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

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 (<20%). 2.2. Fabrication methods of SERS substrate There are many ways to classify the fabrication methods of SERS substrates. The most common are: Top-down and bottom-up fabrication. It should also be noted that, approach with any methods, it is possible to fabricate the two types of SERS substrates mentioned above. 2.2.1. Top-down Laser ablation is a way to create a suspension of nanoparticles in solution. Lithography methods, such as electron beam lithography or focused ion beam lithography give metal nanostructures on solid substrates. Advantages: Creates circulating metal structures with variable dimensions and high purity. Not good: It takes a lot of time. The price is expensive because the use of high- tech equipment is necessary. It is difficult to change the surface morphology. laser ablation E-Lithography The focused ion beam (FIB)) 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 concentration of AgNO3 changed. It is easy to see that the Fig 3.5. SEM images of Ag nanostructures chemically deposited on Si substrates for 15 minutes in 4.8 M HF / AgNO3 solution at room temperature with variable 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 branch structure. . However, when continuing to increase the external voltage to 15V, the structural and order uniformity of AgNDs is now broken and there are some sub- branches that break away from the main branch (Fig 3.9 (d)). Fig 3.9. SEM images of AgNDs on Si substrates fabricated by electrochemical deposition of 15 min in a solution of 4.8 M HF / 20 mM AgNO3 with corresponding external potentials: (a) 5; (b) 10, (c) 12 and (d) 15V. 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). 20 30 40 50 60 70 (220) In te n s it y ( a. u ) 2q (Degree) (111) (200) Fig 3.11. XRD diffraction of HaNDs is electrochemical deposition on Si. Fig 3.12. SEM image of AgNDs 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 use PVP replace of AsA in the deposition solution fabricate AgNFs on Si. Results in Fig 3.17. It can be seen that using of PVP in the deposition solution helps to create the better AgNFs with size of the AgNFs is about 1 µm. Fig 3.15. SEM images of Ag nanostructures chemically deposited on Si in 4,8 M HF/AgNO3/5 mM AsA solution for 10 minutes at room temperature with different AgNO3 concentrations (a) 0.05 mM, (b) 0.1 mM, (c) 0.5 mM and (d) 1 mM. 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 4,8 M HF/1 mM AgNO3-/PVP/10 mM AsA deposition solution with different PVP concentrations: (a) 1 mM, (b) 3 mM, (c ) 5 mM and (d) 7 mM with 10 minutes at room temperature. Fig 3.19. SEM images of AgNFs in 4,8 M HF/1 mM AgNO3/10 mM AsA/5 mM PVP deposition solution with different deposition times: (a) 1 minute, (b) 4 minutes, ( c) 10 minutes and (d) 15 minutes. 13 However, we want AgNFs with sharp points so we used both AsA and PVP in the deposition solution. Results are shown in Fig 3.18. It can be seen that úing both PVP and AsA in the deposition helps to produce tips flower-like structure with the size of the AgNFs about 1 µm to 1.5 µm. Our fabrication results also showed that with deposition time 10 minutes, the flower density was the most uniform as illustrated in Fig 3.19. 20 30 40 50 60 70 In te n si ty ( a. u ) 2q (Degree) (111) (200) (220) Fig 3.20. X-ray diffraction (XRD) of AgNFs on Si 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]. Fig 3.22 Plasmon resonance spectra of AgNPs, AgNFs, AgNDs structures in the excitation wavelength range from 300 nm to 800 nm. For AgNPs structures of average size 70 nm (Fig. 3.4 (b)) there is a peak at 425 nm excitation wavelength. For AgNFs and AgNDs structures we have a wide plasmon band in the entire excited wavelength region. This broad plasmon band is explained by the structure AgNFs and AgNDs are multil-branched structures, each of 300 400 500 600 700 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 A b s o rp ti o n ( a .u ) Wavelength (nm) AgNPs AgNFs AgNDs Fig 3.22. Plasmon resonance spectra 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 on Si were made by electrochemical deposition with current density of 0,05 mA/cm2 for 3 minutes in solutions containing 0.1 mM AgNO3 and 0.14 mM HF. Fig 3.22. SEM images of AuNFs were fabricated by electrochemical deposition with current density of 0,1 mA/cm2 for 10 minutes in a solution containing 0.1 mM HAuCl4 and 0.14 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 spectrum of the “substrate with no organic molecules” resembles a line, proving that our sample washing procedure eliminated most of residue on SERS substrate. As observed in Fig 4.1, the curves and peak intensity at seven different positions are relatively uniform, difficult to observe with the eyes. For more correct results, we perform calculations to calculate the repeatability of the measurement using standard deviation SD and the relative standard deviation RSD. Similarly, we calculated for 600 800 1000 1200 1400 1600 1800 1196 1527 1506 619 1355 1644 VT 2 VT 3 VT 4 VT 6 VT 5 VT 7 In te n s it y ( a .u ) Raman shift (cm-1) 6000 ṽ®¬n VT 1§ tr¾ng 1278 Fig 4.1. SERS spectra of RhB with 1 ppm concentration obtained when using SERS substrate AgNDs was fabricated by current deposition method at seven 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 SERS peak location Type of SERS Peak intensity (a.u) Standard deviation (SD) Relative standard deviation (RSD%) Đỉnh 1278 AgNDs 104.230,213 10.340,316 9,920 AuNFs 73.708,814 5.345,167 7,252 AgNFs 10.359,4419 719,365 6,944 Đỉnh 1644 AgNDs 93.148,3791 10.644,68 11,428 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 Analyte concen- tration SERS peak location five different samples Peak Intensity (a.u) Standard deviation (SD) Relative standard deviation (RSD%) 1 ppm Đỉnh 1278 cm-1 Lô 1 12853.24646 1525,680 11,111 Lô 2 12208.44528 Lô 3 12973.93669 18 Lô 4 14738.91166 Lô 5 15883.47427 9 Lô 1 11704.41479 1283,656 10,331 Lô 2 11416.09864 Lô 3 11430.67329 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 subs