Graduation Thesis Ha Minh Tan Final v2
Transcript of 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
Figure 43: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of
5 mg sample ....................................................................................... 43
Figure 44: Resistance versus concentration of NO 2 of 10 mg sample ............... 44
Figure 45: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of
10 mg sample ..................................................................................... 45
Figure 46: Resistance versus concentration of NO 2 of 20 mg sample ............... 46
Figure 47: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of
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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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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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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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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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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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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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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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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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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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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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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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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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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* *
* *( ) 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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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
Figure 43: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of 5 mg sample
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
Figure 45: Sensitivity versus concentration at 50 oC, 100 oC and 150 oC of 10 mg sample
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 Electrode Electrode
Rw RJ
Rw RJ Rw Electrode Electrode
Rw
(b)
(a)
(c)
(d)
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HA MINH TAN MSE K52 52
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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GRADUATION THESIS
HA MINH TAN MSE K52 53
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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GRADUATION THESIS
HA MINH TAN MSE K52 54
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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HA MINH TAN MSE K52 56
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)
Response timeRecovery time
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)
Response timeRecovery time
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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HA MINH TAN MSE K52 58
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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HA MINH TAN MSE K52 59
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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HA MINH TAN MSE K52 61
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).