Summary of doctor thesis in Material science: Fabrication of aligned carbon nanotubes and graphene materials for biosensor applications

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Objectives of the study: To find out about orientational CNTs and graphene material - the formation, methods of synthesis, properties and applications; optimization of technological conditions to produce high quality oriented CNT and graphene materials using the thermal CVD method.

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Nội dung Text: Summary of doctor thesis in Material science: Fabrication of aligned carbon nanotubes and graphene materials for biosensor applications

1 MINISTRY OF VIETNAM ACADEMY OF EDUCATION AND TRAINING SCIENCE AND TECHNOLOGY GRADATE UNIVERSITY OF SCIENCE AND TECHNOLOGY  CAO THI THANH FABRICATION OF ALIGNED CARBON NANOTUBES AND GRAPHENE MATERIALS FOR BIOSENSOR APPLICATIONS Major : Electronic materials Code : 62.44.01.23 SUMMARY OF DOCTOR THESIS IN MATERIAL SCIENCE HA NOI - 2018
2 The thesis was completed at Key Laboratory for Electronic Materials and Devices, Institute of Materials Science, Vietnam Academy of Science and Technology. Supervisors: 1. Prof. PhD. Tran Dai Lam 2. PhD. Nguyen Van Chuc Reviewer 1: Reviewer 2: Reviewer 3: The dissertation will be defended at Graduate University of Science and Technology, 18 Hoang Quoc Viet street, Hanoi. Time: .............,.............., 2018 The thesis could be found at: - National Library of Vietnam - Library of Graduate University of Science and Technology - Library of Institute of Science Materials
1 INTRODUCTION 1. The thesis necessity Owing to the unique properties such as: larger surface area, high conductivity, high mechanical stability, excellent electronic mobility, inert chemical properties in solution and high compatible biological properties,... carbon nanotubes (CNTs) and graphene materials are promising to apply in many other fields, such as electronics, energy and fabrication of ultra-small sensors. In particular, field effect transistor (FET) and especially ion sensitive field effect transistor (ISFET) using CNTs/graphene are showed that they have high sensitivity, quick response time and very low detection limit. This is because the CNTs/graphene material in the sensor is in direct contact with the analytical material and directly convert the biological reactions on the electrode surface into electrical signals. Therefore, a small change of analytical material could be detected. Some of the sensors based on FET and ISFET configurations using CNTs and graphene materials have been launched in the detection of certain substances such as glucose, DNA, atrazine, E.coli, etc... In order to improve the potential applications of CNTs and graphene materials, especially, applications for biosensors, we need control the quality of materials such as: density, orientation, defection, purity of CNT as well as the layers, uniform of a graphene film. This is also one of the major challenges for many research groups in Vietnam and other groups in the world. For these reasons, we have chosen the topic of thesis is: “Fabrication of aligned carbon nanotubes and graphene materials for biosensor applications”
2 2. Objectives of the study i) To find out about orientational CNTs and graphene material: the formation, methods of synthesis, properties and applications. ii) Optimization of technological conditions to produce high quality oriented CNT and graphene materials using the thermal CVD method. iii) Application of graphene materials in ISFET- based biosensor to detect the atrazine pesticide residues. 3. Main research contents of the thesis i) Fabrication of vertically aligned CNTs (VA-CNTs) and horizontally aligned CNTs (HA-CNTs) on surface of Si substrate by CVD method. The effect of parameters: catalysis source gas, the temperature and catalysis gas source flow on structure, property and quality of orientational CNTs. ii) Fabrication of graphene by thermal CVD. The effect of parameters: temperature, pressure, catalysis gas flow and surface morphology of catalysis material on the quality of graphene. iii) Production of field effect transistor based on graphene material and application of graphene-FET to detect atrazine residues. 4. Thesis structure: This thesis consists of 145 pages: introduction, four chapters in content, conclusion. The main results were published on 10 journals: 07 articles was published on international journal, 02 article was published on national journal and 01 presentation at an international workshop.
3 CHAPTER 1: INTRODUCTION OF CARBON NANOTUBE MATERIAL This chapter aims to present an overview of CNTs and graphene in the areas of synthesis, electronic structure, properties, information and growth mechanisms, analytical technics and applications. We also present an overview of biosensor such as structure, operation principle as well as the detection mechanism of the field effect transistors based biosensor using graphene material to detect atrazine residues. CHAPTER 2: FABRICATION OF ORIENTED CARBON NANOTUBE BY CHEMICAL VAPOR DEPOSITION 2.1 Thermal CVD system used for fabrication of oriented CNTs Thermal CVD system used for fabrication of oriented CNTs consist of following three main parts: furnace, reaction chamber and gases Figure 2.1. The CVD system for fabrication flow control meter CNTs and graphene at Laboratory of carbon as described in fig. nanomaterials, Institute of Materials Science 2.1. 2.2 Fabrication of VA-CNTs material 2.2.1 Preparation of substrate and catalyst material The silicon substrate with SiO2 layer of 90 nm used to deposit the catalyst particles. The substrate were cut into small pieces of 5 mm  5 mm and cleaned before coating catalyst on the surface.
4 Catalysts materials used to fabricate the VA-CNTs materials are cobalt ferrite nanoparticles (CoxFeyO4) which are synthesized by thermal decomposition method as listed in Table 2.1. Table 2.1: The samples of cobalt ferrit catalytic particles used to fabricate VA-CNTs material. Symbol of Formular Component ratio Diameter (nm) samples M1 Fe3O4 Co2+:Fe3+ = 0:3 8.3 ± 0.6 M2 CoFe2O4 Co2+:Fe3+ = 1:2 6.3 ± 0.5 M3 CoFe1.5O4 Co2+:Fe3+ = 1:1.5 5.7 ± 0.5 M4 CoFeO4 Co2+:Fe3+ = 1:1 4.9 ± 0.5 2.2.2 Fabrication process of VA-CNTs The procedure and steps for fabrication of VA-CNTs by thermal CVD method are divided into 6 stages as Figure 2.4: Fabrication process of VA-CNTs by described in fig. thermal CVD method. 2.4 2.2.3. Fabrication results of VA-CNTs 2.2.3.1. Influence of catalyst solution concentration To find suitable catalytic solutions for VA-CNTs growth, we investigated two catalysts: Fe3O4 (M1) and CoFe1.5O4 (M3) dispersed in n- hexane at different concentrations 0.01 g.ml−1, 0.026 g.ml−1, 0.033 g.ml−1 and 0.04 g.ml−1. The results show that the length, density and growth rate of VA-CNTs depend on the concentration of the catalyst solution (fig.
5 2.6). With two Fe3O4 and CoFe1.5O4 catalysts, the optimum solution concentration for the synthesis of VA-CNTs was Figure 2.6: SEM images of VA-CNTs growth by 0.026 g.mL-1 and Fe3O4 and CoFe1,5O4 with the different solution 0.033 g.mL-1, concentrations. respectively. 2.2.3.2. Influence of water vapor In this study, the samples of VA-CNTs were fabricated in two cases: without and with water vapor during CVD process using 0.026 g.ml-1 Fe3O4 (M1) catalyst. The conditions of CVD: 750°C, Ar/H2/C2H2 = 300/100/30 sccm, and 30 min. Water vapor was introduced during the CVD process by bubbling partly argon Figure 2.7. SEM images and size distribution gas supply (60 sccm) of VA-CNTs growth from 0.026 g.ml-1 Fe3O4 through water prior to (M1) catalyst in two cases: a) without water entering the furnace. vapor b) with water vapor
6 The SEM results (fig. 2.7) show that the addition of water vapor during CVD has Figure 2.8: TEM images of two VA-CNTs growth significantly in the same CVD condition in two cases: a) without altered the water vapor; b) with water vapor length, diameter and growth rate of CNTs. The length of the CNTs increased from 6.5 μm to 40.5 μm corresponding without and with water vapor, respectively (corresponding to the growth rate of CNTs increased from 200 nm/min to 1330 nm/min). In addition, the density of CNTs also increased and CNTs became more uniform when water vapor was introduced during the CVD process. Figure 2.8 is a TEM image of two samples of VA-CNTs synthesized without water vapor (Fig. 2.8a) and with water vapor at 60 sccm during CVD process (Fig. 2.8b). The TEM images can clearly distinguish amorphous carbon and structural defects of carbon nanotubes. During CNTs growth, under certain conditions, CNTs are formed in a bamboo structure (as bamboo crops) which are considered undesirable structural defects. Unlike the CNTs grown with H2O vapor, the CNTs have a hollow structure, straight, thin tube, small diameter and uniform. The analysis of Raman spectra evidences increased graphitization of CNTs samples with addition of water vapor during CVD process. In addition, the effect of water vapor flow rate on the growth rate and properties of the VA-CNTs was also investigated. We have grown VA-CNTs using 0.033 g.mL-1 CoFe1.5O4 (M3) catalyst, in the same CVD condition: at 750°C, Ar/H2/C2H2 = 300/100/30 sccm, and CVD time 30 min, with different water vapor flow rates: 20 sccm, 40 sccm, 60 sccm
7 and 80 sccm. The a) b SEM results (Fig. ) 2.10) show that at 60 sccm of water vapor flow rate the length, the c d ) ) density of CNTs is highest and the orentiation of CNTs is also best Figure 2.10: SEM images of VA-CNTs samples among four water VA-CNTs growth from CoFe1,5O4 (M1) 0.033 vapor flow rate g.mL-1catalyst in the same CVD condition with investigated. with different water vapor flow rate 2.2.3.3. Influence of the ratio of catalyst components In this study, we have grown VA-CNTs using four cobalt ferrit samples M1, M2, M3, M4 at different ratios of Co2+:Fe3+ = x:y precursors, in the same 0.033 g.mL-1 concenstration and in the same CVD condition. The results of SEM (fig. 2.12) show that adding the Co2+ component to the catalyst mixture plays a very important role in improving the growth Figure 2.12: SEM images of VA-CNTs rate, length, density or growth from 04 catalysts with component yield of the VA–CNTs. ratios Co2+:Fe3+ = x : y: a) x:y =0:3, b) The highest length of the x:y =1:2, c) x:y =1:1.5, d) x:y =1:1, VA-CNTs was 128.3 ± respectively, in the same CVD condition.
8 5.5 μm on the (M3) catalyst with a Co2+:Fe3+ ratio of 1:1.5 (corresponding to a 40% added Co2+) which is significantly higher than that of the non-Co2+ (M1) catalyst sample. The density of VA-CNTs that are grown on the (M3) catalyst are also much higher than those of others VA-CNTs. This is explained by the differences in physical properties such as transition temperature, melting temperature, and this causes the metal particles to separate, reducing the diffusion and agglomeration of small catalyst particles into large particles of catalyst particles at a high temperature in CVD conditions. This makes the catalyst particles disperse more evenly than the surface of the substrate and keeping the initial small size of the catalyst particle, facilitating the formation and development of the VA-CNT material. However, if too much Co2+ content was added, which meant that the proportion of Fe3+ was reduced, the height and density of CNTs decreased (fig. 2.12d), which reduced the yield of VA- CNTs. 2.3. Fabrication of horizontally aligned CNTs (HA-CNTs) 2.3.1. Preparation of substrate and catalyst material We used a silicon wafer with a 90 nm thick-SiO2 as substrate for the catalysts to fabricate HA-CNTs. We used FeCl3.6H2O salts as catalyst precursors. The salt was dispersed with deionized water at different concentrations of 0.1M, 0.01M, and 0.001M. The solution was then deposited on the clean silicon substrate by spin-coating at a spin Figure 2.18: Fabrication process HA- rate of 6000 rpm. CNTs material by thermal CVD method.
9 2.3.2. Fabrication process of HA-CNTs The procedure and steps for fabrication of HA-CNTs by thermal CVD method are divided into 4 stages as described in fig. 2.18. 2.3.3. Fabrication results of HA-CNTs 2.3.3.1. Influence of catalyst solution concentration We have growth HA-CNTs using a solution of FeCl3 with different solution concentrations: 0.001M, 0.01M, and 0.1M under the same CVD conditions: the growth temperature of 1000°C, Ar/C2H5OH:H2 of 20:30 sccm, and CVD time of 60 min. Figure 2.20 shows the SEM of HA-CNTs grown using FeCl3 with different concentrations of solution: 0.001M, 0.01M, 0.1M. The SEM results show that the density of CNTs increases with increasing the concentration of FeCl3 catalyst solution from 0.001 M to 0.01 M. However, when the solution concentration was too high (fig. 2.20c), the CNTs were not straight, overlapping, coils and quickly end the long growing Figure 2.20: SEM images of HA-CNTs grown process. By using FeCl3 with different solution concentrations: counting the a) 0.001M, b) 0.01M, c) 0.1M. number of CNTs at different distances (1 mm, 5 mm, 10 mm, and 15 mm) from the catalytic, we can plot the density distribution of CNTs by the length of the substrate corresponding to different catalyst solutions. The difference in density and length of the HA-CNTs is due to the difference in size of the catalyst particles when we change the concentration of the catalyst solution. In this study, the FeCl3 catalyst solution concentration of 0.01 M was appropriate. HA-CNTs were formed at this concentration with high density and good orientation.
10 2.3.3.2. Influence of CVD temperature The HA-CNTs were grown at four different temperatures from 850°C to 1000°C with growth conditions: synthesis time of 60 min, Ar/C2H5OH:H2 = 20:30 sccm. The SEM images (fig. 2.23) Figure 2.23: SEM images of HA-CNTs indicates that the density grown at four different temperatures: a) of CNTs increases with 850oC, b) 900oC, c) 950oC, d) 1000oC increasing temperature from 850°C to 950°C. The explanation is that a higher growth temperatures promote a higher density of nanotube nucleations resulting in a higher density of ulralong nanotubes. However, when the temperature was too high, other carbon products, such as amorphous carbon begin to deposit and cover the catalyst particles, and affecting on the germination and growth processes of CNTs. Under our experiment condition, the optimum temperature for the growth process of HA-CNTs was 950oC. 2.3.3.3. Influence of carbon source flow rate We study the influence of ethanol flow rate on the CNTs density and alignment. The results of SEM images (fig. 2.24) show that the CNTs density increases with increasing ethanol flow rate. The highest CNTs density achieves when the ethanol/Ar flow rate was 40 sccm (~150 tubes/mm). However, at this flow rate of ethanol, the density of CNTs decreases rapidly and the ratio of CNTs extending all the length of substrate was low (~25/150 tubes). For others cases, the ethanol flow rate
11 was 30 sccm for the highest density of CNTs (~80 tubes/mm), CNTs have good orientation, high purity and the ratio of CNTs extending all the length of substrate is about 30/80. As a result, the Figure 2.24: SEM images of HA-CNTs optimum flow rate of grown using 0,01 M FeCl3 with different ethanol/Ar to grow the flow rate of ethanol/Ar: a) 10 sccm, b) 20 HA-CNTs was 30 sccm, c) 30 sccm, d) 40 sccm. sccm. 2.3.4. Growth mechanism and structure of HA-CNTs materials To demonstrate the growth mechanism and aligned along the gas streamlines of HA-CNTs in fast- Figure 2.25: a,b) Optical microscopy and SEM heating CVD image of a SiO2/Si substrate with a slit c) SEM method, we image of HA-CNTs SiO2/Si substrate with a gap proceeded to grow of 60 μm HA-CNTs on a SiO2/Si substrate with a slit of 60 μm width and directly growth HA- CNTs on field effect transistor (FET) electrodes consisting of 19 pairs of S-D electrodes (source-drain) with an electrode spacing of 30 μm and the total thickness of the metal layers of the electrode is 188 μm (Cr/Pt = 8/180 μm). The catalyst used to grow HA-CNTs in this case is 0.01M FeCl3 solution with the CVD conditions were optimized: temperature
12 950oC, 60 minutes CVD time and gas Ar/ethanol: H2O flow rate = 30/30 sccm. The results of SEM Figure 2.26: SEM image describes structure of images (fig. 2.25 FET electrode and HA-CNTs grown on the FET and fig. 2.26) shows that the HA-CNTs grown accross the slit and accross the rough surface of FET electrode. The structure of the HA- Figure 2.28: a)Illustration of the experimental CNTs materials was setup of HA-CNTs growing on the TEM grid, determined by image b) SEM images and c)HRTEM image of a HA- analysis of HRTEM, CNT on the TEM grid. Raman spectrum. The results of the HRTEM analysis (fig. 2.28c) and the Raman spectrum (fig. 2.29) show that CNTs have a diameter of 1.5 nm and 70% of HA- CNTs are double Figure 2.29: Raman spectrum of HA-CNTs. wall CNTs (DWCNTs), 30% the remaining are single wall (SWCNTs) and about 50% of them are semiconductor.
13 CHAPTER 3: FABRICATION OF GRAPHENE MATERIAL BY THERMAL CHEMICAL VAPOR DEPOSITION METHOD 3.1. Thermal CVD system used for fabrication of graphene The thermal CVD system used to fabricate graphene films is also the thermal CVD system used to fabricate oriented CNTs materials but has improved the vacuum system. 3.2. Prepare catalyst material Catalyst material for synthesis of graphene films is poly-crystalline Cu foils (25 m-thick) of 99.8% purity (Alfa-Aesar). Cu foils were cut into small pieces of 2-5 cm2 and cleaned before growing graphene. 3.3. Fabrication process of graphene material on Cu substrate The procedure and steps for fabrication of graphene by thermal Figure 3.2: Fabrication process graphene CVD method are material by thermal CVD method divided into four stages atmospheric pressure condition as described in fig. 3.2. 3.4. Fabrication results of graphene films on the Cu substrate 3.4.1. Influence of surface morphology of Cu substrate To study the influence of surface morphology of Cu substrate to the quality of graphene films, we compared the quality of graphene films grown on the Cu substrate treated by two different methods: 5% HNO3 acid for 10 minutes and treatment by the electrochemical polishing method using 85% H3PO4 acid at 1.9 V for 15 minutes. Results of the measurement and calculation are obtained from Raman spectra (fig. 3.8)
14 of graphene show that the graphene films were fabricated on Cu substrate which was treated the surface by the electrochemical polishing method having the highest quality and the lowest Figure 3.8: Raman spectra of graphene number of layers (about films on Cu substrate in three cases: a) 2 layers) in the three before treatment, b) after treatment by HNO3 5% and c) after treatment by the graphene materials electrochemical polishing method. investigated. The quality of graphene is shown via values: I2D/IG = 1.28 is the highest, ID/IG = 0.18 is the lowest, the full width at half maximum (FWHM) = 38.05 cm-1 is the lowest, and the position of 2D = 2731.68 is the lowest. Therefore, electrochemical polishing method was chosen to treat the surface of Cu before synthesizing graphene films in all subsequent analyzes. 3.4.2. Influence of the CVD temperature To study the influence of the growth temperature on the graphene quality, we used five electropolished Cu samples and grown at different temperatures from 850oC to 1030oC with the same CVD condition: CH4 as a carbon source, a synthesis time of 30 minutes, Ar/H2/CH4 flow rates of 1000/300/20 sccm. Raman spectra in Fig. 3.10 show that no graphene growth occurs at 8000C. With a higher temperature 8000C, the appears the graphene character peaks inclusion D peak, G peak, 2D peak at the positions 1370 cm-1, 1590 cm-1 and 2734 cm-1 respectively. Thus, fabricate graphene on the Cu substrate surface with the CH4 catalyst source gas which requests the temperature CVD satisfy the condition
15 higher 8500C. The quality factor (number of layers, uniformity, defect, impurity) of the graphene film was identified via 2D peak position, FWHM, I2D/ID and ID/IG. The results show that the Hình 3.10: Raman spectrum of graphene films CVD temperature on Cu substrate grown at different o o temperatures from 850 C to 1030 C with the 10000C is the suitable same CVD condition for fabricating the graphene fims on the Cu substrate with the CH4 source gas. 3.4.3. Influence of hydrocarbon source flow rate To study the influence of the hydrocarbon source flow rate on the graphene quality, we grown graphene at different flow rates of CH4: 0.5 sccm; 2 sccm; 5 sccm; 10 sccm; 20 sccm and 30 sccm, with the same CVD condition: the growth temperature of 1000°C, a synthesis time of 30 minutes, Ar/H2 flow rates of 1000/300 sccm. Fig. 3.13, and fig. 3.15 present Raman spectra and HRTEM images of graphene films on the Cu Figure 3.13: a) Raman spectra and b-e) Lorentzian lineshape of 2D band of graphene films on the Cu substrate with at different flow rate of CH4
16 substrate with at different flow rate of CH4, respectively. The analyst results Figure 3.15: HRTEM images of graphene films shown that the on the Cu substrate with flow rate of CH4: a) 10 number of layers sccm, b) and c) 30 sccm and quality of graphene films were grately affected by CH4 flow rates. High quality single layer graphene films can be manufactured under atmospheric pressure conditions if the CH4 flow rate is low enough. The number of graphene layers incresed and quality of graphene decresed when the CH4 flow rate was too high. According to our experimental results, the CH4 flow rate of 5 to 10 sccm was the optimum to obtain 1-2 layers of graphene films with high uniform and good quality. 3.4.4. Influence of pressure Pressure has been revealed as key factor during graphene growth. To study the influence of the pressure on the graphene quality, we compared the quality of graphene films grown on the Cu substrate grown by two different conditions: The first grown in atmospheric pressure (APCVD) at 1000°C, a CVD time of 30 minutes, Ar/H2/CH4 flow rates of 1000/300/10 sccm and the second grown in low pressure (LPCVD) at 1000oC, pressure of the reactor of 60 torr, a synthesis time of 30 minutes, H2/CH4 flow rates of 20/0.3 sccm. We also study influence of the reactor vacuum levels on the graphene quality by changing pressure of the reactor from 80 torr to 20 torr. Results of the measurement and calculation are obtained from Raman mapping spectra (fig. 3.16 and fig. 3.17) show that the graphene films synthesized by LPCVD method were higher quality and more uniform than those synthesized by APCVD. The
17 Figure 3.16: Raman spectra of graphene films on the Cu substrate were synthesized by APCVD and LPCVD methods quality and uniformity of the graphene films increases with decreasing the pressure in the reaction chamber. This because of the sublimation of Cu at lower pressure, decrease the number of the sharp structures, thereby making the Cu surface smoother. That is Figure 3.17: Raman spectra of graphene films on the Cu substrate the cause lead to increase the were synthesized with differrent quality and uniformity of pressure of the reaction chamber. graphene films. The lower pressure of the reactor, the lower the density of impurities and residual oxygen. That is also a reason why the graphene films quality increases. Using LPCVD method with pressure of 60 torr, CVD temperature of 1000oC, synthesis time of 30 minutes, H2/CH4 flow rates of 20/0.3 sccm, graphene films is formed with a maximum area about 10 cm2 with a high uniformity and less structural defects. About 70% of the graphene film area is monolayer, 30% of the remaining area is bilayers.
18 CHAPTER 4 ENZYME-GrISFET SENSOR FOR TRACE-DETECTION OF HERBICIDE ATRAZINE 4.1. Basis of the graphene material selection for fabricating the enzyme-GrISFET sensor In this section, we present the basis of the graphene material selection for fabricating the enzyme-GrISFET sensor, including: the synthesis technology, the material properties, the mobility of the carrier charges of the graphene conductive channel and the effective surface area of the material. 4.2. Fabrication of the enzyme-GrISFET sensor The procedure for fabrication of the enzyme-GrISFET sensor as described in fig. 4.3. Fig. 4.9 is an optical microscopy of the enzyme- GrISFET sensor. 4.3. Application of enzyme-GrISFET sensor for detection of pesticide residue atrazine Hình 4.3: Fabrication process of the Figure 4.9: Optical microscopy of enzyme-GrISFET sensor the enzyme-GrISFET sensor The atrazine detection in solution was performed via the competitive inhibition mechanism of itself with respect to the catalytic activity of the enzyme urease. Under the Figure 4.10: Atrazine detection mechanism inhibition of atrazine, the of the enzyme-GrISFET sensor.