Synthesis and Characterization of Au/CdSe & Ag/CdSe Core/Shell
Nanoparticles
A.A. Gadalla*, M. B. Mohamed** & D. A. Hamad*
*Physics
Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
**National Institute of Laser, Cairo University, Cairo, Egypt.
Abstract
Nanoparticles exhibit unique physical properties that are not found in their bulk counterpart. The
synthesis structure which contains Au or Ag core metallic particles and a shell of CdSe semiconductor
has the combined properties of quantum dots and the metallic particles. A new method has been
developed to grow plasmonic semiconductor nanocomposites of Ag/CdSe and Ag/CdSe nanostructure.
The method based on preparing seed of metal nanoparticles which used as a seed using organometllic
pyrolysis followed by adding the semiconductor precursors. Their chemical composition crystal
structure is determined via X-Ray Diffraction The collective optical properties of the plasmonic
semiconductor Nanohybrid has been measured using spectrophotometer techniques and compared to
those individual components.The main features which observed in the hybrid nanostructure is
broodning of Plasmonic band and decrease in its amplitude in addition to shifting excitonic energy
band to higher wave length.The quenching of the emission of Au/CdSe has been observed due to
photoinduced electron injection into metallic seed. The electron transfer processes from CdSe to the
gold is more faster than that of the silver. For this reason, we can consider the CdSe/Au is strong
plasmonic-exitonic coupling but CdSe/Ag is week plasmonic-excitonic coupling.
Introduction
Recently many research efforts have been directed towards the engineering of the
hybrid metal-semiconductor nanostructure1-3, creating a new type of hybrid super
lattices whose new properties offer great promise for applications in magnets,
photovoltaics, electronic device, sensing and chemical biological application.
Particular interest are a class of nanoparticles known as Quantum Dots QDs or
semiconductor nanocrystals that have unique optical and electronic properties, tunable
band gap, sharp emission band with broad excitation and strong resistance to photo
bleaching. In the case of nanocrystals, the particle radius is smaller than Bohers radius
exitons, both the carriers electrons are independently confined. This case is called the
strong confiment regime .In this regime, the band gap decreases by increasing the size
of the particles. Core/shell nanoparticles are nanostructures that have core made of a
material coated with another material. Banin and co-workers4 were able to synthesize
hybrid nanostructure of semiconductor nanorods or tetrapods linked to metallic dots
on the tips and they observe quenching of the emission due to electron transfer from
the semiconductor rods to the Au dots. Surface plasmons SP, excited by the
interaction between light and metal nanoparticles, can increase the density of states
and the spontaneous emission rate of the semiconductor quantum-wells QW when the
SP energy is inherently closed to the band-edge photoluminescence PL of
semiconductor quantum dots QDs5-8. Trials to prepare homogenous sample of AuCdSe core shell particles have been made, but they failed to get control over the size
and shape of the core-shell metallic semiconductor nanoparticles because of the lattice
mismatch between the metal and the semiconductor layer9. Now it is possible to
synthesize these nanocomposites in desired size and shape and with controlled
improved properties such as increased stability, surface area, magnetic, optical and
catalytic properties. Studying metallic-semiconductor nanoparticles with diameter less
than 10 nm became a major disciphancy area of research during the past two decades.
In this work, a method to prepare core/shell metallic-semiconductor nanodots of
controlled size and shape via organometallic pyrolysis method has been developed to
prepare Au/CdSe and Ag/CdSe nanocomposite. Our approach is based on
1
heterogeneous nucleation and growth of the wurtzite CdSe nanodot on preexisting Au
& Ag nanocrystals under mild experimental conditions. We also report the timedependent optical properties of Au/CdSe & Ag/CdSe core shell during the growth
process in organic media.
Experimental
Synthesis of Au/CdSe core/shell nanoparticles
The Au/CdSe nanoparticles have been synthesized with different size using
organometallic pyrolysis in analogies to the procedure which developed by Murray et
al 10. In three necked flask a mixture of 2ml of oleylamine and 1mM of AuHCl4
solution was heated up to 100oC, the color turned to be red which indicates gold
nanoparticle formation. 2gm of TOPO was added to the previous mixture slowly and
the temperate was increased up to 150oC gradually. This temperature was kept to be
constant during the reaction. To form Shell of CdSe nanoparticles around the gold
core, the CdSe precursor (CdO soluble in oleic acid and Se powder soluble in Trioctyl
phosphine) was added drop wise into the reaction mixture. Once the orange color
appears, a sample was taken out by withdrawing approximately 1ml from the reaction
mixture and rapidly was injected in test tube containing 3ml toluene. This last step
was repeated at different time intervals depending on color change observation. Seven
samples with different sizes of Au/CdSe nanodoes could separate at different time
interval to follow the nanoparticle growth.
Synthesis of Ag/CdSe core/shell nanoparticles
In three necked flask, a mixture of 1:1 mole ratio of oleylamine and silver
acetylacetonate (Ag acac) was heated up to 80-90oC. The color turned to be brownish
yellow which indicates formation of silver nanoparticle. Excess of TOPO (2gm) was
added and temperature increased up to 150oC gradually. This temperature was kept to
be constant during the reaction. Cadmium precursor (Cadmium oleate) and Selenium
precursors (Se-TOP) were added to the previous mixture drop by drop with
continuous stirring at the same constant temperature (150oC). Samples were taken out
by withdrawing approximately 1ml from the reaction mixture and rapidly injected in
test tube contained 3ml toluene. All glassware was washed with aqua regain 3:1 ratio
by volume of HCl, HNO3), and rinsed many times with distilled water depending on
color change observation. This last step was repeated at different time intervals
depending on color change observation. Seven samples with different sizes of
Ag/CdSe nanodoes could separated at different time interval.
X-Ray Diffraction Pattern
XRD the structural properties of nanoparticles were carried out with a Philips PW
1700 powder diffractmeter operating with CuKα anode(λ = 0.154183nm). Scans were
done at 6°min−1 for 2θ values between 20 and 90°. The samples were prepared by
precipitating with methanol and washed with toluene solution.
Ultraviolet and visible: UV-visible absorbance spectra of prepared Au/CdSe and
Ag/CdSe core shell were measured with a spectrometer PerkinElmer Lambda 750
double beam spectrophotometer. Diluted solutions of target samples were placed in 1
cm UV quartz and the absorption was recorded within the appropriate scan range. The
spectra were taken against the pure solvent reference of concern for each different
sample. Photoluminescence PL properties of Au/CdSe and Ag/CdSe core/shell are
characterized by PerkinElmer Lambda LS 55 Spectroflorometer to carry out the
emission spectra for different sizes.
2
Result and discussion
Characterization of Au/CdSe core/shell nanocomposite
X-Ray Diffraction
XRD peaks from atomic lattice of gold nanoparticles are seen in figure (1) and the
peaks at 2θ = 38.23, 44.18, 64.82 and 77.85 are due to (111), (200), (220) and (311).
These peaks are perfectly matched with the powder diffraction standard (JCPDS, 04014-0267) which is in conformation of face centered cubic (fcc) structure of gold
crystals. The peak corresponding to (111) plane is more intense than the other planes,
suggesting that the (111) plane is in the predominant orientation. The average particle
size of Au nanoparticle (~20nm) has been estimated by using Debye-Scherrer formula 11, 12.
D = 0.9 λ / β cos θ. Where 'λ' is wave length of X-Ray (0.1541 nm), ‘β’ is FWHM (full width at
half maximum), ‘θ’ is the diffraction angle and ‘D’ is particle diameter size. The calculated
particle size details and ’d’ spacing (nm) are shown in Table (1).
Au (FCC)
Counts (a.u)
(111)
(200)
(311)
(220)
20
30
40
50
60
70
80
2
Figure (1): XRD pattern of gold nanoparticles
2Ѳ of the
intense
peak (deg)
hkl
FWHM of
Intense peak
(β) radians
Size of the
partcle (D)
nm
d-spacing
nm
38.23
111
0.0069
21.33
0.235
44.18
200
0.0066
21.013
0.204
64.82
220
0.0078
20.9
0.143
77.85
311
0.01
18.94
0.122
Table (1): The particle size of gold nanoparticles
(100)
Counts (a.u)
(002)
(110)
(200)
(103)
(111)
(112)
(311)
(220)
(102)
(202)
20
30
40
50
60
70
2
Figure (2): XRD pattern of Au/CdSe nanoparticles
3
80
XRD peaks of Au/CdSe nanoparticles, are seen in figure (2). The peak at 2θ = 24.01,
26.15, 35.74, 42.24, 46.19, 49.75 and 55.62 are due to (100), (002), (102), (110),
(103), (112) and (202) planes of hexagonal CdSe crystal. Average particle size has
been estimated by using Debye-Scherrer formula. The value of d spacing is calculated
using Bragg’s Law13, 2dsinθ = n λ. The calculated cell constant (a) is equal to (4.06
Aº) which is closed to reported data in literature, a= 4.232 Aº (JCPPS file no 04-0119601). Table (2) represents the calculated particle size details and 'd' spacing.
2θ of the
intense
peak (deg)
24.01
hkl
Size of the
partcle (D)
nm
3.53
d-spacing
nm
100
FWHM of
Intense peak
(β) radians
0.04
26.15
002
0.034
4.1
0.340
35.74
102
0.038
3. 83
0.251
42.24
110
0.0358
4.15
0.213
46.19
103
0.0432
3.49
0.1964
49.75
112
0.047
3.25
0.183
55.62
202
0.051
3.07
0.165
0.3706
Table (2): The particles size of Au/CdSe Nanoparticles
Optical Properties
The absortion spectra for Au/CdSe core/shell are shown in figure (3) for deferent shell
thickness. The absorption spectra show one band at 520nm, sample a, due to surface
Plasmon SP, absorption which is the spectral feature of gold spheres. Once CdSe precursors
added, the surface Plasmon band shifted slightly to blue, samples b&c, due to of the dielectric
constant of the surrounding. Different samples of the Au/CdSe hybrid nanostructure have been
withdrawn after at different time interval to follow the nanoparticle growth. As the reaction
time increase, CdSe nanocrystals start to form around gold seeds and the Plasmon band start to
decay as show in the absorption spectra of sample d.
4.8
(a) pure Au
( b,c) AuN.C.
4.2
(d )(4.19nm)
growth time
(e) (4.24nm)
( f) (4.74nm)
3.6
Intensity (a.u)
(g) (4.99nm)
3.0
2.4
1.8
1.2
4
0.6
0.0
400
600
Wave length (nm)
800
Figure (3): Optical absorption spectra for Au /CdSe core/ shell samples.
Increasing the reaction time more lead to complete formation of the CdSe clusters around
gold and the exctonic band of CdSe became predominate, sample “d”. With increasing the
CdSe loading, the broadening of the SPR peak increases and shift to the red direction
comparing with the originals peak position, samples e,f,g. The calculated size of given
prepared samples corresponding to the λmax of absorption spectra are shown in Table
(3). This calculation was carried out by two methods, the polynomial fitting functions
(Yu’s et al14) and effective mass approximation model (Brus's et al15).). The sizing
curves are shown in Figure (4).
Samples
no.
λmax of
UV/vis Peak
(nm)
Shell size Brus's
et al, R(nm)
Shell size Yu’s et
al, D(nm)
d
585.68
3.9
4.19
e
591.90
4.0
4.42
f
599.70
4.4
4.74
g
604.38
4.6
4.96
Table (3): Shell sizes of the prepared Au/ CdSe nanocrystals using Brus’s et al and Yu et al equations.
5.0
Brus et al
Yu et al
4.9
4.8
Dot size (nm)
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4.0
3.9
3.8
585
590
595
600
605
wave length(nm)
Figure (4): Sizing curves for the prepared Au/CdSe nanocrystals
The absorption and the emission spectra for sample of Au/CdSe are shown in Figure
(5) for sample sizes 4.42 & 4.74. The absorption spectra of these samples show the
presence of the surface Plasmon of the gold spheres in addition to the band gap of the
CdSe which is size dependent. The large particles showed the absorption at about 591
& 599 nm due to CdSe absorption .This confirms the formation of hybrid
nanostructure of Au/CdSe. The emission of these samples is quenched completely due
to the electron transfer to the metallic surface. Both absorption and emission spectra
illustrate the strong coupling of Au nanoparticles and CdSe nanocrystals in the
obtained system.
5
4
Sample (f)
absorption
sample (e)
absorption
emission
emission
Intensity (a.u)
Intensity (a.u)
4
2
400
600
800
3
2
400
600
wave length (nm)
wave length(nm)
800
Figure (5): Absorbance & emission spectra of Au/CdSe for particles size (a) 4.42nm & (b) 4.74nm
Characterization of Ag/CdSe core/shell nanocomposit
XRD diffraction Pattern
XRD diffraction of silver nanoparticales deposited on silicon slide is shown in Figure
(6). The XRD shows that silver nanoparticles formed are of fcc crystalline. Four
peaks at 2θ values of 38.3182, 44.4975, 64.6119 and 77.5385 deg corresponding to
(111), (200), (220) and (311) planes of Silver is observed and compared with the
standard powder diffraction card of (JCPPS, 04–014-0266); that was in conformation
of face centered cubic (fcc) structure of silver.
The average core size of pure silver Ag nanoparticle (~15nm) has been estimated by
using Debye-Scherrer formula as shown previously. The calculated particle size
details and 'd' spacing (nm) are shown in Table (4).
Ag ( FCC )
counts (a.u )
(111)
(220)
(200)
(311)
20
30
40
50
60
70
80
90
2
Figure (6): XRD pattern of silver nanoparticles.
6
2Ө of the
intense
peak (deg)
hkl
FWHM of
Intense peak
(β) radians
Size of the
partcle (D)
nm
d-spacing
nm
37.9
111
0.0097
15
0.237
44.49
200
0.0104
15.07
0.203
64. 2
220
0.012
13.86
0.144
77.44
311
0.013
13.7
0.123
Table (4): The calculated particle size details and 'd' spacing (nm) of Ag nanoparticles
Optical Properties
Figure (7) showed the variation of the absorption spectra of Au/CdSe core/shell
nanoparticles, during the course of the chemical growth. Controlling the shell
thickness show a shift of the excitonic CdSe absorption peak to the red as the shell
thickness increase. The Plazmon band of Ag nanodot appears at 400nm, involving
the oscillations of the free electrons in the conduction band that occupy energy states
immediately above the Fermi level. Once CdSe precursors have been added, the
intensity of the Plasmon band of the silver core decreases, and small shoulder due to
the formation of the CdSe was appear at higher wavelength (~512 nm). The loading
of theses oscillations with CdSe shell leads to shift towards the blue and a decrease in
its amplitude in samples. That might be because of the change of the dielectric
constant of the surrounding.
(1&2) AgN.C.
(3) (2.59nm)
(4) (3.20nm)
(5) (3.70nm)
( 6 ) (4.33nm)
(7) (4.73nm)
Intensity (a.u)
5
growth time
0
400
500
600
700
wave length (nm)
Figure (7): Optical absorption spectra for Ag /CdSe core/ shell samples of shell thickness as indicated;
(S 1, 2&3) for Ag core and (S 4,5,6&7) for CdSe shell.
With increasing the CdSe loading, the shell thickness of its layer increases and
broadening of the surface plasmon (SP) peak , that start to appear at 553 nm, and shift
toward lower energy causing a red shift of the excitonic comparing with the original
SP peak position. The plasmonic silver core enhances the optical abortively of the
CdSe shell. The shell thickness calculation was carried out by two methods, the
polynomial fitting functions Yu’s et al10and effective mass approximation model
Brus's et al11. The calculated size of given prepared samples corresponding to the λmax
of absorption spectra are shown in Table (5). And the sizing curve is depicted in
Figure (8).
7
Samples No.
λmax of UV/vis
Peak (nm)
Shell thickness R (nm)
Brus's et al
Shell thickness D ( nm)
Yu’s et al
3
512. 25
2.30
2.59
4
552.80
2.75
3.20
5
570.27
3.59
3.70
6
589.40
4.03
4.33
7
599.60
4.32
4.73
Table (5): Calculated shell size of the prepared Ag/CdSe nanocrystals from the first excitonic
absorption band using effective Brus’s and Yu et al equations.
.
5.0
Brus et al
Yu et al
4.5
Dot size (nm)
4.0
3.5
3.0
2.5
2.0
500
520
540
560
580
600
wave length(nm)
Figure (8): Sizing curves (shell thickness) for the prepared Ag/CdSe nanocrystals using Brus et al and
Yu et al. al equations.
The photoluminescence (PL) spectra of all the samples of Ag-CdSe core shell
nanoparticles shown in Figure (9). The absorption maxima occur at higher energies
than the emission maxima as expected, Stokes shift means the difference between the
absorption and emission wavelengths. The large Stokes shift and narrow size
distribution of these shell nanocrystals, along with the broad emission spectrum,
would alleviate any efficiency loss attributed to self absorption.
1.6
1.4
Sample no.5
Absorption
Absorption
emission
1.2
stoke shift = 10 nm
1.0
1.0
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
400
500
525
550
575
600
625
wave length(nm)
650
675
700
emission
stoke shift =16 nm
0.8
0.0
8
1.2
Intensity (a.u)
Normalized Int. (A.U.)
Sample no.4
1.4
450
500
550
wave length(nm)
600
650
700
2.0
2.4
Sample no.7
Sample no.6
1.8
stoke shift = 11 nm
2.2
Absorption
1.6
emission
Intensity (a.u)
1.8
Intesity(a.u)
1.4
1.2
1.0
0.8
1.6
1.4
1.2
1.0
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
500
Absorption
stoke shift =7nm
2.0
emission
550
600
650
700
0.0
500
550
600
650
700
750
800
850
wave length(nm)
wave length(nm)
Figure (9): The Stoke shift, where absorption (black line) and emission (red line) spectra for all
samples of Ag-CdSe nanocrystal of different sizes equation.
.
Samples
No.
λ max of Abs.
spectra
(Wavelength/nm)
λ max of emission
spectra
(Wavelength / nm)
Stokes shift
(nm)
4
552.80
561.1
10
5
570.27
586.6
16
6
589.44
600.0
11
7
599.60
606.5
7
Table(6): The absorption maxima λAbs. , the emission maxima λPL , and the stokes shift.
The presence of silver metallic core inside the CdSe nanoparticles affects the formed
exciton and the associated phonon contribution. High localized electromagnetic field
generated by the metal surface affect the coulomb interaction force and increases the
intensity of the phonon peak. This reflected in the increasing the intensity of the core
shell (stokes shift = 10 nm), then reaches the maximum effect in (stokes shift = 16
nm). This effect began to decrease again when the shell become thicker (stokes shift =
11&7) . The large shell thickness of CdSe lead to decrease the excitonic band gap
lowers than E-Fermi (Ef). This means that the electron transfers from metal to
semiconductor. The coupling decrease again, and then start to increase more by
increasing the shell thickness. This could be attributed to the fact that the plasmonic
resonance of the silver nanoparticles at 420 nm is far away from the excitonic peaks
of the CdSe nanoparticles which start to appear at 553 nm and shift towards lower
energy by increasing the CdSe size. Table (6) shows the variation of emission maxima
with the absorption maxima. This is linearly relationship of a slope ~ nearly one
which clearly shown in Figure (10).
9
Emission peak maxima (nm)
620
600
580
560
540
520
520
540
560
580
600
620
Absorption peak maxima ( nm)
Figure (10): The linear relation of the absorption vs. emission maximum for Ag-CdSe core s
towards lower energy by increasing the CdSe size. CdSe size.
Conclusion
The Au/CdSe and Ag/CdSe nanoparticles have been synthesized with different sizes
using organometalic pyrolysis method with some modification. The Absorption
spectra for the core/shell Au/CdSe & Ag/CdSe nanoparticles have shown a combined
absorption of metal core SP band and the CdSe nanoshell. The absorption spectrum
for CdSe nanoparticles exhibits remarkable red shift in the surface Plasmon (SP)
energy band with size increase. Increasing the CdSe shelll in the Au/CdSe & Ag/CdSe
nanoparticles gives the opportunity to tune the band gap of the nanoparticles to
harvest a wide range of visible solar spectra.
The emission spectra of Au/CdSe the exciton transition for CdSe leads to rapid
quenching of the CdSe by photo induced electron injection into the Au nanoparticle.
It has been observed that plasmonic gold has more influence on the optical properties
and the quenching rate of the emission of CdSe nanoparticles much more than that of
silver. This could be explained simpley because the plasmonic gold show its plasmon
resonance at 520 nm, which loacated in the same range as that of the optical
absorption of CdSe nanocrystals (band gap absorption is ranged between 500-590
nm). In the mean while, silver nanoparticles absorb at 400 nm, which is far in
resonance of the optical absorption of the CdSe compared to that of gold particles.
Thus, the qunching of the emission in case of CdSe/Au is much more effective, and
the elctron transfer processes from CdSe to the gold is more faster than that of the
silver. For this reason, we can consider that CdSe/Au is strong plasmonic-exciton
coupling but CdSe/Ag is week plasmonic-excitonic coupling. The stoke shift values
of core/shell Ag/CdSe samples were increased with decreasing the shell thickness.
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