................................................... 23

I.5.2. Suggestion to improve sensor performance................................. 26

CHAPTER II: EXPERIMENTAL...................................................................... 27

II.1. Preparation of Interdigitated Electrode (IDE) ......................................... 27

II.2. SnO 2 NWs growth ....................................................................................... 29

II.2.1. Equipment, apparatus and chemical preparation ...................... 29


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II.2.2. Growth procedure of SnO 2 NWs at 800oC ................................. 30

II.3. Structure and morphology of SnO 2 material investigation ....................... 31

II.4. Gas sensitivity investigation ........................................................................ 31

II.4.1. Measurement system .................................................................... 31 a) Heater ................................................................................... 31

b) Gas-mixing part ................................................................... 32

c) Signal receiver and Power supply ....................................... 33

II.4.2. Measurement ................................................................................ 33

CHAPTER III: RESULTS AND DISCUSSION ................................................ 34

III.1. Structure, morphology and density of Nanowires ..................................... 34

III.2. Gas sensing properties ................................................................................ 38

III.2.1. NO 2 sensing at low working temperature .................................... 38

III.2.2. Sensitivity depends on the density of NWs and density of

junction ....................................................................................................... 39

a) 4 mg and 5 mg samples ......................................................... 39

b) 10 mg sample and 20 mg sample ........................................... 43

c) Suggested model for high sensing performance of junction

bridged structure .......................................................................... 47

d) Summary ............................................................................... 54

III.3. Response time and Recovery time .............................................................. 55

III.4. Sensor selectivity ......................................................................................... 58

III.4.1. At low temperature ....................................................................... 58

III.4.2. At high temperature ..................................................................... 60 a) The investigation gas which is able to detected at high

temperature .................................................................................. 60

b) Investigation of sensing property to Ethanol gas ............... 61

III.5. Conclusion and future plan ........................................................................ 64

III.5.1. Conclusion..................................................................................... 64

III.5.2. Future plan ................................................................................... 64

Reference .............................................................................................................. 65


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Table of Figures

Figure 1: Microstructure of tin oxide .................................................................2

Figure 2: The transform of Oxygen on the surface of SnO 2 NWs.....................2Figure 3: Physisorption and chemisorption steps involved in forming

oxygen ion species on SnO 2 surface ....................................................3

Figure 4: The depletion zone at the surface of nanowires and nanobelts .........3

Figure 5: SnO 2 is exposed in NO 2 gas: low temperature (a), high

temperature (b) ...................................................................................4

Figure 6: Direct contact among NW and metal electrode .................................4

Figure 7: NWs junctions and potential barrier at the junction ........................5

Figure 8: Equivalent circuit of total resistance of one networked

nanowires .............................................................................................6

Figure 9: Changing of resistance of sensor when gas is in ................................7

Figure 10: An example of poor sensing characteristics .......................................8

Figure 11: Resistance at different gas concentration of a high

performance sensor .............................................................................9

Figure 12: Two fit-lines and intersection ........................................................... 10

Figure 13: The graph after fitting ...................................................................... 10

Figure 14: An example graph of the sensitivity versus temperature ................ 11

Figure 15: Schematics of NW contact printing involving a) planar and b

c) cylindrical growth (donor) substrates. The SEM images in

the insets of a) and b) show that the grown Ge NWs are

randomly oriented on the growth substrate, resembling aforest. ................................................................................................. 13

Figure 16: SEM images of aligned ZnO nano-rods (a) and not-aligned

ZnO nano-rods (b) between the interdigitated electrodes ............... 17

Figure 17: Experimental procedures to prepare the networked NWs gas

sensor using PDMS patterning ......................................................... 18

Figure 18: VLS mechanism ................................................................................ 20


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Figure 19: The schematic illustration of ZnO-nanowire air bridges over

the SiO 2 /Si substrate. (b) Side- and (d) top-view SEM images

clearly show selective growth of ZnO nanowires on Ti/Pt

electrode. (c) The junction between ZnO nanowires grown onboth electrodes ................................................................................... 21

Figure 20: Top 10 materials form of 1D metal oxide nanostructures used

for gas sensor application in publications 11 ..................................... 23

Figure 21: The sensitivity is very low and the response and recovery time

are also slow 2,6 ................................................................................... 24

Figure 22: a) Ethanol sensing of SnO 2 nanorods upon exposure to ethanol

gas with concentrations of 10 300 ppm at a work temperature

of 300 C. b) The linear relationship between the sensor

sensitivity and the ethanol concentration. 15 ..................................... 25

Figure 23: Sensitivity versus time curves by cycling the dendrites and

nanowires between air and 1000 ppm CO 13 ..................................... 25

Figure 24: The structure of the interdigitated electrode array ......................... 27

Figure 25: Process of IDE ................................................................................... 28

Figure 26: CVD system ....................................................................................... 29

Figure 27: Electrodes after covering contact pads ............................................ 29

Figure 28: Thermal cycle for fabrication SnO 2 ................................................. 30

Figure 29: The dark chamber for gas sensing investigation ............................. 32

Figure 30: Schematic of gas-mixing part ........................................................... 32

Figure 31: (a) Power Supply, (b) The Keithley, (c) Schematic of

measurement ..................................................................................... 33Figure 32: Image of samples (from left to right): 20 mg, 10 mg, 5 mg, 4

mg ...................................................................................................... 34

Figure 33: XRD pattern of SnO 2 NWs at 800oC ............................................... 34

Figure 34: Typical FE-SEM images (magnification 1.5k) of networked

SnO 2 nanowires with 20m -spacing PIEs, depend on mass of

source materials. (a) 2 mg, (b) 4 mg, (c) 5 mg, (d) 10 mg, (e) 20

mg ...................................................................................................... 35
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Figure 35: Typical FE-SEM images of 2 mg sample at magnification 5k ........ 36

Figure 36: Typical FE-SEM images of 4 mg and 5 mg samples at

magnification 5k ................................................................................ 37

Figure 37: Typical FE-SEM images of 10 mg and 20 mg samples atmagnification 5k ................................................................................ 37

Figure 38: Resistance of 5 mg sample sensor in 1 ppm NO 2 gas at various

temperature ....................................................................................... 38

Figure 39: Sensitivity versus temperature at constant 1 ppm NO 2 ................... 39

Figure 40: Resistance versus concentration of NO 2 of 4 mg sample ................. 40

Figure 41: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of

4 mg sample ....................................................................................... 41

Figure 42: Resistance versus concentration of NO 2 of 5 mg sample ................. 42


5 mg sample ....................................................................................... 43

Figure 44: Resistance versus concentration of NO 2 of 10 mg sample ............... 44


10 mg sample ..................................................................................... 45

Figure 46: Resistance versus concentration of NO 2 of 20 mg sample ............... 46


20 mg sample ..................................................................................... 47

Figure 48: (a) and (b): SEM images of sample, (c): Resistance of sensor in

1 ppm of NO 2 ..................................................................................... 48

Figure 49: SEM image cross section: (a) sensor has NWs layer structure,

(b) 10 mg sample ............................................................................... 49Figure 50: Resistance of a sensor based on SnO 2 NWs which has layer

structure ............................................................................................ 50

Figure 51: Equivalent circuits for four cases of NWs bridged: (a) Direct

bridged, (b) Junction bridged, (c) Network Junction bridged,

(d) Combination of Direct and Junction bridged ............................ 51

Figure 52: The 3D graph of sensitivity versus temperature and

concentration ..................................................................................... 54
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Figure 53: Another view of Figure 52 ................................................................ 55

Figure 54: The response time and recovery time of 4 mg sample at 50 oC

(a), 100 oC (b), 150 oC (c) and the summary for 4 mg sample

(d) ....................................................................................................... 56Figure 55: Summary the response time and recovery time of 4 mg sample

(a), 5 mg sample (b), 10 mg sample (c), 20 mg sample (d) ............... 57

Figure 56: Gas response of 4 mg sensor at 100 oC with CO, H 2S, Ethanol

and NH 3 gas ....................................................................................... 59

Figure 57: The resistance versus concentration of 5 mg sample with CO,

H 2S, Ethanol, NH 3 gas at 400oC ....................................................... 60

Figure 58: Sensitivity of 4 mg sample with CO, H 2S, Ethanol and NH 3 ........... 61

Figure 59: Resistance versus Ethanol concentration graph of 4 mg (a), 5

mg (b), 10 mg (c), 20 mg (d) samples at 400 oC ................................ 62

Figure 60: Sensitivity of 5 mg, 10 mg and 20 mg samples ................................. 63


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Table of Tables

Table 1: Sensitivity, response and recovery time of sensors based SnO 2

NWs ...................................................................................................... 24

Table 2: Sensitivity for equivalent circuit ......................................................... 52


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GRADUATION THESIS

HA MINH TAN MSE K52 1

Abstract

Along with economic development is the introduction of urban areas and

industry. This environment is heavily polluted. The levels of pollution by gases

such as CO, CO 2, NO x, SO 2, NH 3, has increased from several to several tens of

times higher than the level allowed by international standards. The measuring,

monitoring and assessment of environmental pollution is necessary, and gas sensor

takes a very important role in this field.

The previous studies that investigated the gas sensitivity of nanostructured

materials, usually of oxide semiconductors such as SnO 2, In 2O3, ZnO, WO 3, TiO 2,

ect. In particular, SnO 2 materials have many advantages such as high sensitivitycapability, low resistance, the rate of research and applications is much greater for

other materials. The sensitivity of sensor based on SnO 2 nanowires (NWs) is

investigated 1 6, but there are some disadvantages in those investigation such as low

sensitivity, slow response and recovery time, high working temperature.

Therefore, goal of this work is improving the performance of sensor based on

SnO 2 to NO 2 gas. Sensors is fabricated by on-chip growth methodology to obtain

junctions bridged structure. The density of NWs and junctions was controlled by

mass of source material. Then, sensing properties to NO 2 of gas sensors was

investigated. The results showed the improvement of NO 2 sensing properties when

junction density decreases. The sensor which has least density of junctions,

corresponding to 4 mg of mass of tin, gives the best performance than the others.

Sensor indicates a very high selectivity to NO 2 gas at working temperature of 100oC with tested gases of CO, H 2S, NH 3, and ethanol. The model for sensitivity of

sensor was suggested. The sensitivity is about 20 at 100 oC, 1 ppm NO 2 and

response, recovery time are below 20 seconds. At higher temperature (300 400oC), the sensor is able to be sensitive to Ethanol gas.

The high sensitivity, fast response and recovery and low working

temperature of NO 2 gas sensor based on SnO 2 NWs give us a way to mass product

high performance and low energy consumption gas sensor.


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GRADUATION THESIS


CHAPTER I

OVERVIEW

I.1. Microstructure and properties of SnO 2

Figure 1: Microstructure of tin oxide

SnO 2 has crystal structure which is tetragonal structure in rutile phase. SnO 2

is n-type semiconductor and the band gap Eg = 3.6 eV. SnO 2 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, nanostructured forms of SnO 2 have been used for gas sensing

applications.

I.2. Sensing mechanism of nanostructured SnO 2

When SnO 2 is exposed to the air, oxygen molecules are adsorbed on the

surface. The adsorbed oxygen molecules extract electrons from SnO 2, forming

oxygen ions on the surface.

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

At low temperature, under 200 oC, Oxygen is in form of 2O . At higher

temperature, Oxygen is in form of O and 2O like equations below:


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GRADUATION THESIS


2 2O e O 2 2O O

O e O 2O e O

Figure 3: Physisorption and chemisorption steps involved in forming oxygen ion species on SnO 2 surface

Since SnO 2 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 4: The depletion zone at the surface of nanowires and nanobelts


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GRADUATION THESIS


When SnO 2 is exposed in NO 2, the mechanism is also the same. NO 2 gas is

adsorb on the surface of SnO 2 and makes depletion zone. At lower temperature, this

equation below occurs:

2( ) 2 22 2gas NO O e NO O

At higher temperature:

2( ) ( ) ( )gas gas surface NO e NO O The processes is demonstrate like figure below

Figure 5: 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 6: Direct contact among NW and metal electrode


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GRADUATION THESIS


Figure 8: Equivalent circuit of total resistance of one networked nanowires

I.3. Characteristics of gas sensing devices

A gas sensor device is evaluated by some parameters like: sensitivity,

response time and recovery time, selectivity, optimal working temperature, and

stability

I.3.1. Sensitivity

Sensitivity is the ability of a sensor to detect a gas with an individual

concentration value of this gas (also known as gas response). Sensitivity is denoted

S and is defined as the ratio:

air

gas

RS

Ror air gas

gas

R RS

Rfor n-type sensor, reducing

gas or p-type sensor, oxidant gas

gas

air

RS

Ror air gas

air

R RS

Rfor n-type sensor, oxidant

gas or p-type sensor, reducing gas

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

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


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GRADUATION THESIS


gas (Rg).

R e s

i s t a n c e

( O h m s

)

Time (seconds)

Ra RaGas in

Gas cut off

Rg

Figure 9: Changing of resistance of sensor when gas is in

We can find the Rg equal to any point of the top, or findthe average of

data rank from the intersection point (see I.3.2 for more detail) to end point of thetop.

I.3.2. Response time and recovery time

Response time is the changing time since the target gas appeared until the

resistance of sensor reached stable value. For calculating, response time is the

changing time to 90% (or 10%, depend on 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 kind of sensor material and

target gas is n-type or p-type) of absolute initial value of sensor resistance

Response time and Recovery time only have meaning when sensor reaches

the final value or return to the initial value. And a sensor device only is useful when


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GRADUATION THESIS


it satisfy this condition. For example,

0 200 400 600 800

2.0k

4.0k

6.0k

8.0k

10.0k

12.0k

14.0k

16.0k

18.0k

R e s

i s t a n c e

( O h m

)

Time (second)

Figure 10: An example of poor sensing characteristics

In this graph, four peaks does not reach the final value, we can see that the

value of resistance will increase if the target gas continued flow. Thus, we cannot

find the maximum (or final value) of resistance, then we cannot find 90% of this

value, mean that we cannot determine the response time. The same with recovery

time, the resistance does not return the initial value so we cannot determine

recovery time.

Therefore, we just can determine response and recovery t ime when sensors

resistance versus concentration characteristics graph look like below


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GRADUATION THESIS


0 200 400 600 800 1000 1200 1400 1600 1800

0

50k

100k

150k

200k

250k

300k

350k

400k

R e s

i s t a n c e

( O h m s

)

Time (seconds)

Figure 11: Resistance at different gas concentration of a high performance sensor

Following this graph, the resistance reaches the stable final value and can

recovery to the initial value. Thus, we can easily calculate the response time and

recovery time. Furthermore, if we calculate the final value by choose a random

point on the peak or calculate the average, then find a point matched 90% value of

final value, this point maybe not accuracy. For more accuracy, I suggest a method

using Origin software. We input data and plot a graph like above, then use a Fitting

Non Linear Curve Function, select a part of graph, for example, from end point of a

base to end point of a top (for response calculating) or from end point of a

top to end point of a base (for recovery calculating).Then, the program will

auto calculate to fit two linear lines. One line is the fitting for rising time, and the

other is the fitting for stable time. Two lines intersect at one point. Then the absciss

of this intersection point is the stable time. It looks like below


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GRADUATION THESIS


Figure 12: Two fit-lines and intersection

Do the same with others part of the graph, we have

0 200 400 600 800 1000 1200 1400 1600 1800

0

50k

100k

150k

200k

250k

300k

350k

400k

R e s

i s t a n c e

( O h m s

)

Time (seconds)

Figure 13: The graph after fitting


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GRADUATION THESIS


Then, find plot data to determine absciss of intersection. The difference

between absciss of intersection and end of base or end of top is response time or

recovery time, respectively.

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

I.3.3. 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.

I.3.4. 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 asTM. Sensitivity depends on the temperature graph usually takes the form shown in

Figure 14

Figure 14: An example graph of the sensitivity versus temperature


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GRADUATION THESIS


I.3.5. 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.4. Some methodologies to fabrication gas sensor devices

I.4.1. Nanowire-Printing 7

Semiconductor NWs with desired atomic composition are readily grown by

CVD. The diameter of the grown NWs is determined by the size of the metal

nanoclusters used as the seeds for the catalytic growth, and can be tuned in the

range d = 10 500 nm. The NWs are typically grown vertically on the substrate but

with random orientation (for the none-pitaxial growth), therefore resembling a

forest, as evident from the scanning electron microscopy (SEM) analysis (Figure

15a ).

Contact printing enables the direct and controllable transfer of NWs from the

growth substrate to the desired support (receiver) substrate as highly aligned,parallel arrays (Figure 15) . This method involves the directional sliding of the NW

growth substrate (either planar or cylindrical) with randomly aligned NWs on top of

a receiver substrate.

During this process, NWs are effectively combed (aligned) by the

directional shear force, and are eventually detached from the donor substrate as they

are anchored by the Van der Waals interactions with the surface of the receiver

substrate, resulting in the direct transfer of aligned NWs to the receiver substrate.


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GRADUATION THESIS


Figure 15: Schematics of NW contact printing involving a) planar and b c) cylindrical growth (donor) substrates. The SEM images in the insets of a) and b)

show that the grown Ge NWs are randomly oriented on the growth substrate, resembling a forest.

The growth substrate can be either planar or cylindrical. Specifically, the

cylindrical growth substrates are used for differential roll printing (DRP) of NWs,

which is a highly scalable process. As shown in Figure 15 b, the DRP approach is

based on the growth of crystalline NWs on a cylindrical substrate (roller) using the

VLS process, and then the directional and aligned transfer of the as-grown NWs

from the donor roller to a receiver substrate by rolling the roller. This approach

minimizes the contact area between the donor and receiver substrates, since the

cylindrical donor substrate rolls over the receiver substrate with only a small

tangent contact area consisting of fresh NWs at any given time. This is highly

beneficial for printing large areas that would otherwise require large planar growth

substrates and long contact-sliding distances. In addition, the roller can be

repetitively used for the NW growth, which is important for a low-cost roll-to-roll


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GRADUATION THESIS


process. Glass, quartz, or stainless-steel tubes with proper outer radius ( rR ~ 0.25

inch, limited by the size of the CVD chamber) were used as the cylindrical growth

substrates with the NWs grown using similar processing conditions as those used in

the synthesis on planar substrates with gold colloids as catalysts. In this case,uniform and dense NW forests are synthesized on the outer surface of the roller.

The roller is then connected to a pair of wheels and mounted on rails that guide the

directional rolling. During the printing process, the roller is brought in contact with

a stationary receiver substrate and rolled at a constant velocity of ~ 5 mm/min. The

receiver substrate is functionalized with amine-terminated monolayers or thin films

of poly- L-lysine. The printing is performed either with or without the application of

a lubricant (which will be described in detail in the later section). It was found that

the NW assembly is relatively insensitive to the rolling speed, but at high velocities

( > 20 mm/min), non-uniform NW printing is attained, arising from the non-

conformal contact between the two substrates. The printing outcome, however,

highly depends on the roller receiver substrate pressure. The optimal pressure for

the set-up shown in Figure 15 b is ~200 g/cm 2, which is tuned by the spring

underneath the stage. At lower pressures, aligned transfer of NWs is not observed,and at higher pressures, mechanically induced damage to the NWs is observed,

resulting in t he assembly of short NWs (< 1 m long). As previously discussed, the

application of shear force is essential for the sliding of the NWs on the receiver

substrate, which results in their eventual aligned transfer. For planar growth

substrates, the shear force is simply attained by the sliding process. For cylindrical

growth substrates (such as DRP process), a mismatch between the roller and wheel

radii ( rR and rW, respectively) is used to result in a linear sliding motion of the roller

relative to the stationary receiver substrate in addition to the rolling motion. The

relative sliding motion for rW rR generates the required shear force for the transfer

of aligned NWs to the receiver substrate, without which negligible density with

random alignment is observed. This differentiates the NW DRP process from the

conventional roll-printing processes where such a mismatch in the radii would be

highly undesirable, as it results in the perturbation of the printing patterns. Notably,


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one rotation of a roller results in a printed region with length equivalent to the

circumference of the roller, 2rW. Moreover, SEM images of the donor substrate

before and after the printing process are investigated, and NWs on the donor

substrate are verified to have been effectively combed by the shear force beforegetting transferred to the receiver substrate. Assuming a NW density of ~5 NW m2

on the donor substrate and a printed density of ~50NW/100m2 on the receiver

substrate, we estimate that only 10% of the NWs are transferred during the contact-

printing process when the donor and the receiver substrates have the same surface

area. Since only 10% of the grown NWs are transferred from the roller to the

receiver substrate after one rotation, in principle, a NW roll can be rotated multiple

times before roller replacement is required. However, detailed studies of the

printed-NW density and uniformity after multiple rotation cycles are needed in the

future.

I.4.2. Dielectrophoresis 8

Dielectrophoresis is a manipulation technique based on Maxwells classical

electromagnetic field theory to make controlled motion of particles in a controlledelectric field between the preset electrode structures. In a spatially non-uniform AC

(alternating current) electric field, dielectric particles experience a translational

force as a consequence of the interaction of the polarization of the particle induced

by the electric field with the non-uniformity in that field. The resulting particle

movement was termed dielectrophoresis by Pohl. According to the theory of

electromagnetism, dielectrophoresis force acting on a spherical particle is given by:

_

22 m DEP rmsF V K E (1)

Where V is the volume of the particle, E rms is the root mean square (rms)

value of the electric field and K() is the real part of what is called the Clausius

Mosotti factor, which is related to the particle dielectric constant p and the medium

dielectric constant m by


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GRADUATION THESIS


* *

* *( ) Re 2 p m

p m

K

(2)Here the asterisk (*) denotes that the dielectric constant is a complex

quantity, and it can be related to the conductivity and the angular frequency through

the standard formula

* i

(3)

For spherical particles, the Claudius Mosotti factor K() can vary between

0.5 and +1.0. When it is positive, particles move toward higher electric field

regions, and this is termed positive dielectrophoresis. When it is negative, the

particles move toward smaller electric regions, and this is termed negative

dielectrophoresis. S ince K() is frequency dependent, both positive and negative

dielectrophoresis can be observed in the same system by varying the frequency.

Though Eq.(1) is induced depending on the spherical particles, prelate particles such

as DNA, nanotubes, nanowires, etc. are more suitable to be manipulated due to their

easier polarization along the axis direction. Bar-shaped materials are employed in

many experiments of dielectrophoretic manipulation.

An interdigitated electrode (IDE) array is made by the same way, conducted

wire from the poles, and then located nano-structured ZnO between interdigitated

electrodes via dielectrophoresis. The distance between the digits is 200m. The

dielectrophoresis was performed with a sinusoidal waveform of 1 MHz frequencyand 8 V amplitude, till all deionized water evaporated. Figure 16 b is a SEM image

of aligned ZnO nano-rods between the digits of the electrode. Figure 16c is a SEM

image of not-aligned ZnO nano-rods, which is presented to draw a comparison

between sensors with and without dielectrophoresis manipulation. Standard

humidity required was produced by saturated salt solutions. Six saturated solutions

including CuSO 4, NaCl, CuCl 2, NaBr, K 2CO 3, MgCl 2 were used with the relative


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humidity of 97.6%RH, 75.29%RH, 68%RH, 57.57%RH, 43.16%RH, 32.78%RH,

respectively, under 1 atm, 25 oC. The impedance of the sensing structure was

measured by an LCR impedance detector.

Figure 16: SEM images of aligned ZnO nano-rods (a) and not-aligned ZnO nano-rods (b) between the interdigitated electrodes

I.4.3. Polydimethylsiloxane (PDMS) patterning and solution deposition 9

A well-defined NWs gas sensor in a networked configuration was suggested

using a PDMS patterning and solution deposition method. The NW density was

manipulated by controlling the coating parameter. The effect of the NW density on

the gas sensing characteristics, such as the gas response (sensitivity) and response

(a)

(b)


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time, were investigated.

Figure 17: Experimental procedures to prepare the networked NWs gas sensor

using PDMS patterning

SnO 2 NWs were grown by a vapor phase transport using Sn metal

(99.999%). The source loaded in the Al 2O3 boat was located in the center of a

quartz tube (inner diameter: 28 mm; length: 800 mm) in a horizontal furnace. A Si

wafer coated with Au (thick- ness: 30 ) was placed 5 cm downstream from the

source. After evacuating the quartz tube to 102

Torr using a rotary pump, thefurnace temperature was increased from room temperature to 750 oC, and the NWs

were formed by a reaction between the source and O 2 gas (0.5 sccm) for 20 min.

For sensor fabrication, the Ti (50 nm) and Pt (300 nm) layers were deposited in

sequence on a 4-in SiO2 (300 nm)/Si wafer by DC sputtering, and comb-like

electrodes with a 500500 m2 area were formed using a lift-off process ( Figure

17a ). The substrate was coated with a solution containing PDMS and hardener (9:1


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by volume) and subsequently hardened at 60 oC for 5h. The PDMS patterns with a

square type hole were formed by cutting the PDMS layer above the electrode area

and its subsequent removal by tweezers ( Figure 17 b). The as-grown 0.01 g SnO 2

NWs were dispersed in a mixture of deionized water and isopropyl alcohol (IPA) (5ml:5 ml) by ultra-sonication. A slurry droplet containing SnO 2 NWs (10l) was

dropped onto the PDMS patterned substrate using a micro-pipette ( Figure 17c ) and

dried gradually (Figure 17d and Figure 17 e). The density of NWs was controlled

from low to high by manipulating the number of droplets deposited. Two sensors

were fabricated by coating one and five droplets of the slurry, which were referred

as low-density nanowires (LD-NWs) and high-density nanowires (HD-NWs)

sensors, respectively. In order to decrease the density of NWs further, 0.005 g SnO 2

NWs were dispersed in a mixture of deionized water and isopropyl alcohol (IPA) (5

ml:5 ml) and 1 droplet of slurry was dropped and dried. This sensor was referred as

very-low-density nanowires (VLD-NWs). The gas sensing characteristics of three

sensors were measured and compared.

The sensor was contained within a quartz tube and heat-treated at 400 oC for

12 h to decompose any residual PDMS that might deteriorate gas sensing

characteristics. And the temperature of furnace was set to the gas sensing

temperature. The gas concentration was controlled by changing the mixing ratio of

the parent gases and dry synthetic air. A flow-through technique with a constant

flow rate of 500 cm 3 /min was used.

I.4.4. On-chip fabrication 10

a) VLS mechanism

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

18 below:


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Figure 18: 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.

b) Fabrication of gas sensor based on ZnO nanowires by on-chip

growth method

Nanowire gas sensors were fabricated by a selective growth of nanowires on

patterned Au catalysts following VLS mechanism, thus forming nanowire air

bridges or nano- bridges between two Pt pillar electrodes.


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Figure 19: The schematic illustration of ZnO-nanowire air bridges over theSiO 2 /Si substrate. (b) Side- and (d) top-view SEM images clearly show selective

growth of ZnO nanowires on Ti/Pt electrode. (c) The junction between ZnO nanowires grown on both electrodes

Figure 19a shows the schematic illustration for a network of ZnO nanowires

floated above SiO2/Si substrate. For the area-selective growth of ZnO nanowires, 2nm-thick Au catalyst film was patterned using the conventional photolithography.

The typical gap between two Au layers was optimized to 5m, taking into account

the length of ZnO nanowire ( 10m). Since the Au layers were used as catalysts

during nanowire growth, we adopted a Pt contact electrode ( 300 nm) with Ti

adhesion-promotion layer ( 20 nm), between the Au layer and SiO2/Si substrate.

ZnO nanowires were synthesized by the carbothermal reduction process


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nanowires on another substrate with prefabricated electrodes.

I.5. Motivation

I.5.1. Historical survey for NO 2 sensor based on metal oxide nanowires

Follow a review 11 , SnO 2 is the most materials is used to form 1D

nanostructure and sensor devices. The characteristics and gas-sensor properties of

devices based SnO 2 are much investigated. SnO 2 is chosen the material for my

research because I can easily find information and articles about this material. From

that, some disadvantage of gas sensor devices which was investigated such as low

sensitivity or high energy consumption or slow response.

Figure 20: Top 10 materials form of 1D metal oxide nanostructures used for gas sensor application in publications 11

In recently papers, they report that sensors based on SnO 2 material are

sensitive with several gas such as NO 22 4,6,12 , CO 13, H 214, Ethanol 15,16 .

In case of NO 2 gas, the sensitivity is poor and slow response time 6 like

Figure 21 below:


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Figure 21: The sensitivity is very low and the response and recovery time are also slow 2,6

Table below is shown the sensitivity, response and recovery time at several

temperature and concentration of NO 2.

Reference Temperature Concentration SensitivityResponse

time

Recovery

time

6

Kim et al.2012

300 oC 10 ppm 1.05 80 s 200 s

4Kim et al.

2011300 oC 10 ppm 8 70 s 600 s

2Choi et al.

2011200 oC 30 ppm 1.2 470 s 460 s

17Hwang et

al. 2006300 oC 1 ppm 7 200 s 200 s

18Choi et al.

2011200 oC 10 ppm 50 > 30 s 100 s

Table 1: Sensitivity, response and recovery time of sensors based SnO 2 NWs


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In case of CO and Ethanol gas, the working temperature is quite high

(around 300 oC - 400 oC).

Figure 22: a) Ethanol sensing of SnO 2 nanorods upon exposure to ethanol gaswith concentrations of 10 300 ppm at a work temperature of 300 C. b) Thelinear relationship between the sensor sensitivity and the ethanol concentration. 15

Figure 23: Sensitivity versus time curves by cycling the dendrites and nanowires between air and 1000 ppm CO 13


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I.5.2. Suggestion to improve sensor performance

According to some report 4,10,12 , on-chip growth methodology is chosen to

fabricate sensor in my experiment because of its advantage. The synthesized SnO 2

nanowires has a single crystal structure 19 , uniform (because SnO 2 NWs is just only

grown on Au layer). And SnO 2 NWs on sensor, which is fabricated on-chip, is more

stable than on sensor which is fabricated by spin-coating, drop-coating or other

methodologies.

NWs junctions bridged structure across electrodes is desired. The density of

junctions is controlled by the mass of source material (tin). The higher mass of

source material is introduced, the higher density of NWs and junctions are received.But the higher density of junctions does not mean that the sensitivity of sensor is

increased. Because the probability of NWs across directly among electrodes is

increased with density of NWs. That decreases the sensitivity of sensor. Meanwhile,

the high density of junctions also decreases the absorption of gas. Finding most

suitable density of junctions or mass of source material is a task of my experiment.

The goal of my experiment is fabricate sensor which is sensitive to NO 2 at

low temperature, fast response and recovery time. Also, the sensitivity to others gas

such as CO and Ethanol at higher temperature have to investigate.


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CHAPTER II

EXPERIMENTAL

II.1. Preparation of Interdigitated Electrode (IDE)

The structure of IDE includes three layers: Au, ITO and Pt on the SiO2/Si

substrate. The dimension of IDE look like Figure 24, the spacing between the digits

is 20m.

Figure 24: The structure of the interdigitated electrode array

The process to fabricate IDE is demonstrated as the figure below


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Figure 25: Process of IDE

Step 1 (Figure 25a ): Oxidase a n-type Si (111) substrate to form a 150nm

SiO2 layer on top

Step 2 (Figure 25 b): Spin-coat a layer of photoresist on substrate

Step 3 ( Figure 25 c): Photolithography with a mass which has the shape like

Figure 24


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Step 3 (Figure 25 d): Sputtering a 300nm layer of Pt

Step 4 (Figure 25 e): Sputtering a 10nm layer of ITO

Step 5 (Figure 25f ): Sputtering a 10nm layer of Au

Step 6 (Figure 25 g): Use Acetone to lift-off photoresist layer

II.2. SnO 2 NWs growth

II.2.1. Equipment, apparatus and chemical preparation

- Equipment and chemical: CVD system, quartz tube, alumina boat, Sn

powder, Ar gas and O 2 gas

Figure 26: CVD system

- Clean apparatus: Clean the quartz tube and alumina boat by soaking in HF

solution 1% on one day. Then use de-ionization water in the last washing. Dry them

by heater.

- Cover IDEs: Place two small pieces of silicon wafer on to the IDEs like

figure below to protect Pt layer.

Figure 27: Electrodes after covering contact pads


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30 minutes.

Step 3: When the furnace up to this temperature, wait 5 min for stabilized

temperature. Flow O 2 with rate of 0.1 sccm. The reaction occurred: Sn + O 2

SnO 2 . Keep furnace at this temperature for 25 minutes for 2 mg, 4 mg and 5 mgsamples; 40 minutes for 10 mg sample; 60 minutes for 20 mg sample. The mass of

source is higher, the reaction time is higher to ensure the source is reacted

completely.

Step 4: Naturally decreasing temperature of the furnace. When the

temperature downs to 600 oC, turn off O 2 and heater.

II.3. Structure and morphology of SnO 2 material investigation

To study the structure of synthetic materials, we use the following analytical

methods:

1. Scanning electron microscopy (FE-SEM) was used to study the surface

morphology of SnO 2 nanowires synthesized.

2. X-ray diffraction: X-ray diffraction method based on the phenomenon of

X-rays scattered by atoms in the crystal. The scattered rays interfere with each otherand imaging on film or on the display device. XDR image gives us information

about the structure and phase of the material.

II.4. Gas sensitivity investigation

An important part of this experiment is investigating the sensing ability of

NWs depend on temperature, kind of gas and structure of NWs.

II.4.1. Measurement system

Measurement system includes three parts: heater, gas-mixing part, signal

receiver and power supply.

a) Heater

The heater keeps the sensor work at a stable temperature. The maximum


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temperature that the heater can provide is 500 oC. The heater is placed in a dark

chamber for avoid temperature noise and electrical noise.

Figure 29: The dark chamber for gas sensing investigation

b) Gas-mixing part

The target gas is mixed with air to form mixed gas in any concentration of

target gas we want. Gas mixing part includes five Mass flow controller(MFC)

used to control flow of target gas and air, combined with mechanical valves and

electrical valve.

Figure 30: Schematic of gas-mixing part


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c) Signal receiver and Power supply

The HP is a power supply. We apply a constant voltage between two

electrodes. Then a Keithley is the signal receiver which measures current between

electrodes. From the current data, we can calculate the resistance of sensor using

Ohms law

U R

I

Figure 31: (a) Power Supply, (b) The Keithley, (c) Schematic of measurement

II.4.2. Measurement

Step 1: Load the sensor into the dark chamber, on the heater. Place the

probes on electrodes. Close the chamber.

Step 2: Turn on heater, tune heater to the desired temperature.

Step 3: Turn on the Keithley, turn on the Power Supply, open Signal

Processing Program on computer.

Step 4: Tune MFCs to get a desired concentration of gas.

Step 5: Turn on and off gas-valve in constant pulse to get data.

(b) (c)

(a)


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CHAPTER III

RESULTS AND DISCUSSION

III.1. Structure, morphology and density of Nanowires

Image of samples after fabrication is below. The white region is layer of

SnO 2 on electrodes.

Figure 32: Image of samples (from left to right): 20 mg, 10 mg, 5 mg, 4 mg

20 25 30 35 40 45 50 55 60 65 70 75

( 1 0 1 )

( 1 1 0 )

( 2 0

2 )

( 3 0 1 )

( 1 1 2 )

( 3 1 0 )

( 0 0 2 ) ( 2

2 0 )

( 2 1 1 )

( 1 1 1 )

( 2 0 0 )

I n t e

n s

i t y

( a

. u . )

2 degree)

Figure 33: XRD pattern of SnO 2 NWs at 800 oC

Figure 33 is the XRD pattern of SnO 2 NWs at 800o

C. According to this


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Figure, SnO 2 NWs is grown at 800oC has rutile phase structure and diffraction

angles is match up to theory. The highest peak at 2 = 26.65o, match up to (110)

surface. Two high others at 2= 33.7 o and 51.7 o, match up to (101) and (211)

surfaces, respectively.These images below are SEM images of sensor devices. They was focused

on the PIEs.

Figure 34: Typical FE-SEM images(magnification 1.5k) of networked SnO 2 nanowires with 20 m-spacing

PIEs, depend on mass of source materials. (a) 2 mg, (b) 4 mg, (c) 5 mg, (d) 10 mg, (e) 20 mg

(a) (b)

(c) (d)

(e)


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Accorsing to Figure 34, the density and length of NWs is increasing with the

mass of source material. In 2 mg sample, the density of NWs is very low. There are

only few wires that are out of the PIEs, and they are not long enough to contact

others and PIEs. The dark lines are silicon wafer which are seen easily. In 4 mg and5 mg samples, the density of NWs is much higher than density of 2 mg sample. And

the length of NWs are long enough to make the NWs are contacted between PIEs.

We can see between 4 mg and 5 mg, just 1mg of mass difference, the density of

NWs are significant increased. The dark lines are still seen. In 10 mg and 20 mg

samples, the density of NWs are very high. The NWs are almost contacted with

others, we can imagine it look like a cloud of NWs on the electrode or a porous

layer of NWs on the electrode. The dark lines can not be seen at this density.

Figure 35: Typical FE-SEM images of 2 mg sample at magnification 5k

In the higher magnification SEM image, we can measure the length of NWs.

In 2 mg sample, the NWs are divided into two class: short NWs and longer NWs.

According to Figure 35, the short NWs have the length about 5m, and the longer

NWs have the length about 9m. Thus, almost the NWs does not contact others. It


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results the 2 mg sample sensor device does not conduct electric.

Figure 36: Typical FE-SEM images of 4 mg and 5 mg samples at magnification 5k

According to Figure 36, there are a lot of contacts (junctions) among NWs in

4 mg sample, and much much of contacts among NWs in 5 mg. The increament of

density, length and contact of NWs are rapid. I also did a 3mg sample, but it still

does not conduct electric, so I do not investagate SEM image of this sample.

Figure 37: Typical FE-SEM images of 10 mg and 20 mg samples at magnification 5k

According to Figure 37, we cannot see the space between digits. The density

of material is crowded. A notable, from 2 mg to 20 mg sample, the rate of nanobelt

is increased.


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III.2. Gas sensing properties

III.2.1. NO 2 sensing at low working temperature

I investigated the sensitivity of 5 mg sample with NO 2 gas at the sameconcentration 1 ppm and different temperature from 50 to 300 oC

Figure 38: Resistance of 5 mg sample sensor in 1 ppm NO 2 gas at various temperature


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concentration of 4 mg sample.

0 300 600 900 1200 1500 1800

0.0

500.0k

1.0M

1.5M

2.0M

2.5M

3.0M

3.5M

4.0M

4.5M

5.0M

5.5MSnO 2 4mg - NO 2 - 50

oC 10 ppm

2.5 ppm

5 ppm

R e s

i s t a n c e

( O h m s

)

Time (seconds)

1 ppm

0 300 600 900 1200 1500 1800

0.0

500.0k

1.0M

1.5M

2.0M

2.5M

3.0M

3.5MSnO 2 4mg - NO 2 - 100

oC 10 ppm

2.5 ppm

5 ppm

R e s

i s t a n c e

( O h m s

)

Time (seconds)

1 ppm

0 300 600 900 1200 1500 1800

0.0

500.0k

1.0M

1.5M

2.0M

2.5M

SnO 2 4mg - NO 2 - 150oC

10 ppm

2.5 ppm

5 ppm

R e s

i s t a n c e

( O h m s

)

Time (seconds)

1 ppm

Figure 40: Resistance versus concentration of NO 2 of 4 mg sample


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GRADUATION THESIS


1 2.5 5 100

20

40

60

80

100

120

140

S e n s

i t i v i t y

Concentration (ppm)

50 oC

100 oC150 oC

Figure 41: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of 4 mg sample

From Figure 40, we can see that 4 mg sample has good response to NO 2 gas.

Resistance of sensor can reach the final stable value when gas in and recovery to thebase when gas off. Note that, at 50 oC, the signal quite noise and 100 oC is better for

low concentration. Figure 41 show us that suitable temperatures for working that are

50 oC and 100 oC.

Figure 42 and Figure 43 below are the resistance versus concentration and

sensitivity versus concentration of 5 mg sample.


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1 2.5 5 10

20

40

60

S e n s

i t i v i t y

Concentration (ppm)

50100150


From Figure 42, we can see that 5 mg sample has good response to NO 2 gas.

Resistance of sensor can reach the final stable value when gas in and recovery to the

base when gas off. According to Figure 41, working temperature 100 oC is still the

best for low concentration.

b) 10 mg sample and 20 mg sample

Because the density of nanowires of 10 mg sample and 20 mg sample are

very high, I think that the sensitivity of them is the same.

Figure 44 and Figure 45 below show us the resistance versus concentration

and sensitivity versus concentration of 10 mg sample.


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1 2.5 5 10

10

20

30

40

50

S e n s

i t i v i t y

Concentration (ppm)

50100150


From Figure 44, we can see that 10 mg sample has quite good response to

NO 2 gas. But at 50oC, the sensor shows the poor recovery, the resistance cannot

return the base. Figure 45 show us that the best temperature for working that is 100oC. At 50 oC and 150 oC, the sensitivities are quite the same at different

concentrations. It is called gas-saturated.

Figure 46 and Figure 47 below are the resistance versus concentration and

sensitivity versus concentration of 20 mg sample


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Figure 51: Equivalent circuits for four cases of NWs bridged: (a) Direct bridged,(b) Junction bridged, (c) Network Junction bridged, (d) Combination of Direct

and Junction bridged

Figure 51 demonstrates equivalent circuits for three cases of NWs bridged.

Called the initial resistance (in air) of a NW is R w, of a junction is R j, neglect the

resistance between NW and electrodes. The increment of resistance of NW and

junctions in NO 2 gas are A and B. Following part I.2, A is very smaller than B.

From that, the table below shows the initial, final resistance and sensitivity of sensor

in NO 2

Rw Electrode Electrode

Rw RJ Rw Electrode Electrode


Rw RJ


Rw

(b)

(a)

(c)

(d)


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Case Initial Resistance Final ResistanceSensitivity

(a) w R

w AR A

(b)2 w j R R 2 w j AR BR

( )2 j

j w

R A B A

R R

(c)3 w j R R 3 w j AR BR

( )3 j

j w

R A B A

R R

(d)(2 )

3w w j

w j

R R R

R R

(2 )

3w w j

w j

AR AR BR

AR BR

12 3 j j

j w j w

R R A B A B A

R R BR AR

Table 2: Sensitivity for equivalent circuit

According to Table 2,

1 ( ) ( )2 3 3 2 j j j j

j w j w j w j w

R R R R A A B A B A A B A A B A R R BR AR R R R R

It means that direct bridged model give very small sensitivity compare to

others. If R j >> R w, sensitivity of equivalent circuit (b) and (c) are B, thus this

assumption is not suitable because from 4 mg to 20 mg sample, the sensitivity is

significant decreased. And the sensitivity of quivalent circuit (d) are A, thus this

assumpption is not acceptable.If R j ~ R w, sensitivity of equivalent circuit (b), (c) and (d) are:

2

3

B A,

3

4

B Aand

4 ( 2 )

3 3

A B A B A

> A

This assumption is acceptable, and it shows us that junction structure always

give better sensitivity than direct bridged.

Model (b) and (c) demonstrate the NWs junctions structure. Model (b) is

single junction bridged; it means that one NW is only contacted to one other NWs


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and one electrode. Model (c) is network junction bridged; it means that one NW can

contact many NWs and just one electrode. Equivalent circuit (c) only demonstrates

case one NW contacts to two NWs and one electrode. The result is model (b) gives

better sensitivity than model (c).Model (d) demonstrate the combination of NWs junctions bridged structure

and direct bridged structure. Equivalent circuit (d) only demonstrates case one

junctions and one direct bridge. The result is model (d) also gives worse sensitivity

than model (b) and (c).

If circuit (b) demonstrates case one NW contacts to n NWs, no direct bridge

and assumse R j ~ R w , the sensitivity of this circuit is stay the same

2

3

B A or

( 2)

3

A (*) where A B

Therefore, the sensitivity of model (b) does not depend on the number of

parallel junction. It depends on A and , but in this case, A isvery smaller than B,

thus is the most important factor in sensitivity of model (b).

If circuit (c) demonstrates case one NW contacts to n NWs, no direct bridge

and assumse R j ~ R w , the sensitivity of this circuit is

( 1)

2

n A Bn

or

( 1 )

2

n An

(**) where A B

If circuit (d) demonstrates case one junction bridged, m direct bridge and

assumse R j ~ R w , the sensitivity of this circuit is

3 1 (2 )

3 (2 1)

m A A Bm A mB

or

(2 )(3 1)

3 (2 ) 1

A m

m

(***) where A B

The value of (**) and (***) approach to A when n and m approach infinity.

That explains why the sensitivity is inversely proportional to density of NWs. The

higher density of NWs, the higher network junction level, the lower sensitivity of

sensors.

Comparing (**) and (***), the value of (***) decreases rapidly when m

increases. The value of (**) is slower decreased when n is increased. Furthermore,

the factor is more affected on (**) than (***). In conclusion, the network junction

structure and combination of junction bridged and direct bridged decrease the


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sensitivity of sensors. Especially, the direct bridged decreases dramatically the

sensityvity of sensors. Therefore, decreasing density of junction and length of NWs

is necessary to enhance the performance of sensor

d) Summary

From data of sensitivity of the samples at various temperatures and

concentration of NO 2, a 3D graph is plotted to show the sensitivity of the samples.

Figure 52: The 3D graph of sensitivity versus temperature and concentration

According Figure 52, we can see easily that the 4 mg sample give the best

sensitivity, next is 5 mg and 10 mg sample, then the last is 20 mg sample. We can


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15 1413

18

8 8

15

22

1 2.5 5 100

10

20

T i m e

( s e c o n

d s )

Concentration (ppm)

Response timeRecovery time

14

18

22

19

119 9

11

1 2.5 5 100

10

20

T i m e

( s e c o n

d s )

Concentration (ppm)


14

108 88

79

8

1 2.5 5 100

10

20

T i m e

( s e c o n

d s

)

Concentration (ppm)


1 2.5 5 106789

101112131415161718192021222324

T i m e

( s e c o n

d s

)

Concentration (ppm)

Response 50 oCResponse 100 oCResponse 150 oCRecovery 50 oCRecovery 100 oCRecovery 150 oC

Figure 54: The response time and recovery time of 4 mg sample at 50 oC (a), 100 oC (b), 150 oC (c) and the summary for 4 mg sample (d)

According to Figure 54 a,b,c, we see that the response and recovery time of 4

mg sample is very short, under 22 seconds, compare with response time 43

second 12, and recovery time 1.5 minutes 3 in others reports. From Figure 54d , the

recovery time almost is equal or less the response time at 100 oC and 150 oC. But

(b)(a)

(c)

(d)


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According to Figure 55, at 50 oC, the response time and recovery time are

slower that at others temperature, in all sample. Specially, in 10 mg sample, at 50oC, the sensor does not recovery to the initial resistance value. And 5 mg and 20 mg

samples show us the extremely response and recovery time, under 5 seconds at 150oC and under 10 seconds at 100 oC. And note that, the response and recovery time

does not much depend on the concentration of NO 2.

In conclusion, 4 mg sample, 10 mg sample and 20 mg sample give us the

best response time and recovery time at 100 oC and 150 oC. Combine with the

conclusion in the previous part; 4 mg samples give best performance: high

sensitivity, fast response time and recovery time and optimal working temperature

at 100 oC.

III.4. Sensor selectivity

This sensor is hoped just be sensitive to NO 2 only at low temperature and

sensitive with other gas at high temperature.

III.4.1. At low temperature

The first, the sensitivity to others gas at low temperature is tested. The 4 mg

sample is used to test, at 100 oC. And CO, H 2S, Ethanol, NH 3 gases are used in this

test.


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0 200 400 600 800 1000 1200 1400 1600

0

100k

200k

300k

400k

500k

600k

700k

800kResistance versus concentration of CO, H 2S, C 2H5OH, NH 3, NO 2

1 ppm

NO2

CO

100 ppm10 ppm 100 ppm

R e s

i s t a n c e

( O h m s

)

Time (seconds)

10 ppm

H2SH2S C2H5OH

Figure 56: Gas response of 4 mg sensor at 100 oC with CO, H 2S, Ethanol and NH 3 gas

According Figure 56, the sensor linearly does not response with theappearance of CO, H 2S, Ethanol and NH 3 gas. The sensitivities are below 1.5. We

can conclude that the sensor is not sensitive to these gases, just only is sensitive to

NO 2 at low temperature (100oC).


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consider the sensitivity of this sensor.

1.5

5.3

10.8

2.8

0

2

4

6

8

10

S e n s i

t i v

i t y

CO

H2SEthanolNH3

Figure 58: Sensitivity of 4 mg sample with CO, H 2S, Ethanol and NH 3

According to Figure 58, this sensor give best sensitivity with H 2S (10.8), but

the sensor cannot recover. It is also sensitive to Ethanol (5.3), and can recover, so I

just investigate the sensing properties of this sensor with Ethanol gas at high

temperature. CO and NH 3 gas are nearly not detected by this sensor.

b) Investigation of sensing property to Ethanol gas

Below are graph about the resistance versus Ethanol concentration of

samples at 400 oC


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244 248 (2008).17. In-Sung Hwang, Young-Jin Choi, Jae-Hwan Park & Jae-Gwan Park Synthesis

of SnO2 Nanowires and Their Gas Sensing Characteristics. Journal of theKorean Physical Society 49, 1229 1233 (2006).

18. Sun-Woo Choi, Sung-Hyun Jung & Sang Sub Kim Significant enhancement of the NO2 sensing capability in networked SnO2 nanowires by Au nanoparticlessynthesized via -ray radiolysis. Journal of Hazardous Materials 193 , 243 248(2011).

19. Hwang, I.-S. et al. Large-scale fabrication of highly sensitive SnO2 nanowirenetwork gas sensors by single step vapor phase growth. Sensors and Actuators

B: Chemical 165 , 97 103 (2012).

graduation thesis ha minh tan final v2

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