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(A) Schematic Diagram of DC Sputtering System; (B) Image of Sputtering System at Itims Institute.

air to increase the contact ability and stability between Ag 2 O nanoparticles and SnO 2 nanowires .

Ag 2 O oxide is a p- type semiconductor oxide . Therefore, in the fabricated sensor, a heterojunction layer of different n-p carrier type is formed at the contact surface between the two oxides, Ag 2 O and SnO 2 .


2.2.3. Fabrication of heterostructured SnO 2 /ZnO nanowires


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To enhance the gas-sensing performance of the SnO 2 nanowire structure , we modified the surface of the SnO 2 nanowire (fabricated as presented in section 2.1) with other metal-semiconductor metal oxide nanoparticles to enhance the effect of the heterojunction layer between the two types of metal-semiconductor oxides on the gas-sensing properties of the sensor.

SnO 2 /ZnO heterostructure nanowires were fabricated by CVD method in two steps. Here, we chose to directly fabricate the heterostructure on the electrode chip. The electrode chip was integrated on a thermally oxidized Si (100) substrate, on which the comb part of the electrode was coated with a 5 nm thick Au layer. SnO 2 /ZnO heterostructure nanowires were grown in two steps as follows:

Step 1: Grow SnO 2 nanowires according to Procedure 2.1


Step 2: Grow the ZnO nano layer from 0.3 g of ZnO and Carbon mixture powder in a 1:1 ratio. Place the SnO 2 nanowire grown in step 1 on the surface of the boat and put it into a quartz tube placed at the center of the furnace. During the ZnO growth, O 2 and Ar gases are continuously supplied with a flow rate of 1 sccm and 30 sccm respectively, the temperature at the center of the furnace is 800 o C. The ZnO layer is grown for 5, 10, 15 minutes to create a structure with different thicknesses of modified layers on the surface of the SnO 2 nanowire .

After the wire growth process, the samples were annealed at 500 ºC for 5 hours to increase the contact stability between ZnO nanoparticles and SnO 2 nanowires . ZnO oxide is an n- type semiconductor oxide . Therefore, in the fabricated sensor, a heterojunction layer of the same type of carrier n- SnO 2 /n- ZnO is formed .

2.2.4. Fabrication of heterostructured SnO 2 /WO 3 nanowires


The SnO 2 /WO 3 heterostructured nanowire sensor was fabricated in two steps. In the first step, the SnO 2 nanowire was grown using the CVD method (as fabricated in section 2.1). In the next step, the WO 3 nanoparticle layer was coated on the surface of the SnO 2 nanowire using the Sputtering method on the DC cathode sputtering system at ITIMS (Figure 2.3). The sputtering technique for fabricating thin metal films or modifying materials is based on the principle of kinetic energy transfer by using inert gas ions accelerated under an electric field to bombard the surface of the target material, transferring kinetic energy to the target atoms to fly towards the substrate and deposit on the substrate. The sputtering system at ITIMS-DKBKHN uses a high voltage DC power source to accelerate the inert gas ions (Ar). Initially, the targets are placed on the cathode in a high vacuum environment with a pressure of approximately 5.10-6 bar thanks to a mechanical pump system and a turbo pump, then the sputtering chamber is filled with Ar gas at a low pressure of ~10-3 bar . An electric field is created to accelerate the Ar gas ions by applying a high voltage between the target (negative electrode) and the sample to be sputtered (positive electrode). This process is a discharge process accompanied by Plasma glow. This method can be applied to many different types of target materials, the adhesion of the films on the substrate is very high. The produced film has low surface roughness and has a much more precise thickness than the thermal evaporation method in vacuum.



Figure 2.6. (A) Schematic diagram of DC sputtering system; (B) Image of sputtering system at ITIMS Institute.

To modify the surface of SnO 2 nanowires by WO 3 nanoparticles by DC sputtering method, we used W as the target material and SnO 2 nanowires prepared in the first step as the substrate. The DC sputtering power was 50 W, the Ar and O 2 gas flows were 15 sccm and the sputtering times were 36, 60, 120, 240 seconds respectively to create WO 3 nanolayers on the surface of SnO 2 nanowires with estimated thicknesses of 3 nm, 5 nm, 10 nm and 20 nm respectively .

After the sputtering process, the sample was annealed at 600 ºC for 5 hours in air to increase the ability to form the WO 3 structure (tungsten oxide can exist in some other structures such as WO 2 , W 2 O 3 , W 2 O 5 , W 18 O 49 ) and the stability of the contact between WO 3 nanoparticles and SnO 2 nanowires . WO 3 oxide is an n- type semiconductor oxide . Therefore, in the fabricated sensor, a heterojunction layer of the same type of carrier n- SnO 2 /n - WO 3 is formed .


2.3. Method of surveying the structure and morphology of the sensor


The synthesized SnO 2 /SMO heterostructure sensors were morphologically examined by field emission scanning electron microscopy (FESEM, Hitachi S-4800, Japan), high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20, FEI), selected area electron diffraction (SAED) and elemental composition analysis using an energy-dispersive X-ray spectrometer (EDS) integrated in the electron microscope. These analytical techniques were performed at the Institute of Advanced Science and Technology (AIST) - Hanoi University of Science and Technology and the Vietnam Academy of Science and Technology. The crystal structure of the sample was determined by X-ray diffraction , using characteristic CuK α radiation with a wavelength of 1.54056 Å at a scanning angle of 0.015%. X-ray diffraction analysis was performed on a Bruker D5005 machine at the Faculty of Physics - University of Science - Vietnam National University, Hanoi.


2.4. Method of surveying gas-sensitive characteristics


To evaluate the parameters of a gas sensor that changes conductivity (resistance) in gas environments or in environments with different gas concentrations, the sensor resistance is often measured continuously over time in a background gas environment (usually

air) to the gas medium to be measured. The H 2 S gas sensitivity characteristics of the fabricated sensors were investigated at different temperatures and gas flow rates by dynamic measurement method.

The dynamic measurement method is a method commonly used in research and development of gas sensors. In this method, gas is continuously blown through the measuring chamber and the resistance of the sensor is continuously measured over time. When switching from the background gas state to the measured gas state by a reversing valve, it must be ensured that the flow of gas blown into the measuring chamber is constant (stable pressure). According to this method [1], the concentration of the gas to be measured C (ppm) is determined by the formula:

𝐶 = 𝐶 𝑓

(2.1)

𝑜 𝑓+𝐹

In which f ( sccm), F (sccm) are the standard gas and carrier gas flow rates respectively, C o (ppm) is the standard gas concentration.

Normally, the carrier gas is clean air, which is also the background gas (when compared) in the measurement, and is also the gas diluted into the gas concentrations to be measured [1]. For the dynamic measurement method, the response and recovery time of the sensor is almost independent of the volume of the measuring chamber but depends on the air flow rate through the measuring chamber. In addition, the response and recovery time of the sensor depends on the nature of the gas-sensitive material layer and on the working temperature of the sensor.

The main components of the gas measurement system for the dynamic measurement method at ITIMS Institute (Figure 2.6) include:

- Gas mixing system: uses gas flow controllers to create the gas concentration to be measured.

- Gas measurement chamber: The sample is placed on a heating furnace connected to a temperature controller to create the temperature to be investigated. The maximum temperature of the furnace is 450 o C. The sample is connected to a Keithley 6002 A source and controlled by the Labview program through the probes placed on the electrode feet. In addition, the gas measurement chamber is also connected to a pump to suck the exhaust gas out.

In this study, to generate the measured gas concentrations we used a one-pass gas mixing system with three mass flow controllers (MFCs) as shown in Figure 2.7.


Figure 2.7. Schematic diagram of the gas measurement system for the dynamic measurement method at ITIMS Institute [1].


The MFC-1 controller is used to control the flow rate (f) of the standard gas, the MFC-2 (F) is used to control the flow rate of the carrier gas (usually air) to mix with the standard gas flow. Just changing the gas flow ratio in these two flow controllers will obtain different gas concentrations while ensuring the total gas flow remains constant (Equation 2.1). The MFC-3 is used to control the baseline flow rate of the carrier gas, which has a flow rate equal to the total gas flow rate of MFC-1 and MFC-2. On the other hand, the gas mixing system requires pressure regulators (1) and (2) to stabilize the pressure before and after the MFCs. Furthermore, a 2-way air valve is used to bring air into the measuring chamber. This valve must ensure the following principle: when switching the valve, the air flow from MFC-1 and MFC-2 enters port 1 and exits 3, while the air from MFC-3 enters from port 2 and exits port 4. At this time, the measured air flow enters the measuring chamber and the background air flow goes to the exhaust pipe. When switching to the recovery measurement state, the air from MFC-1 and MFC-2 goes from port 1 and exits port 4 into the exhaust pipe, while the air flow from MFC-3 enters from port 2 and exits port 3. At this time, the background air enters the measuring chamber. In some cases, it is possible to design 4 electric valves as shown in Figure 2.6 in the red dashed circle. These four valves will have two states: valves 1 and 2 are closed while 3 and 4 are open. When switching states, 3 and 4 are closed, while 1 and 2 are open.

2.5. Conclusion of Chapter 2


In chapter 2, the thesis presented the experimental steps in fabricating SnO 2 /SMO heterostructure nanowire sensors and the method of investigating the structural morphology of the fabricated sensors as well as the method of investigating the gas sensitivity of the sensors:

 Fabrication of SnO 2 nanowires by thermal evaporation method, with Pt electrode coated with 5 nm thick Au catalyst layer.

Modifying the surface of SnO 2 nanowires by electron beam evaporation with Ni metal for different times combined with thermal annealing in air to create heterostructure nanowires with different types of n- SnO 2 / p- NiO carriers with different NiO coating thicknesses.

Modify the surface of SnO 2 nanowires by dipping SnO 2 nanowires in AgNO 3 salt solution to create heterostructure nanowires with different carrier types n- SnO 2 / p- Ag 2 O with different densities of modified nanoparticles.

Modify the surface of SnO 2 nanowires with ZnO layer by CVD method with the growth time of the modified layer being 5, 10, 15 minutes to create heterostructured nanowires with the same type of n- SnO 2 / n- ZnO carriers with different thickness of the modified layer.

Modify the surface of SnO 2 nanowires by DC sputtering method with different metal targets W and sputtering times combined with annealing in air to create heterostructure nanowires with the same type of n- SnO 2 / n- WO 3 carriers with different thicknesses of modified nanolayers.

Present methods for studying the morphology and structure of the manufactured sensors.

Present the method of investigating the sensitivity of H2S gas of sensors.

CHAPTER 3: RESULTS OF RESEARCH ON FABRICATION AND GAS SENSITIVITY OF SEMICONDUCTOR METAL OXIDE NANOWIRE STRUCTURE n- SnO 2 / p- SMO


In this chapter, we present the results of the study on the fabrication of heterostructured semiconductor metal oxide nanowires with n- SnO 2 / p- SMO carriers and the low concentration H 2 S gas sensitivity characteristics (1 ÷ 10 ppm) of the fabricated samples. The study results also show the influence of the thickness of Ag 2 O, NiO modification layer on the surface of SnO 2 nanowires and the working temperature of the sensor on the gas sensitivity properties. The H 2 S gas sensitivity mechanism of the sensor is explained by the energy band theory and the sulfidation process.


n- SnO 2 / p- Ag 2 O structured nanowire sensor


The structure, chemical composition and structural characteristics of SnO 2 nanowires surface modified by Ag 2 O nanoparticles by CVD method combined with dip coating method were studied by scanning electron microscopy (SEM; JEOL 7600F), and energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (HRTEM; JEOL 2100F) and X-ray diffraction (XRD; ….) [2]. After morphological and structural testing, the sensors were examined for their sensitivity to H 2 S gas at concentrations of 0.1 ÷ 1 ppm.


3.1.1. Morphology and structure of the sensor


To investigate the morphology, structure and properties of the material, we selected unmodified SnO 2 nanowire sensor samples (S0), the SnO 2 /Ag 2 O structure created corresponding to the SnO 2 nanowire immersed in AgNO 3 salt solution with a concentration of 0.2 mM, 1 immersion (S2) and the SnO 2 /Ag 2 O structure created corresponding to the AgNO 3 salt concentration of 1 mM, 20 immersions (S5) for SEM, EDS and TEM analysis. Figure 3.1 (A) illustrates the SEM image of the SnO 2 nanowire sensor (S0) grown on an Au-coated Pt electrode . Notably, the comb electrode used had a gap distance of 20 μm, the long grown SnO 2 nanowires bridged the gaps between the two electrodes. SnO 2 nanowires grew mainly on the surface of Pt comb electrodes, but the

Their length is controlled enough to connect between the comb teeth and thus act as channels in the gas measurement of the sensor. The average diameter of the nanowires is about 70 nm. The surface of the unmodified SnO 2 nanowires is smooth, with a single-crystal structure . This result is consistent with the nanowire growth by the vapor-liquid-solid mechanism [98]. Here we used gold as a catalyst in the growth of SnO 2 wires , so the belt-like NWs are obtained in the initial state. The SnO 2 nanowires have a single-crystal structure as presented in the article by NXThai et al. [67]. Composition analysis of SnO 2 nanowires by energy dispersive X-ray spectroscopy (EDS) - Figure 3.1 (B) shows the existence of O 2 , Sn and Pt elements where Pt is from the electrode, while O 2 and Sn are from the SnO 2 nanowires .

The SEM images of SnO 2 NWs after being modified by Ag 2 O nanoparticle layer (S2) are shown in Figure 3.1(C), in which the insets are low magnification SEM images. The comb electrodes were covered with SnO 2 NWs, the nanowire surface was coated with Ag 2 O nanoparticles by dip coating method which maintained the morphology of SnO 2 NWs but the nanowire surfaces were not as smooth as the original sample and very small particles were seen in the SEM image. The high magnification SEM image showed the presence of Ag 2 O nanoparticles on the surface of SnO 2 nanowires . EDS analysis of sample S2 - Figure 3.1(D) confirmed the presence of Ag at the energy level of 2.98 eV. The SEM image of sample S5 is shown in Figure 3.1(E). With the increase of AgNO 3 concentration to 1 mM and the number of dip coating times is 20, the morphology of the modified SnO 2 nanowires changes. The surface of the sensors is found to have increased roughness. Ag 2 O nanoparticles can be seen in the SEM image of sample S5. Continuous or discontinuous Ag 2 O nanoparticles are attached on the surface of SnO 2 nanowires . EDS analysis of sample S5 in Figure 3.1(F) shows that the density of Ag is very high (about 3.5wt%). This result demonstrated that increasing the concentration of AgNO 3 salt solution and the number of dip coating times can increase the density of Ag 2 O nanoparticles on the surface of SnO 2 nanowires .

To further study the modification results of Ag 2 O nanoparticles on the surface of SnO 2 nanowires , we selected samples S0, S2 and S5 to observe by transmission electron microscopy (TEM) (Figure 3.2).

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