Nanoscale Optical Properties ( ) ( )

Nanoscale Optical Properties
When we studied chemical synthesis of nanoparticles, we briefly
discussed nanoscale optical properties:
(1) Semiconductor nanoparticles: Quantum confinement led to
an increase in the energy of optical absorption and emission.
(2) Semiconductor nanowires: Anisotropic structure led to
polarization dependence of optical properties.
P. Mulvaney, et al., MRS Bulletin, 26, 1009 (2001).
Department of Materials Science and Engineering, Northwestern University
(3) Metal nanoparticles: Absorption and scattering of light was
size dependent, leading to color changes in colloidal
solutions.
Department of Materials Science and Engineering, Northwestern University
Localized Surface Plasmon Resonance (LSPR):
collective excitation of the conduction electrons
Nuclear framework
of particle
E-field
Metal
sphere
Size-Dependent Extinction Spectra
Mie theory (1908):
2 3 3/2
24π Na ε ⎡
m
ε ⎤
i
E ( λ ) =
⎢
2
⎥
2
λ ln(10) ⎢⎣( ε
r+ χε
m)
+ ε
i ⎥⎦
Charge cloud of
conduction electrons
e - cloud
Plasmon excitation influences the absorption and
scattering of light from the surfaces of metals.
ε m
ε r , ε i
a
E(λ) = Extinction spectrum = absorption + scattering
χ = shape factor (2 for sphere, > 2 for spheroid)
ε m = external dielectric constant
ε r = real metal dielectric constant
ε i = imaginary metal dielectric constant
Courtesy of Professor Richard Van Duyne
Department of Materials Science and Engineering, Northwestern University
Courtesy of Professor Richard Van Duyne
Department of Materials Science and Engineering, Northwestern University
Size-Tunable Surface Plasmon Resonances
Size-Tunable Surface Plasmon Resonances
Normalized Extinction
120 150 150 95 120 145 145 145 width
42 70 62 48 46 59 55 50 height
shape
426 446 497 565 638 720 747 782 λ
max
Ag/mica
400 500 600 700 800 900
Wavelength (nm)
T. R. Jensen, et al., J. Phys. Chem. B, 104, 10549 (2000).
Department of Materials Science and Engineering, Northwestern University
Calculated Electric Field:
20 nm diameter Ag Sphere in vacuum
Polarization
Propagation
Shape Dependence
Department of Materials Science and Engineering, Northwestern University
48
42
36
30
24
18
12
6
0
Calculated Electric Field:
100 nm Ag Triangle in vacuum
Courtesy of Professor George Schatz
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Nanosphere Lithography
Nanosphere Lithography Size and Spacing Control
J. C. Hulteen, et al., J. Vac. Sci. Technol. A, 13, 1553 (1995).
Department of Materials Science and Engineering, Northwestern University
ip
d ip
d = 1/
( )
D
3 D = 0.577 D
D = 542 nm
a = 126 nm ; d ip = 313 nm
Examples:
3⎛
1 ⎞
a = 3-1- D
2
⎜ ⎟
⎝ 3 ⎠
= 0.233D
D = 400 nm
a = 93 nm ; d ip = 231 nm
Department of Materials Science and Engineering, Northwestern University
a
Nanosphere Lithography: Single Layer Mask
Surface Confined AFM
Released TEM
95
nm
D = 542 nm
a = 126 nm ; d ip = 313 nm
D = 400 nm
a = 93 nm ; d ip = 231 nm
Department of Materials Science and Engineering, Northwestern University
Department of Materials Science and Engineering, Northwestern University
Size-Tunable Ag Nanoparticle Arrays
SL
DL
D
A
A
315 nm
155 nm
540 nm
110 nm
18 nm
18 nm
B
B
231 nm
121 nm
399 nm
97 nm
18 nm
18 nm
542 nm 401 nm 266 nm 165 nm
Department of Materials Science and Engineering, Northwestern University
C
C
153 nm
78 nm
267 nm
64 nm
18 nm
18 nm
D
D
93 nm
42 nm
163 nm
30 nm
10 nm
J. C. Hulteen, D. A. Treichel, M. T. Smith, M. L. Duval, T. R. Jensen,
R. P. Van Duyne, J. Phys. Chem. B, 3854-3863 (1999).
6 nm
Advantages of Nanosphere Lithography
• Massively Parallel: 10 10 nanostructures cm -2
• Simple, inexpensive: < $1 / sample
• High throughput: ~ 10 samples/day/person
• Materials and substrate general
• Minimum feature size is ~ 20 nm
• 1 nm control of nanogaps
• Precision control of nanoparticle size
Department of Materials Science and Engineering, Northwestern University
2

-1
-1
Disadvantages of Nanosphere Lithography
No Arbitrary Patterns
No independent control
of in-plane size and spacing
0.1 – 1% Defects from
Nanosphere Polydispersity
Line Defects
Point Defects
Polycrystalline
Domains
Department of Materials Science and Engineering, Northwestern University
Localized Surface Plasmon Resonance
Spectroscopy of Ag Nanoparticle Array
Extinction
λ max = 586 nm
0.35 ev
400 600
Wavelength (nm)
uv-vis Extinction
PPA
I 0
I
AFM image
2.3 µ m
a = 117 nm
d
ip
= 230 nm
b = 59 nm
d
tt
= 91 nm
Department of Materials Science and Engineering, Northwestern University
800
Jensen, T. R.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. B 1999, 2394-2401.
Near and Mid-Infrared LSPR Spectra
Extinction
Extinction
Wavelength (nm)
600 800 1000 2000
λ max=8888.8 cm -1
max = 8889 cm -1 λ -1
max = 2110 cm -1
0.04 au
16,000 12,000 8,000
-1
Wavenumber (cm )
Wavelength (nm)
600 800 1000 2000
λλ max=7331 -1
max = 7331 cm -1
0.023 au
16,000 12,000 8,000
-1
Wavenumber (cm )
397 nm
559 nm
243 nm
28 nm
323 nm
23 nm
T. R. Jensen, et al., J. Phys. Chem. B, 104, 10549 (2000).
Department of Materials Science and Engineering, Northwestern University
A
B
Extinction
Extinction
3000
3000
Wavelength (nm)
4000 6000 10,000
0.1 au
2000
Wavenumber (cm )
Wavelength (nm)
4000 6000 10,000
0.1 au
λλ max = cm -1
max = 1655 cm -1
2000
Wavenumber (cm )
1000
1000
1555 nm
1390 nm
65 nm
897 nm
830 nm
50 nm
C
D
Extinction Efficiency
Theory vs. Experiment : Effect of Substrate
28 nm height PPA
12
0.28
8
4
DDA
no substrate
DDA
with substrate
Ag
mica
0
0.12
400 500 600 700 800 900
Wavelength (nm)
Department of Materials Science and Engineering, Northwestern University
0.24
0.20
0.16
Measured Extinction
New Stuctural Motifs for Nanosphere Lithography
Angle Resolved Nanosphere Lithography
Department of Materials Science and Engineering, Northwestern University
C. L. Haynes, R. P. Van Duyne, J. Phys. Chem. B, 105, 5599 (2001).
Department of Materials Science and Engineering, Northwestern University
3

E-field Intensity
E-field Intensity
Angle Resolved Nanosphere Lithography
LSPR (nm) vs. Interparticle Spacing (d )
λ max
tt
680
Weak Interparticle Coupling
1 nm spacing
is achievable
experimentally
LSPR (nm)
λ max
640
600
LSPR
λ λ max max = 574 nm
Theory:
DDA
Independent Nanoparticles
AFM
d
tt
= 114 nm
C. L. Haynes, R. P. Van Duyne, J. Phys. Chem. B, 105, 5599 (2001).
Department of Materials Science and Engineering, Northwestern University
560
0 40 80 120
Interparticle Spacing, d tt (nm)
T. R. Jensen, G. C. Schatz, R. P. Van Duyne, J. Phys. Chem. B, 103, 2394-2401 (1999).
Department of Materials Science and Engineering, Northwestern University
Electric Field Enhancement
E
k
852 nm
E
k
57,000
653 nm
2700
Department of Materials Science and Engineering, Northwestern University
4