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Supporting Information
© Wiley-VCH 2007
69451 Weinheim, Germany
High-Yield Synthesis of Single-Crystalline Gold Nanooctahedra
Cuncheng Li, Kevin L. Shuford, Q-Han Park, Weiping Cai, Yue Li, Eun Je Lee, and Sung Oh Cho
24 25 26 27 28 29 30 31 32 33 34 35 36 37 380
10
20
30
40
50
60
70
Num
ber
Edge Length (nm)
A
40 42 44 46 48 50 52 54 56 58 600
10
20
30
40
Num
ber
Edge Length (nm)
B
50 52 54 56 58 60 62 64 66 68 700
10
20
30
40
50
Num
ber
Edge Length (nm)
C
Figure S1. The size distribution of Au nanooctahedra obtained at different reaction time. The edge
lengths (average value ± full width at the half maximum) and the reaction time of the Au nanooctahedra
are (A) 30±1 nm at 6 h, (B) 50±2 nm at 24 h, and (C) 60±2 nm at 48 h, respectively. (D) FESEM images
for Au nanooctahedra obtained at 6 h.
1
1000 800 600 400 200 0
Au4d
Inte
nsity
(a.u
.)
Binding enery (eV)
Au4f
C1sN1s
O1s
A
95 90 85 80
83.6 Au4f7/2
Au4f5/2
Inte
nsity
(a.u
.)
Binding enery (eV)
87.3
B
540 535 530 525
O1s
Inte
nsity
(a.u
.)
Binding enery (eV)
532C
405 400 395 390
N1s
Inte
nsity
(a.u
.)
Binding enery (eV)
399.8D
Figure S2. XPS spectra of the repeatedly washed Au nanooctahedra from a survey scan (A), Au 4f (B), O
1s (C), and N 1s (D) energy region. The XPS spectra were referenced to C 1s at 285 eV. The peaks of
both Au 4f7/2 (83.6 eV) and Au 4f5/2 (87.3 eV) shifted by ca. 0.4 eV toward lower binding energies
relative to bulk Au atoms. The peak of O 1s (532 eV) attributed to carboxyl (C=O) oxygen of PVP shifted
by ~0.7 eV to higher binding energy, while the peak of N 1s (399.8 eV) did not shift compared to those of
pure PVP. These verify that PVP molecules strongly interact with Au atoms on the surface of
nanooctahedra.
2
Figure S3. The products synthesized at other conditions: in the absence of surfactant PVP (A); the
reaction temperature is 100 oC (B) and 150 oC (C) ; the gold precursor was not preheated at 75 oC (D);
the concentration of NaBH4 is 0 mM (E) and 0.75 mM (F). Scale bars for (A) , (B)-(E), (F) are 2 μm, 500
nm, 100 nm, respectively. The amount of the other reagents and experimental processes were controlled
to be identical to that of the Au nanooctahedra.
In order to demonstrate that Au(0) atoms can be oxidized by Au(III) ions, we performed an experiment
to add Au nanoparticles into AuCl3 PEG 600 solution and investigated the color change of the solution.
Au nanoparticles (2~6 nm, Figure S4A) were synthesized by adding 1.2 mL of 500 mM NaBH4 to 20 mL
AuCl3 PEG 600 solution under stirring at room temperature. The color of Au precursor changed rapidly
from yellow to puce, indicating that Au(0) atoms were directly produced from Au(III) ions due to the
high concentration of NaBH4 (the mole ratio of Au/NaBH4 is 1:6). Before using these Au nanoparticles,
the nanoparticle colloid was kept at 75 oC for more than 24 h to decompose superfluous NaBH4.
3
Figure S4. TEM image (A) of the synthesized Au nanoparticles (2 ~ 6 nm in size); FESEM images of the
final products synthesized from sample A (B) and sample B (C), respectively.
When 0.7 mL as-prepared Au colloid was added into 19.6 mL AuCl3 and PEG 600 solution (AuCl3: 0.7
mM, PVP: 100 mM, sample A), the solution color changed from yellow to light puce, which originates
from Au nanoparticles. Subsequently, the solution was heated at 75 oC for 1~2 h and then the solution
became light yellow from light puce, suggesting that the solution mainly consists of Au(III) ions.
Additionally, when 1.4 mL as-prepared Au colloid were added into the solution (sample B), the color of
the solution turned from light puce into colorless after the heating at 75 oC. This indicates that both Au
nanoparticles and Au(III) ions were not the main components in the heated solution. These two results
reflect that Au atoms can be redissolved in the presence of Au(III) ions and that oxidation reaction
between the Au(0) atoms and Au(III) ions can spontaneously occur in our system.
Moreover, high-yield Au nanooctahedra were obtained by further heating sample B at the reaction
temperature of 125 oC (Figure S4C). Whereas, for the sample A, the products had various mixed shapes
(Figure S4B). These results support our argument that almost complete evolution of Au(III) ions into
Au(I) ions before the main reaction is crucial for the selective formation of octahedral Au nanocrystals in
our experiments.
4
Numerical Calculations of the Optical Properties
We have computed the optical properties of octahedral gold nanocrystals using the Discrete Dipole
Approximation (DDA). The DDA method solves Maxwell’s equations by partitioning the particle into
small cubic units represented by dipoles.[S1,S2] Each dipole obtains an oscillating polarization from an
incident field, as well as the fields produced from all of the other dipoles in the array. An iterative
procedure is used to calculate the response of the coupled dipole system. The dipole polarizations are then
used to calculate the optical properties of the scattering object. The DDA equations have been reported
numerous times in the literature and therefore not reproduced here. For a more thorough description of the
method including equations, see the work of Draine and coworkers.[S1-S4] The calculations require
frequency dependent optical constants as input. In this work, we have chosen to use experimentally
determined values for gold.[S5]
The optical properties of an octahedron are dependent upon the orientation of the particle with respect
to the incident field. The polarization of the incident field dictates the spectra for the relatively small
particles studied here, while the direction of propagation is less important because the field amplitude
does not vary significantly across the particle. Figure S5 shows the calculated spectrum of a 30 nm
octahedron for two polarizations that are perpendicular to one another. We define the plasmon excitations
as in-plane and out of plane modes, where the aforementioned plane bisects the octahedron into two
equivalent square pyramids. The collective oscillation of electrons parallel to the bisecting plane is
defined as an in-plane mode. The out of plane mode is defined as the collective oscillation of charge
perpendicular to the bisecting plane. This peak red-shifts by ≈ 30 nm and maintains a similar shape when
the octahedron edge length is increased from 30 to 60 nm. Two in-plane modes are present at 567 and 610
nm that both correspond to dipole plasmon excitations. One can imagine two types of in-plane dipole
excitations. They are an edge dipole mode, where the induced polarizations are parallel to the edges of
two neighboring corners, and a cross dipole mode, where the induced polarizations are oriented
diagonally across the square cross section from one corner towards the opposite corner. These two modes
are strongly coupled by mutual interaction, and both are excited simultaneously to some degree when the
incident field is polarized parallel to the bisecting plane. The true induced polarization that occurs is a
combination of the idealized edge and cross dipole states. The degree of mode mixing depends on the
morphology of the particle and polarization of the incident field as will be discussed below.
Panels B-F in Figure S6 display the evolution of the extinction spectrum as the octahedron edge length
is increased from 30 nm to 100 nm. Two incident polarizations that excite in-plane plasmon modes are
shown in each panel. The blue traces have an incident polarization that is parallel to an edge of the
bisecting plane, while the red traces have an incident polarization that is diagonal as depicted in Panel A.
5
These orientations have been chosen to preferentially excite the edge dipole mode and the cross dipole
mode, respectively.
Figure S5. Extinction spectrum of a 30 nm octahedron in ethanol. In-plane (out of plane) excitation
occurs when the polarization of the incident field is parallel (perpendicular) to the plane bisecting the
octahedron into square pyramids.
Figure S6. Extinction spectra for Au nanooctahedra with various edge sizes. Panel A displays the
polarization of the incident field. The color of the traces in Panels B-F correspond to the orientation
depicted in Panel A. Note that in some panels the traces are not well resolved.
6
Panel B corresponds to a truncated octahedron with a true edge length that is ≈ 24 nm. Octahedra with
truncated tips display only one peak for both incident polarizations as seen in Figure S6 B. It is well
known that large induced polarizations become localized in regions of high curvature, and especially at
tips. Truncating the tip spreads the polarization more evenly around the surface of the particle, which
essentially liminates the interaction of the two modes. Both types of dipole excitations are excited at the
same frequency, yielding a single peak in the spectrum for either incident polarization. Figure S7 shows
the induced polarization and electric field enhancement for the two different orientations at peak
extinction. The induced polarizations are highly uniform, and oriented in a single direction parallel to the
incident field indicating a well-defined dipole excitation. The electric field intensity in Panel C is spread
around the entire surface, and has a maximum enhancement of ≈ 300 occurring at all of the truncated
corners. The field intensity in Panel D shows more localized field enhancements near the corners, with a
maximum intensity of ≈ 600 along the direction of the incident polarization. Note that in this case the
energy carried by the incident field localizes in two regions instead of four leading to greater
enhancements. Figure S7 shows the characteristics of an edge dipole (Figure S7A, C) and a single cross
dipole (Figure S7B, D) in nearly pure states. These vector plots and field patterns can be used to help
analyze the more complex excitations presented below. The calculation results show that no higher-order
multipoles are excited for the nanooctahedra with the sizes from 30 nm to 60 nm.
The mode coupling is substantial in nanoparticles with perfect octahedral shapes, that is, the tips of the
octahedron are not truncated. A perfect 30 nm octahedron produces two peaks in the spectrum as seen in
Figure S6C. The highly polarized regions localized at the tips induce coupling between the two in-plane
modes and thus if one mode is excited, the other mode is also generated. The coupling becomes
maximum when the edge length of octahedron is increased to 60 nm. This can be inferred from the
approximately equal intensities in the spectrum (Figure S6D) and the field patterns that can be seen in
Figure S8. The field enhancements at 582 nm are similar to the edge dipole pattern (Figure S7C), where
the intensity is less localized and distributed more uniformly around the surface. However, a vector plot
of the induced polarization presented in Figure S9A shows that the hot spots on the corners are dipole
moments that are oriented diagonally indicating cross dipole character in addition to edge dipole. These
diagonally oriented polarizations are extremely intense for the resonance at 623 nm as is clearly shown in
the field enhancements in Figure S8C, D. However, again a vector plot of the induced polarization in
Figure S9B shows that some intense dipole moments close to the corners actually do align vertically
indicating edge dipole character in addition to cross dipole character. It is also interesting to note that the
field patterns at interior points of the nanoparticle are consistent for each wavelength regardless of the
incident polarization further supporting the idea that the resulting plasmon excitations are combinations of
the edge and cross dipoles.
7
Figure S7. Properties of a nanooctahedron with a 30 nm edge length (truncated) at 572 nm. The black
(white) arrows indicate the polarization of the incident field. Panels A and B are the induced polarizations
and Panels C and D are electric field intensity.
Figure S6 implies that to some extent the polarization of the incident field can be rotated to selectively
excite a particular in-plane dipole mode. However, the presence of two peaks in the spectrum indicates
that a complete decoupling of the modes is not occurring. The incident fields chosen to selectively excite
a particular dipole mode excites both the edge and cross in-plane modes. This strongly suggests mode
mixing and the coupled mode interpretation presented above. It should also be noted that in addition to
incident polarization and truncation, the size of the nanoparticle affects the coupling. Larger octahedron
favors a state more similar to the cross dipole. This is because the induced polarization becomes localized
at the tips almost exclusively, and these dipoles have a propensity to orient diagonally across the square
cross section. As a result, the amount of edge dipole character decreases and the amount of cross dipole
character increases. This trend can be observed in Figure S6 as the intensity of the blue peak decreasing
and the intensity of the red peak increasing, as well as the relative intensities of both spectral features with
regard to incident polarization.
8
Figure S8. Electric field intensity of an Au octahedron with a 60 nm edge length. The white arrows
indicate the polarization of the incident field.
Figure S9. Induced polarization of a 60 nm Au octahedron. The black arrows indicate the polarization
of the incident field. Panel A corresponds to the field intensity displayed in Figure S8A, and Panel B
corresponds to the field intensity displayed in Figure S8D.
References [S1] B. T. Draine, Astrophys. J. 1988, 333, 848.
[S2] B. T. Draine, P. J. Flatau, J. Opt. Soc. Am. A 1994, 11, 1491.
[S3] B. T. Draine, J. Goodman, Astrophys. J. 1993, 405, 685.
[S4] J. J. Goodman, B. T. Draine, P. J. Flatau, Opt. Lett. 1991, 16, 1198.
[S5] P. B. Johnson, R. W. Christy, Phys. Rev. B 1972, 6, 4370.
9