Franzen, S. “Surface Plasmon Polaritons and Plasma Absorption in Indium Tin
Oxide .... “Surface Plasmon Resonance in Conducting Metal Oxides” J Appl Phys
...
What are surface plasmons?
NC State University
A plasmon is a collective oscillation of the conduction electrons The free electron optical response uses the Drude-Lorentz-Sommerfeld model. The influence of external forces is considered for one electron alone and then the response is multiplied by the number of electrons. All electrons act in phase in this model. 2 me r2 + me r = eE0e – it t t Force + Friction = Driving term
Dipole p = er0 Polarization P = np = ner0 Polarization driven by an electric l i fifield ld P = () ( ) 0E Suscepticility () = () - 1
Electron motion e
E = E0e-it
Electric vector
The plasmon frequency is the resonantt frequency f off frictionally f i ti ll damped p electron motion The forcing term is the electric field of the incident light. 2 2 E ne E – it + = E e t m e 0 0 t 2
–p2 = 2 + i
p =
E = E0e-it ne 2 m e 0
2p 2p =1 – 2 2 + i + 2 + 2 Real part
Imaginary part
Dielectric constants for a f free electron l t conductor d t Imaginary part Out-of-phase Absorption
Real part In-phase Dispersion
Surface selection rules
For c < 0 the p-polarized image charge adds constructively const cti el to the incident field field. +
p polarization + s polarization
+
rp = 1 -
c < 0
Ep = Ei(1 + rp) = Ei(1 – cos2)
For c < 0 the s-polarized image charge adds dd destructively d t ti l to t th the iincident id t field. fi ld p polarization + s polarization
+
-
rs = -1 m < 0
+ Es = Ei(1 + rs) = 0
Kretschmann configuration IRE > s +
+
p polarization s polarization
-
IRE c s
Thin film on a prism
Attenuated total reflection ( > c ) Condition for surface plasmon resonance rp = 1 rS = -1
+
p polarization s polarization +
-
IRE
-
c
s
Evanescent Wave in Medium
Dispersion relations from Drude n cz =
– 2c , n sz = c + s
– 2s s + c
, nx =
c s s + c
Plasmon Dispersion Curve for ITO ωp = 17700 cm-1
Γ = 500 cm -1
k sp = c
c s c + s
Surface Plasmon Resonance on Gold The intensity of the reflected light is reduced at a specific incident angle l producing d i a sharp h di dip-a surface f plasmon l resonance The surface plasmon resonance is related to the material adsorbed onto the thin metal film Visible electromagnetic radiation 2
ne p = m e 0
1/2
Plasma frequency n is charge carrier density me is effective mass
http://www.gwctechnologies.com/images/spreffect.gif
SPR Biosensor Incident Light
Reflected Light
I
Flow Cell
Coupling of the light into the Au thin film requires q ap prism to p provide wavevector matching (Kretschmann configuration).
SPR implementation Incident Light
Reflected Light II
Flow Cell
Molecules in solution induce changes in refractive index and g give rise to a measurable SPR signal when binding occurs.
For c = - 2s the plasmon can be excited resonantly tl tto yield i ld an enhancement h t off the th local field p polarization
+
? +-
m = -2s
Local field Ep = gEi
g=
m–- s0
m++2 20s
Absorption and dispersion in conductors
Franzen, S. “Surface Plasmon Polaritons and Plasma Absorption in Indium Tin Oxide Compared to Silver and Gold” J. Phys. Chem. C 2008, 112, 6027-6032
Biosensing using surface plasmons Known methods: Fluorescence quenching (molecular beacon) Surface plasmon resonance (SPR) Kretschman configuration Nanoparticle plasmon resonance Thermographic detection Proposed methods: Surface enhanced Raman effect Surface enhanced fluorescence Surface enhanced infrared
Fluorescence quenching b plasmons by l Example: Quenching to Ru(bipy)32+
N N
N
Ru
N
N
N
SH
Glomm, Franzen et al. JPC B 2005, 109, 804
Molecular beacon approach
Tsourkas et al. Anal. Chem. 2003, 75, 3697
Plasmon Thermography Excite surface plasmons and detect heating by change h iin bl blackbody kb d radiation: di ti W = T T4
US p patent application pp US2004/0180369A1
Laser-Induced Temperature p Jump p Electrochemistry and Thermography Thermographic array imager
Anodic current for an ITO electrode off
on
ssDNA/gold nanoparticles
ssDNA/ITO
Lowe, Franzen, Feldheim JACS 2003. 125. 14258
Dynamic Range and Limit of Detection of Gold Particles Dropcast p Onto Nylon y Substrates
T T [K]
10
ABI White Nylon Coherent Antares Laser @ 532 nm Laser power: 1 W, beam diameter 2 mm
1 0.33 attomoles/cm2 of particles B k Background d
0.1
0.1
1
10
# of Gold Particles on Surface [amols]
100
Are gold g and silver silver the only SPR substrates? y Fixed charge carrier density: ~1023 electrons/cm3 Limited electrochemical range: Gold is oxidized above 0.8 V Thiol surface chemistry Conducting metal oxides offer possibilities not present on gold –No quenching of fluorescence –Stable electrochemistry over a wide range –Processible Processible surface –Many surface chemistries possible There are hundreds of mixed metal oxide substrates possible.
Indium Tin Oxide (ITO) Composition: 90% Indium Oxide and 10% Tin Oxide Commercial 1700Å thick 8-12 /□ Band Gap 3.7 eV St Structure: t Bi b it cubic Bixbyite bi crystal t l structure t t Tunability: C t i surfaces Customize f ffor specific ifi reactions ti Resistivity changes: Thickness change Alteration of annealling onditions Doping change Crystal orientation changes: Deposition temperature Change in annealling conditions
Type: sp type conductor Common Uses: Heat Shields Flat Panel Displays
Experimental and Calculated Reflectance of ITO Three Phase Fresnel Model (air/ITO/glass)
1 rp12 rp 23 e 2i Rp = rp
2
Reflectan nce, Power R Reflectivity
rp
rp12 rp 23 e 2i
60º, p-polarization 0.8
06 0.6 7.6 square
0.4 9.7 0.2
13.8
0.0 4
6
8
3
10 -11
12
Wavenumbers (x10 ) (cm )
In 2002, Brewer and Franzen predicted that ITO would have a surface plasmon 1. Alloys and Compounds, 2002, 338, 73-79 2. J. Phys. Chem. B. 2002, 106, 12986-12992
Tunable Parameters
• Thickness: - deposition time
• Carrier Concentration: - First Annealing Process: 5%H2/95%N2 (Forming gas) - Second Annealing Process: Varies the Partial Pressure of Oxygen 7x10-7mTorr to 50mTorr •
Mobility: - Sputtering gas gas, Argon: 6mTorr to 20mTorr
• Composition: p - Sn:In ratio 0 – 10%
Fourier-Transform SPR
Steve Weibel, GWC Technologies, Inc. 1. Rhodes C.; Franzen, S.; Maria, J-P.; Losego, M.; Leonard, D.N.; Laughlin, B. ; Duscher G.; Weibel, S.; “Surface Surface Plasmon Resonance in Conducting Metal Oxides” Oxides J. J Appl Appl. Phys Phys. 2006, 2006 100, 100 Art. Art No. No 054905 2. Rhodes, C.L. ; Cerruti, M. ; Efremenko, A. ; Losego, M. ; Aspnes, D.E. ; Maria, J.-P.; Franzen, S. “Dependence of Plasmon Polaritons on the Thickness of Indium Tin Oxide Thin Films” J. Appl. Phys. 2008, 103, Art. No. 093108
Sputtering p g ITO Sputter S t System
Process Parameters Sputter Pressure (Ar) Power Input Power Type (RF vs DC) Substrate Distance Substrate Temperature p Time/Thickness O2/Ar Plasma
Sputtering p g POWER ON Cathode
Ar Ar+ Substrate
Ar+ Ar
Argon
---–-
Target (ITO)
Ar Ar+ Ar+
Ar
Sputtering p g POWER ON
In
In
Sn O ITO Film
Argon
---–-
O
ITO Angle Dip in Air: Single wavenumber representation
40
42
44
46
48
50
Surface Plasmon Resonance on ITO Calculated
Angle Range: 42°-53°
Surface Plasmon Resonance on ITO Experimental
Angle Range: 42°-53°
Capacitive CPR
Surface SPR _ +
Orthogonal
In‐plane
+ _
Optical Resonances Observed in Thin Films Electron Volts (eV) SPR R Reflectanc ce (a.u.)
0.6
0.7
0.8
0.9
1.0
1.1
1.2
Capcitive plasmon resonance: (CPR) • appears in i thi thinner fil films • narrow appearance • weak angle dependence • near-IR near IR range
160nm
100 80 60 40 20 0 5000
6000
7000
8000
9000
10000
-1
Wavenumber (cm )
Surface Plasmon Polariton (SPP) • optimum film thickness 160 nm • strong angle dependence • mid-IR range
SPR R Reflectanc ce (a.u.)
0.6
0.7
ElectronVolts Electron Volts (eV) 0.8
0.9
1.0
1.1
1.2
100 80 60
30nm
40 20 0 5000
6000
7000
8000 -1
Wavenumber (cm )
9000
10000
The planar limit of LSPR as a limiting case of an oblate spheroid
+
+
+
+
+
+
-
-
-
-
-
-
Sphere
Oblate ellipsoid
Planar limit
The planar limit of LSPR as a collection of nanoparticles
+ + + +
+
+
+
- -
-
-
-
-
-
Annealing Controlled Atmosphere Annealing XRD of ITO Films B f Before and d Aft After A Annealing li g
Intensity
Gas Inlet (N2 & H2)
In-situ s tu Transfer
ITO After Aft Anneal A l
Process Parameters
ITO As Deposited
Temperature Time p Atmosphere
10
20
30
40
50
60
70
2 ( )
Losego, S.; Efremenko, A.; Rhodes, C.; Cerruti, M.; Franzen, S.; Maria, J.-P. “Conductive Oxide Thin Films: Model Systems for Understanding and Controlling Surface Plasmon Resonance” J. Appl. Phys. 2009, 106, 024903
80
Plasma frequency is inversely correlated l t d with ith resistivity i ti it Shift in p plasma frequency q y Measured using FT-SPR
Change g in Film Conductivity y with Annealing Atmosphere -4
5 10
-4
2 ne p = m e 0
1/2
Re esistivity ( *cm)
4 10
-4
3 10
-4
2 10
-4
1 10
0
0 10
-18 18
10
-16 16
10
-14 14
10
-12 12
10
-10 10
10
-8 8
10
~Oxygen Partial Pressure of Annealing Atmosphere (Torr)
0.6
Carrier Concentration Series:
0.8
0.9
1.0
1.1
1.2
0.6
Electron Volts (eV)
0.7
0.8
pO2 0.01mTorr A
0.9
1.0
1.1
1.2
Theory pO2 0.01mTorr E
2
2
1 1. Oxygen fills vacancies 2. Sn and O trap e3. n decreases 4. ωp also decreases 5. SPP shifts into the mid-IR
p =
Electron Volts (eV)
0.7
-4
pO2 10 mTorr B
ne 2 0
-4 4
Theory pO2 10 mTorr F
-5
-5
pO2 10 mTorr C
Theory pO2 10 mTorr G -
5
2
-7
Carrier Concentrations cm-3: A. 3.948x1020 B 5 B. 5.659x10 659x1020 C. 7.136x1020 D. 1.120x1021
Theory pO2 10 mTorr H
-7
pO2 10 mTorr D 5
6 7 83 Wavenumber (x10 cm-1)
Experimental
9
10
5
6
7
8
9
Wavenumber (x10
3
Theoretical
-1
cm )
10
AFM Measurement of Grain Size Ar+ 10 mTorr
Ar+ 20 mTorr
Grain Size: 100 nm
Grain Size: < 40 nm
Mobility: ob ty 35 cm2/Vs
Mobility: ob ty 7 cm2/Vs
100 Reflectanc ce (a.u.)
Reflectanc ce (a.u.)
100 80 60 40 20 0
5000 6000 7000 8000 9000 10000 -1 Wavenumber (cm )
80 60 40 20 0
5000 6000 7000 8000 9000 10000 -1 Wavenumber (cm )
Hall Effect Measurements Mobility
21
1.5 10
40
1.5x Change
Mobility ((cm /V*s)
21
1 10
2
-3
Carrier Concentration (cm m )
Carrier Concentration
20
5 10
0 6
8
10
12
14
16
18
20
Sputter Pressure (mTorr)
22
35 30 25 20 15 10
5x Change
5 0 6
8
10
12
14
16
18
20
22
Sputter Pressure (mTorr)
Although sputter pressure affects the carrier concentration, it has a much larger impact on mobility of the charge carriers
0.6
Electron Volts (eV)
0.7
0.8
0.9
1.0
1.1
1.2
0.6
Electron Volts (eV)
0.7
0.8
0.9
M bilit Series: Mobility S i
e me Mobilities cm2/Vs: A. 23.7 B. 30.0 C 21 C. 21.2 2 D. 9.385
6
7
1.2
9mTorr B
Th Theory 9mTorr F
12mTorr C
Theory 12mTorr G
Theory y 15mTorr H
15mTorr D
5
1.1
Theory 6mTorr E
6mTorr A
1. Peaks around 9 mTorr 1. Decreases going away from maxima 2. Damping constant increases 3. Peaks broaden
1.0
8
Wavenumber (x10
Experimental
3
9 -1
cm )
10
5
6
7
8
Wavenumber (x10
Theoretical
3 -1 cm )
9
10
Hybrid plasmons
50 nm Au
50 nm Nano Au
80 nm 80 nm ITO
80 nm 80 nm ITO
1. Franzen. S; Rhodes C.; Cerruti, M.; Efremenko, A.Y.; Gerber, R.W.; Losego, M.; Maria, J.-P.; Aspnes D.; “Equivalences between Gold and Indium Tin Oxide as Plasmonic Materials” Opt. Lett., 2009, 34, 2867-2869 2. Gerber, R.W.; Leonard, D.N.; Franzen. S; “Conductive thin film multilayers of gold on glass formed by self-assembly of multiple size gold nanoparticles” Thin Solid Films, 2009, 517, 6303-6308
Multilayer composite films 12 nm and 2.6 nm particles
Indium tin oxide (ITO) Intermediate thickness: no CPR or SPR
80 nm ITO
Perpendicular polarization
Rp/Rs
CPR
30 nm
ITO
Parallel polarization NR
SPR
160 nm ITO
Indium tin oxide (ITO) Intermediate thickness: no CPR or SPR
80 nm ITO
Quench SPR, Activate CPR
Rp/Rs
CPR
50 50 nm A Au 80 nm ITO
Activate SPR, Quench CPR NR
SPR
50 nm Nano Au Nano Au
80 nm ITO
Comparison of Hybrid and Thickness
Plasmonic amplification: Pl i lifi ti What is possible for a Raman process? p p
Surface-enhanced resonance Raman Spectroscopy (SERS) and heme • Observation of large Raman signals for molecules associated with ith noble bl metals, t l particularly ti l l silver il and d gold. ld First Fi t observed b d in 1974 for pyridine on rough silver electrode. • Electromagnetic and chemical mechanisms. The chemical mechanism could be resonant Raman. • Enhancement factors have increased from 106 to 1015 throughout a year period.
A resonance Raman spectrum is obtained by a laser light scattering experiment Detector Lens Sample Laser
S t Spectrograph h
Inelastic light scattering produces a frequency shift. There is exchange of energy between the vibrations of the molecule and the incident photon. photon
Resonance Raman is a two photon process Incident photon from a laser. h
Scattered photon has an energy shift shift.
The difference is because the molecule is left in an excited vibrational state.
Raman scattering g Raman scattering is an inelastic i l ti lilight ht scattering process. In the resonant picture it involves evolution in the excited state so it also depends on the FC factor and the t transition iti di dipole l moment. On the left a sum-over-states picture is shown.
An optimistic view of enhancement Z + + +
First enhancement E0
r
a X
Second enhancement Y
g= g=
m–- s0
m++2 20s
-
-
-
Static approximation >> a
m–- s0
m++2 20s
Enhancements as large as 1015 ! Local field I = ½ e0 g2 E02 IIncident id t intensity i t it I = ½ e0 E02
Scattered intensity I = ½ e0 g2 g2 E02
O Overall ll enhancement h t th thought ht tto b be g4
Two questions about g4 enhancement: 1 Is this correct? 1. - Experimental SERS increases as |E|2, not |E|4 - Conservation of energy must be satisfied 2. How big is g? - g is can be calculated directly using Drude model - There must be a bandwidth to the SERS effect
A
First enhancement Z + + +
d +
Y
y
x +
X
-
z
r
a
-
Second enhancement
-
E0
B
-
Static approximation >> a
Planar image approx. a >> d
-
Surface Enhancement on NPs • Local Field Treated by Clausius‐Mosotti Relation () ( )–1 (sphere) () + 2 () – 1 (cylinder) g() = 2 () ( )+1 () – 1 g() = (plane) () g() = 3
• g() is the enhancement factor for an applied field ( )i h h f f li d fi ld • Implicitly spheres are treated most often in SERS lit t literature
Generalized Clausius‐Mosotti • The Clausius‐Mosotti relation connects the molecular polarizability with the dielectric function N = 0
() – 1
() + 1/ – 1 • • The parameter is the depolarization factor • The range is 0
