Chế tạo các cấu trúc nano vàng, bạc dạng hoa, lá trên silic để sử dụng trong nhận biết một số phân tử hữu cơ bằng tán xạ raman tăng cường bề mặt tt tiếng anh

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

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

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

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 dendrites (AgNDs), silver flowers (AgNFs) and gold 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

nano-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

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

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

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

dipole also oscillates with the

frequency of the incident light The

vibrating dipole produces an

electromagnetic field (new light

source)

Fig 1.5 Schematic illustration of surface localized plasmon resonance (LSPR) with free conducting electrons in metal nanoparticles that are oriented by oscillation due to strong connection with 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

molecules and metal

Fig 1.6 Three different types of chemical 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:

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

strong SERS enhancement In 2009, P R

Sajanlal et al demonstrated that SERS

Fig 1.7 The dependence of SERS enhancement factor on distance

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-triangular-like; c) silver prism-like

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

enhancement that we

cannot fail to mention are

metal structures in shape

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

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 (<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 tech equipment is necessary It is difficult to change the surface morphology

high-laser ablation E-Lithography The focused ion beam

(FIB))

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

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

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

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

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)

was much stronger than the other peaks, showing that the AgNDs' growth was mainly in the direction of the crystal plane (111)

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