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

1
MINISTRY OF EDUCATION
AND TRAINING

VIETNAM ACADEMY
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


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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


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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 nanoflowers (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.


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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 antiStockes 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.


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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
incident light.
source).
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

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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 prismlike silver nanoparticle was 105.

Fig 1.8. SEM images of nanoparticles with different shapes: a) gold triangularlike; b) silver bow-like; c) silver prism-like.


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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


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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 hightech equipment is necessary. It is difficult to change the surface morphology.

laser ablation

E-Lithography

The focused ion beam
(FIB))


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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.


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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


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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 subbranches 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)


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

Intensity (a.u)

(111)

(220)

(200)

20

30

40

50

60

70

2q (Degree)

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 subbranches. This makes the branch structure a multi-hierarchical structure.


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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
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.


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Intensity (a.u)

However, we want AgNFs with sharp
(111)
points so we used both AsA and PVP
in the deposition solution. Results are
shown in Fig 3.18. It can be seen that
(200)
úing both PVP and AsA in the
(220)
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
2q (Degree)
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
3.0
AgNPs, AgNFs, AgNDs structures in
the excitation wavelength range from
2.5

300 nm to 800 nm. For AgNPs
AgNPs
2.0
structures of average size 70 nm (Fig.
AgNFs
1.5
3.4 (b)) there is a peak at 425 nm
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
of AgNPs, AgNFs, AgNDs structures.
multil-branched structures, each of
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 largerband. 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
Absorption (a.u)

20

30

40

50

60

70


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
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,
§ tr¾ng
difficult to observe with the eyes.
600
800
1000
1200
1400
1600
1800
For more correct results, we perform
Raman shift (cm )
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
1355

6000 ®¬n ṽ

1278

1527

1644

1196

VT 7

Intensity (a.u)

619

1506

VT 6

VT 5

VT 4
VT 3
VT 2
VT 1

-1

Type of SERS

Peak intensity
(a.u)

Standard
deviation (SD)

Relative
standard
deviation
(RSD%)

Đỉnh 1278

AgNDs
AuNFs
AgNFs

104.230,213
73.708,814
10.359,4419

10.340,316
5.345,167
719,365

9,920
7,252
6,944

Đỉnh 1644

AgNDs
AuNFs
AgNFs

93.148,3791
61.130,0398
10.076,8761

10.644,68
5.120,697
807,195

11,428
8,377
8,010

SERS peak
location

Table 4.6. Data were obtained on AgNFs substrates of five different samples
Analyte
concentration

SERS
peak
location

1 ppm

Đỉnh
1278 cm-1

five
different
samples

Peak
Intensity
(a.u)

Lô 1

12853.24646

Lô 2

12208.44528

Lô 3

12973.93669

Standard
deviation
(SD)

Relative
standard
deviation

(RSD%)

1525,680

11,111


18

9

Lô 4

14738.91166

Lô 5

15883.47427

Lô 1

11704.41479

Lô 2

11416.09864

Lô 3

11430.67329

Lô 4

13363.41415

Lô 5

14209.56150

1283,656

10,331

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


19
4.3.2. Phát hiện thuốc trừ sâu pyridaben
4.3.2. Detection of pyridaben
In Vietnam, pyridaben insecticide is widely used on tea plants to destroy red
spiders. Many tea shipments have been returned due to pyridaben concentrations
exceeding the permitted threshold. It can be seen that SERS spectrum of
pyridabene is very complex and we are also one of the first groups to record the
SERS spectrum of pyridabene (Fig 4.9). We also analyzed the SERS spectrum
of pyridabene in the commercial product marketed as Koben 15EC (Fig 4.11).
Although their SERS spectrum is not as beautiful as SERS spectrum of standard
substance. However, we have analyzed pyridaben concentration in Koben 15EC
as low as 0.1 ppm.
635

1218
1106

Intensity (a.u)

670

756 811

780
846

1265
1482
1280

1648

1200

1138
925

Intensity (a.u)

1245
709

1615

944

100 ppm

100 ppm
10 ppm

10 ppm

1 ppm
1 ppm

0.1 ppm
0,1 ppm

600

1000 ppm trªn ® Si

800

1000

1200

1400

1600

1800

Raman shift (cm-1)
600

800

1000

1200

1400

-1

Raman shift (cm

1600

1800

)

Fig 4.11. SERS spectrum of
pyridaben in the insecticide Koben
15EC with different concentrations:
100, 10, 1 and 0.1 ppm

Intensity (a.u)

Fig 4.9. SERS spectra of pyridabene
with different concentrations: 100, 10,
1 and 0.1 ppm
4.3.3. Detection of thiram
Thiram recognized were a
5000 pcs
1386
commercial fungicide ProThiram 80WG (containing the
active ingredient thiram at a
concentration of 80% by

560
weight), manufactured by
1440
1150
Taminco BVBA (Belgium).
1517
344 446
928
Fig 4.12 shows SERS
100 ppm
spectrum of the Pro-Thiram
10 ppm
1 ppm
fungicide
diluted
with
0.1 ppm
0.01 ppm
deionized water to reach a
400
600
800
1000 1200 1400 1600
thiram concentration from 100
Raman shift (cm )
ppm to 0.01 ppm (4,2 x 10-4 Fig 4.12. SERS spectra of thiram at different
M to 4,2 x 10-8 M)
concentrations from 100 ppm to 0.01
-1

20
4.4. Application of silver and gold nano flower-like
4.4.1. Detection of violet crystals
1618
Containers of seafood products
1375
1174
exported from Vietnam rejected in
437
Europe and the US market in recent
800 913
years
because
residues
of
antibiotics are not uncommon. This
has a great impact on the seafood
export industry and the lives of
people working in our country.
Crystal violet (CV) is one of the
banned substances mentioned
Raman shift (cm-1)
above. We analyzed CV at 0.1 ppm Fig 4.14. SERS spectrum of CV molecule
concentration.
4.4.2. Detection of melamine
675
In fact, the nitrogen in melamine is
a non-protein, that is, it is not a
protein but only a protein

982
imitation, so it has no nutritional
579
effects like protein. Because of its
682
high nitrogen content, melamine is
introduced
into
foods
by
"cheating"
manufacturers.
575
980
Melamine is used to "trick" test
method, deceive the examination
600
700
800
900
1000
1100
1200
agencies and of course deceive
Raman shift (cm-1)
consumers. Thus, identification of
Fig 4.15. Raman spectra of melamine
melamine has an important effect
powder and SERS spectrum of melamine
in monitoring and monitoring

at different concentrations from 0.01 to 5
manufacturers when they include
ppm
them in food. We detected
melamine with a concentration of
0.01
ppm
and
a
SERS
enhancement factor of 4,306 x 106.
4.4.3. Detection of xyanide (KCN)
Cyanide is present in industrial and municipal wastes, the most abundant source
of cyanide pollution is from electroplating, metallurgy, steel processing, gold
ore mining and oil-gas industries. The reasons above indicate the urgent need to
control cyanide concentrations in water and in soil. This is especially true in
Vietnam after the death of a number of fish in the central coast of Vietnam in
April 2016.
10000

9000
8000

Intensity (a.u)

7000
6000

10 ppm

5000
4000
3000

1 ppm

2000

0.1 ppm

1000
0

-1000

0.01 ppm x 2

-2000
-3000

Intensity (a.u)

400

600

800

1000

1200

1400

1600

1800

Melamine bét

5 ppm
0,5 ppm
0,1 ppm
0,05 ppm
0,01 ppm


21
30000

(1)

(1)

25000

(1) - 5 ppm
20000

(2) - 1 ppm

(2)

20000

(4) - 0,1 ppm

(3)
15000

(5) - 0,05 ppm

(4)

10000

(2)

(3) - 0,5 ppm

Intensity (a.u)

Intensity (a.u)

(1) - 5 ppm
(2) - 1 ppm
(3) - 0.5 ppm
(4) - 0.1 ppm
(5) - 0.05 ppm
(6) - 0.01 ppm

(7) - 0.005 ppm

25000

(3)

15000

(4)

10000

(5)

(5)
(6)

5000

5000

(7)
0
1800

1900

2000

2100

2200

2300

2400

2500

2600

Raman shift (cm-1)

0
1800

1900

2000

2100

2200

2300

2400

2500

2600

-1

Raman shift (cm )

Fig 4.17. SERS spectrum of cyanide in
ethanol with different concentrations.

Fig 4.18. SERS spectrum of cyanide
in water with different concentrations
The results of SERS signals of cyanide in ethanol are show in Fig 4.17. It can be
seen clearly that KCN has a single peak at 2105 cm-1, which is attributed to CN
bond. Things will be different when KCN is dissolved in water (Fig 4.18). There
is now a peak at 2105 cm-1 but the spectrum leg is extended forward. As KCN
concentration drops lower, shoulder of spectrum becomes more pronounced,
then new-spectral peak is gradually divided into two separate peaks, one peak at
2105 cm-1 and the other at 2140 cm-1.
The peak at 2140 cm-1 appears very clearly when the cyanide concentration
reducing to 0.01 ppm. We identified that change in shape of the spectral peak at
2105 cm-1 in Fig 4.18 compared to Fig 4.17 is due to the cyanide of silver on the
SERS substrate, this process occurs when KCN dissolved in water (instead of
ethanol). The orderly variation of SERS spectrum of KCN as its concentration
changes as shown in Fig 4.18 can be explained, noting that only cyanide (CN)
ions in contact with silver are capable of forming complex with silver. At high
cyanide concentrations, the ratio of [Ag(CN)2]- ions low exposure to silver
hence the 2105 cm-1 peak of the bond (CN) - remains dominant with the spectral
base that tends to elongate. towards higher wavelengths. At low IZ
concentrations, the percentage of cyanide ions in contact with silver increases
with the peak intensity at 2105 cm-1 decreasing continuously, while the peak at

2140 cm-1 appears and gradually becomes separate as shown in Fig 4.18. Thus,
by SERS spectrum, for the first time, we observed the complex formation of Ag
with cyanide in water.


22

Intensity (a.u)

4.4.3. Detection of rhodamine B
5000 cps
In the process of food processing,
1644
1504
619
to give food a beautiful color
1525
1355
1278
industrial colorants are used.
1196
Industrial colors in general,
5 ppm
1 ppm
rhodamine B (RhB) in particular
0.5 ppm
0.1 ppm
are toxic, banned in food because
0.05 ppm
they are difficult to decompose,

0.01 ppm
1 ppb
on the other hand, they also affect
0.1 ppb
the liver, kidneys or long-term
600
800
1000
1200
1400
1600
1800
Raman shift (cm-1)
residues, and cause muscle
damage human body, especially Fig 4.19. SERS spectrum of rhodamine B
can cause cancer. In Vietnam, it is with different concentrations from 0.1 ppb
added to products such as squash
to 5ppm
and melon. We analyzed RhB at as low as 0.1 ppb of concentrations.

Conclusion
1. Successfully fabricated the structures of AgNDs, AgNFs and AuNFs on
Si by chemical deposition and/or electrochemical deposition method
with controllable of morphological and structural parameters. The new
contributions are:
- AgNDs with best branching structure (3 branches) were fabricated
by electrochemical deposition in constant current mode.
- The AgNFs were fabricated by chemical deposition with control of
the sharpness of petals by AsA and PVP.
- AuNFs were fabricated by electrochemical deposition method from

seed, with special feature that seeds are silver nanoparticles (other
authors used gold seeds).
2. The main purpose of the fabrication of nanostructures is using them as
SERS substrates to detect residues in the trace concentrations of
pesticides, toxic additives ... that may be present in food, drinking
water, environment ... To test the activity of the above-mentioned
nanostructures in the role of SERS substrates, they used to detect trace
of some organic molecules.
- AgNDs used to detect herbicide paraquat (PQ), thiram insecticide
(TR) and pyridaben insecticide (PB) with PQ and TR being
detected to 0.01 concentration of ppm. PB can be detected as low
as 0.1 ppm. It should be added that our team was the first group to
publish PB's SERS spectrum.
- AgNFs used to detect traces of organic pigments contemporary was
fungicides crystal violet (CV), food additives melamine (MLM)


23
and toxins or toxics in water as cyanide (CN), with the result of CV
and MLM can be detected as low as 0.01 ppm, and CN as low as 5
ppb of concentrations.
- AuNFs to detect traces of rhodamine B (RhB), with the result RhB
detected can be to 0,1 ppb.
The above SERS spectroscopy results demonstrate that the
nanostructures fabricated can be used as highly efficient SERS
substrates.
3. The above nanostructures tested and evaluated according to the criteria of a
good SERS substrate, including hight SERS enhancement factor (>105),
uniformity of SERS signals at different points on a good SERS substrate is
good (difference <20%), repeatability between different SERS substrates

(difference <20%) ... The results show that AgNDs structure has best SERS
enhancement factor (~ 106), while AgNFs and AuNFs structures had better
uniformity on one substrate and epeatability between different SERS
substrates was better than AgNDs, however even for AgNDs there was a
relative standard deviation on one sole and between different substrate
should not exceed 12%.
LIST PUBLISHED WORKS OF THE THESIS
1. Tran Cao Dao, Truc Quynh Ngan Luong, Tuan Anh Cao, Ngoc Minh
Kieu and Van Vu Le, Application of silver nanodendrites deposited on
silicon in SERS technique forthe trace analysis of paraquat, Adv. Nat.
Sci.: Nanosci. Nanotechnol, 2016, 7, 015007.
2. Kieu Ngoc Minh, Cao Tuan Anh, Luong Truc Quynh Ngan, Le Van
Vu, Dao Tran Cao, Synthesis of Flower-like Silver Nanostructures on
Silicon and Their Application in Surface-enhanced Raman Scattering,
Communications in Physics, 2016, 26, 241-246.
3. Luong Truc Quynh Ngan, Kieu Ngoc Minh, Dao Tran Cao, Cao Tuan
Anh & Le Van Vu, Synthesis of Silver Nanodendrites on Silicon and Its
Application for the Trace Detection of Pyridaben Pesticide Using
Surface Enhanced Raman Spectroscopy, J. Electron. Mater, 2017, 46,
3770-3775.
4. Ngoc Minh Kieu, Tran Cao Dao, Tuan Anh Cao, Van Vu Le and Truc
Quynh Ngan Luong, Fabrication of silver flower-like microstructures
on silicon and their use as surface-enhanced raman scatering
substrates to detect melamine traces, The 6th Asian Symposium on
Advanced Materials: Chemistry, Physics & Biomedicine of Functional
and Novel Materials (ASAM-6), September 27-30, 2017, Hanoi,
Vietnam.