Summary of Material science doctoral thesis: Fabrication of flower like, dendrite like nanostructures of gold and silver on silicon for use in the identification of some organic molecules by surface enhanced Raman scattering

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In this thesis, we research and fabricate AgNDs, AgNFs, AuNFs structures on silicon by chemical deposition and electrochemical deposition method for the main purpose of using as SERS substrate. To this target, we have studied the morphology, structure and some properties of the nanostructures produced.

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Nội dung Text: Summary of Material science doctoral thesis: Fabrication of flower like, dendrite like nanostructures of gold and silver on silicon for use in the identification of some organic molecules by surface enhanced Raman scattering

1 MINISTRY OF EDUCATION VIETNAM ACADEMY AND TRAINING OF SCIENCE AND TECHNOLOGY GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY --------------------------------------------- Kieu Ngoc Minh FABRICATION OF FLOWER-LIKE, DENDRITE-LIKE NANOSTRUCTURES OF GOLD AND SILVER ON SILICON FOR USE IN THE IDENTIFICATION OF SOME ORGANIC MOLECULES BY SURFACE ENHANCED RAMAN SCATTERING Major: Electronic material Code: 9 44 01 23 SUMMARY OF MATERIAL SCIENCE DOCTORAL THESIS Ha Noi – 2020
2 This thesis was accomplished in: Graduated University of Science and Technology – Vietnam Academy of Science and Technology. Supervisor: 1. Prof. Dr. Dao Tran Cao 2. Dr. Cao Tuan Anh Peer reviewer 1: Peer reviewer 2: Peer reviewer 3: This thesis will be defended in: The dissertation will be defended in front of the Institute of Doctoral Dissertation Assessment Council, taking place at the Academy of Science and Technology - Vietnam Academy of Science and Technology at ... hour .... ', day ... month ... year 2020 This thesis will be stored in: - Library of Graduated University of Science and Technology - Vietnam National Library
1 Prologue SERS (surface-enhanced Raman scattering) is a modern analytical technique that is being strongly researched in the world and Vietnam to detection trace (ppm-ppb range) of many different molecules, especially organic and biological molecules. In SERS technique, the most important is the SERS substrate. The SERS substrate is a rugged continuous or discontinuous precious metal (silver or gold) at the nano-scale. When analyte molecules are added to this surface, the signal of Raman scattering of the analyte molecule is greatly enhanced. Thus, it can be said that SERS substrate is the device that amplifies Raman scattering signal of the analyte molecule. In Vietnam, there are some researches on the fabrication of Ag, Au precious metal nanostructures and using as SERS substrates. However, the researches mainly focus fabricate nanoparticle structures and so far, fabrication of silver nano-dendrites (AgNDs), silver nano-flowers (AgNFs) and gold nano- flowers (AuNFs ) very few, especially the statements on fabrication of these structures on silicon. For the purpose of studying and researching AgNDs, AgNFs and AuNFs materials on silicon as well as the properties and applications of this material, I chose the title of the thesis is “Fabrication of flowers-like, dendrites-like nanostructure of silver and gold on silicon for using in detection some organic molecules by surface enhanced Raman scattering” In this thesis, we research and fabricate AgNDs, AgNFs, AuNFs structures on silicon by chemical deposition and electrochemical deposition method for the main purpose of using as SERS substrate. To this target, we have studied the morphology, structure and some properties of the nanostructures produced. Then, we use the nanostructures mentioned above as SERS substrates to detect traces of some toxic organic molecules, to test their effectiveness as a SERS substrate. The scientific significance of the thesis The AgNDs, AgNFs, AuNFs structures on silicon have been successfully fabricated by two methods of chemical deposition and electrochemical deposition with the main purpose for using as SERS substrate. The influence of fabrication parameters on morphology and structure of AgNDs, AgNFs, AuNFs was studied in orderly. The mechanism of formation of the above structures has been studied. Đã nghiên cứu sử dụng các cấu trúc nano nói trên như là đế SERS để phát hiện một số phân tử hữu cơ độc hại ở nồng độ thấp. These nanostructures have been used as SERS substrates to detect some toxic organic molecules in low concentrations.
2 The thesis includes 4 chapters as follows: This thesis includes of 125 pages (excluding references) with the following layout: Introduction: Presenting the reasons for choosing topic, methods, purposes of researching. Chapter 1: Overview of surface enhanced Raman scattering. Chapter 2: Methods to fabricate and investigate SERS substrates. Chapter 3: Fabrication and investigation of silver and gold nanostructures on silicon. Chapter 4: Using gold, silver nanostructures like flowers and dendrites as SERS substrates to detect traces of some organic molecules. Conclusion: Presenting the conclusions drawn from the research results. Chapter 1 Overview of surface enhanced Raman scattering 1.1. Raman scattering Raman scattering is inelastic scattering of a photon with material, discovered by Raman and Krishnan in 1928. The frequency of scattering light changes compared to incident light frequency. This amount of change is exactly equal to the oscillation frequency of the matter molecule and does not depend on the frequency of the incident light. So, Raman scattering is specific to each molecule. Raman scattering include of two types: Stockes Raman and anti- Stockes Raman. It should be noted that the intensity of the Raman effect is usually very low (about 10-8 - one hundred million incident photons then one photon is Raman scattering). 1.2. Surface enhanced Raman scattering. Surface enhancement Raman scattering is a phenomenon that when light fly to the analyte molecule adsorbed on the surface of a rugged metal nanostructure, the intensity of the Raman scattering is greatly increased. The metal nanosurface is called SERS substrate. There are two enhancement mechanisms for SERS, which are electromagnetic enhancement mechanism and chemical enhancement mechanism. In which, electromagnetic enhancement mechanism is main contributor. 1.2.1. Electromagnetic enhancement mechanism Surface localized plasmon resonance (LSPR) occurs when the surface plasmon is confined to a nanostruc-ture that Size is smaller than the wavelength of light. From the Fig 1.5, it can see that the electric field of the incident light is an oscillating electric field. In the first half of the cycle, the incident electric field is directed upwards, which has the effect of causing the conduction electrons to move downwards in metal nanoparticles.
3 Thus, the top part of the metal nanoparticles will be positively charge, resulting the metal nanoparticles becoming an dipole. In the second half of the cycle, the electric field of the incident light changes direction, the dipole also changes direction. As a result, the Fig 1.5. Schematic illustration of surface dipole also oscillates with the localized plasmon resonance (LSPR) frequency of the incident light. The with free conducting electrons in metal vibrating dipole produces an nanoparticles that are oriented by electromagnetic field (new light oscillation due to strong connection with source). incident light. If the new electromagnetic field vibrates with the oscillation frequency of the incident light, then we have a resonance. The result, the incident light field is enhanced by E2 times while the scattering field is also enhanced by E2 times, the total field is enhanced by E4 times. 1.2.2. Chemical enhancement mechanism The presence of chemical mechanism with Raman scattering was observed when plasmonic metals are not used. Studies of non-electromagnetic enhancement mechanisms have shown that resonancing between incident light and metal nanostructures can induce charge transfer between analyte Fig 1.6. Three different types of chemical molecules and metal. enhancement mechanisms in SERS. Charge transmission occurs, the metals and molecules of the analyte must be in direct contact with each other. In other words, charge transmission occurs only when the metals and molecules are close enough that the wave functions overlap. The exact mechanism of charge transfer has not been fully understood until now. 1.3. SERS enhancement factor The SERS enhancement factor used in the thesis is the SERS substrate enhancement factor (SSEF) and is calculated by the following formula: I N SSEF  SERS Normal I Normal N SERS
4 Where, ISERS and INomarl are intensity of Raman spectrum of organic molecule adsorbed on SERS and non-SERS substrate. NNormal, NSERS are the medium number of molecules in the volume scattering (V) of non-SERS measurement, and SERS measurement. 1.4. Dependence of SERS on surface morphology of metal nanostructures Fig.1.7 Simulation dependence of the SERS enhancement factor on the distance between two spherical nanoparticles lying close together. It can be seen that when the distance between the two nanoparticles is 2 nm, the SERS enhancement factor is 108 and the enhancement factor decreased rapidly to only 105 when the distance between the two particles increased to 3 nm. The formation of nanoparticle structures with a narrow between them leading to problems. First, it was difficult to bring the nanoparticles closer together with a distance of 2 nm. Second, analyte molecules into 2 nm gap between particles is also extremely difficult. Therefore, the researchers proceeded to change the shape of the metal nanoparticles in the direction enhancing tips of particles to obtain a Fig 1.7. The dependence of SERS strong SERS enhancement. In 2009, P. R. enhancement factor on distance Sajanlal et al demonstrated that SERS of the spherical nanoparticles. enhancement factor of the triangular gold nanoparticle system was 108 (Fig 1.8 a). L. Feng et al fabricated the bow-like silver nanoparticles and the SERS enhancement factor was 109 (Fig 1.8 b). Comparison of SERS enhancement factor obtained from spherical and prism silver nanostructures was also published by S. H. Ciou et al in 2009 (Fig 1.8 (c)). In this comparison, SERS measurements was in solution. The results showed that enhancement factor of the spherical silver nanoparticle was 103, while enhancement factor of the prism- like silver nanoparticle was 105. Fig 1.8. SEM images of nanoparticles with different shapes: a) gold triangular- like; b) silver bow-like; c) silver prism-like.
5 Fig 1.9. SEM image of metal structures: a) Ag-Cu dendrites; b) silver dendrites on an aluminum substrate; c) silver dendrites on a copper substrate and coated with graphene. Dendritic metal structures have tips more than spherical structures. Dendritic structure of precious metals with different shapes was fabricated as shown in Fig 1.9. X. Chen et al fabricated silver - dendrites on a copper substrate and analyzed R6G to a concentration of 10-6 M (Fig 1.9 (a)). Deposition silver on aluminum substrate, then separate the silver dendritic and cover with a layer of gold and identify 1,2-benzenedithiol at a concentration of 10-4 M (Fig 1.9 (b)). L. Hu et al fabricated silver dendrites on a copper substrate, then coated with graphene oxide on top. They demonstrated that for the same analytes, when coated with graphene oxide on top, enhancement factor is 1.2x107 (Fig 1.9 (c)). One of the metal structures for good SERS enhancement that we cannot fail to mention are metal structures in shape of flowers (Fig 1.10). H. Fig 1.10. SEM images of metal flowers-like: a) Liang et al in 2009 silver nano flower-like; b) gold nano flower-like; successfully fabricated c) Gold nano flower-like with holes. silver flower structures in suspension and used them to detect malachite green with concentrations as low as 10-10 M. Z. Wang et al used electrochemical deposition method to fabricate the gold nanotubes and using this SERS substrate they detected R6G with concentrations as low as 10-10. M. S Ye et al published results for the fabrication of gold nano-structure with holes in the middle and showed that SERS enhancement factor for the biphenyl-4-thiol analyte of this structure was 105. 1.5. Application of SERS During the time since its discovery, SERS has been using as an extremely useful tool for environmental, food, and biomedical analysis. The target molecules analyzed by SERS are also very abundant including pesticides, herbicides, pharmaceutical, chemicals in water, dyes, aromatic chemicals in normal aqueous solutions and in seawater, chlorophenol derivatives and amino acids, war chemicals, soil organic pollutants, and biological molecules such as DNA, RNA. 1.6. Researching of SERS in Vietnam In Vietnam, researching and fabrication on SERS substrates and using of SERS
6 to detect molecules at low concentrations have been starting since 2010. Up to now, in Vietnam, there are several groups has been researching on SERS. Such as, group of GS.VS. Nguyen Van Hieu, Professor's group. Nguyen Quang Liem and Assoc. Ung Thi Dieu Thuy (Institute of Materials Science), Associate's Group. Tran Hong Nhung (Institute of Physics), group of Assoc. Nguyen The Binh (Hanoi University of Science), Assoc. Pham Van Hoi (Institute of Materials Science), group of Professors. Dao Tran Cao (Institute of Materials Science) - this is also the research group that helps me make this thesis. In addition, there are some of other research groups that are also researching on SERS and obtained some good results, we would like to not list here. Chapter 2 Fabrication and investigation methods of SERS substrate 2.1. Introduction to SERS substrates Currently, there are two types of SERS substrates used SERS substrate is suspension of precious metal nanoparticles (Ag, Ag) inside a certain liquid. SERS substrate is a heterogeneous metal surface. Requirements of a good SERS substrate Strong SERS enhancement factor (> 105). Uniformity on the surface and uniformity between samples (
7 2.2.2. Bottom-up There are different methods: - Physical (sputtering, evaporation) - Template, etching - Chemical The chemical reduction method is most used (the metal ion is essentially reduced to atom metal). With the parts in the deposition solution described in the fig include: Reduced substance: usually AgNO3, HAuCl4. Reducing agent (reducing agent): Can be metal, semiconductor, citrate salts, borohydrite (these two salts are most used). Solvent dissolved (most used water, alcohol). Surfactants (most used PVP, CTAB). It should be noted that material can many different roles, for example PVP can make both as a reducing agent and as a surfactant. Deposition can also be performed directly on solid substrates, Al, Cu substrates and in our case Si substrates. Our Si substrate both make as substrate to deposit Ag and Au particles upwards and make as a reducing agent. 2.3. Methods for surveying the structure and properties of SERS substrates SEM imaging: To analyze the morphology of the SERS substrate. X-ray diffraction method (XRD): To analyze the SERS substrate structure. UV-Vis spectrometric method: To analyze plasmon resonance properties of SERS substrate. Raman spectrometric method: To analyze SERS spectrum of toxic organic molecules. Chapter 3 Fabrication of silver and gold nanostructures on Si 3.1. Fabricating of silver nanostructures on Si by chemical deposition and electrochemical deposition The process of deposition of Ag nanoparticles on Si by chemical deposition method is described as Figure 3.1. After the Si substrates are cleaned, they are soaked in a solution containing the chemicals available. After the fabrication, the substrates are removed, washed and air dry, and measured and analyzed. The process of deposition of Ag nanoparticles on Si by electrochemical deposition method is described in Figure 3.2.
8 Fig 3.1. Schematic of steps for fabricating silver nanostructures on Si by chemical deposition method. This process is similar to the deposition process of Ag nanoparticles on Si by chemical deposition method. Another is that after fabrication Si substrate is attach to the cathode of the DC power, the anode made of platinum. Fig 3.1. Schematic of steps for fabricating silver nanostructures on Si by electrochemical deposition method. 3.3. Fabrication of silver nanoparticles on Si by chemical deposition method 3.3.1. Fabrication results Figure 3.4 shows SEM images of samples deposited in a solution containing 0.14 M HF and 0.1 mM AgNO3 in water with different deposition times. AgNPs appeared on Si surface at 3 minutes (Figure 3.4 (a)). When the deposition time increased to 4 minutes, the AgNPs were distributed fairly evenly, spherical or ellipsoid with a diameter of about 70 - 100 nm (Figure 3.4 (b)). When the deposition time continued to increase to 5 minutes, the AgNPs tended to clump together and form larger particles (200 - 250 nm) and the distance between particles increased. Figure 3.4. SEM images of AgNPs on Si by chemical deposition in a solution containing 0.14 M HF / 0.1 mM AgNO3 with deposition time: (a) 3 minutes, (b) 4 minutes and (c) 5 minutes at room temperature. 3.3.2. The mechanism of forming silver nanoparticles on Si that fabricated by chemical deposition method The mechanism for the formation of Ag on Si particles is a galvanic replacement mechanism, in which silver (Ag) replaces Si. Specifically, this process is based
9 on a redox reaction, here, Ag ions in the solution are reduced to atomic silver (Si is reducing agent), while Si is oxidized and dissolved directly following by HF or Si is oxidized by H2O to SiO2, then this SiO2 is dissolved by HF in the solution. Both of these processes occur simultaneously on the Si surface and are represented by the following reaction equations: Cathode: (3.1) Anode: - When Si is oxidized and dissolved directly by HF: (3.2) - When Si is oxidized by H2O and dissolved indirectly by HF: (3.3) (3.4) - The total reaction for both dissolving Si is: (3.5) Here, it is also important to say more about the role of HF in the deposition solution. Specifically, after the reaction (3.3), SiO2 will gradually form on the Si surface. After a certain time this oxide layer will cover the entire Si surface and it prevents the electron transfer from the Si surface to the Ag + ions and stops the deposition. In order for Ag deposition on Si surface to continue, in the sedimentation solution need more HF and HF will dissolve SiO2 layer according to equation (3.4). Once there are Ag atoms, they will link together to form AgNPs. 3.4. Fabrication of silver nanodendrites structures on Si 3.4.1. Fabrication of silver nanodendrites structures on Si by chemical deposition method Fig 3.5 shows the SEM images of the Si sample surface after being chemically deposited Ag for 15 minutes at room temperature in a solution containing 4.8 M HF and AgNO3 with the Fig 3.5. SEM images of Ag nanostructures chemically concentration of deposited on Si substrates for 15 minutes in 4.8 M HF / AgNO3 changed. It is AgNO3 solution at room temperature with variable easy to see that the AgNO3 concentration: (a) 0.25 mM, ( b) 1 mM, (c) 2,5
10 structural morphology mM, (d) 5 mM, (e) 10 mM and (f) 20 mM. of Ag deposited on the Si surface depends on the concentration of AgNO3 in the deposition solution and the AgNDs will also be formed on the Si surface only when the AgNO3 concentration is sufficient. big. Specifically, at a concentration of 20 mM AgNO3 (Fig 3.5 (f)), sub-branches sprouted from Ag nanorods and AgNDs were formed on the surface of Si. It can be seen clearly that the AgNDs structure is a multi-hierarchical structure and that the AgNDs we construct has a quadratic branch structure (a long main branch with short sub-branches growing on either side. ). The diameter of the main branch is about a few hundred nm, and its length is tens of µm, the sub- branches about a few µm long. 3.4.2. Fabrication of Ag nanodendrites on Si by electrochemical deposition method Fig 3.9 shows SEM image of AgNDs on Si fabricated by electrochemical deposition in stable voltage mode with varying potentials (5, 10, 12 and 15V). When the voltage is 12V (Figure 3.9 (c)), now the AgNDs have completely branched to 3 (from the sub-branches to the next ones), creating a pretty and uniform Fig 3.9. SEM images of AgNDs on Si branch structure. . However, when substrates fabricated by electrochemical continuing to increase the external deposition of 15 min in a solution of 4.8 voltage to 15V, the structural and M HF / 20 mM AgNO3 with order uniformity of AgNDs is now corresponding external potentials: (a) 5; broken and there are some sub- (b) 10, (c) 12 and (d) 15V. branches that break away from the main branch (Fig 3.9 (d)). It can be seen that when current density increased to 3 mA/cm2, the AgNDs formed on the Si surface were now almost completely branched and began to have quadratic branching, which makes for a density of branches per branch to become very thick (Fig 3.12 (c)). Next, when current density increased to 4 mA/cm2 (Fig. 3.12 (d)), the AgNDs continued to form and overlapped creating an unevenness on the surface. Formation of branch is too thick leading to several small sub-branches to break. The above results show that a deposition current density of 3 mA/cm2 gives the silver foil the most uniformity. The XRD results of the samples after electrochemical deposition (Fig 3.11) show that AgNDs are monocrystalline with a face-centered cubic structure (FCC). The intensity of the peak Ag (111)
11 was much stronger than the other peaks, showing that the AgNDs' growth was mainly in the direction of the crystal plane (111). (111) Intensity (a.u) (200) (220) 20 30 40 50 60 70 2q (Degree) Fig 3.11. XRD diffraction of HaNDs is electrochemical Fig 3.12. SEM image of AgNDs on Si deposition on Si. substrate fabricated by electrochemical deposition for 15 minutes in aqueous solution containing 4.8 M HF/20 mM AgNO3 with the corresponding current densities: (a) 1; (b) 2; (c) 3; and (d) 4 mA/cm2. 3.4.3. Formation mechanism of silver nanodendrites Formation mechanism of AgNDs so far has not been really clarified. However, most researchers believe that the formation of metallic nanotructures can be explained through the Diffusion-limited aggregation (DLA) model and the oriented attachments. According to the DLA model, first there is one particle, then the other particles continuously diffuse towards the original particle to stick together to form the Dendrites shape. Oriented attachments are believed to be particles that, when coming together, somehow rotate the crystal so that the junction has the same crystal orientation to create a single crystal structure. Therefore, the formation mechanism of AgNDs on Si can be explained as follows. First, AgNPs will be formed on Si surface according to the mechanism presented in Section 3.3. Next, other AgNPs will also diffuse continuously towards these original AgNPs to form AgNPs with larger size. AgNPs clusters will attach oriented to form Ag nanorods and nanowires. The nanorods and nanowires will become the main branches (backbone) of the branches. As the main branch grows, new short sub-branches are continuously formed on the main branch, creating a structure resembling fern leaves. More specifically, these sub-branches can also become a major branch to grow shorter sub- branches. This makes the branch structure a multi-hierarchical structure.
12 3.5. Fabrication of the silver nano flower-like structures on Si 3.5.1. Fabrication results It can be seen that when the concentration of AgNO3 is 1 mM, AgNFs begin to form on the Si surface (Fig 3.15 (d)). AgNFs have relatively uniform sizes (about 700 nm) and their surfaces are rough. According to some authors, the AgNFs can achieve better roughness by adding surfactant polyvinylpyrrolidone (PVP) into deposition solution, so we Fig 3.15. SEM images of Ag use PVP replace of AsA in the nanostructures chemically deposited deposition solution fabricate AgNFs on Si in 4,8 M HF/AgNO3/5 mM AsA on Si. Results in Fig 3.17. It can be solution for 10 minutes at room seen that using of PVP in the temperature with different AgNO3 deposition solution helps to create the concentrations (a) 0.05 mM, (b) 0.1 better AgNFs with size of the AgNFs mM, (c) 0.5 mM and (d) 1 mM. is about 1 µm. Fig 3.17. SEM images of AgNFs fabricated in 4,8 M HF/1 mM AgNO3/PVP deposition solution with PVP concentration varying (a) 5 mM, (b) 10 mM and (c) 15 mM with 10 minutes at room temperature. Fig 3.18. SEM images of AgNFs in Fig 3.19. SEM images of AgNFs in 4,8 M HF/1 mM AgNO3-/PVP/10 mM 4,8 M HF/1 mM AgNO3/10 mM AsA deposition solution with different AsA/5 mM PVP deposition solution PVP concentrations: (a) 1 mM, (b) 3 with different deposition times: (a) 1 mM, (c ) 5 mM and (d) 7 mM with 10 minute, (b) 4 minutes, ( c) 10 minutes minutes at room temperature. and (d) 15 minutes.
13 However, we want AgNFs with sharp (111) points so we used both AsA and PVP in the deposition solution. Results are Intensity (a.u) shown in Fig 3.18. It can be seen that úing both PVP and AsA in the (200) deposition helps to produce tips (220) flower-like structure with the size of the AgNFs about 1 µm to 1.5 µm. Our fabrication results also showed that 20 30 2q (Degree) 40 50 60 70 with deposition time 10 minutes, the Fig 3.20. X-ray diffraction (XRD) of flower density was the most uniform AgNFs on Si as illustrated in Fig 3.19. XRD results of the samples after electrochemical deposition (Fig 3.11) show that AgNFs are crystalline with a face-centered cubic structure (FCC). The direction of crystal development is the direction [111]. 3.5 Fig 3.22 Plasmon resonance spectra of AgNPs, AgNFs, AgNDs structures in 3.0 the excitation wavelength range from 2.5 Absorption (a.u) 300 nm to 800 nm. For AgNPs AgNPs 2.0 structures of average size 70 nm (Fig. AgNFs 3.4 (b)) there is a peak at 425 nm 1.5 excitation wavelength. For AgNFs and 1.0 AgNDs structures we have a wide 0.5 AgNDs plasmon band in the entire excited 0.0 wavelength region. This broad 300 400 500 600 700 800 Wavelength (nm) plasmon band is explained by the structure AgNFs and AgNDs are Fig 3.22. Plasmon resonance spectra multil-branched structures, each of of AgNPs, AgNFs, AgNDs structures. them exhibits its own type of plasmon and is attributed to the hybridization of plasmons relative to the center of the core and sharp vertices around it. Plasmon resonance at longer wavelengths occurs due to a near-field connection between tips when the tips are close together. Due to the heterogeneous size and shape of the core and tip of the AgNDs and AgNFs, the individual plasmon modes of all these sizes and shapes have been coupled together, resulting larger- band. The plasmonic effect is broad and complex as shown in Fig 3.22 and extended to the near infrared band. Plasmon resonance in different excitation wavelength bands of AgNDs and AgNFs structures is also observed when we have recorded SERS spectra of all seven different toxic molecules using both types of steps. Excitation laser wavelength of 633 nm and 785 nm both showed good results. Thus, the characteristic plasmon resonance activity at many
14 different excitation wavelengths is a great advantage over the two structural types AgNDs and AgNFs in SERS analysis. 3.5.2. Formation mechanism of silver flower-like on Si When Si added to the reaction solution containing HF/AgNO3/AsA/PVP, Ag ions are not only reduced by Si (according to reaction equation (3.1)) but also by AsA. The reduction of Ag ions by AsA occurs according to the following reaction equation (2008 Y Wang [169]): C6H8O6 + 2 Ag+ C6H6O6 + 2 Ag + 2 H+ (3.5) According to Equation (3.5), Ag ions will be reduced directly to Ag atom in solution by AsA. Therefore, Ag deposition in AsA-added solution will occur at a faster rate, leading to size of Ag nano formation on Si surface are larger than the AgNPs deposition in solution only HF and AgNO3. When PVP surfactant is added to the deposition solution, PVP will preferentially adsorb onto {100} surfaces over {111} surfaces. Therefore, developing silver nanostructures, PVP will act as a "capping agent" that prevents the particles from approaching to bond on the {100} surfaces so Ag particles will take precedence. linked to the {111} facets. When the PVP concentration in the deposition was low, the coating of PVP on the (100) surface was low leading to growth at {100} and {111} nearly identical surfaces so the flower had a smooth surface. When the PVP concentration is higher, PVP will cover most of the {100}, resulting in the particles being able to only progress to bonding with the {111} surface and create a tips morphology. Thus, the mechanism of formation of AgNFs in deposition solutions containing AsA and PVP can be divided into three phases: i) First stage: In the presence of the AsA reducing agent, the number of Ag atoms is quickly formed and linked together to form the nucleus. ii) Stage two: Silver atoms continue to be produced and the nuclei develop into nanostructures with larger sizes. iii) Final stage: When nanostructures grow to a certain size, the crystal surfaces become large enough for PVP to be adsorbed on surface. PVP will inhibit the growth of Ag structures in [100] direction and Ag particles will approach the link in [111] direction to create AgNFs structures. 3.6. Fabrication of gold nano flower-like structure by electrochemical deposition method 3.6.1. Fabrication of gold nano flower-like on silver's seed Fabrication of flower-like structures (AuNFs) on Si, we separated nucleation and growth. Specifically, we used Ag nanoparticles fabricated by electrochemical deposition on Si surface as the seed to grow AuNFs. It should be noted further that up to now most research groups have used gold nanoparticles to seed the growth of AuNFs. The reason we use Ag seeds to
15 replace Au seeds is because AgNPs will promote the anisotropic development of Au particles on certain crystal axis, so, the AuNFs structure is more easily formed, it is reported of Ujihara authors group new feature in fabrication of Fig 3.21. SEM images of seed of Ag Fig 3.22. SEM images of AuNFs were on Si were made by electrochemical fabricated by electrochemical deposition with current density of deposition with current density of 0,1 0,05 mA/cm2 for 3 minutes in mA/cm2 for 10 minutes in a solution solutions containing 0.1 mM AgNO3 containing 0.1 mM HAuCl4 and 0.14 and 0.14 mM HF. mM HF on Si available Ag seed. AuNFs in our study is used of electrochemical deposition in both the seeding and the AuNFs growth step. The SEM results in Fig 3.21 show that after the deposition process, the Ag seeds generated have almost spherical or ellipsoid morphology with an medium size of about 40 nm and distance between AgNPs is about hundreds of nm is formed on the surface Si. Then, we submerged Si substrate with Ag germs in electrochemical solution containing HauCl4. After deposition time, we obtained AuNFs as illustrated in Fig 3.22. SEM image in Fig 3.22 shows that the AuNFs are uniform on the surface with a diameter about 100-120 nm, the distance between AgNFs is about 10 nm. XRD results of AuNFs samples after electrochemical deposition (Fig 3.11) show that AuNFs are crystals with a face-centered cubic structure (FCC). The preferred direction for growing crystals is the direction [111]. Fig 3.23. a) X-ray diffraction (XRD) of gold nano flower-like structure. 3.6.2. Formation mechanism of gold nano flower-like Formation mechanism of gold nano flower-like on Si is based on redox reaction, where, Au3 + is reduced on Si surface (Si as reducing agent) and Si is
16 oxidized to SiO2 (according to the reaction (3.1) to ( 3.5)). The equation for reducing Au3+ to Au atom is represented by the following equation (2011 L M A Monzon [209]): (3.6) In addition, Au atoms can also be born through an intermediate step according to the equation: (3.7) (3.8) Formation of the Au atom according to equation (3.8) there will be an intermediate reduction reaction (equation (3.7)), where, the ions are reduced before being further reduced to gold atom in the equation (3.8). The process of creating gold atom according to equations (3.7) and (3.8) is much weaker than the process of creating a gold atom according to equation (3.6). According to L M A Monzon et al., equations (3.7) and (3.8) would require a greater amount of energy than equation (3.6) or its deposition solvent should be organic solvent instead of H2O. After, Au elements are present, Au will bond with some definite facets of Ag seed. Finally, Au particles will orientately attach to the Au particles already on some surfaces of the Ag seed forming AuNFs. Chapter 4 Using gold nano flowers-like, silver nano flowers-like, silver nanodendrites structural as SERS substrates to detect traces of some organic molecules 4.1. Reagents are used to analyze SERS and the steps to prepare SERS substrate before measurement Sampling steps for SERS analysis: Preparation of the SERS base (section 3.1); Analytes are premixed with predetermined concentrations (ppm); fixed 25 µl of analyte is applied to SERS substrate surface; spontaneously dry analyte in laboratory environment prior to measuring SERS. There are seven different types of organic molecules that we have used for SERS analysis, including: Paraquat, Pyridaben, Thiram, Crystal violet, Cyanine, Melamine and Rhodamine B. 4.2. Requirements of a good SERS substrate 4.2.1. The uniformity of nano flower-like, dendrites structures of gold and silver In this section we demonstrate the uniformity of SERS substrates fabricated on surface and the uniformity between samples in different fabrications by analyzing SERS via the SERS spectrum of RhB.
17 First, we survey the surface uniformity of AgNDs. SERS spectrum of RhB is shown in Figure 4.1. In Fig 4.1, we can see that SERS 6000 ®¬n ṽ 1355 1506 1527 1278 1644 spectrum of the “substrate with no 619 1196 organic molecules” resembles a line, VT 7 proving that our sample washing Intensity (a.u) procedure eliminated most of VT 6 residue on SERS substrate. As VT 5 observed in Fig 4.1, the curves and VT 4 VT 3 peak intensity at seven different VT 2 positions are relatively uniform, VT 1 § tr¾ng difficult to observe with the eyes. 600 800 1000 1200 1400 1600 1800 For more correct results, we perform Raman shift (cm ) -1 calculations to calculate the Fig 4.1. SERS spectra of RhB with 1 repeatability of the measurement ppm concentration obtained when using using standard deviation SD and the SERS substrate AgNDs was fabricated relative standard deviation RSD. by current deposition method at seven Similarly, we calculated for different positions. structures AgNFs and AuNFs, the results are shown in Table 4.5. The results show that the structures mentioned above have good uniformity. Table 4.5. Comparison of data obtained on AgNDs, AuNFs and AgNFs substrates Relative SERS peak Peak intensity Standard standard Type of SERS location (a.u) deviation (SD) deviation (RSD%) AgNDs 104.230,213 10.340,316 9,920 Đỉnh 1278 AuNFs 73.708,814 5.345,167 7,252 AgNFs 10.359,4419 719,365 6,944 AgNDs 93.148,3791 10.644,68 11,428 Đỉnh 1644 AuNFs 61.130,0398 5.120,697 8,377 AgNFs 10.076,8761 807,195 8,010 Table 4.6. Data were obtained on AgNFs substrates of five different samples five Peak Relative Analyte SERS Standard standard concen- peak different Intensity deviation deviation tration location samples (a.u) (SD) (RSD%) Lô 1 12853.24646 Đỉnh 1 ppm Lô 2 12208.44528 1525,680 11,111 1278 cm-1 Lô 3 12973.93669
18 Lô 4 14738.91166 Lô 5 15883.47427 Lô 1 11704.41479 Lô 2 11416.09864 9 Lô 3 11430.67329 1283,656 10,331 Lô 4 13363.41415 Lô 5 14209.56150 The calculated results for the samples uniformity shown in Table 4.6 are within the permissible range. 4.2.2. Investigation of SERS substrate enhancement factor Table 4.7. Enhancement factor of SERS substrates Type of SERS Peak intensity (a.u) (07 peak) Enhancement factor (EF) AgNDs 104230,20 1,04 x 106 AuNFs 73708,81 0,69 x 106 AgNDs 10538,19 1,05 x 105 The data in Table 4.7 shows that, the SERS substrate reinforcement coefficient of AgNDs substrate is 1,04 x 106, that of AuNFs substrate is 0,69 x 106 and that of AgNF substrates is 1,05 x 105. These numbers show that , SERS substrates have been fabricated to satisfy enhancement factor requirement of a good SERS substrate. 4.3. Application of silver nano-dendrites 4.3.1. Detection of paraquat herbicide It is one of the most widely used herbicides. It is non-selective, killing green plant tissue on contact. Most patients with paraquat poisoning are severe and the mortality rate is very high, estimated at 73%. So the detection of paraquat by SERS is significant. The low paraquat concentration that the AgNDs@Si substrate can detect is 0.01 ppm. Meanwhile, this limit is 5 ppm for Fig 4.8. SERS spectrum of paraquat AgNDs @ Si substrates fabricated by with different concentrations: (1) 1 chemical deposition (Figure 4.8). ppm; (2) 0, 5 ppm; (3) 0.1 ppm; (4) 0.01 ppm