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Clean Energy Lab (CEL)
Towards Plasmonics in Epitaxial Graphene
M.V.S. Chandrashekhar
Department of Electrical and Computer Engineering,
University of South Carolina
1
USC
G.Koley
T.S. Sudarshan
C. Williams
J. Weidner
B.K. Daas
K.M. Daniels
S. Shetu
O. Sabih
A. Obe
CMU
R. Feenstra
N. Srivastava
MPI/Pisa
U. Starke
C. Colletti
Clean Energy Lab (CEL) @ USC
OUTLINE
•What is Graphene?
•Why Plasmonics?
• Viability of IR Plasmonics in EG on SiC
• Infrared carrier transport in EG/SiC
• Molecular doping studies using IR
•Interband processes
•Electrochemical Functionalization of EG
•Summary
WHAT IS GRAPHENE?






Single atomic layer of graphitic carbon “discovered” in 2005Physics Nobel in 2010 Geim & Novoselov, U. Manchester
Electrons behave like they have no mass-am I crazy?
Strongest material known -space elevator E=1.25TPa
Highest thermal conductivity in-plane
It is all surfacesensitive to surroundings
Very transparent and highly conductive-touch screens?
WHAT IS A PLASMON
POLARITON?
Clean Energy Lab (CEL) @ USC
Polariton: Collective oscillation of electrons (Plasmon), generated by the
electromagnetic field that excites the metal/dielectric interface [1]. It is a near-field
phenomenon. Like waves in water.
Electromagnetic wave
Electric or magnetic Dipole
Polariton
(Bosonic-quasiparticles)
Phonon-Polariton (IR photon + Optic phonon)
Exiciton-Polariton ( Visible light + exciton)
Intersubband Polarition (IR photon + intersubband-excition)
Surface plasmon-Polariton , SPP (Surface plasmons +light)
[1] W.L. Barnes, A.Dereux, T.W. Ebbesen, Nature 424 (2003) 824-830
MOTIVATION: THE PLASMONIC
CHIP
Clean Energy Lab (CEL) @ USC
1. Overcome diffraction limit of light (d<λ/2) using SPP
2. Merge electronics and optics together in nano scaled range
3. Important for data processing, super lensing, sensing etc.
Surface Plasmon Polariton at metal/dielectric interface
 p2
 m ( )  1  2

When m<0, K is imaginary
Surface confinement
5
SPP
CHALLENGE: Couple Collective SPP to Single particle excitations
[2] M. Dragoman, D. Dragoman, Nanoelectronics: Principles and Devices, Artech House, Boston, 2006
Clean Energy Lab (CEL) @ USC
HOW DO PLASMONICS WORK?
•SPP propagation mediated by intra band processes
•SPP detection mediated by inter band processes
Graphene
e2
2

 int ra    i
i





dE
f ( E  EF )
]E
E
2
(1  2 )
e2 (  i) 
E
 int er    i
dE
[ f ( E  EF )  f ( E  EF )]
2
2



2
E



i

  

Unlike a metal, there is significant interband conductivity even at low energies.
KEY: How to convert plasmon to e-h pair and vice versa?
-high speed computation
-new paradigm in plasmonic light sources
Clean Energy Lab (CEL) @ USC
SIC SUBSTRATE DIELECTRIC
FUNCTION
 SiC
2
 2  LO
 i1
  SiC ( )    2
2
  TO
 i 2
WLO= Longitudinal optical phonon (972cm-1)
WTO= Transversal optical phonon (796cm-1)
At high frequency  SiC ~6.5 [8]
At low frequency  SiC ~9.52
LST relation:
 (0) L2
 2
 () T
Negative dielectric function
n imaginary, damped wave gives SPP
surface confinement
SiC’s negative dielectric function in restrahlen band
 n is imaginary, damped wave
 confines SPP vertically
Role of metal and dielectric reversed.
[8] Dmitriy Korobkin, Yaroslav Urzhumov, and Gennady Shvets; J. Opt. Soc. Am. B, 23,3,468 (2006)
Clean Energy Lab (CEL) @ USC
Viability of Plasmonics in EG on SiC
TM modes are found by assuming that the electric field
has the form as..
When x>0 Ex  Beiqz Q x and Ez  Aeiqz Q x E y  0
1
1
When x<0 Ex  Deiqz Q x and Ez  Ceiqz Q x
2
2
Ey  0
Dispersion relation for TM mode is given by
1
q 
2
1 2
2

q 
2
c2
 2 2

 ( , q)i
 0
c2
Assuming we are in low q, so q<w/c, SPP
dispersion relation is.
q 
2
2
c2
[1 
1
]
 ( , q)
2
(
 2 )
 0c
Free space dispersion relation is q 
450

c
Fig: SPP dispersion relation plot with free space dispersion
SPP dispersion intersects the free space dispersion -coupling of
SPP into free space radiation- SiC substrate essential.
8
Clean Energy Lab (CEL) @ USC
Viability of Plasmonics in Epitaxial Graphene
Coupling between SPP and Single Particle Excitations
q= wave vector
= frequency

1  vF q
•Intersection between SPP and free space
•Coupling to free space
•Intersection region has to be dominated by
interband scattering
•Energy to create e-h pairs, not heat
•SPP detection
•Potential for tuning this process
•Change Ef by gating to suppress e-h
•SPP guiding.
2  0
q  2k F
2   q  2EF q  2kF
Applying single particle excitation boundary
condition for intra and inter band scattering
Comes from graphene E-k bands
9
(developed by S.Das Sarma)
Clean Energy Lab (CEL) @ USC
MODULATING EPITAXIAL GRAPHENE
PLASMON WAVEGUIDE BY DOPING
‘OFF’: When Ef is low, only
interband transitions allowed.
Can transform plasmon to DC
current
and
vice-versa.
Electrical
manipulation
of
plasmonic signals.
‘ON’: When Ef is high,
interband
transitions
not
allowed. Can propagate signal
without significant damping.
Clean Energy Lab (CEL) @ USC
Graphene
Exfoliated graphene
( single layer)
Epitaxial graphene
(single or multi layer)
Silicon (Si)
GaAs
4H-SiC
Supporting TE
mode
---
---
----
Dispersion
relation
Parabolic
parabolic
parabolic
1.42eV
<8500
3.23eV
<900
parabolic linear –EHP at
any wavelength
0
0
200000
Band gap
1.12eV
Electron Mobility <1400
Metal
(Ag)
No
Graphene
Yes [2]
(cm2/v-s)
RMS roughness
---
----
-------
~1nm
<0.5nm
SPP Detection
and guiding
materials
-----
------
--------
Metal to
guide,
Semi to
detect
Single material
for guiding and
11
detection,
[3] L A Falkovsky “Optical properties of graphene” . Phys.: Conf. Ser., Volume 129, Number 1 (2008)
[4] M.Jablan, H. buljan, M. Soljacic “Plasmonics in Graphene at infrared frequencies” Phy.ReV. B 80 245435 (2009 )
Clean Energy Lab (CEL) @ USC
Epitaxial Graphene Growth
Raman
XPS & ARPES
Graphene
6H-SiC
A
D peak (1345 cm-1)…..due to induced
disorder
B
C
G peak (1585cm-1)… due to in plane
vibration
A
C
2D peak (2670cm-1)…..due to double
resonant process
B
A
A
B
C
FiG: Realization of Graphene from 6H-SiC
ID/IG…Disorder ratio <0.2 [5]
12
[5] A.C Ferrari and J. Robertson “Interpretation of Raman spectra of disordered and amorphous carbon” Phys. Rev B 61 vol 61 num 20 (2000)
[6] P.J.Cumpson; “The Thickogram: a method for easy film thickness measurement in XPS”Surf.Interface.Anal,29,403 (2000)
NON-POLAR FACE GROWTH-6H SIC
EG on Si face
EG on C face
5µm×
5µm
Growth
mechanism is
step flow
mediated [*]
5µm×
5µm
What
happens
in
between?
[*] M. Hupalo, E. Conrad, M. C. Tringides http://arxiv.org/abs/0809.3619
[**] Appl. Phys. Lett. 96, 222103 (2010)
Growth
mechanism is
defect&step
mediated [**]
Clean Energy Lab (CEL) @
USC
13000C
13500C
Si face
A plane
M plane
C face
14000C
14500C
Clean Energy Lab (CEL) @
USC
Raman Characterization
Si
face
What would a H2 etch do?
C face
All peaks are red shifted with increasing temp.
Decreasing stress with temperature increase
2D peaks narrow with increasing temperature
Clean Energy Lab (CEL) @ USC
Surface Plasmon Polariton (SPP) in Epitaxial Graphene
Our approach
Mathematical Model [7]
Experiment:
Blank SiC is used as reference.
2
 2  LO
 i1
 2   2( )    2
2
  TO
 i 2
2
)
2
e2 (  i) 
E
 int er    i
 dE  2E 2     i 2 [ f ( E  EF )  f ( E  EF )]

(1 
e2
2
 int ra    i 
i


Fig: Schematic view of FTIR differential
reflection spectra setup
R




dE

f ( E  EF )
]E
E

 1N ( )  cos(1)

 1N ( )  cos(1)
 1 2 0 /  
 1 2 0 /  

c
n1
sin 1)]2
n2
cos 1
c
2
  1 0
2
  1 0
1  [(
16
[7] T. Stauber, N.M.R Peres, A.K. Geim; “Optical conductivity of graphene in the visible region of the spectrum”Phy.Rev. B 78 085432 (2008)
Clean Energy Lab (CEL) @ USC
Surface Plasmon Polariton (SPP) in Epitaxial Graphene….(Cont.)
Results of developed mathematical model
Fig: Variation of Fermi level
Fig: Variation of number of layer
R



 1N ( )  cos(1)

 1N ( )  cos(1)
 1 2 0 /  
 1 2 0 /  
c
c
2
  1 0
2
  1 0
Variable Parameter
Number of Layer, N
Fermi Energy Ef
Scattering time τ
17
Fig: Variation of scattering time
Clean Energy Lab (CEL) @ USC
Surface Plasmon Polariton (SPP) in EG/SiC interface
Experimental results from FTIR: Evidence of SPP at EG/SiC interface
Fig: AFM image of SiC Substrate
Fig: IR reflection of SiC Substrate with SiC as reference
LO
18
TO
Fig: AFM image of EG (2ML)on SiC
Fig: IR reflection of EG with SiC as reference
Clean Energy Lab (CEL) @ USC
EG transport properties extraction using FTIR
Extracted Parameters:
1.No of Layer N=2-17
2.Fermi Energy Ef=10535meV
3.Scattering time, τ=4-17fs
Interband broadening is assumed
constant=10meV i.e. only intraband
scattering considered.
Extracted No of layer matches well with
XPS measurements.
Fig: IR reflection measurement and mathematical
model are consistent
Clean Energy Lab (CEL) @ USC
EG transport properties extraction using FTIR
B,K. Daas…MVS et al JAP (2012)

Carrier density ns   D( E ) f ( E  EF )dE
0
D ( E )  2 E /  ( vF ) 2
Fig: Fermi level Vs No of layer
  k1(
Fig: Scattering time Vs avg. carrier density
1
) / vF
ns
1
ns
Short range scattering[9]

Coulomb scattering[9]
  ns
Mobility, µ= e vF2 / EF
Mobility (1000-10,000)
cm2/V-s
Fitting value of k1=0.6 suggests our EG is20
dominated by short-range scattering.
[9] L A Falkovsky “Optical properties of graphene” . Phys.: Conf. Ser., Volume 129, Number 1 (2008)
CORRELATION WITH ULTRAFAST
SPECTROSCOPY OF EPITAXIAL
GRAPHENE
If states are occupied by pump,
probe signal will not be absorbed,
transmission increases
85fs, ~10nJ 785nm laser, pump &probe
 Measures ENERGY relaxation time, not momentum
 τenergy>>τmomentum, supports short range scattering

THZ PROBE, OPTICAL PUMP
Non-linear power dependence, quadratic fit works
well-intervalley phonon scattering & Auger dominate
 Explains full behavior, withτrec~200fs , B~1-3cm2/s

MOLECULAR DOPING OF EG-LONG
RANGE?
Clean Energy Lab (CEL) @ USC
Mirror
Collecting
light
signal
Incoming
light
source
Sensing
element
Graphene
SiC Substrate
1.Pure N2 - inert gas
2.15ppm NO2 -electron accepting gas
3.500ppmNH3 -electron donating gas
SPP
Graphene
Fig: Experimental setup
Findings:
Reflection amplitude changes
-Looks like change of thickness
but thickness can’t change
23
Clean Energy Lab (CEL) @ USC
Conductivity Matching:
Optical Conductivity:
2
(1  2 )
e2 (  i) 
E
 int er    i
dE
[ f ( E  EF )  f ( E  EF )]
2
2



 2 E      i 
e2
2

f ( E  EF )

 int ra    i
dE
]E
i 

E


RPA approximation:

RPA
T 0
ns
n F [4rs / (2   rs )]
e2

[
 i
]
 h ni G[4rs / (2   rs )]
4ns
Fig: Dielectric function of SiC
Intraband-low f
rs 
e2
40 SiCvF
x2
G ( x) 
8
Interband high f
2

0
sin 2 
(sin

2
 x)
d
2
x2
F ( x) 
8
2

0
(1  cos  ) 2
(sin

2
 x)
d
2
Here, Γ=h/2πτintra is not taken as constant but is allowed to vary.
This is needed to get a good fit to the data
Extracted parameter ni
Interband scattering
matters even at DC.
Clean Energy Lab (CEL) @ USC
C-FACE IR REFLECTIVITY
•
•
•
•
•
Adsorbed molecules transfer
charge  charged scatterers
As ni increases, inter/intra
band scattering increase
• τ ~1/ni, i.e.
conductivity decreases
Assume each ni is an
adsorbed molecule
From ΔEf, we can extract
carriers induced, n, using
D(E)
0.01e charge donated by
each NO2 molecule
Agrees with Kelvin probe
measurements
Clean Energy Lab (CEL) @ USC
No of Gas
Layer
34
22
9
ni/ML
(cm-2)
N2
Fermi
level
(meV)
25
2x1011
Intra band Avg.
scattering
time (fs)
90-280
185
Inter band
scattering
time(fs)
27-60
NH3
30
6x1012
60-90
75
1.6-2
NO2
N2
NH3
35
45
65
2x1013
3x1011
7.5x1012
2-9
10-17
2-9
5
14
5.5
0.3-0.5
9-17
0.2-2
NO2
N2
NH3
NO2
95
70
90
120
6x1013
5.1x1011
5.5x1013
1.5x1014
0.9
10-20
0.8-1
0.4-0.5
0.9
15
0.9
0.45
0.1-0.2
3-4
0.2-0.5
0.1-0.3
CORRELATION WITH ‘DC’
MEASUREMENTS
4ppm
NO2 makes the C-face more p-type
 Implied δp~1012-13cm-2 -is this possible?

M. Qazi….MVS, Koley et al., Appl. Phys. Exp., 3, 075101 (2010)
CORRELATION WITH KELVIN
PROBE
~60% or more change in conductivity expected
Scattering from impurities not enough to explain
measured change in optical conductivity
Electron affinity of NO2 dominates!
Consistent with F.Schedin’s result of G/SiO2
 Assume ΔEf~10meV for 4ppm. μchem ill-defined.

Clean Energy Lab (CEL) @ USC
No of Gas
Layer
34
22
9
ni/ML
(cm-2)
N2
Fermi
level
(meV)
25
2x1011
Intra band Avg.
scattering
time (fs)
90-280
185
Inter band
scattering
time(fs)
27-60
NH3
30
6x1012
60-90
75
1.6-2
NO2
N2
NH3
35
45
65
2x1013
3x1011
7.5x1012
2-9
10-17
2-9
5
14
5.5
0.3-0.5
9-17
0.2-2
NO2
N2
NH3
NO2
95
70
90
120
6x1013
5.1x1011
5.5x1013
1.5x1014
0.9
100-200
0.8-1
0.4-0.5
0.9
150
0.9
0.45
0.1-0.2
3-4
0.2-0.5
0.1-0.3
From FTIR
From ΔEf, we know δp(n)
 Assume each ni is an NO2 molecule
 So, each NO2 molecule donates δp/ni ~1%e for all
thicknesses-same as SKPM!


~(ΔEf/ΔSWF)2~0.3-2%e over various samples.
ni decrease with thickness-diffusion in C-face?
 NOTE: interband broadening as large as 1eV!

REMEMBER PLASMONICS?
If interband broadening is large, even metallic
graphene plasmons will be damped, must control.
 Periodic structures enable tuning using localized
plasmons-enable conversion of plasmon to e-h pair

SUMMARY FOR PART I
Plasmonic devices possible on EG/SiC
 How clean is as-grown EG?
 Gaseous molecular doping useful for transport
studies over wide energy range near K-point.
 For FET’s, interband scattering could be
important at high carrier concentration, even at
DC. May influence realizing plasmonics.
 Will we be able to convert SPP into e-h pair in
controllable fashion?

PART II:
FUNCTIONALIZATION
ELECTROCHEMICAL
FUNCTIONALIZATION-SI FACE
RMS: 0.57nm
Scale:
8nm
Before
RMS: 1.00nm
Scale:
8nm
After
H+ attracted to graphene cathode 1V, 1hr.
 Can it react? V<1.2V, H2 formation potential
 Goal: Bandgap in diamond-like graphanes.

FUNCTIONALIZATION BY RAMAN
SPECTROSCOPY
 Single
monolayer of graphene is more reactive than bulk
graphite

Up to ten times more reactive than bi-layer and multilayer graphene
 Substrate
enhanced electron transfer
 Emergence of D-peak indicates reaction in graphene
Raman Intensity (arb. units)
1200
D-peak red-shifts 1354-1335
cm-1.
1000
G peak broadens and
slightly blue shifts ~3 cm-1
800
New peak at ~2930
600
400
200
G
D
Indicative of CHbond
Graphene
0
1200
2
GraphaneD
1600
2000
2400
-1
Wavenumber (cm )
34
2800
• R. Sharma, et. al. Anomalously Large Reactivity of Single Graphene Layers and Edges toward Electron Transfer Chemistries, Nano Letters 10, 398-405 (2010)
H-FUNCTIONALIZATION SHOWN BY RAMAN
SLOPE
 Increasing
photoluminescence
background

Increasing hydrogen content
between slope m of the
linear background and the
intensity of the G peak
Raman Intensity
 Ratio
D peak
G peak
S≈ 18µm
Wavenumber
(cm-1)
Florescence is not seen in
carbon only hydrocarbons!!!


m/I(G)
Measure of the bonded H content
 Based
on amourphous carbon
results

maybe dominated by grain
boundaries
•B. Marchon, et.al. Photoluminescence and Raman Spectroscopy in Hydrogenated Carbon Films. IEEE Transactions on Magnetics, Vol. 33, NO. 5, Sept. 1997.
FLUORESCENCE BACKGROUND TO ESTIMATE HCONTENT
Damage distinguished from functionalization by a) damage has
unmesurable slope for a given D/G ratio b) D peak position
36
SUBSTRATE DEPENDENCE
OF
FUNCTIONALIZATION
Table 1: Average Parameters From Each Substrate in Study
Substrate
D-peak
Position
Before (cm1)
D-peak
Position
After (cm1)
D/G
Ratio
Before
D/G
Ratio
After
Normalized
Slope
Before (µm)
Normalized
Slope
After(μm)
SI(1°)
1348
1330
0.21
1.91
3.66
14.4
SI2(on)
1344
1332
0.17
1.32
4.24
18.9
SI3(0.5)
1347
1331
0.13
0.6
3.93
4.42
* All substrate averages contain at least three samples
• Substrate Limited Functionalization
– Possible Causes
• Off-cut angle
• Substrate Resistivity
• Residual Damage in Graphene
 Problem: Issue with conversion control?
 Solution: Enhance reactivity with metal?
37
RAMAN
SPECTRA OF FUNCTIONALIZATION
WITH AND WITHOUT

Chemically Deposited
Platinum

H2PtCl6 · 6H2O + DI water
PT
•
NANOPARTICLES
Raman Shows:
–
–
–
–
Incredibly large D/G ratio~4.5
Emergence of Fluorescence
Addition to D’ shoulder peak
C-H peak at ~2930
38
RESULTS OF EVAPORATED METAL
CATALYSIS FUNCTIONALIZATION

Increased reactivity seen in Au and Pt enhanced conversions
 D/G ratio>1.0 for Au and Pt
 Fluorescence> Noise Threshold (5 µm)
39
SUMMARY: METAL CATALYSIS
D Position D Position ID/IG
Before
After
Ratio
(cm-1)
(cm-1)
Before
ID/IG
Ratio
After
Normalized Normalized
Slope
Slope
Before (µm) After (µm)
SI
1348
1330
0.21
1.91
3.66
14.4
SI2
1344
1332
0.17
1.32
4.24
18.9
SI3
1347
1331
0.13
0.6
3.93
4.42
SI3 Au
Avg
1342
1330
0.22
1.05
4.42
7.86
SI3 Pt
Avg
1364
1330
0.086
1.24
3.81
17.69
 Increased functionalization with metal catalyst
 Increase in fluorescence  bandgap?
40
SCANNING TUNNELING
SPECTROSCOPY
K.M. Daniels, …MVS, R. Feenstra… et.al, presented at EMC2011
accepted, JAP

Evidence of localized states
functionalized
unfunctionalized
*8x8mm
More evidence required to distinguish from damage
What are these states?
41
CYCLIC VOLTAMMETRY
Clear substrate dependence
 Qualitatively different from bulk carbon



Clear peaks, not double-layer charging
Still investigating peak assignments
SUMMARY OF PART II
Electrochemical functionalization possible.
 Evidence for hydrogen incorporation


More clarification needed
Functionalization is substrate dependent
 Metal catalysts enhance functionalization
 Evidence for localized states by STS

MASTER SUMMARY
Plasmonics in EG proposed
 IR transport studies with molecular dopants
 Electrochemical functionalization of EG
 Evidence of localized states

We also gratefully acknowledge the
Southeastern Center for EE Education for support of this work