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.
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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
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